cosmetics
Review
Licorice (Glycyrrhiza glabra, G. uralensis, and G. inflata)
and Their Constituents as Active Cosmeceutical Ingredients
Antonietta Cerulli, Milena Masullo, Paola Montoro
and Sonia Piacente *
Dipartimento di Farmacia, Università degli Studi di Salerno, Via Giovanni Paolo II n. 132, 84084 Fisciano, Italy;
acerulli@unisa.it (A.C.); mmasullo@unisa.it (M.M.); pmontoro@unisa.it (P.M.)
* Correspondence: piacente@unisa.it
Citation: Cerulli, A.; Masullo, M.;
Abstract: The interest in plant extracts and natural compounds in cosmetic formulations is growing.
Natural products may significantly improve cosmetics performance since they have both cosmetic
and therapeutic-like properties, known as cosmeceutical effects. Glycyrrhiza genus, belonging to the
Leguminosae family, comprises more than 30 species, widely distributed worldwide. The rhizomes
and roots are the most important medicinal parts currently used in pharmaceutical industries and
in the production of functional foods and food supplements. In the last few years, the interest in
their potential activities in cosmetic formulations has greatly increased. Glycyrrhiza spp. extracts are
widely implemented in cosmetic products for their good whitening effect. The biological effects of
Glycyrrhiza extracts are especially ascribable to the occurrence of specialized metabolites belonging to
the flavonoid class. This review focuses on the botany and the chemistry of the main investigated
Glycyrrhiza spp. (G. glabra, G. uralensis, and G. inflata) along with their cosmeceutical activities
categorized as skin anti-aging, photoprotective, hair care, and anti-acne. It has been highlighted
how, along with Glycyrrhiza extracts, three main flavonoids namely licochalcone A, glabridin, and
dehydroglyasperin C are the most investigated compounds. It is noteworthy that other molecules
from licorice show potential cosmeceutical effects. These data suggest further investigations to clarify
their potential value for cosmetic industries.
Montoro, P.; Piacente, S. Licorice
(Glycyrrhiza glabra, G. uralensis, and
G. inflata) and Their Constituents as
Keywords: Glycyrrhiza; licorice; cosmeceutical; glabridin; skin anti-aging; photoprotective activity;
hair care; anti-acne activity
Active Cosmeceutical Ingredients.
Cosmetics 2022, 9, 7. https://doi.org/
10.3390/cosmetics9010007
Academic Editor: Piera Di Martino
Received: 29 November 2021
Accepted: 31 December 2021
Published: 5 January 2022
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4.0/).
1. Introduction
Plant extracts and natural compounds derived from plants are considered valuable
materials for the production of cosmetics. Natural products contained in cosmetic formulations may significantly contribute to an improvement of cosmetics performance. They can
also be used as auxiliary substances enhancing the stability or bioavailability of cosmetic
formulations [1]. Plants were the primary source of cosmetics before synthetic compounds,
and now the trend of the cosmetic industry is to look more and more for natural active ingredients. This is due both to consumers’ demand for more natural products and the global
attention for environmentally friendly products. This means that new products containing
herbs will continue to emerge on the market in the future [2]. The term “cosmeceutical”
refers to cosmetics containing active chemicals with drug-like properties. Cosmeceuticals
have beneficial local effects, prevent degenerative skin diseases, and improve skin tone.
Cosmeceuticals are a growing sector of the personal care industry [2].
Glycyrrhiza genus, belonging to the Leguminosae family (also known as Fabaceae),
consists of more than 30 species, widely distributed worldwide. The name “glycyrrhiza”
derives from the Grecian words glykys and rhiza, which mean sweet and root, respectively [3]. It is also called licorice, liquorice, glycyrrhiza, sweet wood, and Liquiritiae
radix [4]. Among the Glycyrrhiza spp., G. glabra L., G. uralensis Fisch., and G. inflata Bat.
are the most investigated species with nutritional and pharmacological benefits, used as
Cosmetics 2022, 9, 7. https://doi.org/10.3390/cosmetics9010007
https://www.mdpi.com/journal/cosmetics
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Radix Glycyrrhizae (licorice) [5]. They are also recorded in Chinese Pharmacopoeia as
medicinal Glycyrrhiza plants [6]. Rhizomes and roots are the most important medicinal
parts of licorice. They have been reported to be used alone or with other herbs to treat
many digestive system disorders, respiratory tract disorders, epilepsy, fever, sexual debility, paralysis, rheumatism, leucorrhoea, psoriasis, prostate cancer, malaria, hemorrhagic
diseases, and jaundice [3]. The extracts are currently used in pharmaceutical industries and
in the manufacture of functional foods and food supplements [4,7,8]. Moreover, they can
be used as a food and beverage flavoring agent [3]. Japan has a wide range of applications
for the licorice chemical constituents, which occur 70% in food products (glycyrrhizin), 26%
in medicinal cosmetics (glabridin), and 4% in tobacco, along with other uses [9].
Researchers focused their attention on exploring mainly G. glabra, G. uralensis, and G.
inflata extracts and isolated pure compounds occurring as active ingredients for cosmetic
purposes, based on their biological activities. Glycyrrhiza spp. extracts are currently used in
cosmetic preparations due to their skin-whitening, anti-sensitizing, and anti-inflammatory
properties [10]. Their use was widely implemented in commercial products, especially in
cosmetic products, for its good whitening effect [11].
Several formulations containing licorice extracts are present in the market places. They
are mostly used daily with SPF (Sun Protection Factor) products containing G. glabra root
extract. The extract is incorporated in the internal aqueous phase of water/oleum emulsion
in cream and serum formulations, claimed for their anti-aging activities, for their effects
on wrinkles, on hyperpigmentation, and as skin lightening. For these effects, the licorice
extracts are also used in the formulation of sunscreens and also for personal care products
such as facial cleansers, make-up removers, toners, and shampoo. Moreover, make-up products such as foundations, concealers, around-the-eye creams, make-up primers, lipsticks,
and BB creams contain licorice extract.
Reviews reported in the literature described the traditional uses, the chemistry, the
chemotaxonomy, the pharmacological activities and the analysis of licorice extracts focusing
mainly on G. glabra [3–6,11–13]. Herein, we describe briefly the botany and the chemistry
of G. glabra, G. inflata, and G. uralensis focusing on the skin anti-aging, photoprotective,
hair care, and anti-acne activities of extracts and bioactive compounds isolated from these
species, never summarized before. In light of all the reports, the potential of licorice extracts
and their specialized metabolites as constituents of cosmeceutical formulations is confirmed
to be very promising.
2. Botanical Description
Glycyrrhiza spp. are herbaceous plants growing in the subtropical and temperate zone.
The plants can reach a maximum height of up to 2 m, while the underground stem can
grow up to 2 m horizontally, generally in fertile and sandy ground [12]. The plants show
pinnate leaves, narrow flowers, lavender to violet in color. The fruit is an oblong legume
containing three-eight brown reniform seeds. The roots are well developed with a brown
color. The pieces of roots break with a fibrous fracture and possess a typical aroma and
a sweet taste [12,14]. The rhizomes and roots are harvested 3–4 years after the planting,
washed to remove buds and rootlets, cut into small pieces, and finally dried [15]. G. glabra,
G. inflata, and G. uralensis are significantly explored for nutritional and pharmacological
benefits among the known species. Three varieties of G. glabra have been reported, grown
in different regions and designated as G. glabra var. violacea (Persian and Turkish), G. glabra
var. gladulifera (Russian), and G. glabra var. typica (Spanish and Italian) [12].
3. Chemistry of Glycyrrhiza
The 50% dry weight of licorice roots is due to water-soluble metabolites and sugars
(5–15% glucose, sucrose, and mannitol), starch (25–30%), glycyrrhizin (10–16%), amines
(1–2% asparagine, betaine, and choline), and sterols (stigmasterol and β-sitosterol) [12].
Thus far, more than 400 phytochemicals have been isolated from the genus Glycyrrhiza.
These molecules can be classified as saponins, flavonoids, chromenes, coumarins, dihy-
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drostilbenes, coumestans, benzofurans, and dihydrophenanthrenes [4,5,12], among which
flavonoids and triterpenoid saponins are abundant in the root or rhizome of licorice [5].
Generally, alkaloids and tannins were not detected [12]. Although the roots represent
the most used parts, phytochemical investigations were also performed on the leaves,
considered an agrochemical waste. These studies demonstrated that certain compounds
present in the roots are also identified in the leaves of G. glabra leaves [16]. This section
will discuss the flavonoids and triterpenoid saponins isolated in G. glabra, G. inflata, and
G. uralensis, the three species investigated for their cosmetic effects, focusing on the main
reported compounds and on compounds tested for the biological activities.
Along with these classes, a coumarin-derivative reported in G. glabra, and G. uralensis,
licoarylcoumarin, will also be discussed for its properties.
3.1. Flavonoids
More than 300 compounds belonging to the class of flavonoids have been isolated and
identified from licorice [4,11]. Flavonoids, generally formed by two benzene rings (A ring
and B ring) through a central tri-carbon chain-generating C ring, are divided into flavonols,
flavones, flavanones, flavanols, dihydro-flavones, chalcones, isoflavones, according to the
occurrence of the C ring, its oxidation degree, and the connection site of the B ring. Several
kinds of flavonoids are representative compounds isolated from G. glabra, G. uralensis, and
G. inflata. The more representative flavonoids, tested for their cosmeceutical activities, have
been categorized as flavanones, flavonols, flavones, isoflavanes, isoflavenes, isoflavones,
and chalcones.
Liquiritin is one of the most abundant flavonoids and is used as a quantitative chemical
marker in the three official medicinal licorice species in Chinese Pharmacopoeia [5]. It consists of liquiritigenin, a flavanone reported in Glycyrrhiza, linked to a β-D-glucopyranosyl
residue at position 4’ via a glycosidic linkage. Pinocembrin and liquiritin apioside, belonging to the flavanone class, have also been reported (Figure 1).
Flavanols such as kaempferol, pratensein, and the flavone chrysoeriol are herein
discussed for their activity.
Glabridin is the principal isoflavane identified, ranging between 0.08% and 0.35% of
G. glabra roots dry weight [4,17]. Chemically, glabridin is a prenylated isoflavane deemed a
typical compound in G. glabra, accounting for 11% of its total flavonoid content [11]. Along
with glabridin, licoricidin (also known as licorisoflavan B), hispaglabridin A, glyasperin
C, glyasperin D, and 3′ -hydroxy-4′ -O-methylglabridin are mentioned. Isoflavenes as
glabrene, dehydroglyasperin C, dehydroglyasperin D, isoflavones as glycyrrhisoflavone,
semilicoisoflavone B, allolicoisoflavone B, isoangustone A, and formononetin, as well as
isoflavanones as dihydrodaidzein and glycyrrhisoflavanone were isolated from Glycyrrhiza
spp. and tested for cosmeceutical properties. All the reported isoflavonoids, except
formononetin, dihydrodaidzein, and pratensein are characterized by a prenyl moiety on
ring A or ring B, which can be free or involved in the formation of a pyran ring. In the case
of glabridin, the prenyl chain at C-8 is involved in the formation of a pyran ring, but this
latter can be fused either to the A ring or B ring of the isoflavonoid skeleton. Hispaglabridin
A is characterized by a prenyl group cyclized to pyrene on the A ring and an additional
prenyl function at the B ring. The isoprenyl groups on the A and B rings make the flavonoid
backbone more lipophilic, resulting in increased affinity with cell membrane structures and
favorable biological activities (Figure 1) [11].
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OH
HO
O
O
O
HO
OH
O
HO
OH O
HO
OH
O
O
allolicoisoflavone B
O
OCH3
dibenzoylmethane
HO
O
O
OCH3
O
O
neolicuraside HOH C
2
HO
CH2OH
licochalcone B
HO
O
OH
O
OH
OH
glyasperin D
O
OH
HO
OH
licuraside
HO
OH
H3CO
licochalcone C
OH
hispaglabridin A
HO
CH2OH
OH
OH
O
O
O
O
OH O
isoliquiritigenin
HO
OH
OH
OCH3
O
OCH3
O
OH
OH
OH
O
OH
OH
OH
isoliquiritin
3'-hydroxy-4'-O-methylglabridin
OH O
OH O
H3CO
glabridin
O
OH O
OCH3
O
O
semilicoisoflavone B
OH
HO
HO
O
glabrene
O
glycyrrhisoflavanone
O
O
OH
OH
glyasperin C
OH
O
HO
O
HO
OCH3
O
OH
HO
O
H3CO
OH
HO
O
OH
HO
OH
licoarylcoumarin
O
HO
OCH3
formononetin
O
OH
licoricidin
O
OCH3
isoangustone A
OH
O
OH
OH O
dihydrodaidzein
HO
OH O
dehydroglyasperin D
OH
O
pinocembrin
OH
OCH3
HO
O
OH
OH
O
OH
dehydroglyasperin C
HO
O
H3CO
OH
OCH3
O
OH O
O
OH O
O
OH
glycyrrhisoflavone
OH
HO
pretensein
kaempferol
OH O
HO
OH
HOH2C OH
O
OH
O
O
HO
HO
O
chrysoeriol
OH
liquiritin apioside
OH
O
O
O
liquiritin
OCH3
OH
HO
OH
O
CH2OH
O
O
HO
O
liquiritigenin
HO
CH2OH
O
O
HO
O
O
O
OH
HOH2C OH
CH2OH
OH
HO
OH
O
OH
OCH3
licochalcone A
HO
OH
O
OH
OCH3
licochalcone D
HO
OH
O
OCH3
licochalcone E
Figure 1. Flavonoid derivatives from Glycyrrhiza spp. as active ingredients for cosmetic formulations.
Chalcones are frequently reported in licorice roots; in this review the activities of
isoliquiritigenin, isoliquiritin, licochalcone A, licochalcone B, licochalcone C, licochalcone
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D, licochalcone E, licuraside, and neolicuroside are discussed. The dibenzoylmethane, a
structural analogue of curcumin (diferuloylmethane) isolated from G. glabra, has also been
investigated for its cosmetic properties.
′
′ ′
Among the flavonoids previously
cited, the main representative compounds
of Gly′
′
′
cyrrhiza are liquiritin (4 ,7-dihydroxyl flavone) and isoliquiritin (2 ,4 ,4-trihydroxyl chalcone)
glycosides (Figure 1).
3.2. Saponins
More than 70 saponins were isolated from Glycyrrhiza roots, and their structures have
been shown in a recent review [13].
Among the oleanane triterpenoid saponins, glycyrrhizinic acid (also known as glyβ
cyrrhizic acid) or its salt glycyrrhizin has been reported as the major secondary metabolite found in the root of Glycyrrhiza spp.; this monodesmosidic
saponin exhibits a 18ββ
glycyrrhetic acid skeletal structure, derived from β-amyrin, linked to a disaccharide unit
made up of two glucuronic acid moieties at position C-3 [18]. Glycyrrhizin along with its
aglycon, glycyrrhetic acid, are the most studied and abundant
compounds
α
β from the roots of
this plant. Glycyrrhetic acid exists as two isomers: 18α-form and 18β-form. As a sweetener,
glycyrrhizin is reported to be 30–50 times sweeter than sucrose [18]. Glycyrrhiza saponins,
composed of aglycone and sugar moiety, can be classified into several classes, in which
glucuronic acid, glucose, rhamnose are the major characteristic parts of sugar moiety [19].
Licorice saponin G2 (also known as 24-hydroxyglycyrrhizin) is shown below (Figure 2).
COOH
COOH
O
O
HOOC
HO
O
O
O
OH
HO
HOOC
O
OH
COOH
OH
glycyrrhizinic acid
HO
O
O
O
OH
HO
OH
O
OH
COOH
OH
licorice saponin G2
Figure 2. Triterpenoid derivatives from Glycyrrhiza spp. as active ingredients for cosmetic formulations.
3.3. Polysaccharides
Among the bioactive ingredients of Glycyrrhiza plants, Glycyrrhiza polysaccharides
are receiving more and more attention. A recent review reports their isolation, structural
characterization, and biological activities [6]. They are heteropolysaccharides mainly
composed of arabinose, glucose, galactose, rhamnose, mannose, xylose, and galacturonic
acid in different proportions and types of glycosidic bonds. A preliminary study on
the moisture retention of polysaccharides highlighted that their water retention ability
was higher than that of glycerol solution, suggesting their potential use as a cosmetic
moisturizing additive [6].
3.4. Species-Specific Markers for G. glabra, G. inflata, and G. uralensis
Several analytical methods have been developed to discriminate the chemical differences of G. glabra, G. inflata, and G. uralensis based on the occurrence or amount of
specialized metabolites [5,20–23]. These works could provide helpful information to the
industry about the choice of Glycyrrhiza species to use. An investigation was performed
by a combined approach using GC–MS, LC–MS, and 1D NMR analysis. Compounds
responsible for the discrimination among the three species were identified: glycyrrhizin,
4-hydroxyphenyl acetic acid, and glycosidic conjugates of liquiritigenin or isoliquiritigenin
along with the amino acid cadaverine were described only in G. inflata [24]. The three
species, identified by DNA barcodes, were further analyzed by LC/UV- or LC/MS/MSbased quantitative analysis, revealing 151 bioactive secondary metabolites, of which 27
were discovered able to differentiate the three species [20]. Principal Component Analy-
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sis (PCA) performed on 1 H NMR spectra, and UHPLC-UV chromatograms highlighted
marked chemical differences among the Glycyrrhiza spp. [25]. An NMR-based metabolomics
analysis, followed by PCA was also performed [26]. The metabolites licochalcone A and
glabridin, which are discussed more in the next paragraphs, were indicated as specific
metabolites of G. inflata, and G. glabra, respectively [20,25]. Indeed, G. uralensis and G.
inflata roots did not contain glabridin, and therefore, glabridin is considered a unique
species-specific marker for G. glabra [12]. Licochalcone A, abundant in G. inflata but present
in small amounts in G. uralensis and G. glabra, along with the occurrence of licochalcone C,
licochalcone E, and licochalcone D, represent a marker for G. inflata. So far, glycycoumarin,
a metabolite reported in higher amounts in G. uralensis, in small amounts in G. inflata, and
in trace amounts in G. glabra, is considered a species-specific metabolite for G. uralensis [12].
A detailed report on the species-specific metabolite markers in different licorice species
was recently published [12]. In light of all these reports, the metabolomic characterization
should be associated with the identification and quantitation of several key markers rather
than the only quantitative analysis of a single ubiquitous Glycyrrhiza constituent.
Different extraction methods have been developed for saponins or flavonoids from
Glycyrrhiza species. They include maceration, countercurrent extraction, supercritical
fluid extraction, extraction by ultrasonics, Soxhlet extraction, and microwave assisted
extraction [5,27]. In this review, most of the cosmeceutical effects described afterward
are attributed to the flavonoid constituents of Glycyrrhiza. Literature data revealed how
methanol and ethanol aqueous solutions were the most commonly used solvents for the
extraction of flavonoids [28]. Indeed a mixture of ethanol/water (30:70, v/v) used for
an extraction time of 60 min under 50 ◦ C of licorice gave a high recovery of glabridin
(72.5%) [28]. To enrich the licorice fractions in licochalcone A, methods including highspeed countercurrent chromatography and treatment by macroporous resin were used [29].
4. Skin Anti-Aging
The thinning epidermal layer and the loss of collagen and elastic fiber lead to wrinkle
formation and cause aging. Aging occurs due to intrinsic factors like genetics, cellular
metabolism, hormone, and metabolic processes, or extrinsic factors like sun exposure,
smoking, diet, and pollution [30]. Many people have chosen natural herbs rather than plastic surgery or laser therapy to look younger and reduce complications in the last few years.
Plants supply nutrients required for healthy skin, helping the biological functioning of the
skin. In addition, phytochemicals derived from plants showed skin beneficial properties
related to UV protection, anti-oxidant action, matrix protection, and skin hydration [31,32].
4.1. Anti-Tyrosinase Activity and Hyperpigmentation Diseases
Glycyrrhiza extracts and their compounds have shown beneficial effects to improve
skin pigmentation. Melanin, synthesized in melanocyte cells by the melanogenesis process,
is responsible for the color of the skin. Different factors play a role in the production and
expression of melanin in the skin, such as exposure to UV radiation, genetic predisposition,
melanocyte size leading to a difference in the amount of melanin produced per cell, as well
as several diseases, including albinism, a genetic inability to produce melanin, and vitiligo, a
progressive loss of melanocytes [33]. Alteration in melanin could bring hyperpigmentation
or hypopigmentation; in particular, a low quantity of melanin can cause local vitiligo and
post-traumatic hypopigmentation. Abnormal amounts of melanin deposits in specific sites
of the skin cause abnormal skin-colored patches like solar lentigos, chloasma, freckles, and
post-inflammatory hyperpigmentation [34,35].
Although different mechanisms are involved in the melanogenesis process, key enzymes responsible for melanin biosynthesis are the polyphenolic oxidase tyrosinases.
Melanogenesis is the physiological process of melanin formation in which tyrosinase, a
copper-dependent enzyme, initiates the first step [36]. In detail, melanogenesis is directly
regulated by three enzymes: tyrosinase, tyrosinase-related protein TRP-1, and TRP-2. Tyrosinase catalyzes the conversion of L-tyrosine to L-DOPA and then to dopachrome, which
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is subsequently polymerized spontaneously to melanin via a series of reactions [37]; consequently, tyrosinase is responsible for skin hyperpigmentation. In the last few years, this
enzyme became an essential target for skin-whitening effects and therapeutic interventions
associated with melanin hyperpigmentation [38]. Thus, molecules acting as tyrosinase
inhibitors play an important role in cosmetic products as skin-whitening agents and in
the treatment of various dermatological disorders. Flavonoids are the most representative
class among natural phenolic compounds acting as tyrosinase inhibitors [35]. Numerous
plants, among which are Glycyrrhiza spp. and natural compounds, have been reported for
tyrosinase inhibitory activity, and they are used in the treatment of skin pigmentation [34]
(Figure 3). Among Glycyrrhiza spp., the most investigated species is represented by G.
glabra. The extract of G. glabra roots is reported for its strong anti-melanogenic activity,
tested by reduction of intracellular tyrosinase and melanin content in B16F10 melanoma
cells. Methanol and ethyl-acetate extracts of licorice roots exhibited significant activity with
low IC50 values (2.1 and 4.7 µg/mL) [38]. Glycyrrhiza glabra extract was also compared to
kojic acid, a molecule currently used as a tyrosinase inhibitor, commercially available. The
extract inhibited tyrosinase activity by 78.45% while kojic acid inhibited it by 99.67%. Unfortunately, kojic acid has the disadvantage of being unstable during storage; G. glabra did not
show any disadvantages during tyrosinase inhibition, suggesting a possible use of licorice
extract in cosmetic formulations [10]. Along with the effects shown by the licorice extracts,
secondary metabolites isolated from G. glabra leaves and roots showed anti-melanogenesis
activity. Flavonoids occurring in Glycyrrhiza are reported for their anti-tyrosinase activity.
Figure 3. Skin depigmentation effect of Glycyrrhiza root extracts and their constituents by inhibition
of tyrosinase enzyme.
Glabridin showed anti melanogenesis activity due to its tyrosinase inhibitory activity.
The structure–activity relationship study highlighted how the hydroxyl groups at 2 and
4 positions seem responsible for the activity. Glabridin inhibited tyrosinase activity in
cultured B16 murine melanoma cells at 0.1 to 1.0 µg/mL, without affecting DNA synthesis [39]. It quenched the intrinsic fluorescence of tyrosinase mainly through a static
quenching procedure, suggesting a generation of a stable glabridin-tyrosinase complex.
Molecular docking calculations were performed to establish the interaction of glabridin
with the tyrosinase enzyme. The results indicated that glabridin did not directly bind to
the active site of tyrosinase [40]. To increase the glabridin water solubility, Hespeler et al.
reported the use of smartPearls technology, aimed at improving dissolution velocity in the
formulations. Glabridin smartPearls displayed a promising perspective if compared to
glabridin raw drug powder, for creating skin products with improved dermal bioavailability [41]. All the features of glabridin smartPearls make it promising for skincare products
with improved glabridin efficacy by simultaneously reducing production costs [41].
Other active compounds, such as glabrene, isoliquiritigenin, licuraside, isoliquiritin,
and licochalcone A, isolated from licorice extracts, were also shown to inhibit tyrosinase
activity [39]. In particular, glabrene and isoliquiritigenin inhibited both mono- and diphenolase tyrosinase activities. The IC50 values for glabrene and isoliquiritigenin were 3.5
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and 8.1 µM, respectively, when tyrosine was used as substrate. The effects of glabrene and
isoliquiritigenin on tyrosinase activity were dose-dependent and correlated to their ability
to inhibit melanin formation in melanocytes [42].
Pinocembrin, the main compound in G. glabra leaves, is reported to have a moderate
inhibitory effect on mushroom tyrosinase [43].
Lin and coworkers reported semilicoisoflavone B, allolicoisoflavone B, and glabridin
for their noticeable tyrosinase inhibitory activities with IC50 of 0.25, 0.80, 0.10 µM, respectively [44]. Successively, Liu et al. developed a method using tyrosinase immobilized
magnetic fishing coupled with high performance liquid chromatography-diode array
detector-tandem mass spectrometry (IMF-HPLC–DAD–MS/MS) to screen and identify
tyrosinase binders from G. uralensis root without isolation of secondary metabolites by the
complex extract. Secondary metabolites of G. uralensis root such as liquiritin apioside, neolicuroside, liquiritigenin, licorice saponin G2, chrysoeriol, dihydrodaidzein, formononetin,
glycyrrhisoflavanone, glycyrrhizinic acid, licoarylcoumarin, and pratensein showed the
capacity to inhibit tyrosinase activity [45].
Literature reports how dehydroglyasperin C could be considered a whitening ingredient against hyperpigmentation in the skin. Dehydroglyasperin C decreased in a
dose-dependent manner intracellular tyrosinase activity and expression of proteins related
to melanin synthesis (TYR and TRP-1) in keratinocytes treated with α-MSH (melanocyte
stimulating hormone) to induce melanogenesis [46]. A series of licochalcones, consisting of
licochalcone A, B, C, and E, normally isolated from the roots of G. inflata, showed tyrosine
phosphatase 1B (PTP1B) inhibitory activities [11].
Multiple signaling pathways involved in melanogenesis were extensively described
by Maddaleno et al., specifically in the regulation of the microphthalmia-associated transcription factor (MITF) [47]. MITF is a basic helix-loop-helix leucine zipper that regulates
the expression of melanogenic enzymes (tyrosinase, TYRP1, and TYRP2) and melanosome
structural proteins (MART-1 and PMEL17) [5].
Dehydroglyasperin C also reduced the downregulation of MITF (melanocyte-specific
transcription factor) through suppression of cAMP-CREB pathway. Phosphorylation of
extracellular signal-regulated kinase (ERK) also decreased MITF by dehydroglyasperin C
treatment [46].
Licochalcone A inhibited melanogenesis through MAPK/ERK pathway by activating
ERK. The MAP kinase family also regulates melanogenesis; phosphorylated p38 can activate microphthalmia-associated transcription factor (MITF), promoting melanin synthesis,
whereas phosphorylated ERK can inhibit the activation of MITF.
A formulation of liquiritin cream (20% of liquiritin) applied at 1 g/day for 4 weeks
showed therapeutical effectiveness in melasma disease. However, this study suggests
that liquiritin probably did not affect tyrosinase, which caused depigmentation by other
mechanisms [48].
4.2. Skin Lightening Activity
Disorders of hyperpigmentation, including post inflammatory hyperpigmentation,
skin problems such as freckles, age spots, acne scars, discoloration related to hormones,
and skin exposure to sunlight, could induce skin pigmentation disorders. These pigmentary skin disorders, such as melasma, can have a consequent psychosocial impact. Skin
lightening creams represent the products that work on skin by reducing melanin. Skin lightening products are also known as whiteners for naturally dark skin and skin brighteners
(Figure 4) [49].
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Figure 4. Prevention of photoaging by Glycyrrhiza root extracts and their constituents.
Ishi et al. reported different types of oil in water (O/W) herbal creams using herbal
extracts such as Curcuma longa, with excellent potential for anti-aging, and G. glabra, known
for the therapeutic effects in skin whitening, plus stearic acid and cetyl alcohol, and other
excipients. The evaluation of formulations highlighted how they were safe to be used for
the skin and could be used as skin lightening and anti-oxidant agents [50].
At the same time, other researchers evaluated the skin lightening capacity of G. glabra
root extracts in cream preparation. In particular, Kirubakaran et al. highlighted the skinlightening properties of cream prepared using G. glabra root extracts and G. indica bark
extracts and physical sun protecting agents such as titanium dioxide. Synergism between
selected extracts generated the melanin inhibition effect through the cellular melanin
inhibition pathway. Consequently, the preparation mentioned above could be used for
skin-whitening for better skin aesthetics [33].
A herbal face cream, in which G. glabra was combined with other herbal extracts,
showed multipurpose effects such as whitening, antiwrinkle, anti-aging, and sunscreen
effect, due to a synergistic effect between all the extracts [51]. In addition to the extracts, the skin-whitening properties were also reported for the bioactive compounds
glycyrrhisoflavone, kaempferol, glyasperin C, and glyasperin D [52,53].
4.3. Antiwrinkle Activity
Among the most frequent phenotypic manifestations of intrinsic and extrinsic aging
is the onset of wrinkles at different depth levels, due to the progressive loss of structural
integrity and physiological function of the skin [54]. The inevitable intrinsic skin aging is
due to physiological aging characterized by the decline of collagen, elastin, and hyaluronic
acid levels, leading to a loss of strength and flexibility in the skin, which results in visible
wrinkles associated with the thickened epidermis, mottled discoloration, laxity, dullness,
and roughness of the skin. Extrinsic skin aging is due to diverse determinants such as sun
exposure, external pollutants, smoking, and diet [55].
Exposure of human skin to ROS (reactive oxygen species) through several factors,
including UV, has been reported to enhance matrix metalloproteinases (MMPs) activity
associated with a notable breakdown of collagen fibers. MMPs, in particular gelatinases
(MMP-2 and -9) that cleave soluble type-IV and type-I collagen, represent the major enzymes responsible for the degradation of the extracellular matrix which contains diverse
biomolecules including collagen and gelatin (Figure 4) [56].
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Ryu et al. reported how 1,3-butylene glycol extract of G. uralensis reduced ROS
production by inhibition of MMP-2 activity along with the consequent increase of collagen
production. Furthermore, the results suggested the use of G. uralensis extract as a cosmetic
ingredient with antiwrinkle and anti-oxidant effects [55]. Antiwrinkle activity by antioxidant mechanism has been proved for G. glabra extract at the dose of 150 mg/kg/day [57].
Moreover, Ciganovic et al. highlighted for G. glabra extract, obtained by a green ultrasoundassisted extraction method using glycerol/water mixtures, a good anti-oxidant activity,
tyrosinase, and elastase inhibitory activity as well as anti-inflammatory activity, leading to
excellent anti-aging properties [58].
Prenylflavonoids dehydroglyasperin C, dehydroglyasperin D, and isoangustone A
showed a superoxide scavenger activity as a mechanism to prevent wrinkles [59].
Moreover, eicosanyl caffeate and docosyl caffeate, two long-chain caffeoyl esters
isolated by ethyl acetate extract of G. glabra roots displayed by a spectrophotometric assay
a potent elastase inhibitory activity, an additional target to prevent aging and wrinkles
formation [60], with IC50 values of 0.99 µg/mL and 1.4 µg/mL, respectively [61].
5. Photoprotective Activity
Ultraviolet (UV) irradiation causes several areas of damage to the skin. Together with
immune suppression, cancer, tanning, and sunburn, it provokes injuries called photoaging,
consisting of connective tissue degradation [62] (Figure 4). UV-B rays are the most dangerous, producing physiological responses connected with oxidative stress, resulting in cell
death at high dosage. On the other hand, minor UV-B irradiation induces oxidative stress
and activates intracellular signal transduction pathways. Several anti-oxidants extracted
from plants are involved in reduced incidence of photocarcinogenesis and photoaging,
and, for these reasons, the relative extracts can be considered for their skin photoprotective
effects [63].
5.1. Anti-Photoaging Effects
Photoaging is the macroscopic and microscopic modification caused by persistent
sun exposure. Most effectors involved in skin photoaging are pro-inflammatory cytokines,
ROS, and effector molecules like MMP-1. Their generation is controlled by NF-kappa B,
produced due to UV exposition (Figure 4) [64].
Afnan et al. in 2012 evaluated the effect of glycyrrhizinic acid on UV-B photoaging
induced by irradiation with a sub-toxic dose of UV-B (10 mj/cm2 ) of human dermal
fibroblasts (HDFs) and its possible mechanism of action. The involvement of glycyrrhizinic
acid on cell viability, matrix metalloproteinase 1 (MMP1), pro-collagen 1, cellular and
nuclear morphology, cell cycle, intracellular ROS, caspase 3, and hyaluronidase inhibition
assays was evaluated. The principal mechanism appeared to be connected with the block
of MMP1 activation by modulating NF-kB signaling [65].
Based on the involvement of MMP in photoaging, a study published in 2017 by
Xuan et al. assessed the anti-photoaging effects of dehydroglyasperin C on MMPs levels in
HaCaT human keratinocytes and tried to elucidate the biological mechanism. Dehydroglyasperin C noticeably repressed UV-B-mediated expression of collagenase (MMP-1) and
gelatinase (MMP-9) by inhibiting ROS generation. Dehydroglyasperin C treatment also
decreased the UV-B irradiation-mediated activation of mitogen-activated protein kinase
(MAPK), c-Jun phosphorylation, and c-fos expression. In addition, the down-regulation
of UV-B-induced c-Jun phosphorylation caused by dehydroglyasperin C treatment was
more intense than the down-regulation of c-fos expression. In conclusion, it appeared that
dehydroglyasperin C may work as a potential anti-photoaging agent by inhibiting UV-Bmediated MMPs expression via suppression of MAPK and AP-1 signaling (Figure 4) [66].
Other phytochemicals extracted from Glycyrrhiza spp. appeared to be involved against
photoaging caused by UV-B. Puri and coworkers in 2017 published a paper on the development of microemulsions of dibenzoylmethane for the treatment of UV-induced photoaging [67]. Dibenzoylmethane exerted sunscreen activity, preventing the damage caused by
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UV rays. It acted as a UV-A screen that prevented the penetration of the UV radiation
in the vital cells and blocked the overproduction of ROS. The authors evaluated in vivo
photoprotection in a mice model of UV-radiation-induced photodamage [67].
Another compound evaluated for its contribution to reduce photoaging was licoricidin.
Its effects were assessed on photoaging of human dermal fibroblasts (HDFs) submitted
to irradiation with UV-A. Licoricidin blocked UV-A-induced photoaging acting as ROS
scavenger. This activity is connected with the modulation of MMP-1 [68].
5.2. Photoprotective Effect against UV-B and Visible Radiation
In addition to the UV protection, Mann et al. in 2020 investigated ROS production
induced by visible radiation and the mechanism of photoprotection of licochalcone A.
The mechanism appeared to involve the stimulation of Nrf2/ARE signaling pathway, as
preliminarily presented in a previous study [69,70]. Randomized clinical trials were carried
out to assess the anti-irritative potential of formulation with licochalcone A on UV-induced
erythema formation. The formulation caused a highly noteworthy reduction in UV-induced
erythema tests, resulting in a powerful inhibition of pro-inflammatory in vitro reactions,
including UV-B-induced PGE2 release by keratinocytes [71]. The activity of licochalcone A
on UV-B-induced erythema was also tested and confirmed on patients with rosacea and
red facial skin, for whom skin tolerance, efficacy, and quality of life were evaluated [72].
Melatonin (N-acetyl-5-methoxytryptamine) is synthesized and secreted by the pineal
gland in vertebrates. The occurrence of melatonin in roots of G. uralensis and the response
of this plant to different light (red, blue, and white) and UV-B irradiation (280–315 nm)
for the synthesis of melatonin were investigated. Production of melatonin in G. uralensis
plants is connected with protection against oxidative damage initiated as a response to UV
irradiation [73].
In a more specific cosmetic approach, a moisturizing cream (oil in water)-based formulation containing an extract of Beta-vulgaris (1%) and an extract of G. glabra (1%) was
developed to provide a UV-A/UV-B protective moisture to be used for post-laser therapy.
In addition, this cream facilitated re-pigmentation by stimulating melanocytic proliferation
and removing stubborn scars and wrinkles [74].
5.3. Anti-Oxidant Effects
Anti-oxidant activity of the functional ingredients in cosmetic products is of great
importance. Functional cosmeceutical ingredients with anti-oxidant activity may have a
more active role in such products [31]. They also offer protection against oxidative damage
of skin macromolecules associated with the effects of free radicals and UV radiation on
the skin [58,75]. The anti-oxidant activity of G. glabra is one of the reasons for its uses
in cosmetics and generally is connected with other activities like photoprotection. The
phenolic content is probably responsible for the observed anti-oxidant activity attributed to
flavonoids, isoflavones, methylated isoflavones, and chalcones [76,77].
The anti-oxidant potential of glabridin was reviewed in a specific paper on the potential
of glabridin and its biological properties [17].
Licochalcones B and D showed a strong scavenging activity in the DPPH assay and
the ability to inhibit microsomal lipid peroxidation. These phenolic compounds appeared
to be effective in protecting biological systems against oxidative stress, being able to inhibit
skin damage [78,79].
The anti-oxidant potential of licorice to be used for preserving cosmetic formulations
was evaluated. The extract was tested for anti-oxidative activity in comparison with
antioxidants (sodium metabisulfite and BHT) at 0.1%, 0.5%, 1.0%, and 2.0% wt./wt. in
a cream formulation with 2% wt./wt. of hydroquinone. The results suggested the use
of licorice extracts at 0.5 and 1.0% as an effective natural anti-oxidant able to preserve
formulations that are susceptible to oxidation [80].
On the other hand, it appears that not only phenolic compounds are involved in
the anti-oxidant effect of licorice extracts. In addition saponins from licorice showed
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anti-oxidant activity, suggesting their possible use in body wash cosmetic products [81].
Polysaccharides isolated from three varieties of G. glabra also exhibited anti-oxidant activity [82]. Due to their good anti-oxidant properties, licorice polysaccharides are suggested
as an additive to delay skin aging and prevent the formation of chloasma in cosmetics [6].
Sometimes Glycyrrhiza extracts were used in polyherbal cosmetic formulations, and
the anti-oxidant effects were generally improved for a synergistic effect [83].
6. Hair Care
Hair, a part of the body connected with physical appeal, is recognized as a health
indicator. Hair treatments and cosmetic products for hair care are continuously under
research. The treatment of hair and scalp mainly involves the use of shampoo for cleansing;
the shampoo is considered not only a cosmetic product having a purifying purpose, but
also a formulation responsible for maintaining the health and beauty of the hair. Herbal
shampoos can be used functionally, and among the different herbal extracts to be used for
this purpose, an interesting position is occupied by licorice extracts as reported in a recent
review [84].
6.1. Hair Growth
Licorice extracts in hair care formulation present an interesting activity to promote hair
growth. A recent investigation evaluated the safety, stability, and hair growth activity of an
ethanol extract of licorice (G. glabra). The hair tonic solutions containing this extract showed
hair growth activity similar to that of the positive control (minoxidil), good physical and
chemical stability, and safe topical use [85].
The beneficial effects of an oriental herbal supplement containing G. uralensis in
addition to Glycine max, and Thuja orientalis were assessed on women’s hair numbers,
hair diameter, scalp moisture and sebum, and scalp conditions, finding a real benefit in
improving hair and scalp conditions [68]. In addition, other formulations containing licorice
showed beneficial effects as a remedy for hair fall [86,87].
The effect on the promotion of hair growth was previously confirmed by a study
on cells, for a mixture composed of extracts of G. uralensis, Angelica gigas, Acoruscalamus,
Cnidium officinale, Panax ginseng, Camellia sinensis, Salvia miltiorrhiza, Zanthoxylum schinifolium, Carthamustinctorius, Prunus persica, and Scrophularia buergeriana. The study was
performed in human hair dermal papilla cells and C57BL/6J cells of mice. The mixture
significantly increased the proliferation of human hair dermal papilla cells in a dose- and
time-dependent manner [88].
6.2. General Hair Care and Dandruff
In the review published in 2020 by Shivakant, the functional effects on scalp care were
reported for a scalp tonic containing licorice [84]. Dandruff is a common scalp problem
connected with flaky and inflamed skin. In a clinical trial 102 subjects (male 56 and 46
female) with moderate to very strong dandruff affliction used a combination of piroctone,
olamine, and licochalcone A. In this study, a cytokine analysis was performed, and the
results proved a significant decrease in pro-inflammatory dandruff markers after treatment
with the tested products. Moreover, the anti-fungal activity of test products was detected,
revealing a significant reduction of Malassezia colony-forming units after treatment with the
anti-dandruff shampoo. The benefit exerted by the combination was primarily based on
the known anti-inflammatory effect of licochalcone A [89]. In a recent investigation, silver
nanoparticles containing G. glabra extract showed antimicrobial effects against dandruff
caused by pathogens. Protein leakage analysis revealed that this formulation disturbed the
solidity of pathogens’ membrane [90].
7. Anti-Acne Potential
Acne vulgaris is a widespread skin syndrome, which is also a chronic inflammatory
disease of pilosebaceous unit that includes the increase of sebum production by sebaceous
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glands and abnormal desquamation of hair follicles that happens in reaction to increasing
of androgen levels with the beginning of puberty. Natural remedies are often more tolerable
and related to minor side effects than synthetic ones [91,92].
Anti-Acne Activity
G. glabra seems to be an interesting remedy against acne. The anti-acne therapeutic
effects of oriental herb extracts among which G. glabra were investigated in terms of
antichemotactic effects on polymorphonuclear leucocytes, antilipogenic, and antibacterial
effects against Propionibacterium acnes. G. glabra showed remarkable antibacterial activity
against P. acnes with a negligible induction of resistance compared to marked development
of resistance in bacteria treated with erythromycin [93].
In an overview of the plants used for skin diseases, the anti-acne effects of G. glabra
were reviewed additionally to the activity of the plant extracts on atopic dermatitis [94].
The anti-acne activity of licorice can be the result of multifactorial effects. The antiacne activity was connected with moisturizing action for several herbal extracts including
G. glabra [95], but the most probable mechanism is the antimicrobial action against acne
bacteria [96,97], although the anti-androgenic activity was proposed as a mechanism of
action connected with the anti-acne final effect [98]. To clarify the plural activities of licorice
on dermatological disorders, a pharmacological study on mice was conducted [99]. In this
study licorice significantly increased epidermal thicknesses as compared to control animals.
The volume of the sebaceous gland and the thickness were significantly increased in the
disease model compared to the control animals and resulted reduced by licorice extract.
It is possible to find several papers reporting poli-herbal formulations in literature,
including Glycyrrhiza with synergistic anti-acne activity. The most recent was proposed
in 2020 by Keshri and Khare [91], but other synergistic formulations were proposed before [96,97,100–103].
Few studies are reported in the literature about the molecules involved in this activity.
The anti-acne activity of licochocalcone A resulted in efficient suppression of the NLPR3
inflammasome [104]. Activation of the nucleotide-binding domain, leucine-rich-containing
family, pyrin domain-containing-3 (NLRP-3) inflammasome by P. acnes is a critical point
for inducing inflammation and aggravating the development of acne lesions [104].
Controlled clinical studies on botanical extracts used in dermatology were reviewed
in 2010 by Reuter and coworkers [94], focusing on clinical trials with botanicals in treating
acne, inflammatory skin diseases, skin infections, UV-induced skin damage, skin cancer,
alopecia, vitiligo, and wounds. In acne therapy, Glycyrrhiza may have the potential to
become a standard treatment [94].
8. Conclusions
In the last years, there has been a growing interest in using extracts and natural compounds from plants instead of synthetic compounds in the cosmetic field. The application
of Glycyrrhiza extracts and natural compounds from licorice, mainly flavonoid compounds,
for their skin anti-aging, photoprotective activity, hair care, and anti-acne activity, is more
and more diffused.
Root extracts are mainly used in cosmetics for the whitening effects. A commercial
formulation containing glabridin is claimed to have a 1000 times stronger whitening effect
than vitamin C. Due to this property, glabridin is known as “whitening gold” and is quite
popular as a whitening ingredient in internationally standard cosmetics [12].
This review has shown a wide array of activities of Glycyrrhiza extracts and their
constituents potentially valuable for cosmetic and dermatologic products.
Glycyrrhiza extracts and flavonoid compounds from licorice exert their whitening
effect as inhibitors of tyrosinase, the central enzymatic system of melanogenesis, become
one of the most important targets for the control of hyperpigmentary disorders (Figure 3).
This review confirmed that despite the diversity of natural inhibitors, many tyrosinase
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inhibitors belong to the phenolic class [35], thus along with the most cited glabridin, other
flavonoids from licorice may contribute to skin depigmentation.
Interestingly, these molecules are mainly represented by isoflavonoids characterized
by the occurrence of prenyl moieties on A ring or B ring. Along with the effects on skin
depigmentation, licorice extract-based formulations may be of value in innovative dermal and
cosmetic products as they counteract oxidative stress damage, maintaining the skin homeostasis
due to their high antioxidant content. In this review it is evident that several investigations were
carried out on G. glabra and G. uralensis, but very few reports account for G. inflata. Therefore,
based on the chemistry of this latter characterized by the presence not only of licochalcone A,
further researches have to be performed to assess its cosmeceutical value.
As evident from literature, licochalcone A, glabridin, and dehydroglyasperin C are
licorice most investigated flavonoids, suggesting further development and application in
cosmetic industries in the future (Figure 5). Other constituents have also shown cosmeceutical properties but further investigations have to be performed to meet industrial purposes.
The first goal could be the selection of Glycyrrhiza species taking into account the occurrence
of the metabolites of interest. Additionally, the choice of the extraction and purification
methods is crucial to obtain a higher amount of the selected constituents. Regarding this
topic, the research is increasingly oriented towards greener alternatives, many of which are
still to be investigated.
Figure 5. Cosmeceutical effects of the most studied bioactive compounds from Glycyrrhiza spp.
A recent review describes the toxicity effects of licorice and glycyrrhizin in acute, subacute, sub-chronic, and chronic states, highlighting their moderate toxicity and the need to
be used with caution during pregnancy. However, their toxicity was mainly evaluated after
oral administration and intraperitoneal, subcutaneous, intravenous, and intramuscular
injection [105]. Therefore, additional investigations should be carried out to assess the
toxicity of topical administration.
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Author Contributions: A.C., M.M., P.M. and S.P. have contributed equally to the elaboration of this
work. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: This work does not present any associated data.
Conflicts of Interest: The authors declare no conflict of interest.
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