The effect of glycyrrhizic acid and aqueous licorice extract on hair growth retardation : in vitro mechanistic and efficacy studies in human
Authors:   , Kiratipaiboon Chayanin 
Published: November 2023

The growth rate of hair is tightly associated with the functions of dermal papilla cells (DPCs), stem cells component of hair follicle. The present study has revealed for the first time that glycyrrhizic acid (GA), the primary active ingredient of licorice or Glycyrrhiza glabra L., and aqueous G. glabra L. extract (GE) significantly reduced the stem cell-related pathways of DPCs. Stem cell features including clonogenic growth and stem cell markers in DPCs were found to be dramatically decreased in response to GA and GE treatments. Regarding molecular mechanism, GA and GE were shown to suppress ATP-dependent tyrosine kinase/glycogen synthase kinase3 beta-dependent (Akt/GSK3 beta) leading to b-catenin down-regulation. Besides, the down-stream stem cell transcription factors of Wnt/beta-catenin pathway, namely, Oct-4, Nanog and Sox2 were significantly down-regulated in the GA-treated and GE-treated cells.  In addition, GA and GE were shown to depress epithelial-mesenchymal transition (EMT) in DPCs as the transcription factors ZEB1, Snail and Slug were markly decreased. The effect of GA and GE on the decline of stem cell features were also exhibited in the primary DPCs directly isolated from human hair follicles. TLC-densitometric method demonstrated GA content of GA and GE samples with value of 80.49% and 11.42%, respectively. The final test formulation for in vivo studies of 15% w/w GA gel was prepared by using propylene glycol and 2% w/w carbomer 940 whereas 15% w/w GE gel was prepared utilizing water : propylene glycol (1:1 w/w) and 1% w/w carbomer 940 as solvent and gelling agent, respectively. Stability testing showed no changes of clarity, color, pH value and viscosity after subjected the gels to 6 cycles of heating-cooling cycle and there was no significant change of GA content in the gels from time zero to 30 days at 30 °C. Furthermore, the gels showed no short term and long term irritation effect on 22 volunteers. After treatment with 15% w/w GA and 15% w/w GE gels, the results indicated that 15% w/w GA and 15% w/w GE gels significantly decreased underarm hair growth in respect of area, thickness, length and number after 14, 21 and 28 days compared with its gel base. Besides, 15% w/w GA gel showed superior hair growth retardation effect than 15% w/w GE gel after 28 days. In order to reduce cost of production, 7.5% w/w GA and 10% w/w GE gel were prepared and tested in the present study. Likewise, the results demonstrated that 7.5% w/w GA and 10% w/w GE gels showed no short term and long term irritation effects (n=22) and significantly decreased underarm hair growth in respect of area, thickness, length and number after 14, 21 and 28 days compared with its gel base. Moreover, hair growth retardation effect of 7.5% w/w GA and 10% w/w GE gels were found to be comparable with 15% w/w GA and 15% w/w GE gel, respectively. Therefore, these findings unveiled a novel molecular mechanism regulating DPCs’ function of GA and GE as well as provided the scientific information supporting the use of these compounds for suppressing the growth of unwanted hair.