An Extract of <i>Withania somnifera</i> Attenuates Endothelin‐1‐stimulated Pigmentation in Human Epidermal Equivalents through the Interruption of PKC Activity Within Melanocytes

Abstract

Redox imbalances have been shown to be closely linked to a variety of altered cellular responses and profoundly affect intracellular signaling pathways, especially the PKC/MAPK pathway which is a major pathway involved in regulating melanogenesis within human melanocytes. To elucidate the effects of redox balance regulation on epidermal hyperpigmentary disorders, an antioxidant-rich herb extract of Withania somnifera was used to assess its effect on endothelin-1 (EDN1)-stimulated pigmentation in human epidermis equivalents and its biological mechanisms analysed. Addition of the Withania somnifera extract (10 µg/mL) elicited a marked depigmenting effect on EDN1 (10 nm)-stimulated pigmentation which was accompanied by a significant decrease in eumelanin content. Real-time RT-PCR and western blotting revealed that the stimulated expression of melanocyte-specific mRNAs and proteins, including microphthalmia associated transcription factor (MITF), was significantly suppressed at days 7-10 of culture by the Withania somnifera extract (10 µg/mL), suggesting an impairment in intracellular signaling upstream of gene expression. Signaling analysis revealed that in Withania somnifera extract (10 µg/mL)-treated human melanoma cells in culture, there was a marked deficiency in EDN1 (10 nm)-stimulated phosphorylation of Raf-1, MEK, ERK, MITF and Cyclic AMP responsive element binding protein (CREB) at 15 min after EDN1 treatment. Consistently, treatment with withaferin A, a major component of the Withania somnifera extract, at concentrations of 10-50 µm also significantly down-regulated the EDN1 stimulated phosphorylation of Raf-1, MEK, ERK, MITF and CREB at 15 min after EDN1 treatment. Since Raf-1 is phosphorylated by protein kinase C (PKC) activity, these findings indicate that the Withania somnifera extract attenuates EDN1-stimulated pigmentation by preferentially inhibiting EDN1-triggered PKC activity.