Induction of Melanogenesis by Rapamycin in Human MNT-1 Melanoma Cells

Abstract

BackgroundMelanogenesis is one of the characteristic parameters of differentiation in melanocytes and melanoma cells. Specific inhibitors of phosphatidylinositol 3-kinase (PI3K), such as wortmannin and LY294002, stimulate melanin production in mouse and in human melanoma cells, suggesting that PI3K and mammalian target of rapamycin (mTOR) might be involved in the regulation of melanogenesis.ObjectiveThe involvement of the mTOR pathway in regulating melanogenesis was examined using human MNT-1 melanoma cells, and the effects of the potent inhibitor of mTOR, rapamycin, in the presence or absence of α-melanocyte-stimulating hormone (α-MSH) were evaluated.MethodsIn cells treated with rapamycin, cell viability, melanin content, and tyrosinase (TYR) activity were measured and compared with untreated controls. Protein levels of TYR, tyrosinase-related protein (TYRP)-1, TYRP-2, and microphthalmia-associated transcription factor (MITF) were also analyzed by Western blot.ResultsIn rapamycin-treated cells, the melanin content increased concomitantly with an elevation in TYR activity, which plays a major role in melanogenesis. There was also an up-regulation of TYR, TYRP-1, and MITF proteins. Combined treatment with rapamycin or wortmannin and α-MSH increased melanogenesis more strongly than α-MSH alone.ConclusionRapamycin-induced melanin formation may be mediated through the up-regulation of TYR protein and activity. Furthermore, rapamycin and wortmannin, inhibitors of mTOR and PI3K, respectively, have co-stimulatory effects with α-MSH in enhancing melanogenesis in melanocyte cells.

Autophagy induction can regulate skin pigmentation by causing melanosome degradation in keratinocytes and melanocytes

“…Our results and those of previous studies suggest that the role of mTOR in autophagy‐mediated melanocyte regulation depends on the melanocyte cell type (Hah et al, ; Katsuyama et al, ). Topical rapamycin, a representative mTOR inhibitor, is reported to substantially improve the appearance of hypopigmented macules in patients with tuberous sclerosis, which presents with distinct mTOR activation (Wataya‐Kaneda et al, ).…”

“…The effect of rapamycin on melanogenesis is equivocal, with variable results depending on the melanocyte cell types (Hah et al, ; Katsuyama, Taira, Yoshioka, Okano, & Masaki, ). To account for the contrasting results regarding melanogenesis, we treated both MNT‐1 cells and primary human melanocytes with rapamycin.…”

Rag GTPases mediate amino acid–dependent recruitment of TFEB and MITF to lysosomes

“…This study also reveals a new and novel role of mTORC1 in the regulation of MITF-1. Two previous studies found that inhibition of mTORC1 leads to melanosome formation and expression of melanogenic genes in MNT-1 melanoma cells, probably through up-regulation of the levels of MITF-4 (Ho et al, 2011; Hah et al, 2012). However, our study is the first one to show that mTORC1 promotes sequestration of this transcription factor in the cytosol.…”

ARP101 inhibits α‐MSH‐stimulated melanogenesis by regulation of autophagy in melanocytes

“…Additionally, treatment with an autophagy activator such as rapamycin resulted in reduced melanin contents in skin-derived NHEKs [12]. Hah et al showed that rapamycin promotes melanogenesis by the up-regulation of tyrosinase protein in melanoma cells [32]. Thus, we further addressed the effect of autophagy on melanogenesis in melanocytes by using ARP101.…”