Oral communication, CS18 / C84

Official XXIst International Pigment Cell Conference website - 21-24 Sept 2011, Bordeaux - France | updated: September 04 2011

Involvement of TGF-beta signaling and the GLI2 transcription factor in M-MITF regulation and pigmentation in melanoma cells

The melanocyte-specific transcription factor M-MITF is involved in numerous aspects of melanoblast lineage biology including pigmentation, survival, and migration. It also plays complex roles at all stages of melanoma progression and metastasis. We have previously shown that expression of M-MITF in human melanoma cell lines is highly variable and inversely correlated with that of GLI2, a Hedgehog mediator identified as a TGF-beta gene target. In this study, we demonstrated that GLI2 overexpression in highly pigmented melanoma cells reduced the expression of M-MITF and that of its gene targets TYR, TYRP1 and DCT, accompanied with a decrease of melanin synthesis. Transient cell transfection experiments with 5’-end M-MITF promoter deletion constructs identified the -317/-280 region as critical for GLI2-driven transcriptional repression. Chromatin immunoprecipitation (ChIP), identified GLI2 bound to the -395/-228 region of the M-MITF promoter. Since GLI2 is a transcriptional target of TGF-beta pathway, we investigated whether the TGF-beta signaling can repress M-MITF expression in melanoma cells. We showed that M-MITF expression was decreased by TGF-beta treatment independently from proteasomal degradation. Inversely, the TbRI (TGF-beta Receptor I) inhibitor SB431542 increased M-MITF expression. TGF-beta induced a strong inhibition of M-MITF promoter activity. ChIP experiments indicated that TGF-beta induces GLI2 binding to the -395/-228 region of the M-MITF promoter in melanoma cells. Yet, TGF-beta still repressed M-MITF promoter activity after GLI2 knockdown, suggesting that other mechanisms are involved to drive TGF-beta effect. Remarkably, TGF-beta inhibited both CREB phosphorylation and PKA activity in melanoma cells and mutation of the cAMP Responsive Element (CRE) located at position -147/-140 of the M-MITF promoter abolished the inhibitory effect exerted by TGF-beta. Taken together, these results suggest that TGF-beta represses M-MITF expression and transcription by two distinct and concomitant means implicating GLI2 induction and inhibition of the cAMP/PKA pathway.

Advertisement from our sponsor: