92 : STAT5 activates the C-Myc proximal promoter: Elevated STAT5 or C-Myc expression relative to STAT1 is linked with reduced survival of melanoma patients given IFNγ therapy

Abstract

Recently, a role for IFN γ in the genesis of melanoma was reported. Here, we define the mechanism whereby IFN γ becomes a growth stimulator of melanoma cells. In this study, IFN γ stimulated STAT5 to activate the expression of c- myc , an oncogene linked with reduced melanoma patient survival. In a preliminary retrospective study of 27 stage III melanoma patients who received IFN α 2 therapy, STAT5A and c- myc expression levels in metastatic lymph node biopsies were highly correlated (Pearson’s r = 0.914; P myc or STAT5 relative to STAT1 were prognostic markers indicating reduced patient survival. In vitro studies showed that STAT5 activated the c- myc proximal promoter via a functional STAT5A binding site, conferring significant inducibility by IFN γ or EGF (but not IFN α ) in luciferase reporter assays of melanoma cells overexpressing STAT5A. IFN γ caused markedly different responses in wild-type A375 cells compared with A375-STAT5A cells, supporting a key role for STAT5A in IFN γ -mediated regulation of c- myc expression in melanoma. ChIP assays confirmed direct STAT5A binding during IFN γ -mediated activation of the c- myc proximal SBE regulatory region. 5′ RACE and Q-PCR showed that increased levels of c- myc in A375-STAT5A cells induced by IFN γ were associated with a shift in the initiation of transcription near to or at the proximal STAT5 binding element. These results indicate that overexpression of STAT5 in cancer cells is responsible for growth-stimulation by IFN γ .