Abstract
Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that strongly influence molecular signalling in normal and cancer cells. Although increasing evidence suggests a role of PPARs in skin carcinogenesis, only expression of PPARgamma has been investigated in human melanoma tissues. Activation of PPARalpha has been shown to inhibit the metastatic potential, whereas stimulation of PPARgamma decreased melanoma cell proliferation. We show here that the third member of the PPAR family, PPARbeta/delta is expressed in human melanoma samples. Specific pharmacological activation of PPARbeta using GW0742 or GW501516 in low concentrations inhibits proliferation of human and murine melanoma cells. Inhibition of proliferation is accompanied by decreased expression of the Wilms' tumour suppressor 1 (WT1), which is implicated in melanoma proliferation. We demonstrate that PPARbeta directly represses WT1 as (1) PPARbeta activation represses WT1 promoter activity; (2) in chromatin immunoprecipitation and electrophoretic mobility shift assays, we identified a binding element for PPARbeta in the WT1 promoter; (3) deletion of this binding element abolishes repression by PPARbeta and (4) the WT1 downstream molecules nestin and zyxin are down-regulated upon PPARbeta activation. Our findings elucidate a novel mechanism of signalling by ligands of PPARbeta, which leads to suppression of melanoma cell growth through direct repression of WT1.