HPV16 E6 Oncoprotein Stimulates the Transforming Growth Factor-β1 Promoter in Fibroblasts through a Specific GC-Rich Sequence

Abstract

Human papillomaviruses (HPV) have been etiologically linked to human cervical cancer. Transforming growth factor-β1 (TGF-β1) is a cytokine which is a potent growth inhibitor of most epithelial, endothelial, lymphoid, and myeloid cells, but is mitogenic for mesenchymal cells and bone cells. In this study, we analyzed the effects of HPV 16 oncoproteins E6 and E7 on the TGF-β1 promoter. The results showed that the HPV 16 E6 significantly induced (sixfold) the TGF-β1 promoter activity while HPV 16 E7 showed no significant effect. The E6 effect was cell type-specific and was observed only in the fibroblast cell lines, not in epithelial cells. Promoter analysis revealed that a 9-bp sequence, GGGGCGGGG, representing the consensus Sp1-binding site between −109 and −100 of the TGF-β1 promoter, was the major target for E6-mediated transactivation. Mutation analysis of the E6 polypeptide showed that the retention of amino acids between 123 and 136 of the HPV 16 E6 protein was critical for the transactivation of the TGF-β1 promoter. Previous studies have shown that the adenovirus 12S E1A oncoprotein represses the TGF-β1 promoter by targeting an adjacent (−90 to −81) but different GC-rich sequence (TGGGTGGGG). These studies provide evidence that variant GC-rich promoter elements are not functionally identical and are differentially regulated by the DNA virus oncoproteins.