Transforming growth factor-beta1 activation and transforming growth factor-beta receptor 2 expression levels in the regulation of the TGF-beta signaling pathway

Colorectal cancer arises from the progressive accumulation of mutations and epigenetic alterations in colon epithelial cells. Such alterations often deregulate signaling pathways, such as the RAS-MAPK and TGF-β pathways, which promotes tumor growth. The tumor promoting effects of mutations in genes, such as APC, have been demonstrated in cancer cell lines and in mouse models of intestinal cancer; however, the biological effects of most epigenetic events identified in colorectal cancer remain unknown. Consequently, we assessed whether the aberrant methylation of TSP1 (Thrombospondin 1), a protein that regulates TGF-β activation, is an epigenetic mechanism for inhibiting the TGF-β signaling pathway. We determined the frequency of aberrantly methylated TSP1 and its effects on TGF-β signaling in colon cancer. We observed methylated TSP1 in colon cancer cell lines (28%), colon adenomas (14%) and colon adenocarcinomas (21%). We demonstrated that methylation-induced silencing of TSP1 expression reduces the concentration of secreted active TGF-β1 and alters TGF-β signaling. Reversal of TSP1 methylation results in increased TSP1 mediated activation of the latent TGF-β:LAP complex and subsequent TGF-β receptor activation. Our results provide evidence that the aberrant methylation of TSP1 is a novel epigenetic mechanism for suppressing TGF-β signaling in colorectal cancer.