Abstract
Colorectal cancer arises from the progressive accumulation of mutations and epigenetic alterations in colon epithelial cells. Such alterations often deregulate signaling pathways that affect the formation of colon cancer, such as the Wnt, RAS-MAPK and TGF-beta pathways. 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, the gene for thrombospondin 1, a regulator of TGF-beta ligand activation, is an epigenetic mechanism for inhibiting the TGF-beta signaling pathway. We found methylated TSP1 occurs in colon cancer cell lines (33%), colon adenomas (14%) and colon adenocarcinomas (21%). In primary colorectal cancers, loss of TSP1 expression correlated with impaired TGF-beta signaling as indicated by decreased Smad2 phosphorylation and nuclear localization. Furthermore, methylation-induced silencing of TSP1 expression reduced the concentration of secreted active TGF-beta1 and attenuated TGF-beta signaling. Reversal of TSP1 methylation resulted in increased TSP1 mediated activation of the latent LAP:TGF-beta complex and subsequent TGF-beta receptor activation. Our results demonstrate that the aberrant methylation of TSP1 has biological consequences and provide evidence that the aberrant methylation of TSP1 is a novel epigenetic mechanism for suppressing TGF-beta signaling in colorectal cancer.