Abstract
During the early stages of colorectal carcinogenesis, the phosphatidylinositol 3-kinase (PI3K)/Akt pathway is activated, enabling the transformed cells to survive and grow in the absence of anchorage to extracellular matrix. Transforming growth factor beta (TGF-beta) is an important tumor suppressor in the colon, and it is inactivated during later stages of colorectal carcinogenesis. The purpose of this study was to determine whether TGF-beta inhibits Akt-induced anchorage-independent growth and resistance to anoikis in gut epithelial cells. Rat intestinal epithelial cells (RIE-1) were infected with a retrovirus containing pLXSN-mAkt, and three independent clones were selected. Anchorage-independent growth was examined by colony formation in soft agar and cell counting in ultralow attachment plates. Anoikis was analyzed with the use of Annexin V staining. All three clones of RIE-1/mAkt formed colonies in soft agar, which were decreased by TGF-beta. TGF-beta induced anoikis and treatment with a general caspase inhibitor, zVAD-fluoromethyl ketone, blocked TGF-beta-mediated decrease in colony formation. TGF-beta attenuated Akt-induced anchorage-independent growth in RIE-1 cells in part by enhancing anoikis. Our data demonstrate a novel tumor-suppressor activity of TGF-beta and provide the molecular justification for the required activation of the PI3K/Akt pathway and the subsequent inactivation of TGF-beta signaling during colorectal carcinogenesis.
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