Transforming growth factor-ß suppresses metastasis in a subset of human colon carcinoma cells.

BACKGROUND: TGFß signaling has typically been associated with suppression of tumor initiation while the role it plays in metastasis is generally associated with progression of malignancy. However, we present evidence here for an anti-metastatic role of TGFß signaling. METHODS: To test the importance of TGFß signaling to cell survival and metastasis we compared human colon carcinoma cell lines that are either non-tumorigenic with TGFß response (FET), or tumorigenic with TGFß response (FETα) or tumorigenic with abrogated TGFß response via introduction of dominant negative TGFßRII (FETα/DN) and their ability to metastasize. Metastatic competency was assessed by orthotopic transplantation. Metastatic colony formation was assessed histologically and by imaging. RESULTS: Abrogation of TGFß signaling through introduction of a dominant negative TGFß receptor II (TGFßRII) in non-metastatic FETα human colon cancer cells permits metastasis to distal organs, but importantly does not reduce invasive behavior at the primary site. Loss of TGFß signaling in FETα-DN cells generated enhanced cell survival capabilities in response to cellular stress in vitro. We show that enhanced cellular survival is associated with increased AKT phosphorylation and cytoplasmic expression of inhibitor of apoptosis (IAP) family members (survivin and XIAP) that elicit a cytoprotective effect through inhibition of caspases in response to stress. To confirm that TGFß signaling is a metastasis suppressor, we rescued TGFß signaling in CBS metastatic colon cancer cells that had lost TGFß receptor expression due to epigenetic repression. Restoration of TGFß signaling resulted in the inhibition of metastatic colony formation in distal organs by these cells. These results indicate that TGFß signaling has an important role in the suppression of metastatic potential in tumors that have already progressed to the stage of an invasive carcinoma. CONCLUSIONS: The observations presented here indicate a metastasis suppressor role for TGFß signaling in human colon cancer cells. This raises the concern that therapies targeting inhibition of TGFß signaling may be imprudent in some patient populations with residual TGFß tumor suppressor activity.

Knockdown of Ron kinase inhibits mutant phosphatidylinositol 3-kinase and reduces metastasis in human colon carcinoma.

Abnormal accumulation and activation of receptor tyrosine kinase Ron (recepteur d'origine nantais) has been demonstrated in a variety of primary human cancers. We show that RNA interference-mediated knockdown of Ron kinase in a highly tumorigenic colon cancer cell line led to reduced proliferation as compared with the control cells. Decreased Ron expression sensitized HCT116 cells to growth factor deprivation stress-induced apoptosis as reflected by increased DNA fragmentation and caspase 3 activation. In addition, cell motility was decreased in Ron knockdown cells as measured by wound healing assays and transwell assays. HCT116 cells are heterozygous for gain of function mutant PIK3CA H1047R. Analysis of signaling proteins that are affected by Ron knockdown revealed that phosphatidylinositol 3-kinase (PI3K) activity of the mutant PI3K as well as AKT phosphorylation was substantially reduced in the Ron knockdown cells compared with the control cells. Moreover, we demonstrated in vivo that knockdown of Ron expression significantly reduced lung metastasis as compared with the control cells in the orthotopic models. In summary, our results demonstrate that Ron plays an essential role in maintaining malignant phenotypes of colon cancer cells through regulating mutant PI3K activity. Therefore, targeting Ron kinase could be a potential strategy for colon cancer treatment, especially in patients bearing gain of function mutant PI3K activity.