Loss of heterozygosity (LOH) analysis and follow up studies of sporadic colon cancer enabled us to show that SMAD4 is the primary target tumor suppressor, localized to the minimally lost region at chromosome 18q21-linked to an advanced stage disease. Subsequent studies by others have confirmed these initial observations and have established that the frequency of SMAD4 mutations/deletions increases as the cancer progresses from adenomas to the metastatic disease. Overall, these findings are consistent with loss of Smad4 function in metastatic colon cancer. In an effort to delineate a molecular basis for an association between SMAD4 deficiency and metastatic colon cancer, we have begun to use appropriately engineered model systems. Our preliminary studies showed that the SMAD4 defect was responsible for an increase in the levels of VEGF, overactivation of MEK-Erk and p38-MAPK auxiliary pathways, enhanced migration of colon cancer cells with a corresponding increase in MMP9, overexpression of GLUT1 under hypoxia, increased aerobic glycolysis and resistance to 5-FU-mediated apoptosis. We also found that overexpression of Smad4 in the model colon cancer cells with SMAD4 deficiency inhibited VEGF reporter activity and Smad4 physically interacts with specific transcription factors (TFs) such as HIF1? to potentially regulat metastatic progression of colon cancer. While the molecular characterizations are consistent with the notion that Smad4 may play a central role in forming a colon cancer metastasis suppressor complex consisting of various transcription factors and cofactors to collectively block colon cancer metastasis, direct targeting for loss of Smad4 function could only account for approximately 30% of tumors that harbor LOH at 18q21. Therefore, metastatic colon cancers that retained intact Smad4 could also progress through inactivation of the other components of the complex that suppresses the metastatic program. In this proposal, we will test the hypothesis that metastatic colon cancers which retain Smad4 exhibit alterations in the other components of the colon cancer metastasis suppressor complex. Thus, we predict that by unraveling the composition of the colon cancer metastasis suppressor complex, one would uncover novel prognostic biomarkers that are alternatively targeted for inactivation in metastatic colon cancer. Here, we outline a strategy using the model cell lines to isolate and characterize the components of the colon cancer metastasis suppressor complex consisting of Smad4 to (1) investigate the nature of the higher order protein complexes that are assembled and dissolved under the conditions of intact and defective Smad4 signaling using proteomic analysis to identify TFs and co-factors as candidate prognostic biomarkers;(2) examine if dysregulation of these other factors of the metastasis suppressor complex disrupts its functionality;and (3) determine if defects/deficiency in the alternate targets of Smad4 metastasis suppressor complex could serve as prognostic biomarkers for colon cancer. In summary, the proposed studies may unravel novel prognostic biomarkers for metastatic colon cancer and it could aid the development of personalized therapy.
Several studies have indicated that consistent loss of heterozygosity (LOH) at chromosome 18q21 is associated with defects in the SMAD4 gene and it is an indicator of poor prognosis and therapeutic resistance in patients with advanced stage colon cancers. While our recent preliminary studies using colon cancer model cell lines is consistent with these findings and suggest that Smad4 imposes an overall metastasis suppressive effect, the molecular basis for colon cancer metastasis in tumors that retained intact Smad4 remain elusive. Here, we propose to identify the components of the Smad4 colon cancer metastasis suppressor complex as alternate targets for inactivation and as novel prognostic biomarkers for colon cancer metastasis in tumors that retain intact Smad4.
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