Crosstalk between Insulin and Wnt Signaling Pathways

Type II diabetes and hyperinsulinemia are associated with increased risks of developing colorectal cancer (CRC). Detailed mechanisms underlying this correlation, however, are yet to be explored. The present study demonstrates that insulin increases the expression of proto-oncogenes c-Myc and cyclin D1 via both translational and transcriptional mechanisms. We show here that insulin stimulates c-Myc gene translation via an Akt/PKB-dependent mechanism involving the mTOR signaling pathway. More importantly, we show for the first time that transcriptional stimulation of c-Myc and cyclin D1 expression by insulin involves a novel Akt/PKB-independent signal crosstalk between insulin and canonical Wnt signaling pathways. We then identified p21-activated protein kianse 1 (PAK-1) as a novel mediator for insulin and Wnt/beta-catenin (b-cat) molecular crosstalk, involving MEK/ERK signaling. Furthermore, we found that insulin treatment leads to increased b-cat phosphorylation at Ser675, and this is associated with increased b-cat nuclear content and increased b-cat interaction with Tcf/Lef-binding elements (TBEs) of the human c-Myc gene promoter. Lastly, we demonstrated that insulin signaling directly alters the expression levels of components of the Wnt signaling pathway, including fizzled homology 4 (Fdz-4) and TCF7L2 (=TCF-4). This study not only demonstrated the existence of signaling crosstalk between insulin and canonical Wnt signaling pathways at multiple levels, it reveals molecular mechanisms for observed correlation between CRC and hyperinsulinemia. The growing evidence implicating PAK-1 in various human tumorigenesis has emerged PAK-1 as a potential therapeutic target. Our discovery of PAK-1 functioning as a novel central mediator for insulin and Wnt signaling crosstalk in intestinal cells suggests that PAK-1 may potentially be a good target candidate for treating patients with CRC, especially those who have Type II diabetes or experience hyperinsulinemia.Ph