Integrin α5/β1 Expression Mediates HER-2 Down-regulation in Colon Cancer Cells

HER-2 is constitutively activated and overexpressed in many cancers, and its inhibition in colon cancer cells diminishes tumorigenicity and induces apoptosis. Little is known about the regulation of HER-2 signaling in colon cancer cells. Integrin α5/β1 expression is frequently lost in colorectal cancer cells compared with normal intestinal epithelium, and colon cancer cells lacking integrin α5/β1 expression utilize HER-2 signaling for proliferation and tumorigenicity. Re-expression of integrin α5/β1 in colon cancer cells abrogated their tumorigenicity, but how this occurs is not well known. Stable expression of integrin α5/β1 in colon cancer cells with little or no detectable integrin α5/β1 protein expression resulted in the post-transcriptional down-regulation of HER-2 protein. Integrin α5/β1 was found to interact with HER-2, and the cytoplasmic domain of integrin α5/β1 was sufficient to mediate HER-2 down-regulation. Integrin α5/β1-mediated down-regulation of HER-2 was the result of increased lysosomal targeting. The inhibition of HER-2 signaling represents a potential mechanism by which integrin α5/β1 exerts its tumor suppressor-like activity in colon cancer cells. These results also suggest that a novel function for integrin α5/β1 is the control of HER-2 expression.