Integrin α6A splice variant regulates proliferation and the Wnt/β-catenin pathway in human colorectal cancer cells

The integrin alpha 6 subunit pre-messenger RNA undergoes alternative splicing to generate two different splice variants, named alpha 6A and alpha 6B, having distinct cytoplasmic domains. In the human colonic gland, these splice variants display different patterns of expression suggesting specific functions for each variant. We have previously found an up-regulation of the alpha 6 beta 4 integrin in colon adenocarcinomas as well as an increase in the alpha 6A/alpha 6B ratio, but little is known about the involvement of alpha 6A beta 4 versus alpha 6B beta 4 in this context. The aim of this study was to elucidate the function of the alpha 6A beta 4 integrin in human colorectal cancer (CRC) cells. Expression studies on a panel of primary CRCs confirmed that the up-regulation of the alpha 6 subunit in CRC is a direct consequence of the increase of the alpha 6A variant. To investigate the functional significance of an alpha 6A up-regulation in CRC, we specifically knocked down its expression in well-established CRC cell lines using a small-hairpin RNA approach. Results showed a growth rate reduction in all alpha 6A knockdown CRC cell lines studied. The alpha 6A silencing was also found to be associated with a significant repression of a number of Wnt/beta-catenin pathway end points. Moreover, it was accompanied by a reduction in the capacity of these cells to develop tumours in xenografts. Taken together, these results demonstrate that the alpha 6A variant is a pro-proliferative form of the alpha 6 integrin subunit in CRC cells and appears to mediate its effects through the Wnt/beta-catenin pathway.