Integrin expression in colon cancer cells is regulated by the cytoplasmic domain of the β6 integrin subunit

摘要

We have previously reported that the αvβ6 integrin upregulates its own expression in a protein kinase C-dependent manner with increasing cell density. The wild-type β6 integrin subunit has also been shown to promote tumour growth in vivo and its growth-enhancing effect is regulated by both a MAP kinase binding motif on β6 and the 11 amino acid C-terminal cytoplasmic extension unique to the β6 subunit. Herein, we show that the 11 amino acid cytoplasmic extension is essential for the cell density-dependent increase in β6 expression and that the 11 amino acid tail exerts a dominant negative effect on cell density- and PKC-mediated β5 expression in αvβ6-expressing colon cancer cells. Cells that express β6 lacking the 11 amino acid tail respond to PKC simulation with increased expression of only the β5 subunit as seen for cells that lack constitutive αvβ6 expression. In contrast, loss of the ERK binding site on β6 markedly impairs cell density- and PKC-dependent expression of either β6 or β5 in the presence or absence of the 11 amino acid tail, respectively. Our findings suggest that in αvβ-expressing cells, a hierarchy of kinase signalling cascades exists and that the β6-ERK2 interaction dominates over PKC-mediated signalling pathways responsible for integrin upregulation with cell confluence. Given the dominance of the β6-ERK2 interaction over PKC-mediated expression of both β5 and β6 integrin subunits, targeting the β6-ERK2 interaction may prove useful as an anticancer strategy in colon cancer.