Involvement of laminin binding integrins in human intestinal epithelial cell functions
In this study we sought to determine the expression of three laminin binding integrins, previously poorly or uncharacterized, in intestinal epithelial cells: [alpha]9[bêta]1, [alpha]7[bêta]1 and [alpha]6[bêta]4. The expression and distribution of each of these integrins was characterized in the developing and mature, human small and large intestine. Complementary studies were carried out using two intestinal epithelial cell models, HIEC, which are crypt-like cells, and Caco-2/15 cells, which are villus-like. These experiments allowed us to associate each individual integrin to a particular cellular function. [alpha]9[bêta]1 was shown to be absent in mature intestinal epithelium. It was however, associated with highly proliferative cells, such as those found in the developing crypts in fetal intestine, as well as in HIEC and Caco-215 cells where, in the latter, [alpha]9[bêta]1 expression was down-regulated as the cells stopped proliferating and undertook their differentiation. This integrin was also found to be reexpressed in an onco-fetal like pattern of expression in a subset of colon cancers. [alpha]7B[bêta]1 was present in the intestine and was found to be restricted to the crypt-villus junction, which suggested a correlation with the onset of differentiation. This pattern of expression was reproduced in Caco-2/15 cells where [alpha]7B protein levels peaked between confluence and five days post-confluence coinciding almost perfectly with laminin-1 deposition, which was shown to be critical for triggering terminal differentiation in these cells. We determined that [alpha]6 associates predominantly with [bêta]4 in HIEC and Caco-2/15 cells and have identified a novel [bêta]4 variant expressed by HIEC, confirming the pattern of expression of [bêta]4 in crypt cells observed in vivo. Moreover, we have determined that the [alpha]6[bêta]4 receptor in HIEC is inactive in terms of adhesion to laminin-5, contrary to Caco-2/15 cells which use [alpha]6[bêta]4 to bind this ligand. Taken together, these results show that intestinal epithelial cells express a number of laminin binding integrins and that each is associated with a distinct cell function.