Integrin α2β1 mediates isoform-specific activation of p38 and upregulation of collagen gene transcription by a mechanism involving the α2 cytoplasmic tail

Two collagen receptors, integrins alpha 1 beta 1 and alpha 2 beta 1,can regulate distinct functions in cells. Ligation of alpha 1 beta 1, unlike alpha 2 beta 1, has been shown to result in recruitment of Shc and activation of the Ras/ERK pathway. To identify the downstream signaling molecules activated by alpha 2 beta 1 integrin, we have overexpressed wild-type alpha 2, or chimeric alpha 2 subunit with alpha 1 integrin cytoplasmic domain in human osteosarcoma cells (Saos-2) lacking endogenous alpha 2 beta 1. The chimeric alpha 2/alpha 1 chain formed a functional heterodimer with pi. In contrast to alpha 2/alpha 1 chimera, forced expression of alpha 2 integrin resulted in upregulation of alpha 1 (I) collagen gene transcription in response to three-dimensional collagen, indicating that the cytoplasmic domain of alpha 2 integrin was required for signaling. Furthermore, signals mediated by alpha 2 beta 1 integrin specifically activated the p38 alpha isoform, and selective p38 inhibitors blocked upregulation of collagen gene transcription. Dominant negative mutants of Cdc42, MKK3, and MKK4 prevented alpha 2 beta 1 integrin-mediated activation of p38 alpha. RhoA had also some inhibitory effect, whereas dominant negative Rac was not effective. Our findings show the isoform-specific activation of p38 by alpha 2 beta 1 integrin ligation and identify Cdc42, MKK3, and MKK4 as possible downstream effecters. These observations reveal a novel signaling mechanism of alpha 2 beta 1 integrin that is distinct from ones previously described for other integrins.