Ambra Pozzi

Integrin alpha1beta1 is a collagen receptor abundantly expressed on microvascular endothelial cells. As well as being the only collagen receptor able to activate the Ras/Shc/mitogen-activated protein kinase pathway promoting fibroblast cell proliferation, it also acts to inhibit collagen and metalloproteinase (MMP) synthesis. We have observed that in integrin alpha1-null mice synthesis of MMP7 and MMP9 was markedly increased compared with that of their wild-type counterparts. As MMP7 and MMP9 have been shown to generate angiostatin from circulating plasminogen, and angiostatin acts as a potent inhibitor of endothelial cell proliferation, we determined whether tumor vascularization was altered in the alpha1-null mice. Tumors implanted into alpha1-null mice showed markedly decreased vascularization, with a reduction in capillary number and size, which was accompanied by an increase in plasma levels of angiostatin due to the action of MMP7 and MMP9 on circulating plasminogen. In vitro analysis of alpha1-null endothelial cells revealed a marked reduction of their proliferation on both integrin alpha1-dependent (collagenous) and independent (noncollagenous) substrata. This reduction was prevented by culturing alpha1-null cells with plasma derived from plasminogen-null animals, thus omitting the source from which to generate angiostatin. Plasma from tumor-bearing alpha1-null animals uniquely inhibited endothelial cell growth, and this inhibition was relieved by the coaddition of either MMP inhibitors, or antibody to angiostatin. Integrin alpha1-deficient mice thus provide a genetically characterized model for enhanced angiostatin production and serve to reveal an unwanted potential side effect of MMP inhibition, increased tumor angiogenesis.

Activation of integrins upon binding to extracellular matrix proteins is believed to be a crucial step for the regulation of cell survival and proliferation. We have used integrin alpha1-null mice to investigate the role of this collagen receptor in the regulation of cell growth and survival in vivo. alpha1-deficient animals, which are viable and fertile, have a hypocellular dermis and a deficiency in dermal fibroblast proliferation as embryos. In vitro analysis of alpha1-null embryonic fibroblasts has revealed that their proliferation rate is markedly reduced when plated on collagenous substrata, despite normal attachment and spreading. Moreover, on the same collagenous matrices, alpha1-null fibroblasts fail to recruit and activate the adaptor protein Shc. The failure to activate Shc is accompanied by a downstream deficiency in recruitment of Grb2 and subsequent mitogen-activated protein kinase activation. Taken together with the growth deficiency observed on collagens, this finding indicates that the alpha1beta1 is the sole collagen receptor which can activate the Shc mediated growth pathway. Thus, integrin alpha1 has a unique role among the collagen receptors in regulating both in vivo and in vitro cell proliferation in collagenous matrices.