Ligation of the alpha2M signalling receptor elevates the levels of p21Ras-GTP in macrophages.

Ligation of the alpha2-macroglobulin signalling receptor (alpha2MSR) with alpha2-macroglobulin (alpha2M)-methylamine or a cloned and expressed receptor binding fragment (RBF) stimulates DNA synthesis. To examine the possible role of the Ras pathway in the mitogenic effects observed on ligating alpha2MSR, we studied the formation of p2 Ras-GTP in murine peritoneal macrophages upon treatment with alpha2M-methylamine and RBF, respectively. Both alpha2M-methylamine (50 pM) and RBF (50 pM) stimulated a 2-3-fold increase in the formation of the p21Ras-GTP complex compared with unstimulated cells. p21Ras-GT32P complex formation was both time and RBF concentration dependent and was comparable to p21Ras-GT32P complex formation induced by EGF (200 ng/mL) and platelet derived growth factor (50 mg/mL). Up-regulation of cells with phorbol dibutyrate prior to stimulation with RBF had no effect on p2 Ras-GT32P formation. However, treatment of macrophages with the tyrosine kinase inhibitor genestein drastically reduced RBF-induced formation of the p21 Ras-GT32P complex. Wortmannin, an inhibitor of phosphatidylinositol-3'-kinase (PI3K), had no effect on p21Ras-GT32P complex formation. It is concluded that the mitogenic effects of ligating alpha2MSR are mediated through a Ras-dependent pathway.