The role of cAMP-dependent signaling in receptor-recognized forms of α2-macroglobulin-induced cellular proliferation

Ligation of alpha(2)-macroglobulin receptors by receptor-recognized forms of alpha(2)-macroglobulin (alpha(2)M*) activates various signaling cascades and promotes cell proliferation. It also elevates cAMP in murine peritoneal macrophages. We now report that a significant elevation of cAMP-response element-binding protein (CREB) occurs in alpha(2)M*-stimulated cells, and this effect is potentiated by isobutylmethylxanthine, dibutyryl-cAMP, or forskolin. An a2M* concentration-dependent rapid increase in phosphorylated CREB at Ser(133) also occurred, a necessary event in its activation. Inhibition of Ca2+/calmodulin kinase, protein kinases A and C, tyrosine kinases, ribosomal S6 kinase, farnesyl transferase, extracellular signal-regulated kinases 1/2, phosphatidylinositol 3-kinase, or p38 mitogen-activated protein kinase markedly reduce alpha(2)M*-induced phosphorylation of CREB, indicating a role for the p21(ras)-dependent and phosphatidylinositol 3-kinase signaling pathways in regulating CREB activation by a2M*. Finally, silencing the CREB gene by transfecting cells with a homologous gene sequence double-stranded RNA drastically reduced the expression of CREB and blocked the ability of a alpha(2)M* to promote macrophage cell division. We conclude that cAMP-dependent signal transduction as well as other signaling cascades are essential for alpha(2)M*-induced cell proliferation.