Differential roles of PI3-kinase and Kit tyrosine 821 in Kit receptor-mediated proliferation, survival and cell adhesion in mast cells

The pleiotropic effects of the Kit receptor system are mediated by Kit-Ligand (KL) induced receptor autophosphorylation and its association with and activation of distinct second messengers, including phosphatidylinositol 3'-kinase (PI3-kinase), p21(ras) and mitogen-activated protein kinase (MAPK). To define the role of PI3-kinase, p21(ras) and MAPK in Kit-mediated cell proliferation, survival and adhesion in bone marrow-derived mast cells (BMMC), mutant Kit receptors were expressed in W(sh)/W(sh) BMMC lacking endogenous c-kit expression. The introduction of both murine Kit(S) and Kit(L) (isoform containing a four amino acid insert) into W(sh)/W(sh) BMMC restored KL-induced proliferation, survival and adhesion to fibronectin, as well as activation of PI3-kinase, p21(ras) and MAPK, and induced expression of c-fos, junB, c-myc and c-myb mRNA. Substitution of tyrosine 719 in the kinase insert with phenylalanine (Y719F) abolished PI3-kinase activation, diminished c-fos and junB induction, and impaired KL-induced adhesion of BMMC to fibronectin. In addition, the Y719F mutation had partial effects on p21(ras) activation, cell proliferation and survival, while MAP kinase activation was not affected. On the other hand, Y821F substitution impaired proliferation and survival without affecting PI3-kinase, p21(ras) and MAPK activation, and induction of c-myc, c-myb, c-fos and c-jun mRNA, while KL-induced cell adhesion to fibronectin remained intact. In agreement with a role for PI3-kinase in Kit-mediated cell adhesion, wortmannin blocked Kit-mediated cell adhesion at concentrations known to specifically inhibit PI3-kinase. We conclude, that association of Kit with p85(PI3-K), and thus with PI3-kinase activity, is necessary for a full mitogenic as well as adhesive response in mast cells. In contrast, tyrosine 821 is essential for Kit-mediated mitogenesis and survival, but not cell adhesion.