Lysophosphatidic acid modulates alpha(1b)-adrenoceptor phosphorylation andfunction: Roles of Gi and phosphoinositide 3-kinase

The effect of lysophosphatidic acid on the phosphorylation and function of alpha(1b)-adrenoceptors transfected into rat-1 fibroblasts was studied. Thi s phospholipid mitogen increased in a concentration-dependent fashion (EC50 similar to 50 nM) the phosphorylation of these adrenoceptors. Lysophosphat idic acid-induced alpha(1b)-adrenoceptor phosphorylation was relatively rap id (t(1/2) similar to 1 min), intense (2.5-fold), and sustained for at leas t 60 min. The effect of lysophosphatidic acid was blocked by pretreatment w ith pertussis toxin. The alpha(1b)-adrenoceptor phosphorylation induced by lysophosphatidic acid was not blocked by genistein, a tyrosine kinase inhib itor, but it was inhibited by inhibitors of protein kinase C (bisindolylmal eimide I, staurosporine, and Ro 31-8220) and phosphoinositide 3-kinase (wor tmannin and LY 294002). The ability of norepinephrine to increase cytosol c alcium concentration was markedly decreased in cells previously challenged with lysophosphatidic acid. Norepinephrine-induced [S-35]GTP gamma S bindin g in membrane preparations was used as an index of the functional coupling of the alpha(1b)-adrenoceptors and G proteins. Norepinephrine-stimulated [S -35]GTP gamma S binding was markedly decreased in membranes from cells pret reated with lysophosphatidic acid. This effect of lysophosphatidic acid was blocked by pretreatment with wortmannin or staurosporine. Our data indicat e that: 1) activation of lysophosphatidic acid receptors induce phosphoryla tion of alpha(1b)-adrenoceptors; 2) this effect is mediated through pertuss is toxin-sensitive G proteins, phosphatidylinositol 3-kinase, and protein k inase C; and 3) the phosphorylation of alpha(1b)-adrenoceptors induced by t he lipid mitogen is associated to adrenoceptor desensitization.