Diversification of cardiac insulin signaling involves the p85[Alpha]/[Beta] subunits of phosphatidylinositol 3-kinase

Diversification of cardiac insulin signaling involves the p85[Alpha]/[Beta] subunits of phosphatidylinositol 3-kinase. Am J Physiol Endocrinol Metab 280: E65-E74, 2001.--Ventricular cardiomyocytes and cardiac tissue of lean and genetically obese (fa/fa) Zucker rats were used 1) to study the role of the p85 regulatory subunit isoforms p85[Alpha] and p85[Beta] for insulin signaling through the phosphatidylinositol (PI) 3-kinase pathway, and 2) to elucidate the implications of these mechanisms for cardiac insulin resistance. Western blot analysis of cardiomyocyte lysates revealed expression of p85[Alpha] and p85[Beta] but no detectable amounts of the splice variants of p85[Alpha]. Essentially no p85[Alpha] subunit of PI 3-kinase was found to be associated with insulin receptor substrate (IRS)-1 or IRS-2 in basal and insulin-stimulated (5 min) cardiomyocytes. Instead, insulin produced a twofold increase in p85[Beta] associated with IRS-1, leading to a three- to fourfold increase in p85[Beta]-associated PI 3-kinase activity. This response was significantly reduced in obese animals. Comparable results were obtained in the intact heart after in vivo stimulation. In GLUT-4-containing vesicles, an increased abundance (3.7 [+ or -] 0.7-fold over basal) of p85[Alpha] was observed after insulin stimulation of lean animals, with no significant effect in the obese group. No p85[Beta] could be detected in GLUT-4-containing vesicles. Recruitment of the p110 catalytic subunit of PI 3-kinase and a twofold increase in enzyme activity in GLUT-4-containing vesicles by insulin was observed only in lean rats. We conclude that, in the heart, p85[Alpha] recruits PI 3-kinase activity to GLUT-4 vesicles, whereas p85[Beta] represents the main regulator of IRS-1- and IRS-2-mediated PI 3-kinase activation. Furthermore, multiple defects of PI 3-kinase activation, involving both the p85[Alpha] and the p85[Beta] adaptor subunits, may contribute to cardiac insulin resistance. GLUT-4-containing vesicles; obesity; insulin resistance; cardiac muscle