Activation of protein kinase B β and γ isoforms by insulin in vivo and by 3-phosphoinositide-dependent protein kinase-1 in vitro:: comparison with protein kinase B α

The regulatory and catalytic properties of the three mammalian isoforms of protein kinase B (PKB) have been compared. All three isoforms (PKB alpha, PKB beta and PKB gamma) were phosphorylated at similar rates and activated to similar extents by 3-phosphoinositide-dependent protein kinase-1 (PDK1). Phosphorylation and activation of each enzyme required the presence of PtdIns(3,4,5)P-3 or PtdIns(3,4)P-2, as well as PDK1. The activation of PKB beta and PKB gamma by PDK1 was accompanied by the phosphorylation of the residues equivalent to Thr(308) in PKB alpha, namely Thr(309) (PKB beta) and Thr(305) (PKB gamma). PKB gamma which had been activated by PDK1 possessed a substrate specificity identical with that of PKB alpha and PKB beta towards a range of peptides, The activation of PKB gamma and its phosphorylation at Thr(305) was triggered by insulin-like growth factor-1 in 293 cells. Stimulation of rat adipocytes or rat hepatocytes with insulin induced the activation of PKB alpha and PKB beta with similar kinetics. After stimulation of adipocytes, the activity of PKB beta was twice that of PKB alpha, but in hepatocytes PKB alpha activity was four-fold higher than PKB beta. Insulin induced the activation of PKB alpha in rat skeletal muscle in vivo, with little activation of PKB beta. Insulin did not induce PKB gamma activity in adipocytes, hepatocytes or skeletal muscle, but PKB gamma was the major isoform activated by insulin in rat L6 myotubes (a skeletal-muscle cell line).