Regulation of Adenosine 3′,5′-Monophosphate-dependent Protein Kinase

Abstract The effects of lipolytic and antilipolytic hormones on the adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase of rat adipose tissue were investigated. Incubation of epididymal fat pads with epinephrine increased the protein kinase activity in tissue extracts assayed in the absence of cAMP but not in the presence of cAMP. This was associated with conversion of the inactive holoenzyme form to the active catalytic subunit form as judged by resolution of these two forms of protein kinase on Sephadex G-100 columns. The time course of formation of the catalytic subunit of protein kinase in general paralleled the epinephrine-induced increase in the cAMP concentration. Concentrations of epinephrine (0.1 to 0.3 µm) which produced small increases in both the cAMP concentration and the catalytic form of protein kinase gave a maximum rate of lipolysis. Higher epinephrine concentrations further increased the cAMP concentration and the percentage of catalytic subunit form of protein kinase. Other lipolytic hormones such as adrenocorticotropic hormone and glucagon also activated the protein kinase. Activation of protein kinase in isolated fat cells by epinephrine in the absence and presence of caffeine was demonstrated. Insulin inhibited by up to 70% the epinephrine of protein kinase in fat pad. This inhibition by insulin was rapid with the maximum effect occurring within 10 min. The insulin effect was associated with a decrease in the relative amount of the catalytic subunit of protein kinase and can be quantitatively accounted for by the decrease in cAMP. Both the epinephrine and insulin effects on protein kinase disappeared when cAMP was removed by chromatography on Sephadex G-25 under conditions that allowed association of the protein kinase subunits. When cAMP was removed under conditions of high ionic strength which prevented the association of the enzyme subunits, the effects of hormones on the protein kinase were retained. The above results are consistent with the hypothesis that epinephrine and insulin do modify the activation state of the cAMP-dependent protein kinase through changes in the cAMP concentration.