Roles of the AMP‐activated and cyclic‐AMP‐dependent protein kinases in the adrenaline‐induced inactivation of acetyl‐CoA carboxylase in rat adipocytes

Abstract

1 In isolated rat adipocytes, acetyl‐CoA carboxylase is inactivated by treatment of the cells with adrenaline or the β‐agonist isoproterenol, but not by the α‐agonist phenylephine. The inactivation is stable during purification in the presence of protein phosphatase inhibitors, and is associated with a 30–40% increase in the labelling of enzyme isolated from ³²P‐labelled cells. 2 Increased phosphorylation occurs within peptide T1, which was identified by sequencing to be the peptide Ser‐Ser⁷⁷‐Met‐Ser⁷⁹‐Gly‐Leu‐His‐Leu‐Val‐Lys, containing Ser‐77 (phosphorylated by cyclic‐AMP‐dependent protein kinase) and Ser‐79 (phosphorylated by the AMP‐activated protein kinase). Analysis of the release of radioactivity as free phosphate during Edman degradation of peptide T1 revealed that all of the phosphate was in Ser‐79 in both basal and hormone‐ or agonist‐stimulated cells. Treatment of adipocytes with various agents which activate cyclic‐AMP‐dependent protein kinase by receptor‐independent mechanisms (forskolin, cyclic AMP analogues, isobutylmethylxanthine) also produced inactivation of acetyl‐CoA carboxylase and increased phosphorylation at Ser‐79. 3 The (Rp)‐[thio]phosphate analogue of cyclic AMP, which is an antagonist of binding of cyclic AMP to the regulatory subunit of cyclic‐AMP‐dependent protein kinase, opposes the effect of adrenaline on phosphorylation and inactivation of acetyl‐CoA carboxylase. Together with the effects of isobutylmethylxanthine and the stimulatory cyclic AMP analogues, this strongly indicates that cyclic‐AMP‐dependent protein kinase is an essential component of the signal transduction pathway, although clearly it does not directly phosphorylate acetyl‐CoA carboxylase. 4 As shown by okadaic acid inhibition, > 95% of the acetyl‐CoA carboxylase phosphatase activity in extracts of rat adipocytes or liver is accounted for by protein phosphatase‐2A, with <5% attributable to protein phosphatase‐1. Inhibition of protein phosphatase‐1 via phosphorylation of inhibitor‐1 is therefore unlikely to be the mechanism by which cyclic‐AMP‐dependent protein kinase indirectly increases phosphorylation of acetyl‐CoA carboxylase. Various other potential mechanisms are discussed.