The AMPK gamma 1 R70Q mutant regulates multiple metabolic and growth pathways in neonatal cardiac myocytes

Although mutations in the gamma -subunit of AMP-activated protein kinase (AMPK) can result in excessive glycogen accumulation and cardiac hypertrophy, the mechanisms by which this occurs have not been well defined. Because >65% of cardiac AMPK activity is associated with the gamma 1-subunit of AMPK, we investigated the effects of expression of an AMPK-activating gamma 1-subunit mutant ( gamma 1 R70Q) on regulatory pathways controlling glycogen accumulation and cardiac hypertrophy in neonatal rat cardiac myocytes. Whereas expression of gamma 1 R70Q displayed the expected increase in palmitate oxidation rates, rates of glycolysis were significantly depressed. In addition, glycogen synthase activity was increased in cardiac myocytes expressing gamma 1 R70Q, due to both increased expression and decreased phosphorylation of glycogen synthase. The inhibition of glycolysis and increased glycogen synthase activity were correlated with elevated glycogen levels in gamma 1 R70Q-expressing myocytes. In association with the reduced phosphorylation of glycogen synthase, glycogen synthase kinase (GSK)-3 beta protein and mRNA levels were profoundly decreased in the gamma 1 R70Q-expressing myocytes. Consistent with GSK-3 beta negatively regulating hypertrophy via inhibition of nuclear factor of activated T cells (NFAT), the dramatic downregulation of GSK-3 beta was associated with increased nuclear activity of NFAT. Together, these data provide important new information about the mechanisms by which a mutation in the gamma -subunit of AMPK causes altered AMPK signaling and identify multiple pathways involved in regulating both cardiac myocyte metabolism and growth that may contribute to the development of the gamma mutant-associated cardiomyopathy.