Role of the atypical protein kinase Czeta in regulation of 5'-AMP-activated protein kinase in cardiac and skeletal muscle.

During metabolic stress, phosphorylation and activation of 5'-AMP-activated protein kinase (AMPK) becomes a major regulator of cellular energy metabolism in heart and skeletal muscle. Despite this, the upstream regulation of AMPK in both heart and muscle is poorly understood. Recent work has implicated the atypical protein kinase Czeta (PKCzeta) as a regulator of AMPK in endothelial cells via phosphorylation of LKB1, an upstream AMPK kinase (AMPKK). Our goal was to determine the potential role PKCzeta plays in regulating AMPK in cardiac and skeletal muscle. Cultures of H9c2 myocytes (cardiac) and C(2)C(12) myotubes (skeletal muscle) were pretreated with a selective PKCzeta pseudosubstrate peptide inhibitor and treated with various AMPK activating agents to determine whether PKCzeta regulates AMPK. PKCzeta activity was also examined in isolated working rat hearts subjected to ischemia. We show that PKCzeta is not involved in regulating threonine 172 AMPK phosphorylation induced by metformin or phenformin in either cardiac or skeletal muscle cells but is involved in 5-aminoimidazole-4-carboxamine-1-beta-D-ribofuranoside (AICAR)-induced AMPK phosphorylation in cardiac muscle cells. Activation of PKCzeta with high palmitate concentrations is also insufficient to increase AMPK phosphorylation. Furthermore, we show that the ischemia-induced activation of AMPK is not accompanied by increased PKCzeta activity. Finally, we show that PKCzeta may actually be a downstream target of AMPK in skeletal muscle, since adenoviral expression of a dominant-negative mutant of AMPK prevented metformin- and AICAR-induced phosphorylation of PKCzeta. We conclude that PKCzeta plays a very minor role in the regulation of AMPK in cardiac and skeletal muscle and may actually be a downstream target of AMPK in skeletal muscle.