Regulation of the cardiocyte growth response by thenPKC isoform, PKC-epsilon

Cardiac hypertrophy is the heart's response to a variety of extrinsic and intrinsic stimuli that impose increased biomechanical stress. While this response is initially compensatory, if prolonged, it becomes an independent risk factor for subsequent cardiovascular morbidity and mortality. There is an extensive body of evidence which suggests a role for the protein kinase C (PKC) family of phospholipid-dependent serine-threonine kinases in models of experimentally-induced hypertrophy. Despite the existence of multiple isoforms of PKC in the heart, a number of transgenic and in vitro studies indicate that the Ca2+-insensitive novel PKC isoform, PKC-&egr;, may play a predominant role in cardiocyte hypertrophy. The focus of the current study was to utilize cardiocyte culture models to examine the necessity of PKC-&egr; activation in the genesis of a cardiocyte hypertrophic phenotype and to identify potential growth-related endpoints mediated by PKC-&egr; signaling. Therefore, we generated replication-defective recombinant adenoviruses that constitutively expressed either wild-type PKC-&egr; (PKC-&egr;/WT) or dominant-negative PKC-&egr; (PKC-&egr;/DN) to selectively modulate PKC-&egr; activity. We demonstrate here that overexpression of PKC-&egr;/DN in neonatal rat cardiocytes was sufficient to block alpha-adrenergic agonist (PE) stimulated growth as measured using two endpoints of the hypertrophic response, namely total protein content and total RNA content. Overexpression of PKC-&egr;/DN in cardiocytes completely inhibited phorbol ester (PMA) stimulated PKC-&egr; kinase activity and reduced PMA stimulated ERK1/2 phosphorylation. However, under these conditions, no effect was observed on the ability of either PKC-alpha or PKC-delta to translocate from soluble to particulate fractions, thereby demonstrating that the PKC-&egr;/DN adenovirus was isoform specific. PKC-&egr;/DN overexpression and pretreatment of cardiocytes with either the PKC-delta inhibitor (Rottlerin) or the PKC-alpha inhibitor (Go6976) revealed that PE mediated ERK1/2 phosphorylation was primarily due to PKC-&egr; activation. Furthermore, PE treatment of cardiocytes was shown to phosphorylate mitogen-activated interacting kinase (MNK-1) through a pathway that involves PKC-&egr;/ERK, but not p38 MAPK. Since MNK-1 activation is linked to translational initiation, we provide novel evidence for a role of PKC-&egr; in the regulation of translational efficiency during G-protein coupled receptor agonist stimulation. Finally, an increase in ribosomal components is critical for sustained cardiocyte growth. (Abstract shortened by UMI.)...