The protein kinase C (PKC) family of serine/threonine kinases functions downstream of nearly all membrane-associated signal transduction pathways. Here we identify PKC-alpha as a fundamental regulator of cardiac contractility and Ca(2+) handling in myocytes. Hearts of Prkca-deficient mice are hypercontractile, whereas those of transgenic mice overexpressing Prkca are hypocontractile. Adenoviral gene transfer of dominant-negative or wild-type PKC-alpha into cardiac myocytes enhances or reduces contractility, respectively. Mechanistically, modulation of PKC-alpha activity affects dephosphorylation of the sarcoplasmic reticulum Ca(2+) ATPase-2 (SERCA-2) pump inhibitory protein phospholamban (PLB), and alters sarcoplasmic reticulum Ca(2+) loading and the Ca(2+) transient. PKC-alpha directly phosphorylates protein phosphatase inhibitor-1 (I-1), altering the activity of protein phosphatase-1 (PP-1), which may account for the effects of PKC-alpha on PLB phosphorylation. Hypercontractility caused by Prkca deletion protects against heart failure induced by pressure overload, and against dilated cardiomyopathy induced by deleting the gene encoding muscle LIM protein (Csrp3). Deletion of Prkca also rescues cardiomyopathy associated with overexpression of PP-1. Thus, PKC-alpha functions as a nodal integrator of cardiac contractility by sensing intracellular Ca(2+) and signal transduction events, which can profoundly affect propensity toward heart failure.

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A calcineurin-nuclear factor of activated T cells (NFAT) regulatory pathway has been implicated in the control of cardiac hypertrophy, suggesting one mechanism whereby alterations in intracellular calcium handling are linked to the expression of hypertrophy-associated genes. Although recent studies have demonstrated a necessary role for calcineurin as a mediator of cardiac hypertrophy, the potential involvement of NFAT transcription factors as downstream effectors of calcineurin signaling has not been evaluated. Accordingly, mice with targeted disruptions in NFATc3 and NFATc4 genes were characterized. Whereas the loss of NFATc4 did not compromise the ability of the myocardium to undergo hypertrophic growth, NFATc3-null mice demonstrated a significant reduction in calcineurin transgene-induced cardiac hypertrophy at 19 days, 26 days, 6 weeks, 8 weeks, and 10 weeks of age. NFATc3-null mice also demonstrated attenuated pressure overload-and angiotensin II-induced cardiac hypertrophy. These results provide genetic evidence that calcineurin-regulated responses require NFAT effectors in vivo.Cardiac hypertrophy is defined by an increase in ventricular wall thickness accompanied by an increase in cardiomyocyte cell volume. Hypertrophic enlargement is precipitated by increased workload or by decreased efficiency within the heart, conditions that are associated with hypertension, ischemic heart disease, valvular insufficiency, neuroendocrine disruptions, or intrinsic defects in contractile proteins (reviewed in reference 30). Although initially compensatory, sustained cardiac hypertrophy predisposes an individual to sudden death, arrhythmias, functional decompensation, and overt heart failure (30).Numerous regulatory pathways have been implicated in the transduction of hypertrophic signaling, linking neuroendocrine and mechanical stress stimuli to altered cardiac gene expression (reviewed in reference 40). Although numerous hypertrophic regulatory pathways have been identified, the recent characterization of the calcium-regulated phosphatase calcineurin as an important signaling factor in the heart has generated considerable interest. Transgenic mice expressing an activated form of calcineurin in the heart developed robust hypertrophy that quickly transitioned to dilation and failure (41). Subsequently, the calcineurin inhibitory drugs cyclosporine (Cs) and FK506 were shown to inhibit or attenuate cardiac hypertrophy or cardiomyopathy in most, but not all, rodent models of heart disease, suggesting a necessary regulatory role for this signaling pathway in the heart (reviewed in reference 39). More recently, transgenic mice expressing either the calcineurin inhibitory domains of Cain, AKAP79, MCIP1, or dominant-negative calcineurin were shown to have attenuated cardiac hypertrophy in response to pathophysiologic stimulation (7,52,72).Perhaps the best-characterized target of calcineurin is the nuclear factor of activated T cells (NFAT) transcription factor family. Calcineurin directly dephosphorylates NFAT transcription fa...

Members of the protein kinase C (PKC) isozyme family are important signal transducers in virtually every mammalian cell type. Within the heart, PKC isozymes are thought to participate in a signaling network that programs developmental and pathological cardiomyocyte hypertrophic growth. To investigate the function of PKC signaling in regulating cardiomyocyte growth, adenoviral-mediated gene transfer of wild-type and dominant negative mutants of PKCα, βII, δ, and ɛ (only wild-type ζ) was performed in cultured neonatal rat cardiomyocytes. Overexpression of wild-type PKCα, βII, δ, and ɛ revealed distinct subcellular localizations upon activation suggesting unique functions of each isozyme in cardiomyocytes. Indeed, overexpression of wild-type PKCα, but not βII, δ, ɛ, or ζ induced hypertrophic growth of cardiomyocytes characterized by increased cell surface area, increased [3H]-leucine incorporation, and increased expression of the hypertrophic marker gene atrial natriuretic factor. In contrast, expression of dominant negative PKCα, βII, δ, and ɛ revealed a necessary role for PKCα as a mediator of agonist-induced cardiomyocyte hypertrophy, whereas dominant negative PKCɛ reduced cellular viability. A mechanism whereby PKCα might regulate hypertrophy was suggested by the observations that wild-type PKCα induced extracellular signal–regulated kinase1/2 (ERK1/2), that dominant negative PKCα inhibited PMA-induced ERK1/2 activation, and that dominant negative MEK1 (up-stream of ERK1/2) inhibited wild-type PKCα–induced hypertrophic growth. These results implicate PKCα as a necessary mediator of cardiomyocyte hypertrophic growth, in part, through a ERK1/2-dependent signaling pathway.