Phorbol 12-myristate 13-acetate alters SR Ca2+-ATPase gene expression in cultured neonatal rat heart cells

Primary cultures of neonatal rat ventricular myocytes were used to examine how the cardiac myocyte cytoplasmic Ca2+ ([Ca2+](i)) transient and sarcoplasmic reticulum Ca2+-ATPase (SERCA2) gene expression change in response to treatment with the protein kinase C activator phorbol 12-myristate 13-acetate (PMA). Exposure of neonatal myocytes to PMA (200 nM, 48-72 h) produced myocyte growth and a 70% prolongation of the half-time for [Ca2+](i) decline induced by potassium depolarization in the absence of extracellular Na+ (in which the sarcoplasmic reticulum Ca2+ pump is the main mechanism responsible for [Ca2+](i) decline). The reduced rate of [Ca2+](i) transient decline corresponded to a 53% reduction in SERCA2 protein levels and a 43% reduction in SERCA2 mRNA levels as compared with control myocytes. Exposure to PMA for as little as 30 min or for as long as 48 h produced a similar degree of SERCA2 mRNA downregulation over time. PMA-induced downregulation of SERCA2 mRNA levels was blocked by either 10 nM staurosporine or 4 mu M chelerythrine, whereas treatment with either agent alone increased SERCA2 mRNA levels as compared with control cells. Actinomycin D mRNA stability assays revealed that PMA treatment appeared to markedly destabilize the relatively long-lived SERCA2 mRNA transcript. Taken together, these results indicate that downregulation of SERCA2 gene by PMA in cultured neonatal myocytes occurs at least in part by alterations in mRNA stability and results in functional alterations in [Ca2+](i) decline that-are similar to that observed in the hypertrophied and failing adult myocardium.