Transcription of early developmental isogenes in cardiac myocyte hypertrophy

We have developed a cell culture system to study molecular mechanisms important in myocardial hypertrophy. alpha 1-Adrenergic receptor stimulation produces hypertrophy of neonatal rat cardiac myocytes. Myocyte hyperplasia is not induced by alpha 1 stimulation, although alpha 1-adrenergic receptor-mediated DNA synthesis and cell division have been observed in other types of cells. The myocyte hypertrophic response does not require contractile activity. Activation of the alpha 1 receptor also produces highly specific alterations in gene expression, as measured at the mRNA and protein levels. In particular, there is selective up-regulation of two contractile protein isogenes that are expressed in vivo during early development and in pressure-load hypertrophy, skeletal alpha-actin and beta-myosin heavy chain. Studies with an in vitro transcription assay indicate that stimulation of the alpha 1-adrenergic receptor leads to a distinctive temporal sequence of transcriptional activation. Transcription of the skeletal alpha-actin isogene is induced preferentially to that of cardiac alpha-actin. Thus, early developmental isogene induction in alpha 1-stimulated hypertrophy reflects a fundamental change in the transcriptional program of the cardiac myocyte nucleus. The goal now is to define an intracellular pathway connecting the alpha 1-adrenergic receptor in the plasma membrane to activation of RNA polymerase II on the skeletal alpha-actin gene in the cardiac myocyte nucleus. There is evidence that protein kinase C may be one component of this pathway. A model for alpha 1-mediated transcription is presented.