Alpha 1-adrenergic receptor stimulation of sarcomeric actin isogene transcription in hypertrophy of cultured rat heart muscle cells.

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Abstract
During pressure-load hypertrophy of the adult heart in vivo, there is up-regulation of the mRNA encoding skeletal alpha-actin, the sarcomeric actin iso-mRNA characteristic of mature skeletal muscle and the fetal/neonatal heart. We have shown previously that during alpha 1-adrenergic receptor-stimulated hypertrophy of cultured rat heart myocytes, the induction of skeletal alpha-actin mRNA is greater than that of the mRNA encoding cardiac alpha-actin, the sarcomeric actin iso-mRNA characteristic of the adult heart. To determine if this actin iso-mRNA switch during cardiac hypertrophy reflects changes in the transcriptional status of the myocyte nucleus, we quantified the rate of transcription of actin mRNAs and total RNA, using an in vitro run-on transcription assay with nuclei isolated from the cultured myocytes after stimulation with norepinephrine (NE). Transcription of skeletal alpha-actin was increased at 3 h after NE, reached a maximum 6.1-fold increase at 12 h, and returned to the control level at 24 h. The EC50 for NE was 200 nM, and pharmacologic studies indicated alpha 1-receptor specificity. Transcription of cardiac alpha-actin was also increased rapidly by NE (maximum 4.6-fold vs. control at 3 h). However, cardiac alpha-actin transcription had returned to the control level at 6 h, when NE-stimulated skeletal alpha-actin transcription was still increasing. Transcription of the cytoskeletal (beta) actin gene was not changed significantly by NE treatment. Total RNA transcription was not increased until 6 h after NE (1.5-fold vs. control) and remained elevated through 24 h. Inhibition of protein synthesis did not attenuate NE-stimulated actin gene transcription. Thus the alpha 1-adrenoceptor mediates a rapid, transient, and selective increase in transcription of the sarcomeric actin isogenes during cardiac myocyte hypertrophy. Skeletal alpha-actin, the fetal/neonatal isogene, is induced preferentially to cardiac alpha-actin, the adult isogene. The different kinetics of actin isogene and total RNA transcription and the independence of transcription from protein synthesis suggest that transcriptional induction via the alpha 1 receptor is complex and may involve preexisting regulatory factors. These results are the first to demonstrate that the alpha 1-adrenergic receptor is a molecular mediator of transcriptional changes underlying an isogene switch that is known to be associated with cardiac myocyte hypertrophy.

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