Cardiac expressions of α- and β-myosin heavy chains and sarcomeric α- actins are regulated through transcriptional mechanisms. Results from nuclear run-on assays in isolated rat cardiac nuclei

Abstract

In the heart, mRNA accumulations for sarcomeric actins and myosin heavy chains (MHC) are subject to diverse regulatorial processes. To study cardiac contractile protein transcriptional regulations, an in vitro transcription system using nonenzymatically isolated rat cardiac nuclei was characterized. Transcription was shown to be rapid and continuous during the first 20 min of incubation and 5.4-fold less than that seen from comparably isolated hepatocyte nuclei. Neither RNase nor DNase activities were detectable. Direct transcriptional analyses of the α- and β-MHC and cardiac and skeletal α- actin genes from cardiac nuclei were performed. In 23-24-day-old rats, significant levels of transcription were seen for α-MHC and for the sarcomeric α-actins. β-MHC was just detectable, and no positive signals were ever seen for fibronectin. We then compared the percentages of MHC and sarcomeric α-actin expressions determined from 1) the transcriptional assays and 2) total isolated RNA (α-MHC: 90.1 ± 4.8% (transcription), 93.0 ± 4.7% (accumulation); β-MHC: 9.9 ± 4.8%, 7.0 ± 4.7%; cardiac α-actin: 84.0 ± 2.5%, 84.9 ± 2.5%; skeletal α-actin: 16.1 ± 2.5%, 15.0 ± 2.5%). The results support the conclusion that the primary mechanisms controlling the accumulations of these gene products are transcriptional. Additionally, we show that an anti-sense mRNA showing strong homology or identity with the 5' end of the β-MHC gene is transcribed in cardiac nuclei but not in hepatocyte nuclei.