REGULATION OF RAT MYOSIN HEAVY CHAIN GENE EXPRESSION BY THYROID HORMONE.

Abstract

Myosin heavy chain (MHC) isoforms in vertebrate striated muscles are encoded by a highly conserved multigene family. Seven sarcomeric MHCs are known to be expressed in cardiac and skeletal muscles of the rat. These isoforms show distinct patterns of tissue-specific expression which are hormonally and developmentally regulated. Thyroid hormone has been shown to regulate the expression of the ventricular isoforms by causing an accumulation of alpha-MHC and a repression of beta-MHC. Alterations in thyroid status have also been reported to affect MHC isoforms in skeletal muscles. The first part of this study describes the development of a dot-blot assay with which to measure the levels of the sarcomeric MHC mRNAs. This assay was utilized in part two of this dissertation to analyze the effects of thyroid hormone administration on the expression of MHC and actin mRNAs in ventricular muscle, as well as in representative fast and slow skeletal muscles of the hypothyroid rat. The results indicate that expression of all of the MHC genes is regulated by thyroid hormone in a highly complex, tissue specific manner. Also, thyroid hormone caused transient increases in the expression of cardiac actin mRNA in the ventricle, but no actin isoform transitions were observed in any muscle type. The third portion of this dissertation describes the development of a cultured cell system that has been used to analyze changes in MHC and alpha-actin gene expression in response to thyroid hormone in vitro. Cultured primary fetal cardiomyocytes were found to express only the cardiac MHC and actin isoforms. In addition, the alpha- and beta-MHC isoforms were found to be regulated by thyroid hormone in a similar manner to that observed in the intact rat ventricle. In the final portion of this study, the promoter region of the thyroid hormone inducible gene, alpha-MHC, was isolated and ligated to the bacterial chloramphenicol acetyltransferase gene. Chimeric genes were utilized in transfection studies and found to be thyroid hormone inducible. The results suggest that DNA sequences in the 5' flanking region of the alpha-MHC gene are sufficient to mediate hormone induction of the gene.