DISTINCT BEHAVIOR OF CARDIAC MYOSIN HEAVY-CHAIN GENE CONSTRUCTS INVIVO - DISCORDANCE WITH INVITRO RESULTS

Transcriptional thyroid hormone responsiveness of the cardiac alpha-myosin heavy chain (alpha-MHC) gene has been demonstrated in transfections into fetal and neonatal cardiomyocytes and in transgenic mice. However, the correspondence between the regulation of MHC expression in dissociated cells with that in the intact heart is unclear. Given the cost and time involved in generating multiple transgenic lines for the characterization of gene regulatory elements, we used direct cardiac gene transfer to identify elements regulating both basal and thyroid hormone responsive cardiac alpha-MHC gene expression in the adult heart in vivo. Sequences upstream of the rat alpha-MHC gene linked to a luciferase reporter gene were injected into the hearts of adult rats subjected to various thyroid manipulations. The 161-bp sequence upstream of the transcription start site, which contains a TATA box, a CCAATT box, and a thyroid hormone response element, was transcriptionally active but not thyroid hormone responsive. The expression of a construct containing 388 bp of upstream sequence was increased by thyroid hormone administration, a response that required an intact thyroid hormone response element. However, expression of this construct failed to decrease to basal levels in a hypothyroid state. To confer complete (positive and negative) thyroid hormone regulation, 2,936 bp of upstream sequence was sufficient. These results demonstrate that, although necessary, the thyroid hormone response element is not sufficient for complete thyroid hormone regulation of this gene in vivo. In addition, DNA sequences regulating the quantitative expression of cardiac alpha-MHC in the euthyroid state have been demonstrated. One sequence, an MEF-2 site, which has been shown to be essential for high levels of expression of at least one other cardiac gene in neonatal cardiocytes, was mutated and found not to affect alpha-MHC expression in the adult heart. These data emphasize the complexity of gene regulation in an intact organ, aspects of which cannot be simulated in culture.