Thyroid hormone and hemodynamic regulation of beta-myosin heavy chain promoter in the heart

Thyroid hormone exerts marked effects on cardiovascular function. Expression of cardiac alpha- and beta-myosin heavy chain (MHC) isoforms can be altered in response to thyroid hormone as well as by hemodynamic changes imposed on the heart. The molecular mechanisms that mediate these changes are not completely known. We studied the contractile and thyroid hormone responsiveness of the beta MHC promoter in both cultured cardiac myocytes and in vivo by direct DNA transfer. Using transient transfection of neonatal rat cardiomyocytes, the activities of recombinant reporter plasmids containing beta MHC 5'-flanking sequences terminating at positions -2250, -1145, -670, and -354 were decreased significantly in cultures containing L-T-3 (50 nM). Similar deletion analysis showed that 5'-flanking regions terminating within -2250 to -151 bp were contractily responsive; however, deletion to position -126 attenuated this response. In vivo beta MHC promoter activity, determined by injecting the recombinant plasmid into the myocardium, was significantly higher by 2-fold in hypothyroid than in euthyroid ventricles (2.47 +/- 0.41 vs. 1.33 +/- 0.25 luciferase(chloramphenicol acetyltransferase; P < 0.05). Increased ventricular workload, produced by aortic coarctation for 5 days, resulted in ventricular hypertrophy (heart/body weight, 4.05 +/- 0.19 us. 3.42 +/- 0.16 mg/g; P < 0.02) and a 3.4-fold increase in beta MHC messenger RNA content. However, beta MHC promoter activity in vivo was not significantly different between rats experiencing aortic coarctation and sham-operated rats (1.49 +/- 0.41 vs. 0.96 +/- 0.27 luciferase/chloramphenicol acetyltransferase, respectively) and was similar to that in euthyroid animals. These results show that beta MHC promoter activity is T-3 responsive in cultured myocytes and in vivo, but that the increase in beta MHC messenger RNA observed in the in vivo pressure-overloaded myocardium cannot be explained entirely by transcription control mechanisms.