Influence of thyroid hormone and retinoic acid on slow sarcoplasmic reticulum Ca2+ ATPase and myosin heavy chain alpha gene expression in cardiac myocytes. Delineation of cis-active DNA elements that confer responsiveness to thyroid hormone but not to retinoic acid.

The mRNA encoding the sarcoplasmic reticulum (SR) Ca2+ ATPase is highly influenced by thyroid hormone (T3) in the hearts of intact animals. We show here that this effect of T3 can be mimicked in primary neonatal rat cardiocytes, both in serum-containing and in serum-free media; the expression of SR Ca2+ ATPase mRNA is myocyte-specific and is also modulated by retinoic acid (RA). RA also induces myosin heavy chain (MHC) alpha-mRNA in this system. The induction of Ca2+ ATPase mRNA is sensitive to T3 (EC50 approximately 30 pM) and less sensitive to RA (EC50 approximately 2 nM). Transient transfection experiments utilizing various segments of the Ca2+ATPase promoter fused to the reporter gene chloramphenicol acetyltransferase (CAT) indicate a minimal thyroid hormone response element (TRE) between nucleotides -262 and -322, while sequences between -322 and -559 are required for maximal trans-activation. RA is not able to regulate these constructs. Likewise, a clear effect of T3 but no effect of RA was observed when the CAT gene was driven by a TRE derived from the rat alpha-MHC gene. In contrast, CAT expression was induced by either hormone when placed under the control of a synthetic palindromic TRE. Taken together, these results indicate that T3 and RA induce gene expression in primary cardiac myocytes, but through distinct response elements and/or mechanisms.