Potential role of helix-loop-helix proteins in cardiac gene expression.

Because helix-loop-helix (HLH) transcription factors appear to play an important role in mesodermal development, we have investigated the potential role of these factors in cardiac gene expression. HLH proteins interact with DNA at consensus "E-box" sites and may be tissue specific or more widely expressed. We have examined cardiac cells for expression and regulation of widely expressed factors Pan1/Pan2 and the inhibitor of differentiation (Id) by RNase protection analysis. The effect of MyoD, Id, and Pan1/Pan2 expression on skeletal and cardiac promoters in cardiac cells was examined by transient cotransfection studies. Our results indicate that neonatal ventricular cells are a functional HLH environment, because MyoD can activate a skeletal muscle-specific promoter in these cells. MyoD, however, has no effect on the expression of several genes that are expressed in cardiac cells. In addition, Id may be an early response gene for signal transduction in cardiac cells, because increases in Id mRNA occurred within 30 minutes of stimulation with serum or phenylephrine. Activities of three cardiac promoter elements in primary ventricular myocytes were not downregulated by Id. Surprisingly, expression of Pan1 and Pan2 exhibited a strong negative effect on cardiac expression of the myosin light chain-2 promoter.