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 po tential role of these factors in cardiac gene expression. HLH proteins interact with DNA at consensus ''E-box'' sites and may be tissue spec ific or more widely expressed. We have examined cardiac cells for expr ession and regulation of widely expressed factors Pan1/Pan2 and the in hibitor of differentiation (Id) by RNase protection analysis. The effe ct of MyoD, Id, and Pan1/Pan2 expression on skeletal and cardiac promo ters in cardiac cells was examined by transient cotransfection studies . Our results indicate that neonatal ventricular cells are a functiona l HLH environment, because MyoD can activate a skeletal muscle-specifi c promoter in these cells. MyoD, however, has no effect on the express ion of several genes that are expressed in cardiac cells. In addition, Id may be an early response gene for signal transduction in cardiac c ells, because increases in Id mRNA occurred within 30 minutes of stimu lation with serum or phenylephrine. Activities of three cardiac promot er elements in primary ventricular myocytes were not downregulated by Id. Surprisingly, expression of Pan1 and Pan2 exhibited a strong negat ive effect on cardiac expression of the myosin light chain-2 promoter.