Rational promoter selection for gene transfer into cardiac cells

Cardiomyocytes (CMCs) are extremely difficult to transfect with non-viral techniques, but they are efficiently infected by adenoviruses. The most commonly used promoters to drive protein expression in cardiac myocytes are of viral origin, since they are believed to be constitutively active and minimally regulated by physiological or pharmacological challenge of cells. In recombinant adenoviruses, we systematically compared three different promoters: the cytomegalovirus (CMV), the Rous sarcoma virus (RSV), and a synthetic promoter with three MEF2 transcription factor-binding sites upstream of the heat-shock protein 68 minimal promoter. We determined their basal activity in primary cardiac cells as well as their possible stimulation by commonly used agonists. The CMV promoter was activated up to 60-fold by the phorbol ester phorbol myristate acetate (PMA) and/or forskolin in neonatal rat CMCs and cardiac fibroblasts. Primary adult rat CMCs had higher basal expression from the CMV promoter that was not activated by PMA or forskolin. The RSV promoter was less affected by agonists and was more active in cardiac myocytes compared to cardiac fibroblasts. The MEF2-responsive promoter showed high basal expression in both myocytes and fibroblasts, and minimal induction by phorbol esters and forskolin. The relevance of reporter gene induction was confirmed with a contractile protein, troponin T (TnT). The CMV promoter driving TnT could be induced more than 15-fold with phenylephrine or forskolin to replace the endogenous protein almost to completion at a multiplicity of infection of 10. These results suggest the following use of the tested promoters: an inducible system (CMV), a myocyte-enriched system (RSV), or a stable control system (MEF2).