Role of E2F1–Cyclin E1-Cyclin E2 Circuit in Human Coronary Smooth Muscle Cell Proliferation and Therapeutic Potential of Its Downregulation by siRNAs

Aberrant coronary vascular smooth muscle cell (CSMC) proliferation is a pivotal event underlying intimal hyperplasia, a phenomenon impairing the long-term efficacy of bypass surgery and angioplasty procedures. Consequently research has become focused on efforts to identify molecules that are able to control CSMC proliferation. We investigated downregulation of CSMC growth by small interfering RNAs (siRNAs) targeted against E2F1, cyclin E1, and cyclin E2 genes, whose contribution to CSMC proliferation is only now being recognized. Chemically synthesized siRNAs were delivered by two different transfection reagents to asynchronous and synchronous growing human CSMCs cultivated either in normo- or hyperglycemic conditions. The depletion of each of the three target genes affected the expression of the other two genes, demonstrating a close regulatory control. The clearest effects associated with the inhibition of the E2F1–cyclin E1/E2 circuit were the reduction in the phosphorylation levels of the retinoblastoma protein pRB and a decrease in the amount of cyclin A2. At the phenotypic level the downmodulation of CSMC proliferation resulted in a decrease of S phase matched by an increase of G1-G0 phase cell amounts. The antiproliferative effect was cell–donor and transfectant independent, reversible, and effective in asynchronous and synchronous growing CSMCs. Importantly, it was also evident in hyperglycemia, a condition that underlies diabetes. No significant aspecific cytotoxicity was observed. Our data demonstrate the interrelation among E2F1–cyclin E1-cyclin E2 and the pivotal role this circuit exerts in CSMC proliferation. Additionally, our work validates the concept of utilizing anti–E2F1–cyclin E1-cyclin E2 siRNAs to develop a potential novel therapy to control intimal hyperplasia.