Abstract P292: ENPP1-Fc Protein Inhibits Proliferation of Human Vascular Smooth Muscle Cells

Background: Arterial stenosis leading to hypertension or heart failure is common in patients with Generalized Arterial Calcification of Infancy (GACI), an ultra-rare disease associated with loss of function mutations in ENPP1, an ectonucleotide pyrophosphatase that hydrolyzes extracellular ATP. GACI is characterized by accelerated calcification and severe myointimal proliferation. The role of ENPP1 in myoinitimal proliferation is unknown. Here, we examined the effect of ENPP1 on proliferation of human vascular smooth muscle cells (hVSMCs). Methods: ENPP1 expression was assessed in primary hVSMCs using qRT-PCR. ENPP1 expression was silenced using siRNA and its activity was verified by a cell based assay or inorganic pyrophosphate (PPi) assay and its effect on VSMC proliferation was determined by 3H-thymidine incorporation. Results: Treatment of primary hVSMCs with siRNA specific to the hENPP1 resulted in ~90% decrease in ENPP1 levels. Cellular enzyme activity correlated with ENPP1 expression in VSMCs. Silencing ENPP1 in hVSMCs led to 1-3 fold increase in proliferation relative to that of cells transfected with control siRNA in 2 out of 2 donors. Peak cell proliferation was observed at 5 days post-transfection. Human iPSC derived VSMCs (iVSMCs) expressed higher levels of ENPP1 than primary hVSMCs. Silencing ENPP1 in iVSMCs resulted in 3-5 fold increase in proliferation relative to that of cells transfected with negative control siRNA in 2 out of 2 donors. Addition of recombinant ENPP1-Fc protein restored ENPP1- associated proliferation in all donors (8099.75 ±134.32 (untreated) vs 1478 ± 55.34 (5ug/ml ENPP1-Fc treated, P Conclusion: For the first time, we have demonstrated that ENPP1 knockdown promotes proliferation of human VSMCs, and treatment with a functionally active ENPP1-Fc protein significantly inhibits ENPP1-associated proliferation. These results suggest that ENPP1 enzyme replacement may be a potential strategy to treat myointimal proliferation in patients with GACI disease.