ObjectiveOverexpression of tissue-nonspecific alkaline phosphatase (TNAP) in endothelium leads to arterial calcification in mice. The purpose of this study was to examine the effect of elevated endothelial TNAP on coronary atherosclerosis. In addition, we aimed to examine endogenous TNAP activity in human myocardium.Approach and resultsA vascular pattern of TNAP activity was observed in human non-failing, ischemic, and idiopathic dilated hearts (5 per group); no differences were noted between groups in this study. Endothelial overexpression of TNAP was achieved in mice harboring a homozygous recessive mutation in the low density lipoprotein receptor (whc allele) utilizing a Tie2-cre recombinase (WHC-eTNAP mice). WHC-eTNAP developed significant coronary artery calcification at baseline compared WHC controls (4312 vs 0μm2 alizarin red area, p<0.001). Eight weeks after induction of atherosclerosis, lipid deposition in the coronary arteries of WHC-eTNAP was increased compared to WHC controls (121633 vs 9330μm2 oil red O area, p<0.05). Coronary lesions in WHC-eTNAP mice exhibited intimal thickening, calcifications, foam cells, and necrotic cores. This was accompanied by the reduction in body weight and left ventricular ejection fraction (19.5 vs. 23.6g, p<0.01; 35% vs. 47%, p<0.05). In a placebo-controlled experiment under atherogenic conditions, pharmacological inhibition of TNAP in WHC-eTNAP mice by a specific inhibitor SBI-425 (30mg*kg-1*d-1, for 5 weeks) reduced coronary calcium (78838 vs.144622μm2) and lipids (30754 vs. 77317μm2); improved body weight (22.4 vs.18.8g) and ejection fraction (59 vs. 47%). The effects of SBI-425 were significant in the direct comparisons with placebo but disappeared after TNAP-negative placebo-treated group was included in the models as healthy controls.ConclusionsEndogenous TNAP activity is present in human cardiac tissues. TNAP overexpression in vascular endothelium in mice leads to an unusual course of coronary atherosclerosis, in which calcification precedes lipid deposition. The prevalence and significance of this mechanism in human atherosclerosis requires further investigations.

Objective: Vascular calcification in asymptomatic individuals is an independent predictor of coronary heart disease (CHD). It is therefore plausible that vascular calcification plays a direct pathophysiological role in atherosclerosis, an underlying cause of CHD. The purpose of this study was to examine the contribution that vascular calcification has on the development of coronary atherosclerosis in a mouse model of familial hypercholesterolemia. Approach and Results: Calcification was induced by overexpression of tissue-nonspecific alkaline phosphatase (TNAP) in endothelial cells of mice harboring a point mutation in the low density lipoprotein receptor ( ldlr, wicked high cholesterol, WHC). Mice were fed an atherogenic diet; echocardiographic and biochemical data were collected longitudinally. Atherosclerosis and vascular calcification were analyzed histologically in the aorta, aortic sinus and coronary arteries. TNAP mice were also treated with a combination of an atherogenic diet and a specific inhibitor of TNAP (SBI-425). Combined with the ldlr mutation and an atherogenic diet, TNAP-driven arterial calcification led to severe atherosclerosis with 100% morbidity characterized by occlusive coronary artery disease, pathological cardiac hypertrophy with dilated LV and reduced ejection fraction (EF). We detected an interaction between vascular calcification and atherosclerosis in mice with endothelial TNAP overexpression. This interaction was particularly prominent in coronary circulation. Targeting TNAP activity therapeutically helped improve survival and heart function of endothelial TNAP overexpressor mice, however the incomplete inhibition of TNAP by SBI-425 was a limitation of this study. Conclusions: Vascular calcification via TNAP overexpression in endothelial cells promotes coronary atherosclerosis and is pathogenic under conditions of hypercholesterolemia.

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