Rôle de la phosphatase alcaline non spécifique du tissu "TNAP" dans la calcification des plaques d’athérosclérose

Although Knowledge of the calcium score (CAC), improves the prediction of cardiovascular events, the situation remains quite controversial regarding the local impact of calcification on the stability of atherosclerotic plaque. The current paradigm suggests that highly calcified plaques are rather stabilized, whereas those containing microcalcifications are probably dangerous: by exacerbating the inflammation of the plaques, they could trigger their rupture. However, no experimental approach has been able to prevent the formation of microcalcifications in a preclinical model in order to determine their real impact. Tissue-nonspecific alkaline phosphatase (TNAP), which is the only enzyme necessary for skeletal and dental mineralization, is strongly suspected to participate in plaque calcification. TNAP is expressed in atherosclerotic plaques, and is also produced in the liver and released in the circulation at levels that correlate with metabolic syndrome and cardiovascular mortality. In this context, we first sought to explore the involvement of TNAP in atherosclerotic plaques in the mousse model of mice deficient in apolipoprotein E (ApoE)-. Second, we set out to study the effect of a recently developed TNAP inhibitor «SBI-425» on atherosclerotic plaque calcification and development. Plaque calcification was explored in 64 ApoE-/- mice fed a high fat diet from 10 weeks of age, and sacrificed every two weeks, from 17 to 31 weeks with 18F-NaF positron emission tomography and computed tomography in vivo, and with the calcium tracer osteosense ex vivo (osteosense fluorescent agent (OS)). TNAP expression and/or activity were investigated in mice and in calcified and non-calcified human carotid plaques. TNAP was inhibited in-vivo in 52 mice by oral administration of SBI-425 (30 mg/kg/day), from 10 to 25 weeks of age. In parallel, it was inhibited in-vitro in human vascular smooth muscle cells (VSMCs) with its parental compound MLS-0038949. In the aortas of ApoE-deficient mice, as well as in human carotids, plaque calcifications consistently colocalizes with local TNAP activity. In-vitro, in human VSMCs, TNAP inhibition prevented calcification. In ApoE-/- mice, TNAP inhibition by SBI-425, prevented plaque calcification, reduced plaque inflammation and limited plaque development (reduced lipid accumulation), in association with reduced levels of serum cholesterol and triglycerides. Importantly, these beneficial effects of TNAP inhibition occurred without significant impact on weight, mortality, bone architecture and kidneys. On the other hand, metabolomics analysis by RMN of liver extracts identified phosphocholine as a likely substrate of liver TNAP, whose reduced dephosphorylation upon TNAP inhibition may have participated in the reduced release of cholesterol and triglycerides into the blood, and impaired plaque development. In conclusion, our findings demonstrate that the systemic inhibition of TNAP for up to 25 weeks ameliorates atherosclerosis acting on both vascular and liver TNAP.