Molecular mechanisms of cardiovascular calcification

Abstract

Cardiovascular calcification is a pathophysiological process characterized by the deposition of calcium-phosphate crystals in the arteries and the leaflets of the heart valves. In the arteries calcification causes arterial stiffness, which may lead to poor cardiac perfusion, systolic hypertension and heart failure. In the aortic valve, calcification causes left ventricular outflow obstruction. Currently, no medical treatment exists to halt or reverse cardiovascular calcification. For that reason, understanding the molecular mechanisms underlying cardiovascular calcification is of particular importance.

Molecularly, cardiovascular calcification is a continuum comprising intertwined physicochemical and biologically active processes. In particular, cardiovascular calcification commences when cells become overburdened by the mineral imbalance typical of chronic kidney disease (CKD), or the unresolved inflammation characteristic of atherosclerosis and aortic valve stenosis (AVS). These alterations in homeostasis lead to changes in the fate and phenotype of structural cells such as vascular smooth muscle cells (VSMCs) and valvular interstitial cells (VICs). This phenotypic switch is characterized by: the loss of calcification inhibitors, an increase in pro-osteogenic signaling, changes in proliferation, abnormal processing and synthesis of extracellular matrix (ECM), and alterations in autophagy.

In the current thesis, three pathways relevant to cardiovascular calcification are discussed. First, in Articles I and II, the G-protein coupled receptor ChemR23 arises as a promoter of a synthetic and proliferative VSMC phenotype, prone to phosphate-induced calcification. Importantly, this phenotype could be reverted by genetic deletion of ChemR23, and calcification was inhibited by the ChemR23 ligands: RvE1 and chemerin. Translationally, chemerin was negatively associated with coronary artery calcification in CKD patients. Moreover, in Article III, ChemR23 expressed in macrophages, promoted the resolution of inflammation, and inhibited VSMC proliferation in a mouse model of intimal hyperplasia. Secondly, Article IV demonstrates that iron, preferentially present in the calcified regions of the aortic valve, accumulated in VICs. This uptake of iron enhanced VIC proliferation and actively contributed to the ECM remodeling. Finally, Article V reveals a detrimental role of the second generation tyrosine kinase inhibitor nilotinib on the aortic valve. In vivo, nilotinib promoted aortic valve thickening. In vitro, nilotinib enhanced VIC osteoblastic trans- differentiation, increased calcification and inhibited autophagy. Mechanistically, nilotinib preferentially inhibited the most abundant collagen sensing tyrosine kinase in the valve: the discoidin domain receptor 2.

Overall the results from this thesis suggests that changes in VSMC and VIC phenotype, as well as alterations in the ECM content and sensing can have profound effects on cardiovascular calcification, and therefore serve as potential therapeutic targets.