Exploring the biology of vascular calcification in chronic kidney disease: what's circulating?

Chronic kidney disease (CKD) is associated with fatal cardiovascular consequences in part due to ectopic calcification of soft tissues particularly arteries, capillaries, and cardiac valves. An increasing body of evidence from experimental studies and in vivo data suggest that (I) a mineral imbalance with hyperphosphatemia and high-circulating calcium x phosphate product, (II) a deficiency of systemic or local calcification inhibitors, (III) death or 'damage' of vascular smooth muscle cells (VSMCs), and/or (IV) phenotypic transformation of VSMCs to osteo/chondrocytic cells may all act in concert to initiate and sustain vascular calcification. In CKD patients inhibitory systems are overwhelmed by a multitude of agents that induce VSMC damage and cell death resulting in the release of vesicles capable of nucleating basic calcium phosphate. Studies with genetically altered mice have identified both local and systemic calcification inhibitors that act to maintain VSMC differentiation or regulate vesicle properties. However, for many of these proteins the mechanisms and sites of action are still under investigation. In particular, it is unclear whether factors present in the circulation have an inhibitory role there and whether circulating levels of these proteins influence or are indicative of underlying disease processes in individual patients. A greater understanding of the origins and roles of potential circulating inhibitors may result in novel strategies aimed at the prevention or reversal of the life-limiting calcifying vasculopathies seen in CKD patients.