Vascular calcification and osteoporosis--from clinical observation towards molecular understanding

Patients with osteoporosis frequently suffer from vascular calcification, which was shown to predict both cardiovascular morbidity/mortality and osteoporotic fractures. Various common risk factors and mechanisms have been suggested to cause both bone loss and vascular calcification, including aging, estrogen deficiency, vitamin D and K abnormalities, chronic inflammation and oxidative stress. Major breakthroughs in molecular and cellular biology of bone metabolism and the characterization of knockout animals with deletion of bone-related genes have led to the concept that common signaling pathways, transcription factors and extracellular matrix interactions may account for both skeletal and vascular abnormalities. For example, mice that lack the cytokine decoy receptor osteoprotegerin or the hormone Klotho display a combined osteoporosis-arterial calcification phenotype. In this review, we summarize the current data and evaluate potential mechanisms of the osteoporosis-arterial calcification syndrome. We propose a unifying hypothesis of vascular calcification that combines both active and passive mechanisms of vascular mineralization with aspects of bone resorption and age-related changes.