Abstract-Vascular calcification is a common problem among the elderly and those with chronic kidney disease (CKD) and diabetes. The process of tunica media vascular calcification in CKD appears to involve a phenotypic change in the vascular smooth muscle cell (VSMC) resulting in cell-mediated mineralization of the extracellular matrix. The bone morphogenetic proteins (BMPs) are important regulators in orthotopic bone formation, and their localization at sites of vascular calcification raises the question of their role. In this review, we will discuss the actions of the BMPs in vascular calcification. Although the role of BMP-2 in vascular calcification is not proven, it has been the most studied member of the BMP family in this disease process. The role of BMP-2 may be through inducing osteoblastic differentiation of VSMCs through induction of MSX-2, or by inducing apoptosis of VSMCs, a process thought critical in the initiation of vascular calcification. Additionally, BMP-2 may be related to loss of regulation of the matrix Gla protein. A second BMP, BMP-7, less studied than BMP-2 may have opposing actions in vascular calcification. In postnatal life, BMP-7 is expressed primarily in the kidney, and expression is diminished by renal injury. BMP-7 is an important regulator of skeletal remodeling and the VSMC phenotype. BMP-7 restores skeletal anabolic balance in animal models of CKD with disordered skeletal modeling, also reducing serum phosphate in the process. BMP-7 also reverses vascular calcification in CKD, and reduction in vascular calcification is due, in part, to reduced serum phosphate, an important inducer of VSMC-mediated vascular mineralization and in part to direct actions on the VSMC. 15,17 or myofibroblasts into the vessel wall accounts for the mineralizing cell population in the medial artery calcification of diabetes. Although the search for the origin of the mineralizing cell goes on, it is also imperative to understand the stimulus that drives it. One possible stimulus is the bone morphogenetic proteins (BMPs), which along with the Wnt family of glycoproteins 18 and sex steroids, are the known important anabolic factors in bone formation and determinants of bone mineral content. 19 -24 Because they are essential to normal bone formation, it is intuitive to consider that the BMPs may also be important in the pathophysiology of vascular calcification. Although definitive evidence to support this is lacking, there is considerable supportive evidence, and we will discuss the basic physiology of the BMPs, concentrating primarily on BMP-2 and -7, in this review. We will discuss how BMP-2 expression in the vasculature may entrain a transcriptional program that leads to an osteoblastlike cellular phenotype and matrix mineralization. Furthermore, we will discuss preliminary studies of the protective actions of another BMP, BMP-7, on vascular calcification. The Bone Morphogenetic ProteinsThe BMPs are a group of at least 30 proteins named for their osteoinductive properties that have important develop...

Observational studies have determined hyperphosphatemia to be a cardiovascular risk factor in chronic kidney disease. Mechanistic studies have elucidated that hyperphosphatemia is a direct stimulus to vascular calcification, which is one cause of morbid cardiovascular events contributing to the excess mortality of chronic kidney disease. This review describes the pathobiology of hyperphosphatemia that develops as a consequence of positive phosphate balance in chronic kidney disease and the mechanisms by which hyperphosphatemia acts on neointimal vascular cells that are stimulated to mineralize in chronic kidney disease. The characterization of hyperphosphatemia of chronic kidney disease as a distinct syndrome in clinical medicine with unique disordered skeletal remodeling, heterotopic mineralization and cardiovascular morbidity is presented.

LDL receptor (LDLR)-null mice fed high-fat/cholesterol diets, a model of the metabolic syndrome, have vascular calcification (VC) worsened by chronic kidney disease (CKD) and ameliorated by bone morphogenetic protein-7 (BMP-7), an efficacious agent in treating animal models of renal osteodystrophy. Here, LDLR؊/؊ high-fat-fed mice without CKD were shown to have significant reductions in bone formation rates, associated with increased VC and hyperphosphatemia. Superimposing CKD resulted in a low turnover osteodystrophy, whereas VC worsened and hyperphosphatemia persisted. BMP-7 treatment corrected the hyperphosphatemia, corrected the osteodystrophy, and prevented VC, compatible with skeletal phosphate deposition leading to reduced plasma phosphate and removal of a major stimulus to VC. A pathologic link between abnormal bone mineralization and VC through the serum phosphorus was supported by the partial effectiveness of directly reducing the serum phosphate by a phosphate binder that had no skeletal action. Thus, in this model of the metabolic syndrome with CKD, a reduction in bone-forming potential of osteogenic cells leads to low bone turnover rates, producing hyperphosphatemia and VC, processes ameliorated by the skeletal anabolic agent BMP-7, in part through deposition of phosphate and increased bone formation. C ardiovascular (CV) mortality in patients with chronic kidney disease (CKD) is extremely high (1,2). Conventional risk factors that are characteristic of the metabolic syndrome (3), such as hypertension, dyslipidemia, insulin resistance, and overt diabetes, are highly prevalent in CKD, but other CV risk factors with additive affects that are more specific to the uremic milieu have also been identified (4). One such is the presence of vascular calcification (VC) (5), a form of heterotopic mineralization that is predictive of CV mortality (6,7) and is both common and severe in CKD (8). J Am Soc NephrolThe pathogenesis of VC in CKD remains under investigation, but hyperphosphatemia is an important risk factor for both VC and CV mortality (9,10). VC and CV mortality are also associated with other abnormalities of calcium and phosphate homeostasis, including hypercalcemia, elevated calcium and phosphate ion products, vitamin D therapy, and hyperparathyroidism (9,10), and these findings suggest a link with renal osteodystrophy (ROD).ROD is virtually ubiquitous in CKD, characterized by a spectrum of histologic abnormalities of bone that contribute to the biochemical abnormalities discussed above (11). At one end of the spectrum, osteitis fibrosa is a high-turnover state driven by secondary hyperparathyroidism, characterized by poorly differentiated osteoblast precursors manifesting a fibroblastic phenotype, and stimulating increased osteoclastic activity. This results in net bone resorption, fibrosis of the bone marrow space, and release of calcium and phosphate into the extracellular fluid (12). At the other end of the spectrum, adynamic bone disorder (ABD) is characterized by quiescent osteoblasts and osteo...

Hyperphosphatemia and vascular calcification have emerged as cardiovascular risk factors among those with chronic kidney disease. This study examined the mechanism by which phosphorous stimulates vascular calcification, as well as how controlling hyperphosphatemia affects established calcification. In primary cultures of vascular smooth muscle cells derived from atherosclerotic human aortas, activation of osteoblastic events, including increased expression of bone morphogenetic protein 2 (BMP-2) and the transcription factor RUNX2, which normally play roles in skeletal morphogenesis, was observed. These changes, however, did not lead to matrix mineralization until the phosphorus concentration of the media was increased; phosphorus stimulated expression of osterix, a second critical osteoblast transcription factor. Knockdown of osterix with small interference RNA (siRNA) or antagonism of BMP-2 with noggin prevented matrix mineralization in vitro. Similarly, vascular BMP-2 and RUNX2 were upregulated in atherosclerotic mice, but significant mineralization occurred only after the induction of renal dysfunction, which led to hyperphosphatemia and increased aortic expression of osterix. Administration of oral phosphate binders or intraperitoneal BMP-7 decreased expression of osterix and aortic mineralization. It is concluded that, in chronic kidney disease, hyperphosphatemia stimulates an osteoblastic transcriptional program in the vasculature, which is mediated by osterix activation in cells of the vascular tunica media and neointima. Chronic kidney disease (CKD) is a fatal illness, and cardiovascular complications are the major causes of morbidity and mortality. 1,2 The causes of the excess cardiovascular mortality associated with CKD are unknown, because the role of the standard risk factors associated with cardiovascular mortality do not account for the increased risk in CKD. 2 There is strong epidemiologic evidence that serum phosphorus is an independent risk factor for cardiovascular events and mortality in CKD. 3,4 The serum phosphorus has been linked to another cardiovascular risk factor, vascular calcification (VC), 3,5,6 an important cause of vascular stiffness in CKD leading to increased pulse wave velocity, increased cardiac work, left ventricular hypertrophy, and decreased coronary artery blood flow. 6 -8 Phosphorus has been further implicated as a cause of VC through studies in vitro that have demonstrated that it induces phenotypic changes in vascular smooth muscle cells (VSMC) by increasing gene transcription of proteins involved in osteoblast function-bone formation 9 and stimulating matrix mineralization. 10 -12 In the uremic calcifying environment, expression of the contractile proteins of VSMC, such as ␣-smooth muscle actin, SM22,

An apparent conflict exists between observational studies that suggest that vitamin D receptor (VDR) activators provide a survival advantage for patients with ESRD and other studies that suggest that they cause vascular calcification. In an effort to explain this discrepancy, we studied the effects of the VDR activators calcitriol and paricalcitol on aortic calcification in a mouse model of chronic kidney disease (CKD)-stimulated atherosclerotic cardiovascular mineralization. At dosages sufficient to correct secondary hyperparathyroidism, calcitriol and paricalcitol were protective against aortic calcification, but higher dosages stimulated aortic calcification. At protective dosages, the VDR activators reduced osteoblastic gene expression in the aorta, which is normally increased in CKD, perhaps explaining this inhibition of aortic calcification. Interpreting the results obtained using this model, however, is complicated by the adynamic bone disorder; both calcitriol and paricalcitol stimulated osteoblast surfaces and rates of bone formation. Therefore, the skeletal actions of the VDR activators may have contributed to their protection against aortic calcification. We conclude that low, clinically relevant dosages of calcitriol and paricalcitol may protect against CKD-stimulated vascular calcification.