Suresh Mathew

Bone Morphogenetic Proteins in Vascular Calcification

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.

Hyperphosphatemia of chronic kidney disease

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.

The Mechanism of Phosphorus as a Cardiovascular Risk Factor in CKD

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.

Vitamin D Receptor Activators Can Protect against Vascular Calcification

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.

Low Turnover Osteodystrophy and Vascular Calcification Are Amenable to Skeletal Anabolism in an Animal Model of Chronic Kidney Disease and the Metabolic Syndrome

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.

Reversal of the Adynamic Bone Disorder and Decreased Vascular Calcification in Chronic Kidney Disease by Sevelamer Carbonate Therapy

A model of chronic kidney disease (CKD)-induced vascular calcification (VC) that complicates the metabolic syndrome was produced. In this model, the metabolic syndrome is characterized by severe atherosclerotic plaque formation, hypertension, type 2 diabetes, obesity, and hypercholesterolemia, and CKD stimulates calcification of the neointima and tunica media of the aorta. The CKD in this model is associated the adynamic bone disorder form of renal osteodystrophy. The VC of the model is associated with hyperphosphatemia, and control of the serum phosphorus both in this animal model and in humans has been preventive in the development of VC. This article reports studies that demonstrate reduction of established VC by the addition of sevelamer carbonate to the diets of this murine metabolic syndrome model with CKD. Sevelamer, besides normalizing the serum phosphorus, surprisingly, reversed the CKD-induced trabecular osteopenia. Sevelamer therapy increased osteoblast surfaces in the metaphyseal trabeculae of the tibia and femur. It also increased osteoid surfaces and, importantly, bone formation rates. In addition, sevelamer was found to be effective in decreasing serum cholesterol levels. These results suggest that sevelamer may have important actions in decreasing diabetic and uremic vasculopathy and that sevelamer carbonate may be capable of increasing bone formation rates that are suppressed by diabetic nephropathy.

CKD Accelerates Development of Neointimal Hyperplasia in Arteriovenous Fistulas

Arteriovenous (AV) access failure resulting from venous neointimal hyperplasia is a major cause of morbidity in patients with ESRD. To understand the role of chronic kidney disease (CKD) in the development of neointimal hyperplasia, we created AV fistulae (common carotid artery to jugular vein in an end-to-side anastomosis) in mice with or without CKD (renal ablation or sham operation). At 2 and 3 wk after operation, neointimal hyperplasia at the site of the AV anastomosis increased 2-fold in animals with CKD compared with controls, but cellular proliferation in the neointimal hyperplastic lesions did not significantly differ between the groups, suggesting that the enhanced neointimal hyperplasia in the setting of CKD may be secondary to a migratory phenotype of vascular smooth muscle cells (VSMC). In ex vivo migration assays, aortic VSMC harvested from mice with CKD migrated significantly greater than VSMC harvested from control mice. Moreover, animals with CKD had higher serum levels of osteopontin, which stimulates VSMC migration. When we treated animals with bone morphogenic protein-7, which promotes VSMC differentiation, before creation of the AV anastomosis, the effect of CKD on the development of neointimal hyperplasia was eliminated. In summary, CKD accelerates development of neointimal hyperplasia at the anastomotic site of an AV fistula, and administration of bone morphogenic protein-7 neutralizes this effect.

Bone morphogenetic protein 7: a novel treatment for chronic renal and bone disease.

Purpose of reviewWhen last reviewed, bone morphogenetic protein 7 was presented as a potential new renal therapeutic agent, with multiple efficacies in chronic kidney disease. The object of this review is to describe progress from many sources since then in support or denial of the hypothesis. Recent findingsBone morphogenetic protein 7 has been shown to be an effective defence in several forms of chronic kidney disease in animal models, and its mechanisms of action have begun to be elucidated. Bone morphogenetic protein 7 inhibits tubular epithelial cell de-differentiation, mesenchymal transformation and apoptosis stimulated by various renal injuries. Bone morphogenetic protein 7 preserves glomerular integrity and inhibits injury-mediated mesangial matrix accumulation. In renal osteodystrophy, bone morphogenetic protein 7 affects osteoblast morphology and number, eliminates peritrabecular fibrosis, decreases bone resorption, and increases bone formation in secondary hyperparathyroidism. Bone morphogenetic protein 7 restores normal rates of bone formation in the adynamic bone disorder. Bone morphogenetic protein 7 is broadly efficacious in renal osteodystrophy, and importantly increases the skeletal deposition of ingested phosphorus and calcium, improving ion homeostasis in chronic kidney disease. Bone morphogenetic protein 7 was shown to prevent vascular calcification in a model of chronic kidney disease associated with the restoration of osteocalcin expression to normal tissue-restricted sites. SummaryBone morphogenetic protein 7 may be a powerful new therapeutic agent for chronic kidney disease, with the novel attribute of not only treating the kidney disease itself, but also directly inhibiting some of the most important complications of the disease state.

Renal osteodystrophy, phosphate homeostasis, and vascular calcification.

New advances in the pathogenesis of renal osteodystrophy (ROD) change the perspective from which many of its features and treatment are viewed. Calcium, phosphate, parathyroid hormone (PTH), and vitamin D have been shown to be important determinants of survival associated with kidney diseases. Now ROD dependent and independent of these factors is linked to survival more than just skeletal frailty. This review focuses on recent discoveries that renal injury impairs skeletal anabolism decreasing the osteoblast compartment of the skeleton and consequent bone formation. This discovery and the discovery that PTH regulates the hematopoietic stem cell niche alters our view of secondary hyperparathyroidism in chronic kidney disease (CKD) from that of a disease to that of a necessary adaptation to renal injury that goes awry. Furthermore, ROD is shown to be an underappreciated factor in the level of the serum phosphorus in CKD. The discovery and the elucidation of the mechanism of hyperphosphatemia as a cardiovascular risk in CKD change the view of ROD. It is now recognized as more than a skeletal disorder, it is an important component of the mortality of CKD that can be treated.

Function and effect of bone morphogenetic protein-7 in kidney bone and the bone-vascular links in chronic kidney disease

In two independent and separate studies, we have shown that renal injury and chronic kidney disease (CKD) directly inhibit skeletal anabolism, and that stimulation of bone formation decreased the serum phosphate. In the first study, the serum Ca PO4, parathyroid hormone (PTH), and calcitriol were maintained normal after renal ablation in mice, and even mild renal injury equivalent to stage 3 CKD decreased bone formation rates. More recently, these observations were rediscovered in low‐density lipoprotein receptor null (LDLR‐/–) mice fed high‐fat/cholesterol diets, a model of the metabolic syndrome (hypertension, obesity, dyslipidemia and insulin resistance). We demonstrated that these mice have vascular calcification (VC) of both the intimal atherosclerotic type and medial calcification. We have also shown that VC is made worse by CKD and ameliorated by bone morphogenetic protein‐7 (BMP‐7). The finding that high‐fat fed LDLR‐/– animals with CKD had hyperphosphatemia which was prevented in BMP‐7‐treated animals lead us to examine the skeletons of these mice. It was found that significant reductions in bone formation rates were associated with high‐fat feeding, and superimposing CKD resulted in the adynamic bone disorder (ABD), while VC was made worse. The effect of CKD to decrease skeletal anabolism (decreased bone formation rates and reduced number of bone modelling units) occurred despite secondary hyperparathyroidism. The BMP‐7 treatment corrected the ABD and hyperphosphatemia, owing to BMP‐7‐driven stimulation of skeletal phosphate deposition reducing plasma phosphate and thereby removing a major stimulus to VC. A pathological link between abnormal bone mineralization and VC through the serum phosphorus was demonstrated by the partial effectiveness of directly reducing the serum phosphate by a phosphate binder that had no skeletal action.