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,

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. 18: 122-130, 200718: 122-130, . doi: 10.1681 P rogression of diabetic nephropathy (DN) generally is considered in terms of progressive loss of kidney function until end-stage kidney failure occurs and renal replacement therapy begins. However, DN is a systemic disease, and it also is fatal. Indeed, more patients with DN die before reaching the need for dialysis than accrue to modalities of renal replacement therapy (1-3). Cardiovascular mortality in patients with chronic kidney disease (CKD) is extremely high (1,4). Conventional risk factors that are characteristic of the metabolic syndrome (5), such as hypertension, dyslipidemia, insulin resistance, and overt diabetes, are highly prevalent in CKD, but other risk factors with additive affects that are more specific to the uremic milieu also have been identified (6 -8). J Am Soc NephrolOne is the presence of vascular calcification (VC) (9), a form of heterotopic mineralization that is predictive of cardiovascular mortality (10,11) and is both common and severe in CKD (12). The VC of the tunica media that is seen in CKD is similar to that observed in type 2 diabetes without DN, and when CKD is added to diabetes through DN, the cardiovascular risk is at least additive of that of CKD plus diabetes; in other words, extreme. We have developed an animal model of VC that is worsened by CKD (13). The model is partial renal ablation in the LDL receptor-deficient (LDLRϪ/Ϫ) mouse that is fed high-fat/ cholesterol diets. This model resembles the clinical situation of CKD's complicating the metabolic syndrome, because the mice have obesity, hypertension, insulin...

The microcolony assay originally described by Withers and Elkind in 1970 (1) has been a useful method for investigating the effects of radiation and various other genotoxic and cytotoxic damaging agents on the intestinal epithelial stem cell population and to assess the ability of a variety of compounds to protect the epithelial stem cell population from the lethal effects of chemical and physical agents (e.g., 2-7). Epithelial stem cells are located near the base of each intestinal crypt and play an important role in normal epithelial renewal and differentiation, epithelial injury-repair, and in neoplastic transformation (8-11). In the adult mouse small intestine these functionally anchored clonogenic stem cells divide rarely to produce a daughter stem cell (self renewal) as well as a more rapidly replicating transit cell. Transit cells, in turn, undergo a number of rapid cell divisions in the proliferative zone located in the lower half of each crypt. Their progeny subsequently differentiate into the mature epithelial cell types found in the small intestine as they migrate away from the proliferative zone in each intestinal crypt (8-11). Following intestinal injury and disruption of the epithelium, epithelial cells adjacent to the wound first migrate over the injured area to reestablish continuity of the epithelium. Stem cells subsequently proliferate to increase their numbers and to give rise to the more rapidly proliferating transit cell population. The transit cell population then expands rapidly to form a regenerative crypt. If the injury has completely destroyed some crypts, the surviving regenerative crypts can subsequently branch and divide to restore near normal numbers of viable crypts (3).

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