Chronic kidney disease (CKD) is regarded as a state of Klotho deficiency and FGF23 excess. In patients with CKD a strong association has been found between increased serum FGF23 and mortality risk, possibly via enhanced atherosclerosis, vascular stiffness, and vascular calcification. The aim of this study was to examine the hypothesis that soluble Klotho and FGF23 exert direct, rapid effects on the vessel wall. We used three in vitro models: mouse aorta rings, human umbilical vein endothelial cells, and human vascular smooth muscle cells (HVSMC). Increasing medium concentrations of soluble Klotho and FGF23 both stimulated aorta contractions and increased ROS production in HVSMC. Klotho partially reverted FGF23 induced vasoconstriction, induced relaxation on phosphate preconstricted aorta and enhanced endothelial NO production in HUVEC. Thus Klotho increased both ROS production in HVSMC and NO production in endothelium. FGF23 induced contraction in phosphate preconstricted vessels and increased ROS production. Phosphate, Klotho and FGF23 together induced no change in vascular tone despite increased ROS production. Moreover, the three compounds combined inhibited relaxation despite increased NO production, probably owing to the concomitant increase in ROS production. In conclusion, although phosphate, soluble Klotho and FGF23 separately stimulate aorta contraction, Klotho mitigates the effects of phosphate and FGF23 on contractility via increased NO production, thereby protecting the vessel to some extent against potentially noxious effects of high phosphate or FGF23 concentrations. This novel observation is in line with the theory that Klotho deficiency is deleterious whereas Klotho sufficiency is protective against the negative effects of phosphate and FGF23 which are additive.
Chronic kidney disease (CKD) is characterized by vascular remodeling and the retention of uremic toxins, several of which are independently associated with the high cardiovascular mortality rate in CKD patients. Whether the association between these uremic toxins and cardiovascular mortality is due to induction of vascular dysfunction and resulting vascular remodeling remains to be determined. This study evaluates the effects of para-cresyl sulfate (PCS), a newly identified uremic toxin, on vascular function and remodeling. PCS acutely induced oxidative stress in both endothelial and vascular smooth muscle cells, with a maximal effect at 0.15 mM, corresponding to the mean "uremic" concentration found in dialysis patients. PCS significantly increased within 30 min phenylephrine-induced contraction of mouse thoracic aorta, through direct activation of rho-kinase, independently of oxidative stress induction, as demonstrated by the capacity of rho-kinase inhibitor Y-27632 to abolish this effect. After exposure of the aorta to PCS for 48 h, we observed inward eutrophic remodeling, a hallmark of uremic vasculopathy characterized by a reduction of the area of both lumen and media, with unchanged media/lumen ratio. In conclusion, elevated PCS concentrations such as those observed in CKD patients, by promoting both vascular dysfunction and vascular remodeling, may contribute to the development of hypertension and to cardiovascular mortality in CKD.
In epidemiological studies of CKD patients and the general population, high phosphate levels are associated with CV disease (CVD). The association is even observed in early-stage CKD and is independent of other traditional CV risk factors. 5, 6 Along with other factors, hyperphosphatemia is involved in the development of vascular calcification (VC) and endothelial dysfunction (ED), 7-9 both of which are major nontraditional CV risk factors for CKD. Phosphate is able to act either directly on these parameters or indirectly via the FGF23/Klotho and PTH axes (Figure 1).We review the various effects of hyperphosphatemia on CV parameters, with a focus on its emerging role in ED. The efficacy of current therapies for improving phosphate-related CV outcomes is also discussed. Direct Effects of Phosphate on CV Calcification and ED Epidemiological and Interventional StudiesVC is deposition of calcium/phosphate, mostly as apatite, in the blood vessels, myocardium and cardiac valves. Calcification of both the intima and media occurs in CKD, 10 resulting in the stiffness of the large arteries (as evidenced by a higher pulse wave velocity) and thus promotion of CV morbidity/ mortality. The results of several epidemiological studies have highlighted an association between serum phosphate levels and VC in stage 5D CKD patients, 11,12 in early-stage CKD patients and in the general population (in whom serum phosphate levels are still within the normal range). 13, 14 Indeed, 2 elegant prospective studies of large cohorts of hosphate, principally provided by food, is involved in many physiological processes: it buffers the intracellular pH, ensures the stability of the skeleton and contributes to many essential biological functions (eg, DNA synthesis, cell membrane phospholipids, energy metabolism and intracellular signaling pathways that are regulated by phosphorylation/dephosphorylation).The physiological concentration of phosphate (0.8-1.5 mmol/L) is tightly regulated by, among others, the fibroblast growth factor 23 (FGF23)/Klotho axis, parathyroid hormone (PTH) and calcitriol (the active form of vitamin D). 1 In chronic kidney disease (CKD), the progressive loss of functional nephrons induces retention of phosphate, which in turn leads to (1) an increase in calcium/phosphate products and (2) FGF23 synthesis. With the aim of preventing hyperphosphatemia, FGF23 decreases circulating levels of calcitriol, thus inhibiting intestinal phosphate and calcium absorption. PTH, the overexpression of which is triggered by hypocalcemia, stimulates bone resorption for the primary purpose of restoring calcemia but in doing so it increases phosphatemia and thus the calcium/phosphate products. During the course of CKD, Klotho's downregulation in both the parathyroid gland and the kidney results in the loss of FGF23 being able to respectively (1) decrease PTH expression and (2) inhibit renal phosphate reabsorption. In late-stage CKD, the few remaining functional nephrons are no longer able to eliminate phosphate and a vicious circle (referred ...
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