Yifu Fang

Early chronic kidney disease-mineral bone disorder stimulates vascular calcification

The chronic kidney disease-mineral and bone disorder (CKD-MBD) syndrome is an extremely important complication of kidney diseases. Here we tested whether CKD-MBD causes vascular calcification in early kidney failure by developing a mouse model of early CKD in a background of atherosclerosis stimulated arterial calcification. CKD equivalent in glomerular filtration reduction to human CKD stage 2 stimulated early vascular calcification and inhibited the tissue expression of α-klotho (klotho) in the aorta. In addition, osteoblast transition in the aorta was stimulated by early CKD as shown by the expression of the critical transcription factor, RUNX2. The ligand associated with the klotho-fibroblast growth factor receptor complex, FGF23, was found to be expressed in the vascular media of sham operated mice. Its expression was decreased in early CKD. Increased circulating levels of the osteocyte secreted proteins, FGF23, and sclerostin may have been related to increased circulating klotho levels. Finally, we observed low turnover bone disease with a reduction in bone formation rates more than bone resorption. Thus, the CKD-MBD, characterized by cardiovascular risk factors, vascular calcification, increased circulating klotho, FGF23 and sclerostin levels, and low turnover renal osteodystrophy, was established in early CKD. Early CKD caused a reduction of vascular klotho, stimulated vascular osteoblastic transition, increased osteocytic secreted proteins, and inhibited skeletal modeling producing the CKD-MBD.

CKD-Induced Wingless/Integration1 Inhibitors and Phosphorus Cause the CKD–Mineral and Bone Disorder

In chronic kidney disease, vascular calcification, renal osteodystrophy, and phosphate contribute substantially to cardiovascular risk and are components of CKD-mineral and bone disorder (CKD-MBD). The cause of this syndrome is unknown. Additionally, no therapy addresses cardiovascular risk in CKD. In its inception, CKD-MBD is characterized by osteodystrophy, vascular calcification, and stimulation of osteocyte secretion. We tested the hypothesis that increased production of circulating factors by diseased kidneys causes the CKD-MBD in diabetic mice subjected to renal injury to induce stage 2 CKD (CKD-2 mice). Compared with non-CKD diabetic controls, CKD-2 mice showed increased renal production of Wnt inhibitor family members and higher levels of circulating Dickkopf-1 (Dkk1), sclerostin, and secreted klotho. Neutralization of Dkk1 in CKD-2 mice by administration of a monoclonal antibody after renal injury stimulated bone formation rates, corrected the osteodystrophy, and prevented CKD-stimulated vascular calcification. Mechanistically, neutralization of Dkk1 suppressed aortic expression of the osteoblastic transcription factor Runx2, increased expression of vascular smooth muscle protein 22-α, and restored aortic expression of klotho. Neutralization of Dkk1 did not affect the elevated plasma levels of osteocytic fibroblast growth factor 23 but decreased the elevated levels of sclerostin. Phosphate binder therapy restored plasma fibroblast growth factor 23 levels but had no effect on vascular calcification or osteodystrophy. The combination of the Dkk1 antibody and phosphate binder therapy completely treated the CKD-MBD. These results show that circulating Wnt inhibitors are involved in the pathogenesis of CKD-MBD and that the combination of Dkk1 neutralization and phosphate binding may have therapeutic potential for this disorder.

Cardiovascular risk factors in chronic kidney disease: does phosphate qualify?

Risk factors for disease states are rigorously defined. This analysis considers the definition of a risk factor as applied to the question of whether the serum phosphorus level is a risk factor for cardiovascular disease. Observational studies strongly suggest that phosphorus is associated with cardiovascular risk, and definitive prospective animal studies are supportive. A plausible mechanism of action has been discovered demonstrating that phosphorus stimulates osteoblastic transition of cells in the neointima of atherosclerotic plaques, which, if prevented, blocks vascular calcification. However, prospective studies demonstrating that modulation of the putative risk factor affects clinical outcomes are lacking, and phosphorus, as yet, does not qualify as a cardiovascular risk factor. This is a clarion call for additional research.Risk factors for disease states are rigorously defined. This analysis considers the definition of a risk factor as applied to the question of whether the serum phosphorus level is a risk factor for cardiovascular disease. Observational studies strongly suggest that phosphorus is associated with cardiovascular risk, and definitive prospective animal studies are supportive. A plausible mechanism of action has been discovered demonstrating that phosphorus stimulates osteoblastic transition of cells in the neointima of atherosclerotic plaques, which, if prevented, blocks vascular calcification. However, prospective studies demonstrating that modulation of the putative risk factor affects clinical outcomes are lacking, and phosphorus, as yet, does not qualify as a cardiovascular risk factor. This is a clarion call for additional research.

The chronic kidney disease — Mineral bone disorder (CKD-MBD): Advances in pathophysiology

The causes of excess cardiovascular mortality associated with chronic kidney disease (CKD) have been attributed in part to the CKD-mineral bone disorder syndrome (CKD-MBD), wherein, novel cardiovascular risk factors have been identified. New advances in the causes of the CKD-MBD are discussed in this review. They demonstrate that repair and disease processes in the kidneys release factors to the circulation that cause the systemic complications of CKD. The discovery of WNT inhibitors, especially Dickkopf 1 (Dkk1), produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. This lead to the discovery that activin A is a second renal repair factor circulating in increased levels during CKD. Activin A derives from peritubular myofibroblasts of diseased kidneys, wherein it stimulates fibrosis, and decreases tubular klotho expression. Activin A binds to the type 2 activin A receptor, ActRIIA, which is variably affected by CKD in the vasculature. In diabetic/atherosclerotic aortas, specifically in vascular smooth muscle cells (VSMC), ActRIIA signaling is inhibited and contributes to CKD induced VSMC dedifferentiation, osteogenic transition and neointimal atherosclerotic calcification. In nondiabetic/nonatherosclerotic aortas, CKD increases VSMC ActRIIA signaling, and vascular fibroblast signaling causing the latter to undergo osteogenic transition and stimulate vascular calcification. In both vascular situations, a ligand trap for ActRIIA prevented vascular calcification. In the skeleton, activin A is responsible for CKD stimulation of osteoclastogenesis and bone remodeling increasing bone turnover. These studies demonstrate that circulating renal repair and injury factors are causal of the CKD-MBD and CKD associated cardiovascular disease.

Ligand trap for the activin type IIA receptor protects against vascular disease and renal fibrosis in mice with chronic kidney disease

The causes of cardiovascular mortality associated with chronic kidney disease (CKD) are partly attributed to the CKD-mineral bone disorder (CKD-MBD). The causes of the early CKD-MBD are not well known. Our discovery of Wnt (portmanteau of wingless and int) inhibitors, especially Dickkopf 1, produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. In the search for such factors, we studied the effects of activin receptor type IIA (ActRIIA) signaling by using a ligand trap for the receptor, RAP-011 (a soluble extracellular domain of ActRIIA fused to a murine IgG-Fc fragment). In a mouse model of CKD that stimulated atherosclerotic calcification, RAP-011 significantly increased aortic ActRIIA signaling assessed by the levels of phosphorylated Smad2/3. Furthermore, RAP-011 treatment significantly reversed CKD-induced vascular smooth muscle dedifferentiation as assessed by smooth muscle 22α levels, osteoblastic transition, and neointimal plaque calcification. In the diseased kidneys, RAP-011 significantly stimulated αklotho levels and it inhibited ActRIIA signaling and decreased renal fibrosis and proteinuria. RAP-011 treatment significantly decreased both renal and circulating Dickkopf 1 levels, showing that Wnt activation was downstream of ActRIIA. Thus, ActRIIA signaling in CKD contributes to the CKD-MBD and renal fibrosis. ActRIIA signaling may be a potential therapeutic target in CKD.

The activin receptor is stimulated in the skeleton, vasculature, heart, and kidney during chronic kidney disease

We examined activin receptor type IIA (ActRIIA) activation in chronic kidney disease (CKD) by signal analysis and inhibition in mice with Alport syndrome using the ActRIIA ligand trap RAP-011 initiated in 75-day-old Alport mice. At 200 days of age, there was severe CKD and associated Mineral and Bone Disorder (CKD-MBD), consisting of osteodystrophy, vascular calcification, cardiac hypertrophy, hyperphosphatemia, hyperparathyroidism, elevated FGF23, and reduced klotho. The CKD-induced bone resorption and osteoblast dysfunction was reversed, and bone formation was increased by RAP-011. ActRIIA inhibition prevented the formation of calcium apatite deposits in the aortic adventitia and tunica media and significantly decreased the mean aortic calcium concentration from 0.59 in untreated to 0.36 mg/g in treated Alport mice. Aortic ActRIIA stimulation in untreated mice increased p-Smad2 levels and the transcription of sm22α and αSMA. ActRIIA inhibition reversed aortic expression of the osteoblast transition markers Runx2 and osterix. Heart weight was significantly increased by 26% in untreated mice but remained normal during RAP-011 treatment. In 150-day-old mice, GFR was significantly reduced by 55%, but only by 30% in the RAP-011-treated group. In 200-day-old mice, the mean BUN was 100 mg/dl in untreated mice compared to 60 mg/dl in the treated group. In the kidneys of 200-day-old mice, ActRIIA and p-Smad2 were induced and MCP-1, fibronectin, and interstitial fibrosis were stimulated; all were attenuated by RAP-011 treatment. Hence, the activation of ActRIIA signaling during early CKD contributes to the CKD-MBD components of osteodystrophy and cardiovascular disease and to renal fibrosis. Thus, the inhibition of ActRIIA signaling is efficacious in improving and delaying CKD-MBD in this model of Alport syndrome.