Phosphate feeding induces arterial medial calcification in uremic mice: role of serum phosphorus, fibroblast growth factor-23, and osteopontin

Recent progress in the treatment of vascular calcification

Vascular calcification is common in patients with advanced chronic kidney disease and is associated with poorer outcomes. Although the pathophysiology is not completely understood, it is clear that it is a multifactorial process involving altered mineral metabolism, as well as changes in systemic and local factors that can promote or inhibit vascular calcification, and all of these are potential therapeutic targets. Current therapy is closely linked to strategies for preventing disordered bone and mineral metabolism in advanced kidney disease and involves lowering the circulating levels of both phosphate and calcium. The efficacy of compounds that specifically target calcification, such as bisphosphonates and thiosulfate, has been shown in animals but only in small numbers of humans, and safety remains an issue. Additional therapies, such as pyrophosphate, vitamin K, and lowering of pH, are supported by animal studies, but are yet to be investigated clinically. As the mineral composition of vascular calcifications is the same as in bone, potential effects on bone must be addressed with any therapy for vascular calcification.

The bone-vascular axis in chronic kidney disease

PURPOSE

This review highlights the most recent publications addressing the relationship between bone and vascular calcification in patients with chronic and end-stage kidney disease.

RECENT FINDINGS

The relatively new term 'chronic kidney disease-mineral bone disorder' reflects the growing reach of chronic kidney disease research into the realm of systems physiology, involving a triad of renal, skeletal, and vascular tissues. Recent studies address underlying mechanisms of the bone and vascular complications of chronic kidney disease and point to a variety of biochemical factors, including phosphatonins (fibroblast growth factor-23, matrix extracellular phosphoglycoprotein), bone morphogenetic protein 7, osteoprotegerin, matrix GLA protein, ectonucleotide pyrophosphatase/phosphodiesterase 1, alkaline phosphatase, and lipid oxidation products. Studies also demonstrate that agents used for treatment of one component of the triad often act on the other components of the triad - beneficially or adversely. These findings emphasize the importance of avoiding the subspecialty, single organ viewpoint when treating individual components of chronic kidney disease-mineral bone disorder.

SUMMARY

The consistent synchrony among chronic kidney disease, aortic calcification, and bone loss offers clues to underlying mechanisms for the systemic abnormalities.

Hyperphosphatemia-induced nanocrystals upregulate the expression of bone morphogenetic protein-2 and osteopontin genes in mouse smooth muscle cells in vitro

Vascular calcification, which contributes to cardiovascular disease in patients with uremic hyperphosphatemia, is associated with vascular cell expression of osteogenic genes, including bone morphogenetic protein (BMP)-2 and osteopontin (OPN). High inorganic phosphate levels in vitro stimulate the osteogenic conversion of smooth muscle cells; however, the mechanism governing this is not clear. We found that high-phosphate medium increased the expression of BMP-2 and OPN in mouse smooth muscle cells in culture. However, this effect was lost in the presence of the mineralization inhibitor, pyrophosphate, suggesting a contribution of calcium phosphate crystals. Addition of 1-2 mmol/l phosphate alone to growth medium was sufficient to induce nanosized crystals after 1 day at 37 °C. Isolated crystals were about 160 nm in diameter and had a calcium to phosphate ratio of 1.35, consistent with the hydroxyapatite precursor octacalcium phosphate. Nanocrystal formation increased fourfold in the absence of serum, was blocked by fetuin-A, and was dependent on time and on the concentrations of phosphate and calcium. Purified synthetic hydroxyapatite nanocrystals and isolated high-phosphate-induced nanocrystals, but not nanocrystal-free high-phosphate medium, also induced BMP-2 and OPN. Thus, our results suggest that BMP-2 and OPN are induced by calcium phosphate nanocrystals, rather than soluble phosphate. This mechanism may contribute, in part, to hyperphosphatemia-related vascular cell differentiation and calcification.

Regulatory mechanisms in vascular calcification

In the past decade, the prevalence, significance, and regulatory mechanisms of vascular calcification have gained increasing recognition. Over a century ago, pathologists recognized atherosclerotic calcification as a form of extraskeletal ossification. Studies are now identifying the mechanism of this remarkable process as a recapitulation of embryonic endochondral and membranous ossification through phenotypic plasticity of vascular cells that function as adult mesenchymal stem cells. These embryonic developmental programs, involving bone morphogenetic proteins and potent osteochondrogenic transcription factors, are triggered and modulated by a variety of inflammatory, metabolic, and genetic disorders, particularly hyperlipidemia, chronic kidney disease, diabetes, hyperparathyroidism, and osteoporosis. They are also triggered by loss of powerful inhibitors, such as fetuin A, matrix Gla protein, and pyrophosphate, which ordinarily restrict biomineralization to skeletal bone. Teleologically, soft-tissue calcification might serve to create a wall of bone to sequester noxious foci such as chronic infections, parasites, and foreign bodies. This Review focuses on atherosclerotic and medial calcification. The capacity of the vasculature to produce mineral in culture and to produce de novo, vascularized, trabecular bone and cartilage tissue, even in patients with osteoporosis, should intrigue investigators in tissue engineering and regenerative biology.

Phosphate and vascular calcification: Emerging role of the sodium-dependent phosphate co-transporter PiT-1

Elevated serum phosphate is a risk factor for vascular calcification and cardiovascular events in kidney disease as well as in the general population. Elevated phosphate levels drive vascular calcification, in part, by regulating vascular smooth muscle cell (VSMC) gene expression, function, and fate. The type III sodium-dependent phosphate co-transporter, PiT-1, is necessary for phosphate-induced VSMC osteochondrogenic phenotype change and calcification, and has recently been shown to have unexpected functions in cell proliferation and embryonic development.

Chondrocyte rather than osteoblast conversion of vascular cells underlies medial calcification in uremic rats

OBJECTIVE

To investigate cell biological changes in calcified aortas of rats that experienced chronic renal failure.

METHODS AND RESULTS

Vascular smooth muscle cells have the potential to transdifferentiate to either chondrocytes or osteoblasts, depending on the molecular pathways that are stimulated. Uremia-related medial calcification was induced by feeding rats an adenine low-protein diet for 4 weeks. Aortic calcification was evaluated biochemically and histochemically and with in vivo micro-computed tomographic scanning. Immunohistochemistry and RT-PCR were applied to analyze the time-dependent aortic expression of molecules involved in the segregation between the chondrocyte versus osteoblast differentiation pathway. After 4 weeks, 85% of the uremic rats had developed distinct aortic medial calcification, which increased to severely calcified lesions during further follow-up. The calcification process was accompanied by a significant time-dependent increase in the expression of the chondrocyte-specific markers sex determining region Y-box 9 (sox9), collagen II, and aggrecan and a nonsignificant trend toward enhanced core binding factor alpha 1 (cbfa1), and collagen I. The expression of the osteoblast marker osterix and both lipoprotein receptor-related protein 6 and beta-catenin, molecules of the wingless-type MMTV integration site family member (Wnt)/beta-catenin pathway induced during osteoblast differentiation, was suppressed.

CONCLUSIONS

In the aorta of uremic rats, medial smooth muscle cells acquire a chondrocyte rather than osteoblast phenotype during the calcification process.

The associations of fibroblast growth factor 23 and uncarboxylated matrix Gla protein with mortality in coronary artery disease: the Heart and Soul Study

BACKGROUND

Fibroblast growth factor 23 (FGF23), uncarboxylated matrix Gla protein (ucMGP), and fetuin-A are regulators of mineral metabolism and inhibitors of vascular calcification. Whether circulating levels of each are associated with cardiovascular disease (CVD) events or mortality in populations without end-stage renal disease is unknown.

OBJECTIVE

To evaluate the associations of FGF23, ucMGP, and fetuin-A with mortality and CVD events.

DESIGN

Observational study.

SETTING

12 outpatient clinics in the San Francisco Bay area.

PATIENTS

833 outpatients with stable coronary artery disease (CAD), recruited from 11 September 2000 to 20 December 2002.

MEASUREMENTS

Fibroblast growth factor 23, ucMGP, and fetuin-A concentrations were measured at baseline. Participants were followed until 1 December 2008 for mortality and CVD events.

RESULTS

During a median follow-up of 6.0 years, 220 participants died and 182 had CVD events. Compared with participants with FGF-23 levels in the lowest tertile, those in the highest tertile had 2-fold greater risk for mortality (hazard ratio [HR], 2.15 [95% CI, 1.43 to 3.24]) and CVD events (HR, 1.83 [CI, 1.15 to 2.91]) after adjustment for traditional CVD risk factors, C-reactive protein levels, and kidney function. The highest ucMGP tertile was associated with lower mortality risk (HR, 0.48 [CI, 0.31 to 0.75]) and showed a nonsignificant trend toward lower CVD event risk by tertile analysis (HR, 0.65 [CI, 0.40 to 1.05])-an association that was significant when modeled continuously (P = 0.029). No significant association of fetuin-A with mortality (HR, 0.84 [CI, 0.55 to 1.27]) or CVD events (HR, 0.99 [CI, 0.64 to 1.55]) was observed.

LIMITATION

Participants had prevalent CAD.

CONCLUSION

In outpatients with stable CAD, higher FGF23 and lower ucMGP levels are independently associated with mortality and CVD events.

PRIMARY FUNDING SOURCE

American Heart Association.

The associations of fibroblast growth factor 23 and uncarboxylated matrix Gla protein with mortality in coronary artery disease: the Heart and Soul Study

BACKGROUND

Fibroblast growth factor 23 (FGF23), uncarboxylated matrix Gla protein (ucMGP), and fetuin-A are regulators of mineral metabolism and inhibitors of vascular calcification. Whether circulating levels of each are associated with cardiovascular disease (CVD) events or mortality in populations without end-stage renal disease is unknown.

OBJECTIVE

To evaluate the associations of FGF23, ucMGP, and fetuin-A with mortality and CVD events.

DESIGN

Observational study.

SETTING

12 outpatient clinics in the San Francisco Bay area.

PATIENTS

833 outpatients with stable coronary artery disease (CAD), recruited from 11 September 2000 to 20 December 2002.

MEASUREMENTS

Fibroblast growth factor 23, ucMGP, and fetuin-A concentrations were measured at baseline. Participants were followed until 1 December 2008 for mortality and CVD events.

RESULTS

During a median follow-up of 6.0 years, 220 participants died and 182 had CVD events. Compared with participants with FGF-23 levels in the lowest tertile, those in the highest tertile had 2-fold greater risk for mortality (hazard ratio [HR], 2.15 [95% CI, 1.43 to 3.24]) and CVD events (HR, 1.83 [CI, 1.15 to 2.91]) after adjustment for traditional CVD risk factors, C-reactive protein levels, and kidney function. The highest ucMGP tertile was associated with lower mortality risk (HR, 0.48 [CI, 0.31 to 0.75]) and showed a nonsignificant trend toward lower CVD event risk by tertile analysis (HR, 0.65 [CI, 0.40 to 1.05])-an association that was significant when modeled continuously (P = 0.029). No significant association of fetuin-A with mortality (HR, 0.84 [CI, 0.55 to 1.27]) or CVD events (HR, 0.99 [CI, 0.64 to 1.55]) was observed.

LIMITATION

Participants had prevalent CAD.

CONCLUSION

In outpatients with stable CAD, higher FGF23 and lower ucMGP levels are independently associated with mortality and CVD events.

PRIMARY FUNDING SOURCE

American Heart Association.

Vascular calcification in chronic kidney disease

VC (vascular calcification) is highly prevalent in patients with CKD (chronic kidney disease), but its mechanism is multifactorial and incompletely understood. In addition to increased traditional risk factors, CKD patients also have a number of non-traditional cardiovascular risk factors, which may play a prominent role in the pathogenesis of arterial calcification, such as duration of dialysis and disorders of mineral metabolism. The transformation of vascular smooth muscle cells into chondrocytes or osteoblast-like cells seems to be a key element in VC pathogenesis, in the context of passive calcium and phosphate deposition due to abnormal bone metabolism and impaired renal excretion. The process may be favoured by the low levels of circulating and locally produced VC inhibitors. VC determines increased arterial stiffness, left ventricular hypertrophy, a decrease in coronary artery perfusion, myocardial ischaemia and increased cardiovascular morbidity and mortality. Although current therapeutic strategies focus on the correction of phosphate, calcium, parathyroid hormone or vitamin D, a better understanding of the mechanisms of abnormal tissue calcification may lead to development of new therapeutic agents, which could reduce VC and improve cardiovascular outcome in CKD patients. The present review summarizes the following aspects: (i) the pathophysiological mechanism responsible for VC and its promoters and inhibitors, (ii) the methods for detection of VC in patients with CKD, including evaluation of arterial stiffness, and (iii) the management of VC in CKD patients.

Regulation of vascular smooth muscle cell calcification by extracellular pyrophosphate homeostasis: synergistic modulation by cyclic AMP and hyperphosphatemia

Vascular calcification is a multifaceted process involving gain of calcification inducers and loss of calcification inhibitors. One such inhibitor is inorganic pyrophosphate (PP(i)), and regulated generation and homeostasis of extracellular PP(i) is a critical determinant of soft-tissue mineralization. We recently described an autocrine mechanism of extracellular PP(i) generation in cultured rat aortic vascular smooth muscle cells (VSMC) that involves both ATP release coupled to the ectophosphodiesterase/pyrophosphatase ENPP1 and efflux of intracellular PP(i) mediated or regulated by the plasma membrane protein ANK. We now report that increased cAMP signaling and elevated extracellular inorganic phosphate (P(i)) act synergistically to induce calcification of these VSMC that is correlated with progressive reduction in ability to accumulate extracellular PP(i). Attenuated PP(i) accumulation was mediated in part by cAMP-dependent decrease in ANK expression coordinated with cAMP-dependent increase in expression of TNAP, the tissue nonselective alkaline phosphatase that degrades PP(i). Stimulation of cAMP signaling did not alter ATP release or ENPP1 expression, and the cAMP-induced changes in ANK and TNAP expression were not sufficient to induce calcification. Elevated extracellular P(i) alone elicited only minor calcification and no significant changes in ANK, TNAP, or ENPP1. In contrast, combined with a cAMP stimulus, elevated P(i) induced decreases in the ATP release pathway(s) that supports ENPP1 activity; this resulted in markedly reduced rates of PP(i) accumulation that facilitated robust calcification. Calcified VSMC were characterized by maintained expression of multiple SMC differentiation marker proteins including smooth muscle (SM) alpha-actin, SM22alpha, and calponin. Notably, addition of exogenous ATP (or PP(i) per se) rescued cAMP + phosphate-treated VSMC cultures from progression to the calcified state. These observations support a model in which extracellular PP(i) generation mediated by both ANK- and ATP release-dependent mechanisms serves as a critical regulator of VSMC calcification.

Characterization of mandibular bone in a mouse model of chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) is a worldwide health problem with increasing prevalence and poor outcomes, including severe cardiovascular disease and renal osteodystrophy. With advances in medical treatment, patients with CKD are living longer and require oral care. The aim of this study is to determine the effects of CKD and dietary phosphate on mandibular bone structure using a uremic mouse model.

METHODS

Uremia (U) was induced in female dilute brown agouti/2 mice by partial renal ablation. Uremic mice received a normal-phosphate (NP) or a high-phosphate (HP) diet. sham surgeries were performed in a control group of mice; half received an NP diet, and the other half was fed an HP diet. At termination, animals were sacrificed, and mandibles were collected for microcomputed tomography (micro-CT) and histologic analysis.

RESULTS

Sera levels of blood urea nitrogen, parathyroid hormone, and alkaline phosphatase were significantly increased in U/NP and U/HP mice versus sham controls, whereas serum calcium was increased in the U/HP group, and no differences were noted in serum phosphate levels among groups. Micro-CT analyses revealed a significant reduction in cortical bone thickness and an increase in trabecular thickness and trabecular bone volume/tissue volume in U/NP and U/HP groups compared to the sham/NP group. A significant reduction in cortical bone thickness was also found in the sham/HP group versus the sham/NP group. Histologic evaluation confirmed increased trabeculation in the U groups.

CONCLUSION

CKD in mice, especially under conditions of HP feeding, results in marked effects on alveolar bone homeostasis.

Chronic mineral dysregulation promotes vascular smooth muscle cell adaptation and extracellular matrix calcification

In chronic kidney disease (CKD) vascular calcification occurs in response to deranged calcium and phosphate metabolism and is characterized by vascular smooth muscle cell (VSMC) damage and attrition. To gain mechanistic insights into how calcium and phosphate mediate calcification, we used an ex vivo model of human vessel culture. Vessel rings from healthy control subjects did not accumulate calcium with long-term exposure to elevated calcium and/or phosphate. In contrast, vessel rings from patients with CKD accumulated calcium; calcium induced calcification more potently than phosphate (at equivalent calcium-phosphate product). Elevated phosphate increased alkaline phosphatase activity in CKD vessels, but inhibition of alkaline phosphatase with levamisole did not block calcification. Instead, calcification in CKD vessels most strongly associated with VSMC death resulting from calcium- and phosphate-induced apoptosis; treatment with a pan-caspase inhibitor ZVAD ameliorated calcification. Calcification in CKD vessels was also associated with increased deposition of VSMC-derived vesicles. Electron microscopy confirmed increased deposition of vesicles containing crystalline calcium and phosphate in the extracellular matrix of dialysis vessel rings. In contrast, vesicle deposition and calcification did not occur in normal vessel rings, but we observed extensive intracellular mitochondrial damage. Taken together, these data provide evidence that VSMCs undergo adaptive changes, including vesicle release, in response to dysregulated mineral metabolism. These adaptations may initially promote survival but ultimately culminate in VSMC apoptosis and overt calcification, especially with continued exposure to elevated calcium.

A blueprint for randomized trials targeting phosphorus metabolism in chronic kidney disease

The diagnosis of chronic kidney disease (CKD) confers dismal clinical outcomes regardless of whether patients are initiating dialysis and face a median survival of only 2-3 years or they have earlier-stage CKD and face a risk of death that is greater than the risk of progression to dialysis. These poor outcomes are driven by extraordinarily high rates of cardiovascular disease that historically have not responded to risk-factor modification strategies proven to attenuate risk in the general population. Nor have measures aimed at increasing the dose or quality of dialysis made an appreciable dent in mortality. Still worse, interventions that were expected to be beneficial resulted in increased mortality in recent trials. Although this apparent lack of progress in advancing the care of CKD is discouraging, resignation is not an option. On the contrary, with the rising rates of CKD worldwide, there is an urgent need to rigorously test novel therapeutic strategies in randomized trials. The breadth of accumulating evidence linking disordered phosphorus metabolism to adverse outcomes spans in vitro, animal, and human studies, and positions phosphorus management as an attractive target for intervention. Although opinion-based practice guidelines promote phosphorus management strategies that are widely accepted in dialysis patients, there is a clear need to perform randomized controlled trials to prove or disprove the benefits of therapy. Perhaps even more important, the discovery of fibroblast growth factor 23 (FGF23) and its potential as a novel diagnostic to identify disordered phosphorus metabolism at an early, subclinical state has presented the opportunity to develop placebo-controlled randomized trials in pre-dialysis CKD patients with normal serum phosphate levels. This commentary considers the justification and challenges for such trials and presents a 'first-draft' blueprint of distinct trial approaches to initiate a dialog that will ultimately culminate in studies aimed at improving survival across the spectrum of CKD.