Phosphate homeostasis and disorders.

Recent studies of inherited disorders of phosphate metabolism have shed new light on the understanding of phosphate metabolism. Phosphate has important functions in the body and several mechanisms have evolved to regulate phosphate balance including vitamin D, parathyroid hormone and phosphatonins such as fibroblast growth factor-23 (FGF23). Disorders of phosphate homeostasis leading to hypo- and hyperphosphataemia are common and have clinical and biochemical consequences. Notably, recent studies have linked hyperphosphataemia with an increased risk of cardiovascular disease. This review outlines the recent advances in the understanding of phosphate homeostasis and describes the causes, investigation and management of hypo- and hyperphosphataemia.

The relationship between intact PTH and biointact PTH (1-84) with bone and mineral metabolism in pre-dialysis chronic kidney disease (CKD).

OBJECTIVES: Abnormalities in PTH are implicated in the pathogenesis of bone abnormalities in chronic kidney disease (CKD)-mineral bone disorder (CKD-MBD). PTH concentrations are important in clinical decision and management. This emphasises the importance of providing an assay which measures biologically active PTH. We compared concentrations of intact PTH with biointact PTH (1-84) in CKD and end stage renal disease (ESRD) and investigated the relationship between the 2 PTH assays with bone and mineral laboratory parameters and bone mineral density (BMD) in CKD. DESIGN AND METHODS: We assessed 140 patients (61 in ESRD and 79 with CKD stages 1-4) in this cross-sectional study. We measured biointact PTH (1-84) as well as routine biochemical parameters on all subjects. In the CKD cohort, bone turnover markers; bone alkaline phosphatase (BAP) and tartrate resistant acid phosphatase (TRACP)-5b and bone mineral density (BMD) were also determined. RESULTS: In ESRD, intact PTH concentration was significantly higher compared to biointact PTH (1-84) (422 [443] v/s 266 [251] pg/mL, (p<0.001) with an average bias of 60%. In CKD, intact PTH concentration was also higher compared to biointact PTH (1-84) (79[55] v/s 68[49] pg/mL p<0.001) with an average bias of 18%. Only the biointact PTH (1-84) assay showed any significant correlation with serum calcium concentrations (r=-0.26, p<0.05) and phosphate (r=0.25, p<0.05) in CKD. Following multilinear regression analysis and adjustment for all significant co-variables, only eGFR, BAP and 25 (OH)vitamin remained significantly associated with intact PTH and biointact PTH (1-84). The strength of association was stronger between BAP and biointact PTH (1-84) (biointact PTH (1-84): p=0.007, intact PTH: p=0.01). In adjusted analyses, only biointact PTH (1-84) was significantly associated with BMD at the fore-arm (FARM) (p=0.049). CONCLUSIONS: The study confirms the differences between intact PTH and biointact PTH (1-84) in ESRD. Whilst there may be similarities in the diagnostic ability of both intact and biointact PTH (1-84), our data suggest that biointact PTH (1-84) assay may better reflect bone metabolism and BMD in CKD. Further longitudinal studies are needed.

Circulating sclerostin and Dickkopf-1 (DKK1) in predialysis chronic kidney disease (CKD): relationship with bone density and arterial stiffness.

Abnormalities of bone metabolism and increased vascular calcification are common in chronic kidney disease (CKD) and important causes of morbidity and mortality. The Wnt signaling pathway may play a role in the bone and vascular disturbances seen in CKD, termed collectively "CKD-MBD." The aim of the study was to investigate the possible association of circulating concentrations of the secreted Wnt signaling inhibitors DKK1 and sclerostin with BMD and arterial stiffness in predialysis CKD. Seventy-seven patients (48 M, 29 F), mean age 57 (SD = 14) years with CKD stages 3B (n = 32) and 4 (n = 45) were studied. Sclerostin, DKK1, PTH, and 1,25(OH)(2)D were analyzed. BMD was measured at the lumbar spine (LS), femoral neck (FN), total hip (TH), and forearm (FARM). Arterial stiffness index was determined by contour analysis of digital volume pulse (SI(DVP)). There was a positive correlation between sclerostin and age (r = 0.47, p < 0.000). Sclerostin was higher in men than women (p = 0.013). Following correction for age and gender, there was a negative association between GFR and sclerostin (p = 0.002). We observed a positive association between sclerostin and BMD at the LS (p = 0.0001), FN (p = 0.004), and TH (p = 0.002). In contrast, DKK1 was negatively associated with BMD at the FN (p = 0.038). A negative association was seen between DKK1 and SI(DVP) (p = 0.027). Our data suggest that the Wnt pathway may play a role in CKD-MBD. Prospective studies are required to establish the clinical relevance of sclerostin and DKK1 as serological markers in CKD.

Association of bone turnover markers and arterial stiffness in pre-dialysis chronic kidney disease (CKD).

Vascular calcification (VC) is highly prevalent in CKD and leads to increased vascular stiffness and cardiovascular disease (CVD). Non-traditional cardiovascular risk factors include abnormal bone turnover and/or dysregulation of the calcification inhibitors, although their relative contribution remains unclear. We investigated the association between bone turnover, the calcification inhibitors (matrix gla protein; MGP and Fetuin-A), and the phosphate regulating hormone; fibroblast growth factor-23 (FGF-23) and arterial stiffness in pre-dialysis CKD patients. One hundred and forty-five patients with CKD stages 1-4 (74 M, 71 F) aged (mean [SD]) 53 [14] years were studied. Bone turnover markers (bone-specific alkaline phosphatase (BALP) and tartrate-resistant acid phosphatase (TRACP)) and MGP, Fetuin-A and FGF-23 were determined. BMD was measured at the lumbar spine (LS), femoral neck (FN), forearm (FARM) and total hip (TH). Arterial stiffness was assessed by contour analysis of digital volume pulse (SI(DVP)). There was a significant positive correlation between TRACP:BALP ratio and SI(DVP) ( r=0.19, p=0.023). Following multi-linear regression analysis, significant associations were seen between serum BALP (p=0.037), TRACP (p=0.009) and TRACP:BALP ratio (p=0.001) and SI(DVP) independently of traditional CVD risk factors. No significant relationship between SI(DVP) and MGP, Fetuin-A and FGF-23 was observed. A significant negative correlation was seen between BMD at the FARM and SI(DVP) in CKD stage 4 (r=-0.35, p=0.024). The association remained significant following correction for age, gender and cardiovascular risk factors (p=0.029). Our data suggest a link between imbalances in bone turnover and arterial stiffness in pre-dialysis CKD. Longitudinal studies are needed to evaluate the clinical usefulness of these bone turnover markers as predictors of CVD in CKD.

Fibroblast growth factor-23 is associated with C-reactive protein, serum phosphate and bone mineral density in chronic kidney disease.

UNLABELLED: We investigated the association between fibroblast growth factor-23 (FGF-23) and (1) the biochemical parameters implicated in chronic kidney disorder (CKD)-bone and mineral disorder (CKD-MBD) and (2) bone mineral density (BMD) in patients with CKD 1-4. C-reactive protein (CRP) and serum phosphate correlated with FGF-23. A significant association was seen between FGF-23 and BMD at the hip. INTRODUCTION: Circulating FGF-23 is elevated in CKD, although the primary stimulus remains unclear. Moreover, it is still unknown whether increase in FGF-23 has a biological effect on bone metabolism. The aim of the study was to investigate the association of FGF-23 with (1) the biochemical parameters linked with CKD-bone and mineral disorder (CKD-MBD) and (2) bone mineral density in CKD. METHODS: We studied 145 patients (74 M, 71 F) aged (mean [SD]) 53 [14] years with CKD 1-4. Serum calcium, phosphate, parathyroid hormone, FGF-23, 25 (OH) vitamin D, 1, 25 (OH)(2) vitamin D, bone turnover markers, CRP were determined. BMD was measured at the lumbar spine, femoral neck (FN), forearm, and total hip (TH). Multivariate analysis was undertaken to explore the association between (1) the biochemical variables and FGF-23 and (2) FGF-23 and BMD. RESULTS: Elevations in FGF-23 occurred in CKD stage 3 compared to CKD stage 1/2, although no significant differences in serum phosphate were observed. Serum phosphate (p<0.001), CRP (p<0.001) and diabetes mellitus (p<0.05) were associated with FGF-23. BMD Z-score was significantly lower at the TH and FN in CKD 4 (p<0.05). A significant association was seen between BMI, FGF-23, bone specific alkaline phosphatase and BMD at the TH (p<0.05). CONCLUSIONS: The data suggest that FGF-23 may be associated with parameters implicated in the complications of CKD. Longitudinal studies are required for further clinical evaluation.