Parathyroid hormone (PTH) has an important role in the maintenance of serum calcium levels. It activates renal 1α-hydroxylase and increases the synthesis of the active form of vitamin D (1,25[OH]₂D₃). PTH promotes calcium release from the bone and enhances tubular calcium resorption through direct action on these sites. Hallmarks of secondary hyperparathyroidism associated with chronic kidney disease (CKD) include increase in serum fibroblast growth factor 23 (FGF-23), reduction in renal 1,25[OH]₂D₃ production with a decline in its serum levels, decrease in intestinal calcium absorption, and, at later stages, hyperphosphatemia and high levels of PTH. In this paper, we aim to critically discuss severe CKD-related hyperparathyroidism, in which PTH, through calcium-dependent and -independent mechanisms, leads to harmful effects and manifestations of the uremic syndrome, such as bone loss, skin and soft tissue calcification, cardiomyopathy, immunodeficiency, impairment of erythropoiesis, increase of energy expenditure, and muscle weakness.

Several factors contribute to renal-function decline in CKD patients, and the role of phosphate content in the diet is still a matter of debate. This study aims to analyze the mechanism by which phosphate, independent of protein, is associated with the progression of CKD. Adult Munich-Wistar rats were submitted to 5/6 nephrectomy (Nx), fed with a low-protein diet, and divided into two groups. Only phosphate content (low phosphate, LoP, 0.2%; high phosphate, HiP, 0.95%) differentiated diets. After sixty days, biochemical parameters and kidney histology were analyzed. The HiP group presented worse renal function, with higher levels of PTH, FGF-23, and fractional excretion of phosphate. In the histological analysis of the kidney tissue, they also showed a higher percentage of interstitial fibrosis, expression of α-actin, PCNA, and renal infiltration by macrophages. The LoP group presented higher expression of beclin-1 in renal tubule cells, a marker of autophagic flux, when compared to the HiP group. Our findings highlight the action of phosphate in the induction of kidney interstitial inflammation and fibrosis, contributing to the progression of renal disease. A possible effect of phosphate on the dysregulation of the renal cell autophagy mechanism needs further investigation with clinical studies.

Fibroblast growth factor 23 (FGF23) is a phosphate-regulating hormone produced by osteocytes. In iron deficiency anemia (IDA) and in chronic kidney disease (CKD), FGF23 is also produced by erythroid cells. Recent studies have suggested that rising circulating FGF23 is negatively associated with erythropoiesis in IDA and CKD. However, the distinct contributions of bone- and erythroid-produced FGF23 to anemia in IDA remain unclear. Using the conditional deletion of Fgf23 in osteocytes (Fgf23Dmp1-cKO) and in erythroid cells (Fgf23HbB-cKO) in mice fed a control (Ctr) or an iron deficient (ID) diet, we first determined that in ID, osteocytes and erythroid cells are distinct sources of circulating intact FGF23 (iFGF23) and FGF23 cleaved peptides, respectively. We further show that erythroid-specific deletion of Fgf23 corrected anemia in ID mice, and overexpression induced anemia in Ctr mice unlike osteocyte-specific deletion or overexpression of Fgf23. Importantly, erythroid-specific deletion of Furin (FurinHbB-cKO), the enzyme responsible for FGF23 cleavage, led to increased production of iFGF23 from erythroid cells and aggravated ID-induced anemia. iFGF23 also dose-dependently blocked the differentiation of erythroid progenitors in culture triggering mitochondrial dysfunction leading to impaired erythropoiesis. These effects were fully suppressed by co-treatment with an FGFR1 inhibitor. Finally, erythroid-specific deletion of Fgf23 in an animal model of progressive CKD prevented the development of anemia of CKD. In aggregate, our results show that erythroid-expressed FGF23 is a negative regulator of erythropoiesis that contributes to anemia via direct paracrine FGFR1 activation in erythroid precursors.

Muscle weakness is a common symptom in CKD patients, and the pathway by which secondary hyperparathyroidism (SHPT) affects muscle function is unknown. Osteopontin (OPN), a bone matrix protein stimulated by PTH and phosphate, has been associated with inflammatory muscle diseases. In this observational and prospective cohort study, we evaluated 30 patients with severe SHPT (39 ± 12 yr; 18 women), before and 6 mo after parathyroidectomy (PTx). We examined the relationships among CKD–mineral and bone disorder parameters; myokine and inflammatory cytokine levels; and changes in resting energy expenditure (REE), muscle function, BMD, and muscle-related proteins. At baseline, the patients showed low gene expression of muscle turnover markers and irisin, as well as high protein expression of OPN, transforming growth factor beta (TGF-β), and fibroblast growth factor 21. Six months after PTx, REE and muscle mass had not changed, but physical performance, muscle strength, and bone mass improved, more so in patients undergoing total PTx. Also, there were reductions in the protein expression of OPN (11 vs 3%, p=.01) and TGF-β (21 vs 7%, p=.002) in muscle, together with a significant increase in irisin muscular levels (30 vs 35 pg/mg, p=.02). The gain in bone mass and the increase in irisin levels correlated with a reduction in PTH. The levels of interleukin (IL)-1β, tumor necrosis factor alpha, and IL-17 (markers of myositis) were also lower after PTx. Our data suggest that SHPT plays a role in CKD-induced muscle dysfunction, indirectly, via release of bone-specific proteins, which is partially reverted with PTx. How secondary hyperparathyroidism (SHPT) affects muscle quality in CKD patients is unclear. This observational, prospective study performed muscle phenotyping by performance evaluation, body composition analysis, and muscle biopsies of 30 patients on dialysis with SHPT, before and after parathyroidectomy (PTx). The main findings include evidence of local tissue inflammation and blunting of muscle turnover in patients compared to healthy individuals. Despite no changes to muscle mass or energy expenditure, PTx improved functionality, bone mass, and influenced the molecular phenotyping of muscle tissue by altering the expression of bone-specific proteins and reducing inflammation. These findings suggest that SHPT plays a role in CKD-induced muscle dysfunction and that PTx has a beneficial effect on uremic myopathy.

Background: Data on parathyroid hormone (PTH) levels at hemodialysis (HD) initiation and during the first year of therapy are still scarce. We hypothesized that high baseline PTH levels contribute to more severe hyperparathyroidism during this period.