Klotho/fibroblast growth factor 23- and PTH-independent estrogen receptor-α-mediated direct downregulation of NaPi-IIa by estrogen in the mouse kidney

Renal tubular estrogen ß receptors are expressed at high levels in small vessel vasculitis and are primarily localized in the distal tubule

This study investigated the possible roles of renal estrogen receptors (ER) in glomerulonephritis associated with small vessel vasculitis. The relationships of ERs were investigated in antineutrophilic cytoplasmic antibody (ANCA)-associated glomerulonephritis and immunoglobulin A (IgA) nephropathy groups, which are small vessel vasculitis subtypes with two different glomerulonephritis development pathophysiologies. The design of this study was prepared as a retrospective cohort study. The study included 42 patients with ANCA-associated vasculitis and 18 with IgA nephropathy in the small vessel vasculitis group. For the control group, intact renal tissues of 28 patients who underwent nephrectomy due to renal cell carcinoma were used. Renal biopsy samples of the groups were stained with ER beta (ß) and ER alpha (α). Tubular ER ß expression score (TERßES) median values were found to be significantly higher in ANCA- associated vasculitis (B = 0.724, OR [95%CI]: 2.064 [1.141-3.731], p = .016) and IgA nephropathy (B = 0.898, OR [95%CI]: 2.454 [1.307-4.609], p = .005) than in intact kidney tissue. It was determined that tubular ERß was most frequently localized in the distal tubule at 57.9% and the second most common in the proximal tubule at 20.4%. The expression of tubular ERß is increased in glomerulonephritis due to small vessel vasculitis. Tubular ERßs are most commonly localized in the distal tubule. Further studies are needed to understand the physiological and pathophysiological effects of altered renal ER levels in small vessel vasculitis.

The basics of phosphate metabolism

Phosphorus is an essential mineral that is, in the form of inorganic phosphate (Pi), required for building cell membranes, DNA and RNA molecules, energy metabolism, signal transduction and pH buffering. In bone, Pi is essential for bone stability in the form of apatite. Intestinal absorption of dietary Pi depends on its bioavailability and has two distinct modes of active transcellular and passive paracellular absorption. Active transport is transporter mediated and partly regulated, while passive absorption depends mostly on bioavailability. Renal excretion controls systemic Pi levels, depends on transporters in the proximal tubule and is highly regulated. Deposition and release of Pi into and from soft tissues and bone has to be tightly controlled. The endocrine network coordinating intestinal absorption, renal excretion and bone turnover integrates dietary intake and metabolic requirements with renal excretion and is critical for bone stability and cardiovascular health during states of hypophosphataemia or hyperphosphataemia as evident from inborn or acquired diseases. This review provides an integrated overview of the biology of phosphate and Pi in mammals.

Renal tubular and glomerular estrogen receptor ß levels are lower in lupus nephritis than in familial Mediterranean fever-associated renal amyloidosis

BACKGROUND

Estrogen has been thought to play an essential role in the disease pathogenesis of systemic lupus erythematosus, which is 9-10 times more prevalent in the female population. It has been shown that irregular estrogen/estrogen receptor signaling pathways may contribute to the pathophysiology of various renal diseases. In this study, we compared renal estrogen receptors between lupus nephritis, familial Mediterranean fever-associated renal amyloidosis, ANCA-associated nephritis, and intact kidney to investigate their role in the pathophysiology of renal diseases.

METHODS

This study was designed as a retrospective cohort study. Thirty systemic lupus erythematosus patients with lupus nephritis, 12 familial Mediterranean fever amyloidosis and 10 ANCA-associated glomerulonephrites, and 14 individuals with normal renal histology were included in the study.

RESULTS

Tubular estrogen receptor ß expression score was found to be significantly higher in the familial Mediterranean fever [5 (1-8)] group than in the lupus nephritis [0 (0-1)] (B = 1.385, OR = 3.996, CI %95 = 1.805-8.846, p = .001) and ANCA [4 (1-6.5)] (B = -1.431, OR = 0.239, CI 95% = 0.093-0.614, p = .003) groups. A significant correlation was found between serum creatinine values and tubular estrogen receptor ß expression score (OR = 0.565, CI 95% = 0.622-1.402, p < .0001). In ANCA-associated glomerulonephritis, a significant relationship was found between fibro cellular crescents in renal biopsy and glomerular estrogen receptor ß expression score (OR = 0.247, CI 95% = 0.11-0.999, p = .045) and tubular estrogen receptor ß expression score (OR = 0.282, CI 95% = -0.180-2.812, p = .026).

CONCLUSIONS

This study showed that tubular estrogen receptor ß expression score was elevated in familial Mediterranean fever amyloidosis and correlated with serum creatinine levels and renal crescents.

Expression of Renal Vitamin D and Phosphatonin-Related Genes in a Sheep Model of Osteoporosis

Simple Summary

Osteoporosis is a significant public health issue around the world, with post-menopausal osteoporosis due to estrogen deficiency resulting in approximately ¾ of cases. Treatment with glucocorticoids is another common cause of osteoporosis in humans. Sheep are a well-established model for osteoporosis in humans. In this study, aged sheep had their ovaries removed (ovariectomy) to simulate estrogen deficiency, and some sheep were also treated with glucocorticoids. The results showed that expression of the gene klotho in the kidney had the most marked difference in ovariectomized sheep treated with glucocorticoids for 2 months followed by a recovery period of 3 months. Klotho is known as the “anti-aging” hormone and is an important regulator of calcium and phosphorus metabolism. It may therefore be involved in the recovery of bone mineral density seen in ovariectomized sheep treated with glucocorticoids for 2 months followed by euthanasia at 5 months. As such, it could be an important treatment target for osteoporosis in humans.

Abstract

Osteoporosis is a significant public health issue around the world, with post-menopausal osteoporosis due to estrogen deficiency resulting in approximately ¾ of cases. In this study, 18 aged Merino ewes were ovariectomized, and 10 were controls. Three of the ovariectomized ewes were treated weekly with 400 mg of methylprednisolone for 5 months and three were treated weekly for 2 months, followed by a 3-month recovery period. At 2 months, five control animals and six ovariectomized animals were euthanized. At 5 months, all the remaining ewes were euthanized. Kidney samples were collected postmortem for qPCR analysis of NPT1, PTH1R, NPT2a, NPT2c, Klotho, FGFR1IIIc, VDR, CYP24A1, CYP27B1, TRPV5, TRPV6, CalD9k, CalD28k, PMCA and NCX1. Ovariectomized sheep had significantly greater VDR expression compared with other groups. Ovariectomized sheep treated with glucocorticoids for 2 months followed by euthanasia at 5 months showed significant differences in TRPV5, CYP24A1 and klotho gene expression compared to other groups. Differences in klotho expression were most marked after adjustment for repeated measures (p = 0.1). Klotho is known as the “anti-aging” hormone and is involved in calcium and phosphorus metabolism. Klotho may be involved in the recovery of bone mineral density in ovariectomized sheep treated with glucocorticoids for 2 months followed by euthanasia at 5 months. Further research on the role of klotho is recommended.

Fibroblast growth factor 23 leads to endolysosomal routing of the renal phosphate cotransporters NaPi-IIa and NaPi-IIc in vivo

Na⁺-dependent phosphate cotransporters NaPi-IIa and NaPi-IIc, located at the brush-border membrane of renal proximal tubules, are regulated by numerous factors, including fibroblast growth factor 23 (FGF23). FGF23 downregulates NaPi-IIa and NaPi-IIc abundance after activating a signaling pathway involving phosphorylation of ERK1/2 (phospho-ERK1/2). FGF23 also downregulates expression of renal 1-α-hydroxylase (Cyp27b1) and upregulates 24-hydroxylase (Cyp24a1), thus reducing plasma calcitriol levels. Here, we examined the time course of FGF23-induced internalization of NaPi-IIa and NaPi-IIc and their intracellular pathway toward degradation in vivo. Mice were injected intraperitoneally with recombinant human (rh)FGF23 in the absence (biochemical analysis) or presence (immunohistochemistry) of leupeptin, an inhibitor of lysosomal proteases. Phosphorylation of ERK1/2 was enhanced 60 min after rhFGF23 administration, and increased phosphorylation was still detected 480 min after injection. Colocalization of phospho-ERK1/2 with NaPi-IIa was seen at 60 and 120 min and partly at 480 min. The abundance of both cotransporters was reduced 240 min after rhFGF23 administration, with a further reduction at 480 min. NaPi-IIa and NaPi-IIc were found to colocalize with clathrin and early endosomal antigen 1 as early as 120 min after rhFGF23 injection. Both cotransporters partially colocalized with cathepsin B and lysosomal-associated membrane protein-1, markers of lysosomes, 120 min after rhFGF23 injection. Thus, NaPi-IIa and NaPi-IIc are internalized within 2 h upon rhFGF23 injection. Both cotransporters share the pathway of clathrin-mediated endocytosis that leads first to early endosomes, finally resulting in trafficking toward the lysosome as early as 120 min after rhFGF23 administration.NEW & NOTEWORTHY The hormone fibroblast growth factor 23 (FGF23) controls phosphate homeostasis by regulating renal phosphate excretion. FGF23 acts on several phosphate transporters in the kidney. Here, we define the time course of this action and demonstrate how phosphate transporters NaPi-IIa and NaPi-IIc are internalized.

Estrogen inhibits renal Na-Pi Co-transporters and improves klotho deficiency-induced acute heart failure

Objective and hypothesis

Klotho is an aging-suppressor gene. Mutation of Klotho gene causes hyperphosphatemia and acute heart failure. However, the relationship of hyperphosphatemia and acute heart failure is unclear. We hypothesize that hyperphosphatemia mediates Klotho deficiency-induced acute heart failure and further that therapeutic reduction of hyperphosphatemia prevents acute heart failure in Klotho mutant (KL(−/−)) mice.

Methods and results

A significant elevation of serum phosphorus levels and a large reduction of heart function were found in KL(−/−) mice by six weeks of age. Normalization of serum phosphorus levels by low phosphate diet (LPD) rescued Klotho deficiency-induced heart failure and extended lifespan in male mice. Klotho deficiency impaired cardiac mitochondrial respiratory enzyme function and increased superoxide production, oxidative stress, and cardiac cell apoptosis in male KL(−/−) mice which can be eliminated by LPD. LPD, however, did not rescue hyperphosphatemia or heart failure in female KL(−/−) mice. LPD did not affect estrogen depletion in female KL(−/−) mice. Normalization of serum estrogen levels by treatment with 17β-estradiol prevented hyperphosphatemia and heart failure in female KL(−/−) mice. Mechanistically, treatment with 17β-estradiol rescued hyperphosphatemia via inhibiting renal Na-Pi co-transporter expression. Normalization of serum phosphorus levels by treatment with 17β-estradiol also abolished cardiac mitochondrial respiratory enzyme dysfunction, ROS overproduction, oxidative stress and cardiac cell apoptosis in female KL(−/−) mice.

Conclusion

Klotho deficiency causes acute heart failure via hyperphosphatemia in male mice which can be prevented by LPD. 17β-estradiol prevents Klotho deficiency-induced hyperphosphatemia and heart failure by eliminating upregulation of renal Na-Pi co-transporter expression in female mice.

Clinicopathological and Prognostic Significance of Klotho and Estrogen Receptors Expression in Human Hepatocellular Carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is male-predominant cancer, but the underlying mechanism remains unclear. This study aimed to investigate the expression of the age-suppressing gene klotho and estrogen receptors (ERs) in HCC patients and analyze their association with clinicopathological variables and their effects on prognosis.

METHODS

The expression patterns of klotho, ERα, and ERβ were determined by tissue microarray and immunohistochemical technique, and their correlations with clinicopathological characteristics were investigated using univariate and multivariate analysis.

RESULTS

Klotho expression was significantly lower in HCC than in the adjacent noncancerous tissues (52.7% (49/93) vs. 90.8% (79/87), P = .000), and its protein level in HCC tissue was negatively correlated with clinical staging, histological grade, and stage of the primary tumor (T) (P < .05). Whereas the expression of nuclear ERα and ERβ was higher in HCC than their corresponding non-neoplastic tissues (55.9% (52/93) vs. 35.6% (31/87), P = .006; 59.1% (55/93) vs. 43.7% (38/87), P = .038), and the level of nuclear ERα and ERβ in HCC tissue was inversely correlated with T stage, tumor size, and clinical staging (P < .05). Correlation analysis showed the expression level of klotho, which is positively correlated with that of nuclear ERα (r = 0.243, P = .019). Patients with klotho-positive tumors had longer survival than those with klotho-negative tumors (P = .002). Cox proportional hazards model analysis demonstrated that positive expression of klotho was an important factor indicating good prognosis (P = .003).

CONCLUSION

Klotho, partially regulated by ERα-mediated estrogen pathway, acts as a tumor suppressor and might be a novel biomarker candidate for predicting progression and prognosis in HCC patients.

Jak1/Stat3 Activation Alters Phosphate Metabolism Independently of Sex and Extracellular Phosphate Levels

INTRODUCTION

Phosphate homeostasis is regulated by a complex network involving the parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and calcitriol acting on several organs including the kidney, intestine, bone, and parathyroid gland. Previously, we showed that activation of the Janus kinase 1 (Jak1)-signal transducer and activator of transcription 3 (Stat3) signaling pathway leads to altered mineral metabolism with higher FGF23 levels, lower PTH, and higher calcitriol levels. Here, we investigated if there are sex differences in the role of Jak1/Stat3 signaling pathway on phosphate metabolism and if this pathway is sensitive to extracellular phosphate alterations.

METHODS

We used a mouse model (Jak1S645P+/-) that resembles a constitutive activating mutation of the Jak1/Stat3 signaling pathway in humans and analyzed the impact of sex on mineral metabolism parameters. Furthermore, we challenged Jak1S645P+/- male and female mice with a high (1.2% w/w) and low (0.1% w/w) phosphate diet and a diet with phosphate with organic origin with lower bioavailability.

RESULTS

Female mice, as male mice, showed higher intact FGF23 levels but no phosphaturia, and higher calcitriol and lower PTH levels in plasma. A phosphate challenge did not alter the effect of Jak1/Stat3 activation on phosphate metabolism for both genders. However, under a low phosphate diet or a diet with lower phosphate availability, the animals showed a tendency to develop hypophosphatemia. Moreover, male and female mice showed similar phosphate metabolism parameters. The only exception was higher PTH levels in male mice than those in females.

DISCUSSION/CONCLUSION

Sex and extracellular phosphate levels do not affect the impact of Jak1/Stat3 activation on phosphate metabolism.

Age and sex effects on FGF23-mediated response to mild phosphate challenge

BACKGROUND

During aging, there is a normal and mild loss in kidney function that leads to abnormalities of the kidney-bone metabolic axis. In the setting of increased phosphorus intake, hyperphosphatemia can occur despite increased concentrations of the phosphaturic hormone FGF23. This is likely from decreased expression of the FGF23 co-receptor Klotho (KL) with age; however, the roles of age and sex in the homeostatic responses to mild phosphate challenges remain unclear.

METHODS

Male and female 16-week and 78-week mice were placed on either normal grain-based chow or casein (higher bioavailable phosphate) diets for 8 weeks. Gene expression, serum biochemistries, micro-computed tomography, and skeletal mechanics were used to assess the impact of mild phosphate challenge on multiple organ systems. Cell culture of differentiated osteoblast/osteocytes was used to test mechanisms driving key outcomes.

RESULTS

Aging female mice responded to phosphate challenge by significantly elevating serum intact FGF23 (iFGF23) versus control diet; males did not show this response. Male mice, regardless of age, exhibited higher kidney KL mRNA with similar phosphate levels across both sexes. However, males and females had similar blood phosphate, calcium, and creatinine levels irrespective of age, suggesting that female mice upregulated FGF23 to maintain blood phosphorus, and compromised renal function could not explain the increased serum iFGF23. The 17β-estradiol levels were not different between groups, and in vivo bone steroid receptor (estrogen receptor 1 [Esr1], estrogen receptor 2 [Esr2], androgen receptor [Ar]) expression was not different by age, sex, or diet. Trabecular bone volume was higher in males but decreased with both age and phosphate challenge in both sexes. Cortical porosity increased with age in males but not females. In vitro studies demonstrated that 17β-estradiol treatment upregulated FGF23 and Esr2 mRNAs in a dose-dependent manner.

CONCLUSIONS

Our study demonstrates that aging female mice upregulate FGF23 to a greater degree during a mild phosphate challenge to maintain blood phosphorus versus young female and young/old male mice, potentially due to direct estradiol effects on osteocytes. Thus, the control of phosphate intake during aging could have modifiable outcomes for FGF23-related phenotypes.

Estrogen and estrogen receptors in kidney diseases

Acute kidney injury (AKI) and chronic kidney disease (CKD) are posing great threats to global health within this century. Studies have suggested that estrogen and estrogen receptors (ERs) play important roles in many physiological processes in the kidney. For instance, they are crucial in maintaining mitochondrial homeostasis and modulating endothelin-1 (ET-1) system in the kidney. Estrogen takes part in the kidney repair and regeneration via its receptors. Estrogen also participates in the regulation of phosphorus homeostasis via its receptors in the proximal tubule. The ERα polymorphisms have been associated with the susceptibilities and outcomes of several renal diseases. As a consequence, the altered or dysregulated estrogen/ERs signaling pathways may contribute to a variety of kidney diseases, including various causes-induced AKI, diabetic kidney disease (DKD), lupus nephritis (LN), IgA nephropathy (IgAN), CKD complications, etc. Experimental and clinical studies have shown that targeting estrogen/ERs signaling pathways might have protective effects against certain renal disorders. However, many unsolved problems still exist in knowledge regarding the roles of estrogen and ERs in distinct kidney diseases. Further research is needed to shed light on this area and to enable the discovery of pathway-specific therapies for kidney diseases.

Acetazolamide causes renal [Formula: see text] wasting but inhibits ammoniagenesis and prevents the correction of metabolic acidosis by the kidney

Carbonic anhydrase (CAII) binds to the electrogenic basolateral Na+-[Formula: see text] cotransporter (NBCe1) and facilitates [Formula: see text] reabsorption across the proximal tubule. However, whether the inhibition of CAII with acetazolamide (ACTZ) alters NBCe1 activity and interferes with the ammoniagenesis pathway remains elusive. To address this issue, we compared the renal adaptation of rats treated with ACTZ to NH4Cl loading for up to 2 wk. The results indicated that ACTZ-treated rats exhibited a sustained metabolic acidosis for up to 2 wk, whereas in NH4Cl-loaded rats, metabolic acidosis was corrected within 2 wk of treatment. [Formula: see text] excretion increased by 10-fold in NH4Cl-loaded rats but only slightly (1.7-fold) in ACTZ-treated rats during the first week despite a similar degree of acidosis. Immunoblot experiments showed that the protein abundance of glutaminase (4-fold), glutamate dehydrogenase (6-fold), and SN1 (8-fold) increased significantly in NH4Cl-loaded rats but remained unchanged in ACTZ-treated rats. Na+/H+ exchanger 3 and NBCe1 proteins were upregulated in response to NH4Cl loading but not ACTZ treatment and were rather sharply downregulated after 2 wk of ACTZ treatment. ACTZ causes renal [Formula: see text] wasting and induces metabolic acidosis but inhibits the upregulation of glutamine transporter and ammoniagenic enzymes and thus suppresses ammonia synthesis and secretion in the proximal tubule, which prevented the correction of acidosis. This effect is likely mediated through the inhibition of the CA-NBCe1 metabolon complex, which results in cell alkalinization. During chronic ACTZ treatment, the downregulation of both NBCe1 and Na+/H+ exchanger 3, along with the inhibition of ammoniagenesis and [Formula: see text] generation, contributes to the maintenance of metabolic acidosis.

Electrolyte Disorders Induced by Antineoplastic Drugs

The use of antineoplastic drugs has a central role in treatment of patients affected by cancer but is often associated with numerous electrolyte derangements which, in many cases, could represent life-threatening conditions. In fact, while several anti-cancer agents can interfere with kidney function leading to acute kidney injury, proteinuria, and hypertension, in many cases alterations of electrolyte tubular handling and water balance occur. This review summarizes the mechanisms underlying the disturbances of sodium, potassium, magnesium, calcium, and phosphate metabolism during anti-cancer treatment. Platinum compounds are associated with sodium, potassium, and magnesium derangements while alkylating agents and Vinca alkaloids with hyponatremia due to syndrome of inappropriate antidiuretic hormone secretion (SIADH). Novel anti-neoplastic agents, such as targeted therapies (monoclonal antibodies, tyrosine kinase inhibitors, immunomodulators, mammalian target of rapamycin), can induce SIADH-related hyponatremia and, less frequently, urinary sodium loss. The blockade of epidermal growth factor receptor (EGFR) by anti-EGFR antibodies can result in clinically significant magnesium and potassium losses. Finally, the tumor lysis syndrome is associated with hyperphosphatemia, hypocalcemia and hyperkalemia, all of which represent serious complications of chemotherapy. Thus, clinicians should be aware of these side effects of antineoplastic drugs, in order to set out preventive measures and start appropriate treatments.

Phosphate Transport in Epithelial and Nonepithelial Tissue

Phosphate is an essential nutrient for life and is a critical component of bone formation, a major signaling molecule, and structural component of cell walls. Phosphate is also a component of high-energy compounds (i.e., AMP, ADP, and ATP) and essential for nucleic acid helical structure (i.e., RNA and DNA). Phosphate plays a central role in the process of mineralization, normal serum levels being associated with appropriate bone mineralization, while high and low serum levels are associated with soft tissue calcification. The serum concentration of phosphate and the total body content of phosphate are highly regulated, a process that is accomplished by the coordinated effort of two families of sodium-dependent transporter proteins. The three isoforms of the SLC34 family (SLC34A1-A3) show very restricted tissue expression and regulate intestinal absorption and renal excretion of phosphate. SLC34A2 also regulates the phosphate concentration in multiple lumen fluids including milk, saliva, pancreatic fluid, and surfactant. Both isoforms of the SLC20 family exhibit ubiquitous expression (with some variation as to which one or both are expressed), are regulated by ambient phosphate, and likely serve the phosphate needs of the individual cell. These proteins exhibit similarities to phosphate transporters in nonmammalian organisms. The proteins are nonredundant as mutations in each yield unique clinical presentations. Further research is essential to understand the function, regulation, and coordination of the various phosphate transporters, both the ones described in this review and the phosphate transporters involved in intracellular transport.

Iron replacement ameliorates hypophosphatemia in autosomal dominant hypophosphatemic rickets: A review of the role of iron

Autosomal dominant hypophosphatemic rickets (ADHR) is remarkable among the hypophosphatemic rickets syndromes for its variable age of presentation and periods of quiescence during which serum phosphate and fibroblast growth factor 23 (FGF 23) levels are normal without therapy. In contrast, hypophosphatemia in X-linked hypophosphatemic rickets (XLH) manifests soon after birth and requires lifelong therapy. This suggests that there are environmental factors which can alter FGF 23 activity in ADHR but not in XLH. We present an adult with ADHR in whom resolution of hypophosphatemia was achieved by correcting iron deficiency without the need for phosphate supplementation. Serial iron and FGF 23 levels revealed an inverse relationship (r=-0.79, p<0.04). All patients with ADHR who present with hypophosphatemia and worsening symptoms should be screened for iron deficiency. If iron deficiency is detected, therapy with a combination of calcitriol and iron supplementation should be considered without phosphate supplementation.

Yield of diagnostic tests in unexplained renal hypophosphatemia: a case series

BACKGROUND

Isolated renal hypophosphatemia may be inherited or acquired. An increasing number of patients with unexplained renal hypophosphatemia is being referred to our clinics, but the optimal diagnostic work-up is not known. Therefore, the aim of this study was to assess the diagnostic yield in these patients.

METHODS

We retrospectively evaluated all patients who were referred because of unexplained isolated renal hypophosphatemia to two academic tertiary referral centers in The Netherlands in the period of 2013-2017.

RESULTS

We evaluated 17 patients. In five female patients renal hypophosphatemia could be attributed to the use of oral contraceptives. The other 12 patients had a median age of 48 years (10 males). There were no other signs of tubulopathy and none of the patients used drugs known to be associated with hypophosphatemia. FGF23 levels were above normal (> 125 RU/ml) in 2/12 patients. Genetic testing, performed in all patients, did not identify a mutation in genes known to be associated with renal phosphate wasting. A scan with a radiolabeled somatostatin analogue was performed in 8 patients. In one patient, with an FGF23 level of 110 RU/ml, an increased uptake of the somatostatin analog was observed due to tumor induced osteomalacia (TIO).

CONCLUSIONS

Oral contraceptive use is an important but under-recognized cause of renal hypophosphatemia. The cause of isolated renal hypophosphatemia remained unexplained in the majority of other patients despite extensive and expensive additional investigations. The pre-test probability for tumor-induced osteomalacia or inherited renal hypophosphatemia in a patient with aspecific complaints and a normal FGF23 level is low. Further research is needed to investigate which patients should be screened for TIO. At present we suggest to perform somatostatin scans only in patients with severe complaints, elevated FGF23 levels, or progressive disease.

Sex steroids and the kidney: role in renal calcium and phosphate handling

Calcium and phosphate are vital for the organism and constitute essential components of the skeleton. Serum levels are tightly hormonally regulated and maintained by exchange with three major sources: the intestines, the kidney and the bone. The effects of sex steroids on the bone have been extensively studied and it is well known that sex steroid deficiency induces bone loss, indirectly influencing renal calcium and phosphate homeostasis. However, it is unknown whether sex steroids also directly regulate renal calcium and phosphate handling, hereby potentially indirectly impacting on bone. The presence of androgen receptors (AR) and estrogen receptors (ER) in both human and rodent kidney, although their exact localization within the kidney remains debated, supports direct effects. Estrogens stimulate renal calcium reabsorption as well as phosphate excretion, while the effects of androgens are less clear. Many of the studies performed with regard to renal calcium and/or phosphate homeostasis do not correct for the calcium and phosphate fluxes from the bone and intestines, which complicates the differentiation between the direct effects of sex steroids on renal calcium and phosphate handling and the indirect effects via the bone and intestines. The objective of this study is to review the literature and current insight of the role of sex steroids in calcium and phosphate handling in the kidney.

X-linked hypophosphatemia and growth

X-Linked hypophosphatemia (XLH) is the most common form of hereditary rickets caused by loss-of function mutations in the PHEX gene. XLH is characterized by hypophosphatemia secondary to renal phosphate wasting, inappropriately low concentrations of 1,25 dihydroxyvitamin D and high circulating levels of fibroblast growth factor 23 (FGF23). Short stature and rachitic osseous lesions are characteristic phenotypic findings of XLH although the severity of these manifestations is highly variable among patients. The degree of growth impairment is not dependent on the magnitude of hypophosphatemia or the extent of legs´ bowing and height is not normalized by chronic administration of phosphate supplements and 1α hydroxyvitamin D derivatives. Treatment with growth hormone accelerates longitudinal growth rate but there is still controversy regarding the potential risk of increasing bone deformities and body disproportion. Treatments aimed at blocking FGF23 action are promising, but information is lacking on the consequences of counteracting FGF23 during the growing period. This review summarizes current knowledge on phosphorus metabolism in XLH, presents updated information on XLH and growth, including the effects of FGF23 on epiphyseal growth plate of the Hyp mouse, an animal model of the disease, and discusses growth hormone and novel FGF23 related therapies.