Sirolimus induced phosphaturia is not caused by inhibition of renal apical sodium phosphate cotransporters

Rapamycin treatment induces tubular proteinuria: role of megalin-mediated protein reabsorption

Introduction: Rapamycin is an immunosuppressor that acts by inhibiting the serine/threonine kinase mechanistic target of rapamycin complex 1. Therapeutic use of rapamycin is limited by its adverse effects. Proteinuria is an important marker of kidney damage and a risk factor for kidney diseases progression and has been reported in patients and animal models treated with rapamycin. However, the mechanism underlying proteinuria induced by rapamycin is still an open matter. In this work, we investigated the effects of rapamycin on parameters of renal function and structure and on protein handling by proximal tubule epithelial cells (PTECs). Methods: Healthy BALB/c mice were treated with 1.5 mg/kg rapamycin by oral gavage for 1, 3, or 7 days. At the end of each treatment, the animals were kept in metabolic cages and renal function and structural parameters were analyzed. LLC-PK1 cell line was used as a model of PTECs to test specific effect of rapamycin. Results: Rapamycin treatment did not change parameters of glomerular structure and function. Conversely, there was a transient increase in 24-h proteinuria, urinary protein to creatinine ratio (UPCr), and albuminuria in the groups treated with rapamycin. In accordance with these findings, rapamycin treatment decreased albumin-fluorescein isothiocyanate uptake in the renal cortex. This effect was associated with reduced brush border expression and impaired subcellular distribution of megalin in PTECs. The effect of rapamycin seems to be specific for albumin endocytosis machinery because it did not modify renal sodium handling or (Na⁺+K⁺)ATPase activity in BALB/c mice and in the LLC-PK1 cell line. A positive Pearson correlation was found between megalin expression and albumin uptake while an inverse correlation was shown between albumin uptake and UPCr or 24-h proteinuria. Despite its effect on albumin handling in PTECs, rapamycin treatment did not induce tubular injury measured by interstitial space and collagen deposition. Conclusion: These findings suggest that proteinuria induced by rapamycin could have a tubular rather than a glomerular origin. This effect involves a specific change in protein endocytosis machinery. Our results open new perspectives on understanding the undesired effect of proteinuria generated by rapamycin.

[Pattern of biochemical markers of mineral and bone disorders in kidney transplant recipients: real-world data]

BACKGROUND

There is a lack of studies providing comprehensive data on the prevalence of mineral and bone disorders (MBD) laboratory abnormalities after kidney transplantation in Russia.

AIM

to obtain real-world data on the prevalence of the main mineral abnormalities among kidney transplant recipients and to revise their concomitant MBD therapy.

METHOD

This cross-sectional study included 236 patients with successful kidney transplantation. Their serum intact parathyroid hormone (iPTH), total calcium (Ca), phosphorus (P), and alkaline phosphatase (ALP) levels were measured.

RESULTS

Only 6.2% of our cohort had all laboratory parameters within the target range, whereas persistent HPT along with hypercalcemia was noted in almost one third of the patients (31%). Normal iPTH levels were observed in 13% cases; 84% of the patients had hyperparathyroidism. The fraction of patients with target iPTH did not differ between the groups with normal and decreased estimated glomerular filtration rate (eGFR) (p=0.118). Hypercalcemia was observed in 29% cases. The serum P level varied significantly in groups with different eGFR (p<0.0001), increasing with declining graft function. Furthermore, 40.7% of patients had ALP above the target range. While 123 patients received active vitamin D (alfacalcidol), 33 received monotherapy with inactive vitamin D (cholecalciferol). The control group consisted of 57 medication-naïve patients. The serum total Ca level varied significantly between the groups (p=0.0006), being higher in patients supplemented with cholecalciferol. The fraction of patients with normocalcemia was lowest in the cholecalciferol group (chi-square, р=0.0018).

CONCLUSION

The prevalence of biochemical abnormalities after kidney transplantation is high. Alfacalcidol usage may be safer than using cholecalciferol to prevent hypercalcemia development.

Role of mTOR Signaling for Tubular Function and Disease

The mechanistic target of rapamycin (mTOR) forms two distinct intracellular multiprotein complexes that control a multitude of intracellular processes linked to metabolism, proliferation, actin cytoskeleton, and survival. Recent studies have identified the importance of these complexes for transport regulation of ions and nutrients along the entire nephron. First reports could link altered activity of these complexes to certain disease entities, i.e. diabetic nephropathy, acute kidney injury or hyperkalemia.

Lack of PTEN in osteocytes increases circulating phosphate concentrations by decreasing intact fibroblast growth factor 23 levels

Fibroblast growth factor 23 (FGF23) has been centric to the regulation of phosphate (Pi) metabolism; however, the regulatory network of FGF23 in osteocytes has not yet been defined in detail. We herein investigated the role of PTEN (phosphatase and tensin homolog deleted from chromosome 10) in this regulation. We created mice lacking PTEN expression mainly in osteocytes by crossing Pten-flox mice with Dmp1-Cre mice. The lack of PTEN in the osteocytes of these mice was associated with decreased skeletal and serum intact FGF23 levels, which, in turn, resulted in reductions of urinary Pi excretion and elevations of serum Pi levels. Mechanistically, the knockdown of PTEN expression in osteoblastic UMR106 cells activated the AKT/mTORC1 (mechanistic target of rapamycin complex 1) pathway and this was associated with reductions in Fgf23 expression. Furthermore, the suppression of Fgf23 expression by PTEN knockdown or insulin simulation in UMR106 cells was partially restored by the treatment with the mTORC1 inhibitor, rapamycin. These results suggest that FGF23 expression in osteoblastic cells is in part regulated through the AKT/mTORC1 pathway and provide new insights into our understanding of the regulatory network of Pi metabolism.

The tripartite interaction of phosphate, autophagy, and αKlotho in health maintenance

Aging-related organ degeneration is driven by multiple factors including the cell maintenance mechanisms of autophagy, the cytoprotective protein αKlotho, and the lesser known effects of excess phosphate (Pi), or phosphotoxicity. To examine the interplay between Pi, autophagy, and αKlotho, we used the BK/BK mouse (homozygous for mutant Becn1F121A ) with increased autophagic flux, and αKlotho-hypomorphic mouse (kl/kl) with impaired urinary Pi excretion, low autophagy, and premature organ dysfunction. BK/BK mice live longer than WT littermates, and have heightened phosphaturia from downregulation of two key NaPi cotransporters in the kidney. The multi-organ failure in kl/kl mice was rescued in the double-mutant BK/BK;kl/kl mice exhibiting lower plasma Pi, improved weight gain, restored plasma and renal αKlotho levels, decreased pathology of multiple organs, and improved fertility compared to kl/kl mice. The beneficial effects of heightened autophagy from Becn1F121A was abolished by chronic high-Pi diet which also shortened life span in the BK/BK;kl/kl mice. Pi promoted beclin 1 binding to its negative regulator BCL2, which impairs autophagy flux. Pi downregulated αKlotho, which also independently impaired autophagy. In conclusion, Pi, αKlotho, and autophagy interact intricately to affect each other. Both autophagy and αKlotho antagonizes phosphotoxicity. In concert, this tripartite system jointly determines longevity and life span.

Therapeutic Use of mTOR Inhibitors in Renal Diseases: Advances, Drawbacks, and Challenges

The mammalian (or mechanistic) target of rapamycin (mTOR) pathway has a key role in the regulation of a variety of biological processes pivotal for cellular life, aging, and death. Impaired activity of mTOR complexes (mTORC1/mTORC2), particularly mTORC1 overactivation, has been implicated in a plethora of age-related disorders, including human renal diseases. Since the discovery of rapamycin (or sirolimus), more than four decades ago, advances in our understanding of how mTOR participates in renal physiological and pathological mechanisms have grown exponentially, due to both preclinical studies in animal models with genetic modification of some mTOR components as well as due to evidence coming from the clinical experience. The main clinical indication of rapamycin is as immunosuppressive therapy for the prevention of allograft rejection, namely, in renal transplantation. However, considering the central participation of mTOR in the pathogenesis of other renal disorders, the use of rapamycin and its analogs meanwhile developed (rapalogues) everolimus and temsirolimus has been viewed as a promising pharmacological strategy. This article critically reviews the use of mTOR inhibitors in renal diseases. Firstly, we briefly overview the mTOR components and signaling as well as the pharmacological armamentarium targeting the mTOR pathway currently available or in the research and development stages. Thereafter, we revisit the mTOR pathway in renal physiology to conclude with the advances, drawbacks, and challenges regarding the use of mTOR inhibitors, in a translational perspective, in four classes of renal diseases: kidney transplantation, polycystic kidney diseases, renal carcinomas, and diabetic nephropathy.

Association of tenofovir disoproxil fumarate with primary allograft survival in HIV-positive kidney transplant recipients

BACKGROUND

Tenofovir disoproxil fumarate (TDF) is an antiretroviral agent frequently used to treat human immunodeficiency virus (HIV). There are concerns regarding its potential to cause acute kidney injury, chronic kidney disease, and proximal tubulopathy. Although TDF can effectively suppress HIV after kidney transplantation, it is unknown whether use of TDF-based antiretroviral therapy (ART) after kidney transplantation adversely affects allograft survival.

METHODS

We examined 104 HIV+ kidney transplant (KT) recipients at our center between 2001 and 2014. We generated a propensity score for TDF treatment using recipient and donor characteristics. We then fit Cox proportional hazards models to investigate the association between TDF treatment and 3-year, death-censored primary allograft failure, adjusting for the propensity score and delayed graft function (DGF).

RESULTS

Of the 104 HIV+ KT candidates who underwent transplantation during the study period, 23 (22%) were maintained on TDF-based ART at the time of transplantation, and 81 (78%) were on non-TDF-based ART. Median age of the cohort was 48 years; 87% were male; 88% were black; and median CD4 count at transplantation was 450 cells/mm³ . Median kidney donor risk index was 1.2. At 3 years post transplantation, primary allograft failure occurred in 26% of patients on TDF-based ART and in 28% of patients on non-TDF-based ART (P=.5). TDF treatment was not associated with primary allograft failure at 3 years post transplant after adjusting for DGF and a propensity score for TDF use (hazard ratio 2.12, 95% confidence interval 0.41-10.9).

CONCLUSIONS

In a large single-center experience of HIV+ kidney transplantation, TDF use following kidney transplantation was not significantly associated with primary allograft failure. These results may help inform management for HIV+ KT recipients in need of TDF therapy for adequate viral suppression.

Roles of mTOR complexes in the kidney: implications for renal disease and transplantation

The mTOR pathway has a central role in the regulation of cell metabolism, growth and proliferation. Studies involving selective gene targeting of mTOR complexes (mTORC1 and mTORC2) in renal cell populations and/or pharmacologic mTOR inhibition have revealed important roles of mTOR in podocyte homeostasis and tubular transport. Important advances have also been made in understanding the role of mTOR in renal injury, polycystic kidney disease and glomerular diseases, including diabetic nephropathy. Novel insights into the roles of mTORC1 and mTORC2 in the regulation of immune cell homeostasis and function are helping to improve understanding of the complex effects of mTOR targeting on immune responses, including those that impact both de novo renal disease and renal allograft outcomes. Extensive experience in clinical renal transplantation has resulted in successful conversion of patients from calcineurin inhibitors to mTOR inhibitors at various times post-transplantation, with excellent long-term graft function. Widespread use of this practice has, however, been limited owing to mTOR-inhibitor- related toxicities. Unique attributes of mTOR inhibitors include reduced rates of squamous cell carcinoma and cytomegalovirus infection compared to other regimens. As understanding of the mechanisms by which mTORC1 and mTORC2 drive the pathogenesis of renal disease progresses, clinical studies of mTOR pathway targeting will enable testing of evolving hypotheses.

mTOR Regulates Endocytosis and Nutrient Transport in Proximal Tubular Cells

Renal proximal tubular cells constantly recycle nutrients to ensure minimal loss of vital substrates into the urine. Although most of the transport mechanisms have been discovered at the molecular level, little is known about the factors regulating these processes. Here, we show that mTORC1 and mTORC2 specifically and synergistically regulate PTC endocytosis and transport processes. Using a conditional mouse genetic approach to disable nonredundant subunits of mTORC1, mTORC2, or both, we showed that mice lacking mTORC1 or mTORC1/mTORC2 but not mTORC2 alone develop a Fanconi-like syndrome of glucosuria, phosphaturia, aminoaciduria, low molecular weight proteinuria, and albuminuria. Interestingly, proteomics and phosphoproteomics of freshly isolated kidney cortex identified either reduced expression or loss of phosphorylation at critical residues of different classes of specific transport proteins. Functionally, this resulted in reduced nutrient transport and a profound perturbation of the endocytic machinery, despite preserved absolute expression of the main scavenger receptors, MEGALIN and CUBILIN. Our findings highlight a novel mTOR-dependent regulatory network for nutrient transport in renal proximal tubular cells.

Uroguanylin modulates (Na++K+)ATPase in a proximal tubule cell line: Interactions among the cGMP/protein kinase G, cAMP/protein kinase A, and mTOR pathways

BACKGROUND

The natriuretic effect of uroguanylin (UGN) involves reduction of proximal tubule (PT) sodium reabsorption. However, the target sodium transporters as well as the molecular mechanisms involved in these processes remain poorly understood.

METHODS

To address the effects of UGN on PT (Na(+)+K(+))ATPase and the signal transduction pathways involved in this effect, we used LLC-PK1 cells. The effects of UGN were determined through ouabain-sensitive ATP hydrolysis and immunoblotting assays during different experimental conditions.

RESULTS

We observed that UGN triggers cGMP/PKG and cAMP/PKA pathways in a sequential way. The activation of PKA leads to the inhibition of mTORC2 activity, PKB phosphorylation at S473, PKB activity and, consequently, a decrease in the mTORC1/S6K pathway. The final effects are decreased expression of the α1 subunit of (Na(+)+K(+))ATPase and inhibition of enzyme activity.

CONCLUSIONS

These results suggest that the molecular mechanism of action of UGN on sodium reabsorption in PT cells is more complex than previously thought. We propose that PKG-dependent activation of PKA leads to the inhibition of the mTORC2/PKB/mTORC1/S6K pathway, an important signaling pathway involved in the maintenance of the PT sodium pump expression and activity.

GENERAL SIGNIFICANCE

The current results expand our understanding of the signal transduction pathways involved in the overall effect of UGN on renal sodium excretion.

Inorganic Phosphate Activates the AKT/mTORC1 Pathway and Shortens the Life Span of an α‑Klotho-Deficient Model

Inorganic phosphate (Pi) has been implicated in the pathogenesis of accelerated aging; however, the underlying mechanisms remain elusive. Herein, we demonstrated in cultured cells and in vivo that increased levels of extracellular Pi activated the AKT/mammalian target of rapamycin complex 1 (mTORC1) pathway by suppressing membrane-bound phosphatase and tensin homolog (PTEN) levels in a manner requiring the sodium-dependent Pi transporter PiT‑1. High levels of extracellular Pi also led to phosphorylation of Ser/Thr clusters in the C‑terminal tail of PTEN, which has been shown to dissociate PTEN from the membrane. Notably, blockade of mTORC1 activity by rapamycin treatment prolonged the life span of hyperphosphatemic α‑Klotho-deficient (Kl(-/-)) mice. Dietary correction of hyperphosphatemia or treatment with rapamycin also rescued the brown adipose tissue dysfunction and oxidative damage observed in Kl(-/-) mice. Furthermore, rapamycin treatment partially rescued these effects and extended the life span when Kl(-/-) mice were maintained on a high-phosphate diet. Finally, rapamycin reduced circulating Pi levels in Kl(-/-) mice, apparently by decreasing the localization of sodium-dependent Pi transport protein 2a at the renal brush border membrane. Therefore, the activation of mTORC1 may create a vicious loop that exacerbates the retention of Pi, which in turn may enhance oxidative damage and ultimately shorten the life span of Kl(-/-) mice. These results demonstrate that Pi has important roles in the aging process, and the blockade of mTORC1 may have therapeutic potential for premature aging-like symptoms associated with hyperphosphatemia.

Renal phosphate transporters

PURPOSE OF REVIEW

Phosphate homeostasis is tightly controlled by the coordinated activity of bone, kidney, intestine, and parathyroid gland. The renal phosphate transporters have emerged as key regulators of both total body phosphate homeostasis and serum phosphate concentration. This review focuses on the latest updates in phosphate transport and transporters with an emphasis on renal phosphate transporters.

RECENT FINDINGS

Structure function analysis of type II sodium phosphate cotransporters has revealed motifs with significant similarity to those seen in other sodium-coupled solute transporters, identifying key amino acid residues important for solute binding and transport. Previously unidentified regulators of these transporters have been found, although their physiologic significance and interaction with more traditional regulators have not been established. Type II and type III sodium phosphate cotransporters play critical roles in bone, choroid plexus, and vascular physiology and pathophysiology.

SUMMARY

Increasing knowledge of structure function relationships for sodium phosphate cotransporters, as well as greater appreciation for the complexity of their regulation and role in renal and nonrenal tissue, brings the promise of newer, more specific treatments for disorders of phosphate homeostasis.

VIDEO ABSTRACT

http://links.lww.com/CONH/A10.

mTOR controls kidney epithelia in health and disease

Renal epithelial function is the cornerstone of key excretory processes performed by our kidneys. Most of these tasks need to be tightly controlled to keep our internal environment in balance. Recently, the mTOR signalling network emerged as a key pathway controlling renal epithelial cells from the glomerular tuft along the entire nephron. Both mTOR complexes, mTORC1 and mTORC2, regulate such diverse processes as glomerular filtration and the fine tuning of tubular electrolyte balance. Most importantly, dysregulation of mTOR signalling contributes to prevalent kidney diseases like diabetic nephropathy and cystic kidney disease. The following review shall summarize our current knowledge of the renal epithelial mTOR signalling system under physiological and pathophysiological conditions.

mTORC1 maintains renal tubular homeostasis and is essential in response to ischemic stress

Mammalian target of rapamycin complex 1 (mTORC1) is a key regulator of cell metabolism and autophagy. Despite widespread clinical use of mTORC1 inhibitors, the role of mTORC1 in renal tubular function and kidney homeostasis remains elusive. By using constitutive and inducible deletion of conditional Raptor alleles in renal tubular epithelial cells, we discovered that mTORC1 deficiency caused a marked concentrating defect, loss of tubular cells, and slowly progressive renal fibrosis. Transcriptional profiling revealed that mTORC1 maintains renal tubular homeostasis by controlling mitochondrial metabolism and biogenesis as well as transcellular transport processes involved in countercurrent multiplication and urine concentration. Although mTORC2 partially compensated for the loss of mTORC1, exposure to ischemia and reperfusion injury exaggerated the tubular damage in mTORC1-deficient mice and caused pronounced apoptosis, diminished proliferation rates, and delayed recovery. These findings identify mTORC1 as an important regulator of tubular energy metabolism and as a crucial component of ischemic stress responses.