Metabolic adaptations of the nephron in renal disease

FGF23 is synthesised locally by renal tubules and activates injury-primed fibroblasts

In kidney disease, higher circulating levels of the mineral-regulating hormone fibroblast growth factor (FGF)-23 are predictive of disease progression but direct pathogenic effects on the kidney are unknown. We sought evidence of local renal synthesis in response to unilateral ureteric obstruction in the mouse, and pro-fibrotic actions of FGF23 on the fibroblast in vitro. Acute tubulointerstitial injury due to unilateral ureteric obstruction stimulated renal FGF23 synthesis by tubules, and downregulated inactivating proprotein convertases, without effects on systemic mineral metabolism. In vitro, FGF23 had divergent effects on fibroblast activation in cells derived from normal and obstructed kidneys. While FGF23 failed to stimulate fibrogenesis in normal fibroblasts, in those primed by injury, FGF23 induced pro-fibrotic signalling cascades via activation of TGF-β pathways. Effects were independent of α-klotho. Tubule-derived FGF23 may amplify myofibroblast activation in acute renal injury, and might provide a novel therapeutic target in renal fibrosis.

Proteomic analysis of rat proximal tubule cells following stretch-induced apoptosis in an in vitro model of kidney obstruction

Urinary tract obstruction (UTO) is a commonly noted disorder on prenatal ultrasound that has the potential to lead to permanent loss of renal function. To study the molecular processes of the disease, an in vitro model has been developed which involves mechanical stretch of proximal tubule cells grown on flexible plates which mimics the physiological conditions during UTO. This study employs a one dimensional SDS-PAGE fractionation procedure, followed by in-gel digest and LC-MS/MS analysis in a semi-quantitative experiment using spectral counting to relatively quantify changes in protein expression following the established model of UTO. Quantitative analysis shows 317 of the 1630 identified proteins express altered abundance, with 135 increased and 182 decreased in abundance as a result of stretch. Gene ontology (GO) and KEGG annotations implicate a number of physiological processes that have been previously shown in addition to some potentially novel processes in UTO. The quantitative proteomic analysis we performed here provides a more complete characterization of changes in protein abundance as a result of stretch than previous studies, and provides a number of previously undescribed proteins in proximal tubule cells that may play a role in UTO.

BIOLOGICAL SIGNIFICANCE

Urinary tract obstruction (UTO) is a commonly noted abnormality on prenatal ultrasound that can either resolve spontaneously or require surgical intervention to prevent permanent renal damage or loss of function. While targeted studies of UTO have shown a number of pathological responses in proximal tubule cells, there are currently no large-scale quantitative studies that aim to elucidate a global cellular response. This study uses a semi-quantitative approach and applies a well characterized model of UTO to determine a number of cellular processes affected by UTO simulation and identifies a number of proteins with altered abundance that have not been noted previously in UTO. This article is part of a Special Issue entitled: Can Proteomics Fill the Gap Between Genomics and Phenotypes?

Efficacy and safety of lanthanum carbonate on chronic kidney disease-mineral and bone disorder in dialysis patients: a systematic review

BACKGROUND

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is a common complication in CKD patients, particularly in those with end-stage renal disease that requires dialysis. Lanthanum carbonate (LC) is a potent, non-aluminum, non-calcium phosphate binder. This systematic review evaluates the efficacy and safety of LC in CKD-MBD treatment for maintenance-dialysis patients.

METHODS

A systematic review and meta-analysis on randomized controlled trials (RCTs) and quasi-RCTs was performed to assess the efficacy and safety of LC in maintenance hemodialysis or peritoneal dialysis patients. Analysis was performed using the statistical software Review Manager 5.1.

RESULTS

Sixteen RCTs involving 3789 patients were identified and retained for this review. No statistical difference was found in all-cause mortality. The limited number of trials was insufficient to show the superiority of LC over other treatments in lowering vascular calcification or cardiovascular events and in improving bone morphology, bone metabolism, or bone turn-over parameters. LC decreased the serum phosphorus level and calcium × phosphate product (Ca × P) as compared to placebo. LC, calcium carbonate (CC), and sevelamer hydrochloride (SH) were comparable in terms of controlling the serum phosphorus, Ca × P product, and intact parathyroid hormone (iPTH) levels. However, LC resulted in a lower serum calcium level and a higher bone-specific alkaline phosphatase level compared with CC. LC had higher total cholesterol and low-density lipoprotein (LDL) cholesterol levels compared with SH. LC-treated patients appeared to have a higher rate of vomiting and lower risk of hypercalcemia, diarrhea, intradialytic hypotension, cramps or myalgia, and abdominal pain. Meta-analysis showed no significant difference in the incidence of other side effects. Accumulation of LC in blood and bone was below toxic levels.

CONCLUSIONS

LC has high efficacy in lowering serum phosphorus and iPTH levels without increasing the serum calcium. Current evidence does not show a higher rate of adverse effects for LC compared with other treatments, except for a higher incidence of vomiting. Moreover, LC accumulation in blood and bone was below toxic levels. Well-designed studies should be conducted to evaluate the long-term effects of LC.

Control of serum phosphate by oral lanthanum carbonate in patients undergoing haemodialysis and continuous ambulatory peritoneal dialysis in a short-term, placebo-controlled study

BACKGROUND

Hyperphosphataemia in dialysis patients is associated with significant morbidity. We assessed the ability of lanthanum carbonate to control phosphate levels in patients undergoing haemodialysis or continuous ambulatory peritoneal dialysis (CAPD) in a short-term, placebo-controlled study.

METHODS

This was a double-blind, placebo-controlled, parallel-group study consisting of three phases: a 2 week washout period; a 4 week, open-label, dose-titration phase; and a 4 week, double-blind, placebo-controlled phase. After washout, patients (n = 59) received lanthanum (375 mg/day), titrated up to a maintenance dose (maximum: 2250 mg) that achieved control of serum phosphate levels between 1.3 and 1.8 mmol/l (4.03-5.58 mg/dl). After titration, patients were randomized to receive their maintenance dose of lanthanum (n = 17) or placebo (n = 19) for 4 weeks. Control of serum phosphate was the primary efficacy assessment. Levels of calcium, parathyroid hormone, calcium x phosphate product and lanthanum as well as adverse events were evaluated.

RESULTS

By the end of titration, 70% of patients had serum phosphate levels < or =1.8 mmol/l. Lanthanum carbonate continued to control serum phosphate levels in the double-blind phase. At the end of the study, 64.7% of lanthanum carbonate-treated patients were controlled compared with 21.4% in the placebo group. Results in patients receiving CAPD were similar to those seen in the group as a whole. Mean parathyroid hormone levels (P = 0.41) and calcium x phosphate product (P<0.001) were both higher in the placebo than the lanthanum carbonate group.

CONCLUSIONS

Lanthanum carbonate is an effective phosphate binder able to control serum phosphate and calcium x phosphate product.

Potassium transport in the mammalian collecting duct

The mammalian collecting duct plays a dominant role in regulating K(+) excretion by the nephron. The collecting duct exhibits axial and intrasegmental cell heterogeneity and is composed of at least two cell types: collecting duct cells (principal cells) and intercalated cells. Under normal circumstances, the collecting duct cell in the cortical collecting duct secretes K(+), whereas under K(+) depletion, the intercalated cell reabsorbs K(+). Assessment of the electrochemical driving forces and of membrane conductances for transcellular and paracellular electrolyte movement, the characterization of several ATPases, patch-clamp investigation, and cloning of the K(+) channel have provided important insights into the role of pumps and channels in those tubule cells that regulate K(+) secretion and reabsorption. This review summarizes K(+) transport properties in the mammalian collecting duct. Special emphasis is given to the mechanisms of how K(+) transport is regulated in the collecting duct.

Transport defects of rabbit medullary thick ascending limb cells in obstructive nephropathy

To characterize the sodium transport defect responsible for salt wasting in obstructive nephropathy, the major sodium transporters in the medullary thick ascending limb (mTAL), the apical Na-K-2Cl cotransporter and the basolateral Na-K-ATPase, were studied in fresh suspensions of mTAL cells and outer medulla plasma membranes prepared from obstructed and untreated kidneys. Oxygen consumption (QO2) studies in intact cells revealed marked reductions in the inhibitory effects of both furosemide and ouabain on QO2 in cells from obstructed, as compared with control animals, indicating a reduction in activities of both the Na-K-2Cl cotransporter and the Na-K-ATPase. Saturable [3H]bumetanide binding was reduced in membranes isolated from obstructed kidneys, but the Kd for [3H]bumetanide was unchanged, indicating a decrease in the number of functional luminal Na-K-2Cl cotransporters in obstructed mTAL. Ouabain sensitive Na-K-ATPase activity in plasma membranes was also reduced, and immunoblots using specific monoclonal antibodies directed against the alpha and beta subunits of rabbit Na-K-ATPase showed decreased amounts of both subunits in outer medullas of obstructed kidney. A significant decrease in [3H]bumetanide binding was detected after 4 h of ureteral obstruction, whereas Na-K-ATPase activity at this time was still not different from control. We conclude that ureteral obstruction reduces the amounts of both luminal Na-K-2Cl cotransporter and basolateral Na-K-ATPase in mTAL of obstructed kidney and that these reductions contribute to the salt wasting observed after release of obstruction.

Ureteral obstruction decreases renal prepro-epidermal growth factor and Tamm-Horsfall expression

Northern and dot-blot analysis of polyadenylated RNAs of kidney cortical and outer medullary tissue was performed in male Sprague-Dawley rats at varying times up to 24 hours after bilateral ureteral obstruction (BUO), after 24 hours of unilateral obstruction (UUO) and at varying periods after release of BUO or UUO. Pre-proEGF (preproEGF) and Tamm-Horsfall (TH) mRNA declined by four hours of BUO to virtually undetectable levels at 24 hours of ureteral obstruction. Upon release of BUO or UUO, preproEGF and TH mRNA returned slowly toward normal but remained below control levels up to four days after release of ureteral obstruction. Urinary EGF excretion paralleled these changes in renal preproEGF mRNA. Although these changes are similar to those observed during nephrotoxic and ischemic renal failure, where the expression of the immediate early genes precedes the fall in preproEGF and TH expression, no such increase in the expression of these genes occurred after bilateral ureteral obstruction. These changes in preproEGF and TH expression could also be dissociated from uremia and high rates of DNA synthesis, suggesting that ureteral obstruction itself is a sufficient cause of the reduced expression. The increase in ureteral pressure and its functional and humoral effects may each play a role in reduced preproEGF and TH expression during ureteral obstruction.

Plasma erythropoietin levels and acquired cystic disease of the kidney in patients receiving regular haemodialysis treatment

Acquired cystic disease of the kidney (ACDK) in patients with end-stage renal failure can be associated with development of polycythaemia. The relationship between plasma erythropoietin levels and ACDK in 17 patients on long-term haemodialysis treatment was studied. There was a significantly higher level of plasma erythropoietin in patients with multiple renal cysts than in those patients with less than five cysts or no cysts. Haemoglobin tended to be higher in the ACDK group, but the difference was not significant. These results indicate that the development of renal cysts results in increased secretion of erythropoietin.

Model of gentamicin-induced nephrotoxicity and its amelioration by calcium and thyroxine

The exact mechanism of gentamicin-induced acute renal failure is presently unknown; various mechanisms have been proposed but there is no proposed commonality between them. In animals, dietary calcium loading and L-thyroxine administration have been shown to ameliorate toxicity, with again no common process. A mechanism of competitive displacement of calcium and other cations from anionic phospholipids at the plasma and organelle membrane level, resulting in a decrease in Na+ -K+ ATPase, adenylate cyclase, mitochondrial function and ATP production, protein synthesis, solute reabsorption and overall cellular function is proposed. A further proposal is dietary calcium loading and thyroxine (which increases intracellular calcium) reverse gentamicin-induced acute renal failure by increasing the calcium and solute flux, thereby competitively inhibiting the primary lesion: anionic phospholipid binding.