Chronic regulation of the renal Na(+)/H(+) exchanger NHE3 by dopamine: translational and posttranslational mechanisms

The intrarenal autocrine/paracrine dopamine (DA) system contributes to natriuresis in response to both acute and chronic Na(+) loads. While the acute DA effect is well described, how DA induces natriuresis chronically is not known. We used an animal and a cell culture model to study the chronic effect of DA on a principal renal Na(+) transporter, Na(+)/H(+) exchanger-3 (NHE3). Intraperitoneal injection of Gludopa in rats for 2 days elevated DA excretion and decreased total renal cortical and apical brush-border NHE3 antigen. Chronic treatment of an opossum renal proximal cell line with DA decreased NHE3 activity, cell surface and total cellular NHE3 antigen, but not NHE3 transcript. The decrease in NHE3 antigen was dose and time dependent with maximal inhibition at 16-24 h and half maximal effect at 3 × 10(-7) M. This is in contradistinction to the acute effect of DA on NHE3 (half maximal at 2 × 10(-6) M), which was not associated with changes in total cellular NHE3 protein. The DA-induced decrease in total NHE3 protein was associated with decrease in NHE3 translation and mediated by cis-sequences in the NHE3 5'-untranslated region. DA also decreased cell surface and total cellular NHE3 protein half-life. The DA-induced decrease in total cellular NHE3 was partially blocked by proteasome inhibition but not by lysosome inhibition, and DA increased ubiquitylation of total and surface NHE3. In summary, chronic DA inhibits NHE3 with mechanisms distinct from its acute action and involves decreased NHE3 translation and increased NHE3 degradation, which are novel mechanisms for NHE3 regulation.

Reduction of renal triglyceride accumulation: effects on proximal tubule Na+/H+ exchange and urinary acidification

One main pathophysiological mechanism underlying the increased risk for uric acid nephrolithiasis in humans with the metabolic syndrome is the excretion of unduly acidic urine, in part because of reduced excretion of the main urinary buffer, ammonium. The Zucker diabetic fatty (ZDF) rat, an established rodent model of the metabolic syndrome, has similar urinary abnormalities, attributed in part to lower expression and activity of the principal mediator of proximal tubule ammonium excretion, brush-border membrane Na+/H+ exchanger 3 (NHE3). These defects are associated with renal tubular steatosis in ZDF rats, but the causal relationship between renal steatosis and defective urinary acidification has not been investigated in vivo. We hypothesized that reduction of renal steatosis would commensurately normalize urinary acidification parameters. We treated ZDF rats with thiazolidinediones to reduce nonadipose tissue steatosis. Four weeks of treatment reduced renal triglyceride accumulation and restored urinary acidification parameters in ZDF rats to levels comparable to their lean littermates; urinary acidification was not affected by treatment in lean rats. To further document the direct effects of fat, we showed that functional abnormalities induced by fat loading in a cell culture model of proximal tubule steatosis and lipotoxicity can be reversed by fat removal but not by thiazolidinediones alone. Together, these findings support the causative role of renal steatosis in the pathogenesis of urinary acidification defects, demonstrate reversibility upon lipid removal, and highlight a potential therapeutic strategy for renal abnormalities in the metabolic syndrome.

Effect of renal lipid accumulation on proximal tubule Na+/H+ exchange and ammonium secretion

Patients with metabolic syndrome have increased risk of uric acid nephrolithiasis due to lower urinary pH and impaired ammonium excretion. The pathophysiology underlying these urinary changes is unknown. We used two animal models and a cell culture model to study whether the alteration in renal acidification is associated with renal fat infiltration (steatosis). Compared with pair-fed lean control rats, Zucker diabetic fatty rats have higher renal triglyceride content, decreased urinary ammonium and pH, and lower levels of brush border membrane Na(+)/H(+) exchanger-3 (NHE3), a major mediator of ammonium excretion. High-fat feeding in Sprague-Dawley rats results in transient lowering of urinary ammonium and pH, with all parameters returning to normal when the animals resumed eating normal chow. This is consistent with an absence of diet-induced renal steatosis in these animals. To examine the direct effect of fat accumulation, we incubated opossum kidney (OKP) cells with a mixture of long-chain fatty acids and found accumulation of intracellular lipids with concomitant dose-dependent decrease in NHE3 activity, surface biotin-accessible NHE3 protein, and ammonium secretion. A lower dose of fatty acids that leads to intracellular lipid accumulation but does not change baseline NHE3 is sufficient to abolish the stimulation of NHE3 by insulin and to partially block the stimulation of NHE3 by glucocorticoid hormones; acid regulation of NHE3 in lipid-loaded OKP cells is not affected. These findings suggest that renal steatosis decreases ammonium secretion in the proximal tubule, in part by reducing NHE3 activity and by impairing the regulation of NHE3 by specific agonists.

A sperm-specific Na+/H+ exchanger (sNHE) is critical for expression and in vivo bicarbonate regulation of the soluble adenylyl cyclase (sAC)

We previously identified a sperm-specific Na(+)/H(+) exchanger (sNHE) principally localized to the flagellum. Disruption of the sNHE gene in mice resulted in absolute male infertility associated with a complete loss of sperm motility. Here, we show that the sNHE-null spermatozoa fail to develop the cAMP-dependent protein tyrosine phosphorylation that coincides with the functional maturation occurring upon incubation in capacitating conditions in vitro. Both the sperm motility defect and the lack of induced protein tyrosine phosphorylation are rescued by the addition of cell-permeable cAMP analogs, suggesting that cAMP metabolism is impaired in spermatozoa lacking sNHE. Our analyses of the bicarbonate-dependent soluble adenylyl cyclase (sAC) signaling pathway in sNHE-null sperm cells reveal that sNHE is required for the expression of full-length sAC, and that it is important for the bicarbonate stimulation of sAC activity in spermatozoa. Furthermore, both codependent expression and coimmunoprecipitation experiments indicate that sNHE and sAC associate with each other. Thus, these two proteins appear to be components of a signaling complex at the sperm flagellar plasma membrane. We propose that the formation of this complex efficiently modulates intracellular pH and bicarbonate levels through the rapid and effective control of sAC and sNHE activities to facilitate sperm motility regulation.

Dopamine acutely decreases apical membrane Na/H exchanger NHE3 protein in mouse renal proximal tubule

BACKGROUND

Dopamine is a principal natriuretic hormone in mammalian Na+ homeostasis. Dopamine acutely alters glomerular filtration rate (GFR) and decreases Na+ absorption in both the proximal and distal nephron. Proximal tubule natriuresis is effected through inhibition of the apical membrane Na/H exchanger NHE3.

METHODS

We examined whether dopamine directly and acutely decreases apical membrane NHE3 protein using renal tissue in two in vitro systems: renal cortical slices and in vitro perfused single tubules. After incubation with dopamine, NHE3 activity was measured by 22Na flux and NHE3 antigen was measured by immunoblot in apical membrane and total cellular membranes.

RESULTS

Direct application of dopamine to either cortical slices or microperfused tubules acutely decreases NHE3 activity and antigen at the apical membrane of the proximal tubule. No change in total cellular NHE3 was detected.

CONCLUSION

One mechanism by which dopamine causes natriuresis is via direct and acute reduction of NHE3 protein at the apical membrane via changes in NHE3 protein trafficking.

Interleukin-10 inhibits ischemic and cisplatin-induced acute renal injury

BACKGROUND

Acute renal failure (ARF) is caused by ischemic and nephrotoxic insults acting alone or in combination. Anti-inflammatory agents have been shown to decrease renal ischemia-reperfusion and cisplatin-induced injury and leukocyte infiltration. Interleukin-10 (IL-10) is a potent anti-inflammatory cytokine that inhibits inflammatory and cytotoxic pathways implicated in acute renal injury. Therefore, we sought to determine if IL-10 inhibits acute renal injury.

METHODS

The effects of IL-10 were studied in mice following cisplatin administration and bilateral renal ischemia-reperfusion, in a rat model of renal transplantation, and in cultured mouse cortical tubule cells.

RESULTS

IL-10 significantly decreased renal injury following cisplatin administration and following renal ischemia/reperfusion. Delay of IL-10 treatment for one hour after cisplatin also significantly inhibited renal damage. IL-10 and alpha-melanocyte stimulating hormone (alpha-MSH) increased recovery following transplantation of a kidney subjected to warm ischemia. To explore the mechanism of action of IL-10, its effects were measured on mediators of leukocyte trafficking and inducible nitric oxide synthase (NOS-II). IL-10 inhibited cisplatin and ischemia-induced increases in mRNA for tumor necrosis factor-alpha (TNF-alpha), intercellular adhesion molecule-1 (ICAM-1), and NOS-II. IL-10 also inhibited staining for markers of apoptosis and cell cycle activity following cisplatin administration, and nitric oxide production in cultured mouse cortical tubules.

CONCLUSIONS

IL-10 protects against renal ischemic and cisplatin-induced injury. IL-10 may act, in part, by inhibiting the maladaptive activation of genes that cause leukocyte activation and adhesion, and induction of iNOS.

Alpha-melanocyte-stimulating hormone inhibits renal injury in the absence of neutrophils

BACKGROUND

We previously showed that alpha-melanocyte stimulating hormone (alpha-MSH) decreases ischemia/reperfusion injury even when started six hours after ischemia. Alpha-MSH inhibits both neutrophil accumulation and nitric oxide production. To determine the relative importance of alpha-MSH on the neutrophil pathway, we examined the effects of alpha-MSH in injury models where neutrophil effects are minimal or absent.

METHODS

We studied the effects of alpha-MSH in (1) intercellular adhesion molecule-1 (ICAM-1) knock-out and background mice that were subjected to 40 minutes of ischemia and 24 hours reperfusion, and (2) isolated kidneys that were subjected to in vivo ischemia for 20 minutes and then perfused ex vivo for one hour without neutrophils. To begin to search for direct tubule effects of alpha-MSH, we studied the effect of alpha-MSH on nitric oxide (NO) in endotoxin/interferon-gamma-treated mouse cortical tubule cells.

RESULTS

ICAM-1 knock-out mice had 75% less neutrophil infiltration than background mice after ischemia. Despite the relative lack of neutrophils, alpha-MSH inhibited renal injury in ICAM-1 knock-out mice. Alpha-MSH also significantly preserved GFR and tubular sodium reabsorption in the isolated perfused ischemic kidney model. Alpha-MSH and a nitric oxide inhibitor did not exhibit synergy. Finally, alpha-MSH inhibited nitrite production by 20% in the mouse cortical tubule cells (MCT), similar to parallel observations in a cultured mouse macrophage line (RAW cells).

CONCLUSIONS

We conclude that alpha-MSH decreases renal injury when neutrophil effects are minimal or absent, indicating that alpha-MSH inhibits neutrophil-independent pathways of renal injury. The preservation of sodium absorption ex vivo and inhibition of nitrite production in cultured MCT cells suggests that alpha-MSH inhibits tubular injury by direct tubular effects.

Alpha-melanocyte-stimulating hormone protects against renal injury after ischemia in mice and rats

Reperfusion after ischemia induces cytokines, chemoattractant chemokines, adhesion molecules, and nitric oxide (NO). The resultant neutrophil adherence and NO potentiates renal injury. alpha-Melanocyte-stimulating hormone (alpha-MSH) is a potent anti-inflammatory agent that inhibits neutrophil migration and production of neutrophil chemokines and NO. Since neutrophils and NO promote renal ischemic injury, we sought to determine if alpha-MSH inhibits renal injury in a model of bilateral renal ischemia. alpha-MSH significantly reduced ischemia-induced renal damage, measured by changes in renal histology and plasma blood urea nitrogen and creatinine in mice. alpha-MSH significantly decreased tubule necrosis, neutrophil plugging, and capillary congestion. Delay of alpha-MSH treatment for 6 h after ischemia also significantly inhibited renal damage. alpha-MSH also significantly inhibited ischemic damage in rats. To begin to determine the mechanism of action of alpha-MSH, we measured its effects on mediators of neutrophil trafficking and induction of the inducible isoform of NO synthase-II. alpha-MSH inhibited ischemia-induced increases in mRNA for the murine neutrophil chemokine KC/IL-8. alpha-MSH also inhibited induction of mRNA for the adhesion molecule ICAM-1, which is known to be critical in renal ischemic injury. alpha-MSH inhibited nitration of kidney proteins and induction of NO synthase-II. We conclude: (a) alpha-MSH protects against renal ischemia/reperfusion injury; and (b) it may act, in part, by inhibiting the maladaptive activation of genes that cause neutrophil activation and adhesion, and induction of NO synthase.