Search

Dietary K intake plays an important role in the regulation of renal K secretion: a high K intake stimulates whereas low K intake suppresses renal K secretion. Our previous studies demonstrated that the Src family protein-tyrosine kinase and mitogen-activated protein kinase (MAPK) are involved in mediating the effect of low K intake on renal K channels and K secretion. However, the molecular mechanism by which low K intake stimulates MAPK is not completely understood. Here we show that inhibitor of growth 4 (ING4), a protein with a highly conserved plant homeodomain finger motif, is involved in mediating the effect of low K intake on MAPK. K restriction stimulates the expression of ING4 in the kidney and superoxide anions, and its related products are involved in mediating the effect of low K intake on ING4 expression. We used HEK293 cells to express ING4 and observed that expression of ING4 increased the phosphorylation of p38 and ERK MAPK, whereas down-regulation of ING4 with small interfering RNA decreased the phosphorylation of p38 and ERK. Immunocytochemistry showed that ING4 was expressed in the renal outer medullary potassium (ROMK)-positive tubules. Moreover, ING4 decreased K currents in Xenopus oocytes injected with ROMK channel cRNA. This inhibitory effect was reversed by blocking p38 and ERK MAPK. These data provide evidence for the role of ING4 in mediating the effect of low K intake on ROMK channel activity by stimulation of p38 and ERK MAPK.

We previously demonstrated that arachidonic acid (AA) inhibits epithelial Na channels (ENaC) through the cytochrome P-450 (CYP) epoxygenase-dependent pathway ( 34 ). In the present study, we tested the hypothesis that low Na intake suppresses the expression of CYP2C23, which is mainly responsible for converting AA to epoxyeicosatrienoic acid (EET) in the kidney ( 11 ) and attenuates the AA-induced inhibition of ENaC. Immunostaining showed that CYP2C23 is expressed in the Tamm-Horsfall protein (THP)-positive and aquaporin 2 (AQP2)-positive tubules. This suggests that CYP2C23 is expressed in the thick ascending limb (TAL) and collecting duct (CD). Na restriction significantly suppressed the expression of CYP2C23 in the TAL and CD. Western blot also demonstrated that the expression of CYP2C23 in renal cortex and outer medulla diminished in rats on Na-deficient diet (Na-D) but increased in those on high-Na diet (4%). Moreover, the content of 11,12-epoxyeicosatrienoic acid (EET) decreased in the isolated cortical CD from rats on Na-D compared with those on a normal-Na diet (0.5%). Patch-clamp study showed that application of 15 μM AA inhibited the activity of ENaC by 77% in the CCD of rats on a Na-D for 3 days. However, the inhibitory effect of AA on ENaC was significantly attenuated in rats on Na-D for 14 days. Furthermore, inhibition of CYP epoxygenase with MS-PPOH increased the ENaC activity in the CCD of rats on a control Na diet. We also used microperfusion technique to examine the effect of MS-PPOH on Na transport in the distal nephron. Application of MS-PPOH significantly increased Na absorption in the distal nephron of control rats but had no significant effect on Na absorption in rats on Na-D for 14 days. We conclude that low Na intake downregulates the activity and expression of CYP2C23 and attenuates the inhibitory effect of AA on Na transport.

It was demonstrated previously that low dietary potassium (K) intake stimulates Src family protein tyrosine kinase (PTK) expression via a superoxide-dependent signaling. This study explored the role of mitogen-activated protein kinase (MAPK) in mediating the effect of superoxide anions on PTK expression and ROMK (Kir 1.1) channel activity. Western blot analysis demonstrated that low K intake significantly increased the phosphorylation of P38 MAPK (P38) and extracellular signal-regulated kinase (ERK) but had no effect on phosphorylation of c-JUN N-terminus kinase in renal cortex and outer medulla. The stimulatory effect of low K intake on P38 and ERK was abolished by treatment of rats with tempol. The possibility that increases in superoxide and related products that are induced by low K intake were responsible for stimulating phosphorylation of P38 and ERK also was supported by the finding that application of H(2)O(2) increased the phosphorylation of ERK and P38 in the cultured mouse collecting duct cells. Simultaneous blocking of ERK and P38 completely abolished the effect of H(2)O(2) on c-Src expression in mouse collecting duct cells. For determination of the role of P38 and ERK in the regulation of ROMK-like small-conductance K (SK) channels, the patch-clamp technique was used to study the effect of inhibiting P38 and ERK on SK channels in the cortical collecting duct from rats that were on a control K diet (1.1%) and on a K-deficient diet for 1 d. Inhibition of ERK, c-JUN N-terminus kinase, or P38 alone had no effect on SK channels. In contrast, simultaneous inhibition of P38 and ERK significantly increased channel activity. The effect of inhibiting MAPK on SK channels was not affected in the presence of herbimycin A, a PTK inhibitor, and was larger in rats that were on a K-deficient diet than in rats that were on a normal-K diet. However, the stimulatory effect of inhibiting ERK and P38 on SK was absent in the cortical collecting duct that was treated with colchicine. It is concluded that low K intake-induced increases in superoxide levels are responsible for stimulation of P38 and ERK and that MAPK inhibit the SK channels by stimulating PTK expression and via a PTK-independent mechanism.

TNF has been shown to be synthesized by the medullary thick ascending limb (mTAL) ( 21 ). In the present study, we used the patch-clamp technique to study the acute effect of TNF on the apical 70-pS K+channel in the mTAL. Addition of TNF (10 nM) significantly stimulated activity of the 70-pS K+channel and increased NPo[a product of channel open probability ( Po) and channel number ( N)] from 0.20 to 0.97. The stimulatory effect of TNF was observed only in cell-attached patches but not in excised patches. Moreover, addition of TNF had no effect on the ROMK-like small-conductance K+channels in the TAL. The dose-response curve of the TNF effect yielded a Kmvalue of 1 nM, a concentration that increased channel activity to 50% maximal stimulatory effect of TNF. The concentrations required for reaching the plateau of the TNF effect were between 5 and 10 nM. The stimulatory effect of TNF on the 70-pS K+channel was observed in the presence of Nω-nitro-l-arginine methyl ester. This indicated that the effect of TNF was not mediated by a nitric oxide-dependent pathway. Also, inhibition of PKA did not affect the stimulatory effect of TNF. In contrast, inhibition of protein tyrosine kinase not only increased activity of the 70-pS K+channel but also abolished the effect of TNF. Moreover, inhibition of protein tyrosine phosphatase (PTP) blocked the stimulatory effect of TNF on the 70-pS K+channel. The notion that the TNF effect results from stimulation of PTP activity is supported by PTP activity assay in which treatment of mTAL cells with TNF significantly increased the activity of PTP. We conclude that TNF stimulates the 70-pS K+channel via stimulation of PTP in the mTAL.

Previous study has demonstrated that superoxide and the related products are involved in mediating the effect of low K intake on renal K secretion and ROMK channel activity in the cortical collecting duct (CCD). This study investigated the role of gp91 phox -containing NADPH oxidase (NOXII) in mediating the effect of low K intake on renal K excretion and ROMK channel activity in gp91(−/−) mice. K depletion increased superoxide levels, phosphorylation of c-Jun, expression of c-Src, and tyrosine phosphorylation of ROMK in renal cortex and outer medulla in wild-type (WT) mice. In contrast, tempol treatment in WT mice abolished whereas deletion of gp91 significantly attenuated the effect of low K intake on superoxide production, c-Jun phosphorylation, c-Src expression, and tyrosine phosphorylation of ROMK. Patch-clamp experiments demonstrated that low K intake decreased mean product of channel number (N) and open probability (P) (NP o ) of ROMK channels from 1.1 to 0.4 in the CCD. However, the effect of low K intake on ROMK channel activity was significantly attenuated in the CCD from gp91(−/−) mice and completely abolished by tempol treatment. Immunocytochemical staining also was used to examine the ROMK distribution in WT, gp91(−/−), and WT mice with tempol treatment in response to K restriction. K restriction decreased apical staining of ROMK in WT mice. In contrast, a sharp apical ROMK staining was observed in the tempol-treated WT or gp91(−/−) mice. Metabolic cage study further showed that urinary K loss is significantly higher in gp91(−/−) mice than in WT mice. It is concluded that superoxide anions play a key role in suppressing K secretion during K restriction and that NOXII is involved in mediating the effect of low K intake on renal K secretion and ROMK channel activity.

We used the patch-clamp technique to study the effect of H2O2 on the apical ROMK-like small-conductance K (SK) channel in the cortical collecting duct (CCD). The addition of H2O2 decreased the activity of the SK channels and the inhibitory effect of H2O2 was larger in the CCD from rats on a K-deficient diet than that from rats on a normal-K or a high-K diet. However, application of H2O2 did not inhibit the SK channels in inside-out patches. This suggests that the H2O2-mediated inhibition of SK channels was not due to direct oxidation of the SK channel protein. Because a previous study showed that H2O2 stimulated the expression of Src family protein tyrosine kinase (PTK) which inhibited SK channels ( 3 ), we explored the role of PTK in mediating the effect of H2O2 on SK channels. The application of H2O2 stimulated the activity of endogenous PTK in M-1 cells and increased tyrosine phosphorylation of ROMK in HEK293 cells transfected with GFP-ROMK1 and c-Src. However, blockade of PTK only attenuated but did not completely abolish the inhibitory effect of H2O2 on SK channels. Since H2O2 has also been demonstrated to activate mitogen-activated protein kinase, P38, and ERK ( 3 ), we examined the role of P38 and ERK in mediating the effect of H2O2 on SK channels. Similar to blockade of PTK, suppression of P38 and ERK did not completely abolish the H2O2-induced inhibition of SK channels. However, combined use of ERK, P38, and PTK inhibitors completely abolished the effect of H2O2 on SK channels. Also, treatment of the CCDs with concanavalin A, an agent which has been shown to inhibit endocytosis ( 19 ), abolished the inhibitory effect of H2O2. We conclude that addition of H2O2 inhibited SK channels by stimulating PTK activity, P38, and ERK in the CCD and that H2O2 enhances the internalization of the SK channels.

We used the patch-clamp technique to examine the effect of DOCA treatment (2 mg/kg) on the apical small-conductance K (SK) channels, epithelial Na channels (ENaC), and the basolateral 18-pS K channels in the cortical collecting duct (CCD). Treatment of rats with DOCA for 6 days significantly decreased the plasma K from 3.8 to 3.1 meq and reduced the activity of the SK channel, defined as NPo, from 1.3 in the CCD of control rats to 0.6. In contrast, DOCA treatment significantly increased ENaC activity from 0.01 to 0.53 and the basolateral 18-pS K channel activity from 0.67 to 1.63. Moreover, Western blot analysis revealed that DOCA treatment significantly increased the expression of the nonreceptor type of protein tyrosine kinase (PTK), cSrc, and the tyrosine phosphorylation of ROMK in the renal cortex and outer medulla. The possibility that decreases in apical SK channel activity induced by DOCA treatment were the result of stimulation of PTK activity was further supported by experiments in which inhibition of PTK with herbimycin A significantly increased NPo from 0.6 to 2.1 in the CCD from rats receiving DOCA. Also, when rats were fed a high-K (10%) diet, DOCA treatment did not increase the expression of c-Src and decrease the activity of the SK channel in the CCD. We conclude that DOCA treatment decreased the apical SK channel activity in rats on a normal-K diet and that an increase in PTK expression may be responsible for decreased channel activity in the CCD from DOCA-treated rats.

We previously demonstrated that low K intake stimulated the expression of c-Src and that stimulation of protein tyrosine kinase inhibited ROMK channel activity (Wei, Y., Bloom, P., Lin, D. H., Gu, R. M., and Wang, W. H. (2001) Am. J. Physiol. 281, F206-F212). Decreases in dietary K content significantly increased O(2)(-) levels and the phosphorylation of c-Jun, a transcription factor, in renal cortex and outer medulla. The role of O(2)(-) and related products such as H(2)O(2) in stimulating the expression of protein tyrosine kinase is suggested by the observation that addition of 50-200 microm H(2)O(2) increased the phosphorylation of c-Jun and the expression of c-Src in M1 cells, a mouse collecting duct principal cell line. The effect of H(2)O(2) on c-Src expression was completely abolished with cyclohexamide or actinomycin D. The treatment of animals on a K-deficient (KD) diet with tempol for 7 days significantly decreased the production of O(2)(-), c-Jun phosphorylation, and c-Src expression. Moreover, low K intake decreased the activity of ROMK-like small conductance channels from 1.37 (control K diet) to 0.5 in the cortical collecting duct and increased the tyrosine phosphorylation of ROMK in the renal cortex and outer medulla. In contrast, the tempol treatment not only increased channel activity to 1.1 in the cortical collecting duct but also decreased the tyrosine phosphorylation of ROMK from rats on a KD diet. Finally, suppressing O(2)(-) production with tempol significantly increased renal K excretion measured with metabolic cage and lowered the plasma K concentration in comparison with those on a KD diet alone without tempol. We conclude that O(2)(-) and related products play a role in mediating the effect of low K intake on c-Src expression and in suppressing ROMK channel activity and renal K secretion.