Protein kinase G activates inwardly rectifying K(+) channel in cultured human proximal tubule cells

Proinflammatory Cytokines and Potassium Channels in the Kidney

Proinflammatory cytokines affect several cell functions via receptor-mediated processes. In the kidney, functions of transporters and ion channels along the nephron are also affected by some cytokines. Among these, alteration of activity of potassium ion (K(+)) channels induces changes in transepithelial transport of solutes and water in the kidney, since K(+) channels in tubule cells are indispensable for formation of membrane potential which serves as a driving force for the transepithelial transport. Altered K(+) channel activity may be involved in renal cell dysfunction during inflammation. Although little information was available regarding the effects of proinflammatory cytokines on renal K(+) channels, reports have emerged during the last decade. In human proximal tubule cells, interferon-γ showed a time-dependent biphasic effect on a 40 pS K(+) channel, that is, delayed suppression and acute stimulation, and interleukin-1β acutely suppressed the channel activity. Transforming growth factor-β1 activated KCa3.1 K(+) channel in immortalized human proximal tubule cells, which would be involved in the pathogenesis of renal fibrosis. This review discusses the effects of proinflammatory cytokines on renal K(+) channels and the causal relationship between the cytokine-induced changes in K(+) channel activity and renal dysfunction.

ANP-induced signaling cascade and its implications in renal pathophysiology

The balance between vasoconstrictor/sodium-retaining and vasodilator/natriuretic systems is essential for maintaining body fluid and electrolyte homeostasis. Natriuretic peptides, such as atrial natriuretic peptide (ANP), belong to the vasodilator/natriuretic system. ANP is produced by the conversion of pro-ANP into ANP, which is achieved by a proteolytical cleavage executed by corin. In the kidney, ANP binds to the natriuretic peptide receptor-A (NPR-A) and enhances its guanylyl cyclase activity, thereby increasing intracellular cyclic guanosine monophosphate production to promote natriuretic and renoprotective responses. In the glomerulus, ANP increases glomerular permeability and filtration rate and antagonizes the deleterious effects of the renin-angiotensin-aldosterone system activation. Along the nephron, natriuretic and diuretic actions of ANP are mediated by inhibiting the basolaterally expressed Na(+)-K(+)-ATPase, reducing apical sodium, potassium, and protein organic cation transporter in the proximal tubule, and decreasing Na(+)-K(+)-2Cl(-) cotransporter activity and renal concentration efficiency in the thick ascending limb. In the medullary collecting duct, ANP reduces sodium reabsorption by inhibiting the cyclic nucleotide-gated cation channels, the epithelial sodium channel, and the heteromeric channel transient receptor potential-vanilloid 4 and -polycystin 2 and diminishes vasopressin-induced water reabsorption. Long-term ANP treatment may lead to NPR-A desensitization and ANP resistance, resulting in augmented sodium and water reabsorption. In mice, corin deficiency impairs sodium excretion and causes salt-sensitive hypertension. Characteristics of ANP resistance and corin deficiency are also encountered in patients with edema-associated diseases, highlighting the importance of ANP signaling in salt-water balance and renal pathophysiology.

Function of cGMP-dependent protein kinase II in volume load-induced diuresis

Atrial natriuretic peptide (ANP)/cGMPs cause diuresis and natriuresis. Their downstream effectors beyond cGMP remain unclear. To elucidate a probable function of cGMP-dependent protein kinase II (cGKII), we investigated renal parameters in different conditions (basal, salt diets, starving, water load) using a genetically modified mouse model (cGKII-KO), but did not detect any striking differences between WT and cGKII-KO. Thus, cGKII is proposed to play only a marginal role in the adjustment of renal concentration ability to varying salt loads without water restriction or starving conditions. When WT mice were subjected to a volume load (performed by application of a 10-mM glucose solution (3% of BW) via feeding needle), they exhibited a potent diuresis. In contrast, urine volume was decreased significantly in cGKII-KO. We showed that AQP2 plasma membrane (PM) abundance was reduced for about 50% in WT upon volume load, therefore, this might be a main cause for the enhanced diuresis. In contrast, cGKII-KO mice almost completely failed to decrease AQP2-PM distribution. This significant difference between both genotypes is not induced by an altered p-Ser256-AQP2 phosphorylation, as phosphorylation at this site decreases similarly in WT and KO. Furthermore, sodium excretion was lowered in cGKII-KO mice during volume load. In summary, cGKII is only involved to a minor extent in the regulation of basal renal concentration ability. By contrast, cGKII-KO mice are not able to handle an acute volume load. Our results suggest that membrane insertion of AQP2 is inhibited by cGMP/cGKII.

Effects of cytokines on potassium channels in renal tubular epithelia

Renal tubular potassium (K(+)) channels play important roles in the formation of cell-negative potential, K(+) recycling, K(+) secretion, and cell volume regulation. In addition to these physiological roles, it was reported that changes in the activity of renal tubular K(+) channels were involved in exacerbation of renal cell injury during ischemia and endotoxemia. Because ischemia and endotoxemia stimulate production of cytokines in immune cells and renal tubular cells, it is possible that cytokines would affect K(+) channel activity. Although the regulatory mechanisms of renal tubular K(+) channels have extensively been studied, little information is available about the effects of cytokines on these K(+) channels. The first report was that tumor necrosis factor acutely stimulated the single channel activity of the 70 pS K(+) channel in the rat thick ascending limb through activation of tyrosine phosphatase. Recently, it was also reported that interferon-γ (IFN-γ) and interleukin-1β (IL-1β) modulated the activity of the 40 pS K(+) channel in cultured human proximal tubule cells. IFN-γ exhibited a delayed suppression and an acute stimulation of K(+) channel activity, whereas IL-1β acutely suppressed the channel activity. Furthermore, these cytokines suppressed gene expression of the renal outer medullary potassium channel. The renal tubular K(+) channels are functionally coupled to the coexisting transporters. Therefore, the effects of cytokines on renal tubular transporter activity should also be taken into account, when interpreting their effects on K(+) channel activity.

Interaction between Calcineurin and Ca/Calmodulin Kinase-II in Modulating Cellular Functions

Roles of calcineurin (CaN), a Ca²⁺/calmodulin- (CaM-) dependent protein phosphatase, and Ca²⁺/CaM-dependent protein kinase-II (CaMKII) in modulating K⁺ channel activity and the intracellular Ca²⁺ concentration ([Ca²⁺]_i) have been investigated in renal tubule epithelial cells. The channel current through the cell membrane was recorded with the patch-clamp technique, and [Ca²⁺]_i was monitored using fura-2 imaging. We found that a CaN-inhibitor, cyclosporin A (CyA), lowered the K⁺ channel activity and elevated [Ca²⁺]_i, suggesting that CyA closes K⁺ channels and opens Ca²⁺-release channels of the cytosolic Ca²⁺-store. Moreover, both of these responses were blocked by KN-62, an inhibitor of CaMKII. It is suggested that the CyA-mediated response results from the activation of CaMKII. Indeed, Western blot analysis revealed that CyA increased phospho-CaMKII, an active form of CaMKII. These findings suggest that CaN-dependent dephosphorylation inhibits CaMKII-mediated phosphorylation, and the inhibition of CaN increases phospho-CaMKII, which results in the stimulation of CaMKII-dependent cellular actions.

Role of calcineurin-mediated dephosphorylation in modulation of an inwardly rectifying K+ channel in human proximal tubule cells

Activity of an inwardly rectifying K(+) channel with inward conductance of about 40 pS in cultured human renal proximal tubule epithelial cells (RPTECs) is regulated at least in part by protein phosphorylation and dephosphorylation. In this study, we examined involvement of calcineurin (CaN), a Ca(2+)/calmodulin (CaM)-dependent phosphatase, in modulating K(+) channel activity. In cell-attached mode of the patch-clamp technique, application of a CaN inhibitor, cyclosporin A (CsA, 5 microM) or FK520 (5 microM), significantly suppressed channel activity. Intracellular Ca(2+) concentration ([Ca(2+)]( i )) estimated by fura-2 imaging was elevated by these inhibitors. Since inhibition of CaN attenuates some dephosphorylation with increase in [Ca(2+)]( i ), we speculated that inhibiting CaN enhances Ca(2+)-dependent phosphorylation, which might result in channel suppression. To verify this hypothesis, we examined effects of inhibitors of PKC and Ca(2+)/CaM-dependent protein kinase-II (CaMKII) on CsA-induced channel suppression. Although the PKC inhibitor GF109203X (500 nM) did not influence the CsA-induced channel suppression, the CaMKII inhibitor KN62 (20 microM) prevented channel suppression, suggesting that the channel suppression resulted from CaMKII-dependent processes. Indeed, Western blot analysis showed that CsA increased phospho-CaMKII (Thr286), an activated CaMKII in inside-out patches, application of CaM (0.6 microM) and CaMKII (0.15 U/ml) to the bath at 10(-6) M Ca(2+) significantly suppressed channel activity, which was reactivated by subsequent application of CaN (800 U/ml). These results suggest that CaN plays an important role in supporting K(+) channel activity in RPTECs by preventing CaMKII-dependent phosphorylation.

Delayed and acute effects of interferon-gamma on activity of an inwardly rectifying K+ channel in cultured human proximal tubule cells

The activity of an inwardly rectifying K(+) channel in cultured human renal proximal tubule cells (RPTECs) is stimulated and inhibited by nitric oxide (NO) at low and high concentrations, respectively. In this study, we investigated the effects of IFN-gamma, one of the cytokines which affect the expression of inducible NO synthase (iNOS), on intracellular NO and channel activity of RPTECs, using RT-PCR, NO imaging, and the cell-attached mode of the patch-clamp technique. Prolonged incubation (24 h) of cells with IFN-gamma (20 ng/ml) enhanced iNOS mRNA expression and NO production. In these cells, a NOS inhibitor, N(omega)-nitro-l-arginine methyl ester (l-NAME; 100 microM), elevated channel activity, suggesting that NO production was so high as to suppress the channel. This indicated that IFN-gamma would chronically suppress channel activity by enhancing NO production. Acute effects of IFN-gamma was also examined in control cells. Simple addition of IFN-gamma (20 ng/ml) to the bath acutely stimulated channel activity, which was abolished by inhibitors of IFN-gamma receptor-associated Janus-activated kinase [P6 (1 microM) and AG490 (10 microM)]. However, l-NAME did not block the acute effect of IFN-gamma. Indeed, IFN-gamma did not acutely affect NO production. Moreover, the acute effect was not blocked by inhibition of PKA, PKG, and phosphatidylinositol 3-kinase (PI3K). We conclude that IFN-gamma exerted a delayed suppressive effect on K(+) channel activity by enhancing iNOS expression and an acute stimulatory effect, which was independent of either NO pathways or phosphorylation processes mediated by PKA, PKG, and PI3K in RPTECs.

Dendroaspis natriuretic peptide relaxes gastric antral circular smooth muscle of guinea-pig through the cGMP/cGMP-dependent protein kinase pathway

AIM: To systematically investigate if cGMP/cGMP-dependent protein kinase G (PKG) signaling pathway may participate in dendroaspis natriuretic peptide (DNP)-induced relaxation of gastric circular smooth muscle.

METHODS: The content of cGMP in guinea pig gastric antral smooth muscle tissue and perfusion solution were measured using radioimmunoassay; spontaneous contraction of gastric antral circular muscles recorded using a 4-channel physiograph; and Ca²⁺-activated K⁺ currents (I_K(Ca)) and spontaneous transient outward currents (STOCs) in isolated gastric antral myocytes were recorded using the whole-cell patch clamp technique.

RESULTS: DNP markedly enhanced cGMP levels in gastric antral smooth muscle tissue and in the perfusion medium. DNP induced relaxation in gastric antral circular smooth muscle, which was inhibited by KT5823, a cGMP-dependent PKG inhibitor. DNP increased I_K(Ca). This effect was almost completely blocked by KT5823, and partially blocked by LY83583, an inhibitor of guanylate cyclase to change the production of cGMP. DNP also increased STOCs. The effect of DNP on STOCs was abolished in the presence of KT5823, but not affected by KT-5720, a PKA-specific inhibitor.

CONCLUSION: DNP activates I_K(Ca) and relaxes guinea-pig gastric antral circular smooth muscle via the cGMP/PKG-dependent singling axis instead of cAMP/PKA pathway.

Dual regulation of the ATP-sensitive potassium channel by activation of cGMP-dependent protein kinase

Adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels couple cellular metabolic status to membrane electrical activity. In this study, we performed patch-clamp recordings to investigate how cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG) regulates the function of K(ATP) channels, using both transfected human SH-SY5Y neuroblastoma cells and embryonic kidney (HEK) 293 cells. In intact SH-SY5Y cells, the single-channel currents of Kir6.2/sulfonylurea receptor (SUR) 1 channels, a neuronal-type K(ATP) isoform, were enhanced by zaprinast, a cGMP-specific phosphodiesterase inhibitor; this enhancement was abolished by inhibition of PKG, suggesting a stimulatory role of cGMP/PKG signaling in regulating the function of neuronal K(ATP) channels. Similar effects of cGMP accumulation were confirmed in intact HEK293 cells expressing Kir6.2/SUR1 channels. In contrast, direct application of purified PKG suppressed rather than activated Kir6.2/SUR1 channels in excised, inside-out patches, while tetrameric Kir6.2LRKR368/369/370/371AAAA channels expressed without the SUR subunit were not modulated by zaprinast or purified PKG. Lastly, reconstitution of the soluble guanylyl cyclase/cGMP/PKG signaling pathway by generation of nitric oxide led to Kir6.2/SUR1 channel activation in both cell types. Taken together, here, we report novel findings that PKG exerts dual functional regulation of neuronal K(ATP) channels in a SUR subunit-dependent manner, which may provide new means of therapeutic intervention for manipulating neuronal excitability and/or survival.

Regulation of an inwardly rectifying K+ channel by nitric oxide in cultured human proximal tubule cells

We investigated the effects of nitric oxide (NO) on activity of the inwardly rectifying K(+) channel in cultured human proximal tubule cells, using the cell-attached mode of the patch-clamp technique. An inhibitor of NO synthases, N(omega)-nitro-L-arginine methyl ester (L-NAME; 100 microM), reduced channel activity, which was restored by an NO donor, sodium nitroprusside (SNP; 10 microM) or 8-bromo-cGMP (8-BrcGMP; 100 microM). However, SNP failed to activate the channel in the presence of an inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM). Similarly, the SNP effect was abolished by a protein kinase G (PKG)-specific inhibitor, KT-5823 (1 microM), but not by a protein kinase A-specific inhibitor, KT-5720 (500 nM). Another NO donor, S-nitroso-N-acetyl-D,L-penicillamine (10 microM), mimicked the SNP-induced channel activation. In contrast to the stimulatory effect of SNP at a low dose (10 microM), a higher dose of SNP (1 mM) reduced channel activity, which was not restored by 8-BrcGMP. Recordings of membrane potential with the slow whole cell configuration demonstrated that l-NAME (100 microM) and the high dose of SNP (1 mM) depolarized the cell by 10.1 +/- 2.6 and 9.2 +/- 1.0 mV, respectively, whereas the low dose of SNP (10 microM) hyperpolarized it by 7.1 +/- 0.7 mV. These results suggested that the endogenous NO would contribute to the maintenance of basal activity of this K(+) channel and hence the potential formation via a cGMP/PKG-dependent mechanism, whereas a high dose of NO impaired channel activity independent of cGMP/PKG-mediated processes.

Role of calcium-activated potassium currents in CNP-induced relaxation of gastric antral circular smooth muscle in guinea pigs

AIM: To investigate ion channel mechanism in CNP-induced relaxation of gastric circular smooth muscle in guinea pigs.

METHODS: Spontaneous contraction of gastric smooth muscle was recorded by a four-channel physiograph. The whole cell patch-clamp technique was used to record calcium-activated potassium currents and membrane potential in the gastric myocytes isolated by collagenase.

RESULTS: C-type natriuretic peptide (CNP) markedly inhibited the spontaneous contraction in a dose-dependent manner in gastric circular smooth muscle in guinea pigs. Ly83583, an inhibitor of guanylate cyclase, weakened CNP-induced inhibition on spontaneous contraction but Zaparinast, an inhibitor of cGMP sensitive phosphoesterase, potentiated CNP-induced inhibition in gastric circular smooth muscles. The inhibitory effects of CNP on spontaneous contraction were blocked by tetrathylammonium (TEA), a nonselective potassium channel blocker. C N P hyperpolarized membrane potential from -60.0 mV ± 2.0 mV to -68.3 mV ± 3.0 mV in a single gastric myocyte. CNP increased calcium-activated potassium currents (I_K(ca)) in a dose-dependent manner in gastric circular myocytes. CNP also increased the spontaneously transient outward currents (STOCs). Ly83583 partly blocked CNP-induced increase of calcium-activated potassium currents, but Zaparinast potented the effect.

CONCLUSION: CNP inhibits spontaneous contraction, and potassium channel may be involved in the process in gastric circular smooth muscle of guinea pigs. CNP-induced increase of I_K(ca) is mediated by a cGMP dependent pathway.

Role of PKC and calcium in modulation of effects of angiotensin II on sodium transport in proximal tubule

It has been well documented that low concentrations of ANG II (10(-11) to 10(-10) M) stimulate, whereas high concentrations of ANG II (10(-8) to 10(-5) M) inhibit Na(+) transport in proximal tubules of rat and rabbit kidneys. Measured ANG II concentration in proximal tubular fluid is in the nanomolar range. In the present study, we investigated the role of PKC, intracellular Ca(2+), and cAMP in modulating the effects of luminal ANG II on Na(+) absorption by microperfusion techniques in rabbit superficial segment of proximal tubules in vitro. We confirmed that ANG II (10(-9) M) had no change on fluid absorption (J(v)); however, fluid absorption increased significantly when 10(-9) M ANG II and 3,4,5-trimethoxybenzoic acid-8-(diethylamino)octyl ester (TMB-8), a blocker of intracellular calcium mobilization, were added together. In contrast, ANG II significantly decreased J(v) when PKC was inhibited. When 10(-9) M ANG II was present together with 1-(5-isoquinolinesulfonyl)-2-mehtylpiperazine and TMB-8, no significant change of J(v) occurred. Inhibition of endogenous cAMP activity by a PKA inhibitor did not change either basal or ANG II-stimulated fluid absorption. Our results indicate that ANG II regulates Na(+) absorption by a cAMP-independent mechanism and that PKC and intracellular calcium both play a critical role in modulating the effects of physiological concentration of ANG II on proximal tubule transport. Balance between these two cytosolic messengers modulates the effects of ANG II on fluid absorption in the proximal tubule.