Inhibition of MAPK stimulates the Ca2+ -dependent big-conductance K channels in cortical collecting duct

G-Protein Inwardly Rectifying Potassium Channel 1 (GIRK1) Knockdown Decreases Beta-Adrenergic, MAP Kinase and Akt Signaling in the MDA-MB-453 Breast Cancer Cell Line

Previous data from our laboratory have indicated that there is a functional link between the beta-adrenergic receptor signaling pathway and the G-protein inwardly rectifying potassium channel (GIRK1) in breast cancer cell lines and that these pathways are involved in growth regulation of these cells. To determine functionality, MDA-MB-453 breast cancer cells were stimulated with ethanol, known to open GIRK channels. Decreased GIRK1 protein levels were seen after treatment with 0.12% ethanol. In addition, serum-free media completely inhibited GIRK1 protein expression. This data indicates that there are functional GIRK channels in breast cancer cells and that these channels are involved in cellular signaling. In the present research, to further define the signaling pathways involved, we performed RNA interference (siRNA) studies. Three stealth siRNA constructs were made starting at bases 1104, 1315, and 1490 of the GIRK1 sequence. These constructs were transfected into MDA-MB-453 cells, and both RNA and protein were isolated. GIRK1, β₂-adrenergic and 18S control levels were determined using real-time PCR 24 hours after transfection. All three constructs decreased GIRK1 mRNA levels. However, β₂ mRNA levels were unchanged by the GIRK1 knockdown. GIRK1 protein levels were also reduced by the knockdown, and this knockdown led to decreases in beta-adrenergic, MAP kinase and Akt signaling.

K restriction inhibits protein phosphatase 2B (PP2B) and suppression of PP2B decreases ROMK channel activity in the CCD

We used Western blot analysis to examine the effect of dietary K intake on the expression of serine/threonine protein phosphatase in the kidney. K restriction significantly decreased the expression of catalytic subunit of protein phosphatase (PP)2B but increased the expression of PP2B regulatory subunit in both rat and mouse kidney. However, K depletion did not affect the expression of PP1 and PP2A. Treatment of M-1 cells, mouse cortical collecting duct (CCD) cells, or 293T cells with glucose oxidase (GO), which generates superoxide anions through glucose metabolism, mimicked the effect of K restriction on PP2B expression and significantly decreased expression of PP2B catalytic subunits. However, GO treatment increased expression of regulatory subunit of PP2B and had no effect on expression of PP1, PP2A, and protein tyrosine phosphatase 1D. Moreover, deletion of gp91-containing NADPH oxidase abolished the effect of K depletion on PP2B. Thus superoxide anions or related products may mediate the inhibitory effect of K restriction on the expression of PP2B catalytic subunit. We also used patch-clamp technique to study the effect of inhibiting PP2B on renal outer medullary K (ROMK) channels in the CCD. Application of cyclosporin A or FK506, inhibitors of PP2B, significantly decreased ROMK channels, and the effect of PP2B inhibitors was abolished by blocking p38 mitogen-activated protein kinase (MAPK) and ERK. Furthermore, Western blot demonstrated that inhibition of PP2B with cyclosporin A or small interfering RNA increased the phosphorylation of ERK and p38 MAPK. We conclude that K restriction suppresses the expression of PP2B catalytic subunits and that inhibition of PP2B decreases ROMK channel activity through stimulation of MAPK in the CCD.

Phorbol ester stimulation of RasGRP1 regulates the sodium-chloride cotransporter by a PKC-independent pathway

The sodium-chloride cotransporter (NCC) is the principal salt-absorptive pathway in the mammalian distal convoluted tubule (DCT) and is the site of action of one of the most effective classes of antihypertensive medications, thiazide diuretics. We developed a cell model system to assess NCC function in a mammalian cell line that natively expresses NCC, the mouse DCT (mDCT) cell line. We used this system to study the complex regulation of NCC by the phorbol ester (PE) 12-O-tetradecanoylphorbol-13-acetate (TPA), a diacylglycerol (DAG) analog. It has generally been thought that PEs mediate their effects on transporters through the activation of PKC. However, there are at least five other DAG/PE targets. Here we describe how one of those alternate targets of DAG/PE effects, Ras guanyl-releasing protein 1 (RasGRP1), mediates the PE-induced suppression of function and the surface expression of NCC. Functional assessment of NCC by using thiazide-sensitive (22)Na(+) uptakes revealed that TPA completely suppresses NCC function. Biotinylation experiments demonstrated that this result was primarily because of decreased surface expression of NCC. Although inhibitors of PKC had no effect on this suppression, MAPK inhibitors completely prevented the TPA effect. RasGRP1 activates the MAPK pathway through activation of the small G protein Ras. Gene silencing of RasGRP1 prevented the PE-mediated suppression of NCC activity, the activation of the H-Ras isoform of Ras, and the activation of ERK1/2 MAPK. This finding confirmed the critical role of RasGRP1 in mediating the PE-induced suppression of NCC activity through the stimulation of the MAPK pathway.

Na+ and K+ transport by the renal connecting tubule

PURPOSE OF REVIEW

The connecting tubule is emerging as a nephron segment critical to the regulation of Na+ and K+ excretion and the maintenance of homeostasis for these ions. The segment is difficult to study, however, and much of the available information we have concerning its functions is indirect. Here, we review the major transport mechanisms and transporters found in this segment and outline several unsolved problems in the field.

RECENT FINDINGS

Recent electrophysiological and immunohistochemical measurements together with theoretical studies provide a more comprehensive view of ion transport in the connecting tubule. New signaling pathways governing Na+ and K+ transport have also been described.

SUMMARY

Key questions about how Na+ and K+ transport are regulated remain unanswered. Is the connecting tubule the site of final regulation of both Na+ and K+ excretion? If so, how are the transport rates of these two ions independently controlled?

BK channels in the kidney

PURPOSE OF REVIEW

Large, BK (calcium-activated potassium) channels are now regarded as relevant players in many aspects of renal physiology, including potassium secretion. This review will highlight recent discoveries regarding the function and localization of BK in the kidney.

RECENT FINDINGS

Patch clamp electrophysiology has revealed BK in cultured podocytes, glomerular mesangial cells, and in several tubule segments including principal cells (connecting tubules/principal cells), and intercalated cells of connecting tubules and cortical collecting ducts. Flow-induced potassium secretion is mediated by BK in the distal nephron and may be partly the result of shear stress-induced increases in cell calcium concentrations. ROMK-/- and wild-type mice on a high potassium diet exhibit BK-mediated potassium secretion, and studies of BK-alpha-/- and BK-beta1-/- mice suggest that flow-induced potassium secretion is mediated by BK-alpha/beta1, which is specifically localized in the apical membrane of the connecting tubule of the mouse and connecting tubule plus initial cortical collecting duct of the rabbit.

SUMMARY

BK channels, located in glomerular cells and in many nephron segments, especially mediate potassium secretion in the combined condition of potassium adaptation and high flow. Understanding the molecular makeup of BK in specific renal cells and the dietary and physiological conditions for their expression can yield improved potassium-sparing compounds.

PGE2 inhibits apical K channels in the CCD through activation of the MAPK pathway

We used the patch-clamp technique and Western blot analysis to explore the effect of PGE(2) on ROMK-like small-conductance K (SK) channels and Ca(2+)-activated big-conductance K channels (BK) in the cortical collecting duct (CCD). Application of 10 microM PGE(2) inhibited SK and BK channels in the CCD. Moreover, either inhibition of PKC or blocking mitogen-activated protein kinase (MAPK), P38 and ERK, abolished the effect of PGE(2) on SK channels in the CCD. The effect of PGE(2) on SK channels was completely blocked in the presence of SC-51089, a specific EP1 receptor antagonist, and mimicked by application of sulprostone, an agonist for EP1 and EP3 receptors. To determine whether PGE(2) stimulates the phosphorylation of P38 and ERK, we treated mouse CCD cells (M-1) with PGE(2). Application of PGE(2) significantly stimulated the phosphorylation of P38 and ERK within 5 min. The dose-response curve of PGE(2) effect shows that 1, 5, and 10 microM PGE(2) increased the phosphorylation of P38 and ERK by 20-21, 50-80, and 80-100%, respectively. The stimulatory effect of PGE(2) on MAPK phosphorylation was not affected by indomethacin but abolished by inhibition of PKC. This suggests that the effect of PGE(2) on MAPK phosphorylation is PKC dependent. Also, the expression of cyclooxygenase II and PGE(2) concentration in renal cortex and outer medulla was significantly higher in rats fed a K-deficient diet than those on a normal-K diet. We conclude that PGE(2) inhibits SK and BK channels and that there is an effect of PGE(2) on SK channels in the CCD through activation of EP1 receptor and MAPK pathways. Also, high concentrations of PGE(2) induced by K restriction may be partially responsible for increasing MAPK activity during K restriction.

Role of gp91phox -containing NADPH oxidase in mediating the effect of K restriction on ROMK channels and renal K excretion

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.

The role of the BK channel in potassium homeostasis and flow-induced renal potassium excretion

The kidney is the major regulator of potassium homeostasis. In addition to the ROMK channels, large conductance Ca(2+)-activated K(+) (BK) channels are expressed in the apical membrane of the aldosterone sensitive distal nephron where they could contribute to renal K(+) secretion. We studied flow-induced K(+) secretion in BK channel alpha-subunit knockout (BK(-/-)) mice by acute pharmacologic blockade of vasopressin V(2) receptors, which caused similar diuresis in wild-type and knockout mice. However, wild-type mice, unlike the BK(-/-), had a concomitant increase in urinary K(+) excretion and a significant correlation between urinary flow rate and K(+) excretion. Both genotypes excreted similar urinary amounts of K(+) irrespective of K(+) diet. This was associated, however, with higher plasma aldosterone and stronger expression of ROMK in the apical membrane of the aldosterone-sensitive portions of the distal nephron in the knockout than in the wild-type under control diet and even more so with the high-K(+) diet. High-K(+) intake significantly increased the renal expression of the BK channel in the wild-type mouse. Finally, despite the higher plasma K(+) and aldosterone levels, BK(-/-) mice restrict urinary K(+) excretion when placed on a low-K(+) diet to the same extent as the wild-type. These studies suggest a role of the BK channel alpha-subunit in flow-induced K(+) secretion and in K(+) homeostasis. Higher aldosterone and an upregulation of ROMK may compensate for the absence of functional BK channels.

Identification and localization of BK-beta subunits in the distal nephron of the mouse kidney

Large-conductance, Ca(2+)-activated K(+) channels (BK), comprised of pore-forming alpha- and accessory beta-subunits, secrete K(+) in the distal nephron under high-flow and high-K(+) diet conditions. BK channels are detected by electrophysiology in many nephron segments; however, the accessory beta-subunit associated with these channels has not been determined. We performed RT-PCR, Western blotting, and immunohistochemical staining to determine whether BK-beta1 is localized to the connecting tubule's principal-like cells (CNT) or intercalated cells (ICs), and whether BK-beta2-4 are present in other distal nephron segments. RT-PCR and Western blots revealed that the mouse kidney expresses BK-beta1, BK-beta2, and BK-beta4. Available antibodies in conjunction with BK-beta1(-/-) and BK-beta4(-/-) mice allowed the specific localization of BK-beta1 and BK-beta4 in distal nephron segments. Immunohistochemical staining showed that BK-beta1 is localized in the CNT but not ICs of the connecting tubule. The localization of BK-beta4 was discerned using an anti-BK-beta4 antibody on wild-type tissue and anti-GFP on GFP-replaced BK-beta4 mouse (BK-beta4(-/-)) tissue. Both antibodies (anti-BK-beta4 and anti-GFP) localized BK-beta4 to the thick ascending limb (TAL), distal convoluted tubule (DCT), and ICs of the distal nephron. It is concluded that BK-beta1 is narrowly confined to the apical membrane of CNTs in the mouse, whereas BK-beta4 is expressed in the TAL, DCT, and ICs.

Ca2+ dependence of flow-stimulated K secretion in the mammalian cortical collecting duct

Apical low-conductance SK and high-conductance Ca(2+)-activated BK channels are present in distal nephron, including the cortical collecting duct (CCD). Flow-stimulated net K secretion (J(K)) in the CCD is 1) blocked by iberiotoxin, an inhibitor of BK but not SK channels, and 2) associated with an increase in [Ca(2+)](i), leading us to conclude that BK channels mediate flow-stimulated J(K). To examine the Ca(2+) dependence and sources of Ca(2+) contributing to flow-stimulated J(K), J(K) and net Na absorption (J(Na)) were measured at slow (approximately 1) and fast (approximately 5 nl.min(-1).mm(-1)) flow rates in rabbit CCDs microperfused in the absence of luminal Ca(2+) or after pretreatment with BAPTA-AM to chelate intracellular Ca(2+), 2-aminoethoxydiphenyl borate (2-APB), to inhibit the inositol 1,4,5-trisphosphate (IP(3)) receptor or thapsigargin to deplete internal stores. These treatments, which do not affect flow-stimulated J(Na) (Morimoto et al. Am J Physiol Renal Physiol 291: F663-F669, 2006), inhibited flow-stimulated J(K). Increases in [Ca(2+)](i) stimulate exocytosis. To test whether flow induces exocytic insertion of preformed BK channels into the apical membrane, CCDs were pretreated with 10 microM colchicine (COL) to disrupt microtubule function or 5 microg/ml brefeldin-A (BFA) to inhibit delivery of channels from the intracellular pool to the plasma membrane. Both agents inhibited flow-stimulated J(K) but not J(Na) (Morimoto et al. Am J Physiol Renal Physiol 291: F663-F669, 2006), although COL but not BFA also blocked the flow-induced [Ca(2+)](i) transient. We thus speculate that BK channel-mediated, flow-stimulated J(K) requires an increase in [Ca(2+)](i) due, in part, to luminal Ca(2+) entry and ER Ca(2+) release, microtubule integrity, and exocytic insertion of preformed channels into the apical membrane.