WNK4 inhibition of ENaC is independent of Nedd4-2-mediated ENaC ubiquitination

A serine-threonine protein kinase, WNK4, reduces Na⁺ reabsorption and K⁺ secretion in the distal convoluted tubule by reducing trafficking of the thiazide-sensitive Na-Cl cotransporter to and enhancing renal outer medullary potassium channel retrieval from the apical membrane. Epithelial sodium channels (ENaC) in the distal nephron also play a role in regulating Na⁺ reabsorption and are also regulated by WNK4, but the mechanism is unclear. In A6 distal nephron cells, transepithelial current measurement and single channel recording show that WNK4 inhibits ENaC activity. Analysis of the number of channel per patch shows that WNK4 reduces channel number but has no effect on channel open probability. Western blots of apical and total ENaC provide additional evidence that WNK4 reduces apical as well as total ENaC expression. WNK4 enhances ENaC internalization independent of Nedd4-2-mediated ENaC ubiquitination. WNK4 also reduced the amount of ENaC available for recycling but has no effect on the rate of transepithelial current increase to forskolin. In contrast, Nedd4-2 not only reduced ENaC in the recycling pool but also decreased the rate of increase of current after forskolin. WNK4 associates with wild-type as well as Liddle's mutated ENaC, and WNK4 reduces both wild-type and mutated ENaC expressed in HEK293 cells.

ENaC is regulated by natriuretic peptide receptor-dependent cGMP signaling

Epithelial sodium channels (ENaCs) located at the apical membrane of polarized epithelial cells are regulated by the second messenger guanosine 3',5'-cyclic monophosphate (cGMP). The mechanism for this regulation has not been completely characterized. Guanylyl cyclases synthesize cGMP in response to various intracellular and extracellular signals. We investigated the regulation of ENaC activity by natriuretic peptide-dependent activation of guanylyl cyclases in Xenopus 2F3 cells. Confocal microscopy studies show natriuretic peptide receptors (NPRs), including those coupled to guanylyl cyclases, are expressed at the apical membrane of 2F3 cells. Single-channel patch-clamp studies using 2F3 cells revealed that atrial natriuretic peptide (ANP) or 8-(4-chlorophenylthio)-cGMP, but not C-type natriuretic peptide or cANP, decreased the open probability of ENaC. This suggests that NPR-A, but not NPR-B or NPR-C, is involved in the natriuretic peptide-mediated regulation of ENaC activity. Also, it is likely that a signaling pathway involving cGMP and nitric oxide (NO) are involved in this mechanism, since inhibitors of soluble guanylyl cyclase, protein kinase G, inducible NO synthase, or an NO scavenger blocked or reduced the effect of ANP on ENaC activity.

Scanning ion conductance microscopy: a nanotechnology for biological studies in live cells

Scanning ion-conductance microscope (SICM), which enables high-resolution imaging of cell surface topography, has been developed for over two decades. However, only recently, a unique scanning mode is increasingly used in biological studies to allow SICM to detect the surface of live cells. More recently, in combination with confocal microscopy and patch-clamp electrophysiological techniques, SICM allows investigators to localize proteins or ion channels in a specific nanostructure at the cell surface. This article will briefly review SICM nanotechnique and summarize the role of SICM in biological studies.

Phosphatidylinositol phosphate-dependent regulation of Xenopus ENaC by MARCKS protein

Phosphatidylinositol phosphates (PIPs) are known to regulate epithelial sodium channels (ENaC). Lipid binding assays and coimmunoprecipitation showed that the amino-terminal domain of the β- and γ-subunits of Xenopus ENaC can directly bind to phosphatidylinositol 4,5-bisphosphate (PIP(2)), phosphatidylinositol 3,4,5-trisphosphate (PIP(3)), and phosphatidic acid (PA). Similar assays demonstrated various PIPs can bind strongly to a native myristoylated alanine-rich C-kinase substrate (MARCKS), but weakly or not at all to a mutant form of MARCKS. Confocal microscopy demonstrated colocalization between MARCKS and PIP(2). Confocal microscopy also showed that MARCKS redistributes from the apical membrane to the cytoplasm after PMA-induced MARCKS phosphorylation or ionomycin-induced intracellular calcium increases. Fluorescence resonance energy transfer studies revealed ENaC and MARCKS in close proximity in 2F3 cells when PKC activity and intracellular calcium concentrations are low. Transepithelial current measurements from Xenopus 2F3 cells treated with PMA and single-channel patch-clamp studies of Xenopus 2F3 cells treated with a PKC inhibitor altered Xenopus ENaC activity, which suggest an essential role for MARCKS in the regulation of Xenopus ENaC activity.

Cathepsin B is secreted apically from Xenopus 2F3 cells and cleaves the epithelial sodium channel (ENaC) to increase its activity

The epithelial sodium channel (ENaC) plays an important role in regulating sodium balance, extracellular volume, and blood pressure. Evidence suggests the α and γ subunits of ENaC are cleaved during assembly before they are inserted into the apical membranes of epithelial cells, and maximal activity of ENaC depends on cleavage of the extracellular loops of α and γ subunits. Here, we report that Xenopus 2F3 cells apically express the cysteine protease cathepsin B, as indicated by two-dimensional gel electrophoresis and mass spectrometry analysis. Recombinant GST ENaC α, β, and γ subunit fusion proteins were expressed in Escherichia coli and then purified and recovered from bacterial inclusion bodies. In vitro cleavage studies revealed the full-length ENaC α subunit fusion protein was cleaved by active cathepsin B but not the full-length β or γ subunit fusion proteins. Both single channel patch clamp studies and short circuit current experiments show ENaC activity decreases with the application of a cathepsin B inhibitor directly onto the apical side of 2F3 cells. We suggest a role for the proteolytic cleavage of ENaC by cathepsin B, and we suggest two possible mechanisms by which cathepsin B could regulate ENaC. Cathepsin B may cleave ENaC extracellularly after being secreted or intracellularly, while ENaC is present in the Golgi or in recycling endosomes.

Molecular approaches to examine the phosphorylation state of the C type natriuretic peptide receptor

The intracellular domain of the C type natriuretic peptide receptor (NPRC) contains one threonine and several serine residues where phosphorylation is thought to occur. Several phosphorylation consensus sequences for various kinases have been identified within the intracellular domain of NPRC, but the exact residues that are phosphorylated and the specific kinases responsible for their phosphorylation have not been thoroughly defined. Here we introduce a recombinant GST fusion protein and a rat gastric mucosa (RGM1) cell line as molecular tools to study the phosphorylation state of NPRC in vitro and in vivo, respectively. We utilize a previously characterized polyclonal antibody against NPRC to probe for total NPRC protein and various phosphospecific and substrate motif antibodies to probe for phosphorylation of NPRC. Phosphoprotein staining reagents were used with a phosphoprotein control set to detect phosphorylation of NPRC at serine and threonine residues. Recombinant GST-NPRC fusion protein was phosphorylated in vitro by RGM1 lysate in the presence of adenosine-5'-triphosphate (ATP). Western blot analysis using a monoclonal phospho-Thr antibody, which exclusively detects phosphorylated threonine residues, and does not cross-react with phosphorylated serine residues revealed NPRC immunoprecipitated from RGM1 lysate is phosphorylated on a threonine residue. Global analysis of the entire rat NPRC sequence using a protein kinase A (PKA) prediction algorithm, identified five putative PKA phosphorylation sites containing a serine residue and one containing a threonine residue, Thr 505. Taken together, the data presented here suggest that rat NPRC is a substrate for PKA and Thr 505 located within the intracellular domain of NPRC is a likely candidate site for the phosphorylation.

The C type natriuretic peptide receptor tethers AHNAK1 at the plasma membrane to potentiate arachidonic acid-induced calcium mobilization

Arachidonic acid (AA) liberated from membrane phospholipids is known to activate phospholipase C gamma1 (PLCgamma1) concurrently with AHNAK in nonneuronal cells. The recruitment of AHNAK from the nucleus is required for it to activate PLCgamma1 at the plasma membrane. Here, we identify the C-type natriuretic peptide receptor (NPR-C), an atypical G protein-coupled receptor, as a protein binding partner for AHNAK1 in various cell types. Mass spectrometry and MASCOT analysis of excised bands from NPR-C immunoprecipitation studies revealed multiple signature peptides corresponding to AHNAK1. Glutathione S-transferase (GST) pulldown assays using GST- AHNAK1 fusion proteins corresponding to each of the distinct domains of AHNAK1 showed the C1 domain of AHNAK1 associates with NPR-C. The role of NPR-C in mediating AA-dependent AHNAK1 calcium signaling was explored in various cell types, including 3T3-L1 preadipocytes during the early stages of differentiation. Sucrose density gradient centrifugation studies showed AHNAK1 resides in the nucleus, cytoplasm, and at the plasma membrane, but small interfering RNA (siRNA)-mediated knockdown of NPR-C resulted in AHNAK1 accumulation in the nucleus. Overexpression of a portion of AHNAK1 resulted in augmentation of intracellular calcium mobilization, whereas siRNA-mediated knockdown of NPR-C or AHNAK1 protein resulted in attenuation of intracellular calcium mobilization in response to phorbol 12-myristate 13-acetate. We characterize the novel association between AHNAK1 and NPR-C and provide evidence that this association potentiates the AA-induced mobilization of intracellular calcium. We address the role of intracellular calcium in the various cell types that AHNAK1 and NPR-C were found to associate.

C-type natriuretic peptide receptor expression in pancreatic alpha cells

Atrial natriuretic peptide (ANP), brain type natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) comprise a family of natriuretic peptides that mediate their biological effects through three natriuretic peptide receptor subtypes, NPR-A (ANP, BNP), NPR-B (CNP) and NPR-C (ANP, BNP, CNP). Several reports have provided evidence for the expression of ANP and specific binding sites for ANP in the pancreas. The purpose of this study was to identify the ANP receptor subtype and to localize its expression to a specific cell type in the human pancreas. NPR-C immunoreactivity, but neither ANP nor NPR-A, was detected in human islets by immunofluorescent staining. No immunostaining was observed in the exocrine pancreas or ductal structures. Double-staining revealed that NPR-C was expressed mainly in the glucagon-containing alpha cells. NPR-C mRNA and protein were detected in isolated human islets by RT-PCR and Western blot analysis, respectively. NPR-C expression was also detected by immunofluorescent staining in glucagonoma but not in insulinoma. ANP, as well as BNP and CNP, stimulated glucagon secretion from perifused human islets (1,111 +/- 55% vs. basal [7.3 fmol/min]; P < 0.001). This response was mimicked by cANP(4-23), a selective agonist of NPR-C. In conclusion, the NPR-C receptor is expressed in normal and neoplastic human alpha cells. These findings suggest a role for natriuretic peptides in the regulation of glucagon secretion from human alpha cells.

Cardiac and kidney hormones cure up to 86% of human small-cell lung cancers in mice

BACKGROUND

Four cardiac hormones synthesized by the same gene, i.e. atrial natriuretic peptide, vessel dilator, long acting natriuretic peptide and kaliuretic peptide, and the kidney hormone urodilatin have anticancer effects in vitro.

MATERIALS AND METHODS

These cardiac hormones and urodilatin were infused subcutaneously for 28 days with weekly fresh hormones since they lose biological effects at body temperature for more than a week at 0.3 nm kg(-1) body weight in athymic mice bearing human small-cell lung carcinomas.

RESULTS

Long acting natriuretic peptide, vessel dilator, kaliuretic peptide, atrial natriuretic peptide and urodilatin eliminated 86%, 71%, 57%, 43% (P < 0.001 for the cardiac hormones) and 25% (P < 0.05; urodilatin) of the human small-cell lung carcinomas. The treated small-cell lung carcinomas that were not cured grew rapidly, similar to the untreated controls, whose volume was 7 fold larger in 1 week, 18-fold increased in 2 weeks, 39-fold increased in 3 weeks, 63-fold increased in 1 month and 97-fold increased in volume in 6 weeks. One vessel dilator treated small-cell lung carcinoma animal developed a large tumour (8428 mm3 volume) on treatment and this tumour was eliminated with utilizing atrial natriuretic peptide and then long acting natriuretic peptide sequentially.

CONCLUSIONS

Four cardiac hormones eliminate up to 86% of human small-cell lung carcinomas in athymic mice. Urodilatin can also eliminate small-cell lung carcinomas but at a lower cure rate of 25%. Unresponsive lesions can be eliminated by utilizing different hormones synthesized by the atrial natriuretic peptide gene in a sequential manner.

Four cardiac hormones eliminate up to two-thirds of human breast cancers in athymic mice

BACKGROUND

Four cardiac hormones i.e. atrial natriuretic peptide (ANP), vessel dilator, long acting natriuretic peptide (LANP) and kaliuretic peptide have anticancer effects in vitro.

MATERIALS AND METHODS

These four cardiac hormones were infused subcutaneously for 28 days with weekly fresh hormones at 3 nM min(-1) kg(-1) body weight in athymic mice bearing human breast adenocarcinomas.

RESULTS

Vessel dilator, LANP, kaliuretic peptide and ANP eliminated 67%, 50%, 67% and 33% of the HTB-132 human breast adenocarcinomas. LANP eliminated 100% and vessel dilator 1/3 of CRL-2327 breast adenocarcinomas. There was no recurrence of the breast cancers in the primary site and no metastasis except in the ANP-treated group in one year post-treatment. The natriuretic peptide receptors-A and -C were decreased 50% and 31%, respectively, in metastatic versus primary ANP-treated breast adenocarcinomas.

CONCLUSION

Four cardiac hormones eliminate up to two-thirds of human breast adenocarcinomas in athymic mice.

Four peptides decrease human colon adenocarcinoma cell number and DNA synthesis via cyclic GMP

BACKGROUND

Mortality from colon cancer is significant with an expected 30,350 colon cancer deaths in 2005 with current treatment(s). Long-acting natriuretic peptide, vessel dilator, kaliuretic peptide, and atrial natriuretic peptide have significant anticancer effects in breast and pancreatic adenocarcinomas.

AIM OF STUDY

Whether these peptide hormones have anticancer effects in colon adenocarcinoma cells and whether these effects are specifically mediated by cyclic GMP has not been determined.

METHODS

These peptide hormones were evaluated for anticancer effects in human colon adenocarcinoma cells and to determine whether their anticancer effects are specifically mediated by cyclic GMP.

RESULTS

There was a 89-97% decrease (p <0.001 for each) in colon adenocarcinoma cells within 24 h with 1 mM of these peptide hormones. There was a significant (p <0.05) decrease in human colon cancer cell number with each 10-fold increase in concentration from 1 to 1,000 microM (i.e., 1 mM) of these four peptide hormones without any proliferation in the 3 d following this decrease. These same hormones decreased DNA synthesis 65-83% (p <0.001). Cyclic GMP antibody inhibited 75- 80% of these peptides' ability to decrease colon adenocarcinoma cell number and inhibited 92-96% of their DNA synthesis effects and 97% of cyclic GMP's effects. Western blots revealed that for the first time natriuretic peptide receptors (NPR) A and C were present in colon adenocarcinoma cells.

CONCLUSIONS

Four peptide hormones eliminate up to 97% of colon cancer cells within 24 h with their DNA effects specifically mediated by cyclic GMP.

Elimination of up to 80% of human pancreatic adenocarcinomas in athymic mice by cardiac hormones

BACKGROUND

Four cardiac hormones have anticancer effects in vitro: i) atrial natriuretic peptide (ANP), ii) vessel dilator, iii) long acting natriuretic peptide (LANP), and iv) kaliuretic peptide.

MATERIALS AND METHODS

These cardiac hormones were infused subcutaneously for 28 days with weekly fresh hormones at 3 nM min(-1) kg(-1) body weight in athymic mice bearing human pancreatic adenocarcinomas.

RESULTS

ANP, vessel dilator, LANP and kaliuretic peptide eliminated 80%, 33%, 20% and 14% of the pancreatic adenocarcinomas. Even in the treated animals which did not have a total cure, their tumor volume decreased to less than 10% (and with vessel dilator to 2%) of that of the untreated animals. The natriuretic peptide receptor (NPR)-A receptor was decreased 33% to 55% in the metastatic lesions compared to the primary pancreatic adenocarcinoma.

CONCLUSION

Four cardiac hormones eliminated up to 80% of human pancreatic adenocarcinomas in athymic mice.

Four cardiac hormones eliminate 4-fold more human glioblastoma cells than the green mamba snake peptide

Within 24h four cardiac hormones, i.e., vessel dilator, kaliuretic peptide, atrial natriuretic peptide, and long acting natriuretic peptide decrease the number of human glioblastoma cells 75%, 68%, 67%, and 65% while Dendroaspis (green mamba) peptide caused a 17% decrease when each were utilized at 100 microM. The four cardiac hormones decreased DNA synthesis 65-87% and increased cyclic GMP 1.3- to 3.8-fold in the glioblastoma cells. Natriuretic peptide receptors (NPR)-A and -C were present.

CONCLUSION

four cardiac hormones eliminate up to 75% of glioblastoma cells via cyclic GMP-mediated up to 87% decrease in DNA synthesis.

Four cardiac hormones eliminate up to 82% of human medullary thyroid carcinoma cells within 24 hours

Four cardiac hormones, i.e., atrial natriuretic peptide, vessel dilator, long-acting natriuretic peptide, and kaliuretic peptide, which have anticancer effects, were evaluated for the first time on any endocrine cancer to determine if they have anticancer effects in an endocrine cancer. These four cardiac hormones were evaluated for their anticancer, DNA synthesis, and receptor status in human medullary thyroid cancer cells. There was a significant (p < 0.001) decrease in human medullary thyroid cancer cells with each 10-fold increase from 1 to 100 microM of the four cardiac hormones. There was an 81%, 68%, 71%, and 66% elimination within 24 h of medullary thyroid cancer cells secondary to vessel dilator, kaliuretic peptide, atrial natriuretic peptide, and long-acting natriuretic peptide, respectively (p < 0.0001). Three days after treatment with these peptide hormones, there was no proliferation of the medullary thyroid cancer cells. These cardiac hormones decreased DNA synthesis in the medullary thyroid cells from 65% to 84% (p < 0.0001). Western blots revealed natriuretic peptide receptors-A and -C were present in human medullary thyroid cancer cells. These results indicate the four cardiac hormones have potent anticancer effects by eliminating up to 82% of human medullary thyroid carcinoma cells within 24 h of treatment.

Urodilatin and four cardiac hormones decrease human renal carcinoma cell numbers

BACKGROUND

Mortality from renal-cell cancer remains a significant problem with an estimated 12,600 deaths in the United States in 2005 even with current treatment(s) of surgery, chemotherapy, radiation and immunotherapy. Four cardiac natriuretic peptides, that is, atrial natriuretic peptide, vessel dilator, long-acting natriuretic peptide and kaliuretic peptide have significant anti-cancer effects in breast, pancreatic, prostate and colon adenocarcinomas.

MATERIALS AND METHODS

These four peptide hormones plus brain natriuretic peptide (BNP), C-natriuretic peptide (CNP) and urodilatin, a peptide hormone formed in the kidney by a different post-translational processing of the atrial natriuretic peptide prohormone, were evaluated for their anti-cancer effects in renal carcinomas.

RESULTS

Dose-response curves revealed a significant (P < 0.0001) decrease in human renal carcinoma cells with each 10-fold increase in concentration from 1 microm to 100 microm of five of these peptide hormones. There was an 81%, 74%, 66%, 70% and 70% elimination within 24 h in renal carcinoma cells secondary to vessel dilator, kaliuretic peptide, urodilatin, atrial natriuretic peptide and long-acting natriuretic peptide, respectively (P < 0.0001 for each), whereas BNP had no effect and CNP decreased renal cancer cell number by 10% (P = 0.04) at their 100 microm concentrations. Three days after treatment with these peptide hormones, the cancer cells began to proliferate again. The four cardiac hormones and urodilatin decreased DNA synthesis from 65-84% (P < 0.00001), whereas BNP and CNP decreased DNA synthesis 3% and 12% (both non-significant). Western blots revealed for the first time natriuretic peptide receptors (NPR)-A, -B and -C were present in the renal cancer cells.

CONCLUSIONS

These results indicate that urodilatin and the four cardiac hormones have potent anti-cancer effects by eliminating up to 81% of renal carcinoma cells within 24 h of treatment.

Four peptide hormones' specific decrease (up to 97%) of human prostate carcinoma cells

BACKGROUND

Mortality from prostate cancer remains a significant problem with current treatment(s), with an expected 30 350 deaths from prostate cancer in 2005. Long-acting natriuretic peptide, vessel dilator, kaliuretic peptide and atrial natriuretic peptide have significant anticancer effects in breast and pancreatic adenocarcinomas. Whether these effects are specific and whether they have anticancer effects in prostate adenocarcinoma cells has not been determined.

MATERIALS AND METHODS

These peptide hormones were evaluated to determine if they have specific anticancer effects in human prostate adenocarcinomas.

RESULTS

Dose-response curves revealed a significant (P < 0.05) decrease in human prostate cancer number with each tenfold increase in the concentration from 1 microM to 1000 microM (i.e. 1 mM) of these four peptide hormones. There was a 97.4%, 87%, 88% and 89% (P < 0.001 for each) decrease in prostate cancer cells secondary to vessel dilator, long-acting natriuretic peptide, kaliuretic peptide and atrial natriuretic peptide, respectively, at their 1-mM concentrations within 24 h, without any proliferation in the 3 days following this decrease. These same hormones decreased DNA synthesis from 68% to 89% (P < 0.001). When utilized with their respective antibodies their ability to decrease prostate adenocarcinoma cells or inhibit their DNA synthesis was completely blocked. Western blots revealed that for the first time natriuretic peptide receptors (NPR) A- and C- were present in prostate cancer cells.

CONCLUSIONS

These results indicate that these peptide hormones' anticancer effects are specific. Furthermore, they have very potent effects of eliminating up to 97% of prostate cancer cells within 24 h of treatment.

Five cardiac hormones decrease the number of human small-cell lung cancer cells

BACKGROUND

Four peptide hormones of a family of six hormones, i.e. atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), C-natriuretic peptide (CNP), long acting natriuretic peptide (LANP), vessel dilator and kaliuretic peptide, significantly decrease the number of adenocarcinoma cells in culture. The present investigation was designed to determine whether these peptide hormones' effects are specific to adenocarcinomas or whether they might decrease the number of cancer cells of a different type of cancer, i.e. small-cell lung cancer.

METHODS AND MATERIALS

These six hormones were evaluated for their ability to decrease the number and/or proliferation of human small-cell lung cancer cells in culture for 24, 48, 72, and 96 h.

RESULTS

Within 24 h, vessel dilator, LANP, kaliuretic peptide, ANP and their intracellular mediator cyclic GMP, each at 1 microM, decreased the number of small-cell lung cancer cells by 63% (P < 0.001), 21% (P < 0.05), 30% (P < 0.05), 39% (P < 0.05), and 31% (P < 0.05), respectively. There was no proliferation in the 3 days following this decrease in cell number. These same hormones decreased DNA synthesis 68% to 82% (P < 0.001). Brain natriuretic peptide and CNP did not decrease the number of small-cell lung cancer cells or inhibit their DNA synthesis at 1 microM or 10 microM concentrations. Dose-response curves revealed that at 100 microM, the vessel dilator decreased 92% of the cancer cells in 24 h while BNP had no effect, but CNP caused a 39% decrease. Western blots revealed that the natriuretic peptide receptors A- and C- were present in these cancer cells.

CONCLUSIONS

Five peptide hormones significantly decrease the number of human small-cell lung cancer cells within 24 h and inhibit their proliferation for at least 96 h. Their mechanism of doing so involves inhibition of DNA synthesis mediated in part by cyclic GMP.