Blocking the salt-inducible kinase 1 network prevents the increases in cell sodium transport caused by a hypertension-linked mutation in human alpha-adducin

The Triad Na+ Activated Na+ Channel (Nax)-Salt Inducible KINASE (SIK) and (Na+ + K+)-ATPase: Targeting the Villains to Treat Salt Resistant and Sensitive Hypertension

The Na⁺-activated Na⁺ channel (Nax) and salt-inducible kinase (SIK) are stimulated by increases in local Na⁺ concentration, affecting (Na⁺ + K⁺)-ATPase activity. To test the hypothesis that the triad Nax/SIK/(Na⁺ + K⁺)-ATPase contributes to kidney injury and salt-sensitive hypertension (HTN), uninephrectomized male Wistar rats (200 g; n = 20) were randomly divided into 4 groups based on a salt diet (normal salt diet; NSD-0.5% NaCl-or high-salt diet; HSD-4% NaCl) and subcutaneous administration of saline (0.9% NaCl) or deoxycorticosterone acetate (DOCA, 8 mg/kg), as follows: Control (CTRL), CTRL-Salt, DOCA, and DOCA-Salt, respectively. After 28 days, the following were measured: kidney function, blood pressure, (Na⁺ + K⁺)-ATPase and SIK1 kidney activities, and Nax and SIK1 renal expression levels. SIK isoforms in kidneys of CTRL rats were present in the glomerulus and tubular epithelia; they were not altered by HSD and/or HTN. CTRL-Salt rats remained normotensive but presented slight kidney function decay. HSD rats displayed augmentation of the Nax/SIK/(Na⁺ + K⁺)-ATPase pathway. HTN, kidney injury, and kidney function decay were present in all DOCA rats; these were aggravated by HSD. DOCA rats presented unaltered (Na⁺ + K⁺)-ATPase activity, diminished total SIK activity, and augmented SIK1 and Nax content in the kidney cortex. DOCA-Salt rats expressed SIK1 activity and downregulation in (Na⁺ + K⁺)-ATPase activity in the kidney cortex despite augmented Nax content. The data of this study indicate that the (Na⁺ + K⁺)-ATPase activity response to SIK is attenuated in rats under HSD, independent of HTN, as a mechanism contributing to kidney injury and salt-sensitive HTN.

Salt Sensation and Regulation

Taste sensation and regulation are highly conserved in insects and mammals. Research conducted over recent decades has yielded major advances in our understanding of the molecular mechanisms underlying the taste sensors for a variety of taste sensations and the processes underlying regulation of ingestion depending on our internal state. Salt (NaCl) is an essential ingested nutrient. The regulation of internal sodium concentrations for physiological processes, including neuronal activity, fluid volume, acid–base balance, and muscle contraction, are extremely important issues in animal health. Both mammals and flies detect low and high NaCl concentrations as attractive and aversive tastants, respectively. These attractive or aversive behaviors can be modulated by the internal nutrient state. However, the differential encoding of the tastes underlying low and high salt concentrations in the brain remain unclear. In this review, we discuss the current view of taste sensation and modulation in the brain with an emphasis on recent advances in this field. This work presents new questions that include but are not limited to, “How do the fly’s neuronal circuits process this complex salt code?” and “Why do high concentrations of salt induce a negative valence only when the need for salt is low?” A better understanding of regulation of salt homeostasis could improve our understanding of why our brains enjoy salty food so much.

The potent roles of salt-inducible kinases (SIKs) in metabolic homeostasis and tumorigenesis

Salt-inducible kinases (SIKs) belong to AMP-activated protein kinase (AMPK) family, and functions mainly involve in regulating energy response-related physiological processes, such as gluconeogenesis and lipid metabolism. However, compared with another well-established energy-response kinase AMPK, SIK roles in human diseases, especially in diabetes and tumorigenesis, are rarely investigated. Recently, the pilot roles of SIKs in tumorigenesis have begun to attract more attention due to the finding that the tumor suppressor role of LKB1 in non-small-cell lung cancers (NSCLCs) is unexpectedly mediated by the SIK but not AMPK kinases. Thus, here we tend to comprehensively summarize the emerging upstream regulators, downstream substrates, mouse models, clinical relevance, and candidate inhibitors for SIKs, and shed light on SIKs as the potential therapeutic targets for cancer therapies.

Suppression of gluconeogenic gene transcription by SIK1-induced ubiquitination and degradation of CRTC1

CRTCs are a group of three transcriptional coactivators required for CREB-dependent transcription. CREB and CRTCs are critically involved in the regulation of various biological processes such as cell proliferation, metabolism, learning and memory. However, whether CRTC1 efficiently induces gluconeogenic gene expression and how CRTC1 is regulated by upstream kinase SIK1 remain to be understood. In this work, we demonstrated SIK1-induced phosphorylation, ubiquitination and degradation of CRTC1 in the context of the regulation of gluconeogenesis. CRTC1 protein was destabilized by SIK1 but not SIK2 or SIK3. This effect was likely mediated by phosphorylation at S155, S167, S188 and S346 residues of CRTC1 followed by K48-linked polyubiquitination and proteasomal degradation. Expression of gluconeogenic genes such as that coding for phosphoenolpyruvate carboxykinase was stimulated by CRTC1, but suppressed by SIK1. Depletion of CRTC1 protein also blocked forskolin-induced gluconeogenic gene expression, knockdown or pharmaceutical inhibition of SIK1 had the opposite effect. Finally, SIK1-induced ubiquitination of CRTC1 was mediated by RFWD2 ubiquitin ligase at a site not equivalent to K628 in CRTC2. Taken together, our work reveals a regulatory circuit in which SIK1 suppresses gluconeogenic gene transcription by inducing ubiquitination and degradation of CRTC1. Our findings have implications in the development of new antihyperglycemic agents.

Inhibition of SIK2 and SIK3 during differentiation enhances the anti-inflammatory phenotype of macrophages

The salt-inducible kinases (SIKs) control a novel molecular switch regulating macrophage polarization. Pharmacological inhibition of the SIKs induces a macrophage phenotype characterized by the secretion of high levels of anti-inflammatory cytokines, including interleukin (IL)-10, and the secretion of very low levels of pro-inflammatory cytokines, such as tumour necrosis factor α. The SIKs, therefore, represent attractive new drug targets for the treatment of macrophage-driven diseases, but which of the three isoforms, SIK1, SIK2 or SIK3, would be appropriate to target remains unknown. To address this question, we developed knock-in (KI) mice for SIK1, SIK2 and SIK3, in which we introduced a mutation that renders the enzymes catalytically inactive. Characterization of primary macrophages from the single and double KI mice established that all three SIK isoforms, and in particular SIK2 and SIK3, contribute to macrophage polarization. Moreover, we discovered that inhibition of SIK2 and SIK3 during macrophage differentiation greatly enhanced the production of IL-10 compared with their inhibition in mature macrophages. Interestingly, macrophages differentiated in the presence of SIK inhibitors, MRT199665 and HG-9-91-01, still produced very large amounts of IL-10, but very low levels of pro-inflammatory cytokines, even after the SIKs had been reactivated by removal of the drugs. Our data highlight an integral role for SIK2 and SIK3 in innate immunity by preventing the differentiation of macrophages into a potent and stable anti-inflammatory phenotype.

Genetics of ion homeostasis in Ménière's Disease

Aim of this work was to assess the role of polymorphisms belonging to genes involved in the regulation of ionic homeostasis in Caucasian patients with Ménière Disease (MD). We recruited 155 patients with definite Ménière Disease and 186 controls (Control Group 1) without a lifetime history of vertigo, overlapping with patients for age and rate of hypertension. We validated the positive results on 413 Caucasian subjects selected from a European general population (Control Group 2). The clinical history for migraine and hypertension was collected; genomic DNA was characterized for a panel of 33 SNPs encoding proteins involved in ionic transport. We found a higher rate of migraineurs in MD subjects compared to Group 1 (46.8 vs 15.5%, p = 0.00005). Four SNPs displayed differences in MD patients compared to Group 1 controls: rs3746951 and rs2838301 in SIK1 gene, rs434082 and rs487119 in SLC8A1; the p values of Chi-squared test for genotype frequencies are 0.009, 0.023, 0.009 and 0.048, respectively. SLC8A1 gene encodes for Na⁺-Ca⁺⁺ exchanger, while SIK1 gene encodes for Salt Inducible Kinase 1, an enzyme associated with Na⁺-K⁺ ATPase function. The validation with Control Group 2 displayed that only rs3746951 and rs487119 are strongly associated to MD (p = 0.001 and p = 0.0004, respectively). These data support the hypothesis that a genetically induced dysfunction of ionic transport may act as a predisposing factors to develop MD.

Lack of salt-inducible kinase 2 (SIK2) prevents the development of cardiac hypertrophy in response to chronic high-salt intake

Cardiac left ventricle hypertrophy (LVH) constitutes a major risk factor for heart failure. Although LVH is most commonly caused by chronic elevation in arterial blood pressure, reduction of blood pressure to normal levels does not always result in regression of LVH, suggesting that additional factors contribute to the development of this pathology. We tested whether genetic preconditions associated with the imbalance in sodium homeostasis could trigger the development of LVH without concomitant increases in blood pressure. The results showed that the presence of a hypertensive variant of α-adducin gene in Milan rats (before they become hypertensive) resulted in elevated expression of genes associated with LVH, and of salt-inducible kinase 2 (SIK2) in the left ventricle (LV). Moreover, the mRNA expression levels of SIK2, α-adducin, and several markers of cardiac hypertrophy were positively correlated in tissue biopsies obtained from human hearts. In addition, we found in cardiac myocytes that α-adducin regulates the expression of SIK2, which in turn mediates the effects of adducin on hypertrophy markers gene activation. Furthermore, evidence that SIK2 is critical for the development of LVH in response to chronic high salt diet (HS) was obtained in mice with ablation of the sik2 gene. Increases in the expression of genes associated with LVH, as well as increases in LV wall thickness upon HS, occurred only in sik2^+/+ but not in sik2^−/− mice. Thus LVH triggered by HS or the presence of a genetic variant of α-adducin requires SIK2 and is independent of elevated blood pressure. Inhibitors of SIK2 may constitute part of a novel therapeutic regimen aimed at prevention/regression of LVH.

A review of nutritional factors in hypertension management

Hypertension is a major health problem worldwide. Its attendant morbidity and mortality complications have a great impact on patient's quality of life and survival. Optimizing blood pressure control has been shown to improve overall health outcomes. In addition to pharmacological therapies, nonpharmacological approach such as dietary modification plays an important role in controlling blood pressure. Many dietary components such as sodium, potassium, calcium, and magnesium have been studied substantially in the past decades. While some of these nutrients have clear evidence for their recommendation, some remain controversial and are still of ongoing study. Dietary modification is often discussed with patients and can provide a great benefit in blood pressure regulation. As such, reviewing the current evidence will be very useful in guiding patients and their physician and/or dietician in decision making. In this review article of nutritional factors in hypertension management, we aim to examine the role of nutritional factors individually and as components of whole dietary patterns.

Possible role of brain salt-inducible kinase 1 in responses to central sodium in Dahl rats

In Dahl salt-sensitive (S) rats, Na+ entry into the cerebrospinal fluid (CSF) and sympathoexcitatory and pressor responses to CSF Na+ are enhanced. Salt-inducible kinase 1 (SIK1) increases Na+/K+-ATPase activity in kidney cells. We tested the possible role of SIK1 in regulation of CSF [Na+] and responses to Na+ in the brain. SIK1 protein and activity were lower in hypothalamic tissue of Dahl S (SS/Mcw) compared with salt-resistant SS.BN13 rats. Intracerebroventricular infusion of the protein kinase inhibitor staurosporine at 25 ng/day, to inhibit SIK1 further increased mean arterial pressure (MAP) and HR but did not affect the increase in CSF [Na+] or hypothalamic aldosterone in Dahl S on a high-salt diet. Intracerebroventricular infusion of Na+-rich artificial CSF caused significantly larger increases in renal sympathetic nerve activity, MAP, and HR in Dahl S vs. SS.BN13 or Wistar rats on a normal-salt diet. Intracerebroventricular injection of 5 ng staurosporine enhanced these responses, but the enhancement in Dahl S rats was only one-third that in SS.BN13 and Wistar rats. Staurosporine had no effect on MAP and HR responses to intracerebroventricular ANG II or carbachol, whereas the specific protein kinase C inhibitor GF109203X inhibited pressor responses to intracerebroventricular Na+-rich artificial CSF or ANG II. These results suggest that the SIK1-Na+/K+-ATPase network in neurons acts to attenuate sympathoexcitatory and pressor responses to increases in brain [Na+]. The lower hypothalamic SIK1 activity and smaller effect of staurosporine in Dahl S rats suggest that impaired activation of neuronal SIK1 by Na+ may contribute to their enhanced central responses to sodium.

Salt-inducible kinase 1 regulates E-cadherin expression and intercellular junction stability

The protein kinase liver kinase B1 (LKB1) regulates cell polarity and intercellular junction stability. Also, LKB1 controls the activity of salt-inducible kinase 1 (SIK1). The role and relevance of SIK1 and its downstream effectors in linking the LKB1 signals within these processes are partially understood. We hypothesize that SIK1 may link LKB1 signals to the maintenance of epithelial junction stability by regulating E-cadherin expression. Results from our studies using a mouse lung alveolar epithelial (MLE-12) cell line or human renal proximal tubule (HK2) cell line transiently or stably lacking the expression of SIK1 (using SIK1 siRNAs or shRNAs), or with its expression abrogated (sik1(+/+) vs. sik1(-/-) mice), indicate that suppression of SIK1 (∼40%) increases the expression of the transcriptional repressors Snail2 (∼12-fold), Zeb1 (∼100%), Zeb2 (∼50%), and TWIST (∼20-fold) by activating cAMP-response element binding protein. The lack of SIK1 and activation of transcriptional repressors decreases the availability of E-cadherin (mRNA and protein expression by ∼100 and 80%, respectively) and the stability of intercellular junctions in epithelia (decreases in transepithelial resistance). Furthermore, LKB1-mediated increases in E-cadherin expression are impaired in cells where SIK1 has been disabled. We conclude that SIK1 is a key regulator of E-cadherin expression, and thereby contributes to the stability of intercellular junctions.

Increases in intracellular sodium activate transcription and gene expression via the salt-inducible kinase 1 network in an atrial myocyte cell line

Cardiac hypertrophy (CH) generally occurs as the result of the sustained mechanical stress caused by elevated systemic arterial blood pressure (BP). However, in animal models, elevated salt intake is associated with CH even in the absence of significant increases in BP. We hypothesize that CH is not exclusively the consequence of mechanical stress but also of other factors associated with elevated BP such as abnormal cell sodium homeostasis. We examined the effect of small increases in intracellular sodium concentration ([Na(+)](i)) on transcription factors and genes associated with CH in a cardiac cell line. Increases in [Na(+)](i) led to a time-dependent increase in the expression levels of mRNA for natriuretic peptide and myosin heavy chain genes and also increased myocyte enhancer factor (MEF)2/nuclear factor of activated T cell (NFAT) transcriptional activity. Increases in [Na(+)](i) are associated with activation of salt-inducible kinase 1 (snflk-1, SIK1), a kinase known to be critical for cardiac development. Moreover, increases in [Na(+)](i) resulted in increased SIK1 expression. Sodium did not increase MEF2/NFAT activity or gene expression in cells expressing a SIK1 that lacked kinase activity. The mechanism by which SIK1 activated MEF2 involved phosphorylation of HDAC5. Increases in [Na(+)](i) activate SIK1 and MEF2 via a parallel increase in intracellular calcium through the reverse mode of Na(+)/Ca(2+)-exchanger and activation of CaMK1. These data obtained in a cardiac cell line suggest that increases in intracellular sodium could influence myocardial growth by controlling transcriptional activation and gene expression throughout the activation of the SIK1 network.

α-Adducin Gly460Trp gene mutation and essential hypertension in a Chinese population: a meta-analysis including 10,960 subjects

Background

The α-adducin Gly460Trp (G460W) gene polymorphism may be associated with susceptibility to essential hypertension (EH), but this relationship remains controversial. In an attempt to resolve this issue, we conducted a meta-analysis.

Methods

Twenty-three separated studies involving 5939 EH patients and 5021 controls were retrieved and analyzed. Four ethnicities were included: Han, Kazakh, Mongolian, and She. Eighteen studies with 5087 EH patients and 4183 controls were included in the Han subgroup. Three studies with 636 EH patients and 462 controls were included in the Kazakh subgroup. The Mongolian subgroup was represented by only one study with 100 EH patients and 50 controls; similarly, only one study with 116 EH patients and 326 controls was available for the She subgroup. The pooled and ethnic group odds ratios (ORs) along with the corresponding 95% confidence intervals (95% CI) were assessed using a random effects model.

Results

There was a significant association between the α-adducin G460W gene polymorphism and EH in the pooled Chinese population under both an allelic genetic model (OR: 1.12, 95% CI: 1.04–1.20, P = 0.002) and a recessive genetic model (OR: 1.40, 95% CI: 1.16–1.70, P = 0.0005). In contrast, no significant association between the α-adducin G460W gene polymorphism and EH was observed in the dominant genetic model (OR: 0.88, 95% CI: 0.72–1.09, P = 0.24). In stratified analysis by ethnicity, significantly increased risk was detected in the Han subgroup under an allelic genetic model (OR: 1.13, 95% CI: 1.04–1.23, P = 0.003) and a recessive genetic model (OR: 1.43, 95% CI: 1.17–1.75, P = 0.0006).

Conclusions

In a Chinese population of mixed ethnicity, the α-adducin G460W gene polymorphism was linked to EH susceptibility, most strongly in Han Chinese.

Gene expression profiling reveals renin mRNA overexpression in human hypertensive kidneys and a role for microRNAs

The kidney has long been invoked in the etiology of essential hypertension. This could involve alterations in expression of specific genes and microRNAs (miRNAs). The aim of the present study was to identify, at the transcriptome-wide level, mRNAs and miRNAs that were differentially expressed between kidneys of 15 untreated hypertensive and 7 normotensive white male subjects of white European ancestry. By microarray technology we found 14 genes and 11 miRNAs that were differentially expressed in the medulla. We then selected and confirmed by real-time quantitative PCR expression differences for NR4A1, NR4A2, NR4A3, PER1, and SIK1 mRNAs and for the miRNAs hsa-miR-638 and hsa-let-7c. Luciferase reporter gene experiments in human kidney (HEK293) cells confirmed the predicted binding of hsa-let-7c to the 3' untranslated region of NR4A2 mRNA. In the renal cortex we found differential expression of 46 genes and 13 miRNAs. We then confirmed expression differences for AIFM1, AMBP, APOE, CD36, EFNB1, NDUFAF1, PRDX5, REN, RENBP, SLC13A1, STX4, and TNNT2 mRNAs and for miRNAs hsa-miR-21, hsa-miR-126, hsa-miR-181a, hsa-miR-196a, hsa-miR-451, hsa-miR-638, and hsa-miR-663. Functional experiments in HEK293 cells demonstrated that hsa-miR-663 can bind to the REN and APOE 3' untranslated regions and can regulate REN and APOE mRNA levels, whereas hsa-miR-181a regulated REN and AIFM1 mRNA. Our data demonstrated for the first time that miRNAs can regulate renin expression. The observed downregulation of 2 miRNAs in hypertension could explain the elevation in intrarenal renin mRNA. Renin, CD36, and other mRNAs, as well as miRNAs and associated pathways identified in the present study, provide novel insights into hypertension etiology.

Long-term regulation of Na+,K+-ATPase in opossum kidney cells by ouabain

Na(+),K(+)-ATPase, a basolateral transporter responsible for tubular reabsorption of Na(+) and for providing the driving force for vectorial transport of various solutes and ions, can also act as a signal transducer in response to the interaction with steroid hormones. At nanomolar concentrations ouabain binding to Na(+),K(+)-ATPase activates a signaling cascade that ultimately regulates several membrane transporters including Na(+),K(+)-ATPase. The present study evaluated the long-term effect of ouabain on Na(+),K(+)-ATPase activity (Na(+) transepithelial flux) and expression in opossum kidney (OK) cells with low (40) and high (80) number of passages in culture, which are known to overexpress Na(+),K(+)-ATPase (Silva et al., 2006, J Membr Biol 212, 163-175). Activation of a signal cascade was evaluated by quantification of ERK1/2 phosphorylation by Western blot. Na(+),K(+)-ATPase activity was determined by electrophysiological techniques and expression by Western blot. Incubation of cells with ouabain induced activation of ERK1/2. Long-term incubation with ouabain induced an increase in Na(+) transepithelial flux and Na(+),K(+)-ATPase expression only in OK cells with 80 passages in culture. This increase was prevented by incubation with inhibitors of MEK1/2 and PI-3K. In conclusion, ouabain-activated signaling cascade mediated by both MEK1/2 and PI-3K is responsible for long-term regulation of Na(+) transepithelial flux in epithelial renal cells. OK cell line with high number of passages is suggested to constitute a particular useful model for the understanding of ouabain-mediated regulation of Na(+) transport.

Transcriptome analysis of aldosterone-regulated genes in human vascular endothelial cell lines stably expressing mineralocorticoid receptor

A series of studies have demonstrated that endothelial cell is one of the target tissues of aldosterone. Here, we have conducted a transcriptome analysis of aldosterone-inducible genes in human endothelial cell lines stably expressing human mineralocorticoid receptor (MR) by retroviral system (MR-EAhy). We found that aldosterone in physiologic concentrations robustly induced MR-dependent transcriptional response in MR-EAhy. By DNA microarray analysis, we validated 12 aldosterone-up-regulated genes among which at least seven were concomitantly associated with increased protein expression. We also found five aldosterone-down-regulated genes. Among 11 aldosterone-up-regulated genes tested, mRNA expressions of three (ESM1, SNF1LK, ANGPTL4) were significantly up-regulated in aortic tissue from aldosterone-induced hypertensive rats compared to those from control rats, suggesting their potential pathophysiologic significance in vivo. In conclusion, using MR stably expressed human endothelial cell lines, we identified a variety of aldosterone-inducible genes, suggesting their possible roles in the development and/or the protection for aldosterone-induced vascular injury.

Salt-inducible kinase 1 is present in lung alveolar epithelial cells and regulates active sodium transport

Salt-inducible kinase 1 (SIK1) in epithelial cells mediates the increases in active sodium transport (Na(+), K(+)-ATPase-mediated) in response to elevations in the intracellular concentration of sodium. In lung alveolar epithelial cells increases in active sodium transport in response to β-adrenergic stimulation increases pulmonary edema clearance. Therefore, we sought to determine whether SIK1 is present in lung epithelial cells and to examine whether isoproterenol-dependent stimulation of Na(+), K(+)-ATPase is mediated via SIK1 activity. All three SIK isoforms were present in airway epithelial cells, and in alveolar epithelial cells type 1 and type 2 from rat and mouse lungs, as well as from human and mouse cell lines representative of lung alveolar epithelium. In mouse lung epithelial cells, SIK1 associated with the Na(+), K(+)-ATPase α-subunit, and isoproterenol increased SIK1 activity. Isoproterenol increased Na(+), K(+)-ATPase activity and the incorporation of Na(+), K(+)-ATPase molecules at the plasma membrane. Furthermore, those effects were abolished in cells depleted of SIK1 using shRNA, or in cells overexpressing a SIK1 kinase-deficient mutant. These results provide evidence that SIK1 is present in lung epithelial cells and that its function is relevant for the action of isoproterenol during regulation of active sodium transport. As such, SIK1 may constitute an important target for drug discovery aimed at improving the clearance of pulmonary edema.

Genetics of salt-sensitive hypertension

The assessment of salt sensitivity of blood pressure is difficult because of the lack of universal consensus on definition. Regardless of the variability in the definition of salt sensitivity, increased salt intake, independent of the actual level of blood pressure, is also a risk factor for cardiovascular morbidity and mortality and kidney disease. A modest reduction in salt intake results in an immediate decrease in blood pressure, with long-term beneficial consequences. However, some have suggested that dietary sodium restriction may not be beneficial to everyone. Thus, there is a need to distinguish salt-sensitive from salt-resistant individuals, but it has been difficult to do so with phenotypic studies. Therefore, there is a need to determine the genes that are involved in salt sensitivity. This review focuses on genes associated with salt sensitivity, with emphasis on the variants associated with salt sensitivity in humans that are not due to monogenic causes. Special emphasis is given to gene variants associated with salt sensitivity whose protein products interfere with cell function and increase blood pressure in transgenic mice.

Association of α-adducin and G-protein β3 genetic polymorphisms with hypertension: a meta-analysis of Chinese populations

Background

Mounting evidence has suggested that α-adducin and G-protein β3 (GNB3) genes are logical candidates for salt-sensitive hypertension. Some, but not all, studies have reported that α-adducin G460T and GNB3 C825T polymorphisms may influence the risk of the disease. To comprehensively address this issue, we performed a meta-analysis to evaluate the influence of these two polymorphisms on hypertension and potential biases in Chinese.

Methods

Data were analyzed using Stata (v11.0) and random-effects model was applied irrespective of between-studies heterogeneity, which was evaluated via subgroup and meta-regression analyses. Study quality was assessed in duplicate. Publication bias was weighed using Egger's test and funnel plot.

Results

36 study populations totaling 9042 hypertensive patients and 8399 controls were finally identified. Overall, in allelic/genotypic/dominant/recessive models, no significant association was identified for both G460T and C825T polymorphisms (P>0.05) and there was possible heterogeneity (I²>25%). Subgroup analyses by study design indicated that the magnitude of association in population-based studies was marginally significantly strengthened for α-adducin G460T allelic model (OR = 1.12; 95% CI: 1:00–1.25; P = 0.043). Moreover, subgroup analyses by geographic distribution indicated comparison of 825T with 825C yielded a marginally significant increased risk in southern Chinese only (OR = 1.48; 95% CI: 1.01–2.16; P = 0.045). Further meta-regression analyses showed that geographic regions were a significant source of between-study heterogeneity for both polymorphisms. There was a possibility of publication bias for G460T, but not for C825T.

Conclusions

Our overall results suggest null association of α-adducin G460T and GNB3 C825T polymorphisms with hypertension in Chinese but indicate local marginal significance of C825T, as a putative salt-sensitive switch, in southern Chinese.

Induction of vertebrate regeneration by a transient sodium current

Amphibians such as frogs can restore lost organs during development, including the lens and tail. To design biomedical therapies for organ repair, it is necessary to develop a detailed understanding of natural regeneration. Recently, ion transport has been implicated as a functional regulator of regeneration. Whereas voltage-gated sodium channels play a well known and important role in propagating action potentials in excitable cells, we have identified a novel role in regeneration for the ion transport function mediated by the voltage-gated sodium channel, Na(V)1.2. A local, early increase in intracellular sodium is required for initiating regeneration following Xenopus laevis tail amputation, and molecular and pharmacological inhibition of sodium transport causes regenerative failure. Na(V)1.2 is absent under nonregenerative conditions, but misexpression of human Na(V)1.5 can rescue regeneration during these states. Remarkably, pharmacological induction of a transient sodium current is capable of restoring regeneration even after the formation of a nonregenerative wound epithelium, confirming that it is the regulation of sodium transport that is critical for regeneration. Our studies reveal a previously undetected competency window in which cells retain their intrinsic regenerative program, identify a novel endogenous role for Na(V) in regeneration, and show that modulation of sodium transport represents an exciting new approach to organ repair.

Intracellular sodium sensing: SIK1 network, hormone action and high blood pressure

Sodium is the main determinant of body fluid distribution. Sodium accumulation causes water retention and, often, high blood pressure. At the cellular level, the concentration and active transport of sodium is handled by the enzyme Na(+),K(+)-ATPase, whose appearance enabled evolving primitive cells to cope with osmotic stress and contributed to the complexity of mammalian organisms. Na(+),K(+)-ATPase is a platform at the hub of many cellular signaling pathways related to sensing intracellular sodium and dealing with its detrimental excess. One of these pathways relies on an intracellular sodium-sensor network with the salt-inducible kinase 1 (SIK1) at its core. When intracellular sodium levels rise, and after the activation of calcium-related signals, this network activates the Na(+),K(+)-ATPase and expel the excess of sodium from the cytosol. The SIK1 network also mediates sodium-independent signals that modulate the activity of the Na(+),K(+)-ATPase, like dopamine and angiotensin, which are relevant per se in the development of high blood pressure. Animal models of high blood pressure, with identified mutations in components of multiple pathways, also have alterations in the SIK1 network. The introduction of some of these mutants into normal cells causes changes in SIK1 activity as well. Some cellular processes related to the metabolic syndrome, such as insulin effects on the kidney and other tissues, also appear to involve the SIK1. Therefore, it is likely that this protein, by modulating active sodium transport and numerous hormonal responses, represents a "crossroad" in the development and adaptation to high blood pressure and associated diseases.

Targeting of renal proximal tubule Na,K-ATPase by salt-inducible kinase

The renal proximal tubule (RPT) is a central locale for Na+ reabsorption, and blood pressure regulation. Na+ reabsorption in the RPT depends upon the Na,K-ATPase, which is controlled by a complex regulatory network, including Salt-Inducible Protein Kinase (SIK). SIKs are recently discovered members of the AMP-activated Protein Kinase (AMPK) family, which regulate salt homeostasis and metabolism in a number of tissues. In the RPT, SIK interacts with the Na,K-ATPase in the basolateral membrane (BM), regulating both the activity and level of Na,K-ATPase in the BM. Thus, Na,K-ATPase activity can be rapidly adjusted in response to changes in Na+ balance. Long-term changes in Na+ intake affect the state of SIK phosphorylation, and as a consequence the phosphorylation of TORCs, Transducers of Regulated CREB (cAMP Regulatory Element Binding Protein). Once phosphorylated, TORCs enter the nucleus, and activate transcription of the ATP1B1 gene encoding for the Na,K-ATPase beta subunit.

Salt, Na+,K+-ATPase and hypertension

Chronic hypertension is characterized by a persistent increase in vascular tone. Sodium-rich diets promote hypertension; however, the underlying molecular mechanisms are not fully understood. Variations in the sodium content of the diet, through hormonal mediators such as dopamine and angiotensin II, modulate renal tubule Na(+),K(+)-ATPase activity. Stimulation of Na(+),K(+)-ATPase activity increases sodium transport across the renal proximal tubule epithelia, promoting Na(+) retention, whereas inhibited Na(+),K(+)-ATPase activity decreases sodium transport, and thereby natriuresis. Diets rich in sodium also enhance the release of adrenal endogenous ouabain-like compounds (OLC), which inhibit Na(+),K(+)-ATPase activity, resulting in increased intracellular Na(+) and Ca(2+) concentrations in vascular smooth muscle cells, thus increasing the vascular tone, with a corresponding increase in blood pressure. The mechanisms by which these homeostatic processes are integrated in response to salt intake are complex and not completely elucidated. However, recent scientific findings provide new insights that may offer additional avenues to further explore molecular mechanisms related to normal physiology and pathophysiology of various forms of hypertension (i.e. salt-induced). Consequently, new strategies for the development of improved therapeutics and medical management of hypertension are anticipated.