WNK1 is a chloride-stimulated scaffold that regulates mTORC2 activity and ion transport.

Mammalian (or mechanistic) target of rapamycin complex 2 (mTORC2) is a kinase complex that targets predominantly Akt family proteins, SGK1 and protein kinase C (PKC), and has well-characterized roles in mediating hormone and growth factor effects on a wide array of cellular processes. Recent evidence suggests that mTORC2 is also directly stimulated in renal tubule cells by increased extracellular K+ concentration, leading to activation of the Na+ channel, ENaC, and increasing the electrical driving force for K+ secretion. We identify here a signaling mechanism for this local effect of K+. We show that an increase in extracellular [K+] leads to a rise in intracellular chloride (Cl-), which stimulates a previously unknown scaffolding activity of the protein 'with no lysine-1' (WNK1) kinase. WNK1 interacts selectively with SGK1 and recruits it to mTORC2, resulting in enhanced SGK1 phosphorylation and SGK1-dependent activation of ENaC. This scaffolding effect of WNK1 is independent of its own kinase activity and does not cause a generalized stimulation of mTORC2 kinase activity. These findings establish a novel WNK1-dependent regulatory mechanism that harnesses mTORC2 kinase activity selectively toward SGK1 to control epithelial ion transport and electrolyte homeostasis.

Aldosterone-independent regulation of K + secretion in the distal nephron.

PURPOSE OF REVIEW: Maintenance of plasma K + concentration within a narrow range is critical to all cellular functions. The kidneys are the central organ for K + excretion, and robust renal excretory responses to dietary K + loads are essential for survival. Recent advances in the field have challenged the view that aldosterone is at the center of K + regulation. This review will examine recent findings and propose a new mechanism for regulating K + secretion. RECENT FINDINGS: Local aldosterone-independent response systems in the distal nephron are increasingly recognized as key components of the rapid response to an acute K + load, as well as playing an essential role in sustained responses to increased dietary K + . The master kinase mTOR, best known for its role in mediating the effects of growth factors and insulin on growth and cellular metabolism, is central to these aldosterone-independent responses. Recent studies have shown that mTOR, particularly in the context of the "type 2" complex (mTORC2), is regulated by K + in a cell-autonomous fashion. SUMMARY: New concepts related to cell-autonomous K + signaling and how it interfaces with aldosterone-dependent regulation are emerging. The underlying signaling pathways and effectors of regulated K + secretion, as well as implications for the aldosterone paradox and disease pathogenesis are discussed.

Potassium Activates mTORC2-dependent SGK1 Phosphorylation to Stimulate Epithelial Sodium Channel: Role in Rapid Renal Responses to Dietary Potassium.

SIGNIFICANCE STATEMENT: Rapid renal responses to ingested potassium are essential to prevent hyperkalemia and also play a central role in blood pressure regulation. Although local extracellular K + concentration in kidney tissue is increasingly recognized as an important regulator of K + secretion, the underlying mechanisms that are relevant in vivo remain controversial. To assess the role of the signaling kinase mTOR complex-2 (mTORC2), the authors compared the effects of K + administered by gavage in wild-type mice and knockout mice with kidney tubule-specific inactivation of mTORC2. They found that mTORC2 is rapidly activated to trigger K + secretion and maintain electrolyte homeostasis. Downstream targets of mTORC2 implicated in epithelial sodium channel regulation (SGK1 and Nedd4-2) were concomitantly phosphorylated in wild-type, but not knockout, mice. These findings offer insight into electrolyte physiologic and regulatory mechanisms. BACKGROUND: Increasing evidence implicates the signaling kinase mTOR complex-2 (mTORC2) in rapid renal responses to changes in plasma potassium concentration [K + ]. However, the underlying cellular and molecular mechanisms that are relevant in vivo for these responses remain controversial. METHODS: We used Cre-Lox-mediated knockout of rapamycin-insensitive companion of TOR (Rictor) to inactivate mTORC2 in kidney tubule cells of mice. In a series of time-course experiments in wild-type and knockout mice, we assessed urinary and blood parameters and renal expression and activity of signaling molecules and transport proteins after a K + load by gavage. RESULTS: A K + load rapidly stimulated epithelial sodium channel (ENaC) processing, plasma membrane localization, and activity in wild-type, but not in knockout, mice. Downstream targets of mTORC2 implicated in ENaC regulation (SGK1 and Nedd4-2) were concomitantly phosphorylated in wild-type, but not knockout, mice. We observed differences in urine electrolytes within 60 minutes, and plasma [K + ] was greater in knockout mice within 3 hours of gavage. Renal outer medullary potassium (ROMK) channels were not acutely stimulated in wild-type or knockout mice, nor were phosphorylation of other mTORC2 substrates (PKC and Akt). CONCLUSIONS: The mTORC2-SGK1-Nedd4-2-ENaC signaling axis is a key mediator of rapid tubule cell responses to increased plasma [K + ] in vivo . The effects of K + on this signaling module are specific, in that other downstream mTORC2 targets, such as PKC and Akt, are not acutely affected, and ROMK and Large-conductance K + (BK) channels are not activated. These findings provide new insight into the signaling network and ion transport systems that underlie renal responses to K +in vivo .

Atypical Glandular Cells of Endometrial Origin and the Risk of Endometrial Cancer.

OBJECTIVES: To assess the risk of endometrial cancer (EC) associated with atypical glandular cells of endometrial origin (AGC-EM) in 2 age groups (age younger than 51 vs 51 years or older). METHODS: A retrospective case series was assembled identifying AGC from a pathology database between January 1, 2005 and January 1, 2009. Demographics, cervical cytology results, and final diagnoses (including clinically significant diseases and cancers) were recorded from the initial AGC diagnosis until August 30, 2011. Data were analyzed using the χ test to compare rates of disease between age groups. RESULTS: Among the 444 patients with AGC, 41% (183/444) had AGC-EM. Women younger than 51 years, compared to those 51 years or older, had significantly lower rates of AGC-EM (35% [105/296] vs 53% [78/148]; p < .001; odds ratio, 0.49; 95% confidence interval, 0.33-0.74). The rate of EC was significantly lower in those younger than 51 years, compared to those aged 51 or older (5% [8/158] vs 19% [18/95]; p < .001; odds ratio, 0.23; 95% confidence interval, 0.09-0.55) in women who underwent endometrial biopsy. In women younger than 51 years who underwent an endometrial biopsy, the rate of EC had a stepwise increase across 3 subclasses of AGC (from AGC of endocervical origin [AGC-EC] to AGC not otherwise specified to AGC-EM) (p = .04). CONCLUSIONS: Women aged 51 years or older who have AGC are more likely to have AGC-EM and EC than women younger than 51 years. In women younger than age 51, AGC-EM is the subclass most associated with EC while compared to 2 other subclasses (AGC not otherwise specified and AGC-EC).

Coordinated Regulation of Renal Glucose Reabsorption and Gluconeogenesis by mTORC2 and Potassium.

Background: The kidney proximal tubule is uniquely responsible for reabsorption of filtered glucose and gluconeogenesis (GNG). Insulin stimulates glucose transport and suppresses GNG in the proximal tubule, however, the signaling mechanisms and coordinated regulation of these processes remain poorly understood. The kinase complex mTORC2 is critical for regulation of growth, metabolism, solute transport, and electrolyte homeostasis in response to a wide array of inputs. Here we examined its role in the regulation of renal glucose reabsorption and GNG. Methods: Rictor, an essential component of mTORC2, was knocked out using the Pax8-LC1 system to generate inducible tubule specific Rictor knockout (TRKO) mice. These animals were subjected to fasting, refeeding, and variation in dietary K + . Metabolic parameters including glucose homeostasis and renal function were assessed in balance cages. Kidneys and livers were also harvested for molecular analysis of gluconeogenic enzymes, mTORC2-regulated targets, and plasma membrane glucose transporters. Results: On a normal chow diet, TRKO mice had marked glycosuria despite indistinguishable blood glucose relative to WT controls. Kidney plasma membrane showed lower SGLT2 and SGLT1 in the fed state, supporting reduced renal glucose reabsorption. Additional metabolic testing provided evidence for renal insulin resistance with elevated fasting insulin, impaired pyruvate tolerance, elevated hemoglobin A1c, and increased renal gluconeogenic enzymes in the fasted and fed states. These effects were correlated with reduced downstream phosphorylation of Akt and the transcription factor FOXO4, identifying a novel role of FOXO4 in the kidney. Interestingly, high dietary K + prevented glycosuria and excessive GNG in TRKO mice, despite persistent reduction in mTORC2 substrate phosphorylation. Conclusion: Renal tubule mTORC2 is critical for coordinated regulation of sodium-glucose cotransport by SGLT2 and SGLT1 as well as renal GNG. Dietary K + promotes glucose reabsorption and suppresses GNG independently of insulin signaling and mTORC2, potentially providing an alternative signaling mechanism in states of insulin resistance. SIGNIFICANCE STATEMENT: The kidney contributes to regulation of blood glucose through reabsorption of filtered glucose and gluconeogenesis. This study shows that mTORC2 and dietary potassium coordinate the regulation of sodium-glucose cotransport and glucose production in the kidney via independent mechanisms. New insights into the regulation of these processes in the kidney offer promising implications for diabetes mellitus management and treatment.

Urinary Plasmin(ogen) as a Prognostic Factor for Hypertension.

INTRODUCTION: Plasmin and its precursor, plasminogen, are detectable in urine from patients with glomerular disease. Urinary plasmin(ogen) levels correlate with blood pressure (BP) and may contribute to renal Na+ retention by activating the epithelial Na+ channel (ENaC). In a longitudinal nested-cohort study, we asked whether urinary plasmin(ogen) levels predict subsequent increase in BP, incident hypertension, or mortality in subjects with type I diabetes, who often develop proteinuria. METHODS: The Pittsburgh Epidemiology of Diabetes Complications (EDC) study followed up type I diabetic subjects for 25 years. Urine specimens from 70 subjects with a spectrum of baseline urinary albumin levels were examined. Outcomes included increased BP after 2 years (≥1 SD over baseline systolic or diastolic BP, examined via logistic regression), 25-year incident hypertension (≥140/90 mm Hg or initiating BP-lowering medications), and all-cause or cardiovascular mortality, examined using Cox regression. RESULTS: Subjects experiencing a 2-year increase in BP had higher baseline urinary plasmin(ogen)/creatinine levels (uPl/Cr) than other subjects (P = 0.04); the difference in baseline urinary albumin/creatinine levels (uAlb/Cr) was similar (P = 0.07). Baseline uPl/Cr was associated with increased 25-year hypertension incidence (hazard ratio = 2.05, P = 0.001), all-cause mortality (HR = 2.05, P = 0.01) and cardiovascular mortality (HR = 3.30, P = 0.005), although not independent of uAlb/Cr. CONCLUSION: This is the first long-term prospective study addressing clinical outcomes associated with increased urinary plasmin(ogen). Findings are consistent with a role for plasmin(ogen) in promoting increased BP, but also demonstrate the difficulty in distinguishing effects due to plasmin(ogen) from those of albuminuria.

Trial of Amiloride in Type 2 Diabetes with Proteinuria.

INTRODUCTION: Renal Na+ retention and extracellular fluid volume expansion are hallmarks of nephrotic syndrome, which occurs even in the absence of activation of hormones that stimulate renal Na+ transporters. Plasmin-dependent activation of the epithelial Na+ channel (ENaC) has been proposed to have a role in renal Na+ retention in the setting of nephrotic syndrome. We hypothesized that the ENaC inhibitor amiloride would be an effective therapeutic agent in inducing a natriuresis and lowering blood pressure in individuals with macroscopic proteinuria. METHODS: We conducted a pilot double-blind randomized cross-over study comparing the effects of daily administration of either oral amiloride or hydrochlorothiazide (HCTZ) to patients with type 2 diabetes and macroscopic proteinuria. Safety and efficacy were assessed by monitoring systolic blood pressure (SBP), kidney function, adherence, weight, urinary Na+ excretion and serum electrolytes. Nine subjects were enrolled in the trial. RESULTS: No significant difference in SBP or weight was seen between HCTZ and amiloride (p≥0.15). Amiloride induced differences in serum K+ (p<0.001), with a 0.88±0.30 mmol/L greater acute increase observed. Two subjects developed acute kidney injury and hyperkalemia when treated with amiloride. Four subjects had readily detectable levels of urinary plasminogen plus plasmin (uPl), and five did not. Changes in SBP in response to amiloride did not differ between individuals with vs. those without detectable uPl. CONCLUSION: In summary, among patients with type 2 diabetes, normal renal function and proteinuria, there were reductions in SBP in groups treated with HCTZ or amiloride. Acute kidney injury and severe hyperkalemia were safety concerns with amiloride.

Vitamin D deficiency and cardiovascular disease in postmenopausal women: contributions from human and nonhuman primate studies.

OBJECTIVE: Cardiovascular disease (CVD) is the leading cause of death among American postmenopausal women and all adult Americans. The medical community and the lay community have recently become intrigued with vitamin D and its potential role in reducing the risk of CVD. Research findings from multiple retrospective studies, few prospective studies, and recent nonhuman primate studies have been inconsistent and conflicting. The objective of this study is to review what is known about the topic, what questions remain unanswered, and where the research community should be focusing. METHODS: A literature search was conducted through PubMed and Google Scholar up to August 1, 2014. One hundred six articles, including 18 double-blind, placebo-controlled, randomized clinical trials, relevant to the study topic were identified. All studies were stratified based on study design and primary outcome. The effects of vitamin D on CVD were reviewed and summarized. RESULTS: Although there is an abundance of observational studies suggesting an association with CVD protection, the most well-controlled randomized human trial data available show no benefit of vitamin D on CVD. However, highly controlled nonhuman primate studies indicate a beneficial relationship. CONCLUSIONS: Well-designed research, with CVD as primary outcome, is needed to help bridge the gap in our knowledge on this topic. In the meantime, caution should be applied to avoid overdiagnosis and overtreatment of vitamin D deficiency.

Age as a predictor of osteoporotic fracture compared with current risk-prediction models.

OBJECTIVE: To compare several fracture risk-prediction models and their predictive values. METHODS: Women older than age 49 years were sent for dual-energy X-ray absorptiometry screening between January 2007 and March 2009. Data collection included multiple osteoporosis risk factors. The ability to identify fractures was analyzed and compared using the North American Menopause Society 2006 and 2010 Position Statements, The Fracture Risk Assessment Tool, along with age alone. The area under the curve (AUC) comparison with chance (AUC 0.50) and paired AUC comparisons between models were used to investigate the efficacy of each model in predicting osteoporotic fractures. RESULTS: Among the 615 women studied, with mean (standard deviation) age of 61.4 (8.3) years and 94.5% being white, 15 have experienced a fracture. All screening approaches were significantly better than chance at predicting fractures. Paired comparisons of the detection ability of fracture prediction models showed no significant differences. Age alone was a significant predictor for fracture (AUC 0.79, 95% confidence interval [CI] 0.67-0.91, P<.001) with the optimal cutoff age of 65 years, which was associated with a sensitivity (95% CI) of 80% (77-83%) and specificity (95% CI) of 73% (70-77%). Compared with young postmenopausal women (younger than 65 years), the odds ratio (95% CI) of fractures in older women (65 years or older) is 10.2 (2.32-44.97). In addition, when age was added, it significantly increased the AUC of each model. CONCLUSION: These data suggest that all current screening modalities are effective in predicting fracture but not significantly better than age alone. Age should be considered carefully while evaluating patients for osteoporosis screening and treatment. LEVEL OF EVIDENCE: II.