Systemic PPARγ deletion causes severe disturbance in fluid homeostasis in mice

Bile acid receptors and renal regulation of water homeostasis

The kidney is the key organ responsible for maintaining the body's water and electrolyte homeostasis. About 99% of the primary urine filtered from the Bowman's capsule is reabsorbed along various renal tubules every day, with only 1-2 L of urine excreted. Aquaporins (AQPs) play a vital role in water reabsorption in the kidney. Currently, a variety of molecules are found to be involved in the process of urine concentration by regulating the expression or activity of AQPs, such as antidiuretic hormone, renin-angiotensin-aldosterone system (RAAS), prostaglandin, and several nuclear receptors. As the main bile acid receptors, farnesoid X receptor (FXR) and membrane G protein-coupled bile acid receptor 1 (TGR5) play important roles in bile acid, glucose, lipid, and energy metabolism. In the kidney, FXR and TGR5 exhibit broad expression across all segments of renal tubules, and their activation holds significant therapeutic potential for numerous acute and chronic kidney diseases through alleviating renal lipid accumulation, inflammation, oxidative stress, and fibrosis. Emerging evidence has demonstrated that the genetic deletion of FXR or TGR5 exhibits increased basal urine output, suggesting that bile acid receptors play a critical role in urine concentration. Here, we briefly summarize the function of bile acid receptors in renal water reabsorption and urine concentration.

Effects of KY-903, a Novel Tetrazole-Based Peroxisome Proliferator-Activated Receptor γ Modulator, in Male Diabetic Mice and Female Ovariectomized Rats

Peroxisome proliferator-activated receptor γ (PPARγ) modulators are expected to exert anti-diabetic effects without PPARγ-related adverse effects, such as fluid retention, weight gain, and bone loss. The present study showed that the novel tetrazole derivative KY-903 exerted similar selective PPARγ partial agonist properties to INT-131, a known PPARγ modulator, in transactivation assays, and decreased plasma glucose and triglyceride levels with increases in adiponectin levels in diabetic KK-Ay mice. These effects were similar to those of pioglitazone. Pioglitazone, but not KY-903, increased adipose tissue and heart weights. In pre-adipocytes (3T3-L1), KY-903, in contrast to pioglitazone, increased adiponectin mRNA levels without adipocyte differentiation, indicating anti-diabetic effects via adiponectin without adipogenesis. In ovariectomized rats fed a high-fat diet (OVX/HFD), KY-903 and pioglitazone decreased plasma triglyceride and non-esterified fatty acid levels and increased adiponectin levels, indicating insulin sensitization via adiponectin. KY-903 reduced body weight gain and adipose tissue weight, while pioglitazone increased heart weight and markedly reduced bone mineral density. In mesenchymal stem cell-like ST2 cells, KY-903 slightly reduced osteoblast differentiation without adipocyte differentiation, while pioglitazone markedly reduced it with adipocyte differentiation. In conclusion, KY-903 is a novel PPARγ modulator that exerts anti-diabetic effects without body weight gain or cardiac hypertrophy in diabetic mice and anti-obesity effects with minor bone loss in OVX/HFD, possibly due to increases in adiponectin levels without adipogenesis.

PPARγ and Its Agonists in Chronic Kidney Disease

Chronic kidney disease (CKD) has become a global healthcare issue. CKD can progress to irreversible end-stage renal diseases (ESRD) or renal failure. The major risk factors for CKD include obesity, diabetes, and cardiovascular diseases. Understanding the key process involved in the disease development may lead to novel interventive strategies, which is currently lagging behind. Peroxisome proliferator-activated receptor γ (PPARγ) is one of the ligand-activated transcription factor superfamily members and is globally expressed in human tissues. Its agonists such as thiazolidinediones (TZDs) have been applied as effective antidiabetic drugs as they control insulin sensitivity in multiple metabolic tissues. Besides, TZDs exert protective effects in multiple other CKD risk disease contexts. As PPARγ is abundantly expressed in major kidney cells, its physiological roles in those cells have been studied in both cell and animal models. The function of PPARγ in the kidney ranges from energy metabolism, cell proliferation to inflammatory suppression, although major renal side effects of existing agonists (including TZDs) have been reported, which limited their application in treating CKD. In the current review, we systemically assess the function of PPARγ in CKDs and the benefits and current limitations of its agonists in the clinical applications.

Obestatin ameliorates water retention in chronic heart failure by downregulating renal aquaporin 2 through GPR39, V2R and PPARG signaling

AIMS

Obestatin regulates water metabolism by inhibiting arginine vasopressin (AVP) release and upregulated obestatin has been detected in patients with chronic heart failure (CHF). However, the significance of obestatin in CHF, particularly with regard to water retention and aquaporin 2 (AQP2) expression, remains unknown.

MAIN METHODS

Using a CHF rat model, the effects of 2-week exogenous obestatin administration were evaluated. Expression of AQP2 was evaluated by immunoblotting, immunohistochemical staining, and quantitative real-time PCR (qPCR) in CHF rat model and mouse inner medullary collecting duct (mIMCD) 3 cell line. Moreover, the influence of obestatin on the genetic transcription profile in mIMCD3 cells was evaluated by microarray, and the potential regulatory mechanisms of obestatin on AQP2 were evaluated by RNA silencing of vasopressin receptor 2 (V2R), peroxisome proliferator-activated receptor gamma (PPARG), and G protein-coupled receptor 39 (GPR39).

KEY FINDINGS

Obestatin increased urinary output and improved expression of CHF biomarker without significantly altering cardiac function, plasma electrolyte concentrations, or the plasma AVP concentration. AQP2 expression was significantly reduced. The results of microarray analyses and qPCR indicated that mRNA levels of Aqp2, Pparg, and V2r were significantly decreased. Inhibition of V2r and Pparg mRNA further reduced the expression of AQP2, while the inhibitory efficacy of obestatin on AQP2 was significantly offset after Gpr39 knockdown.

SIGNIFICANCE

Long-term treatment with obestatin improves water retention in CHF by increasing urinary output through downregulation of AQP2 expression in renal IMCD cells. These effects may be at least partially mediated by regulation of GPR39, V2R and PPARG signaling.

Zhen-wu-tang attenuates Adriamycin-induced nephropathy via regulating AQP2 and miR-92b

Nephrotic syndrome (NS) is characterized by proteinuria, hypoalbuminemia and edema. The disorder of sodium and water metabolism is a critical mechanism regulating the origination and progression of NS. Zhen-wu-tang (ZWT) has been traditionally used to treat edema disease in China and Japan. The present study was carried out to assess the protective effect of ZWT in Adriamycin-induced (ADR) NS rats and investigate the potential anti-NS mechanisms of ZWT. We found that ZWT treatment ameliorate impaired kidney function and regulate water balance of kidney. Importantly, ZWT increased the expression of Aquaporin-2 (AQP2) which play key roles in maintaining body water homeostasis. Additionally, we determined miRNAs expression patterns in NS rats. Using bioinformatics prediction and miR-92b mimic or inhibitor in vitro, we identified miR-92b as a possible modulator of AQP2. Also we found that ZWT can decrease the expression of miR-92b and reverse the effect of miR-92b on AQP2 in vitro. We further demonstrated that miR-92b directly regulated AQP2 expression by targeting 3'-UTR of AQP2. These finding suggest that ZWT may reduce renal edema in Adriamycin-induced nephropathy via regulating AQP2 and miR-92b.

Morus alba Leaf Bioactives Modulate Peroxisome Proliferator Activated Receptor γ in the Kidney of Diabetic Rat and Impart Beneficial Effect

Peroxisome proliferator activated receptor gamma (PPARγ) is a ligand-activated nuclear receptor that can be activated or repressed by several exogenous and endogenous ligands and acts by modulating genes that regulate lipid, glucose, and insulin homeostasis. In kidney, PPARγ is involved in normal kidney development and other physiological functions. In our earlier report, we showed that feeding Morus alba leaves to experimental diabetic rats ameliorated diabetic nephropathy and significantly decreased microalbuminuria. In this paper, we have attempted to look into the molecular mechanism involving PPARγ modulation by mulberry leaf bioactive compounds by in vitro and in vivo methods and its impact on key inflammatory markers. In vitro assay by TR-FRET suggested that mulberry leaf extracts can serve as a putative modulator of PPARγ. High glucose conditions in vitro and in vivo increased PPARγ levels, which were ameliorated by mulberry leaves or their extracts. Interestingly, PPARγ was significantly phosphorylated at Ser112 by upstream kinases ERK42/44 in kidney of diabetic animals on feeding mulberry leaves. In vitro studies using MDCK cell line revealed that increased Ser112 phosphorylation was observed when cells were treated with bound phenolic acid rich extract but not with free phenolic acid rich extracts. HPLC analysis and bioassay-guided activity revealed that coumaric acid was the bioactive molecule within bound phenolic acid rich extract that was responsible for increased ERK42/44-mediated phosphorylation at Ser112. Furthermore, mulberry leaf bioactive compounds showed beneficial effect on the tested inflammatory markers.

The kidney-specific expression of genes can be modulated by the extracellular osmolality

With this study, we wanted to prove the hypothesis that the unique extracellular osmolality within the renal medulla modulates a specific gene expression pattern. The physiologic functions of the kidneys are mediated by the segment-specific expression of key proteins. So far, we have limited knowledge about the mechanisms that control this gene expression pattern. The hyperosmolality in the renal medullary interstitium is of major importance as a driving force for urine concentration. We made use of primarily cultured rat renal inner medullary collecting-duct cells and microarray analysis to identify genes affected by the environmental osmolality of the culture medium. We identified hundreds of genes that were either induced or repressed in expression by hyperosmolality in a time- and osmolality-dependent fashion. Further analysis demonstrated that many of them, physiologically, showed a kidney- and even collecting-duct-specific expression, including secreted proteins, kinases, and transcription factors. On the other hand, we identified factors, down-regulated in expression, that have a diuretic effect. In conclusion, the kidney is the only organ that has such a hyperosmotic environment, and study provides an excellent method for controlling tissue-specific gene expression.-Schulze Blasum, B., Schröter, R., Neugebauer, U., Hofschröer, V., Pavenstädt, H., Ciarimboli, G., Schlatter E., Edemir, B. The kidney-specific expression of genes can be modulated by the extracellular osmolality.

Nuclear Receptor Regulation of Aquaporin-2 in the Kidney

Aquaporin-2 (AQP2) is a vasopressin-regulated water channel responsible for regulating water reabsorption through the apical plasma membrane of the principal cells of renal collecting ducts. It has been found that dysregulation and dysfunction of AQP2 cause many disorders related to water balance in people and animals, including polyuria and dilutional hyponatremia. Classically, AQP2 mRNA and protein expression and its membrane translocation are regulated by systemic vasopressin involving short-term regulation of AQP2 trafficking to and from the apical plasma membrane and long-term regulation of the total amount of the AQP2 protein in the cell. Recently, increasing evidence has demonstrated that collecting duct AQP2 expression and membrane translocation are also under the control of many other local factors, especially nuclear receptors. Here, we briefly review the progress of studies in this area and discuss the role of nuclear receptors in the regulation of water reabsorption via affecting AQP2 expression and function.

Current Advances in the Biochemical and Physiological Aspects of the Treatment of Type 2 Diabetes Mellitus with Thiazolidinediones

The present review summarizes the current advances in the biochemical and physiological aspects in the treatment of type 2 diabetes mellitus (DM2) with thiazolidinediones (TZDs). DM2 is a metabolic disorder characterized by hyperglycemia, triggering the abnormal activation of physiological pathways such as glucose autooxidation, polyol's pathway, formation of advance glycation end (AGE) products, and glycolysis, leading to the overproduction of reactive oxygen species (ROS) and proinflammatory cytokines, which are responsible for the micro- and macrovascular complications of the disease. The treatment of DM2 has been directed toward the reduction of hyperglycemia using different drugs such as insulin sensitizers, as the case of TZDs, which are able to lower blood glucose levels and circulating triglycerides by binding to the nuclear peroxisome proliferator-activated receptor gamma (PPARγ) as full agonists. When TZDs interact with PPARγ, the receptor regulates the transcription of different genes involved in glucose homeostasis, insulin resistance, and adipogenesis. However, TZDs exhibit some adverse effects such as fluid retention, weight gain, hepatotoxicity, plasma-volume expansion, hemodilution, edema, bone fractures, and congestive heart failure, which limits their use in DM2 patients.