Peroxisome proliferator-activated receptor-γ agonists repress epithelial sodium channel expression in the kidney

PPARG agonist pioglitazone influences diurnal kidney medulla mRNA expression of core clock, inflammation-, and metabolism-related genes disrupted by reverse feeding in mice

This study examined the influence of PPARG activation by pioglitazone (PG) on the mRNA of core clock, inflammation- and metabolism-related genes in the mouse kidney medulla as well as urinary sodium/potassium excretion rhythms disrupted by reverse feeding. Mice were assigned to daytime feeding and nighttime feeding groups. PG 20 mg/kg was administered at 7 am or 7 pm. On day 8 of the feeding intervention, mice were killed at noon and midnight. Kidney medulla expression of Arntl, Clock, Nr1d1, Cry1, Cry2, Per1, Per2, Nfe2l2, Pparg, and Scnn1g was determined by qRT PCR. We measured urinary K⁺ , Na⁺ , urine volume, food, and H₂ O intake. The reverse feeding uncoupled the peripheral clock gene rhythm in mouse kidney tissues. It was accompanied by a decreased expression of Nfe2l2 and Pparg as well as an increased expression of Rela and Scnn1g. These changes in gene expressions concurred with an increase in urinary Na⁺ , K⁺ , water excretion, microcirculation disorders, and cell loss, especially in distal tubules. PG induced the restoration of diurnal core clock gene expression as well as Nfe2l2, Pparg, Scnn1g mRNA, and decreased Rela expressions, stimulating Na⁺ reabsorption and inhibiting K⁺ excretion. PG intake at 7 pm was more effective than at 7 am.

Role of the Peroxisome Proliferator Activated Receptors in Hypertension

Nuclear receptors represent a large family of ligand-activated transcription factors which sense the physiological environment and make long-term adaptations by mediating changes in gene expression. In this review, we will first discuss the fundamental mechanisms by which nuclear receptors mediate their transcriptional responses. We will focus on the PPAR (peroxisome proliferator-activated receptor) family of adopted orphan receptors paying special attention to PPARγ, the isoform with the most compelling evidence as an important regulator of arterial blood pressure. We will review genetic data showing that rare mutations in PPARγ cause severe hypertension and clinical trial data which show that PPARγ activators have beneficial effects on blood pressure. We will detail the tissue- and cell-specific molecular mechanisms by which PPARs in the brain, kidney, vasculature, and immune system modulate blood pressure and related phenotypes, such as endothelial function. Finally, we will discuss the role of placental PPARs in preeclampsia, a life threatening form of hypertension during pregnancy. We will close with a viewpoint on future research directions and implications for developing novel therapies.

Peroxisome Proliferator-Activated Receptor γ and Its Role in Adipocyte Homeostasis and Thiazolidinedione-Mediated Insulin Sensitization

Adipose tissue is a dynamic organ that makes critical contributions to whole-body metabolic homeostasis. Although recent studies have revealed that different fat depots have distinct molecular signatures, metabolic functions and adipogenic mechanisms, peroxisome proliferator-activated receptor γ (PPARγ) is still widely viewed as the master regulator of adipogenesis and critical for maintaining mature adipocyte function. Using an inducible, adipocyte-specific knockout system, we explored the role of PPARγ in mature adipocytes in vivo Short-term PPARγ deficiency in adipocytes reduces whole-body insulin sensitivity, but adipocytes are viable both in vitro and in vivo However, after exposure to a high-fat diet, even short-term PPARγ deficiency leads to rapid adipocyte death. When mature adipocytes are depleted of both PPARγ and CCAAT-enhancer-binding protein α (C/EBPα), they are rapidly depleted of lipids and undergo adipocyte death, both in vitro and in vivo Surprisingly, although thiazolidinediones (TZDs; PPARγ agonists) are thought to act mainly on PPARγ, PPARγ in adipocytes is not required for the whole-body insulin-sensitizing effect of TZDs. This offers new mechanistic aspects of PPARγ/TZD action and its effect on whole-body metabolic homeostasis.

Does Thiazolidinedione therapy exacerbate fluid retention in congestive heart failure?

The ever-growing global burden of congestive heart failure (CHF) and type 2 diabetes mellitus (T2DM) as well as their co-existence necessitate that anti-diabetic pharmacotherapy will modulate the cardiovascular risk inherent to T2DM while complying with the accompanying restrictions imposed by CHF. The thiazolidinedione (TZD) family of peroxisome proliferator-activated receptor γ (PPARγ) agonists initially provided a promising therapeutic option in T2DM owing to anti-diabetic efficacy combined with pleiotropic beneficial cardiovascular effects. However, the utility of TZDs in T2DM has declined in the past decade, largely due to concomitant adverse effects of fluid retention and edema formation attributed to salt-retaining effects of PPARγ activation on the nephron. Presumably, the latter effects are potentially deleterious in the context of pre-existing fluid retention in CHF. However, despite a considerable body of evidence on mechanisms responsible for TZD-induced fluid retention suggesting that this class of drugs is rightfully prohibited from use in CHF patients, there is a paucity of experimental and clinical studies that investigate the effects of TZDs on salt and water homeostasis in the CHF setting. In an attempt to elucidate whether TZDs actually exacerbate the pre-existing fluid retention in CHF, our review summarizes the pathophysiology of fluid retention in CHF. Moreover, we thoroughly review the available data on TZD-induced fluid retention and proposed mechanisms in animals and patients. Finally, we will present recent studies challenging the common notion that TZDs worsen renal salt and water retention in CHF.

PPARγ signaling and emerging opportunities for improved therapeutics

Peroxisome proliferator-activated receptor gamma (PPARγ) is a ligand-activated nuclear receptor that regulates glucose and lipid metabolism, endothelial function and inflammation. Rosiglitazone (RGZ) and other thiazolidinedione (TZD) synthetic ligands of PPARγ are insulin sensitizers that have been used for the treatment of type 2 diabetes. However, undesirable side effects including weight gain, fluid retention, bone loss, congestive heart failure, and a possible increased risk of myocardial infarction and bladder cancer, have limited the use of TZDs. Therefore, there is a need to better understand PPARγ signaling and to develop safer and more effective PPARγ-directed therapeutics. In addition to PPARγ itself, many PPARγ ligands including TZDs bind to and activate G protein-coupled receptor 40 (GPR40), also known as free fatty acid receptor 1. GPR40 signaling activates stress kinase pathways that ultimately regulate downstream PPARγ responses. Recent studies in human endothelial cells have demonstrated that RGZ activation of GPR40 is essential to the optimal propagation of PPARγ genomic signaling. RGZ/GPR40/p38 MAPK signaling induces and activates PPARγ co-activator-1α, and recruits E1A binding protein p300 to the promoters of target genes, markedly enhancing PPARγ-dependent transcription. Therefore in endothelium, GPR40 and PPARγ function as an integrated signaling pathway. However, GPR40 can also activate ERK1/2, a proinflammatory kinase that directly phosphorylates and inactivates PPARγ. Thus the role of GPR40 in PPARγ signaling may have important implications for drug development. Ligands that strongly activate PPARγ, but do not bind to or activate GPR40 may be safer than currently approved PPARγ agonists. Alternatively, biased GPR40 agonists might be sought that activate both p38 MAPK and PPARγ, but not ERK1/2, avoiding its harmful effects on PPARγ signaling, insulin resistance and inflammation. Such next generation drugs might be useful in treating not only type 2 diabetes, but also diverse chronic and acute forms of vascular inflammation such as atherosclerosis and septic shock.

Thiazolidinediones and Edema: Recent Advances in the Pathogenesis of Thiazolidinediones-Induced Renal Sodium Retention

Thiazolidinediones (TZDs) are one of the major classes of antidiabetic drugs that are used widely. TZDs improve insulin resistance by activating peroxisome proliferator-activated receptor gamma (PPARγ) and ameliorate diabetic and other nephropathies, at least, in experimental animals. However, TZDs have side effects, such as edema, congestive heart failure, and bone fracture, and may increase bladder cancer risk. Edema and heart failure, which both probably originate from renal sodium retention, are of great importance because these side effects make it difficult to continue the use of TZDs. However, the pathogenesis of edema remains a matter of controversy. Initially, upregulation of the epithelial sodium channel (ENaC) in the collecting ducts by TZDs was thought to be the primary cause of edema. However, the results of other studies do not support this view. Recent data suggest the involvement of transporters in the proximal tubule, such as sodium-bicarbonate cotransporter and sodium-proton exchanger. Other studies have suggested that sodium-potassium-chloride cotransporter 2 in the thick ascending limb of Henle and aquaporins are also possible targets for TZDs. This paper will discuss the recent advances in the pathogenesis of TZD-induced sodium reabsorption in the renal tubules and edema.

PPARγ agonist-induced fluid retention depends on αENaC expression in connecting tubules

BACKGROUND/AIMS

Thiazolidinediones (TZDs, like rosiglitazone (RGZ)) are peroxisome proliferator-activated receptor γ (PPARγ) agonists used to treat type 2 diabetes. Clinical limitations include TZD-induced fluid retention and body weight (BW) increase, which are inhibited by amiloride, an epithelial-sodium channel (ENaC) blocker. RGZ-induced fluid retention is maintained in mice with αENaC knockdown in the collecting duct (CD). Since ENaC in the connecting tubule (CNT) rather than in CD appears to be critical for normal NaCl retention, we aimed to further explore the role of ENaC in CNT in RGZ-induced fluid retention.

METHODS

Mice with conditional inactivation of αENaC in both CNT and CD were used (αENaC lox/lox AQP2-Cre; 'αENaC-CNT/CD-KO') and compared with littermate controls (αENaC lox/lox mice; 'WT'). BW was monitored and total body water (TBW) and extracellular fluid volume (ECF) were determined by bioelectrical impedance spectroscopy (BIS) before and after RGZ (320 mg/kg diet for 10 days).

RESULTS

On regular NaCl diet, αENaC-CNT/CD-KO had normal BW, TBW, ECF, hematocrit, and plasma Na(+), K(+), and creatinine, associated with an increase in plasma aldosterone compared with WT. Challenging αENaC-CNT/CD-KO with a low NaCl diet unmasked impaired NaCl and K homeostasis, consistent with effective knockdown of αENaC. In WT, RGZ increased BW (+6.1%), TBW (+8.4%) and ECF (+10%), consistent with fluid retention. These changes were significantly attenuated in αENaC-CNT/CD-KO (+3.4, 1.3, and 4.3%).

CONCLUSION

Together with the previous studies, the current results are consistent with a role of αENaC in CNT in RGZ-induced fluid retention, which dovetails with the physiological relevance of ENaC in this segment.

A novel MitoNEET ligand, TT01001, improves diabetes and ameliorates mitochondrial function in db/db mice

The mitochondrial outer membrane protein mitoNEET is a binding protein of the insulin sensitizer pioglitazone (5-[[4-[2-(5-ethylpyridin-2-yl)ethoxy]phenyl]methyl]-1,3-thiazolidine-2,4-dione) and is considered a novel target for the treatment of type II diabetes. Several small-molecule compounds have been identified as mitoNEET ligands using structure-based design or virtual docking studies. However, there are no reports about their therapeutic potential in animal models. Recently, we synthesized a novel small molecule, TT01001 [ethyl-4-(3-(3,5-dichlorophenyl)thioureido)piperidine-1-carboxylate], designed on the basis of pioglitazone structure. In this study, we assessed the pharmacological properties of TT01001 in both in vitro and in vivo studies. We found that TT01001 bound to mitoNEET without peroxisome proliferator-activated receptor-γ activation effect. In type II diabetes model db/db mice, TT01001 improved hyperglycemia, hyperlipidemia, and glucose intolerance, and its efficacy was equivalent to that of pioglitazone, without the pioglitazone-associated weight gain. Mitochondrial complex II + III activity of the skeletal muscle was significantly increased in db/db mice. We found that TT01001 significantly suppressed the elevated activity of the complex II + III. These results suggest that TT01001 improved type II diabetes without causing weight gain and ameliorated mitochondrial function of db/db mice. This is the first study that demonstrates the effects of a mitoNEET ligand on glucose metabolism and mitochondrial function in an animal disease model. These findings support targeting mitoNEET as a potential therapeutic approach for the treatment of type II diabetes.

Coordinate regulation between the nuclear receptor peroxisome proliferator-activated receptor-γ and cyclooxygenase-2 in renal epithelial cells

The peroxisome proliferator-activated receptors (PPARs) are ligand-dependent transcription factors involved in lipid metabolism and glucose utilization, in cell growth, differentiation and apoptosis, and in the regulation of pro-inflammatory genes expression such as cyclooxygenase-2 (COX-2). PPARγ is the main isoform in the renal inner medulla where it is believed to possess nephroprotective actions. In this kidney zone, COX-2 acts as an osmoprotective gene and its expression is modulated by changes in interstitial osmolarity. In the present work we evaluated whether hyperosmolar-induced COX-2 expression is modulated by PPARγ in renal epithelial cells MDCK subjected to high NaCl medium. The results presented herein show that ligand-activated PPARγ repressed COX-2 expression. But more important, the present findings show that hyperosmolar medium decreased PPARγ protein and increases the PPARγ phosphorylated form, which is inactive. ERK1/2 and p38 activation precedes PPARγ disappearance and induced-COX-2 expression. Therefore, the decrease in PPARγ expression is required for hyperosmotic induction of COX-2. We also found that PGE2, the main product of COX-2 in MDCK cells, induced these changes in PPARγ protein. Our results may alert on the long term use of thiazolidinediones (TZD) since they could affect renal medullary function that depends on COX-2 for cellular protection against osmotic stress.

Tribbles homolog 3 attenuates mammalian target of rapamycin complex-2 signaling and inflammation in the diabetic kidney

The endoplasmic reticulum (ER) stress response is activated in the diabetic kidney and functions to reduce ER protein accumulation and improve cellular function. We previously showed that tribbles homolog 3 (TRB3), an ER stress-associated protein, is upregulated in the diabetic kidney. Here, we investigated whether absence of TRB3 alters outcomes in diabetic nephropathy. Type 1 diabetes was induced in TRB3 wild-type and knockout ((-/-)) mice by low-dose streptozotocin, and the mice were followed for 12 weeks. Diabetic TRB3(-/-) mice developed higher levels of albuminuria and increased expression of inflammatory cytokine and chemokine mRNA in renal cortices relative to wild-type littermates, despite similar hyperglycemia. Diabetic TRB3(-/-) mice also expressed higher levels of ER stress-associated molecules in both the renal cortices and glomeruli. This change was associated with higher renal cortical phosphorylation of AKT at serine 473 (Ser(473)), which is the AKT site phosphorylated by mammalian target of rapamycin complex-2 (mTORC2). We show in renal tubular cells that TRB3 binds to mTOR and the rapamycin-insensitive companion of mTOR (Rictor), a protein specific to mTORC2. Finally, we demonstrate in murine tubular cells that TRB3 can inhibit secretion of IL-6. Thus, TRB3 reduces albuminuria and inflammatory gene expression in diabetic kidney disease by a mechanism that may involve inhibition of the mTORC2/AKT pathway and may prove to be a novel therapeutic target.

Diuretics prevent thiazolidinedione-induced cardiac hypertrophy without compromising insulin-sensitizing effects in mice

Much concern has arisen regarding critical adverse effects of thiazolidinediones (TZDs), including rosiglitazone and pioglitazone, on cardiac tissue. Although TZD-induced cardiac hypertrophy (CH) has been attributed to an increase in plasma volume or a change in cardiac nutrient preference, causative roles have not been established. To test the hypothesis that volume expansion directly mediates rosiglitazone-induced CH, mice were fed a high-fat diet with rosiglitazone, and cardiac and metabolic consequences were examined. Rosiglitazone treatment induced volume expansion and CH in wild-type and PPARγ heterozygous knockout (Pparg(+/-)) mice, but not in mice defective for ligand binding (Pparg(P465L/+)). Cotreatment with the diuretic furosemide in wild-type mice attenuated rosiglitazone-induced CH, hypertrophic gene reprogramming, cardiomyocyte apoptosis, hypertrophy-related signal activation, and left ventricular dysfunction. Similar changes were observed in mice treated with pioglitazone. The diuretics spironolactone and trichlormethiazide, but not amiloride, attenuated rosiglitazone effects on volume expansion and CH. Interestingly, expression of glucose and lipid metabolism genes in the heart was altered by rosiglitazone, but these changes were not attenuated by furosemide cotreatment. Importantly, rosiglitazone-mediated whole-body metabolic improvements were not affected by furosemide cotreatment. We conclude that releasing plasma volume reduces adverse effects of TZD-induced volume expansion and cardiac events without compromising TZD actions in metabolic switch in the heart and whole-body insulin sensitivity.

Do thiazolidinediones still have a role in treatment of type 2 diabetes mellitus?

Thiazolidinediones have been introduced in the treatment of type 2 diabetes mellitus (T2DM) since the late 1990s. Although troglitazone was withdrawn from the market a few years later due to liver toxicity, both rosiglitazone and pioglitazone gained widespread use for T2DM treatment. In 2010, however, due to increased risk of cardiovascular events associated with its use, the European Medicines Agency recommended suspension of rosiglitazone use and the Food and Drug Administration severely restricted its use. Thus pioglitazone is the only thiazolidinedione still significantly employed for treating T2DM and it is the only molecule of this class still listed in the American Diabetes Association-European Association for the Study of Diabetes 2012 Position Statement. However, as for the other thiazolidinediones, use of pioglitazone is itself limited by several side effects, some of them potentially dangerous. This, together with the development of novel therapeutic strategies approved in the last couple of years, has made it questionable whether or not thiazolidinediones (namely pioglitazone) should still be used in the treatment of T2DM. This article will attempt to formulate an answer to this question by critically reviewing the available data on the numerous advantages and the potentially worrying shortcomings of pioglitazone treatment in T2DM.

Increased susceptibility of db/db mice to rosiglitazone-induced plasma volume expansion: role of dysregulation of renal water transporters

Thiazolidinediones (TZDs), which are synthetic peroxisome proliferator-activated receptor subtype-γ (PPARγ), agonists are highly effective for treatment of type 2 diabetes. However, the side effect of fluid retention has significantly limited their application. Most of the previous studies addressing TZD-induced fluid retention employed healthy animals. The underlying mechanism of this phenomenon is still incompletely understood, particularly in the setting of disease state. The present study was undertaken to examine rosiglitazone (RGZ)-induced fluid retention in db/db mice and to further investigate the underlying mechanism. In response to RGZ treatment, db/db mice exhibited an accelerated plasma volume expansion as assessed by hematocrit (Hct) and fluorescent nanoparticles, in parallel with a greater increase in body weight, compared with lean controls. In response to RGZ-induced fluid retention, urinary Na(+) excretion and urine volume were significantly increased in lean mice. In contrast, the natriuretic and diuretic responses were significantly blunted in db/db mice. RGZ db/db mice exhibited a parallel decrease in plasma Na(+) concentration and plasma osmolality, contrasting to unchanged levels in lean controls. Imunoblotting analysis showed downregulation of renal aquaporin (AQP) 2 expression in response to RGZ treatment in lean mice but not in db/db mice. Renal AQP3 protein expression was unaffected by RGZ treatment in lean mice but was elevated in db/db mice. In contrast, the expression of Na(+)/H(+) exchanger-3 (NHE3) and NKCC2 was unchanged in either mouse strain. Together these results suggest that compared with the lean controls, db/db mice exhibited accelerated plasma volume expansion that was in part due to the inappropriate response of renal water transporters.

PPARγ signaling and metabolism: the good, the bad and the future

Thiazolidinediones (TZDs) are potent insulin sensitizers that act through the nuclear receptor peroxisome proliferator-activated receptor-γ (PPARγ) and are highly effective oral medications for type 2 diabetes. However, their unique benefits are shadowed by the risk for fluid retention, weight gain, bone loss and congestive heart failure. This raises the question as to whether it is possible to build a safer generation of PPARγ-specific drugs that evoke fewer side effects while preserving insulin-sensitizing potential. Recent studies that have supported the continuing physiologic and therapeutic relevance of the PPARγ pathway also provide opportunities to develop newer classes of molecules that reduce or eliminate adverse effects. This review highlights key advances in understanding PPARγ signaling in energy homeostasis and metabolic disease and also provides new explanations for adverse events linked to TZD-based therapy.

Thiazolidinedione-induced fluid retention: recent insights into the molecular mechanisms

Peroxisome proliferator-activated receptor- γ (PPAR γ ) agonists such as rosiglitazone and pioglitazone are used to improve insulin sensitivity in patients with diabetes mellitus. However, thiazolidinediones induce fluid retention, edema, and sometimes precipitate or exacerbate heart failure in a subset of patients. The mechanism through which thiazolidinediones induce fluid retention is controversial. Most studies suggest that this effect results from the increase in tubular sodium and water reabsorption in the kidney, but the role of specific nephron segments and sodium carriers involved is less clear. Some studies suggested that PPAR γ agonist stimulates Na(+) reabsorption in the collecting duct by activating epithelial Na(+) channel (ENaC), either directly or through serum and glucocorticoid-regulated kinase-1 (SGK-1). However, other studies did not confirm this mechanism and even report the suppression of ENaC. Alternative mechanisms in the collecting duct include stimulation of non-ENaC sodium channel or inhibition of chloride secretion to the tubular lumen. In addition, thiazolidinediones may augment sodium reabsorption in the proximal tubule by stimulating the expression and activity of apical Na(+)/H(+) exchanger-3 and basolateral Na(+)-HCO3 (-) cotransporter as well as of Na(+),K(+)-ATPase. These effects are mediated by PPAR γ -induced nongenomic transactivation of the epidermal growth factor receptor and downstream extracellular signal-regulated kinases (ERK).

Role of renal proximal tubule transport in thiazolidinedione-induced volume expansion

Thiazolidinediones (TZDs), pharmacological activators of peroxisome-proliferator-activated receptors γ (PPARγ), significantly improve insulin resistance and lower plasma glucose concentrations. However, the use of TZDs is associated with plasma volume expansion, the mechanism of which has been a matter of controversy. Originally, PPARγ-mediated enhanced transcription of the epithelial Na channel (ENaC) γ subunit was thought to play a central role in TZD-induced volume expansion. However, later studies suggested that the activation of ENaC alone could not explain TZD-induced volume expansion. We have recently shown that TZDs rapidly stimulate sodium-coupled bicarbonate absorption from renal proximal tubule (PT) in vitro and in vivo. TZD-induced transport stimulation was dependent on PPARγ/Src/EGFR/ERK, and observed in rat, rabbit and human. However, this stimulation was not observed in mouse PTs where Src/EGFR is constitutively activated. Analysis in mouse embryonic fibroblast cells confirmed the existence of PPARγ/Src-dependent non-genomic signaling, which requires the ligand binding ability but not the transcriptional activity of PPARγ. The TZD-induced enhancement of association between PPARγ and Src supports an obligatory role for Src in this signaling. These results support the view that TZD-induced volume expansion is multifactorial. In addition to the PPARγ-dependent enhanced expression of the sodium transport system(s) in distal nephrons, the PPARγ-dependent non-genomic stimulation of renal proximal transport may be also involved in TZD-induced volume expansion.