Generalized Cellular Hypertrophy is Induced by a Dual-Acting PPAR Agonist in Rat Urinary Bladder Urothelium In Vivo

Pharmacological Utility of PPAR Modulation for Angiogenesis in Cardiovascular Disease

Peroxisome proliferator activated receptors, including PPARα, PPARβ/δ, and PPARγ, are ligand-activated transcription factors belonging to the nuclear receptor superfamily. They play important roles in glucose and lipid metabolism and are also supposed to reduce inflammation and atherosclerosis. All PPARs are involved in angiogenesis, a process critically involved in cardiovascular pathology. Synthetic specific agonists exist for all PPARs. PPARα agonists (fibrates) are used to treat dyslipidemia by decreasing triglyceride and increasing high-density lipoprotein (HDL) levels. PPARγ agonists (thiazolidinediones) are used to treat Type 2 diabetes mellitus by improving insulin sensitivity. PPARα/γ (dual) agonists are supposed to treat both pathological conditions at once. In contrast, PPARβ/δ agonists are not in clinical use. Although activators of PPARs were initially considered to have favorable effects on the risk factors for cardiovascular disease, their cardiovascular safety is controversial. Here, we discuss the implications of PPARs in vascular biology regarding cardiac pathology and focus on the outcomes of clinical studies evaluating their benefits in cardiovascular diseases.

Urinary crystal formation and urothelial effects of pyroxasulfone administered to male rats

Pyroxasulfone induced a low incidence of urinary bladder tumors in male rats in a 2-year bioassay at 1000 and 2000 ppm, with occasional urinary calculi. No increased incidence of tumors of any tissue occurred in female rats or in mice of either gender. We performed three short-term studies to evaluate early development of pyroxasulfone-induced urinary crystals and urothelial cytotoxicity with consequent regenerative proliferation. First, male rats were treated with dietary 50, 1000 or 2000 ppm pyroxasulfone for 1, 3 or 7 days. The urothelium was examined by light and scanning electron microscopy (LM, SEM) and bromodeoxyuridine labeling index (BrdU LI). In two other studies, male rats were treated with dietary 20 000 ppm pyroxasulfone for 1 week. Urine collected at various times of day was examined by SEM and energy dispersive spectroscopy (EDS) or by LM, SEM, EDS, and infrared spectroscopy (IFS). Urinary crystals were present at various time points. EDS and IFS showed some contained calcium; others contained organic matter. Cytotoxicity was detected by SEM as cellular swelling, craters, and necrosis and by LM as cellular hypertrophy. Increased cell proliferation was detected by LM (hyperplasia), SEM (piling up of round cells), and by increased BrdU LI. There was no evidence of increased apoptosis. These findings support a mode of action for pyroxasulfone-associated bladder tumors in male rats involving formation of urinary crystals leading to urothelial cytotoxicity and regenerative proliferation. This is a high dose phenomenon, therefore, pyroxasulfone is not likely to be carcinogenic to humans at exposure levels that do not cause crystals with subsequent calculi formation in the urinary tract.

A Contemporary Overview of PPARα/γ Dual Agonists for the Management of Diabetic Dyslipidemia

Background:

Diabetes mellitus and concomitant dyslipidemia, being referred to as ‘diabetic dyslipidemia’, are the foremost detrimental factors documented to play a pivotal role in cardiovascular illness. Diabetic dyslipidemia is associated with insulin resistance, high plasma triglyceride levels, low HDL-cholesterol concentration and elevated small dense LDL-cholesterol particles. Maintaining an optimal glucose and lipid levels in patients afflicted with diabetic dyslipidemia could be a major task that might require a well-planned diet-management system and regular physical activity, or otherwise an intake of combined antidiabetic and antihyperlipidemic medications. Synchronized treatment which efficiently controls insulin resistance-associated diabetes mellitus and co-existing dyslipidemia could indeed be a fascinating therapeutic option in the management of diabetic dyslipidemia. Peroxisome proliferator-activated receptors α/γ (PPARα/γ) dual agonists are such kind of drugs which possess therapeutic potentials to treat diabetic dyslipidemia. Nevertheless, PPARα/γ dual agonists like muraglitazar, naveglitazar, tesaglitazar, ragaglitazar and aleglitazar have been reported to have undesirable adverse effects, and their developments have been halted at various stages. On the other hand, a recently introduced PPARα/γ dual agonist, saroglitazar is an emerging therapeutic agent of glitazar class approved in India for the management of diabetic dyslipidemia, and its treatment has been reported to be generally safe and well tolerated.

Conclusion:

Some additional and new compounds, at initial and preclinical stages, have been recently reported to possess PPARα/γ dual agonistic potentials with considerable therapeutic efficacy and reduced adverse profile. This review sheds light on the current status of various PPARα/γ dual agonists for the management of diabetic dyslipidemia.

The Opportunities and Challenges of Peroxisome Proliferator-Activated Receptors Ligands in Clinical Drug Discovery and Development

Peroxisome proliferator-activated receptors (PPARs) are a well-known pharmacological target for the treatment of multiple diseases, including diabetes mellitus, dyslipidemia, cardiovascular diseases and even primary biliary cholangitis, gout, cancer, Alzheimer's disease and ulcerative colitis. The three PPAR isoforms (α, β/δ and γ) have emerged as integrators of glucose and lipid metabolic signaling networks. Typically, PPARα is activated by fibrates, which are commonly used therapeutic agents in the treatment of dyslipidemia. The pharmacological activators of PPARγ include thiazolidinediones (TZDs), which are insulin sensitizers used in the treatment of type 2 diabetes mellitus (T2DM), despite some drawbacks. In this review, we summarize 84 types of PPAR synthetic ligands introduced to date for the treatment of metabolic and other diseases and provide a comprehensive analysis of the current applications and problems of these ligands in clinical drug discovery and development.

Pioglitazone Use and Risk of Bladder Cancer: an In Vitro Study

Aims: Whether pioglitazone (PIO), a peroxisome proliferator-activated receptor-gamma agonist, increases the risk of developing bladder cancer has been debated for several years. The aim of this study was to investigate the in vitro effects of PIO on normal urothelial transitional epithelium (NUTE) cells and bladder cancer (J82) cells to further evaluate the risk. Methods: NUTE cells were obtained from Sprague-Dawley rats. NUTE and J82 cells were treated with different concentrations of PIO for various time periods. Cell proliferation was tested by the MTT assay. Cell apoptosis was evaluated by flow cytometry. The expressions of p53, cyclin D1, Bcl-2, and Bax were determined by qRT-PCR and western blots. Results: After 24 hours, the treatment of NUTE cells with 10 μmol/L PIO led to morphological changes, without changes in J82 cells. Moreover, PIO inhibited the proliferation and induced apoptosis of NUTE cells, but not J82 cells, in a time- and dose-dependent manner. However, PIO did not alter the growth of cells from other tissues. In addition, treatment with PIO for up to 72 hours did not result in changes in the expressions of p53, cyclin D1, Bcl-2, and Bax in NUTE cells and J82 cells. Interestingly, PIO significantly downregulated the protein levels of p53 and cyclin D1 in J82 cells, but not NUTE cells after more than 192 hours of treatment. Conclusions: PIO did not promote malignant alterations of NUTE cells or stimulate proliferation of J82 cells. PIO decreased the expression of p53 and cyclin D1 in J82 cells after long-term culture, which suggested that PIO may be helpful for diabetic patients with bladder cancer.

Deciphering the Roles of Thiazolidinediones and PPAR in Bladder Cancer

The use of thiazolidinedione (TZD) therapy in type II diabetic patients has proven useful in the lowering of blood glucose levels. However, recent investigations have shown that there may be potential health concerns associated, including the risk of developing bladder cancer as well as complications in the cardiovasculature. TZDs are ligands for the nuclear receptor PPARγ, and activation causes lipid uptake and insulin sensitization, both of which are critical processes for diabetic patients whose bodies are unable to utilize insulin effectively. Several studies have shown that PPARγ/TZDs decrease IGF-1 levels and, thus, reduce cancer growth in carcinomas such as the pancreas, colon, liver, and prostate. However, other studies have shed light on the potential of the receptor as a biomarker for uroepithelial carcinomas, particularly due to its stimulatory effect on migration of bladder cancer cells. Furthermore, PPARγ may provide the tumor-promoting microenvironment by de novo synthesis of nutrients that are needed for bladder cancer development. In this review, we closely examine the TZD class of drugs and their effects on PPARγ in patient studies along with additional molecular factors that are positive modulators, such as protein phosphatase 5 (PP5), which may have considerable implications for bladder cancer therapy.

Refocusing Peroxisome Proliferator Activated Receptor-α: A New Insight for Therapeutic Roles in Diabetes

Although glucose-lowering treatment shows some risk lowering effects in cardiovascular diseases, risks of macrovascular and microvascular complications have still remained, and development of new therapeutic strategies is needed. Recent data have shown that peroxisome proliferator activated receptor-α (PPAR-α) plays a pivotal role in the regulation of lipid homeostasis, fatty acid oxidation, cellular differentiation, and immune response such as inflammation or vascularization related to diabetic complication. This review will re-examine the metabolic role of PPAR-α, summarize data from clinical studies on the effect of PPAR-α agonist in diabetes, and will discuss the possible therapeutic role of PPAR-α activation.

Proliferative and nonproliferative lesions of the rat and mouse urinary system

The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying lesions observed in the urinary tract of rats and mice. The standardized nomenclature of urinary tract lesions presented in this document is also available electronically on the Internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous developmental and aging lesions as well as those induced by exposure to test materials. A widely accepted and utilized international harmonization of nomenclature for urinary tract lesions in laboratory animals will decrease confusion among regulatory and scientific research organizations in different countries and provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists.

Idealized PPARγ-Based Therapies: Lessons from Bench and Bedside

The incidence of type 2 (T2D) diabetes and other chronic conditions associated with insulin resistance is increasing at an alarming rate, underscoring the need for effective and safe therapeutic strategies. Peroxisome-proliferator-activated receptor gamma (PPARγ) has emerged as a critical regulator of glucose homeostasis, lipid homeostasis, and vascular inflammation. Currently marketed drugs targeting this receptor, the thiazolidinediones (TZDs), have proven benefits on insulin resistance and hyperglycemia associated with T2D. Unfortunately, they have been associated with long-term unfavorable effects on health, such as weight gain, plasma volume expansion, bone loss, cardiovascular toxicity, and possibly cancer, and these safety concerns have led to reduced interest for many PPARγ ligands. However, over the last years, data from human genetic studies, animal models, and studies with ligands have increased our understanding of PPARγ's actions and provided important insights into how ligand development strategies could be optimized to increase effectiveness and safety of PPARγ-based therapies.

The cardiovascular effects of peroxisome proliferator-activated receptor agonists

Although peroxisome proliferator-activated receptor agonists are prescribed to improve cardiovascular risk factors, their cardiovascular safety is controversial. We therefore reviewed the literature to identify landmark randomized controlled trials evaluating the effect of peroxisome proliferator-activated receptor gamma agonists (pioglitazone and rosiglitazone), alpha agonists (fenofibrate and gemfibrozil), and pan agonists (bezafibrate, muraglitazar, ragaglitazar, tesaglitazar, and aleglitazar) on cardiovascular outcomes. Pioglitazone may modestly reduce cardiovascular events but also may increase the risk of bladder cancer. Rosiglitazone increases the risk of myocardial infarction and has been withdrawn in European and restricted in the United States. Fibrates improve cardiovascular outcomes only in select subgroups: fenofibrate in diabetic patients with metabolic syndrome, gemfibrozil in patients with dyslipidemia, and bezafibrate in patients with diabetes or metabolic syndrome. The cardiovascular safety of the new pan agonist aleglitazar, currently in phase II trials, remains to be determined. The heterogenous effects of peroxisome proliferator-activated receptor agonists to date highlight the importance of postmarketing surveillance. The critical question of why peroxisome proliferator-activated receptor agonists seem to improve cardiovascular risk factors without significantly improving cardiovascular outcomes requires further investigation.

Peroxisome proliferator-activated receptor agonists and bladder cancer: lessons from animal studies

This article reviews available animal studies on the possible link between the use of peroxisome proliferator-activated receptor (PPAR) agonists and bladder cancer, with further discussion on the possible implications to humans. Carcinogenicity studies suggest that the PPARγ agonist pioglitazone and dual PPARα/γ agonists such as ragaglitazar, muraglitazar, and naveglitazar may increase the risk of bladder cancer in a dose-responsive pattern in rats. It is interesting that bladder cancer related to PPAR agonists shows remarkable species- and sex-specificity and has a predilection to occur in the ventral dome of bladder in rodents. While male rats treated with pioglitazone or muraglitazar have a higher propensity to develop bladder cancer than female rats, mice of both sexes do not develop bladder cancer even when exposed to very high doses. Direct genotoxicity or cytotoxicity of PPAR agonists is unlikely to be the mode of action because most of the parent compounds or their metabolites of the PPAR agonists are neither mutagenic nor genotoxic, and they are rarely excreted in the urine; but a receptor-mediated PPAR effect cannot be excluded. Some suggest a "urolithiasis hypothesis" referring to the formation of urinary solids and calculi, which subsequently causes bladder necrosis, regenerative proliferation, hypertrophy, and cancer. However, whether these animal findings could have human relevance is not yet fully understood. Some argue that the urolithiasis-induced bladder cancer might be rat-specific and would probably not be applicable to humans. An effect of increased urinary growth factors induced by PPAR agonists has also been proposed, but this requires more investigations. Before fully clarified, a balance between the risks and benefits of the use of pioglitazone, an approved oral antidiabetic agent that has recently been linked to an increased but not yet confirmed risk of bladder cancer in humans, should be justified for individual use.

Medicinal Chemistry and Actions of Dual and Pan PPAR Modulators

Peroxisome proliferator-activated receptor (PPAR) agonists are used as adjunct therapy in the treatment of diabetes mellitus. Fibrates, including fenofibrate, gemfibrozil, benzafibrate, ciprofibrate, and clofibrate act on PPAR alpha to reduce the level of hypertriglyceridemia. However, agonists (ligands) of PPAR-beta/delta receptors, such as tesaglitazar, muraglitazar, ragaglitazar, imiglitazar, aleglitazar, alter the body's energy substrate preference from glucose to lipids and hence contribute to the reduction of blood glucose level. Glitazones or thiazolidinediones on the other hand, bind to PPAR-gamma receptors located in the nuclei of cells. Activation of PPAR-gamma receptors leads to a decrease in insulin resistance and modification of adipocyte metabolism. They reduce hyperlipidaemia by increasing the level of ATP-binding cassette A1, which modifies extra-hepatic cholesterol into HDL. Dual or pan PPAR ligands stimulate two or more isoforms of PPAR and thereby reduce insulin resistance and prevent short- and long-term complications of diabetes including micro-and macroangiopathy and atherosclerosis, which are caused by deposition of cholesterol. This review examines the chemical structure, actions, side effects and future prospects of dual and pan PPAR agonists.

PPARgamma and MEK Interactions in Cancer

Peroxisome proliferator-activated receptor-gamma (PPARγ) exerts multiple functions in determination of cell fate, tissue metabolism, and host immunity. Two synthetic PPARγ ligands (rosiglitazone and pioglitazone) were approved for the therapy of type-2 diabetes mellitus and are expected to serve as novel cures for inflammatory diseases and cancer. However, PPARγ and its ligands exhibit a janus-face behaviour as tumor modulators in various systems, resulting in either tumor suppression or tumor promotion. This may be in part due to signaling crosstalk to the mitogen-activated protein kinase (MAPK) cascades. The genomic activity of PPARγ is modulated, in addition to ligand binding, by phosphorylation of a serine residue by MAPKs, such as extracellular signal-regulated protein kinases-1/2 (ERK-1/2), or by nucleocytoplasmic compartmentalization through the ERK activators MAPK kinases-1/2 (MEK-1/2). PPARγ ligands themselves activate the ERK cascade through nongenomic and often PPARγ-independent signaling. In the current review, we discuss the molecular mechanisms and physiological implications of the crosstalk of PPARγ with MEK-ERK signaling and its potential as a novel drug target for cancer therapy in patients.

Suppressive effects of acid-forming diet against the tumorigenic potential of pioglitazone hydrochloride in the urinary bladder of male rats

Pioglitazone hydrochloride (PIO), a peroxisome proliferator-activated receptor gamma (PPARγ) agonist, was administered orally for 85 weeks at 16 mg/kg/day to male rats fed either a diet containing 1.5% ammonium chloride (acid-forming diet) or a control diet to investigate the effects of urinary acidification induced by the acid-forming diet on the tumorigenic potential of PIO in the urinary bladder. The surviving animals at the end of the administration period were followed to the end of the 2-year study period without changes in the diet and were subjected to terminal necropsy on Week 104. The number of urinary microcrystals, evaluated by manual counting with light microscopy and by an objective method with a laser diffraction particle size analyzer, was increased by PIO on Weeks 12 and 25 and the increases were markedly suppressed by urinary acidification. Urinary citrate was decreased by PIO throughout the study period, but no changes were seen in urinary oxalate at any timepoint. The incidences of PIO-treated males bearing at least one of the advanced proliferative changes consisting of papillary hyperplasia, nodular hyperplasia, papilloma or carcinoma were significantly decreased from 11 of 82 males fed the control diet to 2 of 80 males fed the acid-forming diet. The acid-forming diet did not show any effects on the toxicokinetic parameters of PIO and its metabolites. Microcrystalluria appears to be involved in the development of the advanced stage proliferative lesions in bladder tumorigenesis induced by PIO in male rats.

PPAR alpha and PPAR gamma coactivation rapidly induces Egr-1 in the nuclei of the dorsal and ventral urinary bladder and kidney pelvis urothelium of rats

To facilitate studies of the rat bladder carcinogenicity of dual-acting PPAR alpha+gamma agonists, we previously identified the Egr-1 transcription factor as a candidate carcinogenicity biomarker and developed rat models based on coadministration of commercially available specific PPAR alpha and PPAR gamma agonists. Immunohistochemistry for Egr-1 with a rabbit monoclonal antibody demonstrated that male vehicle-treated rats exhibited minimal urothelial expression and specifically, no nuclear signal. In contrast, Egr-1 was induced in the nuclei of bladder, as well as kidney pelvis, urothelia within one day (2 doses) of oral dosing of rats with a combination of 8 mg/kg rosiglitazone and 200 mg/kg fenofibrate (specific PPAR gamma and PPAR alpha agonists, respectively). These findings were confirmed by Western blotting using a different Egr-1 antibody. Egr-1 was induced to similar levels in the dorsal and ventral bladder urothelium, arguing against involvement of urinary solids. Egr-1 induction sometimes occurred in a localized fashion, indicating physiological microheterogeneity in the urothelium. The rapid kinetics supported that Egr-1 induction occurred as a result of pharmacological activation of PPAR alpha and PPAR gamma, which are coexpressed at high levels in the rat urothelium. Finally, our demonstration of a nuclear localization supports that the Egr-1 induced by PPAR alpha and PPAR gamma coactivation in the rat urothelium may be biologically active.

Rat Urinary Bladder Carcinogenesis by Dual-Acting PPARalpha + gamma Agonists

Despite clinical promise, dual-acting activators of PPARalpha and gamma (here termed PPARalpha+gamma agonists) have experienced high attrition rates in preclinical and early clinical development, due to toxicity. In some cases, discontinuation was due to carcinogenic effect in the rat urothelium, the epithelial layer lining the urinary bladder, ureters, and kidney pelvis. Chronic pharmacological activation of PPARalpha is invariably associated with cancer in rats and mice. Chronic pharmacological activation of PPARgamma can in some cases also cause cancer in rats and mice. Urothelial cells coexpress PPARalpha as well as PPARgamma, making it plausible that the urothelial carcinogenicity of PPARalpha+gamma agonists may be caused by receptor-mediated effects (exaggerated pharmacology). Based on previously published mode of action data for the PPARalpha+gamma agonist ragaglitazar, and the available literature about the role of PPARalpha and gamma in rodent carcinogenesis, we propose a mode of action hypothesis for the carcinogenic effect of PPARalpha+gamma agonists in the rat urothelium, which combines receptor-mediated and off-target cytotoxic effects. The proposed mode of action hypothesis is being explored in our laboratories, towards understanding the human relevance of the rat cancer findings, and developing rapid in vitro or short-term in vivo screening approaches to faciliate development of new dual-acting PPAR agonist compounds.

Tissue distribution of human AKR1C3 and rat homolog in the adult genitourinary system

Human aldo-keto reductase (AKR) 1C3 (type 2 3alpha-hydroxysteroid dehydrogenase/type 5 17beta-hydroxysteroid dehydrogenase) catalyzes androgen, estrogen, and prostaglandin metabolism. AKR1C3 is therefore implicated in regulating ligand access to the androgen receptor, estrogen receptor, and peroxisome proliferator activating receptor gamma in hormone target tissues. Recent reports on close relationships between ARK1C3 and various cancers including breast and prostate cancers implicate the involvement of AKR1C3 in cancer development or progression. We previously described the characterization of an isoform-specific monoclonal antibody against AKR1C3 that does not cross-react with related, >86% sequence identity, human AKR1C1, AKR1C2, or AKR1C4, human aldehyde reductase AKR1A1, or rat 3alpha-hydroxysteroid dehydrogenase (AKR1C9). In this study, a clone of murine monoclonal antibody raised against AKR1C3 was identified and characterized for its recognition of rat homolog. Tissue distribution of human AKR1C3 and its rat homolog in adult genitourinary systems including kidney, bladder, prostate, and testis was studied by IHC. A strong immunoreactivity was detected not only in classically hormone-associated tissues such as prostate and testis but also in non-hormone-associated tissues such as kidney and bladder in humans and rats. The distribution of these two enzymes was comparable but not identical between the two species. These features warrant future studies of AKR1C3 in both hormone- and non-hormone-associated tissues and identification of the rodent homolog for establishing animal models.

Effects of PPAR agonists on proliferation and differentiation in human urothelium

Systemic treatment of rats with peroxisome proliferator-activated receptor (PPAR) agonists (mainly of dual alpha/gamma activity) has indicated that they may invoke non-genotoxic carcinogenesis in the epithelial lining of the urinary tract (urothelium). Although there is evidence in the male rat to support an indirect effect via a crystaluria-induced urothelial damage response, there is other evidence to indicate a direct signalling effect on the urothelium and hence the full implication for using these drugs in man is unclear. Numerous reports have demonstrated that PPARs are expressed within the urothelium of different species, including man, and from an early developmental stage. We have developed methods to maintain normal human urothelial (NHU) cells in culture, where the cells retain PPAR expression and express a highly proliferative phenotype, mediated via autocrine stimulation of the epidermal growth factor (EGF) receptor. We have shown that specific activation of PPARgamma results in a programme of gene expression changes associated with late/terminal cytodifferentiation, including induction of cytokeratins CK13 and CK20, tight junction-associated claudin 3, and uroplakins UPK1a and UPK2, but this is dependent upon inhibition of the signalling cascade downstream of the EGF receptor. This indicates a subtle balance in the regulation of proliferation and differentiation in urothelium, with PPARgamma agonists promoting differentiation. Our data indicate that human urothelium is a target tissue for PPARgamma signalling, but it has yet to be determined whether dual agonists could have a modulatory effect on the proliferation/differentiation balance.

Urothelial carcinogenesis in the urinary bladder of rats treated with naveglitazar, a gamma-dominant PPAR alpha/gamma agonist: lack of evidence for urolithiasis as an inciting event

Naveglitazar, a gamma-dominant peroxisome proliferator-activated receptor (PPAR) alpha/gamma dual agonist, was tested for carcinogenicity in F344 rats in a 2-year study. Changes in urine composition and urothelial morphology were characterized in a companion 18-month investigative study. A significant increase in neoplasms of the bladder occurred only in females of the high-dose group (14/60) in the carcinogenicity study. Trends toward increased cell proliferation in the urothelium were noted in both sexes at all time points evaluated in the 18-month study. Group means for urothelial mitogenesis were increased statistically significantly only in high-dose females at 12 and 18 months. Urothelial hyperplasia occurred in high-dose females at 18 months. Morphologic changes in the urothelium at earlier time points were limited to hypertrophy and decreased immunolabeling of the superficial cells for cytokeratin 20 (a marker of terminal differentiation in urothelial cells) in both males and females. No treatment-related changes in urinary parameters, including urinary sediments, were associated with the occurrence of urothelial proliferation. Urinary pH was unaffected by treatment in both males and females, but expected diurnal changes were demonstrated. Collectively, these data indicate that naveglitazar was associated with hypertrophic and proliferative effects on the urothelium, but a link with changes in urinary parameters was not demonstrated.

Trans-Species Comparison of PPAR and RXR Expression by Rat and Human Urothelial Tissues

Because some investigational peroxisome proliferator-activated receptors (PPAR) agonists cause tumors in the lower urinary tract of rats, we compared normal human and rat urothelium in terms of PPAR and retinoid X receptor (RXR) expression and proliferation-associated phenotypes. In situ, few human but most rat urothelial cells were Ki67 positive, indicating fundamental differences in cell cycle control. Rat and human urothelia expressed all 3 PPAR and the RXRα and RXRβ isoforms in a predominantly nuclear localization, indicating that they may be biologically active. However, immunolocalization differences were observed between species. First, whereas PPARα and PPARβ/δ were expressed throughout the human bladder or ureteric urothelium, in the rat urothelium PPARα was primarily, and PPARβ/δ exclusively, restricted to superficial cells. Second, RXRβ was restricted to intermediate and superficial layers of the human urothelium but tended to be absent from the rat superficial cells. Third, PPARγ expression was present throughout the urothelia of both species but was most intense in the superficial human urothelium. Species differences were also observed in the expression of PPAR and RXR isoforms between cultured rat and human urothelial cells and in the smooth muscle. Our findings highlight the unique coexpression of multiple PPAR and RXR isoforms by urothelium and suggest that species differences in PPAR function between rat and human urothelia may be explored in an in vitro setting.

Urothelial carcinogenesis in the urinary bladder of male rats treated with muraglitazar, a PPAR alpha/gamma agonist: Evidence for urolithiasis as the inciting event in the mode of action

Muraglitazar, a PPARalpha/gamma agonist, dose-dependently increased urinary bladder tumors in male Harlan Sprague-Dawley (HSD) rats administered 5, 30, or 50 mg/kg/day for up to 2 years. To determine the mode of tumor development, male HSD rats were treated daily for up to 21 months at doses of 0, 1, or 50 mg/kg while being fed either a normal or 1% NH4Cl-acidified diet. Muraglitazar-associated, time-dependent changes in urine composition, urothelial mitogenesis and apoptosis, and urothelial morphology were assessed. In control and treated rats fed a normal diet, urine pH was generally > or = 6.5, which facilitates formation of calcium-and magnesium-containing solids, particularly in the presence of other prolithogenic changes in rat urine. Urinary citrate, an inhibitor of lithogenesis, and soluble calcium concentrations were dose dependently decreased in association with increased calcium phosphate precipitate, crystals and/or microcalculi; magnesium ammonium phosphate crystals and aggregates; and calcium oxalate-containing thin, rod-like crystals. Morphologically, sustained urothelial cytotoxicity and proliferation with a ventral bladder predilection were noted in treated rats by month 1 and urinary carcinomas with a similar distribution occurred by month 9. Urothelial apoptotic rates were unaffected by muraglitazar treatment or diet. In muraglitazar-treated rats fed an acidified diet, urine pH was invariably < 6.5, which inhibited formation of calcium-and magnesium-containing solids. Moreover, dietary acidification prevented the urothelial cytotoxic, proliferative, and tumorigenic responses. Collectively, these data support an indirect pharmacologic mode of urinary bladder tumor development involving alterations in urine composition that predispose to urolithiasis and associated decreases in urine-soluble calcium concentrations.

Peroxisome proliferator-activated receptor gamma as a drug target in the pathogenesis of insulin resistance

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. The activation of PPAR-gamma, an isotype of PPARs, can either increase or decrease the transcription of target genes. The genes controlled by this form of PPAR have been shown to encode proteins or peptides that participate in the pathogenesis of insulin resistance. Insulin resistance is defined as a state of reduced responsiveness to normal circulating concentrations of insulin and it often co-exists with central obesity, hypertension, dyslipidemia, and atherosclerosis. There is substantial evidence that links obesity with insulin resistance and type-2 diabetes. The early phase of obesity-related insulin resistance has 2 components: (a) interruption of lipid homeostasis leading to the increased plasma concentration of fatty acids that is normally suppressed by the activation of PPAR-gamma, and (b) activation of factors such as cytokines depressed by PPAR-gamma that cause insulin resistance. Therefore, it is logical to suggest that activation of PPAR-gamma may partially reverse the state of insulin resistance. Evidently, activation of the nuclear receptor, PPAR-gamma, by thiazolidinediones has been reported to ameliorate insulin resistance. Although hepatotoxity and possibility to induce congestive heart failure (CHF) limit the widely use of thiazolodinediones, they are still powerful weapon to fight against insulin resistance and type-2 diabetes if use properly. This article reviews the physiology of PPAR-gamma and insulin-signaling transduction, the pathogenesis of insulin resistance in obesity-related type-2 diabetes, the pharmacological role of PPAR-gamma in insulin resistance, and additional effects of thiazolidinediones.