Urothelial Carcinogenesis in the Urinary Bladder of Male Rats Treated with Muraglitazar, a PPARα/γ Agonist: Evidence for Urolithiasis as the Inciting Event in the Mode of Action

Abstract

Muraglitazar, a PPARα/γ 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% NH 4 Cl-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 ≥ 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 γ Pathway Targeting in Carcinogenesis: Implications for Chemoprevention

“…In this setting, the combined agonists at pharmacologic concentrations result in bladder and/or kidney carcinogenesis with at least five dual agonists (10). One detailed pathologic study provides evidence that chronic effects of urolithiasis contribute to this effect (65). This has resulted in withdrawal of most of these agents from further testing.…”

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

“…Dual-acting "-glitazars" appear more potent than PPARγ agonists at inducing tumors associated with the lower urinary tract (LUT) in rats. Some studies suggested an indirect effect, in which bladder cancer resulted from regenerative responses to damage induced by precipitated urinary deposits in male rats (Cohen, 2005;Dominick et al, 2006;Tannehill-Gregg et al, 2007). However, others have suggested a direct effect of PPAR agonists on the rat bladder epithelial (urothelial) lining, based on rapid changes in urothelial gene expression and intracellular signaling, as well as early urothelial hypertrophy in treated rats (Egerod et al, 2005;Oleksiewicz et al, 2005).…”

Investigations of Rodent Urinary Bladder Carcinogens: Collection, Processing, and Evaluation of Urine and Bladders

“…It is therefore essential that the urine is collected from nonfasted animals at appropriate times during the day to reflect the various compositions of urine over a 24-hour period (Dominick et al, 2006). For most purposes, this requires fresh void urine collected either at night or shortly after the lights go on in the morning.…”