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.
Metal response element-binding transcription factor-1 (MTF-1) binds specifically to metal response elements (MREs) and transactivates metallothionein (MT) gene expression in response to zinc and cadmium. This investigation contrasts the mechanism of mouse MT gene (mMT-I) promoter activation by cadmium and zinc in IMR-32 human neuroblastoma cells to determine whether MTF-1 binding to the MRE is necessary for activation by these metals. Cadmium activated a mMT-1 promoter (؊150 base pairs) luciferase reporter 20 -25-fold through a MRE-dependent mechanism. In contrast, zinc had little effect on the mMT-1 luciferase reporter. IMR-32 cells lacked MRE binding activity, and treatment with zinc in vitro or in vivo did not generate a MTF-1⅐MRE complex, suggesting that IMR-32 cells lack functional MTF-1. Overexpression of mMTF-1 regenerated a zinc-mediated induction of the MRE without affecting cadmium activation. Because no other transition metals tested activated the MRE, this effect appeared to be cadmium-specific. These data demonstrate that in IMR-32 human neuroblastoma cells, zinc and cadmium can use independent mechanisms for activation of the mMT-I promoter and cadmium-mediated MRE activation is independent of MTF-1 and zinc.
In carcinogenicity studies with PPAR gamma and alpha/gamma agonists, urinary bladder tumors have been reported in Harlan Sprague-Dawley (HSD) and Charles River Sprague-Dawley (SD) but not Wistar (WI) rats, with urolithiasis purported to be the inciting event. In two 3-month studies, the authors investigated strain-related differences in urine composition by sampling urine multiple times daily. Urine pH, electrolytes, creatinine, protein, citrate and oxalate levels, and serum citrate were assessed; urine sediment was analyzed by scanning electron microscopy and energy dispersive x-ray spectroscopy. HSD rats had significantly higher urine calcium than SD or WI rats, primarily as calcium phosphate-containing precipitate. When compared to SD rats, HSD rats had lower urine volume, higher urine protein, and a comparable (week 4) to lower (week 13) burden of MgNH(4)PO(4) aggregates. Relative to WI rats, HSD rats had higher urine protein and magnesium and lower serum and urine citrate. Overall, the susceptibility to urolithiasis in male rats was HSD > SD > WI; this was likely due to strain-related differences in the amount of urine protein (a nidus for crystal formation), lithogenic ions, citrate (an inhibitor of lithogenesis), and/or volume. Strain-related differences in urine composition need to be considered when interpreting the outcome of studies with compounds that alter urine composition.
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