A second hit necessary for renal lithogenesis? The possible role of industrial nephrotoxins and renal tubular apoptosis

AIMS: Calcium nephrolithiasis (CN) is a multifactorial disorder in which environmental and dietary factors play a major role. Environment and diet clearly affect urine supersaturation; however, this is not sufficient to induce CN. Crystal attachment to the tubular lumen is now considered to be necessary for CN formation. However, it is hard to devise how environment and diet can affect the process of crystal attachment to the tubular epithelium. We verified the hypothesis that in the presence of mild urine supersaturation, the contemporaneous damage of the tubular epithelium, which is known to enhance crystal attachment, increases the risk of sever crystalluria. METHODS: To cause urine supersaturation, hyperoxaluria was induced in male Wistar rats by oral ethylene glycol (EG) in tap water (0.50%) for 15 days; to cause the tubular damage, hexachloro-1:3-butadiene (HCBD), 10, 25, 50 mg/kg, was i.p. administered on day 7. Adequate control groups were considered. HCBD is an industrial nephrotoxin largely diffused in the environment. In reference to the EG and HCBD concentrations, they were chosen to span from sub-optimal to very low dosages as far as effects on crystalluria and tubular epithelial damage respectively are considered. Enzymuria, proteinuria, oxaluria, crystalluria and renal pathology were investigated. RESULTS: Even the lowest HCBD dosage was effective in increasing crystalluria; cylinder-shaped crystal cluster were occasionally observed. Interestingly, crystalluria was induced also at the lowest EG concentration which by its own is not sufficient (it just induced very mild hyperoxaluria, and no damage of the tubule was evident at the electron microscope). Of interest, HCBD induced apoptosis of the renal epithelium, and a correlation was shown between crystalluria and apoptosis of the proximal tubule. CONCLUSIONS: The study suggests that: (1) an apoptotic tubular toxic agent can be necessary for crystallogenesis in borderline metabolic condition for crystalluria; (2) apoptosis may be relevant for nephrolithiasis as suggested in vitro; (3) CN may need 2nd hit (further to the metabolic predisposition, 1st “hit”) to occur; (4) the 2nd hit could be an environmental pollutant.