Preneoplastic lesions in kidney and carcinogenesis by non-genotoxic compounds

Molecular characterization of preneoplastic lesions provides insight on the development of renal tumors

Kidneys are the second most frequent site for chemically induced cancers in rats. However, there is still limited information on direct effects of carcinogens on pathways involved in the development of kidney tumors. Since transformed tumor cells have different characteristics than their cell of origin, it was hypothesized that healthy tissue and progressing stages of preneoplastic lesions are differentially influenced by chemical carcinogens. To elucidate this question, TSC2(-/-) Eker rats were gavaged with genotoxic aristolochic acid or nongenotoxic ochratoxin A for 3 and 6 months, respectively. Histopathology and cell proliferation analysis demonstrated a compound- and sex-specific onset of preneoplastic lesions. In contrast, comparable gene expression profiles of laser-microdissected preneoplastic lesions from carcinogen-treated and control rats, including reduced expression of genes involved in carcinogen uptake and metabolism, point to a compound-independent lesion progression. Gene expression profiles and additional immunostaining suggested that clonal expansion of renal lesions appears primarily driven by disturbed mammalian target of rapamycin complex 1 and mammalian target of rapamycin complex 2 pathway regulation. Finally, prolonged carcinogen exposure resulted in only marginal gene expression changes in tubules with normal morphology, indicating that some tubules may have adapted to the treatment. Taken together, these findings indicate that the final outcome of in vivo carcinogenicity studies is primarily determined by time-restricted initial events, while lesion progression may be a compound-independent process, involving deregulated mTOR signaling in the Eker rat model.

Characterization of the genotoxicity of nitrilotriacetic acid

The chelating agent nitrilotriacetic acid (NTA), classified as an epigenetic rodent carcinogen, was assessed in the in vivo rodent Comet assay on isolated kidney cells. Unexpected potent increases in DNA damage were obtained in both the short (3-6 hr) and long-term (22-26 hr) expression times after a single oral treatment at 1,000-2,000 mg/kg bw. NTA was assayed in the Ames test using TA1537, TA98, TA100, and TA102 tester strains, and in the in vitro micronucleus assay on L5178Y mouse lymphoma cells and on CTLL2 and CTLL2/Bcl2 cells coupled to the apoptosis measurement, both with and without metabolic activation by aroclor 1254-induced liver or kidney rat S9-mix. Whatever the S9 origin, neither genotoxicity nor apoptosis was detected, while a strong increase in the micronuclei formation was observed without S9 without any apoptosis induction. The direct genotoxicity of NTA was confirmed in the mouse lymphoma tk+/- gene mutation assay and in the chromosomal aberrations test on human lymphocytes. When tested in combination with an excess of Ca2+, NTA gave negative results on L5178Y mouse lymphoma cells in the in vitro Comet and micronucleus assays but still induced DNA damage on rat primary kidney cells. The higher sensitivity of renal cells to Ca2+ variations could explained the positive response observed in vivo. The carcinogenicity of NTA could be a consequence of the survival of kidney cells to intracellular variations of Ca2+, leading to a local and indirect genotoxicity. This suggests that threshold dose exists beyond which tumor-generating events will be displayed.