Comparative excretion and tissue distribution of selenium in mice and rats following treatment with the chemopreventive agent 1,4-phenylenebis(methylene)selenocyanate.

In a previous preliminary investigation, we reported on the excretion, tissue disposition and metabolism of the chemopreventive agent 1,4-phenylenebis(methylene)selenocyanate (p-XSC) in the rat, but similar studies in the mouse have not been explored. Following the oral administration of p-XSC (50 micromol/kg body weight), selenium excretion in feces was comparable to that in urine in mice, but in rats, feces was the major route of excretion. Tetraselenocyclophane (TSC) was the major metabolite detected in mouse and rat feces. In both species, levels of selenium in exhaled air were negligible. At termination, in the mouse, the stomach had the highest selenium content followed by liver and blood, but lung and kidney contained negligible levels of selenium; in the rat, the selenium level in liver was the highest followed by kidney, stomach, blood and lung. The identification of TSC as a fecal metabolite in both species let us to postulate the following metabolic pathway: p-XSC-->glutathione conjugate (p-XSeSG)-->a selenol (p-XSeH)-->TSC. Since the glutathione conjugate appears to be the proximal precursor for the selenol metabolite that may be an important intermediate in cancer chemoprevention, we report for the first time the synthesis of p-XSeSG and its other potential metabolites, namely the cysteine- and N-acetylcysteine-conjugates of p-XSC. HPLC analysis of the urine and bile showed a few metabolites of p-XSC; none of which eluted with the synthetic standards described above. When we examined the conversion of p-XSC and p-XSeSG in vitro using rat cecal microflora, TSC was formed from p-XSeSG but not from p-XSC. The formation of TSC from p-XSC in vivo but not in vitro suggests that p-XSC needs to be metabolized to p-XSeSG or an intermediate derived from its further metabolism. Thus, p-XSeSG was given orally to rats and the results showed that the pattern of selenium excretion after p-XSeSG treatment was similar to that of p-XSC; TSC was also identified as a fecal metabolite of p-XSeSG. It may be that the conversion of p-XSeSG to TSC is too facile, or the mere conjugation of p-XSC with glutathione does not occur in rats and mice.

Induction of preneoplastic lung lesions in guinea pigs by cigarette smoke inhalation and their exacerbation by high dietary levels of vitamins C and E.

The development of effective chemopreventive agents against cigarette smoke-induced lung cancer could be greatly facilitated by the availability of suitable laboratory animal models. Here we report that male Hartley guinea pigs treated with cigarette smoke by inhalation twice a day for 28 days developed preneoplastic lung lesions, including bronchial hyperplasia, dysplasia and squamous metaplasia, analogous to those found in human smokers. The lesions were accompanied by increased expression of proliferating cell nuclear antigen and activation of the serine/threonine kinase Akt in the bronchial epithelium. In contrast, no lung lesions were found in guinea pigs ('sham smoked') that were submitted to identical procedures but without cigarettes. Compared with a diet low in vitamin C (50 p.p.m.) and vitamin E (15 p.p.m.), a diet high in vitamin C (4000 p.p.m.) and vitamin E (40 p.p.m.) significantly increased the incidence of these lesions. The inclusion of 1,4-phenylenebis(methylene)selenocyanate (p-XSC), a synthetic chemopreventive organoselenium compound, in the high vitamin C-high vitamin E diet at a level of 15 p.p.m. as selenium appeared to decrease the lesion incidence. Administration of (-)-epigallocatechin gallate, a powerful green tea polyphenolic antioxidant, at 560 p.p.m. in the drinking water had no effect. As in human smokers, levels of ascorbate in blood plasma, lung, liver and the adrenal glands were significantly decreased by cigarette smoke inhalation. These results identify a relevant in vivo laboratory model of cigarette smoke-induced lung cancer, suggest that p-XSC may have activity as a chemopreventive agent against cigarette smoke-induced lung lesions and provide additional evidence that very high dietary levels of certain antioxidants can have co-carcinogenic activity in cigarette smoke-induced lung cancer.

Hypomethylation and hypermethylation of DNA in Wilms tumors.

We quantitatively analysed hypermethylation at CpG islands in the 5' ends of 12 genes and one non-CpG island 5' region (MTHFR) in 31 Wilms tumors. We also determined their global genomic 5-methylcytosine content. Compared with various normal postnatal tissues, approximately 40-90% of these pediatric kidney cancers were hypermethylated in four of the genes, MCJ, RASSF1A, TNFRSF12 and CALCA as determined by a quantitative bisulfite-based assay (MethyLight). Interestingly, the non-CpG island 5' region of MTHFR was less methylated in most tumors relative to the normal tissues. By chromatographic analysis of DNA digested to deoxynucleosides, about 60% of the Wilms tumors were found to be deficient in their overall levels of DNA methylation. We also analysed expression of the three known functional DNA methyltransferase genes. No relationship was observed between global genomic 5-methylcytosine levels and relative amounts of RNA for DNA methyltransferases DNMT1, DNMT3A, and DNMT3B. Importantly, no association was seen between CpG island hypermethylation and global DNA hypomethylation in these cancers. Therefore, the overall genomic hypomethylation frequently observed in cancers is probably not just a response or a prelude to hypermethylation elsewhere in the genome. This suggests that the DNA hypomethylation contributes independently to oncogenesis or tumor progression.