Effects of arsenite on p53, p21 and cyclin D expression in normal human fibroblasts -- a possible mechanism for arsenite's comutagenicity.

Arsenite, the most likely environmental carcinogenic form of arsenic, is not significantly mutagenic at non-toxic concentrations, but is able to enhance the mutagenicity of other agents. Evidence suggests that this comutagenic effect of arsenite is due to inhibition of DNA repair, but no specific repair enzyme has been found to be sensitive to low (<1 microM) concentrations of arsenite. To determine whether arsenite affects signaling which might alter DNA repair, this study assesses the effect of arsenite on p53-related signal transduction pathways after ionizing radiation. Long-term (14 day) low dose (0.1 microM) arsenite caused a modest increase in p53 expression in WI38 normal human fibroblasts, while only toxic (50 microM) concentrations increased p53 levels after short-term (18 h) exposure. When cells were irradiated (6 Gy), p53 and p21 protein concentrations were increased after 4h, as expected. Both long-term, low dose and short-term, high dose exposure to arsenite greatly suppressed the radiation-induced increase in p21 abundance. In addition, long-term, low dose (but not short-term, high dose) exposure to arsenite resulted in increased expression of cyclin D1. These results show that in cells treated with arsenite, p53-dependent increase in p21 expression, normally a block to cell cycle progression after DNA damage, is deficient. At the same time, low (non-toxic) exposure to arsenite enhances positive growth signaling. We suggest that the absence of normal p53 functioning, along with increased positive growth signaling in the presence of DNA damage may result in defective DNA repair and account for the comutagenic effects of arsenite.

Fasting-induced depletion of glutathione in the aging mouse.

Previous results indicate that aging is associated with a general deficiency of glutathione (GSH). As fasting is also known to lower hepatic GSH levels, we investigated the combined effects of aging and 24 hr of food deprivation on liver, kidney and blood GSH and cyst(e)ine levels in C57BL/6N mice of ages 6 (young), 12 (mature) and 24 (old) months. No age-related differences in baseline hepatic GSH were observed in these mice, consistent with previous findings where the deficiency in liver is not apparent until about 29 months of age. By 6 hr of fasting, an age-related reduction in hepatic GSH was evident, culminating in a 4-fold greater decrease during maturation, and a 5-fold greater decrease during aging (P < 0.001) compared to young animals. Liver weight also declined, decreasing total liver GSH content by 24% in young, 44% in mature, and 56% in old mice. Renal GSH and hepatic cyst(e)ine concentrations were unaffected by fasting. In young and mature mice, depletion of hepatic GSH was accompanied by a concomitant increase in blood GSH and kidney cyst(e)ine levels after 6 hr of fasting, suggesting enhancement of hepatic GSH efflux. However, in old animals, GSH depletion was associated with decreased blood GSH and kidney cyst(e)ine. Altogether, these results suggest that the stress of fasting reveals aging changes in hepatic GSH homeostasis occurring well before the GSH deficiency of aging is observed. These aging changes are likely due to decreased GSH turnover resulting from impaired biosynthesis.