INDUCTION OF APOPTOTIC CELL-DEATH BY PARTICULATE LEAD CHROMATE - DIFFERENTIAL-EFFECTS OF VITAMIN-C AND VITAMIN-E ON GENOTOXICITY AND SURVIVAL

Certain hexavalent chromium compounds are documented human carcinogens . Exposure of cells to particulate forms of chromium results in cell-e nhanced dissolution of particles in the extracellular microenvironment and chronic production of chromium oxyanions, which are taken up by t he cell through an anion transport system and are genotoxic and clasto genic. It was previously shown that apoptosis is the mode of cell deat h of nearly all of the Chinese hamster ovary cells (CHO-AA8 cell fine) , which die after high-dose, short-term treatments with soluble sodium chromate. In this report the mode of cell killing by particulate lead chromate and of low-dose continuous treaments of soluble sodium chrom ate designed to mimic conditions of ionic chromate uptake after lead c hromate exposure was examined. CHO-AA8 cells were treated for 24 hr wi th doses of sodium chromate or lead chromate which cause a 50% decreas e in survival in colony-forming effeciency assays. Longer treatments ( up to 72 hr) at the same doses did not decrease survival further than the 24-hr exposure. Morphological changes indicative of apoptosis, as well as internucleosomal DNA fragmentation, were detectable by 24 hr a fter treatment with lead chromate or soluble sodium chromate. All of t he cells killed by treatments with lead chromate particles underwent a poptosis as the mode of cell death and this was accurately modeled in cell culture by continuous treatments with low-dose soluble sodium chr omate. Exposure of cells to hexavalent chromium compounds causes a spe ctrum of DNA damage which can be selectively altered by pretreatment o f cells with antioxidant vitamins prior to chromium exposure. Here we show that ascorbate and alpha-tocopherol markedly inhibited the chromo somal aberrations induced by both particulate and soluble chromate com pounds, even though chromium adduct levels were not decreased by eithe r vitamin pretreatment. Cell survival assays showed that ascorbate, bu t not alpha-tocopherol, protected cells from apoptosis induced by sodi um chromate. The results differentiate chromium-induced apoptosis from both chromosomal damage and adduct levels and suggest that other lesi ons sensitive to ascorbate but not tocopherol are the proximal inducin g signal for chromium-induced apoptosis. (C) 1997 Academic Press.