The cellular and molecular effects of hexavalent chromium as related to growth arrest, apoptosis, and neoplastic progression

Certain hexavalent chromium [Cr(VI)] compounds are known occupational respiratory tract toxins and carcinogens. Cr(VI) elicits a broad spectrum of genotoxic lesions, some of which may be involved in cellular responses that lead to cell cycle arrest, cytotoxicity, or neoplastic transformation. Apoptosis is the predominant mode of cell death caused by Cr(VI) exposure, and doses of Cr(VI) that induce some level of apoptosis are usually required to induce neoplastic transformation. Thus, it is important to understand the interrelationship between Cr(VI)-induced genotoxicity, apoptosis, and neoplastic progression. In these studies, we show that Cr(VI) exposure causes dose-dependent Cr-DNA adduct formation in primary human small airway epithelial cells (SAEC) and normal diploid human lung fibroblasts (HLF). Cr-DNA adducts may serve as a toxicologically-salient biomarker and molecular dosimeter for Cr bioavailability and genomic interactions. Thus, we correlated adduct levels with levels of Cr(VI)-induced growth arrest and apoptosis. We also undertook a dynamic population study to assess cumulative cellular fates of cell populations exposed to a narrow range of increasingly genotoxic doses of Cr(VI). We show that the spectrum of possible cellular fates (terminal growth arrest, apoptosis, and continued replicative survival) adjusts as the level of genotoxicity increases. Our results suggest that the regaining of replicative potential after genotoxic damage is due to either escape from, or resistance to, apoptosis. We show that Cr(VI)-induced apoptosis involves the disruption of mitochondrial stability as indicated by disruption of the mitochondrial membrane potential and release of cytochrome c from the mitochondria to the cytosol. Our results suggest that the critical point in the apoptotic pathway in which cells are irreversibly committed to death occurs at the mitochondria. Finally, we show that clonogenic survivors of a genotoxic Cr(VI) exposure have a reduced susceptibility to apoptosis induced by a subsequent Cr(VI) challenge. Our results suggest that these survivor cells have a selective advantage over the rest of the population due to an intrinsic or induced resistance to apoptosis. If early tumor growth requires a net accumulation of cells due to disruption of cell growth/loss homeostasis, then selection of cells with resistance to apoptosis may facilitate the early steps of carcinogenesis.