INDUCTION OF INTERNUCLEOSOMAL DNA FRAGMENTATION BY CARCINOGENIC CHROMATE - RELATIONSHIP TO DNA-DAMAGE, GENOTOXICITY, AND INHIBITION OF MACROMOLECULAR-SYNTHESIS

Hexavalent chromium (Cr) compounds are respiratory carcinogens in humans and animals. Treatment of Chinese hamster ovary cells with 150 and 300 mu M sodium chromate (Na2CrO4) for 2 hr decreased colony-forming efficiency by 46 and 92%, respectively. These treatments induced dose-dependent internucleosomal fragmentation of cellular DNA beyond 24 hr after chromate treatment. This fragmentation pattern is characteristic of apoptosis as a mechanism of cell death. These treatments also induced an immediate inhibition of macromolecular synthesis and delayed progression of cells through S-phase of the cell cycle. Cell growth (as evidenced by DNA synthesis) was inhibited for at least 4 days and transcription remained suppressed for at least 32 hr. Many of the cells that did progress to metaphase exhibited chromosome damage. Chromate caused the dose-dependent formation of DNA single-strand breaks and DNA-protein cross-links, but these were repaired 8 and 24 hr after removal of the treatment, respectively. In contrast, Cr-DNA adducts (up to 1/100 base-pairs) were extremely resistant to repair acid were still detectable even 5 days after treatment. Compared with other regions of the genome, DNA-protein cross-links acid Cr adducts were preferentially associated with the nuclear matrix DNA of treated cells, which was 4.5-fold enriched in actively transcribed genes. Chromium adducts, formed on DNA in vitro at a similar level to that detected in nuclear matrix DNA, arrested the progression of a DNA polymerase in a sequence-specific manner, possibly through the formation of DNA-DNA cross-links. Total RNA and mRNA synthesis and induction of expression of the inducible GRP78 gene were suppressed in a concentration- and time-dependent manner by chromate. The effects of chromate on GRP78 induction correlated most closely with the presence of DNA-protein cross-links but suppression of total RNA and mRNA synthesis correlated with the presence of DNA-Cr adducts in cells. These results suggest that the persistent Cr-DNA adducts may be responsible for the protracted eel cycle delay and transcriptional inhibition caused by chromate. Escape from apoptosis may be one of the steps invoked in chromate-induced carcinogenesis.