Halocarbon hepatotoxicity is not initiated by Ca²⁺-stimulated endonuclease activation

Previous studies from this laboratory have demonstrated that cytosolic Ca²⁺ rapidly rises to supraphysiologic levels in liver cells exposed to the hepatotoxins carbon tetrachloride (CCl₄) and 1,1-dichloroethylene (DCE) in vivo and in vitro. The present study examines whether this increase in intracellular Ca²⁺ activates endonucleases that could initiate or contribute to the ensuing hepatotoxic events. Initial experiments demonstrated that there was no generalized breakdown of hepatic DNA in intact rats exposed to CCl₄ and DCE, as assessed by the appearance of nucleosomal fragments in liver nuclear DNA separated on agarose gels. Nor was generalized fragmentation observed in DNA isolated from primary hepatocyte cultures exposed to halocarbons, except at very late times following loss of plasma membrane integrity. Endonuclease activation was further examined at a more sensitive level by specifically monitoring hypersensitive sites (HSS) in serum albumin gene. Actively transcribed genes, such as albumin in liver tissue, are extremely sensitive to attack by exogenous nucleolytic enzymes at discrete sites. We speculated that subtle halocarbon-induced endonuclease activation would first become evident at these sites. To locate HSS, DNA was digested with restriction enzymes Eco R1 or Hind III, electrophoresed on agarose gels, blotted onto nitrocellulose, and hybridized to a ³²P-labeled 1400 bp rat albumin genomic clone. No cleavage at hypersensitive sites was detected in DNA isolated from rat liver or hepatocyte DNA at early times when elevations of Ca²⁺ were developing. Thus, these data indicate that endonuclease activation by intracellular Ca²⁺ and resultant nucleolytic destruction of DNA is not an early event in the hepatotoxicity produced by halocarbons.