Down‐regulation of particulate protein kinase Cϵ and up‐regulation of nuclear activator protein–1 DNA binding in liver following in vivo exposure of B6C3F1 male mice to heptachlor epoxide

The effects of in vivo administration of the cyclodiene tumor promoter heptachlor epoxide on mouse liver protein kinase C were studied in male B6C3F1 mice by protein kinase C activity assays and Western blotting under conditions known to increase the incidence of hepatocellular carcinoma because protein kinase C is thought to be critical in phorbol ester–induced tumor promotion. Under these test conditions, 20 ppm dietary heptachlor epoxide for 1–20 days increased cytosolic and decreased particulate total protein kinase C activities, while 10 ppm had no effect. Further, total cytosolic and particulate protein kinase C activities were decreased within 1 hour by 10 mg/kg intraperitoneal (i.p.) heptachlor epoxide. Western blotting showed that conventional protein kinase Cα and β isoforms were unaffected by heptachlor epoxide. Particulate novel protein kinase Cϵ, however, was selectively down‐regulated by 1, 10, and 20 ppm dietary heptachlor epoxide, whereas the cytosolic isoform was decreased by 1 and 10 ppm heptachlor epoxide for 10 days. The high‐dose treatment for 24 hours also decreased particulate novel protein kinase Cϵ but increased the cytosolic titer. These results demonstrate that this isoform is unique in its sensitivity to heptachlor epoxide. Activator protein–1 DNA binding, a critical factor in tumor promotion, was substantially increased at 3 and 6 hours with 3.7 mg/kg (i.p.) heptachlor epoxide and at 3 and 10 days with 20 ppm dietary heptachlor epoxide. The effects of heptachlor epoxide on protein kinase C and activator protein–1 are similar to those caused by phorbol ester treatments and correlate well to heptachlor levels found to induce tumors in mice. However, heptachlor epoxide did not initially activate protein kinase C with in vivo treatments or with in vitro treatments of a plasma membrane fraction aimed at demonstrating direct activation, as has been shown for phorbol esters. The ability of heptachlor epoxide to down‐regulate particulate novel protein kinase Cϵ correlates to dosages used in in vivo tumor promotion studies. However, this may represent a negative feedback response rather than a causative effect. © 2001 John Wiley & Sons, Inc. J Biochem Mol Toxicol 15:1–14, 2001