Dose response of hepatocyte replication in rats following continuous exposure to diethylnitrosamine

Chronic exposure to the hepatocellular carcinogen diethylnitrosamine (DEN) causes a dose-dependent accumulation of the promutagenic DNA adduct O4-ethyldeoxythymidine in hepatocytes and increases in the number of initiated hepatocytes as indicated by gamma-glutamyl transpeptidase positive foci. Initiation is thought to be dependent on the quantity of promutagenic DNA adducts, their efficiency for causing base pair mismatch, and the extent of replication in the target tissue. If the extent of replication is also dose dependent, then this dependence could alter the number of promutagenic DNA adducts that mispair prior to repair and enhance the clonal expansion of initiated cells. We have examined the effect of DEN on hepatocellular proliferation over a wide range of doses. Six-week-old male F-344 rats were exposed to drinking water containing 0.4, 1, 4, 10, 40, or 100 ppm DEN for 1, 4, or 10 weeks. Following exposures to DEN, rats were injected i.p. with [3H]thymidine, sections from the left, right median and anterior right lobes of the liver were processed for autoradiography and the labeling index of hepatocytes determined. A progressive increase in hepatocyte replication was induced by exposure to 40 and 100 ppm DEN. This was especially marked in the left lobe where 40 and 100 ppm DEN induced increases of 800 and 1500%, respectively, over controls after 10 weeks of exposure. Exposure to 4 and 10 ppm DEN resulted in a 300 to 400% increase in hepatocyte replication in all lobes, whereas 1 and 0.4 ppm DEN did not significantly increase cell proliferation compared to unexposed controls.