2,3,7,8-Tetrachlorodibenzo-p-dioxin blocks the physiological regulation of hepatic phosphoenolpyruvate carboxykinase activity in primary rat hepatocytes.

It has been previously reported that TCDD dose-dependently reduces the activity of PEPCK, the rate-limiting enzyme of hepatic gluconeogenesis. To further investigate the mechanism, whereby TCDD decreases PEPCK activity, we studied the effect of TCDD on PEPCK activity in primary rat hepatocytes (PRH). PRH were isolated from male Sprague-Dawley rats by collagenase perfusion and incubated on collagen-coated culture dishes in medium M199 containing 1 nM insulin. Cells were pretreated with dexamethasone (100 nM) 8 h before PEPCk induction was initiated by addition of glucagon (10 nM) and concurrent withdrawal of insulin. This hormonal treatment induced the enzymatic activity of PEPCK in control cells about 2-fold within 8 h. This PEPCK induction regimen was used to perform two sets of experiments. In the first set of experiments, rats were pretreated with TCDD (125 micrograms/kg p.o. in corn oil, 4 ml/kg) 4 days prior to isolation of PRH. This resulted in a complete block of the glucagon-dependent induction of PEPCK in PRH from TCDD-pretreated animals. In the second set of experiments, TCDD (100 nM) was added directly to the PRH either 24 or 48 h prior to the induction regimen. Incubation of PRH with TCDD 24 h prior to initiation of the induction regimen resulted in a slight decrease in the degree of PEPCK induction when compared to controls. However, treatment of PRH with TCDD 48 h prior to initiation of the induction regimen almost completely blocked PEPCK induction. It is, therefore, suggested that the effect of TCDD on liver PEPCK activity is due to a direct effect on liver cells and is not mediated by factors from outside the liver.