Uncoupling protein 2 mRNA expression and respiratory parameters in Kupffer cells isolated from euthyroid and hyperthyroid rat livers

Objective: The levels of uncoupling protein 2 (UCP2) mRNA and determinants of respiration (ATP synthesis, proton leak and non-mitochondrial respiration) were evaluated in Kupffer cells isolated from the livers of normal euthyroid, acute hyperthyroid and chronic hyperthyroid rats. Methods: After liver perfusion, Kupffer cells were purified by density-gradient centrifugation followed by counterflow centrifugal elutriation. UCP2 mRNA levels were measured by Northern blot and respiratory parameters by polarographic method. Results: In cells isolated from hyperthyroid (tri-iodothyronine (T3)-treated) rats, the effect of T3 treatment on the UCP2 mRNA level varied: it was more than doubled (P<0.05) in acutely T3- treated rats but, after chronic (3-week) T3 treatment, it was only 30% (not statistically significant) above the control (euthyroid) level. In Kupffer cells from the livers of chronic hyperthyroid rats, we observed an increase in total respiration rate, with an increase in the percentage attributable to the proton leak and a corresponding decrease in the percentage attributable to ATP synthesis (no alteration was observed in the percentage attributable to non-mitochondrial respiration). In the acute hyperthyroid rats, no significant differences were observed in any of the respiratory parameters, although they all tended to increase. Conclusion: These data are indicative of a possible uncoupling effect of UCP2 in Kupffer cells. T3, by enhancing the expression of UCP2, could play a role in the energy homeostasis of these cells.