Role of interleukin 6 and corticosteroids in the regulation of expression of glutathione S-transferases in primary cultures of rat hepatocytes

The effect of recombinant interleukin 6 (rIL-6) on the transcript levels of rat glutathione S-transferase (GST) genes rGSTA2, rGSTP1, rGSTM1 and rGSTM2 was examined in primary cultures of rat hepatocytes. rIL-6 had little effect on the increase in expression of rGSTP1 that occurs in cultured hepatocytes. Dexamethasone (DEX), in contrast, prevented the expression of rGSTP1 by hepatocytes, and rIL-6 in combination with DEX had no additional effect. Neither rIL-6 nor DEX alone had a significant effect on the transcript levels of rGSTA2, rGSTM1 and rGSTM2 in cultured hepatocytes. However, when both were present (15 ng/ml rIL-6 and 10^-7 M DEX) the transcript levels of rGSTA2, rGSTM1 and rGSTM2 decreased significantly (P < 0.05) after 48 h in culture. If the rIL-6 was removed from the cultures after 24 h, the levels of transcripts recovered and were the same at 48 h as cells cultured without rIL-6 for the entire period. Dose-response relationships of rIL-6 with 10^-7 M DEX were determined for transcripts of each GST isoenzyme and the IC₅₀ values were between 1.5 and 7.5 ng/ml. Declines in transcript levels of rGSTA2 were observed with rIL-6 plus 10^-8 or 10^-7 M DEX but not with rIL-6 plus 10^-9, 10^-6, or 10^-5 M DEX. To determine if the cytokine and glucocorticoid effects were mediated by sequences in the 5'-flanking sequence of rGSTA2, a plasmid construct containing a 1.6 kb fragment of the 5'-flanking sequence of the rGSTA2 gene and the chloramphenicol acetyltransferase (CAT) reporter gene was used to transfect rat hepatocytes in primary culture. The addition of rIL-6 and DEX to the culture medium caused a significant (P < 0.05) decrease in CAT activity after 48 h in culture. If rIL-6 was removed after 24 h in culture, CAT activity after an additional 24 h in culture was greater than the CAT activity in cells cultured for 48 h without rIL-6. Therefore cytokines and glucocorticoids may be important physiological regulators of GST expression.