DIVERGENT REGULATION OF THE CLASS-II P-GLYCOPROTEIN GENE IN PRIMARY CULTURES OF HEPATOCYTES VERSUS H35 HEPATOMA BY GLUCOCORTICOIDS

We investigated whether the glucocorticoid-mediated mechanisms control ling P-glycoprotein (pgp2 or mdr1b) are similar in normal hepatocytes compared with the H35 hepatoma cell line. In primary rat hepatocytes, dexamethasone (DEX) caused a dose- and time-dependent decrease in the amount of the pgp2 mRNA, which correlated with functional pgp2 express ion (intracellular accumulation of [H-3]vincristine). The suppression of pgp2 mRNA was specific for glucocorticoids because a representative estrogen and progestin were without effect, and DEX suppression of pg p2 mRNA could be reversed by cotreatment with an anti-glucocorticoid. DEX suppression of pgp2 mRNA appears to be posttranscriptional because following actinomycin D inhibition of new RNA synthesis, the pgp2 tra nscript disappeared at a faster rate in DEX treated versus untreated h epatocytes. Moreover, transcriptional activity of chloramphenicol acet yltransferase plasmids containing the pgp2 promoter in primary rat hep atocytes was unaffected by DEX treatment. Thus, suppression of pgp2 mR NA by glucocorticoids in primary hepatocytes is due to a decrease in p gp2 mRNA stability. In contrast, in the H35 hepatoma cell line, DEX do se dependently increased pgp protein and pgp2 mRNA, effects which para llel transcriptional activation of the pgp2 promoter. Activation of th e pgp2 promoter was specific for glucocorticoids since a representativ e estrogen had no significant effect on transcription of the pgp2 prom oter and RU486 blocked DEX activation of pgp2 transcription. Transcrip tional activation of the pgp2 promoter was not due to a global up-regu lation of basal transcription factors because DEX treatment did not ac tivate either a herpes simplex virus thymidine kinase promoter or the SV40 early gene promoter. Further studies with a panel of pgp2 5' sequ ence deletion plasmids revealed that the minimal promoter (-66 bp) was not activated by DEX. In contrast, inclusion of sequences up to -177 bp restored DEX-dependent transcriptional activation. These are the fi rst studies to demonstrate that glucocorticoids regulate pgp2 by diffe rent mechanisms in normal rat hepatoctyes compared to the H35 hepatoma cell line.