INDUCTION OF PHOSPHOENOLPYRUVATE CARBOXYKINASE GENE-EXPRESSION BY RETINOIC ACID IN AN ADULT-RAT HEPATOCYTE LINE

Regulation of expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene was examined in an adult rat hepatocyte line, RALA255-10G, that was immortalized with an SV40 temperature-sensitive (ts) A mutant. These hepatocytes express a transformed phenotype at the permissive temperature (33 °C) but a differentiated liver phenotype at the nonpermissive temperature (40 °C). We have shown previously that RALA255-10G cells express only low levels of liver-specific genes such as albumin and tyrosine aminotransferase at 33 °C. In the present study, we demonstrated that at 33 °C, PEPCK synthesis and mRNA expression could be detected only in the simultaneous presence of dexamethasone (DEX), retinoic acid, and dibutyryl-cAMP (Bt2cAMP). At 40 °C, PEPCK synthesis and mRNA expression were demonstrated in the presence of Bt2cAMP alone, but not in the presence of either DEX or retinoic acid. However, at 40 °C, PEPCK gene expression was stimulated by the combination of DEX plus retinoic acid; additionally, DEX and retinoic acid potentiated the Bt2cAMP-mediated PEPCK induction. In RALA255-10G cells, optimal PEPCK gene expression required the simultaneous presence of DEX, retinoic acid, and Bt2cAMP; DEX had to be present at all times. Triiodothyronine (T3) also potentiated the Bt2cAMP-mediated PEPCK gene expression but failed to increase further the induction by DEX/retinoic acid/Bt2cAMP. By performing nuclear runoff assays, we demonstrated that the PEPCK gene transcription rate in the absence or presence of inducing agents was closely related to the levels of the corresponding mRNAs. At 40 °C, the PEPCK gene transcription rate in the presence of DEX/retinoic acid/Bt2cAMP was approximately 10-fold higher than that in the presence of DEX/Bt2cAMP, retinoic acid/Bt2cAMP, or DEX/retinoic acid. Thus, regulation of expression of the rat PEPCK gene by retinoic acid as well as cAMP and glucocorticoid hormone is primarily regulated at the transcriptional level. © 1990, American Chemical Society. All rights reserved.