REGULATION OF GENE-EXPRESSION OF BRANCHED-CHAIN KETO ACID DEHYDROGENASE COMPLEX IN PRIMARY CULTURED-HEPATOCYTES BY DEXAMETHASONE AND A CAMPANALOG

The present study demonstrates that dexamethasone and 8-(4-chloropheny lthio)adenosine 3',5'-monophosphate (CPT-cAMP), a cAMP analog, increas e the substrate flux through branched chain keto acid dehydrogenase (B CKDH) in primary rat hepatocytes cultured in defined medium. Maximum r esponse (2.7-fold increase in flux) was observed when hepatocytes were cultured with 1 mu M dexamethasone plus 50 mu M CPT-cAMP for 24 h. Th is increase in the flux rate was accompanied by significant increases in both the basal and total activities of BCKDH (2.2- and 2.0-fold, re spectively), without any significant change in the activity state of t his enzyme. The increase in BCKDH activity was the result of in- creas ed protein mass of E(1) alpha (3.2-fold), E(1) beta (2.9-fold), and E( 2) (1.6-fold) subunits of BCKDH, indicating that E(2) is the limiting subunit for the expression of BCKDH. The relative abundance of mRNAs e ncoding the E(1) alpha, E(1) beta, and E(2) subunits of BCKDH increase d by 7.4-, 21.7-, and 4.8-fold, respectively. We conclude that increas ed flux through BCKDH in hepatocytes cultured with dexamethasone and C PT-cAMP is due to increased expression of BCKDH subunit genes. However , nonstoichiometric expression of individual subunits and the correspo nding mRNAs suggests regulation of BCKDH also at translational and pos t-translational steps.