Expression of the rat liver haptoglobin gene is mediated by isoforms of C/EBPalpha, -beta and -delta proteins

CCAAT/enhancer binding proteins (C/EBP) alpha, -beta and -delta play an important role in mediating I interleukin-6 (IL-6) dependent expression of acute-phase protein (APP) genes in liver during acute-phase (AP) response. Based on the presence of type IL-6 responsive element (IL-6 RE) in the rat haptoglobin (Hp) gene promoter we assumed that some C/EBPalpha, -beta and/or -delta isoforms could mediate the expression of this gene during turpentine-induced AP response. By Western immunoblot and Northern blot assays, we found that turpentine treatment of rats led to a coordinate induction of C/EBPbeta and -delta as well as repression of C/EBPalpha isoforms pool levels in rat liver nuclear extracts (NEs) which was preceded by an adequate alteration of their mRNAs expression in liver. Consequently, results of DNA affinity chromatography revealed that affinity of certain C/EBPalpha isoforms to bind the type I IL-6 RE within the rat Hp gene promoter decreased whereas affinities of certain C/EBPbeta isoforms and C/EBPdelta were increased and induced, respectively. Our data suggest that turpentine-induced alterations of C/EBPalpha, -beta and -delta pool levels and DNA-binding activities can be regarded as an integral part of activation of the Hp gene expression in the course of AP response.     

C/EBPbeta interacts with the P-enolpyruvate carboxykinase adipocyte-specific enhancer.

CCAAT/enhancer binding protein (C/EBP) family members are known to transactivate the gene encoding cytosolic phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) in hepatocytes via promoter proximal C/EBP response elements. PEPCK is also expressed in adipocytes; however, fibroblasts that are homozygous null for C/EBPbeta cannot express PEPCK when induced to differentiate into adipocytes (Tanaka et al., EMBO J. 16, 7432-7443, 1997). This along with our previous observation that an upstream adipocyte-specific enhancer contains multiple putative C/EBP binding elements suggested the possibility that C/EBPbeta transactivates the PEPCK gene in adipocytes via distal elements. We report here that C/EBPbeta transactivates a PEPCK-luciferase chimera in transient transfection assays. C/EBPbeta acted independently of peroxisome proliferator-activated receptor gamma (PPARgamma) which is required for function of the enhancer. C/EBPbeta in nuclear extracts and recombinant C/EBPbeta bound three of the putative C/EBP-binding elements within the enhancer. C/EBPbeta binding to these three elements was strongly cooperative. However, mutation of all three elements did not affect reporter transactivation by C/EBPbeta suggesting that additional elements participate in PEPCK regulation or that the effects of C/EBPbeta are indirect.