Edwards A. Park

The role of the CCAAT/enhancer-binding protein in the transcriptional regulation of the gene for phosphoenolpyruvate carboxykinase (GTP)

Previous studies have identified a region in the promoter of the gene for phosphoenolpyruvate carboxykinase (GTP) (PEPCK) (positions -460 to +73) containing the regulatory elements which respond to cyclic AMP, glucocorticoids, and insulin and confer the tissue- and developmental stage-specific properties to the gene. We report that CCAAT/enhancer-binding protein (C/EBP) binds to the cyclic AMP-responsive element CRE-1 as well as to two regions which have been previously shown to bind proteins enriched in liver nuclei. The DNase I footprint pattern provided by the recombinant C/EBP was identical to that produced by a 43-kDa protein purified from rat liver nuclear extracts, using a CRE oligonucleotide affinity column, which was originally thought to be the CRE-binding protein CREB. Transient contransfection experiments using a C/EBP expression vector demonstrated that C/EBP could trans activate the PEPCK promoter. The trans activation occurred through both the upstream, liver-specific protein-binding domains and the CRE. The CRE-binding protein bound only to CRE-1 and not to the upstream C/EBP-binding sites. The results of this study, along with physiological properties of C/EBP, indicate an important role for this transcription factor in providing the PEPCK gene with several of its regulatory characteristics.

Characterization of the 5' flanking region of the gene encoding rat liver glycogen phosphorylase

A genomic region encompassing 800 bp of the promoter-regulatory region and exon 1 of the gene (LGP) encoding rat liver glycogen phosphorylase has been isolated and characterized. Transcripts of the LGP gene initiate predominantly within an 8-bp region 48-bp upstream from the start codon. Additional transcripts were detected that initiate as far as 95 bp upstream from the start codon. To identify cis-acting sequences involved in regulating transcription, HepG2 cells were transfected with vectors containing serial deletions of the promoter-regulatory region of LGP ligated to the cat reporter gene. Two upstream regions were found to enhance transcription. One of these regions contains an alternating purine-pyrimidine sequence. LGP, which lacks a consensus TATA sequence, is like TATA-less and CAAT-less housekeeping genes in that it contains G + C-rich domains upstream from multiple transcription start points. Nuclear proteins from adult rat tissues bound in a tissue-specific fashion to one of these G + C-rich regions.

Modulation of hormone response elements by promoter environment

Hormone response elements (HREs) are nucleotide sequences that confer onto promoters the ability to alter their transcriptional pace in response to hormones. Growing evidence indicates that the functional activity of HREs can be significantly modulated by their promoter environment, making it possible for genes containing the same HRE to display diversity in their responsiveness to a given hormone signal.