Negative regulation of human insulin gene expression by the 5'-flanking region in non-pancreatic cells

We have examined the effect of the 5'-flanking region of the human insulin gene on its expression in non-pancreatic cells. The presence of the region containing the insulin gene enhancer (-339 to -169 bp) markedly repressed the promoter activity of the insulin gene. This suppressive phenomenon was restored by the addition of forskolin or dibutyryl cAMP, suggesting that this region alone is not sufficient to repress completely insulin gene expression in the presence of extracellular stimuli which increase the intracellular cAMP level. The hypervariable region (HVR) located at -365 bp also repressed the promoter activity. These results show negative regulation of human insulin gene expression in non-pancreatic cells by these regions.     

C/EBP, c-Myb, and PU.1 cooperate to regulate the neutrophil elastase promoter.

The murine neutrophil elastase (NE) gene is expressed specifically in immature myeloid cells. A 91-bp NE promoter region contains three cis elements which are conserved evolutionarily and are essential for activation of the promoter in differentiating 32D cl3 myeloid cells. These elements bound c-Myb (at -49), C/EBPalpha (at -57), and PU.1 (at -82). In NIH 3T3 cells, the NE promoter was activated by c-Myb, C/EBPalpha, and PU.1, via their respective binding sites. Cooperative activation was seen by any combination of c-Myb, C/EBPalpha, and PU.1, including all three together, again via their DNA-binding sites. In CV-1 cells, but not in NIH 3T3 cells, cooperation between Myb and C/EBPalpha depended on the integrity of the PU.1-binding site. In addition to C/EBPalpha, C/EBPdelta strongly activated the NE promoter, alone or with c-Myb, but C/EBPbeta was less active. Either of C/EBPalpha's two transactivation domains cooperatively activated the promoter with c-Myb, in both NIH 3T3 and 32D c13 cells. Synergistic binding to DNA in a gel shift assay between C/EBPalpha, c-Myb, and PU.1 could not be demonstrated. Also, separation of the C/EBP- and c-Myb-binding sites by 5 or 10 bp did not prevent cooperativity. These results suggest that a coactivator protein mediates cooperative activation of the NE promoter by a C/EBP and c-Myb. These factors, together with PU.1, direct restricted expression of the NE promoter to immature myeloid cells.     

Mouse p53 represses the rat brain creatine kinase gene but activates the rat muscle creatine kinase gene.

The creatine kinases (CK) regenerate ATP for cellular reactions with a high energy expenditure. While muscle CK (CKM) is expressed almost exclusively in adult skeletal and cardiac muscle, brain CK (CKB) expression is more widespread and is highest in brain glial cells. CKB expression is also high in human lung tumor cells, many of which contain mutations in p53 alleles. We have recently detected high levels of CKB mRNA in HeLa cells and, in this study, have tested whether this may be due to the extremely low amounts of p53 protein present in HeLa cells. Transient transfection experiments showed that wild-type mouse p53 severely repressed the rat CKB promoter in HeLa but not CV-1 monkey kidney cells, suggesting that, in HeLa but not CV-1 cells, p53 either associates with a required corepressor or undergoes a posttranslational modification necessary for CKB repression. Conversely, mouse wild-type p53 strongly activated the rat CKM promoter in CV-1 cells but not in HeLa cells, suggesting that, in CV-1 cells, p53 may associate with a required coactivator or is modified in a manner necessary for CKM activation. The DNA sequences required for p53-mediated modulations were found to be within bp -195 to +5 of the CKB promoter and within bp -168 to -97 of the CKM promoter. Moreover, a 112-bp fragment from the proximal rat CKM promoter (bp -168 to -57), which contained five degenerate p53-binding elements, was capable of conferring p53-mediated activation on a heterologous promoter in CV-1 cells. Also, this novel p53 sequence, when situated in the native 168-bp rat CKM promoter, conferred p53-mediated activation equal to or greater than that of the originally characterized far-upstream (bp -3160) mouse CKM p53 element. Therefore, CKB and CKM may be among the few cellular genes which could be targets of p53 in vivo. In addition, we analyzed a series of missense mutants with alterations in conserved region II of p53. Mutations affected p53 transrepression and transactivation activities differently, indicating that these activities in p53 are separable. The ability of p53 mutants to transactivate correlated well with their ability to inhibit transformation of rat embryonic fibroblasts by adenovirus E1a and activated Ras.     

Identification of basal and cyclic AMP regulatory elements in the promoter of the phosphoenolpyruvate carboxykinase gene.

Promoter elements important for basal and cyclic AMP (cAMP)-regulated expression of the phosphoenolpyruvate carboxykinase (PEPCK) gene have been identified by analysis of a series of PEPCK promoter mutations in transfection experiments. Fusion genes containing wild-type and mutated PEPCK promoter sequences from -600 to +69 base pairs (bp) fused to the coding sequence for chloramphenicol acetyltransferase were studied. Internal deletion mutations that replaced specific bases with a 10-bp linker within the region from -129 bp to -18 bp of the PEPCK promoter were examined. In addition, wild-type and mutated DNA templates were used as probes in DNase I protection experiments to determine sites of protein-DNA interaction. The PEPCK promoter contains a binding site for nuclear factor 1-CAAT. Deletion of the 5' end of this binding site reduced the size of the DNase I footprint in this region but had no effect on promoter activity. In contrast, deletion or disruption of the 3' end of this binding site completely eliminated protein binding and reduced promoter activity by 50%. Deletion of core sequences of the cAMP regulatory element (CRE) resulted in loss of cAMP responsiveness and an 85% decrease in basal promoter activity, indicating that the CRE also functions as a basal stimulatory element. Mutation of the core sequence of the CRE resulted in loss of the DNase I footprint over the CRE. Internal deletions flanking the CRE showed no loss of induction by cAMP but did have reduced promoter activity. This delimits the CRE to an 18-bp region between nucleotides -100 and -82. Analysis of mutations that disrupted bases between the CRE and the initiation site identified a basal inhibitory element adjacent to a basal stimulatory element, both located just 3' of the CRE, as well as a basal stimulatory element coincident with the TATA consensus sequence centered at -27. These data demonstrate that several cis-acting elements are located within 130 nucleotides of the initiation site of the PEPCK gene and that the CRE is essential for both basal promoter activity and cAMP-regulated expression of this gene.     

Tissue-specific regulation of the ecto-5'-nucleotidase promoter. Role of the camp response element site in mediating repression by the upstream regulatory region.

We have isolated the 5' region of the ecto-5'-nucleotidase (low K(m) 5'-NT) gene and established that a 969-base pair (bp) fragment confers cell-specific expression of a CAT reporter gene that correlates with the expression of endogenous ecto-5'-NT mRNA and enzymatic activity. A 768-bp upstream negative regulatory region has been identified that conferred lymphocyte-specific negative regulation in a heterologous system with a 244-bp deoxycytidine kinase core promoter. DNase I footprinting identified several protected areas including Sp1, Sp1/AP-2, and cAMP response element (CRE) binding sites within the 201-bp core promoter region and Sp1, NRE-2a, TCF-1/LEF-1, and Sp1/NF-AT binding sites in the upstream regulatory region. Whereas the CRE site was essential in mediating the negative activity of the upstream regulatory region in Jurkat but not in HeLa cells, mutation of the Sp1/AP-2 site decreased promoter activity in both cell lines. Electrophoretic mobility shift assay analysis of proteins binding to the CRE site identified both ATF-1 and ATF-2 in Jurkat cells. Finally, phorbol 12-myristate 13-acetate increased the activity of both the core and the 969-bp promoter fragments, and this increase was abrogated by mutations at the CRE site. In summary, we have identified a tissue-specific regulatory region 5' of the ecto-5'-NT core promoter that requires the presence of a functional CRE site within the basal promoter for its suppressive activity.