Identification of multiple promoters within the N-ras proto-oncogene

Transcription of human zinc finger ZNF268 gene requires an intragenic promoter element

Human ZNF268 gene is a typical Krüppel-associated box/C2H2 zinc finger gene whose homolog has been found only in higher mammals and not in lower mammals such as mouse. Its expression profiles have suggested that it plays a role in the differentiation of blood cells during early human embryonic development and the pathogenesis of leukemia. To gain additional insight into the molecular mechanisms controlling the expression of the ZNF268 gene and to provide the necessary tools for further genetic studies of leukemia, we have mapped the 5'-end of the human ZNF268 mRNA by reverse transcription-PCR and primer extension assays. We then cloned the 5'-flanking genomic DNA containing the putative ZNF268 gene promoter and analyzed its function in several different human and mouse tissue culture cell lines. Interestingly, our studies show that the ZNF268 gene lacks a typical eukaryotic promoter that is present upstream of the transcription start site and directs a basal level of transcription. Instead, the functional promoter requires an essential element that is located within the first exon of the gene. Deletion and mutational analysis reveals the requirement for a cAMP response-element-binding protein (CREB)-binding site within this element for promoter function. Gel mobility shift and chromatin immunoprecipitation assays confirm that CREB-2 binds to the site in vitro and in vivo. Furthermore, overexpression of CREB-2 enhances the promoter activity. These results demonstrate that the human ZNF268 gene promoter is atypical and requires an intragenic element located within the first exon that mediates the effect of CREB for its activity.

Cellular response to oncogenic ras involves induction of the Cdk4 and Cdk6 inhibitor p15(INK4b)

The cell cycle inhibitor p15(INK4b) is frequently inactivated by homozygous deletion together with p16(INK4a) and p19(ARF) in some types of tumors. Although the tumor suppressor capability of p15(INK4b) is still questioned, it has been found to be specifically inactivated by hypermethylation in hematopoietic malignancies in the absence of p16(INK4a) alterations. Here we show that, in vitro, p15(INK4b) is a strong inhibitor of cellular transformation by Ras. Surprisingly, p15(INK4b) is induced in cultured cells by oncogenic Ras to an extent similar to that of p16(INK4a), and their expression is associated with premature G(1) arrest and senescence. Ras-dependent induction of these two INK4 genes is mediated mainly by the Raf-Mek-Erk pathway. Studies with activated and dominant negative forms of Ras effectors indicate that the Raf-Mek-Erk pathway is essential for induction of both the p15(INK4b) and p16(INK4a) promoters, although other Ras effector pathways can collaborate, giving rise to a stronger response. Our results indicate that p15(INK4b), by itself, is able to stop cell transformation by Ras and other oncogenes such as Rgr (a new oncogene member of the Ral-GDS family, whose action is mediated through Ras). In fact, embryonic fibroblasts isolated from p15(INK4b) knockout mice are susceptible to transformation by the Ras or Rgr oncogene whereas wild-type embryonic fibroblasts are not. Similarly, p15(INK4b)-deficient mouse embryo fibroblasts are more sensitive than wild-type cells to transformation by a combination of the Rgr and E1A oncogenes. The cell cycle inhibitor p15(INK4b) is therefore involved, at least in some cell types, in the tumor suppressor activity triggered after inappropriate oncogenic Ras activation in the cell.

N-ras protein: frequent quantitative and qualitative changes occur in human colorectal carcinomas

Point mutation and overexpression are recognized mechanisms for ras activation in malignancy. However, little information is available on overexpression of N-ras protein compared with H- or K-ras proteins, as N-ras-specific antibodies have only recently become available. For comparative analyses of ras protein levels, we have probed Western blots of extracts from 9 normal human tissues and 55 pairs of colorectal carcinoma and matched control mucosa, using monoclonal antibodies (MAbs) specific for H-, K- or N-ras proteins. On multi-tissue blots, N-ras protein was more highly expressed in colon than in the other human tissues analyzed, suggesting a role for N-ras in colorectal function. N-ras protein levels in multiple independent extracts of normal colon mucosa were consistently higher than either K-ras protein or H-ras protein levels. In 69% of colon carcinomas, N-ras protein levels were increased an average of 4.8-fold over normal mucosa. Overexpression of K-ras protein was also observed in colon cancers but less frequently (13% of cases) than N-ras protein. H-ras protein levels were too low for comparative studies. Alterations in N-ras protein mobility, possibly reflecting increased post-translational processing, were also detected in 42% of colon carcinomas. N-ras protein, typically present as a single 23 kDa band in normal mucosa, was expressed in some cancers as a 22 kDa band or as multiple bands of 20-23 kDa. Sequencing of N-ras DNA from 6 carcinomas with these variations in protein mobility did not reveal mutations in codons 12, 13, 59 or 61. Thus, frequent quantitative and qualitative changes in N-ras protein expression, which do not appear to correlate with the presence of typical N-ras point mutations, result in abnormal N-ras protein patterns in human colorectal carcinomas.

Effects of melatonin on mammary gland lesions in transgenic mice overexpressing N-ras proto-oncogene

The oncostatic effects of melatonin on the mammary gland have been studied in transgenic mice carrying the N-ras proto-oncogene under the control of the MMTV-LTR. Female (4-week-old) virgin mice with positive transgenic pedigrees were injected with melatonin (200 micrograms/mouse/ day, five times a week) or vehicle late in the evening. After 5 months of treatment, animals were sacrificed and the mammary glands were dissected for whole mounts, histology, and immunohistochemical analysis with a mouse monoclonal antibody specific for N-ras protein. Mammary glands of control transgenic mice showed different densities of hyperplastic alveolar nodules (HANs) consisting primarily of dysplastic epithelial cells with nuclear atypia and prominent nucleoli. The epithelial cells of HANs showed a high expression of N-ras while no immunostaining was detected in the unaffected mammary parenchyma. Only one (10%) of the control transgenic mice presented an infiltrating ductal carcinoma with the neoplastic cells overexpressing N-ras protein. The mammary glands of melatonin treated mice had a lower density of HANs, absence of epithelial dysplastic cells, and weak immunostaining of N-ras protein in comparison to the vehicle-treated group. None of the melatonin treated animals developed mammary carcinomas during the observation period. The lymph nodes of the inguinal mammary glands of all the vehicle-treated transgenic mice presented hyperplasia and two animals even had lymphomas, whereas in melatonin-treated animals there was less hyperplasia (two cases were atrophic) and a lack of lymphomas. We conclude that in the mammary glands of MMTV-LTR/N-ras transgenic female virgin mice, melatonin a) reduces the incidence of HANs and the expression of N-ras protein in focal hyperplastic lesions, b) completely prevents the development of epithelial cell atypia and mammary adenocarcinomas, and c) also reduces the hyperplasia of the mammary lymphoid tissue and prevents the development of lymphomas.

A conserved downstream element defines a new class of RNA polymerase II promoters

Although many TATA-less promoters transcribed by RNA polymerase II initiate transcription at multiple sites, the regulation of multiple start site utilization is not understood. Beginning with the prediction that multiple start site promoters may share regulatory features and using the P-glycoprotein promoter (which can utilize either a single or multiple transcription start site(s)) as a model, several promoters with analogous transcription windows were grouped and searched for the presence of a common DNA element. A downstream protein-binding sequence, MED-1 (Multiple start site Element Downstream), was found in the majority of promoters analyzed. Mutation of this element within the P-glycoprotein promoter reduced transcription by selectively decreasing utilization of downstream start sites. We propose that a new class of RNA polymerase II promoters, those that can utilize a distinctive window of multiple start sites, is defined by the presence of a downstream MED-1 element.