Elements in the immunoglobulin heavy-chain enhancer directly regulate simian virus 40 ori-dependent DNA replication

Wild-type p53 stimulates homologous recombination upon sequence-specific binding to the ribosomal gene cluster repeat

p53 plays a central role in the maintenance of the genome integrity, both as a gatekeeper and a caretaker. Sequence-specific recognition of DNA is underlying the ability of p53 to transcriptionally transactivate target genes during checkpoint control and to regulate DNA replication at the TGCCT repeat from the ribosomal gene cluster (RGC). In contrast, suppression of recombination by p53 has been observed with nonconsensus DNA sequences. In this study, we discovered that wild-type p53 stimulates homologous recombination adjacent to the RGC repeat, whereas downregulation is seen with a mutated version thereof and with a microsatellite repeat sequence. Analysis of the causes possibly underlying the enhancement of homologous recombination revealed that p53 binding to the RGC element delays DNA synthesis. This was demonstrated after integration of the corresponding DNA fragments into our Simian virus 40-based model system, which was used to study recombination on replicating minichromosomes. Differently, with plasmid-based substrates, p53 did not stimulate recombination at the RGC sequence. Thus, in combination with our previous findings, p53 may promote homologous recombination by two separate mechanisms involving either molecular interactions with topoisomerase I or/and by specific binding to certain genomic regions, thereby causing replication fork stalling and recombination.

Expression of NF-kappaB in epidermis and the relationship between NF-kappaB activation and inhibition of keratinocyte growth

BACKGROUND

Nuclear factor-kappaB (NF-kappaB) is a transcription factor involved in a number of signalling pathways in many cell types. NF-kappaB in mice has been implicated as an important regulator of keratinocyte proliferation and differentiation.

OBJECTIVES

To evaluate the role of NF-kappaB in keratinocyte growth in human beings, we examined its expression in keratinocytes both in culture and in situ, and studied the relationship between NF-kappaB activation and the inhibition of keratinocyte proliferation induced by known modulators of keratinocyte growth.

METHODS

The expression of subunits of the NF-kappaB family was examined in human skin, primary cultured keratinocytes and an immortalized keratinocyte line by immunohistochemistry and reverse transcriptase-polymerase chain reaction analysis. NF-kB activation was examined in keratinocytes treated with various modulating agents by electrophoretic mobility shift assay (for DNA-binding activity) and by immunocytochemistry (nuclear translocation). The proliferative capacity of treated keratinocytes was also examined by 3H-thymidine incorporation, cell cycle analysis, and expression of Ki-67, a nuclear marker for cell proliferation. The involvement of NF-kappaB was assessed using sodium salicylate, which inhibits NF-kappaB activation.

RESULTS

The NF-kappaB subunits, p50, p65, RelB, and c-Rel (but not p52), were detected in keratinocytes and in normal epidermis at mRNA and protein levels. The four subunits were expressed in a cytoplasmic (rather than a nuclear) pattern in both basal and suprabasal keratinocytes. Phorbol myristate acetate (PMA), tumour necrosis factor alpha, and interferon gamma each activated NF-kappaB and inhibited keratinocyte proliferation. Lipopolysaccharide and dexamethasone did not activate NF-kappaB and had the least effect on proliferation. Finally, a high concentration of calcium (Ca2+) and retinoic acid each failed to activate NF-kappaB, but were potent inhibitors of keratinocyte proliferation, respectively. PMA-induced cell cycle arrest of keratinocytes was blocked by pretreatment with sodium salicylate.

CONCLUSIONS

NF-kappaB is constitutively expressed in a resting state in both human cultured keratinocytes and the epidermis. Activation of NF-kappaB is required for PMA-induced keratinocyte growth arrest.

Transcriptional activation by a matrix associating region-binding protein. contextual requirements for the function of bright

Bright (B cell regulator of IgH transcription) is a B cell-specific, matrix associating region-binding protein that transactivates gene expression from the IgH intronic enhancer (E mu). We show here that Bright has multiple contextual requirements to function as a transcriptional activator. Bright cannot transactivate via out of context, concatenated binding sites. Transactivation is maximal on integrated substrates. Two of the three previously identified binding sites in E mu are required for full Bright transactivation. The Bright DNA binding domain defined a new family, which includes SWI1, a component of the SWI.SNF complex shown to have high mobility group-like DNA binding characteristics. Similar to one group of high mobility group box proteins, Bright distorts E mu binding site-containing DNA on binding, supporting the concept that it mediates E mu remodeling. Transfection studies further implicate Bright in facilitating spatially separated promoter-enhancer interactions in both transient and stable assays. Finally, we show that overexpression of Bright leads to enhanced DNase I sensitivity of the endogenous E mu matrix associating regions. These data further suggest that Bright may contribute to increased gene expression by remodeling the immunoglobulin locus during B cell development.

Differential DNA replication origin activities in human normal skin fibroblast and HeLa cell lines

A modification of the extrusion method for the isolation of nascent DNA from mammalian cells and a PCR-based assay has been used in order to compare the in vivo activities of DNA replication origins in different cell lines. Conventional PCR was firstly applied to detect the chromosomal activities of several known (origins associated with c-myc, hsp70, beta-globin, immunoglobulin mu-chain enhancer) and putative DNA replication origins (autonomously replicating sequences obtained from enriched libraries of human origins of DNA replication from normal and transformed cells) in four human cell lines (HeLa, NSF, WI-38 and SK-MG-1). Then, in nascent DNA samples from normal skin fibroblast (NSF) and HeLa cells, abundance of DNA sequences in the regions of five of these origins was determined by competitive PCR. Our results suggest that autonomously replicating sequences NOA3, S14, S3 and F15 are associated with functional chromosomal origins of replication. Quantitative comparison of origin activities demonstrates that origins associated with c-myc and NOA3 are approximately twice as active in HeLa cells as in NSF cells. The described approach can facilitate the identification of origins which may be differentially active in normal cells and transformed cells or in different cell types.

Site-specific in situ amplification of the integrated polyomavirus genome: a case for a context-specific over-replication model of gene amplification

The fate of the genome of the polyoma (Py) tumor virus following integration in the chromosomes of transformed rat FR3T3 cells was re-examined. The viral sequences were integrated at a single transformant-specific chromosomal site in each of 22 transformants tested. In situ amplification of the viral sequences was observed in 24 of 34 transformants analyzed. Large T antigen, the unique viral function involved in initiating DNA replication from the viral origin, was essential for the amplification process. There was an absolute requirement for a reiteration of viral sequences and the extent of the reiteration affected the degree of amplification. The reiteration may be important for homologous recombination-mediated resolution of in situ amplified sequences. Among 11 transformants harboring a 1 to 2 kb repeat, the degree of amplification was transformant-specific and varied over a wide range. At the high end of the spectrum, the genome copy number increased 1300-fold at steady state, while at the low end, amplification was below twofold. Some aspect of the host chromatin at the site integration that affected viral gene expression, also directly or indirectly modulated the amplification. Use of high-resolution electrophoresis for the analysis of the integrated amplified sequences revealed a recurring novel pattern, consisting of a ladder with numerous bands separated by a constant distance approximately the size of the Py genome. We suggest that this pattern was generated by conversion of the amplified viral genomes to head to tail linear arrays with cell to cell variations in the number of genome repeats at single, transformant-specific, chromosomal sites. In light of the known "out of schedule" firing of the Py origin, we propose an "onion skin" structure intermediate and present a homologous recombination model for the conversion from onion skins to linear arrays. The relevance of the in situ amplification of the Py genome to cellular gene amplification is discussed. Finally, these results clarify our understanding of the integration of the Py genome in rat cells. They suggest that, in most cases, the multiple bands previously described in Py-transformants are likely to reflect genome amplification rather than multiple independent integration events, as assumed in the past. This interpretation is congruent with the accepted view that the integration of the Py genome is a rare and rate-limiting event in transformation.