Collaboration of signal transducer and activator of transcription 1 (STAT1) and BRCA1 in differential regulation of IFN-γ target genes

Most of the activities of IFN-γ are the result of STAT1-mediated transcriptional responses. In this study, we show that the BRCA1 tumor suppressor acts in concert with STAT1 to differentially activate transcription of a subset of IFN-γ target genes and mediates growth inhibition by this cytokine. After IFN-γ treatment, induction of the cyclin-dependent kinase inhibitor, p21WAF1, was synergistically activated by BRCA1, whereas the IRF-1 gene was unaffected. Importantly, the differential induction of p21WAF1 was impaired in breast cancer cells homozygous for the mutant BRCA1 5382C allele. Biochemical analysis illustrated that the mechanism of this transcriptional synergy involves interaction between BRCA1 aa 502–802 and the C-terminal transcriptional activation domain of STAT1 including Ser-727 whose phosphorylation is crucial for transcriptional activation. Significantly, STAT1 proteins mutated at Ser-727 bind poorly to BRCA1, reinforcing the importance of Ser-727 in the recruitment of transcriptional coactivators by STAT proteins. These findings reveal a novel mechanism for BRCA1 function in the IFN-γ-dependent tumor surveillance system.

Immune response to a differentiation antigen induced by altered antigen: A study of tumor rejection and autoimmunity

Recognition of self is emerging as a theme for the immune recognition of human cancer. One question is whether the immune system can actively respond to normal tissue autoantigens expressed by cancer cells. A second but related question is whether immune recognition of tissue autoantigens can actually induce tumor rejection. To address these issues, a mouse model was developed to investigate immune responses to a melanocyte differentiation antigen, tyrosinase-related protein 1 (or gp75), which is the product of the brown locus. In mice, immunization with purified syngeneic gp75 or syngeneic cells expressing gp75 failed to elicit antibody or cytotoxic T-cell responses to gp75, even when different immune adjuvants and cytokines were included. However, immunization with altered sources of gp75 antigen, in the form of either syngeneic gp75 expressed in insect cells or human gp75, elicited autoantibodies to gp75. Immunized mice rejected metastatic melanomas and developed patchy depigmentation in their coats. These studies support a model of tolerance maintained to a melanocyte differentiation antigen where tolerance can be broken by presenting sources of altered antigen (e.g., homologous xenogeneic protein or protein expressed in insect cells). Immune responses induced with these sources of altered antigen reacted with various processed forms of native, syngeneic protein and could induce both tumor rejection and autoimmunity.

Expression of a Hox gene, Cnox-2, and the division of labor in a colonial hydroid

We report the isolation and expression of the Hox gene, Cnox-2, in Hydractinia symbiolongicarpus, a hydrozoan displaying division of labor. We found different patterns of aboral-to-oral Cnox-2 expression among polyp polymorphs, and we show that experimental conversion of one polyp type to another is accompanied by concordant alteration in Cnox-2 expression. Our results are consistent with the suggestion that polyp polymorphism, characteristic of hydractiniid hydroids, arose via evolutionary modification of proportioning of head to body column.

Efficiency of the u,v method of estimation

Given a pool of motorists, how do we estimate the total intensity of those who had a prespecified number of traffic accidents in the past year? We previously have proposed the u,v method as a solution to estimation problems of this type. In this paper, we prove that the u,v method provides asymptotically efficient estimators in an important special case.

Formation of stable adducts and absence of depurinating DNA adducts in cells and DNA treated with the potent carcinogen dibenzo[a,l]pyrene or its diol epoxides

Polycyclic aromatic hydrocarbons (PAH) are widespread environmental contaminants, and some are potent carcinogens in rodents. Carcinogenic PAH are activated in cells to metabolites that react with DNA to form stable covalent DNA adducts. It has been proposed [Cavalieri, E. L. & Roger, E. G. (1995) Xenobiotica 25, 677–688] that unstable DNA adducts are also formed and that apurinic sites in the DNA resulting from unstable PAH adducts play a key role in the initiation of cancer. The potent carcinogen dibenzo[a,l]pyrene (DB[a,l]P) is activated in cells to (+)-syn- and (−)-anti-DB[a,l]P-11,12-diol-13,14-epoxide (DB[a,l]PDE), which have been shown to form stable adducts with DNA. To evaluate the importance of unstable PAH adducts, we compared stable adduct formation to apurinic site formation. Stable DB[a,l]PDE adducts were determined by 33P-postlabeling and HPLC. To measure apurinic sites they were converted to strand breaks, and these were monitored by examining the integrity of a particular restriction fragment of the dihydrofolate reductase gene. The method easily detected apurinic sites resulting from methylation by treatment of cells or DNA with dimethyl sulfate or from reaction of DNA with DB[a,l]P in the presence of horseradish peroxidase. We estimate the method could detect 0.1 apurinic site in the 14-kb fragment examined. However, apurinic sites were below our limit of detection in DNA treated directly with (+)-syn- or (−)-anti-DB[a,l]PDE or in DNA from Chinese hamster ovary B11 cells so treated, although in these samples the frequency of stable adducts ranged from 3 to 10 per 14 kb. We also treated the human mammary carcinoma cell line MCF-7 with DB[a,l]P and again could not detect significant amounts of unstable adducts. These results indicate that the proportion of stable adducts formed by DB[a,l]P activated in cells and its diol epoxides is greater than 99% and suggest a predominant role for stable DNA adducts in the carcinogenic activity of DB[a,l]P.

Can ends justify the means?: Telomeres and the mechanisms of replicative senescence and immortalization in mammalian cells

Finite replicative lifespan, or senescence, of mammalian cells in culture is a phenomenon that has generated much curiosity since its description. The obvious significance of senescence to organismal aging and the development of cancer has engendered a long-lasting and lively debate about its mechanisms. Recent discoveries concerning the phenotypes of telomerase knockout mice, the consequences of telomerase reexpression in somatic cells, and genes that regulate senescence have provided striking molecular insights but also have uncovered important new questions. The objective of this review is to reconcile old observations with new molecular details and to focus attention on the key remaining puzzles.

The Friend of GATA proteins U-shaped, FOG-1, and FOG-2 function as negative regulators of blood, heart, and eye development in Drosophila

Friend of GATA (FOG) proteins regulate GATA factor-activated gene transcription. During vertebrate hematopoiesis, FOG and GATA proteins cooperate to promote erythrocyte and megakaryocyte differentiation. The Drosophila FOG homologue U-shaped (Ush) is expressed similarly in the blood cell anlage during embryogenesis. During hematopoiesis, the acute myeloid leukemia 1 homologue Lozenge and Glial cells missing are required for the production of crystal cells and plasmatocytes, respectively. However, additional factors have been predicted to control crystal cell proliferation. In this report, we show that Ush is expressed in hemocyte precursors and plasmatocytes throughout embryogenesis and larval development, and the GATA factor Serpent is essential for Ush embryonic expression. Furthermore, loss of ush function results in an overproduction of crystal cells, whereas forced expression of Ush reduces this cell population. Murine FOG-1 and FOG-2 also can repress crystal cell production, but a mutant version of FOG-2 lacking a conserved motif that binds the corepressor C-terminal binding protein fails to affect the cell lineage. The GATA factor Pannier (Pnr) is required for eye and heart development in Drosophila. When Ush, FOG-1, FOG-2, or mutant FOG-2 is coexpressed with Pnr during these developmental processes, severe eye and heart phenotypes result, consistent with a conserved negative regulation of Pnr function. These results indicate that the fly and mouse FOG proteins function similarly in three distinct cellular contexts in Drosophila, but may use different mechanisms to regulate genetic events in blood vs. cardial or eye cell lineages.