Down-regulation of BRCA1 and BRCA2 in human ovarian cancer cells exposed to adriamycin and ultraviolet radiation

TET2 mutations contribute to adverse prognosis in acute myeloid leukemia (AML): results from a comprehensive analysis of 502 AML cases and the Beat AML public database

Despite the high incidence of tet methylcytosine dioxygenase 2 (TET2) mutations in acute myeloid leukemia (AML), the prognostic implications of these mutations in three AML risk groups based on the 2022 ELN AML risk classification are still unclear. A total of 502 consecutive de novo AML patients who had next-generation sequencing data available between March 2011 and July 2021 at the Peking University Institute of Hematology were enrolled in this study. Univariate and multivariate Cox regression analyses were performed to explore the prognostic impact of TET2 mutations in the above cohort and the Beat AML cohort. Of the 502 total AML patients, 76 (15.1%) carried TET2 mutations. Multivariate analysis revealed TET2 mutations as independent risk factor for overall survival (OS) in both the total AML cohort (OR = 1.649, p = 0.009) and in the 2022 ELN intermediate-risk cohort (HR = 1.967, p = 0.05). Analysis of RNA-seq data from the Beat AML study revealed 1042 differentially expressed genes (DEGs) between the TET2-mutant and TET2 wild-type groups. The results of enrichment analysis indicated the DEGs to be notably enriched in categories related to the PI3K-Akt signaling pathway. Collectively, our findings indicate that mutations in TET2 are prognostically disadvantageous in AML patients. Assessment of TET2 mutational status contributes to the stratification of intermediate-risk AML patients. Multiple genes and pathways of potential therapeutic relevance may be differentially modulated by TET2 mutations in AML.

Hippophae rhamnoides mediate gene expression profiles against keratinocytes infection of Staphylococcus aureus

Staphylococcus aureus causes a wide range of skin diseases such as bacterial keratitis, follicles, psoriasis, cellulitis and atopic dermatitis. This study aims to investigate the S. aureus mediated molecular modulation, and the effect of HR in reversing the deleterious impact of S. aureus in keratinocytes. Human keratinocyte (HaCaT) cells were cultured in DMEM, supplemented with 10% fetal bovine serum (FBS) and 1% penicillin/streptomycin. Subcultures were divided into three flasks: control with no S. aureus and extract (C), S. aureus infected (SA) and S. aureus infected cells and extract (SE). RNA was isolated and microarray analysis was performed. The data was annotated using GO functional analysis and DAVID functional annotation. For each comparison group, significant probes were filtered out at significant cut off by fold change (P < 0.05 and/or > twofold change). For SA vs control, SE vs control, and SE vs SA, 204, 9369, 9900 probes were filtered respectively. In SA vs control, TNF signaling, NOD-like receptor and chemokine receptor signaling pathways were upregulated with key genes such as CCL2, CCL20 and BIRC3. The SE vs SA, showed significant expression variations of a number of important genes. Molecular pathways associated with ILs, TNFs, TGFs, IFNs, FGFs, MAPKs, MMPs, caspases and Wnts were either up regulated or downregulated. This effect was reversed by the extract, possibly through downregulating various proinflammatory cytokines and apoptotic pathways. Our study reveals that S. aureus inserts a negative impact on the regulation of various key genes which is apparently reversed by the HR extract.

TET2 Function in Hematopoietic Malignancies, Immune Regulation, and DNA Repair

Over the last decade, investigation of Ten-Eleven Translocation 2 (TET2) gene function and TET2 mutation have become of increasing interest in the field of hematology. This heightened interest was sparked by the seminal discoveries that (1) TET2 mutation is associated with development of hematological malignancies and that (2) the TET family of proteins is critical in promoting DNA demethylation and immune homeostasis. Since then, additional studies have begun to unravel the question “Does TET2 have additional biological functions in the regulation of hematopoiesis?” Here, we present a mini-review focused on the current understanding of TET2 in hematopoiesis, hematological malignancies, and immune regulation. Importantly, we highlight the critical function that TET2 facilitates in maintaining the stability of the genome. Based on our review of the literature, we provide a new hypothesis that loss of TET2 may lead to dysregulation of the DNA repair response, augment genome instability, and subsequently sensitize myeloid leukemia cells to PARP inhibitor treatment.

Impact of Etoposide on BRCA1 Expression in Various Breast Cancer Cell Lines

Breast cancer 1 (BRCA1), as a tumor suppressor, exerts an effective influence on protecting DNA integrity to suppress the development of breast cancer (BC). BRCA1 expression is induced in response to DNA-damaging agents such as etoposide. Germline BRCA1 gene mutations are associated with development of hereditary BC. However, besides BRCA-mutated BCs, some sporadic cancers may also exhibit a BRCA-like phenotype, displaying so-called ‘BRCAness’. This common phenotype may respond to similar therapeutic approaches as BRCA-mutated tumors and may thus have important implications for the clinical management of these cancers. In order to determine whether and how etoposide regulates the protein levels of BRCA1 in BC cells, we exposed a panel of five selected cell lines to etoposide, compared the results to untreated control cells, and then stained the cells with the specific, reliable, and reproducible MS110 antibody directed against phosphorylated Ser1423 BRCA1. By evaluating cytoplasmic BRCA1 protein levels, we were able to distinguish three aggressive BC subtypes with BRCAness characteristics. In addition, determination of early and late apoptosis helped to complete the analysis of BRCA1 functions in the DNA damage pathway of aggressive BC. In conclusion, our study suggested that high cytoplasmic BRCA1 protein levels could be considered as a potential predictive marker for response to chemotherapy in both sporadic and hereditary BC. Tumors with either BRCAness phenotype or germline BRCA1 mutation are both aggressive BCs associated with poor prognosis and could both be subjected to targeted therapies against BRCA1-mutated BC in future clinical management strategies.

Lipoic acid prevents Cr(6+) induced cell transformation and the associated genomic dysregulation

Investigation of the transcription profile of cells transformed by Cr(6+) in vivo was undertaken. The objective was to elucidate genomic changes underlying the mechanism of action of the carcinogenic dose of Cr(6+)and their prevention using metabolic antioxidant lipoic acid (LA). Cr(6+) was administered intraperitoneally to LPS+TPA challenged Swiss albino mice in host mediated cell transformation assay using peritoneal macrophages in vivo. The cell transforming potential of Cr(6+) test doses was validated by gain of anchorage independent growth potential in soft agar and loss of Fc receptor on target cells. LA was administered in equimolar doses. Compared to non-transformed cells, the gene expression profile of transformed cells was found to be dysregulated substantially and in dose dependent manner. Genes showing down regulation were found to be involved in tumour suppression, apoptosis, DNA repair, and cell-cycle. A similar response was noted in the genes pertaining to immune system, morphogenesis, cell-communication, energy-metabolism, and biosynthesis. The co-administration of lipoic acid prevented the transcription dysregulation and cell transformation by Cr(6+) in vivo. The influenced pathways seem to be crucial for progression as well as mitigation of Cr toxicity; and their response to LA indicated their critical role in mechanism of anti-carcinogenic action of LA. Results are of importance to mitigate Cr(6+) induced occupational cancer hazard.

Egr-1 regulates the transcription of the BRCA1 gene by etoposide

The breast cancer susceptibility gene BRCA1 encodes a nuclear protein, which functions as a tumor suppressor and is involved in gene transcription and DNA repair processes. Many families with inherited breast and ovarian cancers have mutations in the BRCA1 gene. However, only a few studies have reported on the mechanism underlying the regulation of BRCA1 expression in humans. In this study, we investigated the transcriptional regulation of BRCA1 in HeLa cells treated with etoposide. We found that three Egr-1-binding sequences (EBSs) were located at -1031, -1005, and -385 within the enhancer region of the BRCA1 gene. Forced expression of Egr-1 stimulated the BRCA1 promoter activity. EMSA data showed that Egr-1 bound directly to the EBS within the BRCA1 gene. Knockdown of Egr-1 through the expression of a small hairpin RNA (shRNA) attenuated etoposide-induced BRCA1 promoter activity. We conclude that Egr-1 targets the BRCA1 gene in HeLa cells exposed to etoposide.

High levels of wild-type BRCA2 suppress homologous recombination

Endogenous levels of the BRCA2 (breast cancer susceptibility 2) protein promote homologous recombination by regulating the essential strand exchange protein RAD51. To examine BRCA2 function in homologous recombination, we expressed human BRCA2 in control mouse hybridoma cells, as well as those that were depleted of endogenous Brca2 by small interfering RNA. With moderate human BRCA2 expression, homologous recombination was stimulated. Conversely, a higher level of BRCA2 reduced homologous recombination and DNA-damage-induced Rad51 foci formation. Cells expressing high levels of BRCA2 feature normal growth, increased sensitivity to mitomycin C, and increased illegitimate recombination. BRCA2-overexpressing cells are also characterized by suppression of p53 transcriptional regulation and a corresponding reduction in the expression of the p53-responsive genes Noxa and p21. Notably, in cells expressing high levels of BRCA2, small interfering RNA depletion of human BRCA2 or ectopic expression of Rad51 increases homologous recombination and decreases illegitimate recombination. Thus, high levels of wild-type BRCA2 perturb Rad51-mediated homologous recombination, and relatively normal recombination responses can be restored by rebalancing recombination factors.

PI 3 kinase related kinases-independent proteolysis of BRCA1 regulates Rad51 recruitment during genotoxic stress in human cells

Background

The function of BRCA1 in response to ionizing radiation, which directly generates DNA double strand breaks, has been extensively characterized. However previous investigations have produced conflicting data on mutagens that initially induce other classes of DNA adducts. Because of the fundamental and clinical importance of understanding BRCA1 function, we sought to rigorously evaluate the role of this tumor suppressor in response to diverse forms of genotoxic stress.

Methodology/Principal Findings

We investigated BRCA1 stability and localization in various human cells treated with model mutagens that trigger different DNA damage signaling pathways. We established that, unlike ionizing radiation, either UVC or methylmethanesulfonate (MMS) (generating bulky DNA adducts or alkylated bases respectively) induces a transient downregulation of BRCA1 protein which is neither prevented nor enhanced by inhibition of PIKKs. Moreover, we found that the proteasome mediates early degradation of BRCA1, BARD1, BACH1, and Rad52 implying that critical components of the homologous recombinaion machinery need to be functionally abrogated as part of the early response to UV or MMS. Significantly, we found that inhibition of BRCA1/BARD1 downregulation is accompanied by the unscheduled recruitment of both proteins to chromatin along with Rad51. Consistently, treatment of cells with MMS engendered complete disassembly of Rad51 from pre-formed ionizing radiation-induced foci. Following the initial phase of BRCA1/BARD1 downregulation, we found that the recovery of these proteins in foci coincides with the formation of RPA and Rad51 foci. This indicates that homologous recombination is reactivated at later stage of the cellular response to MMS, most likely to repair DSBs generated by replication blocks.

Conclusion/Significance

Taken together our results demonstrate that (i) the stabilities of BRCA1/BARD1 complexes are regulated in a mutagen-specific manner, and (ii) indicate the existence of mechanisms that may be required to prevent the simultaneous recruitment of conflicting signaling pathways to sites of DNA damage.

BP1, an isoform of DLX4 homeoprotein, negatively regulates BRCA1 in sporadic breast cancer

Introduction: Several lines of evidence point to an important role for BP1, an isoform of DLX4 homeobox gene, in breast carcinogenesis and progression. BRCA1 is a well-known player in the etiology of breast cancer. While familial breast cancer is often marked by BRCA1 mutation and subsequent loss of heterozygosity, sporadic breast cancers exhibit reduced expression of wild type BRCA1, and loss of BRCA1 expression may result in tumor development and progression.

Methods: The Cister algorithm and Genomatix program were used to identify potential BP1 binding sites in BRCA1 gene. Real-time PCR, Western blot and immunohistochemistry analysis were performed to verify the expression of BRCA1 and BP1 in cell lines and breast cancer tissues. Double-stranded siRNA transfection was carried out for silencing BP1 expression. ChIP and EMSA were used to confirm that BP1 specifically binds to BRCA1.

Results: A putative BP1 binding site was identified in the first intron of BRCA1, which was confirmed by chromatin immunoprecipiation and electrophoresis mobility shift assay. BP1 and BRCA1 expression were inversely correlated in breast cancer cell lines and tissues, suggesting that BP1 may suppress BRCA1 transcription through consensus sequence binding.

Conclusions: BP1 homeoprotein represses BRCA1 expression through direct binding to its first intron, which is consistent with a previous study which identified a novel transcriptional repressor element located more than 500 base pairs into the first intron of BRCA1, suggesting that the first intron plays an important role in the negative regulation of BRCA1. Although further functional studies are necessary to confirm its repressor activity towards BRCA1, the elucidation of the role of BP1 in breast tumorigenesis holds great promise in establishing BP1 as a novel target for drug therapy.

Transcriptional autoregulation by BRCA1

The BRCA1 gene product plays numerous roles in regulating genome integrity. Its role in assembling supermolecular complexes in response to DNA damage has been extensively studied; however, much less is understood about its role as a transcriptional coregulator. Loss or mutation is associated with hereditary breast and ovarian cancers, whereas altered expression occurs frequently in sporadic forms of breast cancer, suggesting that the control of BRCA1 transcription might be important to tumorigenesis. Here, we provide evidence of a striking linkage between the roles for BRCA1 as a transcriptional coregulator with control of its expression via an autoregulatory transcriptional loop. BRCA1 assembles with complexes containing E2F-1 and RB to form a repressive multicomponent transcriptional complex that inhibits BRCA1 promoter transcription. This complex is disrupted by genotoxic stress, resulting in the displacement of BRCA1 protein from the BRCA1 promoter and subsequent upregulation of BRCA1 transcription. Cells depleted of BRCA1 respond by upregulating BRCA1 transcripts, whereas cells overexpressing BRCA1 respond by downregulating BRCA1 transcripts. Tandem chromatin immmunoprecipitation studies show that BRCA1 is regulated by a dynamic coregulatory complex containing BRCA1, E2F1, and Rb at the BRCA1 promoter that is disrupted by DNA-damaging agents to increase its transcription. These results define a novel transcriptional mechanism of autoregulated homeostasis of BRCA1 that selectively titrates its levels to maintain genome integrity in response to genotoxic insult.

Differential modulation of BRCA1 and BARD1 nuclear localisation and foci assembly by DNA damage

The BRCA1/BARD1 heterodimer regulates genomic maintenance and contributes to the DNA damage checkpoint response. We previously reported that BRCA1 and BARD1 can shuttle between nucleus and cytoplasm. In this study, we evaluated the localisation patterns of BRCA1 and BARD1 in response to different types of DNA damaging agents and chemotherapeutic drugs. In MCF-7 cells, endogenous BRCA1 increased transiently in the nucleus at 2h after ionising radiation (IR), whereas BARD1 was unaffected. IR treatment did not induce nuclear export of either protein, in contrast to previous reports. DNA damage by UV radiation, etoposide or camptothecin caused a preferential down-regulation of nuclear BARD1 at 6h post-treatment. The UV-dependent loss of nuclear BARD1 was blocked by the proteasome inhibitor MG132, but not by leptomycin B, indicating a change in BARD1 nuclear degradation rather than nuclear export. MG132 also blocked the dispersal of BARD1/BRCA1 nuclear foci at 6h after UV, implicating the proteasome in repair foci disassembly. In the cytoplasm, BRCA1 and BARD1 were detected at centrosomes but their distribution was not altered by DNA damage. BARD1 displayed a stronger mitochondria accumulation than BRCA1, and became phosphorylated at mitochondria in response to DNA damage. The mitotic spindle poisons vincristine and paclitaxel had no effect on BRCA1 or BARD1 subcellular distribution. We conclude that BARD1 phosphorylation, expression and localisation patterns are regulated in the nucleus and at mitochondria in response to different forms of DNA damage, contributing to the role of BRCA1/BARD1 in DNA repair and apoptotic responses.

Methylation and protein expression of DNA repair genes: association with chemotherapy exposure and survival in sporadic ovarian and peritoneal carcinomas

Background

DNA repair genes critically regulate the cellular response to chemotherapy and epigenetic regulation of these genes may be influenced by chemotherapy exposure. Restoration of BRCA1 and BRCA2 mediates resistance to platinum chemotherapy in recurrent BRCA1 and BRCA2 mutated hereditary ovarian carcinomas. We evaluated BRCA1, BRCA2, and MLH1 protein expression in 115 sporadic primary ovarian carcinomas, of which 31 had paired recurrent neoplasms collected after chemotherapy. Additionally, we assessed whether promoter methylation of BRCA1, MLH1 or FANCF influenced response to chemotherapy or explained alterations in protein expression after chemotherapy exposure.

Results

Of 115 primary sporadic ovarian carcinomas, 39 (34%) had low BRCA1 protein and 49 (42%) had low BRCA2 expression. BRCA1 and BRCA2 protein expression were highly concordant (p < 0.0001). MLH1 protein loss occurred in 28/115 (24%) primary neoplasms. BRCA1 protein loss in primary neoplasms was associated with better survival (p = 0.02 Log Rank test) and remained significant after accounting for either stage or age in a multivariate model (p = 0.04, Cox proportional hazards). In paired specimens, BRCA1 protein expression increased in 13/21 (62%) and BRCA2 protein expression increased in 15/21 (71%) of recurrent carcinomas with low or intermediate protein in the paired primary. In contrast MLH1 expression was rarely decreased in recurrent carcinomas (1/33, 3%). Similar frequencies of MLH1, BRCA1, and FANCF promoter methylation occurred in primary carcinomas without previous chemotherapy, after neoadjuvant chemotherapy, or in recurrent neoplasms.

Conclusion

Low BRCA1 expression in primary sporadic ovarian carcinoma is associated with prolonged survival. Recurrent ovarian carcinomas commonly have increased BRCA1 and/or BRCA2 protein expression post chemotherapy exposure which could mediate resistance to platinum based therapies. However, alterations in expression of these proteins after chemotherapy are not commonly mediated by promoter methylation, and other regulatory mechanisms are likely to contribute to these alterations.

Novel BRCA2-interacting protein BJ-HCC-20A inhibits the induction of apoptosis in response to DNA damage

The major hereditary breast cancer susceptibility gene BRCA2 is associated with familial breast and ovarian cancer. BRCA2 plays a role in DNA repair, transcription, cell cycle regulation, maintenance of genomic stability in response to DNA damage, centrosome regulation, and cytokinesis. To further understand the function of BRCA2, we used a yeast two-hybrid method and identified a novel BRCA2-interacting protein, BJ-HCC-20A, which is reported to be a potential cancer-testis antigen. We confirmed the interaction between endogenous BJ-HCC-20A and BRCA2 in mammalian cells, and showed that BJ-HCC-20A interacts with a portion of the highly conserved region of BRCA2 in various mammals, and M phase-specific phosphorylation of the binding region of BRCA2 modulates BJ-HCC-20A binding. Overexpression of BJ-HCC-20A increases cell growth, and downregulation of endogenous BJ-HCC-20A expression using small interfering RNA suppresses cell growth and leads to the induction of apoptosis. Importantly, the BJ-HCC-20A mRNA level is downregulated by adriamycin (ADR)-induced DNA damage and depletion of BJ-HCC-20A expression by small interfering RNA promotes the reduction of BRCA2 expression and enhances cell apoptosis in response to DNA damage. Additionally, the recovery of BJ-HCC-20A expression in ADR-induced DNA damage inhibits ADR-induced apoptosis. The data suggest that BJ-HCC-20A promotes cell growth and may regulate the induction of cell apoptosis in response to DNA damage in cooperation with BRCA2 in an M phase-dependent manner. Therefore, we speculate that targeting BJ-HCC-20A may aid in the treatment of breast tumors.

Features that determine telomere homolog oligonucleotide-induced therapeutic DNA damage-like responses in cancer cells

Cancer is the second leading cause of death in the USA, with metastatic disease proving a particular management challenge. Treatment modalities for patients with metastatic disease are limited, and survival beyond 5 years is uncommon. We have reported that an 11-base DNA oligonucleotide 100% homologous to the telomere 3' overhang can induce apoptosis, senescence and/or differentiation of several types of malignant cells in vitro and in vivo, while having minimal effect on normal cells. We now report that 22 oligonucleotides, 9-20 bases in length, with or without a 5' phosphate group and with varying homology (40-100%) to the 3' overhang, inhibit growth and induce apoptosis of human cell lines derived from breast cancers, pancreatic and ovarian carcinomas, and malignant melanoma, lines that lack p53 and/or p16 and harbor a variety of other abnormalities in key regulatory signaling pathways. Cytosine (C) content adversely affected oligonucleotide efficacy, decreasing their effect on cellular apoptosis by > or =80%. These data confirm and expand our earlier work suggesting that such telomere homolog oligonucleotides (T-oligos) target an innate anti-cancer defense system in human cells and may provide an effective treatment for cancers of multiple different cellular origins and genetic profile.

Characterization of CSOC 882, a novel immortalized ovarian cancer cell line expressing EGFR, HER2, and activated AKT

OBJECTIVE

Only a small number of comprehensively characterized immortalized ovarian cancer cell lines are available for laboratory studies on ovarian cancer. We describe a new ovarian cancer cell line that arose from primary culture of a stage IC, grade III ovarian carcinoma, designated CSOC 882.

METHODS

We characterized CSOC 882 by karyotyping, growth modeling, immunohistochemical staining, immunoblotting, drug sensitivity testing, and xenografting in nude mice.

RESULTS

CSOC 882 possessed an abnormal tetraploid karyotype including loss of one copy of chromosomes 2, 17, 19, and 21, and deletion of 8p21. Growth of CSOC 882 was best modeled using the logistic growth equation revealing an average doubling time of 31 h. CSOC 882 cells expressed vimentin, cytokeratin, p53, BRCA1, EGF receptor, HER2, androgen receptor, estrogen receptor alpha, and progesterone receptor, while no evidence of estrogen receptor beta or factor VIII was observed. Some but not all CSOC 882 cells were positive for CA125 reflecting the primary tumor, which had patchy CA125 staining. Drug sensitivity assays demonstrated that CSOC 882 was more sensitive to cisplatin and carboplatin than SKOV3 and HCC1937 while CSOC 882 and SKOV3 were both sensitive to paclitaxel unlike HCC1937. CSOC 882 xenografts retained the original characteristics of vimentin, cytokeratin, and factor VIII labeling.

CONCLUSIONS

CSOC 882 is an immortalized cell line that has survived more than 130 passages in culture and retained molecular features of the primary tumor from which it was derived. Compared to the most common ovarian carcinoma cell lines, CSOC 882 is a unique resource for genetic and cellular research on ovarian cancer.

E2F6 negatively regulates ultraviolet-induced apoptosis via modulation of BRCA1

E2F6 is believed to repress E2F-responsive genes and therefore plays an important role in cell-cycle regulation. However, the role of E2F6 in the control of apoptosis remains unknown. We show here that the expression of E2F6 was downregulated with a concurrent increase in BRCA1 mRNA and cleaved protein during ultraviolet (UV)-induced apoptosis in human embryonic kidney 293 cells. Moreover, E2F6 overexpression distinctly inhibited UV-induced apoptosis as well as UV-induced increases in BRCA1 expression and cleavage, accompanied with increases of the full-length BRCA1 and BRCA1 nuclear foci. In contrast, knockdown of E2F6 by small interfering RNA had opposite effects. Furthermore, these effects of E2F6 on BRCA1 depended upon the association of E2F6 with BRCA1 via its C-terminus in a UV-triggered manner and upon the transcriptional repression by E2F6 on the BRCA1 promoter. These findings provide the first demonstration of the important role for E2F6 in the control of apoptosis via targeting of BRCA1.

Staurosporine modulates radiosensitivity and radiation-induced apoptosis in U937 cells

PURPOSE

The present study aims at investigating the involvement of several genes in the cell cycle distribution and apoptosis in U937 cells, a cell line lacking functional p53 protein, after combined treatment with staurosporine and irradiation.

MATERIALS AND METHODS

Using a DNA fragmentation assay, flow cytometry and western blot analysis, the molecular basis for the effects of staurosporine in combination with the irradiation of leukemia cells was investigated.

RESULTS

Our results indicated that combined treatment led to an increased apoptotic cell death in U937 cells, which is correlated with the phosphorylation of the V-Jun sarcoma virus 17 oncogene homolog (c-JUN) NH(2)-terminal kinase protein (JNK), the activation of caspases, the increase in B cell leukemia/lymphoma 2 (Bcl-2) associated X protein (Bax), the decrease in Bcl xL protein (Bcl-XL) levels, the loss of mitochondria membrane potential and the release of cytochrome c.

CONCLUSIONS

Abrogation of the G2 checkpoint should be an effective strategy against p53-deficient leukemia cells to irradiation-induced cell killing.

Expression of common chromosomal fragile site genes, WWOX/FRA16D and FHIT/FRA3B is downregulated by exposure to environmental carcinogens, UV, and BPDE but not by IR

Common chromosomal fragile sites are unstable genomic loci susceptible to breakage, rearrangement, and are highly recombinogenic. Frequent alterations at these loci in tumor cells led to the hypothesis that they may contribute to cancer development. The two most common chromosomal fragile sites FRA16D and FRA3B which harbor WWOX and FHIT genes, respectively, are frequently altered in human cancers. Here we report that environmental carcinogens, ultraviolet (UV) light, and Benzo[a]pyrene diol epoxide (BPDE), significantly downregulate expression of both genes. On the other hand, we observe that ionizing radiation (IR) does not affect expression of these genes, suggesting that the effect of repression exerted by UV and BPDE is not just a consequence of DNA damage but may be a result of different signaling pathways triggered by specific DNA lesions. Such downregulation correlates with an induction of an S-phase delay in the cell cycle. Treatment of UV-irradiated cells with caffeine abrogates the S-phase delay while concomitantly overcoming the repression phenomenon. This suggests the involvement of unique cell cycle checkpoint mechanisms in the observed repression. Therefore, it is hypothesized that protracted downregulation of the putative tumor suppressor genes WWOX and FHIT by environmental carcinogens may constitute an additional mechanism of relevance in the initiation of tumorigenesis.

Development and applications of a real-time quantitative RT-PCR method (QRT-PCR) for BRCA1 mRNA

OBJECTIVES

To develop a real-time quantitative RT-PCR method for BRCA1 mRNA and then use it for the study of BRCA1 gene expression in human MCF-7 breast cancer cells after their exposure to antineoplastic agents and gamma irradiation.

DESIGN AND METHODS

The developed QRT-PCR method is based on the real-time monitoring of a fluorescein-labeled TaqMan probe, specific for BRCA1 mRNA, during PCR in the LightCycler. A BRCA1 PCR amplicon was purified, quantitated and used as a standard of known concentration for the development and analytical evaluation of the assay. The method was applied to study the alteration of BRCA1 gene expression after exposure to taxol, doxorubicin, 5-fluorouracil, etoposide or gamma irradiation in human MCF-7 breast cancer cells.

RESULTS

The developed method is quantitative, highly specific for mRNA and highly sensitive (detection limit of 4 BRCA1 copies per mug of total RNA). We observed a reduction of BRCA1 expression for all antineoplastic agents used, while the gamma irradiated MCF-7 cells had an increase of expression with a peak at the 10 Gy dose.

CONCLUSIONS

The developed BRCA1 QRT-PCR method is quantitative, highly sensitive and specific. The proposed method is rapid, automated, and cost effective and can be used to study BRCA1 expression in a variety of clinical samples.

The DNA double-strand break response pathway: becoming more BRCAish than ever

Breast carcinoma is the leading cause of cancer incidence, and second in cancer mortality to lung cancer, in women of the Western hemisphere. Germ line mutations in the breast cancer susceptibility gene, BRCA1, is responsible for half of all cases of hereditary breast cancer, which constitutes about 5-10% of all cases of breast cancer. Current hypothesis has ascribed a role for Brca1 in maintaining genomic stability, through its involvement in cellular response pathway to the DNA double-strand breaks (DSB). DNA DSB, which are the most deleterious form of DNA damage, are repaired through a series of coordinated steps embedded in a signal transduction pathway that ultimately ensure the elimination of potentially harmful mutations to the genome. This pathway can be crudely divided into a primary and secondary phase. The primary response phase is initiated by sensor proteins that activate transducer protein kinases Atm and Atr, which target downstream effector proteins, such as Chk1 and Chk2, to elicit the secondary response phase. Brca1 has been intimately linked with various aspects of this signaling pathway. However, the precise role of Brca1 in this process remains unclear. In this review, we will provide a simple model in an attempt to clarify the role of Brca1 during cellular response to DNA DSB.

DNA damage induces p53-dependent BRCA1 nuclear export

The tumor suppressor gene BRCA1 plays an important role in the response to DNA damage. BRCA1 function is regulated by a variety of mechanisms including transcriptional control, phosphorylation, and protein-protein interactions. Recent studies have shown that BRCA1 is a nuclear-cytoplasmic shuttle protein. Its subcellular localization is controlled by a nuclear localization signal-mediated nuclear import via the importin receptor pathway and a nuclear export signal-facilitated nuclear export through a CRM1-dependent pathway. Using the human breast cancer cell line, MCF7, the subcellular distribution of BRCA1 was assessed by immunohistochemical staining and Western blotting analyses of fractionated subcellullar extracts. Ionizing radiation stimulated BRCA1 nuclear export in a dose-dependent manner. This DNA damage-induced BRCA1 nuclear export utilized a CRM1-dependent mechanism and also required wild-type p53, whose function was abrogated by the E6 protein in MCF7 cells. In addition, the dependence on p53 was confirmed using a second cell type operating a tetracycline-inducible system. The effect of ionizing radiation on BRCA1 export was observed in every phase of the cell cycle, although BRCA1 localization did vary between the G(1), S, and G(2)/M phases. These results imply that, in addition to ATM-, ATR-, and Chk2-dependent phosphorylations, cytoplasmic relocalization of BRCA1 protein is a mechanism whereby BRCA1 function is regulated in response to DNA damage.

BRCA1 gene in breast cancer

The BRCA1 gene was identified and cloned in 1994 based its linkage to early onset breast cancer and breast-ovarian cancer syndromes in women. While inherited mutations of BRCA1 are responsible for about 40-45% of hereditary breast cancers, these mutations account for only 2-3% of all breast cancers, since the BRCA1 gene is rarely mutated in sporadic breast cancers. However, BRCA1 expression is frequently reduced or absent in sporadic cancers, suggesting a much wider role in mammary carcinogenesis. Since BRCA1 was cloned in 1994, its molecular function has been the subject of intense investigation. These studies have revealed multiple functions of the BRCA1 that may contribute to its tumor suppressor activity, including roles in: cell cycle progression, several highly specialized DNA repair processes, DNA damage-responsive cell cycle check-points, regulation of a set of specific transcriptional pathways, and apoptosis. Many of these functions are linked to protein:protein interactions involving different portions of the 1,863 amino acid (aa) BRCA1 protein. BRCA1 functions in cell cycle progression and the DNA damage response appear to be regulated by distinct and specific phosphorylation events, but the molecular pathways activated by these phosphorylations are only beginning to be unraveled. In addition, the reason that BRCA1 mutation carriers develop specific tumor types (breast and ovarian cancers in women and possibly prostate cancers in men) is not clearly understood. Elucidation of the precise molecular functions of the BRCA1 gene product will greatly enhance our understanding of the pathogenesis of hereditary as well as sporadic mammary carcinogenesis.

p53 mediates repression of the BRCA2 promoter and down-regulation of BRCA2 mRNA and protein levels in response to DNA damage

Adriamycin and other DNA-damaging agents have been shown to reduce BRCA2 mRNA levels in breast cancer cell lines, but the mechanism by which this occurs is unknown. In this study, we show that adriamycin and mitomycin C, but not other DNA-damaging agents, repress BRCA2 promoter activity in a dose- and time-dependent manner. We demonstrate that the effect is dependent on wild type p53 and that adriamycin and p53 mediate repression of the BRCA2 promoter by inhibiting binding of an upstream stimulatory factor protein complex to the promoter. In addition, we present evidence indicating that adriamycin and other DNA-damaging agents reduce BRCA2 mRNA and protein levels by altering both BRCA2 mRNA stability and protein stability. Thus, BRCA2 levels in the cell are regulated by three independent mechanisms in a p53-dependent manner.

Role of BRCA1 in heat shock response

The heat shock response is an evolutionarily conserved response to heat and other stresses that promotes the maintenance of key metabolic functions and cell survival. We report that exposure of human prostate (DU-145) and breast (MCF-7) cancer cells to heat (42 degrees C) caused a rapid disappearance of the breast cancer susceptibility gene-1 (BRCA1) protein, starting at approximately 1 h after the onset of heating and slightly lagging behind the increase in heat shock protein 70 (HSP70) levels. The heat-induced loss of BRCA1 occurred at the protein level, since: (1) BRCA1 mRNA expression was unaffected; and (2) the BRCA1 protein loss was also observed in DU-145 cells that expressed exogenous wild-type BRCA1 (wtBRCA1). In addition to heat regulation of BRCA1 protein levels, we also found that BRCA1 could modulate the heat shock response. Thus, wtBRCA1 overexpressing DU-145 cell clones showed significantly decreased sensitivity to heat-induced cytotoxicity; and Brca1 mutant mouse embryo fibroblasts showed increased sensitivity to heat. The DU-145 wtBRCA1 clones also showed increased expression of the small heat shock protein HSP27; and reporter assays revealed that wtBRCA1 stimulated a two to four-fold increase in HSP27 promoter activity, consistent with its ability to upregulate HSP27 mRNA and protein levels. In studies using epitope-tagged truncated BRCA1 proteins, the ability to stimulate the HSP27 promoter and to mediate heat-induced degradation required the amino-terminus but not the carboxyl-terminus of BRCA1. Although the heat-induced loss of BRCA1 appeared to be due to protein degradation, various protein metabolic agents (or combinations) failed to block this event, including: MG132 (a 26S proteasomal inhibitor), N-acetyl-leucyl-leucyl-norleucinal (a calpain inhibitor), z-VAD-fmk (a pan-caspase inhibitor), and ammonium chloride and chloroquine (which stabilize lysosomes). These findings suggest that in addition to its other functions, BRCA1 may participate in mammalian heat shock response pathways.

Enhanced DNA repair of cyclobutane pyrimidine dimers changes the biological response to UV-B radiation

The goal of DNA repair enzyme therapy is the same as that for gene therapy: to rescue a defective proteome/genome by introducing a substitute protein/DNA. The danger of inadequate DNA repair is highlighted in the genetic disease xeroderma pigmentosum. These patients are hypersensitive to sunlight and develop multiple cutaneous neoplasms very early in life. The bacterial DNA repair enzyme T4 endonuclease V was shown over 25 years ago to be capable of reversing the defective repair in xeroderma pigmentosum cells. This enzyme, packaged in an engineered delivery vehicle, has been shown to traverse the stratum corneum, reach the nuclei of living cells of the skin, and enhance the repair of UV-induced cyclobutane pyrimidine dimers (CPD). In such a system, changes in DNA repair, mutagenesis, and cell signaling can be studied without manipulation of the genome.

Thymic model for examining BRCA2 expression and function

Mutations in the human BRCA2 breast cancer susceptibility gene are associated with increased risks of breast, ovarian, and other cancers. BRCA2 has been hypothesized to function in processes of DNA damage/breakage repair, cell proliferation, and apoptosis. These processes continually occur in the thymus during thymocyte development, and BRCA2 mRNA is highly expressed in thymus relative to most other organs. We therefore used the thymus as a model system to study BRCA2 expression and function. Quantitative reverse transcription polymerase chain reaction experiments showed that highly activated immature CD4(+) CD8(+) double-positive human thymocytes that exhibited high levels of proliferation and apoptosis had increased BRCA2 mRNA levels relative to other thymocyte subsets. BRCA2 mRNA levels were upregulated in thymocytes treated with the DNA-damaging agent etoposide. Only modest increases were associated with proliferation in human peripheral lymphocytes in response to concanavalin A (ConA) mitogen. Mice homozygous for a targeted mutation in Brca2 exon 27 (Brca2(Delta27/Delta27)) showed normal thymic architecture but had 18% decreased thymocyte cellularity compared with wild-type mice. Thymocytes from these Brca2(Delta27/Delta27) mice displayed decreased apoptosis in response to etoposide-induced DNA damage compared with wild-type thymocytes. These studies suggest that BRCA2 mRNA levels are modulated during DNA damage and may be important during apoptosis.

Caspase-3 mediated cleavage of BRCA1 during UV-induced apoptosis

The breast cancer suppressor protein, BRCA1 plays an important role in mediating cell cycle arrest, apoptosis and DNA responses to DNA damage signals. In this study, we show that BRCA1 level is downregulated during UV-induced apoptosis by caspase-3 mediated cleavage. Cleavage of BRCA1 by caspase-3 produced a fragment that contained the C-terminal of the molecule. Accordingly, treatment of cells with caspase-3 inhibitor or mutation of a specific caspase-3 cleavage site (DLLD) at amino acid 1151-1154 of BRCA1 abolished cleavage and consequential accumulation of the BRCA1 C-terminal fragment. Whereas expression of the non-cleavable BRCA1 (D/A 1154) mutant conferred the resistance phenotype to UV-induced cell death, expression of the cleaved BRCA1 C-terminus induced cell death in the absence of UV. Examination of the mechanism of C-terminus-induced cell death revealed that the cleaved fragment triggers the apoptotic response through activation of BRCA1 downstream effectors, GADD45 and JNK. Altogether, results of our study demonstrate a functional role for caspase-3 mediated cleavage of BRCA1 during UV-induced apoptosis.

Inhibition of BRCA1 leads to increased chemoresistance to microtubule-interfering agents, an effect that involves the JNK pathway

We have developed ribozymes (Rz) that inhibit BRCA1 expression in order to study the role of this gene in chemosensitivity. Two Rz, targeting positions 358 or 5282 of the BRCA1 mRNA, were cloned into the retroviral vector LXSN and lipofected into the breast cancer cell-line HBL100. We obtained 79-99% inhibition of BRCA1 expression, as determined by real-time quantitative PCR and by Western blotting. Decreased expression of BRCA1 led to sensitivity to the DNA damaging agents cisplatin and etoposide, resistance to the microtubule-interfering agents (MIA) taxol and vincristine. The molecular mechanism of resistance to MIA was investigated further by determining the status of the JNK pathway. We found that JNK1 expression was elevated, while JNK2 expression was decreased in Rz-expressing clones compared to controls. We have quantified the mRNA levels of BRCA1, JNK1, 2, MEK-4, -7 and c-jun after treatment with MIA. Vincristine treatment of control cells resulted in transcriptional repression of BRCA1, while the JNK1, 2, MEK-4, -7 and c-jun genes were induced. In Rz-treated cells, only JNK1 and MEK-4 were expressed and none was induced after MIA treatment. We then studied the phosphorylation of c-jun, a downstream effector of the JNK pathway. We observed a strong increase in phosphorylated c-jun after MIA treatment of the control cells but not in BRCA1-Rz treated cells, suggesting inhibition of the JNK pathway. These results show that the BRCA1-JNK pathway is involved in the cytotoxic response to MIA treatment, and inhibition of BRCA1 leads to transcriptional modifications of the JNK pathway.

BRCA1 and prostate cancer

The breast cancer susceptibility gene BRCA1 on chromosome 17q21 encodes an 1863 amino acid protein that is important for normal embryonic development. Germline mutations of this gene are linked to a significantly increased lifetime risk for breast and/or ovarian cancer, and recent studies suggest that the same may be true for prostate cancer. Several activities that may contribute to the tumor suppressor function of BRCA1 have been identified via in vitro and experimental animal studies. These include (i) regulation of cell proliferation; (ii) participation in DNA repair/recombination processes related to the maintenance of genomic integrity; (iii) induction of apoptosis in damaged cells; and (iv) regulation of transcription. A second breast cancer susceptibility gene (BRCA2) operates in some of the same molecular pathways as BRCA1, and mutations of this gene predispose to breast and ovarian cancer and probably to other tumor types, including prostate cancer. Finally, recent studies from our laboratory suggest that BRCA1 modulates proliferation, chemosensitivity, repair of DNA strand breaks, apoptosis induction, and expression of certain key cellular regulatory proteins (including BRCA2 and p300) in human prostate cancer cells. These activities are consistent with a putative prostate tumor suppressor function of BRCA1.

Repression of BRCA1 through a feedback loop involving p53

The BRCA1 and p53 tumor suppressors have been shown to interact and cooperate to activate transcription of p53-responsive genes. In this study, we show that BRCA1 is initially up-regulated, followed by a reduction to below basal levels in response to treatment with the DNA-damaging agents adriamycin and mitomycin C, and that the reduction of BRCA1 expression is dependent on the presence of wild-type p53. Elimination of p53 by expression of human papilloma virus E6 resulted in an inability to down-regulate BRCA1 in response to adriamycin. Ectopic expression of p53 resulted in a rapid decrease in BRCA1 protein and RNA levels and BRCA1 promoter-driven luciferase activity even in null p21 cells deficient in p53-dependent G(1) arrest. ATM(-)(/-) lymphoblastoid cells were deficient in their ability to reduce BRCA1 protein in response to DNA damage, whereas the wild-type counterparts reduced BRCA1 protein levels after exposure to adriamycin. These results, in conjunction with others, suggest a loop wherein BRCA1 initially participates in accumulation of p53 protein, whereas later p53 acts to reduce BRCA1 expression.

Increased sensitivity to chromatid aberration induction by bleomycin and neocarzinostatin results from alterations in a DNA damage response pathway

DNA damage response pathways coordinate the cellular response to DNA damage. To investigate the roles of tumor suppressor genes in these pathways, human lymphoblastoid cells (wild-type, p53-/-, ATM-/-) were treated for 1 h with 0-3 microg/ml of the radiomimetic compound bleomycin (BLM), and cells treated in G(2) were analyzed for chromatid aberrations. BLM-induced aberration frequencies were significantly increased, to the greatest extent in the ATM-/- cells and, to a lesser extent, in the p53-/- cells compared to wild-type cells. These observations are consistent with p53 and ATM acting in a damage response pathway activated by DNA strand breaks. The consequences of disrupting this pathway were further investigated by studies using wortmannin, a PI-3 kinase and DNA repair inhibitor. Wortmannin significantly increased the BLM-induced aberration frequencies in all but the ATM-/- cells, elevating the sensitivity of p53-/- cells to ATM-/- levels and that of wild-type cells to intermediate levels. These differential sensitivities suggest that the ATM phenotype is the result of dual cellular defects, one involving p53 and the other a wortmannin-sensitive component. Similar studies in Brca1+/- and Brca2+/- human lymphoblasts showed no increased sensitization to BLM in the absence of inhibitor, and differential sensitization by wortmannin. To determine if there was any substrate specificity for p53- and ATM-mediated DNA damage responses, chromatid aberrations were assessed in wild-type, p53-/-, and ATM-/- cells exposed to 0-0.4 microg/ml neocarzinostatin (NCS) for 1 h. In contrast to results with BLM, the p53-/- cells exhibited a low sensitivity to NCS-induced aberrations, similar to wild-type, while ATM-/- cells remained highly sensitive. This suggests that the response to BLM- and NCS-induced lesions involves different mechanisms.

Altered expression of BRCA1, BRCA2, and a newly identified BRCA2 exon 12 deletion variant in malignant human ovarian, prostate, and breast cancer cell lines

Germline mutations of BRCA1 and BRCA2 predispose to hereditary breast, ovarian, and possibly prostate cancer, yet structural mutations in these genes are infrequent in sporadic cancer cases. To better define the involvement of these genes in sporadic cancers, we characterized expression levels of BRCA1 and BRCA2 transcripts in cancer cell lines derived from neoplasms of the ovary, prostate, and breast and compared them with those expressed in primary cultures of normal epithelial cells established from these organs. We observed upregulation of BRCA1 and/or BRCA2 expression in six of seven ovarian cancer cell lines (OVCA420, OVCA429, OVCA432, ALST, DOV13, and SKOV3) when compared with levels found in normal ovary surface epithelial cells. Furthermore, five cancerous or immortalized prostatic epithelial cell lines (BPH-1, TSU-Pr1, LNCaP, PC-3, and DU145) also expressed higher levels of BRCA1 and/or BRCA2 mRNA than did primary cultures of normal prostatic epithelial cells. In contrast, only the estrogen receptor-positive MCF-7 cell line overexpressed these messages, whereas the estrogen receptor-negative breast cancer cell lines Hs578T, MDA-MB-231, and MDA-MB-468 showed no change in expression levels when compared with normal breast epithelial cells. In addition, expanding on our recent identification of a novel BRCA2 transcript variant carrying an in-frame exon 12 deletion (BRCA2 delta 12), we report increased expression of this variant in several ovarian, prostate, and mammary cancer cell lines (OVCA420, OVCA433, ALST, DOV13, SKOV3, TSU-Pr1, DU145, and MDA-MB-468). Most notably, high levels of BRCA2 delta 12 mRNA were detected in an estrogen receptor-positive breast cancer cell line, MCF-7, and in an androgen-independent prostate cancer cell line, DU-145. Interestingly, the wild-type BRCA2 transcript was barely detectable in DU145, which could be used as a model system for future investigations on BRCA2 delta 12 function. Taken together, our data suggest disruption of BRCA1 and/or BRCA2 gene expression in certain epithelial cancer cell lines of the ovary, prostate, and breast. Because wild-type BRCA1 and BRCA2 gene products increase during cell-cycle progression and are believed to exert growth-inhibitory action, enhanced expression of these genes in cancer cells may represent a negative feedback mechanism for curbing proliferation in fast-growing cells. At present, the functionality of BRCA2 delta 12 remains elusive.

Nuclear localization and cell cycle-specific expression of CtIP, a protein that associates with the BRCA1 tumor suppressor

The BRCA1 tumor suppressor has been implicated in a diverse spectrum of cellular processes, including transcriptional regulation, DNA repair, and cell cycle checkpoint control. CtIP was recently identified as a protein that associates with BRCA1 and two other nuclear factors, CtBP1 and Rb1. To understand the functions of CtIP, we have evaluated its biological properties with respect to those of BRCA1. Our results show that CtIP, like its associated factors, is predominantly a nuclear protein. A subset of the endogenous pool of CtIP polypeptides exists in a protein complex that includes both BRCA1 and the BRCA1-associated RING domain protein (BARD1). At the protein level, CtIP expression varies with cell cycle progression in a pattern identical to that of BRCA1. Thus, the steady-state levels of CtIP polypeptides, which remain low in resting cells and G(1) cycling cells, increase dramatically as dividing cells traverse the G(1)/S boundary. In contrast to BRCA1, however, the G(1)/S induction of CtIP expression is mediated primarily by post-transcriptional mechanisms. Finally, the interaction between CtIP and BRCA1 is shown to be stable in the face of genotoxic stress elicited by treatment with UV light, adriamycin, or hydrogen peroxide. Together, these results indicate that CtIP can potentially modulate the functions ascribed to BRCA1 in transcriptional regulation, DNA repair, and/or cell cycle checkpoint control.

Isolation of a functional copy of the human BRCA1 gene by transformation-associated recombination in yeast

The BRCA1 gene, mutations of which contribute significantly to hereditary breast cancer, was not identified in the existing YAC and BAC libraries. The gene is now available only as a set of overlapping fragments that form a contig. In this work we describe direct isolation of a genomic copy of BRCA1 from human DNA by transformation-associated recombination (TAR) cloning. Despite the presence of multiple repeats, most of the primary BRCA1 YAC isolates did not contain detectable deletions and could be stably propagated in a host strain with conditional RAD52. Similar to other circular YACs, approximately 90kb BRCA1 YACs were efficiently and accurately retrofitted into bacterial artificial chromosomes (BACs) with the Neo(R) mammalian selectable marker and transferred as circular BAC/YACs in E. coli cells. The BRCA1 BAC/YAC DNAs were isolated from bacterial cells and were used to transfect mouse cells using the Neo(R) gene as selectable marker. Western blot analysis of transfectants showed that BRCA1 YACs isolated by a TAR cloning contained a functional gene. The advantage of this expression vector is that the expression of BRCA1 is generated from its own regulatory elements and does not require additional promoter elements that may result in overexpression of the protein. In contrast to the results with cDNA expression vectors, the level of BRCA1 expression from this TAR vector is stable, does not induce cell death, maintains serum regulation, and approximates the level of endogenously expressed BRCA1 in human cells. The entire isolation procedure of BRCA1 described in this paper can be accomplished in approximately 10 days and can be applied to isolation of gene from clinical material. We propose that the opportunity to directly isolate normal and mutant forms of BRCA1 will greatly facilitate analysis of the gene and its contribution to breast cancer.

Establishment and characterization of a breast cell strain containing a BRCA1 185delAG mutation

OBJECTIVE

The aim of this study was to examine whether cells containing the heterozygous form of a BRCA1 185delAG mutation would exhibit abnormal growth or an altered response to DNA damage.

METHODS

A primary culture of human mammary epithelial cells (90P) was obtained from the nontumor breast tissue of a 35-year-old patient who had undergone a mastectomy for removal of a breast tumor. These cells were immortalized (90PE6E7) following retroviral infection with HPV-16 viral E6/E7. genes. Both the 90P cell strain and the cell line were characterized for their ability to grow in culture, form colonies in soft agar, and produce tumors in athymic nude mice compared to normal breast epithelial cells containing wild-type BRCA1. 90P cells were also analyzed for cellular response to gamma radiation and H(2)O(2).

RESULTS

These cells were confirmed to contain a frameshift mutation, 185delAG, of the BRCA1 gene. Despite being heterozygous for wild-type BRCA1, the 220-kDa full-size BRCA1 protein was abundantly expressed. 90P and 90PE6E7 cells grew at a similar rate and were anchorage dependent. 90PE6E7 also failed to form tumors in athymic nude mice. Finally, 90P cells exhibited a survival response similar to that of normal mammary epithelial cells to radiation damage and exposure to oxidative stress.

CONCLUSION

To our knowledge the 90P cells and the 90PE6E7 cells are the first characterized, non-tumor-derived breast epithelial cells that are heterozygous for the BRCA1 germline mutation 185delAG. Our conclusion is that these BRCA1 mutant cells appear to have growth and stress response characteristics similar to those of normal human breast cells, which is consistent with the hypothesis that loss of heterozygosity must occur to impair putative BRCA1 function.

Inhibition of BRCA-1 expression by benzo[a]pyrene and its diol epoxide

The objective of this study was to investigate whether polycyclic aromatic hydrocarbons (PAHs) contribute to the etiology of sporadic breast cancer by altering the expression of BRCA-1. Acute exposure to the PAH benzo[a]pyrene (B[a]P) inhibited in a time- and dose-dependent fashion cell proliferation and levels of BRCA-1 mRNA and protein in estrogen receptor (ER)-positive breast MCF-7 and ovarian BG-1 cancer cells. Moreover, the acute exposure to B[a]P abrogated estrogen induction of BRCA-1 in MCF-7 cells. The loss of BRCA-1 expression was prevented by the aromatic hydrocarbon receptor (AhR) antagonist alpha-naphthoflavone, suggesting participation of the AhR pathway. BRCA-1 exon 1a transcripts were downregulated by B[a]P faster than exon 1b mRNA was. Long-term exposure to B[a]P (40 nM for 15 mo) lowered BRCA-1 mRNA levels in subclones of MCF-7 and BG-1 cells, whereas expression of BRCA-1 in these clones was reverted to normal levels by washing out of B[a]P. The mechanisms of BRCA-1 repression by B[a]P were further investigated by examining the effects of the halogenated aryl hydrocarbon 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and the B[a]P metabolite 7r, 8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE). While TCDD did not influence basal BRCA-1 mRNA and protein levels at any of the doses (from 10 nM to 1 microM) tested in this study, treatment with 50 nM BPDE drastically reduced BRCA-1 mRNA levels, indicating that metabolism of B[a]P to BPDE may contribute to downregulation of BRCA-1. Conversely, ER-negative breast MDA-MB-231 and HBL-100 cancer cells were refractory to treatment with B[a]P or TCDD and expressed constant levels of BRCA-1 mRNA and protein. We conclude that B[a]P may be a risk factor in the etiology of sporadic breast cancer.

Coordinate alterations in the expression of BRCA1, BRCA2, p300, and Rad51 in response to genotoxic and other stresses in human prostate cancer cells

BACKGROUND

BRCA1 and BRCA2 participate in cell cycle progression, apoptosis, and DNA repair pathways. The latter role may be mediated by interaction with DNA recombinase Rad51. The purpose of this study was to evaluate the effects of genotoxic and other cytotoxic agents on expression of DNA damage-response genes (BRCA1, BRCA2, p300, and Rad51) in human prostate cancer cells.

METHODS

Subconfluent proliferating cultures of Tsu-Prl or DU-145 cells were treated with various stressful agents and assayed 24 hr later for alterations in: 1) mRNA expression (by semiquantitative reverse transcription-PCR); 2) cell viability (by trypan blue dye exclusion); and 3) protein expression (by Western blotting).

RESULTS

Of 26 agents screened, BRCA1 and BRCA2 mRNA reductions were observed in both cell lines after exposure to adriamycin (ADR), camptothecin (CPT), sodium selenite (SLN), and ultraviolet radiation (UV), while nitrogen mustard (HN2) caused mRNA reduction in DU-145 but not in Tsu-Prl. Inhibition of BRCA1/2 expression by ADR and HN2 was blocked by cycloheximide, suggesting that this requires new protein synthesis, while inhibition by CPT, SLN, and UV did not require protein synthesis. Reduction of p300 and Rad51 mRNA levels occurred in parallel with that of BRCA1/2, suggesting coordinate regulation of these genes. The ability of an agent to inhibit mRNA expression was not directly correlated with cytotoxicity. ADR, CPT, UV, and SLN also caused reduction of protein levels; but the kinetics of decreases in protein vs. mRNA differed. After ADR treatment, high molecular weight (Mr hyperphosphorylated) BRCA1 decreased more rapidly than the low Mr species. BRCA2 showed a more rapid decrease in protein than mRNA, while Rad51 showed the opposite. By 48 and 72 hr post-ADR, all four mRNAs and proteins were reduced to well below control levels, except for Rad51 protein, which was only moderately decreased.

CONCLUSIONS

Selected DNA-damaging agents (ADR, CPT, and UV) and a reducing agent (SLN) inhibited BRCA1/2, p300, and Rad51 expression in prostate cancer cells, although decreases in mRNA vs. protein did not coincide. We postulate that temporal changes in relative protein levels affect different phases of the stress response, and that the ultimate downregulation of all four genes promotes prostate cancer survival.