Introduction
Germline mutations in the breast and ovarian cancer susceptibility gene BRCA1, which is located on chromosome 17q21, are associated with a predisposition to the development of cancer in these organs [1,2]. Initial analyses [22] suggested that women with germline mutations in the BRCA1 gene and a strong family history of breast or ovarian cancer have 85 and 44% lifetime risks of developing breast and ovarian cancer, respectively. Recent studies [23], however, have suggested that analyses based on women who were not selected for a familial history of cancer indicate that the risk for cancer associated with mutations in these genes is 50 and 16% for breast and ovarian cancers, respectively. No mutations in the BRCA1 gene have been detected in sporadic breast cancer cases; however, mutations have been detected in sporadic cases of ovarian cancer [3,4]. Although there is debate regarding the level of cancer risk associated with mutations in BRCA1 and the significance of the lack of mutations in sporadic tumors, it is possible that alterations in the function of BRCA1 may occur by mechanisms other than mutation. This would lead to an underestimation of risk when it is calculated solely on the basis of mutational analysis. Such alterations cannot be identified until the function and regulation of BRCA1 are better understood.
The BRCA1 gene encodes a 220-kDa nuclear protein that may be regulated by phosphorylation through the cell cycle and in response to DNA damaging agents [5,6,7]. The level of BRCA1 is also regulated in response to estrogen or estrogen-induced growth in breast [8,9,10,11] and ovarian cell lines. BRCA1 has been shown to colocalize in nuclear dots with other cellular proteins, including BARD-1 [24], Rad51, PCNA, and BRCA2 [7,25]. In addition, BRCA1 can act as a transcriptional transactivator in yeast reporter assays [26,27] and binds the RNA polymerase II holoenzyme, a component of the basal transcription machinery [25]. The precise mechanism of action and the specific signaling pathway affected by BRCA1 remain unknown, however.
Studies of BRCA1 expression patterns in mouse tissue reveal that BRCA1 is most highly expressed in tissues undergoing rapid proliferation and differentiation, and that expression in vivo is also hormone responsive. For example, analyses of mammary gland growth and development show high levels of BRCA1 expression in terminal end buds during puberty and in budding alveoli during pregnancy. In addition, hormonal stimulation in ovariectomized mice results in induction of BRCA1 expression in the breast [28]. Attempts to develop homozygous, BRCA1-deleted mouse models have resulted in embryonic lethality [29,30]. For example, when the BRCA1 gene deletion was targeted in exons 5 and 6, mutant mice died before day 7.5 of embryogenesis. Analysis of DNA synthesis in the mutant embryos indicated that cell proliferation was impaired, suggesting that BRCA1 may paradoxically play a positive role in the regulation of embryonic cell growth [29].
Most of the mechanistic BRCA1 studies to date have been conducted in breast carcinoma cell lines; therefore, we decided to conduct a study to determine the effect of BRCA1 expression on the cellular phenotype of an ovarian carcinoma cell line, BG-1. BG-1 cells were derived from a patient with stage III, poorly differentiated ovarian adenocarcinoma [13]. This cell line, which expresses wild-type BRCA1, is estrogen responsive, and withdrawal of estrogen results in eventual cell death. Previous studies suggested that BRCA1 is stimulated as a result of estrogen treatment [8,9,10,11], and that BRCA1 may be involved in the cell death process [14]. Therefore, we examined the effect of reduction of BRCA1 levels in BG-1 cells on the cellular response to estrogen stimulation as well as hormone depletion. Our results suggest that when BG-1 cells are subjected to growth restrictive and hormone-depleted conditions, cells that have even moderately reduced levels of BRCA1 protein have a distinct advantage for survival. In addition, significant reduction in BRCA1 protein level correlates with enhanced estrogen proliferation when compared with cells that express moderate to wild-type BRCA1 levels, grown under optimal growth conditions both in vitro and in vivo.