BRCA1 Regulates PIG3-mediated Apoptosis In A P53 Dependent Manner

Background and objective:The breast cancer susceptibility gene 1(BRCA1) is a tumor suppressor gene located in 17q21 which is first identified by Hall in 1990. The occurrence of genetic mutations in humans is by far the most common hereditary cancer risk of a contributing factor, often caused by breast or ovarian cancer in women. BRCA1 has important roles in multiple cellular processes, including DNA damage repair, cell cycle checkpoint control, transcriptional regulation, chromatin remodeling, and apoptosis. It has been reported that BRCA1 can modulate the transcription of p53. BRCA1 (residues 224-500) interacts with p53 (residues 300-393), leading to increased transcription from p53-responsive promoters such as p21 and bax and induction of cancer cell apoptosis. The p53 inducible gene 3 (PIG3) is a downstream target of p53 and is involved in p53-initiated apoptosis. However, little is known about whether BRCA1 can regulate PIG3-mediated apoptosis and the possible mechanisms have not yet been fully elucidated. In this study, we investigate whether BRCA1 can regulate PIG3 and to reveal the mechanism of BRCA1 in modulating PIG3 and the relationship with OS in breast cancer patients. Provide the basis understanding of BRCA1 that might serve as predictive biomarkers of prognosis in breast cancer.Methods:1. Immunohistochemistry staining was performed to detect the relationship between BRCA1 and PIG3 expression and the association obetween BRCA1 and/or PIGS expression with OS in human breast cancer.2. MTT assay and Flow cytometric analysis were performed to detect the effect of BRCA1 on the proliferation and apoptosis of breast cancer cells3. CHIP、luciferase reporter assays and western blot analysis were performed to observe the binding of PHB1 to the (TGYCC)15 motif of PIG3 promoter and the effect of PHB1 on modulating PIG3 expression.4. The possible mechanisms of BRCA1 in modulating PIG3 was detected by RT-PCR, western blot, Immunofluorescence.Results:1. Immunohistochemistry staining of the BRCA1 and PIG3 in malignant tumor samples from 149 patients showed that high expression of PIGS and/or BRCA1 was associated with better OS in human breast cancer patients(all P<0.05) and significant positive correlation was found between BRCA1 and PIG3 expression in this breast cancer tissue-array(r=0.678, P<0.001).2. BRCA1 inhibits cell proliferation and induces apoptosis of p53 wild-type breast cancer cells(P<0.05), but it has no effect on p53-null breast cancer cells(P>0.05).3. BRCA1 upregulates PIG3 expression in p53 wild-type breast cancer cells(P<0.05), but in p53-null breast cancer cells it has no significant effect on PIG3 expression(P>0.05).4. CHIP and luciferase reporter assays showed that PHB1 can bind to the (TGYCC)15 motif of the PIG3 promoter regardless of p53 status and CPT treatment and therefore activated PIG3 transcription(P<0.05).5. PHB1 can upregulates PIG3 expression in a p53-depentent or -independent manner by transiently transfected assay and Western blot analysis(P<0.05).6. BRCA1 has no significant effect on PHB1 expression regardless of p53 status in breast cancer cells(P>0.05).Conclusion:BRCA1 positively regulates PIG3 expression in a p53-dependent manner, and thereby to inhibit the proliferation of breast cancer cells and promote tumor cell apoptosis. Our clinical data demonstrate high PIG3 and/or BRCA1 expression were associated with better OS in human breast cancer patients.