BCL6 is a negative prognostic factor and exhibits pro-oncogenic activity in ovarian cancer

Abstract

Background: Dysregulation of BCL6 plays critical oncogenic roles and facilitates tumorigenesis in various malignancies. However, whether the aberrant expression of BCL6 in ovarian carcinoma is associated with malignancy, metastasis or prognosis remains unknown. Our study aimed to investigate the expression of BCL6 in ovarian carcinoma and its possible correlation with clinicopathological features as well as patient survival to reveal its biological effects in ovarian tumor progression.

Methods: Immunochemistry analysis was performed in 105 cases of ovarian carcinoma covering the histological types of serous, endometrioid and clear cell. Spearman analysis was used to calculate the correlation between pathological parameters and the expression of BCL6. Kaplan-Meier method and Cox proportional hazards analysis were used to analyze the disease-specific survival (DSS) and disease-free survival (DFS). We also assessed whether overexpression and knockdown of BCL6 influence in vitro cell proliferation, cell cycle progression, as well as tumor cell invasion and migration.

Results: The expression of BCL6 was higher in all three major kinds of ovarian cancer in comparison with paratumorous epithelium. BCL6 expression was tightly correlated with FIGO staging, lymph node metastasis and recurrence. Higher expression of BCL6 led to a significantly poorer DSS and DFS and multivariate analysis revealed that BCL6 was an independent risk factor of DSS and DFS. Enforced overexpression of BCL6 in ovarian tumor cells stimulated proliferation by inducing G1-S transition, and promoted tumor cell invasion and migration. Conversely, RNA interference-mediated silencing BCL6 expression inhibited proliferation by altered cell cycle progression and reduced the ability of the cells to migrate, and invade the extracellular matrix in culture.

Conclusions: Our study suggests that the inappropriate activation of BCL6 predicts poor prognosis and promotes tumor progression in ovarian carcinoma. Targeting BCL6 could be a novel therapeutic choice for treating ovarian carcinoma patients.

Keywords: BCL6; invasion; ovarian carcinoma; prognosis; proliferation.

Figure 1

BCL6 was highly expressed and predicted poor prognosis in ovarian carcinoma. A: Representative immunostaining results of BCL6 in paratumorous epithelium, serous, endometrioid and clear cell carcinoma. B: Representative immunostaining results of BCL6 in paratumorous epithelium, malignant carcinoma of Stage I, II and III. C, D: Kaplan-Meier disease-free survival (DFS) (C) and disease-specific survival (DSS) (D) curves of patients with different expressions of BCL6 in ovarian carcinoma (Low vs. High).

Figure 2

Efficiencies of overexpression and knockdown of BCL6 in ovarian carcinoma. A: The baseline expressions of BCL6 protein in six ovarian tumor cell lines detected by Western blotting. B: The baseline mRNA levels of BCL6 in six ovarian tumor cell lines detected by RT-qPCR. C: The expressions of BCL6 protein in SKOV3 and ES-2 cells transfected with pcDNA3.1-HA-BCL6 and in CAOV3 cells transfected with siBCL6 detected by Western blotting. D: The mRNA levels of BCL6 in SKOV3 and ES-2 cells transfected with pcDNA3.1-HA-BCL6 and in CAOV3 cells transfected with siBCL6 detected by RT-qPCR.

Figure 3

BCL6 stimulated tumor cell proliferation in ovarian carcinoma. A: The CCK8 cell counting assays revealed cell growth curves of indicated cells. B: The EdU imaging results revealed cell growth curves of indicated cells (The scale bars = 100 μm). The EdU positive cells were counted under 200× by immunofluorescence microscope. *p < 0.05.

Figure 4

BCL6 promoted cell cycle progression in ovarian carcinoma. A: Representative Flow-cytometric images with determination of proportion of indicated cells in distinct cell-cycle phases. *p < 0.05. B: The Western blotting results of CyclinB1 and Cdc25B in indicated cells. GAPDH was used as reference.

Figure 5

BCL6 induced tumor cell invasion and migration in ovarian carcinoma. A: Representative images (left) and quantification (right) of transwell invasion assays for indicated cells. (Scale bars = 50 μm). *p < 0.01. B: Representative images (left) and quantification (right) of wound-healing assays for indicated cells. (Scale bars = 400 μm). *p < 0.01. C: The immunoblotting results of N-Cadherin, MMP2 and MMP9 in indicated cells. GAPDH was used as reference. D: The mRNA levels of N-Cadherin, MMP2 and MMP9 in indicated cells. GAPDH was used as reference.