Introduction
Ovarian cancer is the second leading cancer in women and the fifth leading cause of cancer-related deaths in women. Epithelial ovarian carcinoma (EOC) is disproportionately deadly because no sophisticated approach for the early diagnosis makes most ovarian cancers diagnosed at advanced stages [
1]. Treatment may be shortly effective; patients with advanced International Federation of Gynecology and Obstetrics (FIGO) stages still have poor prognosis with 5-year survival rates below 33 % depending on response to chemotherapy [
2]. EOCs are believed to arise from the ovarian epithelium and fallopian tubes, probably due to such risk factors as hereditary, reproductive, hormonal, inflammatory, hereditary, and geographic factors [
3]. Carcinogenesis and progression of ovarian carcinoma are multistage processes, and increased understanding of the changes that occur in gene expression during carcinogenesis may result in the improvement of its diagnosis, treatment, and prevention. Therefore, it is an urgent need to identify more effective and new molecular targeted therapies for EOC.
The human BTG (B-cell translocation gene)/Transducer of ErbB2 gene family comprises of six proteins (BTG1, BTG2/TIS21/PC3, BTG3, BTG4/PC3B, Transducer of ErbB-2, and TOB2), which inhibit cell proliferation and regulate cell-cycle progression and differentiation in a variety of cell types. BTG is a nuclear protein that is imported into the nucleus through a nuclear localization signal (NLS)-mediated mechanism, and its nucleocytoplasmic translocation depends on cell growth states [
4]. The conserved N-terminal BTG domain encompasses 104–106 amino acids and contains Box A and B. It has been reported that the conserved BTG domain mediates interactions with the highly similar Caf1a (CNOT7) and Caf1b (CNOT8) catalytic subunits of the Ccr4–Not deadenylase complex, which was closely associated with the anti-proliferative activity of BTG/TOB proteins. Furthermore, the activity of BTG/TOB proteins in the regulation of mRNA abundance and translation is dependent on Caf1a/Caf1b [
5]. BTG3/ANA/APRO4 (MGC8928, Protein BTG3, Protein Tob5, TOB5, tob55, TOB55, TOFAD) is a member of the anti-proliferative BTG family and has been reported to be a tumor suppressor gene in some malignancies [
6]. The human
BTG3 gene is localized in chromosome 21q21.1, and its two cDNA variants encode two variants with 132-nucleotide deletion by alternative splicing [
7‐
9]. BTG3 is a downstream target of p53 and interacts with E2F1 to suppress the DNA binding of the E2F1–DP1 transcription factor complex through an N-terminal domain including the conserved Box A, suggesting its negative regulatory influence on cellular S-phase progression [
10]. BTG3 is also able to interact with the Smad8 receptor-regulated Smad transcription factor as BTG1 and BTG2 [
11]. BTG3 associates with Src via its C-terminal proline-rich domain to down-regulate Src tyrosine kinase activity and suppresses Ras/MAP kinase signaling.
BTG3 deficiency enhances bone morphogenetic protein-induced ectopic bone formation via transcriptional events [
12]. BTG3 can associate with Caf1 and is a preferred partner of the CCR4 transcription factor-associated protein Caf1 by its amino-terminal half [
13]. Loss of BTG3 in normal cells induced cellular senescence, which was correlated with enhanced ERK-AP1 signaling and elevated expression of the histone H3K27me3 demethylase JMJD3/KDM6B, leading to acute induction of p16(INK4a) [
14].
In mice,
BTG3 mRNA was ubiquitously expressed in adult mice, the level being relatively high in the heart, lung, kidney, and testis, but low in the spleen and skeletal muscle. In human, BTG3 expression is down-regulated in lung, prostate, or renal cancer tissues and cells, and induced by genistein and 5-aza-2′-deoxycytidine, suggesting silenced BTG3 expression is attributable to its epigenetic methylation [
8,
15‐
17]. Long-term observation of BTG3-deficient mice reveals that 8 % of them develop lung tumors (5/66) by 21 months after birth. Exogenous BTG3 protein suppresses the levels of matrix metalloproteinase-2 and plasminogen activator inhibitor-1 expression in lung cancer cells [
15]. Taken together, it is suggested that BTG3 protein might have a negative regulatory effect on tumor progression by suppressing angiogenesis, invasion, and metastasis. In our previous work, aberrant BTG3 expression was found to link to gastric carcinogenesis and its venous invasion (unpublished). To explore the roles of
BTG3 expression in the ovarian carcinogenesis and subsequent progression, we examined the expression of BTG3 mRNA and protein in ovarian normal, benign, and borderline tumor, primary, and metastatic epithelial ovarian carcinoma in omentum, and compared them with clinicopathological parameters of EOCs.
Materials and methods
One hundred cells were randomly selected and counted from five representative fields of each section blindly by two independent observers (Deng BY and Zheng HC). The inconsistent data were confirmed by both persons until final agreements were reached. The expression positivity was graded and counted as follows: 0 = negative, 1 = 1–49 %, 2 = 50–74 %, and 3 ≥75 %. The staining intensity score was graded as follows: 1 = weak, 2 = intermediate, and 3 = strong. The scores for BTG3 positivity and staining intensity were multiplied to obtain a final score, which determines their expression as − = 0, + = 1–2, ++ = 3–4, and +++ = 6–9. The expression positivity of Ki-67 was graded and counted as follows: 0 = negative, 1 = 1–49 %, 2 = 50–74 %, and 3 ≥75 %.
Discussion
Increasing evidence suggests that BTG3 is thought to be a negative regulator of cellular S-phase progression, and it has been shown that its anti-proliferative action is through inhibition of transcription factor E2F1 or interaction with Smad transcription factor as a tumor suppressor [
10,
11]. Here, we for the first time examined in situ BTG3 expression in ovarian normal tissue, benign and borderline tumor, and carcinoma samples. It was found that BTG3 protein was mainly localized in the cytoplasm of ovarian fiber cells, fallopian tube, benign and borderline tumor, and carcinoma cells. It was suggested that the BTG3 expression pattern has cellular specificity, which determines its biological functions. However, the mechanisms of its cell-specific characteristics should be further investigated.
Using western blot and immunochemistry staining, BTG3 expression was reduced in ovarian carcinoma, compared with ovarian normal tissue and benign tumor, indicating that down-regulated BTG3 expression contributes to ovarian epithelial carcinogenesis. In the present study,
BTG3 mRNA level was reduced from ovarian borderline tumor and carcinoma, in line with the immunohistochemical data. Yoneda et al. [
15] also reported that
BTG3 mRNA expression was down-regulated in lung carcinoma tissue or cell lines, consistent with the normal counterparts. Yu et al. [
8] demonstrated that
BTG3 expression was markedly reduced and its promoter region of
BTG3 was hypermethylated without detectable mutations in the promoter and coding region in a wide variety of human breast cancer cell lines, suggesting that hypermethylation might be an important mechanism for inactivation of BTG3. Methylation-mediated down-regulation of
BTG3 was also documented in renal and prostate cancer cells [
16,
17]. Sasajima et al. [
19] suggested that anti-proliferative proteins of the BTG/Tob family might be degraded by the ubiquitin–proteasome system. The ubiquitin-mediated degradation of BTG protein could support the BTG3 hypoexpression in ovarian borderline tumor and EOCs. However, the mechanism of BTG3 down-regulation in EOC will be investigated in the future study.
In the present study, BTG3 protein expression was positively linked to clinicopathological features of ovarian carcinoma, including differentiation and FIGO staging in agreement with the data of our real-time RT–PCR, indicating that BTG3 might be involved in the development and differentiation of EOC and be considered as a good biomarker to indicate the aggressive behaviors of EOC. We also compared the expression of BTG3 and Ki-67 in EOC samples. Ki-67 is a nuclear protein found in G1 phase of cell cycle associated with cell proliferation. As an excellent marker to determine the cell proliferation index, Ki-67 was also associated with EOC progression and prognosis [
20]. Since increased Ki-67 expression revealed an increase in mitotic cell activity and proliferation, the negative correlation we found between BTG3 and Ki-67 expression provided evidence about the suppressive effects of BTG3 on the cell proliferation. Regarding the prognostic significance of other
BTGs, Möllerström et al. [
21] demonstrated high-level BTG2 protein expression correlates with prolonged survival in patients with breast carcinoma. Kamalakaran et al. [
22] identified differential methylation of CpG islands proximal to BTG1 as having prognostic value independent of subtypes and other clinical factors of luminal breast cancers. We revealed the inverse links between BTG3 expression levels and the overall and disease-free survival of patients with EOC in the present study. Multivariate analysis demonstrated that BTG3 protein expression was an independent factor to indicate the favorable prognosis of ovarian carcinoma. Also in our study, FIGO staging was the independent factor of both the overall and disease-free survival of the EOC, as other researchers revealed [
23,
24]. Since Bast et al. [
25] had developed an assay for serum CA125 and found it elevated in 82 % of EOC patients, serum CA125 kinetics during chemotherapy had value in predicting survival of EOCs. Some research also revealed that pre-operative serum CA125 was an independent factor to EOC prognosis [
26]. But in our study, there is no statistical relationship between pre-operative CA125 and prognosis of EOCs. This may be due to many characteristics which could profoundly influence serum CA125, including tumor characteristics such as histology, grade, stage, and presence of ascites together with certain conditions such as peritoneal and mucosal inflammation [
27]. Our findings suggested that BTG3 protein expression could be employed to indicate the prognosis of ovarian carcinoma patients as an independent factor.
In conclusion, our study indicated that BTG3 was down-regulated in epithelial ovarian carcinoma, which might have impact on the clinicopathogenesis of EOC, and should be considered as a good biomarker for ovarian carcinogenesis and subsequent progression. Nevertheless, the biological functions of BTG3 in EOC need further investigation.