ING3 is widely expressed in normal human tissues, such as heart, skeletal muscle, thymus, spleen, kidney, liver, pancreas, colon, ovary, testis, prostate, peripheral blood leukocytes, etc[18]. In normal human tissues, ING3 is principally distributed in the cytoplasm, but it is occasionally observed in both the cytoplasm and nucleus, including tongue, esophagus, lung, skin, bladder, cervix and breast cells[19]. In our study, ING3 was distributed both in the cytoplasm and nucleus in benign breast tumor tissues, which was consistent with the report. Meanwhile, we also observed that, ING3 was mainly expressed in the cytoplasm in breast cancer tissues, and the expression of ING3 in the nucleus was significantly decreased. Similarly, the expression of ING3 decreased in many tumors, including human cutaneous melanoma[11], human head and neck cancers[9] and human primary hepatocellular carcinoma[10]. Although the nuclear ING3 expression was significantly reduced in breast cancer tissues compared with benign breast tumor tissues, it is not clear how the nuclear ING3 is reduced. Studies has shown that there might be a translocation of ING3 from nucleus to cytoplasm in melanoma[11]. However, in our study, we did not notice that the significantly decrease of nuclear ING3 was accompanied by a remarkably increase in the expression of cytoplasmic ING3. One possible explanation for this discrepancy was that the nuclear to cytoplasm translocation of ING3 may be a partial reason for significantly reduce of nuclear ING3 in breast cancer tissues. The nuclear localization of ING3 protein was determined by the nuclear localization sequence (NLS) of ING3 gene[20]. At the same time, Chen G et al found that the decreased expression of ING3 in melanoma was degraded by ubiquitin-proteasome signal pathway[21]. Therefore, the mechanism of the decreased expression of nuclear ING3 in breast cancer needs to be further studied.
In our study, we found that there was a correlation between the downregulation of ING3 in nucleus and clinicopathologic characters in breast cancer. The nuclear ING3 was negatively related with histological grade, which suggests that the decreased expression of ING3 in the nucleus was involved in the differentiation and played a continued role in the advancement and development of breast cancer. As the homologs of ING3, the degree of inhibition of ING4 protein expression was related to clinical stage, and the expression of ING4 in patients with lymph node metastasis was lower than that in patients without lymph node metastasis, indicating that ING4 participated in the occurrence and development of colon cancer[22]. In breast cancer patients with the larger the tumor, the higher the stage, and the lower the expression of ING4 were more prone to lymph node metastasis[14]. On the other hand, the nuclear ING3 was also negatively correlated with lymph node metastasis, which suggested that nuclear ING3 was related to the migration and metastasis of breast cancer. In agreement with our findings, Lu M et al suggested that overexpression of ING3 might inhibit the migration and metastasis of hepatocellular carcinoma cells[23].
In addition, the nuclear ING3 was positively correlated with ER and PR status, which were closely related to endocrine therapy of breast cancer. Previous study had shown that low expression of ING4 reduced the efficacy of tamoxifen in breast cancer, by inhibiting ER activity in hormone-dependent breast cancer[24]. As the homologous of ING4, ING3 might also be related to endocrine therapy of breast cancer. In this study, it had shownthat there were no relationship between expression of ING3 in nucleus and age, TNM stage and HER-2 status. Interestingly, the deletion of ING4 gene was associated with HER-2 status in breast cancer[25]. Meanwhile, the rate of high expression nuclear ING3 in breast cancer with luminal-type was higher than that with HER2-enriched and TNBC, which suggested that nuclear ING3 might play a role in distinguishing different subtypes of breast cancer.
All the above finding ssuggested that nuclear ING3 might play a key role, at least in part, in predicting the prognosis of breast cancer. Prognostic molecular biomarkers are valuable for evaluating the survival status of patients and assisting tumor control. It had been well demonstrated that ING3 might be a positive independent factor in melanoma, human primary hepatocellular carcinoma and head and neck cancer [8, 10, 11]. Similarly, our survival analysis also showed that in luminal-type breast cancer and lymph node metastasis group, the lower nuclear ING3 expression, the poorer 5-DFS and 5-OS. Moreover, the independent prognostic biomarker of nuclear ING3 in breast cancer patients was revealed based on the multivariate Cox regression analysis.