Gliomas, one of the most common malignant brain tumors, showed increasing morbidity and mortality in recent years. But regardless of the unsatisfactory prognosis of gliomas, elucidation of implicit molecular mechanism have enriched our knowledge about tumor and pushed forward the war against tumor. Of the reported critical genes in tumors, RBM5 was a tumor suppressor and enhanced tumor cell apoptosis in a list of tumors. Peng et al. demonstrated that RBM5 underexpression was closely correlated with poor clinicopathological features in pancreatic cancer [
16]. Consistently, RBM5 was shown to be weakly expressed in either gliomas tissues or cell lines here. And RBM5 expression was clinically correlated with tumor stage but not sex. This was inconsistent with the previous study in which gliomas incidence was reported higher in male patients than that in female patients aged between 30 and 40 years [
17]. Maybe a larger cohort of gliomas patients is necessary to support this conclusion. In addition, patient with low RBM5 expression showed worse prognosis than those with high RBM5 expression. These data pointed out the clinical value of RBM5 in gliomas.
To investigate the role of RBM5 in gliomas, we successfully constructed a gliomas cell line stably expressing RBM5 with lentivirus vector. Our data demonstrated that upregulated RBM5 significantly inhibited cell growth and disrupted DNA synthesis in gliomas cells. DNA synthesis is a critical process in cell cycle and keeps the maintenance of genetic information during mitosis which is ensued by proliferation. Further, both migration ability and invasive capacity of gliomas cells were markedly suppressed by RBM5 overexpression. Migration and invasiveness generally precede metastasis during tumor progression. And it is known that tumors are characterized by uncontrolled cell proliferation and potent aggressive ability [
18]. Based on these facts, we concluded that RBM5 played a suppressor role in gliomas.
But how did RBM5 suppress oncogenesis and metastasis in gliomas? The first as well as the most reported mechanism was that RBM5 could activate apoptotic signaling and enhance cell apoptosis [
12]. Apoptosis, also called programmed cell death (PCD), was a complex activity involving multiple genes and was vital to the physiological and pathological process of cell life. Bcl-2 and Bax, often form heterodimers, are two most important regulators in apoptotic signaling pathway [
12]. They are reversely correlated during PCD process which means that Bcl-2 antagonizes the effect of Bax and vice verse. Caspase cascade response directly caused cell PCD, and this process includes sequentially activation of key effectors such as caspase-9 and caspase-3. In lung cancer cells transfected with RBM5, pro-apoptotic protein Bax was increased while anti-apoptotic protein Bcl-2 was decreased reversely [
12]. Meanwhile, mitochondrial apoptotic pathway was also activated by RBM5 as caspase-9 and caspase-3 activity was upregulated greatly. In another study, expression of cleaved caspase-9 and cleaved caspase-3 was upregulated by RBM5 in prostate cancer [
10]. In this study, we demonstrated that Bcl-2 was downregulated while the level of Bax, cleaved caspase-3, and TNF-α was increased significantly by RBM5 in gliomas cells, which suggest that RBM5, at least partly, promoted cell apoptosis in gliomas cells. And apoptosis assay further confirmed the apoptosis-inducing function of RBM5. The second mechanism of RBM5 involves its anti-proliferative effect and cell cycle arrest. RBM5 was reported to inhibit cell growth and colony formation through increased p53 in H1299 cells while caused cell cycle arrest at G1 phase [
19]. At the same time, RBM5 was shown to antagonistically regulate the proliferative ability of cancer cells through alternative splicing of NUMB gene [
8]. However, different with the early reports, we found in this study that Wnt/β-catenin signaling was activated by RBM5 in gliomas cells. Wnt/β-catenin signaling was one typical proliferation-promoting signaling pathway and has been shown to be activated in various kinds of tumors. Besides the above two mechanisms, other signaling may be also involved in the anti-tumor role of RBM5. For example, RBM5 was also shown to downregulate EGFR expression and induce cell autophagy [
20,
21]. Autophagy is another autonomic programmed cell death or type II PCD. But its role in gliomas needs more study. Accordingly, our data proved that RBM5 promoted tumorigenesis of gliomas through activation of Wnt/β-catenin signaling and increased cell apoptosis.