The tumor suppressor gene RBM5 inhibits lung adenocarcinoma cell growth and induces apoptosis

Background: The loss of tumor suppressor gene (TSG) function is a critical step in the pathogenesis of human lung cancer. RBM5 (RNA-binding motif protein 5, also named H37/LUCA-15) gene from chromosome 3p21.3 demonstrated tumor suppressor activity. However, the role of RBM5 played in the occurrence and development of lung cancer is still not well understood. Method: Paired non-tumor and tumor tissues were obtained from 30 adenocarcinomas. The expression of RBM5 mRNA and protein was examined by RT-PCR and Western blot. A549 cell line was used to determine the apoptotic function of RBM5 in vitro. A549 cells were transiently transfected with pcDNA3.1-RBM5. AnnexinV analysis was performed by flow cytometry. Expression of Bcl-2, cleaved caspase-3, caspase-9 and PAPP proteins in A549 lung cancer cells and the A549 xenograft BALB/c nude mice model was determined by Western blot. Tumor suppressor activity of RBM5 was also examined in the A549 xenograft model treated with pcDNA3.1-RBM5 plasmid carried by attenuated Salmonella typhi Ty21a. Result: The expression of RBM5 mRNA and protein was decreased significantly in adenocarcinoma tissues compared to that in the non-tumor tissues. In addition, as compared to the vector control, a significant growth inhibition of A549 lung cancer cells was observed when transfected with pcDNA3.1-RBM5 as determined by cell proliferation assay. We also found that overexpression of RBM5 induced both early and late apoptosis in A549 cells using AnnexinV/PI staining as determined by flow cytometry. Furthermore, the expression of Bcl-2 protein was decreased, whereas the expression of cleaved caspase-3, caspase-9 and PARP proteins was significantly increased in the RBM5 transfected cells; similarly, expression of decreased Bcl-2 and increased cleaved caspase-3 proteins was also examined in the A549 xenograft model. More importantly, we showed that accumulative and stable overexpression of RBM5 in the A549 xenograft BALB/c nude mice model significantly inhibited the tumor growth rate in vivo as compared to that in the control. Conclusion: Our study demonstrates that RBM5 can inhibit the growth of lung cancer cells and induce apoptosis both in vitro and in vivo, which suggests that RBM5 might be used as a potential biomarker or target for lung cancer diagnosis and chemotherapy. Moreover, we propose a novel animal model set up in BALB/c nude mice treated with attenuated Salmonella as a vector carrying plasmids to determine RBM5 function in vivo.