Study On The Mechanism Of Expression Regulation And The Biological Function Of RIG-G Gene

RIG-G gene belongs to an IFN-induced gene family, and it shares high homology with other members in this family, such as IFI54, IFI56 and IFI58. It has been found that RIG-G gene can be rapidly induced by IFN in diverse types of cancer cells, including the leukemia cell lines NB4, HL60, U937, and various solid tumor cells, such as cervical carcinoma HeLa cells, non-small cell lung cancer H460 and A549 cells, epithelial-like WISH cells and head and neck squamous carcinoma cells, suggesting that Rig-G may play an important role in IFN signal transduction. In this study, we tried to explore the mechanism of expression regulation and the biological function of RIG-G gene.We found two well-conserved IFN-stimulated response elements (ISREs) in the promoter of RIG-G gene. By luciferase reporter assay and electrophoretic mobility shift assay (EMSA), we confirmed that ISRE I and ISRE II are the molecular basis of RIG-G gene transcription. We also showed that the transcription factor STAT1 (signal transducer and activator of transcription 1) could bind directly to ISRE I and ISRE II and was a prerequisite for IFN-induced RIG-G expression.To investigate the biological function of Rig-G, we used Tet-off expression system to transfect the ectopic RIG-G gene into U937 cells and established an inducible cell line which could stably express RIG-G gene. By measuring the cell growth, cell morphologic features, cell cycle distribution, cell surface antigen and some cell cycle regulatory proteins, we found that Rig-G could accumulate the cells at G0/G1 phase through increasing the protein levels of cell cycle inhibitors p21 and p27. In addition, an increase of the CD11C expression as well as some morphologic features of partial differentiation were also observed in Rig-G expressing cells, indicating that Rig-G possessed potent abilities to inhibit cell proliferation and promote cell differentiation.Furthermore, we demonstrated that Rig-G was able to interact with JAB1 (Jun activation domain-binding protein-1) and interfere with the normal function of JAB1 by altering its cellular localization and distribution. According to literatures, JAB1 plays key roles in the degradation process of p27. It is the JAB1 that mediates the nuclear export of p27 and facilitates p27 to degrade via ubiquitin/proteasome pathway in cytoplasm. The interaction between Rig-G and JAB1 could result in JAB1 sequestration in the cytoplasm, blocking the nuclear export of p27 and its subsequent proteolysis, finally increasing p27 protein level and inhibiting cell growth. Meanwhile, we found that Rig-G could upregulate p21 at the transcriptional level by decreasing c-Myc expression, therefore inhibiting cell proliferation and facilitating cell differentiation.Taken together, our results indicated that RIG-G was a direct target of STAT1, which exerted a prominent growth inhibitory ability and was a key mediator in the IFN-stimulated antiproliferative signaling pathway. It was likely that the antiproliferative ability of STAT1 was associated with Rig-G. Our study layed some theoretical and experimental grounds to elucidate the cellular and molecular mechanisms of IFN-induced growth inhibition of tumor cells, and clarify the IFN signal transduction pathways.