Functional Characterization of a candidate tumor suppressor gene, Mirror image polydactyly 1, in nasopharyngeal carcinoma.

Abstract

Introduction: Nasopharyngeal carcinoma (NPC) is a malignancy with a high incidence in southern China. Previously, a NPC candidate tumor suppressor gene (TSG), MIPOL1, was identified by utilizing a microcell-mediated chromosome transfer (MMCT) approach. MIPOL1 was found to be down-regulated in NPC cell lines and tumors. Promoter hypermethylation and loss of heterozygosity were confirmed to be critical mechanisms to induce MIPOL1 down-regulation. Functionally, MIPOL1-re-expression suppressed in vivo tumor growth of NPC cells via the up-regulation of p21 and p27. These results provide the impetus for further studying this candidate gene in NPC. Aim: The current study aims to perform comprehensive signaling and functional studies of MIPOL1 in NPC to elucidate its mechanistic functions in NPC development. Methodologies: A total of 119 pairs of matched NPC tumors and normal tissues with different clinical outcomes were used to investigate the level of MIPOL1 in NPC clinical samples. MIPOL1 wild type (WT) and two truncated mutants were expressed in two NPC cell lines, HONE1 and HK1, and the changes in the cancer-related signaling pathways were examined by protein array and western blot. Yeast 2-hybrid (Y2H) and co-immunoprecipitation (CoIP) were performed to identify the MIPOL1 interaction partners. A RhoB activity assay was used to determine the role of MIPOL1 re-expression in the regulation of tumor suppressor, RhoB, in NPC cells. Furthermore, subcutaneous injection and intrasplenic injection were used to examine the suppressive role of MIPOL1 in both in vivo tumor growth and metastasis. Results: MIPOL1 showed down-regulation in 75% of the 119 NPC tumors. In the pathway level, the protein array and western blot revealed that re-expression of MIPOL1 can suppress the cancer-related pathways, including angiogenesis, cell survival (Akt/NFκB), and invasion and migration (FAK/Src). Furthermore, RhoB was confirmed to be a potential interaction partner with MIPOL1 and re-expression of MIPOL1 in NPC cells resulted in increased RhoB activity. The MIPOL1 truncation study shows that ΔN100 and ΔC200-442 could reduce the expression of pAKT, pIκBα, and pFAK in HK1. Also, suppression of the angiogenesis markers could not be observed, when ΔN100 or ΔC200-442 were over-expressed in both HONE1 and HK1. Finally, the in vivo study showed that re-expression of the MIPOL1 WT could suppress the tumor growth in the nude mice. Likewise, the MIPOL1 WT re-expression resulted in inhibition of metastasis in the nude mice. Conclusion: Re-expression of the WT MIPOL1 inhibits tumor growth and metastasis in vivo with the evidence of MIPOL1/RhoB interaction, up-regulation of RhoB activity, and inhibition of the AKT/NFkB, Fak/Src pathways. Acknowledgement: This study was supported by the Research Grants Council of the Hong Kong Special Administrative Region, People's Republic of China: Grant number 17101715.