MiRNA-646-mediated reciprocal repression between HIF-1α and MIIP contributes to tumorigenesis of pancreatic cancer

Migration and invasion inhibitory protein (MIIP) is recently identified as an inhibitor in tumor development. However, the regulatory mechanism and biological contributions of MIIP in pancreatic cancer (PC) have been not elucidated. In this study, we demonstrated a negative feedback of MIIP and hypoxia-induced factor-1 alpha (HIF-1 alpha), which was mediated by a hypoxia-induced microRNA. Compared with paracarcinoma tissues, MIIP was downregulated in PC tissues. Overexpression of MIIP significantly impeded the proliferation and invasion of PC cells both in vitro and in mouse xenograft models. We further verified MIIP was downregulated under hypoxia in a HIF-1 alpha-mediated manner. Interestingly, although MIIP promoter containing two putative hypoxia response elements (HREs), the chromatin immunoprecipitation (ChIP) and luciferase reporter assays did not support an active interaction between HIF-1a and MIIP promoter. Meanwhile, microRNA array revealed a hypoxia-induced microRNA, miR-646, impaired stability of MIIP mRNA and consequently inhibited its expression by targeting the coding sequence (CDS). Coincidently, knockdown of miR-646 significantly repressed proliferation and invasion ability of PC cells both in vitro and in vivo by upregulating MIIP expression. Besides, ChIP and luciferase reporter assays further validated that HIF-1 alpha activated transcription of miR-646 in hypoxia condition. Therefore, these results suggested HIF-1 alpha indirectly regulated MIIP expression in post-transcriptional level through upregulating miR-646 transcription. Conversely, our results further revealed that MIIP suppressed deacetylase ability of histone deacetylase 6 (HDAC6) to promote the acetylation and degradation of HIF-1 alpha, by which impairing HIF-1 alpha accumulation. What is more, a specific relationship between downregulated MIIP and upregulated miR-646 expression was validated in PC samples. Moreover, the dysregulated miR-646 and MIIP expression was correlated with advanced tumor stage, lymphatic invasion, metastasis and shorter overall survival in PC patients. Together, our results highlight that the reciprocal loop of HIF-1 alpha/miR-646/MIIP might be implemented as an applicable target for pancreatic cancer therapy.