| BackgroundPancreatic cancer is a highly malignant digestive tract tumor,and its incidence and mortality are on the rise worldwide.Pancreatic ductal adenocarcinoma(PDAC)accounts for the vast majority of pancreatic neoplasms,and other pancreatic tumors include neuroendocrine tumors,mucinous cystadenomas,intraductal papillary mucinous tumors,solid pseudopapillary tumors,islet cell tumors,and acinar carcinomas.Risk factors for pancreatic cancer include smoking,alcohol consumption,obesity,family history,diabetes,and chronic pancreatitis.PDAC is not easy to find in the early stages.On the one hand,it is because of the anatomical relationship of the pancreas and the usually insidious symptoms of patients.On the other hand,there is still a lack of early appropriate screening and diagnostic methods.When patients have obvious symptoms,most of them are in local progression or late stage.Currently,the standard management option for pancreatic cancer is surgical resection combined with adjuvant therapy.Radical surgery can significantly improve the prognosis of patients,however,most patients are at an advanced stage at the time of diagnosis,so only about 20% of patients can undergo radical surgery.Even after radical treatment,most patients will eventually relapse,and the 5-year survival rate of patients is still unsatisfactory.This may be associated with tumor micrometastasis at an earlier stage,tumor resistance to drugs,and tumor insensitivity to radiotherapy.Micro RNAs(miRNAs)are a set of small non-coding RNAs of 20-24 nucleotides in size that can regulate the expression of target genes at the post transcriptional level.They mainly perform the cleavage or translation inhibition function of the target m RNA in a base-pairing manner with the 3′-untranslated region(3′-UTR)of the target m RNA,so as to inhibit the expression of the target gene.miRNAs are distributed in many species,including mammals,insects,and plants.They can be abnormally expressed in many human malignancies including pancreatic cancer and can act as both tumor promoters and tumor suppressors to influence the occurrence and development of malignant tumors.For example,miR-93 positively regulates CRMP2/MAPRE1/YES1 to facilitate pancreatic cancer cell proliferation,migration,and invasion.While miR-146a-5p down-regulated the TRAF6/NF-κB p65/P-gp signaling axis to suppress pancreatic cancer cell growth and chemoresistance.Besides,miR-148a-3p could directly target Wnt1 and/or Wnt10 b and thus inhibit the Wnt/β-catenin pathway,thereby repressing the epithelial-mesenchymal transition and invasion of of pancreatic cancer cells.miR-877-5p is located on the short arm of human chromosome 6.Current evidence shows that miR-877-5p is aberrantly expressed in various human cancers and participates in the regulation of the occurrence and progression of malignant tumors.In pancreatic cancer,miR-877-5p expression was downregulated.However,the role of miR-877-5p in PDAC progression and the underlying molecular mechanisms remain unclear.FOXM1 is a transcription factor that belongs to the forkhead box protein family.It has been demonstrated that FOXM1 is highly expressed in pancreatic cancer and plays a key regulatory role in various biological behaviors of pancreatic cancer cells.However,the upstream regulatory mechanism of FOXM1 remains unclear.By analyzing and predicting the possible target genes of miR-877-5p,we found that the 3′-UTR of FOXM1 had a binding site with miR-877-5p.In conclusion,we propose a hypothesis that miR-877-5p directly regulates FOXM1 to inhibit the occurrence and progression of PDAC.ObjectiveTo investigate the role of miR-877-5p in PDAC development and its potential molecular mechanism.Methods(1)The expression of miR-877-5p in 29 pairs of PDAC tissues and 4 pancreatic cancer cell lines was examined by quantitative real-time polymerase chain reaction(q RT-PCR).Meanwhile,the correlation between miR-877-5p expression and the clinicopathological parameters of PDAC patients was evaluated.In addition,the relationship between miR-877-5p expression and the prognosis of PDAC patients was analyzed by the OncomiR database.(2)CCK-8,Colony formation and Transwell assays were used to investigate the impacts of miR-877-5p on the proliferation,migration and invasion of pancreatic cancer cells.Cell cycle and apoptosis were illustrated by flow cytometry.Western blot(WB)was performed to detect the expression of three key proteins during epithelial mesenchymal transformation,including E-cadherin,N-cadherin and Vimentin.(3)A nude mouse model of subcutaneously transplanted PDAC tumor was established to observe the effects of miR-877-5p overexpression on tumor formation in vivo.The expressions of miR-877-5p,FOXM1 and Ki-67 in subcutaneous tumor tissues were detected by q RT-PCR,WB and immunohistochemistry,respectively.(4)The expression of FOXM1 in PDAC tissues and cells was tested by q RT-PCR,and the correlation between FOXM1 and miR-877-5p expression was analyzed.After up-and down-regulation of miR-877-5p,q RT-PCR and WB were used to detect the expression of FOXM1 m RNA and protein in each group.The dual-luciferase reporter assay was performed to examine the binding relationship between miR-877-5p and FOXM1.(5)The functional recovery experiment was implemented in two cell lines,and the expression of FOXM1 m RNA and protein in each group was measured by q RT-PCR and WB.CCK-8 and Transwell assays were performed to investigate the effects of both miR-877-5p and FOXM1 co-expression on the proliferation,migration and invasion of pancreatic cancer cells.Results(1)Compared with adjacent normal pancreatic tissue and normal pancreatic ductal epithelial cells,miR-877-5p was lowly expressed in PDAC tissues and cells;and its expression was associated with lymph node metastasis and TNM stage of PDAC patients.Survival analysis showed that the overall survival time of patients in the group with high miR-877-5p expression was remarkably higher than that in the group with low miR-877-5p expression.(2)miR-877-5p overexpression significantly hindered the proliferation,clone formation,migration and invasion of SW1990 cells,induced cell arrest in G1/S phase,and facilitated cell apoptosis;whereas suppression of miR-877-5p remarkably promoted Mia Pa Ca-2 cell proliferation,clone formation,migration and invasion ability,accelerated G1/S phase process,and inhibited cell apoptosis.Furthermore,miR-877-5p overexpression upregulated E-cadherin levels and downregulated the N-cadherin and Vimentin levels;while silencing miR-877-5p presented the opposite consequences.(3)In vivo experimental results indicated that the volume and weight of tumors in the miR-877-5p overexpression group were significantly reduced.Compared with the control group,the expression of miR-877-5p was increased in the miR-877-5p overexpression group,while the expression of FOXM1 and Ki-67 was decreased.(4)FOXM1 was highly expressed in PDAC tissues and cells,and revealed that FOXM1 m RNA expression correlated negatively with miR-877-5p.Up-regulation of miR-877-5p expression significantly decreased the expression of FOXM1,while down-regulation of miR-877-5p expression significantly increased the expression of FOXM1.The dual-luciferase reporter assay demonstrated that miR-877-5p could bind to FOXM1-3’ UTR.(5)The functional recovery experiments showed that miR-877-5p overexpression suppressed the pancreatic cancer cell proliferation,migration and invasion ability,which could be partially reversed by upregulating FOXM1.ConclusionsmiR-877-5p was lowly expressed in PDAC tissues and cells,and its expression was associated with lymph node metastasis,TNM stage and prognosis of PDAC patients.miR-877-5p can suppress the proliferation,epithelial-mesenchymal transition,migration and invasion ability of pancreatic cancer cells,and induce cell cycle arrest and promote cell apoptosis.Taken together,miR-877-5p represses the malignant biological behavior of PDAC by downregulating FOXM1. 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