Downregulation of miR-132 by promoter methylation contributes to pancreatic cancer development

MicroRNAs (miRNAs), which regulate gene expression by partial complementarity to the 3' untranslated region of their target genes, have been implicated in cancer initiation and progression. However, the molecular mechanism underlying the regulation of miRNA expression during pancreatic tumorigenesis has not been extensively reported. In this study, we first compared the miRNA expression in human pancreatic cancers and adjacent normal tissues by miRNA array and identified 12 differentially expressed miRNAs. miR-132, which is downregulated in tumors, was further studied in greater detail. Decreased expression of miR-132 was confirmed in 16 of 20 pancreatic carcinomas (P < 0.0001), compared with their respective benign tissues by TaqMan miRNA assays. miR-132 expression was remarkably influenced by promoter methylation in PANC1 and SW1990 cells. Promoter hypermethylation was observed in tumor samples but not in the normal counterparts, and the expression of miR-132 negatively correlated with its methylation status (P = 0.013). miR-132 was transcribed by RNA polymerase II, and Sp1 played a major role in miR-132 transcription. The expression of Sp1 correlated with that of miR-132 in tissues. Moreover, cancerous tissues showed significantly lower Sp1-binding affinity to the miR-132 promoter, relative to non-tumor samples. Proliferation and colony formation of pancreatic cancer cells were suppressed in cells transfected with miR-132 mimics and enhanced in cells transfected with miR-132 inhibitor by negatively regulating the Akt-signaling pathway. Our present findings illustrate the mechanism driving miR-132 downregulation and the important role of miR-132 in pancreatic cancer development.