Identification of differentially expressed microRNAs related to chemo- and radio-resistance in pancreatic cancer stem cells

Abstract

Pancreatic ductal adenocarcinoma remains an extremely aggressive malignancy that is virtually therapy-resistant. Recent studies suggest that the existence of cancer stem cells was reported in some solid malignancies including breast, brain, prostate, liver, and pancreas cancers. Thus, it is necessary to understand cancer stem cell biology to better treat pancreatic cancer. MicroRNA (miRNA) regulates various cellular processes including development, differentiation, proliferation, maintenance, and apoptosis. Recently, several miRNAs including let-7, mir-200, and mir-181 have been shown to have regulatory functions in cancer stem cells in certain organs; however, the role of miRNAs in other cancers, including pancreatic, has not yet been reported. In the present study, pancreatic cancer stem cell-specific miRNAs and mRNAs were analyzed and identified to investigate their correlations to cancer stem cell biology. To explore the differentially expressed miRNAs in pancreatic cancer stem cells, sphere cultivation method was used to enrich the cancer stem cells and compared its expression with cancer cells or adherent cultured cells using microRNA microarray and obtained 210 present call miRNAs. To identify the target proteins controlled by the cancer stem cell specific microRNAs, correlation among miRNAs with 258 stem cell-associated mRNAs differentially expressed in pancreatic cancer stem cells were analyzed using linear equation. The cross correlation analysis yielded six groups of miRNAs and three groups of mRNAs in clusters that may directly or indirectly control cancer stem cell biology. To determine the possible roles of individual miRNAs, pancreatic cancer stem cells specific miRNAs were either over-expressed or suppressed in HPAC human pancreatic cancer cell line and its effect on proliferation, sphere formation, drug resistance, and radiation treatment were analysis. As a result, differentially expressed mir-99a and mir-125b showed involvement in anchorage independent growth, and chemo- and radio-resistance what cancer stem cells possess. This study further showed that mir-99a over-expression not only suppress previously reported gene, SMARCA5, but also stem cell-associated genes including FOXA1, Gli-2, HOXA1, IGFIR-ß, SOX-2, and Nanog . Cross-correlation results provided insights into possible linkages between clusters of miRNAs and clusters of stem cell-associated mRNAs in cancer stem cells and functional analysis of individual miRNAs suggested possible involvement of cancer stem cell-specific miRNAs in cancer stem cell biology that adjuvant approach may have broad implications in our understanding of cancer stem cells and cancer stem cell-targeted cancer therapy.