Transcriptional regulation of genes associated with drug resistance and growth of pancreatic cancer cells by mucin 1.

摘要

Pancreatic ductal adenocarcinoma (PDA) is the 4th leading cause of cancer-related deaths in the US. An understanding of the molecular pathogenesis of PDA is of utmost importance to be able to improve the current or design new targeted therapies for treatment of PDA. MUC1 (CD227), a membrane tethered mucin glycoprotein is overexpressed in >60% of human pancreatic cancers and 80% of PDA and is associated with poor prognosis, enhanced metastasis, and chemo resistance in PDA. The objective of thesis was to delineate the mechanism by which MUC1 induces drug resistance, and promotes invasion and proliferation in PDA.;We report here for the first time that MUC1 contributes to drug resistance in pancreatic cancer (PC) via upregulating the expression of ABC transporters that reduces intracellular drug concentration inside the cancer cells. We found that MUC1 high PC cells exhibit increased resistance to chemotherapeutic drugs (gemcitabine and etoposide) in comparison to cells that express low levels of MUC1. This chemo resistance is attributed to the enhanced expression of multidrug resistance (MDR) genes including ABCC1, ABCC3, ABCC5 and ABCB1. In particular, levels of MRP1 protein, encoded by the ABCC1 gene is significantly higher in the MUC1-high PDA cells. In human PDA cell lines, MUC1 upregulates MRP1 via an Akt dependent pathway, whereas, in mouse cells, MUC1 mediated MRP1 upregulation is via an Akt independent mechanism. However, in both mouse and human cell lines, the cytoplasmic tail motif of MUC1 associates directly with the promoter region of the Abcc1/ABCC1 gene, indicating a possible role of MUC1 as a transcriptional regulator of this gene. This is the first report to show that MUC1 can directly regulate the expression of MDR genes in PDA cells and thus confer drug resistance.;We also report that human and mouse PDA cell lines expressing high levels of endogenous MUC1 also express high levels of Cox-2 compared to MUC1 null cells. Further, in both mouse and human cell lines, MUC1 upregulates expression of Cox-2/COX-2 gene via an NF-kB dependent mechanism. In MUC1 positive PDA cell lines, MUC1 and NF-kB binds to the 5'UTR of Cox-2/COX-2 gene around the NFkB response element (within 500bp upstream of TSS), which is not observed in MUC1 null PDA cells. The increased expression of Cox-2 gives the MUC1 positive PDA cell lines a growth and/or invasive advantage.;Lastly, we report that MUC1 modulates TGF-beta signaling axis causing TGF-beta1 to act as a tumor promoter in MUC1 high cells and acting as a tumor suppressor in MUC1 null cells. The difference in TGF-beta1 functioning could be partly attributed to difference in the expression profile of the TGF-beta RI and TGF-beta RII and activation of the downstream signaling cascades.