Synergistic induction of the MUC4 mucin gene by interferon-γ and retinoic acid in human pancreatic tumour cells involves a reprogramming of signalling pathways

The transmembrane mucin, MUC4, is aberrantly expressed with a high incidence in human pancreatic adenocarcinomas and plays an important role in the pathogenesis of the disease. Our recent studies have shown that interferon-γ (IFNγ) and retinoic acid (RA) are important regulators of MUC4 in pancreatic tumour cells. Induction of MUC4 by IFNγ occurs via a novel pathway involving upregulation of the signal transducer and activator of transcription 1 (STAT-1), whereas its stimulation by RA requires mediation by the transforming growth factor β-2 (TGFβ-2). In this study, we have investigated the molecular mechanisms underlying the interaction of IFNγ and RA in MUC4 regulation in pancreatic tumour cells. We demonstrate that these reagents exert a synergistic induction of MUC4. Interestingly, while the upregulation of STAT-1 by IFNγ is partially inhibited by RA, IFNγ is shown to repress RA-driven TGFβ-2 induction, pointing to the involvement of alternative mechanism(s) in IFNγ-RA synergism. Moreover, a dose-dependent and cooperative induction of MUC4 promoter activity suggests a regulation at the transcriptional level, most likely by STAT-1 and RAR/RXR (RA receptor/retinoic X receptor) or other IFNγ/RA-induced secondary intermediate effectors. Our findings provide potential mechanisms that may account for the aberrant expression of MUC4 in pancreatic tumour cells and expose a novel molecular mechanism of gene induction, whereby a reprogramming of signalling pathway through alternative route(s) operates during a synergistic interaction of biological modifiers.