Id3 modulates cellular localization of bHLH Ptf1-p48 protein

Ptf1-p48 is a pancreas-specific bHLH transcriptional protein, which, in the normal adult pancreas, shows a restricted expression in acinar cells where it is predominantly localized in the nucleus and activates the transcription of exocrine-specific genes. Ptf1-p48 partners with two proteins to form the PTF1 active complex: a bHLH E-protein and suppressor of hairless RBP-J. Cytoplasmic mislocalization of Ptf1-p48 has been reported in pancreatic pathologies, suggesting its contribution in the early steps of pancreatic carcinogenesis. The aim of the our work was to elucidate the mechanisms regulating Ptf1-p48 subcellular localization. We hypothesized a role of Id proteins acting in a dominant-negative fashion by heterodimerizing with bHLH proteins. We reproduced Ptf1-p48 cytoplasmic mislocalization in acinar AR4-2J cells and demonstrated that a proliferative signal elicited by gastrin leads to increases in Id3 protein expression and levels of Id3/E47 and Id3/Ptf1-p48 interactions, and a decrease in the level of E47/Ptf1-p48 interaction. By contrast, Id3 silencing reversed the cytoplasmic mislocalization of Ptf1-p48 induced by gastrin. As E47 is responsible for the nuclear import of the PTF1 complex, disruption of this complex via Id3 interactions with both E47 and Ptf1-p48 appears to induce cytoplasmic mislocalization of Ptf1-p48. We then found that Ptf1-p48 is either absent or mislocalized in the cytoplasm and Id3 is overexpressed in human and murine pancreatic preneoplastic lesions. Our data provide novel insight into the regulation of Ptf1-p48 function and provide evidence that Ptf1-p48 cytoplasmic mislocalization and Id3 overexpression are early events in pancreatic cancer progression.