A longstanding unsettled question is whether pancreatic beta cells originate from exocrine duct cells. We have now used genetic labeling to fate map embryonic and adult pancreatic duct cells. We show that Hnf1beta+ cells of the trunk compartment of the early branching pancreas are precursors of acinar, duct, and endocrine lineages. Hnf1beta+ cells subsequent form the embryonic duct epithelium, which gives rise to both ductal and endocrine lineages, but not to acinar cells. By the end of gestation, the fate of Hnf1beta+ duct cells is further restrained. We provide compelling evidence that the ductal epithelium does not make a significant contribution to acinar or endocrine cells during neonatal growth, during a 6 month observation period, or during beta cell growth triggered by ligation of the pancreatic duct or by cell-specific ablation with alloxan followed by EGF/gastrin treatment. Thus, once the ductal epithelium differentiates it has a restricted plasticity, even under regenerative settings.

Acinar cells dedifferentiate into an embryonic progenitor-like phenotype upon suspension culture. This is associated with the activation of a senescence programme. Both processes take place in experimental chronic pancreatitis where senescence may contribute to limit tumour progression.

When pancreatic tissue is injured after duct obstruction, acinoductal metaplasia is observed. Similar metaplastic changes occur when exocrine pancreatic cells are isolated and cultured. We demonstrate that under these experimental conditions the exocrine acinar cells lose their differentiated characteristics: expression of the acinar transcription factors p48/ Ptf1␣ and Mist1 is decreased or lost, whereas expression of the embryonic transcription factor Pdx1 is increased. The receptors Notch1 and Notch2, members of the DSL family of Notch ligands, and the target genes in the Notch-signaling pathway Hes1, Hey1, and Hey2 become strongly up-regulated. We noted also reduced expression of Sel1L, a Notch repressor that is normally highly expressed in exocrine pancreas. Stimulation of Notch by its ligand Jagged1 diminished the proliferation of cultured metaplastic exocrine cells. Chemical inhibition of Notch signaling resulted in increased proliferation and induction of the cell-cycle regulator p21Cip1 . This effect seems to be Hes1-independent and mainly coincides with decreased Hey1 and Hey2 mRNA expression. In conclusion , we demonstrate that during acinoductal metaplasia the Notch-signaling pathway is activated concomitantly with changes in transcription factor expression of pancreatic acinar cells. In addition, we show that Notch signaling is implicated in the suppression of proliferation of these metaplastic exocrine cells. The latter may be important in protection from neoplastic transformation.