Understanding the initiation and progression of pancreatic ductal adenocarcinoma (PDAC) may provide therapeutic strategies for this deadly disease. Recently, we and others made the surprising finding that PDAC and its preinvasive precursors, pancreatic intraepithelial neoplasia (PanIN), arise via reprogramming of mature acinar cells. We therefore hypothesized that the master regulator of acinar differentiation, PTF1A, could play a central role in suppressing PDAC initiation. In this study, we demonstrate that PTF1A expression is lost in both mouse and human PanINs, and that this downregulation is functionally imperative in mice for acinar reprogramming by oncogenic KRAS. Loss of Ptf1a alone is sufficient to induce acinar-to-ductal metaplasia, potentiate inflammation, and induce a KRAS-permissive, PDAC-like gene expression profile. As a result, Ptf1a-deficient acinar cells are dramatically sensitized to KRAS transformation, and reduced Ptf1a greatly accelerates development of invasive PDAC. Together, these data indicate that cell differentiation regulators constitute a new tumor suppressive mechanism in the pancreas.DOI: http://dx.doi.org/10.7554/eLife.07125.001

Maintenance of cell type identity is crucial for health, yet little is known of the regulation that sustains the long-term stability of differentiated phenotypes. To investigate the roles that key transcriptional regulators play in adult differentiated cells, we examined the effects of depletion of the developmental master regulator PTF1A on the specialized phenotype of the adult pancreatic acinar cell in vivo. Transcriptome sequencing and chromatin immunoprecipitation sequencing results showed that PTF1A maintains the expression of genes for all cellular processes dedicated to the production of the secretory digestive enzymes, a highly attuned surveillance of unfolded proteins, and a heightened unfolded protein response (UPR). Control by PTF1A is direct on target genes and indirect through a ten-member transcription factor network. Depletion of PTF1A causes an imbalance that overwhelms the UPR, induces cellular injury, and provokes acinar metaplasia. Compromised cellular identity occurs by derepression of characteristic stomach genes, some of which are also associated with pancreatic ductal cells. The loss of acinar cell homeostasis, differentiation, and identity is directly relevant to the pathologies of pancreatitis and pancreatic adenocarcinoma. Loss of cellular identity has long been associated with tissue injury and a first step in cancer progression (for examples, see references 1 and 2). Maintenance of a specific cellular phenotype depends on the continued transcription of cell-type-specific genes, largely through open chromatin architecture (3, 4) maintained by a small group of lineage-restricted DNA-binding transcription factors (TFs) (5, 6) that establish a unique transcriptional regulatory network (7). Many physiologic or pathophysiologic perturbations can affect the differentiated state of a cell quantitatively, but fewer affect the state of differentiation qualitatively. Qualitative changes involve the acquisition of characteristics of another cell type (or types), often defined by one or a few cell-specific markers, in addition to the diminution of the original phenotype. Despite progress with cellular reprogramming (for example, see reference 8), the molecular and genetic mechanisms that maintain cellular identity within the context of adult organs remain incompletely understood. In this report, we show that inactivation of the transcriptional regulatory gene Ptf1a in adult pancreatic acinar cells has pleiotropic effects on gene expression that cause quantitative and multigene qualitative changes of acinar differentiation.The acinar cell of the pancreas has been an informative model of terminal cellular differentiation (9). Common cellular processes are greatly exaggerated in support of the prodigious synthesis, processing, storage, and exocytosis of secretory proteins. The pancreatic acinar cell has the most ribosomes (10) and the highest rate of protein synthesis (11) of any mammalian somatic cell; it synthesizes, stores, and secretes its weight in protein daily. Specialized cellular functions ...

The orphan nuclear receptor NR5A2 is necessary for the stem-like properties of the epiblast of the pre-gastrulation embryo and for cellular and physiological homeostasis of endoderm-derived organs postnatally. Using conditional gene inactivation, we show that Nr5a2 also plays crucial regulatory roles during organogenesis. During the formation of the pancreas, Nr5a2 is necessary for the expansion of the nascent pancreatic epithelium, for the subsequent formation of the multipotent progenitor cell (MPC) population that gives rise to pre-acinar cells and bipotent cells with ductal and islet endocrine potential, and for the formation and differentiation of acinar cells. At birth, the NR5A2-deficient pancreas has defects in all three epithelial tissues: a partial loss of endocrine cells, a disrupted ductal tree and a >90% deficit of acini. The acinar defects are due to a combination of fewer MPCs, deficient allocation of those MPCs to pre-acinar fate, disruption of acinar morphogenesis and incomplete acinar cell differentiation. NR5A2 controls these developmental processes directly as well as through regulatory interactions with other pancreatic transcriptional regulators, including PTF1A, MYC, GATA4, FOXA2, RBPJL and MIST1 (BHLHA15). In particular, Nr5a2 and Ptf1a establish mutually reinforcing regulatory interactions and collaborate to control developmentally regulated pancreatic genes by binding to shared transcriptional regulatory regions. At the final stage of acinar cell development, the absence of NR5A2 affects the expression of Ptf1a and its acinar specific partner Rbpjl, so that the few acinar cells that form do not complete differentiation. Nr5a2 controls several temporally distinct stages of pancreatic development that involve regulatory mechanisms relevant to pancreatic oncogenesis and the maintenance of the exocrine phenotype.

bMuch remains unknown regarding the regulatory networks formed by transcription factors in mature, differentiated mammalian cells in vivo, despite many studies of individual DNA-binding transcription factors. We report a constellation of feed-forward loops formed by the pancreatic transcription factors MIST1 and PTF1 that govern the differentiated phenotype of the adult pancreatic acinar cell. PTF1 is an atypical basic helix-loop-helix transcription factor complex of pancreatic acinar cells and is critical to acinar cell fate specification and differentiation. MIST1, also a basic helix-loop-helix transcription factor, enhances the formation and maintenance of the specialized phenotype of professional secretory cells. The MIST1 and PTF1 collaboration controls a wide range of specialized cellular processes, including secretory protein synthesis and processing, exocytosis, and homeostasis of the endoplasmic reticulum. PTF1 drives Mist1 transcription, and MIST1 and PTF1 bind and drive the transcription of over 100 downstream acinar genes. PTF1 binds two canonical bipartite sites within a 0.7-kb transcriptional enhancer upstream of Mist1 that are essential for the activity of the enhancer in vivo. MIST1 and PTF1 coregulate target genes synergistically or additively, depending on the target transcriptional enhancer. The frequent close binding proximity of PTF1 and MIST1 in pancreatic acinar cell chromatin implies extensive collaboration although the collaboration is not dependent on a stable physical interaction.T he mammalian pancreas is a mixed exocrine and endocrine gland consisting of acini, ducts, and islets. Approximately 90% of the mass of the adult pancreas is exocrine acinar tissue. Pancreatic acinar cells are highly specialized for the synthesis and secretion of digestive enzymes that are flushed via ducts to the intestine for digestion of complex nutrients (1). Abundant rough endoplasmic reticulum (ER) supports an extraordinary level of secretory protein synthesis (2). Maintenance of ER homeostasis without the accumulation of misfolded and unfolded proteins is especially important for acinar function and viability (3). Acinar cells are polarized, with basal rough ER and an extensive supranuclear Golgi apparatus for sorting and condensing newly synthesized secretory proteins into secretory vesicles (zymogen granules) that fill the apical cellular domain nearest the luminal plasma membrane. Secretion is regulated to ensure an appropriate surge of digestive enzyme release in response to feeding (1).Several transcription factors, including PTF1 and MIST1, are known to play crucial roles in the specification, differentiation, and maturation of pancreatic acinar cells (4-6). PTF1 is a complex of three tightly associated DNA-binding subunits: the cell-typerestricted basic helix-loop-helix (bHLH) protein PTF1A, one of the common bHLH E proteins (e.g., E47) (7), and RBPJ or its paralog RBPJL (8, 9). All three subunits contribute to the recognition of an extended bipartite binding sequence consisting of an E box boun...

The lineage-specific basic helix-loop-helix transcription factor Ptf1a is a critical driver for development of both the pancreas and nervous system. How one transcription factor controls diverse programs of gene expression is a fundamental question in developmental biology. To uncover molecular strategies for the program-specific functions of Ptf1a, we identified bound genomic regions in vivo during development of both tissues. Most regions bound by Ptf1a are specific to each tissue, lie near genes needed for proper formation of each tissue, and coincide with regions of open chromatin. The specificity of Ptf1a binding is encoded in the DNA surrounding the Ptf1a-bound sites, because these regions are sufficient to direct tissue-restricted reporter expression in transgenic mice. Fox and Sox factors were identified as potential lineage-specific modifiers of Ptf1a binding, since binding motifs for these factors are enriched in Ptf1a-bound regions in pancreas and neural tube, respectively. Of the Fox factors expressed during pancreatic development, Foxa2 plays a major role. Indeed, Ptf1a and Foxa2 colocalize in embryonic pancreatic chromatin and can act synergistically in cell transfection assays. Together, these findings indicate that lineage-specific chromatin landscapes likely constrain the DNA binding of Ptf1a, and they identify Fox and Sox gene families as part of this process.