Search

A novel role for ATF3 is identified in acinar-to–duct cell metaplasia during pancreatic injury. ATF3 targets transcriptional regulators that affect acinar cell fate and reduces the severity of pancreatic injury. However, by doing so, ATF3 may also increase the susceptibility for progression to pancreatic adenocarcinoma., Pancreatitis is a debilitating disease of the exocrine pancreas that, under chronic conditions, is a major susceptibility factor for pancreatic ductal adenocarcinoma (PDAC). Although down-regulation of genes that promote the mature acinar cell fate is required to reduce injury associated with pancreatitis, the factors that promote this repression are unknown. Activating transcription factor 3 (ATF3) is a key mediator of the unfolded protein response, a pathway rapidly activated during pancreatic insult. Using chromatin immunoprecipitation followed by next-generation sequencing, we show that ATF3 is bound to the transcriptional regulatory regions of >30% of differentially expressed genes during the initiation of pancreatitis. Of importance, ATF3-dependent regulation of these genes was observed only upon induction of pancreatitis, with pathways involved in inflammation, acinar cell differentiation, and cell junctions being specifically targeted. Characterizing expression of transcription factors that affect acinar cell differentiation suggested that acinar cells lacking ATF3 maintain a mature cell phenotype during pancreatitis, a finding supported by maintenance of junctional proteins and polarity markers. As a result, Atf3–/– pancreatic tissue displayed increased tissue damage and inflammatory cell infiltration at early time points during injury but, at later time points, showed reduced acinar-to–duct cell metaplasia. Thus our results reveal a critical role for ATF3 as a key regulator of the acinar cell transcriptional response during injury and may provide a link between chronic pancreatitis and PDAC.

Several animal models have been developed to investigate the pathobiology of pancreatitis, but few studies have examined the effects that altered pancreatic gene expression have in these models. In this study, the sensitivity to secretagogue-induced pancreatitis was examined in a mouse line that has an altered acinar cell environment due to the targeted deletion of Mist1. Mist1 is an exocrine specific transcription factor important for the complete differentiation and function of pancreatic acinar cells. Mice lacking the Mist1 gene [ Mist1 knockout (KO) mice] exhibit cellular disorganization and functional defects in the exocrine pancreas but no gross morphological defects. Following the induction of pancreatitis with caerulein, a CCK analog, we observed elevated serum amylase levels, necrosis, and tissue damage in Mist1 KO mice, indicating increased pancreatic damage. There was also a delay in the regeneration of acinar tissue in Mist1 KO animals. Molecular profiling revealed an altered activation of stress response genes in Mist1 KO pancreatic tissue compared with wild-type (WT) tissue following the induction of pancreatitis. In particular, Western blot analysis for activating transcription factor 3 and phosphorylated eukaryotic initiation factor 2α (eIF2α), mediators of endoplasmic reticulum (ER) stress, indicated limited activation of this pathway in Mist1 KO animals compared with WT controls. Conversely, Mist1 KO pancreatic tissue exhibits increased expression of growth arrest and DNA damage inducible 34 protein, an inhibitor of eIF2α phosphorylation, before and after the induction of pancreatitis. These finding suggest that activation of the ER stress pathway is a protective event in the progression of pancreatitis and highlight the Mist1 KO mouse line as an important new model for studying the molecular events that contribute to the sensitivity to pancreatic injury.

Fibroblast growth factor 21 (FGF21) is a key regulator of metabolism under conditions of stress such as starvation, obesity, and hypothermia. Rapid induction of FGF21 is also observed in experimental models of pancreatitis, and FGF21 reduces tissue damage observed in these models, suggesting a nonmetabolic function. Pancreatitis is a debilitating disease with significant morbidity that greatly increases the risk of pancreatic ductal adenocarcinoma. The goals of this study were to examine the regulation and function of FGF21 in acinar cell injury, specifically in a mouse model of pancreatic injury ( Mist1 −/−). Mist1 −/− mice exhibit acinar cell disorganization, decreased acinar cell communication and exocytosis, and increased sensitivity to cerulein-induced pancreatitis (CIP). Examination of Fgf21 expression in Mist1 −/− mice by qRT-PCR, Northern blot, and Western blot analyses showed a marked decrease in pancreatic Fgf21 expression before and after induction of CIP compared with C57Bl/6 mice. To determine whether the loss of FGF21 accounted for the Mist1 −/− phenotypes, we generated Mist1 −/− mice overexpressing human FGF21 from the ApoE promoter ( Mist1 −/− ApoE-FGF21). Reexpression of FGF21 partially mitigated pancreatic damage in Mist1 −/− tissue based on reduced intrapancreatic enzyme activation, reduced expression of genes involved in fibrosis, and restored cell-cell junctions. Interestingly, alteration of Fgf21 expression in Mist1 −/− tissue was not simply due to a loss of direct transcriptional regulation by MIST1. Chromatin immunopreciptation indicated that the loss of Fgf21 in the Mist1 −/− pancreas is due, in part, to epigenetic silencing. Thus, our studies identify a new role for FGF21 in reducing acinar cell injury and uncover a novel mechanism for regulating Fgf21 gene expression.

Gene expression is affected by modifications to histone core proteins within chromatin. Changes in these modifications, or epigenetic reprogramming, can dictate cell fate and promote susceptibility to disease. The goal of this study was to determine the extent of epigenetic reprogramming in response to chronic stress that occurs following ablation of MIST1 (Mist1-/-), which is repressed in pancreatic disease. Chromatin immunoprecipitation for trimethylation of lysine residue 4 on histone 3 (H3K4Me3) in purified acinar cells from wild type and Mist1-/- mice was followed by Next Generation sequencing (ChIP-seq) or ChIP-qPCR. H3K4Me3-enriched genes were assessed for expression by qRT-PCR in pancreatic tissue before and after induction of cerulein-induced pancreatitis. While most of H3K4Me3-enrichment is restricted to transcriptional start sites, >25% of enrichment sites are found within, downstream or between annotated genes. Less than 10% of these sites were altered in Mist1-/- acini, with most changes in H3K4Me3 enrichment not reflecting altered gene expression. Ingenuity Pathway Analysis of genes differentially-enriched for H3K4Me3 revealed an association with pancreatitis and pancreatic ductal adenocarcinoma in Mist1-/- tissue. Most of these genes were not differentially expressed but several were readily induced by acute experimental pancreatitis, with significantly increased expression in Mist1-/- tissue relative to wild type mice. We suggest that the chronic cell stress observed in the absence of MIST1 results in epigenetic reprogramming of genes involved in promoting pancreatitis to a poised state, thereby increasing the sensitivity to events that promote disease. © 2014 Mehmood et al.

Pancreatic ductal adenocarcinoma (PDAC) has a 5 year survival rate post-diagnosis of5%. Individuals with chronic pancreatitis (CP) are 20-fold more likely to develop PDAC, making it a significant risk factor for PDAC. While the relationship for the increased susceptibility to PDAC is unknown, loss of the acinar cell phenotype is common to both pathologies. Pancreatic acinar cells can dedifferentiate or trans-differentiate into a number of cell types including duct cells, β cells, hepatocytes and adipocytes. Knowledge of the molecular pathways that regulate this plasticity should provide insight into PDAC and CP. MIST1 (encoded by Bhlha15 in mice) is a transcription factor required for complete acinar cell maturation. The goal of this study was to examine the plasticity of acinar cells that do not express MIST1 (Mist1(-/-) ). The fate of acinar cells from C57Bl6 or congenic Mist1(-/-) mice expressing an acinar specific, tamoxifen-inducible Cre recombinase mated to Rosa26 reporter LacZ mice (Mist1(CreERT/-) R26r) was determined following culture in a three-dimensional collagen matrix. Mist1(CreERT/-) R26r acini showed increased acinar dedifferentiation, formation of ductal cysts and transient increases in PDX1 expression compared to wild-type acinar cells. Other progenitor cell markers, including Foxa1, Sox9, Sca1 and Hes1, were elevated only in Mist1(-/-) cultures. Analysis of protein kinase C (PKC) isoforms by western blot and immunofluorescence identified increased PKCε accumulation and nuclear localization of PKCδ that correlated with increased duct formation. Treatment with rottlerin, a PKCδ-specific inhibitor, but not the PKCε-specific antagonist εV1-2, reduced acinar dedifferentiation, progenitor gene expression and ductal cyst formation. Immunocytochemistry on CP or PDAC tissue samples showed reduced MIST1 expression combined with increased nuclear PKCδ accumulation. These results suggest that the loss of MIST1 is a common event during PDAC and CP and events that affect MIST1 function and expression may increase susceptibility to these pathologies.

Background: Alcohol abuse is a leading cause of pancreatitis in humans. However, rodent models suggest that alcohol only sensitizes the pancreas to subsequent insult, indicating that additional factors play a role in alcohol-induced pancreatic injury. The goal of this study was to determine if an absence of MIST1, a transcription factor required for complete differentiation of pancreatic acinar cells in mice, increased the sensitivity to alcohol. Methods: Two to four month-old mice lacking MIST1 (Mist1 -/-) or congenic C57 Bl6 mice were placed on a Lieber-DeCarli diet (36% of total kcal from ethanol and fat), a control liquid diet (36% kcal from fat) or a regular breeding chow diet (22% kcal from fat). After six weeks, pancreatic morphology was assessed. Biochemical and immunofluorescent analysis was used to assess mediators of the unfolded protein response (UPR). Results: Ethanol-fed Mist1 -/- mice developed periductal accumulations of inflammatory cells that did not appear in wild type or control-fed Mist1 -/- mice. Wild type mice fed diets high in ethanol or fat showed enhancement of the UPR based on increased accumulation of peIF2α and spliced XBP1. These increases were not observed in Mist1 -/- pancreatic tissue, which had elevated levels of UPR activity prior to diet exposure. Indeed, exposure to ethanol resulted in a reduction of UPR activity in Mist1 -/- mice. Conclusions: Our findings suggest that an absence of MIST1 increases the sensitivity to ethanol that correlated with decreased activity of the UPR. Therefore, events that affect the expression and/or function of MIST1 may be confounding factors in pancreatitis. © 2011 Alahari et al.

MIST1 is a transcription factor expressed in pancreatic acinar cells and other serous exocrine cells. Mice harboring a targeted deletion of the Mist1 gene (Mist1(-/-)) exhibit alterations in acinar regulated exocytosis and aberrant Ca(2+) signaling that are normally controlled by acinar cell Ca(2+)-ATPases. Previous studies indicated that total sarcoendoplasmic reticulum Ca(2+)-ATPases (SERCA) and plasma membrane Ca(2+)-ATPases (PMCA) remained unaffected in Mist1(-/-) acinar cultures. Therefore, we have assessed the expression of Atp2c2, the gene that encodes the secretory pathway Ca(2+)-ATPase 2 (SPCA2). We revealed a dramatic decrease in pancreatic expression of Atp2a2 mRNA and SPCA2 protein in Mist1(-/-) mice. Surprisingly, this analysis indicated that the acinar-specific Atp2c2 mRNA is a novel transcript, consisting of only the 3' end of the gene and the protein and localizes to the endoplasmic reticulum. Expression of SPCA2 was also lost in Mist1(-/-) secretory cells of the salivary glands and seminal vesicles, suggesting that Atp2c2 transcription is regulated by MIST1. Indeed, inducible MIST1 expression in Mist1(-/-) pancreatic acinar cells restored normal Atp2c2 expression, supporting a role for MIST1 in regulating the Atp2c2 gene. Based on these results, we have identified a new Atp2c2 transcript, the loss of which may be linked to the Mist1(-/-) phenotype.

The seminal vesicle is a male accessory sex organ that develops from segments of the Wolffian duct adjacent to the urogenital sinus. It produces most of the seminal plasma in both humans and rodents. To date, very few transcription factors have been linked to the development and differentiation of seminal vesicles. In this study, we have examined the role of basic helix-loop-helix (BHLH) B8 transcription factor expressed at high levels in the adult seminal vesicle and during seminal gland differentiation. Immunofluorescent studies indicate that BHLHB8 is expressed within the epithelial layer of the seminal layer of the seminal vesicle following branching morphogenesis but prior to full maturation of cell morphology and function. Analysis of mice that do not express BHLHB8 (Bhlhb8(-/-)) indicates no deficiency in the initial development of the seminal vesicle. However, morphological and ultrastructural analysis indicates disruption of the epithelial cellular architecture. The seminal vesicle epithelial layer of 2-mo-old Bhlhb8(-/-) mice shows extensive cellular degeneration based on the appearance of reduced microvilli, altered granule size, and dilated endoplasmic reticulum and Golgi apparatus. The seminal vesicle epithelial cells also degenerate prematurely, as evidenced by disruption of nuclear architecture and significant accumulations of autophagic bodies. These results identify BHLHB8 as a regulator in establishing and stabilizing the secreting epithelial cells of the seminal vesicle.

The pancreas is a complex organ that consists of separate endocrine and exocrine cell compartments. Although great strides have been made in identifying regulatory factors responsible for endocrine pancreas formation, the molecular regulatory circuits that control exocrine pancreas properties are just beginning to be elucidated. In an effort to identify genes involved in exocrine pancreas function, we have examined Mist1, a basic helix-loop-helix transcription factor expressed in pancreatic acinar cells. Mist1-null (Mist1KO) mice exhibit extensive disorganization of exocrine tissue and intracellular enzyme activation. The exocrine disorganization is accompanied by increases in p8, RegI/PSP, and PAP1/RegIII gene expression, mimicking the molecular changes observed in pancreatic injury. By 12 m, Mist1KO mice develop lesions that contain cells coexpressing acinar and duct cell markers. Analysis of the factors involved in cholecystokinin (CCK) signaling reveal inappropriate levels of the CCK receptor A and the inositol-1,4,5-trisphosphate receptor 3, suggesting that a functional defect exists in the regulated exocytosis pathway of Mist1KO mice. Based on these observations, we propose that Mist1KO mice represent a new genetic model for chronic pancreas injury and that the Mist1 protein serves as a key regulator of acinar cell function, stability, and identity.