Silencing Mist1 Gene Expression Is Essential for Recovery from Acute Pancreatitis

Abstract

Acinar cells of the exocrine pancreas are tasked with synthesizing, packaging and secreting vast quantities of pro-digestive enzymes to maintain proper metabolic homeostasis for the organism. Because the synthesis of high levels of hydrolases is potentially dangerous, the pancreas is prone to acute pancreatitis (AP), a disease that targets acinar cells, leading to acinar-ductal metaplasia (ADM), inflammation and fibrosis—events that can transition into the earliest stages of pancreatic ductal adenocarcinoma. Despite a wealth of information concerning the broad phenotype associated with pancreatitis, little is understood regarding specific transcriptional regulatory networks that are susceptible to AP and the role these networks play in acinar cell and exocrine pancreas responses. In this study, we examined the importance of the acinar-specific maturation transcription factor MIST1 to AP damage and organ recovery. Analysis of wild-type and Mist1 conditional null mice revealed that Mist1 gene transcription and protein accumulation were dramatically reduced as acinar cells underwent ADM alterations during AP episodes. To test if loss of MIST1 function was primarily responsible for the damaged status of the organ, mice harboring a Cre-inducible Mist1 transgene (iMist1) were utilized to determine if sustained MIST1 activity could alleviate AP damage responses. Unexpectedly, constitutive iMist1 expression during AP led to a dramatic increase in organ damage followed by acinar cell death. We conclude that the transient silencing of Mist1 expression is critical for acinar cells to survive an AP episode, providing cells an opportunity to suppress their secretory function and regenerate damaged cells. The importance of MIST1 to these events suggests that modulating key pancreas transcription networks could ease clinical symptoms in patients diagnosed with pancreatitis and pancreatic cancer.

MIST1 Links Secretion and Stress as both Target and Regulator of the Unfolded Protein Response

“…Several studies have shown that MIST1 is essential for appropriate responses to diverse cell stimuli, including alcohol and drug-induced inflammation, development, and response to secretagogues (9,10,12,26). Thus, we evaluated if induced Mist1 expression was restricted to ER stress conditions or whether other physiological stresses could influence expression of the Mist1 gene.…”

“…Bioinformatics analyses also suggested that MIST1 regulates key genes during ER stress in response to activation of the IRE pathway. In order to examine this in greater detail, we took advantage of a newly developed Mist1 conditional mouse model where one Mist1 allele is flanked by loxP sites while the other Mist1 allele encodes CreERT2 (26). Tamoxifen-treated Mist1 CreERT/fl mice rapidly delete the remaining Mist1 coding allele, generating an acinar-specific Mist1 null pancreas.…”

“…Studies by Lin et al (39) have shown that sustained ER stress results in the eventual downregulation of Xbp1, a necessary step as cells transition from a prosurvival UPR (governed primarily by XBP1 and ATF6 targets) to a proapoptotic UPR (governed primarily by PERK targets). To determine if MIST1 alters Xbp1 gene expression during pancreatitis, two transgenic mouse models were used in which MIST1 was either conditionally deleted (Mist1 cKO ) or constitutively overexpressed (Mist1 OE ) via acinarspecific Cre-mediated recombination (12,26). Acute pancreatitis (AP) was induced in WT, Mist1 cKO , and Mist1 OE mice by caerulein injection, and pancreatic RNAs were isolated 2 and 4 days post-AP, corresponding to the maximum cell damage and initial recovery phases, respectively (26).…”

“…To determine if MIST1 alters Xbp1 gene expression during pancreatitis, two transgenic mouse models were used in which MIST1 was either conditionally deleted (Mist1 cKO ) or constitutively overexpressed (Mist1 OE ) via acinarspecific Cre-mediated recombination (12,26). Acute pancreatitis (AP) was induced in WT, Mist1 cKO , and Mist1 OE mice by caerulein injection, and pancreatic RNAs were isolated 2 and 4 days post-AP, corresponding to the maximum cell damage and initial recovery phases, respectively (26). Importantly, Mist1 is transiently downregulated at 2 days post-AP in this model (Fig.…”

“…Acute pancreatitis (AP) was triggered at 7 days postTam administration in adult (6-to 8-week-old) C57BL/6 wild-type, Mist1 cKO (26), and constitutively overexpressing-Mist1 (Mist1 OE ) (12) mice via intraperitoneal injection of caerulein (Sigma-Aldrich, St. Louis, MO) at 50 g/kg body weight in eight hourly injections for two consecutive days. Control mice received phosphate-buffered saline (PBS).…”