title: Mist1 regulates pancreatic acinar cell proliferation through p21 CIP1/WAF1.
abstract: OBJECTIVE
  Mist1 is a basic helix-loop-helix (bHLH) transcription factor that is important to the proper development of the exocrine pancreas. The aim of this study was to investigate the role of Mist1 in modulating acinar cell proliferation.
  
  
  METHODS
  Ductal and acinar pancreatic cell lines were engineered to express an inducible Mist1 complementary DNA or to express a short hairpin RNA that targeted endogenous Mist1. Alterations in RNA and protein levels were detected by real-time reverse-transcription polymerase chain reaction and immunoblots. Chromatin immunoprecipitation and reporter gene assays were performed to map Mist1-responsive elements on target genes; the overall proliferation index of acinar cells from Mist1 null pancreata was evaluated by immunohistochemistry.
  
  
  RESULTS
  Expression of Mist1 resulted in a significant decrease in the proliferative potential of cells that was associated with induced expression of p21(CIP1/WAF1). Short hairpin RNA-directed knockdown of p21(CIP1/WAF1) generated cells that were refractory to Mist1 expression, whereas knockdown of Mist1 transcripts or deletion of Mist1 from the mouse genome led to increased cell proliferation and a concomitant decrease in p21(CIP1/WAF1) protein levels. Surprisingly, Mist1-dependent activation of the p21(CIP1/WAF1) promoter was independent of classic basic helix-loop-helix protein binding sites. Instead, Sp1 binding sites were essential for Mist1-dependent transcription, suggesting that Mist1 activates p21(CIP1/WAF1) expression through a unique Sp1 pathway. Indeed, coimmunoprecipitation studies demonstrated that Mist1 and Sp1 were found within the same transcription complex.
  
  
  CONCLUSIONS
  Our results show that Mist1 has a dual role in the development of the exocrine pancreas: controlling cell proliferation and promoting terminal differentiation.
authors.affiliation: Department of Biological Sciences, Purdue Cancer Center, Purdue University, West Lafayette, Indiana 47907-2064, USA.
authors.forename: Di
authors.initials: D
authors.lastname: Jia
authors.forename: Yan
authors.initials: Y
authors.lastname: Sun
authors.forename: Stephen F
authors.initials: SF
authors.lastname: Konieczny