Deciphering the bHLH transcription factor Mist1: Dual roles in pancreas development
Abstract
Mist1 is a basic helix-loop-helix (bHLH) transcription factor that plays important roles in the proper development of the exocrine pancreas. Mice lacking the Mist1 gene exhibit disorganized acinar cell structure, defective intercellular communication and acinar-to-ductal metaplasia. In this thesis, I explored novel functions of Mist1 and studied the mechanisms by which Mist1 exerts these functions in the exocrine pancreas. Expression of Mist1 resulted in a significant decrease in the growth potential of cells, which was associated with induced expression of p21CIP1/WAF1. In contrast, shRNA-directed knock-down of endogenous Mist1 transcripts resulted in increased cell proliferation and a concomitant decrease in p21CIP1/WAF1 protein levels. Increased cell proliferation was also found in Mist1KO pancreata. Mist1-dependent activation of the p21CIP1/WAF1 promoter was independent of classic bHLH protein binding sites (E-box, CANNTG). Instead, Mist1 activated p21CIP1/WAF1 expression through the Sp1 binding sites within the p21CIP1/WAF1 promoter by directly binding to Sp1. These results indicate that Mist1 suppresses pancreatic acinar cell growth through induction of p21CIP1/WAF1. Interestingly, in a MS/MS analysis designed to identify Mist1 binding proteins in acinar cells, Mist1 was found to associate with SAP18, a subunit of the Sin3-HDAC corepressor complex. This leads to the possibility that Mist1 also functions to repress transcription of certain genes in the exocrine pancreas. Indeed, transcript levels of Rnd2 and K19 negatively correlated with Mist1 protein levels in pancreatic cell lines. Coexpression of Mist1 and SAP18 resulted in decreased transcript levels of Rnd2 and K19. Taken together, Mist1 plays multiple roles in the exocrine pancreas: controlling acinar cell proliferation through the transcriptional activation of p21CIP1/WAF1 and maintaining acinar cell identity by repressing the expression of the duct cell marker gene K19.
Degree
Ph.D.
Advisors
Konieczny, Purdue University.
Subject Area
Molecular biology
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