Bmi1 is required for regeneration of the exocrine pancreas in mice

BACKGROUND & AIMS: Bmi1 is a member of the Polycomb protein family and represses transcription by modifying chromatin organization at specific promoters. Bmi1 is implicated in the control of stem cell self-renewal and has been shown to regulate cell proliferation, tissue homeostasis, and differentiation. Bmi1 is present in a subpopulation of self-renewing pancreatic acinar cells and is expressed in response to pancreatic damage. We investigated the role of Bmi1 in regeneration of exocrine pancreas. METHODS: Acute pancreatitis was induced in Bmi1^-/- mice with cerulein; pancreatic cell regeneration, differentiation, and apoptosis were assessed. Cultured Bmi1^-/- and wild-type primary acini were analyzed in vitro to determine acinar-specific consequences of Bmi1 deletion. To investigate cell autonomous versus noncell autonomous roles for Bmi1 in vivo, pancreatitis was induced in Bmi1^-/- mice reconstituted with a wild-type hematopoietic system. RESULTS: Bmi1 expression was up-regulated in the exocrine pancreas during regeneration after cerulein-induced pancreatitis. Exocrine regeneration was impaired following administration of cerulein to Bmi1^-/- mice. Pancreata of Bmi1^-/- mice were hypoplastic, and the exocrine pancreas was replaced with ductal metaplasia that had increased apoptosis and decreased cell proliferation compared with that of wild-type mice. Expression of Cdkn2a and p53-dependent apoptotic genes was markedly up-regulated in Bmi1 ^-/- pancreas compared with wild-type mice after injury. Furthermore, after transplantation of bone marrow from wild-type to Bmi1^-/- mice, the chimeric mice had intermediate levels of pancreatic hypoplasia and significant but incomplete rescue of impaired exocrine regeneration after cerulein injury. CONCLUSIONS: Bmi1 contributes to regeneration of the exocrine pancreas after cerulein-induced injury through cell autonomous mechanisms, in part by regulating Cdkn2a expression, and noncell autonomous mechanisms.