Ectopic expression of the beta-cell specific transcription factor Pdx1 inhibits glucagon gene transcription
- B. Ritz-LaserAffiliated withDiabetes Unit, University Hospital Geneva Email author
- , B. R. GauthierAffiliated withDiabetes Unit, University Hospital Geneva
- , A. EstreicherAffiliated withDiabetes Unit, University Hospital Geneva
- , A. MaminAffiliated withDiabetes Unit, University Hospital Geneva
- , T. BrunAffiliated withDiabetes Unit, University Hospital Geneva
- , F. RisAffiliated withJeantet Research Laboratories
- , P. SalmonAffiliated withDepartment of Microbiology, Centre Médical Universitaire
- , P. A. HalbanAffiliated withJeantet Research Laboratories
- , D. TronoAffiliated withDepartment of Microbiology, Centre Médical Universitaire
- and 1 more
- , J. PhilippeAffiliated withDiabetes Unit, University Hospital Geneva
- Show less
The transcription factor Pdx1 is required for the development and differentiation of all pancreatic cells. Beta-cell specific inactivation of Pdx1 in developing or adult mice leads to an increase in glucagon-expressing cells, suggesting that absence of Pdx1could favour glucagon gene expression by a default mechanism.
We investigated the inhibitory role of Pdx1 on glucagon gene expression in vitro. The glucagonoma cell line InR1G9 was transduced with a Pdx1-encoding lentiviral vector and insulin and glucagon mRNA levels were analysed by northern blot and real-time PCR. To understand the mechanism by which Pdx1 inhibits glucagon gene expression, we studied its effect on glucagon promoter activity in non-islet cells using transient transfections and gel-shift analysis.
In glucagonoma cells transduced with a Pdx1-encoding lentiviral vector, insulin gene expression was induced while glucagon mRNA levels were reduced by 50 to 60%. In the heterologous cell line BHK-21, Pdx1 inhibited by 60 to 80% the activation of the α-cell specific element G1 conferred by Pax-6 and/or Cdx-2/3. Although Pdx1 could bind three AT-rich motifs within G1, two of which are binding sites for Pax-6 and Cdx-2/3, the affinity of Pdx1 for G1 was much lower as compared to Pax-6. In addition, Pdx1 inhibited Pax-6 mediated activation through G3, to which Pdx1 was unable to bind. Moreover, a mutation impairing DNA binding of Pdx1 had no effect on its inhibition on Cdx-2/3. Since Pdx1 interacts directly with Pax-6 and Cdx-2/3 forming heterodimers, we suggest that Pdx1 inhibits glucagon gene transcription through protein to protein interactions with Pax-6 and Cdx-2/3.
Cell-specific expression of the glucagon gene can only occur when Pdx1 expression extinguishes from the early α cell precursor.
KeywordsEndocrine pancreas Pdx1 Pax-6 Cdx-2/3 glucagon transcriptional regulation α-cell
- Ectopic expression of the beta-cell specific transcription factor Pdx1 inhibits glucagon gene transcription
Volume 46, Issue 6 , pp 810-821
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- Endocrine pancreas
- transcriptional regulation
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- A1. Diabetes Unit, University Hospital Geneva, 24, rue Micheli-du-Crest, 1211, Geneva 14, Switzerland
- A2. Jeantet Research Laboratories, Geneva, Switzerland
- A3. Department of Microbiology, Centre Médical Universitaire, Geneva, Switzerland