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Division of Endocrinology and Metabolism, Departments of 1 Medicine and 2 Anatomy, Shiga University of Medical Science, Seta, Otsu, Shiga 520-2192 Japan
Pancreatic
duodenal homeobox-1 (Pdx1) is a transcription factor, and its
phosphorylation is thought to be essential for activation of insulin
gene expression. This phosphorylation is related to a concomitant shift
in molecular mass from 31 to 46 kDa. However, we found that Pdx1 was
modified by SUMO-1 (small ubiquitin-related modifier 1) in
-TC-6 and
COS-7 cells, which were transfected with Pdx1 cDNA. This modification
contributed to the increase in molecular mass of Pdx1 from 31 to
46 kDa. Additionally, sumoylated Pdx1 localized in the nucleus. The
reduction of SUMO-1 protein by use of RNA interference (SUMO-iRNAs)
resulted in a significant decrease in Pdx1 protein in the nucleus. A
34-kDa form of Pdx1 was detected by the cells exposed to SUMO-iRNAs in
the presence of lactacystin, a proteasome inhibitor. Furthermore, the
reduced nuclear sumoylated Pdx1 content was associated with significant lower transcriptional activity of the insulin gene. These findings indicate that SUMO-1 modification is associated with both the localization and stability of Pdx1 as well as its effect on insulin gene activation.
small ubiquitin-related modifier 1; pancreatic duodenal homeobox-1; ribonucleic acid interference
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