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Transcription factors recognizing overlapping C1-A2 binding sites positively regulate insulin gene expression
1Section of Islet Transplantation & Cell Biology, Joslin Diabetes Center, Harvard Medical School, Boston, Massachusetts 02215, USA.
The Journal of Biological Chemistry
|October 12, 2000
Summary
Transcription factors binding the RIPE3b region regulate insulin gene expression. Both C1-A2 elements form the RIPE3b1 activator binding site, with A2-specific activators also identified, impacting glucose response.
Area of Science:
- Molecular Biology
- Endocrinology
- Genetics
Background:
- The insulin enhancer region, RIPE3b, is crucial for beta-cell specific and glucose-responsive insulin gene expression.
- A RIPE3b1-binding complex activator is essential for these functions, with its DNA binding activity modulated by glucose and glucotoxicity.
- The C1 element was previously suggested as the RIPE3b1 activator binding site, and the A2 element shares homology with insulin enhancer A elements, which bind PDX-1 and HNF-1 alpha.
Purpose of the Study:
- To elucidate the roles of the C1 and A2 elements within the RIPE3b region in regulating insulin gene expression.
- To identify the specific transcription factor binding sites and complexes involved in insulin gene regulation.
Main Methods:
- Systematic mutation of the RIPE3b region in an insulin enhancer.
- Analysis of the effects of these mutations on gene expression.
- Identification and characterization of DNA-binding complexes in nuclear extracts from insulinoma cell lines using electrophoretic mobility shift assays.
Main Results:
- Both C1 and A2 elements together form the binding site for the RIPE3b1 activator.
- Three A2-specific binding complexes were identified, with the A2.2 complex being specific to insulin-producing cell lines.
- Two base pairs within the A2 element are critical for the binding of both RIPE3b1 and A2.2 activators.
- Transient transfection assays showed cooperative activation of insulin gene expression by C1-A2 and A2-specific binding activators.
Conclusions:
- The C1-A2 region (RIPE3b) and A2-specific elements bind distinct activators that cooperatively regulate insulin gene expression.
- Differential glucose responsiveness of RIPE3b1 and A2-specific activators suggests a complex regulatory mechanism for insulin gene expression.
- Overlapping binding specificities and functional cooperation of these activators are key to fine-tuning insulin gene expression in response to metabolic cues.