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Repression and activation of transcription of phosphoenolpyruvate carboxykinase gene during liver development
1The Department of Developmental Biochemistry, Hebrew University-Hadassah Medical School, P.O. Box 12272, Jerusalem, Israel.
Insights
The hepatic phosphoenolpyruvate carboxykinase (PEPCK) gene
Area of Science:
- Biochemistry
- Molecular Biology
- Genetics
Background:
- Hepatic phosphoenolpyruvate carboxykinase (PEPCK) gene transcription is critical for initiating hepatic gluconeogenesis at birth.
- Dysregulation of PEPCK expression leads to life-incompatible conditions like hypoglycemia or neonatal diabetes.
Purpose of the Study:
- To investigate the regulatory mechanisms controlling the precise onset of hepatic PEPCK gene transcription.
- To identify DNA sequences and transcription factors involved in PEPCK gene repression and activation.
Main Methods:
- Utilized transgenic mice and transfected hepatoma cells (Hepa1c1c7) to study PEPCK promoter activity.
- Analyzed promoter constructs of varying lengths (597 bp and 2000 bp) and specific DNA sequences.
- Investigated the effects of transcription factors like HNF-1, C/EBP, RXRalpha, and PPARalpha on PEPCK gene expression.
Main Results:
- Transgenes driven by the PEPCK promoter showed premature and over-expression in fetal and neonatal mouse livers, respectively.
- A 597 bp proximal promoter had higher activity than longer constructs in Hepa1c1c7 cells.
- Deletion of a region containing a PPAR/RXR element reduced repression of a 2000 bp promoter.
- HNF-1/C/EBP activated the promoter, while RXRalpha repressed it, an effect reversed by PPARalpha.
Conclusions:
- Upstream sequences of the PEPCK gene play a crucial role in restraining perinatal expression.
- Specific transcription factors and their interactions with regulatory elements dictate the precise transcriptional control of PEPCK.
- Understanding these regulatory mechanisms is vital for comprehending metabolic adaptations at birth.
Abstract:
Transcriptional activation of the hepatic phosphoenolpyruvate carboxykinase (PEPCK) gene at birth is critical since PEPCK appearance initiates hepatic gluconeogenesis. A delayed appearance results in hypoglycemia, while a premature appearance results in neonatal diabetes, both are incompatible with sustaining life. Experiments using transgenic mice and transfected hepatoma cells suggest that both repression and activation underlie the correct onset of hepatic PEPCK gene transcription. In transgenic mice, transgenes driven by the proximal PEPCK promoter are prematurely expressed in the fetal liver and over-expressed in the neonatal liver, indicating that sequences upstream of the proximal promoter restrain perinatal expression. In Hepa1c1c7 cells, which mimic the fetal liver, the proximal PEPCK promoter (597 bp) exhibited a 3. 5-10-fold higher activity than longer promoters. Repression of the longer promoter (2000 bp) was diminished upon deletion of the sequence spanning positions(-840) to(- 1116) which contains a PPAR/RXR recognition element. The intact 2000 bp PEPCK promoter could be markedly activated by co-transfecting the transcription factor HNF-1 together with C/EBP. It could be repressed by co-transfection with RXRalpha and adding PPARalpha relieved this inhibition.