Egr-1 activates basic fibroblast growth factor transcription. Mechanistic implications for astrocyte proliferation

Area of Science:

• Neuroscience
• Molecular Biology
• Cell Biology

Background:

• Astrocyte proliferation mechanisms are not well understood.
• Endogenous neuropeptides endothelin-3 (ET-3) and atrial natriuretic peptide (ANP) modulate astrocyte proliferation.
• ET-3 and ANP regulate the immediate-early gene egr-1, which transactivates basic fibroblast growth factor (bFGF).

Purpose of the Study:

• To determine the involvement of MAP kinase (Erk) activation by ET-3 in egr-1 transcription.
• To elucidate the molecular mechanisms by which Egr-1 transactivates bFGF.
• To understand the inhibitory role of ANP in astrocyte proliferation.

Main Methods:

• Transfection of astrocytes with MAP kinase (MAPK) expression vectors and dominant-negative MAPK mutants.
• Analysis of egr-1 transcription using chloramphenicol acetyltransferase (CAT) constructs.
• Mobility shift assays to study protein-DNA interactions on the bFGF promoter.
• Deletion experiments on the bFGF promoter.

Main Results:

• ET-3-induced egr-1 transcription is mediated by MAP kinase (Erk) activation.
• ET-3 stimulates Egr-1 protein expression, which binds to the bFGF promoter.
• ANP inhibits ET-3-induced egr-1 transcription and Egr-1 binding to the bFGF promoter.
• A specific Egr-1 binding site at -160 bp of the bFGF promoter is crucial for bFGF transactivation.

Conclusions:

• ET-3 stimulates astrocyte proliferation by activating egr-1 transcription via a MAP kinase pathway.
• Egr-1 transactivates bFGF through a specific noncanonical Egr-1 binding site on the promoter.
• ANP exerts anti-proliferative effects by inhibiting these ET-3-mediated signaling steps.