Inflammatory signaling pathways regulating ApoE gene expression in macrophages

Area of Science:

• Molecular Biology
• Immunology
• Cardiovascular Research

Background:

• Apolipoprotein E (apoE) has a crucial atheroprotective role.
• Inflammation, mimicked by lipopolysaccharide (LPS), reduces macrophage apoE expression, promoting atherosclerosis.

Purpose of the Study:

• To investigate signaling pathways mediating LPS-induced repression of apoE gene expression in macrophages.
• To identify key kinases and transcription factors involved in this inflammatory response.

Main Methods:

• Utilized mouse macrophage cell line RAW 264.7 treated with LPS.
• Employed dominant-negative IkappaB, small interfering RNA (siRNA) for c-Jun, transfection, and DNA binding assays.
• Investigated the roles of Tpl-2, MEKK1, ERK, JNK, NF-kappaB, and c-Jun in apoE promoter regulation.

Main Results:

• LPS treatment down-regulates apoE promoter activity via Tpl-2 and MEKK1 kinases.
• These pathways converge on NF-kappaB and activate c-Jun phosphorylation by JNK.
• Both NF-kappaB and c-Jun bind to the apoE core promoter (-55/+73 region) and repress its activity.
• LPS inhibits apoE expression through the Tpl-2/MEK/ERK pathway affecting a distinct promoter region.

Conclusions:

• LPS represses macrophage apoE gene expression through Tpl-2 and MEKK1.
• The mitogen-activated protein kinases ERK and JNK, along with transcription factors AP-1 and NF-kappaB, are central to this repression.
• Distinct signaling arms involving NF-kappaB and c-Jun/AP-1 contribute to the inhibition of apoE promoter activity.