Regulation of human apolipoprotein A-I gene expression by equine estrogens

Area of Science:

• Biochemistry
• Molecular Biology
• Endocrinology

Background:

• Estrogen replacement therapies (CEE) are linked to reduced coronary heart disease risk in postmenopausal women.
• Apolipoprotein A-I (apoA-I) plays a crucial role in reverse cholesterol transport and cardiovascular protection.

Purpose of the Study:

• To investigate how conjugated equine estrogen (CEE) components modulate human apolipoprotein A-I (apoA-I) promoter activity.
• To identify the specific estrogen component and the underlying molecular mechanism responsible for apoA-I gene regulation.

Main Methods:

• Utilized a HepG2 stable cell line (HepG2/S) with a human apoA-I promoter-luciferase reporter.
• Employed nuclear runoff assays to measure gene transcription rates.
• Performed transient transfection and mobility shift analyses to identify cis-acting elements and receptor involvement.

Main Results:

• Equilenin (Eqn), a CEE component, significantly increased apoA-I promoter activity (3-fold) and apoA-I mRNA levels (without affecting degradation).
• Eqn elevated apoA-I gene transcription rates (2.5-fold) and increased apoA-I protein secretion (80%) independently of apoA-II levels.
• The apoA-I electrophile/antioxidant response element (EpRE/ARE) was identified as the key cis-acting element mediating Eqn's effect.
• Estrogen receptors (ERalpha and ERbeta) were not involved in the Eqn-mediated induction of apoA-I promoter activity.

Conclusions:

• Equilenin potently induces human apoA-I gene promoter activity, leading to increased apoA-I mRNA and protein secretion.
• This induction occurs through an estrogen receptor-independent pathway, mediated by the apoA-I EpRE/ARE.
• Findings suggest a novel mechanism by which CEE may exert its cardioprotective effects through apoA-I modulation.