Pro-inflammatory signaling by 24,25-dihydroxyvitamin D3 in HepG2 cells

Area of Science:

• Endocrinology
• Molecular Biology
• Hepatology

Background:

• The vitamin D metabolite 24,25-dihydroxyvitamin D3 (24,25[OH]2D3) is known to induce nongenomic signaling pathways.
• Previous studies showed 24,25[OH]2D3 suppresses apolipoprotein A-I (apo A-I) gene expression and high-density lipoprotein (HDL) secretion in hepatocytes.
• The mechanism by which 24,25[OH]2D3 modulates nongenomic signaling in hepatocytes is not well understood, especially given its low affinity for the vitamin D receptor (VDR).

Purpose of the Study:

• To investigate the capacity of 24,25[OH]2D3 to activate various signaling pathways relevant to apo A-I synthesis in HepG2 cells.
• To elucidate the nongenomic mechanisms underlying 24,25[OH]2D3-mediated regulation of apo A-I expression and related cellular responses.

Main Methods:

• HepG2 cells were treated with 24,25[OH]2D3.
• Gene expression of PPARα and RXRα was analyzed.
• Protein kinase activities (PKCα, JNK1, ERK1/2) and c-jun phosphorylation were assessed.
• AP-1-dependent transcriptional activity and apo A-I promoter activity were measured.
• Expression of pro-inflammatory cytokines (IL-1β, IL-6, IL-8) was quantified.

Main Results:

• Treatment with 24,25[OH]2D3 decreased PPARα and RXRα expression.
• Rapid activation of PKCα, JNK1, and ERK1/2 was observed, correlating with c-jun phosphorylation.
• AP-1-dependent transcriptional activity increased, while apo A-I promoter activity decreased.
• Expression of IL-1β, IL-6, and IL-8 was upregulated by 24,25[OH]2D3 treatment.

Conclusions:

• 24,25[OH]2D3 rapidly suppresses apo A-I gene expression in hepatocytes through nongenomic signaling pathways.
• The mechanism involves activation of pro-inflammatory protein kinases (PKCα, JNK1, ERK1/2) and modulation of AP-1 activity.
• These rapid effects of 24,25[OH]2D3 may impact lipid metabolism and hepatic function through pro-inflammatory cytokine induction.