Pro-inflammatory signaling by 24,25-dihydroxyvitamin D3in HEPG2 cells

Abstract

The vitamin D metabolite 24,25-dihydroxyvitamin D3(24, 25[OH]2D3) was shown to induce nongenomic signaling pathways in resting zone chondrocytes and other cells involved in bone remodeling. Recently, our laboratory demonstrated that 24,25-[OH]2D3but not 25-hydroxyvitamin D3, suppresses apolipoprotein A-I (apo A-I) gene expression and high-density lipoprotein (HDL) secretion in hepatocytes. Since 24,25-[OH]2D3has low affinity for the vitamin D receptor (VDR) and little is known with regard to how 24,25-[OH]2D3modulates nongenomic signaling in hepatocytes, we investigated the capacity of 24,25-[OH]2D3to activate various signaling pathways relevant to apo A-I synthesis in HepG2 cells. Treatment with 24,25-[OH]2D3resulted in decreased peroxisome proliferator-activated receptor alpha (PPARα) expression and retinoid-X-receptor alpha (RXRα) expression. Similarly, treatment of hepatocytes with 50 nM 24,25-[OH]2D3for 1–3 h induced PKCα activation as well as c-jun-N-terminal kinase 1 (JNK1) activity and extracellular-regulated kinase 1/2 (ERK1/2) activity. These changes in kinase activity correlated with changes in c-jun phosphorylation, an increase in AP-1-dependent transcriptional activity, as well as repression of apo A-I promoter activity. Furthermore, treatment with 24,25-[OH]2D3increased IL-1β, IL-6, and IL-8 expression by HepG2 cells. These observations suggest that 24,25-[OH]2D3elicits several novel rapid nongenomicmediated pro-inflammatory protein kinases targeting AP1 activity, increasing proinflammatory cytokine expression, potentially impacting lipid metabolism and hepatic function.