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]₂D₃ but 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]₂D₃ has low affinity for the vitamin D receptor (VDR) and little is known with regard to how 24,25-[OH]₂D₃ modulates nongenomic signaling in hepatocytes, we investigated the capacity of 24,25-[OH]₂D₃ to activate various signaling pathways relevant to apo A-I synthesis in HepG2 cells. Treatment with 24,25-[OH]₂D₃ resulted 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]₂D₃ for 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]₂D₃ increased IL-1β, IL-6, and IL-8 expression by HepG2 cells. These observations suggest that 24,25-[OH]₂D₃ elicits several novel rapid nongenomic-mediated pro-inflammatory protein kinases targeting AP1 activity, increasing pro-inflammatory cytokine expression, potentially impacting lipid metabolism and hepatic function.