BACKGROUND/OBJECTIVES: Endoplasmic reticulum (ER) stress in adipose tissue causes an inflammatory response and leads to metabolic diseases. However, the association between vitamin D and adipose ER stress remains poorly understood. In this study, we investigated whether 1,25-dihydroxyvitamin D (1,25(OH)D) alleviates ER stress in adipocytes.

MATERIALS/METHODS: 3T3-L1 cells were treated with different concentrations (i.e., 10-100 nM) of 1,25(OH)D after or during differentiation (i.e., on day 0-7, 3-7, or 7). They were then incubated with thapsigargin (TG, 500 nM) for an additional 24 h to induce ER stress. Next, we measured the mRNA and protein levels of genes involved in unfold protein response (UPR) and adipogenesis using real-time polymerase chain reaction and western blotting and quantified the secreted protein levels of pro-inflammatory cytokines. Finally, the mRNA levels of UPR pathway genes were measured in adipocytes transfected with siRNA-targeting .

RESULTS

Treatment with 1,25(OH)D during various stages of adipocyte differentiation significantly inhibited ER stress induced by TG. In fully differentiated 3T3-L1 adipocytes, 1,25(OH)D treatment suppressed mRNA levels of , , and and decreased the secretion of monocyte chemoattractant protein-1, interleukin-6, and tumor necrosis factor-α. However, downregulation of the mRNA levels of , , and following 1,25(OH)D administration was not observed in -knockdown adipocytes. In addition, exposure of 3T3-L1 preadipocytes to 1,25(OH)D inhibited transcription of , , , , and and reduced the p-alpha subunit of translation initiation factor 2 (eIF2α)/eIF2α and p-protein kinase RNA-like ER kinase (PERK)/PERK protein ratios. Furthermore, 1,25(OH)D treatment before adipocyte differentiation reduced adipogenesis and the mRNA levels of adipogenic genes.

CONCLUSIONS

Our data suggest that 1,25(OH)D prevents TG-induced ER stress and inflammatory responses in mature adipocytes by downregulating UPR signaling via binding with . In addition, the inhibition of adipogenesis by vitamin D may contribute to the reduction of ER stress in adipocytes.