Regulatory Effect of 25-hydroxyvitamin D(3) on Nitric Oxide Production in Activated Microglia

Microglia are activated by inflammatory and pathophysiological stimuli in neurodegenerative diseases, and activated microglia induce neuronal damage by releasing cytotoxic factors like nitric oxide (NO). Activated microglia synthesize a significant amount of vitamin D(3) in the rat brain, and vitamin D(3) has an inhibitory effect on activated microglia. To investigate the possible role of vitamin D(3) as a negative regulator of activated microglia, we examined the effect of 25-hydroxyvitamin D(3) on NO production of lipopolysaccharide (LPS)-stimulated microglia. Treatment with LPS increased the production of NO in primary cultured and BV2 microglial cells. Treatment with 25-hydroxyvitamin D(3) inhibited the generation of NO in LPS-activated primary microglia and BV2 cells. In addition to NO production, expression of 1-α-hydroxylase and the vitamin D receptor (VDR) was also upregulated in LPS-stimulated primary and BV2 microglia. When BV2 cells were transfected with 1-α-hydroxylase siRNA or VDR siRNA, the inhibitory effect of 25-hydroxyvitamin D(3) on activated BV2 cells was suppressed. 25-Hydroxyvitamin D(3) also inhibited the increased phosphorylation of p38 seen in LPS-activated BV2 cells, and this inhibition was blocked by VDR siRNA. The present study shows that 25-hydroxyvitamin D(3) inhibits NO production in LPS-activated microglia through the mediation of LPS-induced 1-α-hydroxylase. This study also shows that the inhibitory effect of 25-hydroxyvitamin D(3) on NO production might be exerted by inhibiting LPS-induced phosphorylation of p38 through the mediation of VDR signaling. These results suggest that vitamin D(3) might have an important role in the negative regulation of microglial activation.