Dopamine inhibits the expression of proinflammatory cytokines of microglial cells through the formation of dopamine quinone in the mouse striatum.

We previously reported that dopamine (DA) attenuated lipopolysaccharide (LPS)-induced expression of proinflammatory cytokines through the formation of DA quinone (DAQ) in murine microglial cell line BV-2 and primary murine microglial cells. To reveal whether DA inhibits the expression of proinflammatory cytokines of microglial cells through the formation of DAQ in the central nervous system (CNS), in this study, we examined the effect of DAQ on LPS-induced mRNA expression of proinflammatory cytokines in C57BL/6 mouse brain under two experimental conditions: 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration and l-dopa/carbidopa administration. Acute MPTP administration reduced the number of tyrosine hydroxylase-positive cells in the substantia nigra, and decreased the level of quinoprotein, an indicator of DAQ formation, in the striatum. Real-time RT-PCR analysis revealed that intraperitoneal administration of LPS increased the mRNA levels of proinflammatory cytokines, including tumor-necrosis factor-α and interleukin-1ß, in the striatum. These increases were enhanced in MPTP-treated mice. On the other hand, l-dopa/carbidopa administration increased the level of quinoprotein, attenuated the LPS-induced mRNA expression of proinflammatory cytokines, and reduced the LPS-induced increase in the number of microglial cells in the striatum. These results suggest that DA attenuate the expression of proinflammatory cytokines in microglia through the formation of DAQ in the CNS.

Regulation of Iron-Ion Transporter SLC11A2 by Three Identical miRNAs.

Here, we searched for microRNAs (miRNAs) in silico that could interact with SLC11A2 mRNA, a solute carrier (SLC) iron-ion transporter, and investigated their effects on SLC11A2 gene expression using the cultured human colon carcinoma cell line, Caco-2. In silico analysis using the miRWalk2.0 database revealed that several types of miRNAs interact with the human SLC11A2 gene; we focused on three miRNAs, miR-149-5p, miR-362-5p, and miR-539-5p as candidates in this study. We first revealed that the three miRNAs interact with the SLC11A2 3'-untranslated region (3'-UTR) using a luciferase assay in a Caco-2 cell line. We then examined whether the expression of each miRNA affected the expression of SLC11A2 mRNAs and their transcribed transporter proteins. We found transiently expressed miRNAs significantly reduced the reporter activity of the SLC11A2 3'-UTR site in Caco-2 cells by significantly decreasing the SLC11A2 gene and protein expression in the miRNA-transfected Caco-2 cells. Subsequently, we investigated the effects of these miRNAs on SLC11A2's iron-ion transporting activity by measuring iron-ion concentration in Caco-2 cells. Administration of ammonium iron (II) sulfate hexahydrate to Caco-2 cells significantly increased the intracellular iron-ion concentration. However, in iron-ion-pretreated cells, overexpression of each of the three miRNAs resulted in decreased intracellular iron-ion concentration. This indicated that overexpressed miRNAs inhibited iron-ion influx into Caco-2 cells by attenuating SLC11A2 transporting activity. Using in silico analysis, we predicted that three studied miRNAs could bind to the iron-ion influx transporter SLC11A2 and revealed that they regulate SLC11A2 gene expression and iron-ion transporting function in an in vitro system.

Dopamine inhibits lipopolysaccharide-induced nitric oxide production through the formation of dopamine quinone in murine microglia BV-2 cells.

Dopamine (DA) has been suggested to modulate functions of glial cells including microglial cells. To reveal the regulatory role of DA in microglial function, in the present study, we investigated the effect of DA on lipopolysaccharide (LPS)-induced nitric oxide (NO) production in murine microglial cell line BV-2. Pretreatment with DA for 24 h concentration-dependently attenuated LPS-induced NO production in BV-2 cells. The inhibitory effect of DA on LPS-induced NO production was not inhibited by SCH-23390 and sulpiride, D1-like and D2-like DA receptor antagonists, respectively. In addition, pretreatment with (-)-(6aR,12bR)-4,6,6a,7,8,12b-Hexahydro-7-methylindolo[4,3-a]phenanthridin (CY 208-243) and bromocriptine, D1-like and D2-like DA receptor agonists, respectively, did not affect the LPS-induced NO production. N-Acetylcysteine, which inhibits DA oxidation, completely inhibited the effect of DA. Tyrosinase, which catalyzes the oxidation of DA to DA quionone (DAQ), accelerated the inhibitory effect of DA on LPS-induced NO production. These results suggest that DA attenuates LPS-induced NO production through the formation of DAQ in BV-2 cells.

Dopamine attenuates lipopolysaccharide-induced expression of proinflammatory cytokines by inhibiting the nuclear translocation of NF-κB p65 through the formation of dopamine quinone in microglia.

Many reports have indicated that dopamine has immunomodulatory effects on peripheral immune cells. The purpose of this study was to reveal the immunomodulatory effect of dopamine on the expression of proinflammatory cytokines in microglial cells, which are the immune cells of the central nervous system. In murine microglial cell line BV-2 cells, pretreatment with dopamine for 24 h attenuated the lipopolysaccharide (LPS)-induced expression of proinflammatory cytokines such as tumor-necrosis factor-α, interleukin-1ß, and interleukin-6. Neither (5R)-8-chloro-3-methyl-5-phenyl-1,2,4,5-tetrahydro-3-benzazepin-7-ol; hydrochloride (SCH-23390) nor sulpiride, which are dopamine D1-like and D2-like receptor antagonists, respectively, affected the attenuation of LPS-induced expression of cytokines by dopamine. In addition, pretreatment with neither (-)-(6aR,12bR)-4,6,6a,7,8,12b-Hexahydro-7-methylindolo[4,3-a]phenanthridin (CY208-243) nor bromocriptine, dopamine D1-like and D2-like receptor agonists, respectively, was effective in doing so. However, N-acetylcysteine (NAC), which inhibits dopamine oxidation to dopamine quinone, did inhibit this attenuated expression. Dopamine increased the level of quinoproteins, and this increase was inhibited by NAC. Western blot and immunocytochemical analyses revealed that dopamine inhibited LPS-induced nuclear translocation of nuclear factor-kappa B (NF-κB) p65. Dopamine also attenuated the expression of cytokines and the nuclear translocation of NF-κB p65 induced by LPS in mouse microglial cells in primary culture. These results suggest that dopamine attenuated LPS-induced expression of cytokines by inhibiting the nuclear translocation of NF-κB p65 through the formation of dopamine quinone in microglial cells.