Regulation of ubiquitin-specific processing protease 8 suppresses neuroinflammation

In our previous study, we reported that luteolin might exert neuroprotective functions by inhibiting the production of proinflammatory mediators, thereby suppressing microglial activation. In this study, we used two-dimensional gel electrophoresis (2-DE) and mass spectrometry (MS) to study the effect of ubiquitin-specific processing protease 8 (USP8) in luteolin-treated microglia. Western blot analysis verified that USP8 expression is upregulated by luteolin. Researchers have found that USP8 markedly enhanced the stability of neuregulin receptor degradation protein-1 (Nrdp1), which in turn inhibited the production of proinflammatory cytokines in toll-like receptor-triggered macrophages. We next hypothesized that luteolin inhibits microglial inflammation by regulating USP8 gene expression. After transfecting BV2-immortalized murine microglial cells with USP8, a significant reduction in the degradation of Nrdp1 was observed. USP8 overexpression also reduced the production of lipopolysaccharide (LPS)-induced proinflammatory mediators such as inducible nitric oxide synthase (iNOS), nitric oxide (NO), cyclooxygenase-2 (COX-2), and prostaglandin E2 (PGE2). We also found that USP8 siRNA blocked luteolin inhibition of pro-inflammatory gene expression such as iNOS, NO, COX-2, and PGE2. Taken together, our findings suggested that luteolin inhibits microglial inflammation by enhancing USP8 protein production. We concluded that in addition to anti-inflammatory luteolin, USP8 might represent a novel mechanism for the treatment of neuroinflammation and neurodegeneration.