Gillian M. Tannahill

Metformin, a frontline treatment for type II diabetes mellitus, decreases production of the pro-form of the inflammatory cytokine IL-1β in response to LPS in macrophages. We found that it specifically inhibited pro-IL-1β production, having no effect on TNF-α. Furthermore, metformin boosted induction of the anti-inflammatory cytokine IL-10 in response to LPS. We ruled out a role for AMP-activated protein kinase (AMPK) in the effect of metformin because activation of AMPK with A769662 did not mimic metformin here. Furthermore, metformin was still inhibitory in AMKPα1- or AMPKβ1-deficient cells. The activity of NADH:ubiquinone oxidoreductase (complex I) was inhibited by metformin. Another complex I inhibitor, rotenone, mimicked the effect of metformin on pro-IL-1β and IL-10. LPS induced reactive oxygen species production, an effect inhibited by metformin or rotenone pretreatment. MitoQ, a mitochondrially targeted antioxidant, decreased LPS-induced IL-1β without affecting TNF-α. These results, therefore, implicate complex I in LPS action in macrophages. Metformin, a frontline treatment for type II diabetes mellitus, decreases production of the pro-form of the inflammatory cytokine IL-1β in response to LPS in macrophages. We found that it specifically inhibited pro-IL-1β production, having no effect on TNF-α. Furthermore, metformin boosted induction of the anti-inflammatory cytokine IL-10 in response to LPS. We ruled out a role for AMP-activated protein kinase (AMPK) in the effect of metformin because activation of AMPK with A769662 did not mimic metformin here. Furthermore, metformin was still inhibitory in AMKPα1- or AMPKβ1-deficient cells. The activity of NADH:ubiquinone oxidoreductase (complex I) was inhibited by metformin. Another complex I inhibitor, rotenone, mimicked the effect of metformin on pro-IL-1β and IL-10. LPS induced reactive oxygen species production, an effect inhibited by metformin or rotenone pretreatment. MitoQ, a mitochondrially targeted antioxidant, decreased LPS-induced IL-1β without affecting TNF-α. These results, therefore, implicate complex I in LPS action in macrophages.

Cytokine responses can be regulated by a family of proteins termed suppressors of cytokine signaling (SOCS) which can inhibit the JAK/STAT pathway in a classical negative-feedback manner. While the SOCS are thought to target signaling intermediates for degradation, relatively little is known about how their turnover is regulated. Unlike other SOCS family members, we find that SOCS2 can enhance interleukin-2 (IL-2)- and IL-3-induced STAT phosphorylation following and potentiate proliferation in response to cytokine stimulation. As a clear mechanism for these effects, we demonstrate that expression of SOCS2 results in marked proteasome-dependent reduction of SOCS3 and SOCS1 protein expression. Furthermore, we provide evidence that this degradation is dependent on the presence of an intact SOCS box and that the loss of SOCS3 is enhanced by coexpression of elongin B/C. This suggests that SOCS2 can bind to SOCS3 and elongin B/C to form an E3 ligase complex resulting in the degradation of SOCS3. Therefore, SOCS2 can enhance cytokine responses by accelerating proteasome-dependent turnover of SOCS3, suggesting a mechanism for the gigantism observed in SOCS2 transgenic mice.