The effects of metformin on the early LPS response of macrophages

Metformin is a front-line drug in the treatment of type-2 diabetes mellitus (T2DM). In addition to its antigluconeogenic and insulin-sensitizing properties, it has emerged as a potent inhibitor of the inflammatory response of macrophages. Specifically, metformin has been shown to reduce transcript levels of Il1b, the gene encoding the pro-inflammatory cytokine interleukin (IL)-1b, during long-term exposure of macrophages to the bacterial cell-wall component lipopolysaccharide (LPS). However, the extent to which metformin affects the early transcriptional response to LPS has never been investigated. Here, we show that metformin affects transcript levels of a large yet selective subset of LPS-responsive genes after only two hours of LPS exposure, mostly counteracting the effect of LPS rather than enhancing it. The affected genes are implicated in a variety of biological functions, in particular cellular movement and trafficking. Intriguingly, metformin affects transcript levels of Il1b at this early time point as well, but through a molecular mechanism fundamentally different from the regulation observed after longer exposure. While down-regulation of Il1b by metformin during the late stages of the LPS response has been shown to rely on stabilization of hypoxia-inducible factor (HIF)-1α and production of IL-10, Il1b inhibition at the early stage requires AMP-activated protein kinase (AMPK) activation but is independent of HIF-1α and IL-10. These results reveal an unexpected complexity in the anti-inflammatory properties of metformin and demonstrate that Il1b is down-regulated by distinct mechanisms in the early and late stages of the LPS response. Bone-marrow-derived macrophages from WT C57Bl/6 mice were either left untreated, stimulated with 100 ng/ml LPS for 2 h or pretreated with 5 mM metformin for 6 h and then stimulated with 100 ng/ml LPS for 2 h.