The TLR4-NOS1-AP1 signaling axis regulates macrophage polarization

Macrophages polarize to proinflammatory M1 or anti-inflammatory M2 states with distinct physiological functions. This transition within the M1-M2 phenotypes decides the nature, duration and severity of an inflammatory response. Although there is a substantial understanding of the fate of these phenotypes, the underlying molecular mechanism of transition within the M1-M2 phenotypes is not well understood. We have investigated the role of neuronal nitric oxide synthase (NOS1)-mediated regulation of activator protein 1 (AP-1) transcription factor in macrophages as a critical effector of macrophage phenotypic change.

Raw 264.7 and THP1 macrophages were stimulated with LPS (250 ng/ml) to activate the inflammatory signaling pathway. We analyzed the effect of pharmacological NOS1 inhibitor: TRIM (1-(2- Trifluoromethylphenyl) imidazole) on LPS-induced inflammatory response in macrophages.

We determined that NOS1-derived nitric oxide (NO) facilitate Fos and Jun interaction which induces IL-12 & IL-23 expression. Pharmacological inhibition of NOS1 inhibits ATF2 and Jun dimer. Switching of Fos and Jun dimer to ATF2 and Jun dimerization controls phenotype transition from IL-12^high IL-23^high IL-10^low to IL-12^low IL-23^lowIL-10^high phenotype, respectively.

These findings highlight a key role of the TLR4-NOS1-AP1 signaling axis in regulating macrophage polarization.