Caspase-11 contributes to site-1 protease cleavage and SREBP1 activation in the inflammatory response of macrophages.

Sterol regulatory element-binding proteins (SREBPs) are key transcription factors that control fatty acid and cholesterol metabolism. As the major SREBP isoform in macrophages, SREBP1a is also required for inflammatory and phagocytotic functions. However, it is insufficiently understood how SREBP1a is activated by the innate immune response in macrophages. Here, we show that mouse caspase-11 is a novel inflammatory activator of SREBP1a in macrophages. Upon LPS treatment, caspase-11 was found to promote the processing of site-1 protease (S1P), an enzyme that mediates the cleavage and activation of SREBP1. We also determined that caspase-11 directly associates with S1P and cleaves it at a specific site. Furthermore, deletion of the Casp4 gene, which encodes caspase-11, impaired the activation of S1P and SREBP1 as well as altered the expression of genes regulated by SREBP1 in macrophages. These results demonstrate that the caspase-11/S1P pathway activates SREBP1 in response to LPS, thus regulating subsequent macrophage activation.

Cyclic stretch regulates immune responses via tank-binding kinase 1 expression in macrophages.

Macrophages distributed in tissues throughout the body contribute to homeostasis. In the inflammatory state, macrophages undergo mechanical stress that regulates the signal transduction of immune responses and various cellular functions. However, the effects of the inflammatory response on macrophages under physiological cyclic stretch are unclear. We found that physiological cyclic stretch suppresses inflammatory cytokine expression in macrophages by regulating NF-κB activity. NF-κB phosphorylation at Ser536 in macrophages was inhibited, suggesting that tank-binding kinase (TBK1) regulates NF-κB activity during physiological stress. Moreover, TBK1 expression was suppressed by physiological stretch, and TBK1 knockdown by siRNA induced the suppression of NF-κB phosphorylation at Ser536. In conclusion, physiological stretch triggers suppression of a TBK1-dependent excessive inflammatory response, which may be necessary to maintain tissue homeostasis.

Activated cholesterol metabolism is integral for innate macrophage responses by amplifying Myd88 signaling.

Recent studies have shown that cellular metabolism is tightly linked to the regulation of immune cells. Here, we show that activation of cholesterol metabolism, involving cholesterol uptake, synthesis, and autophagy/lipophagy, is integral to innate immune responses in macrophages. In particular, cholesterol accumulation within endosomes and lysosomes is a hallmark of the cellular cholesterol dynamics elicited by Toll-like receptor 4 activation and is required for amplification of myeloid differentiation primary response 88 (Myd88) signaling. Mechanistically, Myd88 binds cholesterol via its CLR recognition/interaction amino acid consensus domain, which promotes the protein's self-oligomerization. Moreover, a novel supramolecular compound, polyrotaxane (PRX), inhibited Myd88­dependent inflammatory macrophage activation by decreasing endolysosomal cholesterol via promotion of cholesterol trafficking and efflux. PRX activated liver X receptor, which led to upregulation of ATP binding cassette transporter A1, thereby promoting cholesterol efflux. PRX also inhibited atherogenesis in Ldlr-/- mice. In humans, cholesterol levels in circulating monocytes correlated positively with the severity of atherosclerosis. These findings demonstrate that dynamic changes in cholesterol metabolism are mechanistically linked to Myd88­dependent inflammatory programs in macrophages and support the notion that cellular cholesterol metabolism is integral to innate activation of macrophages and is a potential therapeutic and diagnostic target for inflammatory diseases.