COMMENTARY How macrophages ring the inflammation alarm Marco E. Bianchia,1 and Angelo A. Manfredib resume their normal occupations (and replen- aDivision of Genetics and Cell Biology and bDivision of Regenerative Medicine, Stem Cells and ish HMGB1 by resynthesis) once the danger Gene Therapy, San Raffaele University and Scientific Institute, 20132 Milan, Italy is over. Examples are HMGB1 secretion by cardiomyocytes in hypoxic conditions follow- ing ischemia (5) or by neural cells following the Inflammation is the immediate response of histones, to form nucleosomes (2). However, our tissues to clear and present danger: HMGB1 is also the archetypal damage-associated depolarization of their plasma membrane (6). microbial invasion, injury, or a serious mal- molecular pattern (DAMP): molecules that, Myeloid cells secrete HMGB1 once they are function of our body’s internal workings. when released from a cell’s interior follow- activated by the presence of pathogens, by Whereas all cells can respond to danger, in- ing its untimely death, alert immune cells to the detection of injured cells, or by cytokines, nate immunity cells have that as their mis- danger (3). Notably, cells that die by apopto- in order to rebroadcast the danger message. sion: They deploy a vast array of receptors to sis (and therefore kill themselves presumably Extracellular HMGB1 plays a central role detect telltale molecules from pathogens or ail- after careful consideration) do not release in inflammation, both acute and chronic, and ing cells and secrete an equally vast number of HMGB1 to activate the immune system (4), indeed in sepsis when innate immunity goes soluble proteins to communicate among them- but rather expose “eat-me” signals to request awry (7). HMGB1 both recruits and activates selves and with the resident cells of the tissue. swift disposal. myeloid cells; however, these are separable In PNAS, Lu et al. report on how myeloid cells Interestingly, the meaning of extracellular processes, which depend on different redox use a key word in this cellular conversation: HMGB1 goes beyond a simple “I’mdead”: states of HMGB1 and on different receptors. high-mobility group box 1 (HMGB1) (1). Most cells can secrete HMGB1 before they HMGB1 contains three cysteines (at positions HMGB1 is a component of eukaryotic die and therefore can cry out for help before 23, 45, and 106): When all are in the reduced chromatin, and within the nucleus, it bends it is too late for them. In fact, cells can secrete (thiol) state, HMGB1 forms a heterocomplex DNA and facilitates its interaction with tran- most of the HMGB1 they contain without with the chemokine stromal cell-derived scription factors and other molecules, including necessarily being committed to die and can factor (SDF-1/CXCL12) and binds to the CXCR4 receptor, acting as a potent chemo- attractant for all motile cells, including innate immunity cells (8, 9). When cysteines 23 and IFNβ LPS IFNAR2 45 form a disulfide bond and the cysteine 106 is in the thiol state, HMGB1 binds to κ IFNAR1 the TLR4 receptor, activating NF- Btodrive TLR4 the transcription of genes involved in inflam- mation, including chemokine and cytokine TRIF genes; when one of the cysteines is terminally TRAM AUTOPHAGY oxidized to a sulfonate, HMGB1 loses activity both as a chemoattractant and as an inflam- HMGB1 IRFs matoryligand(9,10).HMGB1alsobindsto P P secretion other surface molecules, including receptor for advanced glycation end products (RAGE), HMGB1 thrombomodulin,CD24,andTIM-3;RAGE Histone acetylase cytoplasmic accumulation can cooperate both with HMGB1’schemoat- IFNβ tractant and inflammatory activities, whereas IRFs the other molecules limit HMGB’s effects (7). Given HMGB1’s key role in inflammation, understanding how it secreted is of central importance. HMGB1 secretion is unconven- Inflammasome tional, as may be expected for a nuclear pro- activation Danger signals tein that has no leader peptide to direct it to the endoplasmic reticulum and the exocytosis route. Rather, HMGB1 is endowed with nu- clear localization and nuclear export signals Author contributions: M.E.B. and A.A.M. wrote the paper. HMGB1 Acetylated HMGB1 Conflict of interest statement: M.E.B. is founder and part-owner of HMGBiotech, a biotech company that specializes in the HMGB1 field. Fig. 1. Diagrammatic representation of the pathway for HMGB1 secretion induced by LPS, as proposed by Lu et al. See companion article on page 3068. (1). The two subsequent steps, IFN-β secretion on LPS activation of TLR4 and HMGB1 secretion on binding of IFN-β to 1To whom correspondence should be addressed. E-mail: bianchi. its own receptor, are distinguished by arrow color. [email protected]. 2866–2867 | PNAS | February 25, 2014 | vol. 111 | no. 8 www.pnas.org/cgi/doi/10.1073/pnas.1324285111 Downloaded by guest on September 25, 2021 – β (NLS and NES, respectively) (11), which keep of lysine acetylases (KATs), the inhibition of compatible with the finding that anti IFN- COMMENTARY it in constant flux between the nucleus and histone deacetylases (HDACs), or both. Nota- antibodies interfere with LPS-induced HMGB1 the cytoplasm. In basal conditions, almost all bly, all STATs likely bind CBP and p300 (14), secretion. However, the implication is that HMGB1 is nuclear, but removal of positive which are ubiquitous KATs that have been macrophages do not secrete HMGB1 in re- charges from its NLSs via lysine acetylation shown to be capable of acetylating HMGB1 sponse to LPS directly, but only indirectly. (11) or addition of negative charges via serine (11). Moreover, STAT1-mediated transcrip- This is counterintuitive, as macrophages are phosphorylation (12) shifts the balance to tion requires HDACs as coactivators, even well equipped to respond immediately to min- allow the transfer of a fraction of HMGB1 though HDAC activity is usually associated ute amounts of LPS, whereas the response to to the cytoplasm. Tracey and coworkers, with transcriptional repression. It would be their own IFN production would take time; who were the first to realize that HMGB1 interesting to know whether STAT1 binding one wonders why evolution has not favored can be secreted by macrophages (13), now to DNA is required for HMGB1 acetylation a direct response. A second question is whether implicate the JAK/STAT signaling pathway and whether the same recruited coactivator JAK/STAT signaling is always necessary for in determining the acetylation of HMGB1’s complex concomitantly acetylates both HMGB1 secretion. HMGB1 secretion requires NLSs and its eventual secretion (1) (Fig. 1). HMGB1 and histones. If this is the case, its cytoplasmic relocalization, but pathways JAKs and STATs are critical for signaling by HMGB1 might modulate STAT1-mediated alternative to NLS acetylation appear to cytokine transmembrane receptors. After the gene expression, perhaps directly or by com- exist, including calmodulin-mediated ki- receptors associate with their respective cyto- peting with histone tails as a substrate. nase (CAMK) phosphorylation of NLS kine ligand, they undergo a conformational In conclusion, the present findings eluci- serines (19). Lu et al. indeed show that change, bringing two JAKs close enough to date one step in the cascade between macro- HMGB1 relocalization after rapamycin- phosphorylate each other and the receptor. phage activation and HMGB1 secretion and induced autophagy is not blocked by pyridone Phosphorylation of tyrosine residues in the convincingly implicate JAK/STAT signaling. 6. Finally, and related to the second ques- cytoplasmic receptor tail creates docking sites As depicted in Fig. 1, LPS binding to Toll-like tion, it is important to determine whether for molecules of the STAT family. On binding, receptor (TLR)4 promotes IFN transcrip- JAK/STAT signaling is required for HMGB1 the STATs themselves become a JAK substrate, tion, and secreted IFNs bind IFN receptors secretion by all cells or just by macrophages; and phosphorylated STATs form dimers that (IFNARs) and cause STAT1 phosphoryla- critically, the authors show that IFN-β fails translocate to the nucleus and bind enhancers tion by JAKs. Activated STAT1 promotes to direct HMGB1 release from mouse em- and promoters of cytokine responsive genes HMGB1 acetylation and cytoplasmic reloca- bryonic fibroblasts. Cell type specificity is of (14). Receptors of type 1 and 2 IFNs (IFN-α/β tion. Concomitant inflammasome activation paramount importance for therapeutic ap- and IFN-γ, respectively) depend on JAKs for would then cause the actual secretion of plications: Myeloid cells do not appear to con- their function. It had been known that HMGB1. The elucidation of this pathway tribute much to circulating levels of HMGB1 HMGB1’s secretion can be induced by expo- may translate into therapeutic approaches: in LPS endotoxemia, because the level of sure of macrophages to IFNs and that phar- Because extracellular HMGB1 contributes HMGB1 is similar in LPS-challenged mice macological or genetic blockade of the JAK/ decisively to sepsis (18), an inhibitor like whetherornottheirmyeloidcellscontain STAT downstream signaling abrogates pyridone 6 might hold a lot of promise. Sev- HMGB1 (20). Despite all of these uncertain- HMGB1 secretion (15, 16). Lu et al. show eral potential questions and problems remain, ties, the illumination of one side of HMGB1 that pyridone 6, a pan-inhibitor of JAK1/2, however. Foremost is why LPS requires JAK/ biology will undoubtedly help the understand- blocks the LPS-induced secretion of HMGB1 STAT signaling to induce HMGB1 secretion. ing of the remaining dark sides and more from mouse macrophage-like RAW 267.4 As first suggest by Kim et al. (16), LPS would generally of how our body detects and re- cells, whereas it does not decrease LPS-in- induce IFN secretion, which in turn would sponds to its own failings (which most people duced secretion of TNF and other cytokines. cause HMGB1 secretion; this hypothesis is would call illnesses). Cytokines are secreted via the classical exo- cytotic pathway, whereas HMGB1’s secretion requires the acetylation of its NLSs to direct it 1 Lu B, et al.