Inflammation resolution counterbalances excessive inflammation and restores tissue homeostasis after injury. Failure of resolution contributes to the pathology of numerous chronic inflammatory ...diseases. Resolution is mediated by endogenous specialized proresolving mediators (SPMs), which are derived from long-chain fatty acids by lipoxygenase (LOX) enzymes. 5-LOX plays a critical role in the biosynthesis of two classes of SPMs: lipoxins and resolvins. Cytoplasmic localization of the nonphosphorylated form of 5-LOX is essential for SPM biosynthesis, whereas nuclear localization of phosphorylated 5-LOX promotes proinflammatory leukotriene production. We previously showed that MerTK, an efferocytosis receptor on macrophages, promotes SPM biosynthesis by increasing the abundance of nonphosphorylated, cytoplasmic 5-LOX. We now show that activation of MerTK in human macrophages led to ERK-mediated expression of the gene encoding sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2), which decreased the cytosolic Ca
concentration and suppressed the activity of calcium/calmodulin-dependent protein kinase II (CaMKII). This, in turn, reduced the activities of the mitogen-activated protein kinase (MAPK) p38 and the kinase MK2, resulting in the increased abundance of the nonphosphorylated, cytoplasmic form of 5-LOX and enhanced SPM biosynthesis. In a zymosan-induced peritonitis model, an inflammatory setting in which macrophage MerTK activation promotes resolution, inhibition of ERK activation delayed resolution, which was characterized by an increased number of neutrophils and decreased amounts of SPMs in tissue exudates. These findings contribute to our understanding of how MerTK signaling induces 5-LOX-derived SPM biosynthesis and suggest a therapeutic strategy to boost inflammation resolution in settings where defective resolution promotes disease progression.
Clearance of apoptotic cells (ACs) by phagocytes (efferocytosis) prevents post-apoptotic necrosis and dampens inflammation. Defective efferocytosis drives important diseases, including ...atherosclerosis. For efficient efferocytosis, phagocytes must be able to internalize multiple ACs. We show here that uptake of multiple ACs by macrophages requires dynamin-related protein 1 (Drp1)-mediated mitochondrial fission, which is triggered by AC uptake. When mitochondrial fission is disabled, AC-induced increase in cytosolic calcium is blunted owing to mitochondrial calcium sequestration, and calcium-dependent phagosome formation around secondarily encountered ACs is impaired. These defects can be corrected by silencing the mitochondrial calcium uniporter (MCU). Mice lacking myeloid Drp1 showed defective efferocytosis and its pathologic consequences in the thymus after dexamethasone treatment and in advanced atherosclerotic lesions in fat-fed Ldlr−/− mice. Thus, mitochondrial fission in response to AC uptake is a critical process that enables macrophages to clear multiple ACs and to avoid the pathologic consequences of defective efferocytosis in vivo.
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•Continued efferocytosis requires Drp1-mediated mitochondrial fission•Mitochondrial fission allows release of ER Ca2+ into the cytoplasm•Ca2+-mediated vesicular trafficking enables phagocytosis of a second apoptotic cell•Mice lacking myeloid Drp1 have defective efferocytosis and its pathologic sequelae
How are dead cells in our bodies rapidly cleared by phagocytes in order to avoid inflammation and necrosis?
Nonalcoholic steatohepatitis (NASH) is a leading cause of liver disease worldwide. However, the molecular basis of how benign steatosis progresses to NASH is incompletely understood, which has ...limited the identification of therapeutic targets. Here we show that the transcription regulator TAZ (WWTR1) is markedly higher in hepatocytes in human and murine NASH liver than in normal or steatotic liver. Most importantly, silencing of hepatocyte TAZ in murine models of NASH prevented or reversed hepatic inflammation, hepatocyte death, and fibrosis, but not steatosis. Moreover, hepatocyte-targeted expression of TAZ in a model of steatosis promoted NASH features, including fibrosis. In vitro and in vivo mechanistic studies revealed that a key mechanism linking hepatocyte TAZ to NASH fibrosis is TAZ/TEA domain (TEAD)-mediated induction of Indian hedgehog (Ihh), a secretory factor that activates fibrogenic genes in hepatic stellate cells. In summary, TAZ represents a previously unrecognized factor that contributes to the critical process of steatosis-to-NASH progression.
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•TAZ/WWTR1 is increased in hepatocytes in human and mouse NASH liver•Hepatocyte TAZ silencing in steatosis blocks inflammation, cell death, and fibrosis•Hepatocyte TAZ silencing in NASH reverses inflammation, cell death, and fibrosis•Hepatocyte TAZ promotes fibrosis by inducing Ihh, a hepatic stellate cell activator
Hepatic fibrosis is a key feature of nonalcoholic steatohepatitis (NASH) affecting morbidity. Wang et al. investigate the underlying mechanisms for progression from benign steatosis to NASH and show that the transcription factor TAZ/WWTR1 is increased in mouse and human NASH. Silencing TAZ can prevent or reverse NASH features, notably fibrosis.
Nonalcoholic steatohepatitis (NASH) is emerging as a leading cause of chronic liver disease. However, therapeutic options are limited by incomplete understanding of the mechanisms of NASH fibrosis, ...which is mediated by activation of hepatic stellate cells (HSCs). In humans, human genetic studies have shown that hypomorphic variations in MERTK, encoding the macrophage c-mer tyrosine kinase (MerTK) receptor, provide protection against liver fibrosis, but the mechanisms remain unknown. We now show that holo- or myeloid-specific Mertk targeting in NASH mice decreases liver fibrosis, congruent with the human genetic data. Furthermore, ADAM metallopeptidase domain 17 (ADAM17)-mediated MerTK cleavage in liver macrophages decreases during steatosis to NASH transition, and mice with a cleavage-resistant MerTK mutant have increased NASH fibrosis. Macrophage MerTK promotes an ERK-TGFβ1 pathway that activates HSCs and induces liver fibrosis. These data provide insights into the role of liver macrophages in NASH fibrosis and provide a plausible mechanism underlying MERTK as a genetic risk factor for NASH fibrosis.
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•MerTK signaling in liver macrophages promotes liver fibrosis in NASH•Macrophage MerTK activates hepatic stellate cells in NASH by inducing TGF-β1•ATRA-induced MerTK cleavage blocks the TGF-b1-fibrosis pathway in steatosis•ATRA-induced MerTK cleavage becomes defective in NASH
Genome-wide association studies have indicated that MerTK is a risk factor for liver fibrosis in NASH with an unknown mechanism. Cai et al. discovered that MerTK signaling in liver macrophages, which is enhanced in NASH owing to suppression of its cleavage by ADAM17, promotes TGF-β1 production, HSC activation, and liver fibrosis in NASH.
The acute inflammatory response requires a coordinated resolution program to prevent excessive inflammation, repair collateral damage, and restore tissue homeostasis, and failure of this response ...contributes to the pathology of numerous chronic inflammatory diseases. Resolution is mediated in part by long-chain fatty acid-derived lipid mediators called specialized proresolving mediators (SPMs). However, how SPMs are regulated during the inflammatory response, and how this process goes awry in inflammatory diseases, are poorly understood. We now show that signaling through the Mer proto-oncogene tyrosine kinase (MerTK) receptor in cultured macrophages and in sterile inflammation in vivo promotes SPM biosynthesis by a mechanism involving an increase in the cytoplasmic:nuclear ratio of a key SPM biosynthetic enzyme, 5-lipoxygenase. This action of MerTK is linked to the resolution of sterile peritonitis and, after ischemia–reperfusion (I/R) injury, to increased circulating SPMs and decreased remote organ inflammation. MerTK is susceptible to ADAM metallopeptidase domain 17 (ADAM17)-mediated cell-surface cleavage under inflammatory conditions, but the functional significance is not known. We show here that SPM biosynthesis is increased and inflammation resolution is improved in a new mouse model in which endogenous MerTK was replaced with a genetically engineered variant that is cleavage-resistant (MertkCR
). MertkCR
mice also have increased circulating levels of SPMs and less lung injury after I/R. Thus, MerTK cleavage during inflammation limits SPM biosynthesis and the resolution response. These findings contribute to our understanding of how SPM synthesis is regulated during the inflammatory response and suggest new therapeutic avenues to boost resolution in settings where defective resolution promotes disease progression.
Incomplete understanding of how hepatosteatosis transitions to fibrotic non-alcoholic steatohepatitis (NASH) has limited therapeutic options. Two molecules that are elevated in hepatocytes in human ...NASH liver are cholesterol, whose mechanistic link to NASH remains incompletely understood, and TAZ, a transcriptional regulator that promotes fibrosis but whose mechanism of increase in NASH is unknown. We now show that increased hepatocyte cholesterol upregulates TAZ and promotes fibrotic NASH. ASTER-B/C-mediated internalization of plasma membrane cholesterol activates soluble adenylyl cyclase (sAC; ADCY10), triggering a calcium-RhoA-mediated pathway that suppresses β-TrCP/proteasome-mediated TAZ degradation. In mice fed with a cholesterol-rich NASH-inducing diet, hepatocyte-specific silencing of ASTER-B/C, sAC, or RhoA decreased TAZ and ameliorated fibrotic NASH. The cholesterol-TAZ pathway is present in primary human hepatocytes, and associations among liver cholesterol, TAZ, and RhoA in human NASH liver are consistent with the pathway. Thus, hepatocyte cholesterol contributes to fibrotic NASH by increasing TAZ, suggesting new targets for therapeutic intervention.
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•Liver cholesterol and TAZ are elevated in human and mouse fibrotic NASH•Increased hepatocyte cholesterol upregulates TAZ in human and mouse hepatocytes•Cholesterol blocks TAZ proteasomal degradation via an adenylyl cyclase-RhoA pathway•Silencing the pathway in hepatocytes lowers TAZ and fibrosis in experimental NASH
Cholesterol is consistently elevated in human NASH, but its mechanistic link to NASH progression remains incompletely understood. Wang et al. now define a cholesterol-TAZ connection, in which increased hepatocyte cholesterol upregulates TAZ through an adenylyl cyclase-calcium-RhoA pathway and promotes fibrotic NASH.
Efficient clearance of apoptotic cells, termed efferocytosis, critically regulates normal homeostasis whereas defective uptake of apoptotic cells results in chronic and non-resolving inflammatory ...diseases, such as advanced atherosclerosis. Monocyte-derived macrophages recruited into developing atherosclerotic lesions initially display efficient efferocytosis and temper inflammatory responses, processes that restrict plaque progression. However, during the course of plaque development, macrophages undergo cellular reprogramming that reduces efferocytic capacity, which results in post-apoptotic necrosis of apoptotic cells and inflammation. Furthermore, defective efferocytosis in advanced atherosclerosis is a major driver of necrotic core formation, which can trigger plaque rupture and acute thrombotic cardiovascular events. In this review, we discuss the molecular and cellular mechanisms that regulate efferocytosis, how efferocytosis promotes the resolution of inflammation, and how defective efferocytosis leads to the formation of clinically dangerous atherosclerotic plaques.
Endocytic transport necessitates the generation of membrane tubules and their subsequent fission to transport vesicles for sorting of cargo molecules. The endocytic recycling compartment, an array of ...tubular and vesicular membranes decorated by the Eps15 homology domain protein, EHD1, is responsible for receptor and lipid recycling to the plasma membrane. It has been proposed that EHD dimers bind and bend membranes, thus generating recycling endosome (RE) tubules. However, recent studies show that molecules interacting with CasL-Like1 (MICAL-L1), a second, recently identified RE tubule marker, recruits EHD1 to preexisting tubules. The mechanisms and events supporting the generation of tubular recycling endosomes were unclear. Here, we propose a mechanism for the biogenesis of RE tubules. We demonstrate that MICAL-L1 and the BAR-domain protein syndapin2 bind to phosphatidic acid, which we identify as a novel lipid component of RE. Our studies demonstrate that direct interactions between these two proteins stabilize their association with membranes, allowing for nucleation of tubules by syndapin2. Indeed, the presence of phosphatidic acid in liposomes enhances the ability of syndapin2 to tubulate membranes in vitro. Overall our results highlight a new role for phosphatidic acid in endocytic recycling and provide new insights into the mechanisms by which tubular REs are generated.
Rhythmic intraorgan communication coordinates environmental signals and the cell-intrinsic clock to maintain organ homeostasis. Hepatocyte-specific KO of core components of the molecular clock ...Rev-erbα and -β (Reverb-hDKO) alters cholesterol and lipid metabolism in hepatocytes as well as rhythmic gene expression in nonparenchymal cells (NPCs) of the liver. Here, we report that in fatty liver caused by diet-induced obesity (DIO), hepatocyte SREBP cleavage-activating protein (SCAP) was required for Reverb-hDKO-induced diurnal rhythmic remodeling and epigenomic reprogramming in liver macrophages (LMs). Integrative analyses of isolated hepatocytes and LMs revealed that SCAP-dependent lipidomic changes in REV-ERB-depleted hepatocytes led to the enhancement of LM metabolic rhythms. Hepatocytic loss of REV-ERBα and β (REV-ERBs) also attenuated LM rhythms via SCAP-independent polypeptide secretion. These results shed light on the signaling mechanisms by which hepatocytes regulate diurnal rhythms in NPCs in fatty liver disease caused by DIO.
Tissue-type plasminogen activator (tPA) is a major mediator of fibrinolysis and, thereby, prevents excessive coagulation without compromising hemostasis. Studies on tPA regulation have focused on its ...acute local release by vascular cells in response to injury or other stimuli. However, very little is known about sources, regulation, and fibrinolytic function of noninjury-induced systemic plasma tPA. We explore the role and regulation of hepatocyte-derived tPA as a source of basal plasma tPA activity and as a contributor to fibrinolysis after vascular injury. We show that hepatocyte tPA is downregulated by a pathway in which the corepressor DACH1 represses ATF6, which is an inducer of the tPA gene Plat. Hepatocyte-DACH1–knockout mice show increases in liver Plat, circulating tPA, fibrinolytic activity, bleeding time, and time to thrombosis, which are reversed by silencing hepatocyte Plat. Conversely, hepatocyte-ATF6–knockout mice show decreases in these parameters. The inverse correlation between DACH1 and ATF6/PLAT is conserved in human liver. These findings reveal a regulated pathway in hepatocytes that contributes to basal circulating levels of tPA and to fibrinolysis after vascular injury.
•Hepatocyte-derived tPA contributes to basal circulating tPA activity and affects injury-induced fibrinolysis.•Hepatocyte tPA is induced by ATF6 and subjected to negative regulation by the corepressor DACH1.
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