mTOR trains heightened macrophage responses

STAT3 governs the HIF-1α response in IL-15 primed human NK cells

Natural killer (NK) cells mediate innate host defense against microbial infection and cancer. Hypoxia and low glucose are characteristic for these tissue lesions but do not affect early interferon (IFN) γ and CC chemokine release by interleukin 15 (IL-15) primed human NK cells in vitro. Hypoxia inducible factor 1α (HIF-1α) mediates cellular adaption to hypoxia. Its production is supported by mechanistic target of rapamycin complex 1 (mTORC1) and signal transducer and activator of transcription 3 (STAT3). We used chemical inhibition to probe the importance of mTORC1 and STAT3 for the hypoxia response and of STAT3 for the cytokine response in isolated and IL-15 primed human NK cells. Cellular responses were assayed by magnetic bead array, RT-PCR, western blotting, flow cytometry, and metabolic flux analysis. STAT3 but not mTORC1 activation was essential for HIF-1α accumulation, glycolysis, and oxygen consumption. In both primed normoxic and hypoxic NK cells, STAT3 inhibition reduced the secretion of CCL3, CCL4 and CCL5, and it interfered with IL-12/IL-18 stimulated IFNγ production, but it did not affect cytotoxic granule degranulation up on target cell contact. We conclude that IL-15 priming promotes the HIF-1α dependent hypoxia response and the early cytokine response in NK cells predominantly through STAT3 signaling.

NLRP3 and mTOR Reciprocally Regulate Macrophage Phagolysosome Formation and Acidification Against Vibrio vulnificus Infection

The marine bacterium Vibrio vulnificus causes potentially fatal bloodstream infections, typically in patients with chronic liver diseases. The inflammatory response and anti-bacterial function of phagocytes are crucial for limiting bacterial infection in the human hosts. How V. vulnificus affects macrophages after phagocytosis is unclear. In this report, we found that the bactericidal activity of macrophages to internalize V. vulnificus was dependent on mammalian target of rapamycin (mTOR) and NOD-like receptor (NLR) family pyrin domain containing 3 (NLRP3) interaction. Additionally, the NLRP3 expression was dependent on mTORC1 activation. Inhibited mTORC1 or absence of NLRP3 in macrophages impaired V. vulnificus-induced phagosome acidification and phagolysosome formation, leading to a reduction of intracellular bacterial clearance. mTORC1 signaling overactivation could increase NLRP3 expression and restore insufficient phagosome acidification. Together, these findings indicate that the intracellular bactericidal activity of macrophages responding to V. vulnificus infection is tightly controlled by the crosstalk of NLRP3 and mTOR and provide critical insight into the host bactericidal activity basis of clearance of V. vulnificus through lyso/phagosome.

Rheb (Ras Homolog Enriched in Brain 1) Deficiency in Mature Macrophages Prevents Atherosclerosis by Repressing Macrophage Proliferation, Inflammation, and Lipid Uptake

Objective:

Macrophage foam cell formation is an important process in atherosclerotic plaque development. The small GTPase Rheb (Ras homolog enriched in brain 1) regulates endocytic trafficking that is critical for foam cell formation. However, it is unclear whether and how macrophage Rheb regulates atherogenesis, which are the focuses of the current study.

Approach and Results:

Immunofluorescence study confirmed the colocalization of Rheb in F4/80 and Mac-2 (galectin-3)–labeled lesional macrophages. Western blot and fluorescence-activated cell sorting analysis showed that Rheb expression was significantly increased in atherosclerotic lesions of atherosclerosis-prone (apoE −/− [apolipoprotein E deficient]) mice fed with Western diet. Increased Rheb expression was also observed in oxidized LDL (low-density lipoprotein)–treated macrophages. To investigate the in vivo role of macrophage Rheb, we established mature Rheb mKO (macrophage-specific Rheb knockout) mice by crossing the Rheb floxed mice with F4/80-cre mice. Macrophage-specific knockout of Rheb in mice reduced Western diet–induced atherosclerotic lesion by 32%, accompanied with a decrease in macrophage content in plaque. Mechanistically, loss of Rheb in macrophages repressed oxidized LDL–induced lipid uptake, inflammation, and macrophage proliferation. On the contrary, lentivirus-mediated overexpression of Rheb in macrophages increased oxidized LDL–induced lipid uptake and inflammation, and the stimulatory effect of Rheb was suppressed by the mTOR (mammalian target of rapamycin) inhibitor rapamycin or the PKA (protein kinase A) activator forskolin.

Conclusions:

Macrophage Rheb plays important role in Western diet–induced atherosclerosis by promoting macrophage proliferation, inflammation, and lipid uptake. Inhibition of expression and function of Rheb in macrophages is beneficial to prevent diet-induced atherosclerosis.

Short Term Hypoxia Synergizes with Interleukin 15 Priming in Driving Glycolytic Gene Transcription and Supports Human Natural Killer Cell Activities

Natural killer (NK) cells induce apoptosis in infected and transformed cells and are important producers of immunoregulatory cytokines. Therefore, they operate under low oxygen conditions (hypoxia) in inflammatory and tumor environments. In vitro studies of NK cells are, however, commonly performed in ambient air (normoxia). We used global gene expression profiling to evaluate changes in transcriptional pathways in primary human NK cells following short term culture under hypoxia compared with normoxia and in response to interleukin 15 (IL-15) priming using a 2 × 2 factorial design. The largest contrasts observed were priming dependences for associations between hypoxia and the hypoxia-inducible factor (Hif) 1 signaling and glycolysis pathways. RT-PCR confirmed positive synergistic hypoxia/IL-15 interactions for genes of key regulatory and metabolic enzymes. IL-15 primes NK cells for effector functions, which were recently demonstrated to depend on glycolytic switching. We did not, however, observe important increases in glycolytic flux through hypoxia and priming alone. Chemical Hif-1α inhibition suggested equal importance of this transcription factor for glycolysis and energy production under normoxia and hypoxia. Hypoxia promoted secretion of CC chemokines Ccl3/4/5 and macrophage migration inhibitory factor. Unexpectedly, hypoxia also stimulated migration of NK cells through the extracellular matrix and shifted amounts of susceptible leukemia target cells toward late apoptosis in a cell killing assay. We conclude that short term hypoxia supports these activities by positively interacting with NK cell priming at the level of glycolytic gene transcription. Hypoxic conditioning of NK cells may thus benefit their use in cell-based immunotherapy of cancer.

Control of macrophage metabolism and activation by mTOR and Akt signaling

Macrophages are pleiotropic cells that assume a variety of functions depending on their tissue of residence and tissue state. They maintain homeostasis as well as coordinate responses to stresses such as infection and metabolic challenge. The ability of macrophages to acquire diverse, context-dependent activities requires their activation (or polarization) to distinct functional states. While macrophage activation is well understood at the level of signal transduction and transcriptional regulation, the metabolic underpinnings are poorly understood. Importantly, emerging studies indicate that metabolic shifts play a pivotal role in control of macrophage activation and acquisition of context-dependent effector activities. The signals that drive macrophage activation impinge on metabolic pathways, allowing for coordinate control of macrophage activation and metabolism. Here we discuss how mTOR and Akt, major metabolic regulators and targets of such activation signals, control macrophage metabolism and activation. Dysregulated macrophage activities contribute to many diseases, including infectious, inflammatory, and metabolic diseases and cancer, thus a better understanding of metabolic control of macrophage activation could pave the way to the development of new therapeutic strategies.