AKT mediated glycolytic shift regulates autophagy in classically activated macrophages

Autophagy in glioblastoma: A mechanistic perspective

Glioblastoma (GBM) is one of the most lethal malignancies in humans. Even after surgical resection and aggressive radio- or chemotherapies, patients with GBM can survive for less than 14 months. Extreme inter-tumor and intra-tumor heterogeneity of GBM poses a challenge for resolving recalcitrant GBM pathophysiology. GBM tumor microenvironment (TME) exhibits diverse heterogeneity in cellular composition and processes contributing to tumor progression and therapeutic resistance. Autophagy is such a cellular process; that demonstrates a cell-specific and TME context-dependent role in GBM progression, leading to either the promotion or suppression of GBM progression. Autophagy can regulate GBM cell function directly via regulation of survival, migration, and invasion, or indirectly by affecting GBM TME composition such as immune cell population, tumor metabolism, and glioma stem cells. This review comprehensively investigates the role of autophagy in GBM pathophysiology.

Sonodynamic therapy for the treatment of atherosclerosis

Atherosclerosis (AS) is a chronic inflammatory disease of large and medium-sized arteries that leads to ischemic heart disease, stroke, and peripheral vascular disease. Despite the current treatments, mortality and disability still remain high. Sonodynamic therapy (SDT), a non-invasive and localized methodology, has been developed as a promising new treatment for inhibiting atherosclerotic progression and stabilizing plaques. Promising progress has been made through cell and animal assays, as well as clinical trials. For example, the effect of SDT on apoptosis and autophagy of cells in AS, especially macrophages, and the concept of non-lethal SDT has also been proposed. In this review, we summarize the ultrasonic parameters and known sonosensitizers utilized in SDT for AS; we elaborate on SDT's therapeutic effects and mechanisms in terms of macrophages, T lymphocytes, neovascularization, smooth muscle cells, lipid, extracellular matrix and efferocytosis within plaques; additionally, we discuss the safety of SDT. A comprehensive summary of the confirmed effects of SDT on AS is conducted to establish a framework for future researchers.

Functional polarization of tumor-associated macrophages dictated by metabolic reprogramming

Macrophages are highly plastic in different tissues and can differentiate into functional subpopulations under different stimuli. Tumor-associated macrophages (TAMs) are one of the most important innate immune cells implicated in the establishment of an immunosuppressive tumor microenvironment (TME). Recent evidence pinpoints the critical role of metabolic reprogramming in dictating pro-tumorigenic functions of TAMs. Both tumor cells and macrophages undergo metabolic reprogramming to meet energy demands in the TME. Understanding the metabolic rewiring in TAMs can shed light on immune escape mechanisms and provide insights into repolarizing TAMs towards anti-tumorigenic function. Here, we discuss how metabolism impinges on the functional divergence of macrophages and its relevance to macrophage polarization in the TME.

mTOR signaling inhibition decreases lysosome migration and impairs the success of Trypanosoma cruzi infection and replication in cardiomyocytes

Chagas disease is caused by the parasite Trypanosoma cruzi (T. cruzi) and, among all the chronic manifestations of the disease, Chronic Chagas Cardiomyopathy (CCC) is the most severe outcome. Despite high burden and public health importance in Latin America, there is a gap in understanding the molecular mechanisms that results in CCC development. Previous studies showed that T. cruzi uses the host machinery for infection and replication, including the repurposing of the responses to intracellular infection such as mitochondrial activity, vacuolar membrane, and lysosomal activation in benefit of parasite infection and replication. One common signaling upstream to many responses to parasite infection is mTOR pathway, previous associated to several downstream cellular mechanisms including autophagy, mitophagy and lysosomal activation. Here, using human iPSC derived cardiomyocytes (hiPSCCM), we show the mTOR pathway is activated in hiPSCCM after T. cruzi infection, and the inhibition of mTOR with rapamycin reduced number of T. cruzi 48 h post infection (hpi). Rapamycin treatment also reduced lysosome migration from nuclei region to cell periphery resulting in less T. cruzi inside the parasitophorous vacuole (PV) in the first hour of infection. In addition, the number of parasites leaving the PV to the cytoplasm to replicate in later times of infection was also lower after rapamycin treatment. Altogether, our data suggest that host's mTOR activation concomitant with parasite infection modulates lysosome migration and that T. cruzi uses this mechanism to achieve infection and replication. Modulating this mechanism with rapamycin impaired the success of T. cruzi life cycle independent of mitophagy.

Epigallocatechin-3-gallate inhibits osteoclastic differentiation by modulating mitophagy and mitochondrial functions

A natural plant product, epigallocatechin-3-gallate (EGCG), was evaluated for its effectiveness in the regulation of osteoclastogenesis. We found that EGCG inhibited the osteoclast (OC) differentiation in vitro, and in primary bone marrow cells in a dose-dependent manner. Quantitative RT-PCR studies showed that the EGCG reduced the expression of OC differentiation markers. DCFDA, MitoSOX, and JC-1 staining revealed that the EGCG attenuated the reactive oxygen species (ROS), and mitochondrial membrane potential; and flux analysis corroborated the effect of EGCG. We further found that the EGCG inhibited mRNA and protein expressions of mitophagy-related molecules. We confirmed that the OC differentiation was inhibited by EGCG by modulating mitophagy through AKT and p38MAPK pathways. Furthermore, in silico analysis revealed that the binding of RANK and RANKL was blocked by EGCG. Overall, we defined the mechanisms of osteoclastogenesis during arthritis for developing a new therapy using a natural compound besides the existing therapeutics.

miRNA-29a inhibits atherosclerotic plaque formation by mediating macrophage autophagy via PI3K/AKT/mTOR pathway

Background: miR-29a plays a vital role in AS, but the relationship between the miR-29a-targeted PI3K signaling pathway and AS remains unclear. Therefore, this study was carried out.

Methods: Gene expression profiles from the GEO database containing AS samples were analyzed. ApoE^−/− mice and RAW264.7 cells were treated with miR-29a negative control (NC), miR-29a mimic and miR-29a inhibitor to establish the AS model. Then MOVAT staining, TEM, Western blotting, and immunofluorescence staining were adopted for testing target proteins.

Results: DEGs were identified from GSE137578, GSE132651, GSE113969, GSE43292, and GSE97210 datasets. It was found that there were targeted binding sites between miR-29a and PIK3CA. Besides, GO and KEGG analysis demonstrated that autophagy was an enriched pathway in AS. Later, PPI network was depicted, and hub genes were then determined. The results revealed that miR-29a suppressed the areas of plaques and lesional macrophages, but had no impact on VSMCs. TEM results showed the organelles pyknosis of lesional macrophages damaged morphological changes. Furthermore, miR-29a amplified the M2-like macrophages but suppressed the polarization of M1-like macrophages in atherosclerotic plaques. According to mouse and RAW 264.7 cell experiments, miR-29a significantly inhibited the protein expressions of PI3K, p-PI3K, p-AKT, and p-mTOR, which were consistent with the increased expressions of autophagy-related proteins, Beclin 1 and LC3II. However, the miR-29a suppression exhibited the contrary results.

Conclusion: MiR-29a elevation induces the increase of autophagy by down-regulating the PI3K/AKT/mTOR pathway in the progression of AS, indicating that miR-29a is a novel therapeutic strategy for AS.

Autophagy in Tumor Immunity and Viral-Based Immunotherapeutic Approaches in Cancer

Autophagy is a fundamental catabolic process essential for the maintenance of cellular and tissue homeostasis, as well as directly contributing to the control of invading pathogens. Unsurprisingly, this process becomes critical in supporting cellular dysregulation that occurs in cancer, particularly the tumor microenvironments and their immune cell infiltration, ultimately playing a role in responses to cancer therapies. Therefore, understanding "cancer autophagy" could help turn this cellular waste-management service into a powerful ally for specific therapeutics. For instance, numerous regulatory mechanisms of the autophagic machinery can contribute to the anti-tumor properties of oncolytic viruses (OVs), which comprise a diverse class of replication-competent viruses with potential as cancer immunotherapeutics. In that context, autophagy can either: promote OV anti-tumor effects by enhancing infectivity and replication, mediating oncolysis, and inducing autophagic and immunogenic cell death; or reduce OV cytotoxicity by providing survival cues to tumor cells. These properties make the catabolic process of autophagy an attractive target for therapeutic combinations looking to enhance the efficacy of OVs. In this article, we review the complicated role of autophagy in cancer initiation and development, its effect on modulating OVs and immunity, and we discuss recent progress and opportunities/challenges in targeting autophagy to enhance oncolytic viral immunotherapy.

Mesenchymal Stem Cells and Tuberculosis: Clinical Challenges and Opportunities

Tuberculosis (TB) is one of the communicable diseases caused by Mycobacterium tuberculosis (Mtb) infection, affecting nearly one-third of the world's population. However, because the pathogenesis of TB is still not fully understood and the development of anti-TB drug is slow, TB remains a global public health problem. In recent years, with the gradual discovery and confirmation of the immunomodulatory properties of mesenchymal stem cells (MSCs), more and more studies, including our team's research, have shown that MSCs seem to be closely related to the growth status of Mtb and the occurrence and development of TB, which is expected to bring new hope for the clinical treatment of TB. This article reviews the relationship between MSCs and the occurrence and development of TB and the potential application of MSCs in the treatment of TB.

Trehalose limits opportunistic mycobacterial survival during HIV co-infection by reversing HIV-mediated autophagy block

Opportunistic bacterial infections amongst HIV-infected individuals contribute significantly to HIV-associated mortality. The role of HIV-mediated modulation of innate mechanisms like autophagy in promoting opportunistic infections, however, remains obscure. Here we show, HIV reactivation in or infection of macrophages inhibits autophagy and helps the survival of pathogenic Mycobacterium tuberculosis (Mtb) and nonpathogenic non-tuberculous mycobacterial strains (NTMs). The HIV-mediated impairment of xenophagy flux facilitated bacterial survival. Activation of autophagy by trehalose could induce xenophagy flux and kill intracellular Mtb or NTMs either during single or co-infections. Trehalose, we delineate, activates PIKFYVE leading to TFEB nuclear translocation in MCOLN1-dependent manner to induce autophagy. Remarkably, trehalose significantly reduced HIV-p24 levels in ex-vivo-infected PBMCs or PBMCs from treatment-naive HIV patients and also controlled mycobacterial survival within Mtb-infected animals. To conclude, we report leveraging of HIV-mediated perturbed host innate-immunity by opportunistic bacterial pathogens and show an attractive therapeutic strategy for HIV and associated co-morbidities.Abbreviations: AIDS: acquired immune deficiency syndrome; AMPK: AMP-activated protein kinase; ATG5: autophagy related 5; BafA1: bafilomycin A1; CFU: colony forming unit; CTSD: cathepsin D; CD63: CD63 molecule; EGFP: enhanced green fluorescent protein; FRET: Förster resonance energy transfer; GABARAP: gamma-aminobutyric acid receptor-associated protein; GAPDH: glyceraldehyde 3-phosphate dehydrogenase; GLUT: glucose transporter; HIV: human immunodeficiency virus; hMDMs: human monocyte derived macrophages; IL2: interleukin 2; LAMP1: lysosomal-associated membrane protein 1; LC3B-II: lipidated microtubule-associated proteins 1A/1B light chain 3B; Mtb: Mycobacterium tuberculosis; MTOR: mechanistic target of rapamycin; mRFP: monomeric red fluorescent protein; M6PR: mannose-6-phosphate receptor; NAC: N- acetyl- L -cysteine; NTM's: non-tuberculous mycobacteria; PBMC: Peripheral Blood Mononuclear cells; PIKFYVE: phosphoinositide kinase; FYVE-Type Zinc Finger; PHA: phytohemagglutinin; PMA: phorbol 12-myristate 13-acetate; PtdIns(3,5)P2: Phosphatidylinositol 3,5-bisphosphate; ptfLC3: pEGFP-mRFP-LC3; ROS: reactive oxygen species; SQSTM1: sequestosome1; TFEB: transcription factor EB; MCOLN1/TRPML1: mucolipin 1; PIP4P1/TMEM55B: Human trans-membrane Protein 55B; UVRAG: UV Radiation Resistance Associate; VPS35: vacuolar protein sorting associated protein 35; WDR45: WD repeat domain 45; YCAM: Yellow Chameleon.

Autophagy in the Regulation of Tissue Differentiation and Homeostasis

Autophagy is a constitutive pathway that allows the lysosomal degradation of damaged components. This conserved process is essential for metabolic plasticity and tissue homeostasis and is crucial for mammalian post-mitotic cells. Autophagy also controls stem cell fate and defective autophagy is involved in many pathophysiological processes. In this review, we focus on established and recent breakthroughs aimed at elucidating the impact of autophagy in differentiation and homeostasis maintenance of endothelium, muscle, immune system, and brain providing a suitable framework of the emerging results and highlighting the pivotal role of autophagic response in tissue functions, stem cell dynamics and differentiation rates.

Autophagy-dependent mitochondrial function regulates osteoclast differentiation and maturation

Bone homeostasis is maintained by bone remodeling, which involves continuous bone resorption by osteoclasts and bone formation by osteoblasts. Dysregulation of bone turnover, caused by osteoclast overactivation, causes destructive bone diseases. However, the mechanisms underlying the maintenance of osteoclast differentiation and activation are unclear. Herein, we examined the role of autophagy in the maintenance of osteoclast differentiation and maturation. We used in vitro and in vivo assays to evaluate relationships between mitochondrial activity and autophagy during osteoclast differentiation and maturation. Our results indicate that autophagy was enhanced during osteoclast differentiation and maturation, and autophagic activity was positively correlated with osteoclast activity and survival. Maintenance of mitochondrial function, which is critical during osteoclast differentiation and maturation, was controlled by autophagy. Continuous exposure of osteoclasts to glucocorticoids upregulated autophagic processes. Treatment with the autophagic inhibitor chloroquine suppressed prolonged survival of activated osteoclasts and attenuated excessive osteoclast activity. Our study shows that autophagy-dependent mitochondrial function plays an important role in osteoclast differentiation and maturation. Elucidating the mechanisms regulating autophagic activity in osteoclasts, and developing bone-tissue-specific inhibitors of autophagy, will lead to improved understanding of the pathologies involved in destructive bone diseases.

Mesenchymal stem cells offer a drug-tolerant and immune-privileged niche to Mycobacterium tuberculosis

Anti-tuberculosis (TB) drugs, while being highly potent in vitro, require prolonged treatment to control Mycobacterium tuberculosis (Mtb) infections in vivo. We report here that mesenchymal stem cells (MSCs) shelter Mtb to help tolerate anti-TB drugs. MSCs readily take up Mtb and allow unabated mycobacterial growth despite having a functional innate pathway of phagosome maturation. Unlike macrophage-resident ones, MSC-resident Mtb tolerates anti-TB drugs remarkably well, a phenomenon requiring proteins ABCC1, ABCG2 and vacuolar-type H⁺ATPases. Additionally, the classic pro-inflammatory cytokines IFNγ and TNFα aid mycobacterial growth within MSCs. Mechanistically, evading drugs and inflammatory cytokines by MSC-resident Mtb is dependent on elevated PGE2 signaling, which we verify in vivo analyzing sorted CD45^-Sca1⁺CD73⁺-MSCs from lungs of infected mice. Moreover, MSCs are observed in and around human tuberculosis granulomas, harboring Mtb bacilli. We therefore propose, targeting the unique immune-privileged niche, provided by MSCs to Mtb, can have a major impact on tuberculosis prevention and cure.

Non-lethal sonodynamic therapy facilitates the M1-to-M2 transition in advanced atherosclerotic plaques via activating the ROS-AMPK-mTORC1-autophagy pathway

Emerging evidence indicates that macrophage functional polarization is critically involved in the development of atherosclerosis (AS). Here, we examined the role of 5-aminolaevulinic acid (ALA)-mediated non-lethal sonodynamic therapy (NL-SDT) in macrophage-subset polarization and atherosclerotic lesion stability and explored the potential underlying mechanisms. Using Western diet-fed apolipoprotein E (apoE)^−/− and green fluorescent protein (GFP)-positive bone marrow (BM) chimeric mouse models, we demonstrated that NL-SDT promoted phenotypic switching of both BM-derived and resident macrophages from M1 to M2 and significantly inhibited AS progression. Further mechanistic studies indicated that NL-SDT enhanced macrophage differentiation toward the M2 phenotype by activating the reactive oxygen species (ROS)–5′ AMP-activated protein kinase (AMPK)–mammalian target of rapamycin complex 1 (mTORC1)–autophagy signaling pathway in murine BM-derived M1 macrophages (BMDM1s). Moreover, NL-SDT drastically reduced lipid droplets, mainly by promoting apoAI-mediated cholesterol efflux in vitro. Specifically, administration of pharmacological inhibitors to the animal model showed a reciprocal effect on NL-SDT-induced macrophage polarization. These findings indicate that NL-SDT engages a virtuous cycle that enhances M1-to-M2 polarization, cholesterol efflux, and anti-inflammatory reactions in advanced plaque in vivo and in BMDM1s in vitro by activating the ROS–AMPK–mTORC1–autophagy pathway. This discovery might help elucidate the mechanism underlying NL-SDT as a potential treatment to prevent atherothrombotic events.

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NL-SDT enhances M1-to-M2 shift and significantly inhibits atherosclerosis progression in a mouse model.

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NL-SDT induces autophagy by activating the AMPK signaling pathway.

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M2-like macrophages promoted by NL-SDT facilitate cholesterol efflux and attenuate intracellular cholesterol deposition.

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The ROS–AMPK–mTORC1–autophagy pathway is critical for NL-SDT-mediated effects on M2 macrophage polarization.

Comparative proteomic study reveals the enhanced immune response with the blockade of interleukin 10 with anti-IL-10 and anti-IL-10 receptor antibodies in human U937 cells

Blocking cytokine interleukin 10 (IL-10) at the time of immunisation enhances vaccine induced T cell responses and improves control of tumour cell growth in vivo. However, the effect of an IL-10 blockade on the biological function of macrophages has not been explored. In the current paper, a macrophage precursor cell line, U937 cells, was selected to investigate the differential expression of proteins and relevant cell signalling pathway changes, when stimulated with lipopolysaccharide (LPS) in the presence of antibodies to IL-10 or IL-10 receptor. We used a quantitative proteomic strategy to investigate variations in protein profiles of U937 cells following the treatments with LPS, LPS plus human anti-IL10 antibody and anti-IL10R antibody in 24hrs, respectively. The LPS treatment significantly activated actin-related cell matrix formation and immune response pathways. The addition of anti-IL10 and anti-IL10R antibody further promoted the immune response and potentially effect macrophage survival through PI3K/AKT signalling; however, the latter appeared to also upregulated oncogene XRCC5 and Cajal body associated processes.

Glycolysis Is an Intrinsic Factor for Optimal Replication of a Norovirus

Viruses depend on the host cells they infect to provide the machinery and substrates for replication. Host cells are highly dynamic systems that can alter their intracellular environment and metabolic behavior, which may be helpful or inhibitory for an infecting virus. In this study, we show that macrophages, a target cell of murine norovirus (MNV), increase glycolysis upon viral infection, which is important for early steps in MNV infection. Human noroviruses (hNoV) are a major cause of gastroenteritis globally, causing enormous morbidity and economic burden. Currently, no effective antivirals or vaccines exist for hNoV, mainly due to the lack of high-efficiency in vitro culture models for their study. Thus, insights gained from the MNV model may reveal aspects of host cell metabolism that can be targeted for improving hNoV cell culture systems and for developing effective antiviral therapies.

The metabolic pathways of central carbon metabolism, glycolysis and oxidative phosphorylation (OXPHOS), are important host factors that determine the outcome of viral infections and can be manipulated by some viruses to favor infection. However, mechanisms of metabolic modulation and their effects on viral replication vary widely. Herein, we present the first metabolomics and energetic profiling of norovirus-infected cells, which revealed increases in glycolysis, OXPHOS, and the pentose phosphate pathway (PPP) during murine norovirus (MNV) infection. Inhibiting glycolysis with 2-deoxyglucose (2DG) in macrophages revealed that glycolysis is an important factor for optimal MNV infection, while inhibiting the PPP and OXPHOS showed a relatively minor impact of these pathways on MNV infection. 2DG affected an early stage in the viral life cycle after viral uptake and capsid uncoating, leading to decreased viral protein production and viral RNA. The requirement of glycolysis was specific for MNV (but not astrovirus) infection, independent of the type I interferon antiviral response, and unlikely to be due to a lack of host cell nucleotide synthesis. MNV infection increased activation of the protein kinase Akt, but not AMP-activated protein kinase (AMPK), two master regulators of cellular metabolism, implicating Akt signaling in upregulating host metabolism during norovirus infection. In conclusion, our findings suggest that the metabolic state of target cells is an intrinsic host factor that determines the extent of norovirus replication and implicates glycolysis as a virulence determinant. They further point to cellular metabolism as a novel therapeutic target for norovirus infections and improvements in current human norovirus culture systems.

Regulation of the innate immune system by autophagy: monocytes, macrophages, dendritic cells and antigen presentation

Autophagy is well equipped functionally to isolate microbial pathogens in autophagosomes and to carry out their clearance by dismemberment in the course of catabolic processes in the lysosome. Clearly, this is a non-metabolic function of autophagy that impacts strongly on the immune system. While in a preceding article on neutrophils, eosinophils, mast cells, and natural killer cells our focus was on the role of autophagy in regulating innate immune cell differentiation, degranulation, phagocytosis and extracellular trap formation, here we discuss monocytes/macrophages and dendritic cells, specifically, the influence of autophagy on functional cellular responses, such as phagocytosis, antigen presentation, cytokine production, control of inflammasome activation, tolerance and the consequences for overall host defense.

Selective Autophagy and Xenophagy in Infection and Disease

Autophagy, a cellular homeostatic process, which ensures cellular survival under various stress conditions, has catapulted to the forefront of innate defense mechanisms during intracellular infections. The ability of autophagy to tag and target intracellular pathogens toward lysosomal degradation is central to this key defense function. However, studies involving the role and regulation of autophagy during intracellular infections largely tend to ignore the housekeeping function of autophagy. A growing number of evidences now suggest that the housekeeping function of autophagy, rather than the direct pathogen degradation function, may play a decisive role to determine the outcome of infection and immunological balance. We discuss herein the studies that establish the homeostatic and anti-inflammatory function of autophagy, as well as role of bacterial effectors in modulating and coopting these functions. Given that the core autophagy machinery remains largely the same across diverse cargos, how selectivity plays out during intracellular infection remains intriguing. We explore here, the contrasting role of autophagy adaptors being both selective as well as pleotropic in functions and discuss whether E3 ligases could bring in the specificity to cargo selectivity.

Urolithin A attenuates pro-inflammatory mediator production by suppressing PI3-K/Akt/NF-κB and JNK/AP-1 signaling pathways in lipopolysaccharide-stimulated RAW264 macrophages: Possible involvement of NADPH oxidase-derived reactive oxygen species

Urolithin A, a gut microbial metabolite of ellagic acid, is reported to exert anti-inflammatory effects in vitro and in vivo. However, complete mechanisms underlying the regulation of inflammatory responses by urolithin A remain unclear. This study aimed to evaluate the anti-inflammatory potential of urolithin A and its underlying mechanisms in lipopolysaccharide (LPS)-stimulated RAW264 macrophages. Urolithin A significantly attenuated the pro-inflammatory mediator production in LPS-stimulated RAW264 and mouse peritoneal macrophages. This compound significantly suppressed the LPS-elicited nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) activation. The phosphorylation of Akt and c-Jun N-terminal kinase (JNK) was also inhibited by the treatment with urolithin A. Through experiments using kinase inhibitors, urolithin A abolished the LPS-induced phosphatidylinositol 3-kinase (PI3-K)/Akt/NF-κB and JNK/AP-1 signaling pathways, resulting in suppression of pro-inflammatory mediator production. Furthermore, treatment with this compound significantly reduced the intracellular accumulation of reactive oxygen species, which are known to act as secondary messengers in the activation of redox-sensitive transcription factors NF-κB and AP-1. Urolithin A treatment also diminished the LPS-evoked activation of NADPH oxidase (NOX), which is the main source of reactive oxygen species in activated macrophages. The inhibition of this activity by urolithin A led to the prevention of LPS-elicited NF-κB and AP-1 activation as well as Akt and JNK phosphorylation, resulting in the reduction of pro-inflammatory mediator production. Collectively, these results indicate that urolithin A treatment attenuates pro-inflammatory mediator production by suppressing NOX-derived reactive oxygen species-mediated PI3-K/Akt/NF-κB and JNK/AP-1 signaling pathways in LPS-stimulated macrophages.

Akt Signaling Pathway in Macrophage Activation and M1/M2 Polarization

Macrophages become activated initiating innate immune responses. Depending on the signals, macrophages obtain an array of activation phenotypes, described by the broad terms of M1 or M2 phenotype. The PI3K/Akt/mTOR pathway mediates signals from multiple receptors including insulin receptors, pathogen-associated molecular pattern receptors, cytokine receptors, adipokine receptors, and hormones. As a result, the Akt pathway converges inflammatory and metabolic signals to regulate macrophage responses modulating their activation phenotype. Akt is a family of three serine-threonine kinases, Akt1, Akt2, and Akt3. Generation of mice lacking individual Akt, PI3K, or mTOR isoforms and utilization of RNA interference technology have revealed that Akt signaling pathway components have distinct and isoform-specific roles in macrophage biology and inflammatory disease regulation, by controlling inflammatory cytokines, miRNAs, and functions including phagocytosis, autophagy, and cell metabolism. Herein, we review the current knowledge on the role of the Akt signaling pathway in macrophages, focusing on M1/M2 polarization and highlighting Akt isoform-specific functions.

Hypoxia and classical activation limits Mycobacterium tuberculosis survival by Akt-dependent glycolytic shift in macrophages

Cellular reactive oxygen species (ROS) is a major antibacterial defense mechanism used by macrophages upon activation. Exposure of Mycobacterium tuberculosis (Mtb)-infected macrophages to hypoxia is known to compromise the survival of the pathogen. Here we report that the hypoxia-induced control of intracellular Mtb load in RAW 264.7 macrophages was mediated by regulating the cellular ROS levels. We show that similar to classical activation, hypoxia incubation of macrophages resulted in decreased mitochondrial outer membrane potential (MOMP) and a concomitant increase in the cellular ROS levels. Mitochondrial depolarization and consequently higher ROS could be blocked by knocking down Akt using siRNAs, which acted by inhibiting the switch to glycolytic mode of metabolism, an essential adaptive response upon classical activation or hypoxic incubation of macrophages. Moreover, in the classically activated macrophages or in the macrophages under hypoxia incubation, supplementation with additional glucose had similar effects as Akt knockdown. Interestingly, in both the cases, the reversal of phenotype was linked with the ability of the mitochondrial F₀–F₁ ATP synthase activity to maintain the MOMP in the absence of oxidative phosphorylation. Both Akt knockdown and glucose supplementation were also able to rescue Mtb survival in these macrophages upon classical activation or hypoxia incubation. These results provide a framework for better understanding of how the interplay between oxygen supply, which is limiting in the human tubercular granulomas, and nutrient availability could together direct the outcome of infections in vivo.

Targeting Drug-Sensitive and -Resistant Strains of Mycobacterium tuberculosis by Inhibition of Src Family Kinases Lowers Disease Burden and Pathology

The existing treatment regimen for tuberculosis (TB) suffers from deficiencies like high doses of antibiotics, long treatment duration, and inability to kill persistent populations in an efficient manner. Together, these contribute to the emergence of drug-resistant tuberculosis. Recently, several host factors were identified which help intracellular survival of Mycobacterium tuberculosis within the macrophage. These factors serve as attractive targets for developing alternate therapeutic strategies against M. tuberculosis. This strategy promises to be effective against drug-resistant strains. The approach also has potential to considerably lower the risk of emergence of new drug-resistant strains. We explored tyrosine kinase Src as a host factor exploited by virulent M. tuberculosis for intracellular survival. We show that Src inhibition can effectively control tuberculosis in infected guinea pigs. Moreover, Src inhibition ameliorated TB-associated pathology in guinea pigs. Thus, Src inhibitors have strong potential to be developed as possible anti-TB drugs.

In view of emerging drug resistance among bacterial pathogens, including Mycobacterium tuberculosis, the development of novel therapeutic strategies is increasingly being sought. A recent paradigm in antituberculosis (anti-TB) drug development is to target the host molecules that are crucial for intracellular survival of the pathogen. We previously showed the importance of Src tyrosine kinases in mycobacterial pathogenesis. Here, we report that inhibition of Src significantly reduced survival of H37Rv as well as multidrug-resistant (MDR) and extremely drug-resistant (XDR) strains of M. tuberculosis in THP-1 macrophages. Src inhibition was also effective in controlling M. tuberculosis infection in guinea pigs. In guinea pigs, reduced M. tuberculosis burden due to Src inhibition also led to a marked decline in the disease pathology. In agreement with the theoretical framework of host-directed approaches against the pathogen, Src inhibition was equally effective against an XDR strain in controlling infection in guinea pigs. We propose that Src inhibitors could be developed into effective host-directed anti-TB drugs, which could be indiscriminately used against both drug-sensitive and drug-resistant strains of M. tuberculosis.

IMPORTANCE The existing treatment regimen for tuberculosis (TB) suffers from deficiencies like high doses of antibiotics, long treatment duration, and inability to kill persistent populations in an efficient manner. Together, these contribute to the emergence of drug-resistant tuberculosis. Recently, several host factors were identified which help intracellular survival of Mycobacterium tuberculosis within the macrophage. These factors serve as attractive targets for developing alternate therapeutic strategies against M. tuberculosis. This strategy promises to be effective against drug-resistant strains. The approach also has potential to considerably lower the risk of emergence of new drug-resistant strains. We explored tyrosine kinase Src as a host factor exploited by virulent M. tuberculosis for intracellular survival. We show that Src inhibition can effectively control tuberculosis in infected guinea pigs. Moreover, Src inhibition ameliorated TB-associated pathology in guinea pigs. Thus, Src inhibitors have strong potential to be developed as possible anti-TB drugs.