The scaffold protein prohibitin is required for antigen-stimulated signaling in mast cells

PHB2 inhibits WSSV replication by promoting the nuclear translocation of STAT

Prohibitins (PHBs) are ubiquitously expressed conserved proteins in eukaryotes that are associated with apoptosis, cancer formation, aging, stress responses and cell proliferation. However, the function of the PHBs in immune regulation has largely not been determined. In the present study, we identified PHB2 in the red swamp crayfish Procambarus clarkii. PHB2 was found to be widely distributed in several tissues, and its expression was significantly upregulated by white spot syndrome virus (WSSV) challenge. PHB2 significantly reduced the amount of WSSV in crayfish and the mortality of WSSV-infected crayfish. Here, we observed that PHB2 promotes the nuclear translocation of STAT by binding to STAT. After blocking PHB2 or STAT with antibodies or interfering with PHB2 or STAT, the expression levels of the antiviral genes β-thymosin (PcThy-4) and crustin2 (Cru2) decreased. The gene sequence of PHB2 was analyzed and found to contain a nuclear introgression sequence (NIS). After in vivo injection of PHB2 with deletion of NIS (rΔNIS-PHB2), the nuclear translocation of STAT did not change significantly compared to that in the control group. These results suggest that PHB2 promoted the nuclear translocation of STAT through NIS and mediated the expression of antiviral proteins to inhibit WSSV infection.

Mitochondria in biology and medicine - 2023

Mitochondria are an indispensable part of the cell that plays a crucial role in regulating various signaling pathways, energy metabolism, cell differentiation, proliferation, and cell death. Since mitochondria have their own genetic material, they differ from their nuclear counterparts, and dysregulation is responsible for a broad spectrum of diseases. Mitochondrial dysfunction is associated with several disorders, including neuro-muscular disorders, cancer, and premature aging, among others. The intricacy of the field is due to the cross-talk between nuclear and mitochondrial genes, which has also improved our knowledge of mitochondrial functions and their pathogenesis. Therefore, interdisciplinary research and communication are crucial for mitochondrial biology and medicine due to the challenges they pose for diagnosis and treatment. The ninth annual conference of the Society for Mitochondria Research and Medicine (SMRM)- India, titled "Mitochondria in Biology and Medicine" was organized at the Centre for DNA Fingerprinting and Diagnostics (CDFD), Hyderabad, India, on June 21-23, 2023. The latest advancements in the field of mitochondrial biology and medicine were discussed at the conference. In this article, we summarize the entire event for the benefit of researchers working in the field of mitochondrial biology and medicine.

The prohibitin complex regulates macrophage fatty acid composition, plasma membrane packing, and lipid raft-mediated inflammatory signaling

Prohibitins (PHB1 and PHB2) are ubiquitously expressed proteins which play critical roles in multiple biological processes, and together form the ring-like PHB complex found in phospholipid-rich cellular compartments including lipid rafts. Recent studies have implicated PHB1 as a mediator of fatty acid transport as well as a membrane scaffold mediating B lymphocyte and mast cell signal transduction. However, the specific role of PHBs in the macrophage have not been characterized, including their role in fatty acid uptake and lipid raft-mediated inflammatory signaling. We hypothesized that the PHB complex regulates macrophage inflammatory signaling through the formation of lipid rafts. To evaluate our hypothesis, RAW 264.7 macrophages were transduced with shRNA against PHB1, PHB2, or scrambled control (Scr), and then stimulated with lipopolysaccharide (LPS) or tumor necrosis factor-alpha (TNF-α), which activate lipid raft-dependent receptor signaling (CD14/TLR4 and TNFR1, respectively). PHB1 knockdown was lethal, whereas PHB2 knockdown (PHB2kd), which also resulted in decreased PHB1 expression, led to attenuated nuclear factor-kappa-B (NF-κB) activation and subsequent cytokine and chemokine production. PHB2kd macrophages also had decreased cell surface TNFR1, CD14, TLR4, and lipid raft marker ganglioside GM1 at baseline and post-stimuli. Post-LPS, PHB2kd macrophages did not increase the concentration of cellular saturated, monounsaturated, and polyunsaturated fatty acids. This was accompanied by decreased lipid raft formation and modified plasma membrane molecular packing, further supporting the PHB complex's importance in lipid raft formation. Taken together, these data suggest a critical role for PHBs in regulating macrophage inflammatory signaling via maintenance of fatty acid composition and lipid raft structure. SUMMARY: Prohibitins are proteins found in phospholipid-rich cellular compartments, including lipid rafts, that play important roles in signaling, transcription, and multiple other cell functions. Macrophages are key cells in the innate immune response and the presence of membrane lipid rafts is integral to signal transduction, but the role of prohibitins in macrophage lipid rafts and associated signaling is unknown. To address this question, prohibitin knockdown macrophages were generated and responses to lipopolysaccharide and tumor necrosis factor-alpha, which act through lipid raft-dependent receptors, were analyzed. Prohibitin knockdown macrophages had significantly decreased cytokine and chemokine production, transcription factor activation, receptor expression, lipid raft assembly and membrane packing, and altered fatty acid remodeling. These data indicate a novel role for prohibitins in macrophage inflammatory signaling through regulation of fatty acid composition and lipid raft formation.

Prohibitin-1 plays a regulatory role in Leydig cell steroidogenesis

Mitochondria are essential for steroidogenesis. In steroidogenic cells, the initiation of steroidogenesis from cholesterol occurs on the matrix side of the inner mitochondrial membrane by the enzyme P450scc. This requires cholesterol import from the cytoplasm through the outer mitochondrial membrane, facilitated by the StAR protein. The subsequent steps leading to P450scc remain elusive. Here we report that the male transgenic mice that expressed a mutant form of a mitochondrial protein prohibitin-1 (PHB1^Tyr114Phe) from the Fabp-4 gene promoter displayed smaller testes, higher testosterone, and lower gonadotropin levels compared with PHB1-expressing and wild-type mice. A subsequent analysis of the testis and Leydig cells from the mice revealed that PHB1 played a previously unknown regulatory role in Leydig cell steroidogenesis. This includes a role in coordinating cell signaling, cholesterol homeostasis, and mitochondrial biology pertaining to steroidogenesis. The implications of our finding are broad as the initial stages of steroidogenesis are indistinguishable across steroidogenic cells.

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Tyr114Phe-PHB-1 transgenic male mice reveal PHB-1’s role in testosterone production

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PHB-1 coordinates steroidogenic signaling and events in testosterone biosynthesis

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Tyr114 residue in PHB-1 plays a regulatory role in testosterone production

Prohibitin-1 Is a Dynamically Regulated Blood Protein With Cardioprotective Effects in Sepsis

Background In sepsis, circulating cytokines and lipopolysaccharide elicit mitochondrial dysfunction and cardiomyopathy, a major cause of morbidity and mortality with this condition. Emerging research places the PHB1 (lipid raft protein prohibitin-1) at the nexus of inflammation, metabolism, and oxidative stress. PHB1 has also been reported in circulation, though its function in this compartment is completely unknown. Methods and Results Using a wide-ranging approach across multiple in vitro and in vivo models, we interrogated the functional role of intracellular and circulating PHB1 in the heart during sepsis, and elucidated some of the mechanisms involved. Upon endotoxin challenge or sepsis induction in rodent models, PHB1 translocates from mitochondria to nucleus in cardiomyocytes and is secreted into the circulation from the liver in a manner dependent on nuclear factor (erythroid-derived 2)-like 2, a key transcriptional regulator of the antioxidant response. Overexpression or treatment with recombinant human PHB1 enhances the antioxidant/anti-inflammatory response and protects HL-1 cardiomyocytes from mitochondrial dysfunction and toxicity from cytokine stress. Importantly, administration of recombinant human PHB1 blunted inflammation and restored cardiac contractility and ATP production in mice following lipopolysaccharide challenge. This cardioprotective, anti-inflammatory effect of recombinant human PHB1 was determined to be independent of nuclear factor (erythroid-derived 2)-like 2, but partially dependent on PI3K/AKT  signaling in the heart. Conclusions These findings reveal a previously unknown cardioprotective effect of PHB1 during sepsis, and illustrate a pro-survival, protective role for PHB1 in the circulation. Exploitation of circulating PHB1 as a biomarker and/or therapeutic could have widespread benefit in the clinical management of sepsis and other severe inflammatory disorders.

Lipopolysaccharide-induced innate immune responses are exacerbated by Prohibitin 1 deficiency and mitigated by S-adenosylmethionine in murine macrophages

Prohibitin 1 (Phb1) is a pleiotropic protein with multiple functions in mammalian cells including cell cycle regulation and mitochondrial protein stabilization. It has been proposed as a potential therapeutic target for a variety of diseases including inflammatory diseases. In this study, we investigated the potential immune-modulatory functions of Phb1 and anti-inflammatory properties of S-adenosylmethionine (SAMe) using macrophages, which play a major role in the innate immune system. The results showed that expressions of Phb1 mRNA and protein were reduced in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells (p<0.05). Phb1 knockdown further ameliorated the mRNA expression of pro- and anti-inflammatory cytokines such as TNF-α, IL-1α, IL-1β, IL-6, and IL10 in LPS-stimulated RAW 264.7 cells. SAMe significantly attenuated LPS-induced inflammatory responses such as IL-1β, IL-10, Nos2, and NO production in the presence of siPhb1. Luciferase reporter assay was conducted to determine the mechanisms underlying the effects of Phb1 and SAMe on the immune system. The luciferase activity of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) was significantly increased in LPS-treated RAW 264.7 cells. In addition, the luciferase reporter assay showed increased NF-κB activation in Phb1 knockdown RAW 264.7 cells (p<0.1) and SAMe treatment attenuated the NF-κB luciferase activity in Phb1 knockdown RAW 264.7 cells. Based on the results, we concluded that Phb1 possibly modulates the inflammatory response whereas SAMe has an anti-inflammatory effect on Phb1 knockdown macrophage cells. Furthermore, Phb1 expression level has potential properties of affecting on innate immune system by modulating the NF-κB signaling pathway.

Prohibitin ligands: a growing armamentarium to tackle cancers, osteoporosis, inflammatory, cardiac and neurological diseases

Over the last three decades, the scaffold proteins prohibitins-1 and -2 (PHB1/2) have emerged as key signaling proteins regulating a myriad of signaling pathways in health and diseases. Small molecules targeting PHBs display promising effects against cancers, osteoporosis, inflammatory, cardiac and neurodegenerative diseases. This review provides an updated overview of the various classes of PHB ligands, with an emphasis on their mechanism of action and therapeutic potential. We also describe how these ligands have been used to explore PHB signaling in different physiological and pathological settings.

A subset of flavaglines inhibits KRAS nanoclustering and activation

The RAS oncogenes are frequently mutated in human cancers and among the three isoforms (KRAS, HRAS and NRAS), KRAS is the most frequently mutated oncogene. Here, we demonstrate that a subset of flavaglines, a class of natural anti-tumour drugs and chemical ligands of prohibitins, inhibit RAS GTP loading and oncogene activation in cells at nanomolar concentrations. Treatment with rocaglamide, the first discovered flavagline, inhibited the nanoclustering of KRAS, but not HRAS and NRAS, at specific phospholipid-enriched plasma membrane domains. We further demonstrate that plasma membrane-associated prohibitins directly interact with KRAS, phosphatidylserine and phosphatidic acid, and these interactions are disrupted by rocaglamide but not by the structurally related flavagline FL1. Depletion of prohibitin-1 phenocopied the rocaglamide-mediated effects on KRAS activation and stability. We also demonstrate that flavaglines inhibit the oncogenic growth of KRAS-mutated cells and that treatment with rocaglamide reduces non-small-cell lung carcinoma (NSCLC) tumour nodules in autochthonous KRAS-driven mouse models without severe side effects. Our data suggest that it will be promising to further develop flavagline derivatives as specific KRAS inhibitors for clinical applications.

Prohibitin 1 interacts with signal transducer and activator of transcription 3 in T-helper 17 cells

T-helper 17 (Th17) cells are involved in the occurrence and development of several inflammation-associated diseases. Interleukin (IL)-17, the main cytokine secreted by differentiated Th17 cells, mediates immunoreactions and plays important roles in immunological diseases, including psoriasis, rheumatic arthritis, and inflammatory bowel disease. The maturation and stabilization of the differentiated Th17 cell phenotype are associated with the expression of IL-17A, which is induced by the activation of signal transducer and activator of transcription 3 (STAT3). Prohibitin 1 (PHB1) also plays an essential role in T-cell activation. However, the molecular mechanisms underlying Th17 cell differentiation and the role of PHB1 have not yet been completely elucidated, and this information would greatly facilitate the development of therapeutic strategies to prevent or treat immune-associated diseases. Here, we found that STAT3 expression was correlated with PHB1 mRNA expression during Th17 cell differentiation. Double-labeling immunofluorescence assay results showed that exogenous PHB1 and STAT3 proteins were not only located primarily in the nucleus but also in the cytoplasm of human Th17 cells. Co-immunoprecipitation assays of Th17 cells revealed that PHB1 interacted with STAT3 and with activated STAT3 phosphorylated at Ser727 but not at Tyr705. Knocking down PHB1 with specific short hairpin RNAs attenuated both STAT3 and IL-17 expression levels as well as IL-17 secretion in Th17 cells. These results indicate that PHB1 and STAT3 interact to affect IL-17 secretion in Th17 cells and provide important insights for modulating Th17-mediated pathogenic immune responses.

Proteomic Analysis of Lipid Rafts from RBL-2H3 Mast Cells

Lipid rafts are highly ordered membrane microdomains enriched in cholesterol, glycosphingolipids, and certain proteins. They are involved in the regulation of cellular processes in diverse cell types, including mast cells (MCs). The MC lipid raft protein composition was assessed using qualitative mass spectrometric characterization of the proteome from detergent-resistant membrane fractions from RBL-2H3 MCs. Using two different post-isolation treatment methods, a total of 949 lipid raft associated proteins were identified. The majority of these MC lipid raft proteins had already been described in the RaftProtV2 database and are among highest cited/experimentally validated lipid raft proteins. Additionally, more than half of the identified proteins had lipid modifications and/or transmembrane domains. Classification of identified proteins into functional categories showed that the proteins were associated with cellular membrane compartments, and with some biological and molecular functions, such as regulation, localization, binding, catalytic activity, and response to stimulus. Furthermore, functional enrichment analysis demonstrated an intimate involvement of identified proteins with various aspects of MC biological processes, especially those related to regulated secretion, organization/stabilization of macromolecules complexes, and signal transduction. This study represents the first comprehensive proteomic profile of MC lipid rafts and provides additional information to elucidate immunoregulatory functions coordinated by raft proteins in MCs.

Prohibitin: a prime candidate for a pleiotropic effector that mediates sex differences in obesity, insulin resistance, and metabolic dysregulation

Adipocytes and macrophages, the two major constituents of adipose tissue, exhibit sex differences and work in synergy in adipose tissue physiology and pathophysiology, including obesity-linked insulin resistance and metabolic dysregulation. Sex steroid hormones play a major role in sex differences in adipose tissue biology. However, our knowledge of the molecules that mediate these effects in adipose tissue remains limited. Consequently, it remains unclear whether these effector molecules in different adipose and immune cell types are distinct or if there are also pleiotropic effectors. Recently, a protein named prohibitin (PHB) with cell compartment- and tissue-specific functions has been found to play a role in sex differences in adipose and immune functions. Transgenic (Tg) mouse models overexpressing PHB (PHB-Tg) and a phospho-mutant PHB (mPHB-Tg) from the fatty acid binding protein-4 (Fabp-4) gene promoter display sex-neutral obesity; however, obesity-related insulin resistance and metabolic dysregulation are male-specific. Intriguingly, with aging, the male PHB-Tg mice developed hepatic steatosis and subsequently liver tumors whereas the male mPHB-Tg mice developed lymph node tumors and splenomegaly. Unlike the male transgenic mice, the female PHB-Tg and mPHB-Tg mice remain protected from obesity-related metabolic dysregulation and tumor development. In conclusion, the sex-dimorphic metabolic and immune phenotypes of PHB-Tg and mPHB-Tg mice have revealed PHB as a pleiotropic effector of sex differences in adipose and immune functions. In this mini-review, we will discuss the pleiotropic attributes of PHB and potential mechanisms that may have contributed to the sex-dimorphic metabolic phenotypes in PHB-Tg and mPHB-Tg mice, which warrant future research. We propose that PHB is a prime candidate for a pleiotropic mediator of sex differences in adipose and immune functions in both physiology and pathophysiology, including obesity, insulin resistance, and metabolic dysregulation.

Prohibitin: A hypothetical target for sex-based new therapeutics for metabolic and immune diseases

IMPACT STATEMENT

Traditional sex-related biases in research are now obsolete, and it is important to identify the sex of humans, animals, and even cells in research protocols, due to the role of sex as a fundamental facet of biology, predisposition to disease, and response to therapy. Genetic sex, epigenetics and hormonal regulations, generate sex-dimorphisms. Recent investigations acknowledge sex differences in metabolic and immune health as well as chronic diseases. Prohibitin, an evolutionarily conserved molecule, has pleotropic functions in mitochondrial housekeeping, plasma membrane signaling, and nuclear genetic transcription. Studies in adipocytes, macrophages, and transgenic mice indicate that prohibitin interacts with sex steroids and plays a role in mediating sex differences in adipose tissues and immune cell types. Prohibitin may, depending on context, modulate predisposition to chronic metabolic diseases and malignancy and, because of these attributes, could be a target for sex-based therapies of metabolic and immune-related diseases as well as cancer.

Mastocytosis-derived extracellular vesicles exhibit a mast cell signature, transfer KIT to stellate cells, and promote their activation

Extracellular vesicles (EVs) have been implicated in the development and progression of hematological malignancies. We thus examined serum samples from patients with systemic mastocytosis (SM) and found EVs with a mast cell signature including the presence of tryptase, FcεRI, MRGX2, and KIT. The concentration of these EVs correlated with parameters of disease including levels of serum tryptase, IL-6, and alkaline phosphatase and physical findings including hepatosplenomegaly. Given reports that EVs from one cell type may influence another cell's behavior, we asked whether SM-EVs might affect hepatic stellate cells (HSCs), based on the abnormal liver pathology associated with mastocytosis. We found that KIT was transferred from SM-EVs into an HSC line eliciting proliferation, cytokine production, and differentiation, processes that have been associated with liver pathology. These effects were reduced by KIT inhibition or neutralization and recapitulated by enforced expression of KIT or constitutively active D816V-KIT, a gain-of-function variant associated with SM. Furthermore, HSCs in liver from mice injected with SM-EVs had increased expression of α-SMA and human KIT, particularly around portal areas, compared with mice injected with EVs from normal individuals, suggesting that SM-EVs can also initiate HSC activation in vivo. Our data are thus consistent with the conclusion that SM-EVs have the potential to influence cells outside the hematological compartment and that therapeutic approaches for treatment of SM may be effective in part through inhibition of effects of EVs on target tissues, findings important both to understanding complex disease pathology and in developing interventional agents for the treatment of hematologic diseases.

Targeting prohibitins at the cell surface prevents Th17-mediated autoimmunity

T helper (Th)17 cells represent a unique subset of CD4⁺ T cells and are vital for clearance of extracellular pathogens including bacteria and fungi. However, Th17 cells are also involved in orchestrating autoimmunity. By employing quantitative surface proteomics, we found that the evolutionarily conserved prohibitins (PHB1/2) are highly expressed on the surface of both murine and human Th17 cells. Increased expression of PHBs at the cell surface contributed to enhanced CRAF/MAPK activation in Th17 cells. Targeting surface-expressed PHBs on Th17 cells with ligands such as Vi polysaccharide (Typhim vaccine) inhibited CRAF-MAPK pathway, reduced interleukin (IL)-17 expression and ameliorated disease pathology with an increase in FOXP3⁺-expressing Tregs in an animal model for multiple sclerosis (MS). Interestingly, we detected a CD4⁺ T cell population with high PHB1 surface expression in blood samples from MS patients in comparison with age- and sex-matched healthy subjects. Our observations suggest a pivotal role for the PHB-CRAF-MAPK signalling axis in regulating the polarization and pathogenicity of Th17 cells and unveil druggable targets in autoimmune disorders such as MS.

Repositioning of anti-cancer drug candidate, AZD7762, to an anti-allergic drug suppressing IgE-mediated mast cells and allergic responses via the inhibition of Lyn and Fyn

Mast cells are critical effector cells in IgE-mediated allergic responses. The aim of this study was to investigate the anti-allergic effects of 3-[(aminocarbonyl)amino]-5-(3-fluorophenyl)-N-(3S)-3-piperidinyl-2-thiophenecarboxamide (AZD7762) in vitro and in vivo. AZD7762 inhibited the antigen-stimulated degranulation from RBL-2H3 (IC₅₀, ∼27.9 nM) and BMMCs (IC₅₀, ∼99.3 nM) in a dose-dependent manner. AZD7762 also inhibited the production of TNF-α and IL-4. As the mechanism of its action, AZD7762 inhibited the activation of Syk and its downstream signaling proteins, such as Linker of activated T cells (LAT), phospholipase (PL) Cγ1, Akt, and mitogen-activated protein (MAP) kinases (Erk1/2, p38, and JNK) in mast cells. In in vitro protein kinase assay, AZD7762 inhibited the activity of Lyn and Fyn kinases, which are important for the activation of Syk in mast cells. Furthermore, AZD7762 also suppressed the degranulation of LAD2 human mast cells (IC₅₀, ∼49.9 nM) and activation of Syk in a dose-dependent manner. As observed in experiments with mast cells in vitro, AZD7762 inhibited antigen-mediated passive cutaneous anaphylaxis in mice (ED₅₀, ∼35.8 mg/kg). Altogether, these results suggest that AZD7762 could be used as a new therapeutic agent for mast cell-mediated allergic diseases.

Prohibitin: a potential therapeutic target in tyrosine kinase signaling

Prohibitin is a pleiotropic protein that has roles in fundamental cellular processes, such as cellular proliferation and mitochondrial housekeeping, and in cell- or tissue-specific functions, such as adipogenesis and immune cell functions. The different functions of prohibitin are mediated by its cell compartment-specific attributes, which include acting as an adaptor molecule in membrane signaling, a scaffolding protein in mitochondria, and a transcriptional co-regulator in the nucleus. However, the precise relationship between its distinct cellular localization and diverse functions remain largely unknown. Accumulating evidence suggests that the phosphorylation of prohibitin plays a role in a number of cell signaling pathways and in intracellular trafficking. Herein, we discuss the known and potential importance of the site-specific phosphorylation of prohibitin in regulating these features. We will discuss this in the context of new evidence from tissue-specific transgenic mouse models of prohibitin, including a mutant prohibitin lacking a crucial tyrosine phosphorylation site. We conclude with the opinion that prohibitin can be used as a potential target for tyrosine kinase signal transduction-targeting therapy, including in insulin, growth factors, and immune signaling pathways.

Interaction of DJ-1 with Lyn is essential for IgE-mediated stimulation of human mast cells

BACKGROUND

DJ-1 is a redox-sensitive protein with multiple roles in cell homeostasis, levels of which are altered in patients with mast cell (MC)-related disorders. However, whether DJ-1 can regulate human MC function is unknown.

OBJECTIVE

We sought to investigate the potential role of DJ-1 in the responses of human MCs to antigen stimulation.

METHODS

DJ-1 was silenced in human CD34⁺-derived MCs and in the LAD2 MC line by using lentiviral short hairpin RNA constructs. Release of β-hexosaminidase, prostaglandin D₂, and GM-CSF and changes in reactive oxygen species levels were measured after FcεRI engagement. Enzymatic assays, sucrose density gradient centrifugation, immunoprecipitation, dot and Western blotting, and confocal imaging were performed for signaling, cellular localization, and coassociation studies.

RESULTS

DJ-1 knockdown substantially reduced mediator release, as well as Lyn kinase and spleen tyrosine kinase activation and signaling through mechanisms that appeared largely unrelated to DJ-1 antioxidant activity. Following FcεRI activation, nonoxidized rather than oxidized DJ-1 translocated to lipid rafts, where it associated with Lyn, an interaction that appeared critical for maximal Lyn activation and initiation of signaling. Using purified recombinant proteins, we demonstrated that DJ-1 directly bound to Lyn but not to other Src kinases, and this interaction was specific for human but not mouse proteins. In addition, DJ-1 reduced Src homology 2 domain-containing phosphatase 2 phosphatase activity by scavenging reactive oxygen species, thus preventing spleen tyrosine kinase dephosphorylation and perpetuating MC signaling.

CONCLUSION

We demonstrate a novel role for DJ-1 in the early activation of Lyn by FcεRI, which is essential for human MC responses and provides the basis for an alternative target in allergic disease therapy.

Prohibitin - At the crossroads of obesity-linked diabetes and cancer

The promoter of a gene that is selectively expressed in just a few cell types provides unique opportunities to study: (1) the pleiotropic function of a protein in two different cell types including the cell compartment specific function, and (2) the crosstalk between two cell/tissue types at the systemic level. This is not possible with a ubiquitous or a highly specific gene promoter. The adipocyte protein-2 ( aP2) is one such gene. It is primarily expressed in adipocytes, but also selectively in monocytic macrophages and dendritic cells, among various immune cell types. Thus, the adipocyte protein-2 gene promoter provides an opportunity to simultaneously manipulate adipose and immune functions in a transgenic animal. Prohibitin (PHB) is a pleiotropic protein that has roles in both adipocytes and immune cells. Adipocyte specific functions of prohibitin are mediated through its mitochondrial function, whereas its immune functions are mediated in a phosphorylation-dependent manner. We capitalized on this attribute of prohibitin to explore the crosstalk between adipose and immune functions, and to discern mitochondrial and plasma membrane-associated cell signaling functions of prohibitin, by expressing wild type prohibitin (Mito-Ob) and a phospho-mutant form of prohibitin (m-Mito-Ob) from the protein-2 gene promoter, individually. Both transgenic mice develop obesity in a sex-neutral manner, but develop obesity-related metabolic dysregulation in a male sex-specific manner. Subsequently, the male Mito-Ob mice spontaneously developed type 2 diabetes and liver cancer, whereas the male m-Mito-Ob mice developed lymph node tumors or autoimmune diabetes in a context-dependent manner. This review provides a point of view on the role of prohibitin in mediating sex differences in adipose and immune functions at the systemic level. We discuss the unique attributes of prohibitin and provide a new paradigm in adipose-immune crosstalk mediated through a pleiotropic protein. Impact statement Prohibitin (PHB) is ubiquitously expressed and plays a role in adipocyte-immune cell cross-talk. Both male and female transgenic mice expressing wild-type PHB in adipose tissue and in macrophages are obese, but only males develop diabetes and liver cancer. When the mice express PHB mutated on tyrosine-114 in adipocytes and macrophages, both males and females are still obese, but none develops liver cancer; instead, males develop lymph node tumors. Adipocyte specific functions of PHB are mediated through its mitochondrial function, whereas its immune functions are mediated in a phosphorylation-dependent manner. Thus, PHB appears to be an important molecule linking obesity, diabetes, and cancer. In addition, this link appears to be affected by sex steroids. Therefore, targeting PHB may lead to a better understanding of the pathogenesis of obesity, diabetes and cancer.

Role of leishmanial acidocalcisomal pyrophosphatase in the cAMP homeostasis in phagolysosome conditions required for intra-macrophage survival

Exposure of Leishmania donovani to macrophage phagolysosome conditions (PC) (37°C and pH 5.5) led to increased intracellular cAMP and cAMP-mediated responses, which help in intra-macrophage survival pre-requisite for infectivity. In the absence of typical orthologs for G-proteins and G-protein coupled receptors, we sought to study the precise mechanisms for positive modulation of cAMP production during exposure to PC. Amongst two promastigote-stage specific membrane bound receptor adenylate cyclases (LdRAC-A and LdRAC-B), LdRAC-A appeared to function as a major cAMP generator following PC exposure. Pyrophosphate (PPi), an energy storage compound as well as a by-product of cAMP biosynthesis by adenylate cyclise, was found to be decreased following PC exposure. This may be due to microtubule and microfilament-driven translocation of acidocalcisomes near plasma membrane vicinity with concomitant increase of acidocalcisome membrane pyrophosphatase (LdV-H⁺PPase) and acidocalcisomal soluble pyrophosphatase (LdVSP1). Episomal over-expression and conditional silencing demonstrated regulatory role of V-H⁺PPase on cAMP trigger and consequent induction of resistance to macrophage-derived pro-oxidants and parasite killing. Furthermore, immunofluorescence analysis revealed possible co-localization of LdV-H⁺PPase and LdRAC-A during PC exposure. Collectively, these results suggest that translocation of acidocalcisome in membrane vicinity functions as a trigger for LdRAC-A-driven cAMP generation through depletion of PPi pool by LdV-H⁺PPase.

A novel prohibitin-binding compound induces the mitochondrial apoptotic pathway through NOXA and BIM upregulation

We previously described diaryl trifluorothiazoline compound 1a (hereafter referred to as fluorizoline) as a first-in-class small molecule that induces p53-independent apoptosis in a wide range of tumor cell lines. Fluorizoline directly binds to prohibitin 1 and 2 (PHBs), two proteins involved in the regulation of several cellular processes, including apoptosis. Here we demonstrate that fluorizoline-induced apoptosis is mediated by PHBs, as cells depleted of these proteins are highly resistant to fluorizoline treatment. In addition, BAX and BAK are necessary for fluorizoline-induced cytotoxic effects, thereby proving that apoptosis occurs through the intrinsic pathway. Expression analysis revealed that fluorizoline induced the upregulation of Noxa and Bim mRNA levels, which was not observed in PHB-depleted MEFs. Finally, Noxa^−/−/Bim^−/− MEFs and NOXA-downregulated HeLa cells were resistant to fluorizoline-induced apoptosis. All together, these findings show that fluorizoline requires PHBs to execute the mitochondrial apoptotic pathway.

Prohibitin in Adipose and Immune Functions

Prohibitin (PHB) was discovered in a quest to find genes with antiproliferative functions. However, the attribute of PHB that is responsible for its antiproliferative function remains elusive. Meanwhile, recent studies have established PHB as a pleiotropic protein with roles in metabolism, immunity, and senescence. PHB has cell compartment-specific functions, acting as a scaffolding protein in mitochondria, an adaptor molecule in membrane signaling, and a transcriptional coregulator in the nucleus. However, it remains unclear whether different functions and locations of PHB are interrelated or independent from each other, or if PHB works in a tissue-specific manner. Here, we discuss new findings on the role of PHB in adipose-immune interaction and an unexpected role in sex differences in adipose and immune functions.

Analysis of the Rab GTPase Interactome in Dendritic Cells Reveals Anti-microbial Functions of the Rab32 Complex in Bacterial Containment

Dendritic cells (DCs) orchestrate complex membrane trafficking through an interconnected transportation network linked together by Rab GTPases. Through a tandem affinity purification strategy and mass spectrometry, we depicted an interactomic landscape of major members of the mammalian Rab GTPase family. When complemented with imaging tools, this proteomic analysis provided a global view of intracellular membrane organization. Driven by this analysis, we investigated dynamic changes to the Rab32 subnetwork in DCs induced by L. monocytogenes infection and uncovered an essential role of this subnetwork in controlling the intracellular proliferation of L. monocytogenes. Mechanistically, Rab32 formed a persistent complex with two interacting proteins, PHB and PHB2, to encompass bacteria both during early phagosome formation and after L. monocytogenes escaped the original containment vacuole. Collectively, we have provided a functional compartmentalization overview and an organizational framework of intracellular Rab-mediated vesicle trafficking that can serve as a resource for future investigations.

Prohibitin: an unexpected role in sex dimorphic functions

Sex differences are known to exist in adipose and immune functions in the body, and sex steroid hormones are known to be involved in sexually dimorphic biological and pathological processes related to adipose-immune interaction. However, our knowledge of proteins that mediate such differences is poor. Two novel obese mice models, Mito-Ob and m-Mito-Ob, that have been reported recently have revealed an unexpected role of a pleiotropic protein, prohibitin (PHB), in sex differences in adipose and immune functions. This discovery points towards a role of pleiotropic proteins and their potential interplay with sex steroid hormones in mediating sexually dimorphic adipose-immune interaction.

Rab32/38 and the xenophagic restriction of intracellular bacteria replication

Rab GTPases' subversion by intracellular pathogens during infection has been extensively documented. Recent findings have implicated a key intracellular bacterial restriction/containment function for Rab32/38 in Salmonella species in macrophages and Listeria monocytogenes in dendritic cells. Rab32/38 aids the phagolysosome maturation, and mediates a parallel xenophagy mechanism by engaging prohibitins.

Expression of a mutant prohibitin from the aP2 gene promoter leads to obesity-linked tumor development in insulin resistance-dependent manner

A critical unmet need for the study of obesity-linked cancer is the lack of preclinical models that spontaneously develop obesity and cancer sequentially. Prohibitin (PHB) is a pleiotropic protein that has a role in adipose and immune functions. We capitalized on this attribute of PHB to develop a mouse model for obesity-linked tumor. We achieved this by expressing Y114F-PHB (m-PHB) from the aP2 gene promoter for simultaneous manipulation of adipogenic and immune signaling functions. The m-PHB mice develop obesity in a sex-neutral manner, but only male mice develop impaired glucose homeostasis and hyperinsulinemia similar to transgenic mice expressing PHB. Interestingly, only male m-PHB mice develop histiocytosis with lymphadenopathy, suggesting that metabolic dysregulation or m-PHB alone is not sufficient for the tumor development and that both are required for tumorigenesis. Moreover, ovariectomy in female m-PHB mice resulted in impaired glucose homeostasis, hyperinsulinemia and consequently tumor development similar to male m-PHB mice. These changes were not observed in sham-operated control m-Mito-Ob mice, further confirming the role of obesity-related metabolic dysregulation in tumor development in m-PHB mice. Our data provide a proof-of-concept that obesity-associated hyperinsulinemia promotes tumor development by facilitating dormant mutant to manifest and reveals a sex-dimorphic role of PHB in adipose-immune interaction or immunometabolism. Targeting PHB may provide a unique opportunity for the modulation of immunometabolism in obesity, cancer and in immune diseases.

A Novel Class of Small Molecule Compounds that Inhibit Hepatitis C Virus Infection by Targeting the Prohibitin-CRaf Pathway

Identification of novel drug targets and affordable therapeutic agents remains a high priority in the fight against chronic hepatitis C virus (HCV) infection. Here, we report that the cellular proteins prohibitin 1 (PHB1) and 2 (PHB2) are pan-genotypic HCV entry factors functioning at a post-binding step. While predominantly found in mitochondria, PHBs localize to the plasma membrane of hepatocytes through their transmembrane domains and interact with both EGFR and CRaf. Targeting PHB by rocaglamide (Roc-A), a natural product that binds PHB1 and 2, reduced cell surface PHB1 and 2, disrupted PHB-CRaf interaction, and inhibited HCV entry at low nanomolar concentrations. A structure-activity analysis of 32 synthetic Roc-A analogs indicated that the chiral, racemic version of aglaroxin C, a natural product biosynthetically related to Roc-A, displayed improved potency and therapeutic index against HCV infection. This study reveals a new class of HCV entry inhibitors that target the PHB1/2-CRaf pathway.

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Cellular proteins prohibitins 1 and 2 are essential HCV entry factors that function at a post-binding step.

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The natural compound Roc-A potently blocks HCV infection by disrupting prohibitins-CRaf interaction

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The Roc-A derivative, aglaroxin C, displays improved potency and therapeutic index towards HCV infection

Current FDA-approved HCV drugs all target viral proteins. We now demonstrate that a group of small molecules, the rocaglates, potently block HCV entry at low nanomolar concentrations. Roc-A inhibits HCV entry by disrupting the important interaction between two pan-genomic HCV entry factors, PHB1 and 2, and the signaling molecule CRaf. Overall, Roc-A and related rocaglates represent a new class of compounds that hold significant therapeutic promise in treating HCV infection.

Rhamnus davurica leaf extract inhibits Fyn activation by antigen in mast cells for anti-allergic activity

BACKGROUND

Complementary and alternative herbal medicines are recently considered as a promising approach for treating various diseases. We screened approximately 100 plant extracts for anti-allergic activity. Rhamnus davurica leaf extract showed the most potent inhibitory effect on the activation of RBL-2H3 mast cells. Although Rhamnus davurica extract has been used to treat pruritus, dysuresia, and constipation as a traditional herbal medicine in some Asian countries, an anti-allergic effect of Rhamnus davurica has not yet been demonstrated. We aimed to investigate the effect and mechanism of the leaf extract of Rhamnus davurica (LERD) on mast cells in vitro and allergic responses in vivo.

METHODS

The effects of LERD on the activation of mast cells and mast cell-mediated passive cutaneous anaphylaxis (PCA) were measured in mice and two types of mast cells, mouse bone marrow-derived mast cells (BMMCs) and RBL-2H3 cells in vitro. A mechanistic study of its inhibitory effect was performed by using degranulation assay, reverse transcriptase-polymerase chain reaction, enzyme-linked immunosorbent assay, and western blotting analysis.

RESULTS

LERD reversibly suppressed antigen-stimulated degranulation in BMMCs and RBL-2H3 cells, and also inhibited mRNA expression and secretion of TNF-α and IL-4 in a dose-dependent manner. In a PCA animal model, LERD significantly inhibited antigen-induced allergic response and degranulation of ear tissue mast cells. As for the mechanism of action, LERD inhibited the activation of Syk, which is the pivotal signaling protein for mast cell activation by antigen. Furthermore, LERD also impeded the activations of well-known downstream proteins such as LAT, Akt and three MAP kinases (Erk, p38 and JNK). In an in vitro kinase assay, LERD suppressed the activation of Fyn in antigen-stimulated mast cells.

CONCLUSION

This study demonstrated for the first time that LERD has anti-allergic effects through inhibiting the Fyn/Syk pathway in mast cells. Therefore, this study provides scientific evidence for LERD to be used as an herbal medicine or health food for patients with allergic diseases.

Prohibitin is involved in the activated internalization and degradation of protease-activated receptor 1

The protease-activated receptor 1 (PAR1) is a G-protein-coupled receptor that is irreversibly activated by either thrombin or metalloprotease 1. Due this irrevocable activation, activated internalization and degradation are critical for PAR1 signaling termination. Prohibitin (PHB) is an evolutionarily conserved, ubiquitously expressed, pleiotropic protein and belongs to the stomatin/prohibitin/flotillin/HflK/C (SPFH) domain family. In a previous study, we found that PHB localized on the platelet membrane and participated in PAR1-mediated human platelet aggregation, suggesting that PHB likely regulates the signaling of PAR1. Unfortunately, PHB's exact function in PAR1 internalization and degradation is unclear. In the current study, flow cytometry revealed that PHB expressed on the surface of endothelial cells (HUVECs) but not cancer cells (MDA-MB-231). Further confocal microscopy revealed that PHB dynamically associates with PAR1 in a time-dependent manner following induction with PAR1-activated peptide (PAR1-AP), though differently between HUVECs and MDA-MB-231 cells. Depletion of PHB by RNA interference significantly inhibited PAR1 activated internalization and led to sustained Erk1/2 phosphorylation in the HUVECs; however, a similar effect was not observed in MDA-MB-231 cells. For both the endothelial and cancel cells, PHB repressed PAR1 degradation, while knockdown of PHB led to increased PAR1 degradation, and PHB overexpression inhibited PAR1 degradation. These results suggest that persistent PAR1 signaling due to the absence of membrane PHB and decreased PAR1 degradation caused by the upregulation of intracellular PHB in cancer cells (such as MDA-MB-231 cells) may render cells highly invasive. As such, PHB may be a novel target in future anti-cancer therapeutics, or in more refined cancer malignancy diagnostics.

A role for prohibitin in mast cell activation: location matters

Prohibitin 1 (PHB1) and PHB2 are evolutionarily conserved, ubiquitously expressed, pleotropic proteins that control various fundamental cellular processes, including proliferation, migration, metabolism, and death. Studies have unveiled a crucial role for plasma membrane-associated PHBs in regulating tumor metastasis, viral entry, and immune cell activation. A study now identifies a role for PHB1 in the activation of mast cells and allergic reactions mediated by immunoglobulin E (IgE). PHB1 was primarily localized in mast cell granules; however, in response to stimulation with antigen, PHB1 translocated to plasma membrane lipid rafts to form a ternary complex with the high-affinity IgE receptor FcεRIγ and the nonreceptor tyrosine kinase Syk. Syk became activated, which led to the activation of downstream signaling that stimulated mast cell degranulation and the secretion of cytokines. PHB1 was phosphorylated by the Src family tyrosine kinase Lyn, and palmitoylation of PHB1 was required for its association with the plasma membrane. These observations unveil a previously uncharacterized facet of prohibitin biology and shed further light on the proximal events that drive the activation of FcεRI by IgE in mast cells during allergic reactions.