The IgM antigen receptor of B lymphocytes is associated with prohibitin and a prohibitin-related protein

Knockdown of heat shock protein family D member 1 (HSPD1) in lung cancer cell altered secretome profile and cancer-associated fibroblast induction

The crosstalk between lung cancer cells and cancer-associated fibroblast (CAF) is pivotal in cancer progression. Heat shock protein family D member 1 (HSPD1) is a potential prognostic biomarker associated with the tumor microenvironment in lung adenocarcinoma (LUAD). However, the role of HSPD1 in CAF activation remains unclear. This study established stable HSPD1-knockdown A549 lung cancer cells using a lentivirus-mediated shRNA transduction. A targeted label-free proteomic analysis identified six significantly altered secretory proteins in the shHSPD1-A549 secretome compared to shControl-A549. Functional enrichment analysis highlighted their involvement in cell-to-cell communication and immune responses within the tumor microenvironment. Additionally, most altered proteins exhibited positive correlations and significant prognostic impacts on LUAD patient survival. Investigations on the effects of lung cancer secretomes on lung fibroblast WI-38 cells revealed that the shControl-A549 secretome stimulated fibroblast proliferation, migration, and CAF marker expression. These effects were reversed upon the knockdown of HSPD1 in A549 cells. Altogether, our findings illustrate the role of HSPD1 in mediating CAF induction through secretory proteins, potentially contributing to the progression and aggressiveness of lung cancer.

A fungal protein organizes both glycogen and cell wall glucans

Glycogen is a glucose storage molecule composed of branched α-1,4-glucan chains, best known as an energy reserve that can be broken down to fuel central metabolism. Because fungal cells have a specialized need for glucose in building cell wall glucans, we investigated whether glycogen is used for this process. For these studies, we focused on the pathogenic yeast Cryptococcus neoformans, which causes ~150,000 deaths per year worldwide. We identified two proteins that influence formation of both glycogen and the cell wall: glycogenin (Glg1), which initiates glycogen synthesis, and a protein that we call Glucan organizing enzyme 1 (Goe1). We found that cells missing Glg1 lack α-1,4-glucan in their walls, indicating that this material is derived from glycogen. Without Goe1, glycogen rosettes are mislocalized and β-1,3-glucan in the cell wall is reduced. Altogether, our results provide mechanisms for a close association between glycogen and cell wall.

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.

Prohibitin 2: A key regulator of cell function

The PHB2 gene is located on chromosome 12p13 and encodes prohibitin 2, a highly conserved protein of 37 kDa. PHB2 is a dimer with antiparallel coils, possessing a unique negatively charged region crucial for its mitochondrial molecular chaperone functions. Thus, PHB2 plays a significant role in cell life activities such as mitosis, mitochondrial autophagy, signal transduction, and cell death. This review discusses how PHB2 inhibits transcription factors or nuclear receptors to maintain normal cell functions; how PHB2 in the cytoplasm or membrane ensures normal cell mitosis and regulates cell differentiation; how PHB2 affects mitochondrial structure, function, and cell apoptosis through mitochondrial intimal integrity and mitochondrial autophagy; how PHB2 affects mitochondrial stress and inhibits cell apoptosis by regulating cytochrome c migration and other pathways; how PHB2 affects cell growth, proliferation, and metastasis through a mitochondrial independent mechanism; and how PHB2 could be applied in disease treatment. We provide a theoretical basis and an innovative perspective for a comprehensive understanding of the role and mechanism of PHB2 in cell function regulation.

The many "Neurofaces" of Prohibitins 1 and 2: Crucial for the healthy brain, dysregulated in numerous brain disorders

Prohibitin 1 (PHB1) and prohibitin 2 (PHB2) are proteins that are nearly ubiquitously expressed. They are localized in mitochondria, cytosol and cell nuclei. In the healthy CNS, they occur in neurons and non-neuronal cells (oligodendrocytes, astrocytes, microglia, and endothelial cells) and fulfill pivotal functions in brain development and aging, the regulation of brain metabolism, maintenance of structural integrity, synapse formation, aminoacidergic neurotransmission and, probably, regulation of brain action of certain hypothalamic-pituitary hormones.With regard to the diseased brain there is increasing evidence that prohibitins are prominently involved in numerous major diseases of the CNS, which are summarized and discussed in the present review (brain tumors, neurotropic viruses, Alzheimer disease, Down syndrome, Fronto-temporal and vascular dementia, dementia with Lewy bodies, Parkinson disease, Huntington disease, Multiple sclerosis, Amyotrophic lateral sclerosis, stroke, alcohol use disorder, schizophrenia and autism). Unfortunately, there is no PHB-targeted therapy available for any of these diseases.

Essential Protein PHB2 and Its Regulatory Mechanisms in Cancer

Prohibitins (PHBs) are a highly conserved class of proteins and have an essential role in transcription, epigenetic regulation, nuclear signaling, mitochondrial structural integrity, cell division, and cellular membrane metabolism. Prohibitins form a heterodimeric complex, consisting of two proteins, prohibitin 1 (PHB1) and prohibitin 2 (PHB2). They have been discovered to have crucial roles in regulating cancer and other metabolic diseases, functioning both together and independently. As there have been many previously published reviews on PHB1, this review focuses on the lesser studied prohibitin, PHB2. The role of PHB2 in cancer is controversial. In most human cancers, overexpressed PHB2 enhances tumor progression, while in some cancers, it suppresses tumor progression. In this review, we focus on (1) the history, family, and structure of prohibitins, (2) the essential location-dependent functions of PHB2, (3) dysfunction in cancer, and (4) the promising modulators to target PHB2. At the end, we discuss future directions and the clinical significance of this common essential gene in cancer.

Lamprey prohibitin 2 inhibits non-small cell lung carcinoma cell proliferation by down-regulating the expression and phosphorylation levels of cell cycle-associated proteins

Prohibitin 2 (PHB2) is an evolutionarily conserved and functionally diverse protein that plays an important role in multiple cellular functions, including cell proliferation, cell migration, and apoptosis, and is also known to participate in the process of tumorigenesis and development. In this study, the lamprey PHB2 (Lm-PHB2) gene was over-expressed in KRAS (kirsten rat sarcoma viral oncogene homolog)-mutated non-small cell lung carcinoma (NSCLC) cells to investigate its effect on cell proliferation. The effects of Lm-PHB2 protein on the proliferation of NSCLC cells were determined by treating cells with the purified recombinant Lm-PHB2 protein (rLm-PHB2) followed by cell counting kit (CCK) assays and flow cytometry. Analysis showed that rLm-PHB2 blocked cells in the G2 phase and inhibited the cell proliferation of A549, Calu-1, and NCI-H226 to various degrees. The effect on Calu-1 cells was the most obvious and was concentration- and time-dependent. Similarly, cells transfected with the pEGFP-N1-Lm-PHB2 plasmid also resulted in the suppression of proliferation in A549 cells and Calu-1 cells. Quantitative real-time polymerase chain reaction (qRT-PCR) showed that Lm-PHB2 inhibited cell proliferation by repressing the transcription of PLK1 (polo-like kinase 1), Wee1 (wee1 kinase), CCNB1 (cyclin B1), and CDC25C (cell division control protein 25C). According to western blot analysis, Lm-PHB2 not only down-regulated the expression of PLK1, Wee1, CCNB1, and CDC25C but also reduced the phosphorylation levels of CCNB1 and CDC25C, thus blocking Calu-1 cells in G2/M phase. Our findings demonstrate a function of lamprey PHB2 that may inhibit the proliferation of some NSCLC cells by down-regulating the expression and phosphorylation of cell cycle-associated proteins.

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.

PHB2 Maintains the Contractile Phenotype of VSMCs by Counteracting PKM2 Splicing

BACKGROUND

Phenotypic transition of vascular smooth muscle cells (VSMCs) accounts for the pathogenesis of a variety of vascular diseases during the early stage. Recent studies indicate the metabolic reprogramming may be involved in VSMC phenotypic transition. However, the definite molecules that link energy metabolism to distinct VSMC phenotype remain elusive.

METHODS

A carotid artery injury model was used to study postinjury neointima formation as well as VSMC phenotypic transition in vivo. RNA-seq analysis, cell migration assay, collagen gel contraction assay, wire myography assay, immunoblotting, protein interactome analysis, co-immunoprecipitation, and mammalian 2-hybrid assay were performed to clarify the phenotype and elucidate the molecular mechanisms.

RESULTS

We collected cell energy-regulating genes by using Gene Ontology annotation and applied RNA-Seq analysis of transforming growth factor-β or platelet-derived growth factor BB stimulated VSMCs. Six candidate genes were overlapped from energy metabolism-related genes and genes reciprocally upregulated by transforming growth factor-β and downregulated by platelet-derived growth factor BB. Among them, prohibitin 2 has been reported to regulate mitochondrial oxidative phosphorylation. Indeed, prohibitin 2-deficient VSMCs lost the contractile phenotype as evidenced by reduced contractile proteins. Consistently, Phb2^SMCKO mice were more susceptible to postinjury VSMC proliferation and neointima formation compared with Phb2^flox/flox mice. Further protein interactome analysis, co-immunoprecipitation, and mammalian 2-hybrid assay revealed that prohibitin 2, through its C-terminus, directly interacts with hnRNPA1, a key modulator of pyruvate kinase M1/2 (PKM) mRNA splicing that promotes PKM2 expression and glycolysis. Prohibitin 2 deficiency facilitated PKM1/2 mRNA splicing and reversion from PKM1 to PKM2, and enhanced glycolysis in VSMCs. Blocking prohibitin 2-hnRNPA1 interaction resulted in increased PKM2 expression, enhanced glycolysis, repressed contractile marker genes expression in VSMCs, as well as aggravated postinjury neointima formation in vivo.

CONCLUSIONS

Prohibitin 2 maintains VSMC contractile phenotype by interacting with hnRNPA1 to counteract hnRNPA1-mediated PKM alternative splicing and glucose metabolic reprogramming.

Mitochondrial prohibitin complex regulates fungal virulence via ATG24-assisted mitophagy

Prohibitins are highly conserved eukaryotic proteins in mitochondria that function in various cellular processes. The roles of prohibitins in fungal virulence and their regulatory mechanisms are still unknown. Here, we identified the prohibitins ChPhb1 and ChPhb2 in a plant pathogenic fungus Colletotrichum higginsianum and investigated their roles in the virulence of this anthracnose fungus attacking crucifers. We demonstrate that ChPhb1 and ChPhb2 are required for the proper functioning of mitochondria, mitophagy and virulence. ChPhb1 and ChPhb2 interact with the autophagy-related protein ChATG24 in mitochondria, and ChATG24 shares similar functions with these proteins in mitophagy and virulence, suggesting that ChATG24 is involved in prohibitin-dependent mitophagy. ChPhb1 and ChPhb2 modulate the translocation of ChATG24 into mitochondria during mitophagy. The role of ChATG24 in mitophagy is further confirmed to be conserved in plant pathogenic fungi. Our study presents that prohibitins regulate fungal virulence by mediating ATG24-assisted mitophagy.

Prohibitins recruit ChATG24 into the mitochondria to modulate mitophagy, thereby affecting the virulence of Colletotrichum higginsianum.

Structural Requirements for the Binding of a Peptide to Prohibitins on the Cell Surface of Monocytes/Macrophages

The screening of phage peptide libraries resulted in the identification of a sequence (named NW peptide, NWYLPWLGTNDW) that specifically binds to human monocytes and macrophages. Although the NW peptide can be used for the targeted delivery of therapeutics without knowledge of its receptor(s), the identification of-its binding partners will support future clinical applications-Here, we used the biotinylated NW peptide for cross-linking cell surface receptor(s) on live cells or as bait in pull-down assays with membrane proteins isolated from monocytes or human THP-1 cells differentiated into macrophages. Proteomic analysis of the captured proteins identified cell surface prohibitins (PHB1 and PHB2) and modified albumin as binding partners. Using flow cytometry and pull-down methods, we demonstrated that PHB1 and PHB2 interact directly with the NW peptide. Confocal imaging showed co-localization of the peptide with PHB1 on the surface of monocytes. Single replacement of either tryptophan or leucine with alanine completely inhibited binding, whereas the replacement of asparagine at position 1 or 10 and aspartic acid at position 11 with alanine did not affect the binding of the peptide variants. Neutral amino acid replacement of tryptophan at positions 2, 6, and 12 with tyrosine or phenylalanine also abolished the binding, implying that the indole ring of tryptophan is indispensable for the NW peptide to bind. Overall, the data suggest that membrane-associated prohibitins might be a useful target for the delivery of therapeutics to monocytes/macrophages and that tryptophan and leucine are key residues for peptide binding.

PHB2 affects the virulence of Vip3Aa to Sf9 cells through internalization and mitochondrial stability

The vegetative insecticidal proteins (Vip3A) secreted by some Bacillus thuringiensis (Bt) strains during vegetative growth are regarded as a new generation of insecticidal toxins. Like insecticidal crystal proteins, they are also used in transgenic crops to control pests. However, their insecticidal mechanisms are far less defined than those of insecticidal crystal protein. Prohibitin 2 (PHB2) is a potential Vip3Aa binding receptor identified from the membrane of Sf9 cells in our previous work. In this paper, we demonstrated the interaction between Vip3Aa and PHB2 using pull-down, dot blotting, microscale thermophoresis, and co-immunoprecipitation assays. PHB2 is distributed on the cell membrane and in the cytoplasm, and the co-localization of PHB2 and Vip3Aa was observed in Sf9 cells using a confocal laser scanning microscope. Moreover, PHB2 could interact with scavenger receptor-C via its SPFH (stomatin, prohibitin, flotillin, and HflK/C) domain. Downregulation of phb2 expression reduced the degree of internalization of Vip3Aa, exacerbated Vip3Aa-mediated mitochondrial damage, and increased Vip3Aa toxicity to Sf9 cells. This suggested that PHB2 performs two different functions: Acting as an interacting partner to facilitate the internalization of Vip3Aa into Sf9 cells and maintaining the stability of mitochondria. The latter has a more important influence on the virulence of Vip3Aa.

Membrane prohibitin forms a dynamic complex with p56lck to regulate T cell receptor signaling

Prohibitin is a highly conserved ubiquitously expressed protein involved in several key cellular functions. Targeting of this protein in the membrane by the virulence polysaccharide, Vi, of human typhoid-causing pathogen, Salmonella enterica serovar Typhi (S. Typhi), results in suppression of IL-2 secretion from T cells activated through the T-cell receptor (TCR). However, the mechanism of this suppression remains unclear. Here, using Vi as a probe, we show that membrane prohibitin associates with the src-tyrosine kinase, p56^lck (Lck), and actin in human model T cell line, Jurkat. Activation with anti-CD3 antibody brings about dissociation of this complex, which coincides with downstream ERK activation. The trimolecular complex reappears towards culmination of proximal TCR signaling. Engagement of cells with Vi prevents TCR-triggered activation of Lck and ERK by inhibiting dissociation of the former from prohibitin. These findings suggest a regulatory role for membrane prohibitin in Lck activation and TCR signaling.

Two interaction proteins between AtPHB6 and AtSOT12 regulate plant salt resistance through ROS signaling

In the past, the PHB gene function was mainly focused on anti-cell proliferation and antitumor effects. But the molecular mechanism of the PHB gene regarding saline and oxidative stresses is unclear. To study the role of AtPHB6 in salt and oxidative stress, AtPHB6 was cloned from A. thaliana. Bioinformatics analysis showed that AtPHB6 was closely related to AtPHB1 and AtPHB2, which are both type II PHB. RT-qPCR results indicated that the AtPHB6 in the leaves and roots of A. thaliana was obviously induced under different stress treatments. AtPHB6-overexpressing plants were larger and more lush than wild-type and mutant plants when placed under stress treatments during seed germination. The root length and fresh weight of AtPHB6 transgenic plants showed the best resistance compared to wild-type plants under different treatments, in contrast, the AtPHB6 mutants had the worst resistance during the seedling stage. AtSOT12 was an interacting protein of AtPHB6, which screened by yeast two-hybrid system. The interaction between the two proteins were further confirmed using in vitro pull-down experiments and in vivo BiFC experiments. Subcellular localization showed both AtPHB6 and AtSOT12 protein expressed in the nucleus and cytoplasm. The H₂O₂ content in both the transgenic AtPHB6 and AtSOT12 plants were lower than that in the wild type under stresses. Thus, AtPHB6 increased plant resistance to salt stress and interacted with the AtSOT12 protein.

The Identification of Prohibitin in the Rat Heart Mitochondria in Heart Failure

Mitochondria are considered the main organelles in the cell. They play an important role in both normal and abnormal heart function. There is a supramolecular organization between the complexes of the respiratory chain (supercomplexes (SCs)), which are involved in mitochondrial respiration. Prohibitins (PHBs) participate in the regulation of oxidative phosphorylation (OXPHOS) activity and interact with some subunits of the OXPHOS complexes. In this study, we identified a protein whose level was decreased in the mitochondria of the heart in rats with heart failure. This protein was PHB. Isoproterenol (ISO) has been used as a compound to induce heart failure in rats. We observed that astaxanthin (AX) increased the content of PHB in rat heart mitochondria isolated from ISO-injected rats. Since it is known that PHB forms complexes with some mitochondrial proteins and proteins that are part of the complexes of the respiratory chain, the change in the levels of these proteins was investigated under our experimental conditions. We hypothesized that PHB may be a target for the protective action of AX.

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.

SLAMF receptors negatively regulate B cell receptor signaling in chronic lymphocytic leukemia via recruitment of prohibitin-2

We identified a subset of Chronic Lymphocytic Leukemia (CLL) patients with high Signaling Lymphocytic Activation Molecule Family (SLAMF) receptor-related signaling that showed an indolent clinical course. Since SLAMF receptors play a role in NK cell biology, we reasoned that these receptors may impact NK cell-mediated CLL immunity. Indeed, our experiments showed significantly decreased degranulation capacity of primary NK cells from CLL patients expressing low levels of SLAMF1 and SLAMF7. Since the SLAMF^low signature was strongly associated with an unmutated CLL immunoglobulin heavy chain (IGHV) status in large datasets, we investigated the impact of SLAMF1 and SLAMF7 on the B cell receptor (BCR) signaling axis. Overexpression of SLAMF1 or SLAMF7 in IGHV mutated CLL cell models resulted in reduced proliferation and impaired responses to BCR ligation, whereas the knockout of both receptors showed opposing effects and increased sensitivity toward inhibition of components of the BCR pathway. Detailed molecular analyzes showed that SLAMF1 and SLAMF7 receptors mediate their BCR pathway antagonistic effects via recruitment of prohibitin-2 (PHB2) thereby impairing its role in signal transduction downstream the IGHV-mutant IgM-BCR. Together, our data indicate that SLAMF receptors are important modulators of the BCR signaling axis and may improve immune control in CLL by interference with NK cells.

The BCL-2 family members NOXA and BIM mediate fluorizoline-induced apoptosis in multiple myeloma cells

Fluorizoline is a new synthetic molecule that induces apoptosis by selectively targeting prohibitins. In this study, we have assessed the pro-apoptotic effect of fluorizoline in 3 different multiple myeloma cell lines and 12 primary samples obtained from treatment-naïve multiple myeloma patients. Fluorizoline induced apoptosis in both multiple myeloma cell lines and primary samples at concentrations in the low micromolar range. All primary samples were sensitive to fluorizoline. Moreover, fluorizoline increased the mRNA and protein levels of the pro-apoptotic BCL-2 family member NOXA both in cell lines and primary samples analyzed. Finally, NOXA-depletion by CRISPR/Cas9 in cells that do not express BIM conferred resistance to fluorizoline-induced apoptosis in multiple myeloma cells. These results suggest that targeting prohibitins could be a new therapeutic strategy for myeloma multiple.

Importance of thorough tissue and cellular level characterization of targeted drugs in the evaluation of pharmacodynamic effects

Targeted nanoparticle delivery is a promising strategy for increasing efficacy and limiting side effects of therapeutics. When designing a targeted liposomal formulation, the in vivo biodistribution of the particles must be characterized to determine the value of the targeting approach. Peroxisome proliferator-activated receptor (PPAR) agonists effectively treat metabolic syndrome by decreasing dyslipidemia and insulin resistance but side effects have limited their use, making them a class of compounds that could benefit from targeted liposomal delivery. The adipose targeting sequence peptide (ATS) could fit this role, as it has been shown to bind to adipose tissue endothelium and induce weight loss when delivered conjugated to a pro-apoptotic peptide. To date, however, a full assessment of ATS in vivo biodistribution has not been reported, leaving important unanswered questions regarding the exact mechanisms whereby ATS targeting enhances therapeutic efficacy. We designed this study to evaluate the biodistribution of ATS-conjugated liposomes loaded with the PPARα/γ dual agonist tesaglitazar in leptin-deficient ob/ob mice. The ATS-liposome biodistribution in adipose tissue and other organs was examined at the cellular and tissue level using microscopy, flow cytometry, and fluorescent molecular tomography. Changes in metabolic parameters and gene expression were measured by target and off-target tissue responses to the treatment. Unexpectedly, ATS targeting did not increase liposomal uptake in adipose relative to other tissues, but did increase uptake in the kidneys. Targeting also did not significantly alter metabolic parameters. Analysis of the liposome cellular distribution in the stromal vascular fraction with flow cytometry revealed high uptake by multiple cell types. Our findings highlight the need for thorough study of in vivo biodistribution when evaluating a targeted therapy.

Circulating levels of mitochondrial uncoupling protein 2, but not prohibitin, are lower in humans with type 2 diabetes and correlate with brachial artery flow-mediated dilation

BACKGROUND

Excessive reactive oxygen species from endothelial mitochondria in type 2 diabetes individuals (T2DM) may occur through multiple related mechanisms, including production of mitochondrial reactive oxygen species (mtROS), inner mitochondrial membrane (Δψ_m) hyperpolarization, changes in mitochondrial mass and membrane composition, and fission of the mitochondrial networks. Inner mitochondrial membrane proteins uncoupling protein-2 (UCP2) and prohibitin (PHB) can favorably impact mtROS and mitochondrial membrane potential (Δψ_m). Circulating levels of UCP2 and PHB could potentially serve as biomarker surrogates for vascular health in patients with and without T2DM.

METHODS

Plasma samples and data from a total of 107 individuals with (N = 52) and without T2DM (N = 55) were included in this study. Brachial artery flow mediated dilation (FMD) was measured by ultrasound. ELISA was performed to measure serum concentrations of PHB1 and UCP2. Mitochondrial membrane potential was measured from isolated leukocytes using JC-1 dye.

RESULTS

Serum UCP2 levels were significantly lower in T2DM subjects compared to control subjects (3.01 ± 0.34 vs. 4.11 ± 0.41 ng/mL, P = 0.04). There were no significant differences in levels of serum PHB. UCP2 levels significantly and positively correlated with FMDmm (r = 0.30, P = 0.03) in T2DM subjects only and remained significant after multivariable adjustment. Within T2DM subjects, serum PHB levels were significantly and negatively correlated with UCP2 levels (ρ = - 0.35, P = 0.03).

CONCLUSION

Circulating UCP2 levels are lower in T2DM patients and correlate with endothelium-dependent vasodilation in conduit vessels. UCP2 could be biomarker surrogate for overall vascular health in patients with T2DM and merits additional investigation.

Characterization of PHB in the gonadal development of the swimming crab Portunus trituberculatus

Prohibitin (PHB) is an evolutionarily conserved multifunctional protein with ubiquitous expression. In this study, we cloned the PHB gene from the testis of the swimming crab Portunus trituberculatus (PtPHB) and analyzed the deduced amino acid sequence. The expression level of phb mRNA in larvae was analyzed using qRT-PCR. The expression level of phb mRNA and PHB protein in different tissues were analyzed using qRT-PCR and Western blot respectively. Enzyme-linked immunosorbent assay analyses of the PHB protein were conducted with the testis and ovaries from P. trituberculatus specimens at different developmental stages. PHB was localized with mitochondria and ubiquitin in the testis and ovaries. The PtPHB gene was found to contain an open reading frame of 825 bp, encoding a predicted peptide with 275 amino acids, sharing between 65.9% and 96.7% similarity with that of other species. The qRT-PCR and Western blot results showed that the phb gene and PHB protein both expressed less in the testis and ovary than in other tissues, and the phb gene presented the lowest expression in the Z1 stage. Furthermore, the phb gene and PHB protein expression were different in the testis and ovaries at different developmental stages. PHB was mainly found to be co-localized with mitochondria and ubiquitin in cytoplasm and acrosome complex during spermatogenesis and in follicular cells during oogenesis. Interestingly, PHB-mitochondria signals and ubiquitin signal were also found in oocytes. These results indicated that PHB might play important roles during spermatogenesis and oogenesis by regulating mitochondrial activities.

The Arabidopsis PHB3 is a pleiotropic regulator for plant development

Prohibitins (PHBs) are composed of an obviously conserved protein family in eukaryotic cells. Despite the extensive and in-depth research of mammalian PHB1 and PHB2, the plant prohibitins functions are not completely elucidated and little is known about their mechanism of action. This review focuses on the current knowledge about the protein subcellular localization, interaction proteins and target genes of PHB3. Furthermore, we discussed the roles of PHB3 protein in plant growth and development, plant responses to abiotic or biotic stresses and its participation in phytohormonal signaling.

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.

Nuclear Prohibitin3 Maintains Genome Integrity and Cell Proliferation in the Root Meristem through Minichromosome Maintenance 2

The nucleo-mitochondrial dual-localized proteins can act as gene expression regulators; however, few instances of these proteins have been described in plants. Arabidopsis (Arabidopsis thaliana) PROHIBITIN 3 (PHB3) is involved in stress responses and developmental processes, but it is unknown how these roles are achieved at the molecular level in the nucleus. In this study, we show that nucleo-mitochondrial PHB3 plays an essential role in regulating genome stability and cell proliferation. PHB3 is up-regulated by DNA damage agents, and the stress-induced PHB3 proteins accumulate in the nucleus. Loss of function of PHB3 results in DNA damage and defective maintenance of the root stem cell niche. Subsequently, the expression patterns and levels of the root stem cell regulators are altered and down-regulated, respectively. In addition, the phb3 mutant shows aberrant cell division and altered expression of cell cycle-related genes, such as CycB1 and Cyclin dependent kinase 1 Moreover, the minichromosome maintenance (MCM) genes, e.g. MCM2, MCM3, MCM4, MCM5, MCM6, and MCM7, are up-regulated in the phb3 mutant. Reducing the MCM2 expression level substantially recovers the DNA damage in the phb3 mutant and partially rescues the altered cell proliferation and root deficiency of phb3 seedlings. PHB3 acts as a transcriptional coregulator that represses MCM2 expression by competitively binding to the promoter E2F-cis-acting elements with E2Fa so as to modulate primary root growth. Collectively, these findings indicate that nuclear-localized PHB3 acts as a transcriptional coregulator that suppresses MCM2 expression to sustain genome integrity and cell proliferation for stem cell niche maintenance in Arabidopsis.

Prohibitins: A Critical Role in Mitochondrial Functions and Implication in Diseases

Prohibitin 1 (PHB1) and prohibitin 2 (PHB2) are proteins that are ubiquitously expressed, and are present in the nucleus, cytosol, and mitochondria. Depending on the cellular localization, PHB1 and PHB2 have distinctive functions, but more evidence suggests a critical role within mitochondria. In fact, PHB proteins are highly expressed in cells that heavily depend on mitochondrial function. In mitochondria, these two proteins assemble at the inner membrane to form a supra-macromolecular structure, which works as a scaffold for proteins and lipids regulating mitochondrial metabolism, including bioenergetics, biogenesis, and dynamics in order to determine the cell fate, death, or life. PHB alterations have been found in aging and cancer, as well as neurodegenerative, cardiac, and kidney diseases, in which significant mitochondrial impairments have been observed. The molecular mechanisms by which prohibitins regulate mitochondrial function and their role in pathology are reviewed and discussed herein.

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.

Characterization of mitochondrial prohibitin from Boleophthalmus pectinirostris and evaluation of its possible role in spermatogenesis

Prohibitin (PHB) is an evolutionarily conserved mitochondrial membrane protein. It plays a vital role in cell proteolysis, senescence, and apoptosis and is associated with spermatogenesis and sperm quality control in mammals. To study the characteristics of the PHB gene and its potential roles during spermatogenesis in Boleophthalmus pectinirostris, we cloned a 1153-bp full-length cDNA from the testis of B. pectinirostris with an open reading frame of 816 bp, which encodes 272 amino acid residues. Real-time quantitative PCR (qPCR) analysis revealed the presence of phb mRNA in all the tissues examined, with higher expression levels found in the testis, kidney, intestine, and muscle tissues. We examined the localization of phb mRNA during spermatogenesis by in situ hybridization (ISH), showing that phb mRNA was distributed in the periphery of the nucleus in primary and secondary spermatocytes. In spermatid and mature sperm, the phb mRNA gradually moved toward one side, where the flagellum is formed. Immunofluorescence (IF) results showed co-localization of the PHB and mitochondria at different stages during spermatogenesis of B. pectinirostris. The signals obtained for PHB decreased as spermatogenesis proceeded; the strongest detection signal was found in secondary spermatocytes, with lower levels of staining in other stages. Additionally, in the mature germ cells, the PHB signals were weak and aggregate in the midpiece of the flagellum.

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.

Prohibitin promotes androgen receptor activation in ER-positive breast cancer

Prohibitin (PHB) is an evolutionarily conserved protein with multiple functions in both normal and cancer cells. Androgen receptor (AR) was reported to act as a different role in the ER-positive and ER-negative breast cancer. However, little is known about the role of PHB and whether PHB could regulate AR expression in the ER-positive breast cancer. Here, we determined the expression and clinical outcomes of PHB in breast cancer samples using 121 breast cancer tissues and published databases, and investigated the role of PHB in breast cancer cell growth, apoptosis and cell cycle arrest in the ER-positive breast cancer cells. We obtained the expression of PHB is significantly low in breast cancer samples, and low PHB expression positively correlated with poor prognosis of breast cancer. We detected that PHB could inhibit breast cancer cell proliferation, change cell cycle distribution and promote cell apoptosis in the ER-positive breast cancer cells. Moreover, we found PHB could significantly increase AR expression in both mRNA and protein levels in the ER-positive breast cancer cells. Additionally, a significant positive correlation between PHB and AR expression was identified in the 121 breast cancer tissues. PHB and AR expression are associated with prognosis in the ER-positive breast cancer patients. Our results indicate that PHB promotes AR activation in ER-positive breast cancer, making PHB and AR potential molecular targets for ER-positive breast cancer therapy.

Mitochondrial oxidative phosphorylation complexes exist in the sarcolemma of skeletal muscle

Although proteomic analyses have revealed the presence of mitochondrial oxidative phosphorylation (OXPHOS) proteins in the plasma membrane, there have been no in-depth evaluations of the presence or function of OXPHOS I-V in the plasma membrane. Here, we demonstrate the in situ localization of OXPHOS I-V complexes to the sarcolemma of skeletal muscle by immunofluorescence and immunohistochemistry. A portion of the OXPHOS I-V complex proteins was not co-stained with MitoTracker but co-localized with caveolin-3 in the sarcolemma of mouse gastrocnemius. Mitochondrial matrix-facing OXPHOS complex subunits were ectopically expressed in the sarcolemma of the non-permeabilized muscle fibers and C2C12 myotubes. The sarcolemmal localization of cytochrome c was also observed from mouse gastrocnemius muscles and C2C12 myotubes, as determined by confocal and total internal resonance fluorescence (TIRF) microscopy. Based on these data, we conclude that a portion of OXPHOS complexes is localized in the sarcolemma of skeletal muscle and may have non-canonical functions. [BMB Reports 2016; 49(2): 116-121]

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.

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.

Prohibitin( PHB) roles in granulosa cell physiology

Ovarian granulosa cells (GC) play an important role in the growth and development of the follicle in the process known as folliculogenesis. In the present review, we focus on recent developments in prohibitin (PHB) research in relation to GC physiological functions. PHB is a member of a highly conserved eukaryotic protein family containing the repressor of estrogen activity (REA)/stomatin/PHB/flotillin/HflK/C (SPFH) domain (also known as the PHB domain) found in diverse species from prokaryotes to eukaryotes. PHB is ubiquitously expressed in a circulating free form or is present in multiple cellular compartments including mitochondria, nucleus and plasma membrane. In mitochondria, PHB is anchored to the mitochondrial inner membrane and forms complexes with the ATPases associated with proteases having diverse cellular activities. PHB continuously shuttles between the mitochondria, cytosol and nucleus. In the nucleus, PHB interacts with various transcription factors and modulates transcriptional activity directly or through interactions with chromatin remodeling proteins. Many functions have been attributed to the mitochondrial and nuclear PHB complexes such as cellular differentiation, anti-proliferation, morphogenesis and maintenance of the functional integrity of the mitochondria. However, to date, the regulation of PHB expression patterns and GC physiological functions are not completely understood.

The potential function of prohibitin during spermatogenesis in Chinese fire-bellied newt Cynops orientalis

Prohibitin proteins are multifunctional proteins located mainly at the inner membrane of mitochondria expressed in universal species. They play a vital role in mitochondria's function, cell proteolysis, senescence, apoptosis and as a substrate for ubiquitination. In this study, we used PCR cloning, protein and nucleotide acids alignment, protein structure prediction, western blot, in situ hybridization and immunofluorescence to study the characteristics of the prohibitin gene and the potential role of prohibitin in spermatogenesis and spermiogenesis processes in the Chinese fire-bellied newt Cynops orientalis. First, we cloned a 1452-bp full-length cDNA from the testis of Cynops orientalis. Second, we found that the 272 amino acids of prohibitin have a SPFH family domain. Thirdly, the western blots showed high expression of prohibitin in testis while the protein size was approximately 32 kDa. Fourthly, the results of in situ hybridization and immunofluorescence experiments showed that most of the prohibitins travelled with the mitochondria's migration in Cynops orientalis. The quantities of mRNA decreased as spermiogenesis proceeded, although the signals of prohibitins existed during the whole period of spermatogenesis and spermiogenesis. In the mature germ cells, the signals of prohibitins were weak and aggregated at the end of the cell. Finally, we discovered that the Sertoli cells had a large quantity of prohibitins and we made several assumptions of prohibitins' potential roles in those cells. This is the first time that the relationship between mitochondria and prohibitin in different stages of the sperm cells in Cynops orientalis has been examined, which also revealed that Sertoli cells have abundant prohibitins.

Identification of prohibitin 1 as a potential prognostic biomarker in human pancreatic carcinoma using modified aqueous two-phase partition system combined with 2D-MALDI-TOF-TOF-MS/MS

Membrane proteins are an important source of potential targets for anticancer drugs or biomarkers for early diagnosis. In this study, we used a modified aqueous two-phase partition system combined with two-dimensional (2D) matrix-assisted laser desorption ionization (MALDI) time of flight (TOF) mass spectrometry (MS, 2D-MALDI-TOF-TOF-MS/MS) analysis to isolate and identify membrane proteins in PANC-1 pancreatic cancer cells. Using this method, we identified 55 proteins, of which 31 (56.4 %) were membrane proteins, which, according to gene ontology annotation, are associated with various cellular processes including cell signal transduction, differentiation, and apoptosis. Immunohistochemical analysis showed that the expression level of one of the identified mitochondria membrane proteins, prohibitin 1 (PHB1), is correlated with pancreatic carcinoma differentiation; PHB1 is expressed at a higher level in normal pancreatic tissue than in well-differentiated carcinoma tissue. Further studies showed that PHB1 plays a proapoptotic role in human pancreatic cancer cells, which suggests that PHB1 has antitumorigenic properties. In conclusion, we have provided a modified method for isolating and identifying membrane proteins and demonstrated that PHB1 may be a promising biomarker for early diagnosis and therapy of pancreatic (and potentially other) cancers.

Raf-interactome in tuning the complexity and diversity of Raf function

Raf kinases have been intensely studied subsequent to their discovery 30 years ago. The Ras-Raf-mitogen-activated protein kinase/extracellular signal-regulated kinase kinase-extracellular signal-regulated kinase/mitogen-activated protein kinase (Ras-Raf-MEK-ERK/MAPK) signaling pathway is at the heart of the signaling networks that control many fundamental cellular processes and Raf kinases takes centre stage in the MAPK pathway, which is now appreciated to be one of the most common sources of the oncogenic mutations in cancer. The dependency of tumors on this pathway has been clearly demonstrated by targeting its key nodes; however, blockade of the central components of the MAPK pathway may have some unexpected side effects. Over recent years, the Raf-interactome or Raf-interacting proteins have emerged as promising targets for protein-directed cancer therapy. This review focuses on the diversity of Raf-interacting proteins and discusses the mechanisms by which these proteins regulate Raf function, as well as the implications of targeting Raf-interacting proteins in the treatment of human cancer.

Cloning and expression analysis of prohibitin mRNA in canine mammary tumors

Prohibitin is an antiproliferative protein that is a product of a putative tumor suppressor gene. However, there is little information on prohibitins in companion animals. In this study, we cloned canine prohibitin mRNA using RT-PCR and 3′-RACE (Rapid Amplification of cDNA Ends). The sequence was well conserved compared with those of other mammals, including human. The deduced amino acid sequence translated from the open reading frame completely corresponded to the human sequence. Canine prohibitin mRNA was expressed in all normal mammary and tumor samples examined. These results suggest that this protein plays a vital role in cell growth mechanisms and may be related to the occurrence of canine mammary tumors.

Prohibitins role in cellular survival through Ras-Raf-MEK-ERK pathway

Prohibitins are members of a highly conserved protein family containing the stomatin/prohibitin/flotillin/HflK/C (SPFH) domain (also known as the prohibitin [PHB] domain) found in unicellular eukaryotes, fungi, plants, animals, and humans. Two highly homologous members of prohibitins expressed in eukaryotes are prohibitin (PHB; B-cell receptor associated protein-32, BAP-32) and prohibitin 2/repressor of estrogen receptor activity (PHB2, REA, BAP-37). Both PHB and REA/PHB2 are ubiquitously expressed and are present in multiple cellular compartments including the mitochondria, nucleus, and the plasma membrane. Multiple functions have been attributed to the mitochondrial and nuclear PHB and PHB2/REA including cellular differentiation, anti-proliferation, and morphogenesis. One of the major functions of the prohibitins are in maintaining the functional integrity of the mitochondria and protecting cells from various stresses. In the present review, we focus on the recent research developments indicating that PHB and PHB2/REA are involved in maintaining cellular survival through the Ras-Raf-MEK-Erk pathway. Understanding the molecular mechanisms by which the intracellular signaling pathways utilize prohibitins in governing cellular survival is likely to result in development of therapeutic strategies to overcome various human pathological disorders such as diabetes, obesity, neurological diseases, inflammatory bowel disease, and cancer.

The virulence polysaccharide Vi released by Salmonella Typhi targets membrane prohibitin to inhibit T-cell activation

T cells are critical to immunity against pathogenic Salmonella including Salmonella Typhi which causes systemic infection, typhoid, in humans. The strategies that this pathogen employs to keep T-cell mediated immune responses in check during establishment of systemic infection are not completely understood. Here, we show that the virulence polysaccharide Vi, which distinguishes S. Typhi from localized gastroenteritis-producing nontyphoidal Salmonella serovars, is a potent inhibitor of T-cell activation. Vi released by S. Typhi interacts with the membrane prohibitin complex and inhibits IL-2 secretion from T cells stimulated through the T-cell receptor (TCR) but does not affect PMA-activated interleukin 2 (IL-2) secretion. Treatment with Vi suppresses early activation events including TCR down-regulation, actin polymerization, and phosphorylation of ERK. Coadministration of Vi with anti-CD3 Ab reduces secretion of IL-2 and interferon γ in mice. Our findings reveal a mechanism by which S. Typhi may target T-cell immunity during establishment of typhoid.

Identification of prohibitin and prohibiton as novel factors binding to the p53 induced gene 3 (PIG3) promoter (TGYCC)(15) motif

The promoter of p53 induced gene 3 (PIG3) contains a variable number of tandem repeats (VNTRs) of pentanucleotides (TGYCC)n that is known as a p53 binding site. In this study, we investigated whether other potential molecules could bind to this PIG3 promoter (TGYCC)n motif. Ligand-chromatography combined with liquid chromatography-tandem mass spectrometry analyses indicated direct interactions of prohibitin and/or prohibiton with the (TGYCC)15 motif, which was confirmed by electrophoretic mobility shift assay and super-gel shift analysis with anti-prohibitin and anti-prohibiton antibodies. Using the chromatin immunopercipipation assay, we further demonstrated that prohibitin and prohibiton associated with the (TGYCC)15 motif in vivo regardless of the p53 status and apoptotic stress. We also found that prohibitin and prohibiton up-regulated PIG3 transcription independent of p53, although p53 obviously enhanced this process, and that the knock-down of prohibitin and prohibiton inhibited camptothecin-induced apoptosis. Taken together, our findings suggest that prohibitin and prohibiton contribute to PIG3-mediated apoptosis by binding to the PIG3 promoter (TGYCC)15 motif.

Proteomic changes during B cell maturation: 2D-DIGE approach

B cells play a pivotal role in adaptive immune system, since they maintain a delicate balance between recognition and clearance of foreign pathogens and tolerance to self. During maturation, B cells progress through a series of developmental stages defined by specific phenotypic surface markers and the rearrangement and expression of immunoglobulin (Ig) genes. To get insight into B cell proteome during the maturation pathway, we studied differential protein expression in eight human cell lines, which cover four distinctive developmental stages; early pre-B, pre-B, plasma cell and immature B cell upon anti-IgM stimulation. Our two-dimensional differential gel electrophoresis (2D-DIGE) and mass spectrometry based proteomic study indicates the involvement of large number of proteins with various functions. Notably, proteins related to cytoskeleton were relatively highly expressed in early pre-B and pre-B cells, whereas plasma cell proteome contained endoplasmic reticulum and Golgi system proteins. Our long time series analysis in anti-IgM stimulated Ramos B cells revealed the dynamic regulation of cytoskeleton organization, gene expression and metabolic pathways, among others. The findings are related to cellular processes in B cells and are discussed in relation to experimental information for the proteins and pathways they are involved in. Representative 2D-DIGE maps of different B cell maturation stages are available online at http://structure.bmc.lu.se/BcellProteome/.

Loss of prohibitin induces mitochondrial damages altering β-cell function and survival and is responsible for gradual diabetes development

Prohibitins are highly conserved proteins mainly implicated in the maintenance of mitochondrial function and architecture. Their dysfunctions are associated with aging, cancer, obesity, and inflammation. However, their possible role in pancreatic β-cells remains unknown. The current study documents the expression of prohibitins in human and rodent islets and their key role for β-cell function and survival. Ablation of Phb2 in mouse β-cells sequentially resulted in impairment of mitochondrial function and insulin secretion, loss of β-cells, progressive alteration of glucose homeostasis, and, ultimately, severe diabetes. Remarkably, these events progressed over a 3-week period of time after weaning. Defective insulin supply in β-Phb2(-/-) mice was contributed by both β-cell dysfunction and apoptosis, temporarily compensated by increased β-cell proliferation. At the molecular level, we observed that deletion of Phb2 caused mitochondrial abnormalities, including reduction of mitochondrial DNA copy number and respiratory chain complex IV levels, altered mitochondrial activity, cleavage of L-optic atrophy 1, and mitochondrial fragmentation. Overall, our data demonstrate that Phb2 is essential for metabolic activation of mitochondria and, as a consequence, for function and survival of β-cells.

Differential expression and subcellular localization of Prohibitin 1 are related to tumorigenesis and progression of non-small cell lung cancer

Lung cancer remains the leading cause of cancer-related deaths worldwide and non-small cell lung cancer (NSCLC) accounts for approximately 85% of all lung cancer. With a variety of biological functions, Prohibitin1 (PHB1) has been proved tumor-associated. But there are conflicting data regarding the involvement of PHB1 in tumorigenesis and few studies regarding the role of PHB1 in lung cancer. The studies reported herein used a combination of clinical observations and molecular methods to investigate the possible role of PHB1 in NSCLC tissues and cell lines. PHB1 expression was evaluated by RT-PCR, RT-qPCR, Western blotting and immunohistochemistry analysis. Flow cytometric analysis was used to determine the surface expression profiles of PHB1 in lung cell lines. The results showed that PHB1 expression were generally increased in lung cancer tissues when compared with matched noncancerous tissues and closely related with tumor differentiation and lymph node invasion. PHB1 expression levels was also increased in three lung cancer cell lines (SK-MES-1, NCI-H157 and NCI-H292) as compared with BEAS-2B cells. Moreover, there were various subcellular localization of PHB1 in different lung cancer cells and the presence of PHB1 on the surface of lung cancer cells was significantly reduced. In conclusion, PHB1 expression is increased in NSCLC and the up-regulation of PHB1 is associated with clinically aggressive phenotype. The different subcellular localization of PHB1 in NSCLC cells and the loss of the membrane-associated PHB1 probably related to the tumorigenesis and progression of NSCLC and suggests that PHB1 may play different roles in various types of NSCLC.

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.

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

The protein prohibitin (PHB) is implicated in diverse cellular processes, including cell signaling, transcriptional control, and mitochondrial function. We found that PHB was abundant in the intracellular granules of mast cells, which are critical for allergic responses to antigens. Thus, we investigated whether PHB played a role in signaling mediated by the high-affinity receptor for antigen-bound immunoglobulin E (IgE), FcεRI. PHB-specific small interfering RNAs (siRNAs) inhibited antigen-mediated signaling, degranulation, and cytokine secretion by mast cells in vitro. Knockdown of PHB inhibited the antigen-dependent association of the tyrosine kinase Syk with FcεRI and inhibited the activation of Syk. Fractionation studies revealed that PHB translocated from intracellular granules to plasma membrane lipid rafts in response to antigen, and knockdown of PHB suppressed the movement of FcεRIγ and Syk into lipid rafts. Tyrosine phosphorylation of PHB by Lyn was observed early after exposure to antigen, and point mutations in PHB indicated that Tyr(114) and Tyr(259) were required for the recruitment of Syk to FcεRIγ and mast cell activation. In mice, PHB-specific siRNAs inhibited antigen-initiated mast cell degranulation, passive cutaneous anaphylaxis, and passive systemic anaphylaxis. Together, these results suggest that PHB is essential for FcεRI-mediated mast cell activation and allergic responses in vivo, raising the possibility that PHB might serve as a therapeutic target for the treatment of allergic diseases.