Cell cycle activity and expression of prohibitin mRNA

Subcellular distribution of prohibitin 1 in rat liver during liver regeneration and its cellular implication

BACKGROUND

The function of prohibitin 1 (Phb1) during liver regeneration (LR) remains relatively unexplored. Our previous research identified downregulation of Phb1 in rat liver mitochondria 24 h after 70% partial hepatectomy (PHx), as determined by subcellular proteomic analysis.

AIM

To investigate the potential role of Phb1 during LR.

METHODS

We examined changes in Phb1 mRNA and protein levels, subcellular distribution, and abundance in rat liver during LR following 70% PHx. We also evaluated mitochondrial changes and apoptosis using electron microscopy and flow cytometry. RNA-interference-mediated knockdown of Phb1 (PHBi) was performed in BRL-3A cells.

RESULTS

Compared with sham-operation control groups, Phb1 mRNA and protein levels in 70% PHx test groups were downregulated at 24 h, then upregulated at 72 and 168 h. Phb1 was mainly located in mitochondria, showed a reduced abundance at 24 h, significantly increased at 72 h, and almost recovered to normal at 168 h. Phb1 was also present in nuclei, with continuous increase in abundance observed 72 and 168 h after 70% PHx. The altered ultrastructure and reduced mass of mitochondria during LR had almost completely recovered to normal at 168 h. PHBi in BRL-3A cells resulted in increased S-phase entry, a higher number of apoptotic cells, and disruption of mitochondrial membrane potential.

CONCLUSION

Phb1 may contribute to maintaining mitochondrial stability and could play a role in regulating cell proliferation and apoptosis of rat liver cells during LR.

Molecular characterization of prohibitins and their differential responses to WSSV infection in hemocyte subpopulations of Fenneropenaeus chinensis

In our previous work, prohibitin1 (PHB1) was identified to be only expressed in granulocytes of Fenneropenaeus chinensis. In order to elucidate the potential immunological properties of prohibitins in hemocyte subpopulations, in this paper, the full-length cDNAs of PHB1 and PHB2 were firstly cloned from F. chinensis using rapid amplification of cDNA ends approach, and they were designated FcPHB1 and FcPHB2, respectively. Based on the sequence analysis and multiple sequence alignment, FcPHB1 and FcPHB2 were members of SPFH protein family. By quantitative real-time RT-PCR, the higher mRNA transcription levels of FcPHB1 and FcPHB2 were detected in intestine and hemocytes of F. chinensis, and these two genes in hemocytes were significantly up-regulated upon WSSV infection. The FcPHB1 and FcPHB2 were recombinantly expressed in Escherichia coli BL21 (DE3), and employed as immunogens to produce the polyclonal antibodies (PAbs) in rabbits. Indirect immunofluorescence assay (IFA) revealed that the FcPHB1 and FcPHB2 were located both in the cytoplasm and nuclei of hemocytes, which could also be specifically recognized by the PAbs against FcPHB1 or FcPHB2 in Western blot. Interestingly, it was found that FcPHB1 and FcPHB2 were only expressed in the granulocytes of heathy shrimp and highly expressed in the WSSV-infected granulocytes, however only weak expressions of FcPHB1 and FcPHB2 were observed in the hyalinocytes of WSSV-infected shrimp. Meanwhile, silencing of FcPHB1 and FcPHB2 genes were performed by small interfering RNA, and the results showed that the WSSV copies in hemocytes were increased by knockdown of either FcPHB1 or FcPHB2, and the cumulative mortalities of shrimp in the silenced groups were also markedly increased. These results demonstrated that FcPHB1 and FcPHB2 played important roles in anti-WSSV infection, and their differential expression characteristics in hemocyte subpopulations provided a further understanding of the immune functions of granulocytes and hyalinocytes.

Involvement of miR-27a-3p in diabetic nephropathy via affecting renal fibrosis, mitochondrial dysfunction, and endoplasmic reticulum stress

Diabetic nephropathy (DN) is acknowledged as a serious chronic complication of diabetes mellitus. Nevertheless, its pathogenesis is complicated and unclear. Thus, in this study, the role of miR-27a-3p-prohibitin/TMBIM6 signaling axis in the progression of DN was elucidated. Type 2 diabetic db/db mice and high glucose (HG)-challenged HK-2 cells were used as in vivo and in vitro models. Our results showed that miR-27a-3p was upregulated and prohibitin or transmembrane BAX inhibitor motif containing 6 (TMBIM6) was downregulated in the kidney tissues of db/db mice and HG-treated HK-2 cells. Silencing miR-27a-3p enhanced the expression of prohibitin and TMBIM6 in the kidney tissues and HK-2 cells. Inhibition of miR-27a-3p improved functional injury, as evidenced by decreased blood glucose, urinary albumin, serum creatinine, and blood urea nitrogen levels. MiR-27a-3p silencing ameliorated renal fibrosis, reflected by reduced profibrogenic genes (e.g., transforming growth factor β1, fibronectin, collagen I and III, and α-smooth muscle actin). Furthermore, inhibition of miR-27a-3p relieved mitochondrial dysfunction in the kidney of db/db mice, including upregulation of mitochondrial membrane potential, complex I and III activities, adenosine triphosphate, and mitochondrial cytochrome C, as well as suppressing reactive oxygen species production. In addition, miR-27a-3p silencing attenuated endoplasmic reticulum (ER) stress, reflected by reduced expression of p-IRE1α, p-eIF2α, XBP1s, and CHOP. Mechanically, we identified prohibitin and TMBIM6 as direct targets of miR-27a-3p. Inhibition of miR-27a-3p protected HG-treated HK-2 cells from apoptosis, extracellular matrix accumulation, mitochondrial dysfunction, and ER stress by regulating prohibitin or TMBIM6. Taken together, we reveal that miR-27a-3p-prohibitin/TMBIM6 signaling axis regulates the progression of DN, which can be a potential therapeutic target.

Regulatory effects of Prohibitin 1 on proliferation and apoptosis of pulmonary arterial smooth muscle cells in monocrotaline-induced PAH rats

BACKGROUND

Pulmonary arterial hypertension (PAH) is a severe pulmonary vascular disease characterized by unbalanced proliferation and apoptosis of pulmonary arterial smooth muscle cells (PASMCs). Prohibitin 1 (PHB1) is known for its significant anti-proliferative activity. However, the role of PHB1 in PASMCs and PAH have not been elucidated.

METHODS

Monocrotaline (MCT 60 mg/kg) was used to build a PAH model in SD rats. Right ventricular systolic pressure (RVSP) and right ventricle (RV) hypertrophy were measured. Morphology of pulmonary vessels was observed by Hematoxylin-Eosin (HE) staining. Expression of PHB1 in pulmonary arteries and PASMCs was determinated by immunoblot and immunofluorescence. Cell proliferation was detected by CCK8 and EDU when PASMCs were stimulated by PDGF-BB (20 ng/mL). Furthermore, siRNA for PHB1 and Akt inhibitor were conducted to investigate the mechanism behind the role of PHB1 and AKT signaling pathway in PASMCs proliferation and apoptosis.

RESULTS

The protein expression of PHB1 in PAH rats lung tissue was significantly up-regulated accompanied by elevated RVSP and enhanced RV hypertrophy. Immunohistochemistry showed that PHB1 was mainly localized in the pulmonary vascular smooth muscle layer. PDGF-BB significantly up-regulated the expression of PHB1 in rat primary PASMCs in a time- and dose-dependent manner. After PHB1 knock down, PASMCs proliferation was significantly suppressed while apoptosis was significantly recovered. Meanwhile the level of proliferating cell nuclear antigen (PCNA) and P-Akt were significantly down-regulated. Perifosine (Akt inhibitor) also significantly inhibit proliferation of PASMCs.

CONCLUSION

PHB1 contributes to pulmonary vascular remodeling by accelerating proliferation of PASMCs which involves AKT phosphorylation.

Expression pattern of prohibitin, capping actin protein of muscle Z-line beta subunit and tektin-2 gene in Murrah buffalo sperm and its relationship with sperm motility

Objective

The aim of the current study is to investigate the relationship between prohibitin (PHB), capping actin protein of muscle Z-line beta subunit (CAPZB), and tektin-2 (TEKT2) and sperm motility in Murrah buffalo.

Methods

We collected the high-motility and low-motility semen samples, testis, ovary, muscle, kidney, liver, brain and pituitary from Murrah buffalo, and analysed the expression of PHB, CAPZB, and TEKT2 in mRNA (message RNA) and protein level.

Results

Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) result showed that the expression of PHB was higher and CAPZB, TEKT2 were specifically expressed in testis as compared to the other 6 tissues, and that in testis, the expression of TEKT2 was higher than that of CAPZB and PHB. Immunohistochemistry test revealed that all three genes were located on the convoluted seminiferous tubule and enriched in spermatogenic cells. Both qRT-PCR and Western Blot results showed that the expression levels of PHB, CAPZB, and TEKT2 were significantly lower in the low-motility semen group compared to the high-motility semen group (p<0.05).

Conclusion

The expression levels of PHB, CAPZB, and TEKT2 in Murrah buffalo sperm have a high positive correlation with sperm motility. And the three genes may be potential molecular markers for the decline of buffalo sperm motility.

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.

The prohibitin-repressive interaction with E2F1 is rapidly inhibited by androgen signalling in prostate cancer cells

Prohibitin (PHB) is a tumour suppressor molecule with pleiotropic activities across several cellular compartments including mitochondria, cell membrane and the nucleus. PHB and the steroid-activated androgen receptor (AR) have an interplay where AR downregulates PHB, and PHB represses AR. Additionally, their cellular locations and chromatin interactions are in dynamic opposition. We investigated the mechanisms of cell cycle inhibition by PHB and how this is modulated by AR in prostate cancer. Using a prostate cancer cell line overexpressing PHB, we analysed the gene expression changes associated with PHB-mediated cell cycle arrest. Over 1000 gene expression changes were found to be significant and gene ontology analysis confirmed PHB-mediated repression of genes essential for DNA replication and synthesis, for example, MCMs and TK1, via an E2F1 regulated pathway—agreeing with its G1/S cell cycle arrest activity. PHB is known to inhibit E2F1-mediated transcription, and the PHB:E2F1 interaction was seen in LNCaP nuclear extracts, which was then reduced by androgen treatment. Upon two-dimensional western blot analysis, the PHB protein itself showed androgen-mediated charge differentiation (only in AR-positive cells), indicating a potential dephosphorylation event. Kinexus phosphoprotein array analysis indicated that Src kinase was the main interacting intracellular signalling hub in androgen-treated LNCaP cells, and that Src inhibition could reduce this AR-mediated charge differentiation. PHB charge change may be associated with rapid dissociation from chromatin and E2F1, allowing the cell cycle to proceed. The AR and androgens may deactivate the repressive functions of PHB upon E2F1 leading to cell cycle progression, and indicates a role for AR in DNA replication licensing.

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.

Plants grow with a little help from their organelle friends

Chloroplasts and mitochondria are indispensable for plant development. They not only provide energy and carbon sources to cells, but also have evolved to become major players in a variety of processes such as amino acid metabolism, hormone biosynthesis and cellular signalling. As semi-autonomous organelles, they contain a small genome that relies largely on nuclear factors for its maintenance and expression. An intensive crosstalk between the nucleus and the organelles is therefore essential to ensure proper functioning, and the nuclear genes encoding organellar proteins involved in photosynthesis and oxidative phosphorylation are obviously crucial for plant growth. Organ growth is determined by two main cellular processes: cell proliferation and cell expansion. Here, we review how plant growth is affected in mutants of organellar proteins that are differentially expressed during leaf and root development. Our findings indicate a clear role for organellar proteins in plant organ growth, primarily during cell proliferation. However, to date, the role of the nuclear-encoded organellar proteins in the cellular processes driving organ growth has not been investigated in much detail. We therefore encourage researchers to extend their phenotypic characterization beyond macroscopic features in order to get a better view on how chloroplasts and mitochondria regulate the basic processes of cell proliferation and cell expansion, essential to driving growth.

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.

Nuclear cytoplasmic trafficking of proteins is a major response of human fibroblasts to oxidative stress

We have used a subcellular spatial razor approach based on LC-MS/MS-based proteomics with SILAC isotope labeling to determine changes in protein abundances in the nuclear and cytoplasmic compartments of human IMR90 fibroblasts subjected to mild oxidative stress. We show that response to mild tert-butyl hydrogen peroxide treatment includes redistribution between the nucleus and cytoplasm of numerous proteins not previously associated with oxidative stress. The 121 proteins with the most significant changes encompass proteins with known functions in a wide variety of subcellular locations and of cellular functional processes (transcription, signal transduction, autophagy, iron metabolism, TCA cycle, ATP synthesis) and are consistent with functional networks that are spatially dispersed across the cell. Both nuclear respiratory factor 2 and the proline regulatory axis appear to contribute to the cellular metabolic response. Proteins involved in iron metabolism or with iron/heme as a cofactor as well as mitochondrial proteins are prominent in the response. Evidence suggesting that nuclear import/export and vesicle-mediated protein transport contribute to the cellular response was obtained. We suggest that measurements of global changes in total cellular protein abundances need to be complemented with measurements of the dynamic subcellular spatial redistribution of proteins to obtain comprehensive pictures of cellular function.

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, cellular, and network analyses reveal new DUSP3 interactions with nucleolar proteins in HeLa cells

DUSP3 (or Vaccinia virus phosphatase VH1-related; VHR) is a small dual-specificity phosphatase known to dephosphorylate c-Jun N-terminal kinases and extracellular signal-regulated kinases. In human cervical cancer cells, DUSP3 is overexpressed, localizes preferentially to the nucleus, and plays a key role in cellular proliferation and senescence triggering. Other DUSP3 functions are still unknown, as illustrated by recent and unpublished results from our group showing that this enzyme mediates DNA damage response or repair processes. In this study, we sought to identify new interactions between DUSP3 and proteins directly or indirectly involved in or correlated with its biological roles in HeLa cells exposed to gamma or UV radiation. By using GST-DUSP as bait, we pulled down interacting proteins and identified them by LC-MS/MS. Of the 46 proteins obtained, six hits were extensively validated by immune techniques; the proteins Nucleophosmin, HnRNP C1/C2, and Nucleolin were the most promising targets found to directly interact with DUSP3. We then analyzed the DUSP3 interactomes using physical protein-protein interaction networks using our hits as the seed list. The validated hits as well as unvalidated hits fluctuated on the DUSP3 interactomes of HeLa cells, independent of the time post radiation, which confirmed our proteomic and experimental data and clearly showed the proximity of DUSP3 to proteins involved in processes intimately related to DNA repair and senescence, such as Ku70 and Tert, via interactions with nucleolar proteins, which were identified in this study, that regulate DNA/RNA structure and functions.

Follicular stage-dependent regulation of apoptosis and steroidogenesis by prohibitin in rat granulosa cells

BACKGROUND

Follicular growth and atresia are tightly regulated processes, which involve the participation of endocrine, autocrine and paracrine factors at the cellular level. Prohibitin (PHB) is a multifunctional intracellular protein playing an important role in the regulation of proliferation, apoptosis and differentiation. Here we examined the expression of PHB and its regulation by FSH in vitro and studied the role of PHB in the regulation of apoptosis and steroidogenesis in response to the apoptosis inducer staurosporine (STS) and to FSH, respectively.

METHODS

Undifferentiated and differentiated granulosa cells were collected from diethylstilbestrol (DES)- and equine chronic gonadotropin (eCG)-primed immature rats, respectively and then cultured with various treatments (FSH, adenovirus infection, STS) according to experimental design. The apoptosis rate, the production of estradiol and progesterone, and the expression of distinct proteins (PHB, caspase-3, phospho- and total Akt) were assessed.

RESULTS

PHB is anti-apoptotic and its action is dependent on the differentiated state of the granulosa cells. Data from gain- and loss-of-function experiments demonstrate that PHB inhibited STS-induced caspase-3 cleavage and apoptosis in undifferentiated granulosa cells, but was ineffective in differentiated cells. In contrast, PHB suppresses FSH-induced steroidogenesis and this response is evident irrespective of the differentiated state of granulosa cells.

CONCLUSION

These findings suggest that PHB regulates granulosa cell apoptosis and steroidogenesis in a follicular stage-dependent manner and that the dysregulation of PHB expression and action may be relevant to ovarian dysfunction.

Characterization of Cherax quadricarinatus prohibitin and its potential role in spermatogenesis

Prohibitin (PHB) proteins have diverse functions, such as cellular signaling, transcriptional control and mitochondrial biogenesis. In this study, we characterized PHB gene and its protein expression in Cherax quadricarinatus. PHB cDNA comprises 1472 nucleotides with an open reading frame of 828bp, which encodes 275 amino acid residues. The highest transcript levels were found during the spermatogonial developmental phase, with the lowest levels detected during the resting phase in the reproductive cycle. Western blot analysis revealed that PHB is an approximately 30kDa protein, and occurs in a number of unexpected isoforms, ranging from 30kDa to greater than 180kDa in the testes of different developmental phases, which may be the ubiquitinated substrates. The strongest immunolabeling signal was found in spermatogonia, with lower levels of staining in secondary spermatocytes, and weak or absent expression in mature sperm. Immunogold electron microscopy results confirmed the localization of PHB in the inner mitochondrial membranes. The results showed that PHB is a substrate protein for spermatogenesis, with a potential reproductive function involving sperm ubiquitination in invertebrates.

Inhibitory roles of prohibitin and chemerin in FSH-induced rat granulosa cell steroidogenesis

Follicular differentiation is a tightly regulated process involving various endocrine, autocrine, and paracrine factors. The biosynthesis of progesterone and estradiol in response to FSH involves the regulation of multiple steroidogenic enzymes, such as p450 cholesterol side-chain cleavage enzyme and aromatase. Here we demonstrated that prohibitin (PHB), a multifunctional protein, inhibits FSH-induced progesterone and estradiol secretion in rat granulosa cells. The mRNA abundances of cyp11a (coding p450 cholesterol side-chain cleavage enzyme) and cyp19 (coding aromatase) were also suppressed by PHB in a time-dependent manner. It is known that a novel adipokine chemerin suppresses FSH-induced steroidogenesis in granulosa cells. Chemerin up-regulates the content of PHB, and PHB knockdown attenuates the suppressive role of chemerin on steroidogenesis. In addition, inhibition of phosphatidylinositol 3-kinase/Akt pathway enhances the suppressive action of PHB, whereas expression of constitutively active Akt attenuates this response. These findings suggest that PHB is a novel negative regulator of FSH-induced steroidogenesis, and its action with chemerin may contribute to the dysregulation of steroidogenesis in the pathogenesis of polycystic ovarian syndrome.

Signaling pathways of prohibitin and its role in diseases

Prohibitin (PHB), appearing to be a negative regulator of cell proliferation and to be a tumor suppressor, has been connected to diverse cellular functions including cell cycle control, senescence, apoptosis and the regulation of mitochondrial activities. It is a growth regulatory gene that has pleiotropic functions in the nucleus, mitochondria and cytoplasmic compartments. However, in different tissues/cells, the expression of PHB was different, such as that it was increased in most of the cancers, but its expression was reduced in kidney diseases. Signaling pathways might be very important in the pathogenesis of diseases. This review was performed to provide a relatively complete signaling pathways flowchart for PHB to the investigators who were interested in the roles of PHB in the pathogenesis of diseases. Here, we review the signal transduction pathways of PHB and its role in the pathogenesis of diseases.

Lipid rafts association and anti-apoptotic function of prohibitin in ultraviolet B light-irradiated HaCaT keratinocytes

Upon UVB irradiation, an alternation of major lipid raft components can lead to the recruitment/activation of rafts-associated proteins and initiation of downstream apoptotic signalling pathways. We used two-dimensional gel electrophoresis (2-DE) to identify potential regulators of UVB-induced apoptosis and mass spectrometry fingerprint analysis to identify proteins that are altered in the rafts after UVB irradiation. Our data show that levels of several proteins, including prohibitin (PHB), were changed in lipid rafts after UVB irradiation. We also demonstrate that while total PHB expression was not changed, the protein was enriched in lipid rafts after UVB irradiation. Reduced expression of PHB using siRNA knockdown resulted in an increase in cellular apoptosis after UVB irradiation. Based on these results, we propose that PHB protects keratinocytes from UVB-induced apoptosis.

Association of PHB 1630 C>T and MTHFR 677 C>T polymorphisms with breast and ovarian cancer risk in BRCA1/2 mutation carriers: results from a multicenter study

BACKGROUND

The variable penetrance of breast cancer in BRCA1/2 mutation carriers suggests that other genetic or environmental factors modify breast cancer risk. Two genes of special interest are prohibitin (PHB) and methylene-tetrahydrofolate reductase (MTHFR), both of which are important either directly or indirectly in maintaining genomic integrity.

METHODS

To evaluate the potential role of genetic variants within PHB and MTHFR in breast and ovarian cancer risk, 4102 BRCA1 and 2093 BRCA2 mutation carriers, and 6211 BRCA1 and 2902 BRCA2 carriers from the Consortium of Investigators of Modifiers of BRCA1 and BRCA2 (CIMBA) were genotyped for the PHB 1630 C>T (rs6917) polymorphism and the MTHFR 677 C>T (rs1801133) polymorphism, respectively.

RESULTS

There was no evidence of association between the PHB 1630 C>T and MTHFR 677 C>T polymorphisms with either disease for BRCA1 or BRCA2 mutation carriers when breast and ovarian cancer associations were evaluated separately. Analysis that evaluated associations for breast and ovarian cancer simultaneously showed some evidence that BRCA1 mutation carriers who had the rare homozygote genotype (TT) of the PHB 1630 C>T polymorphism were at increased risk of both breast and ovarian cancer (HR 1.50, 95%CI 1.10-2.04 and HR 2.16, 95%CI 1.24-3.76, respectively). However, there was no evidence of association under a multiplicative model for the effect of each minor allele.

CONCLUSION

The PHB 1630TT genotype may modify breast and ovarian cancer risks in BRCA1 mutation carriers. This association need to be evaluated in larger series of BRCA1 mutation carriers.

Characterization of a gene encoding prohibitin in silkworm, Bombyx mori

BACKGROUND

Prohibitin (PHB) is an evolutionarily conserved multifunctional protein with ubiquitous expression. However, its molecular roles are largely unknown.

METHODS

To better understand the function of prohibitin protein in silkworm (BmPHB), its coding sequence was isolated from a cDNA library of silkworm pupae. An His-tagged BmPHB fusion protein was expressed in Escherichia coli Rosetta (DE3) and purified with affinity and reversed-phase chromatography. Purified rBmPHB was used to generate anti-BmPHB polyclonal antibody. The subcellular localization of BmPHB was analysed by immunohistochemistry.

RESULTS

BmPHB gene has an ORF of 825 bp, encoding a predicted peptide with 274 amino acid residues. Immunostaining indicate that prohibitin is expressed in nucleus and predominately in cytoplasm. Western blot analyses indicated that, in the fifth instar larva, BmPHB was expressed descendingly in gonad, malpighian tubule, trachea, fatty body, intestine, and head. However, no expression was detected in larva's silk gland and epidermis. In addition, BmPHB was expressed in the nascent egg, larva and pupa, but not in the moth.

CONCLUSIONS

The expression of BmPHB gene presents differential characteristic in different stage and tissues. It may play important roles in the development of silkworm.

GENERAL SIGNIFICANCE

Studies on prohibitin have been still restricted to a few specific insects and insect cell lines such as Drosophila, Acyrthosiphon pisum and mosquito cell lines, not yet in silkworm. This is a first characterization of prohibitin in silkworm, B. mori.

Prohibitin physically interacts with MCM proteins and inhibits mammalian DNA replication

Prohibitin, a tumor suppressor protein, has been shown to repress E2F-mediated transcription and arrest cell cycle progression. while prohibitin has been proposed to regulate cell cycle progression by repressing transcriptional targets of E2F1, it is not clear whether other mechanisms are also involved in mediating the growth arrest. Here we demonstrate that prohibitin can function as a potent inhibitor of DNA replication by interacting with members of Minichromosome maintenance complex of proteins (MCM2-7). The data presented here indicates that prohibitin can physically interact with MCM2, MCM5 and MCM7 in in vitro GST binding assays as well as in MCF-7 cells as seen by immunoprecipitation-western blot experiments. The association was cell cycle dependent, and more pronounced 4-8 hours after serum stimulation of quiescent cells. Prohibitin associated more robustly with MCM2 and MCM5 compared to MCM7, suggesting that prohibitin mainly interacts with the regulatory subunits of the MCM complex. Confirming these results, prohibitin was found to co-localize with MCM2, MCM5 and MCM7 in MCF-7 cells, as seen by double immunofluorescence experiments. Further, Prohibitin strongly inhibited DNA replication in an in vitro replication assay. These results strongly suggest that prohibitin effectively represses replication by interacting with the components of mammalian replication machinery and this might contribute to the growth regulatory properties of prohibitin.

Prohibitin silencing reverses stabilization of mitochondrial integrity and chemoresistance in ovarian cancer cells by increasing their sensitivity to apoptosis

Current approaches to the treatment of ovarian cancer are limited because of the development of resistance to chemotherapy. Prohibitin (Phb1) is a possible candidate protein that contributes to development of drug resistance, which could be targeted in neoplastic cells. Phb1 is a highly conserved protein that is associated with a block in the G0/G1 phase of the cell cycle and also with cell survival. Our study was designed to determine the role of Phb1 in regulating cellular growth and apoptosis in ovarian cancer cells. Our results showed that Phb1 content is differentially overexpressed in papillary serous ovarian carcinoma and endometrioid ovarian adenocarcinoma when compared to normal ovarian epithelium and was inversely related to Ki67 expression. Immunofluorescence microscopy and Western analyses revealed that Phb1 is primarily associated with the mitochondria in ovarian cancer cells. Over-expression of Phb1 by adenoviral Phb1 infection resulted in an increase in the percentage of ovarian cancer cells accumulating at G0/G1 phase of the cell cycle. Treatment of ovarian cancer cells with staurosporine (STS) induced apoptosis in a time-dependent manner. Phb1 over-expression induced cellular resistance to STS via the intrinsic apoptotic pathway. In contrast, silencing of Phb1 expression by adenoviral small interfering RNA (siRNA) sensitized ovarian cancer cells to STS-induce apoptosis. Taken together, these results suggest that Phb1 induces block at G0/G1 phase of the cell cycle and promotes survival of cancer cells. Furthermore, silencing of the Phb1 gene expression may prove to be a valuable therapeutic approach for chemoresistant ovarian cancer by increasing sensitivity of cancer cells to apoptosis.

Prohibitin-1 maintains the angiogenic capacity of endothelial cells by regulating mitochondrial function and senescence

Prohibitin 1 (PHB1) is a highly conserved protein that is mainly localized to the inner mitochondrial membrane and has been implicated in regulating mitochondrial function in yeast. Because mitochondria are emerging as an important regulator of vascular homeostasis, we examined PHB1 function in endothelial cells. PHB1 is highly expressed in the vascular system and knockdown of PHB1 in endothelial cells increases mitochondrial production of reactive oxygen species via inhibition of complex I, which results in cellular senescence. As a direct consequence, both Akt and Rac1 are hyperactivated, leading to cytoskeletal rearrangements and decreased endothelial cell motility, e.g., migration and tube formation. This is also reflected in an in vivo angiogenesis assay, where silencing of PHB1 blocks the formation of functional blood vessels. Collectively, our results provide evidence that PHB1 is important for mitochondrial function and prevents reactive oxygen species-induced senescence and thereby maintains the angiogenic capacity of endothelial cells.

Molecular modeling of prohibitin domains

Prohibitins comprise a family of highly conserved ubiquitous eukaryotic proteins that mainly localize to the mitochondria. They have been implicated in important cellular processes such as cellular signaling and transcriptional control, apoptosis, cellular senescence, and mitochondrial biogenesis. Using molecular modeling techniques, we have generated structural models of human prohibitins BAP32 and BAP37, which have previously been shown to exist as large ringlike oligomers in the membrane-bound state. The middle domain of prohibitins is evolutionary conserved in the family of SPFH (PHB) domain proteins. On the basis of the known structure of flotillin-2, another member of the SPFH-domain family, we have generated homology models for BAP32 and BAP37, and elucidated the implications for formation of high molecular weight oligomers. A model for the dimeric-building block of BAP32: BAP37 for such assemblies was generated and its stability scrutinized by molecular dynamics simulations. The model of BAP32 was also analyzed as to potential ligand-binding sites and the previously identified ligand melanogenin was docked into a membrane-proximal cavity. The results are discussed in the context of prohibitin interactions with mitochondrial AAA-proteases and we suggest two possible interaction interfaces between the BAP32:BAP37 building block and the protease.

Modulation of the host cell proteome by the intracellular apicomplexan parasite Toxoplasma gondii

To investigate how intracellular parasites manipulate their host cell environment at the molecular level, we undertook a quantitative proteomic study of cells following infection with the apicomplexan parasite Toxoplasma gondii. Using conventional two-dimensional electrophoresis, difference gel electrophoresis (DIGE), and mass spectrometry, we identified host proteins that were consistently modulated in expression following infection. We detected modification of protein expression in key metabolic pathways, including glycolysis, lipid and sterol metabolism, mitosis, apoptosis, and structural-protein expression, suggestive of global reprogramming of cell metabolism by the parasite. Many of the differentially expressed proteins had not been previously implicated in the response to the parasite, while others provide important corroborative protein evidence for previously proposed hypotheses of pathogen-cell interactions. Significantly, over one-third of all modulated proteins were mitochondrial, and this was further investigated by DIGE analysis of a mitochondrion-enriched preparation from infected cells. Comparison of our proteomic data with previous transcriptional studies suggested that a complex relationship exits between transcription and protein expression that may be partly explained by posttranslational modifications of proteins and revealed the importance of investigating protein changes when interpreting transcriptional data. To investigate this further, we used phosphatase treatment and DIGE to demonstrate changes in the phosphorylation states of several key proteins following infection. Overall, our findings indicate that the host cell proteome responds in a dramatic way to T. gondii invasion, in terms of both protein expression changes and protein modifications, and reveal a complex and intimate molecular relationship between host and parasite.

Differential regulation of human YY1 and caspase 7 promoters by prohibitin through E2F1 and p53 binding sites

Prohibitin is a 30 kDa growth suppressive protein that has pleiotropic functions in the cell. Although prohibitin has been demonstrated to have potent transcriptional regulatory functions, it has also been proposed to facilitate protein folding in the mitochondria and promote cell migration in association with Raf-1. Our previous studies have shown that prohibitin physically interacts with the marked-box domain of E2F family members and represses their transcriptional activity; in contrast, prohibitin could bind to and enhance the transcriptional activity of p53. Here, we show that promoters of human YY1 (Yin and Yang 1) as well as caspase 7 genes are modulated by prohibitin. YY1 promoter activity was reduced upon overexpression of prohibitin, while it was enhanced when prohibitin was depleted by small interfering RNA techniques. The repressive effects of prohibitin on the YY1 promoter were mediated through E2F binding sites, as seen by mutational analysis and chromatin immunoprecipitation assays. Further, depletion of E2F1 prevented prohibitin from repressing the YY1 promoter. In contrast with YY1, prohibitin overexpression led to enhanced levels of caspase 7, whereas depletion of prohibitin reduced it. Interestingly, the caspase 7 promoter was found to have p53-binding sites and prohibitin activated this promoter through p53. These studies show that prohibitin can have diverse effects on the expression of different genes and the activity of various cellular promoters is affected by prohibitin. Further, it appears very likely that prohibitin carries out many of its cellular functions by affecting the transcription of different genes.

Prohibitin protects against oxidative stress in intestinal epithelial cells

Prohibitin (PHB) is an evolutionarily conserved and ubiquitously expressed protein whose expression or function in intestinal diseases is not known. In this study, we examined the expression and role of PHB in oxidative stress associated with inflammatory bowel disease. Our results show that PHB primarily localizes to the mitochondria in intestinal epithelial cells. Its expression is down-regulated during active human Crohn's disease, experimental colitis in vivo, and oxidative stress in vitro. PHB overexpression increases the expression of glutathione-S-transferase pi and protects from oxidant-induced depletion of glutathione. Finally, PHB overexpression decreases accumulation of reactive oxygen metabolites, as well as increased permeability induced by oxidative stress in intestinal epithelial cells. Together, these results suggest that PHB constitutes a previously unrecognized cellular defense against oxidant injury. Thus, strategies to modulate PHB levels may constitute a novel therapeutic approach for intestinal inflammatory diseases, wherein oxidative stress plays a critical role in tissue injury and inflammation.

Prohibitin facilitates cellular senescence by recruiting specific corepressors to inhibit E2F target genes

Prohibitin is a growth regulatory gene that has pleiotropic functions in the nucleus, mitochondria, and cytoplasmic compartments. Earlier studies had proposed a role for prohibitin in modulating cellular senescence, but the underlying mechanisms remain unknown. Here we show that senescence induced by DNA-damaging agents causes the localization of prohibitin to specific heterochromatic foci. Prohibitin could bind to heterochromatin protein 1 (HP1) family proteins and colocalized with HP1gamma in senescence-associated heterochromatic foci. Further, HP1gamma could synergize with prohibitin to repress E2F1-mediated transcriptional activity. The depletion of prohibitin by small interfering RNA or antisense techniques led to a reduction in the senescent phenotype, correlating with a reduced expression of senescence-associated beta-galactosidase and fewer numbers of senescence-associated heterochromatic foci. Chromatin immunoprecipitation assays showed that prohibitin is needed for the recruitment of HP1gamma to E2F1-regulated proliferative promoters, leading to their repression. The ablation of prohibitin prevented the recruitment of HPIgamma, but not Suv39H, to the promoters upon senescence. Prohibitin-mediated recruitment of HP1gamma occurred in only senescent cells, not in quiescent cells; thus, there is a dichotomy in the recruitment of different corepressors by prohibitin, depending on the type of growth arrest. These studies show that prohibitin plays a vital role in inducing cellular senescence.

Prohibitin: a potential target for new therapeutics

Prohibitin (PHB) is localized to the mitochondria where it might have a role in the maintenance of mitochondrial function and protection against senescence. There is considerable controversy concerning the function of nuclear-localized PHB. PHB has potential roles as a tumor suppressor, an anti-proliferative protein, a regulator of cell-cycle progression and in apoptosis. PHB might also function as a cell-surface receptor for an as-yet unidentified ligand. Cell-associated PHB in the gastrointestinal tract has been implicated in protection against infection and inflammation and the induction of apoptosis in other tissues. The diverse array of functions of PHB, together with the emerging evidence that its function can be modulated specifically in certain tissues, suggest that targeting PHB would be a useful therapeutic approach for the treatment of variety of disease states, including inflammation, obesity and cancer.

A differential proteome in tumors suppressed by an adenovirus-based skin patch vaccine encoding human carcinoembryonic antigen

We created an anti-tumor vaccine by using adenovirus as a vector which contains a cytomegalovirus early promoter-directed human carcinoembryonic antigen gene (AdCMV-hCEA). In an attempt to develop the skin patch vaccine, we epicutaneously vaccinated Balb/c mice with AdCMV-hCEA. After nine weeks post-immunization, vaccinated mice evoked a robust antibody titer to CEA and demonstrated the capability of suppressing in vivo growth of implanted murine mammay adenocarioma cell line (JC-hCEA) tumor cells derived from a female Balb/c mouse. Proteomic analysis of the tumor masses in the non-vaccinated naive and vaccinated mice reveal that six proteins change their abundance in the tumor mass. The levels of adenylate kinase 1, beta-enolase, creatine kinase M chain, hemoglobin beta chain and prohibitin were statistically increased whereas the level of a creatine kinase fragment, which is undocumented, was decreased in the tumor of vaccinated mice. These proteins may provide a vital link between early-stage tumor suppression and immune response of skin patch vaccination.

Prospective highlights of functional skin proteomics

Although a wide variety of protein profiles have been extensively constructed via proteomic analysis, the comprehensive proteomic profiling of the skin, which is considered to be the largest organ of the human body, is still far from complete. Our efforts to establish the functional skin proteome, a protein database describing the protein networks that underlie biological processes, has set in motion the identification and characterization of proteins expressed in the epidermis and dermis of the BALB/c mice. In this review, we will highlight various cutaneous proteins we have characterized and discuss their biological functions associated with skin distress, immunity, and cancer. This type of research into functional skin proteomics will provide a critical step toward understanding disease and developing successful therapeutic strategies.

Regulation of Prohibitin Expression During Follicular Development and Atresia in the Mammalian Ovary1

Prohibitin is a ubiquitous and highly conserved protein implicated as an important regulator in cell survival. Prohibitin content is inversely associated with cell proliferation, but it increases during granulosa cell differentiation as well as in earlier events of apoptosis in a temperature-sensitive granulosa cell line. In the present study, we have characterized the spatial expression patterns for prohibitin using established in vivo models for the induction of follicular development and atresia in the mammalian ovary. Comparative Western blot analyses of granulosa cell lysates from control ovaries and from ovaries primed with eCG or treated with eCG plus anti-eCG (gonadotropin withdrawal) were conducted. Prohibitin was immunolocalized in rat ovarian sections probed with antibodies against either proliferating cell nuclear antigen (PCNA) or cholesterol side-chain cleavage cytochrome P450 (P450(scc)) or in terminal deoxynucleotidyl transferase-mediated dUTP nick end labeled sections. Additionally, porcine oocytes, zygotes, and blastocyts were also immunolocalized with prohibitin antibody. Immunolocalization revealed the presence of prohibitin in granulosa cells, theca-interstitial cells, and the oocyte. The results indicate that prohibitin protein expression in the gonadotropin-treated cells was upregulated. Immunoreactivity of prohibitin was inversely related to PCNA expression during follicular maturation and colocalized with P450(scc). Prohibitin appeared to be translocated from the cytoplasm to the nucleus in atretic follicles, germinal vesicle-stage oocytes, zygotes, and blastocysts. These results suggest that prohibitin has several functional regulatory roles in granulosa and theca-interstitial cells and in the ovum during follicular maturation and atresia. It is likely that prohibitin may play an important role in determining the fate of these cells and eventual follicular destiny.

BRG1/BRM and prohibitin are required for growth suppression by estrogen antagonists

Estrogen antagonists are universally employed in the breast cancer therapy, although antagonist therapy is limited by the inevitable development of cellular resistance. The molecular mechanisms by which these agents inhibit cellular proliferation in breast cancer cells are not fully defined. Recent studies have shown the involvement of the E2F pathway in tamoxifen-induced growth arrest. We show that an E2F repressor, prohibitin, and the chromatin modifiers Brg1/Brm are required for estrogen antagonist-mediated growth suppression through the estrogen receptor, and that their recruitment to native promoter-bound E2F is induced via a JNK1 pathway. In addition, we demonstrate major mechanistic differences among the signaling pathways initiated by estrogen, estrogen deprivation, and estrogen antagonists. Collectively, these findings suggest that the prohibitin/Brg1/Brm node is a major cellular target for estrogen antagonists, and thereby also implicate prohibitin/Brg1/Brm as potentially important targets for breast cancer therapy.

Hyperphosphorylation of a mitochondrial protein, prohibitin, is induced by calyculin A in a rice lesion-mimic mutant cdr1

The rice (Oryza sativa) lesion-mimic mutants, cell death and resistance (cdr), show spontaneous cell death on the entire leaf and exhibited significant resistance to the rice blast fungus. Our previous studies showed that CDR1 and CDR2 genes negatively regulated the phosphorylation steps leading to the activation of NADPH oxidase, which is associated with oxidative burst. To identify novel factors involved in the phosphorylation steps, the phosphorylation level of total proteins was compared between cdr mutants and wild type using two-dimensional gel electrophoresis. Here, we show that the phosphorylation level of four proteins in cdr1 was increased as compared with the wild type after calyculin A treatment. Partial amino acid sequences revealed that one of the four proteins is homologous to prohibitin (PHB), which has been shown to be associated with senescence and cell death and to function as a chaperone in the assembly of mitochondrial respiratory chain complex in yeast and mammals. Analysis of green fluorescent protein fusions indicated that rice PHB (OsPHB1) was targeted to mitochondria as found in yeast and mammals, suggesting a possibility that PHB is involved in defense response and/or programmed cell death through the mitochondrial function.

Ubiquitination of prohibitin in mammalian sperm mitochondria: possible roles in the regulation of mitochondrial inheritance and sperm quality control

Ubiquitination of the sperm mitochondria during spermatogenesis has been implicated in the targeted degradation of paternal mitochondria after fertilization, a mechanism proposed to promote the predominantly maternal inheritance of mitochondrial DNA in humans and animals. The identity of ubiquitinated substrates in the sperm mitochondria is not known. In the present study, we show that prohibitin, a highly conserved, 30- to 32-kDa mitochondrial membrane protein, occurs in a number of unexpected isoforms, ranging from 64 to greater than 185 kDa in the mammalian sperm mitochondria, which are the ubiquitinated substrates. These bands bind antiubiquitin antibodies, displaying a pattern consistent with polyubiquitinated "ladders." Immunoprecipitation of sperm extracts with antiprohibitin antibodies followed by probing of the resultant immunocomplexes with antiubiquitin yields a banding pattern identical to that observed by antiprohibitin Western blot analysis. In fact, the presumably nonubiquitinated 30-kDa prohibitin band shows no antiubiquitin immunoreactivity. We demonstrate that ubiquitination of prohibitin occurs in testicular spermatids and spermatozoa. Ubiquitinated prohibitin molecules also accumulate in the defective fractions of ejaculated spermatozoa, which are thought to undergo surface ubiquitination during epididymal passage. In such sperm fractions, ubiquitin also coprecipitates with tubulin and microtubule-associated proteins, presumably contributed by the axonemes of defective, ubiquitinated spermatozoa. The results of the present study suggest that prohibitin is one of the ubiquitinated substrates that makes the sperm mitochondria recognizable by the egg's ubiquitin-proteasome dependent proteolytic machinery after fertilization and most likely facilitates the marking of defective spermatozoa in the epididymis for degradation.

Characterization of prohibitin in a newly established rat ovarian granulosa cell line

Prohibitin is an evolutionary conserved protein that is associated with cellular differentiation, atresia, and luteolysis in the rat ovary. However, the specific cellular location and function of prohibitin in ovarian cells has not been clearly elucidated. To characterize the expression of prohibitin during cell proliferation, differentiation, and cell death, we have successfully established a temperature-sensitive granulosa cell line, designated RGA-1. At a permissive temperature of 33 C, RGA-1 cells proliferate, but revert to a differentiated phenotype at a nonpermissive temperature of 39 C. Significant inductions of prohibitin mRNA and protein expression were observed in the differentiated phenotype when compared with proliferating cells. Differentiated RGA-1 cells were found to express inhibin alpha- and beta-transcripts, as well as steroidogenic acute regulatory protein and peripheral-type benzodiazepine receptor proteins in a manner reminiscent of steroidogenic functional responses observed in primary differentiated granulosa cells. Prohibitin expression correlated well with the expression of these steroidogenic proteins. At 39 C, RGA-1 cells also displayed increases in p53 protein levels, indicative of growth arrest in the nonproliferating cells. Confocal and electron microscopic examinations revealed increased prohibitin localization to the mitochondria at 39 C, along with changes in mitochondrial size and shape. These changes were accompanied by marked reductions in cytochrome c oxidase subunit II levels and in unit mitochondrial transmembrane potential. In addition, cell fractionation studies demonstrated that the prohibitin protein was mainly localized to the mitochondrial membrane. Collectively, these findings suggest a role for prohibitin in mitochondrial structure and function during growth and differentiation in ovarian granulosa cells. Prohibitin expression may also be indicative of mitochondrial destabilization during apoptosis-related events.

BAP37 and Prohibitin are specifically recognized by an SV40 T antigen antibody

We have identified two cellular proteins that are specifically immunoprecipitated by an anti-SV40 T antigen monoclonal antibody. This antibody, PAb419, recognizes an epitope contained within a region of T antigen which we have recently demonstrated is required for the initiation of immortalization by SV40 T antigen, but is not essential for maintenance of the immortal state. The two proteins were identified as BAP37 and Prohibitin. Recent results suggest Prohibitin may enhance the transcriptional inactivation of E2F by the retinoblastoma family of pocket proteins (pRb, p107, p130). BAP37 and Prohibitin are specifically recognized by PAb419 and PAb210, another anti-SV40 T antigen monoclonal antibody, which has an overlapping epitope, but not by other anti-SV40 T antigen monoclonal antibodies, demonstrating the specificity of the interaction.

Prohibitins, stomatins, and plant disease response genes compose a protein superfamily that controls cell proliferation, ion channel regulation, and death

Prohibitins, stomatins, and a group of plant defense response genes are demonstrated to belong to a novel protein superfamily. This superfamily is bound by similar primary and secondary predicted protein structures and hydropathy profiles. A PROSITE-formatted regular expression was generated that is highly predictive for identifying members of this superfamily using PHI-BLAST. The superfamily is named PID (proliferation, ion, and death) because prohibitins are involved in proliferation and cell cycle control, stomatins are involved in ion channel regulation, and the plant defense-related genes are involved in cell death. The plant defense gene family is named HIR (hypersensitive induced reaction) because its members are associated with hypersensitive reactions involving cell death and pathogen resistance. For this study, eight novel maize genes were introduced: four closely related to prohibitins (Zm-phb1, Zm-phb2, Zm-phb3, and Zm-phb4), one to stomatins (Zm-stm1), and three to a gene implicated in plant disease responses (Zm-hir1, Zm-hir2, and Zm-hir3). The maize Zm-hir3 gene transcript is up-regulated in a disease lesion mimic mutation (Les9), supporting a role in maize defense responses. Members of this gene superfamily are involved in diverse functions, but their structural similarity suggests a conserved molecular mechanism, which we postulate to be ion channel regulation.

Prohibitins act as a membrane-bound chaperone for the stabilization of mitochondrial proteins

Prohibitins are ubiquitous, abundant and evolutionarily strongly conserved proteins that play a role in important cellular processes. Using blue native electrophoresis we have demonstrated that human prohibitin and Bap37 together form a large complex in the mitochondrial inner membrane. This complex is similar in size to the yeast complex formed by the homologues Phb1p and Phb2p. In yeast, levels of this complex are increased on co-overexpression of both Phb1p and Phb2p, suggesting that these two proteins are the only components of the complex. Pulse-chase experiments with mitochondria isolated from phb1/phb2-null and PHB1/2 overexpressing cells show that the Phb1/2 complex is able to stabilize newly synthesized mitochondrial translation products. This stabilization probably occurs through a direct interaction because association of mitochondrial translation products with the Phb1/2 complex could be demonstrated. The fact that Phb1/2 is a large multimeric complex, which provides protection of native peptides against proteolysis, suggests a functional homology with protein chaperones with respect to their ability to hold and prevent misfolding of newly synthesized proteins.

Biochemical characterization of pKi67 with the identification of a mitotic-specific form associated with hyperphosphorylation and altered DNA binding

Although widely used as an operational marker of proliferation, the cell cycle-regulated Ki67 protein is of unknown function. pKi67 is found predominantly in the nucleolus in cycling interphase cells and moves to become perichromosomal during mitosis. We have performed a detailed immunochemical analysis of pKi67 in HeLa cells and report the existence of a novel hyperphosphorylated form in mitosis. Two isoforms can be identified on immunoblots as a consequence of the previously described alternative splicing. In extracts from mitotic cells both these isoforms have considerably reduced mobility. Treatment with phosphatase converts the mitotic form to the interphase form. Immunoprecipitated pKi67 can be phosphorylated in vitro both by cdc2/cyclin B and by protein kinase C, and treatment by PKC leads to the full mobility shift. Treatment of nocodazole-arrested mitotic HeLa cells with staurosporine causes a dephosphorylation of pKi67 to the interphase state and a concomitant change in the localization of pKi67 with movement away from the perichromosomal layer to cytoplasmic dots that colocalize with nucleolin. These data indicate that pKi67 localization is regulated by the action of cell cycle-specific kinase(s) and phosphatase(s). The data presented here provide a starting point for the analysis of pKi67 function and regulation.