Sequence analysis of a mammalian phospholipid-binding protein from testis and epididymis and its distribution between spermatozoa and extracellular secretions

RKIP localizes to the nucleus through a bipartite nuclear localization signal and interaction with importin α to regulate mitotic progression

Raf kinase inhibitor protein (RKIP) is a multifunctional modulator of intracellular signal transduction. Although most of its functions have been considered cytosolic, we show here that the localization of RKIP is primarily nuclear in both growing and quiescent Madin-Darby canine kidney epithelial cells and in Cal-51 and BT-20 human breast cancer cells. We have identified a putative bipartite nuclear localization signal (NLS) in RKIP that maps to the surface of the protein surrounding a known regulatory region. Like classical NLS sequences, the putative NLS of RKIP is rich in arginine and lysine residues. Deletion of and point mutations in the putative NLS lead to decreased nuclear localization. Point mutation of all the basic residues in the putative NLS of RKIP particularly strongly reduces nuclear localization. We found consistent results in reexpression experiments with wildtype or mutant RKIP in RKIP-silenced cells. A fusion construct of the putative NLS of RKIP alone to a heterologous reporter protein leads to nuclear localization of the fusion protein, demonstrating that this sequence alone is sufficient for import into the nucleus. We found that RKIP interacts with the nuclear transport factor importin α in BT-20 and MDA-MB-231 human breast cancer cells, suggesting importin-mediated active nuclear translocation. Taken together, these findings suggest that a bipartite NLS in RKIP interacts with importin α for active transport of RKIP into the nucleus and that this process may be involved in the regulation of mitotic progression. Evaluating the biological function of nuclear localization of RKIP, we found that the presence of the putative NLS is important for the role of RKIP in mitotic checkpoint regulation in MCF-7 human breast cancer cells.

Low-dose perfluorooctanoic acid stimulates steroid hormone synthesis in Leydig cells: Integrated proteomics and metabolomics evidence

Perfluorooctanoic acid (PFOA), one of the well-known perfluoroalkyl substances (PFASs), has been widespread in the environment and associated with male reproductive toxicity. However, the molecular mechanism involved in low-level PFOA-induced male endocrine disruption remains to be elucidated. In this study, we performed a combined proteomics and metabolomics analysis to investigate the proteomic and metabolic alterations in MLTC-1 Leydig cells responsive to low levels of PFOA exposure. The results showed that PFOA significantly regulated the expressions of 67 proteins and 17 metabolites, among which 18 proteins and 7 metabolites were specifically tied to lipid and fatty acid metabolism as well as testicular steroidogenesis. It is further suggested that low-dose PFOA stimulates steroid hormone synthesis by accelerating fatty acid metabolism and steroidogenic process, which is involved in the repression of p38 and cAMP-dependent ERK signaling pathway. The animal studies also revealed that environmentally relevant levels of PFOA increased serum steroid hormone levels accompanied by the activated cAMP and inhibited p38/ERK pathway in testis, which confirmed our in vitro findings. Overall, the present study will provide novel insights into the toxicological mechanisms of low-level PFOA-mediated steroidogenic disturbance, and may implicate the reproductive health risk of humans with environmental PFOA exposure.

Insights of RKIP-Derived Suppression of Prostate Cancer

Simple Summary

Despite an intensive research effort in the past few decades, prostate cancer (PC) remains a top cause of cancer death in men, particularly in the developed world. The major cause of fatality is the progression of local prostate cancer to metastasis disease. Treatment of patients with metastatic prostate cancer (mPC) is generally ineffective. Based on the discovery of mPC relying on androgen for growth, many patients with mPC show an initial response to the standard of care: androgen deprivation therapy (ADT). However, lethal castration resistant prostate cancers (CRPCs) commonly develop. It is widely accepted that intervention of metastatic progression of PC is a critical point of intervention to reduce PC death. Accumulative evidence reveals a role of RKIP in suppression of PC progression towards mPC. We will review current evidence and discuss the potential utilization of RKIP in preventing mPC progression.

Abstract

Prostate cancer (PC) is a major cause of cancer death in men. The disease has a great disparity in prognosis. Although low grade PCs with Gleason scores ≤ 6 are indolent, high-risk PCs are likely to relapse and metastasize. The standard of care for metastatic PC (mPC) remains androgen deprivation therapy (ADT). Resistance commonly occurs in the form of castration resistant PC (CRPC). Despite decades of research efforts, CRPC remains lethal. Understanding of mechanisms underpinning metastatic progression represents the overarching challenge in PC research. This progression is regulated by complex mechanisms, including those regulating PC cell proliferation, epithelial–mesenchymal transition (EMT), and androgen receptor (AR) signaling. Among this PC metastatic network lies an intriguing suppressor of PC metastasis: the Raf kinase inhibitory protein (RKIP). Clinically, the RKIP protein is downregulated in PC, and showed further reduction in mPC. In xenograft mouse models for PC, RKIP inhibits metastasis. In vitro, RKIP reduces PC cell invasion and sensitizes PC cells to therapeutic treatments. Mechanistically, RKIP suppresses Raf-MEK-ERK activation and EMT, and modulates extracellular matrix. In return, Snail, NFκB, and the polycomb protein EZH2 contribute to inhibition of RKIP expression. In this review, we will thoroughly analyze RKIP’s tumor suppression actions in PC.

RKIP Pleiotropic Activities in Cancer and Inflammatory Diseases: Role in Immunity

Simple Summary

The human body consists of tissues and organs formed by cells. In each cell there is a switch that allows the cell to divide or not. In contrast, cancer cells have their switch on which allow them to divide and invade other sites leading to death. Over two decades ago, Doctor Kam Yeung, University of Toledo, Ohio, has identified a factor (RKIP) that is responsible for the on/off switch which functions normally in healthy tissues but is inactive or absent in cancers. Since this early discovery, many additional properties have been ascribed to RKIP including its role in inhibiting cancer metastasis and resistance to therapeutics and its role in modulating the normal immune response. This review describes all of the above functions of RKIP and suggesting therapeutics to induce RKIP in cancers to inhibit their growth and metastases as well as inhibit its activity to treat non-cancerous inflammatory diseases.

Abstract

Several gene products play pivotal roles in the induction of inflammation and the progression of cancer. The Raf kinase inhibitory protein (RKIP) is a cytosolic protein that exerts pleiotropic activities in such conditions, and thus regulates oncogenesis and immune-mediated diseases through its deregulation. Herein, we review the general properties of RKIP, including its: (i) molecular structure; (ii) involvement in various cell signaling pathways (i.e., inhibition of the Raf/MEK/ERK pathway; the NF-kB pathway; GRK-2 or the STAT-3 pathway; as well as regulation of the GSK3Beta signaling; and the spindle checkpoints); (iii) regulation of RKIP expression; (iv) expression’s effects on oncogenesis; (v) role in the regulation of the immune system to diseases (i.e., RKIP regulation of T cell functions; the secretion of cytokines and immune mediators, apoptosis, immune check point inhibitors and RKIP involvement in inflammatory diseases); and (vi) bioinformatic analysis between normal and malignant tissues, as well as across various immune-related cells. Overall, the regulation of RKIP in different cancers and inflammatory diseases suggest that it can be used as a potential therapeutic target in the treatment of these diseases.

Immuno-histological mapping and functional association of seminal proteins in testis and excurrent ducts with sperm function in buffalo

The region-specific expression of seminal proteins in testis and excurrent duct system determines the quality and function of the spermatozoa. In the present study, localization and expression of some of the seminal proteins such as insulin-like growth factor receptor 1β (IGF-1Rβ), phosphatidylethanolamine-binding protein 4 (PEBP4), α-tubulin and tissue factor pathway inhibitor 2 (TFPI2) were carried out in testis, excurrent duct system and spermatozoa of buffalo. IGF-1Rβ was localized in the cells of the seminiferous tubules of the testis, except in primary spermatocytes. The PEBP4 was localized only in the elongated spermatid, whereas α-tubulin and TFPI2 proteins were localized in all cells of the seminiferous tubule including spermatocyte. In the buffalo spermatozoa, IGF-1Rβ, PEBP4, α-tubulin and TFPI2 were localized in the acrosome region, the post-acrosomal region till the tail end, post-acrosome to the entire tail region and the equatorial region, respectively. The study indicates that IGF-1R, α-tubulin and PEBP4 proteins regulate spermatogenesis, whereas TFPI2 may be involved during the zona binding process of the buffalo spermatozoa.

Phosphatidylethanolamine binding protein 4 (PEBP4) is a secreted protein and has multiple functions

Phosphatidylethanolamine binding proteins (PEBP) represent a superfamily of proteins that are conserved from bacteria to humans. In mammals, four members have been identified, PEBP1-4. To determine the functional differences among PEBP1-4 and the underlying mechanism for their actions, we performed a sequence alignment and found that PEBP4 contains a signal peptide and potential glycosylation sites, whereas PEBP1-3 are intracellular proteins. To test if PEBP4 is secreted, we made constructs with Myc epitope at the amino (N) terminus or carboxyl (C) terminus to mask the signal sequence or keep it free, respectively. Our data revealed that both mouse and human PEBP4 were secreted when the epitope was tagged at their C-terminus. To our surprise, secretion was dependent upon the C-terminal conserved domain in addition to the N-terminal signal sequence. When the epitope was placed to the N-terminus, the recombinant protein failed to secrete and instead, was retained in the cytoplasm. Mass spectrometry detected asparagine (N)-glycosylation on the secreted PEBP4. Although overexpression of N-terminal tagged PEBP4 resulted in an inhibition of ERK activation by EGF, that with a C-terminal epitope tag did not have such an effect. Likewise, transfection of PEBP4 shRNA did not appear to affect ERK activation, suggesting that PEBP4 does not participate in the regulation of this pathway. In contrast, PEBP4 siRNA suppressed phosphorylation of Act at S473. Therefore, our results suggest that PEBP4 is a multifunctional protein and can be secreted. It will be important to investigate the mechanism by which PEBP4 is secreted and regulates cellular events.

RKIP: much more than Raf kinase inhibitory protein

From its discovery as a phosphatidylethanolamine-binding protein in bovine brain to its designation as a physiological inhibitor of Raf kinase protein, RKIP has emerged as a critical molecule for maintaining subdued, well-orchestrated cellular responses to stimuli. The disruption of RKIP in a wide range of pathologies, including cancer, Alzheimer's disease, and pancreatitis, makes it an exciting target for individualized therapy and disease-specific interventions. This review attempts to highlight recent advances in the RKIP field underscoring its potential role as a master modulator of many pivotal intracellular signaling cascades that control cellular growth, motility, apoptosis, genomic integrity, and therapeutic resistance. Specific biological and functional niches are highlighted to focus future research towards an enhanced understanding of the multiple roles of RKIP in health and disease.

Raf kinase inhibitor protein (RKIP) in cancer

Raf kinase inhibitory protein (RKIP) was initially identified as phosphatidylethanolamine binding protein in bovine brain. It was later identified as a protein that inhibits Raf kinase activation of MEK. Further exploration has revealed that RKIP modulates several other signaling pathways including NF-κB and G-protein signaling. A gene array screen revealed that RKIP expression was low in a metastatic compared with non-metastatic prostate cancer cell line. Further experiments revealed that RKIP fits the criteria for a metastasis suppressor gene. RKIP expression has been shown to be downregulated in metastatic tissues, compared with non-metastatic tissue in multiple cancers, suggesting that loss of RKIP metastasis suppressor activity is a broad mechanism leading to metastasis. Additionally, loss of RKIP has been shown to impact therapy through conferring radioresistance and chemoresistance. Taken together, these data indicate understanding RKIP's contributions to cancer may lead to important therapeutic strategies to prevent metastasis and promote therapeutic efficacy.

Transcriptional regulation of RKIP expression by androgen in prostate cells

BACKGROUND/AIMS

Raf kinase inhibitory protein (RKIP) is a scaffolding molecule in the PEBP family that sequesters certain signaling molecules away from their pathways, thereby abrogating intracellular growth signals. RKIP has been assigned multiple functions and is associated with an increasing number of diseases through its involvement with signal transduction pathways. We previously demonstrated that RKIP is highly expressed in human normal prostate epithelial cells and plays a pivotal role during prostate cancer (PCa) progression. Whether RKIP is subject to endocrine regulation has not been reported.

METHODS

The effect of dihydrotestosterone (DHT) on RKIP expression in normal prostate epithelial cells was determined by real-time RT-PCR and Western blot. Report assay was performed to determine whether the regulation of RKIP by androgens is at the transcriptional level. The binding of androgen receptor (AR) to the RKIP promoter was determined by EMSA and Chromatin Immunoprecipitation (ChIP) assays. To determine whether RKIP was regulated by androgen in vivo, we examined RKIP expression level in response to castration in 6-8 week old C57BL/6 male mice.

RESULTS

Here we report that DHT positively regulates the transcription of RKIP in the normal prostate epithelial cells. The anti-androgen bicalutamide blocked androgen-mediated regulation of RKIP, which indicates that this regulation is mediated through AR. Transfection of the cells with a RKIP promoter-driven luciferase reporter vector showed that DHT increased RKIP promoter activity in parallel with changes in expression. EMSA demonstrates that AR binds to a putative ARE in the RKIP promoter, which was further validated by ChIP assay. Importantly, these data are further supported by our in vivo experiment where castrated mice had less RKIP expression in their prostate glands than sham-operated mice.

CONCLUSIONS

Collectively, the results establish RKIP as a novel androgen target gene. Androgens induce RKIP expression through AR-mediated transcriptional modulation of the RKIP promoter in the prostate. This is the first demonstration of endocrine regulation of the metastasis suppressor gene RKIP.

Systematic analysis of proteins from different signaling pathways in the tumor center and the invasive front of colorectal cancer

In colorectal cancer, the functional impact of proteins from different signaling pathways varies between tumor center and tumor front. Our objective was to identify differential protein expression profiles between the tumor center and the tumor front of colorectal cancer. Twenty proteins from different signaling pathways (epidermal growth factor receptor [EGFR], phosphorylated extracellular signal regulated kinase [pERK], receptor for hyalouronic acid mediated motility [RHAMM], Raf-1 kinase inhibitor protein [RKIP], β-catenin, E-cadherin, phosphorylated AK transforming [pAKT], p16, p21, Ki-67, B-cell Lymphoma-2 [BCL2], vascular endothelial growth factor, apoptosis protease activating factor 1 [APAF-1], mucin1 [MUC1], ephrin B2 receptor [EphB2], matrix metalloproteinase 7 [MMP7], phosphorylated mothers against decapentaplegic 2 [pSMAD2], caudal type homeobox transcription factor 2 [CDX2], Laminin5γ2, and mammalian sterile 20-like kinase 1 [MST1]) involved in colorectal cancer progression were studied immunohistochemically on 220 well-characterized patients using a multiple-punch tissue microarray including 437 and 430 samples from the tumor center and the invasive front, respectively. Mean expression between the tumor center and the tumor front varied statistically significantly for pSMAD2, pERK, Raf-1 kinase inhibitor protein, E-cadherin, pAKT, BCL2, vascular endothelial growth factor, EphB2, matrix metalloproteinase 7, CDX2, Laminin5γ2, MST1, and APAF-1. Overexpression of pAKT, BCL2, vascular endothelial growth factor, APAF-1, pERK, EphB2, Raf-1 kinase inhibitor protein, CDX2, E-cadherin, MST1 (P < .001 each), and pSMAD2 (P = .002) was more frequently observed in the tumor center, whereas matrix metalloproteinase 7 and Laminin5γ2 (P < .001 each) overexpression was associated with the invasive front. In multivariate analysis, vascular endothelial growth factor (P < .001), Raf-1 kinase inhibitor protein (P = .009), and Laminin5γ2 (P < .001) were the most relevant proteins with the multimarker phenotypes positive/positive/negative and negative/negative/positive being most discriminating between the tumor center and the tumor front. Moreover, the combination negative/negative/positive vascular endothelial growth factor/Raf-1 kinase inhibitor protein/Laminin5γ2 at the tumor front was associated with vascular/lymphatic invasion (P = .014), distant metastasis (P = .019), higher tumor grade (P < .001), and poorer survival (P = .05). Our findings show that, in colorectal cancer progression, vascular endothelial growth factor overexpression seems to play a role in the tumor center, whereas Laminin5γ2-positivity combined with Raf-1 kinase inhibitor protein loss is associated with tumor invasion at the front.

Loss of Raf-1 kinase inhibitory protein in pancreatic ductal adenocarcinoma

AIMS

Raf-1 kinase inhibitor protein (RKIP) has emerged as a significant metastatic suppressor in a variety of human cancers. The aim of this study was to evaluate RKIP expression and to determine its association with metastasis and prognostic significance in pancreatic cancer patients.

METHODS

Immunohistochemical staining for RKIP was performed on 63 cases of pancreatic ductal adenocarcinoma (PDAC). We investigated whether RKIP expression correlated with clinicopathological parameters and patient outcomes.

RESULTS

The islet cells, acinar cells and ductal epithelial cells of normal pancreas consistently showed strong RKIP immunoreactivity. In contrast, in PDAC, RKIP was lost in 57.1% (37/63) of cases. Loss of RKIP expression was significantly associated with the presence of nodal (p=0.001) and distant (p=0.010) metastases and a higher stage group (p=0.012). Univariate analysis for distant metastasis-free survival (DMFS) showed that the median DMFS of RKIP negative PDAC patients (10 months) was significantly shorter than that of RKIP positive PDAC patients (17 months; p=0.009). Multivariate analysis also revealed that loss of RKIP expression was an independent predictor of worse DMFS in PDAC patients (p=0.015).

CONCLUSIONS

Our results strongly suggest that RKIP is a metastasis suppressor in PDAC.

Metastasis suppressors in human benign prostate, intraepithelial neoplasia, and invasive cancer: their prospects as therapeutic agents

Despite advances in diagnosis and treatment of prostate cancer, development of metastases remains a major clinical challenge. Research efforts are dedicated to overcome this problem by understanding the molecular basis of the transition from benign cells to prostatic intraepithelial neoplasia (PIN), localized carcinoma, and metastatic cancer. Identification of proteins that inhibit dissemination of cancer cells will provide new perspectives to define novel therapeutics. Development of antimetastatic drugs that trigger or mimic the effect of metastasis suppressors represents new therapeutic approaches to improve patient survival. This review focuses on different biochemical and cellular functions of metastasis suppressors known to play a role in prostate carcinogenesis and progression. Ten putative metastasis suppressors implicated in prostate cancer are discussed. CD44s is decreased in both PIN and cancer; Drg-1, E-cadherin, KAI-1, RKIP, and SSeCKS show similar expression between benign epithelia and PIN, but are downregulated in invasive cancer; whereas, maspin, MKK4, Nm23 and PTEN are upregulated in PIN and downregulated in cancer. Moreover, the potential role of microRNA in prostate cancer progression, the understanding of the cellular distribution and localization of metastasis suppressors, their mechanism of action, their effect on prostate invasion and metastasis, and their potential use as therapeutics are addressed.

Role of Raf Kinase Inhibitor Protein in Pathophysiology of Prostate Cancer

Raf kinase inhibitor protein (RKIP) is a small, cytosolic protein named for its ability to block Raf-mediated activation of MAPK and ERK. It also block G-protein signaling and NF-κB activation. An in vitro screen to identify genes that regulate prostate cancer (PCa) metastasis revealed that expression of RKIP was decreased in high versus low metastatic PCa cells. Modulation of RKIP expression revealed that it inhibited invasion and loss of RKIP promoted in vitro invasion. Animal studies were used to demonstrate that RKIP could inhibit PCa metastasis from orthotopically injected tumor cells without an effect on primary tumor growth. Taken together, these results indicated RKIP acted as a PCa metastasis suppressor gene. Evaluation of RKIP expression in clinical cases of PCa revealed that RKIP expression was moderate to high in non-neoplastic prostate, low in 50% of primary prostate cancers, and absent to low in the majority of metastases. Furthermore, low RKIP expression in primary prostate tumors was predictive of early tumor recurrence. Loss of RKIP was shown to induce resistance to radiation in PCa cells in vitro and in an in vivo murine model. Taken together, these studies indicate that RKIP plays multiple roles in PCa pathophysiology, suggesting that a method to increase RKIP expression in PCa may have therapeutic benefits.

Raf-1 kinase inhibitory protein expression in thyroid carcinomas

Raf-1 kinase inhibitory protein (RKIP) has been implicated in several fundamental signal transduction pathways that control cellular growth, differentiation, apoptosis and migration. RKIP is reduced in a variety of human carcinomas, but RKIP expression in thyroid carcinomas has not been analyzed at the protein level. In this study, we examined the immunohistochemical expression of RKIP in various subtypes of thyroid carcinoma. Immunostaining for RKIP was performed on 104 cases of primary thyroid carcinoma (40 papillary, 29 follicular, 11 medullary, 11 poorly differentiated, and 13 anaplastic carcinomas) and 26 cases of nodal metastatic tumor (17 papillary, 4 medullary, and 5 anaplastic carcinomas). Normal thyroid tissue and all cases of follicular, papillary, and medullary carcinomas showed uniform, strong cytoplasmic immunoreactivity for RKIP. With the exception of one case, poorly differentiated carcinomas also revealed strong RKIP expression. In contrast, RKIP expression was completely absent in all anaplastic carcinomas. The transition zone from the differentiated carcinoma component (strong RKIP expression) to the anaplastic carcinoma component (no RKIP expression) demonstrated a completely opposite pattern of RKIP immunoreactivity. This reduction of RKIP expression in anaplastic carcinoma was statistically significant (P < 0.0001). Additionally, RKIP expression of nodal metastatic tumors corresponded with that of primary tumors: metastatic papillary and medullary carcinomas showed uniform, strong cytoplasmic RKIP immunoreactivity, in contrast, in metastatic anaplastic carcinomas, RKIP expression was completely absent. RKIP expression is significantly reduced in anaplastic thyroid carcinoma as compared to other subtypes of thyroid carcinoma. Further studies are necessary to elucidate the precise mechanism of RKIP action in anaplastic thyroid carcinoma.

Raf kinase inhibitor protein correlates with sensitivity of nasopharyngeal carcinoma to radiotherapy

Raf kinase inhibitory protein (RKIP) is a metastasis suppressor whose expression is reduced in nasopharyngeal carcinoma (NPC) tissues and is absent in NPC metastases. To investigate the effect of RKIP on radiosensitivity of NPC, high metastatic 5-8F with low RKIP expression and non-metastatic 6-10B with high RKIP expression were stably transfected with plasmids that expressed sense and antisense RKIP cDNA. Overexpression of RKIP sensitized 5-8F cells to radiation-induced cell death, G(2)-M cell cycle arrest and apoptosis. In contrast, downexpression of RKIP in 6-10B cells protected cells from radiation-induced cell death, G(2)-M cell cycle arrest and apoptosis. In addition, RKIP expression altered the radiosensitivity of NPC cells through MEK and ERK phosphorylation changes of Raf-1/MEK/ERK signaling pathway. We further investigated the RKIP expression in NPC patients and its association with patients' survival after radiotherapy. Downexpression of RKIP was significantly correlated with advanced clinical stage, lymph node metastasis and radioresistance. Furthermore, survival curves showed that patients with RKIP downexpression had a poor prognosis and induced relapse. Multivariate analysis confirmed that RKIP expression was an independent prognostic indicator. The data suggested that RKIP was a potential biomarker for the radiosensitivity and prognosis of NPC, and its dysregulation might play an important role in the radioresistance of NPC.

Reduced expression of Raf-1 kinase inhibitory protein is a significant prognostic marker in patients with gallbladder carcinoma

Gallbladder carcinoma is one of the most aggressive malignancies. It is usually diagnosed at an advanced stage, and the prognosis remains poor despite advances in imaging techniques and aggressive surgical treatment. Because of the lack of reliable prognostic markers, the aim of this study was to investigate the prognostic significance of Raf-1 kinase inhibitory protein expression in gallbladder carcinomas. Immunostaining for Raf-1 kinase inhibitory protein was performed on chronic cholecystitis, adenoma, carcinoma in situ, and primary and nodal metastatic gallbladder carcinoma. Raf-1 kinase inhibitory protein expression was reduced in 68.8% (11/16) and 42.3% (44/104) of nodal metastatic and primary gallbladder carcinoma cases, respectively, but in no case of carcinoma in situ, adenoma, or chronic cholecystitis. The differences in Raf-1 kinase inhibitory protein expression in gallbladder carcinoma versus nongallbladder carcinoma tissues (P < .001), and in nodal metastatic gallbladder carcinoma versus primary gallbladder carcinoma (P = .009), were statistically significant. Kaplan-Meier curves showed that patients with Raf-1 kinase inhibitory protein-negative or weakly positive gallbladder carcinoma had a significantly shorter overall survival than did patients with Raf-1 kinase inhibitory protein-positive gallbladder carcinoma (median, 14 versus 120 months; P = .011). Multivariate survival analysis showed that reduced Raf-1 kinase inhibitory protein expression was an independent prognostic predictor for overall survival (P = .020). Our results suggest that reduction in Raf-1 kinase inhibitory protein expression in gallbladder carcinoma contributes to invasion and metastasis and is a significant prognostic marker in patients with gallbladder carcinoma.

Characterization of the Raf kinase inhibitory protein (RKIP) binding pocket: NMR-based screening identifies small-molecule ligands

BACKGROUND

Raf kinase inhibitory protein (RKIP), also known as phoshaptidylethanolamine binding protein (PEBP), has been shown to inhibit Raf and thereby negatively regulate growth factor signaling by the Raf/MAP kinase pathway. RKIP has also been shown to suppress metastasis. We have previously demonstrated that RKIP/Raf interaction is regulated by two mechanisms: phosphorylation of RKIP at Ser-153, and occupation of RKIP's conserved ligand binding domain with a phospholipid (2-dihexanoyl-sn-glycero-3-phosphoethanolamine; DHPE). In addition to phospholipids, other ligands have been reported to bind this domain; however their binding properties remain uncharacterized.

METHODS/FINDINGS

In this study, we used high-resolution heteronuclear NMR spectroscopy to screen a chemical library and assay a number of potential RKIP ligands for binding to the protein. Surprisingly, many compounds previously postulated as RKIP ligands showed no detectable binding in near-physiological solution conditions even at millimolar concentrations. In contrast, we found three novel ligands for RKIP that specifically bind to the RKIP pocket. Interestingly, unlike the phospholipid, DHPE, these newly identified ligands did not affect RKIP binding to Raf-1 or RKIP phosphorylation. One out of the three ligands displayed off target biological effects, impairing EGF-induced MAPK and metabolic activity.

CONCLUSIONS/SIGNIFICANCE

This work defines the binding properties of RKIP ligands under near physiological conditions, establishing RKIP's affinity for hydrophobic ligands and the importance of bulky aliphatic chains for inhibiting its function. The common structural elements of these compounds defines a minimal requirement for RKIP binding and thus they can be used as lead compounds for future design of RKIP ligands with therapeutic potential.

Behavioral and quantitative mitochondrial proteome analyses of the effects of simvastatin: implications for models of neural degeneration

The 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, simvastatin, is used for lowering elevated low-density lipoprotein cholesterol concentrations. This translates into reduced cardiovascular disease-related morbidity and mortality, while the drugs' anti-oxidant and anti-inflammatory properties have earmarked it as a potential treatment strategy against various neurological conditions. Statins have been shown to protect neurons from degeneration in a number of animal models. Although no mechanism completely explains the multiple benefits exerted by statins, emerging evidence suggests that in some degenerative and brain injury models, mitochondrial impairment may play a contributive rate. However, [corrected] evidence lacks to support a directly influencing role for statins on mitochondria-related proteins and motor behavior. Mitochondrial dysfunction may increase oxygen free radical production, which in turn leaves cells susceptible to energy failure, apoptosis and related events [corrected] which could prove fatal. The potential link between simvastatin treatment and mitochondrial function would be supported if key mitochondrial proteins were altered by simvastatin exposure. Using mass spectroscopy (MS), we identified 24 mitochondrial proteins that differed significantly (P < 0.05) in relative abundancy as a result of simvastatin treatment. The identified proteins represented many facets of mitochondrial integrity, with the majority forming part of the electron transport chain machinery, which is necessary for energy production. In a follow-up study, we then addressed whether simvastatin is capable of altering sensorimotor function in a mitochondrial toxin-induced animal model. Rats were pre-treated with simvastatin for 14 days, followed by a single unihemispheric (substantia nigra; SN) injection of rotenone, a mitochondrial complex I (Co-I) inhibitor. Results showed that simvastatin improved motor performance in rotenone-infused rats. The data are consistent with the possibility that alteration of mitochondrial function may contribute to the beneficial effects associated with statin use.

The epididymal influence on sperm maturation

Consideration of the function of the epididymis has undergone profound changes over the last century during which it has moved from a largely neglected male reproductive organ to one that is an increasingly exploited source of sperm for assisted reproduction strategies. From histological studies in the lizard1 it was considered that, ‘…the cells lining the epididymal canal produce a material necessary for the spermatozoa during their passage through the organ …’ whereas a fertility study with guinea-pigs stated boldly that, ‘… changes undergone [by spermatozoa in the epididymis] are not conditioned by some specific action of epididymal secretion …’. The former view found favour in a review of the literature which concluded that, ‘… there are specific epididymal secretions necessary for sperm maturation and survival …’, although the nature of the secretions were not then known. However, this concept, currently held by most of those studying the epididymis of animals, was again contradicted on the basis of clinical work: ‘… it certainly is possible for sperm that have never passed through any length of the epididymis at all to mature on their own …’.

Expression and characterization of the PEBP homolog genes from Drosophila

The phosphatidylethanolamine binding proteins (PEBPs) family is evolutionarily conserved and involved in different physiological phenomena. PEBPs were found in many species from bacteria to mammals. Despite numerous studies, PEBPs' biological function and mode of action remain elusive. Based on sequence homology, seven PEBP genes were detected in the Drosophila genome. Only one of them, the odorant binding protein (OBP), has been characterized. To date nothing is known concerning the expression pattern and biological roles of the six other PEBP genes. By RT-PCR and Western blot analysis, we examined expression of the PEBPs in different tissues and embryos. The 6 PEBPs were differentially expressed. Only one, CG10298, is specific of only one tissue: the testis. Additionally, by comparing in wild type and male-sterile mutants we show that CG10298 is present only during spermatid differentiation. Furthermore, by comparing structural parameters of the six PEBP proteins with those of human PEBP-1, we have established that PEBP CG10298 is most closely related to human PEBP.

Signaling crossroads: the function of Raf kinase inhibitory protein in cancer, the central nervous system and reproduction

The Raf kinase inhibitory protein 1 (RKIP-1) and its orthologs are conserved throughout evolution and widely expressed in eukaryotic organisms. In its non-phosphorylated form RKIP-1 negatively regulates the Raf/MEK/ERK pathway by interfering with the activity of Raf-1. In its phosphorylated state, RKIP-1 dissociates from Raf-1 and inhibits GRK-2, a negative regulator of G-protein coupled receptors (GPCRs). Available data indicate that the phosphorylation of RKIP-1 by PKC can stimulate both the Raf/MEK/ERK and GPCR pathways. RKIP-1 has also been implicated as a negative regulator of the NF-kappaB pathway. Recent studies have shown that phosphorylated RKIP-1 binds to the centrosomal and kinetochore regions of metaphase chromosomes, where it may be involved in regulating the partitioning of chromosomes and the progression through mitosis. The collective evidence indicates that RKIP-1 regulates the activity and mediates the crosstalk between several important cellular signaling pathways. A variety of ablative interventions suggest that reduced RKIP-1 function may influence metastasis, angiogenesis, resistance to apoptosis, and genome integrity. Attenuation of RKIP-1 may also affect cardiac and neurological functions, spermatogenesis, sperm decapacitation, and reproductive behavior. In this review, the role of RKIP-1 in cellular signaling, and especially its functions revealed using a mouse knockout model, are discussed.

Regulation of tumor cell sensitivity to TRAIL-induced apoptosis by the metastatic suppressor Raf kinase inhibitor protein via Yin Yang 1 inhibition and death receptor 5 up-regulation

Raf-1 kinase inhibitor protein (RKIP) has been implicated in the regulation of cell survival pathways and metastases, and is poorly expressed in tumors. We have reported that the NF-kappaB pathway regulates tumor resistance to apoptosis by the TNF-alpha family via inactivation of the transcription repressor Yin Yang 1 (YY1). We hypothesized that RKIP overexpression may regulate tumor sensitivity to death ligands via inhibition of YY1 and up-regulation of death receptors (DRs). The TRAIL-resistant prostate carcinoma PC-3 and melanoma M202 cell lines were examined. Transfection with CMV-RKIP, but not with control CMV-EV, sensitized the cells to TRAIL-mediated apoptosis. Treatment with RKIP small interfering RNA (siRNA) inhibited TRAIL-induced apoptosis. RKIP overexpression was paralleled with up-regulation of DR5 transcription and expression; no change in DR4, decoy receptor 1, and decoy receptor 2 expression; and inhibition of YY1 transcription and expression. Inhibition of YY1 by YY1 siRNA sensitized the cells to TRAIL apoptosis concomitantly with DR5 up-regulation. RKIP overexpression inhibited several antiapoptotic gene products such as X-linked inhibitor of apoptosis (XIAP), c-FLIP long, and Bcl-x(L) that were accompanied with mitochondrial membrane depolarization. RKIP overexpression in combination with TRAIL resulted in the potentiation of these above effects and activation of caspases 8, 9, and 3, resulting in apoptosis. These findings demonstrate that RKIP overexpression regulates tumor cell sensitivity to TRAIL via inhibition of YY1, up-regulation of DR5, and modulation of apoptotic pathways. We suggest that RKIP may serve as an immune surveillance cancer gene, and its low expression or absence in tumors allows the tumor to escape host immune cytotoxic effector cells.

Identification of novel molecular candidates for acute liver failure in plasma of BALB/c murine model

In an effort to identify proteins involved in the disease process of acute liver failure (ALF), we investigated changes in the plasma proteome associated with d-galactosamine/lipopolysaccharide (GalN/LPS) treatment of BALB/c mice. The plasma samples from mice with ALF and control were screened for potential differences using two-dimensional electrophoresis followed by liquid chromatography-electrospray ionization-tandem mass spectrometry or matrix associated laser desorption ionization-time-of-flight mass spectrometry. The expression levels of candidate protein named phosphatidylethanolamine binding protein (PEBP) in plasma and liver, brain tissues were confirmed by western blot and RT-PCR analyses. Results were confirmed in plasma samples of human beings. Seven proteins existed in plasma of GalN/LPS-treatment animals only but not in controls. They included PEBP, regucalcin, Cu/Zn superoxide dismutase, glyoxalase 1, malate dehydrogenase, proteasome subunit alpha type 1, and HPMS haptoglobin precursor. Two proteins, proteasome subunit alpha type 5 and apolipoprotein A-I precursor, were up-regulated by GalN/LPS, and one protein, HPMS haptoglobin precursor, was down-regulated by this treatment. Western blot analysis confirmed the results that PEBP protein levels increased significantly in plasma and liver tissues only in ALF mice, but not in surviving mice treated with GalN/LPS. Further analysis revealed that GalN/LPS also induced up-regulation of PEBP mRNA levels in liver tissues. Importantly, plasma obtained from ALF patients, but not from healthy volunteers or from hepatitis patients, also contained detectable levels of PEBP. The present study show that PEBP may be a potential plasma biomarker for ALF diagnosis and participate in the pathphysiological process of ALF.

Loss of raf-1 kinase inhibitor protein expression is associated with tumor progression and metastasis in colorectal cancer

Raf-1 kinase inhibitor protein (RKIP) is known as a critical down-regulator of the mitogen-activated protein kinase signaling pathway and a potential molecular determinant of malignant metastasis. The aim of this study was to determine the prognostic significance of RKIP expression in colorectal cancer (CRC). Immunohistochemical staining for RKIP was performed on a tissue microarray comprising 1,197 mismatch repair (MMR)-proficient and 141 MMRdeficient CRCs. The association of RKIP with clinicopathologic features was analyzed. Loss of cytoplasmic RKIP was associated with distant metastasis (P = .038), higher N stage (P = .032), vascular invasion (P = .01), and worse survival (P = .001) in the MMR-proficient group. In MMR-deficient CRCs, loss of cytoplasmic RKIP was associated with distant metastasis (P = .043) and independently predicted worse survival (P = .004). Methylation analysis of 28 cases showed that loss of RKIP expression is unlikely to be due to promoter methylation.Loss of RKIP expression is a marker of tumor progression and distant metastasis in MMR-proficient and MMR-deficient CRCs.

The emerging cardioinhibitory role of the hippocampal cholinergic neurostimulating peptide

Hippocampal cholinergic neurostimulating peptide (HCNP), which derives from phosphatidylethanolamine-binding protein (also named Raf kinase inhibitor protein), enhances acetylcholine synthesis in the hippocampal medial septal nuclei. It is present in the chromaffin secretory granules of the adrenal cells and under stress is cosecreted with peptide hormones and catecholamines. Using the isolated rat heart perfused according to Langendorff to reveal the cardiotropic action of HCNP on the mammalian heart, we showed that rat HCNP exerts, at concentrations of 5x10(-13) to 10(-6) M, a negative inotropism under basal conditions (left ventricular pressure variations ranging from -8.34+/-0.94% to -21+/-3.5%) and enhances the cholinergic-mediated negative inotropy through direct interaction with G-protein-coupled muscarinic receptor pathway. Under adrenergic stimulation (isoproterenol), the peptide exerts an antiadrenergic action. The analysis of the percentage of rate pressure product variations in terms of EC50 values of isoproterenol alone (-8.5+/-0.3; r2=0.90) and in the presence of rat HCNP at 0.01 nM (-6.9+/-0.36; r2=0.88) revealed a competitive type of antagonism of the peptide. HCNP does not affect either heart rate or coronary pressure. The evidence that HCNP in mammals may play a novel role as an inhibitory cardiac modulator throughout an involvement of the myocardial G-protein-coupled receptor pathway provides new insights regarding the neurohumoral control of heart function under normal and physiopathological conditions.

Induction of Raf kinase inhibitor protein contributes to macrophage differentiation

Differential gene expression analysis of human blood monocytes has identified the Raf kinase inhibitor protein (RKIP) as a continuously upregulated gene in macrophage and dendritic cell maturation. Using realtime RT-PCR and Western blot analysis we were able to confirm the initial DNA-microarray findings of RKIP induction on mRNA and protein levels. RKIP upregulation in primary cells and overexpression in THP-1 cells did not alter ERK activity but strongly reduced the amount of the NFkappaB subunit p65 in the nucleus. mRNA levels and cell surface expression of maturation markers including the integrin CD11c and the scavenger receptor CD36 were significantly increased in RKIP transfected THP-1 cells. Our data show for the first time that RKIP is upregulated during macrophage and dendritic cell differentiation on mRNA and protein levels and we conclude that RKIP contributes to the monocytic differentiation process via inhibition of the NFkappaB signaling cascade independent from the canonical Ras/Raf/MEK/ERK pathway.

Metastasis suppressor gene Raf kinase inhibitor protein (RKIP) is a novel prognostic marker in prostate cancer

BACKGROUND

Diminished expression of Raf kinase inhibitor protein (RKIP), an inhibitor of the Raf signaling cascade, promotes prostate cancer (PCa) metastasis in a murine model, suggesting that it is a metastasis suppressor gene. However, the prognostic significance of RKIP expression and its association with metastasis in PCa patients is unknown.

METHODS

To investigate RKIP protein expression is a prognostic marker in PCa we performed immunohistochemical staining for RKIP expression in tissue microarrays consisting of 758 non-neoplastic prostate tissues, primary tumors and metastases from 134 PCa patients. The Cox proportional-hazards model was used to adjust for covariates including Gleason score, tumor volume, tumor weight, clinical stage, digital rectal exam findings, serum PSA level and surgical margins.

RESULTS

RKIP expression was low in approximately 5%, 48%, and 89% of non-neoplastic prostate, primary tumors and metastases, respectively. Low RKIP expression in primary tumors was a strong positive predictive factor for PCa recurrence based on PSA levels. In patients whose primary tumors expressed high RKIP levels, the 7-year PSA recurrence rate was <10%; whereas in patients with tumors with low RKIP expression the recurrence rate was 50% (P<0.001). Multivariate analysis revealed RKIP was an independent prognostic factor (P<0.001).

CONCLUSION

In contrast to increased expression of pro-tumorigenic genes, these results demonstrate decreased protein expression of a gene, for example, RKIP, can serve as a prognostic marker in PCa patients.

The Identification of Mouse Sperm-Surface-Associated Proteins and Characterization of Their Ability to Act as Decapacitation Factors1

Mammalian spermatozoa must undergo capacitation before acquiring the ability to fertilize the oocyte. This process is believed to be initiated following the release of surface-associated decapacitation factors that are elaborated by both the epididymis and the male accessory organs. Herein, we report the identification of a number of proteins that are actively released from the surface of mouse spermatozoa during capacitation in vitro. As anticipated, the addition of these factors back to suspensions of mouse spermatozoa was shown to suppress several correlates of the capacitation process. Specifically, they induced a significant, dose-dependent inhibition of the ability of spermatozoa to undergo a progesterone-induced acrosome reaction and to bind to the zona pellucida in vitro. Inhibition of these functions was associated with the suppression of tyrosine phosphorylation in the sperm plasma membrane but had no effect on the phosphorylation of internal proteins in either the sperm head or tail. This inhibitory activity was attributed to a subset of the isolated proteins compromising at least four putative decapacitation factors. These proteins were identified via tandem-mass spectrometry amino acid sequence analysis as plasma membrane fatty acid binding protein, cysteine-rich secretory protein 1 (CRISP1), phosphatidylethanolamine binding protein 1 (PBP), and an unnamed protein product that we have termed decapacitation factor 10 (DF10). Of these proteins, PBP was identified as a primary candidate for a decapacitation factor.

Decreased phosphatidylethanolamine binding protein expression correlates with Abeta accumulation in the Tg2576 mouse model of Alzheimer's disease

Phosphatidylethanolamine binding protein (PEBP) is a multifunctional protein, with proposed roles as the precursor protein of hippocampal cholinergic neurostimulating peptide (HCNP), and as the Raf kinase inhibitor protein (RKIP). Previous studies have demonstrated a decrease in PEBP mRNA in CA1 region of AD hippocampus. The current study demonstrates that PEBP is decreased in the hippocampus of 11 month Tg2576 mice, in the absence of change in mRNA levels compared to non-transgenic littermates. The level of PEBP in transgenic mouse hippocampus significantly decreases at 11 months (a time point when Abeta begins accumulating) and 15 months (when Abeta plaques have formed). There was a significant correlation between decreased PEBP expression and accumulation of Abeta. Immunohistochemical studies on Tg2576 and AD brain sections demonstrate that PEBP immunoreactivities are present at the periphery of dense multicore Abeta plaques, and in selective astrocytes, primarily surrounding plaques. These findings suggest that PEBP expression may be influenced by accumulation of Abeta. Down-regulation of PEBP may result in lower levels of HCNP or altered coordination of signal transduction pathways that may contribute to neuronal dysfunction and pathogenesis in AD.

The biology of a prostate cancer metastasis suppressor protein: Raf kinase inhibitor protein

Raf kinase inhibitor protein (RKIP) was originally identified as a protein that bound membrane phospholipids and was named phosphatidylethanolamine binding protein-2 (PEBP-2). RKIP was than identified as a protein that bound Raf and blocked its ability to phosphorylate MEK, thus earning its new name of RKIP. Subsequent to identification of its role in the Raf:MEK pathway, RKIP has been demonstrated to regulate several other signaling pathways including G-protein signaling and NF-kappaB signaling. Its involvement in several signaling pathways has engendered RKIP to contribute to several physiological processes including membrane biosynthesis, spermatogenesis, neural development, and apoptosis. RKIP is expressed in many tissues including brain, lung, and liver and thus, dysregulation of RKIP expression or function has potential to contribute to pathophysiology in these tissues. Loss of RKIP expression in prostate cancer cells confers a metastatic phenotype on them. Additionally, restoration of RKIP expression in a metastatic prostate cancer cell line does not effect primary tumor growth, but it does inhibit prostate cancer metastasis. These parameters identify RKIP as a metastasis suppressor gene. In this review, the biology and pathophysiology of RKIP is described.

PROTEIN EXPRESSION ANALYSIS OF RAT TESTES INDUCED TESTICULAR TOXICITY WITH SEVERAL REPRODUCTIVE TOXICANTS

The utilization of safety biomarkers to predict the possibility of compound-related toxicity provides several advantages for drug discovery and development, especially at an early stage. The objectives of this study were to investigate the effects of male reproductive toxicants on protein expression profiles in the rat testes and to identify potential biomarker candidates. Four well-known reproductive toxicants, ethylene glycol monomethyl ether (EGME), cyclophosphamide (CP), sulfasalazine (SASP) and 2,5-hexanedione (2,5-HD), were administered to male rats in a single dose, and protein expression profiles were investigated after 24 hr by two-dimensional gel electrophoresis (2DE). Histopathological examination of the testes and serum concentration analysis were also performed. From the results of the comparison of 2D-gels among different doses of a compound and among compounds, 52, 20, 24 and 111 spots were nominated as differentially expressed spots with EGME, CP, SASP and 2,5-HD treatments, respectively. Several spermatogenesis-involved proteins were identified, including glutathione S-transferase (GST), testis-specific heat shock protein 70-2 (HSP70-2), glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and phosphatidylethanolamine-binding protein (PEBP). Some of them were altered by more than one compound. In summary, remarkable histopathological findings were observed only in the EGME high-dose group, and most of the protein changes were detected before histopathological changes occurred. Therefore, the proteins identified in this study could potentially serve as biomarkers to evaluate male reproductive toxicity at an early stage of drug discovery and development.

Metastasis suppressor genes: a role for raf kinase inhibitor protein (RKIP)

The metastatic cascade is a complicated process that involves many steps from gain of the metastatic phenotype in the primary tumor cells through establishment of macroscopic tumor at the distant target organ. A group of genes, termed metastasis suppressor genes (MSG), encode for proteins that inhibit various steps of the metastatic cascade. Accordingly, loss of MSG promotes the metastatic phenotype. Although several MSG have been identified, the mechanisms through which they enhance metastasis are not clearly defined. Gene array analysis of a low metastatic LNCaP prostate cancer cell line compared to its highly metastatic derivative C4-2B prostate cancer cell line revealed decreased expression of raf kinase inhibitor protein (RKIP) in the C4-2B cell line. RKIP blocks the activation of several signaling pathways including MEK, G-proteins and NFkappaB. Immunohistochemical analysis of prostate cancer primary tumors and metastases revealed that RKIP protein expression was decreased in metastases. Restoration of RKIP expression in the C4-2B cell line diminished metastasis in a murine model. These results demonstrate that RKIP is a MSG. Loss of RKIP enhanced both angiogenesis and vascular invasion, and protected against apoptosis. These findings suggest that targeting the RKIP pathway may diminish the metastatic cascade. However, challenges exist as to the best method to target RKIP expression. Restoration of RKIP expression in all cancer cells in vivo is challenging. A plausible strategy is to use small molecules that target proteins in signaling pathways that are dysregulated due to loss of RKIP.

The role of Raf kinase inhibitor protein (RKIP) in health and disease

Raf kinase inhibitor protein (RKIP) is a member of the phosphatidylethanolamine-binding protein (PEBP) family. RKIP plays a pivotal modulatory role in several protein kinase signaling cascades. RKIP binds inhibits Raf-1-mediated phosphorylation of MEK through binding to Raf-1. Protein kinase C (PKC) phosphorylates RKIP, resulting in release of Raf-1 and activation of MEK and ERK. The phosphorylated RKIP binds to and inhibits G-protein-coupled receptor kinase, resulting in sustained G-protein signaling. The regulatory role that RKIP has in cell signaling is reflected in its role in physiology and pathophysiology. RKIP is involved in neural development, cardiac function and spermatogenesis and appears to have serine protease activity. In addition to its roles in physiology, dysregulated RKIP expression has the potential to contribute to pathophysiological processes including Alzheimer's disease and diabetic nephropathy. RKIP has been shown to fit the criteria of being a metastasis suppressor gene, including having decreased expression in prostate cancer metastases and restoring RKIP expression in a prostate cancer cell line diminishes metastasis in a murine model. Clearly, RKIP has multiple molecular and cellular functions. In this review, RKIP's molecular roles in intracellular signaling, its physiological functions and its role in disease are described.

Differentiation induction of human keratinocytes by phosphatidylethanolamine-binding protein

Phosphatidylethanolamine-binding protein (PEBP) has been demonstrated to bind to Raf-1 and mitogen-activated protein kinase kinase, components of the extracellular signal-regulated protein kinase (ERK) pathway, thereby inhibiting the pathway and resulting in the suppression of cell proliferation. In the present study, we examined whether PEBP is involved in differentiation induction of human keratinocytes. PEBP expression was immunohistochemically examined in normal human skin and skin cancers with different differentiation properties. PEBP was not expressed in the basal layer of the epidermis but was expressed in the spinous and granular layers of normal skin. The protein was expressed in differentiated but not in undifferentiated carcinoma. PEBP expression was also examined in cultured normal human epidermal keratinocytes in which differentiation was induced by calcium treatment. Involucrin was used as a differentiation marker for spinous and granular cells. Northern blotting analysis indicated that both PEBP and involucrin mRNAs were enhanced 6 h after treatment with 2.0 mM CaCl(2). The protein amount of PEBP was also increased by this treatment. To investigate whether PEBP is involved in differentiation induction of keratinocytes, HaCaT keratinocytes were transfected with an expression vector. Fluorescent immunostain revealed that cells expressing PEBP exhibited enlarged and flattened cell shape, and induction of involucrin expression was demonstrated by immunoblot analysis. Although the protein amount of ERK was not altered, phosphorylated ERK levels were decreased and cell proliferation was partly inhibited by PEBP expression. These results indicate that PEBP not only inhibits cell proliferation but also induces differentiation of human keratinocytes.

Molecular diffusion in sperm plasma membranes during epididymal maturation

Fluorescence recovery after photobleaching (FRAP) analysis has been used to measure lipid diffusion in different regions of the sperm plasma membrane. Our goal has been to understand how some membrane components are confined to specific surface domains, whilst others are freely diffusing and in some cases are able to migrate against large concentration gradients. Results with a variety of fluorescent lipid reporter probes (ODAF, NBD-PC, NBD-cholesterol) show that diffusion coefficients (D) are generally three to four times higher on the sperm acrosome than on the principal piece of the tail and increase significantly during epididymal maturation (ram, mouse, goat, dog and monkey sperm). Cholesterol diffusion is approximately 10 times faster on the sperm head than the tail and has a heterogenous distribution when detected with filipin. Lipid diffusion is very temperature sensitive but remarkably insensitive to changes in external pH and osmotic pressure. There was no evidence that the posterior ring or annulus functioned as diffusion barriers to lipids. On this basis it was possible to construct models of increasing complexity to describe the behaviour of a lipid molecule on the sperm surface, beginning with simple linear diffusion progressing to random diffusion and eventually to constrained diffusion.

The Hippocampal Cholinergic Neurostimulating Peptide, the N-terminal Fragment of the Secreted Phosphatidylethanolamine-binding Protein, Possesses a New Biological Activity on Cardiac Physiology

Phosphatidylethanolamine-binding protein (PEBP), alternatively named Raf-1 kinase inhibitor protein, is the precursor of the hippocampal cholinergic neurostimulating peptide (HCNP) corresponding to its natural N-terminal fragment, previously described to be released by hippocampal neurons. PEBP is a soluble cytoplasmic protein, also associated with plasma and reticulum membranes of numerous cell types. In the present report, using biochemistry and cell biology techniques, we report for the first time the presence of PEBP in bovine chromaffin cell, a well described secretion model. We have examined its presence at the subcellular level and characterized this protein on both secretory granule membranes and intragranular matrix. In addition, its presence in bovine chromaffin cell and platelet exocytotic medium, as well as in serum, was reported showing that it is secreted. Like many other proteins that lack signal sequence, PEBP may be secreted through non-classic signal secretory mechanisms, which could be due to interactions with granule membrane lipids and lipid rafts. By two-dimensional liquid chromatography-tandem mass spectrometry, HCNP was detected among the intragranular matrix components. The observation that PEBP and HCNP were secreted with catecholamines into the circulation prompted us to investigate endocrine effects of this peptide on cardiovascular system. By using as bioassay an isolated and perfused frog (Rana esculenta) heart preparation, we show here that HCNP acts on the cardiac mechanical performance exerting a negative inotropism and counteracting the adrenergic stimulation of isoproterenol. All together, these data suggest that PEBP and HCNP might be considered as new endocrine factors involved in cardiac physiology.

Secreted epididymal glycoprotein 2D6 that binds to the sperm's plasma membrane is a member of the beta-defensin superfamily of pore-forming glycopeptides

The plasma membrane of spermatozoa undergoes substantial remodeling during passage through the epididymal duct, principally because of changes in phospholipid composition, exchange of glycoproteins with epididymal fluid, and processing of existing membrane proteins. Here, we describe the interaction of an epididymal glycoprotein recognized by monoclonal antibody 2D6 with the plasma membrane of rat spermatozoa. Our goals have been to understand more about the mechanism of secretion of epididymal glycoproteins, how they interact with the sperm's plasma membrane, and their disposition within it. Reactivity to 2D6 monoclonal antibody was first detectable in principal cells in the distal caput epididymidis and as a soluble high-molecular-weight complex in the secreted fluid. It was not associated with membranous vesicles in the duct lumen. On cauda spermatozoa 2D6 monoclonal antibody recognized a 24-kDa glycoprotein (the subunit of a disulfide cross-linked homodimer of 48 kDa) that was present on the plasma membrane overlying the sperm tail. Binding of 2D6 to immature spermatozoa in vitro was cell-type specific but not species specific, and the antigen could only be extracted from cauda spermatozoa with detergents. Sequencing studies revealed that the 24-kDa glycoprotein was a member of the beta-defensin superfamily of small pore-forming glycopeptides of which several others (ESP13.2, Bin1b, E-2, EP2, HE2) are found in the epididymis. This evidence suggests that some epididymal glycoproteins are secreted into the luminal fluid in a soluble form and bind to specific regions of the sperm's surface via hydrophobic interactions. Given the antimicrobial function of beta-defensins, they have a putative role in protecting spermatozoa and the epididymis from bacterial infections.

Identification of a novel testis-specific member of the phosphatidylethanolamine binding protein family, pebp-2

The phosphatidylethanolamine binding proteins (pebps) are an evolutionarily conserved family of proteins recently implicated in mitogen-activated protein (MAP) kinase pathway regulation, where they are called raf kinase inhibitory proteins. Here, we describe the cloning, cellular localization, and partial characterization of a new member, pebp-2, with potential roles in male fertility. Expression data show that pebp-2 is a testis-specific 21-kDa protein found within late meiotic and haploid germ cells in a stage-specific pattern that is temporally distinct from that of pebp-1. Sequence analyses suggest that pebp-2 forms a distinct subset of the pebp family within mammals. Database analyses revealed the existence of a third subset. Analysis suggests that the specificity/regulation of the distinct pebps subsets is likely to be determined by the amino terminal 40 amino acids or the 3' untranslated region, where the majority of sequence differences occur. Protein homology modeling suggests that pebp-2 protein is, however, topologically similar to other pebps and composed of Greek key fold motifs, a dominant beta-sheet formed from five anti-parallel beta strands forming a shallow groove associated with a putative phosphatidylethanolamine binding site. The pebp-2 gene is intronless and data suggest that it is a retrogene derived from pebp-1. Further, pebp-2 colocalizes with members of the MAP kinase pathway in late spermatocytes and spermatids and on the midpiece of epididymal sperm. These data raise the possibility that pebp-2 is a novel participant in the MAP kinase signaling pathway, with a role in spermatogenesis or posttesticular sperm maturation.

The phosphatidylethanolamine-binding protein is the prototype of a novel family of serine protease inhibitors

Serine proteases are involved in many processes in the nervous system and specific inhibitors tightly control their proteolytic activity. Thrombin is thought to play a role in tissue development and homeostasis. To date, protease nexin-1 is the only known endogenous protease inhibitor that specifically interferes with thrombotic activity and is expressed in the brain. In this study, we report the detection of a novel thrombin inhibitory activity in the brain of protease nexin-1(-/-) mice. Purification and subsequent analysis by tandem mass spectrometry identified this protein as the phosphatidylethanolamine-binding protein (PEBP). We demonstrate that PEBP exerts inhibitory activity against several serine proteases including thrombin, neuropsin, and chymotrypsin, whereas trypsin, tissue type plasminogen activator, and elastase are not affected. Since PEBP does not share significant homology with other serine protease inhibitors, our results define it as the prototype of a novel class of serine protease inhibitors. PEBP immunoreactivity is found on the surface of Rat-1 fibroblast cells and although its sequence contains no secretion signal, PEBP-H(6) can be purified from the conditioned medium upon recombinant expression.

The structure of Antirrhinum centroradialis protein (CEN) suggests a role as a kinase regulator

Expression of the plant protein centroradialis (CEN) leads to a morphological switch between shoot growth and the development of flower structures (inflorescence). We have determined the crystal structure of Antirrhinum CEN to 1.9 A resolution. This structure confirms the CEN proteins as a subset of the family of phosphatidylethanolamine-binding proteins (PEBP), as predicted from sequence homology. Mammalian forms of PEBP have been found to act as inhibitors of MAP kinase signalling, a central signalling cascade regulating cell differentiation. CEN and PEBP proteins share a similar topology dominated by a large central beta-sheet. The strong conservation of a binding pocket at one end of this sheet which is capable of binding phosphoryl ligands, suggests the biological effects of CEN, like PEBP, arise from the ability of this region to form complexes with phosphorylated ligands, hence interfering with kinases and their effectors.

Differential expression of phosphatidylethanol-amine-binding protein in rat hepatoma cell lines: analyses of tumor necrosis factor-alpha-resistant cKDH-8/11 and -sensitive KDH-8/YK cells by two-dimensional gel electrophoresis

To determine intracellular factors influencing the sensitivity of cancer cells to tumor necrosis factor-alpha (TNF-alpha), we studied the expression of intracellular proteins in TNF-alpha-resistant cKDH-8/11 and -sensitive KDH-8/YK rat hepatoma cell lines using the technique of two-dimensional gel electrophoresis (2-DE). From the 2-DE patterns, it was demonstrated that TNF-alpha-resistant cKDH-8/11 cells had increased levels of protein of molecular weight (Mr) 22 500 and isoelectric point (pI) 5.2, compared with TNF-alpha-sensitive KDH-8/YK cells. Therefore, we excised cyanogen bromide (CNBr) fragments of proteins in the spot for N-terminal sequencing. Microsequencing for the CNBr fragments identified the protein as rat phosphatidylethanolamine-binding protein. These findings suggest that the intracellular phosphatidylethanolamine-binding protein could be one of the factors responsible for the resistance of cKDH-8/11 cells to TNF-alpha-induced cell death.

Stability and physicochemical properties of the bovine brain phosphatidylethanolamine-binding protein

The equilibrium behaviour of the bovine phosphatidylethanolamine-binding protein (PEBP) has been studied under various conditions of pH, temperature and urea concentration. Far-UV and near-UV CD, fluorescence and Fourier transform infrared spectroscopies indicate that, in its native state, PEBP is mainly composed of beta-sheets, with Trp residues mostly localized in a hydrophobic environment; these results suggest that the conformation of PEBP in solution is similar to the three-dimensional structure determined by X-ray crystallography. The pH-induced conformational changes show a transition midpoint at pH 3.0, implying nine protons in the transition. At neutral pH, the thermal denaturation is irreversible due to protein precipitation, whereas at acidic pH values the protein exhibits a reversible denaturation. The thermal denaturation curves, as monitored by CD, fluorescence and differential scanning calorimetry, support a two-state model for the equilibrium and display coincident values with a melting temperature Tm = 54 degrees C, an enthalpy change DeltaH = 119 kcal.mol-1 and a free energy change DeltaG(H2O, 25 degrees C) = 5 kcal.mol-1. The urea-induced unfolding profiles of PEBP show a midpoint of the two-state unfolding transition at 4.8 M denaturant, and the stability of PEBP is 4.5 kcal.mol-1 at 25 degrees C. Moreover, the surface active properties indicate that PEBP is essentially a hydrophilic protein which progressively unfolds at the air/water interface over the course of time. Together, these results suggest that PEBP is well-structured in solution but that its conformation is weakly stable and sensitive to hydrophobic conditions: the PEBP structure seems to be flexible and adaptable to its environment.

The novel epididymal secretory protein ESP13.2 in Macaca fascicularis

Newly synthesized mammalian spermatozoa undergo critical modifications as they pass along the epididymis. The modifications endow spermatozoa with fertilizing ability and occur largely as a consequence of epididymal gene expression. With this in mind, we here employed a cDNA cloning strategy designed to identify key epididymal gene products. We describe a novel cynomolgus monkey (Macaca fascicularis) epididymal transcript designated cy-ESP13.2, of 690 nucleotides. The putative human ortholog was cloned and is highly conserved. Both cDNA sequences predict small, secretory proteins with a disulfide-stabilized core. Anti-peptide polyclonal antibodies were raised to a predicted cy-ESP13.2 surface loop. Western blotting with these antibodies revealed high-level, epididymis-specific expression of cy-ESP13.2, consistent with the pattern of cy-ESP13.2 mRNA expression assessed by Northern blotting. cy-ESP13.2 protein was of 30 kDa and was readily detectable in epithelial cells lining the efferent ductules, initial segment, and cauda regions of the epididymis, but not on spermatozoa. Similarities to members of the four-disulfide-core family suggest clues to ESP13.2 function.

Regionalized lipid diffusion in the plasma membrane of mammalian spermatozoa

The plasma membrane of mammalian spermatozoa shows pronounced lateral asymmetry with many glycoproteins restricted to specific domains. Some of these antigens are freely diffusing throughout the membrane whereas others appear static in position. It is not clear whether these concepts also apply to membrane lipids. In this investigation we have used fluorescence recovery after photobleaching (FRAP) techniques to spatially resolve lipid dynamics in various surface domains of 5 species of mammalian spermatozoa (bull, boar, ram, mouse, and guinea pig). Sperm plasma membranes were loaded with 5-(N-octadecanoyl)aminofluorescein (ODAF) reporter probe, and its diffusion was measured in various domains by FRAP analysis. Results showed that in live bull, boar, ram, and mouse spermatozoa, diffusion coefficients (D) were significantly higher over the acrosome and postacrosome than on the midpiece and principal piece of the tail. In dead or permeabilized cells, on the other hand, large immobile phases developed, particularly on the sperm tail, that severely reduced D values. ODAF diffusion was also sensitive to temperature and cross-linking of protein components within the membrane with paraformaldehyde. Guinea pig spermatozoa were different in almost all respects from those of the other species tested. It is concluded that lipid diffusion in the plasma membrane of live spermatozoa varies significantly between surface domains, because of either compositional heterogeneity, or differences in bilayer disposition, or the presence of intramembranous barriers that impede free exchange between domains. This study emphasizes the important role of membrane lipids in regulating polarized migration of sperm surface antigens during developmental processes such as maturation and capacitation.

Crystal structure of the phosphatidylethanolamine-binding protein from bovine brain: a novel structural class of phospholipid-binding proteins

BACKGROUND

Phosphatidylethanolamine-binding protein (PEBP) is a basic protein found in numerous tissues from a wide range of species. The screening of gene and protein data banks defines a family of PEBP-related proteins that are present in a variety of organisms, including Drosophila and inferior eukaryotes. PEBP binds to phosphatidylethanolamine and nucleotides in vitro, but its biological function in vivo is not yet known. The expression of PEBP and related proteins seems to be correlated with development and cell morphogenesis, however. To obtain new insights into the PEBP family and its potential functions, we initiated a crystallographic study of bovine brain PEPB.

RESULTS

The X-ray crystal structure of bovine brain PEBP has been solved using multiple isomorphous replacement methods, and refined to 1.84 A resolution. The structure displays a beta fold and exhibits one nonprolyl cis peptide bond. Analysis of cavities within the structure and sequence alignments were used to identify a putative ligand-binding site. This binding site is defined by residues of the C-terminal helix and the residues His85, Asp69, Gly109 and Tyr119. This site also corresponds to the binding site of phosphorylethanolamine, the polar head group of phosphatidylethanolamine.

CONCLUSIONS

This study shows that PEBP is not related to the G-protein family nor to known lipid-binding proteins, and therefore defines a novel structural family of phospholipid-binding proteins. The PEBP structure contains no internal hydrophobic pocket, as described for lipocalins or small phospholipid-transfer proteins. Nevertheless, in PEBP, a small cavity close to the protein surface has a high affinity for anions, such as phosphate and acetate, and also phosphorylethanolamine. We suggest that this cavity corresponds to the binding site of the polar head group of phosphatidylethanolamine.

Function from structure? The crystal structure of human phosphatidylethanolamine-binding protein suggests a role in membrane signal transduction

BACKGROUND

Proteins belonging to the phosphatidylethanolamine-binding protein (PEBP) family are highly conserved throughout nature and have no significant sequence homology with other proteins of known structure or function. A variety of biological roles have previously been described for members of this family, including lipid binding, roles as odorant effector molecules or opioids, interaction with the cell-signalling machinery, regulation of flowering plant stem architecture, and a function as a precursor protein of a bioactive brain neuropeptide. To date, no experimentally derived structural information has been available for this protein family. In this study we have used X-ray crystallography to determine the three-dimensional structure of human PEBP (hPEBP), in an attempt to clarify the biological role of this unique protein family.

RESULTS

The crystal structures of two forms of hPEBP have been determined: one in the native state (at 2.05 A resolution) and one in complex with cacodylate (at 1.75 A resolution). The crystal structures reveal that hPEBP adopts a novel protein topology, dominated by the presence of a large central beta sheet, and is expected to represent the archaetypal fold for this family of proteins. Two potential functional sites have been identified from the structure: a putative ligand-binding site and a coupled cleavage site. hPEBP forms a dimer in the crystal with a distinctive dipole moment that may orient the oligomer for membrane binding.

CONCLUSIONS

The crystal structure of hPEBP suggests that the ligand-binding site could accommodate the phosphate head groups of membrane lipids, therefore allowing the protein to adhere to the inner leaf of bilipid membranes where it would be ideally positioned to relay signals from the membrane to the cytoplasm. The structure also suggests that ligand binding may lead to coordinated release of the N-terminal region of the protein to form the hippocampal neurostimulatory peptide, which is known to be active in the development of the hippocampus. These studies are consistent with a primary biological role for hPEBP as a transducer of signals from the interior membrane surface.

Expression of phosphatidylethanolamine-binding protein in the male reproductive tract: immunolocalisation and expression in prepubertal and adult rat testes and epididymides

Phosphatidylethanolamine-binding protein (PBP) has been described previously in the male reproductive tract, where it has been implicated in the biogenesis and maintenance of antigen segregation of membranes. In the present study we have used a specific antiserum to PBP to determine its expression and localisation in the adult and prepubertal rat testis and epididymis by Western blotting and immunohistochemistry. In the adult rat testis, PBP was localised to step 17-19 elongating spermatids, residual bodies, and interstitial Leydig cells. In the adult epididymis, PBP was localised to epithelial cells of the caput, corpus, and cauda regions and to the cytoplasmic droplets of spermatozoa in the lumen of the initial segment, caput, and corpus epididymidis. In prepubertal animals, PBP was expressed in both testes and epididymides from day 1 and day 3 postpartum, respectively (day 3 being the earliest epididymal tissue taken). In prepubertal testes, PBP was localised to Leydig cells from day 1 postpartum and was not detected in any other cell type until the differentiation of elongate spermatids, when it was detected in step 17-19 elongating spermatids. These data suggest that PBP may be involved in the organisation of sperm membranes during spermiogenesis. The presence of PBP in Leydig cells, however, suggests diverse roles for this protein as a lipid carrier or binding protein.

The molecular biology of the sperm surface. Post-testicular membrane remodelling

The membrane of testicular spermatozoa undergoes extensive changes in the epididymis, including rearrangement, modification and loss of pre-existing components, addition of new glycoproteins from epididymal secretions, and exchange of lipid constituents. As a result, the membrane of cauda epididymidal spermatozoa has a different composition and different properties, which collectively contribute to male fertility. Special significance has been attributed to sperm surface structures that only appear post-testicularly in the epididymis, the so-called "maturation antigens". Therefore, human post-testicular proteins have been cloned by substractive screening of epididymal cDNA libraries, employing testis as the primary negative control. To date, there is scanty information on their function and mechanism of deposition on the sperm surface. However, the major maturation antigen CD52 seems to bind firmly to the sperm membrane via its GPI anchor. Its synthesis is carefully regulated by the cells of the epididymal epithelium, with temperature and androgens acting synergistically on CD52 mRNA levels.