Characterization and use of monoclonal antibodies for isolation of phosphotyrosyl proteins from retrovirus-transformed cells and growth factor-stimulated cells

Bringing Histidine Phosphorylation into Light: Role of Chemical Tools

Histidine phosphorylation is a historically underexplored post-translational modification (PTM). Once deemed "elusive" due to its chemical lability, phosphohistidine (pHis) has recently come to light thanks to emerging chemical tools─including stable pHis analogs, pHis-specific antibodies, and tailored proteomics workflows─that enable its detection and functional analysis. Together, these innovations have led to a surge in the identification of pHis sites and raised awareness of their roles in both bacterial and mammalian systems. New assay systems have also facilitated the characterization of histidine kinases and phosphatases. This Review summarizes recent breakthroughs in pHis research tools, examines the limitations of current approaches, and outlines future tools needed to fully unravel the potential of histidine phosphorylation.

IL-4 and IL-13 Receptor Signaling From 4PS to Insulin Receptor Substrate 2: There and Back Again, a Historical View

In this historical perspective, written in honor of Dr. William E. Paul, we describe the initial discovery of one of the dominant substrates for tyrosine phosphorylation stimulated by IL-4. We further describe how this “IL-4-induced phosphorylated substrate” (4PS) was characterized as a member of the insulin receptor substrate (IRS) family of large adaptor proteins that link IL-4 and insulin receptors to activation of the phosphatidyl-inositol 3′ kinase pathway as well as other downstream signaling pathways. The relative contribution of the 4PS/IRS pathway to the early models of IL-4-induced proliferation and suppression of apoptosis are compared to our more recent understanding of the complex interplay between positive and negative regulatory pathways emanating from members of the IRS family that impact allergic responses.

pHisphorylation: the emergence of histidine phosphorylation as a reversible regulatory modification

Histidine phosphorylation is crucial for prokaryotic signal transduction and as an intermediate for several metabolic enzymes, yet its role in mammalian cells remains largely uncharted. This is primarily caused by difficulties in studying histidine phosphorylation because of the relative instability of phosphohistidine (pHis) and lack of specific antibodies and methods to preserve and detect it. The recent synthesis of stable pHis analogs has enabled development of pHis-specific antibodies and their use has started to shed light onto this important, yet enigmatic posttranslational modification. We are beginning to understand that pHis has broader roles in protein and cellular function including; cell cycle regulation, phagocytosis, regulation of ion channel activity and metal ion coordination. Two mammalian histidine kinases (NME1 and NME2), two pHis phosphatases (PHPT1 and LHPP), and a handful of substrates were previously identified. These new tools have already led to the discovery of an additional phosphatase (PGAM5) and hundreds of putative substrates. New methodologies are also being developed to probe the pHis phosphoproteome and determine functional consequences, including negative ion mode mass spectroscopy and unnatural amino acid incorporation. These new tools and strategies have the potential to overcome the unique challenges that have been holding back our understanding of pHis in cell biology.

Monoclonal 1- and 3-Phosphohistidine Antibodies: New Tools to Study Histidine Phosphorylation

Histidine phosphorylation (pHis) is well studied in bacteria; however, its role in mammalian signaling remains largely unexplored due to the lack of pHis-specific antibodies and the lability of the phosphoramidate (P-N) bond. Both imidazole nitrogens can be phosphorylated, forming 1-phosphohistidine (1-pHis) or 3-phosphohistidine (3-pHis). We have developed monoclonal antibodies (mAbs) that specifically recognize 1-pHis or 3-pHis; they do not cross-react with phosphotyrosine or the other pHis isomer. Assays based on the isomer-specific autophosphorylation of NME1 and phosphoglycerate mutase were used with immunoblotting and sequencing IgG variable domains to screen, select, and characterize anti-1-pHis and anti-3-pHis mAbs. Their sequence independence was determined by blotting synthetic peptide arrays, and they have been tested for immunofluorescence staining and immunoaffinity purification, leading to putative identification of pHis-containing proteins. These reagents should be broadly useful for identification of pHis substrates and functional study of pHis using a variety of immunological, proteomic, and biological assays.

4-Phosphothiophen-2-yl alanine: a new 5-membered analogue of phosphotyrosine

Polyclonal antibodies raised against 4-phosphothiophen-2-yl alanine 2a, a novel five-membered ring analogue of phosphotyrosine, showed high selectivity for phosphotyrosine and no cross-reactivity with other phosphorylated amino acids. Western blots showed that the polyclonal was similarly effective, but different in selectivity, to a commercially available monoclonal antibody.

4-Phosphopyrazol-2-yl alanine: a non-hydrolysable analogue of phosphohistidine

We report the synthesis of a stable analogue of τ-phosphohistidine: 4-phosphopyrazol-2-yl alanine (pPza).

What pathogens have taught us about posttranslational modifications

Pathogens use various mechanisms to manipulate host processes to promote infection. Decades of research on pathogens have revealed not only the molecular mechanisms that these microbes use to replicate and survive within host cells, but also seminal information on how host signaling machinery regulates cellular processes. Among these discoveries are mechanisms involving posttranslational modifications that alter the activity, localization, or interactions of the modified protein. Herein, we examine how pathogens have contributed to our basic understanding of three posttranslational modifications: phosphorylation, NMPylation, and ubiquitylation. Over the years, technologies, techniques and research tools have developed side by side with the study of pathogens, facilitating the discovery of protein modifications and furthering our understanding of how they contribute to both infection and cellular functions.

A pan-specific antibody for direct detection of protein histidine phosphorylation

Despite its importance in central metabolism and bacterial cell signaling, protein histidine phosphorylation has remained elusive with respect to its extent and functional roles in biological systems because of the lack of adequate research tools. We report the development of the first pan-phosphohistidine (pHis) antibody using a stable pHis mimetic as the hapten. This antibody was successfully used in ELISA, western blotting, dot blot assays and immunoprecipitation and in detection and identification of histidine-phosphorylated proteins from native cell lysates when coupled with MS analysis. We also observed that the amount of protein pHis in Escherichia coli lysates depends on carbon source and nitrogen availability in the growth medium. In particular, we found that the amount of pHis on phosphoenolpyruvate synthase (PpsA) is sensitive to nitrogen availability in vivo and that α-ketoglutarate inhibits phosphotransfer from phosphorylated PpsA to pyruvate. We expect this antibody to open opportunities for investigating other pHis proteins and their functions.

Sperm phosphoproteomics: historical perspectives and current methodologies

Mammalian sperm are differentiated germ cells that transfer genetic material from the male to the female. Owing to this essential role in the reproductive process, an understanding of the complex mechanisms that underlie sperm function has implications ranging from the development of novel contraceptives to the treatment of male infertility. While the importance of phosphorylation in sperm differentiation, maturation and fertilization has been well established, the ability to directly determine the sites of phosphorylation within sperm proteins and to quantitate the extent of phosphorylation at these sites is a recent development that has relied almost exclusively on advances in the field of proteomics. This review will summarize the work that has been carried out to date on sperm phosphoproteomics and discuss how the resulting qualitative and quantitative information has been used to provide insight into the manner in which protein phosphorylation events modulate sperm function. The authors also present the proteomics process as it is most often utilized for the elucidation of protein expression, with a particular emphasis on the way in which the process has been modified for the analysis of protein phosphorylation in sperm.

Subcellular phosphoproteomics

Protein phosphorylation represents one of the most extensively studied post-translational modifications, primarily due to the emergence of sensitive methods enabling the detection of this modification both in vitro and in vivo. The availability of enrichment methods combined with sensitive mass spectrometry instrumentation has played a crucial role in uncovering the dynamic changes and the large expanding repertoire of this reversible modification. The structural changes imparted by the phosphorylation of specific residues afford exquisite mechanisms for the regulation of protein functions by modulating new binding sites on scaffold proteins or by abrogating protein-protein interactions. However, the dynamic interplay of protein phosphorylation is not occurring randomly within the cell but is rather finely orchestrated by specific kinases and phosphatases that are unevenly distributed across subcellular compartments. This spatial separation not only regulates protein phosphorylation but can also control the activity of other enzymes and the transfer of other post-translational modifications. While numerous large-scale phosphoproteomics studies highlighted the extent and diversity of phosphoproteins present in total cell lysates, the further understanding of their regulation and biological activities require a spatio-temporal resolution only achievable through subcellular fractionation. This review presents a first account of the emerging field of subcellular phosphoproteomics where cell fractionation approaches are combined with sensitive mass spectrometry methods to facilitate the identification of low abundance proteins and to unravel the intricate regulation of protein phosphorylation.

Preparation and application of polyclonal and monoclonal sequence-specific anti-phosphoamino acid antibodies

This unit discusses the issues that must be considered in the design, production, and characterization of polyclonal and monoclonal sequence-specific anti-phosphoamino acid antibodies. Protocols are provided for generating and purifying such antibodies, and methods are also provided for producing useful polyclonal antibodies in a non-purified form. Support protocols describe coupling of peptides or phosphotyrosine to a solid support for use in affinity chromatography. An example of the generation, purification, and characterization of two sequence-specific anti-phosphopeptide antibodies specific for different sequences of a single phosphoprotein is described. The cross-reactivity of such antibodies, which is a common problem with anti-peptide antibodies, is also discussed.

Production of antibodies that recognize specific tyrosine-phosphorylated peptides

Phosphorylation of tyrosine residues of various proteins plays an important role in regulation of the functional activities of proteins within the cell. Antibodies to specific tyrosine-phosphorylated peptides are a valuable tool for analysis of the localization and function of individual phosphoproteins. This unit describes the methods used to produce and screen monoclonal and polyclonal antibodies for specificity for only the phosphorylated state of a peptide.

Immunological detection of phosphorylation

Incorporation of phosphates into serine, threonine, and tyrosine acceptors in proteins is a common mechanism for regulating protein function. This unit presents protocols that use specific anti-phosphoamino acid (PAA) and anti-phosphoprotein antibodies to detect protein phosphorylation and protein kinase activity. Immunoblotting to detect protein phosphorylation using either anti-PAA or anti-phosphoprotein antibodies. This is a convenient method that usually yields impressive results. Phosphorylation can also be detected by immunoprecipitation followed by immunoblot analysis or by immunofluorescent staining; these methods are typically more complicated and time consuming. All three methods have been successfully used to detect protein phosphorylation with a wide variety of antibodies and most phosphorylated proteins.

Production of antibodies that recognize specific tyrosine-phosphorylated peptides

It is possible to produce anti-phosphopeptide antibodies (i.e., antibodies recognizing phosphorylated peptides) that recognize a protein only in its phosphorylated state, and that do not cross-react with either the cognate unphosphorylated protein or other phosphoproteins. This unit describes production of antibodies against tyrosine-phosphorylated peptides, with which the authors have the most expertise, but the principles discussed here also apply to peptides phosphorylated on serine and threonine. Protocols are presented for the production of polyclonal and monoclonal anti-phosphopeptide antibodies. Support protocols are provided for the coupling of peptides and phosphotyrosine to an affinity matrix (Affi-Gel 10); BSA-agarose affinity matrix is commercially available.

The hunting of the Src

The non-receptor tyrosine kinase Src is important for many aspects of cell physiology. The viral src gene was the first retroviral oncogene to be identified, and its cellular counterpart was the first proto-oncogene to be discovered in the vertebrate genome. Src has been important, not only as an object of study in itself, but also as an entry point into the molecular genetics of cancer.

Analysis of phosphorylation of unlabeled proteins

Phosphorylation of unlabeled proteins can be detected using immunological or enzymatic techniques. Anti-phosphotyrosine antibodies are used with immunoblots to detect tyrosine phosphorylation. This unit provides a protocol for detection of antibodies by 125I-labeled protein A or enhanced chemiluminescence (ECL). Detection of enzymatic dephosphorylation using a general or phosphoamino acid-specific phosphatase and subsequent SDS-PAGE mobility shift (or a change in activity) is also described.

Identification and purification of vitamin K-dependent proteins and peptides with monoclonal antibodies specific for gamma -carboxyglutamyl (Gla) residues

Novel monoclonal antibodies that specifically recognize gamma-carboxyglutamyl (Gla) residues in proteins and peptides have been produced. As demonstrated by Western blot and time-resolved immunofluorescence assays the antibodies are pan-specific for most or all of the Gla-containing proteins tested (factors VII, IX, and X, prothrombin, protein C, protein S, growth arrest-specific protein 6, bone Gla protein, conantokin G from a cone snail, and factor Xa-like proteins from snake venom). Only the Gla-containing light chain of the two-chain proteins was bound. Decarboxylation destroyed the epitope(s) on prothrombin fragment 1, and Ca(2+) strongly inhibited binding to prothrombin. In Western blot, immunofluorescence, and surface plasmon resonance assays the antibodies bound peptides conjugated to bovine serum albumin that contained either a single Gla or a tandem pair of Gla residues. Binding was maintained when the sequence surrounding the Gla residue(s) was altered. Replacement of Gla with glutamic acid resulted in a complete loss of the epitope. The utility of the antibodies was demonstrated in immunochemical methods for detecting Gla-containing proteins and in the immunopurification of a factor Xa-like protein from tiger snake venom. The amino acid sequences of the Gla domain and portions of the heavy chain of the snake protein were determined.

Adhesion-dependent tyrosine phosphorylation of enabled in Drosophila neuronal cell line

Cell culture consisting of Drosophila BG2-c6 cells and laminin revealed its value for the analysis of the integrin-mediated activity of extracellular matrix (Takagi, Y., et al. (1998) Neurosci. Lett. 244, 149-152). To elucidate Drosophila integrin cascade further, we report here our characterization on the tyrosine phosphorylation in BG2-c6 cells, coupling with their spreading on extracellular matrix. Large-scale culture of Drosophila Kc167 cells provided a sufficient amount of extracellular matrix (including laminin) for performing biochemical analysis on the signal transduction in BG2-c6 cells. Several proteins underwent significant tyrosine phosphorylation in an adhesion-dependent manner. Among them, the heavy phosphorylation of Enabled (a substrate for Abelson tyrosine kinase) was noteworthy because of the proposed function of Enabled in cell adhesion. Together with our previous results, we propose a model for signal transduction activated by cell adhesion for the first time in Drosophila.

Involvement of protein serine and threonine phosphorylation in human sperm capacitation

The involvement of serine and threonine phosphorylation in human sperm capacitation was investigated. Anti-phosphoserine monoclonal antibody (mAb) recognized six protein bands in the 43-55-kDa, 94 +/- 2-kDa, 110-kDa, and 190-kDa molecular regions, in addition to a faint band each in the 18-kDa and 35-kDa regions. Anti-phosphothreonine mAb recognized protein bands in six similar regions, except that the 18-kDa, 35-kDa, and 94 +/- 2-kDa protein bands were sharper and thicker, and an additional band was observed in the 110-kDa molecular region. In the 43-55-kDa molecular region, there was a well-characterized glycoprotein, designated fertilization antigen, that showed a further increase in serine/threonine phosphorylation after exposure to solubilized human zona pellucida. In a cell-free in vitro kinase assay carried out on beads or in solution, four to eight proteins belonging to similar molecular regions, namely 20 +/- 2 kDa, 43-55 kDa, 94 +/- 2 kDa, and 110 +/- 10 kDa, as well as in 80 +/- 4 and 210 +/- 10 kDa regions, were phosphorylated at dual residues (serine/tyrosine and threonine/tyrosine). Capacitation increased the intensity of serine/threonine phosphorylation per sperm cell, increased the number of sperm cells that were phosphorylated, and induced a subcellular shift in the serine/threonine-specific fluorescence. These findings indicate that protein serine/threonine phosphorylation is involved and may have a physiological role in sperm capacitation.

c-Abl proto-oncoprotein is expressed and tyrosine phosphorylated in human sperm cell

The presence and possible role of c-Abl proto-oncoprotein was investigated in human sperm cell. The c-Abl monoclonal antibody (mAb), against the protein tyrosine kinase domain of v-Abl protein, reacted specifically with the acrosomal region of methanol-fixed capacitated and non-capacitated human sperm cell in the indirect immunofluorescence technique. The c-Abl mAb predominantly recognized two protein bands of 145 kD and 95 kD in detergent-solubilized (Triton X-100 and NP-40) sperm and testes preparations in the Western blot procedure. The 95 kD protein band reacted stronger than the 145 kD band and was the only band detected in the lithium diiodosalicylate (LIS)-solubilized sperm preparation, and even in the Triton X-100/NP-40 extracts of sperm of some men. In the in vitro kinase assay using the Triton X-100-solubilized capacitated sperm preparation, the 95 kD protein was autophosphorylated at the tyrosine residues, which was inhibited in the presence of c-Abl mAb. The tyrosine phosphorylation of sperm proteins, especially of the 95 kD protein, has been shown to have a vital role in human sperm function, namely, the sperm capacitation/acrosomal exocytosis and binding to zona pellucida of oocyte. These findings suggest that the c-Abl or c-Abl-like proteins are present in mature sperm cells that are tyrosine autophosphorylated and may have a role in human sperm cell function.

Role of tyrosine specific phosphorylation of cellular proteins, especially EGF receptor and p125FAK in human lung cancer cells

To clarify the role of tyrosine phosphorylation of cellular proteins in human lung cancer cells, phosphotyrosine (PTYR)-containing proteins in lung cancer cell lines and in paired tissues resected from cancerous and normal lungs were studied by immunoblotting with an anti-PTYR antibody. We found that the profiles of protein phosphorylation were very similar among those cell lines which had different histological features. The major PTYR-containing proteins (180-190 KDa, 120-130 KD, and 95-100 KDa) were detected in lung cancer cell lines. The expression of EGF receptor (EGF-r) (p185) and o-erb B2 protein, and tyrosine phosphorylation of p125FAK were examined in cancerous lung tissues and normal lung tissues. In surgical specimens, approximately half of the samples of lung cancer tissues showed clear elevation of tyrosine phosphorylation. In these cancerous tissues, no clear amplification of EGF-r and c-erb B2 protein expression was observed. However, elevation of tyrosine phosphorylation of p125FAK was observed in cancerous lung tissues but not in normal lung tissues, and its phosphorylation was closely correlated with the nodal involvement of cancer and disease-free survival time. These results suggested that the intracellular signaling pathway via tyrosine phosphorylation plays a role in the generation and immortalization of lung cancer, and assessment of tyrosine phosphorylation of cellular proteins. especially p125FAK, may be available clinically as a prognostic factor.

A possible tyrosine phosphorylation of phytochrome

Red/far-red light signal transduction by the phytochrome family of photoreceptors regulates plant growth and development. We investigated the possibility that tyrosine kinases and/or phosphatases are involved in phytochrome-mediated signal transduction using crude extracts of oat seedlings that are grown in the dark. We found that a 124 kDa protein was tyrosine-phosphorylated as determined by Western blotting with a phosphotyrosine-specific monoclonal antibody. The 124 kDa protein was recognized by the anti-phosphotyrosine antibody in anti-phytochrome A immunoprecipitates. The level of anti-phosphotyrosine antibody binding to the 124 kDa protein(s) in phytochrome immunoprecipitates that had been treated with red light prior to immunoprecipitation decreased relative to dark controls. These results suggest that either phytochrome from dark-grown seedlings is tyrosine phosphorylated or that it co-immunoprecipitates with a phosphotyrosine-containing protein of the same molecular weight. The implications of these results in the regulation of (a) the putative Ser/Thr kinase activity of the photoreceptor and (b) the binding of signaling molecules, such as phospholipase C to phytochrome, are discussed.

Fertilization antigen (FA-1) completely blocks human sperm binding to human zona pellucida: FA-1 antigen may be a sperm receptor for zona pellucida in humans

The effects of purified human sperm fertilization antigen-1 (FA-1), affinity-purified monoclonal Fab' antibody to FA-1, and monoclonal Fab' antibody to phosphotyrosine residues on human sperm-zona interaction were investigated. The purified FA-1 antigen completely blocked sperm binding to zona pellucida (P < 0.0001). Also, the monoclonal Fab' antibodies to FA-1 antigen and phosphotyrosine residues significantly (P < 0.05) reduced sperm-zona pellucidae and the antibodies were preincubated with sperm before insemination and not vice versa. These results suggest that the tyrosine phosphorylation especially of FA-1 antigen has an important role in zona pellucida receptor recognition and binding. These findings also suggest that FA-1 antigen may be the sperm receptor involved in zona pellucida binding in humans.

Protein kinases and phosphatases that act on histidine, lysine, or arginine residues in eukaryotic proteins: a possible regulator of the mitogen-activated protein kinase cascade

Phosphohistidine goes undetected in conventional studies of protein phosphorylation, although it may account for 6% of total protein phosphorylation in eukaryotes. Procedures for studying protein N- kinases are described. Genes whose products are putative protein histidine kinases occur in a yeast and a plant. In rat liver plasma membranes, activation of the small G-protein, Ras, causes protein histidine phosphorylation. Cellular phosphatases dephosphorylate phosphohistidine. One eukaryotic protein histidine kinase has been purified, and specific proteins phosphorylated on histidine have been observed. There is a protein arginine kinase in mouse and protein lysine kinases in rat. Protein phosphohistidine may regulate the mitogen-activated protein kinase cascade.

Molecular identities of human sperm proteins that bind human zona pellucida: nature of sperm-zona interaction, tyrosine kinase activity, and involvement of FA-1

The present study was conducted to investigate the molecular identities, nature of interaction, and tyrosine phosphorylation activity of the sperm-zona pellucida binding proteins in humans. Sperm proteins belonging to four major molecular regions, namely 95, 63, 51, and 14-18 kDa, reacted with zona pellucida proteins in the Western blot and immunoprecipitation procedures. In these procedures, zona pellucida protein that reacted strongest with the sperm proteins belonged to the molecular region of 55 kDa (ZP3), besides weakly reacting proteins in the 110-kDa (ZP1/ZP2) and 14-18-kDa molecular regions. The major forces involved in the sperm-zona protein interactions were of hydrophobic and ionic in nature. Three (95, 51, and 14-18 kDa) of the four molecular regions of sperm proteins that bound to the zona pellucida proteins also seem to involve o-phospho-L-tyrosine residues in their interaction, and these proteins demonstrated the presence of phosphotyrosine residues, and the 51-kDa protein also showed autophosphorylating activity in the in vitro kinase assay. The sperm binding zona protein of 55 kDa also demonstrated autophosphorylating activity. Using specific monoclonal antibody to the well characterized sperm-specific glycoprotein, designated FA-1, and the competitive inhibition in the immunoprecipitation procedure, it was found that the 51 kDa protein is indeed FA-1 antigen. Besides elucidating the molecular nature of the sperm-zona interaction, these antigens will find application in the development of a multivalent contraceptive vaccine, and may also help in specific diagnosis and treatment of infertility mediated through defective gamete (sperm or oocyte) function.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

JAK2 associates with the erythropoietin receptor and is tyrosine phosphorylated and activated following stimulation with erythropoietin

Erythropoietin (EPO) regulates the proliferation and differentiation of erythroid cells through interaction with its receptor (EPOR). Although EPOR is a member of the cytokine receptor superfamily and lacks a kinase domain, EPO induces tyrosine phosphorylation, which is correlated with gene transcription and mitogenesis. Here we demonstrate that EPO induces tyrosine phosphorylation of JAK2 kinase and activates its in vitro autophosphorylation. Using EPOR mutants, phosphorylation and activation of kinase activity correlate with the induction of mitogenesis. Furthermore, JAK2 physically associates with a membrane-proximal region of the EPOR cytoplasmic domain that is required for biological activity. The results support the hypothesis that JAK2 is the kinase that couples EPO binding to tyrosine phosphorylation and mitogenesis.

William L. McGuire Memorial Symposium. The role of erbB2 signal transduction pathways in human breast cancer

The erbB2 receptor is expressed at very high levels in nearly 30% of human breast cancer patients and plays an important role in the transformation and the prognosis of breast cancer. While evidence accumulates to support the relationship between erbB2 overexpression and poor overall survival in human breast cancer, understanding of the biological consequence(s) of erbB2 overexpression remains elusive. Our recent discovery, cloning, sequencing, and expression of the erbB2 ligand (gp30) has allowed us to identify a number of related but distinct biological endpoints which appear responsive to signal transduction through the erbB2 receptor. These endpoints of growth, invasiveness, and differentiation have clear implications for the emergence, maintenance, and/or control of malignancy, and represent established endpoints in the assessment of malignant progression in breast cancer. Studies in vitro have shown that gp30 induces a biphasic growth effect (induction of growth at low concentrations and inhibition of growth at high concentrations) on cells with erbB2 over-expression. Strikingly, we have recently observed that the erbB2 signalling pathway can be modulated by estrogen acting through the estrogen receptor (ER). Conversely, we observed that down regulation of erbB2 by estrogen can be blocked by gp30 acting through the erbB2 receptor. Clearly, mechanistic aspects of the erbB2/ligand interaction need to be understood from a therapeutic standpoint, and may furthermore provide additional insights into treatment synergy for particular patients. We think that these studies will facilitate the emergence of erbB2-targeted therapy.

Rapid effects of EGF on cytoskeletal structures and adhesive properties of highly metastatic rat mammary adenocarcinoma cells

In the highly metastatic rat mammary adenocarcinoma cell clone MTLn3, EGF induced increased adhesion to fibronectin while in the human epidermoid carcinoma cell line A431 EGF induced diminished adhesive properties. Flattening of cells with extensive formation of filopodia was observed in MTLn3 cells within 5 min of EGF addition, while in A431 cells EGF induced rounding up and only occasional formation of filopodia. Immunofluorescent analysis revealed extension of microtubules (MT) into the filopodia and Western blot analysis demonstrated an EGF-induced 2- to 3-fold increase in the amount of assembled tubulin in MTLn3 but not in A431 cells. In MTLn3, but only marginally in A431 cells, EGF treatment resulted in phosphorylation of a 280 kD cytoskeleton-associated protein, which was rapid and dose-dependent. These results suggest differential signal transduction pathway of cytoskeleton-associated EGFRs in highly metastatic MTLn3 as compared with A431 cells.

Presence of expression products of c-erbB-1 and c-erbB-2/HER2 genes on mammalian sperm cell, and effects of their regulation on fertilization

The present study was conducted to investigate the presence of expression products of c-erbB-1 and c-erbB-2/HER2 genes on mammalian sperm cell, and study the effects of their antibodies on fertilization. The mature sperm cells from various mammalian species (human, mouse, rabbit and rat) were found to have EGF-receptors but not the p185HER2 molecules by indirect immunofluorescence technique (IFT) and Western blot procedure. Though the EGF-receptors present on sperm cells were functionally active and responded to ligand binding, their activation by EGF or blocking by antibodies did not affect the sperm cells in acquiring their fertilization potential. These results indicate that the products of c-erbB-1 and c-erbB-2/HER2 genes, though they have been shown to have tyrosine kinase enzyme activity, do not seem to play a major role in the development of the fertilizing capacity of sperm cells.

Growth factor-dependent phosphorylation of membrane proteins in cultured human retinal pigment epithelial cells

Recently, growth factors are known to phosphorylate tyrosine residues of proteins to regulate cellular functions. We investigated growth factor-dependent phosphorylation of membrane proteins in cultured human retinal pigment epithelial (RPE) cells. The phosphorylation experiments were done in membrane preparations of cultured RPE cell and the reaction was started by applying [32P]adenosine 5'-triphosphate (ATP) at 0 degrees C, and terminated after 0, 1, 5, 15, and 30 min. The samples were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and were analyzed by autoradiography. Many proteins showed time-dependent phosphorylation. Among them, a 170 kDa protein showed platelet-derived growth factor (PDGF)-specific phosphorylation with both time- (up to 30 min) and dose-(maximal effect at 50 ng/ml) dependence. On the other hand, epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) showed no specific phosphorylation. Phosphoaminoacids of the 170 kDa protein were analyzed by thin layer chromatography and autoradiography. Phosphotyrosine showed much higher radioactivity than phosphoserine or phosphothreonine. Consequently, PDGF induced phosphorylation of the 170 kDa protein which mainly consisted of phosphotyrosine. The data suggest that tyrosine phosphorylation of membrane protein is involved in signal transduction of PDGF in human RPE cells.

Importance of tyrosine phosphatases in the effects of cell-cell contact and microenvironments on EGF-stimulated tyrosine phosphorylation

We have compared the EGF responses of A431 cells when grown as monolayers at a variety of cell densities or as multicellular spheroids in order to investigate the effects of cell contact and 3-dimensional structure on signal transduction. Proliferation of the A431 squamous carcinoma cell line grown in our laboratory was unaffected by EGF when grown in monolayer culture. As 3-dimensional, multicellular spheroids, however, growth was stimulated by EGF. The maximum volume attainable in the presence of EGF was more than 30 times that in its absence. EGF-dependent tyrosine phosphorylation was compared under these conditions by immunohistochemistry and Western blotting. In initial experiments using published procedures, tyrosine phosphorylation was density-dependent in monolayers and undetectable in spheroids. However, the density-dependence was abolished by the addition of high concentrations of protein tyrosine phosphatase inhibitors (1 mM Zn++ and VO4(3)-). The density dependence of EGF-stimulated tyrosine phosphorylation in monolayers was, therefore, largely the result of changes in phosphatase activity rather than kinase. Using high concentrations of phosphatase inhibitors, phosphotyrosine was clearly visible by immunohistochemistry in the outermost cells of spheroids, but it was still not visible in the spheroid center. The lack of response within the spheroid was not related to the presence of EGF receptor nor diffusion of EGF. In companion experiments, we showed that staining for EGF receptor was present homogeneously throughout the spheroid and that EGF penetrated to its center under the conditions of the experiment. Thus, although an increase in tyrosine phosphatase activity was a major factor affecting tyrosine phosphorylation in the outer cells, other factors were important in the inner cells. We concluded that an increase of tyrosine phosphatase activity was the most important component of the adaptation of the EGF signal transduction system to high cell density in monolayer cultures. In spheroids, tyrosine phosphatases are also enhanced, but other factors, such as autocrine synthesis of TGF-alpha and possibly the cellular distribution of EGF receptors and cell shape, play a role.

Aberrant elevation of tyrosine-specific phosphorylation in human gastric cancer cells

Phosphotyrosine-containing proteins in various human cancer cell lines were studied by immunoblotting with anti-phosphotyrosine antibody. Of 29 cell lines derived from oral epidermoid cancer, esophageal cancer, gastric cancer, colon cancer, pancreatic cancer, hepatocellular carcinoma and malignant melanoma, 3 of the 6 gastric cancer cells showed aberrant elevation of tyrosine-specific phosphorylation. On the other hand, both esophageal cancer cells and colon cancer cells, which were reported to have amplified epidermal growth factor receptor and activated p60v-src kinase, respectively, showed no apparent elevation of tyrosine-specific phosphorylation, and their profiles of phosphorylation were similar to that of normal human fibroblasts. Two gastric cancer cells, NUGC-4 and MKN-45, showed similar profiles of phosphorylation but their responses to growth factors differed from each other. Tyrosine phosphorylation in NUGC-4 was strongly activated by treatment with epidermal growth factor and quickly reduced by the acid treatment which is effective in removing growth factors from cellular surface receptors. On the contrary, phosphorylation in MKN-45 did not respond to either growth factor or acid treatment. These results suggest that NUGC-4 and MKN-45 have tyrosine kinases which are activated by different mechanisms but share similar substrates.

Activation of a microtubule-associated protein-2 kinase by insulin-like growth factor-I in bovine chromaffin cells

Treatment of bovine chromaffin cells with insulin-like growth factor-I (IGF-I) caused the activation of a protein kinase that phosphorylates microtubule-associated protein-2 (MAP-2) in vitro. Activation of MAP-2 kinase by IGF-I varied with the time of treatment (maximal at 10-15 min) and the concentration of IGF-I (maximal at 10 nM). The IGF-I-activated MAP-2 kinase was localized to the soluble fraction of chromaffin cell extracts and required Mg2+ for activity. The IGF-I-activated kinase also phosphorylated myelin basic protein, but had little or no activity toward histones or ribosomal S6 protein. To examine the role of protein tyrosine phosphorylation in the activation of the MAP-2 kinase, we isolated phosphotyrosine (PTyr)-containing proteins from chromaffin cells by immunoaffinity adsorption on anti-PTyr-Sepharose beads. Anti-PTyr-Sepharose eluates from IGF-I-treated cells showed increased MAP-2 kinase activity; thus, the MAP-2 kinase (or a closely associated protein) appears to be a PTyr-containing protein. Treatment of anti-PTyr-Sepharose eluates or crude chromaffin cell extracts with alkaline phosphatase significantly decreased kinase activity toward myelin basic protein, indicating that phosphorylation of the IGF-I-activated kinase is required for its activity.

Tyrosine phosphorylation of human urokinase-type plasminogen activator

Immunoblotting analysis of purified human urokinase plasminogen activator (u-PA), gives a positive signal when reacted with anti-phosphotyrosine monoclonal antibodies (MoAb anti-P-Tyr); competition with o-phospho-DL-tyrosine (P-Tyr) but not o-phospho-DL-threonine or serine (P-Treo, P-Ser) completely suppresses this signal. Either the 55 kDa u-PA form and the lower Mw form (33 kDa) derived from the 55 kDa u-PA are Tyr-phosphorylated also the u-PA secreted in the culture media of human fibrosarcoma cells (HT-1080) is phosphorylated in tyrosine as well as u-PA present in tissue extracts of tumors induced in nude mice by HT-1080 cells. These data show that urine purified human u-PA and u-PA produced by human fibrosarcoma cells, in vitro and in vivo, are phosphorylated in tyrosine; furthermore our data show that u-PA is the major Tyr-phosphorylated protein present in these human tumor cells.