Nuclear localization of EGF receptor and its potential new role as a transcription factor

Regulation of cell proliferation by autocrine motility factor/phosphoglucose isomerase signaling

Autocrine motility factor (AMF)/phosphoglucose isomerase (PGI; EC 5.3.1.9) is a housekeeping cytosolic enzyme that plays a key role in both glycolysis and gluconeogenesis pathways. AMF/PGI is also a multifunctional protein that displays cytokine properties, eliciting mitogenic, motogenic, and differentiation activities, and has been implicated in tumor progression and metastasis. Because little is known about AMF/PGI-dependent signaling in general and during tumorigenesis in particular, we sought to study its effect on the cell cycle. To elucidate the functional role of PGI, we stably transfected its cDNA into NIH/3T3 and BALB/c 3T3-A31 fibroblasts. Ectopic overexpression of PGI results in the acquisition of a transformed phenotype associated with an acceleration of G1 to S cell cycle transition. These were manifested by up-regulation of cyclin D1 expression and cyclin-dependent kinase activity and down-regulation of the cyclin-dependent kinase inhibitor p27Kip1. The reduced p27Kip1 protein expression level in PGI-overexpressing cells could be restored to control levels by treatment with proteasome inhibitor. PGI-overexpressing cells also exhibited elevated expression of Skp2 involved in p27Kip1 ubiquitination and elevation in the levels of retinoblastoma protein hyperphosphorylation. Thus, we may conclude that the overexpression of AMF/PGI enhances cell proliferation together with up-regulation of cyclin/cyclin-dependent kinase activities and down-regulation of p27Kip1, whereas the induction of 3T3 fibroblast transformation by PGI is regulated by the retinoblastoma protein pathway.

Localization of importin alpha (Rch1) at the plasma membrane and subcellular redistribution during lymphocyte activation

Rch1 belongs to the importin alpha subfamily and works as an adapter between karyophilic proteins and the nuclear import machinery. Its level of expression varies among species and tissues, and depends on the state of cellular metabolism. In the present study we examined the level of expression of nuclear envelope and nuclear transport proteins (Rch1, importin beta, lamins A/C, lamin B, gp210, p62 and transportin) after human lymphocyte activation with phytohemagglutinin. We observed that the level of Rch1 increases dramatically, especially in larger lymphocytes, in response to activation. Moreover, using immunoelectron microscopy, this nuclear transport factor was found to be localized at the plasma membrane and also in tracks from the cytoplasm through the nuclear envelope into the nucleus. Similar localization was also observed in the human melanoma cell line A375. In addition, metabolic activation led to a redistribution of Rch1 from the cytoplasm to both the plasma membrane and the nuclear interior. These results suggest that, during lymphocyte activation, Rch1 may be involved in a signal transduction pathway that involves the shuttling of karyophilic proteins from the plasma membrane to the nucleus.

Hypertonic induction of COX-2 expression in renal medullary epithelial cells requires transactivation of the EGFR

Hypertonic stress increases expression of cyclooxygenase-2 (COX-2) in renal medullary epithelial and interstitial cells. Because hypertonic COX-2 expression is, in part, sensitive to inhibition of the ERK MAPK, an effector of activated receptor tyrosine kinases such as the EGF receptor, we investigated a role for this receptor in signaling to COX-2 expression. Hypertonic stress increased COX-2 expression at the mRNA and protein levels at 6 and 24 h of hypertonic treatment. Two potent, specific inhibitors of the EGF receptor kinase, AG-1478 and PD-153035, abrogated this effect. These inhibitors also blocked the ability of hypertonic stress to increase PGE2 release; in addition, they partially blocked tonicity-dependent phosphorylation of ERK but not of the related MAPKs, JNK or p38. Pharmacological inhibition of ERK activation partially blocked tonicity-dependent COX-2 expression. Hypertonic induction of COX-2 was likely transcriptionally mediated, as NaCl stress increased luciferase reporter gene activity under control of the human COX-2 promoter, and this effect was also sensitive to inhibition of the EGF receptor kinase. Metalloproteinase action is required for transactivation of the EGF receptor. Pharmacological inhibition of metalloproteinase function blocked tonicity-inducible COX-2 expression. Furthermore, the effect of hypertonicity on COX-2 expression was also evident in the EGF-responsive Madin-Darby canine kidney and 3T3 cell lines but was virtually absent from the EGF-unresponsive (and EGF receptor null) Chinese hamster-derived CHO cell line. Taken together, these data indicate that hypertonicity-dependent COX-2 expression in medullary epithelial cells requires transactivation of the EGF receptor and, potentially, ectodomain cleavage of an EGF receptor ligand.

The role of IFNgamma nuclear localization sequence in intracellular function

Intracellularly expressed interferon gamma (IFNgamma) has been reported to possess biological activity similar to that of IFNgamma added to cells. This study addresses the mechanisms for such similar biological effects. Adenoviral vectors were used to express a non-secreted form of human IFNgamma or a non-secreted mutant form in which a previously demonstrated nuclear localization sequence (NLS), 128KTGKRKR134, was replaced with alanines at K and R positions. With the vector expressing non-secreted wild-type IFNgamma, biological responses normally associated with extracellular IFNgamma, such as antiviral activity and MHC class I upregulation, were observed, although the mutant IFNgamma did not possess biological activity. Intracellular human IFNgamma possessed biological activity in mouse L cells, which do not recognize extracellularly added human IFNgamma. Thus, the biological activity was not due to leakage of IFNgamma to the surroundings and subsequent interaction with the receptor on the cell surface. Biological function was associated with activation of STAT1alpha and nuclear translocation of IFNgamma, IFNGR1 and STAT1alpha. Immunoprecipitation of cellular extracts with antibody to the nuclear transporter NPI-1 showed the formation of a complex with IFNgamma-IFNGR1-STAT1alpha. To provide the physiological basis for these effects we show that extracellularly added IFNgamma possesses intracellular signaling activity that is NLS dependent, as suggested by our previous studies, and that this activity occurs via the receptor-mediated endocytosis of IFNgamma. The data are consistent with previous observations that the NLS of extracellularly added IFNgamma plays a role in IFNgamma signaling.

Activation of epidermal growth factor receptor signals induction of nitric oxide synthase-2 in human optic nerve head astrocytes in glaucomatous optic neuropathy

Glaucoma is an optic neuropathy that is associated with elevated intraocular pressure in most patients. We have previously demonstrated that the mechanism by which pressure damages optic nerve axons involves excessive nitric oxide generated by inducible nitric oxide synthase (NOS-2). We have now found that activation of the epidermal growth factor receptor (EGFR) induces NOS-2 in astrocytes of the human optic nerve head (ONH) in vitro and EGFR is significantly upregulated and tyrosine phosphorylated in reactive astrocytes in human glaucomatous ONHs in vivo. Furthermore, in response to elevated hydrostatic pressure, EGFR rapidly becomes phosphorylated in the nucleus. This pressure-dependent activation of EGFR is necessary for NOS-2 induction. Our results suggest that activation and nuclear localization of EGFR may be needed for induction of NOS-2 in response to elevated intraocular pressure in glaucomatous optic neuropathy. Identification of this key signaling pathway provides new therapeutic approaches to pharmacological neuroprotection for glaucoma.

Signaling receptome: a genomic and evolutionary perspective of plasma membrane receptors involved in signal transduction

Intercellular communication in multicellular organisms requires the relay of extracellular signals by cell surface proteins to the interiors of cells. The availability of genome sequences from humans and several model organisms has facilitated the identification of several human plasma membrane receptor families and allowed the analysis of their phylogeny. This review provides a global categorization of most known signal transduction-associated receptors as enzymes, recruiters, and latent transcription factors. The evolution of known families of human plasma membrane signaling receptors was traced in current literature and validated by sequence relatedness. This global analysis reveals themes that recur during receptor evolution and allows the formulation of hypotheses for the origins of receptors. The human receptor families involved in signaling (with the exception of channels) are presented in the Human Plasma Membrane Receptome database.

Inhibitory role of the somatostatin receptor SST2 on the intracrine-regulated cell proliferation induced by the 210-amino acid fibroblast growth factor-2 isoform: implication of JAK2

The fibroblast growth factor (FGF)-2 isoform of 210 amino acids (HMW FGF-2) contains a nuclear localization sequence (NLS) and is targeted to the nucleus. This FGF-2 isoform allows cells to grow in low serum concentrations through still unknown mechanisms called intracrine regulations. Different peptide hormones and cytokines have been found to be nuclearized through NLS and to induce cell proliferation. The existence of molecules acting as negative regulators of the intracrine-induced cell growth has not been explored. Pancreatic cells AR4-2J were stably transfected to express selectively the HMW FGF-2. We demonstrated that activation of the somatostatin receptor subtype SST2 by the somatostatin analogue RC-160 in serum-deprived medium inhibits the mitogenic effect of the HMW FGF-2, without affecting growth of control cells. The signaling pathway implicates Galphai/JAK2/SHP-1. The Galphai inhibitor pertussis toxin and the JAK2 inhibitor AG490 abrogate the inhibitory effect of RC-160 on HMW FGF-2-induced cell growth. Co-immunoprecipitation studies demonstrate the constitutive association of JAK2 and SHP-1, and RC-160 induces a rapid activation of both proteins followed by the dissociation of the complex. AG490 prevents the RC-160 induced SHP-1 activation indicating the implication of JAK2 in this process. JAK2 and SHP-1 are immunoprecipitated with SST2 in basal conditions indicating the existence of a functional signaling complex at the receptor level. In summary, these data provide the following evidence: 1) the intracrine-induced proliferation can be reversed by extracellular acting polypeptides; 2) SST2 inhibitory signaling may involve the JAK2/SHP-1 pathway.

Strategies to target HER2/neu overexpression for cancer therapy

Amplification or overexpression of the HER2/neu (also known as erbB-2) gene has been noted in various types of human cancers. In addition to malignant transformation, the activation of signaling pathways of HER2/neu enhances various metastasis-associated properties and may render cancer cells resistant to conventional therapies. This, at least partially, contributes to the poor prognosis and lower survival rate of patients. Many studies have demonstrated that repression of HER2/neu overexpression suppresses the malignant phenotypes of cancer cells. Therefore, various novel HER2/neu-blocking agents have been developed, several of which have been tested in clinical trials with satisfactory results, including trastuzumab, a HER2/neu monoclonal antibody that has been approved by the FDA in the treatment of HER2/neu-overexpressing breast cancer patients. In this article, we intend to discuss the biological relevance and significance of HER2/neu overexpression in tumorigenesis, metastasis, and resistance to conventional therapy. We also summarize the currently available strategies and combination therapies targeting HER2/neu-overexpressing cancer cells. Although the optimal treatment for HER2/neu-overexpressing cancer patients remains elusive, the initial success of trastuzumab indicates that HER2/neu is a good target for cancer therapy. Further elucidation of HER2/neu-mediated pathways and downstream molecules is critical to provide alternative therapies, overcome drug resistance, and improve the therapeutic outcome for HER2/neu-overexpressing cancer patients.

Nuclear translocation of insulin receptor substrate-1 by the insulin receptor in mouse embryo fibroblasts

Translocation of the insulin receptor substrate-1 (IRS-1) to the nuclei has been reported to occur in cells stimulated by insulin-like growth factor-1 (IGF-I) or expressing certain viral and cellular oncogenes. We show here that insulin can also induce nuclear translocation of IRS-1 in mouse embryo fibroblasts (MEF), that do not express the type 1 insulin-like growth factor receptor (IGF-IR). Only the A isoform of the insulin receptor (IR) can induce IRS-1 nuclear translocation, which is significant when the receptor is over-expressed. At physiological receptor levels, translocation occurs only in a fraction of cells, and only at high concentrations of ligand.

Epidermal growth factor upregulates beta-adrenergic receptor signaling in a human salivary cell line

The effects of epidermal growth factor (EGF) on the beta-adrenergic receptor-coupled adenylyl cyclase system were studied in a human salivary cell line (HSY). The beta-adrenergic agonist isoproterenol (10(-5) M) stimulated adenylyl cyclase activity by approximately 2-fold, and the isoproterenol response was increased 1.8-fold after prolonged (48 h) exposure to EGF (5 x 10(-10) M). In contrast, enzyme activation via stimulatory prostaglandin receptors and by agents acting on nonreceptor components of the adenylyl cyclase system was not enhanced by EGF. beta-Adrenergic receptor density, assessed by binding of the beta-adrenergic receptor antagonist (-)-[(125)I]iodopindolol, was increased threefold after EGF treatment. Competition binding studies with unlabeled antagonists selective for beta(1)- and beta(2)-adrenergic receptor subtypes indicated that the increase in (-)-[(125)I]iodopindolol binding sites induced by EGF reflected an increased number of beta(2)-adrenergic receptors. Likewise, Northern blot analysis of RNA from EGF-treated cells revealed selective induction of beta(2)-adrenergic receptor mRNA, which was blocked by the RNA synthesis inhibitor actinomycin D. The increase in beta-adrenergic receptor density produced by EGF was unaltered after phorbol ester-induced downregulation of protein kinase C (PKC). Enhancement of isoproterenol-responsive adenylyl cyclase activity and phosphorylation of mitogen-activated protein kinase (MAPK) by EGF were both blocked by the MAPK pathway inhibitor PD-98059. The results suggest that in HSY cells EGF enhances beta-adrenergic responsiveness by upregulating beta(2)-adrenergic receptor expression at the transcriptional level. Moreover, the stimulatory effect of EGF on beta(2)-adrenergic receptor signaling appears to be mediated by the MAPK pathway and independent of PKC activation.

Membrane-initiated steroid signaling (MISS): genomic steroid action starts at the plasma membrane

Plasma membrane (PM) steroid recognition sites are thought to be responsible only for rapid, non-genomic responses without any link to the nuclear receptor-mediated genomic effects of steroids. We focused on a PM "glucocorticoid-importer" (GC-importer) that imports GC into rat liver cells. This site interacts also with particular gestagens (progesterone, P; medroxyprogesterone, MP; ethynodiol, Ethy) and estrogens (ethinylestradiol, EE(2); mestranol), which do not bind to the nuclear GC receptor (GR). To elucidate the role of the GC-importer, we transfected a rat wild-type hepatocyte (CC-1) and a hepatoma cell line, unable to import GC (MH 3924), with a GC<-->GR-responsive luciferase (luc)-reporter gene. Selected steroids were tested for their ability to induce or inhibit luc expression. Corticosterone (B) and dexamethasone (Dex), but also the GC-antagonists cortexolone (Cortex), P and MP, induced luc. Even the PM-impermeable BSA-derivatives of B, Dex and Cortex did so to almost the same extent as the free steroids. MH 3924 cells respond stronger than CC-1 to luc inducing steroids. Luc expression was inhibited by RU 38 486, but also by EE(2) and Ethy. The thiol reactive mesylate-derivatives of B, Dex and Cortex induced to a considerably lesser extent than the free or BSA-steroids. The thiol reagent mersalyl blocks cellular entry of GC and inhibits luc induction in CC-1 cells. Incubation with EE(2) and B of PM-vesicles, isolated from liver cells, resulted in a decrease of the density of two 75 and 52kDa G-proteins reflecting a diminished exchange of GDP by GTP.

CONCLUSION

the PM-residing GC-importer, now renamed "Steroid Hormone Recognition and Effector Complex" (SHREC) is an interdependent part of the complete GC signal propagation in which G-proteins are involved. Free SH-groups of SHREC are a prerequisite for genomic GC activity. Specific interactions between SHREC and GC-agonist/-antagonist trigger steroid-dependent signaling. However, import of the ligand into the cell terminates it. Thus, the PM-related non-genomic steroid responses are clearly linked to the GR-related genomic effects.

Regulation of calcium signals in the nucleus by a nucleoplasmic reticulum

Calcium is a second messenger in virtually all cells and tissues. Calcium signals in the nucleus have effects on gene transcription and cell growth that are distinct from those of cytosolic calcium signals; however, it is unknown how nuclear calcium signals are regulated. Here we identify a reticular network of nuclear calcium stores that is continuous with the endoplasmic reticulum and the nuclear envelope. This network expresses inositol 1,4,5-trisphosphate (InsP3) receptors, and the nuclear component of InsP3-mediated calcium signals begins in its locality. Stimulation of these receptors with a little InsP3 results in small calcium signals that are initiated in this region of the nucleus. Localized release of calcium in the nucleus causes nuclear protein kinase C (PKC) to translocate to the region of the nuclear envelope, whereas release of calcium in the cytosol induces translocation of cytosolic PKC to the plasma membrane. Our findings show that the nucleus contains a nucleoplasmic reticulum with the capacity to regulate calcium signals in localized subnuclear regions. The presence of such machinery provides a potential mechanism by which calcium can simultaneously regulate many independent processes in the nucleus.

Nuclear trafficking of FGFR1: a role for the transmembrane domain

Several members of the fibroblast growth factor (FGF) family lack signal peptide (SP) sequences and are present only in trace amounts outside the cell. However, these proteins contain nuclear localization signals (NLS) and accumulate in the cell nucleus. Our studies have shown that full length FGF receptor 1 (FGFR1) accumulates within the nuclear interior in parallel with FGF-2. We tested the hypothesis that an atypical transmembrane domain (TM) plays a role in FGFR1 trafficking into the nuclear interior. With FGFR1 destined for constitutive fusion with the plasma membrane due to its SP, how the receptor may enter the nucleus is unclear. Sequence analysis identified that FGFR1 has an atypical TM containing short stretches of hydrophobic amino acids (a.a.) interrupted by polar a.a. The beta-sheet is the predicted conformation of the FGFR1 TM, in contrast to the alpha-helical conformation of other single TM tyrosine kinase receptors, including FGFR4. Receptor trafficking in live cells was studied by confocal microscopy via C-terminal FGFR1 fusions to enhanced green fluorescent protein (EGFP) and confirmed by subcellular fractionation and Western immunoblotting. Nuclear entry of FGFR1-EGFP was independent of karyokinessis, and was observed in rapidly proliferating human TE671 cells, in slower proliferating glioma SF763 and post-mitotic bovine adrenal medullary cells (BAMC). In contrast, a chimeric FGFR1/R4-EGFP, where the TM of FGFR1 was replaced with that of FGFR4, was associated with membranes (golgi-ER, plasma, and nuclear), but was absent from the nucleus and cytosol. FGFR1delta-EGFP mutants, with hydrophobic TM a.a. replaced with polar a.a., showed reduced association with membranes and increased cytosolic/nuclear accumulation with an increase in TM hydrophilicity. FGFR1(TM-)-EGFP (TM deleted), was detected in the golgi-ER vesicles, cytosol, and nuclear interior; thus demonstrating that the FGFR1 TM does not function as a NLS. To test whether cytosolic FGFR1 provides a source of nuclear FGFR1, cells were transfected with FGFR1(SP-) (SP was deleted), resulting in cytosolic, non-membrane, protein accumulation in the cytosol and the cell nucleus. Our results indicate that an unstable association with cellular membranes is responsible for the release of FGFR1 into the cytosol and cytosolic FGFR1 constitutes the source of the nuclear receptor.

Nuclear localization and possible functions of receptor tyrosine kinases

Recent data have renewed interest in the possible nuclear localization of receptor tyrosine kinases, as well as their ligands. In one case, that of ErbB-4, the receptor is processed by two membrane-localized proteases to produce a soluble cytoplasmic domain fragment that includes the tyrosine kinase domain. This fragment, generated by a metalloprotease-dependent ectodomain cleavage followed by gamma-secretase cleavage within the transmembrane domain, is also found in the nucleus. Three other receptor tyrosine kinases have been detected in the nucleus in the absence of proteolytic processing. In some instances, nuclear localization of receptor tyrosine kinases is growth-factor-dependent and tentative evidence suggests a role in transcription.

Storage diseases: new insights into sphingolipid functions

Studying human diseases can help us to uncover important processes in normal cells. Cell biologists have recently focused on inherited sphingolipid-storage diseases. Eukaryotic life is characterized by internal membranes of various compositions, and sphingolipids are a small but important part of these membranes. Compositional differences between cellular membranes are maintained by sorting and sphingolipids are thought to organize this process by forming ordered domains of increased thickness in the bilayer. Here, we describe the impact of sphingolipid accumulation on the sorting of endocytic membranes and discuss the proposed basis for the pathology of these diseases at the cellular level.

Critical role of both p27KIP1 and p21CIP1/WAF1 in the antiproliferative effect of ZD1839 ('Iressa'), an epidermal growth factor receptor tyrosine kinase inhibitor, in head and neck squamous carcinoma cells

High expression of the epidermal growth factor receptor (EGFR) has been implicated in the development of squamous-cell carcinomas of head and neck (SCCHN). ZD1839 ('Iressa') is an orally active, selective EGFR-TKI (EGFR-tyrosine kinase inhibitor) that blocks signal transduction pathways implicated in proliferation and survival of cancer cells, and other host-dependent processes promoting cancer growth. We have demonstrated that ZD1839 induces growth arrest in SCCHN cell lines by inhibiting EGFR-mediated signaling. Cell cycle kinetic analysis demonstrated that ZD1839 induces a delay in cell cycle progression and a G1 arrest together with a partial G2/M block; this was associated with increased expression of both p27(KIP1) and p21(CIP1/WAF1) cyclin-dependent kinase (CDK) inhibitors. The activity of CDK2, the main target of CIP/KIP CDK inhibitors, was reduced in a dose-dependent fashion after 24 h of ZD1839 treatment and this effect correlated to the increased amount of p27(KIP1) and p21(CIP1/WAF1) proteins associated with CDK2-cyclin-E and CDK2-cyclin-A complexes. In addition, ZD1839-induced growth inhibition was significantly reduced in cell transfectants expressing p27(KIP1) or p21(CIP1/WAF1) antisense constructs. Overall, these results as well as the timing of the effect of ZD1839 on G1 arrest and p27(KIP1) and p21(CIP1/WAF1) upregulation, suggest a mechanistic connection between these events.

A proteomics strategy to elucidate functional protein-protein interactions applied to EGF signaling

Mass spectrometry-based proteomics can reveal protein-protein interactions on a large scale, but it has been difficult to separate background binding from functionally important interactions and still preserve weak binders. To investigate the epidermal growth factor receptor (EGFR) pathway, we employ stable isotopic amino acids in cell culture (SILAC) to differentially label proteins in EGF-stimulated versus unstimulated cells. Combined cell lysates were affinity-purified over the SH2 domain of the adapter protein Grb2 (GST-SH2 fusion protein) that specifically binds phosphorylated EGFR and Src homologous and collagen (Shc) protein. We identified 228 proteins, of which 28 were selectively enriched upon stimulation. EGFR and Shc, which interact directly with the bait, had large differential ratios. Many signaling molecules specifically formed complexes with the activated EGFR-Shc, as did plectin, epiplakin, cytokeratin networks, histone H3, the glycosylphosphatidylinositol (GPI)-anchored molecule CD59, and two novel proteins. SILAC combined with modification-based affinity purification is a useful approach to detect specific and functional protein-protein interactions.

Comparison of the signal transduction pathways for the induction of gene expression of nitric oxide synthase-2 in response to two different stimuli

Human optic nerve astrocytes induce nitric oxide synthase-2 (NOS-2) in vitro in response to cytokines (interferon-gamma/interleukin-1beta) and elevated hydrostatic pressure. Using relatively specific inhibitors, we have compared induction of NOS-2 in response to these two stimuli to determine whether the same or different signal transduction pathways participate in the responses. Using SN50 and CAGE, which inhibit the NFkappaB pathway, the induction of NOS-2 in response to both cytokines and elevated hydrostatic pressure was blocked. Using SB202190 and SB203580, which inhibit p38 mitogen-activated protein kinase, only the response to cytokines was blocked. In contrast, when inhibitors of epidermal growth factor receptor tyrosine kinase AG 82 and AG 18 were used, the induction of NOS-2 in response to pressure, but not in response to cytokines, was blocked. Signal transduction pathways presumably regulate the synthesis of NOS-2 through downstream events that induce transcription of the NOS-2 gene. Our data suggest that activation of different sites in the promoter region of the NOS-2 gene is needed for these different stimuli to induce NOS-2.

Bidirectional signaling between the estrogen receptor and the epidermal growth factor receptor

Interactions between the estrogen receptor (ER) and the epidermal growth factor receptor (EGFR) contribute to the biological effects of these binding protein families. EGFR stimulates DNA synthesis and gene transcription in the uterus, related in part to estrogen-independent activation of the nuclear ER. This results from signal transduction enacted by the plasma membrane tyrosine kinase growth factor receptor, leading to 1) phosphorylation and activation of the nuclear ER, and 2) phosphorylation of coregulator proteins. More recently, it has been shown that a pool of ERalpha resides in or associates with the plasma membrane as a cytoplasmic protein. These ERs utilize the membrane EGFR to rapidly signal through various kinase cascades that influence both transcriptional and nontranscriptional actions of estrogen in breast cancer cells. This is congruent with a general theme of receptor signaling, where membrane G protein-coupled receptors activate tyrosine kinase growth factor receptors (EGFR, IGF-I receptor) that subsequently signal to MAPKs and other pathways. Overall, the bidirectional cross-talk between EGFR and cellular pools of ER contributes to reproductive organ physiology and pathophysiology.

Neuron-to-glia signaling mediated by excitatory amino acid receptors regulates ErbB receptor function in astroglial cells of the neuroendocrine brain

Hypothalamic astroglial erbB tyrosine kinase receptors are required for the timely initiation of mammalian puberty. Ligand-dependent activation of these receptors sets in motion a glia-to-neuron signaling pathway that prompts the secretion of luteinizing hormone-releasing hormone (LHRH), the neuropeptide controlling sexual development, from hypothalamic neuroendocrine neurons. The neuronal systems that may regulate this growth factor-mediated back signaling to neuroendocrine neurons have not been identified. Here we demonstrate that hypothalamic astrocytes contain metabotropic receptors of the metabotropic glutamate receptor 5 subtype and the AMPA receptor subunits glutamate receptor 2 (GluR2) and GluR3. As in excitatory synapses, these receptors are in physical association with their respective interacting/clustering proteins Homer and PICK1. In addition, they are associated with erbB-1 and erbB-4 receptors. Concomitant activation of astroglial metabotropic and AMPA receptors results in the recruitment of erbB tyrosine kinase receptors and their respective ligands to the glial cell membrane, transactivation of erbB receptors via a mechanism requiring metalloproteinase activity, and increased erbB receptor gene expression. By facilitating erbB-dependent signaling and promoting erbB receptor gene expression in astrocytes, a neuron-to-glia glutamatergic pathway may represent a basic cell-cell communication mechanism used by the neuroendocrine brain to coordinate the facilitatory transsynaptic and astroglial input to LHRH neurons during sexual development.

Role of the ErbB-4 carboxyl terminus in gamma-secretase cleavage

The ErbB-4 receptor tyrosine kinase has a PDZ domain recognition motif at its carboxyl terminus. The first step in ErbB-4 proteolytic processing is a metalloprotease-dependent cleavage of the receptor ectodomain, which is not influenced by deletion of this motif. Metalloprotease cleavage of ErbB-4 produces a membrane-associated 80-kDa fragment that is a substrate for subsequent gamma-secretase cleavage, which releases the cytoplasmic domain from the membrane and allows nuclear translocation of this fragment. Deletion of the PDZ domain recognition motif does abrogate the gamma-secretase cleavage of ErbB-4. The wild-type 80-kDa ErbB-4 fragment forms an association complex with presenilin, thought to be the catalytic moiety of gamma-secretase activity. However, this association is significantly impaired by loss of the PDZ domain recognition motif from ErbB-4. Deletion of this ErbB-4 motif prevents the nuclear localization of the ErbB-4 cytoplasmic domain. Data also show that the basal cleavage of wild-type ErbB-4 by this proteolytic system can produce a sufficient level of ErbB-4 processing to negatively influence cell growth and that loss of the PDZ domain recognition motif abrogates this response.

Nuclear prostaglandin signaling system: biogenesis and actions via heptahelical receptors

Prostaglandins are ubiquitous lipid mediators that play pivotal roles in cardiovascular homeostasis, reproduction, and inflammation, as well as in many important cellular processes including gene expression and cell proliferation. The mechanism of action of these lipid messengers is thought to be primarily dependent on their interaction with specific cell surface receptors that belong to the heptahelical transmembrane spanning G protein-coupled receptor superfamily. Accumulating evidence suggests that these receptors may co-localize at the cell nucleus where they can modulate gene expression through a series of biochemical events. In this context, we have recently demonstrated that prostaglandin E2-EP3 receptors display an atypical nuclear compartmentalization in cerebral microvascular endothelial cells. Stimulation of these nuclear EP3 receptors leads to an increase of eNOS RNA in a cell-free isolated nuclear system. This review will emphasize these findings and describe how nuclear prostaglandin receptors, notably EP3 receptors, may affect gene expression, specifically of eNOS, by identifying putative transducing elements located within this organelle. The potential sources of lipid ligand activators for these intracellular sites will also be addressed. The expressional control of G-protein-coupled receptors located at the perinuclear envelope constitutes a novel and distinctive mode of gene regulation.

An alternative domain containing a leucine-rich sequence regulates nuclear cytoplasmic localization of protein 4.1R

In red blood cells, protein 4.1 (4.1R) is an 80-kDa protein that stabilizes the spectrin-actin network and anchors it to the plasma membrane. The picture is more complex in nucleated cells, in which many 4.1R isoforms, varying in size and intracellular location, have been identified. To contribute to the characterization of signals involved in differential intracellular localization of 4.1R, we have analyzed the role the exon 5-encoded sequence plays in 4.1R distribution. We show that exon 5 encodes a leucine-rich sequence that shares key features with nuclear export signals (NESs). This sequence adopts the topology employed for NESs of other proteins and conserves two hydrophobic residues that are shown to be critical for NES function. A 4.1R isoform expressing the leucine-rich sequence binds to the export receptor CRM1 in a RanGTP-dependent fashion, whereas this does not occur in a mutant whose two conserved hydrophobic residues are substituted. These two residues are also essential for 4.1R intracellular distribution, because the 4.1R protein containing the leucine-rich sequence localizes in the cytoplasm, whereas the mutant protein predominantly accumulates in the nucleus. We hypothesize that the leucine-rich sequence in 4.1R controls distribution and concomitantly function of a specific set of 4.1R isoforms.

Bone growth stimulators. New tools for treating bone loss and mending fractures

In the new millennium, humans will be traveling to Mars and eventually beyond with skeletons that respond to microgravity by self-destructing. Meanwhile in Earth's aging populations growing numbers of men and many more women are suffering from crippling bone loss. During the first decade after menopause all women suffer an accelerating loss of bone, which in some of them is severe enough to result in "spontaneous" crushing of vertebrae and fracturing of hips by ordinary body movements. This is osteoporosis, which all too often requires prolonged and expensive care, the physical and mental stress of which may even kill the patient. Osteoporosis in postmenopausal women is caused by the loss of estrogen. The slower development of osteoporosis in aging men is also due at least in part to a loss of the estrogen made in ever smaller amounts in bone cells from the declining level of circulating testosterone and is needed for bone maintenance as it is in women. The loss of estrogen increases the generation, longevity, and activity of bone-resorbing osteoclasts. The destructive osteoclast surge can be blocked by estrogens and selective estrogen receptor modulators (SERMs) as well as antiosteoclast agents such as bisphosphonates and calcitonin. But these agents stimulate only a limited amount of bone growth as the unaffected osteoblasts fill in the holes that were dug by the now suppressed osteoclasts. They do not stimulate osteoblasts to make bone--they are antiresorptives not bone anabolic agents. (However, certain estrogen analogs and bisphosphates may stimulate bone growth to some extent by lengthening osteoblast working lives.) To grow new bone and restore bone strength lost in space and on Earth we must know what controls bone growth and destruction. Here we discuss the newest bone controllers and how they might operate. These include leptin from adipocytes and osteoblasts and the statins that are widely used to reduce blood cholesterol and cardiovascular damage. But the main focus of this article is necessarily the currently most promising of the anabolic agents, the potent parathyroid hormone (PTH) and certain of its 31- to 38-aminoacid fragments, which are either in or about to be in clinical trial or in the case of Lilly's Forteo [hPTH-(1-34)] tentatively approved by the Food and Drug Administration for treating osteoporosis and mending fractures.

Differential intranuclear localization of fibroblast growth factor-2 isoforms and specific interaction with the survival of motoneuron protein

Fibroblast growth factor 2 (FGF-2) is an important modulator of cell growth and differentiation and a neurotrophic factor. FGF-2 occurs in isoforms, at a low molecular weight of 18,000 and at least two high molecular weight forms (21,000 and 23,000), representing alternative translation products from a single mRNA. In addition to its role as an extracellular ligand, FGF-2 localizes to the nuclei of cells. Here we show differential localization of the 18- and 23-kDa isoforms in the nuclei of rat Schwann cells. Whereas the 18-kDa isoform was found in the nucleoli, nucleoplasm, and Cajal bodies, the 23-kDa isoform localized in a punctuate pattern and associates with mitotic chromosomes suggesting different functional roles of the isoforms. Moreover, we show here that the 23-kDa FGF-2 isoform co-immunoprecipitates specifically with the survival of motor neuron protein (SMN). SMN is an assembly and recycling factor of the splicing machinery and locates to the cytoplasm, the nucleoplasm, and nuclear gems, where it co-localizes with 23-kDa FGF-2. Patients with spinal muscular atrophy suffer from fatal degeneration of motoneurons because of mutations and deletions of the gene for the SMN protein.

Nuclear localization and function of polypeptide ligands and their receptors: a new paradigm for hormone specificity within the mammary gland?

The specific effects triggered by polypeptide hormone/growth factor stimulation of mammary cells were considered mediated solely by receptor-associated signaling networks. A compelling body of new data, however, clearly indicates that polypeptide ligands and/or their receptors are transported into the nucleus, where they function directly to regulate the expression of specific transcription factors and gene loci. The intranuclear function of these complexes may contribute to the explicit functions associated with a given ligand, and may serve as new targets for pharmacologic intervention.

Transport of exogenous growth factors and cytokines to the cytosol and to the nucleus

In recent years a number of growth factors, cytokines, protein hormones, and other proteins have been found in the nucleus after having been added externally to cells. This review evaluates the evidence that translocation takes place and discusses possible mechanisms. As a demonstration of the principle that extracellular proteins can penetrate cellular membranes and reach the cytosol, a brief overview of the penetration mechanism of protein toxins with intracellular sites of action is given. Then problems and pitfalls in attempts to demonstrate the presence of proteins in the cytosol and in the nucleus as opposed to intracellular vesicular compartments are discussed, and some new approaches to study this are described. A detailed overview of the evidence for translocation of fibroblast growth factor, HIV-Tat, interferon-gamma, and other proteins where there is evidence for intracellular action is given, and translocation mechanisms are discussed. It is concluded that although there are many pitfalls, the bulk of the experiments indicate that certain proteins are indeed able to enter the cytosol and nucleus. Possible roles of the internalized proteins are discussed.

Interactions between the juxtamembrane domain of the EGFR and calmodulin measured by surface plasmon resonance

One early response to epidermal growth factor receptor (EGFR) activation is an increase in intracellular calcium. We have used surface plasmon resonance (SPR) to study real-time interactions between the intracellular juxtamembrane (JM) region of EGFR and calmodulin. The EGFR-JM (Met(644)-Phe(688)) was expressed as a GST fusion protein and immobilised on a sensor chip surface. Calmodulin specifically interacts with EGFR-JM in a calcium-dependent manner with a high on and high off rate. Chemical modification of EGFR-JM by using arginine-selective phenylglyoxal or deletion of the basic segment Arg(645)-Arg(657) inhibits the interaction. Phosphorylation of EGFR-JM by protein kinase C (PKC) or glutamate substitution of Thr(654) inhibits the interaction, suggesting that PKC phosphorylation electrostatically interferes with calmodulin binding to basic arginine residues. Calmodulin binding was also inhibited by suramin. Our results suggest that EGFR-JM is essential for epidermal growth factor (EGF)-mediated calcium-calmodulin signalling and for signal integration between other signalling pathways.

Nuclear translocation of insulin receptor substrate-1 by oncogenes and Igf-I. Effect on ribosomal RNA synthesis

The insulin receptor substrate-1 (IRS-1) is one of the major substrates of both the insulin and IGF-I receptors and is generally localized in the cytosol/membrane fraction of the cell. We show here that a substantial fraction of IRS-1 is translocated to the nucleus in mouse embryo fibroblasts (MEF) expressing the simian virus 40 T antigen. Nuclear translocation of IRS-1 occurs also in MEF stimulated with IGF-I or in MEF expressing the oncogene v-src. Nuclear translocation of IRS-1 can be demonstrated by confocal microscopy, immunohistochemistry, or subcellular fractionation. An antibody to IRS-1 immunoprecipitates from nuclear fractions (but not from cytosolic fractions) the upstream binding factor, which is a key regulator of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In agreement with this finding, in 32D murine hemopoietic cells, nuclear translocation of IRS-1 correlates with a markedly increased rRNA synthesis. Our experiments suggest that nuclear IRS-1 may play a specialized role in rRNA synthesis and/or processing.

Spatiotemporal expression of epidermal growth factor receptor messenger RNA and protein in the hamster ovary: follicle stage-specific differential modulation by follicle-stimulating hormone, luteinizing hormone, estradiol, and progesterone

Spatiotemporal expression, endocrine regulation, and activation of epidermal growth factor receptor (EGFR) in the hamster ovary were evaluated by immunofluorescence and in situ hybridization localization. Whereas granulosa cells (GC) of primordial through large preantral (stage 6, 7-8 layers GC) follicles had low immunoreactivity, granulosa cells of antral follicles, theca, and interstitial cells had intense EGFR immunoreactivity. EGFR expression in GC of primordial and small preantral follicles increased progressively from estrous through proestrous, but a significant increase occurred in mural GC of antral follicles following the gonadotropin surge. Interstitial cells around small preantral follicles had strong immunofluorescence, and the intensity increased significantly in fully differentiated thecal cells. Distinct EGFR protein was localized in the nucleus of the oocytes and granulosa cells. FSH significantly stimulated EGFR expression in the GC, especially the mural GC, theca, and interstitial cells in hypophysectomized hamster. Estrogen stimulated EGFR expression in preantral GC as well as in interstitial cells. Progesterone and hCG effect was limited to theca and interstitial cells. EGFR expression correlated well with EGFR activation following endogenous or exogenous gonadotropin exposure. Receptor mRNA expression closely followed the protein expression, with increased mRNA expression in mural GC of antral follicles. These results suggest that low levels of EGF signal as a consequence of low levels of receptors in preantral GC may be critical for cell proliferation, but higher receptor density may evoke increased signal intensity due to activation of other intracellular signal pathways, which activate cellular processes related to granulosa, theca, and interstitial cell differentiation. The spatiotemporal cell type and follicle stage-specific expression of receptor mRNA and protein and EGFR activation is critically regulated by gonadotropins and ovarian steroids, primarily estradiol.

Automated subcellular localization and quantification of protein expression in tissue microarrays

The recent development of tissue microarrays-composed of hundreds of tissue sections from different tumors arrayed on a single glass slide-facilitates rapid evaluation of large-scale outcome studies. Realization of this potential depends on the ability to rapidly and precisely quantify the protein expression within each tissue spot. We have developed a set of algorithms that allow the rapid, automated, continuous and quantitative analysis of tissue microarrays, including the separation of tumor from stromal elements and the sub-cellular localization of signals. Validation studies using estrogen receptor in breast carcinoma show that automated analysis matches or exceeds the results of conventional pathologist-based scoring. Automated analysis and sub-cellular localization of beta-catenin in colon cancer identifies two novel, prognostically significant tumor subsets, not detected by traditional pathologist-based scoring. Development of automated analysis technology empowers tissue microarrays for use in discovery-type experiments (more typical of cDNA microarrays), with the added advantage of inclusion of long-term demographic and patient outcome information.

1,25-Dihydroxyvitamin D down-regulates cell membrane growth- and nuclear growth-promoting signals by the epidermal growth factor receptor

1,25(OH)(2)D(3) antiproliferative properties are widely known. However, the molecular bases of these properties are only partially elucidated. Since 1,25(OH)(2)D(3) effectively arrests growth in many tumors and hyperplastic tissues whose growth is driven by co-expression of EGFR and its ligand TGF-alpha, it was hypothesized that 1,25(OH)(2)D(3) could affect the TGF-alpha/EGFR-autocrine growth loop. This study examined 1,25(OH)(2)D(3) regulation of EGFR-growth signals, using human epidermoid A431 cells, in which the overexpression of EGFR and TGF-alpha constitute the major autocrine mitogenic signal. 1,25(OH)(2)D(3) inhibited autocrine and EGF-induced A431 cell proliferation. Furthermore, 1,25(OH)(2)D(3) changed the cellular localization of both TGF-alpha and EGFR and inhibited ligand-dependent phosphorylation of EGFR and ERK1/2. In addition, 1,25(OH)(2)D(3) impaired autocrine and EGF-induced nuclear translocation of activated EGFR and, consequently, its binding to AT-rich DNA sequences and transcriptional activation of the cyclin D1 promoter. These results demonstrate that 1,25(OH)(2)D(3) alters EGFR membrane trafficking and down-regulates EGFR growth signaling.

Reduced p21, p27 and vitamin D receptor in the nodular hyperplasia in patients with advanced secondary hyperparathyroidism

BACKGROUND

In uremic patients with secondary hyperparathyroidism (2HPT), nodular hyperplasia of parathyroid gland shows a monoclonal pattern of cell proliferation, in which a decreased density of vitamin D receptor (VDR) also is demonstrated. The present study aimed at elucidating the mechanism of parathyroid cell proliferation in relation to cell cycle determinants in patients with advanced 2HPT.

METHODS

The expression of cyclin-dependent kinase inhibitors, p21 and p27, and VDR were examined and compared among four groups of nodular (Nd; N = 23) or diffuse (Df; N = 6) hyperplastic parathyroid glands resected due to 2HPT, primary adenomas (Ad; N = 15), and histologically-normal parathyroid glands (C; N = 20) removed during thyroidectomy. Immunohistochemical analyses for VDR, p21, p27 and Ki67 antigen were performed in formalin-fixed paraffin-embedded tissues by using specific polyclonal antibody. The distribution and the intensity of immunoreactivity was quantified by using NIH imaging, and was expressed as the labeling index (LI) of positive nuclear staining in a random set of 1000 cells.

RESULTS

p21 LI was significantly diminished in both Nd (85 +/- 110; mean +/- SD) and Ad (136 +/- 122) as compared to that in Df (360 +/- 191) or C (359 +/- 228; P < 0.01). p27 LI was also significantly diminished in both Nd (97 +/- 156) and Ad (187 +/- 196) as compared to that in Df (532 +/- 146) or C (631 +/- 170; P < 0.01). VDR LI in Nd (162 +/- 194) was also significantly lower than that in Df (495 +/- 337), Ad (383 +/- 262), or C (659 +/- 234), respectively (P < 0.01). Parathyroid sections with high nuclear VDR expression elicited high p21 and p27 expression. Both p21 and p27 LI in Nd correlated significantly with nuclear VDR LI (r = 0.92; P < 0.01, r = 0.76; P < 0.01), but not with p53 LI, and inversely correlated with the glandular weight (r = 0.44; P < 0.05, r = 0.41; P < 0.05).

CONCLUSIONS

The reduced expression of p21 and p27, in a VDR-dependent manner, is a major pathogenic factor for a nodular parathyroid gland growth.

Resistance to trastuzumab: a necessary evil or a temporary challenge?

The aim of this review article is to examine the potential mechanisms of resistance to trastuzumab. In the clinical setting, when trastuzumab is given as a single agent for first-line treatment of HER2-overexpressing metastatic breast cancer, it is associated with a 40% objective response rate. In the remaining cases, no tumor regression is observed, although HER2 protein is overexpressed and/or the corresponding gene is amplified. Hence, some other factors besides HER2 must play a role in determining the level of sensitivity to trastuzumab. The identification of the potential mechanisms of resistance to trastuzumab can be very helpful for the development of new compounds, which might overcome that resistance and/or have additive/synergistic antitumor effect when given in association with trastuzumab. Moreover, thorough understanding of the HER2 pathway is essential to the identification of new predictive markers of response to trastuzumab that will help to better define the patients who are most likely to benefit from this drug.

Integrin signaling: it's where the action is

Recent advances highlight a critical role for integrin receptors for extracellular matrix in determining where in cells critical signals are transduced. Integrins are shown to activate signaling intermediates at specific surface membrane locations, to promote nuclear translocation of factors that activate gene transcription, and to recruit and augment the signaling power of receptors for growth factors.

Signalling shortcuts: cell-surface receptors in the nucleus?

Do cell-surface growth-factor receptors and their ligands accumulate in the nucleoplasm under physiological conditions? And, if so, how do they get there and what function do they serve in this location? Recent advances have provided tantalizing hints to the answers to these questions, and hold the key to identifying a new mode of signal transduction.

Gamma-secretase-mediated proteolysis in cell-surface-receptor signalling

Many cell-surface receptors transmit signals to the nucleus through complex protein cascades. By contrast, the Notch signalling pathway uses a relatively direct mechanism, in which the intracellular domain of the receptor is liberated by intramembrane cleavage and translocates to the nucleus. This critical cleavage is mediated by the gamma-secretase complex, and new findings reveal that this mechanism is used by various receptors, although many questions remain about the biochemical details.

Nuclear translocation of insulin receptor substrate-1 by the simian virus 40 T antigen and the activated type 1 insulin-like growth factor receptor

32D cells are a murine hemopoietic cell line that undergoes apoptosis upon withdrawal of interleukin-3 (IL-3) from the medium. 32D cells have low levels of the type 1 insulin-like growth factor (IGF-I) receptor and do not express insulin receptor substrate-1 (IRS-1) or IRS-2. Ectopic expression of IRS-1 delays apoptosis but cannot rescue 32D cells from IL-3 dependence. In 32D/IRS-1 cells, IRS-1 is detectable, as expected, in the cytosol/membrane compartment. The SV40 large T antigen is a nuclear protein that, by itself, also fails to protect 32D cells from apoptosis. Co-expression of IRS-1 with the SV40 T antigen in 32D cells results in nuclear translocation of IRS-1 and survival after IL-3 withdrawal. Expression of a human IGF-I receptor in 32D/IRS-1 cells also results in nuclear translocation of IRS-1 and IL-3 independence. The phosphotyrosine-binding domain, but not the pleckstrin domain, is necessary for IRS-1 nuclear translocation. Nuclear translocation of IRS-1 was confirmed in mouse embryo fibroblasts. These results suggest possible new roles for nuclear IRS-1 in IGF-I-mediated growth and anti-apoptotic signaling.

Functional interleukin-7 receptors (IL-7Rs) are expressed by marrow stromal cells: binding of IL-7 increases levels of IL-6 mRNA and secreted protein

DNA spotted microarrays were used to compare gene expression profiles from 2 functionally distinct human marrow stromal cell lines: HS-27a, which supports cobblestone area formation by early hematopoietic progenitors, and HS-5, which secretes multiple cytokines that support the proliferation of committed progenitors. One unexpected result was the high level of interleukin-7 receptor (IL-7R) gene expression in HS-27a stromal cells. Northern blot analysis confirmed the IL-7R RNA expression, and Western blots for the IL-7R protein detected both a full-length (90-kd) IL-7R and a smaller 30-kd fragment in both HS-27a cells and primary stromal cell cultures, whereas only the 90-kd receptor protein was detected in peripheral blood mononuclear cells. Biotinylated IL-7 was shown to bind to HS-27a cells under physiologic conditions, and this binding was inhibited by blocking anti-IL-7 antibodies. Tyrosine phosphorylation of several proteins (55 kd, 30 kd, and 24 kd) in HS-27a cells was rapidly increased after incubation with recombinant IL-7. One of the phosphorylated proteins proved to be the 30-kd IL-7R fragment. Exposure of HS-27a cells to IL-7 resulted in a 10-fold increase in secretion of IL-6 into culture supernatants but no increase in the cytokines stromal cell-derived factor 1, macrophage inflammatory protein 1 alpha, or IL-1 beta. The up-regulation of IL-6 secretion is associated with a rapid but transient increase in detectable levels of IL-6 messenger RNA. These data suggest that IL-7 may function to regulate the milieu of the microenvironment by modulating IL-6 secretion by the IL-7R-expressing stromal elements.

Phosphorylation of calmodulin. Functional implications

Calmodulin (CaM) is phosphorylated in vitro and in vivo by multiple protein-serine/threonine and protein-tyrosine kinases. Casein kinase II and myosin light-chain kinase are two of the well established protein-serine/threonine kinases implicated in this process. On the other hand, within the protein-tyrosine kinases involved in the phosphorylation of CaM are receptors with tyrosine kinase activity, such as the insulin receptor and the epidermal growth factor receptor, and nonreceptor protein-tyrosine kinases, such as several members of the Src family kinases, Janus kinase 2, and p38Syk. The phosphorylation of CaM brings important physiological consequences for the cell as the diverse phosphocalmodulin species have differential actions as compared to nonphosphorylated CaM when acting on different CaM-dependent systems. In this review we will summarize the progress made on this topic as the first report on phosphorylation of CaM was published almost two decades ago. We will emphasize the description of the phosphorylation events mediated by the different protein kinases not only in the test tube but in intact cells, the phosphorylation-mediated changes of CaM activity, its action on CaM-dependent systems, and the functional repercussion of these phosphorylation processes in the physiology of the cell.

Heparin-binding epidermal growth factor-like growth factor: hypoxia-inducible expression in vitro and stimulation of neurogenesis in vitro and in vivo

Heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) is found in cerebral neurons, and its expression is increased after hypoxic or ischemic injury, which also stimulates neurogenesis. To investigate the possible role of HB-EGF in hypoxic-ischemic induction of neurogenesis, we measured its expression, effects, and target receptors in embryonic murine cerebral cortical cultures and in adult rat brain. Hypoxia increased HB-EGF expression by approximately 50% in cortical cultures, where expression was associated with mature and immature neurons. HB-EGF (5-100 ng/ml) stimulated by approximately 80% the incorporation of bromodeoxyuridine (BrdU) into cultured cells that expressed the HB-EGF receptors epidermal growth factor receptor (EGFR)/avian erythroblastic leukemia viral oncogene homolog 1 (ErbB1) and N-arginine dibasic convertase (NRDc). Intracerebroventricular administration of HB-EGF in adult rats increased BrdU labeling in the subventricular zone and in the subgranular zone of dentate gyrus, where EGFR/ErbB1 and NRDc were also expressed and where ischemia-induced neurogenesis is observed. We conclude that HB-EGF stimulates neurogenesis in proliferative zones of the adult brain that are also affected in ischemia and that it does so by interacting with EGFR/ErbB1 and possibly NRDc. Therefore, HB-EGF may help to trigger proliferation of neuronal precursors in brain after hypoxic or ischemic injury.

Identification and characterization of a sphingolipid delta 4-desaturase family

Sphingolipids desaturated at the Delta4-position are important signaling molecules in many eukaryotic organisms, including mammals. In a bioinformatics approach, we now identified a new family of protein sequences from animals, plants, and fungi and characterized these sequences biochemically by expression in Saccharomyces cerevisiae. This resulted in the identification of the enzyme sphingolipid Delta4-desaturase (dihydroceramide desaturase) from Homo sapiens, Mus musculus, Drosophila melanogaster, and Candida albicans, in addition to a bifunctional sphingolipid Delta4-desaturase/C-4-hydroxylase from M. musculus. Among the sequences investigated are the Homo sapiens membrane lipid desaturase, the M. musculus degenerative spermatocyte, and the Drosophila melanogaster degenerative spermatocyte proteins. During spermatogenesis, but not oogenesis of des mutant flies, both cell cycle and spermatid differentiation are specifically blocked at the entry into the first meiotic division, leading to male sterility. This mutant phenotype can be restored to wild-type by complementation with a functional copy of the des gene (Endo, K., Akiyama, T., Kobayashi S., and Okada, M. (1996) Mol. Gen. Genet. 253, 157-165). These results suggest that Delta4-desaturated sphingolipids provide an early signal necessary to trigger the entry into both meiotic and spermatid differentiation pathways during Drosophila spermatogenesis.

VEGF receptor 2 and the adherens junction as a mechanical transducer in vascular endothelial cells

Blood-flow interactions with the vascular endothelium represents a specialized example of mechanical regulation of cell function that has important physiological and pathophysiological cardiovascular consequences. Yet, the mechanisms of mechanostransduction are not understood fully. This study shows that shear stress induces a rapid induction as well as nuclear translocation of the vascular endothelial growth factor (VEGF) receptor 2 and promotes the binding of the VEGF receptor 2 and the adherens junction molecules, VE-cadherin and beta-catenin, to the endothelial cytoskeleton. These changes are accompanied by the formation of a complex containing the VEGF receptor 2-VE-cadherin-beta-catenin. In endothelial cells lacking VE-cadherin, shear stress did not augment nuclear translocation of the VEGF receptor 2 and phosphorylation of Akt1 and P38 as well as transcriptional induction of a reporter gene regulated by a shear stress-responsive promoter. These results suggest that VEGF receptor 2 and the adherens junction act as shear-stress cotransducers, mediating the transduction of shear-stress signals into vascular endothelial cells.

Cytoplasmic transport of Stat3 by receptor-mediated endocytosis

Signal transducer and activator of transcription (STAT) proteins are cytoplasmic transcription factors that translocate to the nucleus and regulate gene expression upon activation of cytokine or growth factor receptors. While this translocation event is essential for gene regulation by STATs, their mechanism of transport through the cytoplasm to the nucleus has remained elusive. We now report that cytoplasmic transport of Stat3 is an active process that requires receptor-mediated endocytosis. Stat3 co-localizes with endocytic vesicles in transit from the cell membrane to the perinuclear region in response to growth factor stimulation. Consistent with a role for receptor endocytosis in growth factor signaling, disruption of endocytosis with specific inhibitors blocks Stat3 nuclear translocation and Stat3-dependent gene regulation. These results indicate that receptor-mediated endocytosis may be a general mechanism of transport through the cytoplasm for a subset of cytoplasmic signaling proteins destined for the nucleus.

c-erbB-3: a nuclear protein in mammary epithelial cells

c-erbB receptors are usually located in cell membranes and are activated by extracellular binding of EGF-like growth factors. Unexpectedly, using immunofluorescence we found high levels of c-erbB-3 within the nuclei of MTSV1-7 immortalized nonmalignant human mammary epithelial cells. Nuclear localization was mediated by the COOH terminus of c-erbB-3, and a nuclear localization signal was identified by site-directed mutagenesis and by transfer of the signal to chicken pyruvate kinase. A nuclear export inhibitor caused accumulation of c-erbB-3 in the nuclei of other mammary epithelial cell lines as demonstrated by immunofluorescence and biochemical cell fractionation, suggesting that c-erbB-3 shuttles between nuclear and nonnuclear compartments in these cells. Growth of MTSV1-7 on permeable filters induced epithelial polarity and concentration of c-erbB-3 within the nucleoli. However, the c-erbB-3 ligand heregulin beta1 shifted c-erbB-3 from the nucleolus into the nucleoplasm and then into the cytoplasm. The subcellular localization of c-erbB-3 obviously depends on exogenous stimuli and on the stage of epithelial polarity and challenges the specific function of c-erbB-3 as a transmembrane receptor protein arguing for additional, as yet unidentified, roles of c-erbB-3 within the nucle(ol)us of mammary epithelial cells.

Elastase-released epidermal growth factor recruits epidermal growth factor receptor and extracellular signal-regulated kinases to down-regulate tropoelastin mRNA in lung fibroblasts

Elastase/anti-elastase imbalance is a hallmark of emphysema, a chronic obstructive pulmonary disease associated with the rupture and inefficient repair of interstitial elastin. We report that neutrophil elastase (NE) at low physiologic concentrations, ranging from 35 nm to 1 microm, invokes transient, peaking at 15 min, activation of extracellular signal-regulated kinases 1 and 2 (ERK) in elastogenic lung fibroblasts. ERK activation is preceded by the release of soluble 25-26-kDa forms of epidermal growth factor (EGF) and transactivation of EGF receptor (EGFR) in NE-exposed cells. The stimulatory effect of NE on ERK is abrogated in the presence of anti-EGF-neutralizing antibodies, EGFR tyrosine kinase inhibitor (AG1478), and ERK kinase inhibitor (PD98059), as well as abolished in both EGFR-desensitized and endocytosis-arrested fibroblasts. Nuclear accumulation of activated ERK is associated with transient, peaking at 30 min, induction of c-Fos and sustained, observed at 24-48 h, decrease of tropoelastin mRNA levels in NE-challenged cells. Pretreatment of fibroblasts with AG1478 or PD98059 abrogates the NE-initiated tropoelastin mRNA suppression. We conclude that proteolytically released EGF signals directly via EGFR and ERK to down-regulate tropoelastin mRNA in NE-challenged lung fibroblasts.

Proteolytic processing of low density lipoprotein receptor-related protein mediates regulated release of its intracellular domain

The low density lipoprotein (LDL) receptor-related protein (LRP) is a multifunctional cell surface receptor that interacts through its cytoplasmic tail with adaptor and scaffold proteins that participate in cellular signaling. Its extracellular domain, like that of the signaling receptor Notch and of amyloid precursor protein (APP), is proteolytically processed at multiple positions. This similarity led us to investigate whether LRP, like APP and Notch, might also be cleaved at a third, intramembranous or cytoplasmic site, resulting in the release of its intracellular domain. Using independent experimental approaches we demonstrate that the cytoplasmic domain is released by a gamma-secretase-like activity and that this event is modulated by protein kinase C. Furthermore, cytoplasmic adaptor proteins that bind to the LRP tail affect the subcellular localization of the free intracellular domain and may regulate putative signaling functions. Finally, we show that the degradation of the free tail fragment is mediated by the proteasome. These findings suggest a novel role for the intracellular domain of LRP that may involve the subcellular translocation of preassembled signaling complexes from the plasma membrane.

Cytochrome P450 2C9-induced endothelial cell proliferation involves induction of mitogen-activated protein (MAP) kinase phosphatase-1, inhibition of the c-Jun N-terminal kinase, and up-regulation of cyclin D1

Cytochrome P450 (CYP)-derived epoxyeicosatrienoic acids (EETs) are important modulators of endothelial cell homeostasis. We investigated the signaling pathway linking the activation of CYP 2C9 to enhanced endothelial cell proliferation. Overexpression of CYP 2C9 in cultured human endothelial cells markedly increased proliferation. This effect was paralleled by an up-regulation of the G(1) phase regulatory protein, cyclin D1. The specific CYP 2C9 inhibitor, sulfaphenazole, prevented both the enhanced cell proliferation and up-regulation of cyclin D1. CYP 2C9 overexpression also decreased the activity of the c-Jun N-terminal kinase (JNK). Coexpression of wild type JNK with CYP 2C9 attenuated the CYP 2C9-induced increase in cyclin D1 expression and abolished the CYP 2C9-induced proliferation response. In contrast, cotransfecting dominant negative JNK with CYP 2C9 restored the CYP 2C9-mediated up-regulation of cyclin D1 and proliferation. The inactivation of JNK is linked to its dephosphorylation by dual specificity mitogen-activated protein (MAP) kinase phosphatases (MKPs). Overexpression of CYP 2C9 significantly increased the expression of MKP-1, as did incubation with 11,12-EET. These data demonstrate that the mitogenic effect of CYP 2C9 is due to the generation of EETs, which promote the MKP-1-mediated dephosphorylation and inactivation of JNK, effects ultimately culminating in the expression of cyclin D1 and endothelial cell proliferation.