Epidermal growth factor induced tyrosine phosphorylation of nuclear proteins associated with translocation of epidermal growth factor receptor into the nucleus

Regulation of ErbB Receptors by the Ca2+ Sensor Protein Calmodulin in Cancer

Overexpression and mutations of the epidermal growth factor receptor (EGFR/ErbB1/HER1) and other tyrosine kinase receptors of the ErbB family (ErbB2/HER2, ErbB3/HER3 and ErbB4/HER4) play an essential role in enhancing the proliferation, the migratory capacity and invasiveness of many tumor cells, leading to cancer progression and increased malignancy. To understand these cellular processes in detail is essential to understand at a molecular level the signaling pathways and regulatory mechanisms controlling these receptors. In this regard, calmodulin (CaM) is a Ca2+-sensor protein that directly interacts with and regulates ErbB receptors, as well as some CaM-dependent kinases that also regulate these receptors, particularly EGFR and ErbB2, adding an additional layer of CaM-dependent regulation to this system. In this short review, an update of recent advances in this area is presented, covering the direct action of Ca2+/CaM on the four ErbB family members mostly in tumor cells and the indirect action of Ca2+/CaM on the receptors via CaM-regulated kinases. It is expected that further understanding of the CaM-dependent mechanisms regulating the ErbB receptors in future studies could identify new therapeutic targets in these systems that could help to control or delay cancer progression.

Nuclear ERK: Mechanism of Translocation, Substrates, and Role in Cancer

The extracellular signal-regulated kinases 1/2 (ERK) are central signaling components that regulate stimulated cellular processes such as proliferation and differentiation. When dysregulated, these kinases participate in the induction and maintenance of various pathologies, primarily cancer. While ERK is localized in the cytoplasm of resting cells, many of its substrates are nuclear, and indeed, extracellular stimulation induces a rapid and robust nuclear translocation of ERK. Similarly to other signaling components that shuttle to the nucleus upon stimulation, ERK does not use the canonical importinα/β mechanism of nuclear translocation. Rather, it has its own unique nuclear translocation signal (NTS) that interacts with importin7 to allow stimulated shuttling via the nuclear pores. Prevention of the nuclear translocation inhibits proliferation of B-Raf- and N/K-Ras-transformed cancers. This effect is distinct from the one achieved by catalytic Raf and MEK inhibitors used clinically, as cells treated with the translocation inhibitors develop resistance much more slowly. In this review, we describe the mechanism of ERK translocation, present all its nuclear substrates, discuss its role in cancer and compare its translocation to the translocation of other signaling components. We also present proof of principle data for the use of nuclear ERK translocation as an anti-cancer target. It is likely that the prevention of nuclear ERK translocation will eventually serve as a way to combat Ras and Raf transformed cancers with less side-effects than the currently used drugs.

SerpinB2 is involved in cellular response upon UV irradiation

Ultraviolet light induced pyrimidine dimer is a helix distortion DNA damage type, which recruits repair complexes. However, proteins of these complexes that take part in both DNA damage recognition and repair have been well-described, the regulation of the downstream steps of nucleotide excision repair (NER) have not been clearly clarified yet. In a high-throughput screen, we identified SerpinB2 (SPB2) as one of the most dramatically upregulated gene in keratinocytes following UV irradiation. We found that both the mRNA and the protein levels of SPB2 were increased upon UV irradiation in various cell lines. Additionally, UV damage induced translocation of SPB2 from the cytoplasm to the nucleus as well as the damage induced foci formation of it. Here we show that SPB2 co-localizes with XPB involved in the NER pathway at UV-induced repair foci. Finally, we demonstrated that UV irradiation promoted the association of SPB2 with ubiquitylated proteins. In basal cell carcinoma tumour cells, we identified changes in the subcellular localization of SPB2. Based on our results, we conclude that SPB2 protein has a novel role in UV-induced NER pathway, since it regulates the removal of the repair complex from the damaged site leading to cancerous malformation.

CAR expression in human embryos and hESC illustrates its role in pluripotency and tight junctions

Coxsackie virus and adenovirus receptor, CXADR (CAR), is present during embryogenesis and is involved in tissue regeneration, cancer and intercellular adhesion. We investigated the expression of CAR in human preimplantation embryos and embryonic stem cells (hESC) to identify its role in early embryogenesis and differentiation. CAR protein was ubiquitously present during preimplantation development. It was localised in the nucleus of uncommitted cells, from the cleavage stage up to the precursor epiblast, and corresponded with the presence of soluble CXADR3/7 splice variant. CAR was displayed on the membrane, involving in the formation of tight junction at compaction and blastocyst stages in both outer and inner cells, and CAR corresponded with the full-length CAR-containing transmembrane domain. In trophectodermal cells of hatched blastocysts, CAR was reduced in the membrane and concentrated in the nucleus, which correlated with the switch in RNA expression to the CXADR4/7 and CXADR2/7 splice variants. The cells in the outer layer of hESC colonies contained CAR on the membrane and all the cells of the colony had CAR in the nucleus, corresponding with the transmembrane CXADR and CXADR4/7. Upon differentiation of hESC into cells representing the three germ layers and trophoblast lineage, the expression of CXADR was downregulated. We concluded that CXADR is differentially expressed during human preimplantation development. We described various CAR expressions: i) soluble CXADR marking undifferentiated blastomeres; ii) transmembrane CAR related with epithelial-like cell types, such as the trophectoderm (TE) and the outer layer of hESC colonies; and iii) soluble CAR present in TE nuclei after hatching. The functions of these distinct forms remain to be elucidated.

Protein kinases and transcription factors activation in response to UV-radiation of skin: implications for carcinogenesis

Solar ultraviolet (UV) radiation is an important environmental factor that leads to immune suppression, inflammation, photoaging, and skin carcinogenesis. Here, we reviewed the specific signal transduction pathways and transcription factors involved in the cellular response to UV-irradiation. Increasing experimental data supporting a role for p38, MAPK, JNK, ERK1/2, and ATM kinases in the response network to UV exposure is discussed. We also reviewed the participation of NF-κB, AP-1, and NRF2 transcription factors in the control of gene expression after UV-irradiation. In addition, we discussed the promising chemotherapeutic intervention of transcription factors signaling by natural compounds. Finally, we focused on the review of data emerging from the use of DNA microarray technology to determine changes in global gene expression in keratinocytes and melanocytes in response to UV treatment. Efforts to obtain a comprehensive portrait of the transcriptional events regulating photodamage of intact human epidermis after UV exposure reveals the existence of novel factors participating in UV-induced cell death. Progress in understanding the multitude of mechanisms induced by UV-irradiation could lead to the potential use of protein kinases and novel proteins as specific targets for the prevention and control of skin cancer.

Ultraviolet irradiation-induces epidermal growth factor receptor (EGFR) nuclear translocation in human keratinocytes

Epidermal growth factor receptor (EGFR) plays a critical role in mediating ultraviolet (UV) irradiation-induced signal transduction and gene expression in human keratinocytes. EGFR activation results from increased phosphorylation on specific tyrosine residues in the C-terminal intracellular domain. It has recently been reported that following growth factor stimulation EGFR translocates from the surface membrane to the nucleus, where it may directly regulate gene transcription. We have investigated the ability of UV irradiation to induce EGFR nuclear translocation in human primary and HaCaT keratinocytes. UV irradiation caused rapid nuclear translocation of EGFR. Significant accumulation of EGFR in the nucleus was observed within 15 min after UV irradiation exposure. Maximal translocation occurred at 30 min post-UV irradiation, and resulted in a 10-fold increase in EGFR in the nucleus, as determined by Western blot analysis of nuclear extracts and confirmed by immunofluorescence. Inhibition of nuclear export by Leptomycin B did not alter UV irradiation-induced nuclear accumulation. EGFR tyrosine kinase inhibitor (PD169540) reduced UV irradiation-induced EGFR nuclear translocation 50%. Mutation of either tyrosine 1148 or tyrosine 1173 reduced nuclear translocation 70%, while mutation of tyrosine 1068 was without effect. In addition, over-expression of receptor type protein tyrosine phosphatase-kappa (RPTP-kappa), which specifically dephosphorylates EGFR tyrosines, decreased UV irradiation-induced EGFR nuclear translocation in human keratinocytes. These data demonstrate that UV irradiation stimulates rapid EGFR nuclear translocation, which is dependent on phosphorylation of specific EGFR tyrosine residues. EGFR nuclear translocation may act in concert with conventional signaling pathways to mediate UV irradiation-induced responses in human keratinocytes.

The direct effect of nuclear pores on nuclear chemotherapeutic concentration in multidrug resistant bladder cancer: the nuclear sparing phenomenon

PURPOSE

Multidrug resistance commonly limits effectiveness in treating malignancy with chemotherapy. Multidrug resistance has classically been described as a cell membrane phenomenon. Multidrug resistant cells are known to specifically exclude chemotherapy from the nucleus, resulting in lower nuclear concentrations than in the cytoplasm. This phenomenon is known as nuclear sparing and little is known of its etiology. We hypothesized that a component of the nuclear membrane, the nuclear pore, is responsible for this phenomenon.

MATERIALS AND METHODS

In this in vitro study we used the drug sensitive urothelial cancer cell line Massachusetts General Hospital urothelial 1 sensitive and its multidrug resistant subline Massachusetts General Hospital urothelial 1 resistant. After quantitative assessment of nuclear pores resistant and sensitive cells were fused using polyethylene glycol and laser scanning confocal microscopy was used to identify if drug resistant and sensitive nuclei can coexist within the same cell. The effect of inhibiting nuclear pore function using the specific pore inhibitor, wheat germ agglutinin, was assessed in whole cells using confocal microscopy and cytotoxicity assay as well as in isolated nuclei.

RESULTS

Nuclear pores appeared more numerous in multidrug resistance cells. Cell fusion experiments showed that multidrug resistance and sensitive nuclei could coexist with the same cell milieu. Wheat germ agglutinin reversed multidrug resistance in whole cells and isolated nuclei.

CONCLUSIONS

Multidrug resistance is a complex phenomenon occurring at many cellular levels, of which all may be potential therapeutic targets. The nuclear pore is involved in this process, which is to our knowledge a previously undescribed phenomenon. These experiments suggest that it may act to export drug from the nucleus, which is a process inhibited by wheat germ agglutinin.

Trafficking and signaling pathways of nuclear localizing protein ligands and their receptors

Interaction of ligands such as epidermal growth factor and interferon-gamma with the extracellular domains of their plasma membrane receptors results in internalization followed by translocation into the nucleus of the ligand and/or receptor. There has been reluctance, however, to ascribe signaling importance to this, the focus instead being on second messenger pathways, including mobilization of kinases and inducible transcription factors (TFs). The latter, however, fails to explain the fact that so many ligands stimulate the same second messenger cascades/TFs, and yet show distinct gene activation profiles. This is particularly apt in the case of the seven STAT TFs that are held to be the mediators of the distinct cellular functions of over 60 ligands. The current review focuses on five representative nuclear localizing ligands for which there is documentation of translocation into the cytosol and nucleus through well-characterized pathways, in addition to a role in gene activation by ligand/receptor in the nucleus.

Functional endothelin receptors are present on nuclei in cardiac ventricular myocytes

Endothelins are thought to act through two specific, plasmalemmal G protein-coupled receptor subtypes, ETAR and ETBR. However, in subfractionated cardiac membranes, ETAR immunoreactivity was detected only in the plasma membrane whereas ETBR immunoreactivity was detected predominantly in membranes of intracellular origin. Confocal microscopy demonstrated the presence of intracellular ETAR and ETBR in ventricular myocytes. ETAR were primarily on plasma membrane (surface membranes and transverse-tubules) and to a lesser extent on the nucleus while ETBR localized primarily to the nuclei. Western blot analysis of nuclei isolated from the heart indicated the presence of endothelin receptors: both ETAR and ETBR copurified with nucleoporin 62, whereas markers of endoplasmic reticulum and Golgi membranes were depleted. Radioligand binding studies revealed that isolated nuclei contain specific [125I]ET-1 binding sites. Specific [125I]ET-1 binding was reduced by 70-80% using the ETAR-selective antagonist BQ610 and 20-30% using the ETBR-specific antagonist BQ788. IRL-1620, an ETBR-specific agonist, also reduced [125I]ET-1 binding. Furthermore, ET-1 and IRL-1620 altered the incorporation of 32P into nuclear proteins and caused a transient increase in nuclear Ca2+ concentration. Hence, cardiac nuclei possess both ETAR and ETBR subtypes, which are functional with respect to ligand binding and are coupled to signaling mechanisms within the nuclear membrane.

Immunohistochemical localization of transforming growth factor alpha in regenerating rat liver

Immunohistochemical localization of TGF-alpha and cell proliferation kinetics during liver regeneration after two-thirds partial hepatectomy (PH) were investigated. Twenty-four to 72 hr after PH, appreciable increase in the number of TGF-alpha-positive hepatocytes was observed in zones 1 and 2. At the peak at 36 hr, almost all positive cells were stained in their nuclei. Considerable increase in the BrdU labeling index was observed 24-36 hr after PH with a peak at 24 hr in zones 1 and 2. These results indicated an association between TGF-alpha expression and hepatocyte regeneration. It is suggested that immunohistochemical localization of TGF-alpha may be a useful marker of cell proliferation activity in rat liver.

A novel mechanism for mitogenic signaling via pro-transforming growth factor alpha within hepatocyte nuclei

Transforming growth factor (TGF) alpha, an important mediator of growth stimulation, is known to act via epidermal growth factor receptor (EGF-R) binding in the cell membrane. Here we show by immunohistology, 2-dimensional immunoblotting, and mass spectrometry of nuclear fractions that the pro-protein of wild-type TGF-alpha occurs in hepatocyte nuclei of human, rat, and mouse liver. Several findings show a close association between nuclear pro-TGF-alpha and DNA synthesis. (1) The number of pro-TGF-alpha+ nuclei was low in resting liver and increased dramatically after partial hepatectomy and after application of hepatotoxic chemicals or the primary mitogen cyproterone acetate (CPA); in any case, S phase occurred almost exclusively in pro-TGF-alpha+ nuclei. The same was found in human cirrhotic liver. (2) In primary culture, 7% of hepatocytes synthesized pro-TGF-alpha, which then translocated to the nucleus; 70% of these nuclei subsequently entered DNA replication, whereas only 2% of pro-TGF-alpha- hepatocytes were in S phase. (3) The frequency of hepatocytes coexpressing pro-TGF-alpha and DNA synthesis was increased by the hepatomitogens CPA or prostaglandin E(2) and was decreased by the growth inhibitor TGF-beta1. (4) Treatment with mature TGF-alpha increased DNA synthesis exclusively in pro-TGF-alpha- hepatocytes, which was abrogated by the EGF-R tyrosine kinase inhibitor tyrphostin A25. In conclusion, TGF-alpha gene products may exert mitogenic effects in hepatocytes via 2 different signaling mechanisms: (1) the "classic" pathway of mature TGF-alpha via EGF-R in the membrane and (2) a novel pathway involving the presence of pro-TGF-alpha in the nucleus.

Proteins encoded by genes involved in chromosomal alterations in lymphoma and leukemia: clinical value of their detection by immunocytochemistry

Acquired chromosomal anomalies (most commonly translocations) in lymphoma and leukemia usually result in either activation of a quiescent gene (by means of immunoglobulin or T-cell-receptor promotors) and expression of an intact protein product, or creation of a fusion gene encoding a chimeric protein. This review summarizes current immunocytochemical studies of these 2 categories of oncogenic protein, with emphasis on the clinical relevance of their detection in diagnostic samples. Among the quiescent genes activated by rearrangement, expression of cyclin D1 (due to rearrangement of the CCND1 [BCL-1] gene) is a near-specific marker of t(11;14) in mantle cell lymphoma; BCL-2 expression distinguishes follicular lymphoma cells from their nonneoplastic counterparts in reactive germinal centers and appears to be an independent prognostic marker in diffuse large cell lymphoma; and TAL-1 (SCL) expression identifies T-cell acute lymphoblastic neoplasms in which this gene is activated. The protein products of other genes activated by chromosomal rearrangement have a role as markers of either lineage (eg, PAX-5 [B-cell-specific activator protein] for B cells, including B-lymphoblastic neoplasms), or maturation stage (eg, BCL-6 for germinal-center and activated B cells and MUM-1/IRF4 for plasma cells). Currently, no hybrid protein encoded by fusion genes is reliably detectable by antibodies recognizing unique junctional epitopes (ie, epitopes absent from the wild-type constituent proteins). Nevertheless, staining for promyelocytic leukemia (PML) protein will detect acute PML with t(15;17) because the microspeckled nuclear labeling pattern for PML-RARalpha is highly distinctive. Similarly, antibodies to the anaplastic lymphoma kinase (ALK) tyrosine kinase are valuable (because wild-type ALK is not found in normal lymphoid tissue) in detecting neoplasms (CD30-positive large T-cell lymphomas) with t(2;5) or its variants. Thus, immunocytochemical detection of the products of many rearranged genes in lymphoma and leukemia can be clinically informative and provide information on cellular and subcellular protein expression that cannot be inferred from studies based on messenger RNA.

Immunohistochemical localization of activated EGF receptor in human eccrine and apocrine sweat glands

Epidermal growth factor (EGF) is secreted into sweat from secretory cells of human sweat glands. The function of EGF in sweat is poorly understood. The biological function of EGF is exerted by the binding of EGF to the receptor (EGFR) and its activation. Therefore, we immunohistochemically localized the activated form of EGFR in human eccrine and apocrine sweat glands to assess the functional importance of the EGF-EGFR system in human sweat glands. Frozen sections of human skin were stained with a monoclonal antibody (MAb) specific for tyrosine-phosphorylated (activated) EGFR and with an MAb that stains both activated and non-activated EGFR. In the secretory portion of eccrine sweat glands, nuclei of the secretory cells were stained with the anti-activated EGFR MAb. In coiled and straight portions of eccrine sweat ducts, nuclei of luminal and peripheral cells were stained with the antibody specific for activated EGFR. Luminal cell membranes and luminal cytoplasm of inner ductal cells possessed non-activated EGFR. In the secretory portion of apocrine sweat glands, activated EGFRs were present in cytoplasm and nuclei of secretory cells. These data suggest that EGF, already known to be present in the cytoplasm of secretory cells in eccrine and apocrine sweat glands, activates EGFR in the nuclei of secretory cells themselves in an intracrine manner. Because ductal cells do not express EGF, EGF in the sweat secreted from the secretory cells should activate EGFR in the ductal cells in a paracrine manner. (J Histochem Cytochem 49:597-601, 2001)

Nuclear localization of epidermal growth factor and epidermal growth factor receptors in human thyroid tissues

Epidermal growth factor (EGF) has widespread growth effects, and in some tissues proliferation is associated with the nuclear localization of EGF and epidermal growth factor receptor (EGFR). In the thyroid, EGF promotes growth but differs from thyrotropin (TSH) in inhibiting rather than stimulating functional parameters. We have therefore studied the occurrence and cellular distribution of EGF and EGFR in normal thyroid, in Graves' disease, where growth is mediated through the thyrotropin receptor (TSHR), and in a variety of human thyroid tumors. In the normal gland the staining was variable, but largely cytoplasmic, for both EGF and EGFR. In Graves' disease there was strong cytoplasmic staining for both EGF and EGFR, with frequent positive nuclei. Nuclear positivity for EGF and particularly for EGFR was also a feature of both follicular adenomas and follicular carcinomas. Interestingly, nuclear staining was almost absent in papillary carcinomas. These findings document for the first time the presence of nuclear EGF and EGFR in thyroid. Their predominant occurrence in tissues with increased growth (Graves' disease, follicular adenoma, and carcinoma) may indicate that nuclear EGF and EGFR play a role in growth regulation in these conditions. The absence of nuclear EGF and EGFR in papillary carcinomas would suggest that the role played by EGF in growth control differs between papillary carcinoma and follicular adenomas/carcinomas of the thyroid.

Upregulation of a raf kinase and a DP-1 family transcription factor in epidermal growth factor (EGF) stimulated filarial parasites

Previously we have shown that in the filarial parasite Brugia malayi, stimulation with murine epidermal growth factor (EGF) upregulated the expression of the nuclear GTPase, Ran. In this paper we provide further evidence that filarial parasites possess the ability to respond to mammalian EGF. Stimulation of B. malayi microfilariae with EGF increased transcription of a Raf kinase, increased the physical interaction between Ran and at least eight unidentified proteins, abolished the association of a putative EGF receptor with the nuclear GTPase Ran and enhanced phosphorylation of native microfilarial proteins. In the cattle filarial parasite Setaria digitata, stimulation of adult worms with EGF was probably responsible for up-regulation of a DP-1 family transcription factor. These data suggest that filarial parasites possess the ability to respond to mammalian EGF and that mammalian growth factors may regulate developmental maturation of filarial parasites.

Nuclear and cytoskeletal translocation and localization of heterotrimeric G-proteins

Heterotrimeric GTP-binding proteins (G-proteins) are involved in a diverse array of signalling pathways. They are generally thought to be membrane-bound proteins, which disassociate on receptor activation and binding of GTP. A model to explain this has been proposed, which is often described as 'the G-protein cycle'. The 'G-protein cycle' is discussed in the present paper in relation to evidence that now exists regarding the non- membranous localization of G-proteins. Specifically, the experimental evidence demonstrating association of G-proteins with the cytoskeleton and the nucleus, and the mechanisms by which G-proteins translocate to these sites are reviewed. Furthermore, the possible effector pathways and the physiological function of G-proteins at these sites are discussed.

Cloning and epitope mapping of a functional partial fusion receptor for human cytomegalovirus gH

A cDNA clone encoding a partial putative human cytomegalovirus (HCMV) gH fusion receptor (CMVFR) was previously identified. In this report, the cDNA sequence of CMVFR was determined and the role of this CMVFR in HCMV/cell fusion was confirmed by rendering fusion-incompetent MOLT-4 cells susceptible to fusion following transfection with receptor cDNA. Blocking experiments using recombinant gH or either of two MAbs (against recombinant gH or purified viral gH:gL) provided additional evidence for the role of gH binding to this protein in virus fusion. An HCMV-binding domain of 12 aa in the middle hydrophilic region of CMVFR was identified by fusion blocking studies using synthetic receptor peptides. The 1368 bp cDNA of CMVFR contained a predicted ORF of 345 aa with two potential membrane-spanning domains and several possible nuclear localization signals. A search of sequence databases indicated that CMVFR is a novel protein. Further characterization of this cell membrane protein that confers susceptibility to fusion with the viral envelope should provide important information about the mechanism by which HCMV infects cells.

The nuclear accumulation of a variant epidermal growth factor receptor (EGFR) lacking the transmembrane domain requires coexpression of a full-length EGFR

Both the epidermal growth factor (EGF) and its receptor (EGFR) accumulate in the nucleoplasm during liver regeneration. This localization in a nonmembraneous compartment presents a challenge in that the standard form of EGFR is a transmembrane protein and suggests the existence of a variant, soluble form of EGFR. To investigate the localization of such a putative EGFR splice variant, we generated a transmembrane-devoid form of EGFR. We placed this transmembrane-negative [TM(-)] EGFR construct and full-length wild-type (wt) EGFR either in a retroviral transfection vector or in an inducible expression vector. Mouse 3T3 cells, which express endogenous EGFR, were transfected with the TM(-) EGFR construct. The expression of these TM(-) EGFR, detected with a specific antibody against human EGFR using a confocal laser-scanning microscope, was predominantly found in the cytoplasm with no nuclear localization. After an overnight incubation with EGF the TM(-) EGFR accumulated in the nucleus. In mouse NR6 cells, which lack endogenous EGFR, transfected TM(-) EGFR were found in the cytoplasm, but incubation with EGF did not result in a nuclear accumulation of TM(-) EGFR. However, NR6 cells transfected with both TM(-) EGFR and wt EGFR showed nuclear accumulation after EGF treatment. These results suggest that both the wt EGFR and the TM(-) EGFR are required for nuclear accumulation of TM(-) EGFR and may implicate a model of homotypic recognition and translocation of a splice variant of EGFR.

Demonstration of new sites of expression of the CCK-B/gastrin receptor in pancreatic acinar AR42J cells using immunoelectron microscopy

The CCK-B/gastrin receptor has been characterised in both normal and tumour tissues. Endocytosis of the CCK-B/gastrin receptor has recently been demonstrated and this has similarly been described for other peptide receptors. In addition, ligand and ligand-receptor translocation to the nucleus has been demonstrated for other peptides. The aim of this study was to identify the sites of expression of the CCK-B/gastrin receptor in the known CCK-B/gastrin receptor bearing pancreatic acinar AR42J cells. The specificity of the CCK-B/gastrin receptor antibody (alpha-CCKBR-Ser antibody) was demonstrated by inhibition ELISA studies, radioligand inhibition studies and immunofluorescence binding studies on AR42J cells. Western blotting and immunogold electron microscopy techniques were used to identify the receptor in AR42J cell preparations. The affinity purified alpha-CCKBR-Ser antibody was shown to be specific for the CCK-B/gastrin receptor. The receptor was expressed on the cell membrane, in the cytoplasm and within the nucleus. Isoforms of the receptor protein identified in extra-nuclear and nuclear extracts ranged in molecular weight from 58 to 66 kDa. We conclude that the CCK-B/gastrin receptor is not only expressed on the cell membrane, but also in the cytoplasm and nucleus of AR42J pancreatic acinar cells.

Cytoplasmic and nuclear cytokine receptor complexes

Much of our understanding on how hormones and cytokines transmit their message into the cell is based on the receptor activation at the plasma membrane. Many experimental in vitro models have established the paradigm for cytokine action based upon such activation of their cell surface receptor. The signaling from the plasma membrane activated cytokine receptor is driven to the nucleus by a rapid ricochet of protein phosphorylation, ultimately integrated as a differentiative, proliferative, or transcriptional message. The Janus kinase (JAK)--signal transducers and activators of transcription (STAT) pathway that was first thought to be cytokine receptor specific now appears to be activated by other noncytokine receptors. Also, evidence is accumulating showing that cytokines modulate the signal transduction machinery of the tyrosine kinase receptors and that of the heterotrimeric guanosine triphosphate (GTP)-binding protein-coupled receptors. Thus cytokine receptor signaling has become much more complex than originally hypothesized, challenging the established model of specificity of the action of a given cytokine. This review is focused on another level of complexity emerging within cytokine receptor superfamily signaling. Over the past 10 years, data from different laboratories have shown that cytokines and their receptors localize to intracellular compartments including the nucleus, and, in some cases, biological responses have been correlated with this unexpected location, raising the possibility that cytokines act as their own messenger through inter-actions with nuclear proteins. Thus, the interplay between cytokine receptor engagement and cellular signaling turns out to be more dynamic than originally suspected. The mechanisms and regulations of intracellular translocation of the cytokines, their receptors, and their signaling proteins are discussed in the context that such compartmentalization provides some of the specificity of the responses mediated by each cytokine.

A cell cycle regulating receptor is localized on cell surface and in nuclei of mitotically and meiotically dividing cells

We previously showed that a heterodimeric surface receptor of molecular weight 65,000 (p65) and 95,000 (p95) is expressed on the surface of proliferating cells such as activated T lymphocytes and neural precursors. This p65/p95 receptor is recognized by a monoclonal antibody and by type 3 reovirus hemagglutinin. Binding of the surface p65/p95 receptor leads to a growth arrest of mitotic cells and a consequent inhibition of proliferation. The p65/p95 receptor was demonstrated to be associated with kinase activity. Because p65/p95 is involved in the regulation of mitotic cell division, we sought to study the cellular distribution of the receptor and its possible role in meiotic cell division. Immunohistochemical labeling and flow cytometry studies were done using adult rat testes and cell lines. All cells undergoing mitotic or meiotic division in the rat testis expressed the p65/p95 receptor; cells that do not divide did not express receptors. Dividing cells had two receptor pools. As previously reported for several mitotically active tissues, a pool of receptors was localized on the cell surface. Interestingly, a pool of receptors was also seen intracellularly over the nucleus of labeled cells. The nuclear label seemed to be associated with chromosomes during specific stages of the mitotic and the two meiotic divisions, suggesting a role in the regulation of nuclear events. Further studies on this receptor and the function of the nuclear pool should provide a better understanding of the control of cell division.

Characterization of a novel amphiregulin-related molecule in 12-O-tetradecanoylphorbol-13-acetate-treated breast cancer cells

Amphiregulin (AR) can be induced at the mRNA level by 17-beta-estradiol (E2) or the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). This study compares the effects of TPA and E2 on the regulation of processing of AR isoforms and on subcellular localization in human MCF-7 breast cancer cells. AR was localized in the nucleus of MCF-7 cells after E2 treatment, whereas it was predominantly secreted after TPA treatment. AR isoforms of 28, 18, and 10 kDa and an additional species of approximately 55-60 kDa were detected in the cellular conditioned media after TPA stimulation. Expression of this unusual AR isoform was inhibited by protein kinase C (PKC) inhibitors such as bryostatin or H-7. The biochemical properties of this isoform are consistent with it being an N-linked glycosylated form of the AR precursor that contains unprocessed mannose residues. The size of this large isoform is reduced to approximately 40 kDa after treating the TPA-induced MCF-7 cells with tunicamycin or treating the conditioned media of such cells with N-glycosidase F or with endoglycosidase H. Moreover, this isoform is able to blind several lectins with specificity for mannose residues. The 55-60 kDa glycosylated AR isoform, like lower Mr AR isoforms, is able to bind to heparin and to stimulate the growth of MCF-10A cells by interacting with the EGF receptor. These data suggest that TPA activation of PKC may be involved in post-translational modifications of AR, such as glycosylation, and in alteration of its subcellular routing to predominantly a secretory pathway.

Nuclear Translocation of fibroblast growth factor (FGF) receptors in response to FGF-2

Members of the FGF family of growth factors localize to the nuclei in a variety of different cell types. To determine whether FGF receptors are also present within nuclei and if this localization is regulated by FGFs, nuclei were prepared from quiescent and FGF-2-treated Swiss 3T3 fibroblasts and examined for the presence of FGF receptors by immunoblotting with an antibody produced against the extracellular domain of FGF receptor-1 (FGFR-1). Little or no FGFR-1 is detected in nuclei prepared from quiescent cells. When cells are treated with FGF-2, however, there is a time- and dose-dependent increase in the association of FGFR-1 immunoreactivity with the nucleus. In contrast, treatment with either EGF or 10% serum does not increase the association of FGFR-1 with the nucleus. When cell surface proteins are labeled with biotin, a biotinylated FGFR-1 is detected in the nuclear fraction prepared from FGF-2-treated, but not untreated, cells indicating that the nuclear-associated FGFR-1 immunoreactivity derives from the cell surface. The presence of FGFR-1 in the nuclei of FGF-2-treated cells was confirmed by immunostaining with a panel of different FGFR-1 antibodies, including one directed against the COOH-terminal domain of the protein. Fractionation of nuclei from FGF-2-treated cells indicates that nuclear FGFR-1 is localized to the nuclear matrix, suggesting that the receptor may play a role in regulating gene activity.

The significance of valine 33 as a ligand-specific epitope of transforming growth factor alpha

Although binding of epidermal growth factor (EGF) and transforming growth factor alpha (TGFalpha) to the EGF receptor (EGFR) is mutually competitive, their binding is not identical, and their biological activities are not always equivalent. To probe for ligand-specific interactions, we have synthesized analogues of TGFalpha with modifications to the residue lying between the fourth and fifth cysteines (the "hinge"). Although this residue lies in a structurally conserved region of the protein, it is not conserved within the EGFR ligand family. Our results show that in TGFalpha there is a preference for a bulky hydrophobic hinge residue; this contrasts with EGF, for which a hydrogen bond donor functionality is preferred. Sequence analysis of the human EGFR ligands revealed that the nature of the hinge residue correlated with the sequence in the B-loop beta-sheet. As this region is an important determinant in recognition of TGFalpha by the chicken EGFR, we assessed the mitogenicity of the TGFalpha hinge mutants, as well as the other EGFR ligands, using chicken embryo fibroblasts. The preference of the chicken EGFR for TGFalpha hinge mutants with hydrophobic side chains paralleled that of the human EGFR. Betacellulin and heparin-binding EGF-like growth factor also possess an hydrophobic hinge; both were at least as potent as TGFalpha for chicken embryo fibroblasts. EGF and amphiregulin, both with hydrogen bond donor functionalities at their hinge, displayed markedly decreased in potency by comparison with TGFalpha. We propose that EGFR ligands can be subclassified into TGFalpha-like and EGF-like and that this is of functional significance, identifying a potential mechanism whereby EGFR can discriminate between its ligands.

Modulation of the receptor binding affinity of amphiregulin by modification of its carboxyl terminal tail

Amphiregulin (AR), a heparin-binding, epidermal growth factor (EGF) receptor ligand has homology with EGF but exhibits a lower affinity for the EGF receptor than EGF. As the mature form of AR is truncated at the C terminus and lacks a conserved leucine residue known to be essential for high affinity binding of EGF to the EGF receptor, wild-type AR (AR1-84), a C-terminally extended AR construct incorporating six residues from the predicted coding sequence of AR (AR1-90) and a similarly extended construct with a Met86 to Leu substitution (AR1-90(leu86)) were expressed as recombinant proteins in yeast, purified by heparin affinity and C18 reverse phase chromatography and their relative biological activities determined. The growth factors were tested in mitogenesis and EGF receptor autophosphorylation assays and their relative order of potencies was found to be leu86 > met86 > wt. The AR1-90(leu86) construct was found to be 50- to 100-fold more active than wild type AR1-84 consistent with previously reported studies of the role of the equivalent C-terminal leucine in EGF or TGF alpha. Significantly, the C-terminally extended form of AR, AR1-90, which utilized six residues from the predicted coding sequence, was 10-times more active than wild type AR1-84. This difference in activity of the C-terminally extended form of AR may be of biological significance since differential proteolytic processing of the AR precursor in vivo could result in production of multiple forms of the growth factor with differing affinities for the EGF receptor and hence differing biological potencies.