Intracellular retention of membrane-anchored v-sis protein abrogates autocrine signal transduction

G protein-coupled receptor signalling in the cardiac nuclear membrane: evidence and possible roles in physiological and pathophysiological function

G protein-coupled receptors (GPCRs) play key physiological roles in numerous tissues, including the heart, and their dysfunction influences a wide range of cardiovascular diseases. Recently, the notion of nuclear localization and action of GPCRs has become more widely accepted. Nuclear-localized receptors may regulate distinct signalling pathways, suggesting that the biological responses mediated by GPCRs are not solely initiated at the cell surface but may result from the integration of extracellular and intracellular signalling pathways. Many of the observed nuclear effects are not prevented by classical inhibitors that exclusively target cell surface receptors, presumably because of their structures, lipophilic properties, or affinity for nuclear receptors. In this topical review, we discuss specifically how angiotensin-II, endothelin, β-adrenergic and opioid receptors located on the nuclear envelope activate signalling pathways, which convert intracrine stimuli into acute responses such as generation of second messengers and direct genomic effects, and thereby participate in the development of cardiovascular disorders.

An autocrine mechanism for constitutive Wnt pathway activation in human cancer cells

Autocrine Wnt signaling in the mouse mammary tumor virus model was the first identified mechanism of canonical pathway activation in cancer. In search of this transformation mechanism in human cancer cells, we identified breast and ovarian tumor lines with upregulation of the uncomplexed transcriptionally active form of beta-catenin without mutations afflicting downstream components. Extracellular Wnt antagonists FRP1 and DKK1 caused a dramatic downregulation of beta-catenin levels in these tumor cells associated with alteration of biological properties and increased expression of epithelial differentiation markers. Colorectal carcinoma cells with knockout of the mutant beta-catenin allele retained upregulated beta-catenin levels, which also could be inhibited by these Wnt antagonists. Together, these findings establish the involvement of autocrine Wnt signaling in human cancer cells.

The nature of intracrine peptide hormone action

Current theory holds that peptide hormone action results from hormone binding to cell-surface receptors, with the generation of intracellular second messengers. However, a growing body of evidence suggests that intracellular peptide hormone, either internalized or synthesized in situ, can exert physiologically relevant effects. These effects are diverse and poorly understood. I propose that such intracrine action can serve to modulate cellular function over time and thereby play a role in biological memory of various sorts, in the maintenance of hormonal responsiveness, and in cellular differentiation.

Removal of the membrane-anchoring domain of epidermal growth factor leads to intracrine signaling and disruption of mammary epithelial cell organization

Autocrine EGF-receptor (EGFR) ligands are normally made as membrane-anchored precursors that are proteolytically processed to yield mature, soluble peptides. To explore the function of the membrane-anchoring domain of EGF, we expressed artificial EGF genes either with or without this structure in human mammary epithelial cells (HMEC). These cells require activation of the EGFR for cell proliferation. We found that HMEC expressing high levels of membrane- anchored EGF grew at a maximal rate that was not increased by exogenous EGF, but could be inhibited by anti-EGFR antibodies. In contrast, when cells expressed EGF lacking the membrane-anchoring domain (sEGF), their proliferation rate, growth at clonal densities, and receptor substrate phosphorylation were not affected by anti-EGFR antibodies. The sEGF was found to be colocalized with the EGFR within small cytoplasmic vesicles. It thus appears that removal of the membrane-anchoring domain converts autocrine to intracrine signaling. Significantly, sEGF inhibited the organization of HMEC on Matrigel, suggesting that spatial restriction of EGF access to its receptor is necessary for organization. Our results indicate that an important role of the membrane-anchoring domain of EGFR ligands is to restrict the cellular compartments in which the receptor is activated.

The road less travelled: c-kit and stem cell factor

Autocrine stimulation of growth factor receptors by autonomously produced ligands regulates different aspects of cellular transformation and progression. In several tumors, including gliomas, multiple autocrine systems are activated and may exert different functions in the malignant transformation process. The c-kit proto-oncogene is widely expressed in human gliomas, and it may be activated by its co-expressed ligand, stem cell factor (SCF). Studies in glioma cell lines as well as different tumor types suggest the possibility of intracellular interactions of c-kit with SCF. Although c-kit and SCF may not play a primary and causal role in the initiation and progression of glial tumors they may still be contributing factors in glioma biology. It can be hypothesized that the parallel activation of several autocrine systems including some of which have found less attention in gliomas, such as c-kit/SCF, could compromise the efficacy of therapies targeting different autocrine loops. A better understanding of the multiplicity and mechanisms of autocrine stimulation has implications for the development of new therapies interfering with autocrine tumor cell growth.

Exit of the pre-TCR from the ER/cis-Golgi is necessary for signaling differentiation, proliferation, and allelic exclusion in immature thymocytes

A major issue is whether surface expression of the pre-TCR is necessary for signaling the development of immature thymocytes. To address this question, we generated transgenic mice expressing a TCRbeta chain that had a strong endoplasmic reticulum (ER) retrieval signal (TCRbetaER) and that was expressed intracellularly but failed to reach the cell surface. In TCRbetaER transgenic mice, there was a failure of allelic exclusion. Also, the transgene failed to rescue the developmental defects observed in TCRbeta-null mice. In contrast, TCRbeta transgenes with a mutant ER retrieval sequence or lacking this sequence signaled efficient allelic exclusion and suppressed the TCRbeta-/- defect. These data show that exit of the pre-TCR from the ER/cis-Golgi is required for progression through the double-negative thymocyte checkpoint.

Transmembrane domain sequence requirements for activation of the p185c-neu receptor tyrosine kinase

The receptor tyrosine kinase p185c-neu can be constitutively activated by the transmembrane domain mutation Val664-->Glu, found in the oncogenic mutant p185neu. This mutation is predicted to allow intermolecular hydrogen bonding and receptor dimerization. Understanding the activation of p185c-neu has assumed greater relevance with the recent observation that achondroplasia, the most common genetic form of human dwarfism, is caused by a similar transmembrane domain mutation that activates fibroblast growth factor receptor (FGFR) 3. We have isolated novel transforming derivatives of p185c-neu using a large pool of degenerate oligonucleotides encoding variants of the transmembrane domain. Several of the transforming isolates identified were unusual in that they lacked a Glu at residue 664, and others were unique in that they contained multiple Glu residues within the transmembrane domain. The Glu residues in the transforming isolates often exhibited a spacing of seven residues or occurred in positions likely to represent the helical interface. However, the distinction between the sequences of the transforming clones and the nontransforming clones did not suggest clear rules for predicting which specific sequences would result in receptor activation and transformation. To investigate these requirements further, entirely novel transmembrane sequences were constructed based on tandem repeats of simple heptad sequences. Activation was achieved by transmembrane sequences such as [VVVEVVA]n or [VVVEVVV]n, whereas activation was not achieved by a transmembrane domain consisting only of Val residues. In the context of these transmembrane domains, Glu or Gln were equally activating, while Lys, Ser, and Asp were not. Using transmembrane domains with two Glu residues, the spacing between these was systematically varied from two to eight residues, with only the heptad spacing resulting in receptor activation. These results are discussed in the context of activating mutations in the transmembrane domain of FGFR3 that are responsible for the human developmental syndromes achondroplasia and acanthosis nigricans with Crouzon Syndrome.

Constitutive receptor activation by Crouzon syndrome mutations in fibroblast growth factor receptor (FGFR)2 and FGFR2/Neu chimeras

Crouzon syndrome is an autosomal dominant condition primarily characterized by craniosynostosis. This syndrome has been associated with a variety of amino acid point mutations in the extracellular domain of fibroblast growth factor receptor 2 (FGFR2). FGFR2/Neu chimeras were generated by substituting the extracellular domain of Neu with that of FGFR2 containing the following Crouzon mutations: Tyr-340-->His; Cys-342-->Tyr; Cys-342-->Arg; Cys-342-->Ser; Ser-354-->Cys: and delta17 (deletion of amino acids 345-361). Each of the mutant chimeric FGFR2/Neu constructs stimulated focus formation in NIH 3T3 cells, indicating that Crouzon mutations can stimulate signal transduction through a heterologous receptor tyrosine kinase. In vitro kinase assay results indicate that FGFR2 receptors containing Crouzon mutations have increased tyrosine kinase activity and, when analyzed under nonreducing conditions, exhibited disulfide-bonded dimers. Thus the human developmental abnormality Crouzon syndrome arises from constitutive activation of FGFR2 due to aberrant intermolecular disulfide-bonding. These results together with our earlier observation that achondroplasia results from constitutive activation of the related receptor FGFR3, leads to the prediction that other malformation syndromes attributed to FGFRs, such as Pfeiffer syndrome and Thanatophoric dysplasia, also arise from constitutive receptor activation.

Constitutive activation of fibroblast growth factor receptor 3 by the transmembrane domain point mutation found in achondroplasia

Achondroplasia, the most common genetic form of dwarfism, is an autosomal dominant disorder whose underlying mechanism is a defect in the maturation of the cartilage growth plate of long bones. Achondroplasia has recently been shown to result from a Gly to Arg substitution in the transmembrane domain of the fibroblast growth factor receptor 3 (FGFR3), although the molecular consequences of this mutation have not been investigated. By substituting the transmembrane domain of the Neu receptor tyrosine kinase with the transmembrane domains of wild-type and mutant FGFR3, the Arg380 mutation in FGFR3 is shown to activate both the kinase and transforming activities of this chimeric receptor. Residues with side chains capable of participating in hydrogen bond formation, including Glu, Asp, and to a lesser extent, Gln, His and Lys, were able to substitute for the activating Arg380 mutation. The Arg380 point mutation also causes ligand-independent stimulation of the tyrosine kinase activity of FGFR3 itself, and greatly increased constitutive levels of phosphotyrosine on the receptor. These results suggest that the molecular basis of achondroplasia is unregulated signal transduction through FGFR3, which may result in inappropriate cartilage growth plate differentiation and thus abnormal long bone development. Achondroplasia may be one of the number of cogenital disorders where constitutive activation of a member of the FGFR family leads to development abnormalities.

The v-sis oncoprotein loses transforming activity when targeted to the early Golgi complex

The location of autocrine interactions between the v-sis protein and PDGF receptors remains uncertain and controversial. To examine whether receptor-ligand interactions can occur intracellularly, we have constructed fusion proteins that anchor v-sis to specific intracellular membranes. Fusion of a cis-Golgi retention signal from a coronavirus E1 glycoprotein to v-sis protein completely abolished its transforming ability when transfected into NIH3T3 cells. Fusion proteins incorporating mutations in this retention signal were not retained within the Golgi complex but instead were transported to the cell surface, resulting in efficient transformation. All chimeric proteins were shown to dimerize properly. Derivatives of some of these constructs were also constructed bearing the cytoplasmic tail from the glycoprotein of vesicular stomatitis virus (VSV-G). These constructs allowed examination of subcellular localization by double-label immunofluorescence, using antibodies that distinguish between the extracellular PDGF-related domain and the VSV-G cytoplasmic tail. Colocalization of sis-E1-G with Golgi markers confirmed its targeting to the early Golgi complex. The sis-E1 constructs, targeted to the early Golgi complex, exhibited no proteolytic processing whereas the mutant forms of sis-E1 exhibited normal proteolytic processing. Treatment with suramin, a polyanionic compound that disrupts ligand/receptor interactions at the cell surface, was able to revert the transformed phenotype induced by the mutant sis-E1 constructs described here. Our results demonstrate that autocrine interactions between the v-sis oncoprotein and PDGF receptors within the early Golgi complex do not result in functional signal transduction. Another v-sis fusion protein was constructed by attaching the transmembrane domain and COOH-terminus of TGN38, a protein that localizes to the trans-Golgi network (TGN). This construct was primarily retained intracellularly, although some of the fusion protein reached the surface. Deletion of the COOH-terminal region of the TGN38 retention signal abrogated the TGN-localization, as evidenced by very prominent cell surface localization, and resulted in increased transforming activity. The behavior of the sis-TGN38 derivatives is discussed within the context of the properties of TGN38 itself, which is known to recycle from the cell surface to the TGN.

Signal-transduction therapy. A novel approach to disease management

In the past decade it has become apparent that many diseases result from aberrations in signaling pathways. These include proliferative diseases such as cancers, atherosclerosis and psoriasis and inflammatory conditions such as sepsis, rheumatoid arthritis and tissue rejection. These findings refocused the research of the medical community to seek new modalities for disease management which essentially consist of designing drugs which intercept cell signaling. In this review, the emerging success in using tyrosine kinase blockers and other signal interceptors, such as protein kinase C blockers, Ras blockers, Ca2+ signaling inhibitors and estrogen antagonists which inhibit growth of cancer cells in vitro and in vivo, will be discussed. These signal interceptors, especially tyrosine-kinase blockers, are also able to block inflammatory responses and the proliferation of vascular smooth muscle cells and psoriatic keratinocytes. The utility of signal interceptors in analyzing signal-transduction pathways is also discussed.

The efficacy and distribution of suramin in the treatment of the 9L gliosarcoma

Suramin inhibits the stimulation of brain tumor deoxyribonucleic acid synthesis in vitro at concentrations of 200 to 400 mg/ml. This report evaluates suramin in the rodent 9L tumor model. Survival was analyzed by treating 10 tumor-bearing animals with suramin (7 mg/kg/d intraperitoneally) for 7 days, beginning 1 week after implantation, and compared with 20 untreated animals. Tissue distribution was analyzed with reverse-phase high-pressure liquid chromatography in homogenized organs of normal animals. Tumor concentration was measured over time in animals treated with a range of suramin doses, beginning 2 weeks after implantation. Suramin imparted no benefit as tumor-bearing control animals and treated animals survived 24.7 +/- 3.4 days and 24.5 +/- 1.5 days, respectively. In the animals receiving 7 mg/kg/d, renal concentrations of suramin were highest--339.8 +/- 30.9 mg/g as late as 25 days after treatment. Concentration in the brain peaked at only 3.3 +/- 1.3 mg/g after 10 days. Concentration in the tumor peaked at 74.4 +/- 16.5 mg/g the day of the last injection, significantly less than estimated by in vitro studies of efficacy. After injections of 35 mg/kg/d, tumor levels reached 230.9 +/- 139.2 mg/g with no evidence of inhibition of tumor progression. The response to a 7 mg/kg direct brain inoculation of suramin was assessed and compared with saline as a control. Animals treated with suramin died after 1 to 3 hours. Intracerebral hematoma volume at the injection site was 13.9 +/- 10.7 mm3 and 1.9 +/- 3.32 mm3 in the suramin-treated and control animals, respectively (P = 0.02), confirming the reported anticoagulant activity of suramin. Suramin is without efficacy in the 9L model because of poor systemic delivery. Alternative direct inoculation results in lethal local hemorrhage. Further consideration is necessary before the broad clinical application of this drug.

Dominant-negative mutants of platelet-derived growth factor revert the transformed phenotype of human astrocytoma cells

Malignant astrocytoma is the most common primary human brain tumor. Most astrocytomas express a combination of platelet-derived growth factor (PDGF) and PDGF receptor which could close an autocrine loop. It is not known whether these autocrine loops contribute to the transformed phenotype of astrocytoma cells or are incidental to that phenotype. Here we show that dominant-negative mutants of the PDGF ligand break the autocrine loop and revert the phenotype of BALB/c 3T3 cells transformed by the PDGF-A or PDGF-B (c-sis) gene. Then, we show that these mutants are selective in that they do not alter the phenotype of 3T3 cells transformed by an activated Ha-ras or v-src gene or by simian virus 40. Finally, we show that these mutants revert the transformed phenotype of two independent human astrocytoma cell lines. They have no effect on the growth of human medulloblastoma, bladder carcinoma, or colon carcinoma cell lines. These observations are consistent with the view that PDGF autocrine loops contribute to the transformed phenotype of at least some human astrocytomas.

Dominant-negative mutants of platelet-derived growth factor revert the transformed phenotype of human astrocytoma cells

Malignant astrocytoma is the most common primary human brain tumor. Most astrocytomas express a combination of platelet-derived growth factor (PDGF) and PDGF receptor which could close an autocrine loop. It is not known whether these autocrine loops contribute to the transformed phenotype of astrocytoma cells or are incidental to that phenotype. Here we show that dominant-negative mutants of the PDGF ligand break the autocrine loop and revert the phenotype of BALB/c 3T3 cells transformed by the PDGF-A or PDGF-B (c-sis) gene. Then, we show that these mutants are selective in that they do not alter the phenotype of 3T3 cells transformed by an activated Ha-ras or v-src gene or by simian virus 40. Finally, we show that these mutants revert the transformed phenotype of two independent human astrocytoma cell lines. They have no effect on the growth of human medulloblastoma, bladder carcinoma, or colon carcinoma cell lines. These observations are consistent with the view that PDGF autocrine loops contribute to the transformed phenotype of at least some human astrocytomas.