Regulation of signal transduction and signal diversity by receptor oligomerization

Expression of Discoidin Domain Receptor 2 (DDR2) in the developing heart

Interactions between cells and the surrounding extracellular matrix are important for a number of developmental events. In the heart, cardiac fibroblasts produce the majority of extracellular matrix proteins, particularly collagen types I and III. Cells originating from the proepicardial organ migrate over the surface of the heart, invade the underlying myocardium and ultimately give rise to smooth muscle cells, fibroblasts, and coronary endothelium. Although integrin expression in the developing heart has been well characterized, the expression of Discoidin Domain Receptor 2 (DDR2) remains to be defined. Using confocal microscopy, the expression of DDR2 was examined at several points during cardiac development. Initially, DDR2 expression was detected on the epicardial surface of the heart and on endothelial and mesenchymal cells within the cardiac cushions. As development progressed, DDR2 expression increased at localized regions in the apex and atrioventricular sulcus, although this expression decreased from epicardial to endocardial surface. Eventually, DDR2 expression spanned the myocardial free wall and was detected within the septum. Not until postnatal development was DDR2 expression detected uniformly throughout the myocardium and this distribution was maintained in the adult heart. In summary, the data presented demonstrate that the distribution of DDR2-positive cells changes within the heart during development.

The transmembrane domains of ErbB receptors do not dimerize strongly in micelles

The epidermal growth factor receptors (erbB) constitute an important class of single pass transmembrane receptors involved in the transduction of signals important for cell proliferation and differentiation. Receptor association is a key event in the signal transduction process, but the molecular basis of this interaction is not fully understood. Previous biochemical and genetic studies have suggested that the single transmembrane helices of these receptor proteins might play a role in stabilizing the receptor complexes. To determine if the erbB transmembrane domains could provide a driving force to stabilize the receptor dimers, we carried out a thermodynamic study of these domains expressed as C-terminal fusion proteins with staphylococcal nuclease. Similar fusion constructs have been used successfully to investigate the oligomerization and association thermodynamics of a number of transmembrane sequences, including that of glycophorin A. Using SDS-PAGE analysis and sedimentation equilibrium analytical ultracentrifugation, we do not find strong, specific homo or hetero-interactions between the transmembrane domains of the erbB receptors in micellar solutions. Our results indicate that any preferential interactions between these domains in micellar solutions are extremely modest, of the order of 1 kcal mol(-1) or less. We applied a thermodynamic formalism to assess the effect of weakly interacting TM segments on the behavior of a covalently attached soluble domain. In the case of the ligand-bound EGFR ectodomain, we find that restriction of the ectodomain to the micellar phase by a hydrophobic TM, even in the absence of strong specific interactions, is largely sufficient to account for the previously reported increase in dimerization affinity.

Ligand-receptor and receptor-receptor interactions act in concert to activate signaling in the Drosophila toll pathway

In Drosophila, the signaling pathway mediated by the Toll receptor is critical for the establishment of embryonic dorso-ventral pattern and for innate immune responses to bacterial and fungal pathogens. Toll is activated by high affinity binding of the cytokine Spätzle, a dimeric ligand of the cystine knot family. In vertebrates, a related family of Toll-like receptors play a critical role in innate immune responses. Despite the importance of this family of receptors, little is known about the biochemical events that lead to receptor activation and signaling. Here, we show that Spätzle binds to the N-terminal region of Toll and, using biophysical methods, that the binding is complex. The two binding events that cause formation of the cross-linked complex are non-equivalent: the first Toll ectodomain binds Spätzle with an affinity 3-fold higher than the second molecule suggesting that pathway activation involves negative cooperativity. We further show that the Toll ectodomains are able to form low affinity dimers in solution and that juxtamembrane sequences of Toll are critical for the activation or derepression of the pathway. These results, taken together, suggest a mechanism of signal transduction that requires both ligand-receptor and receptor-receptor interactions.

Synthesis and initial characterization of FGFR3 transmembrane domain: consequences of sequence modifications

Receptor Tyrosine Kinases (RTKs) conduct biochemical signals via lateral dimerization in the plasma membrane, and defects in their dimerization lead to unregulated signaling and disease. RTK transmembrane (TM) domains are proposed to play an important role in the process, underscored by the finding that single amino acids mutations in the TM domains can induce pathological phenotypes. Therefore, many important questions pertaining to the mode of signal transduction and the mechanism of pathology induction could be answered by studying the chemical-physical basis behind RTK TM domain dimerization and the interactions of RTK TM domains with lipids in model bilayer systems. As a first step towards this goal, here we report the synthesis of the TM domain of fibroblast growth factor receptor 3 (FGFR3), an RTK that is crucial for skeletal development. We have used solid phase peptide synthesis to produce two peptides: one corresponding to the membrane embedded segment and the naturally occurring flanking residues at the N- and C-termini (TMwt), and a second one in which the flanking residues have been substituted with diLysines at the termini (TMKK). We have demonstrated that the hydrophobic FGFR3 TM domain can be synthesized for biophysical studies with high yield. The protocol presented in the paper can be applied to the synthesis of other RTK TM domains. As expected, the Lys flanks decrease the hydrophobicity of the TM domain, such that TMKK elutes much earlier than TMwt during reverse phase HPLC purification. The Lysines have no effect on peptide solubility in SDS and on peptide secondary structure, but they abolish peptide dimerization on SDS gels. These results suggest that caution should be exercised when modifying RTK TM domains to render them more manageable for biophysical studies.

Integrating basic science and clinical research in bladder cancer: update from the first bladder Specialized Program of Research Excellence (SPORE)

PURPOSE OF REVIEW

To review the progress of the genitourinary SPORE (Specialized Program of Research Excellence) in bladder cancer.

RECENT FINDINGS

The optimal management of bladder cancer depends on the accurate assessment of the biological potential of the disease. Methotrexate, vincristine, adriamycin and cisplatin (M-VAC) chemotherapy has been the standard of therapy for over a decade. However, there has been no improvement in patient survival. Encouraging preclinical data have resulted in the rapid translation of epidermal growth factor receptor antagonists into the clinic. However, phase I and II single-agent clinical trials in head and neck, lung, and colon cancer failed to match the hope generated by laboratory investigations since only a minority of patients seemed to benefit from this approach. Nonetheless, recent data revealed that non-small-cell lung cancer tumors that responded to single-agent Iressa possessed activating epidermal growth factor receptor mutations. These findings have generated refound interest for epidermal growth factor receptor-dependent tumors that are identified by molecular and pharmacodynamic approaches prior to or early in the course of therapy.

SUMMARY

Targeted therapy against epidermal growth factor receptor has become one of the primary focuses of the genitourinary SPORE in bladder cancer. The SPORE grant scheme is designed to encourage rapid development of new and innovative cancer research in areas of high priority, in this case bladder cancer. The SPORE has facilitated the advancement of novel epidermal growth factor receptor-targeted therapy, such as the monoclonal antibody IMC-225 and the tyrosine kinase inhibitor ZD1839 (Iressa), from the laboratory to clinical trials. The integration of these new biological agents in combination with chemotherapy, in order to abrogate the progression of advanced bladder cancer, is the prime directive of our current phase II Iressa/docetaxel trial.

Sex differences in expression of transforming growth factor-α and epidermal growth factor receptor mRNA in waved-1 and C57Bl6 mice

A reduction of transforming growth factor-alpha (TGFalpha) expression in the spontaneous Waved-1 (Wa-1) mutant mouse causes specific behavioral and anatomical changes, including reduced fear learning and stress response and enlarged lateral ventricles. These alterations are observed predominantly in male Wa-1 mice after puberty. We hypothesized that regional differences in the expression of TGFalpha and its receptor, epidermal growth factor receptor (EGFR), may regulate the sexual dimorphism of the brain structures and functions during postnatal development. In general, fear learning-associated structures, including hippocampus and amygdala, showed maximum expression before puberty, regardless of genotype. In contrast, an overall temporal delay in the rise of both transcript levels, which peaked around or after puberty onset, was observed for the major stress regulatory hypothalamo-pituitary-adrenal axis. This pattern of expression was reversed for amygdala EGFR and hypothalamus TGFalpha and EGFR transcripts in males. When regional TGFalpha expression was compared between control and Wa-1 mice, far more complex patterns than expected were observed that revealed sex- and structure-dependent differences. In fact, the amygdala, hypothalamus, and pituitary TGFalpha expression pattern in Wa-1 exhibited a clear sex dependency across various age groups. Surprisingly, there was no compensatory up-regulation of the EGFR transcript in Wa-1 mice. The observed expression patterns of the TGFalpha signaling system during normal development and in the Wa-1 mutant mouse suggest complex sex- and age-dependent transcription regulatory mechanisms.

The geminivirus nuclear shuttle protein is a virulence factor that suppresses transmembrane receptor kinase activity

Despite the large number of leucine-rich-repeat (LRR) receptor-like-kinases (RLKs) in plants and their conceptual relevance in signaling events, functional information is restricted to a few family members. Here we describe the characterization of new LRR-RLK family members as virulence targets of the geminivirus nuclear shuttle protein (NSP). NSP interacts specifically with three LRR-RLKs, NIK1, NIK2, and NIK3, through an 80-amino acid region that encompasses the kinase active site and A-loop. We demonstrate that these NSP-interacting kinases (NIKs) are membrane-localized proteins with biochemical properties of signaling receptors. They behave as authentic kinase proteins that undergo autophosphorylation and can also phosphorylate exogenous substrates. Autophosphorylation occurs via an intermolecular event and oligomerization precedes the activation of the kinase. Binding of NSP to NIK inhibits its kinase activity in vitro, suggesting that NIK is involved in antiviral defense response. In support of this, infectivity assays showed a positive correlation between infection rate and loss of NIK1 and NIK3 function. Our data are consistent with a model in which NSP acts as a virulence factor to suppress NIK-mediated antiviral responses.

Ultrafine carbon black particles stimulate proliferation of human airway epithelium via EGF receptor-mediated signaling pathway

Exposure to ambient ultrafine particles induces airway inflammatory reactions and tissue remodeling. In this experiment, to determine whether ultrafine carbon black (ufCB) affects proliferation of airway epithelium and, if so, what the mechanism of action is, we studied human primary bronchial epithelial cell cultures. Incubation of cells in the serum-free medium with ufCB increased incorporations of [3H]thymidine and [3H]leucine into cells in a time- and dose-dependent manner. This effect was attenuated by Cu- and Zn-containing superoxide dismutase (Cu/Zn SOD) and apocynin, an inhibitor of NADPH oxidase, and completely inhibited by pretreatment with the epidermal growth factor receptor (EGF-R) tyrosine kinase inhibitors AG-1478 and BIBX-1382, and the mitogen-activated protein kinase kinase inhibitor PD-98059. Transfection of a dominant-negative mutant of H-Ras likewise abolished the effect ufCB. Stimulation with ufCB also induced processing of membrane-anchored proheparin-binding (HB)-EGF, release of soluble HB-EGF into the medium, association of phosphorylated EGF-R and Shc with glutathione- S-transferase-Grb2 fusion protein, and phosphorylation of extracellular signal-regulated kinase (ERK). Pretreatment with AG-1478, [Glu52] Diphtheria toxin, a specific inhibitor of HB-EGF, neutralizing HB-EGF antibody, Cu/Zn SOD, and apocynin each inhibited ufCB-induced ERK activation. These results suggest that ufCB causes oxidative stress-mediated proliferation of airway epithelium, involving processing of HB-EGF and the concomitant activation of EGF-R and ERK cascade.

Insulin inhibits platelet-derived growth factor-induced cell proliferation

Cellular behavior can be considered to be the result of a very complex spatial and temporal integration of intracellular and extracellular signals. These signals arise from serum-soluble factors as well as from cell-substrate or cell-cell interactions. The current approach in mitogenesis studies is generally to analyze the effect of a single growth factor on serum-starved cells. In this context, a metabolic hormone such as insulin is found to be a mitogenic agent in many cellular types. In the present study, we have considered the effect of insulin stimulation in platelet-derived growth factor (PDGF)-activated NIH-3T3 and C2C12 cells. Our results show that insulin is able to inhibit strongly both NIH-3T3 and C2C12 cell growth induced by PDGF, one of the most powerful mitotic agents for these cell types. This inhibitory effect of insulin is due primarily to a premature down-regulation of the PDGF receptor. Thus, when NIH-3T3 or C2C12 cells are stimulated with both PDGF and insulin, we observe a decrease in PDGF receptor phosphorylation with respect to cells treated with PDGF alone. In particular, we find that costimulation with insulin leads to a reduced production of H2O2 with respect to cell stimulation with PDGF alone. The relative low concentration of H2O2 in PDGF/insulin-costimulated cell leads to a limited down-regulation of protein tyrosine phosphatases, and, consequently, to a reduced PDGF receptor phosphorylation efficiency. The latter is very likely to be responsible for the insulin-dependent inhibition of PDGF-receptor mitogenic signaling.

Expression of fatty acid synthase, ErbB2 and Ki-67 in head and neck squamous cell carcinoma. A clinicopathological study

Fatty acid synthase (FAS) is a multifunctional enzyme responsible for the synthesis of saturated fatty acids using acetyl-CoA and malonyl-CoA as substrates. Overexpression of FAS has been reported in several human malignancies and suggested as a potential prognostic factor. ErbB2 (Her-2/neu), a transmembrane tyrosine kinase member of the ErbB receptor family, is known to be overexpressed in a variety of tumors and was recently shown to regulate FAS production in breast epithelial cell lines. Herein we analyzed by immunohistochemistry the expression of FAS, ErbB2, and the proliferation marker Ki-67 in 62 head and neck squamous cell carcinoma (HNSCC) samples. Approximately 78% of the cases were positive for FAS or ErbB2 at the cell membrane and 70% of the tumors that showed a high expression of FAS were also strongly positive for ErbB2 (Fisher's exact test, p = 0.01). The immunolabeling for both FAS and ErbB2 was stronger in histologically well-differentiated lesions. Additionally, Ki-67 expression was significantly associated with a poor prognosis (log-rank test, p = 0.03). Taken together, the results presented here suggest that ErbB2 regulates FAS expression in HNSCC and point out Ki-67 as a useful prognostic marker for these tumors.

Active and inactive conformations of the epidermal growth factor receptor

The members of the EGFR (epidermal growth factor receptor) family of RTKs (receptor tyrosine kinases), also known as the ErbB or HER family, have been implicated in many human cancers. Structural studies of the EGFR extracellular region (sEGFR) have led to the proposal of a novel mechanism for ligand-induced receptor dimerization. In this model EGF binding induces a dramatic conformational change in EGFR, exposing a dimerization site that is normally occluded in the inactivated conformation, and thus promoting the formation of an entirely receptor-mediated dimer. It is well established that antibodies against the extracellular region of EGFR that prevent ligand binding and/or receptor signalling can inhibit tumour growth in vivo. At least five such anti-EGFR antibodies are currently in clinical trials and one, C225/cetuximab (Erbitux™), was recently approved in the U.S. and Europe for use in advanced colorectal cancers. Recent structural studies of ErbB2 in complex with anti-ErbB2 antibodies (trastuzumab/Herceptin™ and pertuzumab/Omnitarg™) have provided significant insights into how these drugs function. There have been no such studies for similar EGFR-targeted drugs to date. The implications of this model for the possible mechanisms of antibody-mediated inhibition of EGFR are discussed.

Dexamethasone delays ulcer healing by inhibition of angiogenesis in rat stomachs

Using the non-ulcerogenic doses of dexamethasone, we explored the action of glucocorticoids on ulcer healing and its relationship with angiogenic factors in the gastric mucosa. We applied dexamethasone (0.1 or 0.2 mg/kg/day) intragastrically in rats with acetic acid-induced gastric ulcer. The mucosal prostaglandin E(2) level and protein expressions of basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) at the ulcer margin were determined. Ulcer induction significantly increased protein expressions of bFGF, VEGF, and prostaglandin E(2) level at the ulcer margin together with angiogenesis at the ulcer margin and base. The non-ulcerogenic doses of dexamethasone inhibited angiogenesis at the ulcer margin and ulcer base and delayed ulcer healing. These were associated with a significant decrease of prostaglandin E(2) level and VEGF expression, but not the bFGF expression. Supplementation with prostaglandin E(2) attenuated the inhibitory action of dexamethasone on VEGF expression and reversed the adverse effects of dexamethasone on angiogenesis and ulcer healing, without influencing bFGF expression. We concluded that dexamethasone given at non-ulcerogenic doses could decrease angiogenesis and delay acetic acid-induced ulcer healing; these actions were at least, in part, due to depletion of prostaglandin E(2) level followed by down-regulation of VEGF at the ulcer margin of the stomach.

Preparation and characterization of Alexa Fluor 594-labeled epidermal growth factor for fluorescence resonance energy transfer studies: application to the epidermal growth factor receptor

We have prepared and characterized a new fluorescent derivative of murine epidermal growth factor (EGF), Alexa Fluor 594-labeled EGF (A-EGF), for fluorescence studies of EGF-EGF receptor interactions. We describe the synthesis of this derivative and its physical and biological characterization. The significant overlap between the excitation and the emission spectra of A-EGF makes this probe well suited to fluorescence resonance energy homo-transfer. Using time-resolved fluorescence to examine the oligomeric state of the EGF receptor, we have observed resonance energy homo-transfer of A-EGF bound to EGF receptors in cells, but not of A-EGF bound to EGF receptors in membrane vesicles. Our results, interpreted in the context of recent crystallographic studies of the ligand-binding domains of EGF receptors, suggest that observed fluorescence resonance energy transfer does not result from transfer within receptor dimers, but rather results from transfer within higher-order oligomers. Furthermore, our results support a structural model for oligomerization of EGF receptors in which dimers are positioned head-to-head with respect to the ligand-binding site, consistent with the head-to-head interactions observed between adjacent receptor dimers by X-ray crystallography.

A phenylalanine zipper mediates APS dimerization

The APS, SH2-B and LNK proteins are adapters that activate and modulate receptor tyrosine kinase and JAK/STAT signaling. We now show that a conserved N-terminal domain mediates APS homodimerization. We determined the crystal structure of the dimerization domain at a resolution of 1.7 A using bromide ion MAD phasing. Each molecule contributes two helices to a compact four-helix bundle having a bisecting-U topology. Its most conspicuous feature is a stack of interdigitated phenylalanine side chains at the domain core. These residues create a new motif we refer to as a 'phenylalanine zipper,' which is critical to dimerization. A newly developed bridging yeast tri-hybrid assay showed that APS dimerizes JAK2, insulin receptor and IGF1 receptor kinases using its SH2 and dimerization domains. Dimerization via the phenylalanine zipper domain provides a mechanism for activating and modulating tyrosine kinase activity even in the absence of extracellular ligands.

Adenovirus-mediated expression of a soluble fibroblast growth factor receptor inhibits in vitro growth of prostate DU145 cells

BACKGROUND

Fibroblast growth factor (FGF) family plays a key role in prostate cancer. The soluble FGF receptor (sFGFR) has been studied with regards to inhibiting cancer growth and was shown to have a dominant negative effect on cellular signaling and function. Using replication deficient adenovirus-mediated gene transfer, we tested if sFGFR expression may have a suppressive effect on in vitro growth of prostate cancer cells.

METHODS

Western analysis was used to verify expression of sFGFR1 and to examine the effect of sFGFR1 on MAP kinase phosphorylation. The effect on proliferation and invasiveness of DU145 cells was examined using the WST-1 and Matrigel Invasion assay, respectively.

RESULTS

Activation of MAP kinase (pERK1 and 2) by exogenous FGF1, 2, and 7 was suppressed to baseline levels by sFGFR, which was not seen with EGF. Proliferation and invasion of DU145 cells were significantly suppressed by sFGFR.

CONCLUSIONS

A replication deficient adenoviral vector system reproducibly expresses sFGFR in prostate cells. Suppression of in vitro growth in DU145 cells by sFGFR provides the basis of a novel therapeutic approach.

Expression Profiles of Tyrosine Kinases in Cultured Follicular Papilla Cells Versus Dermal Fibroblasts

Tyrosine kinases play crucial roles in cell differentiation and proliferation. Using degenerative primed PCR followed by differential display, we analyzed the tyrosine kinase expression profiles of cultured rat follicular papilla (FP) cells versus dermal fibroblasts. We showed that c-met, cdc2, and tec were preferentially expressed in cultured FP cells, whereas alpha-platelet-derived growth factor receptor (alpha-PDGFR) was preferentially expressed in cultured fibroblasts. The cell type specificity of these tyrosine kinases was confirmed by semi-quantitative RT-PCR using both rat and human cultured cells. Consistent with these results, hepatocyte growth factor preferentially stimulated the growth of rat FP cells, whereas PDGF-AA preferentially stimulated rat fibroblasts. High concentrations of some these kinases are also found in the follicular matrix keratinocytes as revealed by in situ hybridization. The expression of specific tyrosine kinases in FP and matrix cells may play roles in regulating hair growth and cycling.

Heparin regulates survival and differentiation of mesencephalic progenitors mediated via FGF2 in vitro

Heparin plays an important role in the survival and differentiation of mesencephalic progenitors mediated by FGF-2 in vitro. If the heparin concentration is gradually increased, cell survival mediated by FGF-2 can be greatly enhanced, to a maximum concentration of 20 ng/ml FGF-2 from 5 microg/ml heparin. However, differentiation of FGF-2 responsive mesencephalic progenitors is inhibited by heparin. When cortical, mesencephalic and hippocampal astrocytes were primed with FGF-2 and heparin, the latter two astrocytes promoted the differentiation of TH-positive neurons from mesencephalic progenitors. RT-PCR analysis showed that FGFR1, FGFR2 and FGFR3 were expressed in the cortical astrocytes, but only FGFR1 and FGFR3 were expressed in the mesencephalic and hippocampal astrocytes.

Epidermal growth factor receptor expression correlates with histologic grade in resected esophageal adenocarcinoma

Activation of the epidermal growth factor receptor (EGFR) has a role in oncogenesis and may correlate with prognosis. The aim of this study was to examine EGFR expression in esophageal adenocarcinoma and correlate EGFR status with pathologic and clinical prognostic features. An exploratory retrospective review of 38 patients with surgically resected esophageal adenocarcinoma was performed. All patients underwent an esophagogastrectomy with regional lymphadenectomy; 24 patients underwent primary resection and 14 patients had surgery after preoperative chemoradiation therapy. Immunohistochemical analysis was performed on paraffin-embedded tissue samples using an EGFR monoclonal antibody. Low- and moderate-grade tumors were positive for EGFR expression in 2 of 15 patients; poorly differentiated tumors were positive for EGFR expression in 13 of 23 patients (p=0.02). The median survival was 35 months (confidence interval [CI]: 29-40) for EGFR negative patients (n=23) and 16 months (CI: 10-22) for EGFR positive patients (n=13) (p=0.10). Disease recurred in 3 of 21 EGFR negative patients and 6 of 13 EGFR positive patients (p=0.06). Poorly differentiated adenocarcinomas of the esophagus demonstrated higher EGFR expression compared to low-grade tumors based upon immunohistochemical analysis. A trend toward improved disease-free and overall survival was seen in EGFR negative patients.

Combining trastuzumab (Herceptin) with hormonal therapy in breast cancer: what can be expected and why?

Hormonal therapy and the humanised anti-HER2 monoclonal antibody trastuzumab (Herceptin) represent one of the oldest and one of the newest treatment modalities for breast cancer, respectively. Recent data have suggested that HER2 overexpression is associated with resistance to hormonal therapy and there is considerable preclinical evidence to support the existence of interaction or 'cross talk' between HER2 and estrogen-receptor (ER) signalling pathways in breast cancer. Preclinical data also demonstrate that adding trastuzumab to hormonal therapy results in greater antitumour activity than either agent alone. The existence of an inverse relationship between ER expression and HER2 overexpression has also been well established clinically. Thus, a range of clinical trials are now ongoing to determine whether the addition of trastuzumab to hormonal therapy will provide breast cancer patients with benefits in clinical practice. This review describes the rationale for these trials and discusses the potential of therapeutic regimens combining trastuzumab with hormonal therapy.

A Toll-like receptor in horseshoe crabs

Non-self-recognition of invading microbes relies on the pattern-recognition of pathogen-associated molecular patterns (PAMPs) derived from microbial cell-wall components. Insects and mammals conserve a signaling pathway of the innate immune system through cell-surface receptors called Tolls and Toll-like receptors (TLRs). Bacterial lipopolysaccharides (LPSs) are an important trigger of the horseshoe crab's innate immunity to infectious microorganisms. Horseshoe crabs' granular hemocytes respond specifically to LPS stimulation, inducing the secretion of various defense molecules from the granular hemocytes. Here, we show a cDNA which we named tToll, coding for a TLR identified from hemocytes of the horseshoe crab Tachypleus tridentatus. tToll is most closely related to Drosophila Toll in both domain architecture and overall length. Human TLRs have been suggested to contain numerous PAMP-binding insertions located in the leucine-rich repeats (LRRs) of their ectodomains. However, the LRRs of tToll contained no obvious PAMP-binding insertions. Furthermore, tToll was non-specifically expressed in horseshoe crab tissues. These observations suggest that tToll does not function as an LPS receptor on granular hemocytes.

Distinct domains in the SHP-2 phosphatase differentially regulate epidermal growth factor receptor/NF-kappaB activation through Gab1 in glioblastoma cells

The transcription factor nuclear factor kappaB (NF-kappaB) plays an important role in inflammation and cancer, is activated by a variety of stimuli including tumor necrosis factor alpha, interleukin-1, UV irradiation, and viruses, as well as receptor tyrosine kinases, such as epidermal growth factor receptor (EGFR). Although previous studies suggest that EGFR can induce NF-kappaB, the mechanism of this activation remains unknown. In this study, we identify the components of the EGFR-induced signalosome in human glioblastoma cells required to regulate NF-kappaB activation. Immunoprecipitation analyses with ErbB-modulated cells indicate that association between SHP-2 and Grb2-associated binder 1 (Gab1) is the critical step in the formation of the signalosome linking EGFR to NF-kappaB activation. We also show that EGFR-induced NF-kappaB activation is mediated by the PI3-kinase/Akt activation loop. Overexpression of SHP-2, Gab1, and myristoylated Akt significantly upregulated NF-kappaB transcriptional activity and DNA binding activity in glioblastoma cells. Interestingly, overexpression of either one of the two SH2 domain mutants of SHP-2, R32E or R138E, slightly reduced NF-kappaB activity relative to that of wild-type SHP-2, indicating that the SH2 domains of SHP-2 are required for EGFR-induced NF-kappaB activation. On the other hand, ectopic overexpression of either a Gab1 mutant incapable of binding to SHP-2 (Y627F) or a phosphatase-inactive SHP-2 mutant (C459S) caused a significant increase in NF-kappaB activity. Moreover, SHP-2 C459S-expressing cells displayed higher Gab1 phosphotyrosine content, suggesting that SHP-2 regulates Gab1 phosphorylation through its phosphatase domain, which confers a negative regulatory effect on NF-kappaB activity. These results indicate that SHP-2/Gab1 association is critical for linking EGFR to NF-kappaB transcriptional activity via the PI3-kinase/Akt signaling axis in glioblastoma cells and that SHP-2 acts as a dual regulator of NF-kappaB activation.

Mechanism of action of anti-HER2 monoclonal antibodies: scientific update on trastuzumab and 2C4

The HER family of transmembrane tyrosine kinase receptors is composed of four members, BER1 to HER4. HER2 is a ligand-orphan receptor expressed in many human tumors and overexpressed in 25-30% of breast cancers. HER2 amplifies the signal provided by other receptors of the HER family by forming heterodimers. The essential role of HER2 in the HER signaling network led to the development of anti-HER2 monoclonal antibodies (MAbs) for cancer therapy. In particular, the humanized MAb trastuzumab (Herceptin) has antitumor activity against HER2-overexpressing human breast tumor cells and is widely used for the treatment of women with HER2 overexpressing breast cancers. Trastuzumab induces HER2 receptor downmodulation and, as a result, inhibits critical signalling pathways (i.e. ras-Raf-MAPK and PI3K/Akt) and blocks cell cycle progression by inducing the formation of p27/Cdk2 complexes. Trastuzumab also inhibits HER2 cleavage, preceding antibody-induced receptor downmodulation, and this effect might contribute to its antitumor activity in some cancers. In vivo, trastuzumab inhibits angiogenesis and induces antibody-dependent cellular cytotoxicity. A limitation of trastuzumab is that its activity is largely restricted to breast cancers with the highest level of HER2 overexpression or HER2 gene amplification. However, there is a large population of breast cancers and of many other tumors that have low or moderate HER2 expression. In such tumors, HER2 functions as a preferred coreceptor to form heterodimers with HER1 (EGFR), HER3 or HER4. For this reason, a humanized monoclonal antibody, called 2C4, that targets the role of HER2 as a coreceptor is under active development. 2C4 binds to a different epitope of HER2 ectodomain than trastuzumab and sterically hinders HER2 recruitment in heterodimers with other HER receptors. This results in the inhibition of signalling by HER2-based heterodimers both in cells with low and high HER2 expression. In vitro and in vivo antitumor activity has been reported in a range of breast and prostate tumor models. Therefore, 2C4 may have potential against a wide variety of solid tumors. Phase I trials are underway.

A review of small-molecule epidermal growth factor receptor-specific tyrosine kinase inhibitors in development for non-small cell lung cancer

Overexpression of the epidermal growth factor receptor is commonly seen in a variety of epithelial tumors including lung cancer, and it is particularly common in the non-small cell histologic variant. Despite intense research efforts, therapeutic advances to date have failed to result in a significant survival benefit for patients with advanced disease. The lack of overall survival benefit and high toxicity associated with cytotoxic chemotherapy indicates the need for novel therapies that have a favorable effect on survival with minimal toxicity. Greater understanding of molecular biology and the intricate cellular pathways has resulted in the development of a new field of targeted therapeutics that will arrest dysregulated cell growth in malignant cells. Tyrosine kinases represent an important category of signaling proteins that catalyze phosphorylation of tyrosine residues leading to reactions ultimately resulting in cell growth, differentiation, proliferation, and death. Inhibition of tyrosine kinase activity represents a logical targeted approach in malignancies with overexpression of tyrosine kinase. Several small-molecule epidermal growth factor receptor tyrosine kinase inhibitors have been developed and are currently undergoing clinical trials. This article will review several of these agents as well as their role in the treatment of non-small cell lung cancer.

Dear old dad

The origin and frequency of spontaneous mutations that occur with age in humans have been a topic of intense discussion. The mechanisms by which spontaneous mutations arise depend on the parental germ line in which a mutation occurs. In general, paternal mutations are more likely than maternal mutations to be base substitutions. This is likely due to the larger number of germ cell divisions in spermatogenesis than in oogenesis. Maternal mutations are more often chromosomal abnormalities. Advanced parental age seems to influence some mutations, although it is not a factor in the creation of others. In this review, we focus on patterns of paternal bias and age dependence of mutations in different genetic disorders, and the various mechanisms by which these mutations arise. We also discuss recent data on age and the frequency of these mutations in the human male germ line and the impact of these data on this field of research.

The Role of Signal Transducer and Activator of Transcription Factors in Leukemogenesis

Human leukemias are frequently associated with the aberrant expression of activated fusion tyrosine kinases or activated protein tyrosine kinases carrying insertional or point mutations. The activated kinase enzymes typically phosphorylate one or more signal transducer and activator of transcription (STAT) factors, which translocate to the cell nucleus and regulate the expression of genes associated with survival and proliferation. The phosphorylation and activation of STAT family members has been described in a wide range of human leukemias. Furthermore, animal models of leukemia have demonstrated the pivotal contribution of STAT activation to leukemic pathogenesis. This review discusses evidence for the functional importance of STAT activation in the biology of leukemia and current opportunities for modulating STAT proteins in the therapy of this group of diseases.

Mitogenic signaling pathways in airway smooth muscle

Increased airway smooth muscle mass has been demonstrated in patients with asthma, bronchopulmonary dysplasia and most recently, cystic fibrosis. These observations emphasize the need for further knowledge of the events involved in airway smooth muscle mitogenesis and hypertrophy. Workers in the field have developed cell culture systems involving tracheal and bronchial myocytes from different species. An emergent body of literature indicates that mutual signal transduction pathways control airway smooth muscle cell cycle entry across species lines. This article reviews what is known about mitogen-activated signal transduction in airway myocytes. The extracellular signal regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI 3-kinase) pathways appear to be key positive regulators of airway smooth muscle mitogenesis; recent studies have also demonstrated specific roles for reactive oxygen and the JAK/STAT pathway. It is also possible that growth factor stimulation of airway smooth muscle concurrently elicits signaling through negative regulatory intermediates such as p38 mitogen-activated protein (MAP) kinase and protein kinase C (PKC) delta, conceivably as a defense against extreme growth.

Long-distance conformational changes in a protein engineered by modulated sequence duplication

There are few, if any, known instances in which a biological signal is transmitted via a large conformational change through the body of a protein. We describe here a mutant of T4 lysozyme that was engineered to permit structural change at a distance. The design uses a tandem sequence repeat that makes it possible to transmit large-scale structural changes from one end of an alpha-helix to the other over a distance of 17-25 A. The method should be of general applicability and may make it possible to introduce a mutation at one site in a protein that will induce large-scale changes in the structure at a spatially remote site.

An integrated model of epidermal growth factor receptor trafficking and signal transduction

Endocytic trafficking of many types of receptors can have profound effects on subsequent signaling events. Quantitative models of these processes, however, have usually considered trafficking and signaling independently. Here, we present an integrated model of both the trafficking and signaling pathway of the epidermal growth factor receptor (EGFR) using a probability weighted-dynamic Monte Carlo simulation. Our model consists of hundreds of distinct endocytic compartments and approximately 13,000 reactions/events that occur over a broad spatio-temporal range. By using a realistic multicompartment model, we can investigate the distribution of the receptors among cellular compartments as well as their potential signal transduction characteristics. Our new model also allows the incorporation of physiochemical aspects of ligand-receptor interactions, such as pH-dependent binding in different endosomal compartments. To determine the utility of this approach, we simulated the differential activation of the EGFR by two of its ligands, epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha). Our simulations predict that when EGFR is activated with TGF-alpha, receptor activation is biased toward the cell surface whereas EGF produces a signaling bias toward the endosomal compartment. Experiments confirm these predictions from our model and simulations. Our model accurately predicts the kinetics and extent of receptor downregulation induced by either EGF or TGF-alpha. Our results suggest that receptor trafficking controls the compartmental bias of signal transduction, rather than simply modulating signal magnitude. Our model provides a new approach to evaluating the complex effect of receptor trafficking on signal transduction. Importantly, the stochastic and compartmental nature of the simulation allows these models to be directly tested by high-throughput approaches, such as quantitative image analysis.

A novel mechanism for Wnt activation of canonical signaling through the LRP6 receptor

LDL receptor-related protein 6 (LRP6) is a Wnt coreceptor in the canonical signaling pathway, which plays essential roles in embryonic development. We demonstrate here that wild-type LRP6 forms an inactive dimer through interactions mediated by epidermal growth factor repeat regions within the extracellular domain. A truncated LRP6 comprising its transmembrane and cytoplasmic domains is expressed as a constitutively active monomer whose signaling ability is inhibited by forced dimerization. Conversely, Wnts are shown to activate canonical signaling through LRP6 by inducing an intracellular conformational switch which relieves allosteric inhibition imposed on the intracellular domains. Thus, Wnt canonical signaling through LRP6 establishes a novel mechanism for receptor activation which is opposite to the general paradigm of ligand-induced receptor oligomerization.

Oligomerization-induced modulation of TPR-MET tyrosine kinase activity

Phosphorylation, although necessary, may not be sufficient to fully activate many receptor tyrosine kinases (RTKs). Oligomerization-induced conformational changes may be necessary to modulate the kinetic properties of RTKs and render them fully functional. To investigate this regulatory mechanism, recombinant TPR-MET, a functionally active oncoprotein derivative of the RTK c-MET, has been expressed and purified for quantitative enzymatic analysis. This naturally occurring oncoprotein contains the cytoplasmic domain of c-MET fused to a coiled coil motif from the nuclear pore complex (TPR). cytoMET, the monomeric analog of TPR-MET, has also been expressed and purified for comparative enzymatic analysis. ATP and peptide substrates have been kinetically characterized for both TPR-MET and cytoMET. Significantly, phosphorylated TPR-MET has smaller Km values for ATP (Km,ATP) and peptide substrates (Km,peptide) and a larger kcat relative to phosphorylated cytoMET. This provides the first direct evidence that receptor oligomerization and not simply activation loop phosphorylation modulates RTK enzymatic activity. The ATP dissociation constants (Kd,ATP) for the two enzymes also displayed significant differences. In contrast, the KI values for the ATP competitive inhibitor staurosporin are similar for the two phosphorylated enzymes. These results suggest that much of the oligomerization-induced kinetic changes occur with respect to peptide substrate binding or catalytic efficiency. The possibility that oligomerization-induced conformational changes occur within the cytoplasmic domain of receptor tyrosine kinases has significant implications for structure-based design of RTK inhibitors and the development of a detailed mechanistic model of RTK activation.

Mitogenic signaling cascades in glial tumors

Gliomas are primary central nervous system tumors that arise from astrocytes, oligodendrocytes, or their precursors. Gliomas can be classified into several groups according to histological features. A number of genetic alterations have been identified in human gliomas; these generally affect either signal transduction pathways activated by receptor tyrosine kinases or cell cycle growth arrest pathways. These observed genetic alterations are now being used to complement histopathological diagnosis. The aim of the present review is to give a broad overview of the receptor tyrosine kinase signaling machinery involved in gliomagenesis, with an emphasis on the cooperative interaction between receptor tyrosine kinase signaling and the cell cycle-regulatory machinery. Understanding molecular features of primary glial tumors will eventually allow for target-selective intervention in distinct glioma subsets and a more rational approach to adjuvant therapies for these refractory diseases.

Structure and intermolecular interactions of the luminal dimerization domain of human IRE1alpha

Accumulation of unfolded proteins in the lumen of the endoplasmic reticulum activates a signal transduction cascade that culminates in the transcriptional induction of genes encoding adaptive functions. One proximal sensor for this unfolded protein response is the protein kinase/endoribonuclease IRE1alpha. IRE1alpha is a type-I transmembrane glycoprotein for which the N-terminal luminal domain (NLD) senses the accumulation of unfolded proteins. Previously we demonstrated that the NLD forms a stable ligand-independent dimer linked by disulfide bridges. In this report we have identified the cysteine residues responsible for intermolecular disulfide bonding. However, this covalent interaction was not required for dimerization and/or signaling, suggesting that a cryptic dimer interface exists in the NLD that is independent of covalent disulfide interactions. Limited proteolysis of the NLD revealed characteristic fragments, all retaining the same N-terminal sequences as full-length NLD. Biochemical and functional studies using NLD truncation mutants indicated that the dimerization domain of the NLD is confined to the conserved motifs at the N-terminal regions where putative hydrophobic interactions exist. In addition, the peptide binding domain of the endoplasmic reticulum protein chaperone BiP interacted with the N-terminal region within the NLD. Our findings suggest that the NLD has at least two distinct types of interactions mediating dimerization and function in signaling, i.e. covalent interactions involving disulfide bond formation and hydrophobic interactions, with the hydrophobic interaction being the driving force for dimerization.

The involvement of heparan sulfate (HS) in FGF1/HS/FGFR1 signaling complex

Fibroblast growth factor (FGF) signaling begins with the formation of a ternary complex of FGF, FGF receptor (FGFR), and heparan sulfate (HS). Multiple models have been proposed for the ternary complex. However, major discrepancies exist among those models, and none of these models have evaluated the functional importance of the interacting regions on the HS chains. To resolve the discrepancies, we measured the size and molar ratio of HS in the complex and showed that both FGF1 and FGFR1 simultaneously interact with HS; therefore, a model of 2:2:2 FGF1.HS.FGFR1 was shown to fit the data. Using genetic and biochemical methods, we generated HSs that were defective in FGF1 and/or FGFR1 binding but could form the signaling ternary complex. Both genetically and chemically modified HSs were subsequently assessed in a BaF3 cell mitogenic activity assay. The ability of HS to support the ternary complex formation was found to be required for FGF1-stimulated cell proliferation. Our data also proved that specific critical groups and sites on HS support complex formation. Furthermore, the molar ratio of HS, FGF1, and FGFR1 in the ternary complex was found to be independent of the size of HS, which indicates that the selected model can take place on the cell surface proteoglycans. Finally, a mechanism for the FGF.FGFR signaling complex formation on cell membrane was proposed, where FGF and FGFR have their own binding sites on HS and a distinct ternary complex formation site is directly responsible for mitogenic activity.

Tandem gramicidin channels cross-linked by streptavidin

The interaction of biotin-binding proteins with biotinylated gramicidin (gA5XB) was studied by monitoring single-channel activity and sensitized photoinactivation kinetics. It was discovered that the addition of streptavidin or avidin to the bathing solutions of a bilayer lipid membrane (BLM) with incorporated gA5XB induced the opening of a channel characterized by approximately doubled single-channel conductance and extremely long open-state duration. We believe that the deceleration of the photoinactivation kinetics observed here with streptavidin and previously (Rokitskaya, T.I., Y.N. Antonenko, E.A. Kotova, A. Anastasiadis, and F. Separovic. 2000. Biochemistry. 39:13053-13058) with avidin reflects the formation of long-lived channels of this type. Both opening and closing of the double-conductance channels occurred via a transient sub-state of the conductance coinciding with that of the usual single-channel transition. The appearance of the double-conductance channels after the addition of streptavidin was preceded by bursts of fast fluctuations of the current with the open state duration of the individual events of 60 ms. The streptavidin-induced double-conductance channels appeared to be inherent only to the gramicidin analogue with a biotin group linked to the COOH terminus through a long linker arm. Including biotinylated phosphatidylethanolamine into the BLM prevented the formation of the double-conductance channels even with the excess streptavidin. In view of the results obtained here, it is suggested that the double-conductance channel represents a tandem of two neighboring gA5XB channels with their COOH termini being cross-linked by the bound streptavidin at both sides of the BLM. The finding that streptavidin induces the formation of the tandem gramicidin channel comprising two channels functioning in concert is considered to be relevant to the physiologically important phenomenon of ligand-induced receptor oligomerization.

Solution-phase synthesis of combinatorial libraries designed to modulate protein-protein or protein-DNA interactions

A short personal perspective on the development of an approach to the solution-phase synthesis of combinatorial libraries for modulating cellular signaling by inhibiting, promoting, or mimicking protein-protein or protein-DNA interactions is provided.

Investigation of early events in Fc epsilon RI-mediated signaling using a detailed mathematical model

Aggregation of Fc epsilon RI on mast cells and basophils leads to autophosphorylation and increased activity of the cytosolic protein tyrosine kinase Syk. We investigated the roles of the Src kinase Lyn, the immunoreceptor tyrosine-based activation motifs (ITAMs) on the beta and gamma subunits of Fc epsilon RI, and Syk itself in the activation of Syk. Our approach was to build a detailed mathematical model of reactions involving Fc epsilon RI, Lyn, Syk, and a bivalent ligand that aggregates Fc(epsilon)RI. We applied the model to experiments in which covalently cross-linked IgE dimers stimulate rat basophilic leukemia cells. The model makes it possible to test the consistency of mechanistic assumptions with data that alone provide limited mechanistic insight. For example, the model helps sort out mechanisms that jointly control dephosphorylation of receptor subunits. In addition, interpreted in the context of the model, experimentally observed differences between the beta- and gamma-chains with respect to levels of phosphorylation and rates of dephosphorylation indicate that most cellular Syk, but only a small fraction of Lyn, is available to interact with receptors. We also show that although the beta ITAM acts to amplify signaling in experimental systems where its role has been investigated, there are conditions under which the beta ITAM will act as an inhibitor.

Clinical significance of the epidermal growth factor receptor and HER2 receptor in resectable gastric cancer

BACKGROUND

Epidermal growth factor receptor (EGFR or HER1) and its homolog c-erbB-2 (HER2) are membrane receptors. Both EGFR and HER2 genes are overexpressed in a variety of solid human cancers and are related to poor prognosis of the patients. The objective of this work was to evaluate the EGFR and HER2 contents in resectable gastric cancer, their possible relationship with clinicopathologic parameters of tumors, and their prognostic significance.

METHODS

This was a prospective analysis of 63 patients with resectable gastric carcinomas, with a mean follow-up period of 40.7 months. Membranous EGFR levels were examined by radioligand binding assays, and cytosolic HER2 levels were examined by means of an immunoenzymatic assay.

RESULTS

There was a wide variability of EGFR (1-1,239 fmol/mg of protein) and HER2 (7-20,863 NHU/mg of protein) levels in tumors. There was no significant correlation between these levels and patient or tumor characteristics. However, high levels of EGFR and HER2 were significantly associated with a shorter overall survival period (P =.03 and P =.02, respectively).

CONCLUSIONS

There is a wide variability in membranous EGFR levels and in cytosolic HER2 levels in gastric cancer, which seems to be related to the biological heterogeneity of these tumors. In addition, high tumor EGFR and HER2 levels were associated with an unfavorable outcome in patients with resectable gastric cancer.

Comparison of proto-oncogenic and mutant forms of the transmembrane region of the Neu receptor in TFE

A single mutation within the transmembrane region of the Neu receptor (Val664-->Glu) is known to enhance tyrosine kinase activity, by promoting receptor dimerization. In order to gain insight into potential structural changes that arise as a result of the mutation, peptides corresponding to the complete transmembrane domain of proto-oncogenic and mutant forms of Neu have been studied by 1H nuclear magnetic resonance in the solvent trifluoroethanol (TFE). The chemical shifts are similar for both forms of the peptide, with the exception of amide residues close to the mutation site. Both peptides adopt a helical conformation, with a distinct bend one turn downstream of the mutation site. This deformation gives rise to several nuclear Overhauser effects, the majority of which were detected in both peptides, that are atypical for a straight canonical alpha-helix. Our data in this solvent do not support a conformational change in the transmembrane domain of monomeric Neu as a result of the mutation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis indicates that proto-oncogenic Neu peptides have a higher propensity to oligomerize in the solvent TFE than the Glu664 oncogenic form.

CD4 dimers constitute the functional component required for T cell activation

The CD4 molecule plays a key role in the development and activation of helper T cells. Dimerization and oligomerization is often a necessary step in the function of several cell surface receptors. Herein, we provide direct biochemical evidence confirming the presence of CD4 as dimers in transfected cells from hemopoetic and fibroblastic origin as well as in primary T cells. Such dimers are also observed with murine CD4 confirming selective pressure during evolution to maintain such a structure. Using a series of point mutations, we have precisely mapped the dimerization site at residues K318 and Q344 within the fourth extracellular domain of CD4. These residues are highly conserved and their mutation results in interference with dimer formation. More importantly, we demonstrate that dimer formation is essential for the coligand and coreceptor functions of CD4 in T cell activation. These data strongly suggest that CD4 dimerization is necessary for helper T cell function.

Structure and stability of an acidic fibroblast growth factor from Notophthalmus viridescens

The three-dimensional solution structure of an acidic fibroblast growth factor (nFGF-1) from the newt (Notophthalmus viridescens) is determined using multidimensional NMR techniques. Complete assignment of all the atoms ((1)H, (15)N, and (13)C) has been achieved using a variety of triple resonance experiments. 50 structures were calculated using hybrid distance geometry-dynamical simulated annealing technique with a total of 1359 constraints. The atomic root mean square distribution for the backbone atoms in the structured region is 0.60 A. The secondary structural elements include 12 beta-strands arranged antiparallely into a beta-barrel structure. The protein (nFGF-1) exists in a monomeric state upon binding to the ligand, sucrose octa sulfate (SOS), in a stoichiometric ratio of 1:1. The SOS binding site consists of a dense cluster of positively charged residues located at the C-terminal end of the molecule. The conformational stabilities of nFGF-1 and its structural and functional homologue from the human source (hFGF-1) are drastically different. The differential stabilities of nFGF-1 and hFGF-1 are attributed to the differences in the number of hydrogen bonds and the presence of solvent inaccessible cavities in the two proteins.

Inhibition of aortic allograft vasculopathy by local delivery of platelet-derived growth factor receptor tyrosine-kinase blocker AG-1295

BACKGROUND

Signal transduction through the platelet-derived growth factor (PDGF)/PDGF-receptor (PDGFR) system has been linked to vascular smooth muscle cell migration and proliferation leading to allograft vasculopathy. This study describes the effect of the tyrphostin AG-1295, a specific PDGFR tyrosine-kinase inhibitor, on neointimal formation in this disease.

METHODS AND RESULTS

Rat aortic allografts transplanted from dark agouti (RT1 ) donors to Wistar-Furth (RT1 ) recipients were assessed in a new treatment model for local drug delivery from polymeric carrier matrices precoated with AG-1295. Matrices were wrapped around the graft immediately after transplantation. The recipients received no background immunosuppression. At day 80 posttransplantation, intimal thickness in AG-1295-treated grafts was reduced when compared to controls (11.8+/-9.1% intimal thickness vs. 23.7+/-6.4% intimal thickness; P=0.042). This finding corresponded to inhibition of intimal PDGFR-beta expression in AG-1295-treated grafts at day 20 posttransplantation (P =0.029 vs. allogeneic controls).

CONCLUSIONS

The tyrphostin AG-1295 reduces neointimal formation in aortic allograft vasculopathy by inhibition of PDGFR-beta-triggered tyrosine phosphorylation. Local drug release of specific tyrosine-kinase inhibitors from perivascularly co-implanted polymeric carrier matrices is effective in the prophylaxis of allograft vasculopathy under selected experimental conditions.

Comparative proteome bioinformatics: identification of a whole complement of putative protein tyrosine kinases in the model flowering plant Arabidopsis thaliana

Phosphorylation by protein tyrosine kinases is crucial to the control of growth and development of multicellular eukaryotes, including humans, and it also seems to play an important role in multicellular prokaryotes. A plant tyrosine-specific kinase has not been identified yet; hence, plants have been suggested to share with unicellular eukaryote yeast a tyrosine phosphorylation system where a limited number of stress proteins are tyrosyl-phosphorylated only by a few dual-specificity (serine/threonine and tyrosine) kinases. However, preliminary evidence obtained so far suggests that tyrosine phosphorylation in plants depends on the developmental conditions. Since sequencing of the genome of the model flowering plant Arabidopsis thaliana has been recently completed, we have performed a bioinformatic screening of the whole Arabidopsis proteome to identify a model complement of bona fide protein tyrosine kinases. In silico analyses suggest that < 4% of Arabidopsis kinases are tyrosine-specific kinases, whose gene expression has been assessed by a preliminary polymerase chain reaction screening of an Arabidopsis cDNA library. Finally, immunological evidence confirms that the number of Arabidopsis proteins specifically phosphorylated on tyrosine residues is much higher than in yeast.

Human cytomegalovirus infection inhibits epidermal growth factor (EGF) signalling by targeting EGF receptors

Infection with human cytomegalovirus (HCMV) is known to involve complex interactions between viral and cellular factors resulting in perturbation of a number of cellular functions. Specifically, HCMV infection targets control of the cell cycle, cellular transcription and immunoregulation, presumably to optimize the cellular environment for virus persistence and productive infection. Here, we show that HCMV infection also prevents external signalling to the cell by disrupting the function of epidermal growth factor receptor (EGFR). Infection with HCMV resulted in a decrease in cell-surface expression of EGFR. This decrease was correlated with a concomitant decrease in steady-state levels of EGFR protein. Consistent with this, HCMV inhibited EGF-mediated receptor autophosphorylation. Infection with a mutant HCMV deleted of all viral gene products known to be involved in down-regulation of MHC Class I receptors still resulted in this down-regulation, implying that EGFR down-regulation by HCMV is mediated by a novel virus function. We suggest that a primary goal of HCMV is to 'isolate' the infected cell from host-mediated signals so that the cell responds solely to an array of virus-specific signals which optimize the cell for virus production.

Crystal structure of a truncated epidermal growth factor receptor extracellular domain bound to transforming growth factor alpha

We report the crystal structure, at 2.5 A resolution, of a truncated human EGFR ectodomain bound to TGFalpha. TGFalpha interacts with both L1 and L2 domains of EGFR, making many main chain contacts with L1 and interacting with L2 via key conserved residues. The results indicate how EGFR family members can bind a family of highly variable ligands. In the 2:2 TGFalpha:sEGFR501 complex, each ligand interacts with only one receptor molecule. There are two types of dimers in the asymmetric unit: a head-to-head dimer involving contacts between the L1 and L2 domains and a back-to-back dimer dominated by interactions between the CR1 domains of each receptor. Based on sequence conservation, buried surface area, and mutagenesis experiments, the back-to-back dimer is favored to be biologically relevant.

Epithelial growth factor receptor interacting agents

The data reviewed here have further established the promise of anti-EGFR-targeted therapies. This statement is supported by the evidence of antitumor activity of the TK inhibitors ZD1839 and OSI-774 against several tumor types and by the ability of the monoclonal antibody IMC-C225 to reverse clinical chemotherapy resistance. These results are further supported by an emerging number of compounds, monoclonal antibodies, and TK inhibitors directed at the EGFR that are in clinical development (see Fig. 2, Table 1). Among the TK inhibitors, these compounds can be further categorized by their receptor specificity and reversibility of binding. In the case of anti-EGFR monoclonal antibodies, compounds in clinical development include chimeric, humanized, and bispecific antibodies. The fundamental observation is that these compounds have shown activity in several tumor types, including NSCL cancer, prostate carcinoma, colorectal carcinoma, ovarian carcinoma, renal cell carcinoma, and head and neck cancers. These findings observed with different agents and in different tumor types validate EGFR as a target for cancer therapy. The results of ongoing studies with these agents in diverse indications and tumor types may establish the role of these promising therapies to our current cancer treatments.

Identification of a region within the ErbB2/HER2 intracellular domain that is necessary for ligand-independent association

Ligand-independent ErbB2 activation occurs principally by two distinct mechanisms: overexpression and mutation. Overexpression of ErbB2 at the plasma membrane drives receptor self-association in a concentration-dependent manner, which in turn leads to constitutive receptor activation. Subsets of human breast cancers contain a molecular alteration that leads to erbB2 gene amplification and subsequent protein overexpression. Although not recognized to occur in human cancers, mutation can also lead to increased ErbB2 association. A well characterized mutant of the rodent ortholog neu involves substitution of glutamate for valine within the transmembrane domain. In each case, a number of explanations have been proposed to explain the resulting ErbB2 activation. These include stabilization of receptor oligomers, release of negative constraints, and altered receptor conformations. Here we define a short amino acid segment comprising amino acids 966-968 in the intracellular domain that seemingly disrupts receptor-receptor association that is driven either by overexpression or mutation in the transmembrane region. Because of the hydrophobic nature of these amino acids (VVI), we propose that alteration of this segment likely results in a global conformational change in an area that has been proposed previously to be a dimerization motif for ErbB homomeric association.

All signaling is local?

The mechanism of signal transmission following ligand stimulation of receptor tyrosine kinases in living cells is poorly understood. Recent studies have visualized the spatio-temporal pattern of EGF signaling, indicating that receptor density is an important factor in the mechanism of lateral propagation of local EGF signaling.

Targeting epidermal growth factor receptor in lung cancer

Among the most promising agents in clinical development to treat non-small-cell lung cancer (NSCLC) are the epidermal growth factor receptor (EGFR) targeting agents. A series of recent studies have demonstrated the activity of anti-EGFR targeted therapies for NSCLC. In advanced NSCLC that is refractory to chemotherapy, antitumor responses have been reported with EGFR tyrosine kinase inhibitors (ZD1839 and OSI-774). The role of ZD1839 and OSI-774 as possible additions to standard chemotherapy in the first-line setting has also been evaluated, and the studies conducted to date should respond to the question of whether these compounds could provide a survival benefit. Other areas of research involve looking at the role of EGFR tyrosine kinase inhibitors in the neoadjuvant treatment of stage III NSCLC and the planning of chemoprevention studies. These exciting results and plans are further complemented by an emerging number of compounds in clinical development, including both monoclonal antibodies (ie, IMC-C225) and other tyrosine kinase inhibitors, directed at the EGFR.

Landing of immune receptors and signal proteins on lipid rafts: a safe way to be spatio-temporally coordinated?

In the past decade one of the cell biology's breakthroughs was discovery of membrane microdomains (rafts, caveolae) and recognition of their important in cellular signaling and protein traffic. In the present minireview a short comprehensive overview is given about physico-chemical, structural and functional properties of rafts. In addition to the classical immunochemical techniques the latest physcial and biophysical technologies that can be used to study these microdomains are also described briefly. The funcational significance of rafts in signaling of multichain immune recognition receptors (MIRRs), the IL-2R and ErbB family factor receptors is also discussed herein together with the still open questions and future prospects of the raft hypothesis.