Regulation of survivin by ErbB2 signaling: therapeutic implications for ErbB2-overexpressing breast cancers

In breast cancer, overexpression of ErbB2 or aberrant regulation of survivin, a member of the inhibitor of apoptosis family, is associated with resistance to chemo/hormone therapy and predicts for a poor clinical outcome. A functional link between the two predictive factors has not been previously shown. Here, using genetic and pharmacologic approaches to block ErbB2 signaling, we show that ErbB2 regulates survivin protein expression in ErbB2-overexpressing breast cancer cells. Selective knockdown of ErbB2 using small interfering RNA markedly reduced survivin protein, resulting in apoptosis of ErbB2-overexpressing breast cancer cell lines such as BT474. Alternatively, inhibition of ErbB2 signaling using lapatinib (GW572016), a reversible small-molecule inhibitor of ErbB1/ErbB2 tyrosine kinases, at pharmacologically relevant concentrations, leads to marked inhibition of survivin protein with subsequent apoptosis. The effect of lapatinib on survivin seems to be predominantly posttranslational, mediated by ubiquitin-proteosome degradation as lactacystin, a proteosome inhibitor, reverses these effects. Furthermore, lapatinib down-regulated the expression of His-tagged survivin, which was under the transcriptional control of a heterologous promoter, providing additional evidence supporting a posttranslational mechanism of regulation. In contrast, trastuzumab and gefitinib failed to down-regulate survivin in ErbB2-overexpressing breast cancer cells. Importantly, the clinical relevance of these findings was illustrated in patients with ErbB2-overexpressing breast cancer whose clinical response to lapatinib was associated with marked inhibition of survivin in their tumors. These findings shed new light on the mechanism by which ErbB2 overexpression protects against apoptotic stimuli in breast cancer and identifies therapeutic interventions to improve clinical outcomes in these aggressive tumors.

Neuregulin-1 and myelination

The trigger for the process of myelin formation during neural development has long been a mystery. Recent studies suggest that this trigger is the growth factor neuregulin-1, expressed on the surface of axons, which signals through ErbB2 and ErbB3 receptors expressed on the surface of myelin-forming glia.

FAK signaling is critical for ErbB-2/ErbB-3 receptor cooperation for oncogenic transformation and invasion

The overexpression of members of the ErbB tyrosine kinase receptor family has been associated with cancer progression. We demonstrate that focal adhesion kinase (FAK) is essential for oncogenic transformation and cell invasion that is induced by ErbB-2 and -3 receptor signaling. ErbB-2/3 overexpression in FAK-deficient cells fails to promote cell transformation and rescue chemotaxis deficiency. Restoration of FAK rescues both oncogenic transformation and invasion that is induced by ErbB-2/3 in vitro and in vivo. In contrast, the inhibition of FAK in FAK-proficient invasive cancer cells prevented cell invasion and metastasis formation. The activation of ErbB-2/3 regulates FAK phosphorylation at Tyr-397, -861, and -925. ErbB-induced oncogenic transformation correlates with the ability of FAK to restore ErbB-2/3–induced mitogen-activated protein kinase (MAPK) activation; the inhibition of MAPK prevented oncogenic transformation. In contrast, the inhibition of Src but not MAPK prevented ErbB–FAK-induced chemotaxis. In migratory cells, activated ErbB-2/3 receptors colocalize with activated FAK at cell protrusions. This colocalization requires intact FAK. In summary, distinct FAK signaling has an essential function in ErbB-induced oncogenesis and invasiveness.

Ovarian cancer: Homeobox genes, autocrine/paracrine growth, and kinase signaling

Epithelial ovarian cancer, the fourth leading cause of cancer deaths in American women, is currently classified by surgical and histologic appearance. However, the predictive value of this classification is limited. The risk of epithelial ovarian cancer increases with the number of ovulatory events. It is now thought that different ovarian tumors are derived from a single ovarian surface epithelial precursor cell with the degree and pattern of differentiation determined by combinatorial expression of homeobox genes normally involved in differentiation of the female genital tract. This aberrant differentiation occurs in association with histology-specific genomic aberrations, genomic instability, and resultant chromosomal changes, and may be triggered by prolonged abnormal or excessive exposure of surface epithelial cells to autocrine/paracrine stimulation by sex steroids and other growth factors. As the disease progresses, activation of kinase pathways and continued abnormal autocrine/paracrine stimulation contribute to genomic instability but also identify potential targets for novel therapeutic intervention.

The diverse signaling network of EGFR, HER2, HER3 and HER4 tyrosine kinase receptors and the consequences for therapeutic approaches

The HER family of receptor tyrosine kinase couples binding of extracellular growth factor ligands to intracellular signal transduction pathways, contributing in this fashion to the ability of the cell to respond correctly to its environment. The HER family and its ligands are critically involved in the carcinogenesis of the mammary gland. Abnormal function of the members of HER family resulting in receptor hyper-activation (due to gene amplification, protein overexpression or abnormal transcriptional regulation) has been linked with breast cancer prognosis. It is also extensively studied as the predictive factor and target for therapy. There are clinical indications supporting the concept that none of the receptors: EGFR, HER2, HER3 and HER4 can be considered as the stand-alone receptor in breast cancer development and clinical course of the disease. There is a growing body of evidence that cooperation between them contributes to more aggressive tumor phenotype and influences the response to therapy. This underlines the importance of quantification of all HER family members and indicates the urgent need for implementation of methods that can efficiently and reliably examine four HER receptors as a whole panel in breast cancer patients.

The ErbB receptors and their ligands in cancer: an overview

This review article provides an overview on the most recent advances on the role of ErbB receptors and growth factors of the epidermal growth factor (EGF)-family of peptides in cancer pathogenesis and progression. The ErbB tyrosine kinases and the EGF-like peptides form a complex system. In fact, the interactions occurring between receptors and ligands of these families affect the type and the duration of the intracellular signals that derive from receptor activation. Interestingly, activation of ErbB receptors is also driven by different classes of membrane receptor, suggesting that ErbB kinases can amplify growth promoting signals carried by different pathways. The importance of ErbB receptors and EGF-like peptides in development of organs and tissues has been demonstrated by using different mouse models. In vitro and in vivo studies have also shown that ErbB receptors and their ligands can act as transforming genes. However, evidence suggests that cooperation of different receptors and ligands is necessary to induce a fully transformed phenotype. Indeed, co-expression of different ErbB receptors and EGF-like growth factors is a common phenomenon in human primary carcinomas. This observation suggests that the growth and the survival of carcinoma cells is sustained by a network of receptors/ligands of the ErbB family. In this respect, the contemporary expression of different ErbB tyrosine kinases and/or EGF-like growth factors in human carcinomas might also affect tumor response to target based agents directed against the ErbB receptor/ligand system.

Tissue interaction mediated by neuregulin-1 and ErbB receptors regulates epithelial morphogenesis of mouse embryonic submandibular gland

Dimerization and activation of ErbB receptors by their ligands play crucial roles in organogenesis. Epithelial morphogenesis of embryonic mouse submandibular gland (SMG) has been shown to depend on intraepithelial signaling mediated by the epidermal growth factor (EGF) family of molecules and the EGF receptor (ErbB1). Here, we report on the neuregulin (NRG) -1 protein and its receptors ErbB2 and ErbB3 in the developing SMG. The expression of these molecules was demonstrated by reverse transcriptase-polymerase chain reaction and Western blot analysis. Immunofluorescence microscopy showed that the two ErbB receptors as well as ErbB1 were expressed mainly in the epithelium, whereas NRG-1 was exclusively found in the mesenchyme. Epithelial morphogenesis was retarded by anti-NRG-1 neutralizing antibody and promoted by recombinant NRG-1 protein. We suggest that, in the developing SMG, both mesenchyme-derived NRG molecules and epithelium-derived EGF molecules regulate ErbB signaling in the epithelium to participate in tissue morphogenesis.

Concomitant overexpression of cyclooxygenase-2 in HER-2-positive on Smad4-reduced human gastric carcinomas is associated with a poor patient outcome

PURPOSE

The expression of cyclooxygenase-2 (COX-2) is known to be involved in gastric carcinogenesis and tumor progression, but little is known about the mechanisms responsible for the up-regulation of COX-2. We examined the involvement of two growth factor-signaling systems, HER-2 and transforming growth factor (TGF)-beta, in the induction of COX-2 in human gastric cancer tissue.

EXPERIMENTAL DESIGN

COX-2 expression was detected by immunohistochemistry in surgical specimens obtained from 166 patients with advanced gastric cancer; possible correlations between the expression of COX-2 and the expression of HER-2, TGF-beta1, and Smad4, an intracellular mediator that transmits the TGF-beta signal, were then analyzed.

RESULTS

COX-2 protein was overexpressed in 91 (54.8%) tumors; COX-2 overexpression was correlated with a differentiated histologic type, deep invasion, and positive lymph node metastasis. COX-2 was frequently overexpressed in HER-2-positive tumors (19 of 22, 86.4%) and in Smad4-reduced tumors (67 of 104, 64.4%) but irrelevant to the TGF-beta1 expression status. The expression levels of COX-2 and HER-2 and the reduction in Smad4 were all associated with a poor patient outcome. A multivariate analysis demonstrated a significantly poor outcome for the concomitant overexpression of COX-2 in patients with Smad4-reduced tumors.

CONCLUSIONS

These results support the possibility that signal transduction via HER-2 and the TGF-beta/Smad system may be implicated in COX-2 expression and that the reduction of Smad4 may be, in part, of causal significance in the TGF-beta-initiated overexpression of COX-2, which is associated with a poor prognosis for patients with gastric cancer.

ErbB-3 mediates phosphoinositide 3-kinase activity in gefitinib-sensitive non-small cell lung cancer cell lines

Therapies that target the EGF receptor (EGFR), such as gefitinib (IRESSA), are effective in a subset of patients with advanced non-small cell lung cancer (NSCLC). The differences in intracellular signaling networks between gefitinib-sensitive and -resistant NSCLCs remain poorly understood. In this study, we observe that gefitinib reduces phospho-Akt levels only in NSCLC cell lines in which it inhibits growth. To elucidate the mechanism underlying this observation, we compared immunoprecipitates of phosphoinositide 3-kinase (PI3K) between gefitinib-sensitive and -resistant NSCLC cell lines. We observe that PI3K associates with ErbB-3 exclusively in gefitinib-sensitive NSCLC cell lines. Gefitinib dissociates this complex, thereby linking EGFR inhibition to decreased Akt activity. In contrast, gefitinib-resistant cells do not use ErbB-3 to activate the PI3K/Akt pathway. In fact, abundant ErbB-3 expression is detected only in gefitinib-sensitive NSCLC cell lines. Two gefitinib-sensitive NSCLC cell lines with endogenous distinct activating EGFR mutations (L858R and Del747-749), frequently observed in NSCLC patients who respond to gefitinib, also use ErbB-3 to couple to PI3K. Down-regulation of ErbB-3 by means of short hairpin RNA leads to decreased phospho-Akt levels in the gefitinib-sensitive NSCLC cell lines, Calu-3 (WT EGFR) and H3255 (L858R EGFR), but has no effect on Akt activation in the gefitinib-resistant cell lines, A549 and H522. We conclude that ErbB-3 is used to couple EGFR to the PI3K/Akt pathway in gefitinib-sensitive NSCLC cell lines harboring WT and mutant EGFRs.

H-RAS V12-induced radioresistance in HCT116 colon carcinoma cells is heregulin dependent

The abilities of mutated active K-RAS and H-RAS proteins, in an isogenic human carcinoma cell system, to modulate the activity of signaling pathways following exposure to ionizing radiation is unknown. Loss of K-RAS D13 expression in HCT116 colorectal carcinoma cells blunted basal extracellular signal-regulated kinase 1/2 (ERK1/2), AKT, and c-Jun NH2-terminal kinase 1/2 activity. Deletion of the allele to express K-RAS D13 also enhanced expression of ERBB1, ERBB3, and heregulin but nearly abolished radiation-induced activation of all signaling pathways. Expression of H-RAS V12 in HCT116 cells lacking an activated RAS molecule (H-RAS V12 cells) restored basal ERK1/2 and AKT activity to that observed in parental cells but did not restore or alter basal c-jun NH2-terminal kinase 1/2 activity. In parental cells, radiation caused stronger ERK1/2 pathway activation compared with that of the phosphatidylinositol 3-kinase (PI3K)/AKT pathway, which correlated with constitutive translocation of Raf-1 into the plasma membrane of parental cells. Inhibition of mitogen-activated protein kinase/ERK1/2, but not PI3K, radiosensitized parental cells. In H-RAS V12 cells, radiation caused stronger PI3K/AKT pathway activation compared with that of the ERK1/2 pathway, which correlated with H-RAS V12-dependent translocation of PI3K into the plasma membrane. Inhibition of PI3K, but not mitogen-activated protein kinase/ERK1/2, radiosensitized H-RAS V12 cells. Radiation-induced activation of the PI3K/AKT pathway in H-RAS V12 cells 2 to 24 hours after exposure was dependent on heregulin-stimulated ERBB3 association with membrane-localized PI3K. Neutralization of heregulin function abolished radiation-induced AKT activation and reverted the radiosensitivity of H-RAS V12 cells to those levels found in cells lacking expression of any active RAS protein. These findings show that H-RAS V12 and K-RAS D13 differentially regulate radiation-induced signaling pathway function. In HCT116 cells expressing H-RAS V12, PI3K-dependent radioresistance is mediated by both H-RAS-dependent translocation of PI3K into the plasma membrane and heregulin-induced activation of membrane-localized PI3K via ERBB3.

HER2/HER3 heterodimers in prostate cancer: Whither HER1/EGFR?

In this issue of Cancer Cell, Mellinghoff et al. (2004) demonstrate that a small molecule inhibitor of the EGF receptor (EGFR) and the HER2/ErbB2/c-Neu kinase blocks signaling to the androgen receptor by a mechanism that involves HER2/HER3 heterodimerization. Surprisingly, the EGFR is peripheral to this signaling mechanism. These results have implications for the design of targeted therapy for hormone-refractory prostate cancer.

Neuregulin 1-erbB signaling and the molecular/cellular basis of schizophrenia

Schizophrenia is a devastating psychiatric disease that affects 0.5-1% of the world's adult population. The hypothesis that this disease is a developmental disorder of the nervous system with late onset of its characteristic symptoms has been gaining acceptance in past years. However, the anatomical, cellular and molecular bases of schizophrenia remain unclear. Numerous studies point to alterations in different aspects of brain development as possible causes of schizophrenia, including defects in neuronal migration, neurotransmitter receptor expression and myelination. Recently, the gene that encodes neuregulin-1 (NRG1) has been identified as a potential susceptibility gene for schizophrenia, and defects in the expression of erbB3, one of the NRG1 receptors, have been shown to occur in the prefrontal cortex of schizophrenic patients, suggesting that NRG1-erbB signaling is involved in the pathogenesis of schizophrenia. These findings open new approaches to defining the molecular and cellular basis of schizophrenia in more mechanistic terms.

ErbB receptors: directing key signaling networks throughout life

The epidermal growth factor (EGF)-related peptides bind the ErbB receptors, inducing the formation of different homo- and heterodimers. Receptor dimerization promotes activation of the intrinsic kinase, leading to phosphorylation of specific tyrosines located in the ErbB's cytoplasmic region. These phosphorylated residues serve as docking sites for a variety of signaling molecules whose recruitment stimulates intracellular signaling cascades, which ultimately control diverse genetic programs. Particular ligand-receptor complexes have essential roles in embryonic development as well as in the adult. Finally, ErbB receptors are being pursued as therapeutic targets because aberrant ErbB activity has been observed in many human cancers. In this review, we discuss these data in more detail, illustrating the importance of tightly regulated ErbB signaling throughout life.

Axonal neuregulin-1 regulates myelin sheath thickness

In the nervous system of vertebrates, myelination is essential for rapid and accurate impulse conduction. Myelin thickness depends on axon fiber size. We use mutant and transgenic mouse lines to show that axonal Neuregulin-1 (Nrg1) signals information about axon size to Schwann cells. Reduced Nrg1 expression causes hypomyelination and reduced nerve conduction velocity. Neuronal overexpression of Nrg1 induces hypermyelination and demonstrates that Nrg1 type III is the responsible isoform. We suggest a model by which myelin-forming Schwann cells integrate axonal Nrg1 signals as a biochemical measure of axon size.

Emerging treatments in oncology: focus on tyrosine kinase (erbB) receptor inhibitors

OBJECTIVE

To describe the role of tyrosine kinase (TK) and its subreceptors in the development of cancer and the role of TK inhibitors in cancer treatment.

DATA SOURCES

Published articles identified through MEDLINE using search terms such as tyrosine kinase, erbB1, erbB2, erbB3, erbB4, epidermal growth factor receptors (EGFR), and EGFR inhibitors. Additional sources were identified from bibliographies in the articles and from Web sites and reports from the National Cancer Institute, American Society of Clinical Oncology, and European Organization for Research and Treatment of Cancer.

DATA SYNTHESIS

Progress in identifying the biochemical and molecular causes of cancer has led to discovery of abnormalities that characterize cancer cells and represent targets for development of drug therapies. TK receptors represent one such target when these are present in elevated quantities and/or aberrant forms. Abnormalities in these cell surface receptors have been correlated with development and progression of cancer, poor response to chemotherapeutic agents, and low survival rates. Several subtypes of TK receptors have been identified, and mutations in these have been associated with neoplasms of the breast, lung, colon, ovaries, and other organs. Approved agents, such as trastuzumab (Herceptin-Genentech) work by blocking a subtype of TK receptors that has been associated with breast cancer growth. Gefitinib (Iressa-AstraZeneca) was recently approved for treatment of non-small-cell lung cancer in patients who have failed treatment with traditional chemotherapeutic agents. Other agents such as cetuximab, erlotinib, and canertinib are in advanced stages of clinical trials and may be available for general clinical use in the next few years.

CONCLUSION

Cancer continues to be a difficult disease to treat, but newly identified cellular targets have resulted in new medications, and these promising agents are giving hope for additional options for patients with various solid tumors.

Update on HER-kinase-directed therapy in prostate cancer

The human epidermal growth factor receptor (HER) family of receptor tyrosine kinases is part of a network of pathways that are involved in the development and progression of prostate cancer. HER-kinase receptors include epidermal growth factor receptor (EGFR), HER2, HER3, and HER4, which must combine as dimers to affect signaling. Different combinations of receptors produce different qualities and levels of pathway activation. Among HER-family receptors, HER2 activation is particularly important in breast cancer, as HER2 gene amplification is associated with a distinct clinical course and response to treatment with a HER2-directed therapy (trastuzumab). Although HER2 can be over-expressed in prostate cancer, there is no clinical data to support the use of trastuzumab for prostate cancer patients. Preclinical and clinical data show that the activation of the HER-kinase axis is important for the progression of prostate cancer to androgen-independent disease. Data points towards the importance of inhibiting multiple members of the HER-kinase family to achieve more complete blockade of this axis for cancers other than HER2-overexpressing breast cancer. Multiple pharmaceutical agents that block the HER-kinase axis are currently being tested for patients with prostate cancer. These include antibodies, tyrosine kinase inhibitors, and novel strategies which seek to decrease HER2 expression.

[HER-ErbB family of receptors and their ligands: mechanisms of activation, signals and deregulation in cancer]

The topic of this review is first to analyze in normal conditions the signal transduction pathways induced by members of the HER-ErbB receptor family and their ligands, and second, to decipher some deregulations occurring in various cancer types. As a result, new therapeutic opportunities will be mentioned.

Role of neuregulin-1 beta in the developing lung

Neuregulins play a critical role in the developing heart, nervous, and mammary systems. Neuregulin-1-induced cardiac, neuronal, and mammary differentiation is based on a cell-cell communication model, where the ligand neuregulin-1 is produced and secreted by one cell type, which does not express its receptors erbB3 and erbB4 and acts on neighboring cell types that do express these receptors. We proposed that neuregulin-1 affects fetal lung maturation through a similar mechanism. Immunostaining showed neuregulin-1 in fetal lung that increased in fibroblasts at the onset of surfactant synthesis. Neuregulin-1 beta was found to be secreted by the fetal lung fibroblast and stimulated type II cell surfactant synthesis. Both fetal lung fibroblast-conditioned media and neuregulin-1 stimulated erbB2 receptor phosphorylation in type II cells. The effects of neuregulin-1 and of fibroblast-conditioned media on both surfactant synthesis and type II cell erbB2 phosphorylation were specifically blocked by antibody to neuregulin-1. Thus, neuregulin-1 beta may control fetal lung maturation through mesenchymal-epithelial interactions in a paracrine mechanism similar to that described for the developing heart, brain, and mammary systems.

Regulation of neuregulin/ErbB signaling by contractile activity in skeletal muscle

Putative roles of neuregulin (NRG) and the ErbB receptors in skeletal muscle biology include myogenesis, ACh receptor expression, and glucose transport. To date, however, the physiological regulation of NRG/ErbB signaling has not been examined. We tested the hypothesis that contractile activity in vivo induces NRG/ErbB activation. Rat hindlimb muscle contraction was elicited with a single bout of electrical stimulation (RX) or treadmill running (EX). Western blot and immunofluorescence confirmed the expression of multiple NRG isoforms and the ErbB2, ErbB3, and ErbB4 receptors in adult skeletal muscle. Both RX and EX significantly increased phosphorylation of all NRG receptors. Furthermore, contraction induced a shift in the expression profile of NRG, consistent with proteolytic processing of a transmembrane isoform. Thus two distinct modes of exercise activated processing of NRG with concomitant stimulation of ErbB2, ErbB3, and ErbB4 signaling in vivo. To our knowledge, this is the first demonstration of physiological regulation of NRG/ErbB signaling in any organ and implicates this pathway in the metabolic and proliferative responses of skeletal muscle to exercise.

The ErbB receptors and their role in cancer progression

The involvement of the ErbB receptor tyrosine kinases in human cancer, as well as their essential role in a variety of physiological events during normal development, have motivated the interest in this receptor family. Approaches taken to block the activity of ErbB receptors in cancer cells have not only proven that they drive in vitro tumor cell proliferation, but have also become clinically relevant for targeting tumors with deregulated ErbB signaling. The mechanisms and downstream effectors through which the ErbB receptors influence processes linked to malignant development, including proliferation, cell survival, angiogenesis, migration, and invasion, are, however, only now becoming apparent. Our particular emphasis in this review will be on how ErbB receptors, in particular ErbB1 and ErbB2, contribute to processes linked to cancer progression. Importantly, in keeping with the emerging theme that ErbB receptors do not function in isolation, we will focus on receptor cooperativity, i.e., ErbB1 cooperates with other classes of receptors, and the ligand-less ErbB2 functions as a heterodimer with other ErbBs.

The deaf and the dumb: the biology of ErbB-2 and ErbB-3

ErbB-2 (also called HER2/neu) and ErbB-3 are closely related to the epidermal growth factor receptor (EGFR/ErbB-1), but unlike EGFR, ErbB-2 is a ligandless receptor, whereas ErbB-3 lacks tyrosine kinase activity. Hence, both ErbB-2 and ErbB-3 are active only in the context of ErbB heterodimers, and ErbB-2. ErbB-3 heterodimers, which are driven by neuregulin ligands, are the most prevalent and potent complexes. These stringently controlled heterodimers are repeatedly employed throughout embryonic development and dictate the establishment of several cell lineages through mesenchyme-epithelial inductive processes and the interactions of neurons with muscle, glia, and Schwann cells. Likewise, the potent combination of signaling pathways engaged by the heterodimers drives an aggressive phenotype of tumors of secretory epithelia, including breast and lung cancers. This review highlights recent structural insights into the mechanism of ligand-induced heterodimer formation, and concentrates on signaling pathways employed by ErbB-2 and ErbB-3 in normal and in malignant cells.

Activation of ErbB3-PI3-kinase pathway is correlated with malignant phenotypes of adenocarcinomas

Signet-ring cell carcinomas are malignant dedifferentiated carcinomas, which are frequently found in the stomach. We previously demonstrated that a 200 kDa protein is often constitutively phosphorylated on tyrosine and bound to phosphatidylinositol 3-kinase (PI3-kinase) in signet-ring cell carcinoma cells. In this study, we purified the 200 kDa protein from an extract of NUGC-4 cells, a cell line of signet-ring cell carcinoma, and identified it as ErbB3. ErbB3 was found to be phosphorylated selectively in dedifferentiated adenocarcinoma cell lines among various gastric cancer cell lines. Expression of a constitutively active chimeric receptor consisting of ErbB2 and ErbB3 in HCC2998 cells, a highly differentiated adenocarcinoma cell line, revealed that the signaling triggered by phosphorylation of ErbB3 was important for dedifferentiated phenotypes such as loss of cell-cell interaction and high expression of MUC1/DF3 antigen, a marker of the malignant tumors. Taken together, activation of ErbB3 pathway may contribute to the development of dedifferentiated carcinomas.

ErbB receptor tyrosine kinase inhibitors as therapeutic agents

The ErbB family of receptor tyrosine kinases comprise four members: EGFR, ErbB2, ErbB3 and ErbB4. All are essential for normal development and participate in the functioning of normal cells. ErbB receptors, particularly EGFR and ErbB2 are commonly deregulated in certain prevalent forms of human cancer. Recently a number of small molecule inhibitors of the tyrosine kinase activity of these receptors have been developed. Some of these agents, known as TKIs, are progressing through clinical trials in patients with aberrant ErbB receptor expression in their tumors. This article provides a brief overview on the structure and biology of ErbB receptors and their ligands before discussing in detail the development and current status of ErbB receptor TKIs. These agents are shown to inhibit multiple features of cancer cells including proliferation, survival, invasion and angiogenesis. It is clear from recent studies that not all cancer cells that overexpress ErbB receptors will be sensitive to TKIs. Potential explanations for resistance to these molecules are reviewed. Finally the prospect of using TKIs in combination with existing chemotherapeutic agents is discussed.

ErbB2 and ErbB3 do not quantitatively modulate ligand-induced ErbB4 tyrosine phosphorylation

The ErbB family of receptor tyrosine kinases consists of four members: the epidermal growth factor receptor (EGFR/ErbB1), ErbB2/HER2/Neu, ErbB3/HER3, and ErbB4/HER4. ErbB2 is an "orphan" for which there is no naturally occurring, soluble ligand. ErbB3 lacks tyrosine kinase activity. Thus, we hypothesized that ErbB2 enhances ligand-induced ErbB family receptor signalling through mass action. In contrast, we hypothesized that ErbB3 reduces ligand-induced ErbB family receptor signalling by forming receptor heterodimers that cannot undergo bidirectional cross-phosphorylation. We tested these hypotheses using three cell lines that express equal levels of ErbB4. One expresses ErbB4 alone, the second expresses ErbB2 and ErbB4, and the third expresses ErbB3 and ErbB4. We treated the cells with the ErbB4 ligands betacellulin (BTC) and neuregulin1beta (NRG1 beta) and assayed ErbB4 tyrosine phosphorylation. ErbB2 and ErbB3 do not affect the amount of ligand-induced ErbB4 tyrosine phosphorylation. We will discuss these findings within the context of a model for ErbB receptor signalling.

Hyaluronan: genetic insights into the complex biology of a simple polysaccharide

It is appropriate that this review should appear in a volume dedicated to Mert Bernfield. Much of my interest in the cell biology of the extracellular matrix, particularly during development, echoes Mert's pioneering studies. His kind but provocative questioning during meetings is especially missed. The glycosaminoglycan hyaluronan is ubiquitous, and is especially abundant during embryogenesis. Hydrated matrices rich in hyaluronan expand the extracellular space, facilitating cell migration. The viscoelastic properties of hyaluronan are also essential for proper function of cartilage and joints. Recent understanding of hyaluronan biology has benefited from the identification of genes encoding hyaluronan synthases and hyaluronidases, genetic analysis of the roles of hyaluronan during development, elucidation of the biochemical mechanisms of hyaluronan synthesis, and by studies of human genetics and tumors. This review focuses on recent studies utilizing hyaluronan-deficient, gene targeted mice with null alleles for the principal source of hyaluronan during mid-gestation, hyaluronan synthase-2 (has-2).