Proteolytic cleavage and phosphorylation of a tumor-associated ErbB4 isoform promote ligand-independent survival and cancer cell growth

Mitochondrial impairment induced by 3-nitropropionic acid is enhanced by endogenous metalloprotease activity inhibition in cultured rat striatal neurons

Metalloproteases from the metzincin family mediate molecule processing at the cell membrane termed ectodomain shedding (ES). This mechanism enables the generation of intracellular and extracellular fragments from cell membrane molecules that exert additional functions involved in cell processes including cell death, beyond those of full length molecules. Micotoxin 3-nitropropionic acid (3-NP) induces striatal neuronal degeneration in vivo and in vitro through mitochondrial complex II inhibition. In this study, we hypothesized that metalloproteases regulate mitochondrial activity in cultured rat striatal neurons undergoing degeneration. To test this idea, striatal neuronal cultures characterized by NeuN and GAD-67 expression were treated with 3-NP together with the metalloprotease inhibitor GM6001 and their mitochondrial activity was evaluated by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. Our results showed that metalloprotease inhibition potentiated mitochondrial activity impairment induced by 3-NP whereas the inhibitor alone had no effect. These results indicate that metalloproteases regulate and promote mitochondrial functionality in striatal neurons undergoing degeneration induced by 3-NP. Since NMDA receptor is involved in the excitotoxic neuronal death triggered by 3-NP and is known to undergo ES, we analyzed NMDAR subunit NR1 phenotypic distribution by immunofluorescence. 3-NP and GM6001 induced abnormal perinuclear NR1 accumulation that was not observed with 3-NP or GM6001 alone. This observation suggests that metalloproteases are involved in NR1 cellular reorganization induced by 3-NP, and that their inhibition results in abnormal NR1 distribution. Together results indicate that endogenous metalloproteases are activated during striatal neurodegeneration induced by 3-NP eliciting an adaptative or compensatory response that protects mitochondrial functionality.

Upregulation of epidermal growth factor receptor 4 in oral leukoplakia

In the present study, we investigate the expression profile of the epidermal growth factor receptor family, which comprises EGFR/ErbB1, HER2/ErbB2, HER3/ErbB3 and HER4/ErbB4 in oral leukoplakia (LP). The expression of four epidermal growth factor receptor (EGFR) family genes and their ligands were measured in LP tissues from 14 patients and compared with levels in 10 patients with oral lichen planus (OLP) and normal oral mucosa (NOM) from 14 healthy donors by real-time polymerase chain reaction (PCR) and immunohistochemistry. Synchronous mRNA coexpression of ErbB1, ErbB2, ErbB3 and ErbB4 was detected in LP lesions. Out of the receptors, only ErbB4 mRNA and protein was more highly expressed in LP compared with NOM tissues. These were strongly expressed by epithelial keratinocytes in LP lesions, as shown by immunohistochemistry. Regarding the ligands, the mRNA of Neuregulin2 and 4 were more highly expressed in OLP compared with NOM tissues. Therefore, enhanced ErbB4 on the keratinocytes and synchronous modulation of EGFR family genes may contribute to the pathogenesis and carcinogenesis of LP.

Relationship between promoter methylation status and protein expression of the wwox gene in hepatocellular carcinoma

AIM: To investigate the promoter methylation status and protein expression of the wwox gene in hepatocellular carcinoma (HCC).

METHODS: Promoter methylation status and protein expression of the wwox gene were analyzed in 60 cases of HCC tissues and matched tumor-adjacent liver tissues by methylation specific polymerase chain reaction (MSP) and immunohistochemistry (IHC), respectively.

RESULTS: The positive rate of wwox gene promoter methylation in tumor tissues was significantly higher than that in matched tumor-adjacent liver tissues (41.67% vs 8.33%, P = 0.000), while the positive rate of WWOX protein in tumor tissues was significantly lower than that in matched tumor-adjacent liver tissues (35.00% vs 70.00%, P = 0.001). Promoter methylation of the wwox gene was correlated with extrahepatic metastasis, tumor diameter, and tumor differentiation. WWOX protein expression was significantly correlated with clinical stage, tumor diameter, and tumor differentiation. Promoter methylation and protein expression of the wwox gene showed a significantly negative correlation (γ = -0.408, P = 0.001).

CONCLUSION: Promoter methylation is a crucial mechanism of inactivation of the wwox gene. Promoter methylation of the wwox gene might be involved in carcinogenesis, development, and progression of HCC.

Neuregulin-4 is a survival factor for colon epithelial cells both in culture and in vivo

Expression of the ErbB4 tyrosine kinase is elevated in colonic epithelial cells during inflammatory bowel disease, whereas ErbB4 overexpression in cultured colonocytes blocks TNF-induced apoptosis in a ligand-dependent manner. Together, these observations suggest that ErbB4 induction may be a protective response. However, the effects of ErbB4 signaling in the colonic epithelium in vivo are not known. Furthermore, previous work on ErbB4 used ligands shared with other receptors, raising the question of whether the observed responses are explicitly due to ErbB4. In this study, we used the ErbB4-specific ligand neuregulin-4 (NRG4) to activate ErbB4 and define its role in colonocyte biology. NRG4 treatment, either in cultured cells or in mice, blocked colonic epithelial apoptosis induced by TNF and IFN-γ. It was also protective in a murine experimental colitis model. NRG4 stimulated phosphorylation of ErbB4 but not other ErbB receptors, indicating that this is a specific response. Furthermore, in contrast to related ligands, NRG4 enhanced cell survival but not proliferation or migration, and stimulated phosphorylation of the anti-apoptotic mediator Akt but not ERK MAPK. Pharmacological inhibition of PI3K/Akt signaling reversed the anti-apoptotic effects of NRG4, confirming the role of this cascade in NRG4-induced cell survival. With regard to the potential clinical importance of this pathway, NRG4 expression was decreased in human inflammatory bowel disease samples and mouse models of colitis, suggesting that activation of ErbB4 is altered in disease. Thus, exogenous NRG4 may be beneficial for disorders in which epithelial apoptosis is part of the pathology.

Phosphoproteomics: A possible route to novel biomarkers of breast cancer

Proteomics is rapidly transforming the way that cancer and other pathologies are investigated. The ability to identify hundreds of proteins and to compare their abundance in different clinical samples presents a unique opportunity for direct identification of novel disease markers. Furthermore, recent advances allow us to analyse and compare PTMs. This gives an additional dimension for defining a new class of protein biomarker based not only on abundance and expression but also on the occurrence of covalent modifications specific to a disease state or therapy response. Such modifications are often a consequence of the activation/inactivation of a particular disease related pathway. In this review we evaluate the available information on breast cancer related protein-phosphorylation events, illustrating the rationale for investigating this PTM as a target for breast cancer research with eventual clinical relevance. We present a critical survey of the published experimental strategies to study protein phosphorylation on a system wide scale and highlight recent specific advances in breast cancer phosphoproteomics. Finally we discuss the feasibility of establishing novel biomarkers for breast cancer based on the detection of patterns of specific protein phosphorylation events.

Proteolytic processing of ErbB4 in breast cancer

ErbB4 is a receptor tyrosine kinase that can signal by a mechanism involving proteolytic release of intracellular and extracellular receptor fragments. Proteolysis-dependent signaling of ErbB4 has been proposed to be enhanced in breast cancer, mainly based on immunohistochemical localization of intracellular epitopes in the nuclei. To more directly address the processing of ErbB4 in vivo, an ELISA was developed to quantify cleaved ErbB4 ectodomain from serum samples. Analysis of 238 breast cancer patients demonstrated elevated quantities of ErbB4 ectodomain in the serum (≥40 ng/mL) in 21% of the patients, as compared to 0% of 30 healthy controls (P = 0.002). Significantly, the elevated serum ectodomain concentration did not correlate with the presence of nuclear ErbB4 immunoreactivity in matched breast cancer tissue samples. However, elevated serum ectodomain concentration was associated with the premenopausal status at diagnosis (P = 0.04), and estradiol enhanced ErbB4 cleavage in vitro. A 3.4 Å X-ray crystal structure of a complex of ErbB4 ectodomain and the Fab fragment of anti-ErbB4 mAb 1479 localized the binding site of mAb 1479 on ErbB4 to a region on subdomain IV encompassing the residues necessary for ErbB4 cleavage. mAb 1479 also significantly blocked ErbB4 cleavage in breast cancer cell xenografts in vivo, and the inhibition of cleavage was associated with suppression of xenograft growth. These data indicate that ErbB4 processing is enhanced in breast cancer tissue in vivo, and that ErbB4 cleavage can be stimulated by estradiol and targeted with mAb 1479.

Protein inhibitor of activated STAT3 (PIAS3) protein promotes SUMOylation and nuclear sequestration of the intracellular domain of ErbB4 protein

ErbB4 is a receptor tyrosine kinase implicated in the development and homeostasis of the heart, central nervous system, and mammary gland. Cleavable isoforms of ErbB4 release a soluble intracellular domain (ICD) that can translocate to the nucleus and function as a transcriptional coregulator. In search of regulatory mechanisms of ErbB4 ICD function, we identified PIAS3 as a novel interaction partner of ErbB4 ICD. In keeping with the small ubiquitin-like modifier (SUMO) E3 ligase function of protein inhibitor of activated STAT (PIAS) proteins, we showed that the ErbB4 ICD is modified by SUMO, and that PIAS3 stimulates the SUMOylation. Upon overexpression of PIAS3, the ErbB4 ICD generated from the full-length receptor accumulated into the nucleus in a manner that was dependent on the functional nuclear localization signal of ErbB4. In the nucleus, ErbB4 colocalized with PIAS3 and SUMO-1 in promyelocytic leukemia nuclear bodies, nuclear domains involved in regulation of transcription. Accordingly, PIAS3 overexpression had an effect on the transcriptional coregulatory activity of ErbB4, repressing its ability to coactivate transcription with Yes-associated protein. Finally, knockdown of PIAS3 with siRNA partially rescued the inhibitory effect of the ErbB4 ICD on differentiation of MDA-MB-468 breast cancer and HC11 mammary epithelial cells. Our findings illustrate that PIAS3 is a novel regulator of ErbB4 receptor tyrosine kinase, controlling its nuclear sequestration and function.

Conformational transition and energy landscape of ErbB4 activated by neuregulin1β: one microsecond molecular dynamics simulations

ErbB4, a receptor tyrosine kinase of the ErbB family, plays crucial roles in cell growth and differentiation, especially in the development of the heart and nervous system. Ligand binding to its extracellular region could modulate the activation process. To understand the mechanism of ErbB4 activation induced by ligand binding, we performed one microsecond molecular dynamics (MD) simulations on the ErbB4 extracellular region (ECR) with and without its endogenous ligand neuregulin1β (NRG1β). The conformational transition of the ECR-ErbB4/NRG1β complex from a tethered inactive conformation to an extended active-like form has been observed, while such large and function-related conformational change has not been seen in the simulation on the ECR-ErbB4, suggesting that ligand binding is indeed the active inducing force for the conformational transition and further dimerization. On the basis of MD simulations and principal component analysis, we constructed a rough energy landscape for the conformational transition of ECR-ErbB4/NRG1β complex, suggesting that the conformational change from the inactive state to active-like state involves a stable conformation. The energy barrier for the tether opening was estimated as ~2.7 kcal/mol, which is very close to the experimental value (1-2 kcal/mol) reported for ErbB1. On the basis of the simulation results, an atomic mechanism for the ligand-induced activation of ErbB4 was postulated. The present MD simulations provide a new insight into the conformational changes underlying the activation of ErbB4.

Interaction with ErbB4 promotes hypoxia-inducible factor-1α signaling

The receptor-tyrosine kinase ErbB4 was identified as a direct regulator of hypoxia-inducible factor-1α (HIF-1α) signaling. Cleaved intracellular domain of ErbB4 directly interacted with HIF-1α in the nucleus, and stabilized HIF-1α protein in both normoxic and hypoxic conditions by blocking its proteasomal degradation. The mechanism of HIF stabilization was independent of VHL and proline hydroxylation but dependent on RACK1. ErbB4 activity was necessary for efficient HRE-driven promoter activity, transcription of known HIF-1α target genes, and survival of mammary carcinoma cells in vitro. In addition, mammary epithelial specific targeting of Erbb4 in the mouse significantly reduced the amount of HIF-1α protein in vivo. ERBB4 expression also correlated with the expression of HIF-regulated genes in a series of 4552 human normal and cancer tissue samples. These data demonstrate that soluble ErbB4 intracellular domain promotes HIF-1α stability and signaling via a novel mechanism.

Nuclear ErbB4 signaling through H3K9me3 that is antagonized by EGFR-activated c-Src

The ErbB family of receptor tyrosine kinases comprises four members: EGFR (epidermal growth factor receptor)/ErbB1, HER2/ErbB2, ErbB3 and ErbB4, and plays roles in signal transduction at the plasma membrane upon ligand stimulation. Stimulation with neuregulin-1 (NRG-1) cleaves ErbB4 and releases the ErbB4 intracellular domain (4ICD) that translocates into the nucleus to control gene expression. However, little is known about the regulation of 4ICD nuclear signaling through tyrosine phosphorylation. We show here that 4ICD nuclear signaling is antagonized by EGF-induced c-Src activation via EGFR. Generation of 4ICD by NRG-1 leads to increased levels of trimethylated histone H3 on lysine 9 (H3K9me3) in a manner dependent on 4ICD's nuclear accumulation and its tyrosine kinase activity. Once EGF activates c-Src downstream of EGFR concomitantly with NRG-1-induced ErbB4 activation, c-Src associates with phospho-Tyr950 and phospho-Tyr1056 on 4ICD, thereby decreasing nuclear accumulation of 4ICD and inhibiting an increase of H3K9me3 levels. Moreover, 4ICD-induced transcriptional repression of the human telomerase reverse transcriptase (hTERT) is inhibited by EGF-EGFR-Src signaling. Thus, our findings reveal c-Src-mediated inhibitory regulation of ErbB4 nuclear signaling upon EGFR activation.

ErbB4 modulates tubular cell polarity and lumen diameter during kidney development

ErbB4 receptor tyrosine kinase contributes to the development of the heart, the central nervous system, and the lactating mammary gland, but whether it has a role in the development of the kidney epithelium is unknown. Here, we found that expression of Erbb4 isoforms JM-a CYT-1 and JM-a CYT-2 was first detectable around embryonic day 13 in the mouse, mainly in the collecting ducts and both the proximal and distal tubules. In vitro, overexpression of a relevant ErbB4 isoform promoted proliferation and disturbed polarization of kidney epithelial cells when cultured as three-dimensional structures. We examined ErbB4 function in developing kidney tubules in vivo with Pax8-Cre-mediated conditional overexpression of Rosa26 locus-targeted ERBB4 and with conditional Erbb4 knock-out mice. The Pax8-Cre-driven ERBB4 overexpression enhanced proliferation in the collecting ducts, reduced the size of epithelial duct lumens, and promoted formation of cortical tubular cysts. These defects were associated with changes in the subcellular distribution of markers of epithelial cell polarity. Similarly, the Pax8-Cre-mediated Erbb4 knock-out mice manifested dysfunctional kidneys with larger duct lumens and epithelial cell mispolarization. Taken together, these data suggest that ErbB4 signaling modulates proliferation and polarization, cellular functions critical for the development of epithelial ducts in the kidney.

EGF and TGF-β1 Effects on Thyroid Function

Normal epithelial thyroid cells in culture are inhibited by TGF-β1. Instead, transformed thyroid cell lines are frequently resistant to its growth inhibitory effect. Loss of TGF-β responsiveness could be due to a reduced expression of TGF-β receptors, as shown in transformed rat thyroid cell lines and in human thyroid tumors, or to alterations of other genes controlling TGF-β signal transduction pathway. However, in thyroid neoplasia, a complex pattern of alterations occurring during transformation and progression has been identified. Functionally, TGF-β1 acts as a tumor suppressor in the early stage of transformation or as a tumor promoter in advanced cancer. This peculiar pleiotropic behaviour of TGF-β may result from cross-talk with signalling pathways mediated by other growth factors, among which EGF-like ligands play an important role. This paper reports evidences on TGF-β1 and EGF systems in thyroid tumors and on the cross-talk between these growth factors in thyroid cancer.

Nuclear ErbB2 enhances translation and cell growth by activating transcription of ribosomal RNA genes

Aberrant regulation of rRNA synthesis and translation control can facilitate tumorigenesis. The ErbB2 growth factor receptor is overexpressed in many human tumors and has been detected in the nucleus, but the role of nuclear ErbB2 is obscure. In this study, we defined a novel function of nuclear ErbB2 in enhancing rRNA gene transcription by RNA polymerase-I (RNA Pol I). Nuclear ErbB2 physically associates with β-actin and RNA Pol I, coinciding with active RNA Pol I transcription sites in nucleoli. RNA interference-mediated knockdown of ErbB2 reduced pre-rRNA and protein synthesis. In contrast, wild-type ErbB2 augmented pre-rRNA level, protein production, and cell size/cell growth, but not by an ErbB2 mutant that is defective in nuclear translocation. Chromatin immunoprecipitation assays revealed that ErbB2 enhances binding of RNA Pol I to rDNA. In addition, ErbB2 associated with rDNA, RNA Pol I, and β-actin, suggesting how it could stimulate rRNA production, protein synthesis, and increased cell size and cell growth. Finally, ErbB2-potentiated RNA Pol I transcription could be stimulated by ligand and was not substantially repressed by inhibition of PI3-K and MEK/ERK (extracellular signal regulated kinase), the main ErbB2 effector signaling pathways. Together, our findings indicate that nuclear ErbB2 functions as a regulator of rRNA synthesis and cellular translation, which may contribute to tumor development and progression.

Fulvestrant-induced expression of ErbB3 and ErbB4 receptors sensitizes oestrogen receptor-positive breast cancer cells to heregulin β1

Introduction

We have previously reported that induction of epidermal growth factor receptor and ErbB2 in response to antihormonal agents may provide an early mechanism to allow breast cancer cells to evade the growth-inhibitory action of such therapies and ultimately drive resistant cell growth. More recently, the other two members of the ErbB receptor family, ErbB3 and ErbB4, have been implicated in antihormone resistance in breast cancer. In the present study, we have investigated whether induction of ErbB3 and/or ErbB4 may provide an alternative resistance mechanism to antihormonal action in a panel of four oestrogen receptor (ER)-positive breast cancer cell lines.

Methods

MCF-7, T47D, BT474 and MDAMB361 cell lines were exposed to fulvestrant (100 nM) for seven days, and effects on ErbB3/4 expression and signalling, as well as on cell growth, were assessed. Effects of heregulin β1 (HRGβ1) were also examined in the absence and presence of fulvestrant to determine the impact of ER blockade on the capacity of this ErbB3/4 ligand to promote signalling and cell proliferation.

Results

Fulvestrant potently reduced ER expression and transcriptional activity and significantly inhibited growth in MCF-7, T47D, BT474 and MDAMB361 cells. However, alongside this inhibitory activity, fulvestrant also consistently induced protein expression and activity of ErbB3 in MCF-7 and T47D cells and ErbB4 in BT474 and MDAMB361 cell lines. Consequently, fulvestrant treatment sensitised all cell lines to the actions of the ErbB3/4 ligand HRGβ1 with enhanced ErbB3/4-driven signalling activity, reexpression of cyclin D1 and significant increases in cell proliferation being observed when compared to untreated cells. Indeed, in T47D and MDAMB361 HRGβ1 was converted from a ligand having negligible or suppressive growth activity into one that potently promoted cell proliferation. Consequently, fulvestrant-mediated growth inhibition was completely overridden by HRGβ1 in all four cell lines.

Conclusions

These findings suggest that although antihormones such as fulvestrant may have potent acute growth-inhibitory activity in ER-positive breast cancer cells, their ability to induce and sensitise cells to growth factors may serve to reduce and ultimately limit their inhibitory activity.

Gene expression profiling and cancer-related pathways in type I endometrial carcinoma

INTRODUCTION

Malignant transformation of type I endometrium involves alteration in gene expression with subsequent uncontrolled proliferation of altered cells.

OBJECTIVE

The main objective of the present study was to identify the cancer-related genes and gene pathways in the endometrium of healthy and cancer patients.

MATERIALS AND METHODS

Thirty endometrial tissues from healthy and type I EC patients were subjected to total RNA isolation. The RNA samples with good integrity number were hybridized to a new version of Affymetrix Human Genome GeneChip 1.0 ST array. We analyzed the results using the GeneSpring 9.0 GX and the Pathway Studio 6.1 software. For validation assay, quantitative real-time polymerase chain reaction was used to analyze 4 selected genes in normal and EC tissue.

RESULTS

Of the 28,869 genes profiled, we identified 621 differentially expressed genes (2-fold) in the normal tissue and the tumor. Among these genes, 146 were up-regulated and 476 were down-regulated in the tumor as compared with the normal tissue (P < 0.001). Up-regulated genes included the v-erb-a erythroblastic leukemia viral oncogene homolog 3 (ErbB3), ErbB4, E74-like factor 3 (ELF3), and chemokine ligand 17 (CXCL17). The down-regulated genes included signal transducer and activator transcription 5B (STAT5b), transforming growth factor A receptor III (TGFA3), caveolin 1 (CAV1), and protein kinase C alpha (PKCA). The gene set enrichment analysis showed 10 significant gene sets with related genes (P < 0.05). The quantitative polymerase chain reaction of 4 selected genes using similar RNA confirmed the microarray results (P < 0.05).

CONCLUSIONS

Identification of molecular pathways with their genes related to type I EC contribute to the understanding of pathophysiology of this cancer, probably leading to identifying potential biomarkers of the cancer.

Ligand stimulation of ErbB4 and a constitutively-active ErbB4 mutant result in different biological responses in human pancreatic tumor cell lines

Pancreatic cancer is the fourth leading cause of cancer death in the United States. Indeed, it has been estimated that 37,000 Americans will die from this disease in 2010. Late diagnosis, chemoresistance, and radioresistance of these tumors are major reasons for poor patient outcome, spurring the search for pancreatic cancer early diagnostic and therapeutic targets. ErbB4 (HER4) is a member of the ErbB family of receptor tyrosine kinases (RTKs), a family that also includes the Epidermal Growth Factor Receptor (EGFR/ErbB1/HER1), Neu/ErbB2/HER2, and ErbB3/HER3. These RTKs play central roles in many human malignancies by regulating cell proliferation, survival, differentiation, invasiveness, motility, and apoptosis. In this report we demonstrate that human pancreatic tumor cell lines exhibit minimal ErbB4 expression; in contrast, these cell lines exhibit varied and in some cases abundant expression and basal tyrosine phosphorylation of EGFR, ErbB2, and ErbB3. Expression of a constitutively-dimerized and -active ErbB4 mutant inhibits clonogenic proliferation of CaPan-1, HPAC, MIA PaCa-2, and PANC-1 pancreatic tumor cell lines. In contrast, expression of wild-type ErbB4 in pancreatic tumor cell lines potentiates stimulation of anchorage-independent colony formation by the ErbB4 ligand Neuregulin 1β. These results illustrate the multiple roles that ErbB4 may be playing in pancreatic tumorigenesis and tumor progression.

Cell death or survival promoted by alternative isoforms of ErbB4

The report demonstrates that two distinct isoforms of the ErbB4 receptor tyrosine kinase stimulate either proliferation or apoptosis by mechanisms involving differential transcriptional regulation of the PDGFRA gene. These data have implications for developing approaches to target ErbB4 signaling in cancer.

The significance of ErbB4 in tumor biology is poorly understood. The ERBB4 gene is alternatively spliced producing juxtamembrane (JM-a and JM-b) and cytoplasmic (CYT-1 and CYT-2) isoforms. Here, signaling via the two alternative ErbB4 JM isoforms (JM-a CYT-2 and JM-b CYT-2) was compared. Fibroblasts expressing ErbB4 JM-a demonstrated enhanced ErbB4 autophosphorylation, growth, and survival. In contrast, cells overexpressing ErbB4 JM-b underwent starvation-induced death. Both pro- and antisurvival responses to the two ErbB4 isoforms were sensitive to an ErbB kinase inhibitor. Platelet-derived growth factor receptor-alpha (PDGFRA) was identified as an ErbB4 target gene that was differentially regulated by the two ErbB4 isoforms. The soluble intracellular domain of ErbB4, released from the JM-a but not from the JM-b isoform, associated with the transcription factor AP-2 and promoted its potential to enhance PDGFRA transcription. Survival of cells expressing JM-a was suppressed by targeting either PDGFR-α or AP-2, whereas cells expressing JM-b were rescued from cell death by the PDGFR-α agonist, PDGF-BB. These findings indicate that two alternative ErbB4 isoforms may promote antagonistic cellular responses and suggest that pharmacological inhibition of ErbB4 kinase activity may lead to either suppression or promotion of cellular growth.

The fibroblast-derived paracrine factor neuregulin-1 has a novel role in regulating the constitutive color and melanocyte function in human skin

Interactions between melanocytes and neighboring cells in the skin are important in regulating skin color in humans. We recently demonstrated that the less pigmented and thicker skin on the palms and soles is regulated by underlying fibroblasts in those areas, specifically via a secreted factor (DKK1) that modulates Wnt signaling. In this study, we tested the hypothesis that dermal fibroblasts regulate the constitutive skin color of individuals ranging from very light to very dark. We used microarray analysis to compare gene expression patterns in fibroblasts derived from lighter skin types compared to darker skin types, with a focus on secreted proteins. We identified a number of genes that differ dramatically in expression and, among the expressed proteins, neuregulin-1, which is secreted by fibroblasts derived from dark skin, effectively increases the pigmentation of melanocytes in tissue culture and in an artificial skin model and regulates their growth, suggesting that it is one of the major factors determining human skin color.

Ectodomain shedding of interleukin-2 receptor beta and generation of an intracellular functional fragment

Interleukin-2 (IL-2) regulates different functions of various lymphoid cell subsets. These are mediated by its binding to the IL-2 receptor (IL-2R) composed of three subunits (IL2-Ralpha, -beta, and -gamma(c)). IL-2Rbeta is responsible for the activation of several signaling pathways. Ectodomain shedding of membrane receptors is thought to be an important mechanism for down-regulation of cell surface receptor abundance but is also emerging as a mechanism that cell membrane-associated molecules require for proper action in vivo. Here, we demonstrate that IL-2Rbeta is cleaved in cell lines of different origin, including T cells, generating an intracellular 37-kDa fragment (37beta ic) that comprises the full intracellular C-terminal and transmembrane domains. Ectodomain shedding of IL-2Rbeta decreases in a mutant deleted of the juxtamembrane region, where cleavage is predicted to occur, and is inhibited by tissue inhibitor of metalloproteases-3. 37Beta ic is tyrosine-phosphorylated and associates with STAT-5, a canonic signal transducer of IL-2R. Finally, lymphoid cell transfection with a truncated form of IL-2Rbeta mimicking 37beta ic increases their proliferation. These data indicate that IL-2Rbeta is subject to ectodomain shedding generating an intracellular fragment biologically functional, because (i) it is phosphorylated, (ii) it associates with STAT5A, and (iii) it increases cell proliferation.

A growing family: adding mutated Erbb4 as a novel cancer target

As the upward spiral of novel cancer gene discoveries and novel molecular compounds continues to accelerate, a repetitive theme in molecular drug development remains the lack of activity of initially promising agents when given to patients in clinical trials. It is however invigorating that a few targeted agents directed against a select group of a few 'cancer gene superfamilies' have escaped this all to common fate, and have evolved into novel, clinically meaningful molecular therapy strategies. Targeting dysregulated signaling of the epidermal growth factor family of transmembrane receptors (Erbb family) has encompassed over the last decade an ever increasing role in personalized treatment approaches in an increasing number of human malignancies. Erbbs are receptor tyrosine kinases that are important regulators of several signaling pathways. Two of its family members (Erbb1/EGFR and Erbb2/HER2) have previously been shown to be somatically mutated in large fraction of human cancers. To determine if this family is somatically mutated in melanoma, its sequences were recently analyzed and one of its members, Erbb4, was found to be somatically mutated in 19% of melanoma cases. Functional analysis of seven of its mutations was shown to increase its catalytic and transformation abilities as well as providing essential survival signals. Similar to other Erbb family members, mutant Erbb4 seems to confer 'oncogene addiction' on melanoma cells, making it an attractive therapeutic target. Gaining further understanding into the oncogenic mechanism of Erbb4 may not only help in the development of targeted therapy in melanoma patients but might accelerate the acceptance of a novel taxonomy of cancer which is based on the genomic perturbations in cancer genes and cancer gene families and their response to targeted agents.

Potential of ErbB4 antibodies for cancer therapy

Antibodies targeting the extracellular domains of ErbB receptors have been extensively studied for cancer drug development. This work has led to clinical approval of monoclonal antibodies against the well-known oncogenes EGFR and ErbB2. Here we discuss the biological activities of ErbB4, a less-studied member of the EGFR/ErbB growth factor receptor family and speculate on the potential clinical relevance of antibodies targeting ErbB4. In addition to their significance as therapeutics, the role of ErbB4 antibodies in prognostic and predictive applications is surveyed.

It's all in for the HER family in tumorigenesis

The EGF receptor family is a group of receptor tyrosine kinases that have been implicated in the development of a variety of malignancies. As such, they have been targeted in the generation of pharmacologic agents, several of which have been approved as anti-tumor therapeutics. The lone exception is ERBB4, for which the function and relationship to cancer are not yet clear and no targeted therapies exist. The paper under evaluation demonstrates a role for ERBB4 mutations in the development of melanoma. It identifies ERBB4 mutations present in melanomas that augment proliferation and cell survival and thus contribute to dysregulated growth. Furthermore, it shows that agents targeting the EGF receptor family can reduce the proliferation of melanoma cells harboring these mutations. These findings further emphasize the role of the ERBB subfamily in tumorigenesis and establish ERBB4 as a new target in the development of anti-tumor strategies.

The pan-ErbB receptor tyrosine kinase inhibitor canertinib induces ErbB-independent apoptosis in human leukemia (HL-60 and U-937) cells

Epidermal growth factor (EGF) receptor tyrosine kinase inhibitors have recently been shown to display anti-neoplastic effects in human malignant myeloid cells. Our study was initiated in order to determine the effect of the pan-ErbB receptor tyrosine kinase inhibitor, canertinib (CI-1033), on growth and survival of human leukemia (HL-60 and U-937) cells. We show that treatment of HL-60 and U-937 cells with canertinib significantly inhibits growth of both cell lines in a dose-dependent manner; half maximal effective dose (IC(50)) in HL-60 and U-937 cells was approximately 2.5 microM and 1.0 microM, respectively. Treatment with 2 microM canertinib promoted a G(1) cell cycle arrest, whereas doses of 5 microM or more induced apoptosis as determined by the Annexin V method and cleavage of poly-(ADP-ribose) polymerase (PARP). HL-60 and U-937 cells lacked EGF-receptor transcript but expressed ErbB2-4 mRNA as determined by RT-PCR. However, none of the corresponding ErbB-receptor proteins could be detected by Western blot analysis. We conclude that canertinib induces apoptosis in HL-60 and U-937 cells devoid of functional ErbB1-4 receptors. Our results suggest that canertinib could be of potential clinical interest in the treatment of acute myeloid leukemia.

ErbB receptor tyrosine kinases contribute to proliferation of malignant melanoma cells: inhibition by gefitinib (ZD1839)

Members of the epidermal growth factor (EGF) family of structurally related tyrosine kinase receptors, known as the ErbB receptors (EGFR/ErbB1/HER1, ErbB2/HER2/neu, ErbB3/HER3 and ErbB4/HER4) and their respective ligands, have been suggested to be involved in the development and progression of malignant melanoma. Here we investigate the effects of the ErbB1 tyrosine kinase inhibitor gefitinib (ZD1839, Iressa) on human malignant melanoma cells (RaH3 and RaH5) in vitro. ZD1839 inhibited proliferation of exponentially growing RaH3 and RaH5 cells in a dose-dependent manner with a half-maximally effective dose of 3.5 and 2.0 micromol/l, respectively. Cell growth was inhibited at 0.1 micromol/l ZD1839 in both cell lines. Maximal inhibition was accomplished at 10 micromol/l ZD1839; however, the effect was not complete as both cell lines showed a continuous slow growth during the treatment period. Flow cytometry analysis of cell-cycle distribution showed that ZD1839 treatment caused accumulation of RaH3 and RaH5 cells in the G1 phase. The growth arrest induced by ZD1839 coincided with upregulation of the cyclin-dependent kinase inhibitor p27 KIP 1. There was no increase in apoptosis as determined by analysis of plasma phosphatidyl serine redistribution. Western blot analysis revealed that ZD1839 substantially reduced tyrosine phosphorylation of ErbB1 as well as ErbB2 and ErbB3. This was accompanied by a concomitant decrease in Akt-phosphorylation, Erk1/2-phosphorylation, and Stat3-phosphorylation. Our results show that ZD1839 interferes with the growth of human malignant melanoma cells by cytostatic effects. These findings indicate the possible use of ErbB receptor kinase inhibitors as a novel treatment strategy in malignant melanoma.

Presence of HER4 associates with increased sensitivity to Herceptin in patients with metastatic breast cancer

INTRODUCTION

HER2 overexpression, or rather HER2 gene amplification, is indicative for Herceptin therapy in both metastatic and pre-metastatic breast cancer patients. Patient's individual sensitivity to Herceptin treatment, however, varies enormously and spans from effectual responsiveness over acquired insensitivity to complete resistance from the outset. Thus no predictive information can be deduced from HER2 determination so that molecular biomarkers indicative for Herceptin sensitivity or resistance need to be identified. Both ErbB receptor-dependent signalling molecules as well as HER2-related ErbB receptor tyrosine kinases, known to mutually interact and to cross-regulate each other are prime candidates to be involved in cellular susceptibility to Herceptin.

METHODS

Using immunohistochemistry and fluorescence in situ hybridisation, we retrospectively investigated primary breast cancer tissues from 48 patients who were under Herceptin treatment. We quantified the gene copy numbers of all HER receptors and evaluated their coexpression profile. Moreover the HER2 phosphorylation state, the ratio of native to truncated HER2, p27(kip1) and PTEN expression were objects of this study.

RESULTS

Above all markers investigated in this study Kaplan-Meier and Cox regression analysis revealed a significant positive impact of HER4 (co-)expression on overall survival from beginning of antibody therapy. Both HER4 expression and HER4 gene amplification emerged as independent prognostic markers in Herceptin-treated breast cancer patients and responsiveness to Herceptin turned out to be more efficient if tumour cells show HER4 expression.

CONCLUSIONS

Although HER4 is known to potentially exert a tumour cell killing activity and in turn to have a favourable impact in breast cancer patients we demonstrate here the first time that HER4 expression prolongs overall survival in Herceptin-treated patients. Elucidating HER4 receptor function in the context of Herceptin treatment will advance the design of highly efficient receptor targeting. By then we need to extend the analysis of breast cancer by allowing for HER2/HER4 coexpression by which valuable additional prognostic and predictive information might possibly be revealed.

ErbB4 splice variants Cyt1 and Cyt2 differ by 16 amino acids and exert opposing effects on the mammary epithelium in vivo

Data concerning the prognostic value of ErbB4 in breast cancer and effects on cell growth have varied in published reports, perhaps due to the unknown signaling consequences of expression of the intracellular proteolytic ErbB4 s80(HER4) fragment or due to differing signaling capabilities of alternatively spliced ErbB4 isoforms. One isoform (Cyt1) contains a 16-residue intracellular sequence that is absent from the other (Cyt2). We expressed s80(Cyt1) and s80(Cyt2) in HC11 mammary epithelial cells, finding diametrically opposed effects on the growth and organization of colonies in three-dimensional matrices. Whereas expression of s80(Cyt1) decreased growth and increased the rate of three-dimensional lumen formation, that of s80(Cyt2) increased proliferation without promoting lumen formation. These results were recapitulated in vivo, using doxycycline-inducible, mouse breast-transgenic expression of s80(Cyt1) amd s80(Cyt2). Expression of s80(Cyt1) decreased growth of the mammary ductal epithelium, caused precocious STAT5a activation and lactogenic differentiation, and increased cell surface E-cadherin levels. Remarkably, ductal growth inhibition by s80(Cyt1) occurred simultaneously with lobuloalveolar growth that was unimpeded by s80(Cyt1), suggesting that the response to ErbB4 may be influenced by the epithelial subtype. In contrast, expression of s80(Cyt2) caused epithelial hyperplasia, increased Wnt and nuclear beta-catenin expression, and elevated expression of c-myc and cyclin D1 in the mammary epithelium. These results demonstrate that the Cyt1 and Cyt2 ErbB4 isoforms, differing by only 16 amino acids, exhibit markedly opposing effects on mammary epithelium growth and differentiation.

WW domain containing E3 ubiquitin protein ligase 1 targets the full-length ErbB4 for ubiquitin-mediated degradation in breast cancer

ErbB4, a member of the epidermal growth factor receptor family, plays a role in normal breast and breast cancer development by regulating mammary epithelial cell proliferation, survival and differentiation. In this study, we show that WWP1, a C2-WW-HECT type E3 ubiquitin ligase, binds, ubiquitinates and destructs ErbB4-CYT1, but much less efficiently for CYT2, isoforms (both JMa and JMb). The protein-protein interaction occurs primarily between the first and third WW domains of WWP1 and the second PY motif of ErbB4. Knockdown of WWP1 by two different small interfering RNAs increases the endogenous ErbB4 protein levels in both MCF7 and T47D breast cancer cell lines. In addition, overexpression of the wild type, but not the catalytic inactive WWP1, dramatically decreases the endogenous ErbB4 protein levels in MCF7. Importantly, we found that WWP1 negatively regulates the heregulin-beta1-stimulated ErbB4 activity as measured by the serum response element report assay and the BRCA1 mRNA expression. After a systematic screening of all WWP1 family members by small interfering RNA, we found that AIP4/Itch and HECW1/NEDL1 also negatively regulate the ErbB4 protein expression in T47D. Interestingly, the protein expression levels of both WWP1 and ErbB4 are higher in estrogen receptor-alpha-positive than in estrogen receptor-alpha-negative breast cancer cell lines. These data suggest that WWP1 and its family members suppress the ErbB4 expression and function in breast cancer.

Overexpression of WWP1 is associated with the estrogen receptor and insulin-like growth factor receptor 1 in breast carcinoma

WWP1, a HECT type E3 ubiquitin ligase frequently amplified and overexpressed in breast cancer, has the potential to become a useful clinical biomarker and therapeutic target in breast cancer. Here, we performed immunohistochemical staining in formalin-fixed and paraffin-embedded tissue sections from 187 cases of primary invasive mammary carcinoma [137 ductal carcinomas (IDC) and 50 lobular carcinomas (ILC)] by using a monoclonal anti-WWP1 antibody. The normal breast epithelium and adjacent benign epithelium are essentially negative for WWP1. Cytoplasmic WWP1 immunoreactivity was observed in 76/187 (40.6%) tumors and showed a positive correlation with ERalpha (p = 0.05) and IGF-1R proteins (p = 0.001) in this cohort. The positive correlations between WWP1 and ER/IGF-1R were also observed in a panel of 12 breast cancer cell lines by Western blot. Interestingly, the ER levels are decreased when WWP1 is silenced in ER positive MCF7 and T47D breast cancer cell lines. Finally, WWP1 ablation collectively inhibits cell proliferation with tamoxifen in MCF7 and T47D, as measured by (3)H-thymidine incorporation assays. These findings suggest that WWP1 may play an important role in ER positive breast cancer.

Nedd4 mediates ErbB4 JM-a/CYT-1 ICD ubiquitination and degradation in MDCK II cells

ErbB4, a type I transmembrane receptor tyrosine kinase, is a member of the epidermal growth factor receptor family. Its cleavage releases an intracellular C-terminal domain (ICD), which can be either degraded following ubiqitination or translocated to the nucleus and regulate gene expression. There are 2 ErbB4 ICD isoforms: CYT-1 and CYT-2. We and others have previously reported that following cleavage, CYT-2 selectively translocates to the nucleus. In the current study we found that following cleavage, the intracellular levels of CYT-1 ICD decreased rapidly, while levels of CYT-2 ICD remained relatively stable. CYT-1 ICD degradation could be prevented by administration of either the proteasome inhibitor lactacystin or the lysosome inhibitor chloroquine, indicating both proteasomal and lysosomal degradation. Further studies implicated Nedd4, an E3 ubiquitin ligase, as a mediator of CYT-1 ubiquitination and degradation. The interaction of Nedd4 with CYT-1 was shown by coimmnunoprecipitation, an in vitro direct binding assay, and an in vitro ubiquitination assay. Three PPxY or PY motifs present in the CYT-1 C terminus are necessary for binding by Nedd4 WW domains, because impaired interactions are seen in mutation of any of the PY motifs. Nedd4-CYT-1 binding was associated with increased CYT-1 ubiquitination following proteasome inhibitor treatment. Impaired Nedd4 binding to CYT-1 by PY motif mutations led to increased CYT-1 ICD stability, whereas only one of the PY motif mutations (Y1056A), which disrupts the binding sites for both a WW domain and an SH2 domain of PI3 kinase, demonstrated enhanced nuclear translocation following HB-EGF treatment. These studies indicate that Nedd4 mediates ErbB4 CYT-1 ICD ubiquitination and degradation, and the prevention of both WW binding and PI3 kinase activity are required for ErbB4 nuclear translocation.

Removal of cell surface heparan sulfate increases TACE activity and cleavage of ErbB4 receptor

Background

Nuclear localization of proteolytically formed intracellular fragment of ErbB4 receptor tyrosine kinase has been shown to promote cell survival, and nuclear localization of ErbB4 receptor has been described in human breast cancer. Tumor necrosis factor alpha converting enzyme (TACE) initiates the proteolytic cascade leading to ErbB4 intracellular domain formation. Interactions between matrix metalloproteases and heparan sulfate have been described, but the effect of cell surface heparan sulfate on TACE activity has not been previously described.

Results

As indicated by immunodetection of increased ErbB4 intracellular domain formation and direct enzyme activity analysis, TACE activity was substantially amplified by enzymatic removal of cell surface heparan sulfate but not chondroitin sulfate.

Conclusion

In this communication, we suggest a novel role for cell surface heparan sulfate. Removal of cell surface heparan sulfate led to increased formation of ErbB4 intracellular domain. As ErbB4 intracellular domain has previously been shown to promote cell survival this finding may indicate a novel mechanism how HS degradation active in tumor tissue may favor cell survival.

Suppression of breast cancer cell growth by a monoclonal antibody targeting cleavable ErbB4 isoforms

ErbB4 isoforms mediate different cellular activities depending on their susceptibility to proteolytic cleavage. The biological significance of ErbB4 cleavage in tumorigenesis, however, remains poorly understood. Here, we describe characterization of a monoclonal antibody (mAb 1479) that selectively recognizes the ectodomain of cleavable ErbB4 JM-a isoforms both in vitro and in vivo. mAb 1479 was used to analyse ErbB4 JM-a expression and ectodomain shedding in a series of 17 matched breast cancer/histologically normal peripheral breast tissue pairs. ErbB4 ectodomain was observed in 75% of tumors expressing ErbB4 but only in 18% of normal breast tissue samples expressing ErbB4. Difference in the relative quantity of ErbB4 ectodomain between normal and tumor tissue pairs was statistically significant (P=0.015). Treatment with mAb 1479 suppressed ErbB4 function by inhibiting ErbB4 tyrosine phosphorylation and ectodomain shedding, and by stimulating ErbB4 downregulation and ubiquitination. mAb 1479 suppressed both anchorage-dependent and -independent growth of human breast cancer cell lines that naturally express cleavable ErbB4 JM-a. These findings indicate that ErbB4 ectodomain shedding is enhanced in breast cancer tissue in vivo, and that mAb 1479 represents a potential drug candidate that suppresses breast cancer cell growth by selectively binding cleavable ErbB4 isoforms.

Systems-level analysis of ErbB4 signaling in breast cancer: a laboratory to clinical perspective

Although expression of the ErbB4 receptor tyrosine kinase in breast cancer is generally regarded as a marker for favorable patient prognosis, controversial exceptions have been reported. Alternative splicing of ErbB4 pre-mRNAs results in the expression of distinct receptor isoforms with differential susceptibility to enzymatic cleavage and different downstream signaling protein recruitment potential that could affect tumor progression in different ways. ErbB4 protein expression from nontransfected cells is generally low compared with ErbB1 in most cell lines, and much of our knowledge of the role of ErbB4 in breast cancer is derived from the ectopic overexpression of the receptor in non-breast-derived cell lines. One of the primary functions of ErbB4 in vivo is in the maturation of mammary glands during pregnancy and lactation induction. Pregnancy and extended lactation durations have been correlated with reduced risk of breast cancer, and the role of ErbB4 in tumor suppression may therefore be linked with its role in lactation. Most reports are consistent with a role for ErbB4 in reversing growth stimuli triggered by other ErbB family members during puberty. In this report, we provide a systems-level examination of several reports highlighting the seemingly opposing roles of ErbB4 in breast cancer and potential explanations for the discrepancies and draw the conclusion that future studies examining the function of ErbB4 in breast cancer should also take into account the pregnancy history, lactation status, and hormone supplementation or ablation history of the patient from whom the tumor or tumor cells are derived.

The ADAM metalloproteinases

The ADAMs (a disintegrin and metalloproteinase) are a fascinating family of transmembrane and secreted proteins with important roles in regulating cell phenotype via their effects on cell adhesion, migration, proteolysis and signalling. Though all ADAMs contain metalloproteinase domains, in humans only 13 of the 21 genes in the family encode functional proteases, indicating that at least for the other eight members, protein–protein interactions are critical aspects of their biological functions. The functional ADAM metalloproteinases are involved in “ectodomain shedding” of diverse growth factors, cytokines, receptors and adhesion molecules. The archetypal activity is shown by ADAM-17 (tumour necrosis factor-α convertase, TACE), which is the principal protease involved in the activation of pro-TNF-α, but whose sheddase functions cover a broad range of cell surface molecules. In particular, ADAM-17 is required for generation of the active forms of Epidermal Growth Factor Receptor (EGFR) ligands, and its function is essential for the development of epithelial tissues. Several other ADAMs have important sheddase functions in particular tissue contexts. Another major family member, ADAM-10, is a principal player in signalling via the Notch and Eph/ephrin pathways. For a growing number of substrates, foremost among them being Notch, cleavage by ADAM sheddases is essential for their subsequent “regulated intramembrane proteolysis” (RIP), which generates cleaved intracellular domains that translocate to the nucleus and regulate gene transcription. Several ADAMs play roles in spermatogenesis and sperm function, potentially by effecting maturation of sperm and their adhesion and migration in the uterus. Other non-catalytic ADAMs function in the CNS via effects on guidance mechanisms. The ADAM family are thus fundamental to many control processes in development and homeostasis, and unsurprisingly they are also linked to pathological states when their functions are dysregulated, including cancer, cardiovascular disease, asthma, Alzheimer’s disease. This review will provide an overview of current knowledge of the human ADAMs, discussing their structure, function, regulation and disease involvement.

HER4 intracellular domain (4ICD) activity in the developing mammary gland and breast cancer

The HER4 receptor tyrosine kinase was the final member of the EGFR-family to be discovered. In contrast to the other three members of this receptor family which function primarily as mitogenic effectors in the breast, HER4 appears to have multiple divergent functions in the normal and malignant breast. Interestingly, the majority of HER4 activities in the breast including pregnancy induced differentiation and lactation initiation, transcriptional activation, tumor cell proliferation, growth suppression, and induction of apoptosis appear to be mediated by an independently signaling soluble HER4 intracellular domain (4ICD). The 4ICD can accumulate within the nucleus or mitochondria and subcellular localization of 4ICD in part determines the physiological response of breast cells to 4ICD action. Here I will discuss the evidence supporting the role of 4ICD as the critical effector of HER4 signaling in the breast. In addition a developmental and temporal model of 4ICD action in the normal breast and during the progression of breast cancer will be presented to explain the paradox of divergent HER4 and 4ICD activities.

ErbB4/HER4: role in mammary gland development, differentiation and growth inhibition

The ErbB receptor tyrosine kinase family has often been associated with increased growth of breast epithelial cells, as well as malignant transformation and progression. In contrast, ErbB4/HER4 exhibits unique attributes from a two step proteolytic cleavage which releases an 80 kilodalton, nuclear localizing, tyrosine kinase to a signal transduction mechanism that slows growth and stimulates differentiation of breast cells. This review provides an overview of ErbB4/HER4 in growth and differentiation of the mammary epithelium, including its physiologic role in development, the contrasting growth inhibition/tumor suppression and growth acceleration of distinct ErbB4/HER4 isoforms and a description of the unique cell cycle regulated pattern of nuclear HER4 ubiquitination and destruction.

The role of NRG3 in mammary development

The Neuregulin gene family encodes EGF-containing ligands which mediate their effects by binding to the ERBB receptor tyrosine kinases, a signalling network with important roles in both mammary gland development and breast cancer. Neuregulin3 (NRG3), a ligand for ERBB4, promotes early mammary morphogenesis and acts during specification of the mammary placode, an aggregate of epithelial cells that forms during mid-embryogenesis. Recent studies have shown that NRG3 can alter the cell fate of other epidermal progenitor populations when NRG3 is mis-expressed throughout the basal layer of the developing epidermis with the K14 promoter. Here evidence for a key function for NRG3 in promoting early mammary morphogenesis and the implication for the role of NRG3 in breast cancer and establishment of the mammary lineage are discussed.

Role of ErbB4 in breast cancer

Members of the ErbB subfamily of receptor tyrosine kinases are important regulators of normal mammary gland physiology, and aberrations in their signaling have been associated with breast tumorigenesis. Therapeutics targeting epidermal growth factor receptor (EGFR = ErbB1) or ErbB2 in breast cancer have been approved for clinical use. In contrast, relatively little is known about the biological significance of ErbB4 signaling in breast cancer. This review focuses on recent advances in our understanding about the role of ErbB4 in breast carcinogenesis, as well as in the potential clinical relevance of ErbB4 in breast cancer prognostics and therapy.

Activation of ErbB3, EGFR and Erk is essential for growth of human breast cancer cell lines with acquired resistance to fulvestrant

Seven fulvestrant resistant cell lines derived from the estrogen receptor alpha positive MCF-7 human breast cancer cell line were used to investigate the importance of epidermal growth factor receptor (ErbB1-4) signaling. We found an increase in mRNA expression of EGFR and the ErbB3/ErbB4 ligand heregulin2 (hrg2) and a decrease of ErbB4 in all resistant cell lines. Western analyses confirmed the upregulation of EGFR and hrg2 and the downregulation of ErbB4. Elevated activation of EGFR and ErbB3 was seen in all resistant cell lines and the ErbB3 activation occurred by an autocrine mechanism. ErbB4 activation was observed only in the parental MCF-7 cells. The downstream kinases pAkt and pErk were increased in five of seven and in all seven resistant cell lines, respectively. Treatment with the EGFR inhibitor gefitinib preferentially inhibited growth and reduced the S phase fraction in the resistant cell lines concomitant with inhibition of Erk and unaltered Akt activation. In concert, inhibition of Erk with U0126 preferentially reduced growth of resistant cell lines. Treatment with ErbB3 neutralizing antibodies inhibited ErbB3 activation and resulted in a modest but statistically significant growth inhibition of two resistant cell lines. These data indicate that ligand activated ErbB3 and EGFR, and Erk signaling play important roles in fulvestrant resistant cell growth. Furthermore, the decreased level of ErbB4 in resistant cells may facilitate heterodimerization of ErbB3 with EGFR and ErbB2. Our data support that a concerted action against EGFR, ErbB2 and ErbB3 may be required to obtain complete growth suppression of fulvestrant resistant cells.

Alteration of BACE1-dependent NRG1/ErbB4 signaling and schizophrenia-like phenotypes in BACE1-null mice

beta-Site APP-cleaving enzyme 1 (BACE1) is required for the penultimate cleavage of the amyloid-beta precursor protein (APP) leading to the generation of amyloid-beta peptides that is central to the pathogenesis of Alzheimer's disease. In addition to its role in endoproteolysis of APP, BACE1 participates in the proteolytic processing of neuregulin 1 (NRG1) and influences the myelination of central and peripheral axons. Although NRG1 has been genetically linked to schizophrenia and NRG1(+/-) mice exhibit a number of schizophrenia-like behavioral traits, it is not known whether altered BACE1-dependent NRG1 signaling can cause similar behavioral abnormalities. To test this hypothesis, we analyze the behaviors considered to be rodent analogs of clinical features of schizophrenia in BACE1(-/-) mice with impaired processing of NRG1. We demonstrate that BACE1(-/-) mice exhibit deficits in prepulse inhibition, novelty-induced hyperactivity, hypersensitivity to a glutamatergic psychostimulant (MK-801), cognitive impairments, and deficits in social recognition. Importantly, some of these manifestations were responsive to treatment with clozapine, an atypical antipsychotic drug. Moreover, although the total amount of ErbB4, a receptor for NRG1 was not changed, binding of ErbB4 with postsynaptic density protein 95 (PSD95) was significantly reduced in the brains of BACE1(-/-) mice. Consistent with the role of ErbB4 in spine morphology and synaptic function, BACE1(-/-) mice displayed reduced spine density in hippocampal pyramidal neurons. Collectively, our findings suggest that alterations in BACE1-dependent NRG1/ErbB4 signaling may participate in the pathogenesis of schizophrenia and related psychiatric disorders.

Isoform-specific monoubiquitination, endocytosis, and degradation of alternatively spliced ErbB4 isoforms

Endocytosis and subsequent lysosomal degradation serve as a well characterized mechanism to fine-tune and down-regulate EGFR signaling. However, other members of the EGFR/ErbB receptor family have been reported to be endocytosis-impaired. Here we demonstrate that endocytosis of ErbB4 is regulated in an isoform-specific manner: CYT-1 isoforms were efficiently endocytosed whereas CYT-2 isoforms were endocytosis-impaired. CYT-1 isoforms in endocytic vesicles colocalized with Rab5 and Rab7 indicating trafficking via early endosomes to late endosomal/lysosomal structures. A PPXY motif within the CYT-1-specific sequence that lacks from CYT-2 was necessary both for ubiquitination and endocytosis of CYT-1 isoforms and provided a binding site for a WW domain-containing ubiquitin ligase Itch. Itch catalyzed ubiquitination of ErbB4 CYT-1, promoted its localization into intracellular vesicles, and stimulated degradation of ErbB4 CYT-1. Dominant negative Itch suppressed ErbB4 CYT-1 endocytosis and degradation. These data indicate that ErbB4 isoforms differ in endocytosis and degradation by a mechanism mediated by CYT-1-specific PPXY motif interacting with a WW domain-containing E3 ubiquitin ligase.

gamma-Secretase: a multifaceted regulator of angiogenesis

Physiological angiogenesis is essential for development, homeostasis and tissue repair but pathological neovascularization is a major feature of tumours, rheumatoid arthritis and ocular complications. Studies over the last decade have identified γ-secretase, a presenilin-dependent protease, as a key regulator of angiogenesis through: (i) regulated intramembrane proteolysis and transmembrane cleavage of receptors (e.g. VEGFR-1, Notch, ErbB-4, IGFI-R) followed by translocation of the intracellular domain to the nucleus, (ii) translocation of full length membrane-bound receptors to the nucleus (VEGFR-1), (iii) phosphorylation of membrane bound proteins (VEGFR-1 and ErbB-4), (iv) modulation of adherens junctions (cadherin) and regulation of permeability and (v) cleavage of amyloid precursor protein to amyloid-β which is able to regulate the angiogenic process. The γ-secretase-induced translocation of receptors to the nucleus provides an alternative intracellular signalling pathway, which acts as a potent regulator of transcription. γ-secretase is a complex composed of four different integral proteins (presenilin, nicastrin, Aph-1 and Pen-2), which determine the stability, substrate binding, substrate specificity and proteolytic activity of γ-secretase. This seeming complexity allows numerous possibilities for the development of targeted γ-secretase agonists/antagonists, which can specifically regulate the angiogenic process. This review will consider the structure and function of γ-secretase, the growing evidence for its role in angiogenesis and the substrates involved, γ-secretase as a therapeutic target and future challenges in this area.

Association of Wwox with ErbB4 in breast cancer

WWOX, WW domain-containing oxidoreductase, is a tumor suppressor that is altered in many human cancers, including breast cancer. Wwox interacts with the ErbB4 receptor, reduces nuclear translocation of the cleaved intracellular domain of ErbB4, and inhibits its transactivation function mediated through Yes-associated protein. Here, we assessed the clinical significance of the Wwox-ErbB4 association. We determined Wwox protein expression by immunohistochemistry in a series of 556 breast cancers. Wwox expression was absent in 36% of the cancers, and loss of Wwox expression was associated with unfavorable outcome (P = 0.02). Membranous location of ErbB4 was associated with favorable survival compared with women whose cancer lacked such ErbB4 expression (P = 0.02). Wwox expression was strongly associated with membranous ErbB4 localization (P = 0.0003) and with overall ErbB4 expression (P = 0.0002). Coexpression of membranous ErbB4 and Wwox was associated with favorable outcome compared with cases with membranous ErbB4 and no Wwox immunoreactivity (P = 0.002). In vitro, Wwox associated with the two ErbB4 isoforms, JM-a CYT-1 and JM-a CYT-2, expressed in breast cancer. Moreover, expression of Wwox both in vitro and in vivo led to accumulation of total full-length membrane-associated ErbB4. These results suggest that expression of Wwox is associated with ErbB4 expression and that their coexpression has prognostic significance in breast cancer.

The antibody sc-33040-R fails to specifically recognize phosphorylation of ErbB4 on tyrosine1056

Phosphorylation state specific antibodies are important reagents for characterizing protein phosphorylation and signaling. However, these antibodies require proper validation to determine that they do not cross-react with the unphosphorylated peptide or with other phosphoproteins. We have previously shown that phosphorylation of tyrosine1056 of ErbB4 is critical for it to inhibit colony formation on plastic by human tumor cell lines. Thus, an antibody directed against this site would be useful for studying ErbB4 signaling and coupling to biological responses. Here, we demonstrate that a commercially available antibody raised against a phosphopeptide corresponding to the carboxyl-terminal domain of the ErbB4 receptor tyrosine kinase fails to exhibit appropriate specificity. Thus, this antibody does not appear to be suitable for studying ErbB4 phosphorylation or signaling.

Neuregulin3 alters cell fate in the epidermis and mammary gland

BACKGROUND

The Neuregulin family of ligands and their receptors, the Erbb tyrosine kinases, have important roles in epidermal and mammary gland development as well as during carcinogenesis. Previously, we demonstrated that Neuregulin3 (Nrg3) is a specification signal for mammary placode formation in mice. Nrg3 is a growth factor, which binds and activates Erbb4, a receptor tyrosine kinase that regulates cell proliferation and differentiation. To understand the role of Neuregulin3 in epidermal morphogenesis, we have developed a transgenic mouse model that expresses Nrg3 throughout the basal layer (progenitor/stem cell compartment) of mouse epidermis and the outer root sheath of developing hair follicles.

RESULTS

Transgenic females formed supernumerary nipples and mammary glands along and adjacent to the mammary line providing strong evidence that Nrg3 has a role in the initiation of mammary placodes along the body axis. In addition, alterations in morphogenesis and differentiation of other epidermal appendages were observed, including the hair follicles. The transgenic epidermis is hyperplastic with excessive sebaceous differentiation and shows striking similarities to mouse models in which c-Myc is activated in the basal layer including decreased expression levels of the adhesion receptors, alpha6-integrin and beta1-integrin.

CONCLUSION

These results indicate that the epidermis is sensitive to Nrg3 signaling, and that this growth factor can regulate cell fate of pluripotent epidermal cell populations including that of the mammary gland. Nrg3 appears to act, in part, by inducing c-Myc, altering the proliferation and adhesion properties of the basal epidermis, and may promote exit from the stem cell compartment. The results we describe provide significant insight into how growth factors, such as Nrg3, regulate epidermal homeostasis by influencing the balance between stem cell renewal, lineage selection and differentiation.

ErbB4 isoforms selectively regulate growth factor induced Madin-Darby canine kidney cell tubulogenesis

ErbB4, a member of the epidermal growth factor (EGF) receptor family that can be activated by heregulin beta1 and heparin binding (HB)-EGF, is expressed as alternatively spliced isoforms characterized by variant extracellular juxtamembrane (JM) and intracellular cytoplasmic (CYT) domains. ErbB4 plays a critical role in cardiac and neural development. We demonstrated that ErbB4 is expressed in the ureteric buds and developing tubules of embryonic rat kidney and in collecting ducts in adult. The predominant isoforms expressed in kidney are JM-a and CYT-2. In ErbB4-transfected MDCK II cells, basal cell proliferation and hepatocyte growth factor (HGF)-induced tubule formation were decreased by all four isoforms. Only JM-a/CYT-2 cells formed tubules upon HB-EGF stimulation. ErbB4 was activated by both HRG-beta1 and HB-EGF stimulation; however, compared with HRG-beta1, HB-EGF induced phosphorylation of the 80-kDa cytoplasmic cleavage fragment of the JM-a/CYT-2 isoform. HB-EGF also induced early activation of ERK1/2 in JM-a/CYT-2 cells and promoted nuclear translocation of the JM-a/CYT-2 cytoplasmic tail. In summary, our data indicate that JM-a/CYT-2, the ErbB4 isoform that is proteinase cleavable but does not contain a PI3K-binding domain in its cytoplasmic tail, mediates important functions in renal epithelial cells in response to HB-EGF.