Role of the ErbB-4 carboxyl terminus in gamma-secretase cleavage

The Yin and Yang of ERBB4: Tumor Suppressor and Oncoprotein

ERBB4 (HER4) is a member of the ERBB family of receptor tyrosine kinases, a family that includes the epidermal growth factor receptor (EGFR/ERBB1/HER1), ERBB2 (Neu/HER2), and ERBB3 (HER3). EGFR and ERBB2 are oncoproteins and validated targets for therapeutic intervention in a variety of solid tumors. In contrast, the role that ERBB4 plays in human malignancies is ambiguous. Thus, here we review the literature regarding ERBB4 function in human malignancies. We review the mechanisms of ERBB4 signaling with an emphasis on mechanisms of signaling specificity. In the context of this signaling specificity, we discuss the hypothesis that ERBB4 appears to function as a tumor suppressor protein and as an oncoprotein. Next, we review the literature that describes the role of ERBB4 in tumors of the bladder, liver, prostate, brain, colon, stomach, lung, bone, ovary, thyroid, hematopoietic tissues, pancreas, breast, skin, head, and neck. Whenever possible, we discuss the possibility that ERBB4 mutants function as biomarkers in these tumors. Finally, we discuss the potential roles of ERBB4 mutants in the staging of human tumors and how ERBB4 function may dictate the treatment of human tumors. SIGNIFICANCE STATEMENT: This articles reviews ERBB4 function in the context of the mechanistic model that ERBB4 homodimers function as tumor suppressors, whereas ERBB4-EGFR or ERBB4-ERBB2 heterodimers act as oncogenes. Thus, this review serves as a mechanistic framework for clinicians and scientists to consider the role of ERBB4 and ERBB4 mutants in staging and treating human tumors.

Irreversible modifications of receptor tyrosine kinases

Each group of the 56 receptor tyrosine kinases (RTK) binds with one or more soluble growth factors and coordinates a vast array of cellular functions. These outcomes are tightly regulated by inducible post-translational events, such as tyrosine phosphorylation, ubiquitination, ectodomain shedding, and regulated intramembrane proteolysis. Because of the delicate balance required for appropriate RTK function, cells may become pathogenic upon dysregulation of RTKs themselves or their post-translational covalent modifications. For example, reduced ectodomain shedding and decreased ubiquitination of the cytoplasmic region, both of which enhance growth factor signals, characterize malignant cells. Whereas receptor phosphorylation and ubiquitination are reversible, proteolytic cleavage events are irreversible, and either modification might alter the subcellular localization of RTKs. Herein, we focus on ectodomain shedding by metalloproteinases (including ADAM family proteases), cleavage within the membrane or cytoplasmic regions of RTKs (by gamma-secretases and caspases, respectively), and complete receptor proteolysis in lysosomes and proteasomes. Roles of irreversible modifications in RTK signaling, pathogenesis, and pharmacology are highlighted.

The role of ADAM17 in tumorigenesis and progression of breast cancer

A disintegrin and metalloproteinase (ADAM) family members are known to process the target membrane-bound molecules through the quick induction of their protease activities under interaction with other molecules, which have diverse roles in tissue morphogenesis and pathophysiological remodeling. Among these, ADAM17 is a membrane-bound protease that sheds the extracellular domain of various receptors or its ligands from the cell membrane and subsequently activates downstream signaling transduction pathways. Importantly, breast cancer remains a mainspring of cancer-induced death in women, and numerous regulatory pathways have been implicated in the formation of breast cancer. Substantial evidence has demonstrated that an obvious increased in ADAM17 cell surface expression has been discovered in breast cancer and was shown to be associated with mammary tumorigenesis, invasiveness, and drug resistance. Over the last decades, it has received more than its share of attention that ADAM17 plays a potential role in breast cancer, including cell proliferation, invasion, angiogenesis, apoptosis, and trastuzumab resistance. In our review, we discuss the mechanisms through which ADAM17 acts on breast cancer tumorigenesis and progression. Thus, this will provide further impetus for exploiting ADAM17 as a new target for breast cancer treatment.

Intrinsic HER4/4ICD transcriptional activation domains are required for STAT5A activated gene expression

The epidermal growth factor receptor family member HER4 undergoes proteolytic processing at the cell surface to release the HER4 intracellular domain (4ICD) nuclear protein. Interestingly, 4ICD directly interacts with STAT5 and functions as an obligate STAT5 nuclear chaperone. Once in the nucleus 4ICD binds with STAT5 at STAT5 target genes, dramatically potentiating STAT5 transcriptional activation. These observations raise the possibility that 4ICD directly coactivates STAT5 gene expression. Using both yeast and mammalian transactivation reporter assays, we performed truncations of 4ICD fused to a GAL4 DNA binding domain and identified two independent 4ICD transactivation domains located between residues 1022 and 1090 (TAD1) and 1192 and 1225 (TAD2). The ability of the 4ICD DNA binding domain fusions to transactivate reporter gene expression required deletion of the intrinsic tyrosine kinase domain. In addition, we identified the 4ICD carboxyl terminal TVV residues, a PDZ domain binding motif (PDZ-DBM), as a potent transcriptional repressor. The transactivation activity of the HER4 carboxyl terminal domain lacking the tyrosine kinase (CTD) was significantly lower than similar EGFR or HER2 CTD. However, deletion of the HER4 CTD PDZ-DBM enhanced HER4 CTD transactivation to levels equivalent to the EGFR and HER2 CTDs. To determine if 4ICD TAD1 and TAD2 have a physiologically relevant role in STAT5 transactivation, we coexpressed 4ICD or 4ICD lacking TAD2 or both TAD1 and TAD2 with STAT5 in a luciferase reporter assay. Our results demonstrate that each 4ICD TAD contributes additively to STAT5A transactivation and the ability of STAT5A to transactivate the β-casein promoter requires the 4ICD TADs. Taken together, published data and our current results demonstrate that both 4ICD nuclear chaperone and intrinsic coactivation activities are essential for STAT5 regulated gene expression.

A fast growing spectrum of biological functions of γ-secretase in development and disease

γ-secretase, which assembles as a tetrameric complex, is an aspartyl protease that proteolytically cleaves substrate proteins within their membrane-spanning domain; a process also known as regulated intramembrane proteolysis (RIP). RIP regulates signaling pathways by abrogating or releasing signaling molecules. Since the discovery, already >15 years ago, of its catalytic component, presenilin, and even much earlier with the identification of amyloid precursor protein as its first substrate, γ-secretase has been commonly associated with Alzheimer's disease. However, starting with Notch and thereafter a continuously increasing number of novel substrates, γ-secretase is becoming linked to an equally broader range of biological processes. This review presents an updated overview of the current knowledge on the diverse molecular mechanisms and signaling pathways controlled by γ-secretase, with a focus on organ development, homeostasis and dysfunction. This article is part of a Special Issue entitled: Intramembrane Proteases.

Long-term depression-inducing stimuli promote cleavage of the synaptic adhesion molecule NGL-3 through NMDA receptors, matrix metalloproteinases and presenilin/γ-secretase

Long-term depression (LTD) reduces the functional strength of excitatory synapses through mechanisms that include the removal of AMPA glutamate receptors from the postsynaptic membrane. LTD induction is also known to result in structural changes at excitatory synapses, including the shrinkage of dendritic spines. Synaptic adhesion molecules are thought to contribute to the development, function and plasticity of neuronal synapses largely through their trans-synaptic adhesions. However, little is known about how synaptic adhesion molecules are altered during LTD. We report here that NGL-3 (netrin-G ligand-3), a postsynaptic adhesion molecule that trans-synaptically interacts with the LAR family of receptor tyrosine phosphatases and intracellularly with the postsynaptic scaffolding protein PSD-95, undergoes a proteolytic cleavage process. NGL-3 cleavage is induced by NMDA treatment in cultured neurons and low-frequency stimulation in brain slices and requires the activities of NMDA glutamate receptors, matrix metalloproteinases (MMPs) and presenilin/γ-secretase. These results suggest that NGL-3 is a novel substrate of MMPs and γ-secretase and that NGL-3 cleavage may regulate synaptic adhesion during LTD.

Receptor tyrosine kinases in the nucleus

To date, 18 distinct receptor tyrosine kinases (RTKs) are reported to be trafficked from the cell surface to the nucleus in response to ligand binding or heterologous agonist exposure. In most cases, an intracellular domain (ICD) fragment of the receptor is generated at the cell surface and translocated to the nucleus, whereas for a few others the intact receptor is translocated to the nucleus. ICD fragments are generated by several mechanisms, including proteolysis, internal translation initiation, and messenger RNA (mRNA) splicing. The most prevalent mechanism is intramembrane cleavage by γ-secretase. In some cases, more than one mechanism has been reported for the nuclear localization of a specific RTK. The generation and use of RTK ICD fragments to directly communicate with the nucleus and influence gene expression parallels the production of ICD fragments by a number of non-RTK cell-surface molecules that also influence cell proliferation. This review will be focused on the individual RTKs and to a lesser extent on other growth-related cell-surface transmembrane proteins.

Landscape of EGFR signaling network in human cancers: biology and therapeutic response in relation to receptor subcellular locations

The epidermal growth factor receptor (EGFR) pathway is one of the most dysregulated molecular pathways in human cancers. Despite its well-established importance in tumor growth, progression and drug-resistant phenotype over the past several decades, targeted therapy designed to circumvent EGFR has yielded only modest clinical success in cancer patients, except those with non-small cell lung cancer (NSCLC) carrying EGFR activation mutations. However, almost all of these NSCLC patients eventually developed resistance to small molecule EGFR kinase inhibitors. These disappointing outcomes are, in part, due to the high complexity and the interactive nature of the EGFR signaling network. More recent compelling evidence further indicates that EGFR functionality can be dependent on its subcellular location. In this regard, EGFR undergoes translocation into different organelles where it elicits distinctly different functions than its best known activity as a plasma membrane-bound receptor tyrosine kinase. EGFR can be shuttled into the cell nucleus and mitochondrion upon ligand binding, radiation, EGFR-targeted therapy and other stimuli. Nuclear EGFR behaves as transcriptional regulator, tyrosine kinase, and mediator of other physiological processes. The role of mitochondrial EGFR remains poorly understood but it appears to regulate apoptosis and autophagy. While studies using patient tumors have shown nuclear EGFR to be an indicator for poor clinical outcomes in cancer patients, the impact of mitochondrial EGFR on tumor behavior and patient prognosis remains to be defined. Most recently, several lines of evidence suggest that mislocated EGFR may regulate tumor response to therapy and that plasma membrane-bound EGFR elicits survival signals independent of its kinase activity. In light of these recent progresses and discoveries, we will outline in this minireview an emerging line of research that uncovers and functionally characterizes several novel modes of EGFR signaling that take center stage in the cell nucleus, mitochondrion and other subcellular compartments. We will also discuss the clinical implications of these findings in the rationale design for therapeutic strategy that overcomes tumor drug resistance.

Targeting the EGFR signaling pathway in cancer therapy

INTRODUCTION

Cancer is a devastating disease; however, several therapeutic advances have recently been made, wherein EGFR and its family members have emerged as useful biomarkers and therapeutic targets. EGFR, a transmembrane glycoprotein is a member of the ERBB receptor tyrosine kinase superfamily. EGFR binds to its cognate ligand EGF, which further induces tyrosine phosphorylation and receptor dimerization with other family members leading to enhanced uncontrolled proliferation. Several anti-EGFR therapies such as monoclonal antibodies and tyrosine kinase inhibitors have been developed, which has enabled clinicians to identify and treat specific patient cohorts.

AREAS COVERED

This review covers the basic mechanism of EGFR activation and the role of EGFR signaling in cancer progression. Furthermore, current developments made toward targeting the EGFR signaling pathway for the treatment of epithelial cancers and a summary of the various anti-EGFR therapeutic agents that are currently in use are also presented in this review.

EXPERT OPINION

EGFR signaling is a part of a complex network that has been the target of effective cancer therapies. However, a further understanding of the system is required to develop an effective anticancer regimen. A combination therapy that comprises an anti-EGFR and a chemotherapeutic/chemopreventive agent will exhibit a multi-pronged approach that can be developed into a highly attractive and specific molecular oriented remedy.

EGF receptor exposed to oxidative stress acquires abnormal phosphorylation and aberrant activated conformation that impairs canonical dimerization

Crystallographic studies have offered understanding of how receptor tyrosine kinases from the ErbB family are regulated by their growth factor ligands. A conformational change of the EGFR (ErbB1) was shown to occur upon ligand binding, where a solely ligand-mediated mode of dimerization/activation was documented. However, this dogma of dimerization/activation was revolutionized by the discovery of constitutively active ligand-independent EGFR mutants. In addition, other ligand-independent activation mechanisms may occur. We have shown that oxidative stress (ox-stress), induced by hydrogen peroxide or cigarette smoke, activates EGFR differently than its ligand, EGF, thereby inducing aberrant phosphorylation and impaired trafficking and degradation of EGFR. Here we demonstrate that ox-stress activation of EGFR is ligand-independent, does not induce "classical" receptor dimerization and is not inhibited by the tyrosine kinase inhibitor AG1478. Thus, an unprecedented, apparently activated, state is found for EGFR under ox-stress. Furthermore, this activation mechanism is temperature-dependent, suggesting the simultaneous involvement of membrane structure. We propose that ceramide increase under ox-stress disrupts cholesterol-enriched rafts leading to EGFR re-localization into the rigid, ceramide-enriched rafts. This increase in ceramide also supports EGFR aberrant trafficking to a peri-nuclear region. Therefore, the EGFR unprecedented and activated conformation could be sustained by simultaneous alterations in membrane structure under ox-stress.

Estrogen-induced upregulation of Sftpb requires transcriptional control of neuregulin receptor ErbB4 in mouse lung type II epithelial cells

Estrogen is known for its positive stimulatory effects on surfactant proteins. ErbB4 receptor and its ligand neuregulin (NRG) positively stimulate lung development. ErbB receptors interact with nuclear receptors and ErbB4 co-regulates estrogen receptor (ER)α expression in breast cells. ERβ is highly expressed in pneumocytes and its deletion leads to fewer alveoli and reduced elastic recoil. A similar picture was seen in ErbB4-deleted lungs. We hypothesized that estrogen signals its effect on surfactant protein B (Sftpb) expression through interactions of ERβ and ErbB4. Estrogen and NRG treatment decreased cell numbers and stimulated Sftpb expression in type II cells. Estrogen and NRG both stimulated phosphorylation of ERβ and co-localization of both receptors. Overexpression of ERβ increased the cell number and Sftpb expression, which was further augmented by estrogen and NRG. Finally, estrogen and NRG stimulated ERβ and ErbB4 binding to the Sftpb promoter. Overexpression of these receptors stimulated Sftpb promoter activation, which was further enhanced by estrogen and NRG. The stimulatory effect of estrogen and NRG was abolished in ErbB4 deletion and reconstituted by re-expression of full-length ErbB4 in fetal ErbB4-deleted type II cells. Estrogen-induced nuclear translocation of ErbB4 required the intact γ-secretase cleavage site but not the nuclear localization sequence of the ErbB4 receptor, suggesting that ERβ might function as a nuclear chaperone for ErbB4. These studies demonstrate that estrogen effects on Sftpb expression require an interaction of ERβ and ErbB4. We speculate that the stimulatory effects of estrogen on Sftpb are under transcriptional control of ErbB4.

Decorin, erythroblastic leukaemia viral oncogene homologue B4 and signal transducer and activator of transcription 3 regulation of semaphorin 3A in central nervous system scar tissue

Scar tissue at sites of traumatic injury in the adult central nervous system presents a combined physical and molecular impediment to axon regeneration. Of multiple known central nervous system scar associated axon growth inhibitors, semaphorin 3A has been shown to be strongly expressed by invading leptomeningeal fibroblasts. We have previously demonstrated that infusion of the small leucine-rich proteoglycan decorin results in major suppression of several growth inhibitory chondroitin sulphate proteoglycans and growth of adult sensory axons across acute spinal cord injuries. Furthermore, decorin treatment of leptomeningeal fibroblasts significantly increases their ability to support neurite growth of co-cultured adult dorsal root ganglion neurons. In the present study we show that decorin has the ability to suppress semaphorin 3A expression within adult rat cerebral cortex scar tissue and in primary leptomeningeal fibroblasts in vitro. Infusion of decorin core protein for eight days resulted in a significant reduction of semaphorin 3A messenger RNA expression within injury sites compared with saline-treated control animals. Both in situ hybridization and immunostaining confirmed that semaphorin 3A messenger RNA expression and protein levels are significantly reduced in decorin-treated animals. Similarly, decorin treatment decreased the expression of semaphorin 3A messenger RNA in cultured rat leptomeningeal fibroblasts compared with untreated cells. Mechanistic studies revealed that decorin-mediated suppression of semaphorin 3A critically depends on erythroblastic leukaemia viral oncogene homologue B4 and signal transducer and activator of transcription 3 function. Collectively, our studies show that in addition to suppressing the levels of inhibitory chondroitin sulphate proteoglycans, decorin has the ability to suppress semaphorin 3A in the injured central nervous system. Our findings provide further evidence for the use of decorin as a potential therapy for promoting axonal growth and repair in the injured adult mammalian brain and spinal cord.

Interactions of ErbB4 and Kap1 connect the growth factor and DNA damage response pathways

ErbB4 is unusual among receptor tyrosine kinases because some isoforms can be efficiently cleaved at the plasma membrane to release a soluble intracellular domain. The cleavage product has high kinase activity and homes to the nucleus. A screen for proteins that associate with the ErbB4 intracellular domain identified candidate interactors including ITCH, WWP2, Nucleolin, and Krab-associated protein 1 (Kap1). Kap1 binds to multiple isoforms of ErbB4 but does not require ErbB4 kinase activity for binding, nor is it an ErbB4 substrate. Kap1 reduces ERBB4 transcription and either directly or indirectly modulates the expression of genes that are themselves regulated by ErbB4. Upregulation of ErbB4 and suppression of MDM2 jointly enhance and accelerate the accumulation of p21(CIP1) in response to DNA damage. Overall, these findings further substantiate the role of ErbB4 in conjoint regulation of growth factor signaling and DNA damage responses.

Nuclear localization of orphan receptor protein kinase (Ror1) is mediated through the juxtamembrane domain

Background

Several receptor tyrosine kinases (RTKs) such as EGFR, FGFR, TRK, and VEGFR are capable of localizing in the cell nucleus in addition to their usual plasma membrane localization. Recent reports also demonstrate that nuclear-localized RTKs have important cellular functions such as transcriptional activation. On the basis of preliminary bioinformatic analysis, additional RTKs, including receptor tyrosine kinase-like orphan receptor 1 (Ror1) were predicted to have the potential for nuclear subcellular localization. Ror1 is a receptor protein tyrosine kinase that modulates neurite growth in the central nervous system. Because the nuclear localization capability of the Ror1 cytoplasmic domain has not been reported, we examined the cellular expression distribution of this region.

Results

The Ror1 cytoplasmic region was amplified and cloned into reporter constructs with fluorescent tags. Following transfection, the nuclear distribution patterns of transiently expressed fusion proteins were observed. Serial deletion constructs were then used to map the juxtamembrane domain of Ror1 (aa_471-513) for this nuclear translocation activity. Further site-directed mutagenesis suggested that a KxxK-16 aa-KxxK sequence at residues 486-509 is responsible for the nuclear translocation interaction. Subsequent immunofluorescence analysis by cotransfection of Ran and Ror1 implied that the nuclear translocation event of Ror1 might be mediated through the Ran pathway.

Conclusions

We have predicted several RTKs that contain the nuclear localization signals. This is the first report to suggest that the juxtamembrane domain of the Ror1 cytoplasmic region mediates the translocation event. Ran GTPase is also implicated in this event. Our study might be beneficial in future research to understand the Ror1 biological signaling pathway.

The role of protease activity in ErbB biology

Proteases are now recognized as having an active role in a variety of processes aside from their recognized metabolic role in protein degradation. Within the ErbB system of ligands and receptors, proteases are known to be necessary for the generation of soluble ligands from transmembrane precursors and for the processing of the ErbB4 receptor, such that its intracellular domain is translocated to the nucleus. There are two protease activities involved in the events: proteases that cleave within the ectodomain of ligand (or receptor) and proteases that cleave the substrate within the transmembrane domain. The former are the ADAM proteases and the latter are the gamma-secretase complex and the rhomboid proteases. This review discusses the roles of each of these protease systems within the ErbB system.

Histopathological and molecular heterogeneity among individuals with dementia associated with Presenilin mutations

Background

Mutations in the presenilin (PSEN) genes are associated with early-onset familial Alzheimer's disease (FAD). Biochemical characterizations and comparisons have revealed that many PSEN mutations alter γ-secretase activity to promote accumulation of toxic Aβ42 peptides. In this study, we compared the histopathologic and biochemical profiles of ten FAD cases expressing independent PSEN mutations and determined the degradation patterns of amyloid-β precursor protein (AβPP), Notch, N-cadherin and Erb-B4 by γ-secretase. In addition, the levels of Aβ40/42 peptides were quantified by ELISA.

Results

We observed a wide variation in type, number and distribution of amyloid deposits and neurofibrillary tangles. Four of the ten cases examined exhibited a substantial enrichment in the relative proportions of Aβ40 over Aβ42. The AβPP N-terminal and C-terminal fragments and Tau species, assessed by Western blots and scanning densitometry, also demonstrated a wide variation. The Notch-1 intracellular domain was negligible by Western blotting in seven PSEN cases. There was significant N-cadherin and Erb-B4 peptide heterogeneity among the different PSEN mutations.

Conclusion

These observations imply that missense mutations in PSEN genes can alter a range of key γ-secretase activities to produce an array of subtly different biochemical, neuropathological and clinical manifestations. Beyond the broad common features of dementia, plaques and tangles, the various PSEN mutations resulted in a wide heterogeneity and complexity and differed from sporadic AD.

Altered growth factor signaling pathways as the basis of aberrant stem cell maturation in schizophrenia

In recent years evidence has accumulated that the activity of the signaling cascades of Neuregulin-1, Wnt, TGF-beta, BDNF-p75 and DISC1 is different between control subjects and patients with schizophrenia. These pathways are involved in embryonic and adult neurogenesis and neuronal maturation. A review of the clinical data indicates that in schizophrenia the Wnt pathway is most likely hypoactive, whereas the Nrg1-ErbB4, the TGF-beta- and the BDNF-p75-pathways are hyperactive. Haplo-insuffiency of the DISC1 gene is currently the best established schizophrenia risk factor. Preclinical experiments indicate that suppression of DISC1 signaling leads to accelerated dendrite development in neuronal stem cells, accelerated migration and aberrant integration into the neuronal network. Other preclinical experiments show that increasing NRG1-, BDNF- and TGF-beta signaling and decreasing Wnt signaling, also promotes adult neuronal differentiation and migration. Thus deviations in these pathways detected in schizophrenia could contribute to premature neuronal differentiation, accelerated migration and inappropriate insertion into the neuronal network. Initial clinical findings are confirmatory: neuronal stem cells isolated from nasal biopsies from schizophrenia patients display signs of accelerated development, whilst increased erosion of telomeres and bone age provide further support for accelerated cell maturation in schizophrenia.

Trafficking of receptor tyrosine kinases to the nucleus

It has been known for at least 20 years that growth factors induce the internalization of cognate receptor tyrosine kinases (RTKs). The internalized receptors are then sorted to lysosomes or recycled to the cell surface. More recently, data have been published to indicate other intracellular destinations for the internalized RTKs. These include the nucleus, mitochondria, and cytoplasm. Also, it is recognized that trafficking to these novel destinations involves new biochemical mechanisms, such as proteolytic processing or interaction with translocons, and that these trafficking events have a function in signal transduction, implicating the receptor itself as a signaling element between the cell surface and the nucleus.

Prolactin and ErbB4/HER4 signaling interact via Janus kinase 2 to induce mammary epithelial cell gene expression differentiation

Differentiation of mammary epithelium in vivo requires signaling through prolactin and ErbB4/HER4-dependent mechanisms. Although stimulation of either the prolactin receptor or ErbB4/HER4 results in activation of the transcription factor signal transducer and activator of transcription 5A (STAT5A) and induction of lactogenic differentiation, how these pathways intersect is unknown. We show herein that prolactin signaling in breast cells cooperates with and is substantially enhanced by the receptor tyrosine kinase ErbB4/HER4. Prolactin and the ErbB4/HER4 ligand heparin-binding epidermal growth factor each induced STAT5A tyrosine phosphorylation and nuclear translocation; each pathway required the intracellular tyrosine kinase Janus kinase 2 (JAK2). We found that full prolactin-mediated STAT5A activation and binding to the endogenous beta-casein promoter required ErbB4/HER4 but did not require ErbB1/epidermal growth factor receptor. For example, prolactin-induced STAT5A activity was markedly diminished in cells overexpressing kinase inactive HER4, in cells transfected with small interfering RNAs to specifically knock down endogenous ErbB4/HER4 expression and in cells treated with a small molecule inhibitor that targets ErbB4 kinase. Interestingly, prolactin caused ErbB4/HER4 tyrosine phosphorylation in a JAK2 kinase-dependent manner. Finally, prolactin receptor, ErbB4/HER4, and JAK2 were coimmunoprecipitated from prolactin-treated but not untreated cells. These results suggest that prolactin signaling engages the ErbB4 pathway via JAK2 and that ErbB4 provides an important component of STAT5A-dependent lactogenic differentiation; this pathway integration may help explain the similar deficit in mammary development observed in gene-targeted mice deficient in prolactin receptor, JAK2, ErbB4, or STAT5A.

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.

HER4 D-box sequences regulate mitotic progression and degradation of the nuclear HER4 cleavage product s80HER4

Heregulin-mediated activation of HER4 initiates receptor cleavage (releasing an 80-kDa HER4 intracellular domain, s80(HER4), containing nuclear localization sequences) and results in G(2)-M delay by unknown signaling mechanisms. We report herein that s80(HER4) contains a functional cyclin B-like sequence known as a D-box, which targets proteins for degradation by anaphase-promoting complex (APC)/cyclosome, a multisubunit ubiquitin ligase. s80(HER4) ubiquitination and proteasomal degradation occurred during mitosis but not during S phase. Inhibition of an APC subunit (APC2) using short interfering RNA knockdown impaired s80(HER4) degradation. Mutation of the s80(HER4) D-box sequence stabilized s80(HER4) during mitosis, and s80(HER4)-dependent growth inhibition via G(2)-M delay was significantly greater with the D-box mutant. Polyomavirus middle T antigen-transformed HC11 cells expressing s80(HER4) resulted in smaller, less proliferative, more differentiated tumors in vivo than those expressing kinase-dead s80(HER4) or the empty vector. Cells expressing s80(HER4) with a disrupted D-box did not form tumors, instead forming differentiated ductal structures. These results suggest that cell cycle-dependent degradation of s80(HER4) limits its growth-inhibitory action, and stabilization of s80(HER4) enhances tumor suppression, thus providing a link between HER4-mediated growth inhibition and cell cycle control.

Targeting signal transduction in pancreatic cancer treatment

Pancreatic cancer is a lethal disease with a 5-year survival rate of 4%. The only opportunity for improved survival continues to be complete surgical resection for those with localized disease. Although chemotherapeutic options are limited for the few patients with resectable disease, this problem is even more magnified in the majority (85%) of patients with unresectable or metastastic disease. Therefore, there is an urgent need for improved therapeutic options. The recent success of inhibitors of signal transduction for the treatment of other cancers supports the need to identify and validate aberrant signaling pathways important for pancreatic tumor growth. This review focuses on the validation of specific signaling networks and the present status of inhibitors of these pathways as therapeutic approaches for pancreatic cancer treatment.

Differential nuclear localization and kinase activity of alternative ErbB4 intracellular domains

Cleavable isoforms of the ErbB4 receptor tyrosine kinase release a soluble intracellular domain (ICD) that may translocate to the nucleus and regulate signaling. However, ErbB4 gene is alternatively spliced generating CYT-1 and CYT-2 isoforms with different cytoplasmic tails. Here, we addressed whether the two alternative ErbB4 ICDs of either CYT-1 (ICD1) or CYT-2 (ICD2) type differ in signaling to the nucleus. Confocal microscopy and extraction of nuclear cell fractions indicated that significantly more ICD2 translocated to the nuclei when compared to ICD1. Unlike the membrane-anchored 80 kDa fragments derived from full-length ErbB4 isoforms, the two ICDs did not differ from each other in metabolic stability or ubiquitylation. However, ICD2 was phosphorylated at tyrosine residues to a higher extent and demonstrated greater in vitro kinase activity than ICD1. Mutating the ATP-binding site within ICD2 kinase domain (ICD2 K751R) blocked its tyrosine phosphorylation and significantly reduced its nuclear translocation. When expressed in the context of full-length ErbB4, ICD2 was also more efficient than ICD1 in promoting transcriptional activation of the STAT5 target gene beta-casein. These findings indicate that the two alternative ICDs of ErbB4 differ in their nuclear accumulation, and that the mechanism involves differential kinase activity but not ubiquitin-regulated ICD stability.

Amyloid Precursor Protein and Presenilin1 Interact with the Adaptor GRB2 and Modulate ERK 1,2 Signaling

The amyloid precursor protein (APP) and the presenilins 1 and 2 are genetically linked to the development of familial Alzheimer disease. APP is a single-pass transmembrane protein and precursor of fibrillar and toxic amyloid-beta peptides, which are considered responsible for Alzheimer disease neurodegeneration. Presenilins are multipass membrane proteins, involved in the enzymatic cleavage of APP and other signaling receptors and transducers. The role of APP and presenilins in Alzheimer disease development seems to be related to the formation of amyloid-beta peptides; however, their physiological function, reciprocal interaction, and molecular mechanisms leading to neurodegeneration are unclear. APP and presenilins are also involved in multiple interactions with intracellular proteins, the significance of which is under investigation. Among the different APP-interacting proteins, we focused our interest on the GRB2 adaptor protein, which connects cell surface receptors to intracellular signaling pathways. In this study we provide evidence by co-immunoprecipitation experiments, confocal and electron microscopy, and by fluorescence resonance energy transfer experiments that both APP and presenilin1 interact with GRB2 in vesicular structures at the centrosome of the cell. The final target for these interactions is ERK1,2, which is activated in mitotic centrosomes in a PS1- and APP-dependent manner. These data suggest that both APP and presenilin1 can be part of a common signaling pathway that regulates ERK1,2 and the cell cycle.

Widespread expression of ErbB2, ErbB3 and ErbB4 in non-human primate brain

Neuregulin (NRG) signaling proteins interact with ErbB receptors leading to the proliferation, differentiation and migration of neurons and glia in the developing brain. NRG-1/ErbB4 are susceptibility genes for schizophrenia, yet little is known about the neuroanatomical expression of ErbB receptors specifically in primates. We find widespread expression of ErbB2, ErbB3 and ErbB4 receptor mRNAs throughout the telencephalon of juvenile and adult monkeys with in situ hybridization, with ErbB2 and ErbB4 mRNA more abundant than ErbB3 mRNA. ErbB2 and ErbB4 mRNA are expressed at higher levels in grey matter compared to white matter, whereas ErbB3 mRNA is expressed at low levels in both grey and white matter. We also characterized ErbB protein expression with immunoblotting and immunohistochemistry. In frontal cortex, ErbB2, ErbB3 and ErbB4 antibodies immunostained neuronal soma and nuclei. The ErbB2 antibody also immunostained glia at the pial surface. Within white matter, ErbB3 and ErbB4 proteins were localized to putative interstitial white matter neurons while ErbB2 protein was found in glia. Western blotting revealed immunopositive bands at approximately 180-200 kDa for each ErbB, which is consistent with the size of full-length ErbBs. Smaller immunopositive bands were also identified for each ErbB receptor in whole brain homogenates and separate cytoplasmic and nuclear extracts suggesting nuclear ErbB-back-signaling capacity in the brain. The ubiquitous expression of ErbB receptors indicates that many cell populations throughout the brain of juvenile and adult primates have the potential to respond to NRG-1 in a variety of ways.

Presenilin/gamma-secretase and alpha-secretase-like peptidases cleave human MHC Class I proteins

HLA (human leucocyte antigen)-A2 is an MHC Class I protein with primary functions in T-cell development and initi-ation of immune cell responses. MHC I proteins also play roles in intercellular adhesion, apoptosis, cell proliferation and neuronal plasticity. By utilizing a sequence comparison analysis, we recently identified HLA-A2 as a potential substrate for the Alzheimer's disease-associated PS1 (presenilin 1)/gamma-secretase. alpha-Secretase-like membrane metalloproteinases are responsible for an initial shedding event, partially mediated by ADAM (a disinteg-rin and metalloproteinase)-10. Accordingly, activation or inhibition of alpha-secretase-like membrane metalloproteinases directly modulated levels of a 14 kDa HLA-A2 CTF (C-terminal frag-ment) in CHO (Chinese-hamster ovary) cells. To show that the HLA-A2 CTF is subsequently cleaved by PS1/gamma-secretase, we re-duced its activity in cell lines stably expressing HLA-A2 and in Jurkat T-cells expressing endogenous MHC I. Treatment with specific PS1/gamma-secretase inhibitors or expression of a dominant-negative construct led to a significant accumulation of HLA-A2 CTFs. We also identified the PS1/gamma-secretase cleavage product of HLA-A2 CTF, termed HLA-A2 intracellular domain, in cell-free and cell-based experiments. In the absence of proteasome inhibitors, HLA-A2 intracellular domain underwent rapid degrad-ation. These data indicate that MHC I proteins undergo extra-cellular domain cleavage mediated by alpha-secretases and the cleavage product is subsequently cleaved by PS1/gamma-secretase.

EGFR signaling pathway in breast cancers: from traditional signal transduction to direct nuclear translocalization

Aberrant epidermal growth factor receptor (EGFR) signaling is a major characteristic of many human malignancies including breast cancer. Since the discovery of EGF in 1960's and its receptor in 1980's, our understanding of the EGF/EGFR pathway has been significantly advanced and consequently, EGFR is considered as a major oncogenic factor and an attractive therapeutic target. The well-established traditional function of EGFR is known to transmit extra-cellular mitogenic signals, such as EGF and transforming growth factor-alpha (TGF-alpha), through activating a number of downstream signaling cascades. These include signaling modules that involve phospholipase C-gamma, Ras, and phosphatidylinositol-3 kinase (PI-3K). In cancer cells, the common outcomes following the activation of the EGFR-mediated downstream pathways are altered gene activities, leading to un-controlled tumor proliferation and apoptosis. Interestingly, emerging evidences suggest the existence of a direct mode of the EGFR pathway that is distinct from the traditional transduction pathway. This new mode of EGFR signaling involves cellular transport of EGFR from the cell-surface to the cell nucleus, association of nuclear EGFR complex with gene promoters, and transcriptional regulation of the target genes. Although the nature and pathological consequences of the nuclear EGFR pathway remain elusive, accumulating evidences suggest its association with increased tumor cell proliferation and poor survival rate in breast cancer patients. While several anti-EGFR agents are being tested in breast cancer patients clinically and others under pre-clinical development, a better understanding of the traditional and the nuclear EGFR pathways will facilitate the identification of patients that are likely to respond to these agents as well as future development of more effective anti-EGFR therapeutic interventions.

Nuclear trafficking of secreted factors and cell-surface receptors: new pathways to regulate cell proliferation and differentiation, and involvement in cancers

Secreted factors and cell surface receptors can be internalized by endocytosis and translocated to the cytoplasm. Instead of being recycled or proteolysed, they sometimes translocate to the nucleus. Nuclear import generally involves a nuclear localization signal contained either in the secreted factor or its transmembrane receptor, that is recognized by the importins machinery. In the nucleus, these molecules regulate transcription of specific target genes by direct binding to transcription factors or general coregulators. In addition to the transcription regulation, nuclear secreted proteins and receptors seem to be involved in other important processes for cell life and cellular integrity such as DNA replication, DNA repair and RNA metabolism.

Nuclear secreted proteins and transmembrane receptors now appear to induce new signaling pathways to regulate cell proliferation and differentiation. Their nuclear localization is often transient, appearing only during certain phases of the cell cycle. Nuclear secreted and transmembrane molecules regulate the proliferation and differentiation of a large panel of cell types during embryogenesis and adulthood and are also potentially involved in wound healing. Secreted factors such as CCN proteins, EGF, FGFs and their receptors are often detected in the nucleus of cancer cells. Nuclear localization of these molecules has been correlated with tumor progression and poor prognosis for patient survival. Nuclear growth factors and receptors may be responsible for resistance to radiotherapy.

Nuclear-cytoplasmic transport of EGFR involves receptor endocytosis, importin beta1 and CRM1

Many receptor tyrosine kinases (RTKs) can be detected in the cell nucleus, such as EGFR, HER-2, HER-3, HER-4, and fibroblast growth factor receptor. EGFR, HER-2 and HER-4 contain transactivational activity and function as transcription co-factors to activate gene promoters. High EGFR in tumor nuclei correlates with increased tumor proliferation and poor survival in cancer patients. However, the mechanism by which cell-surface EGFR translocates into the cell nucleus remains largely unknown. Here, we found that EGFR co-localizes and interacts with importins alpha1/beta1, carriers that are critical for macromolecules nuclear import. EGFR variant mutated at the nuclear localization signal (NLS) is defective in associating with importins and in entering the nuclei indicating that EGFR's NLS is critical for EGFR/importins interaction and EGFR nuclear import. Moreover, disruption of receptor internalization process using chemicals and forced expression of dominant-negative Dynamin II mutant suppressed nuclear entry of EGFR. Additional evidences suggest an involvement of endosomal sorting machinery in EGFR nuclear translocalization. Finally, we found that nuclear export of EGFR may involve CRM1 exportin as we detected EGFR/CRM1 interaction and markedly increased nuclear EGFR following exposure to leptomycin B, a CRM1 inhibitor. Collectively, these data suggest the importance of receptor endocytosis, endosomal sorting machinery, interaction with importins alpha1/beta1, and exportin CRM1 in EGFR nuclear-cytoplasmic trafficking. Together, our work sheds light into the nature and regulation of the nuclear EGFR pathway and provides a plausible mechanism by which cells shuttle cell-surface EGFR and potentially other RTKs through the nuclear pore complex and into the nuclear compartment.

The intracellular domain of ErbB4 induces differentiation of mammary epithelial cells

Differentiation of mammary epithelium in vivo requires signaling through prolactin- and ErbB4/HER4-dependent mechanisms; how these pathways intersect is unknown. We show herein that HC11 mouse mammary cells undergo ErbB4-dependent lactational differentiation. Prolactin and the ErbB4 ligand HB-EGF each induced STAT5A activation, expression of lactogenic differentiation markers, and lumen formation in three-dimensional Matrigel cultures in HC11 cells. ErbB4 undergoes ligand-dependent transmembrane domain cleavage at Val-675, releasing a soluble 80-kDa intracellular domain (s80(HER4)) that localizes to nuclei; the physiological relevance of s80(HER4) is unknown. A HER4(V675A) mutant abolishing transmembrane cleavage impaired STAT5A activity, lactogenic gene expression, and lumen formation. Kinase-dead HER4(KD) was neither cleaved nor able to induce differentiation of HC11 cells. Without treating HC11 cells with prolactin or HB-EGF, s80(HER4) (expressed from a cDNA construct) localized to the nucleus, activated STAT5A, and induced three-dimensional lumen formation. Nuclear localization of exogenous s80(HER4) required intact kinase activity of s80(HER4), as did activation of STAT5A. In contrast, nuclear localization of s80(HER4) and STAT5A activation did not require the 16-amino acid region of the ErbB4 intracellular domain specific to the Cyt-1 isoform of ErbB4, and absent in the Cyt-2 isoform. These results suggest that s80(HER4) formation contributes to ErbB4-dependent differentiation of mammary epithelial cells.

Presenilin-dependent intramembrane cleavage of ephrin-B1

Background

Presenilin-dependent γ-secretase cleavage of several transmembrane proteins, including amyloid-β precursor protein and Notch, mediates the intramembrane proteolysis to liberate their intracellular domains that are involved in cellular signaling. Considering γ-secretase inhibitors as therapeutics for Alzheimer's disease, understanding the physiologically and biologically important substrate for γ-secretase activity in brains is emerging issue. To elucidate the molecular mechanism and physiological role of γ-secretase, we screened candidate molecules for γ-secretase substrates.

Results

We show that ephrin-B1, that participates in cell-cell repulsive and attractive signaling together with its Eph receptor, constitutively undergoes ectodomain shedding and that the residual membrane-tethered fragment is sequentially cleaved by γ-secretase to release the intracellular domain. Furthermore, overexpression of membrane-tethered ephrin-B1 caused protrusion of numerous cellular processes consisted of F-actin, that required the preservation of the most C-terminal region of ephrin-B1. In contrast, soluble intracellular domain translocated into the nucleus and had no effect on cell morphology.

Conclusion

Our findings suggest that ephrin-B is a genuine substrate for γ-secretase and regulates the cytoskeletal dynamics through intramembrane proteolysis.

Nuclear EGFR signalling network in cancers: linking EGFR pathway to cell cycle progression, nitric oxide pathway and patient survival

Emerging evidences suggest the existence of a new mode of epidermal growth factor receptor (EGFR) signalling pathway in which activated EGFR undergoes nuclear translocalization and subsequently regulates gene expression and potentially mediates other cellular processes. This signalling route is distinct from the better-characterized, traditional EGFR pathway that involves transduction of mitogenic signals through activation of multiple signalling cascades. Transcriptional activity of nuclear EGFR appears to depend on its C-terminal transactivation domain and its physical and functional interaction with other transcription factors that contain DNA-binding activity. Likely via its ability to upregulate gene expression, nuclear EGFR pathway is associated with major characteristics of more aggressive tumours: increased proliferative potential, nitric oxide synthesis, and accelerated G1/S cell cycle progression. A role of nuclear EGFR in prognostic prediction is further suggested in patients with breast carcinomas and oropharyngeal squamous cell carcinomas. It is noted that significant advances were made towards the knowledge of the nuclear EGFR pathway; however, many aspects of this new pathway remain unresolved and will be discussed in this review. As a number of other receptor tyrosine kinases (RTKs) and cytokine receptors also undergo similar nuclear translocalization, a better understanding of the physiological and malignant nature of the nuclear EGFR pathway will likely shed light into the biology of cancer with nuclear RTKs.

The involvement of ErbB4 with schizophrenia: association and expression studies

Neuregulin 1 (NRG1) has been found to be associated with schizophrenia in several populations. Consistently, mutant mice heterozygous for either NRG1 or its receptor, ErbB4, show a behavioral phenotype that overlaps with mouse models for schizophrenia. These observations raised the hypothesis that impaired NRG1-ErbB4 signaling may contribute to schizophrenia susceptibility. Nineteen SNPs encompassing the ErbB4 gene were selected from the HapMap database and genotyped in genomic DNA isolated from 59 Ashkenazi schizophrenia patients and 130 matched controls. Expression analysis of ErbB4 splice variants was performed on postmortem DLPFC samples obtained from Caucasian patients and controls by real-time PCR. We found a highly significant difference between patient and control groups in three SNPs from one linkage disequilibrium (LD) block both in allele (P = 0.013, 0.0045, 0.0049) and genotype frequencies (P = 0.00013, 0.000021, 0.00018), as well as a risk haplotype (P = 0.00044). Expression analysis indicated that the CYT-1 isoform is overexpressed in patients (P = 0.047) and that juxtamembrane (JM)-a displays a similar trend (P = 0.081). This study provides a direct link between ErbB4 and the disease. We propose that NRG1 and its receptor ErbB4 are components of a biological pathway, involved in the pathophysiology of schizophrenia.

Cellular stress reactions as putative cholinergic links in Alzheimer's disease

Alzheimer's disease involves normal cellular aging and chronic cellular stress events, leading to interrelated changes in gene expression and subsequent neurodegeneration. Premature death of cholinergic neurons and the formation of amyloid fibrils separately initiated the cholinergic and amyloid hypotheses of Alzheimer's disease. Here, we present evidence to the fact that these two distinct phenomena both associate with specific changes in acetylcholinesterase (AChE) gene expression within cholinergic neurons. For example, calcium misregulation promotes aberrant transcription and pro-apoptotic events, as well as AChE-induced modifications in cellular signal cascades. These reciprocally intercept with AChE regulation at the Endoplasmic Reticulum, modifying AChE gene expression, folding and signaling. Altered AChE properties may reflect changes in the enzymatic and/or non-enzymatic features of the multiple AChE splice variants. Under chronic cellular stress, aberrant AChE regulation may thus facilitate apoptotic pathways, promoting plaque formation, cognitive impairments and degeneration of cholinergic nerve cells.

Shedding and gamma-secretase-mediated intramembrane proteolysis of the mucin-type molecule CD43

CD43 is a transmembrane molecule that contains a 123-aminoacids-long cytoplasmic tail and a highly O-glycosylated extracellular domain of mucin type. Endogenous CD43 expressed in COLO 205, K562 and Jurkat cells revealed a membrane-associated, 20 kDa CD43-specific cytoplasmic tail fragment (CD43-CTF) upon inhibition of gamma-secretase. This fragment was formed by an extracellular cleavage, as it was not accumulated after treating cells with 1,10-phenanthroline, a metalloprotease inhibitor. When CD43 was transfected into HEK-293 cells expressing dominant-negative PS1 (presenilin-1), the CD43-CTF was accumulated, but not in cells with wild-type PS1. Owing to its accumulation in the presence of a non-functional PS variant, it may thus be a novel gamma-secretase substrate. This CTF is formed by an extracellular cleavage close to the membrane, is a fragment that can be concluded to be a substrate for gamma-secretase. However, the intracellular gamma-secretase product has not been possible to detect, suggesting a quick processing of this product. During normal growth the CTF was not found without gamma-secretase inhibition, but when the cells (COLO 205) were very confluent the fragment could be detected. The intracellular domain of CD43 has previously been shown to contain a functional nuclear localization signal, and has been suggested to be involved in gene activation. From this and the present results, a novel way to explain how mucin-type molecules may transduce intracellular signals can be proposed.

A novel transmembrane topology of presenilin based on reconciling experimental and computational evidence

The transmembrane topology of presenilins is still the subject of debate despite many experimental topology studies using antibodies or gene fusions. The results from these studies are partly contradictory and consequently several topology models have been proposed. Studies of presenilin-interacting proteins have produced further contradiction, primarily regarding the location of the C-terminus. It is thus impossible to produce a topology model that agrees with all published data on presenilin. We have analyzed the presenilin topology through computational sequence analysis of the presenilin family and the homologous presenilin-like protein family. Members of these families are intramembrane-cleaving aspartyl proteases. Although the overall sequence homology between the two families is low, they share the conserved putative active site residues and the conserved 'PAL' motif. Therefore, the topology model for the presenilin-like proteins can give some clues about the presenilin topology. Here we propose a novel nine-transmembrane topology with the C-terminus in the extracytosolic space. This model has strong support from published data on gamma-secretase function and presenilin topology. Contrary to most presenilin topology models, we show that hydrophobic region X is probably a transmembrane segment. Consequently, the C-terminus would be located in the extracytosolic space. However, the last C-terminal amino acids are relatively hydrophobic and in conjunction with existing experimental data we cannot exclude the possibility that the extreme C-terminus could be buried within the gamma-secretase complex. This might explain the difficulties in obtaining consistent experimental evidence regarding the location of the C-terminal region of presenilin.

Got RIP? Presenilin-dependent intramembrane proteolysis in growth factor receptor signaling

A number of cell surface growth factor receptors are subject to presenilin-dependent regulated intramembrane proteolysis (PS-RIP) after ligand binding and/or ectodomain cleavage. PS-RIP is mediated by a highly conserved multi-component membrane-bound protease, termed gamma-secretase, responsible for generating Alzheimer's disease (AD)-associated Abeta peptide from its membrane-bound beta-amyloid precursor protein (APP), as well as for cleaving a number of other type-I membrane receptors. PS-RIP is a conserved cellular process by which cells transmit signals from one compartment to another, including the liberation of membrane-bound transcription factors. Recent studies indicate that PS-RIP also mediates the proteolytic inactivation of heteromeric receptor complexes by removing the transmembrane domains required for receptor-receptor interaction. Thus, PS-RIP appears to regulate diverse cellular pathways either by generating soluble effectors from membrane-bound precursors, or by removing the transmembrane domain of a membrane-tethered signaling component.

Presenilin-dependent gamma-secretase processing regulates multiple ERBB4/HER4 activities

Transmembrane receptors typically transmit cellular signals following growth factor stimulation by coupling to and activating downstream signaling cascades. Reports of proteolytic processing of cell surface receptors to release an intracellular domain (ICD) has raised the possibility of novel signaling mechanisms directly mediated by the receptor ICD. The receptor tyrosine kinase ERBB4/HER4 (referred to here as ERBB4) undergoes sequential processing by tumor necrosis factor-alpha converting enzyme and presenilin-dependent gamma-secretase to release the ERBB4 ICD (4ICD). Our recent data suggests that regulation of gene expression by the ERBB4 nuclear protein and the proapoptotic activity of ERBB4 involves the gamma-secretase release of 4ICD. To determine the role gamma-secretase processing plays in ERBB4 signaling, we generated an ERBB4 allele with the transmembrane residue substitution V673I (ERBB4-V673I). We demonstrate that ERBB4-V673I fails to undergo processing by gamma-secretase but retains normal cell surface signaling activity. In contrast to wild-type ERBB4, however, ERBB4-V673I was excluded from the nuclei of transfected cells and failed to activate STAT5A stimulation of the beta-casein promoter. These results support the contention that gamma-secretase processing of ERBB4 is necessary to release a functional 4ICD nuclear protein which directly regulates gene expression. We also demonstrate that 4ICD failed to accumulate within mitochondria of ERBB4-V673I transfected cells and the potent proapoptotic activity of ERBB4 was completely abolished in cells expressing ERBB4-V673I. Our results provide the first formal demonstration that proteolytic processing of ERBB4 is a critical event regulating multiple receptor signaling activities.

The carboxyl terminus of VEGFR-2 is required for PKC-mediated down-regulation

Vascular endothelial growth factor receptor-2 (VEGFR-2/Flk-1) is a receptor tyrosine kinase (RTK) whose activation regulates angiogenesis. The regulatory mechanisms that attenuate VEGFR-2 signal relay are largely unknown. Our study shows that VEGFR-2 promotes phosphorylation of c-Cbl, but activation, ubiquitylation, and down-regulation of VEGFR-2 are not influenced by c-Cbl activity. A structure-function analysis of VEGFR-2 and pharmacological approach revealed that down-regulation of VEGFR-2 is mediated by a distinct mechanism involving PKC. A tyrosine mutant VEGFR-2, defective in PLC-gamma1 activation underwent down-regulation efficiently in response to ligand stimulation, suggesting that activation of classical PKCs are not involved in VEGFR-2 down-regulation. Further studies showed that the ectodomain of VEGFR-2 is dispensable for PKC-dependent down-regulation. Progressive deletion of the carboxyl-terminal domain showed that at least 39 amino acids within the carboxyl-terminal domain, immediately C-terminal to the kinase domain, is required for efficient PKC-mediated down-regulation of VEGFR-2. Mutation of serine sites at 1188 and 1191, within this 39 amino acid region, compromised the ability of VEGFR-2 to undergo efficient ligand-dependent down-regulation. Altogether the results show that the regulatory mechanisms involved in the attenuation of VEGFR-2 activation is mediated by nonclassical PKCs and the presence of serine sites in the carboxyl terminal of VEGFR-2.

The ERBB4/HER4 receptor tyrosine kinase regulates gene expression by functioning as a STAT5A nuclear chaperone

In the lactating breast, ERBB4 localizes to the nuclei of secretory epithelium while regulating activities of the signal transducer and activator of transcription (STAT) 5A transcription factor essential for milk-gene expression. We have identified an intrinsic ERBB4 NLS (residues 676–684) within the ERBB4 intracellular domain (4ICD) that is essential for nuclear accumulation of 4ICD. To determine the functional significance of 4ICD nuclear translocation in a physiologically relevant system, we have demonstrated that cotransfection of ERBB4 and STAT5A in a human breast cancer cell line stimulates β-casein promoter activity. Significantly, nuclear localization of STAT5A and subsequent stimulation of the β-casein promoter requires nuclear translocation of 4ICD. Moreover, 4ICD and STAT5A colocalize within nuclei of heregulin β1 (HRG)-stimulated cells and both proteins bind to the endogenous β-casein promoter in T47D breast cancer cells. Together, our results establish a novel molecular mechanism of transmembrane receptor signal transduction involving nuclear cotranslocation of the receptor intracellular domain and associated transcription factor. Subsequent binding of the two proteins at transcription factor target promoters results in activation of gene expression.

Frequent overexpression of multiple ErbB receptors by head and neck squamous cell carcinoma contrasts with rare antibody immunity in patients

In an effort to elucidate the role of ErbB receptors in human head and neck squamous cell carcinoma (HNSCC), expression abnormalities and subcellular localization of epidermal growth factor receptor (EGFR), ErbB2, ErbB3, and ErbB4 were investigated along with EGF and tenascin by immunohistochemistry in 38 carcinomas as compared to adjacent normal mucosa of 24 cases. Although tumour-specific overexpression affected each ErbB receptor (EGFR 47%, ErbB2 29%, ErbB3 21%, ErbB4 26%), EGFR abnormalities were most prevalent. The latter, and overexpression of more than two ErbB receptors in the same tumour, which always included EGFR, correlated with metastatic disease. ErbB products were specifically detected on the cell membrane and in the cytoplasm. In contrast, ErbB4 was uniquely localized to the nucleus in 7 carcinomas and a tumour-derived cell line, indicating a role for regulated intramembrane proteolysis resulting in nuclear ErbB4 translocation in HNSCC. Expression of prototype ligand EGF or low-affinity stromal activator tenascin correlated significantly with EGFR overexpression, implying chronic EGFR activation. Simultaneous overexpression of additional ErbB receptors in most of these cases suggested recurrent involvement of receptor heterodimers. In spite of frequent ErbB receptor alterations, autologous ErbB serum antibodies were rare, with only 1 of 38 tumour patients exhibiting an ErbB2-specific immune response. Based on upregulation of several known immunosuppressive molecules, scarcity of ErbB-specific antibodies is consistent with attenuation of natural tumour-specific immune responses in HNSCC.

Release of a membrane-bound death domain by gamma-secretase processing of the p75NTR homolog NRADD

Neurotrophin receptor alike death domain protein (NRADD) is a death-receptor-like protein with a unique ectodomain and an intracellular domain homologous to p75(NTR). Expression of NRADD results in apoptosis, but only in certain cell types. This paper characterizes the expression and proteolytic processing of the mature 55 kDa glycoprotein. N-terminally truncated NRADD is processed by a gamma-secretase activity that requires presenilins and has the same susceptibility to gamma-secretase inhibitors as the secretion of amyloid beta (A beta). The ectodomain of endogenous NRADD is shed by activation of metalloproteinases. Inhibitor studies provide evidence that NRADD is cleaved in two steps typical of regulated intramembrane proteolysis (RIP). Inhibition of gamma-secretase abrogates both the production of the soluble intracellular domain of NRADD and the appearance of NRADD in subnuclear structures. Thus, solubilized death domains with close homology to p75(NTR) might have a nuclear function. Furthermore, presenilin deficiency leads to abnormally glycosylated NRADD and overexpression of presenilin 2 inhibits NRADD maturation, which is dependent on the putative active site residue D366 but not on gamma-secretase activity. Our results demonstrate that NRADD is an additional gamma-secretase substrate and suggest that drugs against Alzheimer's disease will need to target gamma-secretase in a substrate-specific manner.

Ligand-regulated association of ErbB-4 to the transcriptional co-activator YAP65 controls transcription at the nuclear level

It has been proposed that ligand-dependent Regulated Intramembrane Proteolysis (RIP) of ErbB-4 receptors generates 80 kDa Intra-Cellular Domains (E4.ICDs) that relocate to the nuclear compartments where they implement the signaling abilities of the ErbB-4 receptors. The E4.ICD may directly regulate gene transcription or, in an alternative scenario, the tyrosine kinase activity of E4.ICDs may target proteins involved in transcriptional regulation upon its relocation into the nucleus. We have identified the transcriptional coactivator YAP65, here referred as YAP (Yes Associated Protein), as binding partner of ErbB-4 in a two hybrid screening in yeast. Interaction between YAP and ErbB-4 occurs via the WW domain of YAP and the PPPPY at positions 1297-1301 and the PPPAY at positions 1052-1056 of the amino acid sequence of the Cyt-1 isoform of ErbB-4. Stechiometry of binding is regulated by the ligand-dependent phosphorylation of Tyr 1056 in the PPPAYTPM module that function as "biochemical switch" to decrease the association of YAP to ErbB-4. In principle, this novel interaction highlights new mechanisms of signaling propagation from the ErbB-4 receptors, offering supporting evidences that the E4.ICDs forms released following ligand-receptor engagement may recruit YAP and relocate to the nucleus to implement or regulate transcription.

Epidermal growth factor and trail interactions in epithelial-derived cells

In healthy tissues, there is a balance between cell survival and death. This balance ensures epithelial cells survive in the right milieu, but undergo programmed cell death (apoptosis) when the environment is no longer supportive. Cells sense these changes primarily through receptors on the cell surface that bind to specific ligands present in the extracellular environment. These receptors, through signal transduction pathways, lead to promotion of cell survival or induction of cell death. One of the most important types of receptors regulating cell survival is the epidermal growth factor (EGF) receptors, while one of the most important types of receptors regulating apoptosis is the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) death receptors. EGF receptors activate survival signaling pathways including PI3K/AKT, Ras/MAPK, and JAK/STAT signaling pathways leading to cell survival. TRAIL activates apoptotic signaling pathways leading to caspase activation and mitochondrial dysfunction. The balance between these two signaling pathways determine whether a cell survives or dies. In disease states, this balance is altered. For example, epithelial-derived cancer cells often have increased expression of EGF receptors and are resistant to apoptosis. Understanding the interactions between survival and apoptotic signaling pathways mediated by EGF receptors and TRAIL death receptors will be essential to explain the role these pathways play in healthy and diseased cells.

Neuregulin-2 is developmentally regulated and targeted to dendrites of central neurons

Neuregulin-1 (NRG-1) regulates numerous aspects of neural development and synaptic plasticity; the functions of NRG-2 and NRG-3 are presently unknown. As a first step toward understanding how NRGs contribute to distinct aspects of neural development and function, we characterized their regional and subcellular expression patterns in developing brain. The expression of NRG-1-3 mRNAs was compared postnatally (P0, P7, adult) by using in situ hybridization. NRG-1 expression is highest at birth, whereas NRG-2 mRNA levels increase with development; expression of both genes is restricted to distinct brain regions. In contrast, NRG-3 transcripts are abundant in most brain regions throughout development. NRG-2 antibodies were generated to analyze protein processing, expression, and subcellular distribution. As with NRG-1, the transmembrane NRG-2 proprotein is proteolytically processed in transfected HEK 293 cells and in neural tissues, and its ectodomain is exposed and accumulates on the neuron surface. Despite the structural similarities between NRG-1 and NRG-2, we unexpectedly found that NRG-2 colocalizes with MAP2 in proximal primary dendrites of hippocampal neurons in culture and in vivo, although it is not detectable in axons or in axon terminals. These findings were confirmed with NRG-2 ectodomain antisera and epitope-tagged recombinant protein. In cerebellum, NRG-2 colocalizes with calbindin in proximal dendrites and soma of Purkinje cells. In contrast, NRG-1 is highly expressed in axons of dissociated hippocampal neurons, as well as in somas and dendrites. The distinct temporal, regional, and subcellular expression of NRG-2 suggests its unique and nonredundant role in neural function.

WW domain-containing protein YAP associates with ErbB-4 and acts as a co-transcriptional activator for the carboxyl-terminal fragment of ErbB-4 that translocates to the nucleus

The ErbB-4 receptor protein-tyrosine kinase is proteolytically processed by membrane proteases in response to the ligand or 12-O-tetradecanoylphorbol-13-acetate stimulation resulting in the cytoplasmic fragment translocating to the cell nucleus. The WW domain-containing co-transcriptional activator Yes-associated protein (YAP) associates physically with the full-length ErbB-4 receptor and functionally with the ErbB-4 cytoplasmic fragment in the nucleus. The YAP.ErbB4 complex is mediated by the first WW domain of YAP and the most carboxyl-terminal PPXY motif of ErbB-4. In human tissues, we documented the expression of YAP1 with a single WW domain and YAP2 with two WW domains. It is known that the COOH-terminal fragment of ErbB4 does not have transcriptional activity by itself; however, we show here that in the presence of YAP its transcriptional activity is revealed. There is a difference in the extent of transactivation activity among YAP isoforms: YAP2 is the stronger activator compared with YAP1. This transactivation is abolished by mutations that abrogate the YAP.ErbB4 complex formation. The unphosphorylatable mutation that increases the nuclear localization of YAP increases transcription activity. The COOH-terminal fragment of ErbB-4 and full-length YAP2 overexpressed in cells partially co-localize to the nucleus. Our data indicate that YAP is a potential signaling partner of the full-length ErbB4 receptor at the membrane and of the COOH-terminal fragment of ErbB-4 that translocates to the nucleus to regulate transcription.

Abeta42-lowering nonsteroidal anti-inflammatory drugs preserve intramembrane cleavage of the amyloid precursor protein (APP) and ErbB-4 receptor and signaling through the APP intracellular domain

Epidemiological studies indicate that long term use of nonsteroidal anti-inflammatory drugs (NSAIDs) confers protection from Alzheimer's disease, and some NSAIDs were shown to specifically decrease production of the amyloidogenic Abeta42 peptide, most likely by direct modulation of gamma-secretase activity. In contrast to gamma-secretase inhibitors, Abeta42-lowering NSAIDs do not impair S3 cleavage in the NOTCH receptor and release of the NOTCH intracellular domain, a finding with conceptual implications for the development of safer drugs targeting Abeta production through gamma-secretase modulation. Intramembrane cleavage and release of an intracellular signaling domain has recently been demonstrated in a number of additional gamma-secretase substrates. We now show in cell-based assays that intramembrane cleavage of APP and ErbB-4 receptor is not impaired by the Abeta42-lowering NSAIDs, sulindac sulfide and ibuprofen. Generation of the APP intracellular domain (AICD) was further not inhibited in a cell-free assay at concentrations far exceeding those effective in reducing Abeta42 production. Closer inspection of AICD signaling showed that stabilization of the AICD peptide by FE65 and AICD-mediated transcription were also retained at Abeta42-lowering concentrations. These results demonstrate that S3-like/intramembrane cleavage is preserved by Abeta42-lowering NSAIDs in at least three substrates of gamma-secretase APP, ErbB-4, and NOTCH and underline the striking specificity by which these drugs target Abeta42 production.