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

ERBB Receptors and Their Ligands in the Developing Mammary Glands of Different Species: Fifteen Characters in Search of an Author

The ERBB tyrosine kinase receptors and their ligands belong to a complex family that has diverse biological effects and expression profiles in the developing mammary glands, where its members play an essential role in translating hormone signals into local effects. While our understanding of these processes stems mostly from mouse models, there is the potential for differences in how this family functions in the mammary glands of other species, particularly in light of their unique histomorphological features. Herein we review the postnatal distribution and function of ERBB receptors and their ligands in the mammary glands of rodents and humans, as well as for livestock and companion animals. Our analysis highlights the diverse biology for this family and its members across species, the regulation of their expression, and how their roles and functions might be modulated by varying stromal composition and hormone interactions. Given that ERBB receptors and their ligands have the potential to influence processes ranging from normal mammary development to diseased states such as cancer and/or mastitis, both in human and veterinary medicine, a more complete understanding of their biological functions should help to direct future research and the identification of new therapeutic targets.

The role of the NDRG1 in the pathogenesis and treatment of breast cancer

Breast cancer (BC) is the leading cause of cancer death in women. This disease is heterogeneous, with clinical subtypes being estrogen receptor-α (ER-α) positive, having human epidermal growth factor receptor 2 (HER2) overexpression, or being triple-negative for ER-α, progesterone receptor, and HER2 (TNBC). The ER-α positive and HER2 overexpressing tumors can be treated with agents targeting these proteins, including tamoxifen and pertuzumab, respectively. Despite these treatments, resistance and metastasis are problematic, while TNBC is challenging to treat due to the lack of suitable targets. Many studies examining BC and other tumors indicate a role for N-myc downstream-regulated gene-1 (NDRG1) as a metastasis suppressor. The ability of NDRG1 to inhibit metastasis is due, in part, to the inhibition of the initial step in metastasis, namely the epithelial-to-mesenchymal transition. Paradoxically, there are also reports of NDRG1 playing a pro-oncogenic role in BC pathogenesis. The oncogenic effects of NDRG1 in BC have been reported to relate to lipid metabolism or the mTOR signaling pathway. The molecular mechanism(s) of how NDRG1 regulates the activity of multiple signaling pathways remains unclear. Therapeutic strategies that up-regulate NDRG1 have been developed and include agents of the di-2-pyridylketone thiosemicarbazone class. These compounds target oncogenic drivers in BC cells, suppressing the expression of multiple key hormone receptors including ER-α, progesterone receptor, androgen receptor, and prolactin receptor, and can also overcome tamoxifen resistance. Considering the varying role of NDRG1 in BC pathogenesis, further studies are required to examine what subset of BC patients would benefit from pharmacopeia that up-regulate NDRG1.

EphA1 receptor tyrosine kinase is localized to the nucleus in rhabdomyosarcoma from multiple species

While the typical role of receptor tyrosine kinases is to receive and transmit signals at the cell surface, in some cellular contexts (particularly transformed cells) they may also act as nuclear proteins. Aberrant nuclear localization of receptor tyrosine kinases associated with transformation often enhances the transformed phenotype (i.e. nuclear ErbBs promote tumor progression in breast cancer). Rhabdomyosarcoma (RMS), the most common soft tissue tumor in children, develops to resemble immature skeletal muscle and has been proposed to derive from muscle stem/progenitor cells (satellite cells). It is an aggressive cancer with a 5-year survival rate of 33% if it has metastasized. Eph receptor tyrosine kinases have been implicated in the development and progression of many other tumor types, but there are only two published studies of Ephs localizing to the nucleus of any cell type and to date no nuclear RTKs have been identified in RMS. In a screen for protein expression of Ephs in canine RMS primary tumors as well as mouse and human RMS cell lines, we noted strong expression of EphA1 in the nucleus of interphase cells in tumors from all three species. This localization pattern changes in dividing cells, with EphA1 localizing to the nucleus or the cytoplasm depending on the phase of the cell cycle. These data represent the first case of a nuclear RTK in RMS, and the first time that EphA1 has been detected in the nucleus of any cell type.

EGFR, HER2, and HER3 protein expression in paired primary tumor and lymph node metastasis of colorectal cancer

Due to the difficulty in sampling of metastatic tumors, patient selection is commonly based on results of primary tumor samples when metastatic samples are not available. However, due to tumor heterogeneity, metastatic tumors may be different from primary tumors in their phenotypes. The aim of this study was to investigate the expression of EGFR, HER2, and HER3 between primary and lymph node metastatic lesions of colorectal cancer. Paired primary tumors and lymph node metastases from 79 patients with colorectal cancer were retrospectively collected and analyzed for EGFR, HER2, and HER3 expression. High EGFR, HER2, and HER3 expression (2+ and 3+) was found in 64.2%, 66.0%, and 85.0% of primary tumors, and 56.8%, 46.0%, and 76.0% of lymph node metastases, respectively. Correlation rates between primary and metastatic lesions were 67.1%, 63.3%, and 74.7% for EGFR, HER2, and HER3, respectively. Stage IV tumors (with distant metastasis) had higher correlation rates of HER2 expression compared to stage III tumors (without distant metastasis) (P = 0.050). Moderate correlation rates in EGFR, HER2, and HER3 expression were observed between primary and metastatic lesions of colorectal cancer. Tumor stage or existence of distant metastasis could serve as potential predictive markers for the correlation of HER2 expression between primary tumors and lymph node metastases of colorectal cancer.

"Shedding" light on HER4 signaling in normal and malignant breast tissues

Receptor Tyrosine Kinases of the Epidermal Growth Factor Receptor Family play a pivotal role as drivers of carcinogenesis and uncontrolled cell growth for a variety of malignancies, not least for breast cancer. Besides the estrogen receptor, the HER2 receptor was and still is a representative marker for advanced taxonomic sub-differentiation of breast cancer and emerged as one of the first therapeutic targets for antibody based therapies. Since the approval of trastuzumab for the therapy of HER2-positive breast cancer in 1998 anti-HER2 treatment strategies are being modified, refined, and successfully combined with complementary treatments, nevertheless there is still potential for improvement. The HER2 relatives, namely HER1 (i.e., EGFR), HER3 and HER4 share a high degree of molecular homology and together form a functional unit for signal transmission. Under regular conditions, receptor coexpression patterns and receptor interaction represent key parameters for signaling robustness, which ensures cellular growth control and enables tissue differentiation. In addition, treatment efficiency of e.g., an anti-HER2 targeting is substantially determined by the expression pattern of HER receptors on target cells. Within the receptor family, the HER4 plays a particular role and is engaged in exceptional signaling activities. A favorable prognostic impact has been attributed to HER4 expression in breast cancer under specific molecular conditions. HER4-specific cellular effects are initially determined by a ligand-dependent or -independent receptor activation. Essential processes as cell growth and proliferation, cell differentiation, and apoptotic cell death can be initiated by this receptor. This review gives an overview of the role of HER4 in normal and malignant breast epithelial cells and tissues. Specific mechanism of HER4 activation and subsequent intracellular signaling will be described by taking a focus on effects provoked by receptor shedding. HER4 activities and specific effects will be correlated to breast cancer subtypes and the impact of HER4 on course and outcome of disease will be considered. Moreover, current and potential therapeutic approaches will be discussed.

ErbB4 alternative splicing mediates fetal mouse alveolar type II cell differentiation in vitro

BACKGROUND

Alternative splicing (AS) creates different protein isoforms, an important mechanism regulating cell-specific function. Little is known about AS in lung development, particularly in alveolar type II (ATII) cells. ErbB4 receptor isoforms Jma and Jmb have significant and opposing functions in the brain, heart, and lung development and/or disease. However, the regulators of ErbB4 AS are unknown. ErbB4 AS regulators in fetal mouse ATII cells control its function in ATII cell maturation.

METHODS

Candidate ErbB4 AS regulators were found using in silico analysis. Their developmental expression was studied in fetal mouse ATII cells. The effects of splice factor downregulation and upregulation on ATII cell maturation were analyzed.

RESULTS

ErbB4-Jma increased significantly in ATII cells after gestation E16.5. In silico analysis found four candidate splice factors: FOX2, CUG/CELF1, TIAR, and HUB. Fetal ATII cells expressed these factors in distinct developmental profiles. HUB downregulation in E17.5 ATII cells increased Jma isoform levels and Sftpb gene expression and decreased Jmb. HUB overexpression decreased Jma and Sftpb.

CONCLUSIONS

ErbB4 AS is developmentally controlled by HUB in fetal ATII cells, promoting ATII differentiation. Regulated AS expression during ATII cell differentiation suggests novel therapeutic strategies to approach human disease.

IMPACT

Alternative splicing (AS) of the ErbB4 receptor, involving mutually exclusive exon inclusion, creates Jma and Jmb isoforms with distinct differences in receptor processing and function. The Jma isoform of ErbB4 promotes differentiation of fetal lung alveolar type II cells. The AS is mediated in part by the RNA-binding protein HUB. The molecular mechanism of AS for ErbB4 has not been previously described. The regulation of ErbB4 AS has important implications in the development of organs, such as the lung, brain, and heart, and for disease, including cancer.

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.

Identification of the anti-breast cancer targets of triterpenoids in Liquidambaris Fructus and the hints for its traditional applications

BACKGROUND

Liquidambaris Fructus is the infructescences of Liquidambar formosana Hance and it has been used to treat some breast disease in Traditional Chinese Medicine. In the previous study we found the anti-breast cancer effect of triterpenoid in Liquidambaris Fructus. This study is a further investigation of the triterpenoids in Liquidambaris Fructus and aims to identify their anti-breast cancer targets, meanwhile, to estimate the rationality of the traditional applications of Liquidambaris Fructus.

METHODS

Triterpenoids in Liquidambaris Fructus were isolated and their structures were identified by NMR spectrums. Potential targets of these triterpenoids were predicted using a reverse pharmacophore mapping strategy. Associations between these targets and the therapeutic targets of breast cancer were analyzed by constructing protein-protein interaction network, and targets played important roles in the network were identified using Molecular Complex Detection method. Binding affinity between the targets and triterpenoids was studied using molecular docking method. Gene ontology enrichment analysis was conducted to reveal the biological process and signaling pathways that the identified targets were involved in.

RESULTS

Thirteen triterpenoids were identified and 6 of them were the first time isolated from Liquidambaris Fructus. Predicted ADME properties revealed a good druggability of these triterpenoids. We identified 18 protein targets which were closely related to breast cancer progression, especially triple-negative, basal-like or advanced stage breast cancers. The triterpenoids could bind with these targets as their inhibitors: hydrophobic skeleton is a favorable factor for them to stabilize at binding site and polar C17- or C3- substituent was necessary for binding. GO enrichment analysis indicated that inhibition of protein tyrosine kinases autophosphorylation might be the primary mechanism for the anti-breast cancer effect of the triterpenoids, and ErbB4 and EGFR were the most relevant targets.

CONCLUSIONS

The study revealed that triterpenoids from Liquidambaris Fructus might exert anti-breast cancer effect by directly inhibit multiple protein targets and signaling pathways, especially ErbB4 and EGFR and related pathways. This study also brings up another hint that the traditional applications of Liquidambaris Fructus on hypogalactia should be reassessed systematically because it might suppress rather than promote lactation by inhibiting the activity of ErbB4.

Her4 promotes cancer metabolic reprogramming via the c-Myc-dependent signaling axis

Despite the growing recognition of metabolic reprogramming as an important hallmark of cancer in the past few years, the molecular mechanisms underlying metabolic alterations during tumorigenesis remain unclear. In this study, we identified a critical role of Her4 in rewiring cancer metabolism toward tumor-promoting metabolic processes, including increased glycolysis, glutaminolysis, mitochondrial biogenesis, and oxidative phosphorylation, which may in part cooperate to promote tumorigenesis. We found that overexpression of Her4 promoted the stabilization of c-Myc through a CIP2A-mediated increase in c-Myc^S62 phosphorylation and GSK3β-mediated decrease in c-Myc^T58 phosphorylation, both of which decreased c-Myc degradation. Furthermore, Her4 was found to increase glucose uptake and tumor growth in an osteosarcoma xenograft model. Overall, these findings provide a better understanding of the involvement of Her4 in tumorigenesis and document its potential role in metabolic reprogramming for the first time. We believe that our study might lead to promising opportunities for targeted metabolic therapy for cancer.

Coordinated dysregulation of cancer progression by the HER family and p21-activated kinases

Most epithelial cancer types are polygenic in nature and are driven by coordinated dysregulation of multiple regulatory pathways, genes, and protein modifications. The process of coordinated regulation of cancer promoting pathways in response to extrinsic and intrinsic signals facilitates the dysregulation of several pathways with complementary functions, contributing to the hallmarks of cancer. Dysregulation and hyperactivation of cell surface human epidermal growth factor receptors (HERs) and cytoskeleton remodeling by p21-activated kinases (PAKs) are two prominent interconnected aspects of oncogenesis. We briefly discuss the discoveries and significant advances in the area of coordinated regulation of HERs and PAKs in the development and progression of breast and other epithelial cancers. We also discuss how initial studies involving heregulin signaling via HER3-HER2 axis and HER2-overexpressing breast cancer cells not only discovered a mechanistic role of PAK1 in breast cancer pathobiology but also acted as a bridge in generating a broader cancer research interest in other PAK family members and cancer types and catalyzed establishing the role of PAKs in human cancer, at-large. In addition, growth factor stimulation of the PAK pathway also helped to recognize new facets of PAKs, connecting the PAK pathway to oncogenesis, nuclear signaling, gene expression, mitotic progression, DNA damage response, among other phenotypic responses, and shaped the field of PAK cancer research. Finally, we recount some of the current limitations of HER- and PAK-directed therapeutics in counteracting acquired therapeutic resistance and discuss how cancer's as a polygenic disease may be best targeted with a polygenic approach.

Clinicopathologic significance of nuclear HER4 and phospho-YAP(S127) in human breast cancers and matching brain metastases

Background

Human epidermal growth factor receptor-4 (HER4) and yes-associated protein-1 (YAP) are candidate therapeutic targets in oncology. YAP's transcriptional coactivation function is modulated by the HER4 intracellular domain (HER4-ICD) in vitro, but the clinical relevance of this has not been established. This study investigated the potential for targeting the HER4-YAP pathway in brain metastatic breast cancer.

Methods

We performed immuno-phenotypic profiling of pathway markers in a consecutive breast cancer series with 25 years of clinical follow up (n = 371), and patient-matched breast and metastatic brain tumours (n = 91; 30 pairs).

Results

Membrane localisation of phospho-HER4 [pHER4(Y¹¹⁶²)] was infrequent in primary breast cancer, but very frequent in brain metastases (5.9% versus 75% positive), where it was usually co-expressed with pHER3(Y¹²⁸⁹) (p < 0.05). The presence of YAP in tumour cell nuclei was associated directly with nuclear pERK5(T²¹⁸/Y²¹⁰) (p = 0.003). However, relationships with disease-specific survival depended on oestrogen receptor (ER) status. Nuclear pYAP(S¹²⁷) was associated with smaller, good prognostic ER+ breast tumours (log-rank hazard-ratio 0.53; p = 9.6E^-03), but larger, poor prognostic triple-negative cancers (log-rank hazard-ratio 2.78; p = 1.7E^-02), particularly when co-expressed with nuclear HER4-ICD (p = 0.02). This phenotype was associated with stemness and mitotic instability markers (vimentin, SOX9, ID1, SPAG5, TTK, geminin; p < 0.05). YAP expression in brain metastases was higher than matched primary tumours; specifically, nuclear pYAP(S¹²⁷) in ER-negative cases (p < 0.05). Nuclear YAP was detected in ~70% of ER-negative, HER4-activated brain metastases.

Discussion

Our findings suggest that the canonical-mechanism where Hippo pathway-mediated phosphorylation of YAP ostensibly excludes it from the nucleus is dysfunctional in breast cancer. The data are consistent with pYAP(S¹²⁷) having independent transcriptional functions, which may include transducing neuregulin signals in brain metastases. Consistent with mechanistic studies implicating it as an ER co-factor, nuclear pYAP(S¹²⁷) associations with breast cancer clinical outcomes were dependent on ER status.

Conclusion

Preclinical studies investigating HER4 and nuclear YAP combination therapy strategies are warranted.

HER family in cancer progression: From discovery to 2020 and beyond

The human epidermal growth factor receptor (HER) family of receptor tyrosine kinases (RTKs) are among the first layer of molecules that receive, interpret, and transduce signals leading to distinct cancer cell phenotypes. Since the discovery of the tooth-lid factor-later characterized as the epidermal growth factor (EGF)-and its high-affinity binding EGF receptor, HER kinases have emerged as one of the commonly upregulated or hyperactivated or mutated kinases in epithelial tumors, thus allowing HER1-3 family members to regulate several hallmarks of cancer development and progression. Each member of the HER family exhibits shared and unique structural features to engage multiple receptor activation modes, leading to a range of overlapping and distinct phenotypes. EGFR, the founding HER family member, provided the roadmap for the development of the cell surface RTK-directed targeted cancer therapy by serving as a prototype/precursor for the currently used HER-directed cancer drugs. We herein provide a brief account of the discoveries, defining moments, and historical context of the HER family and guidepost advances in basic, translational, and clinical research that solidified a prominent position of the HER family in cancer research and treatment. We also discuss the significance of HER3 pseudokinase in cancer biology; its unique structural features that drive transregulation among HER1-3, leading to a superior proximal signaling response; and potential role of HER3 as a shared effector of acquired therapeutic resistance against diverse oncology drugs. Finally, we also narrate some of the current drawbacks of HER-directed therapies and provide insights into postulated advances in HER biology with extensive implications of these therapies in cancer research and treatment.

Biasing human epidermal growth factor receptor 4 (HER4) tyrosine kinase signaling with antibodies: Induction of cell death by antibody-dependent HER4 intracellular domain trafficking

Human epidermal growth factor receptor 4 (HER4) isoforms have oncogenic or tumor suppressor functions depending on their susceptibility to proteolytic cleavage and HER4 intracellular domain (4ICD) translocation. Here, we report that the neuregulin 1 (NRG1) tumor suppressor mechanism through the HER4 JMa/CYT1 isoform can be mimicked by the agonist anti‐HER4 Ab C6. Neuregulin 1 induced cleavage of poly(ADP‐ribose) polymerase (PARP) and sub‐G₁ DNA fragmentation, and also reduced the metabolic activity of HER3⁻/HER4⁺ cervical (C‐33A) and ovarian (COV318) cancer cells. This effect was confirmed in HER4 JMa/CYT1‐, but not JMa/CYT2‐transfected BT549 triple‐negative breast cancer cells. Neuregulin 1 favored 4ICD cleavage and retention in mitochondria in JMa/CYT1‐transfected BT549 cells, leading to reactive oxygen species (ROS) production through mitochondrial depolarization. Similarly, the anti‐HER4 Ab C6, which binds to a conformational epitope located on a.a. 575‐592 and 605‐620 of HER4 domain IV, induced 4ICD cleavage and retention in mitochondria, and mimicked NRG1‐mediated effects on PARP cleavage, ROS production, and mitochondrial membrane depolarization in cancer cells. In vivo, C6 reduced growth of COV434 and HCC1187 tumor cell xenografts in nude mice. Biasing 4ICD trafficking to mitochondria with anti‐HER4 Abs to mimic NRG1 suppressor functions could be an alternative anticancer strategy.

Epidermal Growth Factor Receptor Family and its Role in Gastric Cancer

Despite the gradual decrease in incidence, gastric cancer is still the third leading cause of cancer death worldwide. Although chemotherapy enhances overall survival and quality of life in advanced disease, the median overall survival is < 12 months. In recent years, the human epidermal growth factor receptor (ErbB) family has been extensively investigated in gastric cancer. The ErbB family is composed of four closely-related members: ErbB-1 (HER1 or epidermal growth factor receptor, EGFR), ErbB-2 (HER2), ErbB-3 (HER3), and ErbB-4 (HER4), all of which play a critical role in regulating cell growth, proliferation and migration of tumors. It is well known that gastric cancer overexpresses HER in a heterogeneous pattern, especially EGFR, and HER2. HER3 is another important member of the ErbB family that preferentially activates the phosphatidylinositol 3-kinase (PI3K) pathway. Furthermore, its heterodimerization with HER2 seems fundamental for steering HER2-overexpressing breast cancer tumor growth. Less is known about the impact of HER4 on gastric cancer. Improved survival from the use of trastuzumab has paved the way for ErbB receptor family-targeted treatments in gastric cancer. However, unlike trastuzumab, ErbB receptor-targeted drugs have not consistently maintained the encouraging results obtained in preclinical and early clinical trials. This may be attributable to the intrinsic heterogeneity of gastric cancer and/or to the lack of standardized test quality for established biomarkers used to evaluate these biological targets. This review presents an overview of the most recent clinical studies on agents targeting the ErbB family in gastric cancer.

Target HER four in breast cancer?

The HER4 receptor tyrosine kinase is known to have promiscuous activity in malignant cells, last but not least in breast cancer. Evidently, the prognostic and predictive impact of HER4 expression depends on the expression of different receptor isotypes, the way of receptor activation (ligand dependent vs. independent), and on the complex interaction of the HER4 intracellular domain (4ICD) with intracellular regulative molecules which results in either oncogenic or rather tumor suppressive HER4 activity. Recent data suggest that HER4 unfavorably affects the endocrine treatment of postmenopausal breast cancer patients with tamoxifen and therefore might represent an additional therapeutic target in luminal breast cancer.

Role for tyrosine phosphorylation of SUV39H1 histone methyltransferase in enhanced trimethylation of histone H3K9 via neuregulin-1/ErbB4 nuclear signaling

Protein-tyrosine kinases transmit signals by phosphorylating their substrates in diverse cellular events. The receptor-type tyrosine kinase ErbB4, a member of the epidermal growth factor receptor subfamily, is activated and proteolytically cleaved upon ligand stimulation, and the cleaved ErbB4 intracellular domain (4ICD) is released into the cytoplasm and the nucleus. We previously showed that generation of nuclear 4ICD by neuregulin-1 (NRG-1) stimulation enhances the levels of trimethylation of histone H3 at lysine 9 (H3K9me3). However, it remains unclear how nuclear 4ICD enhances H3K9me3 levels. Here we show that the histone H3K9 methyltransferase SUV39H1 associates with NRG-1/ErbB4-mediated H3K9me3. Knockdown of SUV39H1 blocked NRG-1-mediated enhancement of the levels of H3K9me3. Nuclear 4ICD was found to phosphorylate SUV39H1 primarily at Tyr-297, -303, and -308 that are conserved among humans, mice, and flies. Furthermore, knockdown-rescue experiments showed that the unphosphorylatable SUV39H1 mutant (3 YF) was incapable of enhancing the levels of H3K9me3 upon NRG-1 stimulation. These results suggest that nuclear ErbB4 enhances H3K9me3 levels through tyrosine phosphorylation of SUV39H1 in NRG-1/ErbB4 signal-mediated chromatin remodeling.

Dynamics of Axl Receptor Shedding in Hepatocellular Carcinoma and Its Implication for Theranostics

Signaling of the receptor tyrosine kinase Axl and its ligand Gas6 is crucially involved in the development of liver fibrosis and hepatocellular carcinoma (HCC) by activation of hepatic stellate cells and modulation of hepatocyte differentiation. Shedding of Axl’s ectodomain leads to the release of soluble Axl (sAxl), which is increased in advanced fibrosis and in early-to-late stage HCC in the presence and absence of cirrhosis. Here, we focus on the dynamics of Axl receptor shedding and delineate possible scenarios how Axl signaling might act as driver of fibrosis progression and HCC development. Based on experimental and clinical data, we discuss the consequences of modifying Axl signaling by sAxl cleavage, as well as cellular strategies to escape from antagonizing effects of Axl shedding by the involvement of the hepatic microenvironment. We emphasize a correlation between free Gas6 and free sAxl levels favoring abundant Gas6/Axl signaling in advanced fibrosis and HCC. The raised scenario provides a solid basis for theranostics allowing the use of sAxl as an accurate diagnostic biomarker of liver cirrhosis and HCC, as well as Axl receptor signaling for therapeutic intervention in stratified HCC patients.

HER4 expression in estrogen receptor-positive breast cancer is associated with decreased sensitivity to tamoxifen treatment and reduced overall survival of postmenopausal women

BACKGROUND

The sensitivity of estrogen receptor-positive breast cancers to tamoxifen treatment varies considerably, and the molecular mechanisms affecting the response rates are manifold. The human epidermal growth factor receptor-related receptor HER2 is known to trigger intracellular signaling cascades that modulate the activity of coregulators of the estrogen receptor which, in turn, reduces the cell sensitivity to tamoxifen treatment. However, the impact of HER2-related receptor tyrosine kinases HER1, HER3, and, in particular, HER4 on endocrine treatment is largely unknown.

METHODS

Here, we retrospectively evaluated the importance of HER4 expression on the outcome of tamoxifen- and aromatase inhibitor-treated estrogen receptor-positive breast cancer patients (n = 258). In addition, we experimentally analyzed the efficiency of tamoxifen treatment as a function of HER4 co-expression in vitro.

RESULTS

We found a significantly improved survival in tamoxifen-treated postmenopausal breast cancer patients in the absence of HER4 compared with those with pronounced HER4 expression. In accordance with this finding, the sensitivity to tamoxifen treatment of estrogen and HER4 receptor-positive ZR-75-1 breast cancer cells can be significantly enhanced by HER4 knockdown.

CONCLUSION

We suggest an HER4/estrogen receptor interaction that impedes tamoxifen binding to the estrogen receptor and reduces treatment efficiency. Whether the sensitivity to tamoxifen treatment can be enhanced by anti-HER4 targeting needs to be prospectively evaluated.

Mechanisms Underlying the Action and Synergism of Trastuzumab and Pertuzumab in Targeting HER2-Positive Breast Cancer

Human epidermal growth factor receptor (HER) 2 (HER2) is overexpressed in 20⁻30% of breast cancers. HER2 is a preferred target for treating HER2-positive breast cancer. Trastuzumab and pertuzumab are two HER2-targeted monoclonal antibodies approved by the Food and Drug Administration (FDA) to use as adjuvant therapy in combination with docetaxel to treat metastatic HER2-positive breast cancer. Adding the monoclonal antibodies to treatment regimen has changed the paradigm for treatment of HER2-positive breast cancer. Despite improving outcomes, the percentage of the patients who benefit from the treatment is still low. Continued research and development of novel agents and strategies of drug combinations is needed. A thorough understanding of the molecular mechanisms underlying the action and synergism of trastuzumab and pertuzumab is essential for moving forward to achieve high efficacy in treating HER2-positive breast cancer. This review examined and analyzed findings and hypotheses regarding the action and synergism of trastuzumab and pertuzumab and proposed a model of synergism based on available information.

Subtyping Of Triple Negative Breast Carcinoma On The Basis Of RTK Expression

Background: "Triple-negative breast cancers" (TNBC) comprise a heterogeneous group of about 15% of invasive BCs lacking the expression of estrogen and progesterone receptors (ER, PR) and the expression of HER2 (ERBB2) and are therefore no established candidates for targeted treatment options in BC, i.e., endocrine and anti-HER2 therapy. The aim of the present study was to use gene expression profiling and immunohistochemical (IHC) characterization to identify receptor tyrosine kinase (RTK) profiles that would allow patient stratification for the purposes of target-oriented personalized tumor therapy in TNBC. Methods: Twenty-nine cases of TNBC selected according to routine diagnostic IHC/cytogenetic criteria were examined by reverse transcription polymerase chain reaction (RT-PCR). RTK mRNA expression profiles were generated for a total of 31 tumor-relevant biomarkers, mainly belonging to the IGF- and EGF-receptor families but also including biomarkers related to downstream signaling. Protein expression of selected biomarkers was investigated by IHC. Results: Hierarchical cluster analysis revealed a dichotomous differentiation pattern amongst TNBCs. A significant difference in gene expression was observed for 16 of the 31 RTK-associated tumor relevant biomarkers between the two newly identified TNBC subgroups. The findings were verified at the posttranslational level by the IHC data. The RTKs HER4, IGF-1R and IGF-2R and the hormone receptors ER and PR below the IHC detection limit play a central role in the differentiation of the two TNBC subgroups. Observed survival was reported as Kaplan-Meier estimates and point towards an improved survival of patients with RTK-high with superior three-year survival rate of 100% compared to RTK-low gene signatures with superior three-year survival rate of 60% (log-rank test, p-value = 0.022). Conclusion: Gene-expression and IHC analysis of the EGF and IGF receptor families and biomarkers associated with downstream signaling point to the existence of two distinct TNBC subtypes. The RTKs HER4, IGF-1R, IGF-2R and the hormone receptors ER and PR appear to be of particular importance here. Based on survival analysis the differentiation of TNBC with RTK-high and RTK-low gene signatures seems to be of prognostic relevance. Additionally, correlation analysis of the relationship between RTKs and ER suggests co-regulatory mechanisms that may have potential significance in new therapeutic approaches.

Human epidermal growth factor receptor targeted inhibitors for the treatment of ovarian cancer

Ovarian cancer is the second most lethal gynecological cancer worldwide and while most patients respond to initial therapy, they often relapse with resistant disease. Human epidermal growth factor receptors (especially HER1/EGFR and HER2/ERBB2) are involved in disease progression; hence, strategies to inhibit their action could prove advantageous in ovarian cancer patients, especially in patients resistant to first line therapy. Monoclonal antibodies and tyrosine kinase inhibitors are two classes of drugs that act on these receptors. They have demonstrated valuable antitumor activity in multiple cancers and their possible use in ovarian cancer continues to be studied. In this review, we discuss the human epidermal growth factor receptor family; review emerging clinical studies on monoclonal antibodies and tyrosine kinase inhibitors targeting these receptors in ovarian cancer patients; and propose future research possibilities in this area.

SUMOylation regulates nuclear accumulation and signaling activity of the soluble intracellular domain of the ErbB4 receptor tyrosine kinase

Erb-B2 receptor tyrosine kinase 4 (ErbB4) is a kinase that can signal via a proteolytically released intracellular domain (ICD) in addition to classical receptor tyrosine kinase-activated signaling cascades. Previously, we have demonstrated that ErbB4 ICD is posttranslationally modified by the small ubiquitin-like modifier (SUMO) and functionally interacts with the PIAS3 SUMO E3 ligase. However, direct evidence of SUMO modification in ErbB4 signaling has remained elusive. Here, we report that the conserved lysine residue 714 in the ErbB4 ICD undergoes SUMO modification, which was reversed by sentrin-specific proteases (SENPs) 1, 2, and 5. Although ErbB4 kinase activity was not necessary for the SUMOylation, the SUMOylated ErbB4 ICD was tyrosine phosphorylated to a higher extent than unmodified ErbB4 ICD. Mutation of the SUMOylation site compromised neither ErbB4-induced phosphorylation of the canonical signaling pathway effectors Erk1/2, Akt, or STAT5 nor ErbB4 stability. In contrast, SUMOylation was required for nuclear accumulation of the ErbB4 ICD. We also found that Lys-714 was located within a leucine-rich stretch, which resembles a nuclear export signal, and could be inactivated by site-directed mutagenesis. Furthermore, SUMOylation modulated the interaction of ErbB4 with chromosomal region maintenance 1 (CRM1), the major nuclear export receptor for proteins. Finally, the SUMO acceptor lysine was functionally required for ErbB4 ICD-mediated inhibition of mammary epithelial cell differentiation in a three-dimensional cell culture model. Our findings indicate that a SUMOylation-mediated mechanism regulates nuclear localization and function of the ICD of ErbB4 receptor tyrosine kinase.

Genome-wide screen of gamma-secretase-mediated intramembrane cleavage of receptor tyrosine kinases

Receptor tyrosine kinases (RTKs) have been demonstrated to signal via regulated intramembrane proteolysis, in which ectodomain shedding and subsequent intramembrane cleavage by gamma-secretase leads to release of a soluble intracellular receptor fragment with functional activity. For most RTKs, however, it is unknown whether they can exploit this new signaling mechanism. Here we used a system-wide screen to address the frequency of susceptibility to gamma-secretase cleavage among human RTKs. The screen covering 45 of the 55 human RTKs identified 12 new as well as all nine previously published gamma-secretase substrates. We biochemically validated the screen by demonstrating that the release of a soluble intracellular fragment from endogenous AXL was dependent on the sheddase disintegrin and metalloprotease 10 (ADAM10) and the gamma-secretase component presenilin-1. Functional analysis of the cleavable RTKs indicated that proliferation promoted by overexpression of the TAM family members AXL or TYRO3 depends on gamma-secretase cleavage. Taken together, these data indicate that gamma-secretase-mediated cleavage provides an additional signaling mechanism for numerous human RTKs.

ErbB Receptors and Cancer

The ErbB receptor family, also known as the EGF receptor family or type I receptor family, includes the epidermal growth factor (EGF) receptor (EGFR) or ErbB1/Her1, ErbB2/Her2, ErbB3/Her3, and ErbB4/Her4. Among all RTKs, EGFR was the first RTK identified and the first one linked to cancer. Thus, EGFR has also been the most intensively studied among all RTKs. ErbB receptors are activated after homodimerization or heterodimerization. The ErbB family is unique among the various groups of receptor tyrosine kinases (RTKs) in that ErbB3 has impaired kinase activity, while ErbB2 does not have a direct ligand. Therefore, heterodimerization is an important mechanism that allows the activation of all ErbB receptors in response to ligand stimulation. The activated ErbB receptors bind to many signaling proteins and stimulate the activation of many signaling pathways. The specificity and potency of intracellular signaling pathways are determined by positive and negative regulators, the specific composition of activating ligand(s), receptor dimer components, and the diverse range of proteins that associate with the tyrosine phosphorylated C-terminal domain of the ErbB receptors. ErbB receptors are overexpressed or mutated in many cancers, especially in breast cancer, ovarian cancer, and non-small cell lung cancer. The overexpression and overactivation of ErbB receptors are correlated with poor prognosis, drug resistance, cancer metastasis, and lower survival rate. ErbB receptors, especially EGFR and ErbB2 have been the primary choices as targets for developing cancer therapies.

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.

Direct coupling of the HER4 intracellular domain (4ICD) and STAT5A signaling is required to induce mammary epithelial cell differentiation

The HER4 receptor tyrosine kinase and STAT5A cooperate to promote mammary luminal progenitor cell maturation and mammary epithelial cell differentiation. Coupled HER4 and STAT5A signaling is mediated, in part, through association of the HER4 intracellular domain (4ICD) with STAT5A at STAT5A target gene promoters where 4ICD functions as a STAT5A transcriptional coactivator. Despite an essential role for coupled 4ICD and STAT5A signaling in mammary gland development, the mechanistic basis of 4ICD and STAT5A cooperative signaling remains unexplored. Here we show for the first time that 4ICD and STAT5A directly interact through STAT5A recruitment and binding to HER4/4ICD residue Y984. Accordingly, altering the 4ICD Y984 to phenylalanine results in a dramatic reduction of STAT5A and 4ICD-Y984F interacting complexes coimmunoprecipitated with HER4 or STAT5A specific antibodies. We further show that disrupting the 4ICD and STAT5A interaction has an important physiological impact on mammary epithelial cell differentiation. HC11 mammary epithelial cells with stable expression of 4ICD undergo differentiation with significantly increased expression of the STAT5A target genes and differentiation markers β-casein and WAP. In contrast, HC11 cells stably expressing 4ICD-Y984F failed to undergo differentiation with basal expression levels of β-casein and WAP. Differentiation in this cell system was induced in the absence of exogenous prolactin indicating that 4ICD activity is sufficient to induce mammary epithelial cell differentiation. Finally, we show that suppression of STAT5A expression abolishes the ability of 4ICD to induce HC11 differentiation and activate β-casein or WAP expression. Taken together our results demonstrate for the first time that direct coupling of 4ICD and STAT5A is both necessary and sufficient to drive mammary epithelial differentiation. In conclusion, our findings that 4ICD and STAT5A directly interact to form a physiologically important transcriptional activation complex, provide a mechanistic basis for the in vivo observations that HER4/4ICD and STAT5A cooperate to promote mammary gland progenitor cell maturation and initiate lactation at parturition.

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HER4/4ICD tyrosine 984 mediates a direct interaction with STAT5A.

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4ICD expression with an intact Y984 is sufficient to induce mammary differentiation.

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Mammary differentiation is abolished by disrupting the 4ICD and STAT5A interaction.

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STAT5 expression is required for 4ICD-induced mammary differentiation.

Targeting HER family in HER2-positive metastatic breast cancer: potential biomarkers and novel targeted therapies

HER2-targeted therapies have radically changed the prognosis of HER2-positive breast cancer over the last few years. However, resistance to these therapies has been a constant, leading to treatment-failure and new tumor progression. Recently, the kinase-impaired HER3 emerged as a pivotal player in oncogenic signaling, with an important role in both non-treated progression and treatment response. HER2/HER3 dimerization is required for full signaling potential and constitutes the key oncogenic unit. Also, when inhibiting PI3K/AKT pathway (as with anti-HER2 drugs) feedback mechanisms lead to a rebound in HER3 activity, which is one of the main roads to resistance. As current strategies to treat HER2-positive breast cancer are unable to inhibit this feedback response, two great promises emerged: the combination of targeted-therapies and drugs targeting HER3. In this article HER2 and HER3-targeted drugs and possible combinations between them, as well as the biomarkers to predict and monitor these drugs effect, are reviewed.

Deregulation of the EGFR/PI3K/PTEN/Akt/mTORC1 pathway in breast cancer: possibilities for therapeutic intervention

The EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway plays prominent roles in malignant transformation, prevention of apoptosis, drug resistance and metastasis. The expression of this pathway is frequently altered in breast cancer due to mutations at or aberrant expression of: HER2, ERalpha, BRCA1, BRCA2, EGFR1, PIK3CA, PTEN, TP53, RB as well as other oncogenes and tumor suppressor genes. In some breast cancer cases, mutations at certain components of this pathway (e.g., PIK3CA) are associated with a better prognosis than breast cancers lacking these mutations. The expression of this pathway and upstream HER2 has been associated with breast cancer initiating cells (CICs) and in some cases resistance to treatment. The anti-diabetes drug metformin can suppress the growth of breast CICs and herceptin-resistant HER2+ cells. This review will discuss the importance of the EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway primarily in breast cancer but will also include relevant examples from other cancer types. The targeting of this pathway will be discussed as well as clinical trials with novel small molecule inhibitors. The targeting of the hormone receptor, HER2 and EGFR1 in breast cancer will be reviewed in association with suppression of the EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway.

Roles of signaling pathways in drug resistance, cancer initiating cells and cancer progression and metastasis

The EGFR/PI3K/PTEN/Akt/mTORC pathway plays prominent roles in malignant transformation, prevention of apoptosis, drug resistance, cancer initiating cells (CICs) and metastasis. The expression of this pathway is frequently altered in breast and other cancers due to mutations at or aberrant expression of: HER2, EGFR1, PIK3CA, and PTEN as well as other oncogenes and tumor suppressor genes. miRs and epigenetic mechanisms of gene regulation are also important events which regulate this pathway. In some breast cancer cases, mutations at certain components of this pathway (e.g., PIK3CA) are associated with a better prognosis than breast cancers lacking these mutations. The expression of this pathway has been associated with CICs and in some cases resistance to therapeutics. We will review the effects of activation of the EGFR/PI3K/PTEN/Akt/mTORC pathway primarily in breast cancer and development of drug resistance. The targeting of this pathway and other interacting pathways will be discussed as well as clinical trials with novel small molecule inhibitors as well as established drugs that are used to treat other diseases. In this manuscript, we will discuss an inducible EGFR model (v-ERB-B:ER) and its effects on cell growth, cell cycle progression, activation of signal transduction pathways, prevention of apoptosis in hematopoietic, breast and prostate cancer models.

Neuregulin-3 regulates epithelial progenitor cell positioning and specifies mammary phenotype

Mutation of Neuregulin-3 (Nrg3) results in defective embryonic mammary gland development. Here, we investigate functions of Nrg3 signaling in embryonic mammary morphogenesis. Nrg3 regulates the distribution of epithelial progenitor cells within the presumptive mammary-forming region during early mammary morphogenesis. Basal and suprabasal epithelial cells are significantly smaller within the hypoplastic mammary primordium (MP) that forms in Nrg3 mutants, indicative of failure to acquire mammary epithelial cell (MEC) morphological phenotype. Activation of Erbb4 JM-a CYT-1, an Erbb4 isoform expressed in the developing MP, leads to MEC spreading and migration. Nrg3 promotes the accumulation of epithelial progenitor cells at the MP site in embryo explant cultures. Our results implicate Nrg3 signaling in mediating key events of mammary mesenchyme specification, including mesenchymal condensation, mitosis, and induction of mammary marker expression. Taken together, our results show Nrg3 has a major role in conferring specification of the mammary phenotype to both epithelial and mesenchymal progenitor cells.

The ins and outs of fibroblast growth factor receptor signalling

FGFR (fibroblast growth factor receptor) signalling plays critical roles in embryogensis, adult physiology, tissue repair and many pathologies. Of particular interest over recent years, it has been implicated in a wide range of cancers, and concerted efforts are underway to target different aspects of FGFR signalling networks. A major focus has been identifying the canonical downstream signalling pathways in cancer cells, and these are now relatively well understood. In the present review, we focus on two distinct but emerging hot topics in FGF biology: its role in stromal cross-talk during cancer progression and the potential roles of FGFR signalling in the nucleus. These neglected areas are proving to be of great interest clinically and are intimately linked, at least in pancreatic cancer. The importance of the stroma in cancer is well accepted, both as a conduit/barrier for treatment and as a target in its own right. Nuclear receptors are less acknowledged as targets, largely due to historical scepticism as to their existence or importance. However, increasing evidence from across the receptor tyrosine kinase field is now strong enough to make the study of nuclear growth factor receptors a major area of interest.

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.

The prognostic value of Her4 receptor isoform expression in triple-negative and Her2 positive breast cancer patients

BACKGROUND

Not only four but rather seven different human epidermal growth factor receptor related (Her) receptor tyrosine kinases (RTKs) have been described to be expressed in a variety of normal and neoplastic tissues: Her1, Her2, Her3, and additionally four Her4 isoforms have been identified. A differential expression of Her4 isoforms does not, however, play any role in either the molecular diagnostics or treatment decision for breast cancer patients. The prognostic and predictive impact of Her4 expression in breast cancer is basically unclear.

METHODS

We quantified the Her4 variants JM-a/CYT1, JM-a/CYT2, JM-b/CYT1, and JM-b/CYT2 by isoform-specific polymerase chain reaction (qPCR) in (i) triple-negative, (ii) Her2 positive breast cancer tissues and (iii) in benign breast tissues.

RESULTS

In all three tissue collectives we never found the JM-b/CYT1 or the JM-b/CYT2 isoform expressed. In contrast, the two JM-a/CYT1 and JM-a/CYT2 isoforms were always simultaneously expressed but at different ratios. We identified a positive prognostic impact on overall survival (OS) in triple-negative and event-free survival (EFS) in Her2 positive patients. This finding is independent of the absolute JM-a/CYT1 to JM-a/CYT2 expression ratio. In Her2 positive patients, Her4 expression only has a favorable effect in estrogen-receptor (ER)-positive but not in ER-negative individuals.

CONCLUSION

In summary, JM-a/CYT1 and JM-a/CYT2 but not JM-b isoforms of the Her4 receptor are simultaneously expressed in both triple-negative and Her2 positive breast cancer tissues. Although different expression ratios of the two JM-a isoforms did not reveal any additional information, Her4 expression basically indicates a prolonged EFS and OFS. An extended expression analysis that takes all Her receptor homologs, including the Her4 isoforms, into account might render more precisely the molecular diagnostics required for the development of optimized targeted therapies.

Multiple Functional Motifs Are Required for the Tumor Suppressor Activity of a Constitutively-Active ErbB4 Mutant

ErbB4 (HER4) is a member of the ErbB family of receptor tyrosine kinases, which includes the Epidermal Growth Factor Receptor (EGFR/ErbB1), ErbB2 (HER2/Neu), and ErbB3 (HER3). Mounting evidence indicates that ErbB4, unlike EGFR or ErbB2, functions as a tumor suppressor in many human malignancies. Previous analyses of the constitutively-dimerized and -active ErbB4 Q646C mutant indicate that ErbB4 kinase activity and phosphorylation of ErbB4 Tyr1056 are both required for the tumor suppressor activity of this mutant in human breast, prostate, and pancreatic cancer cell lines. However, the cytoplasmic region of ErbB4 possesses additional putative functional motifs, and the contributions of these functional motifs to ErbB4 tumor suppressor activity have been largely underexplored. Here we demonstrate that ErbB4 BH3 and LXXLL motifs, which are thought to mediate interactions with Bcl family proteins and steroid hormone receptors, respectively, are required for the tumor suppressor activity of the ErbB4 Q646C mutant. Furthermore, abrogation of the site of ErbB4 cleavage by gamma-secretase also disrupts the tumor suppressor activity of the ErbB4 Q646C mutant. This last result suggests that ErbB4 cleavage and subcellular trafficking of the ErbB4 cytoplasmic domain may be required for the tumor suppressor activity of the ErbB4 Q646C mutant. Indeed, here we demonstrate that mutants that disrupt ErbB4 kinase activity, ErbB4 phosphorylation at Tyr1056, or ErbB4 cleavage by gamma-secretase also disrupt ErbB4 trafficking away from the plasma membrane and to the cytoplasm. This supports a model for ErbB4 function in which ErbB4 tumor suppressor activity is dependent on ErbB4 trafficking away from the plasma membrane and to the cytoplasm, mitochondria, and/or the nucleus.

ErbB4 is an upstream regulator of TTF-1 fetal mouse lung type II cell development in vitro

TTF-1 is an important transcription factor in lung development and lung disease and is essential for lung cell differentiation, specifically surfactant protein (Sftp) expression. The molecular mechanisms that drive the expression and transcriptional control of TTF-1 are not fully understood. In the fetal lung, ErbB4 functions as a transcriptional co-factor and regulates the timely onset of fetal Sftp expression. We speculate that ErbB4 is an upstream regulator of TTF-1 and regulates Sftpb expression via this pathway in alveolar type II cells. Neuregulin-induced ErbB4 and TTF-1 signaling interactions were studied by co-immunoprecipitation and confocal microscopy. Overexpression of ErbB4 and TTF-1 was analyzed in its effect on cell viability, Sftpb expression, TTF-1 expression, and Sftpb and TTF-1 promoter activity. The effect of ErbB4 deletion and ErbB4 nuclear translocation on TTF-1 expression was studied in primary fetal type II epithelial cells, isolated from transgenic HER4(heart(-/-)) mice. ErbB4 ligand neuregulin induces ErbB4 and TTF-1 co-precipitation and nuclear colocalization. Combined ErbB4 and TTF-1 overexpression inhibits cell viability, while promoting Sftpb expression more than single overexpression of each protein. NRG stimulates TTF-1 expression in ErbB4-overexpressing epithelial cells, while this effect is absent in ErbB4-depleted cells. In primary fetal type II cells, ErbB4 nuclear translocation is critical for its regulation of TTF-1-induced Sftpb upregulation. TTF-1 overexpression did not overcome this important requirement. We conclude that ErbB4 is a critical upstream regulator of TTF-1 in type II epithelial cells and that this interaction is important for Sftpb regulation.

Overexpression of epithelial growth factor receptor (EGFR) predicts better response to neo-adjuvant chemotherapy in patients with triple-negative breast cancer

Background

Triple negative breast cancer (TNBC) occurs in approximately 10% to 25% of all patients with breast cancer and is associated with poor prognosis. Neo-adjuvant chemotherapy has been reported to produce a higher pathologic complete response (pCR) rate in TNBC. If pCR is achieved, patients with TNBC had a similar survival with non-TNBC patients. The aim of our study was to investigate the protein expression of epithelial growth factor receptor (EGFR) and response to neo-adjuvant chemotherapy and clinical outcome in patients with TNBC compared with non-TNBC.

Methods

A total of 198 locally advanced breast cancer patients who received neo-adjuvant chemotherapy were studied. Immunohistochemistry (IHC) was carried out to detect the protein expression of EGFR in tumor samples. Clinical and pathological parameters, pCR rate and survival data were compared between 40 TNBCs and 158 non-TNBCs.

Results

In 198 cases who received neo-adjuvant chemotherapy, significant differences exist in surgical therapy (P=0.005) and pCR rate (P=0.012) between patients with TNBCs and non-TNBCs. Overexpression of EGFR was significantly associated with pCR rate in patients with TNBCs (P < 0.001). Survival analysis revealed that patients with TNBCs had worse DFS and OS than those with non-TNBCs (P = 0.001, P < 0.001 respectively). Furthermore, for patients with non-TNBCs, those who acheived pCR had better DFS and OS than those who acheived RD (both P < 0.001).

Conclusions

Our results suggested that patients with TNBCs had increased pCR rates compared with non-TNBC. Overexpression of EGFR predicted better response to neo-adjuvant chemotherapy in patients with TNBCs.

ErbB2 Is Necessary for ErbB4 Ligands to Stimulate Oncogenic Activities in Models of Human Breast Cancer

ErbB4 is a member of the ErbB family of receptor tyrosine kinases. This family includes ErbB2 (HER2/Neu), a validated therapeutic target in breast cancer. Several studies indicate that ErbB4 functions as a tumor suppressor in breast cancer, whereas others indicate that ErbB4 functions as an oncogene. Here the authors explore the context in which ErbB4 functions as an oncogene. Silencing expression of either ErbB2 or ErbB4 in breast tumor cell lines results in reduced stimulation of anchorage independence and cell motility by the ErbB4 agonist neuregulin 2β. ErbB2 tyrosine kinase activity, but not ErbB4 tyrosine kinase activity, is required for neuregulin 2β to stimulate cell proliferation. Moreover, sites of ErbB4 tyrosine phosphorylation, but not sites of ErbB2 tyrosine phosphorylation, are required for neuregulin 2β to couple to cell proliferation. These data suggest that targeting ErbB2 expression or tyrosine kinase activity may be effective in treating ErbB4-dependent breast tumors, even those tumors that lack ErbB2 overexpression.

The Q43L mutant of neuregulin 2β is a pan-ErbB receptor antagonist

The ErbB4 receptor tyrosine kinase possesses both tumour suppressor and oncogenic activities. Thus pharmacological agents are needed to help elucidate ErbB4 functions. However, limitations of existing ErbB4 agonists and antagonists have led us to seek novel ErbB4 antagonists. The Q43L mutant of the ErbB4 agonist NRG2β (neuregulin 2β) stimulates ErbB4 tyrosine phosphorylation, yet fails to stimulate ErbB4 coupling to cell proliferation. Thus in the present paper we hypothesize that NRG2β/Q43L may be an ErbB4 antagonist. NRG2β/Q43L competitively antagonizes agonist stimulation of ErbB4 coupling to cell proliferation. NRG2β/Q43L stimulates less ErbB4 tyrosine phosphorylation than does NRG2β. In addition, NRG2β stimulation of cell proliferation requires PI3K (phosphoinositide 3-kinase) activity and NRG2β stimulates greater Akt phosphorylation than does NRG2β/Q43L. Moreover, EGFR [EGF (epidermal growth factor) receptor] kinase activity (but not that of ErbB4) is critical for coupling ErbB4 to proliferation. Experiments utilizing ErbB4 splicing isoforms and mutants suggest that NRG2β and NRG2β/Q43L may differentially stimulate ErbB4 coupling to the transcriptional co-regulator YAP (Yes-associated protein). Finally, NRG2β/Q43L competitively antagonizes agonist stimulation of EGFR and ErbB2/ErbB3, indicating that NRG2β/Q43L is a pan-ErbB antagonist. Thus we postulate that NRG2β/Q43L and other antagonistic ligands stimulate ErbB tyrosine phosphorylation on a set of residues distinct from that stimulated by agonists, thus suggesting a novel mechanism of ErbB receptor regulation. Moreover, NRG2β/Q43L and related ligand-based antagonists establish a paradigm for the discovery of anti-ErbB therapeutics.

Nuclear functions and subcellular trafficking mechanisms of the epidermal growth factor receptor family

Accumulating evidence suggests that various diseases, including many types of cancer, result from alteration of subcellular protein localization and compartmentalization. Therefore, it is worthwhile to expand our knowledge in subcellular trafficking of proteins, such as epidermal growth factor receptor (EGFR) and ErbB-2 of the receptor tyrosine kinases, which are highly expressed and activated in human malignancies and frequently correlated with poor prognosis. The well-characterized trafficking of cell surface EGFR is routed, via endocytosis and endosomal sorting, to either the lysosomes for degradation or back to the plasma membrane for recycling. A novel nuclear mode of EGFR signaling pathway has been gradually deciphered in which EGFR is shuttled from the cell surface to the nucleus after endocytosis, and there, it acts as a transcriptional regulator, transmits signals, and is involved in multiple biological functions, including cell proliferation, tumor progression, DNA repair and replication, and chemo- and radio-resistance. Internalized EGFR can also be transported from the cell surface to several intracellular compartments, such as the Golgi apparatus, the endoplasmic reticulum, and the mitochondria, in addition to the nucleus. In this review, we will summarize the functions of nuclear EGFR family and the potential pathways by which EGFR is trafficked from the cell surface to a variety of cellular organelles. A better understanding of the molecular mechanism of EGFR trafficking will shed light on both the receptor biology and potential therapeutic targets of anti-EGFR therapies for clinical application.

Slow down to stay alive: HER4 protects against cellular stress and confers chemoresistance in neuroblastoma

BACKGROUND

Neuroblastoma (NBL) is a common pediatric solid tumor, and outcomes for patients with advanced neuroblastoma remain poor despite extremely aggressive treatment. Chemotherapy resistance at relapse contributes heavily to treatment failure. The poor survival of patients with high-risk NBL prompted this investigation into novel treatment options with the objective of gaining a better understanding of resistance mechanisms. On the basis of previous work and on data from publicly available studies, the authors hypothesized that human epidermal growth factor receptor 4 (Her4) contributes to resistance.

METHODS

Her4 expression was reduced with small-hairpin RNA (shRNA) to over express intracellular HER4, and the authors tested its impact on tumor cell survival under various culture conditions. The resulting changes in gene expression after HER4 knockdown were measured by using a messenger RNA (mRNA) array.

RESULTS

HER4 expression was up-regulated in tumor spheres compared with the expression in monolayer culture. With HER4 knockdown, NBL cells became less resistant to anoikis and serum starvation. Moreover, HER4 knockdown increased the chemosensitivity of NBL cells to cisplatin, doxorubicin, etoposide, and activated ifosfamide. In mRNA array analysis, HER4 knockdown predominately altered genes related to cell cycle regulation. In NBL spheres compared with monolayers, cell proliferation was decreased, and cyclin D expression was reduced. HER4 knockdown reversed cyclin D suppression. Overexpressed intracellular HER4 slowed the cell cycle and induced chemoresistance.

CONCLUSIONS

The current results indicated that HER4 protects NBL cells from multiple exogenous apoptotic stimuli, including anoikis, nutrient deficiency, and cytotoxic chemotherapy. The intracellular fragment of HER4 was sufficient to confer this phenotype. HER4 functions as a cell cycle suppressor, maintaining resistance to cellular stress. The current findings indicate that HER4 overexpression may be associated with refractory disease, and HER4 may be an important therapeutic target.

DHHC-7 and -21 are palmitoylacyltransferases for sex steroid receptors

Extranuclear sex steroid receptors require palmitoylation to traffic to the plasma membrane, where they activate signal transduction cascades. We identify DHHC-7 and -21 palmitoylacyltransferases as conserved enzymes for the three classes of sex steroid receptors.

Classical estrogen, progesterone, and androgen receptors (ERs, PRs, and ARs) localize outside the nucleus at the plasma membrane of target cells. From the membrane, the receptors signal to activate kinase cascades that are essential for the modulation of transcription and nongenomic functions in many target cells. ER, PR, and AR trafficking to the membrane requires receptor palmitoylation by palmitoylacyltransferase (PAT) protein(s). However, the identity of the steroid receptor PAT(s) is unknown. We identified the DHHC-7 and -21 proteins as conserved PATs for the sex steroid receptors. From DHHC-7 and -21 knockdown studies, the PATs are required for endogenous ER, PR, and AR palmitoylation, membrane trafficking, and rapid signal transduction in cancer cells. Thus the DHHC-7 and -21 proteins are novel targets to selectively inhibit membrane sex steroid receptor localization and function.

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.

Ectodomain shedding and remnant peptide signalling of EGFRs and their ligands

Both receptor tyrosine kinases epidermal growth factor receptors (EGFRs) and their ligands are transmembrane proteins. It has been known that ligand binding activates cytoplasmic tyrosine kinase domains of EGFRs, resulting in the transduction of signals for cell proliferation, migration, differentiation or survival. In an EGFRs-ligands system, however, signal transduction occurs not only unidirectionally but also bidirectionally, which is regulated by cell-cell contact and proteolytic cleavage. Recent studies of proteolytic cleavage 'ectodomain shedding' of EGFRs and their ligands mediated by membrane-type metalloproteinases, a disintegrin and metalloproteinases have been unveiling novel functions and molecular mechanism of their remnant peptides. In addition, the study of the remnant peptide signalling would be essential for understanding the physiological and pathological relevance of anti-shedding therapeutic strategies for diseases such as cancer.

Extracellular signals in young and aging breast epithelial cells and possible connections to age-associated breast cancer development

Aging of human breast tissue is accompanied by certain structural and functional variations and several studies suggest a possible contribution of these changes to an aging-related breast cancer development. At the cellular level, aging of human mammary epithelial cells is associated with significant morphological and functional alterations such as an increased cell size and a reduced proliferation. Cellular senescence of HMEC cannot be explained by a single mechanism but represents an interaction of numerous extra- and intracellular events and the complexity of such orchestrating pathways is still hardly understood. Besides the contribution of reactive oxygen species and telomere dysfunction to aging, it is the aim of this mini-review, to compare distinct changes to extracellular signals by certain matrix metalloproteinases including MMP-7 and associated growth factor pathways mediated by HB-EGF activation in young and aging HMEC. Such changes can alter hormone receptor levels within aged HMEC, induce tissue fibrosis and promote epithelial-to-mesenchymal transition as a potential prerequisite for breast cancer development. Moreover, an accumulation of aging cells during the normal life span of the breast tissue may also substantially effect and interact with adjacent neighboring populations in the local microenvironment to provide optimized growth conditions which would also support neoplastic cells.

The therapeutic potential of targeting the EGFR family in epithelial ovarian cancer

The epithelial growth factor receptor (EGFR) family of receptor tyrosine kinases has been reported to have an active role in a number of malignancies. Amplifications and overexpression of various EGFR family members, including EGFR, Her2, and ErbB3, have been reported in epithelial ovarian cancer. Although anti-EGFR-targeted therapy has shown limited clinical activity in ovarian cancer to date, a recent report suggests that activation of ErbB3, one of the members of the EGFR family, may support the growth and proliferation of ovarian cancer cells and that ErbB3 may therefore serve as a potential therapeutic target in this disease. Here, we review the EGFR family and the clinical experience with anti-EGFR family member-directed therapies in ovarian cancer to date.

The ERBB4 intracellular domain (4ICD) regulates NRG1-induced gene expression in hippocampal neurons

The NRG1 growth factor and ERBB4 receptor have been identified as leading schizophrenia risk genes. Although NRG1 and ERBB4 have been shown to modulate neuronal functions involved in schizophrenia, including both GABAergic and glutamatergic synapses, the exact molecular mechanisms remain poorly understood. Here we investigated ERBB4 intracellular domain, 4ICD, transactivator function in rat hippocampal cultures by inhibiting γ-secretase mediated ERBB4 regulated intramembrane proteolysis (RIP). NRG1 stimulation resulted in a dramatic increase in the number of hippocampal cells displaying nuclear 4ICD which was abolished in cultures pretreated with the γ-secretase inhibitor compound E (CE). To identify NRG1-4ICD transactivated genes we compared global gene expression profiles of hippocampal cultures stimulated with NRG1 in the absence or presence of CE. In concordance with the contribution of NRG1-ERBB4 signaling to dendritic spine maturation and schizophrenia, global gene expression analysis followed by Ingenuity Pathway Analysis of the dataset identified NRG1-4ICD regulated genes significantly represented in semaphorin signaling and actin cytoskeletal plasticity and multiple genes with confirmed roles in dendritic spine morphogenesis. Using the power of global gene expression analysis our data provides a proof-of-concept supporting a role for non-canonical NRG1-4ICD signaling in the regulation of gene expression contributing to normal and schizophrenic neuronal function.

Neuregulin receptor ErbB4 functions as a transcriptional cofactor for the expression of surfactant protein B in the fetal lung

Sufficient pulmonary surfactant production is required for the fetal-neonatal transition, especially in preterm infants. Neuregulin (NRG) and its transmembrane receptor ErbB4 positively regulate the onset of fetal surfactant synthesis. Details of this signaling process remain to be elucidated. ErbB4 is known to regulate gene expression in the mammary gland, where the receptor associates with the signal transducer and activator of transcription Stat5a to transactivate the β-casein gene promoter. We hypothesized that in the fetal lung, ErbB4 functions as a transcriptional regulator for surfactant protein B (Sftpb), the most critical surfactant protein gene. Re-expressing full-length ErbB4 in primary fetal ErbB4-depleted Type II epithelial cells led to an increased expression of Sftpb mRNA. This stimulatory effect required the nuclear translocation of ErbB4 and association with Stat5a, with the resultant binding to and activation of the Sftpb promoter. We conclude that ErbB4 directly regulates important aspects of fetal lung maturation that help prepare for the fetal-neonatal transition.

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.

E3 ubiquitin ligases in ErbB receptor quantity control

Signaling through ErbB family growth factor receptor tyrosine kinases is necessary for the development and homeostasis of a wide variety of tissue types. However, the intensity of receptor-mediated cellular signaling must fall within a precise range; insufficient signaling can lead to developmental abnormalities or tissue atrophy, while over-signaling can lead to hyperplastic and ultimately neoplastic events. While a plethora of mechanisms have been described that regulate downstream signaling events, it appears that cells also utilize various mechanisms to regulate their ErbB receptor levels. Such mechanisms are collectively termed "ErbB receptor quantity control." Notably, studies over the past few years have highlighted roles for post-transcriptional processes, particularly protein degradation, in ErbB quantity control. Here the involvement of ErbB-directed E3 ubiquitin ligases is discussed, including Nrdp1-mediated ErbB3 degradation, ErbB4 degradation mediated by Nedd4 family E3 ligases, and CHIP-mediated ErbB2 degradation. The hypothesis is forwarded that protein degradation-based ErbB quantity control mechanisms play central roles in suppressing receptor overexpression in normal cells, and that the loss of such mechanisms could facilitate the onset or progression of ErbB-dependent tumors.