HER3 is required for the maintenance of neuregulin-dependent and -independent attributes of malignant progression in prostate cancer cells

Eph Receptors in Cancer

Eph receptor tyrosine kinases play critical functions during development, in the formation of tissue and organ borders, and the vascular and neural systems. Uniquely among tyrosine kinases, their activities are controlled by binding to membrane-bound ligands, called ephrins. Ephs and ephrins generally have a low expression in adults, functioning mainly in tissue homeostasis and plasticity, but are often overexpressed in cancers, where they are especially associated with undifferentiated or progenitor cells, and with tumour development, vasculature, and invasion. Mutations in Eph receptors also occur in various tumour types and are suspected to promote tumourigenesis. Ephs and ephrins have the capacity to operate as both tumour promoters and tumour suppressors, depending on the circumstances. They have been demonstrated to impact tumour cell proliferation, migration, and invasion in vitro, as well as tumour development, angiogenesis, and metastases in vivo, making them potential therapeutic targets. However, successful development of therapies will require detailed understanding of the opposing roles of Ephs in various cancers. In this review, we discuss the variations in Eph expression and functions in a variety of malignancies. We also describe the multiple strategies that are currently available to target them in tumours, including preclinical and clinical development.

HER3 in cancer: from the bench to the bedside

The HER3 protein, that belongs to the ErbB/HER receptor tyrosine kinase (RTK) family, is expressed in several types of tumors. That fact, together with the role of HER3 in promoting cell proliferation, implicate that targeting HER3 may have therapeutic relevance. Furthermore, expression and activation of HER3 has been linked to resistance to drugs that target other HER receptors such as agents that act on EGFR or HER2. In addition, HER3 has been associated to resistance to some chemotherapeutic drugs. Because of those circumstances, efforts to develop and test agents targeting HER3 have been carried out. Two types of agents targeting HER3 have been developed. The most abundant are antibodies or engineered antibody derivatives that specifically recognize the extracellular region of HER3. In addition, the use of aptamers specifically interacting with HER3, vaccines or HER3-targeting siRNAs have also been developed. Here we discuss the state of the art of the preclinical and clinical development of drugs aimed at targeting HER3 with therapeutic purposes.

Inhibition of GATA2 in prostate cancer by a clinically available small molecule

Castration-resistant prostate cancer (CRPC) remains highly lethal and in need of novel, actionable therapeutic targets. The pioneer factor GATA2 is a significant prostate cancer (PC) driver and is linked to poor prognosis. GATA2 directly promotes androgen receptor (AR) gene expression (both full-length and splice-variant) and facilitates AR binding to chromatin, recruitment of coregulators, and target gene transcription. Unfortunately, there is no clinically applicable GATA2 inhibitor available at the moment. Using a bioinformatics algorithm, we screened in silico 2650 clinically relevant drugs for a potential GATA2 inhibitor. Validation studies used cytotoxicity and proliferation assays, global gene expression analysis, RT-qPCR, reporter assay, reverse phase protein array analysis (RPPA), and immunoblotting. We examined target engagement via cellular thermal shift assay (CETSA), ChIP-qPCR, and GATA2 DNA-binding assay. We identified the vasodilator dilazep as a potential GATA2 inhibitor and confirmed on-target activity via CETSA. Dilazep exerted anticancer activity across a broad panel of GATA2-dependent PC cell lines in vitro and in a PDX model in vivo. Dilazep inhibited GATA2 recruitment to chromatin and suppressed the cell-cycle program, transcriptional programs driven by GATA2, AR, and c-MYC, and the expression of several oncogenic drivers, including AR, c-MYC, FOXM1, CENPF, EZH2, UBE2C, and RRM2, as well as of several mediators of metastasis, DNA damage repair, and stemness. In conclusion, we provide, via an extensive compendium of methodologies, proof-of-principle that a small molecule can inhibit GATA2 function and suppress its downstream AR, c-MYC, and other PC-driving effectors. We propose GATA2 as a therapeutic target in CRPC.

The Metastasis Suppressor NDRG1 Directly Regulates Androgen Receptor Signaling in Prostate Cancer

N-myc-downregulated gene 1 (NDRG1) has potent anticancer effects and inhibits cell growth, survival, metastasis, and angiogenesis. Previous studies suggested that NDRG1 is linked to the androgen signaling network, but this mechanistic relationship is unclear. Considering the crucial role of the androgen receptor (AR) in prostate cancer (PCa) progression, here we examined for the first time the effect of NDRG1 on AR expression, activation, and downstream signaling in LNCaP, 22Rv1, and C4-2B PCa cell types. We demonstrate that NDRG1 effectively promotes interaction of AR with the chaperone HSP90, which in turn stabilizes the AR while decreasing its androgen-mediated activation. The expression of NDRG1 suppressed: (1) AR activation, as measured by p-AR^Ser213 and p-AR^Ser81; (2) expression of a major AR transcriptional target, prostate-specific antigen (PSA); and (3) AR transcriptional activity, probably via inhibiting the c-Jun-AR interaction by reducing c-Jun phosphorylation (p-c-Jun^Ser63). NDRG1 was also demonstrated to inhibit multiple key molecules involved in androgen-dependent and -independent signaling (namely EGFR, HER2, HER3, PI3K, STAT3, and NF-κB), which promote the development of castration-resistant prostate cancer. We also identified the cysteine-rich secretory protein/antigen 5/pathogenesis related-1 (CAP) domain of NDRG1 as vital for inhibition of AR activity. Examining NDRG1 and p-NDRG1 in PCa patient specimens revealed a significant negative correlation between NDRG1 and PSA levels in prostatectomy patients that went on to develop metastasis. These results highlight a vital role for NDRG1 in androgen signaling and its potential as a key therapeutic target and biomarker in PCa.

The Role of the Eph Receptor Family in Tumorigenesis

Simple Summary

The Eph receptor family is implicated in both tumour promotion and suppression, depending on the tissue-specific context of available receptor interactions with ligands, adaptor proteins and triggered downstream signalling pathways. This complex interplay has not only consequences for tumorigenesis but also offers a basis from which new cancer-targeting strategies can be developed. This review comprehensively summarises the current knowledge of Eph receptor implications in oncogenesis in a tissue- and receptor-specific manner, with the aim to develop a better understanding of Eph signalling pathways for potential targeting in novel cancer therapies.

Abstract

The Eph receptor tyrosine kinase family, activated by binding to their cognate ephrin ligands, are important components of signalling pathways involved in animal development. More recently, they have received significant interest due to their involvement in oncogenesis. In most cases, their expression is altered, affecting the likes of cell proliferation and migration. Depending on the context, Eph receptors have the potential to act as both tumour promoters and suppressors in a number of cancers, such as breast cancer, colorectal cancer, lung cancer, prostate cancer, brain cancer and Kaposi’s sarcoma (KS), the latter being intrinsically linked to EphA2 as this is the receptor used for endothelial cell entry by the Kaposi’s sarcoma-associated herpesvirus (KSHV). In addition, EphA2 deregulation is associated with KS, indicating that it has a dual role in this case. Associations between EphA2 sequence variation and KSHV infection/KS progression have been detected, but further work is required to formally establish the links between EphA2 signalling and KS oncogenesis. This review consolidates the available literature of the role of the Eph receptor family, and particularly EphA2, in tumorigenesis, with the aim to develop a better understanding of Eph signalling pathways for potential targeting in novel cancer therapies.

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.

High ErbB3 activating activity in human blood is not due to circulating neuregulin-1 beta

Neuregulin-1β (NRG-1) is a membrane-bound or secreted growth and differentiation factor that mediates its action by binding to ErbB receptors. Circulating levels of NRG-1 are characterized by large inter-individual variability with the range of absolute values covering two orders of magnitude, from hundreds to tens of thousands of picograms per milliliter of blood. NRG-1 signaling via ErbB receptors contributes to the cell survival and downregulation of the inflammatory response. A higher level of circulating NRG-1 may indicate increased shedding of membrane-bound NRG-1, which in turn can contribute to better protection against cardiovascular stress or injury. However, it is unknown whether circulating NRG-1 can induce activation of ErbB receptors. In the current study, we performed an analysis of circulating NRG-1 functional activity using a cell-based ELISA measuring phosphorylation of ErbB3 induced by blood plasma obtained from healthy donors. We found high levels of ErbB3 activating activity in human plasma. No correlations were found between the levels of circulating NRG-1 and plasma ErbB3 activating activity. To determine the direct effect of circulating NRG-1, we incubated plasma with neutralizing antibody, which prevented the stimulatory effect of recombinant NRG-1 on activation of ErbB3. No effect of the neutralizing antibody was found on plasma-induced phosphorylation of ErbB3. We also found that a significant portion of circulating NRG-1 is comprised of full-length NRG-1 associated with large extracellular vesicles. Our results demonstrate that circulating NRG-1 does not contribute to plasma-induced ErbB3 activating activity and emphasizes the importance of functional testing of NRG-1 proteins in biological samples.

Neuropeptide bombesin receptor activation stimulates growth of lung cancer cells through HER3 with a MAPK-dependent mechanism

Despite recent advances in treatment of non-small cell lung cancer (NSCLC), prognosis still remains poor and new therapeutic approaches are needed. Studies demonstrate the importance of the EGFR/HER-receptor family in NSCLC growth, as well as that of other tumors. Recently, HER3 is receiving increased attention because of its role in drug resistance and aggressive growth. Activation of overexpressed G-protein-coupled receptors (GPCR) can also initiate growth by transactivating EGFR/HER-family members. GPCR transactivation of EGFR has been extensively studied, but little is known of its ability to transactivate other EGFR/HER-members, especially HER3. To address this, we studied the ability of bombesin receptor (BnR) activation to transactivate all EGFR/HER-family members and their principal downstream signaling cascades, the PI3K/Akt- and MAPK/ERK-pathways, in human NSCLC cell-lines. In all three cell-lines studied, which possessed EGFR, HER2 and HER3, Bn rapidly transactivated EGFR, HER2 and HER3, as well as Akt and ERK. Immunoprecipitation studies revealed Bn-induced formation of both HER3/EGFR- and HER3/HER2-heterodimers. Specific EGFR/HER3 antibodies or siRNA-knockdown of EGFR and HER3, demonstrated Bn-stimulated activation of EGFR/HER members is initially through HER3, not EGFR. In addition, specific inhibition of HER3, HER2 or MAPK, abolished Bn-stimulated cell-growth, while neither EGFR nor Akt inhibition had an effect. These results show HER3 transactivation mediates all growth effects of BnR activation through MAPK. These results raise the possibility that targeting HER3 alone or with GPCR activation and its signal cascades, may be a novel therapeutic approach in NSCLC. This is especially relevant with the recent development of HER3-blocking antibodies.

ErbB1 and ErbB3 co-over expression as a prognostic factor in gastric cancer

BACKGROUND

Epidermal growth factor receptor family members such as ErbB1 and ErbB3 are involved in tumor progression and metastasis. Although, there are various reports about the prognostic value of EGFR members separately in gastric cancer, there is not any report about the probable correlation between ErbB1 and ErbB3 co-expression and gastric cancer prognosis. In present study, we assessed the correlation between ErbB1 and ErbB3 co-overexpression (in the level of mRNA and protein expression) and gastric cancer prognosis for the first time.

METHODS

ErbB1 and ErbB3 expressions were analyzed by immunohistochemistry and real-time PCR in 50 patients with gastric cancer. Parametric correlations were done between the ErbB1 and ErbB3 expression and clinicopathological features. Multivariate and logistic regression analyses were also done to assess the roles of ErbB1 and ErbB3 in tumor prognosis and survival.

RESULTS

There were significant correlations between ErbB1/ErbB3 co-overexpression and tumor size (p = 0.026), macroscopic features (p < 0.05), tumor differentiation (p < 0.05), stage of tumor (p < 0.05), and recurrence (p < 0.05). Moreover, ErbB1/ErbB3 co-overexpression may predict the survival status of patients (p < 0.05).

CONCLUSION

ErbB1 and ErbB3 co-overexpression is accompanied with the poor prognosis and can be used efficiently in targeted therapy of gastric cancer patients.

Phage Display Selection, In Vitro Characterization, and Correlative PET Imaging of a Novel HER3 Peptide

PURPOSE

HER3 (ERBB3) is a receptor tyrosine kinase that is implicated in treatment resistance across multiple cancers, including those of the breast, lung, and prostate. Overexpression of HER3 following targeted therapy can occur rapidly and heterogeneously both within a single lesion and across sites of metastasis, making protein quantification by biopsy highly challenging. A global, non-invasive methodology such as positron emission tomography (PET) imaging can permit serial quantification of HER3, providing a useful approach to monitor HER3 expression across the entire tumor burden both prior to and following treatment. PET imaging of HER3 expression may permit a more personalized approach to targeted therapy by allowing for detection of HER3-mediated resistance, in addition to informing clinical trial patient selection for novel therapies targeting HER3.

PROCEDURES

Phage display selection targeting the HER3 extracellular domain was performed in order to develop a peptide with optimal blood clearance and highly accurate HER3 quantification.

RESULTS

The selection converged to a consensus peptide sequence that was subsequently found to bind HER3 with an affinity of 270 ± 151 nM. The peptide, termed HER3P1, was bound with high selectivity to HER3 over other similar receptor tyrosine kinases such as EGFR and HER2. Furthermore, HER3P1 was able to distinguish between high and low HER3-expressing cells in vitro. The peptide was radiolabeled with Ga-68 and demonstrated to specifically bind HER3 by in vivo PET imaging. Uptake of [⁶⁸Ga]HER3P1 was highly specific for HER3-positive tumors, with tumor-to-background ratios ranging from 1.59-3.32, compared to those of HER3-negative tumors, ranging from 0.84-0.93. The uptake of [⁶⁸Ga]HER3P1 also demonstrated high (P < 0.001) correlation with protein expression as quantified by Western blot and confirmed by biodistribution.

CONCLUSIONS

HER3P1 accurately quantifies expression of HER3 by PET imaging and has potential utility as a clinical imaging agent.

Evaluation of the Therapeutic Potential of a HER3-Binding Affibody Construct TAM-HER3 in Comparison with a Monoclonal Antibody, Seribantumab

Human epidermal growth factor receptor type 3 (HER3) is recognized to be involved in resistance to HER-targeting therapies. A number of HER3-targeting monoclonal antibodies are under clinical investigation as potential cancer therapeutics. Smaller high-affinity scaffold proteins are attractive non-Fc containing alternatives to antibodies. A previous study indicated that anti-HER3 affibody molecules could delay the growth of xenografted HER3-positive tumors. Here, we designed a second-generation HER3-targeting construct (TAM-HER3), containing two HER3-specific affibody molecules bridged by an albumin-binding domain (ABD) for extension of blood circulation. Receptor blocking activity was demonstrated in vitro. In mice bearing BxPC-3 xenografts, the therapeutic efficacy of TAM-HER3 was compared to the HER3-specific monoclonal antibody seribantumab (MM-121). TAM-HER3 inhibited heregulin-induced phosphorylation in a panel of HER3-expressing cancer cells and was found to be equally as potent as seribantumab in terms of therapeutic efficacy in vivo and with a similar safety profile. Median survival times were 60 days for TAM-HER3, 54 days for seribantumab, and 41 days for the control group. No pathological changes were observed in cytopathological examination. The multimeric HER3-binding affibody molecule in fusion to ABD seems promising for further evaluation as candidate therapeutics for treatment of HER3-overexpressing tumors.

Differential Expression Patterns of Eph Receptors and Ephrin Ligands in Human Cancers

Eph receptors constitute the largest family of receptor tyrosine kinases, which are activated by ephrin ligands that either are anchored to the membrane or contain a transmembrane domain. These molecules play important roles in the development of multicellular organisms, and the physiological functions of these receptor-ligand pairs have been extensively documented in axon guidance, neuronal development, vascular patterning, and inflammation during tissue injury. The recognition that aberrant regulation and expression of these molecules lead to alterations in proliferative, migratory, and invasive potential of a variety of human cancers has made them potential targets for cancer therapeutics. We present here the involvement of Eph receptors and ephrin ligands in lung carcinoma, breast carcinoma, prostate carcinoma, colorectal carcinoma, glioblastoma, and medulloblastoma. The aberrations in their abundances are described in the context of multiple signaling pathways, and differential expression is suggested as the mechanism underlying tumorigenesis.

Positive prognostic value of HER2-HER3 co-expression and p-mTOR in gastric cancer patients

Background

The HER2-HER3 heterodimer significantly decreases survival in breast cancer patients. However, the prognostic value of HER2-HER3 overexpression remains unknown in gastric cancer (GC).

Methods

The expression levels of HER2, HER3, Akt, p-Akt, mTOR and p-mTOR were examined in specimens from 120 GC patients by immunohistochemistry and quantitative reverse transcription-PCR. The associations of HER proteins, PI3K/Akt/mTOR pathway-related proteins, clinicopathological features of GC, and overall survival (OS) were assessed. To comprehensively evaluate the prognostic values of pathway-related proteins, meta-analyses were conducted with STATA 11.0.

Results

HER2 overexpression was significantly associated with HER3 levels (P = 0.02). HER3 was highly expressed in gastric cancer tissues. High HER2 and HER3 levels were associated with elevated p-Akt and p-mTOR amounts (P < 0.05). Furthermore, HER2-HER3 co-expression was associated with high p-Akt and p-mTOR (P < 0.05) levels. Meanwhile, p-mTOR overexpression was tightly associated with differentiation, depth of invasion, lymph node metastasis, TNM stage and OS (P < 0.05). By meta-analyses, Akt, p-Akt, and mTOR levels were unrelated to clinicopathological characters. HER3 overexpression was associated with depth of invasion (OR = 2.39, 95%CI 1.62–3.54, P < 0.001) and lymph node metastasis (OR = 2.35, 95%CI 1.34–4.11, P = 0.003). Further, p-mTOR overexpression was associated with patient age, tumor location, depth of invasion (OR = 1.63, 95%CI 1.08–2.45, P = 0.02) and TNM stage (OR = 1.73, 95%CI 1.29–2.32, P < 0.001). In addition, HER2-HER3 overexpression corresponded to gradually shortened 5-year OS (P < 0.05), and significant relationships were shown among HER3, p-mTOR overexpression, and 1-, 3-, 5-year OS (P < 0.05).

Conclusions

HER2-HER3 co-expression may potentially enhance mTOR phosphorylation. HER2-HER3 co-expression and p-mTOR are both related to the prognosis of GC patients.

Electronic supplementary material

The online version of this article (10.1186/s12885-017-3851-y) contains supplementary material, which is available to authorized users.

An ERBB1-3 Neutralizing Antibody Mixture With High Activity Against Drug-Resistant HER2+ Breast Cancers With ERBB Ligand Overexpression

Background

Plasticity of the ERBB receptor network has been suggested to cause acquired resistance to anti-human epidermal growth factor receptor 2 (HER2) therapies. Thus, we studied whether a novel approach using an ERBB1-3-neutralizing antibody mixture can block these compensatory mechanisms of resistance.

Methods

HER2+ cell lines and xenografts (n ≥ 6 mice per group) were treated with the ERBB1-3 antibody mixture Pan-HER, trastuzumab/lapatinib (TL), trastuzumab/pertuzumab (TP), or T-DM1. Downregulation of ERBB receptors was assessed by immunoblot analysis and immunohistochemistry. Paired pre- and post-T-DM1 tumor biopsies from patients (n = 11) with HER2-amplified breast cancer were evaluated for HER2 and P-HER3 expression by immunohistochemistry and/or fluorescence in situ hybridization. ERBB ligands were measured by quantitative reverse transcription polymerase chain reaction. Drug-resistant cells were generated by chronic treatment with T-DM1. All statistical tests were two-sided.

Results

Treatment with Pan-HER inhibited growth and promoted degradation of ERBB1-3 receptors in a panel of HER2+ breast cancer cells. Compared with TL, TP, and T-DM1, Pan-HER induced a similar antitumor effect against established BT474 and HCC1954 tumors, but was superior to TL against MDA-361 xenografts (TL mean = 2026 mm 3 , SD = 924 mm 3 , vs Pan-HER mean = 565 mm 3 , SD = 499 mm 3 , P = .04). Pan-HER-treated BT474 xenografts did not recur after treatment discontinuation, whereas tumors treated with TL, TP, and T-DM1 did. Post-TP and post-T-DM1 recurrent tumors expressed higher levels of neuregulin-1 (NRG1), HER3 and P-HER3 (all P < .05). Higher levels of P-HER3 protein and NRG1 mRNA were also observed in HER2+ breast cancers progressing after T-DM1 and trastuzumab (NRG1 transcript fold change ± SD; pretreatment = 2, SD = 1.9, vs post-treatment = 11.4, SD = 10.3, P = .04). The HER3-neutralizing antibody LJM716 resensitized the drug-resistant cells to T-DM1, suggesting a causal association between the NRG1-HER3 axis and drug resistance. Finally, Pan-HER treatment inhibited growth of HR6 trastuzumab- and T-DM1-resistant xenografts.

Conclusions

These data suggest that upregulation of a NRG1-HER3 axis can mediate escape from anti-HER2 therapies. Further, multitargeted antibody mixtures, such as Pan-HER, can simultaneously remove and/or block targeted ERBB receptor and ligands, thus representing an effective approach against drug-sensitive and -resistant HER2+ cancers.

A comprehensive review of heregulins, HER3, and HER4 as potential therapeutic targets in cancer

Heregulins (HRGs) bind to the receptors HER3 or HER4, induce receptor dimerization, and trigger downstream signaling that leads to tumor progression and resistance to targeted therapies. Increased expression of HRGs has been associated with worse clinical prognosis; therefore, attempts to block HRG-dependent tumor growth have been pursued. This manuscript summarizes the function and signaling of HRGs and review the preclinical evidence of its involvement in carcinogenesis, prognosis, and treatment resistance in several malignancies such as colorectal cancer, non-small cell lung cancer, ovarian cancer, and breast cancer. Agents in preclinical development and clinical trials of novel therapeutics targeting HRG-dependent signaling are also discussed, including anti-HER3 and -HER4 antibodies, anti-metalloproteinase agents, and HRG fusion proteins. Although several trials have indicated an acceptable safety profile, translating preclinical findings into clinical practice remains a challenge in this field, possibly due to the complexity of downstream signaling and patterns of HRG, HER3 and HER4 expression in different cancer subtypes. Improving patient selection through biomarkers and understanding the resistance mechanisms may translate into significant clinical benefits in the near future.

Evaluation of a radiocobalt-labelled affibody molecule for imaging of human epidermal growth factor receptor 3 expression

The human epidermal growth factor receptor 3 (HER3) is involved in the development of cancer resistance towards tyrosine kinase-targeted therapies. Several HER3‑targeting therapeutics are currently under clinical evaluation. Non-invasive imaging of HER3 expression could improve patient management. Affibody molecules are small engineered scaffold proteins demonstrating superior properties as targeting probes for molecular imaging compared with monoclonal antibodies. Feasibility of in vivo HER3 imaging using affibody molecules has been previously demonstrated. Preclinical studies have shown that the contrast when imaging using anti-HER3 affibody molecules can be improved over time. We aim to develop an agent for PET imaging of HER3 expression using the long-lived positron-emitting radionuclide cobalt-55 (55Co) (T1/2=17.5 h). A long-lived cobalt isotope 57Co was used as a surrogate for 55Co in this study. The anti-HER3 affibody molecule HEHEHE-ZHER3-NOTA was labelled with radiocobalt with high yield, purity and stability. Biodistribution of 57Co-HEHEHE-ZHER3-NOTA was measured in mice bearing DU145 (prostate carcinoma) and LS174T (colorectal carcinoma) xenografts at 3 and 24 h post injection (p.i.). Tumour-to-blood ratios significantly increased between 3 and 24 h p.i. (p<0.05). At 24 h p.i., tumour-to-blood ratios were 6 for DU145 and 8 for LS174T xenografts, respectively. HER3‑expressing xenografts were clearly visualized in a preclinical imaging setting already 3 h p.i., and contrast further improved at 24 h p.i. In conclusion, the radiocobalt-labelled anti-HER3 affibody molecule, HEHEHE-ZHER3-NOTA, is a promising tracer for imaging of HER3 expression in tumours.

A Standardized Wedelia chinensis Extract Overcomes the Feedback Activation of HER2/3 Signaling upon Androgen-Ablation in Prostate Cancer

Crosstalk between the androgen receptor (AR) and other signaling pathways in prostate cancer (PCa) severely affects the therapeutic outcome of hormonal therapy. Although anti-androgen therapy prolongs overall survival in PCa patients, resistance rapidly develops and is often associated with increased AR expression and upregulation of the HER2/3-AKT signaling pathway. However, single agent therapy targeting AR, HER2/3 or AKT usually fails due to the reciprocal feedback loop. Previously, we reported that wedelolactone, apigenin, and luteolin are the active compounds in Wedelia chinensis herbal extract, and act synergistically to inhibit the AR activity in PCa. Here, we further demonstrated that an herbal extract of W. chinensis (WCE) effectively disrupted the AR, HER2/3, and AKT signaling networks and therefore enhanced the therapeutic efficacy of androgen ablation in PCa. Furthermore, WCE remained effective in suppressing AR and HER2/3 signaling in an in vivo adapted castration-resistant PCa (CRPC) LNCaP cell model that was insensitive to androgen withdrawal and second-line antiandrogen, enzalutamide. This study provides preclinical evidence that the use of a defined, single plant-derived extract can augment the therapeutic efficacy of castration with significantly prolonged progression-free survival. These data also establish a solid basis for using WCE as a candidate agent in clinical studies.

Engineered Multivalency Enhances Affibody-Based HER3 Inhibition and Downregulation in Cancer Cells

The receptor tyrosine kinase HER3 has emerged as a therapeutic target in ovarian, prostate, breast, lung, and other cancers due to its ability to potently activate the PI3K/Akt pathway, especially via dimerization with HER2, as well as for its role in mediating drug resistance. Enhanced efficacy of HER3-targeted therapeutics would therefore benefit a wide range of patients. This study evaluated the potential of multivalent presentation, through protein engineering, to enhance the effectiveness of HER3-targeted affibodies as alternatives to monoclonal antibody therapeutics. Assessment of multivalent affibodies on a variety of cancer cell lines revealed their broad ability to improve inhibition of Neuregulin (NRG)-induced HER3 and Akt phosphorylation compared to monovalent analogues. Engineered multivalency also promoted enhanced cancer cell growth inhibition by affibodies as single agents and as part of combination therapy approaches. Mechanistic investigations revealed that engineered multivalency enhanced affibody-mediated HER3 downregulation in multiple cancer cell types. Overall, these results highlight the promise of engineered multivalency as a general strategy for enhanced efficacy of HER3-targeted therapeutics against a variety of cancers.

Elucidation of Resistance Mechanisms to Second-Generation ALK Inhibitors Alectinib and Ceritinib in Non-Small Cell Lung Cancer Cells

Crizotinib is the first anaplastic lymphoma kinase (ALK) inhibitor to have been approved for the treatment of non-small cell lung cancer (NSCLC) harboring an ALK fusion gene, but it has been found that, in the clinic, patients develop resistance to it. Alectinib and ceritinib are second-generation ALK inhibitors which show remarkable clinical responses in both crizotinib-naive and crizotinib-resistant NSCLC patients harboring an ALK fusion gene. Despite their impressive activity, clinical resistance to alectinib and ceritinib has also emerged. In the current study, we elucidated the resistance mechanisms to these second-generation ALK inhibitors in the H3122 NSCLC cell line harboring the EML4-ALK variant 1 fusion in vitro. Prolonged treatment of the parental H3122 cells with alectinib and ceritinib led to two cell lines which are 10 times less sensitive to alectinib and ceritinib than the parental H3122 cell line. Although mutations of ALK in its kinase domain are a common resistance mechanism for crizotinib, we did not detect any ALK mutation in these resistant cell lines. Rather, overexpression of phospho-ALK and alternative receptor tyrosine kinases such as phospho-EGFR, phospho-HER3, and phospho-IGFR-1R was observed in both resistant cell lines. Additionally, NRG1, a ligand for HER3, is upregulated and responsible for resistance by activating the EGFR family pathways through the NRG1-HER3-EGFR axis. Combination treatment with EGFR inhibitors, in particular afatinib, was shown to be effective at overcoming resistance. Our study provides new mechanistic insights into adaptive resistance to second-generation ALK inhibitors and suggests a potential clinical strategy to combat resistance to these second-generation ALK inhibitors in NSCLC.

Affibody-mediated PET imaging of HER3 expression in malignant tumours

Human epidermal growth factor receptor 3 (HER3) is involved in the progression of various cancers and in resistance to therapies targeting the HER family. In vivo imaging of HER3 expression would enable patient stratification for anti-HER3 immunotherapy. Key challenges with HER3-targeting are the relatively low expression in HER3-positive tumours and HER3 expression in normal tissues. The use of positron-emission tomography (PET) provides advantages of high resolution, sensitivity and quantification accuracy compared to SPECT. Affibody molecules, imaging probes based on a non-immunoglobulin scaffold, provide high imaging contrast shortly after injection. The aim of this study was to evaluate feasibility of PET imaging of HER3 expression using (68)Ga-labeled affibody molecules. The anti-HER3 affibody molecule HEHEHE-Z08698-NOTA was successfully labelled with (68)Ga with high yield, purity and stability. The agent bound specifically to HER3-expressing cancer cells in vitro and in vivo. At 3 h pi, uptake of (68)Ga-HEHEHE-Z08698-NOTA was significantly higher in xenografts with high HER3 expression (BT474, BxPC-3) than in xenografts with low HER3 expression (A431). In xenografts with high expression, tumour-to-blood ratios were >20, tumour-to-muscle >15, and tumour-to-bone >7. HER3-positive xenografts were visualised using microPET 3 h pi. In conclusion, PET imaging of HER3 expression is feasible using (68)Ga-HEHEHE-Z08698-NOTA shortly after administration.

Cellular prostatic acid phosphatase, a PTEN-functional homologue in prostate epithelia, functions as a prostate-specific tumor suppressor

The inactivation of tumor suppressor genes (TSGs) plays a vital role in the progression of human cancers. Nevertheless, those ubiquitous TSGs have been shown with limited roles in various stages of diverse carcinogenesis. Investigation on identifying unique TSG, especially for early stage of carcinogenesis, is imperative. As such, the search for organ-specific TSGs has emerged as a major strategy in cancer research. Prostate cancer (PCa) has the highest incidence in solid tumors in US males. Cellular prostatic acid phosphatase (cPAcP) is a prostate-specific differentiation antigen. Despite intensive studies over the past several decades on PAcP as a PCa biomarker, the role of cPAcP as a PCa-specific tumor suppressor has only recently been emerged and validated. The mechanism underlying the pivotal role of cPAcP as a prostate-specific TSG is, in part, due to its function as a protein tyrosine phosphatase (PTP) as well as a phosphoinositide phosphatase (PIP), an apparent functional homologue to phosphatase and tensin homolog (PTEN) in PCa cells. This review is focused on discussing the function of this authentic prostate-specific tumor suppressor and the mechanism behind the loss of cPAcP expression leading to prostate carcinogenesis. We review other phosphatases' roles as TSGs which regulate oncogenic PI3K signaling in PCa and discuss the functional similarity between cPAcP and PTEN in prostate carcinogenesis.

Neuregulin-1 overexpression and Trp53 haploinsufficiency cooperatively promote de novo malignant peripheral nerve sheath tumor pathogenesis

Malignant peripheral nerve sheath tumors (MPNSTs) are Schwann cell-derived malignancies that arise from plexiform neurofibromas in patients with mutation of the neurofibromin 1 (NF1) gene. We have shown that the growth factor neuregulin-1 (NRG1) also contributes to human neurofibroma and MPNST pathogenesis and that outbred C57BL/6J × SJL/J transgenic mice overexpressing NRG1 in Schwann cells (P0-GGFβ3 mice) recapitulate the process of neurofibroma-MPNST progression. However, it is unclear whether NRG1 acts predominantly within NF1-regulated signaling cascades or instead activates other essential cascades that cooperate with NF1 loss to promote tumorigenesis. We now report that tumorigenesis is suppressed in inbred P0-GGFβ3 mice on a C57BL/6J background. To determine whether NRG1 overexpression interacts with reduced Nf1 or Trp53 gene dosage to "unmask" tumorigenesis in these animals, we followed cohorts of inbred P0-GGFβ3;Nf1+/−, P0-GGFβ3;Trp53+/− and control (P0-GGFβ3, Nf1+/− and Trp53+/−) mice for 1 year. We found no reduction in survival or tumors in control and P0-GGFβ3;Nf1+/− mice. In contrast, P0-GGFβ3;Trp53+/− mice died on average at 226 days, with MPNSTs present in 95 % of these mice. MPNSTs in inbred P0-GGFβ3;Trp53+/− mice arose de novo from micro-MPNSTs that uniformly develop intraganglionically. These micro-MPNSTs are of lower grade (WHO grade II-III) than the major MPNSTs (WHO grade III-IV); array comparative genomic hybridization showed that lower grade MPNSTs also had fewer genomic abnormalities. Thus, P0-GGFβ3;Trp53+/− mice represent a novel model of low- to high-grade MPNST progression. We further conclude that NRG1 promotes peripheral nervous system neoplasia predominantly via its effects on the signaling cascades affected by Nf1 loss.

An engineered bivalent neuregulin protects against doxorubicin-induced cardiotoxicity with reduced proneoplastic potential

BACKGROUND

Doxorubicin (DOXO) is an effective anthracycline chemotherapeutic, but its use is limited by cumulative dose-dependent cardiotoxicity. Neuregulin-1β is an ErbB receptor family ligand that is effective against DOXO-induced cardiomyopathy in experimental models but is also proneoplastic. We previously showed that an engineered bivalent neuregulin-1β (NN) has reduced proneoplastic potential in comparison with the epidermal growth factor-like domain of neuregulin-1β (NRG), an effect mediated by receptor biasing toward ErbB3 homotypic interactions uncommonly formed by native neuregulin-1β. Here, we hypothesized that a newly formulated, covalent NN would be cardioprotective with reduced proneoplastic effects in comparison with NRG.

METHODS AND RESULTS

NN was expressed as a maltose-binding protein fusion in Escherichia coli. As established previously, NN stimulated antineoplastic or cytostatic signaling and phenotype in cancer cells, whereas NRG stimulated proneoplastic signaling and phenotype. In neonatal rat cardiomyocytes, NN and NRG induced similar downstream signaling. NN, like NRG, attenuated the double-stranded DNA breaks associated with DOXO exposure in neonatal rat cardiomyocytes and human cardiomyocytes derived from induced pluripotent stem cells. NN treatment significantly attenuated DOXO-induced decrease in fractional shortening as measured by blinded echocardiography in mice in a chronic cardiomyopathy model (57.7±0.6% versus 50.9±2.6%, P=0.004), whereas native NRG had no significant effect (49.4±3.7% versus 50.9±2.6%, P=0.813).

CONCLUSIONS

NN is a cardioprotective agent that promotes cardiomyocyte survival and improves cardiac function in DOXO-induced cardiotoxicity. Given the reduced proneoplastic potential of NN versus NRG, NN has translational potential for cardioprotection in patients with cancer receiving anthracyclines.

Eph receptors and their ligands: promising molecular biomarkers and therapeutic targets in prostate cancer

Although at present, there is a high incidence of prostate cancer, particularly in the Western world, mortality from this disease is declining and occurs primarily only from clinically significant late stage tumors with a poor prognosis. A major current focus of this field is the identification of new biomarkers which can detect earlier, and more effectively, clinically significant tumors from those deemed "low risk", as well as predict the prognostic course of a particular cancer. This strategy can in turn offer novel avenues for targeted therapies. The large family of Receptor Tyrosine Kinases, the Ephs, and their binding partners, the ephrins, has been implicated in many cancers of epithelial origin through stimulation of oncogenic transformation, tumor angiogenesis, and promotion of increased cell survival, invasion and migration. They also show promise as both biomarkers of diagnostic and prognostic value and as targeted therapies in cancer. This review will briefly discuss the complex roles and biological mechanisms of action of these receptors and ligands and, with regard to prostate cancer, highlight their potential as biomarkers for both diagnosis and prognosis, their application as imaging agents, and current approaches to assessing them as therapeutic targets. This review demonstrates the need for future studies into those particular family members that will prove helpful in understanding the biology and potential as targets for treatment of prostate cancer.

HER3 overexpression is a prognostic indicator of extrahepatic cholangiocarcinoma

Members of the HER (ERBB) receptor protein tyrosine kinase family play an important role in regulating cellular division, proliferation, differentiation, and migration and have prognostic significance in a number of cancers. Here, we sought to define their role in extrahepatic cholangiocarcinoma (EHCC). HER2 and HER3 protein expression was studied in 230 EHCC cases using a tissue microarray and compared with clinicopathological variables, including the survival of EHCC patients. HER3 was predominantly localized to the cytoplasm, whereas HER2 exhibited a membranous expression pattern. Overexpression of HER2 and HER3 was observed in 6 % (13/224) and 39 % (90/230) of EHCCs, respectively. Membranous HER2 overexpression occurred more frequently in intraductal papillary neoplasms with an associated invasive carcinoma than in tubular adenocarcinomas (P = 0.02). HER3 protein was more commonly overexpressed in nodular and infiltrative than in papillary tumors (P = 0.03). HER3 overexpression was associated with decreased survival in both univariate (P = 0.01) and multivariate (P = 0.008) analyses, whereas HER2 overexpression was not associated with survival. HER2 and HER3 are overexpressed in subsets of EHCC patients. Notably, HER3 overexpression is correlated with decreased patient survival, suggesting that HER3 constitutes a prognostic factor as well as a potential candidate for targeted therapy.

Androgen-independent prostate cancer cells circumvent EGFR inhibition by overexpression of alternative HER receptors and ligands

The deregulation of the epidermal growth factor receptor (EGFR) pathway plays a major role in the pathogenesis of prostate cancer (PCa). However, therapies targeting EGFR have demonstrated limited effectiveness in PCa. A potential mechanism to overcome EGFR blockade in cancer cells is the autocrine activation of alternative receptors of the human EGFR (HER) family through the overexpression of the HER receptors and ligands. In the present study, we were interested in analyzing if this intrinsic resistance mechanism might contribute to the inefficacy of EGFR inhibitors in PCa. To this end, we selected two androgen-independent human prostate carcinoma cell lines (DU145 and PC3) and established DU145 erlotinib-resistant cells (DUErR). Cells were treated with three EGFR inhibitors (cetuximab, gefinitib and erlotinib) and the sensitivity to each treatment was assessed. The gene expression of the four EGFR/HER receptors and seven ligands of the HER family was analyzed by real-time PCR prior to and after each treatment. The receptors expression and activation were further characterized by flow cytometry and western blot analysis. EGFR inhibition rapidly induced enhanced gene expression of the EGF, betacellulin and neuregulin-1 ligands along with HER2, HER3 and HER4 receptors in the DU145 cells. In contrast, slight changes were observed in the PC3 cells, which are defective in the phosphatase and tensin homolog (PTEN) tumor suppressor gene. In the erlotinib-resistant DUErR cells, the expression of HER2 and HER3 was increased at mRNA and protein levels together with neuregulin-1, leading to enhanced HER3 phosphorylation and the activation of the downstream PI3K/Akt survival pathway. HER3 blockage by a monoclonal antibody restored the cytostatic activity of erlotinib in DUErR cells. Our results confirm that the overexpression and autocrine activation of HER3 play a key role in mediating the resistance to EGFR inhibitors in androgen-independent PCa cells.

Monoclonal antibodies for the treatment of cancer

Over the past decade, the clinical utility of monoclonal antibodies has been realized and antibodies are now a mainstay for the treatment of cancer. Antibodies have the unique capacity to target and kill tumor cells while simultaneously activating immune effectors to kill tumor cells through the complement cascade or antibody-dependent cellular cytotoxicity (ADCC). This multifaceted mechanism of action combined with target specificity underlies the capacity of antibodies to elicit anti-tumor responses while minimizing the frequency and magnitude of adverse events. This review will focus on mechanisms of action, clinical applications and putative mechanisms of resistance to monoclonal antibody therapy in the context of cancer.

Small-molecule protein tyrosine kinase inhibitors for the treatment of metastatic prostate cancer

The microenvironment is critical to the growth of prostate cancer (PCa) in the bone. Thus, for clinical efficacy, therapies must target tumor-microenvironment interactions. Several protein tyrosine kinases have been implicated in the development and growth of PCa bone metastasis. In this review, specific protein tyrosine kinases that regulate these complex interactions, including PDGFR, the EGFR family, c-Src, VEGFR, IGF-1R, FGFR and c-Met will be discussed, with an emphasis on why these kinases are promising therapeutic targets for metastatic PCa treatment. For each of these kinases, small-molecule inhibitors have reached clinical trials. Current results of these trials and future prospects for the use of tyrosine kinase inhibitors for the treatment of PCa bone metastases are also discussed.

Toward the prognostic significance and therapeutic potential of HER3 receptor tyrosine kinase in human colon cancer

PURPOSE

Abnormal accumulation and dysregulation of the epidermal growth factor receptor family member HER3 is associated with the development of various human cancers including those of the breast, lung, and ovary. We have previously shown that in melanoma HER3 is frequently overexpressed and is associated with poor prognosis. However, the importance of HER3 in colon cancer and its putative prognostic significance is still unknown.

EXPERIMENTAL DESIGN

HER3 expression was analyzed in primary colon tumors from 110 patients by immunohistochemistry and correlated with time of progression. Parallel to this, the influence of HER3 overexpression on cell proliferation, migration, invasion, and apoptosis was investigated in four different colon cancer cell lines including DLD-1, LoVo, CaCO2, and T-84.

RESULTS

HER3 was detected at high frequency and exclusively at the membrane of the primary tumors. Elevated HER3 expression levels may serve as a putative prognostic marker because it associates with cell proliferation and decreased time to disease progression. High HER3 protein expression as well as phosphorylation levels were detected in tested cells. HER3 downregulation by RNA interference abrogated cell proliferation, migration, and invasion. In addition, suppression of HER3 resulted in a G(2)-M cell-cycle arrest, induced apoptosis, and led to morphologic changes in colon cancer cell lines. Furthermore, application of a monoclonal antibody specific to the extracellular portion of the receptor reduced heregulin-β1-induced migration and invasion and also induced apoptosis in colon cancer cell lines.

CONCLUSION

We postulate that HER3 is critically involved in colon cancer progression and may serve as a novel target for therapeutic intervention.

Targeting ErbB3: the New RTK(id) on the Prostate Cancer Block

Most prostate cancers (PCa) are critically reliant on functional androgen receptor (AR) signaling. At its onset, PCa is androgen-dependent and although temporarily halted by surgically or pharmacologically blocking the AR (androgen ablation), the disease ultimately recurs as an aggressive, fatal castration resistant prostate cancer (CRPC). FDA-approved treatments like docetaxel, a chemotherapeutic agent, and Provenge, a cancer vaccine, extend survival by a scant 3 and 4 months, respectively. It is clear that more effective drugs targeting CRPC are urgently needed. The ErbB family (EGFR/ErbB1, ErbB2/HER2/neu, ErbB3/HER3 and ErbB4/HER4) of receptor tyrosine kinases (RTKs) have long been implicated in PCa initiation and progression, but inhibitors of ErbB1 and ErbB2 (prototypic family members) fared poorly in PCa clinical trials. Recent research suggests that another family member ErbB3 abets emergence of the castration-resistant phenotype. Considerable efforts are being directed towards understanding ErbB3-mediated molecular mechanisms of castration resistance and searching for novel ways of inhibiting ErbB3 activity via rational drug design. Antibody-based therapy that prevents ligand binding to ErbB3 appears promising and fully-humanized antibodies that inhibit ligand-induced phosphorylation of ErbB3 are currently in early development. Small molecule tyrosine kinase inhibitors are also being vigorously pursued, as are siRNA-based approaches and combination treatment strategies- the simultaneous suppression of ErbB3 and its signaling partners or downstream effectors - with the primary purpose of undermining the resiliency of ErbB3-mediated signal transduction. This review summarizes the existing literature and reinforces the importance of ErbB3 as a therapeutic target in the clinical management of prostate cancer.

Engineered bivalent ligands to bias ErbB receptor-mediated signaling and phenotypes

The ErbB receptor family is dysregulated in many cancers, and its therapeutic manipulation by targeted antibodies and kinase inhibitors has resulted in effective chemotherapies. However, many malignancies remain refractory to current interventions. We describe a new approach that directs ErbB receptor interactions, resulting in biased signaling and phenotypes. Due to known receptor-ligand affinities and the necessity of ErbB receptors to dimerize to signal, bivalent ligands, formed by the synthetic linkage of two neuregulin-1β (NRG) moieties, two epidermal growth factor (EGF) moieties, or an EGF and a NRG moiety, can potentially drive homotypic receptor interactions and diminish formation of HER2-containing heterodimers, which are implicated in many malignancies and are a prevalent outcome of stimulation by native, monovalent EGF, or NRG. We demonstrate the therapeutic potential of this approach by showing that bivalent NRG (NN) can bias signaling in HER3-expressing cancer cells, resulting in some cases in decreased migration, inhibited proliferation, and increased apoptosis, whereas native NRG stimulation increased the malignant potential of the same cells. Hence, this new approach may have therapeutic relevance in ovarian, breast, lung, and other cancers in which HER3 has been implicated.

Targeting tyrosine kinases and autophagy in prostate cancer

Tyrosine kinases play significant roles in tumor progression and therapy resistance. Inhibitors of tyrosine kinases are on the forefront of targeted therapy. For prostate cancer, tyrosine kinases play an additional role in the development of castration-resistant disease state, the most troubling aspect of prostate cancinogenesis which presently defies any effective treatment. Among the 30 or so tyrosine kinases expressed in a typical prostate cancer cell, nearly one third of them have been implicated in prostate carcinogenesis. Interestingly, most of them channel signals through a trio of non-receptor tyrosine kinases, Src/Etk/FAK, referred here as Src tyrosine kinase complex. This complex has been shown to play a significant role in the aberrant activation of androgen receptor (AR) mediated by growth factors (e.g., epidermal growth factor (EGF)), cytokines (interleukin (IL)-6), chemokines (IL-8), and neurokines (gastrin-releasing peptide). These factors are induced and released from the prostate cancer to the stromal cells upon androgen withdrawal. The Src kinase complex has the ability to phosphorylate androgen receptor, resulting in the nuclear translocation and stabilization of un-liganded androgen receptor. Indeed, tyrosine kinase inhibitors targeting Src can inhibit androgen-independent growth of prostate cancer cells in vitro and in preclinical xenograft model. While effective in inducing growth arrest and inhibiting metastasis of castration-resistant tumors, Src inhibitors rarely induce a significant level of apoptosis. This is also reflected by the general ineffectiveness of tyrosine kinase inhibitors as monotherapy in clinical trials. One of the underlying causes of apoptosis resistance is "autophagy," which is induced by tyrosine kinase inhibitors and by androgen withdrawal. Autophagy is a self-digesting process to regenerate energy by removal of long-lived proteins and retired organelles to provide a survival mechanism to cells encountering stresses. Excessive autophagy, sometimes, could lead to type II programmed cell death. We demonstrated that autophagy blockade sensitizes prostate cancer cells toward Src tyrosine kinase inhibitor. Thus, a combination therapy based on Src tyrosine kinase inhibitor and autophagy modulator deserves further attention as a potential treatment for relapsed prostate cancer.

ErbB3 expression promotes tumorigenesis in pancreatic adenocarcinoma

Historically, ErbB3 has been overlooked within the ErbB receptor family due to its perceived lack of tyrosine kinase activity. We have previously demonstrated that in pancreatic cancer ErbB3 is the preferred dimerization partner of EGFR, ErbB3 protein expression level directly correlates with the anti-proliferative effect of erlotinib (an EGFR-specific tyrosine kinase inhibitor), and transient knockdown of ErbB3 expression results in acquired resistance to EGFR-targeted therapy. In this study, we develop a stable isogenic model of ErbB3 expression in an attempt to decipher ErbB3's true contribution to pancreatic cancer tumorigenesis and to examine how this receptor affects cellular sensitivity to EGFR-targeted therapy. Analysis of the EGFR-ErbB3 heterodimer demonstrates that ligand-induced PI3K-AKT signaling is limited to ErbB3-expressing cells and that this signaling cascade can be partially abrogated by inhibiting EGFR function with erlotinib. Using our model of exogenous ErbB3 expression we showed a direct relationship between ErbB3 protein levels and increased pancreatic cancer cell proliferation in vitro. In vivo, ErbB3(+)PANC-1 xenografts had a significantly larger tumor volume than PANC-1 control xenografts (ErbB3-PANC-1) and displayed increased sensitivity to EGFR-targeted therapy. In pancreatic cancer, ErbB3 appears to be critically involved in EGFR signaling as evidenced by its profound effect on cellular proliferation and its ability to influence response to EGFR-targeted therapy.

The role of HER3, the unpretentious member of the HER family, in cancer biology and cancer therapeutics

Many types of human cancer are characterized by deregulation of the human epidermal growth factor receptor (HER) family of tyrosine kinase receptors. In some cancers, genomic events causing overactivity of individual HER family members are etiologically linked with the pathogenesis of these cancers, and constitute the driving signaling function underlying their tumorigenic behavior. HER3 stands out among this family as the only member lacking catalytic kinase function. Cancers with driving HER3 amplifications or mutations have not been found, and studies of its expression in tumors have been only weakly provocative. However, substantial evidence, predominantly from experimental models, now suggest that its non-catalytic functions are critically important in many cancers driven by its' HER family partners. Furthermore, new insights into the mechanism of activation in the HER family has provided clear evidence of functionality in the HER3 kinase domain. The convergence of structural, mechanistic, and experimental evidence highlighting HER3 functions that may be critical in tumorigenesis have now led to renewed efforts towards identification of cancers or subtypes of cancers wherein HER3 function may be important in tumor progression or drug resistance. It appears now that its failure to earn the traditional definition of an oncogene has allowed the tumor promoting functions of HER3 to elude the effects of cancer therapeutics. But experimental science has now unmasked the unpretentious role of HER3 in cancer biology, and the next generation of cancer therapies will undoubtedly perform much better because of it.