Endocytosis and sorting of ErbB2 and the site of action of cancer therapeutics trastuzumab and geldanamycin

Quantitative assessment of antibody internalization with novel monoclonal antibodies against Alexa fluorophores

Antibodies against cell surface antigens may be internalized through their specific interactions with these proteins and in some cases may induce or perturb antigen internalization. The anti-cancer efficacy of antibody-drug conjugates is thought to rely on their uptake by cancer cells expressing the surface antigen. Numerous techniques, including microscopy and flow cytometry, have been used to identify antibodies with desired cellular uptake rates. To enable quantitative measurements of internalization of labeled antibodies, an assay based on internalized and quenched fluorescence was developed. For this approach, we generated novel anti-Alexa Fluor monoclonal antibodies (mAbs) that effectively and specifically quench cell surface-bound Alexa Fluor 488 or Alexa Fluor 594 fluorescence. Utilizing Alexa Fluor-labeled mAbs against the EphA2 receptor tyrosine kinase, we showed that the anti-Alexa Fluor reagents could be used to monitor internalization quantitatively over time. The anti-Alexa Fluor mAbs were also validated in a proof of concept dual-label internalization assay with simultaneous exposure of cells to two different mAbs. Importantly, the unique anti-Alexa Fluor mAbs described here may also enable other single- and dual-label experiments, including label detection and signal enhancement in macromolecules, trafficking of proteins and microorganisms, and cell migration and morphology.

A fluorescent imaging method for analyzing the biodistribution of therapeutic monoclonal antibodies that can distinguish intact antibodies from their breakdown products

Many monoclonal antibodies have been developed for therapy over the last 2 decades. In the development of therapeutic antibodies, the preclinical assessment of an antibody's biodistribution is important for the prediction of the antibody's efficacy and safety. For imaging analyses of such biodistributions, radioisotope (RI) labeling and fluorescence labeling methods are typically used, but the resulting data are limited because these methods cannot distinguish breakdown products from intact antibodies. To resolve this problem, we developed a novel method using fluorescent resonance energy transfer (FRET)-type labeling and a spectral unmixing tool. With FRET-type labeling (labeling with 2 species of fluorophore), different fluorescence properties of labeled intact antibodies and their breakdown products (the hydrolyzed/digested type of breakdown products) are made visible. With the spectral unmixing tool, the fluorescence of a solution containing the intact antibody and its breakdown products could be unmixed in proportion to their contents. Moreover, when labeled antibodies that targeted either human epidermal growth factor receptor-2 or epidermal growth factor receptor were injected into nude mice implanted subcutaneously with tumor cells, the accumulation of the injected labeled antibodies and their breakdown products in the tumor could be separately analyzed by both whole-mouse imaging and a tumor homogenate analysis. These results suggest that our method using FRET-type labeling and a spectral unmixing tool could be useful in distinguishing breakdown products from intact antibodies.

Development of an enzymatic pretargeting strategy for dual-modality imaging

A pretargeted imaging strategy based on the HaloTag dehalogenase enzyme is described. Here, a HaloTag-Trastuzumab conjugate has been used as the primary agent targeting HER2 expression, and three new radiolabelled HaloTag ligands have been used as secondary agents, two of which offer dual-modality (SPECT/optical) imaging capability.

Polypeptide-based nanogels co-encapsulating a synergistic combination of doxorubicin with 17-AAG show potent anti-tumor activity in ErbB2-driven breast cancer models

ErbB2-driven breast cancers constitute 20-25% of the cases diagnosed within the USA. The humanized anti-ErbB2 monoclonal antibody, Trastuzumab (Herceptin™; Genentech), with chemotherapy is the current standard of treatment. Novel agents and strategies continue to be explored, given the challenges posed by Trastuzumab-resistance development in most patients. The HSP90 inhibitor, 17-allylaminodemethoxygeldanamycin (17-AAG), which induces ErbB2 degradation and attenuates downstream oncogenic signaling, is one such agent that showed significant promise in early phase I and II clinical trials. Its low water solubility, potential toxicities and undesirable side effects observed in patients, partly due to the Cremophor-based formulation, have been discouraging factors in the advancement of this promising drug into clinical use. Encapsulation of 17-AAG into polymeric nanoparticle formulations, particularly in synergistic combination with conventional chemotherapeutics, represents an alternative approach to overcome these problems. Herein, we report an efficient co-encapsulation of 17-AAG and doxorubicin, a clinically well-established and effective modality in breast cancer treatment, into biodegradable and biocompatible polypeptide-based nanogels. Dual drug-loaded nanogels displayed potent cytotoxicity in a breast cancer cell panel and exerted selective synergistic anticancer activity against ErbB2-overexpressing breast cancer cell lines. Analysis of ErbB2 degradation confirmed efficient 17-AAG release from nanogels with activity comparable to free 17-AAG. Furthermore, nanogels containing both 17-AAG and doxorubicin exhibited superior antitumor efficacy in vivo in an ErbB2-driven xenograft model compared to the combination of free drugs. These studies demonstrate that polypeptide-based nanogels can serve as novel nanocarriers for encapsulating 17-AAG along with other chemotherapeutics, providing an opportunity to overcome solubility issues and thereby exploit its full potential as an anti-cancer agent.

D-Amino acid peptide residualizing agents bearing N-hydroxysuccinimido- and maleimido-functional groups and their application for trastuzumab radioiodination

INTRODUCTION

Proteins that undergo receptor-mediated endocytosis are subject to lysosomal degradation, requiring radioiodination methods that minimize loss of radioactivity from tumor cells after this process occurs. To accomplish this, we developed the residualizing radioiodination agent N(ϵ)-(3-[(*)I]iodobenzoyl)-Lys(5)-N(α)-maleimido-Gly(1)-D-GEEEK (Mal-D-GEEEK-[(*)I]IB), which enhanced tumor uptake but also increased kidney activity and necessitates generation of sulfhydryl moieties on the protein. The purpose of the current study was to synthesize and evaluate a new D-amino acid based agent that might avoid these potential problems.

METHODS

N(α)-(3-iodobenzoyl)-(5-succinimidyloxycarbonyl)-D-EEEG (NHS-IB-D-EEEG), which contains 3 D-glutamates to provide negative charge and a N-hydroxysuccinimide function to permit conjugation to unmodified proteins, and the corresponding tin precursor were produced by solid phase peptide synthesis and subsequent conjugation with appropriate reagents. Radioiodination of the anti-HER2 antibody trastuzumab using NHS-IB-D-EEEG and Mal-D-GEEEK-IB was compared. Paired-label internalization assays on BT474 breast carcinoma cells and biodistribution studies in athymic mice bearing BT474M1 xenografts were performed to evaluate the two radioiodinated D-peptide trastuzumab conjugates.

RESULTS

NHS-[(131)I]IB-D-EEEG was produced in 53.8%±13.4% and conjugated to trastuzumab in 39.5%±7.6% yield. Paired-label internalization assays with trastuzumab-NHS-[(131)I]IB-D-EEEG and trastuzumab-Mal-D-GEEEK-[(125)I]IB demonstrated similar intracellular trapping for both conjugates at 1h ((131)I, 84.4%±6.1%; (125)I, 88.6%±5.2%) through 24h ((131)I, 60.7%±6.8%; (125)I, 64.9%±6.9%). In the biodistribution experiment, tumor uptake peaked at 48 h (trastuzumab-NHS-[(131)I]IB-D-EEEG, 29.8%±3.6%ID/g; trastuzumab-Mal-D-GEEEK-[(125)I]IB, 45.3%±5.3%ID/g) and was significantly higher for (125)I at all time points. In general, normal tissue levels were lower for trastuzumab-NHS-[(131)I]IB-D-EEEG, with the differences being greatest in kidneys ((131)I, 2.2%±0.4%ID/g; (125)I, 16.9%±2.8%ID/g at 144 h).

CONCLUSION

NHS-[(131)I]IB-D-EEEG warrants further evaluation as a residualizing radioiodination agent for labeling internalizing antibodies/fragments, particularly for applications where excessive renal accumulation could be problematic.

Systematic evaluation of antibody-mediated siRNA delivery using an industrial platform of THIOMAB-siRNA conjugates

Delivery of siRNA is a key hurdle to realizing the therapeutic promise of RNAi. By targeting internalizing cell surface antigens, antibody–siRNA complexes provide a possible solution. However, initial reports of antibody–siRNA complexes relied on non-specific charged interactions and have not been broadly applicable. To assess and improve this delivery method, we built on an industrial platform of therapeutic antibodies called THIOMABs, engineered to enable precise covalent coupling of siRNAs. We report that such coupling generates monomeric antibody–siRNA conjugates (ARCs) that retain antibody and siRNA activities. To broadly assess this technology, we generated a battery of THIOMABs against seven targets that use multiple internalization routes, enabling systematic manipulation of multiple parameters that impact delivery. We identify ARCs that induce targeted silencing in vitro and extend tests to target prostate carcinoma cells following systemic administration in mouse models. However, optimal silencing was restricted to specific conditions and only observed using a subset of ARCs. Trafficking studies point to ARC entrapment in endocytic compartments as a limiting factor, independent of the route of antigen internalization. Our broad characterization of multiple parameters using therapeutic-grade conjugate technology provides a thorough assessment of this delivery technology, highlighting both examples of success as well as remaining challenges.

Microtubule inhibitor-based antibody-drug conjugates for cancer therapy

The specificity of monoclonal antibodies represents a potential therapeutic advantage, but their use as single agents in oncology has proven limited to date. The development of antibody-drug conjugates (ADCs) takes advantage of the specificity of the monoclonal antibody and potent cytotoxic effect of chemotherapy, leading to enhanced cytotoxicity in target cells and limiting toxicity to normal tissue. Microtubules represent a validated oncologic target in a range of tumor types, with a number of anti-microtubule targeting cytotoxic drugs approved for cancer use. The systemic use of potent microtubule-binding agents is limited by their effects in normal cells, which leads to toxicity including myelosuppression and peripheral neuropathy. Linking these agents to monoclonal antibodies may limit toxicity to normal tissues and increase drug concentration in target tissues, also allowing the use of more potent agents which would be too toxic to administer in their unbound form. Two such ADCs have been approved for clinical use and many others are in development. Here we review the characteristics of each of the ADC components that have led to efficacious therapies and discuss some of the tubulin inhibitor-based ADCs in development for cancer therapy.

Antibody-targeted drugs and drug resistance--challenges and solutions

Antibody-based therapy of various human malignancies has shown efficacy in the past 30 years and is now one of the most successful and leading strategies for targeted treatment of patients harboring hematological malignancies and solid tumors. Antibody-drug conjugates (ADCs) aim to take advantage of the affinity and specificity of monoclonal antibodies (mAbs) to selectively deliver potent cytotoxic drugs to antigen-expressing tumor cells. Key parameters for ADC include choosing the optimal components of the ADC (the antibody, the linker and the cytotoxic drug) and selecting the suitable cell-surface target antigen. Building on the success of recent FDA approval of brentuximab vedotin (Adcetris) and ado-trastuzumab emtansine (Kadcyla), ADCs are currently a class of drugs with a robust pipeline with clinical applications that are rapidly expanding. The more ADCs are being evaluated in preclinical models and clinical trials, the clearer are becoming the parameters and the challenges required for their therapeutic success. This rapidly growing knowledge and clinical experience are revealing novel modalities and mechanisms of resistance to ADCs, hence offering plausible solutions to such challenges. Here, we review the key parameters for designing a powerful ADC, focusing on how ADCs are addressing the challenge of multiple drug resistance (MDR) and its rational overcoming.

Role of Erbin in ErbB2-dependent breast tumor growth

ErbB2 (v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 2), a receptor tyrosine kinase of the ErbB family, is overexpressed in around 25% of breast cancers. In addition to forming a heterodimer with other ErbB receptors in response to ligand stimulation, ErbB2 can be activated in a ligand-independent manner. We report here that Erbin, an ErbB2-interacting protein that was thought to act as an antitumor factor, is specifically expressed in mammary luminal epithelial cells and facilitates ErbB2-dependent proliferation of breast cancer cells and tumorigenesis in MMTV-neu transgenic mice. Disruption of their interaction decreases ErbB2-dependent proliferation, and deletion of the PDZ domain in Erbin hinders ErbB2-dependent tumor development in MMTV-neu mice. Mechanistically, Erbin forms a complex with ErbB2, promotes its interaction with the chaperon protein HSP90, and thus prevents its degradation. Finally, ErbB2 and Erbin expression correlates in human breast tumor tissues. Together, these observations establish Erbin as an ErbB2 regulator for breast tumor formation and progression.

UIM domain-dependent recruitment of the endocytic adaptor protein Eps15 to ubiquitin-enriched endosomes

BACKGROUND

Eps15 is an endocytic adaptor protein that stimulates clathrin-mediated endocytosis. Among other interactions, Eps15 binds ubiquitin via UIM domains, recruiting ubiquitinated cargo into clathrin-coated vesicles. In EGF-treated cells, Eps15 also localizes to endosomes. The basis of this localization is not known.

RESULTS

We show that accumulation of ubiquitinated cargo can recruit Eps15 to endosomes via UIM domain interactions. First, treatment of SK-Br-3 breast cancer cells, which overexpress the EGFR family member ErbB2, with geldanamycin to promote receptor ubiquitination and endosomal transport, recruited FLAG-Eps15 to endosomes. Two in-frame ubiquitin constructs, PM-GFP-Ub (retained in endosomes after endocytosis), and GFP-FYVE-UbΔGG (targeted directly to endosomes) also recruited Eps15 to endosomes, as did slowing endosome maturation with constitutively-active Rab5-Q79L. Endosomal recruitment required the UIM domains, but not the N-terminal EH domains or central coiled-coil domains, of Eps15. Silencing of the endosomal Eps15 binding partner Hrs did not affect recruitment of Eps15 to ubiquitin-enriched endosomes. In fact, Hrs silencing itself modestly recruited Eps15 to endosomes, probably by accumulating endogenous ubiquitinated cargo. Eps15 silencing did not affect lysosomal degradation of ubiquitinated ErbB2; however, GFP-FYVE-UbΔGG overexpression inhibited internalization of EGFR and transferrin receptor.

CONCLUSIONS

We show for the first time that ubiquitin is sufficient for Eps15 recruitment to endosomes. We speculate that Eps15 recruitment to ubiquitin-rich endosomes may reduce the level of Eps15 at the plasma membrane, slowing endocytosis to allow time for processing of ubiquitinated cargo in endosomes.

A homogeneous fluorescence-based method to measure antibody internalization in tumor cells

We have developed a simple fluorescence-based method to monitor antibody internalization. Panitumumab was dual-labeled with the fluorophore IRDye 800CW and quencher IRDye QC-1 to yield the biomolecular probe Pan800QC. The fluorescence of IRDye 800CW is quenched by IRDye QC-1 on the same intact antibody. After incubation with epidermal growth factor receptor (EGFR)-expressing cells, internalization of Pan800QC was detected by an increase in fluorescence signal due to enzymatic digestion of the antibody and separation of IRDye 800CW and IRDye QC-1. By optimizing reaction conditions, a signal-to-background ratio of 8.5 was obtained. This homogeneous assay can be applied in the characterization and screening of internalizing antibodies.

A kinase inhibitor screen reveals protein kinase C-dependent endocytic recycling of ErbB2 in breast cancer cells

ErbB2 overexpression drives oncogenesis in 20-30% cases of breast cancer. Oncogenic potential of ErbB2 is linked to inefficient endocytic traffic into lysosomes and preferential recycling. However, regulation of ErbB2 recycling is incompletely understood. We used a high-content immunofluorescence imaging-based kinase inhibitor screen on SKBR-3 breast cancer cells to identify kinases whose inhibition alters the clearance of cell surface ErbB2 induced by Hsp90 inhibitor 17-AAG. Less ErbB2 clearance was observed with broad-spectrum PKC inhibitor Ro 31-8220. A similar effect was observed with Go 6976, a selective inhibitor of classical Ca(2+)-dependent PKCs (α, β1, βII, and γ). PKC activation by PMA promoted surface ErbB2 clearance but without degradation, and ErbB2 was observed to move into a juxtanuclear compartment where it colocalized with PKC-α and PKC-δ together with the endocytic recycling regulator Arf6. PKC-α knockdown impaired the juxtanuclear localization of ErbB2. ErbB2 transit to the recycling compartment was also impaired upon PKC-δ knockdown. PMA-induced Erk phosphorylation was reduced by ErbB2 inhibitor lapatinib, as well as by knockdown of PKC-δ but not that of PKC-α. Our results suggest that activation of PKC-α and -δ mediates a novel positive feedback loop by promoting ErbB2 entry into the endocytic recycling compartment, consistent with reported positive roles for these PKCs in ErbB2-mediated tumorigenesis. As the endocytic recycling compartment/pericentrion has emerged as a PKC-dependent signaling hub for G-protein-coupled receptors, our findings raise the possibility that oncogenesis by ErbB2 involves previously unexplored PKC-dependent endosomal signaling.

Mechanistic pharmacokinetic/pharmacodynamic modeling of in vivo tumor uptake, catabolism, and tumor response of trastuzumab maytansinoid conjugates

PURPOSE

Trastuzumab emtansine (T-DM1), an antibody-drug conjugate (ADC) comprised of trastuzumab linked to the antimitotic agent DM1, has shown promising results in patients with human epidermal growth factor receptor 2-positive metastatic breast cancer. Investigations of the mechanisms of the action of ADCs, including T-DM1, have been primarily descriptive or semiquantitative. However, quantitative pharmacokinetic/pharmacodynamic (PK/PD) analysis may provide insights into their complex behavior. The analyses described herein applied PK/PD modeling to nonclinical studies of maytansinoid conjugates.

METHODS

The maytansinoid conjugates T-DM1 and T-SPP-DM1, with thioether and disulfide linkers, respectively, were tested in mouse efficacy, PK, and tumor uptake studies. (3)[H]DM1-bearing ADCs were used to facilitate the quantitation of the ADCs in plasma, as well as ADC and ADC catabolites in tumors. Three mechanistic PK/PD models were used to characterize plasma ADC, tumor ADC, and tumor catabolite concentrations. Tumor catabolite concentrations were used to fit tumor response. Model parameters were estimated using R software and nonlinear least squares regression.

RESULTS

Plasma ADC-associated DM1 concentrations of T-DM1 decreased more slowly than those of T-SPP-DM1, likely due to slower DM1 release. A comparison of the mechanistic models found that the best model allowed catabolism and catabolite exit rates to differ between ADCs, that T-DM1 exhibited both faster tumor catabolism and catabolite exit rate from tumors than T-SPP-DM1; findings inconsistent with expected behavior based on the physicochemical nature of the respective catabolites. Tumor catabolite concentrations adequately described tumor response with both ADCs showing similar potency.

CONCLUSION

Mechanistic PK/PD studies described herein provided results that confirmed and challenged current hypotheses, and suggested new areas of investigation.

High-content analysis of antibody phage-display library selection outputs identifies tumor selective macropinocytosis-dependent rapidly internalizing antibodies

Many forms of antibody-based targeted therapeutics, including antibody drug conjugates, utilize the internalizing function of the targeting antibody to gain intracellular entry into tumor cells. Ideal antibodies for developing such therapeutics should be capable of both tumor-selective binding and efficient endocytosis. The macropinocytosis pathway is capable of both rapid and bulk endocytosis, and recent studies have demonstrated that it is selectively up-regulated by cancer cells. We hypothesize that receptor-dependent macropinocytosis can be achieved using tumor-targeting antibodies that internalize via the macropinocytosis pathway, improving potency and selectivity of the antibody-based targeted therapeutic. Although phage antibody display libraries have been utilized to find antibodies that bind and internalize to target cells, no methods have been described to screen for antibodies that internalize specifically via macropinocytosis. We hereby describe a novel screening strategy to identify phage antibodies that bind and rapidly enter tumor cells via macropinocytosis. We utilized an automated microscopic imaging-based, High Content Analysis platform to identify novel internalizing phage antibodies that colocalize with macropinocytic markers from antibody libraries that we have generated previously by laser capture microdissection-based selection, which are enriched for internalizing antibodies binding to tumor cells in situ residing in their tissue microenvironment (Ruan, W., Sassoon, A., An, F., Simko, J. P., and Liu, B. (2006) Identification of clinically significant tumor antigens by selecting phage antibody library on tumor cells in situ using laser capture microdissection. Mol. Cell. Proteomics. 5, 2364–2373). Full-length human IgG molecules derived from macropinocytosing phage antibodies retained the ability to internalize via macropinocytosis, validating our screening strategy. The target antigen for a cross-species binding antibody with a highly active macropinocytosis activity was identified as ephrin type-A receptor 2. Antibody-toxin conjugates created using this macropinocytosing IgG were capable of potent and receptor-dependent killing of a panel of EphA2-positive tumor cell lines in vitro. These studies identify novel methods to screen for and validate antibodies capable of receptor-dependent macropinocytosis, allowing further exploration of this highly efficient and tumor-selective internalization pathway for targeted therapy development.

Poly (ADP-ribose) polymerase inhibition enhances trastuzumab antitumour activity in HER2 overexpressing breast cancer

AIM

Poly (ADP-ribose) polymerase (PARP) inhibitors have shown promising results in Breast Cancer (BRCA) deficient breast cancer, but not in molecularly unselected patient populations. Two lines of research in this field are needed: the identification of novel subsets of patients that could potentially benefit from PARP inhibitors and the discovery of suitable targeted therapies for combination strategies.

METHODS

We tested PARP inhibition, alone or combined with the anti-HER2 antibody trastuzumab on HER2+ breast cancer. We used two PARP inhibitors in clinical development, olaparib and rucaparib, as well as genetic downmodulation of PARP-1 for in vitro studies. DNA damage was studied by the formation of γH2AX foci and comet assay. Finally, the in vivo anti-tumour effect of olaparib and trastuzumab was examined in nude mice subcutaneously implanted with BT474 cells.

RESULTS

In a panel of four HER2 overexpressing breast cancer cell lines, both olaparib and rucaparib significantly decreased cell growth and enhanced anti-tumour effects of trastuzumab. Cells exposed to olaparib and trastuzumab had greater DNA damage than cells exposed to each agent alone. Mechanistic exploratory assays showed that trastuzumab downmodulated the homologous recombination protein proliferating cell nuclear antigen (PCNA). Combination treatment in the BT474 xenograft model resulted in enhanced growth inhibition, reduced tumour cell proliferation, and increased DNA damage and apoptosis.

CONCLUSION

Taken together, our results show that PARP inhibition has antitumour effects and increases trastuzumab activity in HER2 overexpressing breast cancer. These findings make this novel combination a promising strategy for clinical development.

The Mysterious Ways of ErbB2/HER2 Trafficking

The EGFR- or ErbB-family of receptor tyrosine kinases consists of EGFR/ErbB1, ErbB2/HER2, ErbB3/HER3 and ErbB4/HER4. Receptor activation and downstream signaling are generally initiated upon ligand-induced receptor homo- or heterodimerization at the plasma membrane, and endocytosis and intracellular membrane transport are crucial for regulation of the signaling outcome. Among the receptors, ErbB2 is special in several ways. Unlike the others, ErbB2 has no known ligand, but is still the favored dimerization partner. Furthermore, while the other receptors are down-regulated either constitutively or upon ligand-binding, ErbB2 is resistant to down-regulation, and also inhibits down-regulation of its partner upon heterodimerization. The reason(s) why ErbB2 is resistant to down-regulation are the subject of debate. Contrary to other ErbB-proteins, mature ErbB2 needs Hsp90 as chaperone. Several data suggest that Hsp90 is an important regulator of factors like ErbB2 stability, dimerization and/or signaling. Hsp90 inhibitors induce degradation of ErbB2, but whether Hsp90 directly makes ErbB2 endocytosis resistant is unclear. Exposure to anti-ErbB2 antibodies can also induce down-regulation of ErbB2. Down-regulation induced by Hsp90 inhibitors or antibodies does at least partly involve internalization and endosomal sorting to lysosomes for degradation, but also retrograde trafficking to the nucleus has been reported. In this review, we will discuss different molecular mechanisms suggested to be important for making ErbB2 resistant to down-regulation, and review how membrane trafficking is involved when down-regulation and/or relocalization of ErbB2 is induced.

A bispecific HER2-targeting FynomAb with superior antitumor activity and novel mode of action

Upregulation of HER2 is a hallmark of 20% to 30% of invasive breast cancers, rendering this receptor an attractive target for cancer therapy. Although HER2-targeting agents have provided substantial clinical benefit as cancer therapeutics, there is a need for the development of new agents aiming at circumventing anti-HER2 resistance. On the basis of the approved antibody pertuzumab, we have created a panel of bispecific FynomAbs, which target two epitopes on HER2. FynomAbs are fusion proteins of an antibody and a Fyn SH3-derived binding protein. One bispecific FynomAb, COVA208, was characterized in detail and showed a remarkable ability to induce rapid HER2 internalization and apoptosis in vitro. Moreover, it elicited a strong inhibition of downstream HER2 signaling by reducing HER2, HER3, and EGFR levels in vitro and in vivo. Importantly, COVA208 demonstrated superior activity in four different xenograft models as compared with the approved antibodies trastuzumab and pertuzumab. The bispecific FynomAb COVA208 has the potential to enhance the clinical efficacy and expand the scope of HER2-directed therapies, and delineates a paradigm for designing a new class of antibody-based therapeutics for other receptor targets.

Systems analysis of drug-induced receptor tyrosine kinase reprogramming following targeted mono- and combination anti-cancer therapy

The receptor tyrosine kinases (RTKs) are key drivers of cancer progression and targets for drug therapy. A major challenge in anti-RTK treatment is the dependence of drug effectiveness on co-expression of multiple RTKs which defines resistance to single drug therapy. Reprogramming of the RTK network leading to alteration in RTK co-expression in response to drug intervention is a dynamic mechanism of acquired resistance to single drug therapy in many cancers. One route to overcome this resistance is combination therapy. We describe the results of a joint in silico, in vitro, and in vivo investigations on the efficacy of trastuzumab, pertuzumab and their combination to target the HER2 receptors. Computational modelling revealed that these two drugs alone and in combination differentially suppressed RTK network activation depending on RTK co-expression. Analyses of mRNA expression in SKOV3 ovarian tumour xenograft showed up-regulation of HER3 following treatment. Considering this in a computational model revealed that HER3 up-regulation reprograms RTK kinetics from HER2 homodimerisation to HER3/HER2 heterodimerisation. The results showed synergy of the trastuzumab and pertuzumab combination treatment of the HER2 overexpressing tumour can be due to an independence of the combination effect on HER3/HER2 composition when it changes due to drug-induced RTK reprogramming.

Trastuzumab-induced recruitment of Csk-homologous kinase (CHK) to ErbB2 receptor is associated with ErbB2-Y1248 phosphorylation and ErbB2 degradation to mediate cell growth inhibition

The inhibitory effect of trastuzumab, a humanized monoclonal antibody directed against the extracellular domain of ErbB2, is associated with its ability to induce ErbB2-Y1248 phosphorylation, and the status of phosphorylated ErbB2-Y1248 (ErbB2-pY1248) may correlate with the sensitivity of breast cancers to trastuzumab. The mechanisms of which remain unclear. Here, we show that binding of trastuzumab to ErbB2 activates ErbB2 kinase activity and enhances ErbB2-Y1248 phosphorylation in trastuzumab-sensitive breast cancer cells. This in turn increases the interaction between ErbB2 and non-receptor Csk-homologous kinase (CHK), leading to growth inhibition of breast cancer cells. Overexpression of CHK mimics trastuzumab treatment to mediate ErbB2-Y1248 phosphorylation, Akt downregulation, and growth inhibition of trastuzumab-sensitive breast cancer cells. CHK overexpression combined with trastuzumab exerts an additive effect on cell growth inhibition. We further demonstrate that positive ErbB2-pY1248 staining in ErbB2-positive breast cancer biopsies correlates with the increased trastuzumab response in trastuzumab neoadjuvant settings. Collectively, this study highlights an important role for ErbB2-pY1248 in mediating trastuzumab-induced growth inhibition and trastuzumab-induced interactions between CHK and ErbB2-pY1248 is identified as a novel mechanism of action that mediates the growth inhibition of breast cancer cells. The novel mechanistic insights into trastuzumab action revealed by this study may impact the design of next generation of therapeutic monoclonal antibodies targeting receptor tyrosine kinases, as well as open new avenues to identify novel targets for the treatment of ErbB2-positive cancers.

Engagement of immune effector cells by trastuzumab induces HER2/ERBB2 downregulation in cancer cells through STAT1 activation

Introduction

Trastuzumab has been widely used for the treatment of human epidermal growth factor receptor 2 (HER2) overexpressing breast cancer for more than a decade. However, reports on the involvement of HER2 downregulation in trastuzumab’s mechanism of action are inconsistent. The aim of this study is to investigate if the dependence of trastuzumab-mediated cancer cell HER2 downregulation on immune effector cells represents a novel mechanism of action for trastuzumab.

Methods

HER2 expression was evaluated by Western blotting, flow cytometry, and real-time polymerase chain reaction (PCR) in cell lysates from co-cultures of multiple cancer cell lines with peripheral blood mononuclear cells (PBMCs) in the presence or absence of trastuzumab. The engagement of immune cells by trastuzumab through Fc gamma receptors (FcγRs) was tested using three trastuzumab variants with compromised or no Fc (fragment crystallizable) functions and FcγRs blocking experiments. The engagement of immune cells by trastuzumab in HER2 downregulation was also evaluated in in vivo mouse xenograft tumor models.

Results

HER2 downregulation of cancer cells by trastuzumab occurred only when trastuzumab was actively engaged with immune cells and cancer cells, as demonstrated consistently in co-cultures of cancer cell lines with PBMCs and in vivo mouse xenograft tumor models. We further demonstrated that HER2 downregulation in cancer cells by immune-cell-engaged trastuzumab was at the transcriptional level, not through the HER2 degradation pathway. Activation of signal transducer and activator of transcription 1 (STAT1) in cancer cells by the increased interferon gamma (IFN-γ) production in immune cells played an important role in downregulating HER2 in cancer cells upon engagement of immune cells by trastuzumab. Furthermore, HER2 downregulation in cancer cells induced by trastuzumab engagement of immune cells was correlated with the antibody’s antitumor efficacy in vivo.

Conclusions

This study reveals that engagement of immune effector cells by trastuzumab induces HER2 downregulation in HER2-expressing cancer cells, which represents a new function of immune cells in trastuzumab-mediated antitumor efficacy and serves as a novel mechanism of action for trastuzumab. Our results imply that HER2 downregulation in cancer cells treated by trastuzumab may predict active engagement of immune effector cells in tumor microenvironment.

Targeting of preexisting and induced breast cancer stem cells with trastuzumab and trastuzumab emtansine (T-DM1)

The antibody trastuzumab (Herceptin) has substantially improved overall survival for patients with aggressive HER2-positive breast cancer. However, about 70% of all treated patients will experience relapse or disease progression. This may be related to an insufficient targeting of the CD44^highCD24^low breast cancer stem cell subset, which is not only highly resistant to chemotherapy and radiotherapy but also a poor target for trastuzumab due to low HER2 surface expression. Hence, we explored whether the new antibody-drug conjugate T-DM1, which consists of the potent chemotherapeutic DM1 coupled to trastuzumab, could improve the targeting of these tumor-initiating or metastasis-initiating cells. To this aim, primary HER2-overexpressing tumor cells as well as HER2-positive and HER2-negative breast cancer cell lines were treated with T-DM1, and effects on survival, colony formation, gene and protein expression as well as antibody internalization were assessed. This revealed that CD44^highCD24^lowHER2^low stem cell-like breast cancer cells show high endocytic activity and are thus particularly sensitive towards the antibody-drug conjugate T-DM1. Consequently, preexisting CD44^highCD24^low cancer stem cells were depleted by concentrations of T-DM1 that did not affect the bulk of the tumor cells. Likewise, colony formation was efficiently suppressed. Moreover, when tumor cells were cocultured with natural killer cells, antibody-dependent cell-mediated cytotoxicity was enhanced, and EMT-mediated induction of stem cell-like properties was prevented in differentiated tumor cells. Thus our study reveals an unanticipated targeting of stem cell-like breast cancer cells by T-DM1 that may contribute to the clinical efficacy of this recently approved antibody-drug conjugate.

Trastuzumab emtansine: mechanisms of action and drug resistance

Trastuzumab emtansine (T-DM1) is an antibody-drug conjugate that is effective and generally well tolerated when administered as a single agent to treat advanced breast cancer. Efficacy has now been demonstrated in randomized trials as first line, second line, and later than the second line treatment of advanced breast cancer. T-DM1 is currently being evaluated as adjuvant treatment for early breast cancer. It has several mechanisms of action consisting of the anti-tumor effects of trastuzumab and those of DM1, a cytotoxic anti-microtubule agent released within the target cells upon degradation of the human epidermal growth factor receptor-2 (HER2)-T-DM1 complex in lysosomes. The cytotoxic effect of T-DM1 likely varies depending on the intracellular concentration of DM1 accumulated in cancer cells, high intracellular levels resulting in rapid apoptosis, somewhat lower levels in impaired cellular trafficking and mitotic catastrophe, while the lowest levels lead to poor response to T-DM1. Primary resistance of HER2-positive metastatic breast cancer to T-DM1 appears to be relatively infrequent, but most patients treated with T-DM1 develop acquired drug resistance. The mechanisms of resistance are incompletely understood, but mechanisms limiting the binding of trastuzumab to cancer cells may be involved. The cytotoxic effect of T-DM1 may be impaired by inefficient internalization or enhanced recycling of the HER2-T-DM1 complex in cancer cells, or impaired lysosomal degradation of trastuzumab or intracellular trafficking of HER2. The effect of T-DM1 may also be compromised by multidrug resistance proteins that pump DM1 out of cancer cells. In this review we discuss the mechanism of action of T-DM1 and the key clinical results obtained with it, the combinations of T-DM1 with other cytotoxic agents and anti-HER drugs, and the potential resistance mechanisms and the strategies to overcome resistance to T-DM1.

Incorporating Target Shedding Into a Minimal PBPK-TMDD Model for Monoclonal Antibodies

Shedding of a pharmacological target from cells, giving rise to a soluble target that can also bind therapeutic proteins, is a common phenomenon. In this study, a minimal physiologically based pharmacokinetic model was used to simulate the pharmacokinetics of trastuzumab and the simultaneous binding of the compound to soluble (in blood and tissue interstitial space) and membrane-bound (in the tissue interstitial space) forms of human epidermal growth factor receptor 2 (HER2). The parameter values describing binding of trastuzumab to HER2 were largely derived from in vitro data, and the effects of varying HER2 levels, the affinity difference between membrane-bound HER2 and shed antigen, and slow binding kinetics were investigated. The model simulates a sharp decrease in trough drug concentrations at concentrations of soluble target between 500 and 1,000 ng/ml in plasma. This corresponds with the clinical concentration range of soluble target wherein changes in half-life of trastuzumab have been observed.

HER2 monoclonal antibodies that do not interfere with receptor heterodimerization-mediated signaling induce effective internalization and represent valuable components for rational antibody-drug conjugate design

The human epidermal growth factor receptor (HER)2 provides an excellent target for selective delivery of cytotoxic drugs to tumor cells by antibody-drug conjugates (ADC) as has been clinically validated by ado-trastuzumab emtansine (Kadcyla(TM)). While selecting a suitable antibody for an ADC approach often takes specificity and efficient antibody-target complex internalization into account, the characteristics of the optimal antibody candidate remain poorly understood. We studied a large panel of human HER2 antibodies to identify the characteristics that make them most suitable for an ADC approach. As a model toxin, amenable to in vitro high-throughput screening, we employed Pseudomonas exotoxin A (ETA') fused to an anti-kappa light chain domain antibody. Cytotoxicity induced by HER2 antibodies, which were thus non-covalently linked to ETA', was assessed for high and low HER2 expressing tumor cell lines and correlated with internalization and downmodulation of HER2 antibody-target complexes. Our results demonstrate that HER2 antibodies that do not inhibit heterodimerization of HER2 with related ErbB receptors internalize more efficiently and show greater ETA'-mediated cytotoxicity than antibodies that do inhibit such heterodimerization. Moreover, stimulation with ErbB ligand significantly enhanced ADC-mediated tumor kill by antibodies that do not inhibit HER2 heterodimerization. This suggests that the formation of HER2/ErbB-heterodimers enhances ADC internalization and subsequent killing of tumor cells. Our study indicates that selecting HER2 ADCs that allow piggybacking of HER2 onto other ErbB receptors provides an attractive strategy for increasing ADC delivery and tumor cell killing capacity to both high and low HER2 expressing tumor cells.

Endocytosis and cancer

Endocytosis entails selective packaging of cell-surface proteins, such as receptors for cytokines and adhesion components, in cytoplasmic vesicles (endosomes). The series of sorting events that determines the fate of internalized proteins, either degradation in lysosomes or recycling back to the plasma membrane, relies on intrinsic sequence motifs, posttranslational modifications (e.g., phosphorylation and ubiquitination), and transient assemblies of both Rab GTPases and phosphoinositide-binding proteins. This multicomponent process is enhanced and skewed in cancer cells; we review mechanisms enabling both major drivers of cancer, p53 and Ras, to bias recycling of integrins and receptor tyrosine kinases (RTKs). Likewise, cadherins and other junctional proteins of cancer cells are constantly removed from the cell surface, thereby disrupting tissue polarity and instigating motile phenotypes. Mutant forms of RTKs able to evade Cbl-mediated ubiquitination, along with overexpression of the wild-type forms and a variety of defective feedback regulatory loops, are frequently detected in tumors. Finally, we describe pharmacological attempts to harness the peculiar endocytic system of cancer, in favor of effective patient treatment.

Synergistic targeting of cell membrane, cytoplasm, and nucleus of cancer cells using rod-shaped nanoparticles

Design of carriers for effective delivery and targeting of drugs to cellular and subcellular compartments is an unmet need in medicine. Here, we report pure drug nanoparticles comprising camptothecin (CPT), trastuzumab (TTZ), and doxorubicin (DOX) to enable cell-specific interactions, subcellular accumulation, and growth inhibition of breast cancer cells. CPT is formulated in the form of nanorods which are coated with TTZ. DOX is encapsulated in the TTZ corona around the CPT nanoparticle. Our results show that TTZ/DOX-coated CPT nanorods exhibit cell-specific internalization in BT-474 breast cancer cells, after which TTZ is recycled to the plasma membrane, leaving CPT nanorods in the perinuclear region and delivering DOX into the nucleus of the cells. The effects of CPT-TTZ-DOX nanoparticles on growth inhibition are synergistic (combination index = 0.17 ± 0.03) showing 10-10 000-fold lower inhibitory concentrations (IC50) compared to those of individual drugs. The design of antibody-targeted pure drug nanoparticles offers a promising design strategy to facilitate intracellular delivery and therapeutic efficiency of anticancer drugs.

Trends in cancer-targeted antibody-drug conjugates

Better knowledge of engineered antibodies and tumour biology has led to the development of novel targeted therapies, such as antibody-drug conjugates (ADCs). ADCs combine a monoclonal antibody, directed toward specific antigen highly expressed on the cancer cell, to potent cytotoxic drug through a stable linker. ADCs are designed to bind selectively to cancer cells and to deliver cytotoxic drugs into the cancer cell, which may preserve normal cells. ADCs should be stable and non-toxic in circulation. Upon binding to antigen, ADCs are internalized by different processes, followed by the intracellular release of an active form of the cytotoxic drug, which in turn kills the cancer cell. This technology has the potential to further improve the anticancer activity while limiting toxicity. First results from ongoing clinical trials are encouraging. Favourable pharmacokinetic profile was observed showing good stability in circulation. Clinical studies demonstrated that ADCs provide clinical efficacy with an acceptable safety profile. Objective responses and clinical benefits were demonstrated with the investigated ADCs. Major toxicities frequently associated to chemotherapy were barely or not reported with ADCs. Taken together, ADCs may become the new wave of anticancer drugs in the future.

V-ATPase inhibition overcomes trastuzumab resistance in breast cancer

The HER2 oncogene targeting drug trastuzumab shows remarkable efficacy in patients overexpressing HER2. However acquired or primary resistance develops in most of the treated patients why alternative treatment strategies are strongly needed. As endosomal sorting and recycling are crucial steps for HER2 activity and the vacuolar H⁺-ATPase (V-ATPase) is an important regulator of endocytotic trafficking, we proposed that targeting V-ATPase opens a new therapeutic strategy against trastuzumab-resistant tumor cells in vitro and in vivo. V-ATPase inhibition with archazolid, a novel inhibitor of myxobacterial origin, results in growth inhibition, apoptosis and impaired HER2 pro-survival signaling of the trastuzumab-resistant cell line JIMT-1. This is accompanied by a decreased expression on the plasma membrane and accumulation of HER2 in the cytosol, where it colocalizes with endosomes, lysosomes and autophagosomes. Importantly, microscopic analysis of JIMT-1 xenograft tumor tissue of archazolid treated mice confirms the defect in HER2-recycling which leads to reduced tumor growth. These results suggest that V-ATPase inhibition by archazolid induces apoptosis and inhibits growth of trastuzumab-resistant tumor cells by retaining HER2 in dysfunctional vesicles of the recycling pathway and consequently abrogates HER2-signaling in vitro as well as in vivo. V-ATPase inhibition is thus suggested as a promising strategy for treatment of trastuzumab-resistant tumors.

Autophagy-related gene 12 (ATG12) is a novel determinant of primary resistance to HER2-targeted therapies: utility of transcriptome analysis of the autophagy interactome to guide breast cancer treatment

The autophagic process, which can facilitate breast cancer resistance to endocrine, cytotoxic, and molecularly targeted agents, is mainly regulated at the post-translational level. Although recent studies have suggested a possible transcriptome regulation of the autophagic genes, little is known about either the analysis tools that can be applied or the functional importance of putative candidate genes emerging from autophagy-dedicated transcriptome studies. In this context, we evaluated whether the constitutive activation of the autophagy machinery, as revealed by a transcriptome analysis using an autophagy-focused polymerase chain reaction (PCR) array, might allow for the identification of novel autophagy-specific biomarkers for intrinsic (primary) resistance to HER2-targeted therapies. Quantitative real-time PCR (qRT-PCR)-based profiling of 84 genes involved in autophagy revealed that, when compared to trastuzumab-sensitive SKBR3 cells, the positive regulator of autophagic vesicle formation ATG12 (autophagy-related gene 12) was the most differentially up-regulated gene in JIMT1 cells, a model of intrinsic cross-resistance to trastuzumab and other HER1/2-targeting drugs. An analysis of the transcriptional status of ATG12 in > 50 breast cancer cell lines suggested that the ATG12 transcript is commonly upregulated in trastuzumab-unresponsive HER2-overexpressing breast cancer cells. A lentiviral-delivered small hairpin RNA stable knockdown of the ATG12 gene fully suppressed the refractoriness of JIMT1 cells to trastuzumab, erlotinib, gefitinib, and lapatinib in vitro. ATG12 silencing significantly reduced JIMT1 tumor growth induced by subcutaneous injection in nude mice. Remarkably, the outgrowth of trastuzumab-unresponsive tumors was prevented completely when trastuzumab treatment was administered in an ATG12-silenced genetic background. We demonstrate for the first time the usefulness of low-density, autophagy-dedicated qRT-PCR-based platforms for monitoring primary resistance to HER2-targeted therapies by transcriptionally screening the autophagy interactome. The degree of predictive accuracy warrants further investigation in the clinical situation.

The success story of trastuzumab emtansine, a targeted therapy in HER2-positive breast cancer

Trastuzumab emtansine is a unique antibody-drug conjugate targeting selectively human epidermal growth factor receptor 2 (HER2)-positive breast cancer cells, thus conferring high efficacy with minimal systemic toxicities. Trastuzumab emtansine consists of a monoclonal antibody trastuzumab and potent cytotoxic agent DM1, combined together through a stable thioether bond. First-in-man phase I study set the maximum tolerated dose at 3.6 mg/kg given intravenously on a 3-weekly regimen. In phase II studies, trastuzumab emtansine at 3.6 mg/kg provided objective tumour responses and clinical benefit with an encouraging safety profile. Over these studies, trastuzumab emtansine had favourable pharmacokinetics. No accumulation of trastuzumab emtansine or catabolites was observed even after repeated dosing and free DM1 was very low in circulation. The stability of trastuzumab emtansine in circulation justifies the minimal systemic toxicity observed. Recently, a randomised international open-label phase III study confirmed the efficacy and safety of trastuzumab emtansine versus lapatinib plus capecitabine in patients with HER2-positive locally advanced or metastatic breast cancer. Overall survival was significantly improved in the trastuzumab emtansine arm. Safety outcomes were also favourable. The adverse events traditionally related to chemotherapy were markedly lower or absent with trastuzumab emtansine. Cardiotoxicity, frequently observed in HER2-directed therapy, was not reported. Although thrombocytopenia and elevations in hepatic enzymes were reported with trastuzumab emtansine, these events were reversible and manageable. Ongoing trials investigating trastuzumab emtansine as a single-agent or in combination with other agents, will determine the place of trastuzumab emtansine in the current therapeutic strategies deployed for HER2-metastatic breast cancer.

Insights into cell entry and intracellular trafficking of peptide and protein drugs provided by electron microscopy

For widening the arsenal of protein and peptide therapeutics that act within cells, their cell-entry mechanisms, intracellular trafficking and distribution need to be characterized in detail. Immunofluorescence microscopy has been a prevalent tool for these studies. However, due to the limited resolution, it is often complemented with other methods. This article focuses on the perspectives of electron microscopy in tracking the intracellular delivery and trafficking of proteins, peptides and their carriers. This review introduces the electron microscopy techniques and labeling methods currently used for studying the cellular whereabouts of peptides and proteins with a focus on their intracellular trafficking. Since cell-penetrating peptides have widely been harnessed as carriers for proteins and peptides, and their usage is rapidly expanding, a particular emphasis has been placed on their applications and cell-entry mechanisms.

Identification of caveolin-1 as a potential causative factor in the generation of trastuzumab resistance in breast cancer cells

The oncogenic tyrosine kinase receptor ErbB2 is a prognostic factor and target for breast cancer therapeutics. In contrast with the other ErbB receptors, ErbB2 is hardly internalized by ligand induced mechanisms, indicating a prevalent surface expression. Elevated levels of ErbB2 in tumor cells are associated with its defective endocytosis and down regulation. Here we show that caveolin-1 expression in breast cancer derived SKBR-3 cells (SKBR-3/Cav-1) facilitates ligand induced ErbB2 endocytosis using an artificial peptide ligand EC-eGFP. Similarly, stimulation with humanized anti ErbB2 antibody Trastuzumab (Herceptin) was found to be internalized and co-localized with caveolin-1 in SKBR-3/Cav-1 cells. Internalized EC-eGFP and Trastuzumab in SKBR-3/Cav-1 cells were then delivered via caveolae to the caveolin-1 containing early endosomes. Consequently, attenuated Fc receptor mediated ADCC functions were observed when exposed to Trastuzumab and EC-Fc (EC-1 peptide conjugated to Fc part of human IgG). On the other hand, this caveolae dependent endocytic synergy was not observed in parental SKBR-3 cells. Therefore, caveolin-1 expression in breast cancer cells could be a predictive factor to estimate how cancer cells are likely to respond to Trastuzumab treatment.

Receptor tyrosine kinase ErbB2 translocates into mitochondria and regulates cellular metabolism

It is well known that ErbB2, a receptor tyrosine kinase, localizes on the plasma membrane. Here we describe a novel observation that ErbB2 also localizes in mitochondria of cancer cells and patient samples. We found that ErbB2 translocates into mitochondria through the association with mtHSP70. Additionally, mitochondrial ErbB2 (mtErbB2) negatively regulates mitochondrial respiratory functions. Oxygen consumption and activities of complexes of the mitochondrial electron transport chain were decreased in mtErbB2-overexpressing cells. Mitochondrial membrane potential and the cellular ATP level also were decreased. In contrast, mtErbB2 enhanced cellular glycolysis. The translocation of ErbB2 and its impact on mitochondrial function are kinase dependent. Interestingly, cancer cells with higher levels of mtErbB2 were more resistant to ErbB2 targeting antibody trastuzumab. Our study provides a novel perspective on the metabolic regulatory function of ErbB2 and reveals that mtErbB2 plays an important role in the regulation of cellular metabolism and cancer cell resistance to therapeutics.

Impact of tumor HER2/ERBB2 expression level on HER2-targeted liposomal doxorubicin-mediated drug delivery: multiple low-affinity interactions lead to a threshold effect

Numerous targeted nanotherapeutics have been described for potential treatment of solid tumors. Although attention has focused on antigen selection and molecular design of these systems, there has been comparatively little study of how cellular heterogeneity influences interaction of targeted nanoparticles with tumor cells. Antigens, such as HER2/ERBB2, are heterogeneously expressed across different indications, across patients, and within individual tumors. Furthermore, antigen expression in nontarget tissues necessitates optimization of the therapeutic window. Understanding the performance of a given nanoparticle under different regimens of antigen expression has the ability to inform patient selection and clinical development decisions. In this work, HER2-targeted liposomal doxorubicin was used as a model-targeted nanoparticle to quantitatively investigate the effect of HER2 expression levels on delivery of doxorubicin to the nucleus. We find quantitatively greater nuclear doxorubicin delivery with increasing HER2 expression, exhibiting a threshold effect at approximately 2 × 10(5) HER2 receptors/cell. Kinetic modeling indicated that the threshold effect arises from multiple low-affinity interactions between the targeted liposome and HER2. These results support previous data showing little or no uptake into human cardiomyocytes, which express levels of HER2 below the threshold. Finally, these results suggest that HER2-targeted liposomal doxorubicin may effectively target tumors that fall below traditional definitions of HER2-positive tumors, thereby expanding the potential population of patients that might benefit from this agent.

Pertuzumab counteracts the inhibitory effect of ErbB2 on degradation of ErbB3

Overexpression of ErbB2 and ErbB3 is found in several human cancers, and ErbB2-ErbB3 heterodimers are known as the most potent signaling units among ErbB dimers. While ErbB2 probably undergoes weak endocytosis, ErbB3 is readily internalized even in the absence of added ligand and without requirement for kinase activity. Overexpression of ErbB2 has been demonstrated to inhibit epidermal growth factor-induced internalization and degradation of epidermal growth factor receptor. This happens due to epidermal growth factor receptor-ErbB2 dimerization and can be counteracted by the anti-ErbB2 antibody pertuzumab, which binds the dimerization arm of ErbB2. Pertuzumab does also inhibit ErbB2-ErbB3 dimerization, but to what extent this has effect on constitutive and/or ligand-induced downregulation of ErbB3 is not known. In this study, we demonstrate that expression of ErbB2 as such did not block constitutive internalization of ErbB3, but that heregulin-induced degradation of ErbB3 was significantly slowed in cells expressing high levels of ErbB2. Incubation with pertuzumab did, however, counteract this effect. This indicates that the formation of ErbB2-ErbB3 heterodimers inhibits downregulation of ErbB3 and supports the notion that pertuzumab inhibits ErbB2 dimerization. The inhibitory effect of pertuzumab on ligand-induced ErbB2-ErbB3 heterodimerization was confirmed by the observation that pertuzumab inhibited heregulin-induced phosphorylation of ErbB3 in cells expressing ErbB2 and efficiently reduced heregulin-induced downstream signaling in cells expressing low levels of ErbB2. Altogether the results indicate that pertuzumab can be a valuable therapeutic agent not only in cancers overexpressing ErbB2 but also in cancers co-expressing ErbB2 and ErbB3.

The Huntington disease protein accelerates breast tumour development and metastasis through ErbB2/HER2 signalling

In Huntington disease (HD), polyglutamine expansion in the huntingtin protein causes specific neuronal death. The consequences of the presence of mutant huntingtin in other tissues are less well understood. Here we propose that mutant huntingtin influences breast cancer progression. Indeed, we show that mammary tumours appear earlier in mouse breast cancer models expressing mutant huntingtin as compared to control mice expressing wild-type huntingtin. Tumours bearing mutant huntingtin have a modified gene expression pattern that reflects enhanced aggressiveness with the overexpression of genes favouring invasion and metastasis. In agreement, mutant huntingtin accelerates epithelial to mesenchymal transition and enhances cell motility and invasion. Also, lung metastasis is higher in HD conditions than in control mice. Finally, we report that in HD, the dynamin dependent endocytosis of the ErbB2/HER2 receptor tyrosine kinase is reduced. This leads to its accumulation and to subsequent increases in cell motility and proliferation. Our study may thus have important implications for both cancer and HD.

Targeting receptor-mediated endocytotic pathways with nanoparticles: rationale and advances

Targeting of drugs and their carrier systems by using receptor-mediated endocytotic pathways was in its nascent stages 25 years ago. In the intervening years, an explosion of knowledge focused on design and synthesis of nanoparticulate delivery systems as well as elucidation of the cellular complexity of what was previously-termed receptor-mediated endocytosis has now created a situation when it has become possible to design and test the feasibility of delivery of highly specific nanoparticle drug carriers to specific cells and tissue. This review outlines the mechanisms governing the major modes of receptor-mediated endocytosis used in drug delivery and highlights recent approaches using these as targets for in vivo drug delivery of nanoparticles. The review also discusses some of the inherent complexity associated with the simple shift from a ligand-drug conjugate versus a ligand-nanoparticle conjugate, in terms of ligand valency and its relationship to the mode of receptor-mediated internalization.

Caveolin-1 is involved in radiation-induced ERBB2 nuclear transport in breast cancer cells

This study examined the radiation-induced ERBB2 nuclear transport in the BT474 breast cancer cell line and the relationship between caveolin-1 and radiation-induced ERBB2 nuclear transport. The BT474 cells were treated with herceptin (200 nmol/L), PP2 (a caveolin-1 inhibitor, 100 nmol/L) and irradiation combined or alone. Confocal microscopy was used to observe the nuclear import of ERBB2 and caveolin-1 after irradiation. Western blotting was employed to detect the expression of ERBB2, caveolin-1 and DNA-PKcs after irradiation, and immunoprecipitation to identify the ERBB2 and caveolin-1 complex before perinuclear ERBB2 localization. Confocal microscopy showed the transport of ERBB2 and caveolin-1 from the cell membrane to the nucleus 15 min after irradiation and the proteins accumulated at the perinuclear region within 45 min. Western blotting revealed that the expression levels of ERBB2, caveolin-1 and DNA-PKcs were increased after irradiation and reached a peak 45 min later. Both herceptin and PP2 treatments were found to decrease ERBB2 expression. An immune complex composed of ERBB2 and caveolin-1 was found in the herceptin group after irradiation. It was concluded that after irradiation, ERBB2 may be transported from the cell membrane to the nucleus and activate DNA-PKcs to trigger DNA double-strand break (DSB) repair; caveolin-1 may participate in this process. Treatments involving the downregulation of caveolin-1 may increase the radiosensitization of breast cancer cells.

The HSP90 inhibitor geldanamycin perturbs endosomal structure and drives recycling ErbB2 and transferrin to modified MVBs/lysosomal compartments

The ErbB2 receptor is a validated cancer target whose internalization and trafficking remain poorly understood. The authors propose that ErbB2 internalization upon geldanamycin (GA) occurs predominantly via clathrin-mediated endocytosis and that GA affects endosomal structure and sorting, forcing recycling cargoes toward mixed endo/lysosomal compartments, irrespective of their HSP90 interaction.

The ErbB2 receptor is a clinically validated cancer target whose internalization and trafficking mechanisms remain poorly understood. HSP90 inhibitors, such as geldanamycin (GA), have been developed to target the receptor to degradation or to modulate downstream signaling. Despite intense investigations, the entry route and postendocytic sorting of ErbB2 upon GA stimulation have remained controversial. We report that ErbB2 levels inversely impact cell clathrin-mediated endocytosis (CME) capacity. Indeed, the high levels of the receptor are responsible for its own low internalization rate. GA treatment does not directly modulate ErbB2 CME rate but it affects ErbB2 recycling fate, routing the receptor to modified multivesicular endosomes (MVBs) and lysosomal compartments, by perturbing early/recycling endosome structure and sorting capacity. This activity occurs irrespective of the cargo interaction with HSP90, as both ErbB2 and the constitutively recycled, HSP90-independent, transferrin receptor are found within modified endosomes, and within aberrant, elongated recycling tubules, leading to modified MVBs/lysosomes. We propose that GA, as part of its anticancer activity, perturbs early/recycling endosome sorting, routing recycling cargoes toward mixed endosomal compartments.