Tumor-initiating cells of HER2-positive carcinoma cell lines express the highest oncoprotein levels and are sensitive to trastuzumab

Dynamic changes in CD44v-positive cells after preoperative anti-HER2 therapy and its correlation with pathologic complete response in HER2-positive breast cancer

Chemotherapy has been reported to increase the proportion of cancer stem cells (CSCs) and to promote epithelial-mesenchymal transition (EMT) phenotype changes. Anti-HER2 therapy may provide a strategy for eliminating CSC and EMT, which contribute to therapeutic resistance. No study has determined the changes in the quantity or characteristics of CSCs or circulating tumor cells (CTCs) with EMT phenotype during preoperative anti-HER2 therapy, and whether these changes correlate to response to dual anti-HER2 therapy. In a prospective clinical trial to evaluate pharmacodynamic biomarkers, 18 patients with operable primary HER2-positive breast cancer received dual anti-Her2 preoperative therapy with trastuzumab and lapatinib with paclitaxel. Proportions of tumor cells with CSC characteristics and EMT markers in CTC's were estimated at baseline, after 6 and 18 weeks of preoperative therapy to determine the quantitative cutoff value to predict pathologic complete response (pCR). Out of 18 patients, 8 (44%) had a pCR; 5 of these 8 patients (62%) were positive for CD44v at baseline and none were positive on the 6-week biopsy. In contrast, 6 of the 10 patients without pCR exhibited persistent levels, or enrichment of CD44v proportion and expression at 6 and 18 weeks (p=0.0128). Other biomarkers were not statistically significant predictors of pCR. Enrichment of CD44v-positive tumor cells after dual anti-HER2 therapy alone may predict poor response to dual anti-HER2 therapy plus chemotherapy.

Eliminating Cancer Stem Cells by Targeting Embryonic Signaling Pathways

Dramatic advances have been made in the understanding of cancer over the past decade. Prime among those are better appreciation of the biology of cancer and the development of targeted therapies. Despite these improvements, however, most tumors remain refractory to anti-cancer medications and frequently recur. Cancer Stem Cells (CSCs), which in some cases express markers of pluripotency (e.g., Oct-4), share many of the molecular features of normal stem cells. These cells have been hypothesised to play a role in tumor resistance and relapse. They exhibit dependence on many primitive regulatory pathways and may be best viewed in the context of embryonic signaling pathways. In this article, we review important embryonic signaling cascades and their differential expression in CSCs. We also discuss these pathways as actionable targets for novel therapies in hopes that eliminating cancer stem cells will lead to an improvement in overall survival for patients.

Notch Signaling in Development, Tissue Homeostasis, and Disease

Notch signaling is an evolutionarily highly conserved signaling mechanism, but in contrast to signaling pathways such as Wnt, Sonic Hedgehog, and BMP/TGF-β, Notch signaling occurs via cell-cell communication, where transmembrane ligands on one cell activate transmembrane receptors on a juxtaposed cell. Originally discovered through mutations in Drosophila more than 100 yr ago, and with the first Notch gene cloned more than 30 yr ago, we are still gaining new insights into the broad effects of Notch signaling in organisms across the metazoan spectrum and its requirement for normal development of most organs in the body. In this review, we provide an overview of the Notch signaling mechanism at the molecular level and discuss how the pathway, which is architecturally quite simple, is able to engage in the control of cell fates in a broad variety of cell types. We discuss the current understanding of how Notch signaling can become derailed, either by direct mutations or by aberrant regulation, and the expanding spectrum of diseases and cancers that is a consequence of Notch dysregulation. Finally, we explore the emerging field of Notch in the control of tissue homeostasis, with examples from skin, liver, lung, intestine, and the vasculature.

Polymer-lipid hybrid anti-HER2 nanoparticles for targeted salinomycin delivery to HER2-positive breast cancer stem cells and cancer cells

PURPOSE

Breast cancer stem cells (CSCs) are responsible for the initiation, recurrence, and metastasis of breast cancer. Sufficient evidence has established that breast cancer cells can spontaneously turn into breast CSCs. Thus, it is essential to simultaneously target breast CSCs and cancer cells to maximize the efficacy of breast cancer therapy. HER2 has been found to be overexpressed in both breast CSCs and cancer cells. We developed salinomycin-loaded polymer-lipid hybrid anti-HER2 nanoparticles (Sali-NP-HER2) to target both HER2-positive breast CSCs and cancer cells.

METHODS

The antitumor activity of Sali-NP-HER2 constructed by conjugating anti-HER2 antibodies to polymer-lipid salinomycin nanoparticles was evaluated in vitro and in vivo.

RESULTS

Sali-NP-HER2 efficiently bound to HER2-positive breast CSCs and cancer cells, resulting in enhanced cytotoxic effects compared with non-targeted nanoparticles or salinomycin. In mice bearing breast cancer xenografts, administration of Sali-NP-HER2 exhibited superior efficacy in inhibiting tumor growth. Sali-NP-HER2 reduced the breast tumorsphere formation rate and the proportion of breast CSCs more effectively than non-targeted nanoparticles or salinomycin alone.

CONCLUSION

Sali-NP-HER2 represents a promising approach in treating HER2-positive breast cancer by targeting both breast CSCs and cancer cells.

Radiolabeled pertuzumab for imaging of human epidermal growth factor receptor 2 expression in ovarian cancer

PURPOSE

Human epidermal growth factor receptor 2 (HER2) is over-expressed in over 30% of ovarian cancer cases, playing an essential role in tumorigenesis and metastasis. Non-invasive imaging of HER2 is of great interest for physicians as a mean to better detect and monitor the progression of ovarian cancer. In this study, HER2 was assessed as a biomarker for ovarian cancer imaging using 64Cu-labeled pertuzumab for immunoPET imaging.

METHODS

HER2 expression and binding were examined in three ovarian cancer cell lines (SKOV3, OVCAR3, Caov3) using in vitro techniques, including western blot and saturation binding assays. PET imaging and biodistribution studies in subcutaneous models of ovarian cancer were performed for non-invasive in vivo evaluation of HER2 expression. Additionally, orthotopic models were employed to further validate the imaging capability of 64Cu-NOTA-pertuzumab.

RESULTS

HER2 expression was highest in SKOV3 cells, while OVCAR3 and Caov3 displayed lower HER2 expression. 64Cu-NOTA-pertuzumab showed high specificity for HER2 (Ka = 3.1 ± 0.6 nM) in SKOV3. In subcutaneous tumors, PET imaging revealed tumor uptake of 41.8 ± 3.8, 10.5 ± 3.9, and 12.1 ± 2.3%ID/g at 48 h post-injection for SKOV3, OVCAR3, and Caov3, respectively (n = 3). In orthotopic models, PET imaging with 64Cu-NOTA-pertuzumab allowed for rapid and clear delineation of both primary and small peritoneal metastases in HER2-expressing ovarian cancer.

CONCLUSIONS

64Cu-NOTA-pertuzumab is an effective PET tracer for the non-invasive imaging of HER2 expression in vivo, rendering it a potential tracer for treatment monitoring and improved patient stratification.

Release of HER2 repression of trefoil factor 3 (TFF3) expression mediates trastuzumab resistance in HER2+/ER+ mammary carcinoma

HER2+/ER+ breast cancer, a subset of the luminal B subtype, makes up approximately 10% of all breast cancers. The bidirectional crosstalk between HER2 and estrogen receptor (ER) in HER2+/ER+ breast cancer contributes to resistance towards both anti-estrogens and HER2-targeted therapies. TFF3 promotes breast cancer progression and has been implicated in anti-estrogen resistance in breast cancer. Herein, we investigated the cross-regulation between HER2 and estrogen-responsive TFF3, and the role of TFF3 in mediating trastuzumab resistance in HER2+/ER+ breast cancer. TFF3 expression was decreased by HER2 activation, and increased by inhibition of HER2 with trastuzumab in HER2+/ER+ breast cancer cells, partially in an ERα-independent manner. In contrast, the forced expression of TFF3 activated the entire HER family of receptor tyrosine kinases (HER1-4). Hence, HER2 negatively regulates its own signalling through the transcriptional repression of TFF3, while trastuzumab inhibition of HER2 results in increased TFF3 expression to compensate for the loss of HER2 signalling. In HER2+/ER+ breast cancer cells with acquired trastuzumab resistance, TFF3 expression was markedly upregulated and associated with a corresponding decrease in HER signalling. siRNA mediated depletion or small molecule inhibition of TFF3 decreased the survival and growth advantage of the trastuzumab resistant cells without re-sensitization to trastuzumab. Furthermore, TFF3 inhibition abrogated the enhanced cancer stem cell-like behaviour in trastuzumab resistant HER2+/ER+ breast cancer cells. Collectively, TFF3 may function as a potential biomarker and therapeutic target in trastuzumab resistant HER2+/ER+ breast cancer.

HER2 in Breast Cancer Stemness: A Negative Feedback Loop towards Trastuzumab Resistance

HER2 receptor tyrosine kinase that is overexpressed in approximately 20% of all breast cancers (BCs) is a poor prognosis factor and a precious target for BC therapy. Trastuzumab is approved by FDA to specifically target HER2 for treating HER2+ BC. However, about 60% of patients with HER2+ breast tumor develop de novo resistance to trastuzumab, partially due to the loss of expression of HER2 extracellular domain on their tumor cells. This is due to shedding/cleavage of HER2 by metalloproteinases (ADAMs and MMPs). HER2 shedding results in the accumulation of intracellular carboxyl-terminal HER2 (p95HER2), which is a common phenomenon in trastuzumab-resistant tumors and is suggested as a predictive marker for trastuzumab resistance. Up-regulation of the metalloproteinases is a poor prognosis factor and is commonly seen in mesenchymal-like cancer stem cells that are risen during epithelial to mesenchymal transition (EMT) of tumor cells. HER2 cleavage during EMT can explain why secondary metastatic tumors with high percentage of mesenchymal-like cancer stem cells are mostly resistant to trastuzumab but still sensitive to lapatinib. Importantly, many studies report HER2 interaction with oncogenic/stemness signaling pathways including TGF-β/Smad, Wnt/β-catenin, Notch, JAK/STAT and Hedgehog. HER2 overexpression promotes EMT and the emergence of cancer stem cell properties in BC. Increased expression and activation of metalloproteinases during EMT leads to proteolytic cleavage and shedding of HER2 receptor, which downregulates HER2 extracellular domain and eventually increases trastuzumab resistance. Here, we review the hypothesis that a negative feedback loop between HER2 and stemness signaling drives resistance of BC to trastuzumab.

HR+HER2− breast cancers with growth factor receptor–mediated EMT have a poor prognosis and lapatinib downregulates EMT in MCF-7 cells

Despite an overall good prognosis, a significant proportion of patients with hormone receptor positive human epidermal growth factor receptor 2 negative breast cancers develop distant metastases. The metastatic potential of epithelial cells is known to be regulated by tumor–stromal interaction and mediated by epithelial-to-mesenchymal transition. Hormone receptor positive human epidermal growth factor receptor 2 negative tumors were used to estimate markers of epithelial-to-mesenchymal transition, and the luminal breast cancer cell line MCF-7 was used to examine the interactions between integrins and growth factor receptors in causation of epithelial-to-mesenchymal transition. A total of 140 primary tumors were sub-divided into groups enriched for the markers of epithelial-to-mesenchymal transition (snail family transcriptional repressor 2 and integrin β6) versus those with low levels. Within the epithelial-to-mesenchymal transition+ tumors, there was a positive correlation between the transcripts of integrin β6 and growth factor receptors—human epidermal growth factor receptor 2 and epidermal growth factor receptor. In tumors enriched for epithelial-to-mesenchymal transition markers, patients with tumors with the highest quartile of growth factor receptor transcripts had a shorter disease-free survival compared to patients with low growth factor receptor expression by Kaplan–Meier analysis (log rank, p = 0.03). Epithelial-to-mesenchymal transition was induced in MCF-7 cells by treatment with transforming growth factor beta 1 and confirmed by upregulation of SNAI1 and SNAI2 transcripts, increase of vimentin and integrin β6 protein, and repression of E-cadherin. Treatment of these cells with the dual-specificity tyrosine-kinase inhibitor lapatinib led to downregulation of epithelial-to-mesenchymal transition as indicated by lower levels of SNAI1 and SNAI2 transcripts, integrin αvβ6, and matrix metalloproteinase 9 protein. The results suggest that synergistic interactions between growth factor receptors and integrin β6 could mediate epithelial-to-mesenchymal transition and migration in a subset of luminal breast cancers and lapatinib might be effective in disrupting this interaction.

Nanomedicine strategies for sustained, controlled and targeted treatment of cancer stem cells

Cancer stem cells (CSCs) are original cancer cells that are of characteristics associated with normal stem cells. CSCs are toughest against various treatments and thus responsible for cancer metastasis and recurrence. Therefore, development of specific and effective treatment of CSCs plays a key role in improving survival and life quality of cancer patients, especially those in the metastatic stage. Nanomedicine strategies, which include prodrugs, micelles, liposomes and nanoparticles of biodegradable polymers, could substantially improve the therapeutic index of conventional therapeutics due to its manner of sustained, controlled and targeted delivery of high transportation efficiency across the cell membrane and low elimination by intracellular autophagy, and thus provide a practical solution to solve the problem encountered in CSCs treatment. This review gives briefly the latest information to summarize the concept, strategies, mechanisms and current status as well as future promises of nanomedicine strategies for treatment of CSCs.

NOTCH1 is a poor prognostic factor for breast cancer and is associated with breast cancer stem cells

Recently, the human gene NOTCH1 has been found to be implicated in cancer cell metastasis and the maintenance of cancer stem cells. However, for breast cancer in particular, an association between NOTCH1 levels and metastasis has not been determined. In this study, we investigated the expression status and correlation of NOTCH1 with clinically important factors related to metastasis and the cancer stem cell marker ALDH1. NOTCH1 and ALDH1 levels in 115 tumor tissues from primary lesions were determined by immunohistochemical staining. Most tissues were stained positive for both NOTCH1 and ALDH1, and NOTCH1 positivity was significantly associated with ALDH1 levels. NOTCH1 levels were significantly associated with TNM stage, metastasis, and triple-negative breast cancer. Moreover, both univariate and multivariate regression analyses revealed that basal-like features and NOTCH1 positivity were associated with disease-free survival as independent predictors. These analyses indicated that breast cancer patients testing positive for NOTCH1 had shorter disease-free survival. Our findings suggest that NOTCH1 may be involved in metastasis and is closely correlated with breast cancer stem cells.

Pathobiological implications of the d16HER2 splice variant for stemness and aggressiveness of HER2-positive breast cancer

We have previously shown that the d16HER2 splice variant is linked to HER2-positive breast cancer (BC) tumorigenesis, progression and response to Trastuzumab. However, the mechanisms by which d16HER2 contributes to HER2-driven aggressiveness and targeted therapy susceptibility remain uncertain. Here, we report that the d16HER2-positive mammary tumor cell lines MI6 and MI7, derived from spontaneous lesions of d16HER2 transgenic (tg) mice and resembling the aggressive features of primary lesions, are enriched in the expression of Wnt, Notch and epithelial–mesenchymal transition pathways related genes compared with full-length wild-type (WT) HER2-positive cells (WTHER2_1 and WTHER2_2) derived from spontaneous tumors arising in WTHER2 tg mice. MI6 cells exhibited increased resistance to anoikis and significantly higher mammosphere-forming efficiency (MFE) and self-renewal capability than the WTHER2-positive counterpart. Furthermore, d16HER2-positive tumor cells expressed a higher fraction of CD29^High/CD24⁺/SCA1^Low cells and displayed greater in vivo tumor engraftment in serial dilution conditions than WTHER2_1 cells. Accordingly, NOTCH inhibitors impaired mammosphere formation only in MI6 cells. A comparative analysis of stemness-related features driven by d16HER2 and WTHER2 in ad hoc engineered human BC cells (MCF7 and T47D) revealed a higher MFE and aldehyde dehydrogenase-positive staining in d16HER2- vs WTHER2-infected cells, sustaining consistent BC-initiating cell enrichment in the human setting. Moreover, marked CD44 expression was found in MCF7_d16 and T47D_d16 cells vs their WTHER2 and Mock counterparts. Clinically, BC cases from two distinct HER2-positive cohorts characterized by high levels of expression of the activated-d16HER2 metagene were significantly enriched in the Notch family and signal transducer genes vs those with low levels of the metagene.

Cancer stem cell-driven efficacy of trastuzumab (Herceptin): towards a reclassification of clinically HER2-positive breast carcinomas

Clinically HER2+ (cHER2+) breast cancer (BC) can no longer be considered a single BC disease entity in terms of trastuzumab responsiveness. Here we propose a framework for predicting the response of cHER2+ to trastuzumab that integrates the molecular distinctions of intrinsic BC subtypes with recent knowledge on cancer stem cell (CSC) biology. First, we consider that two interchangeable populations of epithelial-like, aldehyde dehydrogenase (ALDH)-expressing and mesenchymal-like, CD44+CD24-/low CSCs can be found in significantly different proportions across all intrinsic BC subtypes. Second, we overlap all the intrinsic subtypes across cHER2+ BC to obtain a continuum of mixed phenotypes in which one extreme exhibits a high identity with ALDH+ CSCs and the other extreme exhibits a high preponderance of CD44+CD24-/low CSCs. The differential enrichment of trastuzumab-responsive ALDH+ CSCs versus trastuzumab-refractory CD44+CD24-/low CSCs can explain both the clinical behavior and the primary efficacy of trastuzumab in each molecular subtype of cHER2+ (i.e., HER2-enriched/cHER2+, luminal A/cHER2+, luminal B/cHER2+, basal/cHER2+, and claudin-low/cHER2+). The intrinsic plasticity determining the epigenetic ability of cHER2+ tumors to switch between epithelial and mesenchymal CSC states will vary across the continuum of mixed phenotypes, thus dictating their intratumoral heterogeneity and, hence, their evolutionary response to trastuzumab. Because CD44+CD24-/low mesenchymal-like CSCs distinctively possess a highly endocytic activity, the otherwise irrelevant HER2 can open the door to a type of "Trojan horse" approach by employing antibody-drug conjugates such as T-DM1, which will allow a rapid and CSC-targeted delivery of cytotoxic drugs to therapeutically manage trastuzumab-unresponsive basal/cHER2+ BC. Contrary to the current dichotomous model used clinically, our model proposes that a reclassification of cHER2+ tumors based on the spectrum of molecular BC subtypes might inform on their CSC-determined sensitivity to trastuzumab, thus providing a better delineation of the predictive value of cHER2+ in BC by incorporating CSCs-driven intra-tumor heterogeneity into clinical decisions.

Improved Aptamers for the Diagnosis and Potential Treatment of HER2-Positive Cancer

Aptamers provide a potential source of alternative targeting molecules for existing antibody diagnostics and therapeutics. In this work, we selected novel DNA aptamers targeting the HER2 receptor by an adherent whole-cell SELEX approach. Individual aptamers were identified by next generation sequencing and bioinformatics analysis. Two aptamers, HeA2_1 and HeA2_3, were shown to bind the HER2 protein with affinities in the nanomolar range. In addition, both aptamers were able to bind with high specificity to HER2-overexpressing cells and HER2-positive tumor tissue samples. Furthermore, we demonstrated that aptamer HeA2_3 is being internalized into cancer cells and has an inhibitory effect on cancer cell growth and viability. In the end, we selected novel DNA aptamers with great potential for the diagnosis and possible treatment of HER2-positive cancer.

NANOMEDICINE: will it offer possibilities to overcome multiple drug resistance in cancer?

This review is written with the purpose to review the current nanomedicine literature and provide an outlook on the developments in utilizing nanoscale drug constructs in treatment of solid cancers as well as in the potential treatment of multi-drug resistant cancers. No specific design principles for this review have been utilized apart from our active choice to avoid results only based on in vitro studies. Few drugs based on nanotechnology have progressed to clinical trials, since most are based only on in vitro experiments which do not give the necessary data for the research to progress towards pre-clinical studies. The area of nanomedicine has indeed spark much attention and holds promise for improved future therapeutics in the treatment of solid cancers. However, despite much investment few targeted therapeutics have successfully progressed to early clinical trials, indicating yet again that the human body is complicated and that much more understanding of the fundamentals of receptor interactions, physics of nanomedical constructs and their circulation in the body is indeed needed. We believe that nanomedical therapeutics can allow for more efficient treatments of resistant cancers, and may well be a cornerstone for RNA based therapeutics in the future given their general need for shielding from the harsh environment in the blood stream.

Cancer stem cells and HER2 positive breast cancer: The story so far

Human Epidermal Growth Factor Receptor type 2 (HER2) gene amplification and/or protein overexpression is observed in patients suffering from HER2+ breast cancer. This subtype of breast cancer has improved prognosis due to availability of anti-HER2 therapy. However, drug resistance and tumor recurrence still remains a major concern. Cancer Stem Cells (CSCs) are believed to constitute the subset of cell population that is resistant to drug treatment and possesses characteristics of stem cells. CSCs enable the tumors to thrive despite major insults. This review provides a comprehensive idea about the concept of CSCs in context of HER2+ breast cancer by providing the description of the markers that are used for the identification of CSCs and by elucidating the signaling pathways that are associated with HER2+ breast CSCs. Furthermore, the review also describes the interaction of HER2 with those signaling pathways and the future of targeting CSCs in HER2+ breast cancer.

Targeting Peptidyl-Prolyl Isomerase Pin1 to Inhibit Tumor Cell Aggressiveness

Purpose

Because the peptidyl-prolyl isomerase PIN1 interacts with multiple protein kinases and phosphoproteins into a network orchestrating the cellular response to various stimuli, there is an increasing interest in exploiting its potential as therapeutic target. In the present study, the effect of targeting PIN1 was investigated in 2 human cancer cell lines characterized by increased aggressive potential, high expression of erbB receptor family members, and defective p53.

Methods

PIN1 silencing was carried out in skin squamous cell carcinoma A431 cells displaying elevated EGFR/HER1 levels and in ovarian adenocarcinoma SKOV-3 cells displaying high levels of erbB2 (HER2). Nonoverlapping siRNA duplexes targeting different regions of PIN1 mRNA were transfected in tumor cells, which were analyzed using Western blotting for the expression of selected proteins. In vivo tumorigenicity studies were carried out in athymic nude mice.

Results

A431 and SKOV-3 cell systems were found to be a source of cells with increased aggressive potential, i.e., cancer stem cell-like cells, as defined by the capability to grow as spheres. A marked decrease of PIN1 levels and of sphere-forming capability was observed in PIN1-silenced cells. The expression of phospho-p38 decreased following PIN1 silencing in A431 and SKOV-3 cells, as well as phospho-EGFR levels in A431 - silenced cells. PIN1 inhibition prolonged latency and reduced tumor take and growth of SKOV-3 cells in nude mice.

Conclusions

Our results support that PIN1 may be a valuable target to hit in cancer cells characterized by increased aggressive potential, overexpression of erbB receptor family members, and defective p53.

A Role for Notch Signalling in Breast Cancer and Endocrine Resistance

Over the past decade, there has been growing interest in the Notch signalling pathway within the breast cancer field. This interest stemmed initially from the observation that Notch signalling is aberrantly activated in breast cancer and its effects on various cellular processes including proliferation, apoptosis, and cancer stem cell activity. However more recently, elevated Notch signalling has been correlated with therapy resistance in oestrogen receptor-positive breast cancer. As a result, inhibiting Notch signalling with therapeutic agents is being explored as a promising treatment option for breast cancer patients.

Recurrence and metastasis of breast cancer is influenced by ovarian hormone's effect on breast cancer stem cells

Cancer stem cells (CSCs) have recently attracted great interest because of their emerging role in initiation, progression and metastasis, combined with their intrinsic resistance to chemotherapy and radiation therapy. CSCs and its interaction with hormones in breast cancer are currently being investigated with the aim of uncovering the molecular mechanisms by which they evade conventional treatment regimens. In this review, we discuss recent experimental data and new perspectives in the area of steroid hormones and their cross-talk with breast CSCs. We have covered literature associated with biomarkers, hormone receptors and hormone responsive signaling pathways in breast CSC. In addition, we also discuss the role of miRNAs in hormone mediated regulation of breast CSCs.

Synchronized Targeting of Notch and ERBB Signaling Suppresses Melanoma Tumor Growth through Inhibition of Notch1 and ERBB3

Despite significant advances in melanoma therapy, melanoma remains the deadliest form of skin cancer, with a 5-year survival rate of only 15%. Thus, novel treatments are required to address this disease. Notch and ERBB are evolutionarily conserved signaling cascades required for the maintenance of melanocyte precursors. We show that active Notch1 (Notch1(NIC)) and active (phosphorylated) ERBB3 and ERBB2 correlate significantly and are similarly expressed in both mutated and wild-type BRAF melanomas, suggesting these receptors are co-reactivated in melanoma to promote survival. Whereas blocking either pathway triggers modest effects, combining a ?-secretase inhibitor to block Notch activation and a tyrosine kinase inhibitor to inhibit ERBB3/2 elicits synergistic effects, reducing cell viability by 90% and hampering melanoma tumor growth. Specific inhibition of Notch1 and ERBB3 mimics these results, suggesting these are the critical factors triggering melanoma tumor expansion. Notch and ERBB inhibition blunts AKT and NF?B signaling. Constitutive expression of NF?B partially rescues cell death. Blockade of both Notch and ERBB signaling inhibits the slow cycling JARID1B-positive cell population, which is critical for long-term maintenance of melanoma growth. We propose that blocking these pathways is an effective approach to treatment of melanoma patients regardless of whether they carry mutated or wild-type BRAF.

Ductal carcinoma in situ of the breast: the importance of morphologic and molecular interactions

Ductal carcinoma in situ (DCIS) of the breast is a lesion characterized by significant heterogeneity, in terms of morphology, immunohistochemical staining, molecular signatures, and clinical expression. For some patients, surgical excision provides adequate treatment, but a subset of patients will experience recurrence of DCIS or progression to invasive ductal carcinoma (IDC). Recent years have seen extensive research aimed at identifying the molecular events that characterize the transition from normal epithelium to DCIS and IDC. Tumor epithelial cells, myoepithelial cells, and stromal cells undergo alterations in gene expression, which are most important in the early stages of breast carcinogenesis. Epigenetic modifications, such as DNA methylation, together with microRNA alterations, play a major role in these genetic events. In addition, tumor proliferation and invasion is facilitated by the lesional microenvironment, which includes stromal fibroblasts and macrophages that secrete growth factors and angiogenesis-promoting substances. Characterization of DCIS on a molecular level may better account for the heterogeneity of these lesions and how this manifests as differences in patient outcome and response to therapy. Molecular assays originally developed for assessing likelihood of recurrence in IDC are recently being applied to DCIS, with promising results. In the future, the classification of DCIS will likely incorporate molecular findings along with histologic and immunohistochemical features, allowing for personalized prognostic information and therapeutic options for patients with DCIS. This review summarizes current data regarding the molecular characterization of DCIS and discusses the potential clinical relevance.

Lead Optimization of 2-Cyclohexyl-N-[(Z)-(3-methoxyphenyl/3-hydroxyphenyl) methylidene]hydrazinecarbothioamides for Targeting the HER-2 Overexpressed Breast Cancer Cell Line SKBr-3

Lead derivatives of 2-cyclohexyl-N-[(Z)-(3-methoxyphenyl/3-hydroxyphenyl) methylidene]hydrazinecarbothioamides 1–18 were synthesized, characterized and evaluated in vitro against HER-2 overexpressed breast cancer cell line SKBr-3. All the compounds showed activity against HER-2 overexpressed SKBr-3 cells with IC₅₀ = 17.44 ± 0.01 µM to 53.29 ± 0.33 µM. (2Z)-2-(3-Hydroxybenzylidene)-N-(3-methoxyphenyl)hydrazinecarbothioamide (12, IC₅₀ = 17.44 ± 0.01 µM) was found to be most potent compound of this series targeting HER-2 overexpressed breast cancer cells compared to the standard drug 5-fluorouracil (5-FU) (IC₅₀ = 38.58 ± 0.04 µM). Compound 12 inhibited the cellular proliferation via DNA degradation.

The EGFR-HER2 module: a stem cell approach to understanding a prime target and driver of solid tumors

The epidermal growth factor receptor (EGFR) and a coreceptor denoted HER2/ERBB2 are frequently overexpressed or mutated in solid tumors, such as carcinomas and gliomas. In line with driver roles, cancer drugs intercepting EGFR or HER2 currently outnumber therapies targeting other hubs of signal transduction. To explain the roles for EGFR and HER2 as prime drivers and targets, we take lessons from invertebrates and refer to homeostatic regulation of several mammalian tissues. The model we infer ascribes to the EGFR-HER2 module pivotal functions in rapid clonal expansion of progenitors called transient amplifying cells (TACs). Accordingly, TACs of tumors suffer from replication stress, and hence accumulate mutations. In addition, several lines of evidence propose that in response to EGF and related mitogens, TACs might undergo dedifferentiation into tissue stem cells, which might enable entry of oncogenic mutations into the stem cell compartment. According to this view, antibodies or kinase inhibitors targeting EGFR-HER2 effectively retard some solid tumors because they arrest mutation-enriched TACs and possibly inhibit their dedifferentiation. Deeper understanding of the EGFR-HER2 module and relations between cancer stem cells and TACs will enhance our ability to control a broad spectrum of human malignancies.

PKCα Attenuates Jagged-1-Mediated Notch Signaling in ErbB-2-Positive Breast Cancer to Reverse Trastuzumab Resistance

PURPOSE

Breast cancer is the second leading cause of cancer mortality among women worldwide. The major problem with current treatments is tumor resistance, recurrence, and disease progression. ErbB-2-positive breast tumors are aggressive and frequently become resistant to trastuzumab or lapatinib. We showed previously that Notch-1 is required for trastuzumab resistance in ErbB-2-positive breast cancer.

EXPERIMENTAL DESIGN

Here, we sought to elucidate mechanisms by which ErbB-2 attenuates Notch signaling and how this is reversed by trastuzumab or lapatinib.

RESULTS

The current study elucidates a novel Notch inhibitory mechanism by which PKCα downstream of ErbB-2 (i) restricts the availability of Jagged-1 at the cell surface to transactivate Notch, (ii) restricts the critical interaction between Jagged-1 and Mindbomb-1, an E3 ligase that is required for Jagged-1 ubiquitinylation and subsequent Notch activation, (iii) reverses trastuzumab resistance in vivo, and (iv) predicts better outcome in women with ErbB-2-positive breast cancer.

CONCLUSIONS

The clinical impact of these studies is PKCα is potentially a good prognostic marker for low Notch activity and increased trastuzumab sensitivity in ErbB-2-positive breast cancer. Moreover, women with ErbB-2-positive breast tumors expressing high Notch activation and low PKCα expression could be the best candidates for anti-Notch therapy.

Prognostic values of Notch receptors in breast cancer

Notch receptors are frequently deregulated in several human malignancies including human breast cancer. Activation of Notch has been reported to cause mammary carcinomas in mice. However, the prognostic value of individual Notch receptors in breast cancer (BC) patients remains elusive. In the current study, we investigated the prognostic value of Notch receptors in human BC patients. More specifically, we investigated the prognostic value of four Notch receptors in breast cancer patients through "the Kaplan-Meier plotter" (KM plotter) database, in which updated gene expression data and survival information are from a total of 3554 breast cancer patients. Our results showed that Notch1 messenger RNA (mRNA) high expression was correlated to worsen overall survival (OS) in PgR-negative BC patients. Notch2, Notch3, and Notch4 mRNA high expressions were found to be correlated to better OS for all breast cancer patients. Notch2 was also found to be correlated to better OS in lymph node-negative breast cancer patients and HER2-positive breast cancer patients. These results will be useful for better understanding of the heterogeneity and complexity in the molecular biology of breast cancer and for developing tools to more accurately predict their prognosis and design their customized treatment strategies.

Cancer stem cells: a potential target for cancer therapy

Current evidence indicates that a subpopulation of cancer cells, named cancer stem cells (CSCs) or tumor-initiating cells, are responsible for the initiation, growth, metastasis, therapy resistance and recurrence of cancers. CSCs share core regulatory pathways with normal stem cells; however, CSCs rely on distinct reprogrammed pathways to maintain stemness and to contribute to the progression of cancers. The specific targeting of CSCs, together with conventional chemotherapy or radiotherapy, may achieve stable remission or cure cancer. Therefore, the identification of CSCs and a better understanding of the complex characteristics of CSCs will provide invaluable diagnostic, therapeutic and prognostic targets for clinical application. In this review, we will introduce the dysregulated properties of CSCs in cancers and discuss the possible challenges in targeting CSCs for cancer treatment.

Expression of stemness genes in primary breast cancer tissues: the role of SOX2 as a prognostic marker for detection of early recurrence

The events leading to breast cancer (BC) progression or recurrence are not completely understood and new prognostic markers aiming at identifying high risk-patients and to develop suitable therapy are highly demanded. Experimental evidences found in cancer cells a deregulated expression of some genes involved in governance of stem cell properties and demonstrated a relationship between stemness genes overexpression and poorly differentiated BC subtypes. In the present study 140 primary invasive BC specimens were collected. The expression profiles of 13 genes belonging to the OCT3/SOX2/NANOG/KLF4 core circuitry by RT-PCR were analyzed and any correlation between their expression and the BC clinic-pathological features (CPfs) and prognosis was investigated. In our cohort (117 samples), NANOG, GDF3 and SOX2 significantly correlated with grade 2, Nodes negative status and higher KI67 proliferation index, respectively (p=0.019, p=0.029, p= 0.035). According to multivariate analysis, SOX2 expression resulted independently associated with increased risk of recurrence (HR= 2,99; p= p=0,004) as well as Nodes status (HR=2,44; p=0,009) and T-size >1 (HR=1,77; p=0,035). Our study provides further proof of the suitable use of stemness genes in BC management. Interestingly, a prognostic role of SOX2, which seems to be a suitable marker of early recurrence irrespective of other clinicopathological features.

Cancer stem cells: are they responsible for treatment failure?

Overcoming resistance to standard anticancer treatments represents a significant challenge. The interest regarding cancer stem cells, a cellular population that has the ability to self-renew and to propagate the tumor, was prompted by experimental evidence delineating the molecular mechanisms that are selectively activated in this cellular subset in order to survive chemotherapy. This has also stimulated combination strategies aimed at rendering cancer stem cells vulnerable to anticancer agents. Moreover, cancer stem cells offer a unique opportunity for modeling human cancers in mice, thus emerging as a powerful tool for testing novel drugs and combinations in a simulation of human disease. These novel animal models may lay the foundation for a new generation of clinical trials aimed at anticipating the benefit to patients of anticancer therapies.

Identifying and targeting tumor-initiating cells in the treatment of breast cancer

Breast cancer is the most common cancer in women (excluding skin cancer), and it is the second leading cause of cancer-related deaths. Although conventional and targeted therapies have improved survival rates, there are still considerable challenges in treating breast cancer, including treatment resistance, disease recurrence, and metastasis. Treatment resistance can be either de novo - because of traits that tumor cells possess before treatment - or acquired - because of traits that tumor cells gain in response to treatment. A recently proposed mechanism of de novo resistance invokes the existence of a specialized subset of cancer cells defined as tumor-initiating cells (TICs), or cancer stem cells (CSCs). TICs have the capacity to self-renew and to generate new tumors that consist entirely of clonally derived cell types present in the parental tumor. There are data to suggest that TICs are resistant to many conventional cancer therapies and that they can survive treatment in spite of dramatic shrinkage of the tumor. Residual TICs can then eventually regrow, which results in disease relapse. It has also been hypothesized that TIC may be responsible for metastatic disease. If these hypotheses are correct, targeting TICs may be imperative for achieving a cure. In the present review, we discuss evidence for breast TICs and their apparent resistance to conventional chemotherapy and radiotherapy as well as to various targeted therapies. We also address the potential impact of breast TIC plasticity and metastatic potential on therapeutic strategies. Finally, we describe several genes and signaling pathways that appear to be important for TIC function and may represent promising therapeutic targets.

Improving cancer therapy by targeting cancer stem cells: Directions, challenges, and clinical results

Cancer stem cells (CSC) are a rare cell population within a tumor characterized by the ability to form tumors following injection into an immunocompromised host. While the role of CSC has been clearly established in animal models, evidence of their clinical relevance has been harder to demonstrate. A number of markers, or combination thereof, have been used to detect and measure, although non-specifically, CSC in almost all human tumors. Several pathways have been identified as crucial for, but not necessarily unique to, CSC survival and proliferation, and novel agents have been designed to target such pathways. A number of such agents have entered early phase development. Further, drugs that have long been marketed for non-oncological indications have been redirected to oncology as they appear to affect one or more of such pathways. This article aims to review the available evidence on the clinical relevance of CSC from a drug development standpoint and the results of early phase clinical trials of agents interfering with the above pathways. It also discusses limitations of current clinical trial design and endpoints to demonstrate anti-CSC activity as well as possible strategies to overcome these limitations.

The telomerase inhibitor imetelstat alone, and in combination with trastuzumab, decreases the cancer stem cell population and self-renewal of HER2+ breast cancer cells

Cancer stem cells (CSCs) are thought to be responsible for tumor progression, metastasis, and recurrence. HER2 overexpression is associated with increased CSCs, which may explain the aggressive phenotype and increased likelihood of recurrence for HER2(+) breast cancers. Telomerase is reactivated in tumor cells, including CSCs, but has limited activity in normal tissues, providing potential for telomerase inhibition in anti-cancer therapy. The purpose of this study was to investigate the effects of a telomerase antagonistic oligonucleotide, imetelstat (GRN163L), on CSC and non-CSC populations of HER2(+) breast cancer cell lines. The effects of imetelstat on CSC populations of HER2(+) breast cancer cells were measured by ALDH activity and CD44/24 expression by flow cytometry as well as mammosphere assays for functionality. Combination studies in vitro and in vivo were utilized to test for synergism between imetelstat and trastuzumab. Imetelstat inhibited telomerase activity in both subpopulations. Moreover, imetelstat alone and in combination with trastuzumab reduced the CSC fraction and inhibited CSC functional ability, as shown by decreased mammosphere counts and invasive potential. Tumor growth rate was slower in combination-treated mice compared to either drug alone. Additionally, there was a trend toward decreased CSC marker expression in imetelstat-treated xenograft cells compared to vehicle control. Furthermore, the observed decrease in CSC marker expression occurred prior to and after telomere shortening, suggesting that imetelstat acts on the CSC subpopulation in telomere length-dependent and -independent mechanisms. Our study suggests addition of imetelstat to trastuzumab may enhance the effects of HER2 inhibition therapy, especially in the CSC population.

Mutant p53 Amplifies Epidermal Growth Factor Receptor Family Signaling to Promote Mammary Tumorigenesis

The EGFR family (ErbB2/Her2 and EGFR/ErbB1/Her1) often modulates the transcriptional program involved in promoting mammary tumorigenesis. In humans, the majority of ErbB2-positive sporadic breast cancers harbor p53 mutations, which correlate with poor prognosis. Also, the extremely high incidence of ErbB2-positive breast cancer in women with p53 germline mutations (Li-Fraumeni syndrome) suggests a key role of mutant p53 specifically in ErbB2-mediated mammary tumorigenesis. To examine the role of mutant p53 during ErbB2-mediated mammary tumorigenesis, a mutant p53 allele (R172H) was introduced into the (MMTV)-ErbB2/Neu mouse model system. Interestingly, we show in heterozygous p53 mice that mutant p53 R172H is a more potent activator of ErbB2-mediated mammary tumorigenesis than simple loss of p53. The more aggressive disease in mutant p53 animals was reflected by earlier tumor onset, increased mammary tumor multiplicity, and shorter survival. These in vivo and in vitro data provide mechanistic evidence that mutant p53 amplifies ErbB2 and EGFR signaling to promote the expansion of mammary stem cells and induce cell proliferation.

IMPLICATIONS

This study identifies mutant p53 as an essential player in ErbB2 and EGFR-mediated mammary tumorigenesis and indicates the potential translational importance of targeting mutant p53 in this subset of patients with breast cancer.

New targeted therapies for breast cancer: A focus on tumor microenvironmental signals and chemoresistant breast cancers

Breast cancer is the most frequent female malignancy worldwide. Current strategies in breast cancer therapy, including classical chemotherapy, hormone therapy, and targeted therapies, are usually associated with chemoresistance and serious adverse effects. Advances in our understanding of changes affecting the interactome in advanced and chemoresistant breast tumors have provided novel therapeutic targets, including, cyclin dependent kinases, mammalian target of rapamycin, Notch, Wnt and Shh. Inhibitors of these molecules recently entered clinical trials in mono- and combination therapy in metastatic and chemo-resistant breast cancers. Anticancer epigenetic drugs, mainly histone deacetylase inhibitors and DNA methyltransferase inhibitors, also entered clinical trials. Because of the complexity and heterogeneity of breast cancer, the future in therapy lies in the application of individualized tailored regimens. Emerging therapeutic targets and the implications for personalized-based therapy development in breast cancer are herein discussed.

Recent advances reveal IL-8 signaling as a potential key to targeting breast cancer stem cells

Breast cancer stem-like cells (CSCs) are an important therapeutic target as they are purported to be responsible for tumor initiation, maintenance, metastases, and disease recurrence. Interleukin-8 (IL-8) is upregulated in breast cancer compared with normal breast tissue and is associated with poor prognosis. IL-8 is reported to promote breast cancer progression by increasing cell invasion, angiogenesis, and metastases and is upregulated in HER2-positive cancers. Recently, we and others have established that IL-8 via its cognate receptors, CXCR1 and CXCR2, is also involved in regulating breast CSC activity. Our work demonstrates that in metastatic breast CSCs, CXCR1/2 signals via transactivation of HER2. Given the importance of HER2 in breast cancer and in regulating CSC activity, a pathway driving the activation of these receptors would have important biological and clinical consequences, especially in tumors that express high levels of IL-8 and other CXCR1/2-activating ligands. Here, we review the IL-8 signaling pathway and the role of HER2 in maintaining an IL-8 inflammatory loop and discuss the potential of combining CXCR1/2 inhibitors with other treatments such as HER2-targeted therapy as a novel approach to eliminate CSCs and improve patient survival.

Biological and clinical significance of cancer stem cell plasticity

In the past decade, the traditional view of cancers as a homogeneous collection of malignant cells is being replaced by a model of ever increasing complexity suggesting that cancers are complex tissues composed of multiple cell types. This complex model of tumorigenesis has been well supported by a growing body of evidence indicating that most cancers including those derived from blood and solid tissues display a hierarchical organization of tumor cells with phenotypic and functional heterogeneity and at the apex of this hierarchy are cells capable of self-renewal. These "tumor imitating cells" or "cancer stem cells" drive tumorigenesis and contribute to metastasis, treatment resistance and tumor relapse. Although tumor stem cells themselves may display both genetic and phenotypic heterogeneity, recent studies have demonstrated that cancer stem cells maintain plasticity to transition between mesenchymal-like (EMT) and epithelial-like (MET) states, which may be regulated by the tumor microenvironment. These stem cell state transitions may play a fundamental role in tumor progression and treatment resistance. In this review, we discuss the emerging knowledge regarding the plasticity of cancer stem cells with an emphasis on the signaling pathways and noncoding RNAs including microRNAs (miRNA) and long non-coding RNAs (lncRNAs) in regulation of this plasticity during tumor growth and metastasis. Lastly, we point out the importance of targeting both the EMT and MET states of CSCs in order to eliminate these lethal seeds of cancers.

Epidermal growth factor signaling in transformed cells

Members of the epidermal growth factor receptor (EGFR/ErbB) family play a critical role in normal cell growth and development. However, many ErbB family members, especially EGFR, are aberrantly expressed or deregulated in tumors and are thought to play crucial roles in cancer development and metastatic progression. In this chapter, we provide an overview of key mechanisms contributing to aberrant EGFR/ErbB signaling in transformed cells, which results in many phenotypic changes associated with the earliest stages of tumor formation, including several hallmarks of epithelial-mesenchymal transition (EMT). These changes often occur through interaction with other major signaling pathways important to tumor progression, causing a multitude of transcriptional changes that ultimately impact cell morphology, proliferation, and adhesion, all of which are crucial for tumor progression. The resulting mesh of signaling networks will need to be taken into account as new regimens are designed for targeting EGFR for therapeutic intervention. As new insights are gained into the molecular mechanisms of cross talk between EGFR signaling and other signaling pathways, including their roles in therapeutic resistance to anti-EGFR therapies, a continual reassessment of clinical therapeutic regimes and strategies will be required. Understanding the consequences and complexity of EGF signaling and how it relates to tumor progression is critical for the development of clinical compounds and establishing clinical protocols for the treatment of cancer.

Targeting HER-2 over expressed breast cancer cells with 2-cyclohexyl-N-[(Z)-(substituted phenyl/furan-2-yl/thiophene-2-yl)methylidene]hydrazinecarbothioamide

Cyclohexyl thiosemicarbazone derivatives (C1-14) were synthesized, characterized and evaluated against HER-2 over expressed breast cancer cells. The synthesized compounds were screened in vitro against four breast cancer cell lines; SKBr-3, MCF-7, MDA-MB-468 and MDA-MB-231. All the compounds showed activity against HER-2 over expressed SKBr-3 cells with (IC₅₀ = 25.6 ± 0.07 μM-61.6 ± 0.4 μM). The most active compounds inhibit ALDH⁺ breast cancer stem cells more effectively than the cancer stem cells specific agent Salinomycin. Immunohistochemistry staining also confirmed that these compounds inhibit the expression of HER-2 on SKBr-3 cells. Compound C2 significantly inhibited the cell migration and cell adhesion of breast cancer cell lines. Compound C2 was found to most active compound of this series targeting HER-2 over expressed breast cancer cells.

Top Notch cancer stem cells by paracrine NF-κB signaling in breast cancer

Cancer stem cells are likely to play critical roles in metastasis, therapy resistance, and recurrence of hematological and solid malignancies. It is well known that the stem cell niche plays a key role for asymmetric division and homeostasis of normal stem cells, whereas cancer stem cells seem to use these niches. Among many pathways involved in self-renewal of cancer stem cells, nuclear factor-kappa B (NF-κB) signaling has been documented to promote their expansion in a cell-autonomous fashion. A recent study, however, suggests that paracrine NF-κB activation promotes the expansion of cancer stem cells through the activation of Notch in basal-type breast cancer cells.

Implications of stemness-related signaling pathways in breast cancer response to therapy

There is accumulating evidence that breast cancer may arise from a small subpopulation of transformed mammary stem/progenitor cells, termed breast cancer-initiating cells (BCICs), responsible for initiation and maintenance of cancer. BCICs have been identified in clinical specimens based on CD44(+)/CD24(-/low) membrane expression and/or enzymatic activity of aldehyde dehydrogenase 1 (ALDH1+), or isolated and in vitro propagated as non-adherent spheres. This cell population has been demonstrated to be able to recreate, when injected in mice even at very low concentrations, the same histopathological features of the tumor they were derived from and to escape from current therapeutic strategies. Alterations in genes involved in stemness-related pathways, such as Wnt, Notch, and Sonic Hedgehog, have been proven to play a role in breast cancer progression. Targeting these key elements represents an attractive option, with a solid rationale, although possible concerns may derive from the poor knowledge of tolerance and efficacy of inhibiting these mechanisms without inducing severe side effects. In addition, efforts to develop alternative BCIC-targeted therapies against stemness markers (CD44 and ALDH1) and molecules involved in regulating EMT- and HER2-related pathways, or able to reverse the multi-drug resistance phenotype, or to induce differentiation and to control cell survival pathways are currently ongoing and encouraging results from pre-clinical studies have already been obtained using in vitro and in vivo models.

Vaccination with ErbB-2 peptides prevents cancer stem cell expansion and suppresses the development of spontaneous tumors in MMTV-PyMT transgenic mice

ErbB-2 has been implicated as a target for cancer-initiating cells in breast and other cancers. ErbB-2-directed peptide vaccines have been shown to be effective in prevention of spontaneous tumorigenesis of breast in neu transgenic mouse model, and cellular immunity is proposed as a mechanism for the anti-tumor efficacy. However, there has been no explanation as to how immunity suppresses tumorigenesis from the early stage carcinogenesis, when ErbB-2 expression in breast is low. Here, we investigated a peptide-based vaccine, which consists of two MHC class II epitopes derived from murine ErbB-2, to prevent the occurrence of spontaneous tumors in breast and assess immune impact on breast cancer stem cells. Female MMTV-PyMT transgenic mice were immunized with either ErbB-2 peptide vaccine, or a peptide from tetanus toxoid, or PBS in immune adjuvant. ErbB-2 peptides vaccine completely suppressed spontaneous breast tumors, and the efficacy was correlated with antigen-specific T-cell and antibody responses. In addition, immune serum from the mice of ErbB-2 vaccine group had an inhibitory effect on mammosphere-forming capacity and signaling through ErbB-2 and downstream Akt pathway in ErbB-2 overexpressing mouse mammary cancer cells. We provide evidence that multi-epitope class II peptides vaccine suppresses tumorigenesis of breast potentially by inhibiting the growth of cancer stem cells. We also suggest that a strategy of inducing strong immune responses using multi-epitope ErbB-2-directed helper vaccine might be useful in preventing breast cancer recurrence.

Therapeutic targeting of ERBB2 in breast cancer: understanding resistance in the laboratory and combating it in the clinic

ERBB2 gene amplification occurs in about one quarter of breast carcinomas (BCs) and identifies a distinct clinical subset of BC. The introduction in the clinic of Trastuzumab, a humanized monoclonal antibody (mAb) directed to the ERBB2 extracellular domain, has had a great impact on the therapeutic management of ERBB2+ BC. Yet, not all patients respond to Trastuzumab and resistance develops also among patients that initially benefit from Trastuzumab-based regimens. Pre-clinical studies have discovered several mechanisms through which tumor cells may escape from Trastuzumab-mediated ERBB2 inhibition. These include rewiring of the ErbB signaling network, loss of ERBB2 expression, expression of ERBB2 isoforms refractory to Trastuzumab inhibition, vicarious signaling by non-ErbB tyrosine kinases and constitutive activation of downstream signaling routes, such as the PI3K pathway. While the relative contribution of each of these mechanisms to establishing Trastuzumab resistance in the clinical setting is not fully understood, much attention has been focused on abating resistance by achieving complete blockade of ERBB2-containing dimers. This approach, propelled by the development of novel anti-ERBB2 therapeutics, has led to the recent approval of Lapatinib, Pertuzumab and T-DM1 as additional anti-ERBB2 therapeutics in BC. However, full success is far from being achieved and resistance to ERBB2 targeting remains a relevant problem in the clinical management of BC. Herein, we provide an overview of biological and molecular bases underpinning resistance to ERBB2 therapeutics in BC, discuss outstanding issues in the field of ERBB2 therapeutic targeting and elaborate on future directions of translational research on ERBB2+ breast cancer.

Breast cancer stem cells: Multiple capacities in tumor metastasis

Breast cancer is the leading cause of cancer death among women worldwide. Accumulating evidence indicates that the local recurrent and/or distant metastatic tumors, the major causes of lethality in the clinic, are related to the aggressive phenotype of a small fraction of cancer cells loosely termed as cancer stem cells (CSCs), tumor initiating cells (TICs), or cancer metastasis-initiating cells (CMICs). Breast cancer stem cells (BCSCs) are shown to exhibit unique growth abilities including self-renewal, differentiation potential, and resistance to most anti-cancer agents including chemo- and/or radiotherapy, all of which are believed to contribute to the development and overall aggressiveness of the recurrent or metastatic lesions. It is in the urgent need not only to further define the nature of heterogeneity in each tumor but also to characterize the precise mechanisms governing tumor-host cross-talk which is assumed to be initiated by BCSCs. In this review, we will focus on recently identified key factors, including the BCSCs among circulating tumor cells, interaction of BCSCs with the host, epithelial mesenchymal transition (EMT), tumor microenvironment, the intrinsic resistance due to HER2 expression, potential biomarkers of BCSCs and cancer cell immune signaling. We believe that new evidence coming from both bench and clinical research will help to develop more effective approaches to control or significantly reduce the aggressiveness of metastatic tumors.

Heterogeneity in ERK activity as visualized by in vivo FRET imaging of mammary tumor cells developed in MMTV-Neu mice

Human epidermal growth factor receptor2/Neu, which is overexpressed in about 30% of human breast cancers, transduces growth signals in large part via the Ras-Raf-MEK-ERK pathway. Nevertheless, it is a matter of controversy whether high ERK activity in breast cancer tissues correlates with better or worse prognosis, leaving the role of ERK activity in the progression of breast cancers unresolved. To address this issue, we live-imaged ERK activity in mammary tumors developed in mouse mammary tumor virus-Neu transgenic mice, which had been crossed with transgenic mice expressing a Förster resonance energy transfer biosensor for ERK. Observation of the tumor by two-photon microscopy revealed significant heterogeneity in ERK activity among the mammary tumor cells. The level of ERK activity in each cell was stable up to several hours, implying a robust mechanism that maintained the ERK activity within a limited range. By sorting the mammary tumor cells on the basis of their ERK activity, we found that ERK(high) cells less efficiently generated tumorspheres in vitro and tumors in vivo than did ERK(low) cells. In agreement with this finding, the expressions of the cancer stem cell markers CD49f, CD24 and CD61 were decreased in ERK(high) cells. These observations suggest that high ERK activity may suppress the self-renewal of mammary cancer stem cells.

Nonlinear growth kinetics of breast cancer stem cells: implications for cancer stem cell targeted therapy

Cancer stem cells (CSCs) have been identified in primary breast cancer tissues and cell lines. The CSC population varies widely among cancerous tissues and cell lines, and is often associated with aggressive breast cancers. Despite of intensive research, how the CSC population is regulated within a tumor is still not well understood so far. In this paper, we present a mathematical model to explore the growth kinetics of CSC population both in vitro and in vivo. Our mathematical models and supporting experiments suggest that there exist non-linear growth kinetics of CSCs and negative feedback mechanisms to control the balance between the population of CSCs and that of non-stem cancer cells. The model predictions can help us explain a few long-standing questions in the field of cancer stem cell research, and can be potentially used to predict the efficicacy of anti-cancer therapy.

Metformin selectively targets tumor-initiating cells in ErbB2-overexpressing breast cancer models

Metformin is an oral biguanide used for type II diabetes. Epidemiologic studies suggest a link between metformin use and reduced risk of breast and other types of cancers. ErbB2-expressing breast cancer is a subgroup of tumors with poor prognosis. Previous studies demonstrated that metformin is a potent inhibitor of ErbB2-overexpressing breast cancer cells; metformin treatment extends the life span and impedes mammary tumor development in ErbB2 transgenic mice in vivo. However, the mechanisms of metformin associated antitumor activity, especially in prevention models, remain unclear. We report here for the first time that systemic administration of metformin selectively inhibits CD61(high)/CD49f(high) subpopulation, a group of tumor-initiating cells (TIC) of mouse mammary tumor virus (MMTV)-ErbB2 mammary tumors, in preneoplastic mammary glands. Metformin also inhibited CD61(high)/CD49f(high) subpopulation in MMTV-ErbB2 tumor-derived cells, which was correlated with their compromised tumor initiation/development in a syngeneic tumor graft model. Molecular analysis indicated that metformin induced downregulation of ErbB2 and EGFR expression and inhibited the phosphorylation of ErbB family members, insulin-like growth factor-1R, AKT, mTOR, and STAT3 in vivo. In vitro data indicate that low doses of metformin inhibited the self-renewal/proliferation of cancer stem cells (CSC)/TICs in ErbB2-overexpressing breast cancer cells. We further demonstrated that the expression and activation of ErbB2 were preferentially increased in CSC/TIC-enriched tumorsphere cells, which promoted their self-renewal/proliferation and rendered them more sensitive to metformin. Our results, especially the in vivo data, provide fundamental support for developing metformin-mediated preventive strategies targeting ErbB2-associated carcinogenesis.