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Introduction Breast cancer is the second most common cause of death for women behind lung cancer and the most common cause of cancer deaths for women aged 45–55 years old (CDC.gov 2012). Although there continue to be enormously large numbers of disease incidence, deaths have been declining due to the disease with two hallmark time frames. The first occurred during the mid to late 1980’s when hormonal therapy was introduced as a treatment for ER/PR positive breast cancer. The second occurred in the late 1990’s when trastuzumab was introduced in treating HER2 positive breast cancer. These remarkable accomplishments in developing novel targeted therapies for breast cancer, along with a better understanding of the disease biology have improved disease outcome over the past 20 years. This article reviews the data presented at 2012 American Society of Clinical Oncology and 2012 San Antonio Breast Cancer Symposium regarding progress made in the field of HER2 positive breast cancer and examines the future of HER2 targeted therapy.

The role of epithelial-to-mesenchymal transition (EMT) in metastasis is a longstanding source of debate, largely owing to an inability to monitor transient and reversible EMT phenotypes in vivo . Here we establish an EMT lineage-tracing system to monitor this process in mice, using a mesenchymal-specific Cre-mediated fluorescent marker switch system in spontaneous breast-to-lung metastasis models. We show that within a predominantly epithelial primary tumour, a small proportion of tumour cells undergo EMT. Notably, lung metastases mainly consist of non-EMT tumour cells that maintain their epithelial phenotype. Inhibiting EMT by overexpressing the microRNA miR-200 does not affect lung metastasis development. However, EMT cells significantly contribute to recurrent lung metastasis formation after chemotherapy. These cells survived cyclophosphamide treatment owing to reduced proliferation, apoptotic tolerance and increased expression of chemoresistance-related genes. Overexpression of miR-200 abrogated this resistance. This study suggests the potential of an EMT-targeting strategy, in conjunction with conventional chemotherapies, for breast cancer treatment. An epithelial-to-mesenchymal transition (EMT) lineage-tracing system in a mouse model of breast-to-lung metastasis reveals that although some cells undergo EMT in a primary epithelial tumour, the lung metastases mainly arise from cells that have not undergone EMT; in addition, cells that have undergone EMT appear more resistant to chemotherapy. No requirement for EMT in metastasis It has been suggested that epithelial-to-mesenchymal transition (EMT), in which epithelial cells depolarize and adopt a fibroblast-like morphology, is a requirement for metastasis to occur. Other studies imply that the importance of EMT relies on cell-culture-based manipulation of EMT regulators. In this issue of Nature , two groups present results that suggest that EMT is not a prerequisite for metasasis. Dingcheng Gao and colleagues trace the fate of cells that have undergone EMT in mouse model for breast-to-lung metastasis. They find that although some cells undergo EMT in a primary epithelial tumour, the lung metastases mainly contain cells that have not undergone EMT. However, cells that have undergone EMT appear more resistant to chemotherapy. A microRNA that targets key EMT regulators is shown not to affect metastasis, but to reduce survival of EMT cells following chemotherapy. Raghu Kalluri and colleagues delete Twist or Snail — transcription factors that induce EMT — in a mouse model for pancreatic ductal adenocarcinoma. This leads to an increase in cell proliferation, and a greater sensitivity to chemotherapeutic agent gemcitabine, with no effect on invasion and metastasis.

Until now, the Food and Drug Administration (FDA)-approved iron supplement ferumoxytol and other iron oxide nanoparticles have been used for treating iron deficiency, as contrast agents for magnetic resonance imaging and as drug carriers. Here, we show an intrinsic therapeutic effect of ferumoxytol on the growth of early mammary cancers, and lung cancer metastases in liver and lungs. In vitro , adenocarcinoma cells co-incubated with ferumoxytol and macrophages showed increased caspase-3 activity. Macrophages exposed to ferumoxytol displayed increased mRNA associated with pro-inflammatory Th1-type responses. In vivo , ferumoxytol significantly inhibited growth of subcutaneous adenocarcinomas in mice. In addition, intravenous ferumoxytol treatment before intravenous tumour cell challenge prevented development of liver metastasis. Fluorescence-activated cell sorting (FACS) and histopathology studies showed that the observed tumour growth inhibition was accompanied by increased presence of pro-inflammatory M1 macrophages in the tumour tissues. Our results suggest that ferumoxytol could be applied ‘off label’ to protect the liver from metastatic seeds and potentiate macrophage-modulating cancer immunotherapies. The Food and Drug Administration (FDA)-approved iron supplement ferumoxytol, which contains iron oxide nanoparticles, can suppress growth of early mammary cancers and lung cancer metastasis by inducing pro-inflammatory M1 type macrophage polarization in the tumour tissue, offering a new ‘off label’ application for an approved drug.

Cytotoxic chemotherapy is an effective treatment for invasive breast cancer. However, experimental studies in mice also suggest that chemotherapy has pro-metastatic effects. Primary tumours release extracellular vesicles (EVs), including exosomes, that can facilitate the seeding and growth of metastatic cancer cells in distant organs, but the effects of chemotherapy on tumour-derived EVs remain unclear. Here we show that two classes of cytotoxic drugs broadly employed in pre-operative (neoadjuvant) breast cancer therapy, taxanes and anthracyclines, elicit tumour-derived EVs with enhanced pro-metastatic capacity. Chemotherapy-elicited EVs are enriched in annexin A6 (ANXA6), a Ca 2+ -dependent protein that promotes NF-κB-dependent endothelial cell activation, Ccl2 induction and Ly6C + CCR2 + monocyte expansion in the pulmonary pre-metastatic niche to facilitate the establishment of lung metastasis. Genetic inactivation of Anxa6 in cancer cells or Ccr2 in host cells blunts the pro-metastatic effects of chemotherapy-elicited EVs. ANXA6 is detected, and potentially enriched, in the circulating EVs of breast cancer patients undergoing neoadjuvant chemotherapy. Keklikoglou et al. report that cytotoxic drugs induce tumour-derived extracellular vesicles that facilitate monocyte expansion through annexin A6 and thus lung metastasis in breast cancer.

The allosteric binding of MSI-1436 to the intrinsically disordered C-terminal region of PTP1B promotes a conformational change to generate a compact inactive structure, validating the use of MSI-1436 to inhibit HER2-mediated tumorigenesis. PTP1B, a validated therapeutic target for diabetes and obesity, has a critical positive role in HER2 signaling in breast tumorigenesis. Efforts to develop therapeutic inhibitors of PTP1B have been frustrated by the chemical properties of the active site. We define a new mechanism of allosteric inhibition that targets the C-terminal, noncatalytic segment of PTP1B. We present what is to our knowledge the first ensemble structure of PTP1B containing this intrinsically disordered segment, within which we identified a binding site for the small-molecule inhibitor MSI-1436. We demonstrate binding to a second site close to the catalytic domain, with cooperative effects between the two sites locking PTP1B in an inactive state. MSI-1436 antagonized HER2 signaling, inhibited tumorigenesis in xenografts and abrogated metastasis in the NDL2 mouse model of breast cancer, validating inhibition of PTP1B as a therapeutic strategy in breast cancer. This new approach to inhibition of PTP1B emphasizes the potential of disordered segments of proteins as specific binding sites for therapeutic small molecules.

ACSL4 is critical for induction of ferroptosis, a programmed form of necrotic cell death, through the production of long polyunsaturated fatty acids that can be inhibited in an in vivo ferroptosis model with a small molecule ACSL4 inhibitor. Ferroptosis is a form of regulated necrotic cell death controlled by glutathione peroxidase 4 (GPX4). At present, mechanisms that could predict sensitivity and/or resistance and that may be exploited to modulate ferroptosis are needed. We applied two independent approaches—a genome-wide CRISPR-based genetic screen and microarray analysis of ferroptosis-resistant cell lines—to uncover acyl-CoA synthetase long-chain family member 4 (ACSL4) as an essential component for ferroptosis execution. Specifically, Gpx4 – Acsl4 double-knockout cells showed marked resistance to ferroptosis. Mechanistically, ACSL4 enriched cellular membranes with long polyunsaturated ω6 fatty acids. Moreover, ACSL4 was preferentially expressed in a panel of basal-like breast cancer cell lines and predicted their sensitivity to ferroptosis. Pharmacological targeting of ACSL4 with thiazolidinediones, a class of antidiabetic compound, ameliorated tissue demise in a mouse model of ferroptosis, suggesting that ACSL4 inhibition is a viable therapeutic approach to preventing ferroptosis-related diseases.

Cancer and stromal cells actively exert physical forces (solid stress) to compress tumour blood vessels, thus reducing vascular perfusion. Tumour interstitial matrix also contributes to solid stress, with hyaluronan implicated as the primary matrix molecule responsible for vessel compression because of its swelling behaviour. Here we show, unexpectedly, that hyaluronan compresses vessels only in collagen-rich tumours, suggesting that collagen and hyaluronan together are critical targets for decompressing tumour vessels. We demonstrate that the angiotensin inhibitor losartan reduces stromal collagen and hyaluronan production, associated with decreased expression of profibrotic signals TGF-β1, CCN2 and ET-1, downstream of angiotensin-II-receptor-1 inhibition. Consequently, losartan reduces solid stress in tumours resulting in increased vascular perfusion. Through this physical mechanism, losartan improves drug and oxygen delivery to tumours, thereby potentiating chemotherapy and reducing hypoxia in breast and pancreatic cancer models. Thus, angiotensin inhibitors —inexpensive drugs with decades of safe use — could be rapidly repurposed as cancer therapeutics. Hyaluronan is a component of the tumour extracellular matrix. Here, Chauhan et al . show that hyaluronan increases blood pressure in collagen-rich tumours by compressing vessel walls, and that reducing the level of hyaluranon with an angiotensin II inhibitor increases blood flow and drug penetrance in tumours.

Formate overflow coupled to mitochondrial oxidative metabolism\\ has been observed in cancer cell lines, but whether that takes place in the tumor microenvironment is not known. Here we report the observation of serine catabolism to formate in normal murine tissues, with a relative rate correlating with serine levels and the tissue oxidative state. Yet, serine catabolism to formate is increased in the transformed tissue of in vivo models of intestinal adenomas and mammary carcinomas. The increased serine catabolism to formate is associated with increased serum formate levels. Finally, we show that inhibition of formate production by genetic interference reduces cancer cell invasion and this phenotype can be rescued by exogenous formate. We conclude that increased formate overflow is a hallmark of oxidative cancers and that high formate levels promote invasion via a yet unknown mechanism. Serine catabolism to formate supplies one-carbon units for biosynthesis. Here the authors show that formate production in murine cancers with high oxidative metabolism exceeds the biosynthetic demand and that high formate levels promotes invasion of cancer cells.

The blood vessels of cancerous tumours are leaky 1 , 2 , 3 and poorly organized 4 , 5 , 6 , 7 . This can increase the interstitial fluid pressure inside tumours and reduce blood supply to them, which impairs drug delivery 8 , 9 . Anti-angiogenic therapies—which ‘normalize’ the abnormal blood vessels in tumours by making them less leaky—have been shown to improve the delivery and effectiveness of chemotherapeutics with low molecular weights 10 , but it remains unclear whether normalizing tumour vessels can improve the delivery of nanomedicines. Here, we show that repairing the abnormal vessels in mammary tumours, by blocking vascular endothelial growth factor receptor-2, improves the delivery of smaller nanoparticles (diameter, 12 nm) while hindering the delivery of larger nanoparticles (diameter, 125 nm). Using a mathematical model, we show that reducing the sizes of pores in the walls of vessels through normalization decreases the interstitial fluid pressure in tumours, thus allowing small nanoparticles to enter them more rapidly. However, increased steric and hydrodynamic hindrances, also associated with smaller pores, make it more difficult for large nanoparticles to enter tumours. Our results further suggest that smaller (∼12 nm) nanomedicines are ideal for cancer therapy due to their superior tumour penetration. Repairing the blood vessels in cancerous tumours can improve the delivery of small nanoparticles.

Matrix metalloproteinases (MMPs) degrade and modify the extracellular matrix (ECM) as well as cell-ECM and cell-cell contacts, facilitating detachment of epithelial cells from the surrounding tissue. MMPs play key functions in embryonic development and mammary gland branching morphogenesis, but they are also upregulated in breast cancer, where they stimulate tumorigenesis, cancer cell invasion and metastasis. MMPs have been investigated as potential targets for cancer therapy, but clinical trials using broad-spectrum MMP inhibitors yielded disappointing results, due in part to lack of specificity toward individual MMPs and specific stages of tumor development. Epithelial-mesenchymal transition (EMT) is a developmental process in which epithelial cells take on the characteristics of invasive mesenchymal cells, and activation of EMT has been implicated in tumor progression. Recent findings have implicated MMPs as promoters and mediators of developmental and pathogenic EMT processes in the breast. In this review, we will summarize recent studies showing how MMPs activate EMT in mammary gland development and in breast cancer, and how MMPs mediate breast cancer cell motility, invasion, and EMT-driven breast cancer progression. We also suggest approaches to inhibit these MMP-mediated malignant processes for therapeutic benefit.