Mazzieri R, Pucci F, Moi D, Zonari E, Ranghetti A, Bert A, et al. Targeting the ANG2/TIE2 axis inhibits tumor growth and metastasis by impairing angiogenesis and disabling rebounds of proangiogenic myeloid cells. Cancer Cell 2011; In Press.

DeNardo DG, Brennan DJ, Rexhepaj E, Ruffell B, Shiao SL, Madden SF, et al. Leukocyte complexity predicts breast cancer survival and functionally regulates response to chemotherapy. Cancer Discovery; Published OnlineFirst April 3, 2011;1(1) doi:10.1158/2159-8274.CD-10-0028.

Macrophages and antiangiogenic agents

Myeloid cells and blood vessels are both major targets in the tumor microenvironment. However, the hypoxia induced by existing antiangiogenic treatments can generate an influx of myeloid cells that promotes tissue repair, revascularization, and tumor recurrence. This phenomenon is becoming a major barrier to the translation of tumor angiogenesis research into successful and enduring cancer treatments. Mazzieri and colleagues describe one way around this problem-targeting angiopoietin-2 (ANG2). ANG2 is highly expressed in tumor endothelial cells and is further upregulated by hypoxia. ANG2 is thought to act by sensitizing endothelial cells to other angiogenic factors, but its role in tumor growth, and its potential as a therapeutic target, has not been well defined- until now. Using a fully humanized monoclonal antibody against ANG2 (both not the related ANG1) Mazzieri and colleagues convincingly demonstrate that ANG2 blockade inhibits angiogenesis and induces vascular regression in genetic and transplantable murine cancer models-even those resistant to VEGF/VEGF receptor inhibitors. The antibody inhibited tumor growth and limited metastatic dissemination. Critically, ANG2 blockade also prevented upregulation of TIE2, the ANG2 receptor on macrophages, preventing these macrophages from stimulating the tumor vasculature. The authors conclude that the TIE2 receptor on perivascular macrophages is a crucial regulator of ANG2-mediated proangiogenic programs, at least in these mouse models of cancer. Because these macrophages are known to counteract the benefits of other antiangiogenic therapies, it seems that the ANG2/TIE2 axis is a promising therapeutic target. Given that these results were obtained with a fully humanized monoclonal antibody that can presumably be used for both preclinical experiments and clinical trials, translation of this research into the clinic appears to be feasible and desirable.

Macrophages and chemotherapy

Tumor-associated leukocytes are not only implicated in escape from antiangiogenic treatments as described above, but they may also have a profound influence on a tumor's response to chemotherapy. In an article in the inaugural issue of Cancer Discovery, DeNardo and colleagues demonstrate how common cytotoxic drugs recruit tumor-promoting monocytes/macrophages into the tumor microenvironment. This research was stimulated by a clinically relevant observation: an immune cell profile of high CD68+, high CD4+, and low CD8+ leukocytes in pretreatment biopsies from 2 large cohorts of breast cancer patients was an independent predictor of decreased overall and relapse-free survival in patients whose tumors had already spread to lymph nodes. These findings led the group to hypothesize that blocking macrophage infiltration into tumors might facilitate CD8+ T-cell infiltration enhance antitumor T-cell responses. Consistent with this hypothesis there was an inverse correlation between CD68+ macrophages and CD8+ T cells in breast cancer tissues. Moving into a genetic mouse model of breast cancer, the group found that paclitaxel chemotherapy upregulated the macrophage chemotactic factors CSF1, CCL8, and IL34 that led to an increase in colony stimulating factor 1 (CSF1) receptor-expressing macrophages in the tumor microenvironment. Blockade of macrophage recruitment with inhibitors of CSF1, in combination with chemotherapy, enhanced therapeutic activity, and inhibited metastases. The late-stage carcinomas that did develop during this combination therapy contained large areas of necrosis, paralleled by reduced vessel density in the treated tumors. As predicted by the original hypothesis, inhibition of the macrophage infiltrate also increased T cells in the tumors and mRNA for a number of cytotoxic effector molecules such as granzyme A and B and perforin-1. Depleting CD8+ cytotoxic effector T cells in the mice abrogated the positive effects of macrophage depletion. Consistent with these findings in the mice, human breast cancers revealed that the extent of the CD8+ T-cell infiltrate positively correlates with good prognosis in many cancers, and macrophage infiltrates usually show a negative correlation in the same cancers. This article gives us important mechanistic insights into the role of the tumor's immune profile in the response to chemotherapy and hence the outcome for an individual patient. Importantly, this association seems to be true for different breast cancer subtypes, suggesting that targeting leukocytes in the tumor microenvironment is an exciting option that will be widely applicable, especially in those patients who have an unfavorable immune profile at presentation.

Wertz IE, Kusam S, Lam C, Okamoto T, Sandoval W, Anderson DJ, et al. Sensitivity to antitubulin chemotherapeutics is regulated by MCL1 and FBW7. Nature 2011;471:110–4.

Inuzuka H, Shaik S, Onoyama I, Gao D, Tseng A, Maser RS, et al. SCFFBW7 regulates cellular apoptosis by targeting MCL1 for ubiquitylation and destruction. Nature 2011;471:104–9.

Vincristine and paclitaxel are standard of care in many solid tumors and hematopoietic malignancies. Although we know that these agents block microtubules and govern apoptosis, the details linking microtubule blockade and cell death remain elusive. Reporting separately in the same journal issue, Wertz and colleagues and Inuzuka and colleagues showed that antitubulin chemotherapeutics led to phosphorylation of the BCL-2 family antiapoptotic protein MCL1. Phosphorylated MCL1 is a substrate for the E3 ligase FBW7, targeting MCL1 for degradation. Cancer cell lines lacking FBW7 had elevated levels of MCL1, associated with resistance to both antitubulin and to anti-BCL2 therapeutics, but were sensitive to the multikinase inhibitor sorafenib, known to repress MCL1. These results provide insights into the mechanisms through which antitubulin agents promote cell death, point to MCL1 and FBW7 as therapeutic biomarkers for a wide range of cancers, and provide a therapeutic rationale for targeted therapy in FBW7-deificient cancers.

Sun T, Aceto N, Meerbrey KL, Kessler JD, Zhou C, Migliaccio I, et al. Activation of multiple proto-oncogenic kinases in breast cancer via loss of PTPN12 phosphatase. Cell 2011;144:703–18.

Although targeted therapies have shown clear efficacy in HER2 and hormone receptor-positive breast cancer, triplenegative breast cancers remain a significant therapeutic challenge. Sun and colleagues identified the protein tyrosine phosphatase PTPN12 as a tumor suppressor in triplenegative breast cancers. PTPN12 constrained a number of oncogenic kinases, including epidermal growth factor receptor and HER2. PTPN12 was inactivated in over 20% of primary breast cancers and breast cancer cell lines and in nearly 15% of lung cancers examined, with hypomorphic mutants and variants also enriched in triple-negative breast cancers. Interestingly, the tumor suppressor RE1-silencing transcription factor (REST) blocks expression of mir-124, which negatively regulates expression of PTPN12. Loss of REST also was demonstrated as a second mechanism through which triple-negative breast cancers drove mir-124, blocking expression of PTPN12. In mouse models of triple-negative breast cancer, expression of PTPN12 blocked both tumorigenicity and metastasis. Thus, PTPN12 plays a prominent role in constraining multiple oncogenic kinases, with loss of PTPN12 contributing to tumorigenesis and likely driving metastatic spread in triple-negative breast cancer.

Muthana M, Giannoudis A, Scott SD, Fang HY, Coffelt SB, Morrow FJ, et al. Use of macrophages to target therapeutic adenovirus to human prostate tumors. Cancer Res 2011;71:1805–15.

Direct administration of therapeutic adenoviruses, including both replication-incompetent and replication–competent adenoviruses, has shown limited therapeutic efficacy. Two factors contributing to this lack of objective clinical benefit include trapping of therapeutic adenoviruses in the liver and neutralization by the immune system. A number of approaches have been used to circumvent these problems. Muthana and colleagues used a cell-based delivery approach involving macrophages that extravasate from the bloodstream into tumors, where they accumulate in hypoxic regions, to selectively deliver a conditionally replication–competent adenovirus to prostate tumors. This strategy uses cotransduction of a hypoxia-regulated E1A/B construct with an E1A-dependent oncolytic adenovirus whose proliferation is restricted to prostate tumor cells using prostate-specific promoter elements from the TARP, PSA, and PMSA genes. When transduced macrophages reach areas of extreme hypoxia, viral replication is induced, resulting in release of infectious virus that infects neighboring tumor cells. This strategy was tested as a proof of principle in mice bearing subcutaneous or orthotopic prostate tumors, resulting in a significant inhibition of tumor growth and reduction of pulmonary metastases. This scheme provides 3 levels of tumor specificity, including the ability of macrophages to “home” into hypoxic tumor sites, restricted hypoxia-induced proliferation of the therapeutic adenovirus in host macrophages, and selective target-specific replication of oncolytic virus in prostate tumor cells.

Note:Breaking Advances are written by Cancer Research Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.