Platelets as Metastasis Promoters
Schumacher D, Strilic B, Sivaraj KK, Wettschureck N, Offermanns S. Platelet-derived nucleotides promote tumor-cell transendothelial migration and metastasis via P2Y2 receptor. Cancer Cell 2013;24:130–37.
Platelets are often abnormal in cancer. A raised platelet count in a routine blood test can be a sign of an occult malignancy, and depletion of platelets reduces metastases in many experimental cancer models. Schumacher and colleagues reveal a novel mechanism by which platelets facilitate metastases independent of their role in clotting. They found that adenine nucleotides released from tumor-cell activated platelets resulted in opening of the endothelial barrier and stimulating endothelial transmigration of tumor cells with the platelet-derived ATP acting through the ATP receptor P2Y2 (P2RY2) on the endothelial cells. Blockade of either ATP release from platelets or P2Y2 receptor on endothelial cells had a profound effect on experimental metastases of the transplantable LLC or B16 mouse tumor cell lines. A general blockade of platelet activation might reduce metastasis, but it could also increase the risk of bleeding. These results suggest that inhibiting the P2Y2 receptor might inhibit platelet-promoted metastasis without disabling the normal functions of platelets, especially as the P2Y2 knockout mice were relatively healthy. Recent reports of epidemiologic studies in large patient cohorts show that long-term low-dose aspirin reduces the risk of metastasis. Because aspirin is known to inhibit platelet function, the current study could provide one explanation for this effect.
The Perivascular Niche and Tumor Dormancy
One of the perplexing questions of cancer biology is why some patients develop metastatic disease years, even decades, after initial diagnosis and treatment. In many human and experimental cancers, evidence exists to show that malignant cells disseminate early, maybe even before the primary tumor is apparent. What keeps these malignant cells dormant, and what eventually stimulates them to proliferate and form metastases? Ghajar and colleagues provide one plausible explanation. They reasoned that as breast cancer cells disseminate in the blood to such sites as the lung and bone marrow, endothelial cells and factors deposited in their basement membrane may be involved in malignant cell dormancy. In a series of experiments in mice, orthotopic models of human microvasculature, and zebrafish, Ghahar and colleagues demonstrated how stable microvasculature constitutes a dormant niche, whereas sprouting neovasculature stimulates micrometastatic outgrowth. In murine breast cancer models they found dormant malignant cells in the microvasculature of the lung, bone marrow, and brain. Using organotypic models of lung and bone marrow microvasculature, they identified thrombospondin-1 (TSP-1; THBS1), as an endothelium-derived tumor suppressor. If the vessels were sprouting, TSP-1 levels were decreased. Moreover, tumor growth was accelerated around sprouting tips of blood vessels. These areas were rich in such tumor promoters as active TGF-β1 and periostin. Hence, preserving vascular homeostasis may be critical to maintaining dormancy of disseminated tumor cells. Whether this is true for other cancers and if there is evidence for this phenomenon in human cancer patients remain to be seen. However, this early work begins to suggest possible ways of preventing relapse.
ELTD1 as a Key Regulator of Tumor Angiogenesis
Masiero M, Simões FC, Han HD, Snell C, Peterkin T, Bridges E, et al. A core human primary tumor angiogenesis signature identifies the endothelial orphan receptor ELTD1 as a key regulator of angiogenesis. Cancer Cell 2013;24:1–13.
Despite extensive preclinical studies and recognition that angiogenesis is critical for tumor formation and progression, clinical evidence to date shows limited efficacy of antiangiogenic therapies. Thus, a better understanding of the biology and potential therapeutic targets that regulate the process of angiogenesis may be required if new therapies are to be successful. To address this issue, Masiero and colleagues studied gene expression profiles from over 1,000 samples to identify regulators of angiogenesis in human tumors. They used, as initial “seeds,” a set of genes known to be important in cancer and also involved in angiogenesis. Using data from head and neck squamous carcinoma, breast cancer, and clear cell renal cell carcinoma, they observed that gene signatures of angiogenesis differ among these tumors, but that a set of 43 angiogenesis-associated genes was common to these tumor types. Then they studied the modulation of these 43 genes in response to antiangiogenic therapies, including bevacizumab and DBZ (a γ-secretase inhibitor/NOTCH pathway inhibitor) using preclinical models. They found that DBZ treatment regulated their 43-gene signature in their experimental model. Using cultured endothelial cells stimulated with DLL4 (to activate NOTCH signaling) they further narrowed the gene list to those reciprocally regulated by DBZ and DLL4. One of these genes was ELTD1, an orphan G-protein–coupled receptor on which the authors directed their focus. Immunohistochemical analysis for ELTD1 showed increased staining in tumor-associated endothelial cells in a variety of cancer types. Increased staining was found to be associated with improved clinical outcome in renal cancer and ovarian cancer, which is consistent with the fact that the hypoxic state is associated with treatment resistance. As supporting evidence, they noted that ELDT1 expression levels were inversely associated with a hypoxia gene signature and with the hypoxia inducible gene CA9, which is expressed in the vasculature of zebrafish embryos. Moreover, with the use of antisense morpholinos, severe vascular defects became evident upon gene silencing. In animal tumor models, silencing of ELTD1 blocked sprouting of endothelial cells and vessel formation and improved survival, and ELTD1 was found to be negatively regulated by DLL4 but positively regulated by the proangiogenic factors bFGF and VEGF. These findings identify ELTD1 as a key regulator of tumor angiogenesis and as a target for therapy.
Naked Mole Rats Evade Cancer in a Slippery Manner
For a rodent, the naked mole rat displays considerable longevity, in part due to being cancer free. In their exploration of the mechanism by which naked mole rats evade cancer, Tian and colleagues noticed that the conditioned media of naked mole rat fibroblasts cultured in vitro was unusually viscous. This viscosity was the result of increased amounts of high-molecular-mass hyaluronan, an unbranched disaccharide found in the extracellular matrix of many tissues. High levels of hyaluronan were also observed in naked mole rat tissues. The authors went on to show that the inability of naked mole rat fibroblasts, transformed by large T antigen and HRas-V12, to grow in an anchorage-independent manner or form tumors in nude mice was dependent on the synthesis and increased stability of high-molecular-mass hyaluronan. They also identified the signaling mechanism used by high-molecular-mass hyaluronan in its anticancer capacity, acting through the transmembrane glycoprotein CD44 and NF2 to regulate the cell-cycle inhibitor p16INK4a. The authors suggest that the use of high-molecular-mass hyaluronan, inhibitors of the enzyme that degrade it in the extracellular space, or activators of the CD44 signaling pathway might also act as chemopreventive agents in humans.
Myoferlin: A Potential Breast Cancer Therapeutic Target
Epidermal growth factor receptors (EGFR) play a pivotal role in human tumorigenesis by facilitating cellular growth and metastasis. Less is known about the mechanism of its internalization (endocytosis) and intracellular targeting. Turtoi and colleagues identified myoferlin (MYOF), a ferlin family protein on chromosome 10q24, as a key regulator of EGFR activity in breast cancer cells. Tumor-specific overexpression of myoferlin in the plasma membrane and in the cytoplasm was confirmed in human breast adenocarcinoma tissues. Depletion of myoferlin markedly inhibited EGF-induced migration and epithelial-to-mesenchymal transition of breast cancer cells. Myoferlin was also found to regulate EGFR fate upon EGF-mediated receptor activation, although it did not affect the distribution of cellular clathrin in the breast cancer cells. Interestingly, myoferlin was crucial for proper assembly of caveolin in caveolae in the context of activated EGFR endocytosis. Moreover, depletion of myoferlin markedly affected migration, invasion, and survival of MDA-MB-231 cells in vitro and in vivo. This study identifies myoferlin as a novel regulator of EGFR activity and endocytosis and thus presents a potential therapeutic target for breast and other cancers with elevated expression levels of myoferlin.
Lymph Node Metastasis: Not So Passive, Not So Simple
Das S, Sarrou E, Podgrabinska S, Cassella M, Mungamuri SK, Feirt N, et al. Tumor cell entry into the lymph node is controlled by CCL1 chemokine expressed by lymph node lymphatic sinuses. J Exp Med 2013;210:1509–28.
The presence of metastatic lesions in lymph nodes across different solid tumor types is an indicator of poor prognosis. This is because it is thought to be a reliable measure of tumor cell intravasation at primary sites and target organ lodging. The dogma holds that after active and specific lymphatic intravasation the lymph carries the disseminating tumor cells passively to the subcapsular sinus (SCS) of the lymph node and into the cortex, where growth eventually ensues. Das and colleagues have shifted our understanding of this process by showing that access and colonization of lymph nodes is a highly regulated and complex mechanism. The authors show that the chemokine receptor CCR8 is expressed in melanoma and breast cancer cells that are efficient in lymph node metastasis. CCL1, the ligand for CCR8, is expressed specifically in the lymphatic endothelial cells at the SCS (not in skin lymphatics where the primary tumors grew). Blocking CCL1 completely restricts entry into the SCS, and CCR8low cells had impaired signaling through this G-protein coupled receptor and via motility pathways that regulate chemotaxis. Detailed in situ and live imaging revealed that tumor cells arriving at the narrow space of the SCS attach to the endothelium and migrate actively and slowly through the SCS. In this location, CCR8low tumor cells became entrapped in the afferent collecting lymphatic vessel at the junction with the SCS and were unable to actively access the lymph node cortex, dramatically decreasing lymph node metastasis. Instead, these tumor cells developed “in-transit metastasis” that grew within the afferent lymphatic vessel. These results show for the first time that the CCL1+ lymphatic endothelial cells in the SCS serve as gatekeepers of the lymph nodes, and only tumor cells expressing CCR8 are able to enter this site. Given the tight regulation of this entry to the lymph nodes, a potential implication of these findings is that lymph nodes that are negative for metastasis are not necessarily negative for systemic dissemination because intravasation may still occur. Tumor cells negative for CCR8 or impaired in this pathway may display restricted entry into the lymph node but may still reach other sites.
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.