MINI REVIEW Macrophage migration and gene expression in response to tumor hypoxia Craig Murdoch 1 and Claire E. Lewis 1 * 1 Tumor Targeting Group, Academic Unit of Pathology, Division of Genomic Medicine, University of Sheffield Medical School, Sheffield S10 2RX, United Kingdom Monocytes are recruited into tumors from the circulation along defined chemotactic gradients and they then differentiate into tumor-associated macrophages (TAMs). Recent evidence has shown that large numbers of TAMs are attracted to and retained in avascular and necrotic areas, where they are exposed to tumor hypoxia. At these sites, TAMs appear to undergo marked pheno- typic changes with activation of hypoxia-inducible transcription factors, dramatically upregulating the expression of a large num- ber of genes encoding mitogenic, proangiogenic and prometastatic cytokines and enzymes. As a consequence, high TAMs density has been correlated with increased tumor growth and angiogenesis in various tumor types. Since hypoxia is a hallmark feature of malig- nant tumors and hypoxic tumor cells are relatively resistant to radio- and chemotherapy, these areas have become a target for novel forms of anticancer therapy. These include hypoxia- targeted gene therapy in which macrophages are armed with therapeutic genes that are activated by hypoxia-responsive pro- moter elements. This restricts transgene expression to hypoxic areas, where the gene product is then released and acts on neigh- boring hypoxic tumor cells or proliferating blood vessels. In this way, the responses of macrophages to tumor hypoxia can be exploited to deliver potent antitumor agents to these poorly vascu- larized, and thus largely inaccessible, areas of tumors. ' 2005 Wiley-Liss, Inc. Key words: macrophage; tumor-associated macrophages (TAMs); angiogenesis; hypoxia; tumor Monocyte recruitment into tumors Tumors are composed of an array of cell types, including not only neoplastic cells but also fibroblasts, endothelial cells and var- ious infiltrating leukocytes. For over a hundred years it has been known that the latter make up a significant component of almost all tumors and this has led to the concept that inflammation may be an important element in tumor progression. Many tumors arise from sites of infection, chronic irritation and perpetual inflamma- tion, as these are thought to provide the appropriate microenviron- ment to either promote or facilitate tumor development. 1 As with all sites of inflammation, leukocyte populations, such as neutro- phils and mast cells, are recruited to and infiltrate into tumors, where they are thought to contribute to tumor angiogenesis and metastasis. 2,3 However, in most malignant tumors the most promi- nent component of this leukocytic infiltrate are macrophages (often called tumor-associated macrophages or ‘‘TAMs’’), which in some instances, comprise up to 70% of the cell tumor mass. 4 These cells are an essential cellular component of the innate immune system and are derived from myeloid progenitor cells in the bone marrow. Throughout the lifespan of an individual these stem cells develop into promonocytes and are released into the cir- culation, where they then differentiate into monocytes. The latter then migrate into almost all tissues of the body, where they differ- entiate into ‘‘resident’’ tissue macrophages and help to protect these sites from infection and injury. In addition to this role in innate immunity, recent evidence suggests that macrophages also play an important role in the regulation of angiogenesis in both normal and diseased tissues, including malignant tumors. 5 It is thought that TAMs are almost entirely derived from periph- eral blood monocytes recruited into the tumor from the local circu- lation rather than the reservoir of resident macrophages already in the healthy tissue before tumor development. 6 Elevated expression of a number of monocyte chemoattractants by both tumor and stromal cells within tumors has been shown to positively correlate with increased TAMs numbers in many human tumors (reviewed in Ref. 7 ) and these are thought to drive the recruitment of mono- cytes from the bloodstream. Such chemoattractants include CC chemokines, of which CCL2 (formally monocyte chemoattractant protein-1, MCP-1) and CCL5 (RANTES) appear to be the most important. 8,9 However, overexpression of CCL3 (macrophage inflammatory protein-1a), CCL4 (macrophage inflammatory pro- tein-1b), CCL8 (monocyte chemotactic protein-2) and CCL22 (macrophage-derived chemokine) have also been found in some tumor types, suggesting that these may also be involved. 10 In addition, a number of cytokines and growth factors have also been implicated in the recruitment of monocytes into tumors. These include colony stimulating factor-1 (CSF-1), 6,11 vascular endothelial growth factor (VEGF), 12,13 endothelial monocyte-acti- vating polypeptide II (EMAPII) 14 and endothelin-1. 15 It should be noted that much of the data linking the role of these chemoattrac- tants to monocyte recruitment into tumors have come from descriptive studies correlating their expression with macrophage accumulation in various forms of human tumors. Studies using knock-out mice or blocking the activity of these molecules with neutralizing antibodies are warranted so as to determine whether each molecule is essential in monocyte recruitment. Upon extrava- sation into tumors, TAMs are thought to acquire the characteristics of polarized type ‘‘M2’’ macrophages. 16 Type ‘‘M1’’ and ‘‘M2’’ macrophages differ in terms of receptor expression, cytokine and chemokine production, and effector functions. Type M2 cells tune inflammatory and adaptive immune responses, scavenge cellular debris, and promote angiogenesis and tissue remodeling. In con- trast, M1 macrophages are potent effector cells that kill pathogens and tumor cells, and secrete numerous proinflammatory cyto- kines. 16 Tam accumulate in hypoxic regions of tumors Polarographic oxygen microelectrodes and hypoxia-specific markers have been used to demonstrate that multiple areas of hypoxia are present in most forms of invasive human carcinomas. Normal tissues typically have median oxygen tensions of 40–70 mmHg, while most solid tumors exhibit median values of less than 10 mmHg. Hypoxic areas form when tumor-cell proliferation out- strips the development of new blood vessels in a given area or when such new blood vessels collapse or fail to function properly. Consequently, in these areas blood flow is sluggish or highly irreg- ular and the delivery of oxygen and nutrients and the removal of waste products are poor. 17 The presence of these hypoxic areas often correlates with prognosis in cancer, with the presence of widespread hypoxia in tumors being associated with reduced sur- vival following surgery and radio- or chemotherapy. 18 This is because hypoxic tumor cells are significantly less responsive to radiotherapy than are their well-oxygenated counterparts because *Correspondence to: Fax: 144-114-271-2903. E-mail: Claire.lewis@sheffield.ac.uk Received 19 January 2005; Accepted after revision 21 June 2005 DOI 10.1002/ijc.21422 Published online 16 August 2005 in Wiley InterScience (www.interscience. wiley.com). Int. J. Cancer: 117, 701–708 (2005) ' 2005 Wiley-Liss, Inc. Publication of the International Union Against Cancer