The Chemokine CCL 2 Increases Prostate Tumor Growth and Bone Metastasis through Macrophage and Osteoclast Recruitment 1

CC chemokine ligand 2 (CCL2, also known as monocyte chemoattractant protein-1) has been demonstrated to recruit monocytes to tumor sites. Monocytes are capable of being differentiated into tumor-associated macrophages (TAMs) and osteoclasts (OCs). TAMs have been shown to promote tumor growth in several cancer types. Osteoclasts have also been known to play an important role in cancer bone metastasis. To investigate the effects of CCL2 on tumorigenesis and its potential effects on bone metastasis of human prostate cancer, CCL2 was overexpressed into a luciferase-tagged human prostate cancer cell line PC-3. In vitro, the conditioned medium of CCL2 overexpressing PC-3luc cells (PC-3) was a potent chemoattractant for mouse monocytes in comparison to a conditioned medium from PC-3. In addition, CCL2 overexpression increased the growth of transplanted xenografts and increased the accumulation of macrophages in vivo. In a tumor dissemination model, PC-3 enhanced the growth of bone metastasis, which was associated with more functional OCs. Neutralizing antibodies targeting both human and mouse CCL2 inhibited the growth of PC-3, which was accompanied by a decrease in macrophage recruitment to the tumor. These findings suggest that CCL2 increases tumor growth and bone metastasis through recruitment of macrophages and OCs to the tumor site. Neoplasia (2009) 11, 1235–1242 Introduction Prostate cancer is the most commonly diagnosed lethal malignancy in men in the United States [1]. Although androgen-dependent metastatic prostate cancer is initially treatable by castration, advanced prostate cancer remains incurable owing to the inevitable emergence of androgen-independent cells [2]. New approaches to the treatment of metastatic prostate cancer are needed. One such approach is to identify novel targets based on cancer cell–tumor microenvironment interactions [3–7]. CC chemokine ligand 2 (CCL2, also known as monocyte chemotactic protein-1) is a member of the CC beta chemokine family that is produced by macrophages, fibroblasts, and endothelial cells to stimulate chemotaxis of monocyte/macrophages and other inflammatory cells through its receptor, CCR2 [8]. Monocytes recruited to the tumor site adopt a macrophage phenotype that is dictated by the presence of specific cytokines. Mature macrophages are divided into M1 (classically activated) and M2 (alternatively activated) phenotypes. Tumorassociated macrophages (TAMs) of the M2 phenotype secrete several growth factors (transforming growth factor β, vascular endothelial growth factor, etc.) that promote tumor growth [9–13]. In several cancers, CCL2 has previously been shown to be an important determinant of tumor growth [14–16]. CCL2 promotes prostate cancer cell Address all correspondence to: Kenneth J. Pienta, MD, 7303 CCGC 1500 Medical Center Dr, Ann Arbor, MI 48109-0946. E-mail: kpienta@med.umich. edu K.J. Pienta is supported by a National Institutes of Health grant PO1 CA093900, an American Cancer Society Clinical Research Professorship, National Institutes of Health Specialized Program of Research Excellence in prostate cancer grant P50 CA69568, Cancer Center support grant P30 CA46592, SouthWest Oncology Group CA32102, and the Prostate Cancer Foundation. Received 11 June 2009; Revised 31 July 2009; Accepted 4 August 2009 Copyright © 2009 Neoplasia Press, Inc. All rights reserved 1522-8002/09/$25.00 DOI 10.1593/neo.09988 www.neoplasia. com Volume 11 Number 11 November 2009 pp. 1235–1242 1235