Tumor-associated macrophages (TAMs) have multifaceted roles in tumor development, particularly linked with tumor angiogenesis and invasion, but the molecular mechanism underlying this association remains unclear. In this study, we report that lack of osteopontin (OPN) suppresses melanoma growth in opn(-/-) mice and macrophages are the crucial component responsible for OPN-regulated melanoma growth. In tumor microenvironment, OPN activates macrophages and influences angiogenesis by enhancing cyclooxygenase-2 (COX-2)-dependent prostaglandin E2 (PGE2) production in an autocrine manner. Furthermore, we identify α9β1 integrin as a functional receptor for OPN that mediates its effect and activates ERK and p38 signaling, which ultimately leads to COX-2 expression in macrophages. The major role played by OPN and PGE2 in angiogenesis are further amplified by upregulation of MMP-9. OPN-activated macrophages promote the migration of endothelial and cancer cells via PGE2. These findings provide evidence that TAMs serve as source of key components such as OPN and COX-2-derived PGE2 and MMP-9 in melanoma microenvironment. Clinical specimens analyses revealed that increased infiltration of OPN-positive TAMs correlate with melanoma growth and angiogenesis. These data provide compelling evidence that OPN and COX-2 expressing macrophages are obligatory factors in melanoma growth. We conclude that OPN signaling is involved in macrophage recruitment into tumor, and our results emphasize the potential role of macrophage in modulation of tumor microenvironment via secretion of OPN, PGE2 and MMP-9, which trigger angiogenesis and melanoma growth. Thus, blockade of OPN and its regulated signaling network provides unique strategy to eradicate melanoma by manipulating TAMs.
Hypoxia is a salient feature of most solid tumors, and hypoxic adaptation of cancer cells has crucial implications in propagation of malignant clonal cell population. Osteopontin (OPN) has been identified as a hypoxia-responsive gene, but the mechanistic and regulatory role of OPN under hypoxia is less characterized. The present study identifies the existence of a positive inter-regulatory loop between hypoxia and OPN. We have shown that hypoxia induces OPN expression in breast cancer cells; however, the expression was found to be HIF1α independent. OPN enabled transcriptional upregulation of HIF1α expression both under normoxia and hypoxia, whereas stability of HIF1α protein in breast cancer cells remained unaffected. Moreover, we have shown that OPN induces integrin-linked kinase (ILK)/Akt-mediated nuclear factor (NF)-κB p65 activation leading to HIF1α-dependent vascular endothelial growth factor (VEGF) expression and angiogenesis in response to hypoxia. These in vitro data are biologically important as OPN expressing cells induce greater tumor growth and angiogenesis through enhanced expressions of proangiogenic molecules as compared with control. Immunohistochemical analysis of human breast cancer specimens revealed significant correlation between OPN and HIF1α but not HIF2α. Elevated expression of HIF1α and OPN was observed in pre-neoplastic and early stage infiltrating ductal carcinoma implicating the role of these proteins in neoplastic progression of breast cancer. Together, our results substantiate the prime role of OPN in cellular adaptation through ILK and NF-κB-mediated HIF1α-dependent VEGF expression in response to hypoxia that ultimately controls breast cancer progression and angiogenesis. Our study reinforces the fact that targeting OPN and its regulated signaling network hold important therapeutic implications.
A better understanding of the signalling mechanism by which osteopontin promotes tumourigenesis may be useful in crafting novel osteopontin -based anticancer therapy. The role of osteopontin in promoting cancer progression is the subject of in depth investigation and thus targeting osteopontin might be a suitable therapeutic approach for the treatment of cancer.
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