The Notch signaling pathway regulates a broad spectrum of cell-fate decisions during development and postnatal life (Artavanis-Tsakonas et al., 1999
). The pathway is activated when a signal-sending cell expressing a Notch ligand physically interacts with a signal-receiving cell expressing a Notch receptor. Upon ligand binding, the transmembrane Notch receptor is cleaved sequentially, first by an extracellular matrix metalloprotease and then by the protease complex γ-secretase, releasing the Notch intracellular domain (NICD
). After being liberated, NICD
translocates to the nucleus where it interacts with the DNA-binding protein CSL (Rbp-Jκ in mice; CBF1 in humans), converting it from a transcriptional repressor to activator by recruiting cofactors such as Mastermind-like proteins. The most prominent targets of the Notch pathway include a set of basic helix-loop-helix factors of the Hes and Hey families (Kopan and Ilagan, 2009
Although classically known for its role in embryonic development, the Notch pathway is now being recognized for its aberrant activation in cancer. An oncogenic role for Notch was first discovered in T-cell acute lymphoblastic leukemia (T-ALL), and then extended to other malignancies including lung, ovary, breast and skin cancers (reviewed by Rizzo et al., 2008
). Only recently has Notch signaling been associated with cancer progression; it was shown to regulate mediators of invasion in pancreatic cancer (Wang et al., 2006
) and promote epithelial-mesenchymal transition (Leong et al., 2007
). Interestingly, the Notch ligand Jagged1 is also associated with cancer progression as it is overexpressed in poor prognosis prostate and breast cancer patients (Reedijk et al., 2005
; Santagata et al., 2004
). Despite these advances, the functional mechanism of the Notch pathway in breast cancer metastasis is poorly defined.
Bone metastasis affects over 70% of metastatic breast cancer with debilitating bone fractures, severe pain, nerve compression, and hypercalcemia (Mundy, 2002
). The development and outgrowth of these secondary lesions depends on the intricate cellular and molecular interactions between breast tumor cells and stromal cells of the bone microenvironment. In particular, the ability of tumor cells to disrupt the bone homeostatic balance maintained by two resident bone cell types, osteoclasts and osteoblasts, has been shown to drive bone destruction and metastatic tumor growth (Mundy, 2002
). Tumor cells secrete signaling proteins, such as parathyroid hormone-related peptide (PTHrP) (Guise et al., 1996
), to promote osteoclast differentiation and activity, either directly or indirectly by altering osteoblast production of receptor activator of nuclear factor-κB ligand (RANKL), an essential osteoclast differentiation cytokine, and its antagonist osteoprotegerin (OPG). The resultant bone destruction releases a number of growth factors stored in the bone matrix, such as transforming growth factor-β (TGFβ), to further stimulate the malignancy of tumor cells, completing the so called “vicious cycle” in bone metastasis. Although several molecular contributors of bone metastasis have been identified, effective therapies still await a more comprehensive understanding of the complex molecular and cellular network of tumor-stromal interactions in bone metastasis. In this study, we investigated the role of Notch signaling in the development of osteolytic bone metastasis of breast cancer.