Hedgehog Signaling Induced by Breast Cancer Cells Promotes Osteoclastogenesis and Osteolysis

Abstract

Bone integrity is maintained by a dynamic equilibrium between the activities of bone-forming osteoblasts and boneresorbing osteoclasts. Osteolytic lesions are a painful consequence of metastasis of breast cancer cells to bone in an overwhelming majority of breast cancer patients. Factors secreted by breast cancer cells propel a cascade of events that trigger osteoclastogenesis and elevated bone resorption. In the present study, we show that the Hedgehog (Hh) ligands secreted by breast cancer cells promote osteoclast differentiation and potentiate the activity of mature osteoclasts. Paracrine Hh signaling induced by breast cancer cells mediates a detrimental chain of events by the up-regulation of osteopontin (OPN), which in turn enhances osteoclastic activity by up-regulating cathepsin K and MMP9. Hh signaling is essential for osteoclasts because blocking the Hh pathway using the pharmacological Hh inhibitor, cyclopamine, results in an overall decrease in osteoclastogenesis and resorptive activity. Our studies suggest that inhibiting Hh signaling interferes with the ability of pre-osteoclasts to respond to the stimulatory effects of the breast cancer cells, indicating that Hh signaling is vital to osteoclast activity.Breast cancer cells preferentially metastasize to the bone. Once within bone, an interaction ensues between breast cancer cells and the cells within the bone microenvironment. Breast cancer cells secrete various factors that stimulate osteoblasts and osteoclasts and other cells within the bone; these in turn secrete factors that stimulate the tumor cells, creating a vicious cycle that nurtures the development and propagation of bone metastases (1). Seminal studies by Kang et al. (2) showed that osteopontin (OPN) 2 forms a component of a "bone metastasis signature" of breast cancer cells, i.e. breast cancer cells that metastasized to bone had increased OPN expression. Furthermore, OPN functionally enhanced incidence of bone metastases by breast cancer cells in concert with interleukin-11.OPN is one of the abundant non-collagenous proteins in bone. It is a bone matrix protein that promotes osteoclast function and is consistently overexpressed in highly metastatic cells. Ultrastructural immunocytochemical studies show that the most prominent accumulation of OPN is seen at cement lines in remodeling bone and at laminae limitantes at bone surfaces (3). It is localized to cell-matrix and matrixmatrix interfaces in mineralized tissue, where it is deposited as the result of osteoclast action. Moreover, OPN appears to be an important component in the communication between osteoclasts and osteoblasts, and there is strong evidence for the involvement of OPN in the formation, migration, and attachment of osteoclasts and in their resorptive activity (4, 5). Importantly, interfering with the adhesion of osteoclasts to osteopontin by RGD peptides abolishes their resorptive activity (6, 7).Nearly 42% of primary breast tumors express moderate to strong levels of OPN, and 83% of bone metastases resulting from these...

New paradigms for the Hedgehog signaling network in mammary gland development and breast Cancer

“…The vicious cycle is a process in breast tumor metastasis to bone involving signaling which increases osteoclast activity and enables metastasis growth in a feed-forward response. These results are consistent with findings that tumor-derived Shh and/or Ihh induces osteoclast activity [105,106]. Specifically, Ihh may be required for Runx2 induction of PTHrP mRNA (parathyroid hormone related protein) to increase osteoclast activity [105].…”

What Is Breast in the Bone?

“…The molecular mechanism, in part, involved the PRL-induced upregulation of SHH at the protein level from breast cancer cells. Breast cancer-secreted SHH is capable of inducing osteoclast differentiation both directly and indirectly via osteoclasts or osteoblasts [88,89]. This is the first PRL-based molecular mechanism that has been established in breast cancer metastases, in particular to breast cancer metastases to the bone.…”

Breast Cancer Metastases to Bone: Role of the Microenvironment

“…Runx2 has also been shown to regulate TGF-influenced PTHrP levels, as well as upregulate Indian hedgehog (127). Breast cancer cells express Hedgehog ligands that activate osteopontin expression in osteoclasts, promoting osteoclast maturation and resorptive activity through upregulated Cathepsin K and MMP9 (128,129). Of interest, expression of anti-resorptive OPG has been demonstrated to correlate with increased bonespecific homing and colonization potential in breast cancer cells (122), and to promote cancer cell survival (130,131).…”