Metastasis is a complex, multi-step process by which primary tumour cells invade adjacent tissue. These cells enter the systemic circulation (intravasate), translocate through the vasculature, and arrest in distant capillaries where they extravasate into the surrounding tissue parenchyma, and these microscopic growths (micrometastases) proliferate into macroscopic secondary tumours [
82]. Metastasis is the result of several sequential steps and represents a highly organized, non-random and organ-selective process [
83] that involves interactions from a variety of proteolytic enzymes, growth factors, and cell-cell and cell-substrate adhesion molecules [
84].
In numerous models of breast cancer associated invasion and metastasis, activated TGF-β signaling induces increased aggressiveness. For example, in mice overexpressing the Neu oncogene, activated TGF-β signaling increases the number of lung metastases even while decreasing the growth of the primary tumour [
35]. Likewise, ablation of TGF-β signaling in the same model decreases lung metastasis while also decreasing the latency of primary tumour growth, again emphasizing the dual functions of TGF-β signaling in tumourigenesis [
37]. Additionally, clinical evidence suggests a correlation between expression of the TGF-β ligands and poor patient outcome [
16‐
18,
39]. Furthermore, activated TGF-β signaling has been observed in breast cancer bone metastases and contributes to the establishment of these lesions [
19,
84,
85]. There have also been many specific studies to analyse the role of TGF-β in tumour metastasis to lung [
29].
Bone metastases are common in patients with advanced breast cancer. Tumour cells co-opt bone cells to drive a feed-forward cycle which disrupts normal bone remodeling to result in abnormal bone destruction or formation and tumour growth in bone [
86,
87]. There is abundant evidence to support the role of TGF-β as a major bone-derived factor. TGF-β promotes a feed-forward cycle responsible for tumour growth and (Fig.
2) in bone. Osteolytic bone destruction is caused when tumours in bone secrete osteolytic factors, such as parathyroid hormone-related protein (PTHrP) and interleukin 11 (IL-11) [
88]. TGF-β is released and activated from the mineralized bone matrix by osteoclastic resorption and further induces tumour production of osteolytic and prometastatic factors including PTHrP and IL-11 [
89]. Human breast cancer bone metastases have increased PTHrP expression, more so than primary breast cancers [
90]. PTHrP is a central mediator of TGF-β induced osteolytic metastases; PTHrP neutralizing antibodies blocked the development and progression of breast cancer bone metastases in mouse models [
91]. Another paramount study in 1999 showed that a dominant negative TβRII stably expressed in the breast cancer cell line MDA-MB-231 rendered the cells unresponsive to TGF-β, inhibited PTHrP secretion induced by TGF-β and suppressed bone metastases in a mouse model [
92]. TGF-β increases PTHrP secretion from MDA-MB-231 cells via Smad and p38 MAP kinase pathways [
93]. Furthermore, TGF-β released during bone resorption is likely to have direct effects on bone cells, stimulating osteoclastic bone resorption and inhibiting osteoblast differentiation.
The complexity in the origin of bone metastases has been exemplified by recent transcriptional profiling of subpopulations of human breast cancer cells with an aggressive bone metastases phenotype [
94,
95]. Many of these genes, such as IL-11, connective tissue growth factor (CTGF), C-X-C chemokine receptor type 4 (CXCR4), and MMP1 have effects on bone cells [
96], which could promote bone metastases. Bone resorption is stimulated by IL-11 and MMP-1 causing an increase in osteoblast production of receptor activator of nuclear factor kappa-B ligand (RANKL) [
97]. CXCR4, a chemokine receptor that binds to the osteoblast product stromal-derived factor-1 (SDF-1) produced by osteoblasts to promote homing of cancer cells to bone [
98‐
100]. CTGF stimulates osteoblast proliferation as well as angiogenesis [
101]. These genes act cooperatively when expressed together, to cause osteolytic metastasis by promoting homing to bone, angiogenesis, and invasion [
102]. Among the bone metastasis genes identified, Kang et al
. showed that IL-11 and CTGF were regulated by TGF-β via the classical TGF-β/Smad pathway in metastatic cells [
84]. Other studies indicate that CXCR4 and MMP-1 are also regulated by TGF-β [
98,
99].
Since these first studies into TGF-β and the bone microenvironment, there have been many advances. Recent evidence has suggested that Gli2, a Hedgehog signaling molecule, is required for TGF-β to stimulate PTHrP expression and that blocking Hedgehog-independent Gli2 activity will inhibit tumour-induced bone destruction [
103]. Using a murine syngeneic model that mimics osteolytic changes associated with human breast cancer, one laboratory has examined the role of tumour-bone interaction in tumour-induced osteolysis and malignant growth in the bone microenvironment [
104]. TβRII was identified as a commonly upregulated gene at the tumour-bone interface. Moreover, nuclear localization of phospho-Smad2 was higher in tumour cells and osteoclasts at the tumour-bone interface as compared to the tumour-alone area [
104]. A mouse model sing Cre/LoxP technology, with the WAP promoter driving transgenic expression of Cre recombinase (Cre), ablated the TβRII expression specifically within mouse mammary alveolar progenitors [
105]. Transgenic expression of the polyoma virus middle T antigen, under control of the mouse mammary tumour virus (MMTV) enhancer/promoter, was used to produce mammary tumours in the absence or presence of Cre or TβRII. The loss of TGF-β signaling was significantly correlated with increased tumour size and enhanced carcinoma cell survival.
Human breast cancer bone metastases show active Smad signaling in bone metastasis by accumulation of phosphorylated Smad2 in the nucleus of tumour cells [
84].
Knockdown of Smad4 expression in breast cancer cells reduced growth of bone metastases in a mouse model [
47,
84]. Different studies in mouse models of bone metastases, using live imaging of tumour cells by bioluminescence, have shown that TGF-β signaling is activated in the bone metastases, but not in metastases to adrenal glands [
12,
84,
106]. In this preclinical model, either anti-TGF-β therapy with a small molecule inhibitor of TβRI kinase activity or a bisphosphonate inhibitor of bone resorption was effective to decrease TGF-β signaling activity in these bone metastases [
106]. These data indicate that TGF-β signaling is prominent in bone metastases compared with other metastatic sites and that inhibiting either the TGF-β pathway or osteoclastic bone resorption can impair this activity.
Further investigation of the specific functions of Smad2 and Smad3 in TGF-β-induced responses in breast cancer cells in vitro and in vivo for breast cancer metastasis have recently been undertaken. Studies have shown that Smad2 and Smad3 differentially affect breast cancer bone metastasis formation in vivo [
107]. Knockdown of Smad3 in breast cancer cells in vivo resulted in prolonged latency and delayed growth of bone metastasis. However, Smad2 knockdown resulted in a more aggressive phenotype compared with controls. Furthermore, the data suggest that bone-derived TGF-β, released as a consequence of osteoclastic bone resorption, is the major source of TGF-β to act on tumour cells in bone. Overexpression of BMP-7 in breast cancer cells decreased the development of bone metastases in mice, but had no effect on orthotopic tumours [
25,
108]. BMP7 was found to antagonize TGF-β/Smad signaling. Therefore, BMP-7 may be useful as an inhibitor of bone metastases [
109‐
111].
Another unique aspect of the bone microenvironment is hypoxia. Bone is a hypoxic microenvironment and hypoxia has also been implicated to enhance tumour growth and metastasis [
112]. TGF-β and hypoxia signaling pathways in breast cancer cells were additive to induce vascular endothelial growth factor (VEGF) and CXCR4, via hypoxia-inducible factor-1α (HIF-1α) in vitro. HIF-1α and TGF-β pathways were inhibited in breast tumour cells using shRNA against HIF-1α and dominant negative TβRII approaches [
99,
113]. In vivo, inhibition of either pathway decreased bone metastasis, but there was no additional effect on the development of bone metastasis with a double blockade. In contrast, treatment with pharmacologic inhibitors targeting both pathways decreased bone metastases more than either alone [
99].
Preclinical studies have indicated that tumour cells express a number of genes which encode for proteins that act at different sites of the metastatic cascade as well as at the bone site. For example, several studies have shown that tumours cells produce adhesive molecules that promote binding to marrow stromal cells and bone matrix. These adhesive interactions increase tumour production of angiogenic and bone resorbing factors that enhance tumour growth in bone [
114,
115].
There is evidence that TGF-β can primes breast cancer cells for metastasis to the lungs. This is based on the study by Padua et al
. which showed that the process is dependent on the induction of angiopoietin-like 4 (ANGPTL4) by TGF-β via the Smad signaling pathway [
29]. TGF-β induction of Angptl4 in cancer cells that are about to enter the circulation enhances their subsequent retention in the lungs, but not in the bone. Tumour cell-derived Angptl4 disrupts vascular endothelial cell-cell junctions, increases the permeability of lung capillaries, and facilitates the trans-endothelial passage of tumour cells [
116]. It is suggested that the primary breast tumour microenvironment induces the expression of cytokines in departing tumour cells, enabling these cells to disrupt lung capillary walls and seed pulmonary metastases [
29].
Functional studies have demonstrated that Id1 and its closely related family member Id3 are required for tumour initiating functions, both in the context of primary tumour formation and during metastatic colonization of the lung microenvironment [
117]. In vivo characterization of lung metastatic progression revealed that Id1 and Id3 facilitate sustained proliferation during the early stages of metastatic colonization, subsequent to extravasation into the lung parenchyma. Sadej et al. have shown that attenuation of TGF-β1-induced responses correlated with reduced retention in the lung vascular bed, inhibition of pneumocyte-induced scattering of breast cancer cells in three-dimensional Matrigel, and decrease in experimental metastasis to the lungs. These results identify CD151 as a positive regulator of TGF-β1-initiated signaling and highlight the important role played by this tetraspanin in TGF-β1-induced breast cancer metastasis [
118].
The role of TGF-β coreceptor endoglin has also been studied in breast lung metastasis. Ectopic expression of endoglin in a breast cancer cell line blocked TGF-β-enhanced cell motility and invasion and reduced lung colonization in an in vivo metastasis model [
119]. Endoglin does not modulate Smad-mediated TGF-β signaling in breast cells but attenuates the cytoskeletal remodeling to impair cell migration and invasion [
120].