Since the initial study showing the expression of IL-11 in breast tumor tissues more than 25 years ago
, numerous investigations have been subsequently undertaken to address the regulation and pathological significance of IL-11 expression in breast cancer and, in particular, in the tumor-induced osteolysis
[5, 13, 28–34]. Collectively, these studies have led to two important observations: a) IL-11 is not only expressed in a significant number of breast cancers but also has the potential to serve as a prognostic factor in human breast cancer, and b) IL-11 plays an important role in breast cancer-mediated osteolysis by promoting osteoclastogenesis and bone resorption. Notably, several studies have demonstrated that breast tumor cells can also target osteoblasts to stimulate their production of IL-11
[11, 17], further increasing IL-11 concentrations in the bone microenvironment. Therefore, elucidation of the molecular mechanism by which IL-11 increases osteoclastogenesis and bone resorption in breast cancer bone metastasis may help guide development of effective drugs and/or therapeutic regimens for preventing and treating breast cancer-induced osteolysis.
Early studies on the role of IL-11 in osteoclast formation and function involved the use of the co-culture system containing bone marrow cells and calvarial osteoblasts
[16, 35]; the key finding of these early investigations was that IL-11-mediated osteoclastogenesis requires the presence of osteoblasts, but the precise reason for the dependence of IL-11-mediated osteoclastogenesis on osteoblasts was not fully understood. After the discovery of the RANKL/RANK/OPG system in the late 1990s, it then became clear that osteoblasts in the co-culture system primarily serve as a source of RANKL and IL-11 stimulates osteoblasts to produce RANKL
[36, 37]. This led to the notion that IL-11 can promote osteoclastogenesis indirectly by stimulation osteoblast production of RANKL. On the other hand, it was shown that osteoclasts express IL-11R
, suggesting that IL-11 may also directly target osteoclasts and/or its precursors to regulate osteoclast formation and/or function. Intriguingly, one study demonstrated that IL-11 directly target osteoclast precursors to stimulate osteoclastogenesis and it does so independent of RANKL
. However, this finding is inconsistent with the early studies showing that IL-11-mediated osteoclastogenesis requires the presence of osteoblasts, which is a known source of RANKL.
In this work, we independently carried out a series of in vitro studies to further address the role of IL-11 in osteoclastogenesis. First we determined that the conditioned media of MDA-MB-231, a breast cancer cell line expressing IL-11
[13, 20], gave rise to a population of cells which can form osteoclasts in response to RANKL and M-CSF treatment (Figure
1), indicating that IL-11 may play an important role in osteoclastogenesis by regulating the development and/or survival of osteoclast progenitor cells. Because the MDA-MB-231 also secrete other factors that play a role in osteoclastogenesis it was necessary to look specifically at IL-11 function. Importantly, the ability of the breast cancer conditioned media to generate a population of osteoclast progenitor cells was significantly inhibited by a neutralizing anti-IL-11 antibody (Figure
3). These findings suggest that tumor-derived IL-11 may increase the extent of osteoclastogenesis by promoting the development of a population of osteoclast progenitor cells. To verify the specificity of IL-11, we found that culturing of murine bone marrow cells with IL-11 for 6 days is able to give rise to a pool of osteoclast progenitor cells (Figure
We then investigated other ways that IL-11 may play a role in osteoclastogenesis. We found that IL-11 does not exert any effect on osteoclast survival (Figure
4). We then examined if IL-11 is able to promote osteoclast formation in the absence of RANKL and our data demonstrate that IL-11 cannot induce osteoclastogenesis in tissue culture dishes or on bone slices in the absence of RANKL (Figure
5). We and others have demonstrated that while IL-1 and TNF-α cannot promote osteoclastogenesis in the absence of RANKL, they can do so with suboptimal levels of RANKL or from RANKL-pretreated BMMs
[21–26]. As such, we then investigated whether IL-11 can act in a similar manner. Our data show that IL-11 is not able to promote osteoclastogenesis in the presence of suboptimal levels of RANKL (Figure
6) or from RANKL-pretreated BMMs (Figure
Based on these new findings and those reported previously
[16, 34, 36, 37], we propose that IL-11-expressing breast cancer cells cause increased osteoclast formation and bone resorption by two distinct mechanisms: a) the tumor cells produce IL-11 which in turn stimulate the production of RANKL by stromal cells/osteoblasts in the bone microenvironment, and b) tumor cell-derived IL-11 also augments the pool of osteoclast progenitor cells to increase the extent of osteoclastogenesis. Therefore, our work has led to a better understanding of the action of IL-11 in breast cancer-induced osteolysis. However, the precise mechanism by which IL-11 promotes the development of a population of osteoclast progenitor cells remains unclear. While it is possible that IL-11 does so by stimulating the differentiation, proliferation and/or survival of osteoclast progenitor cells, this cytokine may exert the impact on osteoclast progenitor cell population indirectly through other cell types in the bone marrow. Further studies are needed to elucidate how exactly IL-11 promote the development of a pool of osteoclast progenitor cells.
Moreover, our new data may help guide the development of better therapeutic regimens for preventing and treating breast cancer-induced osteolysis. Particularly, denosumab, a humanized anti-RANKL developed by Amgen Inc, has been approved by the FDA to treat breast cancer-mediated osteolysis. For IL-11 positive tumors, denosumab may be effective only in blocking the RANKL-dependent action of IL-11. In contrast, it is likely that an efficient inhibition of IL-11 can block the IL-11-mediated increase of the pool of osteoclast progenitor cells as well as the RANKL-dependent pathway, thus having the potential to give rise to better efficacy. Future animal model studies need to be undertaken to address the therapeutic potential of targeting IL-11.