Cysteine cathepsin activity suppresses osteoclastogenesis of myeloid-derived suppressor cells in breast cancer.

Laura E. Edgington-Mitchell,Matthew Bogyo,Wouter A. van der Linden,Martijn Verdoes,Krishnath M. Jayatilleke,Jai Rautela,Belinda S. Parker,Hendrika M. Duivenvoorden

doi:10.18632/oncotarget.4714

Abstract

Cysteine cathepsin proteases contribute to many normal cellular functions, and their aberrant activity within various cell types can contribute to many diseases, including breast cancer. It is now well accepted that cathepsin proteases have numerous cell-specific functions within the tumor microenvironment that function to promote tumor growth and invasion, such that they may be valid targets for anti-metastatic therapeutic approaches. Using activity-based probes, we have examined the activity and expression of cysteine cathepsins in a mouse model of breast cancer metastasis to bone. In mice bearing highly metastatic tumors, we detected abundant cysteine cathepsin expression and activity in myeloid-derived suppressor cells (MDSCs). These immature immune cells have known metastasis-promoting roles, including immunosuppression and osteoclastogenesis, and we assessed the contribution of cysteine cathepsins to these functions. Blocking cysteine cathepsin activity with multiple small-molecule inhibitors resulted in enhanced differentiation of multinucleated osteoclasts. This highlights a potential role for cysteine cathepsin activity in suppressing the fusion of osteoclast precursor cells. In support of this hypothesis, we found that expression and activity of key cysteine cathepsins were downregulated during MDSC-osteoclast differentiation. Another cysteine protease, legumain, also inhibits osteoclastogenesis, in part through modulation of cathepsin L activity. Together, these data suggest that cysteine protease inhibition is associated with enhanced osteoclastogenesis, a process that has been implicated in bone metastasis.

Highlights

Breast cancer metastasis to lung and bone poses a serious threat to life
Given the increased cathepsin B activity in (myeloid-derived suppressor cell) RANK ligand (RANKL) (MDSC) derived from bone marrow compared to lung, we examined whether cathepsin activity was required for the differentiation of MDSCs into multinucleated osteoclasts
Cysteine cathepsins have been widely studied in the context of cancer; their site-specific functions are only just beginning to be understood [39]

Summary

Introduction

Breast cancer metastasis to lung and bone poses a serious threat to life. Cysteine cathepsins comprise a family of proteases that have been widely implicated in primary breast tumor invasion and progression [1,2,3,4,5]. Blocking protease activity may have varied effects depending on the target cell, timing of administration, and the specificity of any inhibitor for a particular target www.impactjournals.com/oncotarget protease For this reason, protease inhibitors have largely failed as therapeutic agents in the clinic [11, 12]. The 4T1 series of murine breast cancer cells is ideal for studying cell-specific roles of proteases since it utilizes syngeneic immunocompetent mice [13,14,15] This allows for accurate cross-talk between tumor and host cells, including immune cells. This isogenic series comprises cells that are highly metastatic to lung and bone (4T1.2) and those that are non-metastatic (67NR) [14], allowing assessment of the contribution of tumor and stromal cysteine cathepsins to metastatic progression

Methods

Results

Conclusion