The Role of Secreted Proteins and Exosomes in Cancer Metastasis

Abstract

Metastasis, the spread of cancer cells to distant organs, is the deadliest and most poorly understood feature of malignant diseases. It remains a clinical challenge due to lack of adequate diagnostic and prognostic tests, and effective therapies. Hence, identification and validation of clinically useful biomarkers and novel therapeutic targets provides a vi-able strategy to detect and combat this deadly disease. In the present dissertation, I pro-vide strategies to uncover novel markers and mediators of lung and bone metastasis, fur-ther defining the mechanisms by which tumor cells disseminate and helping to translate experimental findings into the clinic.

The cancer secretome, the collection of all macromolecules secreted by tumor cells, consti-tutes a valuable source for biomarker and drug target discovery. In the first study, I em-ployed quantitative proteomics to comprehensively profile lung metastasis secretomes of breast cancer and melanoma cells, and identify cancer-specific signatures associated with lung metastatic progression. The latter displayed prognostic value in patient datasets, and, despite little protein overlap, exposed a striking overlap of enriched pathways among the two cancer types, suggesting that common biological processes are executed by different sets of proteins to enable lung metastasis. Nidogen 1 (NID1), elevated in secretomes of aggressively lung metastatic cells relative to their weakly metastatic counterparts, corre-lated with clinical and experimental lung metastasis in breast cancer and melanoma mod-els. In vitro functional analysis further revealed the pro-metastatic role of NID1 by en-hancing cancer cell migration and invasion, adhesion to the endothelium, and tube for-mation capacity. As NID1 expression in clinical datasets correlated with poor prognosis and lung relapse, it may become a biomarker for disease progression in breast cancer and melanoma.

Exosomes, small (30 – 100 nm) membrane-enclosed vesicles containing a variety of mac-romolecules, are released at particularly high levels from cancer cells, and constitute a specific subset of the cancer secretome. In the second study, I examined the role of exo-somes in bone homeostasis and bone metastasis of breast tumors. Under physiological conditions, breast cancer-derived exosomes increased the number of osteoclasts, and by promoting osteolysis led to a decrease in bone mass. Further in vitro analyses showed the effect on osteoclastogenesis to be direct, as pre-osteoclasts and monocytes were able to take up exosomes and exhibit increased differentiation. In the context of breast cancer metastasis, exogenous exosomes promoted bone metastasis of breast cancer cells, while a reduction in exosomes secretion diminished the bone metastatic ability of the latter. Final-ly, the role of exosomal ICAM1 in exosome-induced osteoclastogenesis was explored. The results indicated that exosomal ICAM1 was, at least partially, responsible for the ex-osome mediated increase in osteoclastogenesis. These findings establish breast cancer ex-osomes and exosomal ICAM1 as mediators in creating a favorable microenvironment for metastasizing breast tumor cells in the bone, and highlight their potential value as thera-peutic targets and clinical biomarkers of osteolytic bone metastasis.