Metastasis remains a leading cause of morbidity and mortality from solid tumors. Lack of comprehensive systems to study the progression of metastasis contributes to the low success of treatment. We developed a novel three-dimensional in vitro reconstructed metastasis (rMet) model that incorporates extracellular matrix (ECM) elements characteristic of the primary (breast, prostate, or lung) and metastatic (bone marrow, BM) sites. A cytokine-rich liquid interphase separates the primary and distant sites, further recapitulating circulation. Similar to main events underlying the metastatic cascade, the rMet model fractionated human tumor cell lines into sub-populations with distinct invasive and migratory abilities: (i) a primary tumor-like fraction mainly consisting of non-migratory spheroids; (ii) an invasive fraction that invaded through the primary tumor ECM, but failed to acquire anchorage-independence and reach the BM; and (iii) a highly migratory BM-colonizing population that invaded the primary ECM, survived in the "circulation-like" media, and successfully invaded and proliferated within BM ECM. BM-colonizing fractions successfully established metastatic bone lesions in vivo, whereas the tumor-like spheroids failed to engraft the bones, showing the ability of the rMet model to faithfully select for highly aggressive sub-populations with a propensity to colonize a metastatic site. By applying the rMet model to study real-time ECM remodeling, we show that tumor cells secrete collagenolytic enzymes for invading the primary site ECM but not for entering the BM ECM, indicating possible differences in ECM remodeling mechanisms at primary tumor versus metastatic sites.
Keywords: Breast cancer; Extracellular matrix; Metastasis; Solid tumors; Tumor microenvironment.
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