Cancer stemness in bone marrow micrometastases of human breast cancer

Cancer cells metastasize to the bone marrow to create the premetastatic niche. Cancer stemness (expression of stem cell characteristics) is regulated by the tumor microenvironment and associated with self-renewal and poor clinical outcomes. Osteopontin induces mesenchymal stem cells in the tumor microenvironment to adopt a cancer-associated fibroblast phenotype to potentiate cancer growth and metastasis. The mechanisms by which cancer cells and tumor microenvironment regulate stemness in the bone marrow premetastatic niche is unknown.

Human breast cancer cell lines, MDA-MB-231 and MCF-7 were used in an orthotopic murine xenograft model. NOD-scid mice were implanted with 2 × 10⁶ tumor cells in the presence and absence of human mesenchymal stem cells-green fluorescent protein cells and/or osteopontin aptamer, which blocks and inactivates extracellular osteopontin, or mutant aptamer (osteopontin mutant aptamer). In select instances, MCF-7 cells transfected to express osteopontin were coimplanted instead of MCF-7. Stemness markers (Nanog, Oct4, Sox2) in the tumor cells and cancer-associated fibroblast (α-smooth muscle actin, Vimentin) markers in the mesenchymal stem cells were measured in femoral bone marrow via real-time polymerase chain reaction. Cell number was determined by titrating cell number to Ct value in vitro.

Tumor cells and mesenchymal stem cells migrate from the primary tumor site to the bone marrow. Migration of mesenchymal stem cells is osteopontin dependent. In both MDA-MB-231 and MCF-7 cell lines, levels of both cancer-associated fibroblast and stemness markers were 3 to 4 times greater under conditions wherein mesenchymal stem cells were present with osteopontin. Inactivation of extracellular osteopontin with an aptamer decreased migration of mesenchymal stem cells and expression of both cancer-associated fibroblast and stemness markers. Cancer cells exhibited a significantly increased stem cell profile in the presence of cancer-associated fibroblast in the bone marrow. In the presence and absence of osteopontin, Sox2 knockdown abolished expression of both Nanog and Oct4.

We conclude that osteopontin-dependent migration of cancer-associated fibroblast is required for increased cancer cell stemness in the bone marrow premetastatic niche.