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  1. Polyploidal giant cancer cells (PGCCs) are multinucleated chemoresistant cancer cells found in heterogeneous solid tumors. Due in part to their apparent dormancy, the effect of PGCCs on cancer progression has remained largely unstudied. Recent studies have highlighted the critical role of PGCCs as aggressive and chemoresistant cancer cells, as well as their ability to undergo amitotic budding to escape dormancy. Our recent study demonstrated the unique biophysical properties of PGCCs, as well as their unusual migratory persistence. Here we unveil the critical function of vimentin intermediate filaments (VIFs) in maintaining the structural integrity of PGCCs and enhancing their migratory persistence. We performed in-depth single-cell analysis to examine the distribution of VIFs and their role in migratory persistence. We found that PGCCs rely heavily on their uniquely distributed and polarized VIF network to enhance their transition from a jammed to an unjammed state to allow for directional migration. Both the inhibition of VIFs with acrylamide and small interfering RNA knockdown of vimentin significantly decreased PGCC migration and resulted in a loss of PGCC volume. Because PGCCs rely on their VIF network to direct migration and to maintain their enlarged morphology, targeting vimentin or vimentin cross-linking proteins could provide a therapeutic approach to mitigate the impact of these chemoresistant cells in cancer progression and to improve patient outcomes with chemotherapy.

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  2. Age is a leading risk factor for developing breast cancer. This may be in part to the time required for acquiring sufficient cancer mutations; however, stromal cells that accumulate in tissues and undergo senescence eventually develop a senescence-associated secretory phenotype that alters the microenvironment to promote cancer. Our focus is on mesenchymal stem cells (MSCs) – stromal cells recruited to tumors due to their natural tropism for inflammatory tissues; MSCs have been shown to enhance the metastatic potential of tumor cells through direct interactions or paracrine signaling within the tumor. In the tumor, MSCs can differentiate into carcinoma-associated fibroblasts that play a central role in tumor growth and matrix remodeling. We recently investigated the molecular and mechanical differences in pre- and post- senescent MSCs and how their interactions with MDA-MB-231 breast cancer cells contribute to malignancy. Our data show post-senescent MSCs are larger and less motile, with more homogeneous mechanical properties than pre-senescent MSCs. In-depth omics analysis revealed differentially regulated genes and peptides including factors related to inflammatory cytokines, cell adhesion to the extracellular matrix, and cytoskeletal regulation. A 3D co-culture model was used to assess the effects of pre- and post- senescent MSCs on collagen matrix remodeling. Although post-senescent MSCs were far less motile than pre-senescent MSCs and less contractile with the matrix, they profoundly altered matrix protein deposition and crosslinking, which resulted in local matrix stiffening effects. Post-senescent MSCs also induced an invasive breast cancer cell phenotype, characterized by increased proliferation and invasion of breast cancer cells. This invasive breast cancer cell behavior was further amplified when MDA-MB-231 was co-cultured with a mixture of pre- and post- senescent MSCs; this result was attributed to matrix remodeling and soluble factor secretion effects of post-senescent MSCs, which enhanced the migration of pre-senescent MSCs allowing them to form tracks in the collagen network for cancer cells to follow. Finally, molecular inhibitors targeting actomyosin contractility and adhesion were used to alter MSC interactions with breast cancer cells. Actin depolymerizing agent and focal adhesion kinase inhibitor were most efficient and completely able to block the effects of post-senescent MSCs on MDA-MB-231 invasion in collagen gels. This comprehensive approach can be used to identify molecular pathways regulating heterotypic interactions of post-senescent MSCs with other cells in the tumor. Furthermore, the local matrix stiffening effect of post-senescent MSCs may play a critical role in breast cancer progression. 
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  3. Senescence is a potent tumor-suppressive mechanism that irreversibly arrests the growth of damaged cells. However, senescent cells that accumulate in tissues eventually develop a senescence-associated secretory phenotype (SASP) that alters the microenvironment to promote cancer. Paracrine factors in the SASP may also contribute to the formation of rare giant polyploidal cancer cells (GPCCs). A single-cell mechanical approach was used to profile cytoskeletal and nuclear mechanics, morphology, motility, and adhesion for breast cancer cells treated with conditioned media from senescent fibroblasts. Our study showed that a small but significant population of MDA-MB-231 breast cancer cells (less than 5%) treated with conditioned media from senescent LF-1 fibroblasts develop an enlarged morphology, chromosomal instability, and polyploidy, a phenotype associated with GPCCs. Although GPCCs are highly invasive and chemoresistant, little is known about their biophysical properties. First, we developed a method for identifying the small subpopulation of GPCCs in a heterogeneous population of cancer cells based on increased nuclear area and confirmed that GPCCs are more resistant to paclitaxel than normal-size MDA-MB-231 cells (NCCs). We then compared critical biophysical properties of NCCs and GPCCs, including cytoskeletal and nuclear mechanics, cell and nuclear morphology, motility, and adhesion. Cells were stained for cytoskeletal proteins actin, tubulin, and vinculin. Cytoskeletal organization was dramatically altered in GPCCs compared to NCCs. GPCCs displayed more disorganized microtubule structure, dense actin stress fibers, and mature focal adhesions. Intracellular particle tracking microrheology was used to measure cytoskeletal and nuclear mechanics. These studies demonstrated that although GPCCs are thought to be highly invasive cancer cells, they are inherently stiffer than NCCs, in terms of both their cytoskeletal and nuclear mechanics. This was surprising since more invasive cancer cells are often more compliant than less invasive cancer cells. This result may be in part to the ability for GPCCs to behave like activated stromal cells that stiffen in the tumor; we confirmed that GPCCs display similar adhesive behavior as activated stromal cells. To determine how mechanics correlates with cell migration, we used time-lapse nuclear tracking to measure cell motility. The average cell speed was higher for NCCs than for GPCCs; however, GPCCs moved longer distances over time because their motion was more directional. These findings highlight the unusual biophysical behavior of GPCCs. To develop pharmacologic tools that target GPCCs, it is imperative to understand their biophysical properties. 
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