(178f) Tumor-Secreted Soluble Proteins Mediate Mesenchymal Stem Cell Migration to Tumors by Rapidly Changing Cytoskeletal Rigidity

Mesenchymal stem cells (MSCs) are highly proliferative adult stem cells that are involved in wound healing and tissue regeneration. MSCs spontaneously migrate from the bone marrow and infiltrate wounded tissues and tumors¹; however, ex vivo manipulation of MSCs alters their morphology limiting their mobility after reinfusion². In vivo, the recruitment of MSCs to diseased or tumor tissues is mediated by pro-angiogenic proteins that are released by tumors or wound tissues and circulate in the blood³. Previous studies have demonstrated that MSC treatment with pro-angiogenic proteins results in increased in vitro cell migration⁴. However, little is known about the effects of these proteins on the mechanical properties of MSCs. We hypothesized that pro-angiogenic proteins, secreted by tumor or wound tissues, stimulate MSCs, inducing their migration, through rapid modification in cytoskeletal rigidity. To test this hypothesis, we investigated the time-dependent effects of tumor conditioned media (TCM, filtered media isolated from tumor cells in culture) on the mechanical properties of human MSCs.

Using multiple particle tracking microrheology, which involves measuring the microscopic displacements of standard-sized nanoparticles (embedded in the cytoskeleton) with high resolution video microscopy combined with particle tracking software, we quantified the time-dependent effects of TCM on human MSC viscoelasticity. MSC treatment with TCM resulted in rapid changes in the transport rates of injected nanoparticles. Prior to MSC treatment with TCM, the ensemble average MSDs varied almost linearly with time; however, 60 minutes after treatment with TCM, the ensemble average MSDs were nearly constant, indicating that particle transport was much more restricted. The ensemble average MSD at t=1 s was used to determine the average elastic (G') and viscous (G?) moduli at ω=1 s^-1. Within 60 minutes after MSC treatment with TCM, G' was increased from 140 ± 36 dyn/cm² to 424 ± 50 dyn/cm², indicating that the cytoskeleton had become much more rigid. Transwell migration assays were used to determine if microrheological changes were correlated with increased cell motility. Within 2 hours, MSC migration toward TCM was significantly increased. Increased migration persisted for more than 24 hours.

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