Lawrence S. Gazda, Ph.D. (Committee Co-Chair); Madhavi Kadakia, Ph.D. (Committee Co-Chair); Weiwen Long, Ph.D. (Committee Member); Michael Markey, Ph.D. (Committee Member); David Cool, Ph.D. (Committee Member)
Doctor of Philosophy (PhD)
Tumors are heterogeneous systems, whose growth is influenced by intrinsic properties of malignant cells, external systemic factors (i.e. immune, neural, endocrine, etc.), and the dynamic interactions between tumor cells and their microenvironment. Given the inherent complexity of cancers, combined with the continual evolution of tumors and the development of treatment resistance, a precision medicine approach may not provide an optimal clinical response. Exploring a new paradigm that focuses on regulating cancer as a system may not only control tumor progression but also address the extraordinary challenges of tumor heterogeneity and disease recurrence in order to improve clinical outcomes. As a group of discrete, growth-restricted tumor colonies that regulate their own growth and secrete a large number of tumor-inhibitory signals, RENCA macrobeads function as a biological-system, providing the opportunity for a systems-therapeutic approach to cancer management. Previous work has demonstrated that RENCA macrobeads restrict the growth of various cancer cells both in vitro as well as in preclinical and clinical studies; however, the molecular mechanism(s) of this inhibition is unknown. In this study, we demonstrated that factors secreted by RENCA macrobeads significantly altered the transcript levels of multiple MEF2 isoforms in targeted tumor cells. Suppression of various MEF2 isoforms markedly reduced the growth inhibitory effect of RENCA macrobeads and abrogated macrobead induced S-phase arrest. Importantly, we identified an essential role for the MEF2D isoform in mediating RENCA macrobead-induced inhibition. In addition, the cell-surface receptor, EGFR, was shown to be involved in the anti-proliferative response to RENCA macrobeads. Growth inhibition was more robust in cells overexpressing EGFR and was associated with cell accumulation in S-phase. In cell lines with reduced EGFR kinase activity or low-levels of cell-surface receptor, we demonstrated that RENCA macrobeads inhibited growth, although to a lesser degree and exhibited G2/M arrest, supporting the notion that factors secreted by RENCA macrobeads regulate multiple cell cycle checkpoints. Lastly, we identified three proteins in conditioned media of RENCA macrobeads (RTN4, TSP1, TIMP2) that partially contribute to growth regulation of external tumor cells with functional EGFR activity. Moreover, we identified a novel role for these proteins in modulating MEF2 activity and regulating MEF2 expression, particularly the MEF2D isoform. Overall, these studies support a mechanism by which RENCA macrobeads, at least partially, regulate tumor growth external to the macrobead. These findings could identify patients most likely to benefit from RENCA macrobead therapy.
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