(158d) A Kinetic Model of Tumor-Induced Bone Disease Predicts Complex Response Dynamics to Bone-Specific Mechanical Forces and Drug Perturbations

Tumor cells alter signaling in bone cells, typically resulting in net bone loss, a condition known as tumor-induced bone disease (TIBD) [1]. Drugs that inhibit bone resorption in patients with TIBD improve quality of life but have minimal impact on survival [2], emphasizing the need for new and improved therapeutic approaches. At the molecular level, TIBD is due to secretion of tumor-derived factors, including parathyroid hormone-related protein (PTHrP), which leads to activation of osteoclast-mediated bone destruction. The result is a "vicious cycle" [1], whereby TGF-β is released from the bone matrix, further stimulating PTHrP secretion, in part through non-canonical Hedgehog signaling that activates the transcription factor Gli2 [3]. Co-localization of TGF-β Receptor type II (TGFBRII) and integrin β3 sub-unit has been shown to induce GLI2 and PTHRP expression in bone-destructive cell lines cultured on rigid, bone-like substrates but not on compliant, collagen-like substrates [4]. Furthermore, mechanical forces intrinsic to bone can trigger transition to the bone-destructive phenotype in cell lines of various tumor origin [4]. However, due to the complexity of the bone-tumor microenvironment [5], the mechanisms by which intracellular signaling is altered when tumors establish in bone remain largely unknown.

Here, we model the crosstalk between the TGF-β and integrin β3 signaling pathways in tumor cells to understand how bone-specific mechanical forces and cell line-specific intracellular factors (rate constants) control the transition to the bone-destructive phenotype. The model features a feedback loop wherein integrin β3 stimulates its own expression, resulting in complex response dynamics to perturbations. Specifically, signaling through TGFBRII (activated by TGF-β ligand) and integrin β3 converge on Gli2, which is hypothesized to act as a transcription factor for ITGB3 (encoding the β3 sub-unit) and PTHRP. Binding of integrin β3 to TGFBRII on the cell membrane under bone-specific mechanical forces, which are modeled by varying the binding affinity between receptor and integrin, significantly enhances activity through the integrin β3 branch. Examples of complex dynamics predicted by the model include: (i) self sustaining Gli2 levels after removal of external forces, (ii) transient spikes in Gli2 expression following TGF-β inhibition, and (iii) rebound of Gli2 to steady state levels at low Gli2 inhibitor concentrations. Parameter values are determined by Monte Carlo-based calibration to time-resolved proteomics data and a sensitivity analysis is performed to identify potential targets for modulating system response. Data from microfluidics shear flow experiments and in vitro drug treatments are also presented to test the main predictions of the model. Future work includes expanding the model to include additional molecular details of the signaling pathways underlying TIBD (e.g., Wnt, β-catenin) as well as additional bone microenvironment cell populations (e.g., osteoclasts and osteoblasts) that contribute to disease progression. Over time, we expect the model to grow in size and complexity and ultimately provide an in silico experimental platform for discovering novel drug targets that will improve treatment outcomes for patients with TIBD.

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