Matrix rigidity regulates the transition of tumor cells to a bone-destructive phenotype through integrin β3 and TGF-β receptor type II

Cancer patients frequently develop skeletal metastases that significantly impact quality of life. Since bone metastases remain incurable, a clearer understanding of molecular mechanisms regulating skeletal metastases is required to develop new therapeutics that block establishment of tumors in bone. While many studies have suggested that the microenvironment contributes to bone metastases, the factors mediating tumors to progress from a quiescent to a bone-destructive state remain unclear. In this study, we hypothesized that the soil of the bone microenvironment, specifically the rigid mineralized extracellular matrix, stimulates the transition of the tumor cells to a bone-destructive phenotype. To test this hypothesis, we synthesized 2D polyurethane (PUR) films with elastic moduli ranging from the basement membrane (70 MPa) to cortical bone (3800 MPa) and measured expression of genes associated with mechanotransduction and bone metastases. We found that expression of Integrin beta 3 (I beta 3), as well as tumor-produced factors associated with bone destruction (Gli2 and parathyroid hormone related protein (PTHrP)), significantly increased with matrix rigidity, and that blocking I beta 3 reduced Gli2 and PTHrP expression. To identify the mechanism by which I beta 3 regulates Gli2 and PTHrP (both are also known to be regulated by TGF-beta), we performed Forster resonance energy transfer (FRET) and immunoprecipitation, which indicated that I beta 3 co-localized with TGF-beta Receptor Type 11 (TGF-beta RII) on rigid but not compliant films. Finally, transplantation of tumor cells expressing 1133 shRNA into the tibiae of athymic nude mice significantly reduced PTHrP and Gli2 expression, as well as bone destruction, suggesting a crucial role for tumor-produced I beta 3 in disease progression. This study demonstrates that the rigid mineralized bone matrix can alter gene expression and bone destruction in an I beta 3/TGF-beta-dependent manner, and suggests that I beta 3 inhibitors are a potential therapeutic approach for blocking tumor transition to a bone destructive phenotype. Published by Elsevier Ltd.