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 b 3( Ib3), 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 Ib3 reduced Gli2 and PTHrP expression. To identify the mechanism by which Ib3 regulates Gli2 and PTHrP (both are also known to be regulated by TGF-b), we performed F€ resonance energy transfer (FRET) and immunoprecipitation, which indicated that Ib3 co-localized with TGF-b Receptor Type II (TGF-b RII) on rigid but not compliant films. Finally, transplantation of tumor cells expressing Ib3 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 Ib3 in disease progression. This study demonstrates that the rigid mineralized bone matrix can alter gene expression and bone destruction in an Ib3/TGF-b-dependent manner, and suggests that Ib3 inhibitors are a potential therapeutic approach for blocking tumor transition to a bone destructive phenotype. Published by Elsevier Ltd.