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

Abstract

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 β3 (Iβ3), 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 Iβ3 reduced Gli2 and PTHrP expression. To identify the mechanism by which Iβ3 regulates Gli2 and PTHrP (both are also known to be regulated by TGF-β), we performed Förster resonance energy transfer (FRET) and immunoprecipitation, which indicated that Iβ3 co-localized with TGF-β Receptor Type II (TGF-β RII) on rigid but not compliant films. Finally, transplantation of tumor cells expressing Iβ3 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 Iβ3 in disease progression. This study demonstrates that the rigid mineralized bone matrix can alter gene expression and bone destruction in an Iβ3/TGF-β-dependent manner, and suggests that Iβ3 inhibitors are a potential therapeutic approach for blocking tumor transition to a bone destructive phenotype.

Transient mechanical strain promotes the maturation of invadopodia and enhances cancer cell invasion in vitro

“…Nonetheless it was surprising that a known mechanoreceptor would be downregulated in response to mechanical stimulation. However, other studies have shown that expression of integrin β3 is variable in response to other forms of mechanical stimuli in various types of cancer (Felding-Habermann et al, 2001;Page et al, 2015). Additionally, a recent study found that HT1080 cells exhibit an MMP-independent 'nuclear piston' mechanism of invasion that requires integrin β3 activity (Petrie et al, 2017).…”

Hallmarks of Bone Metastasis

“…First, do the tumor cells sense the stiffness of the bone matrix, or are these mechanotransduction signals mediated through the bone lining cells? In vitro evidence strongly supports direct effects of tissue modulus on tumor cell gene expression in bone metastatic breast and lung cancer cells cultured alone on substrates of varying modulus [46–48]. It is difficult to test whether tumor cells come into direct contact with the bone matrix in vivo, but more importantly, must we know?…”

Bone-Induced Expression of Integrin β3 Enables Targeted Nanotherapy of Breast Cancer Metastases

“…Integrin β3 and αvβ3 expression has been observed on human breast cancer bone metastases (25-27), and ectopic overexpression of tumoral β3 on breast cancer has been demonstrated to enhance tumor establishment in bone (26,28). However, for the purpose of targeting αvβ3, physiological expression on breast cancer during tumor growth and metastasis has not been defined.…”