(347g) Tumor Cells Alter Expression Patterns in Response to Bone Mechanical Properties

It is well-established that the mechanical properties of the cell microenvironment can alter gene expression and morphology of various cell types. Specifically, rigid microenvironments can lead to an invasive morphology in mammary epithelial cells. To date, previous studies have focused on tissues with elastic moduli in the kPa range (soft tissue). Breast cancer, however, has a predilection for bone, which provides a much more rigid microenvironment (GPa range) than soft tissues. We hypothesize that the rigidity of bone modulates the expression of factors important in breast tumor growth and behavior in the bone microenvironment.

To determine the role of tissue rigidity in the pathophysiology of bone metastases, we focused on MDA-MB-231 human breast cancer cells that cause osteolytic bone metastases in vivo. Biocompatible polyurethane substrates ranging in elastic modulus (E) from 1.5 GPa to 20 MPa were used to represent tissues of different modulus in vitro. Proteins known to contribute to tumor-induced osteolysis such as PTHrP, TGF-β, Gli2 and TNF-α were assessed both in vitro and in vivo.

We confirmed clinical studies demonstrating that PTHrP is preferentially expressed at the bone metastatic site by measuring PTHrP concentration via an immunoradiometric assay in the serum of tumor bearing mice. We found that mice bearing osteolytic metastases had significantly higher serum PTHrP concentration than mice bearing primary tumors. In vitro, we found that MDA-MB-231 cells cultured in stiff microenvironments equivalent to bone (E ~ GPa) secreted more PTHrP than tumor cells grown on soft substrates. This increase was specific to PTHrP, as the expression of other non-related factors such as TNF-α did not change.

Moreover, this differential PTHrP expression was absent when the contractile mechanism of the tumor cells was inhibited with Blebbistatin (50mM), suggesting that tissue modulus affects PTHrP expression by mechanotransduction.

We then sought to determine the mechanism by which mechanotransduction influences the pathophysiology of bone metastases. Transfection with the Gli luciferase reporter construct showed that Hedgehog signaling, an upstream regulator of PTHrP expression, is increased on rigid substrates. Taken together, these data suggest that the rigidity of bone contributes to the activation of the hedgehog signaling pathway and thus initiating the expression of osteolytic factors in the vicious cycle of bone metastases.