Abstract 4877: MMP-3 generates a novel PTHrP peptide that impacts osteoblast behavior and contributes to metastatic tumor growth in bone
Authors: Frieling Jeremy Steven , Pamen Lizzie Atomi , Yang Shengyu , Lynch Conor C. 
Published: May 2015

Prostate cancer commonly metastasizes to bone, generating incurable mixed lesions that are both osteolytic and osteogenic. To identify new therapeutic targets we performed gene expression analysis and found that matrix metalloproteinase-3 (MMP-3), also known as stromelysin-1 was highly expressed in the prostate tumor-bone microenvironment. In keeping with the literature, immunohistochemical analysis of human bone metastases revealed MMP-3 was largely localized to the stromal compartment. To test whether stromal/host derived MMP-3 contributed to prostate cancer progression in bone, we generated immunocompromized MMP-3 null mice.

Using an in vivo intratibial model of prostate to bone metastases (PaIII), we found that tumor growth, as measured by luminescence, and tumor-induced bone remodeling (μCT, histomorphometry) were significantly mitigated (p&lt;0.05) in MMP-3 null mice compared to wild type controls. Using a candidate approach to examine potential mechanisms, we focused on parathyroid hormone-related protein (PTHrP), a powerful regulator of osteoblast behavior in the vicious cycle. We identified that MMP-3 processes mature PTHrP1-36 to yield unique PTHrP1-17, PTHrP18-26, and PTHrP27-36 fragments in vitro. To test the biological significance of the fragments we focused on testing their impact on stromal cell behavior. Using Boyden chamber migration assays, we observed a significant increase in migration of murine MSCs and MC3T3-E1 pre-osteoblasts in response to 10nM PTHrP1-17 compared to that of PTHrP1-36. Further analysis via confocal microscopy revealed a reduction in actin stress fibers as well as variations in lamellipodia and vinculin organization of pre-osteoblasts and primary osteoblasts treated with PTHrP1-17, and live cell microscopy indicated that these effects were transient with the cells reverting to the usual phenotype in a 24-hour period. Reduced phosphorylation of Rho-associated protein kinase (ROCK) substrates such as the myosin light chain suggest that PTHrP1-17 may mediate pre-osteoblast migration by modulating ROCK activity.

Our data demonstrate that host MMP-3 contributes to prostate tumor growth and tumor induced changes in bone remodeling in vivo. Further, we have identified that mature PTHrP1-36 is subject to cleavage by MMP-3 resulting in PTHrP1-17, PTHrP18-26, and PTHrP27-36 fragments of which PTHrP1-17 significantly stimulates migration of murine MSCs and MC3T3-E1 pre-osteoblasts. Collectively, these data indicate that specific inhibition of MMP-3 and/or targeting MMP generated neo-epitopes would be efficacious for the treatment of prostate to bone metastases.

Citation Format: Jeremy Steven Frieling, Lizzie Atomi Pamen, Shengyu Yang, Conor C. Lynch. MMP-3 generates a novel PTHrP peptide that impacts osteoblast behavior and contributes to metastatic tumor growth in bone. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4877. doi:10.1158/1538-7445.AM2014-4877