Abstract C18: Mesenchymal stem cells in the prostate cancer-bone microenvironment increase osteogenesis and generate apoptotic resistant cancer cells
Authors: McGuire Jeremy , Cook Leah , Frieling Jeremy , Lynch Conor 
Published: October 2016

Introduction: Bone metastatic prostate cancer is common, incurable and, hallmarked by extensive osteoblast driven bone formation that greatly impacts the patient's quality of life. To generate new cures a better understanding of the cellular and molecular mechanisms governing prostate cancer-bone interaction are required. Mesenchymal stem cells (MSCs) give rise to multiple cell types including osteoblasts. The bone is a natural reservoir for MSCs and therefore we reasoned that MSCs contribute to the prostate cancer progression and pathophysiology in bone.

Rationale/Hypothesis: Our in vitro data identified that MSCs are recruited to prostate cancer derived signals and that human and murine specimens of bone metastatic prostate cancer have an increased amount of MSCs compared tumor naïve controls (α-smooth muscle actin positive). Based on this data, we hypothesized that MSCs contribute to prostate cancer induced osteogenesis.

Methodology: Primary MSCs isolated from bone and characterized as CD29+, SCA-1+, CD45- and differentiation capacity were utilized. To mimic bone metastatic prostate cancer, we used an in vivo mouse model (PAIII) of osteogenic bone metastatic prostate cancer. Mice were intratibially inoculated with PAIIIs, PAIIIs/MSCs (1:1), or MSCs (n=8/group, 1x104total cells). Tumor growth was measured at time points using luminescence imaging. Subsequently the tibias were collected for imaging and histological analysis. In vitro, cleaved caspase-3, immunoblot and immunofluorescence were employed to identify mechanisms of MSC-prostate cancer interaction. A resistant clone of the PaIII cell line was derived by multiple rounds of treatment with MSC conditioned media and subsequently used for comparison in vitro and in vivo.

Results: Surprisingly, we initially observed that MSCs suppressed PAIII prostate cancer growth in-vivo compared to PAIII alone at day 11 (p&lt;0.05). However, between days 11 and 15, this trend reversed to where MSCs significantly contributed to PAIII growth (p&lt;0.05) suggesting that MSCs could be contributing to the evolution of a resistant or faster growing clone. Tumor bearing tibias were X-rayed and an overall decrease in tumor-induced osteolysis was observed between the PAIII/MSC and PAIII groups. μCT scans and histomorphometry of tumor bearing bones show a significant increase in bone volume in bones injected with PAIII/MSCs when compared to PAIIIs alone (p&lt;0.05). In vitro, PAIII conditioned media promoted MSC differentiation into osteoblasts while MSC conditioned media inhibited PAIII growth by inducing apoptosis. In fact 5-hour exposure significantly enhanced apoptosis as measured by and increase in cleaved caspase-3. Cytokine array analysis and follow up validation indicate roles for MSC derived Fas ligand and IL-28 in promoting prostate cancer cell apoptosis. Of note, successive rounds of MSC conditioned media exposure yielded PAIII cancer cells that were resistant not only to MSC conditioned media but also etoposide and docetaxel induced apoptosis. Further, we observed that in-vivo the MSC educated PAIII cells grew at significantly faster rates than parental PAIII cells.

Conclusions: Our data identify MSCs as a major component of osteogenic bone metastatic prostate cancers. Prostate cancer cells promote MSC differentiation into osteoblasts that in turn contributes to the osteogenic nature of the disease. Conversely, MSCs promote the evolution of apoptosis resistant clones that ultimately contribute to tumor progression. Understanding the molecular mechanisms underpinning prostate cancer-MSC interaction could yield exciting therapeutic opportunities to eradicate incurable bone metastatic prostate cancer.

Citation Format: Jeremy McGuire, Leah Cook, Jeremy Frieling, Conor Lynch. Mesenchymal stem cells in the prostate cancer-bone microenvironment increase osteogenesis and generate apoptotic resistant cancer cells. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr C18.