Abstract 1564: Reactive endosteum in prostate cancer bone metastases: role of tenascin-C in regulating cancer cell adhesion and proliferation

The objective of this study is to assess the regulatory roles of reactive endosteum associated with foci of bone metastatic prostate cancer cells. Previous studies evaluated tissue arrays of human prostate cancer bone metastases and we identified a “reactive endosteum” spatially associated with foci of prostate cancer cells on trabecular bone. Reactive endosteum was characterized by elevated expression of tenascin-C, a glycoprotein deposited in the extracellular matrix of reactive stroma at sites of wound repair and in the primary tumor microenvironment in adult tissues. In general, tenascin-C is stromal derived in adult tissues. Of interest, tenascin-C is also deposited at sites of fracture repair and at sites of Brodie abscess osteomyelitis-associated inflammation in human bone. Tenascin-C has many diverse functions: it regulates cell adhesion, migration and proliferation in different pathological states while playing an important role in neuropatterning and osteogenesis during development. Moreover, tenascin-C has been shown to regulate several signal transduction pathways. To evaluate prostate cancer-bone interactions, we developed an in vitro, 3D, osteogenic organoid model composed of human mesenchymal stem cells induced to osteogenesis that were combined with human prostate VCaP cells in organ culture. In this model, foci of VCaP cells associated preferentially to regions of high tenascin-C deposition. VCaP cells also preferentially bound to purified human tenascin-C deposited on culture plates in a dose-dependent manner. VCaP cells also exhibited an elevated growth rate on osteo-mimetic plates coated with human tenascin-C as compared to control. Moreover, VCaP cells preferentially bound to human tenascin-C coated bovine trabecular bone cubes in vitro and initiated colony formation. Evaluation of potential mediators identified integrin alpha 9 beta 1 as the key mediator of attachment to human tenascin-C. Neutralization of this integrin inhibited adhesion. Evaluation of potential signaling pathways have implicated activation of EGF receptor (EGFR), Wnk1, and STAT6 pathways. Human tenascin-C exhibits EGF-like repeats and fibronectin type III domains that may mediate activation of these pathways. Of interest, adhesion and growth of VCaP cells on tenascin-C also induced elevated tenascin-C expression in these cells, a novel finding. Additional 3D organoid and in vivo xenograft studies with other cell types support the finding of induced expression of tenascin-C in epithelial cells associated with reactive stroma or matrix. In summary, our studies characterize elevated tenascin-C at sites of a reactive endosteum associated with metastatic prostate cancer foci. Data suggests that tenascin-C mediates adhesion and proliferation of cancer cells via activation of several pathways. These data may aid in developing novel therapeutic approaches to treat metastatic disease. Citation Format: Rebeca San Martin, Kenneth Pienta, David R. Rowley. Reactive endosteum in prostate cancer bone metastases: role of tenascin-C in regulating cancer cell adhesion and proliferation. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1564.