Osteopontin Mediates Tumorigenic Transformation Of A Murine Promotable Cell Line Through Promoting Cell Survival By An Rgd-Dependent Suppression Of Caspase 8 Activity

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Osteopontin (OPN) is a cell adhesive, matricellular protein shown to be an important rate-limiting factor in tumorigenesis in vivo. Further, we have previously shown that addition of exogenous OPN promoted colony formation in an anchorage-independent growth assay of JB6 Cl41.5a cells, a preneoplastic mouse promotable cell line. In addition, down-regulation of the TPA (12-0-tetradecanoylphorbol-13-acetate)-induced OPN expression markedly suppressed TPA-induced colony formation of these cells. Collectively, these findings support the role of osteopontin in mediating neoplastic transformation of JB6 cells in vitro. In the present study, we hypothesize that induced-OPN stimulates the proliferation and/or prevents anoikis of JB6 cells grown in soft-agar and consequently, facilitates colony formation. To determine whether OPN promotes cell proliferation, cell cycle analyses by flow cytometry and direct cell counting of JB6 cells treated with soluble OPN were performed. Data from both of these analyses indicated that OPN-treated cells did not promote cell proliferation compared to non-treated controls. To determine whether OPN prevents anoikis of JB6 cells, annexin V and caspases activities were assesses on cells incubated with or without OPN and suspended on agar. Annexin V analyses showed that OPN significantly decreased anoikis of JB6 cells at 24 h by 41%. In the absence of OPN, suspended JB6 cells underwent anoikis through a sequential activation of caspases 8, 9 and 3 by 1, 3 and 18 h, respectively. Also, the addition of soluble OPN to suspended JB6 cells significantly suppressed caspase 8 activation when compared to non-treated cells. We next determined whether OPN's ability to suppress caspase 8 activity was mediated through its RGD- and/or DRVLFRI-binding motifs as they are known to transduce signals through integrin and CD44 receptors, respectively. Incubation of soluble RGD peptides with suspended JB6 cells suppressed caspase 8 activity. Similar results were obtained when suspended JB6 cells were incubated with OPN only or with RGD peptides and OPN. In contrast, RGE (non-specific peptides) and DRVLFRI peptides alone did not suppress caspase 8 activity of suspended JB6 cells unless OPN was added to these peptides. These findings suggest that OPN suppression of caspase 8 activity of suspended JB6 cells is mediated through its RGD cell-binding motif interacting with integrin receptors. In conclusion, osteopontin promotes tumorigenic transformation of JB6 cells by enhancing the survival of these cells through its RGD-dependent suppression of caspase 8 activity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2735. doi:10.1158/1538-7445.AM2011-2735