Abstract 5730: Co-targeting toll-like receptor and PI3K survival signaling pathways in stem-like castration resistant prostate cancer cells

The development of castration-resistant prostate cancer (CRPCs) has been linked to the ability of prostate cancer stem cells (PCSCs) to either resist or survive the inhibitory effect of androgen deprivation and radiation therapies. Dysregulation of the PI3K/Akt/mTOR pathway is a common phenomenon in CRPCs, resulting in survival, growth enhancement, resistance to apoptosis and altered metabolism. Toll-like receptors (TLRs) have been shown to be surprisingly expressed in prostate cancer cells, which when activated could lead to activation of innate immune response and activation of other cell signaling pathways. Since, recent studies have identified links between TLR signaling and prostate cancer, TLR polymorphisms and prostate cancer risk, as well as possible cross-talks between TLR signaling and the PI3K pathway in immune cells, we hypothesized that there is a link between PI3K signaling pathway and TLR signaling in CRPCs. Our aim is to determine the synergism between targeting PI3K and TLR signaling in a combination regimen. Methods: PTEN -/- (PC3) and PTEN -/+ (DU145) castration-resistant prostate cancer cell lines (CRPCs) were used for this experiment. CRPCs cells were cultured and enriched for CD44+ spheroid cells either by low dose radiation or in serum-free media. CRPCs and stem-like spheroid cells were then treated with different concentration of PI3K, AKT, and mTOR inhibitors, as well as TLR agonists /antagonists as monotherapy. Combination regimens were also done as assess the synergistic effects of PI3K/AKT/mTOR inhibitors and/or TLR ligands (agonists)/antagonists in CRPCs and/or spheroid cells. CRPCs or spheroid cells were assessed for treatment-induced effects such as cell viability (MTT assay), cell cytotoxicity (LDH), mode of cell death (fluorescent staining), protein expression profile (Western blot analysis), and redox activity (NBT assay). We also tested to assess treatment-induced inflammatory response via ELISA assay. Results: Our result shows that TLR 3, 4 and 9, were significantly differentially overexpressed in PTEN -/- CRPCs compared to PTEN -/+ CRPCs. The activation of these pathway results in PI3K signaling downstream. The co-targeting of PI3K signaling pathway with both PI3K inhibitors and TLR signaling modulators appears to be beneficial in preventing proliferation and stemness in PTEN -/- CRPCs compared to PTEN -/+ CRPCs. Co-administration of PI3K and TLR 4/9 agonist appears to be antagonistic but become synergistic when TLRs 4 and 9 signaling were inhibited using siRNAs. Conclusion: One of the advantages of this combination treatment method is that it proffers a solution to the often-encountered feedback loop observed when PI3K signaling inhibitors are used as monotherapy. This study has provided more insights on how targeting can be beneficial in the eradication of CRPCs when used along with PI3K signaling inhibitors. Citation Format: Saheed Oluwasina Oseni, Rolando Branly, Mirjana Pavlovic, James Kumi-Diaka. Co-targeting toll-like receptor and PI3K survival signaling pathways in stem-like castration resistant prostate cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5730.