High Density Lipoprotein-like Nanoparticles Synergize with Akt and Btk Inhibitors to Induce Cell Death in Diffuse Large B Cell Lymphomas

Introduction: We previously reported that our bio-inspired, synthetic high-density lipoprotein-like nanoparticles (HDL NP) induced apoptosis in B cell lymphoma cells expressing scavenger receptor type B1 (SCARB1), the high-affinity receptor for cholesterol-rich HDLs. HDL NPs consist of a 5nm gold nanoparticle core surface functionalized with the HDL-defining apolipoprotein A1 and a phospholipid bilayer, and bind specifically to SCARB1, inducing the efflux of free cholesterol and inhibiting cholesteryl ester influx. SCARB1 is overexpressed in a subset of follicular and diffuse large B cell lymphomas (DLBCL), and resides in cholesterol-rich plasma membrane microdomains called lipid rafts, similar to the B cell receptor (BCR) and its associated signaling kinases. Upon binding to natural HDL, SCARB1 activates a number of pro-survival signaling kinases, including Akt and PI3K. Both Akt and PI3K are also involved in B cell receptor-mediated signaling in germinal center-derived (GC) DLBCL, through tonic BCR signaling, and activated B cell (ABC) DLBCL, through chronic active BCR signaling. Additionally, PI3K was recently shown to play a role in recruitment and activation of Btk, a crucial survival kinase downstream of the BCR. We hypothesized that small molecule inhibitors against pro-survival kinases, specifically Akt and Btk, will synergize with HDL NPs against B cell lymphomas.

Methods: Burkitt's lymphoma (Ramos), GC DLBCL (SUDHL4) and ABC DLBCL (TMD8 and HBL-1) cell lines were treated with the Akt inhibitor GDC-0068 or the Btk inhibitor Ibrutinib, in the absence or presence of HDL NPs, and synergy was calculated using the Calcusyn software. Phos-flow was used to assay for changes in the phosphorylation status of Akt and Btk.

Results: The Burkitt's lymphoma and GC DLBCL cell lines were more sensitive to HDL NP induced cell death compared to the ABC DLBCL cell lines (Ramos HDL NP IC₅₀ = 1.5nM; SUDHL4 HDL NP IC₅₀ = 2.1nM; TMD8 HDL NP IC₅₀ = 31.4nM; HBL-1 HDL NP IC₅₀ = 89nM). HDL NPs synergized with GDC-0068 in the Ramos, SUDHL4 and TMD8 cell lines (all combination indexes < 1). Correspondingly, HDL NPs dose-dependently decreased phosphorylation of Akt in Ramos and TMD8 cells. Ibrutinib synergized with the HDL NPs in all cell lines tested (all combination indexes < 1). In TMD8 cells, HDL NPs decreased p-Btk levels comparable to treatment with 10nM Ibrutinib. Addition of the PI3K inhibitor Pilaralisib (XL147) demonstrated mild synergy in the Ramos cell line, but not the SUDHL4, TMD8 or HBL-1 cell lines (all combination index values >1). Treatment of Ramos and SUDHL4 cells with an inhibitor of PTEN, a phosphatase responsible for acting in opposition to PI3K leading to inactivation of Akt, rescued the cells from HDL NP-induced cell death. TMD8 cells treated with the PTEN inhibitor demonstrated a smaller increase in survival when HDL NPs were applied, suggesting that PI3K may not play a major role in HDL NP-induced cell death in activated B cell DLBCLs. PTEN activity is influenced by the level of cholesterol and cholesteryl esters present in the cell, with increasing levels correlating with decreased PTEN activity. Cholesterol levels were higher in the ABC DLBCL cell lines compared to the other B cell lymphoma cell lines. HDL NPs significantly reduced the cholesterol content of Ramos cells, but not the TMD8 or HBL-1 cells, suggesting that the ability of the HDL NPs to alter cellular cholesterol homeostasis correlates with their ability to induce lymphoma cell death.

Conclusion: HDL NPs demonstrated synergy with inhibitors to the pro-survival kinases Akt and Btk, suggesting that HDL NPs act to disrupt second messenger signaling pathways in lymphoma cells by directly altering signaling through SCARB1, modulating cellular cholesterol homeostasis, and/or through disruption of membrane raft organization. HDL NPs represent an innovative, targeted therapeutic, with great potential, to add to existing combination chemotherapy regimens.