Histone Deacetylase Inhibitor (HDACi) PCI-24781 and Bortezomib Result in Synergistic Apoptosis in Hodgkin Lymphoma (HL) and Non-Hodgkin’s Lymphoma (NHL) Cell Lines: Investigation of Cell Death and NFKB-Mediated Pathways.

HDACi block cancer cell proliferation by mechanisms that involve epigenetic gene regulation leading to cell growth arrest, differentiation, and apoptosis. Bortezomib inhibits NFKB signaling and induces apoptosis. Furthermore, anti-tumor activity of HDACi and bortezomib both depend in part on reactive oxygen species (ROS)-mediated pathways. Both have activity in NHL. We reasoned that these agents may be synergistic in part due to their dependence on overlapping pathways. We investigated the biology of PCI-24781, a pan-HDACi currently in clinical trials, and bortezomib both alone, and in combination, in HL (L428) and NHL cell lines (HF1, Ramos, & SUDHL4). Cells were incubated with increasing concentrations of PCI-24781 and bortezomib (0.25–2.0μM and 2.5–20nM, respectively) for 24–72 hour (hr). Apoptosis was determined by fluorescence-activated cell sorting (FACS) using AnnexinV-FITC/propidium iodide (AnnexinV⁺/PI⁺) staining. Reactive oxygen species (ROS) were measured by oxidation of 2′7′dichlorofluorescein diacetate (H₂DCFDA) to DCF and detected by FACS. Downstream targets of NFKB such as NFKB1, Myc and IL-8 were measured in Ramos using quantitative real time polymerase chain reaction (RT-PCR) following 24 hr incubation of cells with PCI-24781 and bortezomib alone, and in combination. Dose-dependent apoptosis was seen with PCI-24781 and bortezomib alone in all HL and NHL cell lines. IC70 (dose to achieve 70% AnnexinV⁺/PI⁺) was 1μM for PCI-24781 and 2μM for L428. With bortezomib, the IC50 was 10nM in Ramos, HF1, and SUDHL4 and 20 nM in L428. The combination of PCI-24781 and bortezomib resulted in synergistic apoptosis (combination index <0.2) in all 3 NHL cell lines (IC80=0.25μM PCI-24781/5nM bortezomib) and L428 (IC80=0.5μM PCI-24781/10nM bortezomib) compared with minimal cell death using each agent alone at those concentrations. Furthermore, immunoblots of L428 and Ramos showed enhanced caspase 3 and caspase 8 cleavage with the combination of PCI-24781 and bortezomib compared to either agent alone, suggesting that the synergy seen was in part caspase-dependent. HL and NHL cell lines showed a 3- to 4-fold increase in ROS with PCI-24781 or bortezomib alone and in combination at 24hr. Moreover, we found that hyperacetylation of histone-3 and histone-4 on immunoblots of cells treated with combination PCI-2478/bortezomib was significantly increased compared to PCI-24781 alone. Finally, we found that in Ramos cells PC-24781/bortezomib together resulted in downregulation of NFKB targets NFKB1 and Myc, but not IL-8. We conclude that PCI-24781 and bortezomib are active in lymphoma cell lines and that the combination results in synergistic apoptosis. Apoptosis was accompanied by caspase activation and synergistic downregulation of the NFkB pathway. These data have important clinical implications for NHL and HL.