Abstract
Background: Activation of NF-kB pathways are a hallmark feature of TCL and HL, making proteasome inhibition an attractive therapeutic target. Previous studies have demonstrated prominent in vivo efficacy of ixazomib, an oral proteasome inhibitor for both TCL and HL. Among the common significantly regulated genes identified via systems biology approach include upregulation of genes encoding for ubiquitin proteasome subunits (Ravi et al. Cancer Res. 2016). Further combination studies with HDACi belinostat were synergistic in Jurkat, HH and L428 cell lines, and NRF2 was discovered as mediator of proteasome gene expression. We hypothesized that HDACi abrogates NRF2 mediated proteasome recovery leading to synergistic effects on cell viability in combination with ixazomib.
Methods: Global transcriptome analysis was performed on RNA isolated from multiple cell lines include TCL (Jurkat) and HL (L540, L428) treated with ixazomib and control, as well as Jurkat cell lines treated with single agent ixazomib, belinostat and combination. Significant genes were determined by applying a one-way ANOVA with an adjusted Bonferroni correction for a false discovery rate (FDR) < 0.05. Further pathway analysis from significant genes was performed by using a fold change greater than ±1.2 comparing all samples to each other and observing pathway relationships using Ingenuity Pathway Analysis. Gene Set Enrichment Analysis was performed with FDR <0.05 for functional analysis. Proteasome-Glo cell based assay was used to evaluate caspase-, chymotrypsin, and trypsin-like activity. Proteasome activity was measured at 24 and 72 hours after Jurkat cell lines were treated with control, ixazomib or belinostat as single agents, and in combination. SiRNA knockdown experiments were performed in Jurkat cell line with NRF2 and non-targeting (NT) SiRNA transfection. Real-time quantitative PCR (qPCR) for proteasome subunit and NRF2 genes was performed on RNA isolated from treated cells.
Results: Transcriptome analysis revealed upregulation of proteasome genes in ixazomib treated cell lines Jurkat, L540 and L428 at 24 hours. In Jurkat TCL, ixazomib caused decreased caspase-like and chymotrypsin-like proteasome activity at 24 hours that was followed by recovery of these activities at 72 hours. The combination of ixazomib and belinostat significantly decreased proteasome activity for chymotrypsin-like, caspase-like and trypsin-like activity at 72 hours compared to single agent ixazomib or belinostat. In Jurkat cells, NRF2 was identified as a transcriptional regulator involved in proteasome gene regulation, showing upregulation of proteasomal genes and NRF2 with ixazomib single agent, downregulation with belinostat single agent and in combination with ixazomib. These results were confirmed with qPCR for NRF2 and proteasome genes in Jurkat and L428. SiRNA knockdown for NRF2 in Jurkat cells resulted in decreased cell viability, NRF2 and proteasome gene expression compared with NT SiRNA following ixazomib treatment.
Conclusions: Treatment with single agent ixazomib induced prominent proteasome gene expression in all TCL and HL cell lines. In Jurkat, recovery of chymotrypsin and caspase-like proteasome activity occurred by 72 hours suggesting that transcriptional changes induced by proteasome inhibition contributed to proteasome function recovery. Combination therapy with belinostat resulted in downregulation of proteasome genes in Jurkat and L428 and prevented functional recovery of the proteasome observed in Jurkat. Our results suggest that targeting the proteasome itself with ixazomib and preventing the induced recovery of proteasome genes with belinostat contributes to synergistic effects observed on proteasome function and cell viability in TCL and HL. Further studies with CRISPR/Cas to confirm the effect of NRF2 on proteasome gene and functional recovery in the context of proteasome inhibition are ongoing and will be reported.
Evens:Takeda: Other: Advisory board.
Author notes
Asterisk with author names denotes non-ASH members.
