Abstract
Introduction: Diffuse large B-cell lymphoma (DLBCL) is subdivided into the germinal center B-like (GCB) and activated B cell-like (ABC) subtypes by gene expression profiling, and these subtypes exhibit different clinical outcomes and signaling pathway deregulations. Compared to the GCB, the ABC-DLBCL subtype displays a more aggressive clinical course and shorter patient survival. Constitutive nuclear factor kappa-B (NF-kB) activity is often associated with the ABC-DLBCL subtype, however recent studies suggest that NF-kB signaling activation is also observed to a lower extent in the GCB-DLBCL subtype (Lina Odqvist et al. 2014). miRNAs have diagnostic and prognostic value in disease classification, and growing evidence implicates miRNAs in tumorigenesis, tumor maintenance, and dissemination through their ability to modulate the expression of critical genes and signaling networks. We previously demonstrated that miRNA-181a expression correlates with longer survival in patients treated with R-CHOP, independent of established clinical and molecular predictors. However, the molecular and cellular mechanisms underlying the association between miRNA-181a expression and improved prognosis in DLBCL patients are currently unknown. Herein we analyzed the role of miRNA-181a in DLBCL pathogenesis.
Results:Quantitative RT-PCR analyses demonstrate higher endogenous miRNA-181a levels in centroblasts than in plasmablasts. Concordantly, endogenous miRNA-181a levels were significantly higher in GCB DLBCL cell lines and primary tumors compared with ABC DLBCL. These expression differences could not be attributed to distinct DNA methylation signatures in the miRNA-181a promoters (Chromosomes 1, 9) or regulatory elements as analyzed by Mass Array Sequenom Epityping. In search for putative miRNA-181a targets we identified 5 genes (CARD11, NFKB1A (IKBα), NFKB1 (p105/p50), RELA (p65), and REL (CREL)) within the NF-kB signaling pathway. Analyses of these targets show a decrease in the levels of these proteins and mRNAs in ABC and GCB DLBCL cell lines ectopically expressing miRNA-181a compared with scramble control plasmid. Luciferase reporter analyses encoding the respective wild type or mutated 3′UTR sequences demonstrate direct and specific targeting of these transcripts with the exception of RELA. Analysis of the net effect of miRNA-181a on NF-kB signaling using NF-kB luciferase reporter demonstrate significant decrease in NF-kB signaling. Concordantly, anti-miRNA-181a transfection led to increased NF-kB luciferase reporter activity. Moreover, western blot analyses of cytoplasmic and nuclear fractions showed a decrease in the levels of the transcription factors CREL and p50 in both cellular compartments, a decrease in the binding to DNA at NF-kB binding motifs, and a consequent decrease in NF-kB target gene transcription in the miRNA-181a expressing cells compared with scramble control. Together these studies point to miRNA-181a-mediated repression of NF-kB signaling in DLBCLs. Ectopic miRNA-181a expression led to a decrease in cell proliferation and an increase in cell death in both DLBCL subtypes, but this effect was more pronounced in the ABC DLBCL cell lines. The miRNA-181a-mediated increase in cell apoptosis could not be rescued by BCL2 co-transfection, an anti-apoptotic protein that was previously established as a direct miRNA-181a target. Analyses of miRNA-181a effects in NOD/SCID mice demonstrated that in vivo miRNA-181a induction in GCB and ABC human DLBCL xenografts led to decreased tumor growth and significantly longer animal survival. Notably, survival was prolonged in both GCB and ABC DLBCL bearing animals.
Figure 1 Figure 1.
Conclusions: miRNA-181a directly suppress the NF-kB signaling pathway and lead to increased tumor cell death in both DLBCL subtypes suggesting that NF-kB deregulation is present in both tumor subtypes. However, the lower miRNA-181a expression level in the ABC DLBCL subtype may contribute to the higher NF-kB signaling activity that is observed in this subtype. Furthermore, our study provides a plausible explanation for the association between high miRNA-181a expression and longer survival of DLBCL patients.
Disclosures
No relevant conflicts of interest to declare.
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