Epigenetic gene regulation by Janus kinase 1 in diffuse large B-cell lymphoma.

Janus kinases (JAKs) classically signal by activating STAT transcription factors but can also regulate gene expression by epigenetically phosphorylating histone H3 on tyrosine 41 (H3Y41-P). In diffuse large B-cell lymphomas (DLBCLs), JAK signaling is a feature of the activated B-cell (ABC) subtype and is triggered by autocrine production of IL-6 and IL-10. Whether this signaling involves STAT activation, epigenetic modification of chromatin, or both mechanisms is unknown. Here we use genetic and pharmacological inhibition to show that JAK1 signaling sustains the survival of ABC DLBCL cells. Whereas STAT3 contributed to the survival of ABC DLBCL cell lines, forced STAT3 activity could not protect these cells from death following JAK1 inhibition, suggesting epigenetic JAK1 action. JAK1 regulated the expression of nearly 3,000 genes in ABC DLBCL cells, and the chromatin surrounding many of these genes was modified by H3Y41-P marks that were diminished by JAK1 inhibition. These JAK1 epigenetic target genes encode important regulators of ABC DLBCL proliferation and survival, including IRF4, MYD88, and MYC. A small molecule JAK1 inhibitor cooperated with the BTK inhibitor ibrutinib in reducing IRF4 levels and acted synergistically to kill ABC DLBCL cells, suggesting that this combination should be evaluated in clinical trials.

Loss of SIRT3 Provides Growth Advantage for B Cell Malignancies.

B cell malignancies comprise a diverse group of cancers that proliferate in lymph nodes, bone marrow, and peripheral blood. SIRT3 (sirtuin 3) is the major deacetylase within the mitochondrial matrix that promotes aerobic metabolism and controls reactive oxygen species (ROS) by deacetylating and activating isocitrate dehydrogenase 2 (IDH2) and superoxide dismutase 2 (SOD2). There is controversy as to whether SIRT3 acts as an oncogene or a tumor suppressor, and here we investigated its role in B cell malignancies. In mantle cell lymphoma patient samples, we found that lower SIRT3 protein expression was associated with worse overall survival. Further, SIRT3 protein expression was reduced in chronic lymphocytic leukemia primary samples and malignant B cell lines compared to primary B cells from healthy donors. This lower level of expression correlated with hyperacetylation of IDH2 and SOD2 mitochondrial proteins, lowered enzymatic activities, and higher ROS levels. Overexpression of SIRT3 decreased proliferation and diminished the Warburg-like phenotype in SIRT3-deficient cell lines, and this effect is largely dependent on deacetylation of IDH2 and SOD2. Lastly, depletion of SIRT3 from malignant B cell lines resulted in greater susceptibility to treatment with an ROS scavenger but did not result in greater sensitivity to inhibition of the hypoxia-inducible factor-1α pathway, suggesting that loss of SIRT3 increases proliferation via ROS-dependent but hypoxia-inducible factor-1α-independent mechanisms. Our study suggests that SIRT3 acts as a tumor suppressor in B cell malignancies, and activating the SIRT3 pathway might represent a novel therapeutic approach for treating B cell malignancies.

Withaferin A disrupts ubiquitin-based NEMO reorganization induced by canonical NF-κB signaling.

The NF-κB family of transcription factors regulates numerous cellular processes, including cell proliferation and survival responses. The constitutive activation of NF-κB has also emerged as an important oncogenic driver in many malignancies, such as activated B-cell like diffuse large B cell lymphoma, among others. In this study, we investigated the impact and mechanisms of action of Withaferin A, a naturally produced steroidal lactone, against both signal-inducible as well as constitutive NF-κB activities. We found that Withaferin A is a robust inhibitor of canonical and constitutive NF-κB activities, leading to apoptosis of certain lymphoma lines. In the canonical pathway induced by TNF, Withaferin A did not disrupt RIP1 polyubiquitination or NEMO-IKKß interaction and was a poor direct IKKß inhibitor, but prevented the formation of TNF-induced NEMO foci which colocalized with TNF ligand. While GFP-NEMO efficiently formed TNF-induced foci, a GFP-NEMO(Y308S) mutant that is defective in binding to polyubiquitin chains did not form foci. Our study reveals that Withaferin A is a novel type of IKK inhibitor which acts by disrupting NEMO reorganization into ubiquitin-based signaling structures in vivo.

Immunohistochemical evaluation of MYC expression in mantle cell lymphoma.

AIM: To assess the validity and potential clinical utility of evaluating MYC expression by immunohistochemistry (IHC) in mantle cell lymphoma (MCL). METHODS AND RESULTS: MYC IHC was scored on a tissue microarray containing 62 MCLs and 29 controls by two pathologists. Inter-observer correlation was high (intra-class correlation of 0.98). MYC IHC scores correlated with MYC expression (Spearman's rank correlation 0.69, P < 0.0001) and weakly with Ki67 proliferation index (Spearman's rank correlation 0.30, P = 0.03). Six blastic MCLs did not have higher mean MYC IHC scores or MYC mRNA expression than non-blastic MCLs. None of 57 cases assessed, including all of the blastic cases, showed MYC rearrangement by fluorescence in-situ hybridization. Multivariate analysis with backward selection from potential predictors including age, lactate dehydrogenase, leukocyte count, MIPI score, ECOG performance status, blastic morphology and Ki67 index showed that MYC IHC score is an independent predictor of progression-free survival (hazard ratio 2.34, 95% CI 1.42-3.88, P = 0.0009) and overall survival (hazard ratio 1.90, 95% CI 1.05-3.43, P = 0.034). CONCLUSIONS: We show that a new monoclonal anti-MYC antibody can enable accurate and reproducible visual assessment of MYC expression that is independently predictive of clinical outcomes in MCL.