Reversing Metabolic and Epigenetic Cellular Alterations to Overcome Chemo-Resistance in Aggressive B Cell Lymphomas

Abstract 1305

Cancer cells exist in a stressed environment, mainly due to lack of nutrients and oxygen, particularly during chemotherapy, and rely on metabolic homeostatic regulatory mechanisms for protection against these lethal challenges. Increasing glucose metabolism and continuous reactive oxygen species (ROS) production is one strategy of metabolic adaptation utilized by tumor cells to relieve this stress. Thioredoxin interacting protein (TXNIP) is a negative regulator for both redox thioredoxin (ROS production) and cellular glucose uptake, not well understood but found to be repressed in various cancers, including diffuse large B-cell lymphomas (DLBCL), the most common form of non-Hodgkin lymphoma that continues increasing in incidence and remains incurable in many cases, primarily due to development of chemo-resistance. The molecular mechanisms by which TXNIP expression is down-regulated during cancer progression and chemo-resistance development have not been completely elucidated. Since key gene silencing events have now been identified in the pathogenesis of DLBCL, recent therapeutic interest has focused on dysregulated histone modifications as potentially important therapeutic targets, for developing strategies that can reactivate silenced tumor suppressor genes. Enhancer of zeste homolog 2 (EZH2), the catalytic subunit of the polycomb repressive complex 2 (PRC2), is a highly conserved histone methyltransferase that targets lysine-27 of histone H3 (H3K27). Studies in human tumors show that EZH2 is frequently over-expressed in a wide variety of tumors, including lymphomas. More importantly, recent studies using whole-genome sequencing in primary DLBCL, identified frequent mutations in the EZH2 gene that leads to critical gene silencing in DLBCL pathophysiology.

Our study showed that EZH2 is either over-expressed or mutated in representative DLBCL cell lines and primary DLBCL cells, and that down-regulation of EZH2 with siRNA leads to the reactivation of TXNIP, with subsequent inhibition of tumor cell growth and survival mediated through both thioredoxin and glucose metabolism in DLBCL. We also found that histone deacetylation (HDAC) is also involved in EZH2-mediated silencing of TXNIP in DLBCL. Pharmacologic agents aimed at reactivating TXNIP genes include histone methylation inhibitor 3-Deazaneplanocin A (DZNep) that targets EZH2, as well as HDAC inhibitor Vorinostat. DZNep is currently the only histone methylation inhibitor that is commercially available. Our data indicated that DZNep is highly effective in inhibiting cell growth in various DLBCL cell lines, particularly in chemo-resistant DLBCL cell lines. Vorinostat, on the other hand, has been a good drug and is currently in clinical trial for relapsed DLBCL and has been FDA approved for treating cutaneous T-cell lymphoma patients. Our data showed synergistic activity of DZNep and Vorinostat in reactivating TXNIP gene expression and inhibiting DLBCL cell growth and survival.

We also discovered that EZH2 controls constitutive NF-κB activity through both, the canonical and alternative NF-κB pathways in DLBCL. This function of EZH2 is independent of its histone methyltransferase activity. These findings reveal that EZH2 and NF-κB, the two oncogenic factors display functional crosstalk in DLBCL cells. Our findings have indicated that deregulated EZH2 leads to constitutive NF-kB activation and to epigenetic silencing of TXNIP, resulting in uncontrolled tumor cell growth and survival mediated through both thioredoxin and glucose metabolism in DLBCL, and that targeting this pathway represents a novel, rational, and effective therapeutic approach to selectively reverse chemoresistance in DLBCL patients, particularly relapsed/refractory patients.