Cyclin-Dependent Kinase-9 Is a Therapeutic Target in MYC-Expressing Diffuse Large B-Cell Lymphoma.

Deregulation of the MYC transcription factor is a key driver in lymphomagenesis. MYC induces global changes in gene expression that contribute to cell growth, proliferation, and oncogenesis by stimulating the activity of RNA polymerases. A key feature in its ability to stimulate RNA Pol II activity is recruitment of pTEFb, an elongation factor whose catalytic core comprises CDK9/cyclin T complexes. Hence, MYC expression and function may be susceptible to CDK9 inhibition. We conducted a pre-clinical assessment of AZ5576, a selective CDK9 inhibitor, in diffuse large B-cell lymphoma (DLBCL). The in vitro and in vivo effects of AZ5576 on apoptosis, cell cycle, Mcl-1, and MYC expression were assessed by flow cytometry, immunoblotting, qPCR and RNA-Seq. We demonstrate that, in addition to depleting Mcl-1, targeting CDK9 disrupts MYC oncogenic function. Treatment with AZ5576 inhibited growth of DLBCL cell lines in vitro and in vivo, independent of cell-of-origin. CDK9 inhibition downregulated Mcl-1 and MYC mRNA transcript and protein in a dose-dependent manner. MYC-expressing cell lines demonstrated enhanced susceptibility to AZ5576. CDK9 inhibition promoted turnover of MYC protein, and decreased MYC phosphorylation at the stabilizing Ser62 residue and downregulated MYC transcriptional targets in DLBCL cells, a finding confirmed in a functional reporter assay, suggesting that CDK9 may govern MYC protein turnover, thus regulating its expression through multiple mechanisms. Our data suggest that targeting CDK9 is poised to disrupt MYC oncogenic activity in DLBCL and provide rationale for clinical development of selective CDK9 inhibitors.

Targeting ubiquitin-activating enzyme induces ER stress-mediated apoptosis in B-cell lymphoma cells.

Alterations in the ubiquitin proteasome system (UPS) leave malignant cells in heightened cellular stress, making them susceptible to proteasome inhibition. However, given the limited efficacy of proteasome inhibitors in non-Hodgkin lymphoma (NHL), novel approaches to target the UPS are needed. Here, we show that TAK-243, the first small-molecule inhibitor of the ubiquitin activating enzyme (UAE) to enter clinical development, disrupts all ubiquitin signaling and global protein ubiquitination in diffuse large B-cell lymphoma (DLBCL) cells, thereby inducing endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). Activation of the ER stress response protein kinase R (PKR)-like ER kinase and phosphorylation of eukaryotic translation initiator factor 2α led to upregulation of the proapoptotic molecule C/EBP homologous protein and cell death across a panel of DLBCL cell lines independent of cell of origin. Concurrently, targeting UAE led to accumulation of Cdt1, a replication licensing factor, leading to DNA rereplication, checkpoint activation, and cell cycle arrest. MYC oncoprotein sensitized DLBCL cells to UAE inhibition; engineered expression of MYC enhanced while genetic MYC knockdown protected from TAK-243-induced apoptosis. UAE inhibition demonstrated enhanced ER stress and UPR and increased potency compared with bortezomib in DLBCL cell lines. In vivo treatment with TAK-243 restricted the growth of xenografted DLBCL tumors, accompanied by reduced cell proliferation and apoptosis. Finally, primary patient-derived DLBCL cells, including those expressing aberrant MYC, demonstrated susceptibility to UAE inhibition. In sum, targeting UAE may hold promise as a novel therapeutic approach in NHL.