SOX11 Inhibitors Are Cytotoxic in Mantle Cell Lymphoma

PURPOSE

Mantle cell lymphoma (MCL) is a fatal subtype of non-Hodgkin lymphoma. SOX11 transcription factor is overexpressed in the majority of nodal MCL. We have previously reported that B cell-specific overexpression of SOX11 promotes MCL pathogenesis via critically increasing BCR signaling in vivo. SOX11 is an attractive target for MCL therapy; however, no small-molecule inhibitor of SOX11 has been identified to date. Although transcription factors are generally considered undruggable, the ability of SOX11 to bind to the minor groove of DNA led us to hypothesize that there may exist cavities at the protein-DNA interface that are amenable to targeting by small molecules.

EXPERIMENTAL DESIGN

Using a combination of in silico predictions and experimental validations, we report here the discovery of three structurally related compounds (SOX11i) that bind SOX11, perturb its interaction with DNA, and effect SOX11-specific anti-MCL cytotoxicity.

RESULTS

We find mechanistic validation of on-target activity of these SOX11i in the inhibition of BCR signaling and the transcriptional modulation of SOX11 target genes, specifically, in SOX11-expressing MCL cells. One of the three SOX11i exhibits relatively superior in vitro activity and displays cytotoxic synergy with ibrutinib in SOX11-expressing MCL cells. Importantly, this SOX11i induces cytotoxicity specifically in SOX11-positive ibrutinib-resistant MCL patient samples and inhibits Bruton tyrosine kinase phosphorylation in a xenograft mouse model derived from one of these subjects.

CONCLUSIONS

Taken together, our results provide a foundation for therapeutically targeting SOX11 in MCL by a novel class of small molecules.

A Non-ATP-Competitive Dual Inhibitor of JAK2 and BCR-ABL Kinases: Elucidation of a Novel Therapeutic Spectrum Based on Substrate Competitive Inhibition

Here we report the discovery of ON044580, an α-benzoyl styryl benzyl sulfide that possesses potent inhibitory activity against two unrelated kinases, JAK2 and BCR-ABL, and exhibits cytotoxicity to human tumor cells derived from chronic myelogenous leukemia (CML) and myelodysplasia (MDS) patients or cells harboring a mutant JAK2 kinase. This novel spectrum of activity is explained by the non-ATP-competitive inhibition of JAK2 and BCR-ABL kinases. ON044580 inhibits mutant JAK2 kinase and the proliferation of JAK2(V617F)-positive leukemic cells and blocks the IL-3-mediated phosphorylation of JAK2 and STAT5. Interestingly, this compound also directly inhibits the kinase activity of both wild-type and imatinib-resistant (T315I) forms of the BCR-ABL kinase. Finally, ON044580 effectively induces apoptosis of imatinib-resistant CML patient cells. The apparently unrelated JAK2 and BCR-ABL kinases share a common substrate, STAT5, and such substrate competitive inhibitors represent an alternative therapeutic strategy for development of new inhibitors. The novel mechanism of kinase inhibition exhibited by ON044580 renders it effective against mutant forms of kinases such as the BCR-ABL(T315I) and JAK2(V617F). Importantly, ON044580 selectively reduces the number of aneuploid cells in primary bone marrow samples from monosomy 7 MDS patients, suggesting another regulatory cascade amenable to this agent in these aberrant cells. Data presented suggest that this compound could have multiple therapeutic applications including monosomy 7 MDS, imatinib-resistant CML, and myeloproliferative neoplasms that develop resistance to ATP-competitive agents.

Jak/STAT pathways in cytokine signaling and myeloproliferative disorders: approaches for targeted therapies

Hematopoiesis is the cumulative result of intricately regulated signaling pathways that are mediated by cytokines and their receptors. Studies conducted over the past 10 to 15 years have revealed that hematopoietic cytokine receptor signaling is largely mediated by a family of tyrosine kinases termed Janus kinases (JAKs) and their downstream transcription factors, termed STATs (signal transducers and activators of transcription). Aberrations in these pathways, such as those caused by the recently identified JAK2(V617F) mutation and translocations of the JAK2 gene, are underlying causes of leukemias and other myeloproliferative disorders. This review discusses the role of JAK/STAT signaling in normal hematopoiesis as well as genetic abnormalities associated with myeloproliferative and myelodisplastic syndromes. This review also summarizes the status of several small molecule JAK2 inhibitors that are currently at various stages of clinical development. Several of these compounds appear to improve the quality of life of patients with myeloproliferative disorders by palliation of disease-related symptoms. However, to date, these agents do not seem to significantly affect bone marrow fibrosis, alter marrow histopathology, reverse cytopenias, reduce red cell transfusion requirements, or significantly reduce allele burden. These results suggest the possibility that additional mutational events might be associated with the development of these neoplasms, and indicate the need for combination therapies as the nature and significance of these additional molecular events is better understood.

Abstract 643: Targeting cancer with a selective ATP-mimetic inhibitor of polo like kinase-2

The Polo like kinases play key roles in mitosis. While the upregulation of PLK1 in cancer is well documented and PLK3 has been demonstrated to be a tumor suppressor, little is known about the oncogenic significance of PLK2. PLK2 kinase activity is essential for centriolar duplication and is also believed to play a regulatory role in the survival pathway by physically stabilizing the TSC1/2 complex in tumor cells under hypoxic conditions. Protein tyrosine hyper-phosphorylation and the upregulation of serine/threonine kinases associated with cell cycle progression have been linked to the etiology of cancer. In our attempt to develop ATP-mimetic compounds that are cytotoxic against a panel of cancer cell lines, we identified several sulfonyl pyridopyrimidines that possess nanomolar activity. The most potent of these compounds, ON1231320, was found to be a specific PLK2 inhibitor when profiled against a panel of 288 wild-type, 55 mutant and 12 special kinases. Further in vitro testing revealed that ON1231320 is a selective inhibitor of PLK2 with no inhibitory activity against PLK1, PLK3 and PLK4. Confirmation of this interaction and its selectivity was obtained from pull down studies using lysates of tumor cells and a biotinylated analog of ON1231320. The cytotoxic effect of the drug is mediated by apoptosis as evidenced by the induction of Caspase 3/7 activity and by the cleavage of PARP in a dose dependent manner. ON1231320 affects cell cycle progression by blocking tumor cells in the G2/M phase however it does not affect normal human fibroblasts. ON123120 exhibits an excellent safety profile with no overt signs of toxicity, no loss of body weight and 100% survival in mice given a single peritoneal dose of 200 mg/kg. Our ongoing efforts include efficacy studies in nude mouse models, identification of the structural determinants of the interaction between ON1231320 and PLK2 by computational and crystallographic methods and the identification of novel PLK2 substrates to elucidate its role in cancer biology.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 643. doi:10.1158/1538-7445.AM2011-643

Expression of the antiviral protein MxA in cells transiently perturbs endocytosis

MxA is an interferon-induced antiviral protein. Viral replication relies on the trafficking machinery of the host cell. Overexpression of MxA was found to perturb trafficking of internalized transferrin resulting in its accumulation in cells. Interestingly, this perturbation of endocytic trafficking was transient--with a maximal effect being seen 5-6 h after transfection. By 12 h after transfection the perturbation of endocytosis was seen to have subsided although MxA protein levels remained elevated even 24 h after transfection. The accumulation of transferrin is due to a block in transferrin recycling. It is further shown that MxA can physically associate with the endocytic protein dynamin, possibly accounting for the observed effect of MxA expression on transferrin endocytosis. These results uncover a hitherto unknown aspect of MxA action on trafficking processes within cells.

Dynamin interacts with members of the sumoylation machinery

Dynamin is a GTP-binding protein whose oligomerization-dependent assembly around the necks of lipid vesicles mediates their scission from parent membranes. Dynamin is thus directly involved in the regulation of endocytosis. Sumoylation is a post-translational protein modification whereby the ubiquitin-like modifier Sumo is covalently attached to lysine residues on target proteins by a process requiring the concerted action of an activating enzyme (ubiquitin-activating enzyme), a conjugating enzyme (ubiquitin carrier protein), and a ligating enzyme (ubiquitin-protein isopeptide ligase). Here, we show that dynamin interacts with Sumo-1, Ubc9, and PIAS-1, all of which are members of the sumoylation machinery. Ubc9 and PIAS-1 are known ubiquitin carrier protein and ubiquitin-protein isopeptide ligase enzymes, respectively, for the process of sumoylation. We have identified the coiled-coil GTPase effector domain (GED) of dynamin as the site on dynamin that interacts with Sumo-1, Ubc9, and PIAS-1. Although we saw no evidence of covalent Sumo-1 attachment to dynamin, Sumo-1 and Ubc9 are shown here to inhibit the lipid-dependent oligomerization of dynamin. Expression of Sumo-1 and Ubc9 in mammalian cells down-regulated the dynamin-mediated endocytosis of transferrin, whereas dynamin-independent fluid-phase uptake was not affected. Furthermore, using high resolution NMR spectroscopy, we have identified amino acid residues on Sumo-1 that directly interact with the GED of dynamin. The results suggest that the GED of dynamin may serve as a scaffold that concentrates the sumoylation machinery in the vicinity of potential acceptor proteins.

Conformational analysis of the GTP-binding protein MxA using limited proteolysis

Guanosine triphosphate (GTP)-binding proteins are known to function as molecular switches that cycle between GTP-bound and guanosine diphosphate (GDP)-bound states. Switching is achieved by the fact that G-proteins in the GTP-bound conformation can interact with a certain set of effector molecules while they interact with a different set of partners in their GDP-bound conformation. The antiviral properties of the interferon-induced MxA protein are critically dependent on the ability of MxA to bind GTP. Using limited proteolysis we analyzed the conformations of the MxA protein under nucleotide-free, GDP-bound, and GTP-bound conditions. We find that whereas the conformations of nucleotide-free MxA and GDP-bound MxA are essentially similar, GTP-binding causes a dramatic change in the conformation of MxA.