High activation of STAT5A drives peripheral T-cell lymphoma and leukemia

Recurrent gain-of-function mutations in the transcription factors STAT5A and much more in STAT5B were found in hematopoietic malignancies with the highest proportion in mature T- and natural killer-cell neoplasms (peripheral T-cell lymphoma, PTCL). No targeted therapy exists for these heterogeneous and often aggressive diseases. Given the shortage of models for PTCL, we mimicked graded STAT5A or STAT5B activity by expressing hyperactive Stat5a or STAT5B variants at low or high levels in the hematopoietic system of transgenic mice. Only mice with high activity levels developed a lethal disease resembling human PTCL. Neoplasia displayed massive expansion of CD8⁺ T cells and destructive organ infiltration. T cells were cytokine-hypersensitive with activated memory CD8⁺ T-lymphocyte characteristics. Histopathology and mRNA expression profiles revealed close correlation with distinct subtypes of PTCL. Pronounced STAT5 expression and activity in samples from patients with different subsets underline the relevance of JAK/STAT as a therapeutic target. JAK inhibitors or a selective STAT5 SH₂ domain inhibitor induced cell death and ruxolitinib blocked T-cell neoplasia in vivo. We conclude that enhanced STAT5A or STAT5B action both drive PTCL development, defining both STAT5 molecules as targets for therapeutic intervention.

Abstract 1184: Characterization of novel STAT5 inhibitors to interfere with the oncogenic activities of STAT5 in hematopoietic diseases

Activation of the transcription factor STAT5 is essential for the pathogenesis of acute myeloid leukemia (AML) containing the FLT3 internal tandem duplication (ITD). FLT3 ITD is a constitutively active tyrosine kinase (TK) that drives the activation of STAT5 leading to the growth and survival of AML cells. Although there has been some success in identifying TK inhibitors that block the function of FLT3 ITD, treatment options are still limited. This is mainly due to drug resistance development by mutations that allow for the continued activation of STATs. Since STAT5 represents a critical mediator of malignant cellular behavior and sits at the convergence point of many kinase pathways, the direct targeting of STAT5 may be an effective means of overcoming this resistance to TK inhibitors. First, we screened a library of potential STAT5 inhibitors for specific SH2 domain binders using a fluorescent polarization assay. Thereby, we identified small inhibitory molecules, called AC-3-019 and AC-4-130, that bind to the SH2 domain of STAT5, subsequently resulting in the disruption of the reciprocal STAT5-phosphopeptide interactions. They efficiently blocked kinase-mediated phosphorylation, dimer formation, nuclear translocation, DNA binding and STAT5 mediated target gene expression. Further, we observed a time- and dose-dependent impairment of proliferation, blocked cell cycle progression and increased apoptosis. Studies with human AML patient-derived samples similarly showed an induction of apoptotic cell death and decreased colony forming capabilities. A combinatorial drug screen revealed synergistic effects with TK inhibitors, as well as with drugs standardly used in the treatment of AML patients, e.g. Cytarabine. Finally, AC-3-019 and AC-4-130 significantly suppressed tumor growth in vivo without general toxicity in healthy organs. Overall, our findings indicate that AC-3-019 and AC-4-130 are potent and selective inhibitors of STAT5. These compounds provide lead structures for further chemical modifications and clinical development for the identification of compounds to improve existing therapies.

Citation Format: Bettina Wingelhofer, Barbara Maurer, Elizabeth C. Heyes, Patricia Freund, Abbarna A. Cumaraswamy, Jisung Park, Stefan Kubicek, Peter Valent, Patrick T. Gunning, Richard Moriggl. Characterization of novel STAT5 inhibitors to interfere with the oncogenic activities of STAT5 in hematopoietic diseases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1184. doi:10.1158/1538-7445.AM2017-1184

Combined targeting of STAT3 and STAT5: a novel approach to overcome drug resistance in chronic myeloid leukemia

In chronic myeloid leukemia, resistance against BCR-ABL1 tyrosine kinase inhibitors can develop because of BCR-ABL1 mutations, activation of additional pro-oncogenic pathways, and stem cell resistance. Drug combinations covering a broad range of targets may overcome resistance. CDDO-Me (bardoxolone methyl) is a drug that inhibits the survival of leukemic cells by targeting different pro-survival molecules, including STAT3. We found that CDDO-Me inhibits proliferation and survival of tyrosine kinase inhibitor-resistant BCR-ABL1⁺ cell lines and primary leukemic cells, including cells harboring BCR-ABL1^T315I or T315I⁺ compound mutations. Furthermore, CDDO-Me was found to block growth and survival of CD34⁺/CD38⁻ leukemic stem cells (LSC). Moreover, CDDO-Me was found to produce synergistic growth-inhibitory effects when combined with BCR-ABL1 tyrosine kinase inhibitors. These drug-combinations were found to block multiple signaling cascades and molecules, including STAT3 and STAT5. Furthermore, combined targeting of STAT3 and STAT5 by shRNA and STAT5-targeting drugs also resulted in synergistic growth-inhibition, pointing to a new efficient concept of combinatorial STAT3 and STAT5 inhibition. However, CDDO-Me was also found to increase the expression of heme-oxygenase-1, a heat-shock-protein that triggers drug resistance and cell survival. We therefore combined CDDO-Me with the heme-oxygenase-1 inhibitor SMA-ZnPP, which also resulted in synergistic growth-inhibitory effects. Moreover, SMA-ZnPP was found to sensitize BCR-ABL1⁺ cells against the combination ‘CDDO-Me+ tyrosine kinase inhibitor’. Together, combined targeting of STAT3, STAT5, and heme-oxygenase-1 overcomes resistance in BCR-ABL1⁺ cells, including stem cells and highly resistant sub-clones expressing BCR-ABL1^T315I or T315I-compound mutations. Whether such drug-combinations are effective in tyrosine kinase inhibitor-resistant patients with chronic myeloid leukemia remains to be elucidated.

STAT5 Is a Key Regulator in NK Cells and Acts as a Molecular Switch from Tumor Surveillance to Tumor Promotion

Natural killer (NK) cells are tightly regulated by the JAK-STAT signaling pathway and cannot survive in the absence of STAT5. We now report that STAT5-deficient NK cells can be rescued by overexpression of BCL2. Our experiments define STAT5 as a master regulator of NK-cell proliferation and lytic functions. Although NK cells are generally responsible for killing tumor cells, the rescued STAT5-deficient NK cells promote tumor formation by producing enhanced levels of the angiogenic factor VEGFA. The importance of VEGFA produced by NK cells was verified by experiments with a conditional knockout of VEGFA in NK cells. We show that STAT5 normally represses the transcription of VEGFA in NK cells, in both mice and humans. These findings reveal that STAT5-directed therapies may have negative effects: In addition to impairing NK-cell-mediated tumor surveillance, they may even promote tumor growth by enhancing angiogenesis.

SIGNIFICANCE

The importance of the immune system in effective cancer treatment is widely recognized. We show that the new signal interceptors targeting the JAK-STAT5 pathway may have dangerous side effects that must be taken into account in clinical trials: inhibiting JAK-STAT5 has the potential to promote tumor growth by enhancing NK-cell-mediated angiogenesis.

Progress towards direct inhibitors of Stat5 protein

Abstract Molecular approaches to inhibit STAT5 signaling have been hailed as a viable targeted anticancer therapy. In particular, many drugs and drug candidates have been developed to successfully inhibit upstream effectors of STAT5 by indirectly targeting cell surface receptors and protein kinases (FLT-3, JAK2, and BCR-ABL). Indirect strategies have yielded potent agents, such as imatinib, AC2207, and EXEL823 which effectively silence STAT5 activity but which suffer from off-target effects and toxicity. This article will focus on reviewing the current literature pertaining to direct inhibitors of STAT5 protein and assess the prospects for a future STAT5-targeting therapeutic.

Nanomolar-Potency Small Molecule Inhibitor of STAT5 Protein

We herein report the design and synthesis of the first nanomolar binding inhibitor of STAT5 protein. Lead compound 13a, possessing a phosphotyrosyl-mimicking salicylic acid group, potently and selectively binds to STAT5 over STAT3, inhibits STAT5–SH2 domain complexation events in vitro, silences activated STAT5 in leukemic cells, as well as STAT5′s downstream transcriptional targets, including MYC and MCL1, and, as a result, leads to apoptosis. We believe 13a represents a useful probe for interrogating STAT5 function in cells as well as being a potential candidate for advanced preclinical trials.