Abstract
The STAT family of cytoplasmic transcription factors function to transmit signals from external stimuli such as growth factors and cytokines. STAT activation is normally transient, however it has now been extensively demonstrated that several members of the STAT family, particularly STAT3 and STAT5, are aberrantly activated in a number of human cancers. STAT3 induces the expression of genes known to be involved in tumorigenesis, implicating its dysregulation in a number of hallmark oncogenic processes including tumor cell survival, proliferation, angiogenesis, and drug resistance. Aberrant STAT3 signaling is prevalent in hematologic malignancies including Waldenstrom Macroglobulinemia (WM), a rare form of B cell non-Hodgkin lymphoma. Analysis of the gene and protein expression profiles of WM patients suggests that aberrant STAT3 signaling plays a critical role in WM, providing rationale for the therapeutic use of STAT3 inhibitors.
We have demonstrated pre-clinical efficacy of a novel, highly specific and potent small molecule STAT3/5 inhibitor, BP-1–102, in both hyper-IgM secreting B cell lymphoma cell lines (Mec-1 and RL) and two WM cell lines (MWCL-1 and BCWM-1). BP-1–102 is a phosphotyrosine mimetic designed to directly bind the STAT3 SH2 domain (required for STAT3 phosphorylation, dimerization and nuclear localization), and therefore functions to suppress transcriptional activation of STAT3 target genes. We have confirmed that BP-1–102 directly interacts with STAT3's SH2 domain and is one of the most effective disruptors of STAT3 activity described to date. Treatment of cell lines with low M doses of BP-1–102 induced dose-dependent decreases in constitutive and cytokine-stimulated (IL6/IL10) STAT3 phosphorylation (pSTAT3) as well as pSTAT3 nuclear localization. We further compared the potency of BP-1–102 against STAT5 and the potentially pro-apoptotic STAT1 compared to STAT3 using phospho-flow cytometry. BP-1–102 effectively inhibited GM-CSF-induced pSTAT5 in AML2 cells at low doses (< 12.5 M) but only weakly inhibited IFN -induced pSTAT1 in U937 leukemic cells at similar dose concentrations confirming the selectivity of BP-1–102 for STAT3/5. Using a STAT3 dependent luciferase reporter construct, we confirmed repression of STAT3 transcriptional activity which correlated with a dose-dependent decrease in the expression of STAT3 target genes (Mcl-1, Bcl-xL, Survivin and c-Myc). Inhibition of pSTAT3 resulted in decreased cell viability as assessed by MTT assay (IC50: 6–10μM). In addition, treatment of cells with BP-1–102 resulted in caspase-dependent apoptosis which correlated with caspase-3 and PARP cleavage. Interestingly, co-culture of Mec-1, RL and BCWM-1 with bone marrow stroma cells (BMSC) reduced the cytotoxicity of BP-1–102 suggesting BMSC-conferred resistance, while BP-1–102 was equally cytotoxic in MWCL-1 regardless of either culture condition. Preliminary investigation suggests that the efflux system, used by cells to extrude toxic substances and linked to drug resistance in cancer, may be responsible for the observed BMSC-mediate resistance to BP-1–102.
Collectively, these findings demonstrate a critical role for STAT3 signaling in the molecular pathology of WM and provide the rationale for further development of STAT3 inhibitors for the treatment of WM.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A201.
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