Gyeong-Hun Baeg

Background: Human or animals lacking either JAK3 or the common gamma chain (γc) expression display severe combined immunodeficiency disease, indicating the crucial role of JAK3 in T-cell development and the homeostasis of the immune system. JAK3 has also been suggested to contribute to the pathogenesis of tumorigenesis. Recent studies identified activatingJAK3mutations in patients with various hematopoietic malignancies, including acute megakaryoblastic leukemia. Importantly, functional analyses of some of thoseJAK3mutations have been shown to cause lethal hematopoietic malignancies in animal models. These observations make JAK3 an ideal therapeutic target for the treatment of various human diseases. To identify novel small molecule inhibitors of JAK3, we performed structure-based virtual screen using the 3D structure of JAK3 kinase domain and the NCI diversity set of compounds.Results: We identified NSC114792 as a lead compound. This compound directly blocked the catalytic activity of JAK3 but not that of other JAK family membersin vitro. In addition, treatment of 32D/IL-2Rβ cells with the compound led to a block in IL-2-dependent activation of JAK3/STAT5 but not IL-3-dependent activation of JAK2/STAT5. Consistent with the specificity of NSC114792 for JAK3, it selectively inhibited persistently-activated JAK3, but failed to affect the activity of other JAK family members and other oncogenic kinases in various cancer cell lines. Finally, we showed that NSC114792 decreases cell viability by inducing apoptosis through down-regulating anti-apoptotic gene expression only in cancer cells harboring persistently-active JAK3.Conclusions: NSC114792 is a lead compound that selectively inhibits JAK3 activity. Therefore, our study suggests that this small molecule inhibitor of JAK3 can be used as a starting point to develop a new class of drugs targeting JAK3 activity, and may have therapeutic potential in various diseases that are caused by aberrant JAK3 activity.

Summary: In order to identify Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling inhibitors, a cell‐based high throughput screening was performed using a plant extract library that identified Nb‐(α‐hydroxynaphthoyl)serotonin called MS‐1020 as a novel JAK3 inhibitor. MS‐1020 potently inhibited persistently‐active STAT3 in a cell type‐specific manner. Further examination showed that MS‐1020 selectively blocked constitutively‐active JAK3 and consistently suppressed interleukin‐2‐induced JAK3/STAT5 signalling but not prolactin‐induced JAK2/STAT5 signalling. Furthermore, MS‐1020 affected cell viability only in cancer cells harbouring persistently‐active JAK3/STATs, and in vitro kinase assays showed MS‐1020 binds directly with JAK3, blocking its catalytic activity. Therefore, the present study suggested that this reagent selectively inhibits JAK3 and subsequently leads to a block in STAT signalling. Finally, MS‐1020 decreased cell survival by inducing apoptosis via down‐regulation of anti‐apoptotic gene expression. These results suggest that MS‐1020 may have therapeutic potential in the treatment of cancers harbouring aberrant JAK3 signalling.

Inappropriate activation of JAK/STAT signaling occurs with high frequency in human cancers and is associated with cancer cell survival and proliferation. Therefore, the development of pharmacologic STAT signaling inhibitors has therapeutic potential in the treatment of human cancers. Here, we report 2-[(3,5-bis-trifluoromethyl-phenyl)-hydroxy-methyl]-1-(4-nitro-phenylamino)-6-phenyl-1,2,4a,7a-tetrahydro-pyrrolo[3,4-b]-pyridine-5,7-dione (AUH-6-96) as a novel small-molecule inhibitor of JAK/STAT signaling that we initially identified through a cell-based high-throughput screening using cultured Drosophila cells. Treatment of Drosophila cells with AUH-6-96 resulted in a reduction of Unpaired-induced transcriptional activity and tyrosine phosphorylation of STAT92E, the sole Drosophila STAT homologue. In human cancer cell lines, AUH-6-96 inhibited both constitutive and interleukin-6-induced STAT3 phosphorylation. Specifically, in Hodgkin lymphoma L540 cells, treatment with AUH-6-96 resulted in reduced levels of tyrosine phosphorylated STAT3 and of the STAT3 downstream target gene SOCS3 in a dose- and time-dependent manner. In addition, AUH-6-96-treated L540 cells showed decreased expression of persistently activated JAK3, suggesting that AUH-6-96 inhibits the JAK/STAT pathway signaling in L540 cells by affecting JAK3 activity and subsequently blocking STAT3 signaling. Importantly, AUH-6-96 selectively affected cell viability only of cancer cells harboring aberrant JAK/STAT signaling. In support of the specificity of AUH-6-96 for inhibition of JAK/STAT signaling, treatment with AUH-6-96 decreased cancer cell survival by inducing programmed cell death by down-regulating the expression of STAT3 downstream target antiapoptotic genes, such as Bcl-xL. In summary, this study shows that AUH-6-96 is a novel small-molecule inhibitor of JAK/STAT signaling and may have therapeutic potential in the treatment of human cancers harboring aberrant JAK/STAT signaling. [Mol Cancer Ther 2008;7(9):2672–80]