Borrelidin, a small molecule nitrile-containing macrolide inhibitor of threonyl-tRNA synthetase, is a potent inducer of apoptosis in acute lymphoblastic leukemia.

Due to the poor prognosis and limited therapeutic options for adult patients with acute lymphoblastic leukemia (ALL), development of novel therapies is much needed to prolong patient survival and increase the efficacy of their treatment. Malignant T cells need high levels of nutrients to maintain their proliferation rate. Borrelidin, a small molecule nitrile-containing macrolide, is an inhibitor of bacterial and eukaryal threonyl-tRNA synthetase. Borrelidin-mediated inhibition of aminoacyl-tRNA synthesis, leads to an induction in the levels of uncharged tRNA, nutritional stress and ultimately inhibition of protein synthesis. The aim of the present study was to investigate whether borrelidin treatment inhibits the proliferation of malignant ALL cell lines, Jurkat and CEM cells, and study the mechanism by which this drug acts. Our results show that borrelidin was able to potently inhibit the proliferation of ALL cell lines with a half maximal inhibitory concentration of 50 ng/ml. Borrelidin showed a greater inhibitory effect on ALL cell lines compared to primary fibroblasts. Flow cytometry and western blot analysis indicated that borrelidin was able to increase the level of apoptosis and cause G(1) arrest in ALL cell lines. Activation of the general control nonderepressible-2 (GCN2) kinase stress responsive pathway and induction of CHOP protein was significantly higher in ALL cell lines treated with borrelidin. These findings collectively suggest for the first time that borrelidin targets ALL cell lines by inducing apoptosis and mediating G(1) arrest and that borrelidin treatment in ALL cell lines is correlated with activation of the GCN2 kinase pathway.

TYK2-STAT1-BCL2 pathway dependence in T-cell acute lymphoblastic leukemia.

UNLABELLED: Targeted molecular therapy has yielded remarkable outcomes in certain cancers, but specific therapeutic targets remain elusive for many others. As a result of two independent RNA interference (RNAi) screens, we identified pathway dependence on a member of the Janus-activated kinase (JAK) tyrosine kinase family, TYK2, and its downstream effector STAT1, in T-cell acute lymphoblastic leukemia (T-ALL). Gene knockdown experiments consistently showed TYK2 dependence in both T-ALL primary specimens and cell lines, and a small-molecule inhibitor of JAK activity induced T-ALL cell death. Activation of this TYK2-STAT1 pathway in T-ALL cell lines occurs by gain-of-function TYK2 mutations or activation of interleukin (IL)-10 receptor signaling, and this pathway mediates T-ALL cell survival through upregulation of the antiapoptotic protein BCL2. These findings indicate that in many T-ALL cases, the leukemic cells are dependent upon the TYK2-STAT1-BCL2 pathway for continued survival, supporting the development of molecular therapies targeting TYK2 and other components of this pathway. SIGNIFICANCE: In recent years, "pathway dependence" has been revealed in specific types of human cancer, which can be important because they pinpoint proteins that are particularly vulnerable to antitumor-targeted inhibition (so-called Achilles' heel proteins). Here, we use RNAi technology to identify a novel oncogenic pathway that involves aberrant activation of the TYK2 tyrosine kinase and its downstream substrate, STAT1, which ultimately promotes T-ALL cell survival through the upregulation of BCL2 expression