Constitutive activation of the transcription factor STAT3 contributes to the pathogenesis of many cancers, including multiple myeloma (MM). Since STAT3 is dispensable in most normal tissue, targeted inhibition of STAT3 is an attractive therapy for patients with these cancers. To identify STAT3 inhibitors, we developed a transcriptionally based assay and screened a library of compounds known to be safe in humans. We found the drug nifuroxazide to be an effective inhibitor of
IntroductionSignal transducers and activators of transcription (STATs) are a family of transcription factors essential for the pathogenesis of many cancers. 1 In normal cells, signaling initiated by cytokines and growth factors leads to the transient activation of STATs by tyrosine kinases, such as members of the Janus kinase (Jak) family. Activation involves phosphorylation of a tyrosine residue near the carboxyl terminus followed by dimerization of 2 STAT monomers. These activated STATs enter the nucleus, and regulate gene transcription by binding to specific DNA regulatory elements. STATs have long been known for their ability to up-regulate genes; however, emerging evidence demonstrates that STATs can also down-regulate key target genes. 2,3 Soon after activation, STATs are typically dephosphorylated, ending the signaling process. In cancer, STAT activation is often constitutive. 4 This can occur through a variety of mechanisms, such as positive signaling from mutated kinases, or by a lack of negative regulation by phosphatases or other feedback inhibitors. This continuously activated state leads to the inappropriate regulation of genes that are important for survival and proliferation, and that are known to contribute to the malignant phenotype.STAT3 in particular has been found to be a critical mediator in the pathogenesis of many tumors. STAT3 is activated by several factors including IL-6, a cytokine essential for the survival of many cell lineages, including B lymphocytes and plasma cells. Multiple myeloma (MM) is a plasma cell malignancy often characterized by the constitutive activation of STAT3. 5 In the bone marrow environment, cytokines such as IL-6 can be secreted by stromal cells or the myeloma cells themselves, and the presence of these cytokines can promote the continued survival and proliferation of MM cells. 6 Thus, the pathogenesis of MM depends on STAT3 signaling, and interruption of this signaling pathway may lead to the death of these cancer cells.To develop targeted inhibitors of STAT3 function for the treatment of MM and other forms of cancer, we developed a cell-based assay to measure STAT3-dependent transcriptional activity. To accelerate the identification of agents that could be used in clinical trials, we used this system to perform a high-throughput screen of compounds known to be bioactive, including many drugs presently in clinical use for a variety of diseases.
Methods
Cell cultureRPMI 8266, H929, and U266 myeloma cells were obtained from ATCC (Manassas, VA) and were grown in RPMI media containing 10% fetal...
