Therapeutic inhibition of HAUSP-PTEN network in chronic lymphocytic leukemia

Abstract

findings we wondered whether when present at high levels STAT3 induces apoptosis of CLL cells. To test this theory we first overexpress STAT3 in MM1 cells and found that overexpression of STAT3 upregulated caspase3 levels and induced apoptosis of MM1 cells. Because sequence analysis revealed that the promoter of caspase3 harbors putative STAT3 binding sites, we sought to determine whether STAT3 activates caspase-3. Chromatin immunoprecipitation (ChIP) and an electrophoretic mobility shift essay (EMSA) confirmed that STAT3 binds the caspase-3 promoter, and a Luciferase assay validated that STAT3 activates the caspase-3 promoter in IL-6-treated MM1 cells. To assess STAT3’s binding affinity to the promoter of caspase-3, we prepared serial dilutions of CLL cell DNA and, using ChIP and EMSA, found that STAT3’s binding affinities to p21 and c-Myc were 8 and 4 times higher than STAT’s binding affinity to caspase-3, suggesting that at high levels STAT3 are required to activate caspase-3. Taken together, these findings suggest that STAT3 has a previously unknown pro-apoptotic function. When present at high levels, STAT3 activates Caspase3 and induces apoptosis rather than providing CLL cells with survival advantage.