Chk1 Inhibition Potently Blocks STAT3 Tyrosine705 Phosphorylation, DNA-Binding Activity, and Activation of Downstream Targets in Human Multiple Myeloma Cells

The relationship between the checkpoint kinase Chk1 and the STAT3 pathway was examined in multiple myeloma cells. Gene expression profiling of U266 cells exposed to low (nmol/L) Chk1 inhibitor [PF-477736 (PF)] concentrations revealed STAT3 path-way-related gene downregulation (e.g., BCL-XL, MCL-1, c-Myc), findings confirmed by RT-PCR. This was associated with marked inhibition of STAT3 Tyr705 (but not Ser727) phosphorylation, dimerization, nuclear localization, DNA binding, STAT3 promoter activity by chromatin immunoprecipitation assay, and downregu-lation of STAT-3-dependent proteins. Similar findings were obtained in other multiple myeloma cells and with alternative Chk1 inhibitors (e.g., prexasertib, CEP3891). While PF did not reduce GP130 expression or modify SOCS or PRL-3 phosphorylation, the phosphatase inhibitor pervanadate antagonized PF-mediated Tyr705 dephosphorylation. Significantly, PF attenuated Chk1-mediated STAT3 phosphorylation in in vitro assays. Surface plasmon resonance analysis suggested Chk1/STAT3 interactions and PF reduced Chk1/STAT3 co-immunoprecipitation. Chk1 CRISPR knockout or short hairpin RNA knockdown cells also displayed STAT3 inactivation and STAT3-dependent protein downregulation. Constitutively active STAT3 diminished PF-mediated STAT3 inactivation and downregulate STAT3-dependent proteins while significantly reducing PF-induced DNA damage (gH2A.X for-mation) and apoptosis. Exposure of cells with low basal phospho-STAT3 expression to IL6 or human stromal cell conditioned medium activated STAT3, an event attenuated by Chk1 inhibitors. PF also inactivated STAT3 in primary human CD138 thorn multiple myeloma cells and tumors extracted from an NSG multiple mye-loma xenograft model while inhibiting tumor growth.