Drug-repositioning screening identified piperlongumine as a direct STAT3 inhibitor with potent activity against breast cancer.

Signal transducer and activator of transcription (STAT) 3 regulates many cardinal features of cancer including cancer cell growth, apoptosis resistance, DNA damage response, metastasis, immune escape, tumor angiogenesis, the Warburg effect and oncogene addiction and has been validated as a drug target for cancer therapy. Several strategies have been used to identify agents that target Stat3 in breast cancer but none has yet entered into clinical use. We used a high-throughput fluorescence microscopy search strategy to identify compounds in a drug-repositioning library (Prestwick library) that block ligand-induced nuclear translocation of Stat3 and identified piperlongumine (PL), a natural product isolated from the fruit of the pepper Piper longum. PL inhibited Stat3 nuclear translocation, inhibited ligand-induced and constitutive Stat3 phosphorylation, and modulated expression of multiple Stat3-regulated genes. Surface plasmon resonance assay revealed that PL directly inhibited binding of Stat3 to its phosphotyrosyl peptide ligand. Phosphoprotein antibody array analysis revealed that PL does not modulate kinases known to activate Stat3 such as Janus kinases, Src kinase family members or receptor tyrosine kinases. PL inhibited anchorage-independent and anchorage-dependent growth of multiple breast cancer cell lines having increased pStat3 or total Stat3, and induced apoptosis. PL also inhibited mammosphere formation by tumor cells from patient-derived xenografts. PL's antitumorigenic function was causally linked to its Stat3-inhibitory effect. PL was non-toxic in mice up to a dose of 30 mg/kg/day for 14 days and caused regression of breast cancer cell line xenografts in nude mice. Thus, PL represents a promising new agent for rapid entry into the clinic for use in treating breast cancer, as well as other cancers in which Stat3 has a role.

Primary substrate specificity determinants for the H4-specific protease-activated protein phosphotransferase.

The specificity of the histone-H4-specific, protease-activated protein kinase (H4-PK) was examined using two series of synthetic peptides corresponding to the phosphorylation sites in histone H4 and pyruvate kinase. Optimum kinetic constants for phosphorylation were observed using the peptide Val-Lys-Arg-Ile-Ser-Gly-Leu. Peptides in which the Lys was replaced by Arg or the Lys-Arg sequence was transposed were phosphorylated with less favorable kinetics. Peptides with either basic residue deleted did not serve as substrates. Only the H4 peptide, containing an Arg-Arg sequence, was phosphorylated by the cyclic-AMP-dependent protein kinase (CA-PK). Distinct specificity determinants for H4-PK and CA-PK were also observed using the pyruvate kinase peptide (Leu-Arg-Arg-Ala-Ser-Leu-Gly). Collectively the data indicated that the primary substrate specificity determinants for H4-PK are Lys-Arg-Xaa-Ser whereas the CA-PK selectively phosphorylates the sequence Arg-Arg-Xaa-Ser.