Stat3 is an Src homology (SH)2-containing protein constitutively activated in a wide variety of human cancers following its recruitment to YXXQ-containing motifs, which results in resistance to apoptosis. Despite resolution of the crystal structure of Stat3 homodimer bound to DNA, the structural basis for the unique specificity of Stat3 SH2 for YXXQ-containing phosphopeptides remains unresolved. We tested three models of this interaction based on computational analysis of available structures and sequence alignments, two of which assumed an extended peptide configuration and one in which the peptide had a ␤-turn. By using peptide immunoblot affinity assays and mirror resonance affinity analysis, we demonstrated that only phosphotyrosine (Tyr(P)) peptides containing ؉3 Gln (not Leu, Met, Glu, or Arg) bound to wild type Stat3. Examination of a series of wild type and mutant Stat3 proteins demonstrated loss of binding to pYXXQ-containing peptides only in Stat3 mutated at Lys-591 or Arg-609, whose side chains interact with the Tyr(P) residue, and Stat3 mutated at Glu-638, whose amide hydrogen bonds with oxygen within the ؉3 Gln side chain when the peptide ligand assumes a ␤-turn. These findings support a model for Stat3 SH2 interactions that could form the basis for anticancer drugs that specifically target Stat3.

Signal transducer and activator of transcription 3 (STAT3) is an oncogene and a critical regulator of multiple cell-fate decisions, including myeloid cell differentiation. Two isoforms of STAT3 have been identified: ␣ (p92) and ␤ (p83). These differ structurally in their C-terminal transactivation domains, resulting in distinct functional activities. The cis genetic elements that regulate the ratio of ␣ to ␤ messenger RNA (mRNA) are unknown. In this study, cloning, sequencing, and splicing analysis of the human and murine STAT3 genes revealed a highly conserved 5 donor site for generation of both ␣ and ␤ mRNA and distinct branch-point sequences, polypyrimidine tracts, and 3 acceptor sites (ASs) for each. The ␤ 3 AS was found to be located 50 nucleotides downstream of the ␣ 3 AS in exon 23. IntroductionAlthough there is only one signal transducer and activator of transcription 3 (STAT3) gene in mice and humans, 2 protein isoforms have been identified in both species: STAT3␣ (p92) 1,2 and STAT3␤ (p83). [3][4][5] The messenger RNA (mRNA) encoding STAT3␤ has a 50-nucleotide deletion at the 3Ј end that is presumably due to alternative mRNA splicing and results in a protein's missing the C-terminal 55 amino acid residues of STAT3␣. In contrast to other STAT protein ␤ isoforms, in which the C-terminal transactivation domain is simply deleted, in STAT3␤, the 55 amino acid residues of STAT3␣ are replaced by 7 unique amino acid residues at its C-terminal. These residues are encoded by 21 nucleotides spliced in the ϩ2 reading frame downstream of the deletion. The Cterminal 55 amino acid residues comprise the transactivation domain of STAT3␣ and contain serine 727, whose phosphorylation results in enhanced transcriptional activity. 6 This same region of STAT3␣ also influences STAT3␣ dimerization, since the DNAbinding activity of STAT3␣ was shown to be reduced by 15 to 25 fold compared with that of STAT3␤. 7,8 The reduced DNA-binding activity of STAT3␣ was attributed to a reduced stability of STAT3␣ homodimers compared with STAT3␤ homodimers.The ratio of STAT3␣ to STAT3␤ varies in cells and tissues, ranging from 3:1 to 10:1 at the mRNA level and 1:3 to 10:1 at the protein level. 5,9 This variation may have important biologic consequences because the functions of the 2 isoforms do not overlap. When STAT3␤ and STAT3␣ were overexpressed in Cos cells, STAT3␤, but not STAT3␣, was constitutively able to cooperate with c-Jun to activate a reporter construct containing ␣ 2 -macroglobulin. 3 In contrast, STAT3␤ activated downstream of the interleukin 5 receptor inhibited the ability of STAT3␣ to activate a reporter construct containing an intercellular adhesion molecule 1 promoter. 4 In studies examining the distinct biologic functions of the STAT3 isoforms, STAT3␣ enhanced whereas STAT3␤ inhibited v-Src-mediated fibroblast transformation. 10 More relevant for myeloid development, we and others found that overexpression of STAT3␣ inhibited myeloid differentiation mediated by gp130 and granulocyte colony-stimulating factor receptor 1...

G-CSFR cytoplasmic tyrosine (Y) residues (Y704, Y729, Y744, and Y764) become phosphorylated upon ligand binding and recruit specific Src homology 2 domain-containing proteins that link to distinct yet overlapping programs for myeloid cell survival, differentiation, proliferation, and activation. The structural basis for recruitment specificity is poorly understood but could be exploited to selectively target deleterious G-CSFR-mediated signaling events such as aberrant Stat3 activation demonstrated in a subset of acute myeloid leukemia patients with poor prognosis. Recombinant Stat3 bound to G-CSFR phosphotyrosine peptide ligands pY704VLQ and pY744LRC with similar kinetics. Testing of three models for Stat3 Src homology 2-pY ligand binding in vitro and in vivo revealed unique determinants for Stat3 recruitment and activation by the G-CSFR, the side chain of Stat3 R609, which interacts with the pY ligand phosphate group, and the peptide amide hydrogen of E638, which bonds with oxygen/sulfur within the + 3 Q/C side chain of the pY ligand when it assumes a β turn. Thus, our findings identify for the first time the structural basis for recruitment and activation of Stat3 by the G-CSFR and reveal unique features of this interaction that can be exploited to target Stat3 activation for the treatment of a subset of acute myeloid leukemia patients.

No abstract