Induction of differentiation by IL-6-type cytokines is impaired in myeloid leukaemia cells unable to activate Stat5a

Stat5a and Stat5b: fraternal twins of signal transduction and transcriptional activation

Stat5a and Stat5b are discretely encoded transcription factors that mediate signals for a broad spectrum of cytokines. Their activation is often an integral component of redundant cytokine signal cascades involving complex cross-talk and pleiotropic gene regulation by Stat5 has been implicated in cellular functions of proliferation, differentiation and apoptosis with relevance to processes of hematopoiesis and immunoregulation, reproduction, and lipid metabolism. Although Stat5a and Stat5b show peptide sequence similarities of > 90%, targeted gene disruptions in mice yield distinctive phenotypes. Prolactin-directed mammary gland maturation fails without functional Stat5a, while disruption of Stat5b in males mitigates growth hormone effects on hepatic function and body mass. The molecular basis for this biologic dichotomy is probably multifaceted. Limited structural dissimilarities between the Stat5a and Stat5b transactivation domains, or subtle differences in the DNA-binding affinities of Stat5 dimer pairs undoubtedly influence gene regulation, but cell-dependent asymmetries in availability of phosphorylated Stat5 can be an underlying factor. Differences in serine phosphorylation(s) of Stat5a and Stat5b, or Stat5 associations with adaptor proteins or co-transcription factors are other potential sources of functional disparity and the signal amplitude, frequency or duration also can be significant. In addition to Stat5 signal attenuation by phosphatase actions or classical feedback inhibition, truncated forms of Stat5 lacking in transactivation capacity may compete upstream for activation and diminish access of full length molecules to DNA binding sites.

Reconstitution of IL6-inducible differentiation of a myeloid leukemia cell line by activated Stat factors

Differentiation of the myeloid leukemia cell line M1 by treatment with IL6-type cytokines depends on activation of the Jak/Stat (Janus kinase/signal transducer and activator of transcription) pathway. Defects in this cascade are correlated with an impaired cytokine-inducible differentiation of various other myeloid cell lines. Although treatment with IL-6 increased the amount of activated transcription factor Stat3 in the myeloid leukemia line C, differentiation was not induced. However, after cotransfection with expression constructs for the tyrosine kinase Jak2 and Stat factors 3 or 5a, treatment of the cells with IL-6 caused a decrease in the number of viable cells. In parallel, an increase in the percentage of differentiated cells occurred. These findings are consistent with the hypothesis that the Jak/Stat signaling cascade plays an important role in cytokine-induced differentiation of myeloid leukemia cells.

Cytoplasmic domains of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor beta chain (hbetac) responsible for human GM-CSF-induced myeloid cell differentiation

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates differentiation, survival, and proliferation of myeloid progenitor cells. The biologic actions of GM-CSF are mediated by its binding to the alpha and beta subunits of the GM-CSF receptor (GM-CSFRalpha and betac, respectively). To determine whether identical regions of the betac protein mediate both cell growth and differentiation, we expressed cDNA constructs encoding the human wild-type (897 amino acids) and truncated betac (hbetac) subunits along with the wild-type human GM-CSFRalpha subunit in the murine WT19 cell line, an FDC-P1-derived cell line that differentiates toward the monocytic lineage in response to murine GM-CSF. Whereas the WT19 cell line carrying the C-terminal deleted hbetac subunit of 627 amino acids was still able to grow in human GM-CSF (hGM-CSF), 681 amino acids of the hbetac were necessary for cell differentiation. The addition of hGM-CSF to WT19 cell lines containing the hbetac627 subunit stimulated the phosphorylation of ERK (extracellular signal-regulated kinase) and induced the tyrosine-phosphorylation of SHP-2 and STAT5, suggesting that the activation of these molecules is insufficient to mediate the induction of differentiation. A point mutation of tyrosine 628 to phenylalanine (Y628F) within hbetac681 abolished the ability of hGM-CSF to induce differentiation. Our results indicate that the signals required for hGM-CSF-induced differentiation and cell growth are mediated by different regions of the hbetac subunit.

Modulation of the activation status of Stat5a during LIF-induced differentiation of M1 myeloid leukemia cells

Treatment of M1 myeloid leukemia cells with leukemia inhibitory factor (LIF) causes activation of transcription factors Stat1, Stat3 and Stat5a (signal transducers and activators of transcription). DNA-binding of Stat proteins was detectable for extended periods of time in LIF-treated M1 cells, which simultaneously underwent terminal differentiation. The relative composition of Stat factors in the protein-DNA complexes changed during time. Whereas Stat3 was activated up to 36 h during treatment with LIF, Stat5a was activated only short-termed. Similarly, high expression of the immediate early gene CIS (cytokine-inducible SH2-containing protein), a known target gene of Stat5 in hematopoietic cells, occurred only during the onset of differentiation. This suggests a role of Stat5a in the early phase of LIF-induced differentiation and growth arrest of M1 cells.

Transcription factor Stat5 is an early marker of differentiation of murine embryonic stem cells

Embryonic stem (ES) cells are pluripotent descendants of the inner cell mass of blastocysts capable of differentiating into progenitor cells of most if not all tissues. The pluripotency of ES cells is maintained by leukemia inhibitory factor (LIF), a member of the family of interleukin-6-type cytokines. These cytokines activate Janus tyrosine kinases and signal transducer and activator of transcription factors (Stat) via the signalling receptor component gp130. Pluripotent ES1 cells proliferating in the presence of LIF were known from previous studies to contain Stat3 and Stat1 capable of transcriptional activation. Here we report that the level of tyrosine-phosphorylated Stat3 decreases rapidly during differentiation induced by treatment of ES1 cells either with retinoic acid (RA) or by withdrawal of LIF. In line with this finding, the DNA-binding activity of Stat3 decreased during differentiation. In contrast, Stat5 was absent from pluripotent proliferating ES cells, but appeared early after induction of differentiation. The positive correlation between induction of differentiation and expression of Stat5 mRNA was confirmed for three independent ES cell lines. Stat5 transcripts were detectable in ES1 cells as early as 12 h after treatment with RA and 36 h after withdrawal of LIF. Stat5 protein was detectable 2 days after the onset of differentiation. These results establish Stat5 as a novel marker of very early stages of differentiation of ES cells.