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Myeloid differentiation is a highly regulated process governed by various cytokines, such as EPO, TPO, G-CSF, IL-3, IL-5 and GM-CSF. These cytokines act in part through activation of the STAT transcription factor family. In particular, various isoforms of STAT3 and STAT5 are activated during myeloid differentiation in a cell-type and maturation-state dependent fashion. In vitro studies have shown that STAT proteins are essential for cytokine-regulated processes such as cellular proliferation, differentiation as well as survival. Similarly, various STAT knock-outs have highlighted the role of STATs in myeloid differentiation in vivo. STATs also appear to play an important role in various myeloid malignancies, which are characterized by arrested maturation and cytokine-independent proliferation of myeloid progenitors. Constitutive activation of STAT3 and/or STAT5 resulting in enhanced transcription of anti-apoptotic- cell-cycle progression genes is likely to contribute to the pathogenesis of various myeloid leukemia's. Oncogene (2000) 19, 2511–2522 [ABSTRACT FROM AUTHOR]

The binding of insulin to its receptor initiates multiple signal transduction pathways regulating such diverse processes as proliferation, differentiation, glucose transport, and glycogen metabolism. The STAT-family of transcription factors has been demonstrated to play a critical role in gene induction by a variety of hemopoietic cytokines and hormones. Furthermore, constitutive activation of STATs is observed in transformed cells. Here we describe activation of a transcriptional complex binding to a consensus STAT-transcriptional element in response to insulin challenge. This complex is induced rapidly after tyrosine autophosphorylation of the insulin receptor, and is sustained for several hours. Supershift analysis of the insulin-induced complex reveals that it specifically contains the transcription factor Stat3. DAN binding of this complex is inhibited by pre-incubation with tyrosine, but not serine/threonine protein kinase inhibitors, whereas transcriptional activation is inhibited by both. Utilising a dominant negative mutant of p21ras we demonstrate that both insulin-induced Stat3 DNA-binding and also transactivation do not require p21ras. Furthermore, although previous studies have suggested a role for MAP kinases (ERKs) and PI-3K in STAT activation, utilising the specific MEK inhibitor PD098059 and the PI-3K inhibitor wortmannin, we demonstrate that activation of ERKs or PI-3K are not required for insulin induced Stat3 phosphorylation or transactivation. [ABSTRACT FROM AUTHOR]

Most cancer therapeutics fails to eradicate cancer because cancer cells rapidly develop resistance to its proapoptotic effects. The underlying mechanisms remain incompletely understood. Here we show that three representative apoptotic stimuli, that is, serum starvation, a mitochondrial toxin, and a DNA-damaging agent (etoposide), rapidly induce several distinct classes of prosurvival molecules, in particular, Bcl-2/Bcl-XL and superoxide dismutase (SOD; including both MnSOD and Cu/ZnSOD). At the population level, the induction of these prosurvival molecules occurs prior to or concomitant with the induction of proapoptotic molecules such as Bim and Bak. Blocking the induction using siRNAs of the prosurvival or proapoptotic molecules facilitates or inhibits apoptosis, respectively. One master transcription factor, FOXO3a, is involved in the transcriptional activation of some of these prosurvival (e.g., MnSOD) and proapoptotic (e.g., Bim) molecules. Interestingly, in all three apoptotic systems, FOXO3a itself is also upregulated at the transcriptional level. Mechanistic studies indicate that reactive oxygen species (ROS) are rapidly induced upon apoptotic stimulation and that ROS inhibitors/scavengers block the induction of FOXO3a, MnSOD, and Bim. Finally, we show that apoptotic stimuli also upregulate prosurvival molecules in normal diploid human fibroblasts and at subapoptotic concentrations. Taken together, these results suggest that various apoptotic inducers may rapidly mobilize prosurvival mechanisms through ROS-activated master transcription factors such as FOXO3a. The results imply that effective anticancer therapeutics may need to combine both apoptosis-inducing and survival-suppressing strategies.Oncogene (2005) 24, 2020-2031. doi:10.1038/sj.onc.1208385 Published online 24 January 2005 [ABSTRACT FROM AUTHOR]

Granulocyte macrophage colony stimulating factor (GM-CSF), interleukin-3 (IL-3) and interleukin-5 (IL-5 belong to a family of cytokines that regulate proliferation, differentiation and function of haematopoietic cells. Their receptor consists of a ligand specific α-chain and a signal transducing β-chain (βc). While, the role of phosphotyrosine residues in the βc as mediators of downstream signalling cascades has been established, little is known about non-phosphotyrosine mediated events. To identify proteins interacting with βc, we screened a yeast two-hybrid library with the intracellular domain of βc. We found that RACK1, a molecule associating with activated PKC, PLCγ and Src kinases, associated with the membrane proximal region of βc in both yeast two-hybrid, immunoprecipitation and GST-pull-down assays. The association of RACK1 was constitutive, demonstrating no alteration upon cellular stimulation. Furthermore, upon stimulation of cells with IL-5 or PMA, a complex of βc and PKCβ was found. Together, these findings suggest a novel role for RACK1 as a possible adapter molecule associating with the intracellular domain of cytokine receptors. [ABSTRACT FROM AUTHOR]

Interleukin (IL)-3, IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) regulate proliferation, differentiation and apoptosis of target cells. Receptors for these cytokines consist of a cytokine-specific α subunit and a common shared βc subunit. Tyrosine phosphorylation of the βc is thought to play a critical role in mediating signal transduction events. We have examined the effect of mutation of βc tyrosines on the activation of multiple signal transduction pathways. Activation of protein kinase B (PKB) required JAK2 and was inhibited by dominant-negative phosphatidylinositol 3-kinase (P13K). Overexpression of JAK2 was sufficient to activate both protein kinase B (PKB) and extracellular regulated kinase-1 (ERK1). Tyrosine 577 and 612 were found to be critical for the activation of PKB and ERK1, but not activation of STAT transcription factors. Activation of both PKB and ERK have been implicated in the regulation of proliferation and apoptosis. We generated GM-CSFR stable cell lines expressing receptor mutants to evaluate their effect on these processes. Activation of both PKB and ERK was perturbed, while STAT activation remained unaffected. Tyrosines 577 and 612 were necessary for optimal proliferation, however, mutation of these tyrosine residues did not affect GM-CSF mediated rescue from apoptosis. These data demonstrate that while phosphorylation of βc tyrosine residues 577 and 612 are important for optimal cell proliferation, rescue from apoptosis can be mediated by alternative signalling routes apparently independent of PKB or ERK activation. [ABSTRACT FROM AUTHOR]