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1.  Changes in Translational Control after Pro-Apoptotic Stress 
In stressed cells, a general decrease in the rate of protein synthesis occurs due to modifications in the activity of translation initiation factors. Compelling data now indicate that these changes also permit a selective post-transcriptional expression of proteins necessary for either cell survival or completion of apoptosis when cells are exposed to severe or prolonged stress. In this review, we summarize the modifications that inhibit the activity of the main canonical translation initiation factors, and the data explaining how certain mRNAs encoding proteins involved in either cell survival or apoptosis can be selectively translated.
PMCID: PMC3565257  PMID: 23344027
translation initiation; eIF2; eIF4F; 4E-BPs; apoptosis; uORF; IRES
2.  A Switch of G Protein-Coupled Receptor Binding Preference from Phosphoinositide 3-Kinase (PI3K)–p85 to Filamin A Negatively Controls the PI3K Pathway 
Molecular and Cellular Biology  2012;32(5):1004-1016.
Frequent oncogenic alterations occur in the phosphoinositide 3-kinase (PI3K) pathway, urging identification of novel negative controls. We previously reported an original mechanism for restraining PI3K activity, controlled by the somatostatin G protein-coupled receptor (GPCR) sst2 and involving a ligand-regulated interaction between sst2 with the PI3K regulatory p85 subunit. We here identify the scaffolding protein filamin A (FLNA) as a critical player regulating the dynamic of this complex. A preexisting sst2-p85 complex, which was shown to account for a significant basal PI3K activity in the absence of ligand, is disrupted upon sst2 activation. FLNA was here identified as a competitor of p85 for direct binding to two juxtaposed sites on sst2. Switching of GPCR binding preference from p85 toward FLNA is determined by changes in the tyrosine phosphorylation of p85- and FLNA-binding sites on sst2 upon activation. It results in the disruption of the sst2-p85 complex and the subsequent inhibition of PI3K. Knocking down FLNA expression, or abrogating FLNA recruitment to sst2, reversed the inhibition of PI3K and of tumor growth induced by sst2. Importantly, we report that this FLNA inhibitory control on PI3K can be generalized to another GPCR, the mu opioid receptor, thereby providing an unprecedented mechanism underlying GPCR-negative control on PI3K.
PMCID: PMC3295190  PMID: 22203038
3.  NAD(P)H Quinone-Oxydoreductase 1 Protects Eukaryotic Translation Initiation Factor 4GI from Degradation by the Proteasome ▿  
Molecular and Cellular Biology  2009;30(4):1097-1105.
The eukaryotic translation initiation factor 4GI (eIF4GI) serves as a central adapter in cap-binding complex assembly. Although eIF4GI has been shown to be sensitive to proteasomal degradation, how the eIF4GI steady-state level is controlled remains unknown. Here, we show that eIF4GI exists in a complex with NAD(P)H quinone-oxydoreductase 1 (NQO1) in cell extracts. Treatment of cells with dicumarol (dicoumarol), a pharmacological inhibitor of NQO1 known to preclude NQO1 binding to its protein partners, provokes eIF4GI degradation by the proteasome. Consistently, the eIF4GI steady-state level also diminishes upon the silencing of NQO1 (by transfection with small interfering RNA), while eIF4GI accumulates upon the overexpression of NQO1 (by transfection with cDNA). We further reveal that treatment of cells with dicumarol frees eIF4GI from mRNA translation initiation complexes due to strong activation of its natural competitor, the translational repressor 4E-BP1. As a consequence of cap-binding complex dissociation and eIF4GI degradation, protein synthesis is dramatically inhibited. Finally, we show that the regulation of eIF4GI stability by the proteasome may be prominent under oxidative stress. Our findings assign NQO1 an original role in the regulation of mRNA translation via the control of eIF4GI stability by the proteasome.
PMCID: PMC2815573  PMID: 20028737
4.  Direct regulation of pituitary proopiomelanocortin by STAT3 provides a novel mechanism for immuno-neuroendocrine interfacing 
Journal of Clinical Investigation  2000;106(11):1417-1425.
Neuroendocrine ACTH secretion responds to peripheral inflammatory and stress signals. We previously demonstrated that the proinflammatory cytokine, leukemia inhibitory factor (LIF), affects the hypothalamo-pituitary-adrenal axis (HPA) by stimulating in vitro and in vivo pituitary proopiomelanocortin (POMC) gene expression and ACTH secretion and by potentiating the action of hypothalamic corticotropin releasing hormone (CRH). Whereas pathways shown thus far to regulate POMC expression exclusively involve cAMP or calcium, we here describe a direct and indirect STAT3-dependent regulation of POMC transcription by LIF. Using progressive 5′-deletions of POMC promoter, we identified a LIF-responsive –407/–301 region that contains two juxtaposed sequences within –399/–379 related to a STAT3 DNA-binding motif. Each sequence within –399/–379 separately corresponds to a low-affinity and direct binding site for STAT3, but, in combination, these sequences bind STAT3 cooperatively and with high affinity. Moreover, LIF-activated STAT3 indirectly mediates LIF corticotroph action by inducing and potentiating CRH-induced c-fos and JunB expression and binding to the POMC AP-1 element. We therefore conclude that both a direct and indirect route mediate LIF-induced STAT3 activation of POMC transcription. Demonstration of STAT3-dependent regulation of the POMC gene represents a powerful mechanism for immuno-neuroendocrine interfacing and implies a direct stimulation of ACTH secretion by inflammatory and stress-derived STAT3-inducing cytokines.
PMCID: PMC381469  PMID: 11104795
5.  Inhibitory roles for SHP-1 and SOCS-3 following pituitary proopiomelanocortin induction by leukemia inhibitory factor 
Journal of Clinical Investigation  1999;104(9):1277-1285.
Leukemia inhibitory factor (LIF) is a pleiotropic cytokine that stimulates the hypothalamo-pituitary-adrenal (HPA) axis through JAK-STAT activation. We show here that LIF-induced JAK2 and STAT3 tyrosine phosphorylation is transient, disappearing within 20 and 40 minutes, respectively. LIF activates the SH2 domain–containing tyrosine phosphatase, SHP-1, with maximal stimulation observed at 30 minutes. SHP-1 is constitutively associated with JAK2, and LIF induces recruitment of phosphorylated STAT3 to this complex. Overexpression of wild-type or dominant negative forms of SHP-1 shows decreased or increased LIF-induced proopiomelanocortin (POMC) promoter activity, respectively. LIF-induced JAK2 and STAT3 dephosphorylation is delayed until after 60 minutes in cells that overexpress the mutant SHP-1. In addition, SOCS-3, a negative regulator of LIF signaling, binds to JAK2 after 60 minutes of LIF stimulation, after which the complex is degraded by the proteasome. SOCS-3 overexpression blocks LIF-induced JAK2 tyrosine phosphorylation, confirming a role for SOCS-3 in deactivating JAK2 by direct association. Using SOCS-3 fusion proteins, we also define regions of the SOCS-3 protein that are critical for inhibition of LIF-induced POMC promoter activity. Corticotrophic signaling by LIF is thus subject to 2 forms of negative autoregulation: dephosphorylation of JAK2 and STAT3 by the SHP-1 tyrosine phosphatase, and SOCS-3–dependent inactivation of JAK2.
PMCID: PMC409825  PMID: 10545526

Results 1-5 (5)