S6K1- and βTRCP-mediated degradation of PDCD4 promotes protein translation and cell growth

University of Virginia, Charlottesville, Virginia, United States
Science (Impact Factor: 31.48). 11/2006; 314(5798):467-71. DOI: 10.1126/science.1130276
Source: PubMed

ABSTRACT The tumor suppressor programmed cell death protein 4 (PDCD4) inhibits the translation initiation factor eIF4A, an RNA helicase
that catalyzes the unwinding of secondary structure at the 5′ untranslated region (5′UTR) of messenger RNAs (mRNAs). In response
to mitogens, PDCD4 was rapidly phosphorylated on Ser67 by the protein kinase S6K1 and subsequently degraded via the ubiquitin ligase SCFβTRCP. Expression in cultured cells of a stable PDCD4 mutant that is unable to bind βTRCP inhibited translation of an mRNA with
a structured 5′UTR, resulted in smaller cell size, and slowed down cell cycle progression. We propose that regulated degradation
of PDCD4 in response to mitogens allows efficient protein synthesis and consequently cell growth.

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    • "Therefore, we examined how PDCD4 could be regulated by ethanol in cortical neuroblasts. mTOR/p70S6Kinase-mediated phosphorylation of PDCD4 results in degradation of PDCD4 by the ubiquitin ligase βTRCP [50]. Since PDCD4 protein is a target for degradation, we first investigated whether ETOH-induced increase in PDCD4 protein levels is due to protein stability using cycloheximide (CHX) experiments. "
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    ABSTRACT: Ingestion of ethanol (ETOH) during pregnancy induces grave abnormalities in developing fetal brain. We have previously reported that ETOH induces programmed cell death 4 (PDCD4), a critical regulator of cell growth, in cultured fetal cerebral cortical neurons (PCNs) and in the cerebral cortex in vivo and affect protein synthesis as observed in Fetal Alcohol Spectrum Disorder (FASD). However, the mechanism which activates PDCD4 in neuronal systems is unclear and understanding this regulation may provide a counteractive strategy to correct the protein synthesis associated developmental changes seen in FASD. The present study investigates the molecular mechanism by which ethanol regulates PDCD4 in cortical neuroblasts, the immediate precursor of neurons. ETOH treatment significantly increased PDCD4 protein and transcript expression in spontaneously immortalized rat brain neuroblasts. Since PDCD4 is regulated at both the post-translational and post-transcriptional level, we assessed ETOH's effect on PDCD4 protein and mRNA stability. Chase experiments demonstrated that ETOH does not significantly impact either PDCD4 protein or mRNA stabilization. PDCD4 promoter-reporter assays confirmed that PDCD4 is transcriptionally regulated by ETOH in neuroblasts. Given a critical role of glycogen synthase kinase 3β (GSK-3β) signaling in regulating protein synthesis and neurotoxic mechanisms, we investigated the involvement of GSK-3β and showed that multifunctional GSK-3β was significantly activated in response to ETOH in neuroblasts. In addition, we found that ETOH-induced activation of PDCD4 was inhibited by pharmacologic blockade of GSK-3β using inhibitors, lithium chloride (LiCl) and SB-216763 or siRNA mediated silencing of GSK-3β. These results suggest that ethanol transcriptionally upregulates PDCD4 by enhancing GSK-3β signaling in cortical neuroblasts. Further, we demonstrate that canonical Wnt-3a/GSK-3β signaling is involved in regulating PDCD4 protein expression. Altogether, we provide evidence that GSK-3β/PDCD4 network may represent a critical modulatory point to manage the protein synthetic anomalies and growth aberrations of neural cells seen in FASD.
    PLoS ONE 05/2014; 9(5):e98080. DOI:10.1371/journal.pone.0098080 · 3.23 Impact Factor
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    • "A study of human T98G cells showed that the presence of serum rapidly phosphorylated PDCD4 on Ser67 by the protein kinase p70-S6K and subsequently degraded it via the ubiquitin ligase SCFβ-TrCP [18], which was confirmed in HEK293 cells exposed to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate [19]. In this study, PDCD4 protein level was completely rescued under pulsatile shear stress with use of the proteasome inhibitors MG-132 and lactacystin, which suggests that degradation of PDCD4 protein via proteasomes might be the most important path through which pulsatile shear stress negatively regulates PDCD4. "
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    ABSTRACT: We recently found low level of tumor suppressor programmed cell death 4 (PDCD4) associated with reduced atherosclerotic plaque area (unpublished). We investigated whether atheroprotective unidirectional pulsatile shear stress affects the expression of PDCD4 in endothelial cells. En face co-immunostaining of the mouse aortic arch revealed a low level of PDCD4 in endothelial cells undergoing pulsatile shear stress. Application of unidirectional pulsatile shear stress to human umbilical vein endothelial cells (HUVECs) decreased PDCD4 protein but not mRNA level. Immunoprecipitation revealed that pulsatile shear stress induced the coupling of ubiquitin with PDCD4 expression. The phosphatidyl inositol 3-kinase (PI3K)/Akt pathway was involved in this ubiquitin-proteasome-mediated degradation of PDCD4. Gain of function and loss of function experiments showed that PDCD4 induced turnover (proliferation and apoptosis) of HUVECs. Low PDCD4 level was associated with reduced proliferation but not apoptosis or phosphorylation of endothelial nitric oxide synthase caused by pulsatile shear stress to help maintain the homeostasis of endothelial cells. Pulsatile shear stress induces ubiquitin-proteasome-mediated degradation of PDCD4 via a PI3K/Akt pathway in HUVECs. PDCD4 induces turnover (proliferation and apoptosis) of HUVECs. Low PDCD4 level is associated with reduced proliferation for maintenance of HUVEC homeostasis under pulsatile shear stress.
    PLoS ONE 03/2014; 9(3):e91564. DOI:10.1371/journal.pone.0091564 · 3.23 Impact Factor
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    • "One mechanism by which eIF4A may be limiting for eIF4F assembly is via its association with PDCD4 - a tumor suppressor gene product whose association with eIF4A is under PI3K/mTOR regulation [28]. Activation of the PI3K/mTOR pathway has been reported to lead to phosphorylation and degradation of PDCD4 in a proteasome-dependent manner with a concomitant increase in eIF4A availability [28]. However, PDCD4 levels do not significantly change during C2C12 differentiation (Fig. 1B) making it unlikely that destabilization of PDCD4 is responsible for the increase in eIF4AIIc that we observed upon activation of myogenesis. "
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    ABSTRACT: Gene expression during muscle cell differentiation is tightly regulated at multiple levels, including translation initiation. The PI3K/mTOR signalling pathway exerts control over protein synthesis by regulating assembly of eukaryotic initiation factor (eIF) 4F, a heterotrimeric complex that stimulates recruitment of ribosomes to mRNA templates. One of the subunits of eIF4F, eIF4A, supplies essential helicase function during this phase of translation. The presence of two cellular eIF4A isoforms, eIF4AI and eIF4AII, has long thought to impart equivalent functions to eIF4F. However, recent experiments have alluded to distinct activities between them. Herein, we characterize distinct regulatory mechanisms between the eIF4A isoforms during muscle cell differentiation. We find that eIF4AI levels decrease during differentiation whereas eIF4AII levels increase during myofiber formation in a MyoD-dependent manner. This study characterizes a previously undefined mechanism for eIF4AII regulation in differentiation and highlights functional differences between eIF4AI and eIF4AII. Finally, RNAi-mediated alterations in eIF4AI and eIF4AII levels indicate that the myogenic process can tolerate short term reductions in eIF4AI or eIF4AII levels, but not both.
    PLoS ONE 01/2014; 9(1):e87237. DOI:10.1371/journal.pone.0087237 · 3.23 Impact Factor
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