Andreas I. Papadakis

The eIF2α Kinases PERK and PKR Activate Glycogen Synthase Kinase 3 to Promote the Proteasomal Degradation of p53

Phosphorylation of eukaryotic initiation factor 2α (eIF2α) is mediated by a family of kinases that respond to various forms of environmental stress. The eIF2α kinases are critical for mRNA translation, cell proliferation, and apoptosis. Activation of the tumor suppressor p53 results in cell cycle arrest and apoptosis in response to various types of stress. We previously showed that, unlike the majority of stress responses that stabilize and activate p53, induction of endoplasmic reticulum stress leads to p53 degradation through an Mdm2-dependent mechanism. Here, we demonstrate that the endoplasmic reticulum-resident eIF2α kinase PERK mediates the proteasomal degradation of p53 independently of translational control. This role is not specific for PERK, because the eIF2α kinase PKR also promotes p53 degradation in response to double-stranded RNA. We further establish that the eIF2α kinases induce glycogen synthase kinase 3 to promote the nuclear export and proteasomal degradation of p53. Our findings reveal a novel cross-talk between the eIF2α kinases and p53 with implications in cell proliferation and tumorigenesis.

PKR and PKR-like Endoplasmic Reticulum Kinase Induce the Proteasome-dependent Degradation of Cyclin D1 via a Mechanism Requiring Eukaryotic Initiation Factor 2α Phosphorylation

Cyclin D1 plays a critical role in controlling the G1/S transition via the regulation of cyclin-dependent kinase activity. Several studies have indicated that cyclin D1 translation is decreased upon activation of the eukaryotic initiation factor 2α (eIF2α) kinases. We examined the effect of activation of the eIF2α kinases PKR and PKR-like endoplasmic reticulum kinase (PERK) on cyclin D1 protein levels and translation and determined that cyclin D1 protein levels decrease upon the induction of PKR and PERK catalytic activity but that this decrease is not due to translation. Inhibition of the 26 S proteasome with MG132 rescued cyclin D1 protein levels, indicating that rather than inhibiting translation, PKR and PERK act to increase cyclin D1 degradation. Interestingly, this effect still requires eIF2α phosphorylation at serine 51, as cyclin D1 remains unaffected in cells containing a non-phosphorylatable form of the protein. This proteasome-dependent degradation of cyclin D1 requires an intact ubiquitination pathway, although the ubiquitination of cyclin D1 is not itself affected. Furthermore, this degradation is independent of phosphorylation of cyclin D1 at threonine 286, which is mediated by the glycogen synthase kinase 3β and mitogen-activated protein kinase pathways as described in previous studies. Our study reveals a novel functional cross-talk between eIF2α phosphorylation and the proteasomal degradation of cyclin D1 and that this degradation is dependent upon eIF2α phosphorylation during short, but not prolonged, periods of stress.

Phosphorylation of eIF2α at Serine 51 Is an Important Determinant of Cell Survival and Adaptation to Glucose Deficiency

Glucose deficiency leads to the induction of eIF2α phosphorylation at serine 51, which results in a global inhibition of protein synthesis. Phosphorylation of eIF2α is an adaptive process that establishes a cytoprotective state in glucose-deficient cells, with possible implications in biological responses that interfere with glucose metabolism.

PKR and PERK induce the proteasome-dependent degradation of cyclin D1 via a mechanism requiring eIF2α phosphorylation

Cyclin D1 plays a critical role in controlling the G1/S transition via the regulation of cyclin-dependent kinase activity. Several studies have indicated that cyclin D1 translation is decreased upon activation of the eukaryotic initiation factor 2α (eIF2α) kinases. We examined the effect of activation of the eIF2α kinases PKR and PKR-like endoplasmic reticulum kinase (PERK) on cyclin D1 protein levels and translation and determined that cyclin D1 protein levels decrease upon the induction of PKR and PERK catalytic activity but that this decrease is not due to translation. Inhibition of the 26 S proteasome with MG132 rescued cyclin D1 protein levels, indicating that rather than inhibiting translation, PKR and PERK act to increase cyclin D1 degradation. Interestingly, this effect still requires eIF2α phosphorylation at serine 51, as cyclin D1 remains unaffected in cells containing a non-phosphorylatable form of the protein. This proteasome-dependent degradation of cyclin D1 requires an intact ubiquitination pathway, although the ubiquitination of cyclin D1 is not itself affected. Furthermore, this degradation is independent of phosphorylation of cyclin D1 at threonine 286, which is mediated by the glycogen synthase kinase 3β and mitogen-activated protein kinase pathways as described in previous studies. Our study reveals a novel functional cross-talk between eIF2α phosphorylation and the proteasomal degradation of cyclin D1 and that this degradation is dependent upon eIF2α phosphorylation during short, but not prolonged, periods of stress.

Phosphorylation of the translation initiation factor eIF2α at serine 51 determines the cell fate decisions of Akt in response to oxidative stress

Phosphorylation of the α subunit of the translation initiation factor eIF2 at serine 51 (eIF2αP) is a master regulator of cell adaptation to various forms of stress with implications in antitumor treatments with chemotherapeutic drugs. Herein, we demonstrate that genetic loss of the eIF2α kinases PERK and GCN2 or impaired eIF2αP by genetic means renders immortalized mouse fibroblasts as well as human tumor cells increasingly susceptible to death by oxidative stress. We also show that eIF2αP facilitates Akt activation in cells subjected to oxidative insults. However, whereas Akt activation has a pro-survival role in eIF2αP-proficient cells, the lesser amount of activated Akt in eIF2αP-deficient cells promotes death. At the molecular level, we demonstrate that eIF2αP acts through an ATF4-independent mechanism to control Akt activity via the regulation of mTORC1. Specifically, eIF2αP downregulates mTORC1 activity, which in turn relieves the feedback inhibition of PI3K resulting in the upregulation of the mTORC2-Akt arm. Inhibition of mTORC1 by rapamycin restores Akt activity in eIF2αP-deficient cells but renders them highly susceptible to Akt-mediated death by oxidative stress. Our data demonstrate that eIF2αP acts as a molecular switch that dictates either cell survival or death by activated Akt in response to oxidative stress. Hence, we propose that inactivation of eIF2αP may be a suitable approach to unleash the killing power of Akt in tumor cells treated with pro-oxidant drugs.

Doxorubicin bypasses the cytoprotective effects of eIF2α phosphorylation and promotes PKR-mediated cell death.

The eukaryotic cell responds to various forms of environmental stress by adjusting the rates of mRNA translation thus facilitating adaptation to the assaulting stress. One of the major pathways that control protein synthesis involves the phosphorylation of the α-subunit of eukaryotic initiation factor eIF2 at serine 51. Different forms of DNA damage were shown to induce eIF2α phosphorylation by using PERK, GCN2 or PKR. However, the specificity of the eIF2α kinases and the biological role of eIF2α phosphorylation pathway in the DNA damage response (DDR) induced by chemotherapeutics are not known. Herein, we show that PKR is the eIF2α kinase that responds to DDR induced by doxorubicin. We show that activation of PKR integrates two signaling pathways with opposing biological outcomes. More specifically, induction of eIF2α phosphorylation has a cytoprotective role, whereas activation of c-jun N-terminal kinase (JNK) by PKR promotes cell death in response to doxorubicin. We further show that the proapoptotic effects of JNK activation prevail over the cytoprotection mediated by eIF2α phosphorylation. These findings reveal that PKR can be an important inducer of cell death in response to chemotherapies through its ability to act independently of eIF2α phosphorylation.

eIF2α Kinase PKR Modulates the Hypoxic Response by Stat3-Dependent Transcriptional Suppression of HIF-1α

Hypoxia within the tumor microenvironment promotes angiogenesis, metabolic reprogramming, and tumor progression. In addition to activating hypoxia-inducible factor-1α (HIF-1α), cells also respond to hypoxia by globally inhibiting protein synthesis via serine 51 phosphorylation of translation eukaryotic initiation factor 2α (eIF2α). In this study, we investigated potential roles for stress-activated eIF2α kinases in regulation of HIF-1α. Our investigations revealed that the double-stranded RNA-dependent protein kinase R (PKR) plays a significant role in suppressing HIF-1α expression, acting specifically at the level of transcription. HIF-1α transcriptional repression by PKR was sufficient to impair the hypoxia-induced accumulation of HIF-1α and transcriptional induction of HIF-1α-dependent target genes. Inhibition of HIF-1A transcription by PKR was independent of eIF2α phosphorylation but dependent on inhibition of the signal transducer and activator of transcription 3 (Stat3). Furthermore, HIF-1A repression required the T-cell protein tyrosine phosphatase, which acts downstream of PKR, to suppress Stat3. Our findings reveal a novel tumor suppressor function for PKR, which inhibits HIF-1α expression through Stat3 but is independent of eIF2α phosphorylation.

SMARCE1 suppresses EGFR expression and controls responses to MET and ALK inhibitors in lung cancer

Recurrent inactivating mutations in components of SWI/SNF chromatin-remodeling complexes have been identified across cancer types, supporting their roles as tumor suppressors in modulating oncogenic signaling pathways. We report here that SMARCE1 loss induces EGFR expression and confers resistance to MET and ALK inhibitors in non-small cell lung cancers (NSCLCs). We found that SMARCE1 binds to regulatory regions of the EGFR locus and suppresses EGFR transcription in part through regulating expression of Polycomb Repressive Complex component CBX2. Addition of the EGFR inhibitor gefitinib restores the sensitivity of SMARCE1-knockdown cells to MET and ALK inhibitors in NSCLCs. Our findings link SMARCE1 to EGFR oncogenic signaling and suggest targeted treatment options for SMARCE1-deficient tumors.

Evidence for eIF2α phosphorylation-independent effects of GSK2656157, a novel catalytic inhibitor of PERK with clinical implications.

The endoplasmic reticulum (ER)-resident protein kinase PERK is a major component of the unfolded protein response (UPR), which promotes the adaptation of cells to various forms of stress. PERK phosphorylates the α subunit of the translation initiation factor eIF2 at serine 51, a modification that plays a key role in the regulation of mRNA translation in stressed cells. Several studies have demonstrated that the PERK-eIF2α phosphorylation pathway maintains insulin biosynthesis and glucose homeostasis, facilitates tumor formation and decreases the efficacy of tumor treatment with chemotherapeutic drugs. Recently, a selective catalytic PERK inhibitor termed GSK2656157 has been developed with anti-tumor properties in mice. Herein, we provide evidence that inhibition of PERK activity by GSK2656157 does not always correlate with inhibition of eIF2α phosphorylation. Also, GSK2656157 does not always mimic the biological effects of the genetic inactivation of PERK. Furthermore, cells treated with GSK2656157 increase eIF2α phosphorylation as a means to compensate for the loss of PERK. Using human tumor cells impaired in eIF2α phosphorylation, we demonstrate that GSK2656157 induces ER stress-mediated death suggesting that the drug acts independent of the inhibition of eIF2α phosphorylation. We conclude that GSK2656157 might be a useful compound to dissect pathways that compensate for the loss of PERK and/or identify PERK pathways that are independent of eIF2α phosphorylation.

The eIF2α serine 51 phosphorylation-ATF4 arm promotes HIPPO signaling and cell death under oxidative stress

The HIPPO pathway is an evolutionary conserved regulator of organ size that controls both cell proliferation and death. This pathway has an important role in mediating cell death in response to oxidative stress through the inactivation of Yes-associated protein (YAP) and inhibition of anti-oxidant gene expression. Cells exposed to oxidative stress induce the phosphorylation of the alpha (α) subunit of the translation initiation factor eIF2 at serine 51 (eIF2αP), a modification that leads to the general inhibition of mRNA translation initiation. Under these conditions, increased eIF2αP facilitates the mRNA translation of activating transcription factor 4 (ATF4), which mediates either cell survival and adaptation or cell death under conditions of severe stress. Herein, we demonstrate a functional connection between the HIPPO and eIF2αP-ATF4 pathways under oxidative stress. We demonstrate that ATF4 promotes the stabilization of the large tumor suppressor 1 (LATS1), which inactivates YAP by phosphorylation. ATF4 inhibits the expression of NEDD4.2 and WWP1 mRNAs under pro-oxidant conditions, which encode ubiquitin ligases mediating the proteasomal degradation of LATS1. Increased LATS1 stability is required for the induction of cell death under oxidative stress. Our data reveal a previously unidentified ATF4-dependent pathway in the induction of cell death under oxidative stress via the activation of LATS1 and HIPPO pathway.