The RAX/PACT-PKR stress response pathway promotes p53 sumoylation and activation, leading to G₁ arrest

Protein kinase double-stranded RNA-dependent (PKR) in antiviral defence in fish and mammals

The interferon-inducible double-stranded RNA-dependent protein kinase (PKR) is one of the key antiviral arms of the innate immune system. Upon binding of viral double stranded RNA, a viral Pattern Associated Molecular Pattern (PAMP), PKR gets activated and phosphorylates the eukaryotic initiation factor 2α (eIF2α) resulting in a protein shut-down that limits viral replication. Since its discovery in the mid-seventies, PKR has been shown to be involved in multiple important cellular processes including apoptosis, proinflammatory and innate immune responses. Viral subversion mechanisms of PKR underline its importance in the antiviral response of the host. PKR activation pathways and its mechanisms of action were previously identified and characterised mostly in mammalian models. However, fish Pkr and fish-specific paralogue Z-DNA-dependent protein kinase (Pkz) also play key role in antiviral defence. This review gives an update on the current knowledge on fish Pkr/Pkz, their conditions of activation and their implication in the immune responses to viruses, in comparison to their mammalian counterparts.

Luteolin protects DYT-PRKRA cells from apoptosis by suppressing PKR activation

DYT-PRKRA is a movement disorder caused by mutations in the PRKRA gene, which encodes for PACT, the protein activator of interferon-induced, double-stranded RNA (dsRNA)-activated protein kinase PKR. PACT brings about PKR's catalytic activation by a direct binding in response to stress signals and activated PKR phosphorylates the translation initiation factor eIF2α. Phosphorylation of eIF2α is the central regulatory event that is part of the integrated stress response (ISR), an evolutionarily conserved intracellular signaling network essential for adapting to environmental stresses to maintain healthy cells. A dysregulation of either the level or the duration of eIF2α phosphorylation in response to stress signals causes the normally pro-survival ISR to become pro-apoptotic. Our research has established that the PRKRA mutations reported to cause DYT-PRKRA lead to enhanced PACT-PKR interactions causing a dysregulation of ISR and an increased sensitivity to apoptosis. We have previously identified luteolin, a plant flavonoid, as an inhibitor of the PACT-PKR interaction using high-throughput screening of chemical libraries. Our results presented in this study indicate that luteolin is markedly effective in disrupting the pathological PACT-PKR interactions to protect DYT-PRKRA cells against apoptosis, thus suggesting a therapeutic option for using luteolin to treat DYT-PRKRA and possibly other diseases resulting from enhanced PACT-PKR interactions.

PACT promotes the metastasis of basal-like breast cancer through Rac1 SUMOylation and activation

Most basal-like breast cancers (BLBCs) are triple-negative breast cancers (TNBCs), which is associated with high malignancy, high rate of recurrence and distant metastasis, and poor prognosis among all types of breast cancer. However, there are currently no effective therapies for BLBC. Furthermore, chemoresistance limits the therapeutic options for BLBC treatment. In this study, we screen out protein activator of the interferon-induced protein kinase (PACT) as an essential gene in BLBC metastasis. We find that high PACT expression level was associated with poor prognosis among BLBC patients. In vivo and in vitro investigations indicated that PACT could regulate BLBC metastasis by interacting with SUMO-conjugating enzyme Ubc9 to stimulate the SUMOylation and thus consequently the activation of Rac1. BLBC patients receiving chemotherapy presents poorer prognosis with PACT high expression, and PACT disruption sensitizes experimental mammary tumor metastases to chemotherapy, thus providing insights to consider PACT as a potential therapeutic target to overcome acquired chemoresistance in BLBC.

Long non-coding RNA DARS-AS1 promotes tumor progression by directly suppressing PACT-mediated cellular stress

Cancer cells evolve various mechanisms to overcome cellular stresses and maintain progression. Protein kinase R (PKR) and its protein activator (PACT) are the initial responders in monitoring diverse stress signals and lead to inhibition of cell proliferation and cell apoptosis in consequence. However, the regulation of PACT-PKR pathway in cancer cells remains largely unknown. Herein, we identify that the long non-coding RNA (lncRNA) aspartyl-tRNA synthetase antisense RNA 1 (DARS-AS1) is directly involved in the inhibition of the PACT-PKR pathway and promotes the proliferation of cancer cells. Using large-scale CRISPRi functional screening of 971 cancer-associated lncRNAs, we find that DARS-AS1 is associated with significantly enhanced proliferation of cancer cells. Accordingly, knocking down DARS-AS1 inhibits cell proliferation of multiple cancer cell lines and promotes cancer cell apoptosis in vitro and significantly reduces tumor growth in vivo. Mechanistically, DARS-AS1 directly binds to the activator domain of PACT and prevents PACT-PKR interaction, thereby decreasing PKR activation, eIF2α phosphorylation and inhibiting apoptotic cell death. Clinically, DARS-AS1 is broadly expressed across multiple cancers and the increased expression of this lncRNA indicates poor prognosis. This study elucidates the lncRNA DARS-AS1 directed cancer-specific modulation of the PACT-PKR pathway and provides another target for cancer prognosis and therapeutic treatment.

A loss-of-function screen reveals a role for lncRNA DARS-AS1 in promoting cancer cell proliferation and further experiments shows DARS-AS1 regulates the PACT-PKR pathway, overall suggesting it as a potential target for cancer therapy and prognosis.

Inhibiting the aberrant PACT-p53 axis activation ameliorates spinal cord ischaemia-reperfusion injury in rats

Spinal cord ischaemia-reperfusion injury (SCII) induces multiple molecular and cellular changes, resulting in dyskinesia. Recently, it is reported that the p53 network plays a vital role in SCII. However, the roles of the PACT/PRKRA (interferon-inducible double-stranded RNA-dependent protein kinase activator A)-p53 axis in SCII are still unclear. The aim of this study was to elucidate the roles of the PACT-p53 axis in SCII. A Sprague-Dawley rat model of SCII was established by subjecting rats to a 14-min occlusion of the aortic arch. The Tarlov criteria, Western blotting, double immunofluorescence staining, haematoxylin and eosin (HE) staining, and transferase dUTP nick end labelling (TUNEL) assay were performed after SCII. Here, spinal cord ischaemia-reperfusion (SCI) caused hindlimb motor functional deficits as assessed by the Tarlov criteria. The protein expression of PACT was substantially upregulated at 48 h after SCII. Increased PACT fluorescence was mainly localized to neurons. Si-PACT pretreatment improved hindlimb motor function, ameliorated histological changes, and attenuated cell apoptosis after SCII. Si-PACT pretreatment reduced the protein expression of PACT, p53, Caspase-8 and IL-1β and the number of double-labelled PACT and p53. Taken together, inhibiting the aberrant PACT-p53 axis activation by si-PACT pretreatment ameliorates SCI-induced neuroapoptosis and neuroinflammation in rats. Silencing PACT expression is promising new therapeutic strategy for SCII.

Cell Death and the p53 Enigma During Mammalian Embryonic Development

Twelve forms of programmed cell death (PCD) have been described in mammalian cells, but which of them occurs during embryonic development and the role played by the p53 transcription factor and tumor suppressor remains enigmatic. Although p53 is not required for mouse embryonic development, some studies conclude that PCD in pluripotent embryonic stem cells from mice (mESCs) or humans (hESCs) is p53-dependent whereas others conclude that it is not. Given the importance of pluripotent stem cells as models of embryonic development and their applications in regenerative medicine, resolving this enigma is essential. This review reconciles contradictory results based on the facts that p53 cannot induce lethality in mice until gastrulation and that experimental conditions could account for differences in results with ESCs. Consequently, activation of the G2-checkpoint in mouse ESCs is p53-independent and generally, if not always, results in noncanonical apoptosis. Once initiated, PCD occurs at equivalent rates and to equivalent extents regardless of the presence or absence of p53. However, depending on experimental conditions, p53 can accelerate initiation of PCD in ESCs and late-stage blastocysts. In contrast, DNA damage following differentiation of ESCs in vitro or formation of embryonic fibroblasts in vivo induces p53-dependent cell cycle arrest and senescence.

Identification of key genes and pathways at the downstream of S100PBP in pancreatic cancer cells by integrated bioinformatical analysis

Background

The aim of the present study was to identify key genes and pathways downstream of S100PPBP in pancreatic cancer cells.

Methods

The microarray datasets GSE35196 (S100PBP knockdown) and GSE35198 (S100PBP overexpression) were downloaded from the Gene Expression Omnibus (GEO). Differentially expressed genes (DEGs) were obtained separately from GEO2R, and heatmaps showing clustering analysis of DEGs were generated using R software. Gene Ontology and pathway enrichment analyses were performed for identified DEGs using the Database for Annotation, Visualization, and Integrated Discovery and Kyoto Encyclopedia of Genes and Genomes, respectively. A protein-protein interaction (PPI) network was created using the Search Tool for the Retrieval of Interacting Genes and Cytoscape software. Relevant expression datasets of key identified genes were downloaded from The Cancer Genome Atlas, and overall survival (OS) analysis was performed with R software. Finally, Gene Expression Profiling Interactive Analysis was used to evaluate the expression of key DEGs in pancreatic cancer tissues.

Results

A total of 34 DEGs (11 upregulated and 23 downregulated) were screened out from the two datasets. Gene Ontology enrichment analysis revealed that the identified DEGs were mainly functionally enriched in ATPase activity, production of siRNA involved in RNA interference, and production of miRNAs involved in gene silencing by miRNA. The pathway enrichment analysis of the identified DEGs showed enrichment mainly in apoptosis, non-homologous end-joining, and miRNA pathways in cancer. The protein–protein interaction network was composed of 21 nodes and 30 edges. After survival analysis and gene expression analysis, 4 genes associated with poor prognosis were selected, including LMNB1, PRKRA, SEPT2, and XRCC5.

Conclusions

LMNB1, PRKRA, SEPT2, and XRCC5 could be key downstream genes of the S100PBP gene in the inhibition of pancreatic cancer cell adhesion.

Inhibition of PKR by Viruses

Cells respond to viral infections through sensors that detect non-self-molecules, and through effectors, which can have direct antiviral activities or adapt cell physiology to limit viral infection and propagation. Eukaryotic translation initiation factor 2 alpha kinase 2, better known as PKR, acts as both a sensor and an effector in the response to viral infections. After sensing double-stranded RNA molecules in infected cells, PKR self-activates and majorly exerts its antiviral function by blocking the translation machinery and inducing apoptosis. The antiviral potency of PKR is emphasized by the number of strategies developed by viruses to antagonize the PKR pathway. In this review, we present an update on the diversity of such strategies, which range from preventing double-stranded RNA recognition upstream from PKR activation, to activating eIF2B downstream from PKR targets.

Resveratrol inhibits neural apoptosis and regulates RAX/P-PKR expression in retina of diabetic rats

PURPOSE

This study was to investigate the effect of resveratrol (RSV) administration on diabetes-induced neural apoptosis and on RNA-dependent-protein-kinase (PKR)-associated protein X (RAX), PKR and phosphorylated PKR (P-PKR) expression and distribution in retina of diabetic rats.

METHODS

Retina was obtained from normal and diabetic Sprague-Dawley rats with or without RSV (5 and 10 mg/kg/d) treatment at 30-, 32-, 34- and 36-weeks. Apoptosis of retinal neural cells and distribution of RAX/P-PKR was assessed by TUNEL and immunofluorescence methods. Expression of RAX, PKR and P-PKR was evaluated by qRT-PCR and western-blotting methods.

RESULTS

Our study showed that the TUNEL-positive cells were mainly localized in ganglion cells layer (GCL), inner nuclear layer (INL) and outer nuclear layer (ONL) of the diabetic rat's retina at 30-, 32-, 34- and 36-weeks. RSV administration effectively suppressed the neural apoptosis in GCL, INL and ONL. Almost no TUNEL-positive cells were observed in retina of normal control and RSV-treated normal control rats. Our study also showed that the expression level of RAX, P-PKR in diabetic rats retina at 30-, 32-, 34-, and 36-weeks was elevated. With supplementation of 5 and 10 mg/kg/d RSV, the expression level of RAX and P-PKR was decreased (P < 0.05). The expression level of RAX and P-PKR in the retina of normal control rats was not altered by RSV. The expression level of PKR was not altered by streptozotocin injection and RSV treatment.

CONCLUSIONS

Our results suggested that RSV attenuates retinal neural apoptosis in diabetic rats retina may be via regulation RAX/P-PKR expression.

Developmental Acquisition of p53 Functions

Remarkably, the p53 transcription factor, referred to as “the guardian of the genome”, is not essential for mammalian development. Moreover, efforts to identify p53-dependent developmental events have produced contradictory conclusions. Given the importance of pluripotent stem cells as models of mammalian development, and their applications in regenerative medicine and disease, resolving these conflicts is essential. Here we attempt to reconcile disparate data into justifiable conclusions predicated on reports that p53-dependent transcription is first detected in late mouse blastocysts, that p53 activity first becomes potentially lethal during gastrulation, and that apoptosis does not depend on p53. Furthermore, p53 does not regulate expression of genes required for pluripotency in embryonic stem cells (ESCs); it contributes to ESC genomic stability and differentiation. Depending on conditions, p53 accelerates initiation of apoptosis in ESCs in response to DNA damage, but cell cycle arrest as well as the rate and extent of apoptosis in ESCs are p53-independent. In embryonic fibroblasts, p53 induces cell cycle arrest to allow repair of DNA damage, and cell senescence to prevent proliferation of cells with extensive damage.

Endogenous Double-Stranded RNA

The birth of long non-coding RNAs (lncRNAs) is closely associated with the presence and activation of repetitive elements in the genome. The transcription of endogenous retroviruses as well as long and short interspersed elements is not only essential for evolving lncRNAs but is also a significant source of double-stranded RNA (dsRNA). From an lncRNA-centric point of view, the latter is a minor source of bother in the context of the entire cell; however, dsRNA is an essential threat. A viral infection is associated with cytoplasmic dsRNA, and endogenous RNA hybrids only differ from viral dsRNA by the 5' cap structure. Hence, a multi-layered defense network is in place to protect cells from viral infections but tolerates endogenous dsRNA structures. A first line of defense is established with compartmentalization; whereas endogenous dsRNA is found predominantly confined to the nucleus and the mitochondria, exogenous dsRNA reaches the cytoplasm. Here, various sensor proteins recognize features of dsRNA including the 5' phosphate group of viral RNAs or hybrids with a particular length but not specific nucleotide sequences. The sensors trigger cellular stress pathways and innate immunity via interferon signaling but also induce apoptosis via caspase activation. Because of its central role in viral recognition and immune activation, dsRNA sensing is implicated in autoimmune diseases and used to treat cancer.

Dystonia 16 (DYT16) mutations in PACT cause dysregulated PKR activation and eIF2α signaling leading to a compromised stress response

Dystonia 16 (DYT16) is caused by mutations in PACT, the protein activator of interferon-induced double-stranded RNA-activated protein kinase (PKR). PKR regulates the integrated stress response (ISR) via phosphorylation of the translation initiation factor eIF2α. This post-translational modification attenuates general protein synthesis while concomitantly triggering enhanced translation of a few specific transcripts leading either to recovery and homeostasis or cellular apoptosis depending on the intensity and duration of stress signals. PKR plays a regulatory role in determining the cellular response to viral infections, oxidative stress, endoplasmic reticulum (ER) stress, and growth factor deprivation. In the absence of stress, both PACT and PKR are bound by their inhibitor transactivation RNA-binding protein (TRBP) thereby keeping PKR inactive. Under conditions of cellular stress these inhibitory interactions dissociate facilitating PACT-PACT interactions critical for PKR activation. While both PACT-TRBP and PKR-TRBP interactions are pro-survival, PACT-PACT and PACT-PKR interactions are pro-apoptotic. In this study we evaluate if five DYT16 substitution mutations alter PKR activation and ISR. Our results indicate that the mutant DYT16 proteins show stronger PACT-PACT interactions and enhanced PKR activation. In DYT16 patient derived lymphoblasts the enhanced PACT-PKR interactions and heightened PKR activation leads to a dysregulation of ISR and increased apoptosis. More importantly, this enhanced sensitivity to ER stress can be rescued by luteolin, which disrupts PACT-PKR interactions. Our results not only demonstrate the impact of DYT16 mutations on regulation of ISR and DYT16 etiology but indicate that therapeutic interventions could be possible after a further evaluation of such strategies.

The Integrated Stress Response and Phosphorylated Eukaryotic Initiation Factor 2α in Neurodegeneration

The proposed molecular mechanisms underlying neurodegenerative pathogenesis are varied, precluding the development of effective therapies for these increasingly prevalent disorders. One of the most consistent observations across neurodegenerative diseases is the phosphorylation of eukaryotic initiation factor 2α (eIF2α). eIF2α is a translation initiation factor, involved in cap-dependent protein translation, which when phosphorylated causes global translation attenuation. eIF2α phosphorylation is mediated by 4 kinases, which, together with their downstream signaling cascades, constitute the integrated stress response (ISR). While the ISR is activated by stresses commonly observed in neurodegeneration, such as oxidative stress, endoplasmic reticulum stress, and inflammation, it is a canonically adaptive signaling cascade. However, chronic activation of the ISR can contribute to neurodegenerative phenotypes such as neuronal death, memory impairments, and protein aggregation via apoptotic induction and other maladaptive outcomes downstream of phospho-eIF2α-mediated translation inhibition, including neuroinflammation and altered amyloidogenic processing, plausibly in a feed-forward manner. This review examines evidence that dysregulated eIF2a phosphorylation acts as a driver of neurodegeneration, including a survey of observations of ISR signaling in human disease, inspection of the overlap between ISR signaling and neurodegenerative phenomenon, and assessment of recent encouraging findings ameliorating neurodegeneration using developing pharmacological agents which target the ISR. In doing so, gaps in the field, including crosstalk of the ISR kinases and consideration of ISR signaling in nonneuronal central nervous system cell types, are highlighted.

Glycogen synthase kinase (GSK)-3 and the double-strand RNA-dependent kinase, PKR: When two kinases for the common good turn bad

Glycogen synthase kinase (GSK)-3α/β and the double-stranded RNA-dependent kinase PKR are two sentinel kinases that carry-out multiple similar yet distinct functions in both the cytosol and the nucleus. While these kinases belong to separate signal transduction cascades, they demonstrate an uncanny propensity to regulate many of the same proteins either through direct phosphorylation or by altering transcription/translation, including: c-MYC, NF-κB, p53 and TAU, as well as each another. A significant number of studies centered on the GSK3 kinases have led to the identification of the GSK3 interactome and a number of substrates, which link GSK3 activity to metabolic control, translation, RNA splicing, ribosome biogenesis, cellular division, DNA repair and stress/inflammatory signaling. Interestingly, many of these same pathways and processes are controlled by PKR, but unlike the GSK3 kinases, a clear picture of proteins interacting with PKR and a complete listing of its substrates is still missing. In this review, we take a detailed look at what is known about the PKR and GSK3 kinases, how these kinases interact to influence common cellular processes (innate immunity, alternative splicing, translation, glucose metabolism) and how aberrant activation of these kinases leads to diseases such as Alzheimer's disease (AD), diabetes mellitus (DM) and cancer.

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GSK3α/β and PKR are major regulators of cellular homeostasis and the response to stress/inflammation and infection.

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GSK3α/β and PKR interact with and/or modify many of the same proteins and affect the expression of similar genes.

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A balance between AKT and PKR nuclear signaling may be responsible for regulating the activation of nuclear GSK3β.

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GSK3α/β- and PKR-dependent signaling influence major molecular mechanisms of the cell through similar intermediates.

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Aberrant activation of GSK3α/β and PKR is highly involved in cancer, metabolic disorders, and neurodegenerative diseases.

PACT/PRKRA and p53 regulate transcriptional activity of DMRT1

The transcription factor DMRT1 (doublesex and mab-3 related transcription factor) has two distinct functions, somatic-cell masculinization and germ-cell development in some vertebrate species, including mouse and the African clawed frog Xenopus laevis. However, its transcriptional regulation remains unclear. We tried to identify DMRT1-interacting proteins from X. laevis testes by immunoprecipitation with an anti-DMRT1 antibody and MS/MS analysis, and selected three proteins, including PACT/PRKRA (Interferon-inducible double-stranded RNA dependent protein kinase activator A) derived from testes. Next, we examined the effects of PACT/PRKRA and/or p53 on the transcriptional activity of DMRT1. In transfected 293T cells, PACT/PRKRA and p53 significantly enhanced and repressed DMRT1-driven luciferase activity, respectively. We also observed that the enhanced activity by PACT/PRKRA was strongly attenuated by p53. Moreover, in situ hybridization analysis of Pact/Prkra mRNA in tadpole gonads indicated high expression in female and male germline stem cells. Taken together, these findings suggest that PACT/PRKRA and p53 might positively and negatively regulate the activity of DMRT1, respectively, for germline stem cell fate.

eIF2α promotes vascular remodeling via autophagy in monocrotaline-induced pulmonary arterial hypertension rats

PURPOSE

Eukaryotic initiation factor 2α (eIF2α) plays important roles in the proliferation and survival of pulmonary artery smooth muscle cells (PASMCs) in animal hypoxia-induced pulmonary hypertension models. However, the underlying mechanism remains unknown at large. Autophagy has been reported to play a key role in the vascular remodeling in pulmonary arterial hypertension (PAH). The purposes of this study are to determine the functions of eIF2α and autophagy in the vascular remodeling of the monocrotaline-induced PAH rats and to clarify the correlation between eIF2α and autophagy.

METHODS

We established a rat model of monocrotaline-induced PAH, and we established a cell model of platelet derived growth factor (PDGF)-induced PASMCs proliferation. The vascular morphology and the expression of eIF2α, LC3B, and p62 were assessed in the pulmonary arterial tissue of Sprague-Dawleyrats and PDGF-induced PASMCs.

RESULTS

Autophagy was significantly active in monocrotaline model group (MCT)-induced PAH rats, which obviously promotes vascular remodeling in MCT-induced PAH rats. Furthermore, the proliferation of PASMCs was induced by PDGF in vitro. The expression of LC3B, eIF2α was increased in the PDGF-induced PASMCs proliferation, and the expression of p62 was reduced in the PDGF-induced PASMCs proliferation. Moreover, eIF2α siRNA downregulated the expression of eIF2α and LC3B, and upregulated the expression of p62 in PDGF-induced PASMCs proliferation. eIF2α siRNA inhibited the PDGF-induced PASMCs proliferation. Finally, chloroquine can upregulate the protein expression of LC3B and p62, it also can inhibit proliferation in PDGF-induced PASMCs.

CONCLUSION

Based on these observations, we conclude that eIF2α promotes the proliferation of PASMCs and vascular remodeling in monocrotaline-induced PAH rats through accelerating autophagy pathway.

Expression of the double-stranded RNA-dependent kinase PKR influences osteosarcoma attachment independent growth, migration, and invasion

Osteosarcoma (OS) is a rare, insidious tumor of mesenchymal origin that most often affects children, adolescents, and young adults. While the primary tumor can be controlled with chemotherapy and surgery, it is the lung metastases that are eventually fatal. Multiple studies into the initial drivers of OS development have been undertaken, but few of these have examined innate immune/inflammatory signaling. A central figure in inflammatory signaling is the innate immune/stress-activated kinase double-stranded RNA-dependent protein kinase (PKR). To characterize the role of PKR in OS, U2OS, and SaOS-2 osteosarcoma cell lines were stably transfected with wild-type or dominant-negative (DN) PKR. Overexpression of PKR enhanced colony formation in soft agar (U2OS and SaOS-2), enhanced cellular migration (U2OS), and invasive migration (SaOS-2). In contrast, overexpression of DN-PKR inhibited attachment-independent growth, migration and/or invasion. These data demonstrate a role for inflammatory signaling in OS formation and migration/invasion and suggest the status of PKR expression/activation may have prognostic value.

Clinical and therapeutic potential of protein kinase PKR in cancer and metabolism

The protein kinase R (PKR, also called EIF2AK2) is an interferon-inducible double-stranded RNA protein kinase with multiple effects on cells that plays an active part in the cellular response to numerous types of stress. PKR has been extensively studied and documented for its relevance as an antiviral agent and a cell growth regulator. Recently, the role of PKR related to metabolism, inflammatory processes, cancer and neurodegenerative diseases has gained interest. In this review, we summarise and discuss the involvement of PKR in several cancer signalling pathways and the dual role that this kinase plays in cancer disease. We emphasise the importance of PKR as a molecular target for both conventional chemotherapeutics and emerging treatments based on novel drugs, and its potential as a biomarker and therapeutic target for several pathologies. Finally, we discuss the impact that the recent knowledge regarding PKR involvement in metabolism has in our understanding of the complex processes of cancer and metabolism pathologies, highlighting the translational research establishing the clinical and therapeutic potential of this pleiotropic kinase.

The protein activator of protein kinase R, PACT/RAX, negatively regulates protein kinase R during mouse anterior pituitary development

The murine double-stranded RNA-binding protein termed protein kinase R (PKR)-associated protein X (RAX) and the human homolog, protein activator of PKR (PACT), were originally characterized as activators of PKR. Mice deficient in RAX show reproductive and developmental defects, including reduced body size, craniofacial defects and anterior pituitary hypoplasia. As these defects are not observed in PKR-deficient mice, the phenotype has been attributed to PKR-independent activities of RAX. Here we further investigated the involvement of PKR in the physiological function of RAX, by generating rax(-/-) mice deficient in PKR, or carrying a kinase-inactive mutant of PKR (K271R) or an unphosphorylatable mutant of the PKR substrate eukaryotic translation initiation factor 2 α subunit (eIF2α) (S51A). Ablating PKR expression rescued the developmental and reproductive deficiencies in rax(-/-) mice. Generating rax(-/-) mice with a kinase-inactive mutant of PKR resulted in similar rescue, confirming that the rax(-/-) defects are PKR dependent; specifically that the kinase activity of PKR was required for these defects. Moreover, generating rax(-/-) mice that were heterozygous for an unphosphorylatable mutant eIF2α provides partial rescue of the rax(-/-) defect, consistent with mutation of one copy of the Eif2s1 gene. These observations were further investigated in vitro by reducing RAX expression in anterior pituitary cells, resulting in increased PKR activity and induction of the PKR-regulated cyclin-dependent kinase inhibitor p21(WAF1/CIP1). These results demonstrate that PKR kinase activity is required for onset of the rax(-/-) phenotype, implying an unexpected function for RAX as a negative regulator of PKR in the context of postnatal anterior pituitary tissue, and identify a critical role for the regulation of PKR activity for normal development.

Activated PKR inhibits pancreatic β-cell proliferation through sumoylation-dependent stabilization of P53

Double-stranded RNA-dependent protein kinase (PKR) is intimately involved in type 2 diabetes due to its role in insulin resistance in peripheral tissues and anti-proliferative effect on pancreatic β-cells. Activated PKR was found to inhibit β-cell proliferation, partially through accumulation of P53. However the molecular mechanisms underlying PKR-dependent upregulation of P53 remain unknown. The results of the present study showed that PKR can be specifically activated in PKR overexpressing β-cells by a low dosage of the previously synthesized compound 1H-benzimidazole1-ethanol,2,3-dihydro-2-imino-a-(phenoxymethyl)-3-(phenylmethyl)-,monohydrochloride (BEPP), and this led to upregulation of P53 through sumoylation-dependent stability. Activated PKR was found to interact with sumo-conjugating enzyme Ubc9, and P53 sumoylation relies on a PKR-Ubc9 protein-protein interaction. Additionally, a ceramide signal was needed for PKR activation to be triggered by glucolipotoxicity and TNFα stimulation, and stabilization of P53 required endogenous ceramide accumulation. Glucolipotoxicity and pro-inflammatory cytokines therefore promote the sumoylation-dependent stability of P53 via the ceramide/PKR/Ubc9 signalling pathway that is involved in pancreatic β-cell proliferation inhibition in the development of type 2 diabetes.

PKR inhibits the DNA damage response, and is associated with poor survival in AML and accelerated leukemia in NHD13 mice

Increased expression of the interferon-inducible double-stranded RNA-activated protein kinase (PKR) has been reported in acute leukemia and solid tumors, but the role of PKR has been unclear. Now, our results indicate that high PKR expression in CD34(+) cells of acute myeloid leukemia (AML) patients correlates with worse survival and shortened remission duration. Significantly, we find that PKR has a novel and previously unrecognized nuclear function to inhibit DNA damage response signaling and double-strand break repair. Nuclear PKR antagonizes ataxia-telangiectasia mutated (ATM) activation by a mechanism dependent on protein phosphatase 2A activity. Thus, inhibition of PKR expression or activity promotes ATM activation, γ-H2AX formation, and phosphorylation of NBS1 following ionizing irradiation. PKR transgenic but not PKR null mice demonstrate a mutator phenotype characterized by radiation-induced and age-associated genomic instability that was partially reversed by short-term pharmacologic PKR inhibition. Furthermore, the age-associated accumulation of somatic mutations that occurs in the Nup98-HOXD13 (NHD13) mouse model of leukemia progression was significantly elevated by co-expression of a PKR transgene, whereas knockout of PKR expression or pharmacologic inhibition of PKR activity reduced the frequency of spontaneous mutations in vivo. Thus, PKR cooperated with the NHD13 transgene to accelerate leukemia progression and shorten survival. Taken together, these results indicate that increased nuclear PKR has an oncogenic function that promotes the accumulation of potentially deleterious mutations. Thus, PKR inhibition may be a therapeutically useful strategy to prevent leukemia progression or relapse, and improve clinical outcomes.

A Critical Role of the mTOR/eIF2α Pathway in Hypoxia-Induced Pulmonary Hypertension

Enhanced proliferation of pulmonary arterial vascular smooth muscle cells (PASMCs) is a key pathological component of vascular remodeling in hypoxia-induced pulmonary hypertension (HPH). Mammalian targeting of rapamycin (mTOR) signaling has been shown to play a role in protein translation and participate in the progression of pulmonary hypertension. Eukaryotic translation initiation factor-2α (eIF2α) is a key factor in regulation of cell growth and cell cycle, but its role in mTOR signaling and PASMCs proliferation remains unknown. Pulmonary hypertension (PH) rat model was established by hypoxia. Rapamycin was used to treat rats as an mTOR inhibitor. Proliferation of primarily cultured rat PASMCs was induced by hypoxia, rapamycin and siRNA of mTOR and eIF2α were used in loss-of-function studies. The expression and activation of eIF2α, mTOR and c-myc were analyzed. Results showed that mTOR/eIF2α signaling was significantly activated in pulmonary arteries from hypoxia exposed rats and PASMCs cultured under hypoxia condition. Treatment with mTOR inhibitor for 21 days attenuated vascular remodeling, suppressed mTOR and eIF2α activation, inhibited c-myc expression in HPH rats. In hypoxia-induced PASMCs, rapamycin and knockdown of mTOR and eIF2α by siRNA significantly abolished proliferation and increased c-myc expression. These results suggest a critical role of the mTOR/eIF2αpathway in hypoxic vascular remodeling and PASMCs proliferation of HPH.

Protein-Binding Function of RNA-Dependent Protein Kinase Promotes Proliferation through TRAF2/RIP1/NF-κB/c-Myc Pathway in Pancreatic β cells

Double-stranded RNA-dependent protein kinase (PKR), an intracellular pathogen recognition receptor, is involved both in insulin resistance in peripheral tissues and in downregulation of pancreatic β-cell function in a kinase-dependent manner, indicating PKR as a core component in the progression of type 2 diabetes. PKR also acts as an adaptor protein via its protein-binding domain. Here, the PKR protein-binding function promoted β-cell proliferation without its kinase activity, which is associated with enhanced physical interaction with tumor necrosis factor receptor-associated factor 2 (TRAF2) and TRAF6. In addition, the transcription of the nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB)-dependent survival gene c-Myc was upregulated significantly and is necessary for proliferation. Upregulation of the PKR protein-binding function induced the NF-κB pathway, as observed by dose-dependent degradation of IκBα, induced nuclear translocation of p65 and elevated NF-κB-dependent reporter gene expression. NF-κB-dependent reporter activity and β-cell proliferation both were suppressed by TRAF2-siRNA, but not by TRAF6-siRNA. TRAF2-siRNA blocked the ubiquitination of receptor-interacting serine/threonine-protein kinase 1 (RIP1) induced by PKR protein binding. Furthermore, RIP1-siRNA inhibited β-cell proliferation. Proinflammatory cytokines (TNFα) and glucolipitoxicity also promoted the physical interaction of PKR with TRAF2. Collectively, these data indicate a pivotal role for PKR's protein-binding function on the proliferation of pancreatic β cells through TRAF2/RIP1/NF-κB/c-Myc pathways. Therapeutic opportunities for type 2 diabetes may arise when its kinase catalytic function, but not its protein-binding function, is downregulated.

dsRNA binding protein PACT/RAX in gene silencing, development and diseases

PACT (Protein kinase, interferon-inducible double stranded RNA dependent activator) and its murine ortholog RAX (PKR-associated protein X) were originally identified as a protein activator for the dsRNA-dependent, interferon-inducible protein kinase (PKR). Endogenous PACT/RAX activates PKR in response to diverse stress signals such as serum starvation, and peroxide or arsenite treatment. PACT/RAX heterodimerized with PKR and activated it with its third motif in the absence of dsRNA. The activation of PKR leads to enhanced eIF2α phosphorylation followed by apoptosis or inhibition of growth. Besides the role of activating PKR, PACT is associated with a ~500 kDa complex that contains Dicer, hAgo2, and TRBP (TAR RNA binding protein) and it associates with Dicer to facilitate the production of small interfering RNA. PACT/RAX plays an important role in diverse physiological and pathological processes. Pact^-/- mice exhibit notable developmental abnormalities including microtia, with craniofacial ear, and hearing defects. Pact^-/- mice had smaller body sizes and fertility defects, both of which were caused by defective pituitary functions. It was found that dRAX disrupted fly embryos homozygous, displayed highly abnormal commissural axon structure of the central nervous system, and 70% of the flies homozygous for the mutant allele died prior to adulthood. Using high density SNP genotyping arrays, it was found that a mutation in PRKRA (the PACT/RAX gene) is the causative genetic mutation in DYT16, a novel autosomal recessive dystonia-parkinsonism syndrome in Brazilian patients.

PKR is activated by cellular dsRNAs during mitosis and acts as a mitotic regulator

dsRNA-dependent protein kinase R (PKR) plays a key role in innate immunity. PKR binds viral dsRNA and undergoes autophosphorylation, which leads to translational repression and signaling pathway modulation in infected cells. Kim et al. now show that PKR is activated during mitosis in uninfected cells. PKR interacts with dsRNAs formed by inverted Alu repeats, which become accessible to PKR during mitosis. Phosphorylated PKR then suppresses translation and coordinates mitosis. This study unveils a novel function of PKR and endogenous dsRNA mitosis in uninfected cells.

dsRNA-dependent protein kinase R (PKR) is a ubiquitously expressed enzyme well known for its roles in immune response. Upon binding to viral dsRNA, PKR undergoes autophosphorylation, and the phosphorylated PKR (pPKR) regulates translation and multiple signaling pathways in infected cells. Here, we found that PKR is activated in uninfected cells, specifically during mitosis, by binding to dsRNAs formed by inverted Alu repeats (IRAlus). While PKR and IRAlu-containing RNAs are segregated in the cytosol and nucleus of interphase cells, respectively, they interact during mitosis when nuclear structure is disrupted. Once phosphorylated, PKR suppresses global translation by phosphorylating the α subunit of eukaryotic initiation factor 2 (eIF2α). In addition, pPKR acts as an upstream kinase for c-Jun N-terminal kinase and regulates the levels of multiple mitotic factors such as CYCLINS A and B and POLO-LIKE KINASE 1 and phosphorylation of HISTONE H3. Disruption of PKR activation via RNAi or expression of a transdominant-negative mutant leads to misregulation of the mitotic factors, delay in mitotic progression, and defects in cytokinesis. Our study unveils a novel function of PKR and endogenous dsRNAs as signaling molecules during the mitosis of uninfected cells.

HIV-1 translation and its regulation by cellular factors PKR and PACT

The synthesis of proteins from viral mRNA is the first step towards viral assembly. Viruses are dependent upon the cellular translation machinery to synthesize their own proteins. The synthesis of proteins from the human immunodeficiency virus (HIV) type 1 and 2 RNAs utilize several alternative mechanisms. The regulation of viral protein production requires a constant interplay between viral requirements and the cell response to viral infection. Among the antiviral cell responses, the interferon-induced RNA activated protein kinase, PKR, regulates the cellular and viral translation. During HIV-1 infection, PKR activation is highly regulated by viral and cellular factors. The cellular TAR RNA Binding Protein, TRBP, the Adenosine Deaminase acting on RNA, ADAR1, and the PKR Activator, PACT, play important roles. Recent data show that PACT changes its function from activator to inhibitor in HIV-1 infected cells. Therefore, HIV-1 has evolved to replicate in cells in which TRBP, ADAR1 and PACT prevent PKR activation to allow efficient viral protein synthesis. This proper translation will initiate the assembly of viral particles.

SIM-dependent enhancement of substrate-specific SUMOylation by a ubiquitin ligase in vitro

The present study demonstrates that the Saccharomyces cerevisiae ubiquitin ligase Rad18 can unexpectedly catalyse conjugation of SUMO to its target, PCNA, with high selectivity under in vitro conditions by means of a SUMO-interacting motif that is normally involved in substrate recognition.

Sumoylation in gene regulation, human disease, and therapeutic action

Similar to ubiquitination, sumoylation covalently attaches a small ubiquitin-like modifier (SUMO) protein (92-97 amino acids) to the ε-amino group of a lysine residue. This is quite different from the classically defined post-translational modifications, such as phosphorylation, acetylation, and methylation, which typically add a small chemical group to the targeted residue. Sumoylation has been well studied at the molecular and cellular levels, focusing mostly on site-specific conjugation of human SUMO1, SUMO2, and SUMO3, as well as their homologues in various species. In this short review, we will discuss some recent examples to highlight (a) emerging trends about the coordinated regulation of sumoylation and other post-translational modifications in modulating the function of some transcription factors and pathway-specific regulators, (b) diverse roles of sumoylation in gene regulation implicated in stem cells and different pathogenic conditions, and (c) potential therapeutic strategies related to some of the diseases stated above.

PKR negatively regulates leukemia progression in association with PP2A activation, Bcl-2 inhibition and increased apoptosis

Reduced expression and activity of the proapoptotic, double-stranded RNA-dependent protein kinase, PKR (protein kinase R) is observed in breast, lung and various leukemias, suggesting that loss of PKR potentiates transformation. Now we report that decreased PKR activity inhibits chemotherapy-induced apoptosis of leukemia cells both in vitro and in vivo. Inhibition of PKR expression or activity reduces protein phosphatase 2A (PP2A) activity, a B-cell lymphoma 2 (Bcl-2) phosphatase, resulting in enhanced Bcl-2 phosphorylation. Thus, inhibition of PKR activity leads to hyperphosphorylation of Bcl-2, stabilization of Bcl-2/Bax interaction and decreased Bax insertion into the outer mitochondrial membrane. Treatment with the PP2A activator, FTY720, restores Bcl-2 dephosphorylation and apoptosis in cells with reduced PKR expression following stress. Significantly, xenografts of REH leukemic cells with reduced PKR display significantly increased tumor volume, increased resistance to doxorubicin treatment and shorter survival. Importantly, FTY720 treatment restores sensitivity to chemotherapy and prolongs overall survival of these mice. Collectively, these findings suggest that PP2A activation is a downstream target of PKR and the PKR/PP2A signaling axis is required for rapid and potent stress-induced apoptosis. Importantly, loss of PKR promotes leukemia progression and may serve as a biomarker for predicting chemosensitivity.

SUMOylation of p53 mediates interferon activities

There is growing evidence that many host proteins involved in innate and intrinsic immunity are regulated by SUMOylation, and that SUMO contributes to the regulatory process that governs the initiation of the type I interferon (IFN) response. The tumor suppressor p53 is a modulator of the IFN response that plays a role in virus-induced apoptosis and in IFN-induced senescence. Here we demonstrate that IFN treatment increases the levels of SUMOylated p53 and induces cellular senescence through a process that is partially dependent upon SUMOylation of p53. Similarly, we show that vesicular stomatitis virus (VSV) infection induces p53 SUMOylation, and that this modification favors the control of VSV replication. Thus, our study provides evidence that IFN signaling induces p53 SUMOylation, which results in the activation of a cellular senescence program and contributes to the antiviral functions of interferon.

HIPK1 drives p53 activation to limit colorectal cancer cell growth

HIPK1 (homeodomain interacting protein kinase 1) is a serine/threonine kinase that belongs to the CMGC superfamily. Emerging data point to the role of HIPK1 in cancer, but it is still not clear whether it acts as a tumor suppressor or promoter. Here we identified HIPK1 as a kinase that is significantly overexpressed in colorectal cancer (CRC) and whose expression is stage-dependent. Being abundantly expressed at the onset of the disease, the HIPK1 level gradually decreased as tumor stage progressed. To further uncover how this factor regulates tumorigenesis and establish whether it constitutes an early factor necessary for neoplastic transformation or for cellular defense, we studied the effect of its overexpression in vitro by investigating various cancer-related signaling cascades. We found that HIPK1 mostly regulates the p53 signaling pathway both in HCT116 and HeLa cells. By phosphorylating p53 on its serine-15, HIPK1 favored its transactivation potential, which led to a rise in p21 protein level and a decline in cell proliferation. Assuming that HIPK1 could impede CRC growth by turning on the p53/p21 pathway, we then checked p21 mRNA levels in patients. Interestingly, p21 transcripts were only increased in a subset of patients expressing high levels of HIPK1. Unlike the rest of the cohort, the majority of these patients hosted a native p53 protein, meaning that such a pro-survival pathway (HIPK1+ > p53 > p21) is active in patients, and that HIPK1 acts rather as a tumor suppressor.

STAT1 requirement for PKR-induced cell cycle arrest in vascular smooth muscle cells in response to heparin

Interferons (IFNs) are a family of cytokines that exhibit antiviral, antiproliferative, and immunomodulatory properties. PKR (protein kinase, RNA activated) is of central importance in mediating the antiproliferative actions of IFNs. Our research has established that PKR inhibits vascular smooth muscle cell (VSMC) proliferation by regulating G1 to S transition. Many cardiovascular diseases result from complications of atherosclerosis, a chronic and progressive inflammatory condition often characterized by excessive proliferation of VSMC. Thus, an effective method for inhibiting VSMC proliferation is likely to arrest atherosclerosis and restenosis at early stages. Our research establishes that PKR activation in VSMC leads to a G1 arrest brought about by an inhibition of cyclin-dependent kinase 2 (Cdk2) activity by p27(kip1). In quiescent VSMC, p27(kip1) levels are high and when stimulated by serum/growth factors, p27(kip1) levels drop by destabilization of the protein. Under conditions that lead to activation of PKR, there is a marked inhibition of p27(kip1) down-regulation due to increased stability of p27(kip1) protein. In order to understand the mechanism of heparin-induced stabilization of p27(kip1) in VSMC, we examined the involvement of the Signal Transducer and Activator of Transcription-1 (STAT1), which is an important player in mediating antiproliferative effects of IFNs. Our results demonstrate that PKR overexpression in VSMC leads to an increase in p27(kip1) protein levels and this increase requires the catalytic activity of PKR. PKR activation induced by antiproliferative agent heparin leads to phosphorylation of STAT1 on serine 727, which is essential for the cell cycle block. STAT1 null VSMCs are largely defective in heparin-induced cell cycle arrest and in PKR null cells the STAT1 phosphorylation in response to heparin was absent. These results establish that heparin causes STAT1 phosphorylation on serine 727 via activation of PKR and that this event is required for the G1 arrest in VSMC.

PKR regulates proliferation, differentiation, and survival of murine hematopoietic stem/progenitor cells

Protein kinase R (PKR) is an interferon (IFN)-inducible, double-stranded RNA-activated kinase that initiates apoptosis in response to cellular stress. To determine the role of PKR in hematopoiesis, we developed transgenic mouse models that express either human PKR (TgPKR) or a dominant-negative PKR (TgDNPKR) mutant specifically in hematopoietic tissues. Significantly, peripheral blood counts from TgPKR mice decrease with age in association with dysplastic marrow changes. TgPKR mice have reduced colony-forming capacity and the colonies also are more sensitive to hematopoietic stresses. Furthermore, TgPKR mice have fewer hematopoietic stem/progenitor cells (HSPCs), and the percentage of quiescent (G0) HSPCs is increased. Importantly, treatment of TgPKR bone marrow (BM) with a PKR inhibitor specifically rescues sensitivity to growth factor deprivation. In contrast, marrow from PKR knockout (PKRKO) mice has increased potential for colony formation and HSPCs are more actively proliferating and resistant to stress. Significantly, TgPKR HSPCs have increased expression of p21 and IFN regulatory factor, whereas cells from PKRKO mice display mechanisms indicative of proliferation such as reduced eukaryotic initiation factor 2α phosphorylation, increased extracellular signal-regulated protein kinases 1 and 2 phosphorylation, and increased CDK2 expression. Collectively, data reveal that PKR is an unrecognized but important regulator of HSPC cell fate and may play a role in the pathogenesis of BM failure.

dsRNA-dependent protein kinase PKR and its role in stress, signaling and HCV infection

The double-stranded RNA-dependent protein kinase PKR plays multiple roles in cells, in response to different stress situations. As a member of the interferon (IFN)‑Stimulated Genes, PKR was initially recognized as an actor in the antiviral action of IFN, due to its ability to control translation, through phosphorylation, of the alpha subunit of eukaryotic initiation factor 2 (eIF2α). As such, PKR participates in the generation of stress granules, or autophagy and a number of viruses have designed strategies to inhibit its action. However, PKR deficient mice resist most viral infections, indicating that PKR may play other roles in the cell other than just acting as an antiviral agent. Indeed, PKR regulates several signaling pathways, either as an adapter protein and/or using its kinase activity. Here we review the role of PKR as an eIF2α kinase, its participation in the regulation of the NF-κB, p38MAPK and insulin pathways, and we focus on its role during infection with the hepatitis C virus (HCV). PKR binds the HCV IRES RNA, cooperates with some functions of the HCV core protein and may represent a target for NS5A or E2. Novel data points out for a role of PKR as a pro-HCV agent, both as an adapter protein and as an eIF2α-kinase, and in cooperation with the di-ubiquitin-like protein ISG15. Developing pharmaceutical inhibitors of PKR may help in resolving some viral infections as well as stress-related damages.

Increased expression of the dsRNA-activated protein kinase PKR in breast cancer promotes sensitivity to doxorubicin

It has been reported that the expression and activity of the interferon-inducible, dsRNA-dependent protein kinase, PKR, is increased in mammary carcinoma cell lines and primary tumor samples. To extend these findings and determine how PKR signaling may affect breast cancer cell sensitivity to chemotherapy, we measured PKR expression by immunohistochemical staining of 538 cases of primary breast cancer and normal tissues. Significantly, PKR expression was elevated in ductal, lobular and squamous cell carcinomas or lymph node metastases but not in either benign tumor specimens or cases of inflammation compared to normal tissues. Furthermore, PKR expression was increased in precancerous stages of mammary cell hyperplasia and dysplasia compared to normal tissues, indicating that PKR expression may be upregulated by the process of tumorigenesis. To test the function of PKR in breast cancer, we generated MCF7, T-47D and MDA-MB-231 breast cancer cell lines with significantly reduced PKR expression by siRNA knockdown. Importantly, while knockdown of PKR expression had no effect on cell proliferation under normal growth conditions, MCF7, T-47D or MDA-MB-231 cells with reduced PKR expression or treated with a small molecule PKR inhibitor were significantly less sensitive to doxorubicin or H₂O₂-induced toxicity compared to control cells. In addition, the rate of eIF2α phosphorylation following treatment with doxorubicin was delayed in breast cancer cell lines with decreased PKR expression. Significantly, treatment of breast cancer lines with reduced PKR expression with either interferon-α, which increases PKR expression, or salubrinal, which increases eIF2α phosphorylation, restored doxorubicin sensitivity to normal levels. Taken together these results indicate that increased PKR expression in primary breast cancer tissues may serve as a biomarker for response to doxorubicin-containing chemotherapy and that future therapeutic approaches to promote PKR expression/activation and eIF2α phosphorylation may be beneficial for the treatment of breast cancer.

DBC1 does not function as a negative regulator of SIRT1 in liver cancer

The putative tumor suppressor, DBC1 (deleted in breast cancer-1), was recently found to negatively regulate SIRT1 in vitro and in vivo, but the mechanism whereby DBC1 regulates SIRT1 in liver cancer remains to be elucidated. In this study, it was found that although the expression of DBC1 and SIRT1 was not aberrantly regulated in a large cohort of human hepatocellular carcinoma (HCC) patients, these proteins were highly overexpressed in a subset of HCC tissues compared with surrounding non-cancer tissues. In liver cancer, DBC1 and SIRT1 were found to be positively correlated. Inactivation of DBC1 or SIRT1 reduced SNU-182 (a liver cancer cell line) proliferation as determined by MTT viability assays. Notably, although DBC1 functions as a negative regulator of SIRT1 in A549 lung cancer cells since it suppresses the deacetylase activity of the p53 protein, it did not affect the p53 deacetylase activity of SIRT1 in SNU-182 cells. Taken together, we conclude that DBC1 is associated with SIRT1 in HCC, but that it does not inhibit SIRT1.