Novel insights in the interplay between inflammation and metabolic diseases: A role for the pathogen sensing kinase PKR
Authors: Nicolas Marsollier, Pascal Ferré, Fabienne Foufelle
Journal: Journal of Hepatology (2011)

Double-stranded RNA-dependent protein kinase links pathogen sensing with stress and metabolic homeostasis. Nakamura T, Furuhashi M, Li P, Cao H, Tuncman G, Sonenberg N, Gorgun CZ, Hotamisligil GS. Cell 140, 338–348, February 5, 2010. Copyright 2010. Abstract reprinted with permission from Elsevier.http://www.ncbi.nlm.nih.gov/pubmed/20144759Abstract: As chronic inflammation is a hallmark of obesity, pathways that integrate nutrient- and pathogen sensing pathways are of great interest in understanding the mechanisms of insulin resistance, type 2 diabetes, and other chronic metabolic pathologies. Here, Nakamura et al. provide evidence that double-stranded RNA-dependent protein kinase (PKR) can respond to nutrient signals as well as endoplasmic reticulum (ER) stress and coordinate the activity of other critical inflammatory kinases such as the c-Jun N-terminal kinase (JNK) to regulate insulin action and metabolism. PKR also directly targets and modifies insulin receptor substrate and hence integrates nutrients and insulin action with a defined pathogen response system. Dietary and genetic obesity features marked activation of PKR in adipose and liver tissues and absence of PKR alleviates metabolic deterioration due to nutrient or energy excess in mice. These findings demonstrate PKR as a critical component of an inflammatory complex that responds to nutrients and organelle dysfunction. Double-stranded RNA-dependent protein kinase links pathogen sensing with stress and metabolic homeostasis. Nakamura T, Furuhashi M, Li P, Cao H, Tuncman G, Sonenberg N, Gorgun CZ, Hotamisligil GS. Cell 140, 338–348, February 5, 2010. Copyright 2010. Abstract reprinted with permission from Elsevier. http://www.ncbi.nlm.nih.gov/pubmed/20144759 Abstract: As chronic inflammation is a hallmark of obesity, pathways that integrate nutrient- and pathogen sensing pathways are of great interest in understanding the mechanisms of insulin resistance, type 2 diabetes, and other chronic metabolic pathologies. Here, Nakamura et al. provide evidence that double-stranded RNA-dependent protein kinase (PKR) can respond to nutrient signals as well as endoplasmic reticulum (ER) stress and coordinate the activity of other critical inflammatory kinases such as the c-Jun N-terminal kinase (JNK) to regulate insulin action and metabolism. PKR also directly targets and modifies insulin receptor substrate and hence integrates nutrients and insulin action with a defined pathogen response system. Dietary and genetic obesity features marked activation of PKR in adipose and liver tissues and absence of PKR alleviates metabolic deterioration due to nutrient or energy excess in mice. These findings demonstrate PKR as a critical component of an inflammatory complex that responds to nutrients and organelle dysfunction. In the past decades, lifestyle changes characterized by high caloric abundance and reduced physical activity have dramatically increased the incidence of obesity in Western but also in developing countries. This worldwide increase in obesity is associated with an array of health problems, including insulin resistance, type 2 diabetes, fatty liver disease, atherosclerosis, and even cancers. Numerous evidences indicate that metabolic diseases are also closely associated with a chronic ‘low grade’ inflammation characterized by abnormal cytokine production and activation of a network of inflammatory signaling pathway [[1]Hotamisligil G.S. Erbay E. Nutrient sensing and inflammation in metabolic diseases.Nat Rev Immunol. 2008; 8: 923-934Crossref PubMed Scopus (767) Google Scholar].The inflammatory signals converge to disrupt insulin action causing insulin resistance in insulin sensitive tissues, thus altering nutrient metabolism. Insulin receptor substrates (IRS), which are normally phosphorylated on tyrosine residues in order to transmit the insulin message, are phosphorylated on serine residues in inflammatory states, and this in turn abrogates the insulin signal. Among the IRS-modifying enzymes, stress activated kinases such as c-Jun N-terminal kinase (JNK), inhibitor of kappa B kinase (IKK), and protein kinase C (PKC) have been reported to inhibit insulin action by serine phosphorylation of IRS-1 [[2]Samuel V.T. Petersen K.F. Shulman G.I. Lipid-induced insulin resistance: unravelling the mechanism.Lancet. 2010; 375: 2267-2277Abstract Full Text Full Text PDF PubMed Scopus (812) Google Scholar]. This mechanism is critical in the onset of insulin resistance observed in obesity (Fig. 1). Interestingly, sepsis or severe trauma, which both present inflammatory processes, are also characterized by marked insulin resistance leading to an activation of lipolysis, of hepatic gluconeogenesis, and fatty acid oxidation [[3]Goldstein S.A. Elwyn D.H. The effects of injury and sepsis on fuel utilization.Annu Rev Nutr. 1989; 9: 445-473Crossref PubMed Scopus (76) Google Scholar]. In other words, there is a close interaction between immune response pathways through inflammatory processes and the regulation of nutrient metabolism.In the Cell issue of February 2010, Nakamura et al. identified double stranded RNA-dependent protein kinase (PKR) as a new actor that integrates inflammatory signals and lipid metabolism [[4]Nakamura T. Furuhashi M. Li P. Cao H. Tuncman G. Sonenberg N. et al.Double-stranded RNA-dependent protein kinase links pathogen sensing with stress and metabolic homeostasis.Cell. 2010; 140: 338-348Abstract Full Text Full Text PDF PubMed Scopus (374) Google Scholar]. PKR was initially identified as a trigger of cell defense against viral infection. Indeed, PKR initiates innate immune responses by limiting general protein synthesis, thus impairing the synthesis of new viral proteins [[5]Samuel C.E. The eIF-2 alpha protein kinases, regulators of translation in eukaryotes from yeasts to humans.J Biol Chem. 1993; 268: 7603-7606Abstract Full Text PDF PubMed Google Scholar]. PKR is activated by autophosphorylation due to the presence of double-stranded RNA (dsRNA) or by treatment with interferon α [[5]Samuel C.E. The eIF-2 alpha protein kinases, regulators of translation in eukaryotes from yeasts to humans.J Biol Chem. 1993; 268: 7603-7606Abstract Full Text PDF PubMed Google Scholar]. The major substrate of PKR is the eukaryotic translation initiation factor 2α (eiF2α), which regulates protein synthesis. The PKR-mediated inhibition of translation is considered as an important aspect of the antiviral activity of interferon [[6]Meurs E.F. Watanabe Y. Kadereit S. Barber G.N. Katze M.G. Chong K. et al.Constitutive expression of human double-stranded RNA-activated p68 kinase in murine cells mediates phosphorylation of eukaryotic initiation factor 2 and partial resistance to encephalomyocarditis virus growth.J Virol. 1992; 66: 5805-5814Crossref PubMed Google Scholar]. Interestingly, eiF2α phosphorylation is also a key event in the adaptation to endoplasmic reticulum stress (ER stress), another cellular stress which is activated in the tissues of obese subjects. Nakamura and colleagues show that PKR activation is observed in the liver and adipose tissue of genetically or diet-induced obese mice. Interestingly, treatment with palmitate of mouse embryo fibroblasts (MEF), as well as in vivo lipid infusion to mice, led to a significant increase in PKR activity, linking activation of PKR to nutrient excess and metabolic stress. Since JNK, which is activated by excess of lipids, ER stress, or inflammation, is a key actor of insulin resistance, the authors then addressed the potential relationship between PKR and JNK activation.Using MEF deficient in PKR activity, they demonstrate that palmitate as well as thapsigargin (an agent that causes ER stress) fails to activate JNK in the absence of PKR. Surprisingly, the RNA binding motifs of PKR are necessary for the palmitic and ER stress-induced PKR activation. However, the most striking observation is that PKR directly interacts with IRS-1 leading to its phosphorylation on Ser307. Thus PKR could induce an insulin resistance by acting directly on IRS-1 or indirectly by activation of JNK. The role for PKR in the control of insulin sensitivity is supported by further in vivo studies in PKR−/− mice which are characterized by a greater sensitivity to insulin and higher glucose disposal rate after lipid infusion, compared to control mice.In summary, Nakamura et al. demonstrated that PKR is a novel key protein at the crossroad of inflammation and metabolic pathways and could be a critical target for the treatment of metabolic diseases (Fig. 1).This study paves the way for further investigations. Palmitate induces the production of ceramides, which are known to promote insulin resistance. Ceramides are potential stimulators of PKR through the activation of the RAX protein, which associates with PKR [[7]Ruvolo P.P. Gao F. Blalock W.L. Deng X. May W.S. Ceramide regulates protein synthesis by a novel mechanism involving the cellular PKR activator RAX.J Biol Chem. 2001; 276: 11754-11758Crossref PubMed Scopus (68) Google Scholar]. Studying a role of ceramides in lipid-induced PKR activation is certainly worthwhile as well as the potential effect of other fatty acid types. Toll-Like Receptors (TLR) are known to promote both innate immunity and fatty acid-induced insulin resistance [[8]Shi H. Kokoeva M.V. Inouye K. Tzameli I. Yin H. Flier J.S. TLR4 links innate immunity and fatty acid-induced insulin resistance.J Clin Invest. 2006; 116: 3015-3025Crossref PubMed Scopus (2640) Google Scholar]. Likewise, it has been shown that TLR4 is able to activate PKR through the cellular adaptor TRIF in macrophages [[9]Hsu L.C. Park J.M. Zhang K. Luo J.L. Maeda S. Kaufman R.J. et al.The protein kinase PKR is required for macrophage apoptosis after activation of Toll-like receptor 4.Nature. 2004; 428: 341-345Crossref PubMed Scopus (317) Google Scholar]. Although, the authors rule out a role for TLR4 in palmitate-induced PKR activation, it would certainly be interesting to further study a potential involvement of other members of the TLR family.Finally, since viral infection has frequently been associated with the development of insulin resistance or insulin secretion defects, it is tempting to speculate that PKR may be the missing link between infection and dysregulation of glucose homeostasis; an activation of PKR has been observed in liver biopsies of patients with chronic hepatitis C [[10]Asselah T. Bièche I. Laurendeau I. Paradis V. Vidaud D. Degott C. et al.Liver gene expression signature of mild fibrosis in patients with chronic hepatitis C.Gastroenterology. 2005; 129: 2064-2075Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar]. It now remains to be demonstrated that this activation is responsible for the insulin resistance that is often present in VHC patients.Conflict of interestThe authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript. In the past decades, lifestyle changes characterized by high caloric abundance and reduced physical activity have dramatically increased the incidence of obesity in Western but also in developing countries. This worldwide increase in obesity is associated with an array of health problems, including insulin resistance, type 2 diabetes, fatty liver disease, atherosclerosis, and even cancers. Numerous evidences indicate that metabolic diseases are also closely associated with a chronic ‘low grade’ inflammation characterized by abnormal cytokine production and activation of a network of inflammatory signaling pathway [[1]Hotamisligil G.S. Erbay E. Nutrient sensing and inflammation in metabolic diseases.Nat Rev Immunol. 2008; 8: 923-934Crossref PubMed Scopus (767) Google Scholar]. The inflammatory signals converge to disrupt insulin action causing insulin resistance in insulin sensitive tissues, thus altering nutrient metabolism. Insulin receptor substrates (IRS), which are normally phosphorylated on tyrosine residues in order to transmit the insulin message, are phosphorylated on serine residues in inflammatory states, and this in turn abrogates the insulin signal. Among the IRS-modifying enzymes, stress activated kinases such as c-Jun N-terminal kinase (JNK), inhibitor of kappa B kinase (IKK), and protein kinase C (PKC) have been reported to inhibit insulin action by serine phosphorylation of IRS-1 [[2]Samuel V.T. Petersen K.F. Shulman G.I. Lipid-induced insulin resistance: unravelling the mechanism.Lancet. 2010; 375: 2267-2277Abstract Full Text Full Text PDF PubMed Scopus (812) Google Scholar]. This mechanism is critical in the onset of insulin resistance observed in obesity (Fig. 1). Interestingly, sepsis or severe trauma, which both present inflammatory processes, are also characterized by marked insulin resistance leading to an activation of lipolysis, of hepatic gluconeogenesis, and fatty acid oxidation [[3]Goldstein S.A. Elwyn D.H. The effects of injury and sepsis on fuel utilization.Annu Rev Nutr. 1989; 9: 445-473Crossref PubMed Scopus (76) Google Scholar]. In other words, there is a close interaction between immune response pathways through inflammatory processes and the regulation of nutrient metabolism. In the Cell issue of February 2010, Nakamura et al. identified double stranded RNA-dependent protein kinase (PKR) as a new actor that integrates inflammatory signals and lipid metabolism [[4]Nakamura T. Furuhashi M. Li P. Cao H. Tuncman G. Sonenberg N. et al.Double-stranded RNA-dependent protein kinase links pathogen sensing with stress and metabolic homeostasis.Cell. 2010; 140: 338-348Abstract Full Text Full Text PDF PubMed Scopus (374) Google Scholar]. PKR was initially identified as a trigger of cell defense against viral infection. Indeed, PKR initiates innate immune responses by limiting general protein synthesis, thus impairing the synthesis of new viral proteins [[5]Samuel C.E. The eIF-2 alpha protein kinases, regulators of translation in eukaryotes from yeasts to humans.J Biol Chem. 1993; 268: 7603-7606Abstract Full Text PDF PubMed Google Scholar]. PKR is activated by autophosphorylation due to the presence of double-stranded RNA (dsRNA) or by treatment with interferon α [[5]Samuel C.E. The eIF-2 alpha protein kinases, regulators of translation in eukaryotes from yeasts to humans.J Biol Chem. 1993; 268: 7603-7606Abstract Full Text PDF PubMed Google Scholar]. The major substrate of PKR is the eukaryotic translation initiation factor 2α (eiF2α), which regulates protein synthesis. The PKR-mediated inhibition of translation is considered as an important aspect of the antiviral activity of interferon [[6]Meurs E.F. Watanabe Y. Kadereit S. Barber G.N. Katze M.G. Chong K. et al.Constitutive expression of human double-stranded RNA-activated p68 kinase in murine cells mediates phosphorylation of eukaryotic initiation factor 2 and partial resistance to encephalomyocarditis virus growth.J Virol. 1992; 66: 5805-5814Crossref PubMed Google Scholar]. Interestingly, eiF2α phosphorylation is also a key event in the adaptation to endoplasmic reticulum stress (ER stress), another cellular stress which is activated in the tissues of obese subjects. Nakamura and colleagues show that PKR activation is observed in the liver and adipose tissue of genetically or diet-induced obese mice. Interestingly, treatment with palmitate of mouse embryo fibroblasts (MEF), as well as in vivo lipid infusion to mice, led to a significant increase in PKR activity, linking activation of PKR to nutrient excess and metabolic stress. Since JNK, which is activated by excess of lipids, ER stress, or inflammation, is a key actor of insulin resistance, the authors then addressed the potential relationship between PKR and JNK activation. Using MEF deficient in PKR activity, they demonstrate that palmitate as well as thapsigargin (an agent that causes ER stress) fails to activate JNK in the absence of PKR. Surprisingly, the RNA binding motifs of PKR are necessary for the palmitic and ER stress-induced PKR activation. However, the most striking observation is that PKR directly interacts with IRS-1 leading to its phosphorylation on Ser307. Thus PKR could induce an insulin resistance by acting directly on IRS-1 or indirectly by activation of JNK. The role for PKR in the control of insulin sensitivity is supported by further in vivo studies in PKR−/− mice which are characterized by a greater sensitivity to insulin and higher glucose disposal rate after lipid infusion, compared to control mice. In summary, Nakamura et al. demonstrated that PKR is a novel key protein at the crossroad of inflammation and metabolic pathways and could be a critical target for the treatment of metabolic diseases (Fig. 1). This study paves the way for further investigations. Palmitate induces the production of ceramides, which are known to promote insulin resistance. Ceramides are potential stimulators of PKR through the activation of the RAX protein, which associates with PKR [[7]Ruvolo P.P. Gao F. Blalock W.L. Deng X. May W.S. Ceramide regulates protein synthesis by a novel mechanism involving the cellular PKR activator RAX.J Biol Chem. 2001; 276: 11754-11758Crossref PubMed Scopus (68) Google Scholar]. Studying a role of ceramides in lipid-induced PKR activation is certainly worthwhile as well as the potential effect of other fatty acid types. Toll-Like Receptors (TLR) are known to promote both innate immunity and fatty acid-induced insulin resistance [[8]Shi H. Kokoeva M.V. Inouye K. Tzameli I. Yin H. Flier J.S. TLR4 links innate immunity and fatty acid-induced insulin resistance.J Clin Invest. 2006; 116: 3015-3025Crossref PubMed Scopus (2640) Google Scholar]. Likewise, it has been shown that TLR4 is able to activate PKR through the cellular adaptor TRIF in macrophages [[9]Hsu L.C. Park J.M. Zhang K. Luo J.L. Maeda S. Kaufman R.J. et al.The protein kinase PKR is required for macrophage apoptosis after activation of Toll-like receptor 4.Nature. 2004; 428: 341-345Crossref PubMed Scopus (317) Google Scholar]. Although, the authors rule out a role for TLR4 in palmitate-induced PKR activation, it would certainly be interesting to further study a potential involvement of other members of the TLR family. Finally, since viral infection has frequently been associated with the development of insulin resistance or insulin secretion defects, it is tempting to speculate that PKR may be the missing link between infection and dysregulation of glucose homeostasis; an activation of PKR has been observed in liver biopsies of patients with chronic hepatitis C [[10]Asselah T. Bièche I. Laurendeau I. Paradis V. Vidaud D. Degott C. et al.Liver gene expression signature of mild fibrosis in patients with chronic hepatitis C.Gastroenterology. 2005; 129: 2064-2075Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar]. It now remains to be demonstrated that this activation is responsible for the insulin resistance that is often present in VHC patients. Conflict of interestThe authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript. The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.