Mechanisms of Ageing and Development 114 (2000) 123 – 132 www.elsevier.com/locate/mechagedev Tissue specific expression of PKR protein kinase in aging B6D2F1 mice Warren Ladiges a,b,*, John Morton a, Collin Blakely c, Michael Gale c,1 a The Nathan Shock Center for Excellence in the Biology of Aging, School of Medicine, Uni6ersity of Washington, Seattle, WA, USA b Department of Comparati6e Medicine (357190), Uni6ersity of Washington, Seattle, WA 98195, USA c Department of Microbiology, School of Medicine, Uni6ersity of Washington, Seattle, WA, USA Received 5 January 2000; received in revised form 11 February 2000; accepted 11 February 2000 Abstract A decline in the rate of protein synthesis is a common biochemical change observed with aging in a wide variety of cells and organisms. The double stranded RNA-dependent protein kinase PKR has been shown to phosphorylate eukaryotic initiation factor 2 alpha (eIF-2a), a well-characterized factor for down-regulating protein synthesis, in response to environmental stress conditions. Therefore, we were interested in evaluating the role of PKR in the aging process. Tissues from 2- and 20-month-old B6D2F1 male mice were evaluated by Western blot analysis. PKR was detected in all tissues of aging mice confirming its ubiquitous nature. Tissues examined from young mice showed little evidence of PKR expression, suggesting an age-associated up-regulation. P58IPK, a cellular inhibitor of PKR, was expressed in tissues from both age groups but to a greater extent in tissues of aging mice suggesting an up-regulation to control PKR activity. Hyperphosphorylated eIF-2a was increased in selected tissues from older mice compared with tissues from younger mice indicating a possible correlation between PKR expression and kinase function. The data suggest that translational activity is slowing down in a tissue specific manner during the aging process in mice, possibly as the result of increased levels of PKR, and could be a factor in the reduction This work was supported by the Nathan Shock Center for Excellence in the Biology of Aging, NIA AG13280, and the Helen Hey Whitney Foundation. * Corresponding author. Tel.: + 1-206-6853260; fax: + 1-206-6853006. E-mail address: wladiges@u.washington.edu (W. Ladiges) 1 Present address: Department of Microbiology, Southwestern Medical Center, University of Texas, Dallas, TX, USA. 0047-6374/00/$ - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S 0 0 4 7 - 6 3 7 4 ( 0 0 ) 0 0 0 9 7 - X 124 W. Ladiges et al. / Mechanisms of Ageing and De6elopment 114 (2000) 123–132 of the rate of protein synthesis during senescence seen in specific tissues of many organisms. © 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Aging; PKR protein kinase; Mouse 1. Introduction A gradual decrease in the rate of total protein synthesis is a common biochemical change observed with aging in a wide variety of cells and organisms including humans (Richardson and Semsei, 1987; Rattan, 1996). There are multiple implications and consequences of slower rates of protein synthesis including a decrease in production and secretion of hormones, neurotransmitters, antibodies, and components of the extracellular matrix, an increased inefficiency of intracellular and intercellular signaling pathway mechanisms and a decline in the concentration of enzymes available for cellular maintenance, repair, and normal metabolic functioning. The molecular mechanisms responsible for such alterations are still poorly understood. Protein kinases, which catalyze phosphorylation in order to regulate several biological processes, including protein synthesis, are receiving attention as factors in age-related alterations of gene expression (Stein et al., 1990; Richter et al., 1991; Dulic et al., 1993). Phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF-2a) is one of the best characterized mechanisms for down-regulating protein synthesis in higher eukaryotes in response to various stress conditions (Merrick, 1992). The phosphorylation of eIF-2a results in the shutdown of protein synthesis by regulating both global as well as specific mRNA translation (Sonenberg, 1996). It has been shown that conditions like starvation, heat shock, and viral infections can induce the phosphorylation of eIF-2a through a protein kinase known as double stranded RNA-dependent protein kinase (PKR) (Brostrom et al., 1996). PKR is a serine/threonine protein kinase and one of the best characterized of the many proteins induced by type I interferon (IFN). It was proposed early on that PKR plays an important role in the cellular antiviral response, and indeed it inhibits the replication of several viruses in transfected cell lines over-expressing the gene product (Katze, 1995). Recent studies have also shown that PKR may play an important role in mediating the response of uninfected cells to physiologic stress (Hinnebusch, 1994). Conditions that damage proteins, cause protein misfolding, or inhibit protein processing, trigger the onset of protective homeostatic mechanisms resulting in stress responses in mammalian cells. These responses are manifested in a number of ways including an acute inhibition of mRNA translation, a subsequent induction of various protein chaperones, and the recovery of mRNA translation. The production of heat shock proteins (hsp) has been described as part of a cytoplasmic stress response system (Welch, 1992). Induction of hsp occurs in response to elevated temperature, oxidative free radicals, and heavy metals, or from the synthesis of aberrant cytoplasmic proteins in response to amino acid analogs. A recent report indicates that physiologic insults, such as treatment with sodium W. Ladiges et al. / Mechanisms of Ageing and De6elopment 114 (2000) 123–132 125 arsenite, dithiothreitol, or t-butylhydroperoxide, that induce cytoplasmic hsp responses in cells, also cause inhibition of protein synthesis through activation of PKR (Brostrom et al., 1996). The mechanism of this response may involve inactivation of P58IPK, a cellular inhibitor of PKR (Lee et al., 1994) by another cellular protein recently identified as the molecular chaperone hsp40 (Melville et al., 1997a). Hsp40 is involved in mediating the correct folding and assembly of polypeptide chains and may provide a link between hsp-type signaling mechanisms and the regulation of protein synthesis via P58IPK and PKR. Because phosphorylation of regulatory and structural proteins is the major mechanism by which intracellular control mechanisms operate in eukaryotes, we were interested in evaluating the role of the PKR system in aging. Oxidative damage to subcellular components is a reported mechanism of aging (Harmon, 1992; Sohal and Brink, 1992), and a purported factor in the development of chronic degenerative diseases (Ames et al., 1993). Such damage is a constant occurrence at the cellular level and accumulates over time, especially if the body’s defenses against oxidative stress are diminished. We are suggesting that the PKR regulatory pathway is a stress response system which operates to protect cellular resources against environmental stress, including oxidative stress, at the pretranslational and possibly transcriptional levels. This report describes our preliminary findings of expression of PKR in tissues of aging mice but not in tissues of young mice. The expression of P58IPK was seen in tissues from both age groups but generally to a greater extent in tissues of aging mice. The alpha subunit of eIF2 was generally expressed to a lesser degree in all tissues tested, except for liver and lung, of aging mice compared with tissues in younger mice. 2. Methods 2.1. Animals Five of each of 2- and 20-month-old male B6D2F1 mice were obtained from a contract vendor for the National Institute on Aging. Mice were housed under specific pathogen free conditions for 3 weeks, and then euthanitized by cervical dislocation and tissues immediately collected. Colon, liver, kidney, testes, pancreas, lungs, heart, and brain were collected from five mice of each age group and quickly placed in ice-cold saline, blotted dry, weighed, and homogenized in 4 volumes of ice-cold buffer (20 mM Tris, pH7.5, 1 mM dithiothreitrol, 35 mM sucrose, 0.1 mM EDTA containing 125 mM KCl). The homogenates were then quick frozen in liquid nitrogen and stored at −72°C until analyzed. There was no evidence of clinical, serological, or pathological disease or infection in any of the mice. All procedures relevant to the study were approved by the University of Washington Animal Care and Use Committee. The University of Washington is fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International. 126 W. Ladiges et al. / Mechanisms of Ageing and De6elopment 114 (2000) 123–132 2.2. Determination of PKR, P58 IPK, and eIF-2h protein expression Protein concentrations of tissue extracts were determined by using a protein assay as described by the manufacturer (Bio-Rad Hercules, CA). Fifty ug of protein from indicated tissue extracts were separated by SDS-polyacrylamide gel electrophoresis and transferred to nitrocellulose membranes. Membranes were incubated for one hour in blocking buffer and subsequently probed with the indicated primary monoclonal antibodies. After a washing step, membranes were probed with a horseradish peroxidase-conjugated secondary antibody (Jackson ImmunoResearch Lab, Inc, West Grove, PA). Bound antibodies were detected by enhanced chemiluminescence (NEN Life Science Products, Boston, MA). Detection of mouse PKR, P58IPK and eIF-2a, respectively, utilized antibody B10 (Santa Cruz Biotechnology, Santa Cruz, CA), antibody 9F10 (Barber et al., 1994), and an antibody received from Dr Michael Clemons. To control for errors in gel loading or protein concentration assays, all blots were also probed with an antibody specific for actin. Band intensities were quantitated from autoradiograms using a Bio-Rad GS 700 imaging densitometer and computer software supplied by the manufacturer. 2.3. Isoelectric focusing Isolectric focusing of eIF-2a from mouse tissues was conducted as described (Savinova and Jagus, 1997) except that tissue homogenates were supplemented with 1 mM okadaic acid, 15 mM EDTA, 1 mM PMSF, 1 mM DTT, 50 mM NaF, 35 mM b-glycerolphosphate and 10 mM 2-aminopurine. Proteins (20 mg) were separated by vertical isoelectric focusing and blotted to nitrocellulose membranes. eIF-2a was detected by immunoblot analysis using a mouse monoclonal antibody as described (Gale et al., 1998). 3. Results and discussion The protein expression of PKR was seen in tissues of five aging mice but generally not in tissues from five young mice (Table 1 and Fig. 1). Expression in all tissues examined from aging mice confirmed the ubiquitous nature of the enzyme. The pancreas, heart, and lungs appeared to have fairly strong levels of PKR, while levels were quite low in the liver, testes and brain. These tissue specific variations in PKR levels in tissues from older mice may be a reflection of metabolic or functional demands. Expression of P58IPK was seen in tissues of both young and older mice but the bands were generally more intense for the tissues from the older mice. The pancreas showed a robust expression in both age groups consistent with the over-expression of the P58IPK mRNA in this tissue (Korth et al., 1996). Expression of eIF-2a was seen in tissues from both age groups, with generally stronger levels seen in tissues from the young mice, in contrast to findings with PKR and P58IPK. An argument might be made that the difference in expression of the three proteins is merely a reflection of a shift in cell population from young to old. W. Ladiges et al. / Mechanisms of Ageing and De6elopment 114 (2000) 123–132 127 However, the differences are consistent in the eight tissues examined suggesting the findings are mechanistic in nature. It has recently been shown that protein kinases are generally up-regulated in senescent fetal lung fibroblast cultures (Blumenthal et al., 1993) and in long-lived mutants of Caenorhabditis elegans (Vanfleteren and De Vreese, 1997) thus supporting our findings in aging mice. However, at this point we can only speculate on the mechanistic events that may be occurring in relation to different levels of expression of these proteins. If we make the assumption that increased levels of expression in aging tissues indicate an increased function, then it is intriguing to suggest that PKR activity increases to compensate for a continual bombardment of stress-inducing ‘hits’ during the course of aging. For example, oxidative ‘hits’ and the subsequent oxidative damage are cumulative over the course of time as part of growing old (Statdman, 1992). The best characterized function of PKR is an antiviral effect based on its ability to phosphorylate eIF-2a resulting in inactivation and subsequent inhibition of protein synthesis (Katze, 1995). The anti-stress response of PKR may occur through a similar mechanism as suggested by studies using NIH 3T3 cell cultures exposed to sodium arsenite (Brostrom et al., 1996). Extracts from arsenitetreated cells displayed greater degrees of phosphorylation of PKR and eIF-2a than did control extracts resulting in inhibition of translation initiation. In addition, there was a subsequent activation of heat shock protein (hsp) chaperone activity. Table 1 Expression of PKR, P58IPK and eIF-2a in tissues from five paired young and old B6D2 male mice, respectivelya Tissue Age (months) PKR P58IPK eIF-2a Kidney 2 20 2 20 2 20 2 20 2 20 2 20 2 20 2 20 1 39+4* 6 1 20+3* 6 1 41+6* 6 1 34+8* 6 1 27+6* 6 1 222+7* 6 1 181+9* 6 2+1 6 118+5* 6 84+6 6 100+9 6 200+9 6 268+12* 6 69+7 6 79+5 6 12+7 6 41+8* 6 100+9 6 188+7* 6 705+12 6 684+27 6 8+3 6 99+4* 6 1 25+7* 6 76+4 6 11+6** 6 125+9 6 318+10** 6 140+9 6 110+11 6 114+8 6 87+7 6 166+6 6 107+8** 6 230+10 6 210+9 6 84+6 6 155+16* 6 220+6 6 49+8** 6 Liver Colon Brain Testes Pancreas Lung Heart a Values are the average of imaging densitometry readings of immunoblot analysis normalized to percentage actin control levels for each age group. * Significant increase, PB0.05–0.5. Determined by 2 analysis. ** Significant decrease, PB0.1–0.5. Determined by 2 analysis. 128 W. Ladiges et al. / Mechanisms of Ageing and De6elopment 114 (2000) 123–132 Fig. 1. Representative expression of PKR regulatory pathway components in tissues from young and old mice. Paired tissue homogenates prepared from five 2-month-old (Y) and five 20-month-old (O) B6D2F1 specific pathogen free mice were subjected to immunoblot analysis using monoclonal antibodies specific for murine PKR (panel 1), P58IPK (panel 2, or eIF-2a (panel 3). Fig. 1 is representative of the results obtained from each of the five mouse pairs (summarized in Table 1). Each lane contains 50 mg of protein. For PKR, P58IPK, and eIF-2a, the level of each band was quantitated by imaging densitometry, normalized as a percentage of the corresponding level of the actin control (% actin), and used to calculate data for Table 1. Additional evidence that PKR is part of a cellular stress response pathway was recently reported by Melville et al. (1997a) who identified a molecular chaperone known as hsp40 which maintains P58IPK, the inhibitor of PKR, in an inactive state. It may be that some viruses, such as influenza virus, are able to block the interferon antiviral effect mediated by PKR by recruiting the hsp40–P58IPK –PKR cellular stress response pathway (Melville et al., 1997b). P58IPK is the best described of several cellular regulators of PKR (Tang et al., 1996). P58IPK has been identified and cloned (Korth et al., 1996) and found to be a member of the tetratricopeptide repeat family of proteins (Lee et al., 1994). In addition, the C-terminal portion of P58IPK shares homology with the J region of the DNAJ heat shock protein of E coli placing it within the heat shock family of proteins (Melville et al., 1997a). P58IPK appears to be present in all tissues examined so far in both mice and humans, with especially high levels in the liver and pancreas (Korth et al., 1996). We had similar findings, with very strong expression in the pancreas of both young and older mice. In general, P58IPK was expressed in tissues from both young and old mice but at a more intense level in the older mice. Our explanation for the correlating levels of expression of PKR and P58IPK in tissues from young and older mice is the need for additional P58IPK to continually guard against overproduction of PKR. It may be that PKR, at this age in the older B6D2F1 mice, is still able to strongly compensate for oxidative ‘hits’, or other types of stress exposure, and still requires P58IPK to prevent overcompensation and W. Ladiges et al. / Mechanisms of Ageing and De6elopment 114 (2000) 123–132 129 complete shutdown of protein synthesis. Evaluation of even older mice might possibly show exhausted levels of PKR and P58IPK as suggested by Miller (1994) who saw a decline in both serine/threonine and tyrosine specific protein kinase signals after activation of T lymphocytes in aging mice. Expression of eIF-2a was detected in all tissues, as expected, similar to expression of PKR and P58IPK. In contrast to the increased expression of PKR and P58IPK in tissues of older mice compared with younger mice, eIF-2a levels were generally decreased in tissues, except for liver and lung, of older mice relative to tissues of younger mice, similar to findings in tissues from aging rats (Kimball et al., 1992). However, isoelectric focusing showed that eIF-2a in its hyperphosphorylated form was actually increased in selected tissues (kidney and liver) from the older mice compared with tissues in younger mice (Fig. 2) indicating a correlation between PKR expression and kinase function. This would suggest that translational activity is slowing down during the aging process in mice possibly as the result of increased levels of PKR, and could be a factor in the reduction of the rate of protein synthesis during senescence seen in other organisms (Sohal et al., 1993). For example, it has recently been reported that the rate of synthesis of total tissue protein is slower in skeletal muscle of healthy older humans compared with skeletal muscle of young adults (Welle et al., 1997). There was no difference in total mRNA between young and old muscle suggesting that the slower protein synthesis might be the result of decreased translation activity by eIF-2a. The concept that down-regulation of protein translation is mediated by phosphorylation of eIF-2a in aging mouse tissues fits best with our data for kidney, colon, brain, and testis. Several tissues showed inconsistencies. Lung and liver from old mice expressed higher levels of non-phosphorylated eIF-2a than respective tissues from young mice. It may be that liver and lung have different environmental demands reflective of increased need for high levels of eIF-2a associated with aging in these tissues. There was definitely evidence of kinase activity in liver tissue from old mice as indicated by the presence of phosphorylated eIF-2a. Lung tissue was Fig. 2. Isoelectric focusing of eIF-2a in tissue homogenates from kidney and liver (arbitrarily selected as representative tissues) from two 2-month-old (Y) and two 20-month-old (O) B6D2F1 specific pathogen free male mice. Lane 1 is a positive control prepared from a rabbit reticulocyte lysate. Lanes 2 – 9 each represent kidney or liver tissue from a single young or old mouse. The more acidic hyperphosphorylated eIF-2a (eIF-2a-P) can be seen as the upper row of bands, while the more basic hypophosphorylated eIF-2a can be seen as the lower row of bands. Each lane contains 20 mg protein. 130 W. Ladiges et al. / Mechanisms of Ageing and De6elopment 114 (2000) 123–132 not tested for kinase activity but it would be interesting to determine if levels were similar to liver. Our data suggest some unique aspect of protein translation is associated with aging in the liver and lung of mice. In addition, pancreas showed a strong age-associated increase of PKR, a high level of P58IPK that did not change with age, and an abundant level of eIF-2a that showed minimal change. Heart tissue also showed an age-associated up-regulation of PKR and P58IPK, but a dramatic loss of eIF-2a. These are likely characteristics unique to specific cellular functions of these tissues, and further experiments are necessary to determine relevant translation activity. Our explanation for the difference in expression of PKR in tissues of aging versus young mice has focused on the cellular stress pathway, specifically oxidative stress, leading to an inhibition, or at least slowing, of mRNA translation and protein synthesis. However, it has recently been shown that PKR is an activator of apoptosis (Williams, 1997), and that PKR-dependent apoptosis requires eIF-2a phosphorylation (Srivistava et al., 1996). The process of programmed cell death, or apoptosis, is activated as a defensive mechanism for eliminating damaged or harmful cells within a multicellular organism. It may be that aging increases the requirement for PKR in mediating different forms of stress-related apoptosis. Another possible mechanism that may be a functional part of PKR activity during aging is the ability to phosphorylate the inhibitor of nuclear factor-kB (IkB) (Der et al., 1997). IkB, when phosphorylated, is prevented from inhibiting NF-kB, thus allowing the activation of this ubiquitous transcription factor (Baeuerle and Baltimore, 1988). NF-kB stimulates the expression of inflammatory cytokines, immunoregulatory receptors, and acute phase proteins. 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