The glutathione antioxidant system is enhanced in ...

J Comp Physiol B (2007) 177:413–422 DOI 10.1007/s00360-006-0140-5

O RI G I NAL PAPE R

The glutathione antioxidant system is enhanced in growth hormone transgenic coho salmon (Oncorhynchus kisutch) Rosalind A. Leggatt · Colin J. Brauner · George K. Iwama · Robert H. Devlin

Received: 5 October 2006 / Revised: 3 December 2006 / Accepted: 10 December 2006 / Published online: 16 January 2007  Springer-Verlag 2007

Abstract Insertion of a growth hormone (GH) transgene in coho salmon results in accelerated growth, and increased feeding and metabolic rates. Whether other physiological systems within the Wsh are adjusted to this accelerated growth has not been well explored. We examined the eVects of a GH transgene and feeding level on the antioxidant glutathione and its associated enzymes in various tissues of coho salmon. When transgenic and control salmon were fed to satiation, transgenic Wsh had increased tissue glutathione, increased hepatic glutathione reductase activity, decreased hepatic activity of the glutathione synthesis enzyme !-glutamylcysteine synthetase, and increased intestinal activity of the glutathione catabolic enzyme !-glutamyltranspeptidase. However, these diVerences were mostly abolished by ration restriction and fasting, indicating that upregulation of the glutathione antioxidant system was due to accelerated growth, and Communicated by H.V. Carey. R. A. Leggatt Faculty of Land and Food Systems, University of British Columbia, 2357 Main Mall, Vancouver, BC, V6T 1Z4 Canada C. J. Brauner · R. H. Devlin Department of Zoology, University of British Columbia, 6270 University Blvd., Vancouver, BC, V6T 1Z4 Canada G. K. Iwama Faculty of Science, Acadia University, Wolfville, NS, B4P 2R6 Canada R. H. Devlin (&) Fisheries and Oceans Canada, 4160 Marine Drive, West Vancouver, BC, V7V 1N6 Canada e-mail: [email protected]

not to intrinsic eVects of the transgene. Increased food intake and ability to digest potential dietary glutathione, and not increased activity of glutathione synthesis enzymes, likely contributed to the higher levels of glutathione in transgenic Wsh. Components of the glutathione antioxidant system are likely upregulated to combat potentially higher reactive oxygen species production from increased metabolic rates in GH transgenic salmon. Keywords Antioxidant · Coho salmon · Glutathione · Growth hormone transgene · Metabolism Abbreviations CAT Catalase DTNB 5,5!-Dithiobis(2-nitrobenzoic acid) GCS !-Glutamylcysteine synthetase GH Growth hormone GPx Glutathione peroxidase GR Glutathione reductase GSH Glutathione GSSG Oxidized glutathione !GT !-Glutamyltranspeptidase ROS Reactive oxygen species SOD Superoxide dismutase

Introduction Growth rate, controlled in part by growth hormone (GH), is associated with whole animal costs such as increased rate of aging and decreased lifespan (see Rollo et al. 1996; Bartke 1998; Rollo 2002). However, the physiological consequences of, and resulting responses to, accelerated growth rate are not fully

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understood. For example, increased growth rate and GH levels are associated with increased metabolic demands (see Bartke 1998; Carlson et al. 1999). Metabolic rate in turn has been correlated with production of reactive oxygen species (ROS) from the mitochondria (reviewed by Muradian et al. 2002; Fridovich 2004). If left unchecked, this can result in oxidative damage to a variety of macromolecules and cause cell damage or death (reviewed by Kidd 1997). To combat this, ROS production is balanced by a suite of coordinated antioxidants, such as glutathione (GSH), and the antioxidant enzymes glutathione peroxidase (GPx), catalase (CAT) and superoxide dismutase (SOD). Whether accelerated growth rate increases antioxidant levels is poorly studied, although the eVect of GH on antioxidant status has been examined with conXicting results. ROS production is found to be proportional to GH levels, as GH transgenic mice have increased superoxide production (Rollo et al. 1996), and Ames dwarf mice deWcient in GH produce less hydrogen peroxide from the mitochondria than wildtype mice (Brown-Borg et al. 2001). In addition, oxidative stress is, in part, proportional to GH, as GH transgenic mice have increased protein and lipid oxidation, and Ames dwarf mice have less protein, but not lipid, oxidation compared with wild-type mice (Rollo et al. 1996; Carlson et al. 1999; Brown-Borg et al. 2001). Corresponding to increased ROS production and oxidative stress, antioxidant levels are altered by GH and growth rate to varying degrees depending on the species and strain, life-stage, administration method of GH, and antioxidant examined. For example, Kolataj et al. (1998) found mice selected for high weight gain had higher tissue GSH levels than control mice. GH treatment, over-expression, or transgenesis increased Cu/ Zn SOD and glutathione-S-transferase in rats (Beyea et al. 2005), increased GSH levels in various tissues of Ames dwarf mice (Brown-Borg and Rakoczy 2003), increased CAT, SOD, GPx, and the glutathione synthesis enzyme !-glutamylcysteine synthetase (GCS) in mammalian carcinoma cells (Zhu et al. 2005), and increased production of GSH in tumor cells (Cherbonnier et al. 2003). In contrast, GH treatment or transgenesis decreased CAT, GPx, and SOD in some tissues of wild-type and Ames dwarf mice (Hauck and Bartke 2001; Brown-Borg et al. 2002; Brown-Borg and Rakoczy 2003), suggesting GH treatment or transgenesis can both enhance or impair antioxidant defense, depending on the study. In these studies, it was not determined whether alterations in antioxidant status were due to increased growth rate or intrinsic eVects of the GH, and as such the speciWc relationship between growth rate and antioxidant status remains elusive.

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To explore the relationship between growth rate, GH, and antioxidants, and to examine these relationships in a poikilotherm, we have examined the glutathione antioxidant system in GH transgenic and wild-type coho salmon (Oncorhynchus kisutch) fed diVerent ration levels to control for growth rate. GH transgenic coho salmon have accelerated growth rate, and corresponding increased feeding rates (Devlin et al. 1999; Devlin et al. 2004; Raven et al. 2006), oxygen consumption (Leggatt et al. 2003), and increased expression of mitochondrial electron transport chain enzymes (Rise et al. 2006). Growth rate restriction in transgenic Wsh by food rationing or fasting diminishes or prevents metabolic rate increases (Leggatt et al. 2003; Rise et al. 2006). Consequently, these animals make an interesting model for examining the physiological adjustments to accelerated growth in vertebrates. We examined the eVect of a GH transgene and consequent accelerated growth on the GSH antioxidant system in coho salmon. GSH is a particularly important antioxidant as it is often present in mM concentrations, and thus represents a major redox buVer in cells (reviewed by Arrigo 1999). GSH, alone or as a cofactor for GPx enzymes, reduces ROS and products of oxidative stress such as lipid hydroperoxides. In the process, GSH is oxidized (oxidized glutathione—GSSG), and is returned to its reduced form by GSSG reductase (GR), utilizing NADPH as a cofactor. The GSH antioxidant system was measured in Wve groups of size-matched Wsh: transgenic Wsh fed to satiation (maximum growth rate), size-matched control Wsh fed to satiation, transgenic Wsh fed equal rations as control Wsh (control growth rate), as well as transgenic and control Wsh fasted for one month. Transgenic Wsh fed equal rations as control Wsh were of equal age and had growth rates similar to control Wsh, while transgenic Wsh fed to satiation had faster growth rates than control Wsh and were therefore necessarily younger. GSH levels, oxidation, activities of GPx and GR, and activities of the key GSH synthesis enzyme GCS, and the GSH catabolic enzyme !-glutamyltranspeptidase (!GT) were measured in various tissues to determine if a GH transgene and accelerated growth alters GSH dynamics in coho salmon.

Materials and methods Experimental design Transgenic Wsh used in this experiment were produced and raised in a secure and contained transgenic facility at the DFO/UBC Centre for Aquaculture and

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Environmental Research (CAER), West Vancouver, BC, Canada. The GH transgenic gene construct used was OnMTGH1 (containing type-1 GH) fused to a metallothionein-B promoter, both from sockeye salmon (Devlin et al. 1994). The gene was inserted into coho salmon derived from Chehalis River wild stock and all subsequent generations bred with normal wild Chehalis coho salmon. Control Wsh were non-transgenic wild Chehalis River coho salmon. Fish were cultured in oxygen saturated, 10°C well water, with water Xow greater than 1 l min¡1 kg¡1 Wsh and a density less than 5 kg m¡3. Fish were fed stage-speciWc trout food from Skretting (Vancouver, BC, Canada) twice a day until start of the experiment. The experiment took place from September–October. The eVects of a GH transgene and feeding level on GSH levels, oxidation and activity of associated enzymes were Wrst determined in three groups of Wsh: transgenic coho salmon (2005 brood year) fed to satiation throughout their lifetime, control coho salmon (2004 brood year) fed to satiation throughout their lifetime, and transgenic coho salmon (2004 brood year) fed a restricted ration throughout their lifetime that was equal to control salmon feeding level on a per kg body weight basis. 2004 brood-year generation nontransgenic and transgenic salmon were half-sib progeny from the same wild-type brood females (n = 10) produced from wild-type and homozygous transgenic sires, respectively. Although feed intake of the three groups was not measured, GH transgenic coho salmon of the gene construct used here generally have 2–3 times the feed intake of size-matched non-transgenic salmon when both are fed to satiation (Devlin et al. 1999; Stevens and Devlin 2005). As transgenic Wsh fed to satiation grow at a much more rapid rate than transgenic and control Wsh fed at a control satiation level, in order to size-match the Wsh, the former Wsh were by necessity one year older than the latter two groups (9 vs. 21 months old, respectively). While this raises the concern that diVerences between transgenic salmon fed to satiation and the other two groups may be due to age, and not growth rate, GH transgenic salmon fed to satiation show precocious smoltiWcation and onset of sexual maturation, suggesting a compression of the normal lifespan (Devlin et al. 2004). As size appears to be the main cue for physiological events such as smoltiWcation, size-matched transgenic and control salmon were likely of similar physiological age, although diVered in numerical age. While the addition of a control group of equal age as transgenic Wsh fed to satiation may have addressed this problem, the slow growth rate of these Wsh would result in them being much smaller in size and earlier physiological age than all

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other groups, making meaningful comparisons diYcult. In addition, the GSH antioxidant system was measured in control and transgenic Wsh, originally fed to satiation, that were fasted for one month. As these Wsh were of diVerent ages, but approximately equal growth rate (e.g., negligible growth due to lack of feeding) for 1 month, they could be used to address potential eVects of age versus growth rate on GSH dynamics in Wsh. Eight Wsh from each of three groups (transgenic Wsh fed to satiation, control Wsh fed to satiation, and transgenic Wsh rationed to control feed intake) were sacriWced by an overdose of anaesthetic (2 g l¡1 tricaine methanesulfonate, 4 g l¡1 sodium bicarbonate), weight and length recorded and Wsh sampled for liver, muscle, posterior kidney, intestinal mucosa and blood. After sampling, remaining control and transgenic Wsh fed to satiation were implanted with intraperitoneal passive integrated transponders (PIT-tags), then combined into one tank and fasted for one month. Eight Wsh from each group were then identiWed by PIT-tag, weight and length recorded, and sampled for liver, posterior kidney, intestinal mucosa, and blood. To sample Wsh, whole blood was removed by caudal severage, centrifuged at 6,800£g for 3 min, and plasma removed and snap-frozen in liquid nitrogen. Tissues were then excised, rinsed in ice-cold phosphate buVered saline, blotted dry and snap-frozen in liquid nitrogen. To sample intestinal mucosa, the posterior inch of intestine was removed, cut open, rinsed in ice-cold phosphate buVered saline, gently blotted dry and the mucosa scraped oV the musculature with a razor blade. All tissues were stored at ¡80°C until analysis. All tissues were analyzed for total GSH (tGSH = GSH + 2 £ GSSG), liver analyzed for GSSG and activity of GR (EC 1.8.1.7), GPx (EC 1.11.1.9), and GCS (EC 6.3.2.2), and posterior kidney and intestinal mucosa sampled for !GT (EC 2.3.2.2). Tissue preparation and analysis Tissues were homogenized (muscle tissue) or sonicated (all other tissue) in various buVers containing protease inhibitors (1 mM EDTA, 1 "M pepstatin, 1 "M leupeptin, 0.15 "M aprotinin, and 0.5 mM phenylmethylsulfonyl Xuoride), depending on the requirements of the Wnal enzyme assay. For intestinal mucosa and kidney !GT activity and GSH, the buVer used was 100 mM Tris–HCl (pH 8.0). For liver GR activity and GSH, as well as muscle GSH, the buVer used was 125 mM sodium phosphate buVer (pH 7.5), and for liver GPx and GCS activity, the buVer used was 125 mM sodium phosphate (pH 7.5), supplemented with 1 mM dithiothreitol. Tissues were homogenized at an approximate

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ratio of 0.1 g ml¡1 and supernatant obtained by centrifugation at 11,600£g at 4°C for 5 min. A portion of supernatant was removed for total protein, and GCS, GR, GPx and !GT activity analyses. The remaining supernatant or plasma was added to an equal volume of 10% sulfosalicylic acid, centrifuged at 10,000£g at 4°C for 10 min, and the supernatant was removed for tGSH and GSSG analysis. Preliminary studies found no eVect on tGSH or GSSG values when tissues were sonicated in various buVers before addition of sulfosalicylic acid, compared with sonicating in sulfosalicylic acid alone. All analyses were performed in triplicate on 96-well microplates and measured using a SpectraMax spectrophotometer equipped with SoftmaxPro Software (Molecular Devices Corporation). Protein content was analyzed using the bicinchoninic acid method as per Smith et al. (1985), using bovine serum albumin as standards and analyzed at 516 nm. tGSH and GSSG analyses were modiWed from GriYth (1980) as follows. For tGSH analyses, 6 "l of undiluted triethanolamine was added per 100-"l samples and GSSG standards. A total of 10 "l of samples or GSSG standard and 200 "l of a reaction mixture (0.22 mM NADPH, 0.62 mM 5,5!-dithiobis(2-nitrobenzoic acid) (DTNB), 0.56% triethanolamine and 0.5 U ml¡1 GR in 125 mM sodium phosphate, 6 mM EDTA buVer, pH 7.4) were added to a 96 well plate, and absorbance monitored at 412 nm for 5 min. For GSSG analyses, 6 "l of 2-vinylpyridine and 20 "l of 10% triethanolamine in phosphate buVered saline were added per 100 "l of sample or GSSG standard. Samples and standards were mixed vigorously for 1 min and then incubated for 50 min. GSSG analyses were then as per tGSH above except that the concentration of DTNB in the reaction mixture was 1 "M. GR and GPx activity of liver homogenates were analyzed by procedures modiWed from Stephensen et al. (2002), and GCS activity was analyzed by a procedure modiWed from Seelig and Meister (1985). Homogenates were diluted to 5 mg protein ml¡1 with 125 mM sodium phosphate, 6 mM EDTA buVer (pH 7.4) containing either 0.1% bovine serum albumin (GR) or 1 mM dithiothreitol (GCS, GPx). For GR analyses 10 "l of homogenate or buVer blank and 200 "l of a reaction mixture (0.1 mM DTNB, 0.63 mM NADPH) was added to each well, and background absorbance measured at 405 nm for 5 min. Final GR enzyme activity was calculated using an extinction coeYcient of 9.599 OD ml "mol¡1 DTNB. A total of 10 "l 0.33 M GSSG was then added to each well and the absorbance monitored at 405 nm for 5 min. For GPx analyses, 10 "l of homogenate or buVer blank and 200 "l of a reaction mixture (3.5 mM GSH, 1 mM

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sodium azide, 2 U ml¡1 GR and 0.12 mM NADPH) was added to each well. The reaction was initiated by the addition of 10 "l of 0.03% hydrogen peroxide and the change in absorbance monitored at 340 nm for 5 min. Final GPx enzyme activity was calculated using an extinction coeYcient of 2.757 OD ml "mol¡1 NADPH. For GCS analyses, 10 "l of homogenate or buVer blank and 100 "l of a reaction mixture (Wnal concentrations of 100 mM KCl, 60 mM MgCl2, 20 mM EDTA, 0.4 mM NADPH, 6.2 U ml¡1 pyruvate kinase, 8.8 U ml¡1 lactate dehydrogenase, 10 mM phosphoenol pyruvate and 10 mM L-glutamic acid, in 100 mM Tris–HCl, pH 8.0) was added to each well and background absorbance was monitored for 5 min at 340 nm. A total of 100 "l of reaction starter (Wnal concentrations 10 mM ATP, 5 mM dithiothreitol and 10 mM cysteine, in 100 mM Tris–HCl, pH 8.0) was added to each well, and the change in absorbance monitored at 340 nm for 5 min. Final GCS enzyme activity was calculated using an extinction coeYcient of 5.447 OD ml "mol¡1 NADPH. !GT activity of tissue homogenates were analyzed by a procedure modiWed from Meister et al. (1981). A total of 10 "l of tissue homogenate and 200 "l of a reaction mixture (0.25 M glycine–glycine, 1.25 mM L-!-glutamic acid p-nitroanilide, in 100 mM Tris–HCl, pH 8.0) was added to each well, and the change in absorbance monitored at 410 nm over 5 min. Final !GT enzyme activity was calculated using an extinction coeYcient of 4.140 OD ml "mol¡1 p-nitroaniline. Statistical analysis GSH dynamics were Wrst analyzed in transgenic and control salmon fed to satiation or fasted for one month using two-way ANOVA, with factors being GH transgenesis versus control, and fed to satiation versus fasting. All groups, including transgenic salmon rationed at control feeding levels, were then compared by one-way ANOVA, followed by Tukey’s post hoc test using SigmaStat (SPSS Inc.). DiVerences were considered signiWcant if P · 0.05. All data are presented as mean § standard error of the mean, n = 8. Results Weight and length of transgenic and control coho salmon under diVerent feeding and fasting regimes are given in Table 1. There were no factor eVects of transgenesis (P = 0.345) or satiation versus fasting (P = 0.685) when Wsh weight was analyzed by two-way ANOVA, and no diVerences in weight between any

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Table 1 Weight and length of growth hormone transgenic coho salmon fed to satiation, fed a Wxed ration or fasted for 1 month, and control coho salmon fed to satiation or fasted for 1 month Group Fed Transgenic satiation Control satiation Transgenic Wxed ration Fasted Transgenic satiation Control satiation

Weight (g)

Length (cm)

72.9 § 5.0 81.0 § 5.3 74.3 § 6.5

17.1 § 0.4a 18.6 § 0.4ab 19.0 § 0.5b

73.1 § 6.0 76.1 § 6.4

18.6 § 0.5ab 19.1 § 0.4b

Weights and lengths are of sampled Wsh at the end of either the fed or fasted periods. SigniWcant diVerences in length among groups are indicated by diVerent letters (a, b). There were no signiWcant diVerences in weight among any Wsh groups

individual groups of Wsh (P = 0.863). However, there were signiWcant factor eVects of transgenesis (P = 0.024) and satiation versus fasting (P = 0.030) on Wsh length, where control Wsh were 1.06 times longer than transgenic Wsh, and fasted Wsh were 1.06 times longer than Wsh fed to satiation. However, the only individual diVerences between Wsh groups were transgenic Wsh fed to satiation were signiWcantly smaller in length than transgenic Wsh fed a Wxed ration and control Wsh fasted for one month (P = 0.024). Although growth rates of Wsh were not calculated, a previous study with similar groups of Wsh found speciWc growth rates of transgenic Wsh fed to satiation were eight times greater than control Wsh fed to satiation, and four times greater than transgenic Wsh fed a control ration (Rise et al. 2006). In the liver, there were signiWcant factor eVects of both transgenesis (P < 0.001) and satiation versus fasting (P < 0.001) on tGSH levels, where transgenic Wsh had 1.9 times greater hepatic tGSH than control Wsh, and Wsh fed to satiation had 1.9 times greater tGSH levels than fasted Wsh (Fig. 1a). However, when Wsh groups were compared individually, hepatic tGSH levels only diVered between satiated and fasted Wsh in transgenic Wsh, and between transgenic and control Wsh when fed to satiation. Transgenic Wsh fed a control ration had similar levels to both transgenic and control Wsh fed to satiation, although they had greater hepatic tGSH levels than fasted control Wsh. In the posterior kidney, there was a signiWcant interaction between transgenesis and satiation versus fasting in tGSH levels (P = 0.010, Fig. 1b). Transgenic and control Wsh fasted for one month had 4.3 times greater kidney tGSH levels than fed Wsh, regardless of transgenesis or feeding levels (P < 0.001). In the intestinal mucosa, there was a signiWcant factor eVect of satiation versus fasting (P = 0.002), but not transgenesis (P = 0.053), where fasted Wsh had 1.4 times greater intestinal tGSH than

fed Wsh (Fig. 1c). However, the only diVerences between individual groups were fasted control Wsh had greater tGSH than transgenic Wsh fed to satiation or fed a control ration (P < 0.001). In the plasma, there was a signiWcant interaction between transgenesis and satiation versus fasting on tGSH levels (P = 0.040, Fig. 1d). When Wsh groups were compared individually, transgenic Wsh fed to satiation had 1.4 times greater plasma tGSH levels than all other groups (P < 0.001). In the muscle, tGSH levels were signiWcantly higher in transgenic Wsh fed to satiation than control Wsh (P = 0.01) or transgenic Wsh fed a Wxed ration (P < 0.001, Fig. 1e). There were no muscle samples available after one month of fasting. GSSG levels and GCS, GR, and GPx activity were measured in the liver as the organ with generally the highest capacity for GSH-dependent detoxiWcation. There was a signiWcant interaction between transgenesis and satiation versus fasting on hepatic GCS activity (P < 0.001, Fig. 2). Transgenic and control Wsh fed to control satiation levels had 1.4 times greater GCS activity than transgenic Wsh fed to satiation (P < 0.001). As well, transgenic Wsh rationed to control levels had 1.2 times greater GCS activity than control Wsh fasted for one month. There was a signiWcant factor eVect of transgenesis on hepatic GSSG levels (as % of tGSH, P < 0.001), where control Wsh had 2.0 times greater GSSG than transgenic Wsh (Fig. 3a). However, when individual Wsh groups were examined, this diVerence was not signiWcant between control and transgenic Wsh fed to satiation. Transgenic Wsh rationed to a control feeding levels did not diVer in GSSG from any other group. Reduced GSH levels (tGSH—2 £ GSSG) in the liver were similar to tGSH levels, with the exception that transgenic Wsh fed a Wxed ration had signiWcantly lower GSH levels than transgenic Wsh fed to satiation (P = 0.049, Fig. 3b). This indicates that the diVerences observed between Wsh groups were primarily due to increased GSH, and not GSSG levels. There was a signiWcant factor eVect of transgenesis (P < 0.001), but not of satiation versus fasting (P = 0.061), on hepatic GR activity, where transgenic Wsh had 1.4 times greater GR activity than control Wsh while fed to satiation or after fasting (Fig. 4a). Transgenic Wsh fed a control ration level had similar GR activity to fed and fasted control Wsh, and 58% the activity of transgenic Wsh fed to satiation or fasted (P < 0.001). There was a signiWcant factor eVect of satiation versus fasting (P < 0.001) but not transgenesis (P = 0.573) on hepatic GPx activity (Fig. 4b). Fed Wsh, including transgenic Wsh fed a control ration, had 2.7 times greater GPx activity than fasted Wsh, regardless of transgenesis (P < 0.001).

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There was a signiWcant interaction between transgenesis and satiation versus fasting on intestinal mucosa !GT activity (P = 0.002, Fig. 5a). Transgenic Wsh fed to satiation had greater intestinal !GT activity than all other groups, and control and transgenic Wsh fed to control satiation levels had greater activity than

Transgenic rationed

Control satiation

0.5

Fig. 2 !-Glutamylcysteine synthetase (GCS) activity in the liver of the following groups of control and growth hormone transgenic coho salmon: transgenic salmon fed to satiation, control salmon fed to satiation, transgenic salmon fed an equal ration as control salmon, transgenic salmon fasted for one month, and control salmon fasted for one month. Where letters diVer (a, b, c) signiWcant diVerences among Wsh groups exist

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Fig. 1 Total GSH levels (nmol mg¡1 protein unless otherwise stated) in a liver, b posterior kidney, c intestinal mucosa, d plasma, and e white muscle in the following groups of control and growth hormone transgenic coho salmon: transgenic salmon fed to satiation, control salmon fed to satiation, transgenic salmon fed an equal ration as control salmon, transgenic salmon fasted for one month, and control salmon fasted for one month. SigniWcant diVerences among groups, within tissues, are indicated by diVerent letters (a, b, c)

fasted control and transgenic Wsh (P < 0.001). There was a signiWcant factor eVect of satiation versus fasting (P < 0.001) but not transgenesis (P = 0.157) on posterior kidney !GT activity (Fig. 5b). However, the only individual signiWcant diVerences were that all fed Wsh, including transgenic Wsh fed a control ration, had 1.5 times greater activity than fasted control Wsh (P < 0.001).

Discussion This is the Wrst study to examine the eVects of GH combined with ration restriction on antioxidant defenses in vertebrates, and the Wrst study to examine the eVect of GH on antioxidant defenses in a non-mammalian model. GH transgenic coho salmon had higher liver, muscle and plasma tGSH, lower liver GSSG levels and GCS activity, higher liver GR activity, and higher intestinal !GT activity than control Wsh when fed to satiation, although did not diVer from controls in kidney and intestinal tGSH, liver GPx activity or kidney !GT activity. Most of these diVerences were due to increased feeding and growth rate, and not necessarily to intrinsic eVects of over-expression of GH associated

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Fig. 3 a Oxidized (GSSG) and b reduced (GSH) glutathione in liver of the following groups of control and growth hormone transgenic coho salmon: transgenic salmon fed to satiation, control salmon fed to satiation, transgenic salmon fed an equal ration as control salmon, transgenic salmon fasted for one month, and control salmon fasted for one month. Where letters diVer (a, b, c) signiWcant diVerences among Wsh groups exist

with the transgene, as transgenic Wsh fed control rations had similar levels to fed control Wsh, and fasting eliminated most signiWcant diVerences between transgenic and control Wsh. These results are consistent with a mammalian study that found mice selected for increased weight gain had high kidney and liver GSH during feeding, but not after 24 h starvation (Kolataj et al. 1998). While we did not measure metabolic rate in this study, increased growth rate in transgenic salmon has been associated with increased metabolic rates in other studies (Cook et al. 2000; Leggatt et al. 2003). As such, the GSH antioxidant system in GH transgenic salmon may have been upregulated to cope with increased ROS production from high metabolic demand of accelerated growth rate in these Wsh. The high tGSH levels observed in transgenic Wsh were likely driven by increased supply of amino acid precursors due to high feeding levels. In most vertebrates, dietary GSH is not taken up directly by tissues, but is broken down extracellularly by !GT, transported as amino acids or dipeptides, and resynthesized intracellularly (see Meister 1995). Transgenic Wsh fed to satiation had higher !GT activity per mg protein or per unit intestine (data not shown) than other groups, as


Control satiation

Transgenic rationed

Fig. 4 a Glutathione reductase (GR) and b glutathione peroxidase (GPx) activities (nmol min¡1 mg¡1 protein) in the liver of the following groups of control and growth hormone transgenic coho salmon: transgenic salmon fed to satiation, control salmon fed to satiation, transgenic salmon fed an equal ration as control salmon, transgenic salmon fasted for 1 month, and control salmon fasted for 1 month. Where letters diVer (a, b) signiWcant diVerences among Wsh groups exist

well as greater gut surface areas (Stevens and Devlin 2000, 2005). High intestinal !GT activity would increase the ability of transgenic Wsh to metabolize any GSH present in the diet to its transportable amino acid precursors, and thus increase supply of these precursors for GSH synthesis within tissues. While we did not measure GSH levels in the diet, GSH is present in all living cells, and is therefore likely present in Wsh diet via Wsh or plant meal. In mammals, GSH synthesis capacity is primarily limited by activity of the GSH synthesis enzyme GCS (see Meister 1995), while in Wsh, GSH synthesis capacity may be limited primarily by precursor supply (Leggatt 2006). As such, increased GSH precursor supply from high food intake and intestinal !GT activity may have increased the capacity for GSH synthesis in various tissues. This is further conWrmed by the lack of correlation between GSH levels and activity of GCS in transgenic and control Wsh. The method by which increased growth rate altered GSH enzyme activities does not appear to be by increased gene expression, as there were no signiWcant diVerences in hepatic GCS or GR gene expression between control and transgenic Wsh fed to satiation or trans-

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Transgenic rationed

Fig. 5 !-Glutamyltranspeptidase activity (!GT—nmol min¡1 mg¡1 protein) in a intestinal mucosa and b posterior kidney of the following groups of control and growth hormone transgenic coho salmon: transgenic salmon fed to satiation, control salmon fed to satiation, transgenic salmon fed an equal ration as control salmon, transgenic salmon fasted for one month, and control salmon fasted for one month. Where letters diVer (a, b, c) signiWcant diVerences among Wsh groups, within tissues, exist

genic Wsh fed a control ration (Rise et al. 2006). In mammalian studies, GH upregulates GPx activity through post-translational modiWcations (see Romanick et al. 2004), suggesting GH, or accelerated growth rate, may modify other GSH-related proteins by similar pathways. However, Rise et al. (2006) reported that transgenic Wsh fed a Wxed ration had greater hepatic gene expression of phospholipid GPx than control Wsh fed a similar ration, which was not observed in enzyme activity in the current study. There are multiple enzymes that have GPx activity in mammals (see Hayes and McLellan 1999) and possibly Wsh (Nakano et al. 1992). While expression of one GPx enzyme may be increased in ration-restricted transgenic Wsh, downregulation of other GPx enzymes may have resulted in the lack of change in overall GPx activity in the current study. GSH is the major redox regulator in cells (Arrigo 1999), and thus the percent of total GSH in oxidized form can indicate tissue redox balance. In mammals, GSSG is less than 1% of tGSH in healthy cells, and levels of 10–50% can indicate oxidative stress (Forman and Liu 1997; Arrigo 1999; Pastore et al. 2003). GH transgenic salmon have high metabolic rates and

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expression of metabolic enzymes (Cook et al. 2000; Leggatt et al. 2003; Rise et al. 2006), and therefore potentially higher ROS production than control Wsh. As such, transgenic Wsh were expected to have higher % GSSG than control Wsh. However, transgenic Wsh had lower hepatic % GSSG than controls while fed to satiation and after one month of fasting, although this was only signiWcant after fasting. Decreased % GSSG in transgenic Wsh was likely due to observed higher GR activity, as GR is responsible for returning GSSG to its reduced and antioxidant active form. Transgenic Wsh fed to satiation also had higher gene expression of pentose phosphate shunt enzymes, which could increase NADPH production that fuels GSSG reduction (Rise et al. 2006). These data indicate that transgenic Wsh were able to compensate for potential metabolically induced oxidative shifts in the GSH redox balance. Indeed, high GSH and low GSSG suggest transgenic Wsh fed to satiation were better able to maintain liver redox status than control Wsh, and maintained this advantage after one of month of fasting. In mammals, increased age is associated with higher GSSG levels (e.g., Donahue et al. 2006), and the lower GSSG observed in transgenic Wsh may be due to their younger age than control Wsh (9 vs. 21 months, respectively). However, transgenic Wsh fed to satiation have accelerated physiological aging (e.g., smolt at an earlier age, Devlin et al. 2004), and transgenic Wsh fed a control ration did not diVer in % GSSG from those fed to satiation, although had lower GR activity, despite being 1 year older. As such, age may not be a major factor determining GSSG levels in transgenic Wsh, but may be a factor in determining GR activity. In contrast to GR activity, GPx activity was not aVected by feeding level or the transgene, indicating this enzyme is either insensitive to GH and accelerated growth rate, or basal levels are suYcient to cope with increased metabolic rate of transgenic Wsh. The eVect of the GH transgene on tGSH levels in this study are consistent with Brown-Borg and Rakoczy (2003), who found GH injection in Ames dwarf mice increased GSH and decreased GSSG levels in liver, muscle and brain. However, unlike the current study, GH administration decreased !GT activity of Ames mice in several tissues, but had inconsistent eVects on GCS activity, indicating the source for high GSH levels was due to decreased GSH catabolism and not increased synthesis in this organism (Brown-Borg et al. 2005). As well, GH injection decreased GPx and had no eVect on GR activity in Ames dwarf mice (Brown-Borg and Rakoczy 2003), and GH transgenesis decreased GPx in normal mice (Hauck and Bartke 2001), although the eVects of high feeding and growth

J Comp Physiol B (2007) 177:413–422

were not separated from other eVects of GH in these studies. DiVerences between mice studies and the current study indicate the eVects of GH and growth rate on GSH-associated enzymes may diVer between mammalian and Wsh models. Fasting decreased liver tGSH, GSH and GPx, and intestinal and kidney !GT in both transgenic and control Wsh, and decreased plasma tGSH in transgenic Wsh. Similar eVects were reported in several studies in Wsh, reptiles and mammals that found starvation decreased GSH levels in various tissues (Dziubek 1987; Ogasawara et al. 1989; Hum et al. 1991; Gallagher et al. 1992; Di Simplicio et al. 1997; Szkudelski et al. 2004; Leggatt et al. 2006). In contrast to the current study, previous studies found starvation increased % GSSG or lipid peroxidation levels, increased GPx activity, and had inconsistent eVects on GR activity (Ogasawara et al. 1989; Di Simplicio et al. 1997; Pascual et al. 2003; Morales et al. 2004; Altan et al. 2005). High GSSG and lipid peroxidation in previous studies suggest starvation results in oxidative stress in most organisms. Although lipid peroxidation or other end products of oxidative stress were not measured, fasting did not alter % GSSG in the liver of coho salmon, indicating Wsh maintained redox balance in this tissue. Transgenic and control coho salmon reportedly have lower metabolic rates during starvation (Leggatt et al. 2003), which may result in decreased ROS production and consequently decrease demand for antioxidants in this species. Surprisingly, kidney tGSH levels increased after fasting in both transgenic and control Wsh. This may be due to decreased activity of kidney !GT, and consequently decreased catabolism of GSH in this tissue.

Conclusions Accelerated feeding and growth rates associated with the insertion of a GH transgene in coho salmon resulted in upregulation of the GSH antioxidant system. This upregulation was partially maintained after one month of fasting, but not during long-term ration restriction. High GSH levels are likely fueled by increased dietary intake of GSH precursors, including an increased ability to metabolize dietary GSH. The GSH antioxidant system may be upregulated to combat increased metabolic production of ROS arising during accelerated growth in GH transgenic coho salmon, and preliminary data suggest antioxidant upregulation results in maintenance of redox balance during accelerated growth in GH transgenic coho salmon.

421 Acknowledgments This project was supported by a Canadian Regulatory System for Biotechnology grant to RH Devlin, and Natural Sciences and Engineering Research Council of Canada (NSERC) grants to CJ Brauner and GK Iwama. We are indebted to Dionne Sakhrani and Carlo Biagi for assistance with this project, Dr. Patricia Schulte for feedback on this project, and to Dr. Matthew Rise for supplying gene array data. This experiment complied with current laws of the country (Canada) in which experiments were performed.

References Altan O, Oguz I, Erbayraktar Z, Yimaz O, Bayraktarl H, Sis B (2005) The eVect of prolonged fasting and refeeding on GSH-Px activity, plasma glucose and cholesterol levels and welfare of older hens from diVerent genotypes. Archiv GeXügelkunde 69:185–191 Arrigo A-P (1999) Gene expression and the thiol redox state. Free Radic Biol Med 27:936–944. PII S0891-5849(99)00175-6 Bartke A (1998) Growth hormone and aging. Endocrine 8:103– 108. PII S0891-5849(99)00175-6 Beyea JA, Olson DM, Harvey S (2005) Growth hormone (GH) action in the developing lung: changes in lung proteins after adenoviral GH overexpression. Dev Dyn 234:404–412. doi: 10.1002/dvdy.20538 Brown-Borg HM, Johnson WT, Rakoczy SG, Romanick MA (2001) Mitochondrial oxidant generation and oxidative damage in Ames dwarf and GH transgenic mice. Age 24:85–100 Brown-Borg HM, Rakoczy SG, Romanick MA, Kennedy MA (2002) EVects of growth hormone and insulin-like growth factor-1 on hepatocyte antioxidant enzymes. Exp Biol Med 227:94–104 Brown-Borg HM, Rakoczy SG (2003) Growth hormone administration to long-living dwarf mice alters multiple components of the antioxidative defense system. Mech Ageing Dev 124:1013–1024. doi: 10.1016/j.mad.2003.07.001 Brown-Borg HM, Rakoczy SG, Uthus EO (2005) Growth hormone alters methionine and glutathione metabolism in Ames dwarf mice. Mech Ageing Dev 126:389–398. doi: 10.1016/j.mad.2004.09.005 Carlson JC, Bharadwaj R, Bartke A (1999) Oxidative stress in hypopituitary dwarf mice and in transgenic mice overexpressing human and bovine GH. Age 22:181–186 Cherbonnier C, Déas O, Carvalho G, Vassal G, Dürrbach A, HaeVner A, Charpentier B, Bénard J, Hirsch F (2003) Potentiation of tumor apoptosis by human growth hormone via glutathione production and decreased NF-#B activity. Br J Cancer 89:1108–1115. doi: 10.1038/sj.bjc.6601223 Cook JT, McNiven MA, Sutterlin AM (2000) Metabolic rate of pre-smolt growth-enhanced transgenic Atlantic salmon (Salmo salar). Aquaculture 188:33–45. PII S00448486(00)00332-X Devlin R, Yesaki T, Biagi C, Donaldson E (1994) Extraordinary salmon growth. Nature 371:209-210. doi: 10.1038/371209a0 Devlin RH, Johnsson JI, Smailus DE, Biagi CA, Jönsson E, Björnsson BT (1999) Increased ability to compete for food by growth hormone-transgenic coho salmon Oncorhynchus kisutch (Walbaum). Aquac Res 30:479–482 Devlin RH, Biagi CA, Yesaki TY (2004) Growth, viability and genetic characteristics of GH transgenic coho salmon strains. Aquaculture 236:607–632. doi: 10.1016/j.aquaculture.2004.02.026 Di Simplicio P, Rossi R, Falcinelli S, Ceserani R, Formento ML (1997) Antioxidant status in various tissues of the mouse

123

422 after fasting and swimming stress. Eur J Appl Physiol 76:302–307 Donahue AN, Aschner M, Lash LH, Syversen T, Sonntag WE (2006) Growth hormone administration to aged animals reduces disulWde glutathione levels in hippocampus. Mech Ageing Dev 127:57–63. doi: 10.1016/j.mad.2005.09.003 Dziubek K (1987) Glutathione metabolism in selected organs of Rana temporaria L. in annual cycle and under diVerent stressors. Part II. Starvation and temperature eVects. Acta Biologica Cracoviensia ser. Zoologica 29:55–68 Forman HF, Liu R-M (1997) Glutathione cycling in oxidative stress. In: Clerch B, Massaro DJ (eds) Oxygen, gene expression and cellular function. Dekker, New York, pp 99–121 Fridovich I (2004) Mitochondria: are they the seat of senescence? Aging Cell 3:13–16. doi: 10.1111/j.1474-9728.2003.00075.x Gallagher EP, Hasspieler BM, Di Giulio RT (1992) EVects of buthionine sulfoximine and diethyl maleate on glutathione turnover in the channel catWsh. Biochem Pharmacol 43:2209–2215 GriYth OW (1980) Determination of glutathione and glutathione disulWde using glutathione reductase and 2-vinylpyridine. Anal Biochem 106:207–212 Hauck SJ, Bartke A (2001) Free radical defenses in the liver and kidney of human growth homone transgenic mice: possible mechanisms of early mortality. J Gerontol A 56:B153–B162 Hayes JD, McLellan LI (1999) Glutathione and glutathionedependent enzymes represent a co-ordinately regulated defense against oxidative stress. Free Radic Res 31:273–300 Hum S, Robitaille L, HoVer LJ (1991) Plasma glutathione turnover in the rat: eVect of fasting and buthionine sulfoximine. Can J Physiol Pharmacol 69:581–587 Kidd PM (1997) Glutathione: systemic protectant against oxidative and free radical damage. Alternat Mede Rev 2:155–176 Kolataj A, Ryijska J, Flak P (1998) The inXuence of selection on stress reactivity in mice. Part VI. EVect of starvation and T3 injection on the level of glutathione in liver and kidney. J Anim Breed Genet 115:407–411 Leggatt RA (2006) Glutathione in Wsh: transport, inXuence of temperature and growth rate, and interactions with the stress response. Thesis (Ph.D.) University of British Colmbia, Vancouver Leggatt RA, Devlin RH, Farrell AP, Randall DJ (2003) Oxygen uptake of growth hormone transgenic coho salmon during starvation and feeding. J Fish Biol 62:1053–1066. doi: 10.1046/j.1095-8649.2003.00096.x Leggatt RA, Scheer KW, Afonso LOB, Iwama GK (2006) Triploid and diploid rainbow trout do not diVer in their stress response to transportation. North Am J Aquac 68:1–8. doi: 10.1577/A05-035.1 Meister A (1995) Strategies for increasing cellular glutathione. In: Packer L, Cadenas E (eds) Biothiols in health and disease. Dekker, New York, pp 165–188 Meister A, Tate SS, GriYth OW (1981) !-Glutamyl transpeptidase. Meth Enzymol 77:237–253 Morales AE, Pérez-Jiménez A, Hidalgo MC, Abellán E, Cardenete G (2004) Oxidative stress and antioxidant defenses after prolonged starvation in Dentex dentex liver. Comp Biochem Physiol C 139:153–161. doi: 10.1016/j.cca.2004.10.008 Muradian KK, Utko NA, Mozzhukhina TG, Litoshenko AY, Pishel IN, Bezrukov VV, FraiWeld VE (2002) Pair-wise linear and 3D nonlinear relationships between the liver antioxidant enzyme activities and the rate of body oxygen consumption

123

J Comp Physiol B (2007) 177:413–422 in mice. Free Radic Biol Med 33:1736–1739. PII S08915849(02)01196-6 Nakano T, Sato M, Takeuchi M (1992) Partial puriWcation and properties of glutathione peroxidase from carp hepatopancreas. Comp Biochem Physiol B 102:31–35 Ogasawara T, Ohnhaus EE, Hoensch HP (1989) Glutathione and its related enzymes in the small intestinal-mucosa of rats— eVects of starvation and diet. Res Exp Med 189:195–204 Pascual P, Pedrajas JR, Toribio F, López-Barea J, Peinado J (2003) EVect of food deprivation on oxidative stress biomarkers in Wsh (Sparus aurata). Chem Biol Interact 145:191–199. doi: 10.1016/S0009-2797(03)00002-4 Pastore A, Federici G, Bertini E, Piemonte F (2003) Analysis of glutathione: implication in redox and detoxiWcation. Clin Chim Acta 333:19–39. doi: 10.1016/S0009-8981(03)00200-6 Raven PA, Devlin RH, Higgs DA (2006) InXuence of dietary digestible energy content on growth, protein and energy utilization and body composition of growth hormone transgenic and non-transgenic coho salmon (Oncorhynchus kisutch). Aquaculture 254:730–747. doi: 10.1016/j.aquaculture.2005.11.009 Rise ML, Douglas SE, Sakhrani D, Williams J, Ewart KV, Rise M, Davidson WS, Koop BF, Devlin RH (2006) Multiple microarray platforms utilized for hepatic gene expression proWling of growth hormone transgenic coho salmon with and without ration restriction. J Mol Endocrinol 37:259–282 Rollo CD (2002) Growth negatively impacts the life span of mammals. Evol Dev 4:55–61 Rollo CD, Carlson JC, Sawada M (1996) Accelerated aging of giant transgenic mice is associated with elevated free radical processes. Can J Zool 74:606–620 Romanick MA, Rakoczy SG, Brown-Borg HM (2004) Long-lived Ames dwarf mouse exhibits increased antioxidant defense in skeletal muscle. Mech Ageing Dev 125:269–281. doi: 10.1016/ j.mad.2004.02.001 Seelig GF, Meister A (1985) Glutathione biosynthesis—gammaglutamylcysteine synthetase from rat-kidney. Meth Enzymol 113:379–390 Smith PK, Krohn RI, Hemanson GT, Mallia AK, Gartner FH, Provenzano MD, Fujimoto EK, Goeke NM, Olson BJ, Klenk DC (1985) Measurement of protein using bicinchoninic acid. Anal Biochem 150:76–85 Stephensen E, Sturve J, Förlin L (2002) EVects of redox cycling compounds on glutathione content and activity of glutathione-related enzymes in rainbow trout liver. Comp Biochem Physiol C 133:435–442. doi: 10.1016/S1532-0456(02)00129-1 Stevens ED, Devlin RH (2000) Intestinal morphology in growth hormone transgenic coho salmon. J Fish Biol 56:191–195. doi: 10.1111/j.1095-8649.2000.tb02094.x Stevens ED, Devlin RH (2005) Gut size in GH-transgenic coho salmon is enhanced by both the GH transgene and increased food intake. J Fish Biol 66:1633–1648. doi: 10.1111/j.10958649.2005.00707.x Szkudelski T, Okulicz M, Bialik I, Szkudelska K (2004) The inXuence of fasting on liver sulfhydryl groups, glutathione peroxidase and glutathione-S-transferase activities in the rat. J Physiol Biochem 60:1–6 Zhu Z, Mukhina S, Zhu T, Mertani HC, Lee K-O, Lobie PE (2005) p44/42 MAP kinase-dependent regulation of catalase by autocrine human growth hormone protects human mammary carcinoma cells from oxidative stress-induced apoptosis. Oncogene 34:3774–3785. doi: 10.1038/sj.onc.1208541