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Division of Endocrinology, Metabolism and Nutrition, Mayo Clinic, 200 First Street SW, 5-194 Joseph, Rochester, Minnesota 55905, USA

(Correspondence should be addressed to C A Conover; Email: conover{at}mayo.edu)

	Abstract

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Mice born with the deletion of the gene for pregnancy-associated plasma protein-A (PAPP-A), a model of reduced local IGF activity, live 30% longer than their wild-type (WT) littermates. In this study, we investigated metabolic consequences of PAPP-A gene deletion and possible relationship to lifespan extension. Specifically, we determined whether 18-month-old PAPP-A knockout (KO) mice when compared with their WT littermates have reduced energy expenditure and/or altered glucose–insulin sensitivity. Food intake, and total energy expenditure and resting energy expenditure as measured by calorimetry were not different between PAPP-A KO and WT mice when subjected to the analysis of covariance with body weight as the covariate. However, there was an increase in spontaneous physical activity in PAPP-A KO mice. Both WT and PAPP-A KO mice exhibited mild insulin resistance with age, as assessed by fasting glucose/insulin ratios. Oral glucose tolerance and insulin sensitivity were not significantly different between the two groups of mice, although there appeared to be a decrease in the average size of the pancreatic islets in PAPP-A KO mice. Thus, neither reduced ‘rate of living’ nor altered glucose–insulin homeostasis can be considered key determinants of the enhanced longevity of PAPP-A KO mice. These findings are discussed in the context of those from other long-lived mouse models.

	Introduction

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Pregnancy-associated plasma protein-A (PAPP-A) is a recently discovered metalloproteinase whose major physiological function, as so far identified, is to increase local insulin-like growth factor (IGF) bioavailability through the cleavage of inhibitory IGF binding proteins (reviewed in Boldt & Conover 2007). Reduced IGF signaling is associated with a prolonged lifespan in a variety of species (Kenyon 2001, Barbieri et al. 2003, Holzenberger et al. 2004, Richardson et al. 2004). Indeed, mice born with the deletion of the PAPP-A gene live 30% longer than their wild-type (WT) littermates (Conover & Bale 2007). However, the mechanism(s) underlying this enhanced longevity have yet to be determined. With this study, we initiated investigation into key factors involved in lifespan extension in a mammalian model of reduced local IGF action.

Together, with insulin, IGFs are important regulators of cell metabolism and growth. The ‘rate of living’ theory of aging posits that a decrease in overall metabolic rate decreases the production of reactive oxygen species (ROS) during respiration, thereby slowing the aging process (reviewed in Masoro 2005). If so, then reduced IGF signaling might result in decreases in metabolism that could explain the increase in lifespan in PAPP-A knockout (KO) mice. Decreased body temperature and lowered metabolism in the Ames and Snell dwarf mice, long-lived growth hormone (GH)- deficient mouse models, would support this theory (Bartke et al. 2001). It is of note that these mice have low circulating IGF-I due to the loss of GH-dependent hepatic IGF-I expression. Studies of caloric restriction in rats provide evidence for an association between lifespan, circulating IGFs, and metabolic rate (Krystal & Yu 1994). Caloric restriction in rhesus monkeys also results in decreased core temperature and 24 h energy expenditure (Lane et al. 1996). On the other hand, there are several examples of reduced insulin and/or IGF-I signaling where the organism does not have to be metabolically compromised to age slowly (Barbieri et al. 2003, Holzenberger et al. 2003, Hulbert et al. 2004).

Furthermore, a progressive rise in insulin resistance is associated with aging (Ferrannini et al. 1996, Facchini et al. 2001), and a direct cause and effect relationship has been suggested. The ‘altered glucose–insulin system’ hypothesis suggests that the lifetime maintenance of low levels of glucose and insulin can explain life extension (Masoro 2005). Invertebrate models have been particularly informative (Broughton et al. 2005). However, animal models of extended lifespan are somewhat controversial in this regard. In support of this hypothesis, GH-deficient and GH-resistant dwarf mice with reduced circulating IGF-I show decreased fasting levels of glucose and insulin and increased insulin sensitivity (Dominici et al. 2002, Coschigano et al. 2003, Bonkowski et al. 2006). There is a similarly reduced glucose–insulin profile with caloric restriction (Richardson et al. 2004). Seemingly against a direct relationship between aging and insulin resistance are the long-lived Klotho transgenic mice. These mice have an increase in lifespan and are insulin and IGF-I resistant (Kurosu et al. 2005).

In this study, we focused on the basic metabolic and activity profile of PAPP-A KO mice, a model of reduced local IGF-I action in the context of normal circulating IGF-I levels (Conover et al. 2003, Conover & Bale 2007). Specifically, we tested the hypothesis that aged PAPP-A KO mice have reduced energy expenditure and/or altered glucose–insulin sensitivity when compared with WT littermates.

	Materials and Methods

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PAPP-A KO mice

Mice with the targeted deletion of the PAPP-A gene were generated through homologous recombination in embryonic stem cells (Conover et al. 2003). These mice were on a mixed C57BL/6 and 129 background. WT and PAPP-A KO littermates from heterozygous breeding were used in studies. Genotyping was performed by PCR as described previously (Bale & Conover 2005). For all experimental animals, genotypes were reconfirmed on tail DNA at time of sacrifice. Data from mice with obvious tumors (one 18-month-old WT mouse) were eliminated from analyses. All mice were kept on a 12 h light:12 h darkness cycle with the light phase beginning at 0600 h.

Food intake, energy expenditure, and spontaneous activity

For food intake, mice were housed for 7 days in a controlled environment within individual plexiglas cages that matched the chamber used for energy expenditure and activity measurements. These cages have ceramic bowls designed not to tip allowing daily measurement of ad libitum food intake, which was performed for 5 consecutive days. Energy expenditure and physical activity were measured as previously reported (Novak et al. 2006, Barbosa et al. 2007). At least 24 h before the time of measurement, each mouse was allowed to acclimate to the testing conditions in a cylindrical chamber (7 l; 30 cm diameterx10 cm high) in the testing area. To measure energy expenditure, custom made small animal calorimeters with a 7 l cylindrical chamber (Columbus Instruments; Columbus, OH, USA) were used. For calorimetry, the sample flow rate was set at 0.4 l per min (lpm), and the chamber flow rate at which room air pumped through the chamber was set at 0.45–0.7 lpm, depending on the size of the mouse. Samples were collected once every 60 s for 25 h, except for the reference samples collected for 5 min after every 30 samples throughout the 24-h measurement period. Data for each calorimeter were monitored and compared with ensure that the data collected and reference values were comparable between sets of equipment. Data collected included VO₂ and VCO₂ (both in ml/kg per h), respiratory exchange ratio (RER, VCO₂/VO₂), and energy expenditure ((3.815+1.232xRER)xVO₂) in kcal/h). Food and water were supplied ad libitum. The calorimetry and physical activity data were calculated every minute, and the energy expenditure data were 60 s behind the activity data, due to the time needed to move and process the air from the calorimetry chamber. To calculate resting energy expenditure (REE), we calculated the total daily energy expenditure (TDEE) for the minutes where there were no activity counts for the current and previous 5 min (see physical activity, below) and averaged them. When the REE is subtracted from the TDEE, this yields energy expenditure of activity (EEA).

Attached to the calorimeters were Opto-M Varimex Minor activity monitors. These devices contain 45 collimated infrared activity sensors that gather data regarding physical activity in three axes (horizontal x and y axes, plus vertical activity) as well as ambulatory activity (which excluded repetitive signals from a single beam). Data for physical activity and energy expenditure were collected simultaneously every minute, allowing correlation of the two variables in each animal tested.

Oral glucose tolerance and insulin sensitivity testing

For oral glucose tolerance testing, food was removed from cages at 2100 h, and glucose (0.01 ml of a 10% glucose solution per g body weight) administered by gavage at 0900 h the next morning. Blood was collected from the tail vein at 0, 5, 10, 15, 30, 45, 60, 90, and 120 min. Glucose was measured using a One Touch Ultra glucose meter (LifeScan, Milpitas, CA, USA). Insulin was measured at 0, 5, 10, 15, and 30 min using an Ultra Sensitive Rat Insulin ELISA Kit from Crystal Chem Inc. (Downers Grove, IL, USA). Area under the curve was measured using the trapezoid method. For insulin sensitivity testing, overnight-fasted mice were given food at 0830 h for 30 min. At 1200 h, mice then received an i.p. injection of regular insulin (75 mU/kg body weight) with blood collections at 0, 20, 40, and 60 min for glucose measurements. Insulin sensitivity was assessed by comparing rate-of-decay curves.

Pancreas immunohistochemistry

The pancreas was carefully dissected, fixed in formalin, embedded, and processed as described by Devedjian et al. (2000). Briefly, the entire pancreas was cut in 5 µm sections every 150 µm and every other section stained for insulin by the Tissue and Cell Molecular Analysis Core Lab using guinea pig anti-swine insulin antibody from Dako (Carpinteria, CA, USA). The Nikon Microphot-FXA and Nikon ACT-1 Version 2.6.2 (Melville, NY, USA) were used to photo-capture the slides. The number of islets and islet cell mass were measured using the polygonal lasso tool in Adobe PhotoShop 6.0.1.

	Results

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Food intake, energy expenditure, and physical activity

Food intake, energy expenditure, and spontaneous physical activity were measured in 18-month-old WT and PAPP-A KO mice. Both male and female mice were studied, and the results are presented in Table 1. Food intake, expressed as g/day, and TDEE and REE, expressed as kcal/day, were significantly decreased in male and female PAPP-A KO when compared with WT mice. However, these calculations do not take into account the significantly smaller size of the PAPP-A KO mouse (Conover et al. 2003, Bale & Conover 2005). The normalization of the data for body weight indicated significantly increased food intake (males only), TDEE, and REE in PAPP-A KO mice when compared with WT littermates. However, in this case dividing by body weight may overcorrect energy expenditure values of the larger animals (Packard & Boardman 1999, Arch et al. 2006). Therefore, the data were subjected to the analysis of covariance which uses the group (genotype) comparison of regression lines to determine whether the groups differ when body weight is a covariate. The results of these analyses are presented in Table 2. Differences in TDEE and REE were accounted for by body weight and not by genotype. EEA was not significantly different in WT and PAPP-A KO mice and could not be accounted for by body weight. However, vertical activity was increased 30–110% in PAPP-A KO mice, reaching significance in males (Table 1). Thus, there was no decrease in energy expenditure in 18-month-old PAPP-A KO mice when compared with WT mice which would suggest that a compensatory slowing of metabolism was the direct link to longevity. On the contrary, there may be an increase in physical activity in the PAPP-A KO mice.

Fasting glucose and insulin levels were not significantly different between WT and PAPP-A KO mice at 4 months or 18 months of age (Table 3). In general, female mice had lower fasting insulin levels than male mice, which were independent of genotype. There was an increase in insulin levels in the older animals to maintain the same glucose levels, suggesting mild insulin resistance. This increase tended to be less in the PAPP-A KO mice but the data did not reach statistical significance. Oral glucose tolerance testing was performed on 12 h fasted WT and PAPP-A KO mice, both males and females at 18 months of age. There was no significant difference in the endogenous glucose or insulin response to the glucose challenge between PAPP-A KO and WT mice, either male (Fig. 1A) or female (Fig. 1B). Total pancreatic islet area and number were not significantly different in 18-month-old WT and PAPP-A KO mice (Table 4). However, there was a significantly smaller, by 30%, average size of islets in PAPP-A KO when compared with WT mice. Sensitivity to exogenously administered insulin was not significantly different in 18-month-old WT and PAPP-A KO mice, either male (Fig. 2A) or female (Fig. 2B).

View larger version (25K): [in this window] [in a new window]   Figure 1 Oral glucose tolerance curves from (A) male and (B) female PAPP-A KO and WT mice. Eighteen-month-old PAPP-A KO (closed symbols, solid lines) and WT (open symbols, dashed lines) mice underwent an oral glucose tolerance test as described in Materials and Methods. Thicker lines represent the glucose response curve and thinner lines represent the insulin response curve. Values are mean±S.E.M., n=10. There were no significant differences in AUC for glucose or insulin between WT and PAPP-A KO groups.

View larger version (11K): [in this window] [in a new window]   Figure 2 Insulin sensitivity in (A) male and (B) female PAPP-A KO and WT mice. Eighteen-month-old PAPP-A KO (closed symbols, solid lines) and WT (open symbols, dashed lines) mice underwent insulin sensitivity testing as described in Materials and Methods. Values are mean±S.E.M., n=6–12. There were no significant differences in rate-of-decay curves for WT and PAPP-A KO mice.

	Discussion

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There was no decrease in total or REE in 18-month-old PAPP-A KO mice when compared with WT mice when the significantly smaller size of the PAPP-A KO mice was taken into account (Packard & Boardman 1999, Arch et al. 2006). Methods of normalization for body weight can lead to different conclusions and may explain some of the studies with caloric restriction and apparent decreases in metabolic rate. Furthermore, recent studies have suggested that, after an initial reduction, caloric restriction leads to equal or higher metabolic rates than ad libitum fed animals (Faulks et al. 2006). It has been assumed that a decrease in metabolic rate would prolong lifespan by deceasing exposure to ROS produced during respiration. Alternatively, under normal metabolic conditions, tissues from PAPP-A KO mice may be less vulnerable to oxidative damage by the virtue of increased ability to eliminate ROS or show increased mitochondrial efficiency thereby reducing ROS production. These have been suggested as contributors to longevity in other model systems (Balaban et al. 2005).

Furthermore, the PAPP-A KO mice had increased physical activity in the vertical axis. This reached statistical significance in males, although a similar trend for increased activity was seen in the females as well. What this behavior might mean can only be speculated upon at this time. Mice were monitored individually so it is not likely to be aggressive posturing, but rather spontaneous physical activity. There have been few studies monitoring physical activity in the various mouse models associated with aging research. In caloric-restricted mice and IGF-I receptor heterozygous mutant mice, there was no difference in total physical activity when compared to controls, although dimensional aspects were not recorded (Holzenberger et al. 2003, Faulks et al. 2006). Interestingly, spontaneously wiggling Caenorhabditis elegans were longer-lived than their counterparts that needed to be prodded to move (Herndon et al. 2002). Apfeld et al. (2004) suggest that in this model system there is a direct link between activity levels, insulin-like signals and lifespan. Thus, this aspect of the longevity profile warrants further study.

Fasting insulin/glucose ratios increased significantly with age in both WT and PAPP-A KO mice, suggesting mild insulin resistance. Notably, for the purpose of this study, it occurred in both groups and in both sexes. Thus, PAPP-A deficiency does not necessarily translate into a ‘youthful’ insulin–glucose profile, and there was no evidence for sexual dimorphism in insulin resistance as was seen in liver-specific IGF-I deficient mice with reduced circulating IGF-I levels (Tang et al. 2005). Eighteen-month-old WT and PAPP-A KO mice challenged with glucose had similar glucose tolerance curves. Insulin secretory capacity was assessed by immunohistochemistry of insulin-producing β-cells in pancreatic islets. Multiple sections of the total pancreas were stained for insulin to try to account for heterogeneity of islet distribution and size. Interestingly, the average size but not the number or total mass of pancreatic islets was significantly decreased in PAPP-A KO mice. β-Cell-specific IGF-I receptor KO mice exhibit normal development of β-cells but defective glucose-stimulated insulin secretion and impaired glucose tolerance (Kulkarni et al. 2002). Thus, local IGF signaling does not appear to be essential for normal growth of pancreatic islets, which may partially explain the normal islet mass and number in PAPP-A KO mice. However, secretion of insulin from pancreatic β-cells is dependent on IGF-I, and a diminished (but not complete) loss of local IGF-I action in PAPP-A KO mice may affect optimal secretory response without major impact on overall glucose homeostasis. Comparable studies in other mouse models of aging indicated decreased glucose tolerance in Ames and GH-resistant dwarf mice that was due to a decrease in total volume of the islets (Parsons et al. 1995, Liu et al. 2004), perhaps reflecting reduced GH and/or circulating IGF-I. Circulating IGF-I, which is not decreased in PAPP-A KO mice, is an important component of overall insulin action in peripheral tissues (Yakar et al. 2001). This may account for unaltered insulin sensitivity in PAPP-A KO mice. Caloric-restricted mice maintain glucose homeostasis with markedly less insulin, which may be associated with increased insulin sensitivity (Richardson et al. 2004). The co-existence of insulin resistance and enhanced longevity in Klotho male transgenic mice (Kurosu et al. 2005) makes sense, as discussed below, if insulin and/or insulin sensitivity per se was not a key determinant of enhanced lifespan. Indeed, the extended lifespan of fat-specific insulin receptor KO mice may not be primarily due to reduced insulin signaling in adipose tissue, but rather a result of decreased fat mass (Bluher et al. 2003). The percentage fat is similar in WT and PAPP-A KO mice (data not shown). Thus, although we cannot completely rule out a role for reduced insulin signaling as a contributing factor to the longevity of PAPP-A KO mice, the similar fasting and fed levels of insulin ad libitum in WT and PAPP-A KO mice and the similar insulin sensitivity curves in the longer-lived PAPP-A KO mice indicate that this role would not be a decisive one.

Metabolic and activity data from different mouse models of longevity are summarized in Table 5. The many information gaps indicated in the table make it difficult to come to firm conclusions. However, there is no consistency across the seven models in metabolic rate, physical activity, fasting insulin/glucose, glucose tolerance, insulin sensitivity, GH, or circulating IGF-I levels. Of course this could be due to the specifics of the models and the testing methods. In addition, the age of the mice at the time of determination of metabolic parameters was not always apparent in the published articles in the different longevity models. This could be important for antagonist pleiotropic systems such as the IGF system, which can have beneficial effects early in life and detrimental effects late in life (Rincon et al. 2004). We chose 18 months as our ‘aged’ mouse group because after this time the WT mice tended to develop tumors, which could impact metabolic measurements. It is possible that alterations in metabolism occur in younger mice that could impact lifespan. However, this did not occur in those systems that were examined across ages (Coschigano et al. 2003). Uncertainties notwithstanding, one could propose decreased local IGF-I action as the common denominator in these long-lived mouse models. PAPP-A KO, Klotho transgenic, and IGF-I receptor heterozygous mutant mice have diminished local IGF-I signaling (Conover et al. 2003, Holzenberger 2004, Kurosu et al. 2005). It may be that Ames, Snell, and GH receptor-deficient mice have decreased local IGF-I action due to partial GH-dependence of IGF-I expression in peripheral tissues, although there is currently no evidence that these mice exhibit the loss of function of IGF-I signaling. Caloric restriction likewise has been shown to decrease IGF-I expression in hepatic and non-hepatic cells (Masternak et al. 2005, Papaconstantinou et al. 2005). Thus, a decrease in effective local IGF-I signaling, whether through decrease in IGF expression (GH deficiency/resistance, caloric restriction), decrease in IGF receptor signaling (IGF-I receptor mutation, Klotho overexpression), or diminished IGF bioavailability (PAPP-A deficiency), can increase lifespan in mice. Further studies are necessary to establish underlying mechanism(s) specific to the PAPP-A KO model that could enhance our overall understanding of lifespan determinants and suggest therapeutic targets to promote healthy aging.

	Declaration of interest

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	Funding

 
This work was supported by NIH Grant AG028141 and the Ellison Medical Foundation (to C A C), NINDS 55859, 0635113N from the American Heart Association, and a grant from the Minnesota Obesity Consortium (to C M N), and NIH grants DK56650, DK63226, DK66270, and R04-0771 (to J A L).

	References

Top Abstract Introduction Materials and Methods Results Discussion Declaration of interest References

 
Apfeld J, O'Connor G, McDonagh T, DiStefano PS & Curtis R 2004 The AMP-activated protein kinase AAK-2 links energy levels and insulin-like signals to lifespan in C. elegans. Genes and Development 18 3004–3009.[Abstract/Free Full Text]

Arch JRS, Hislop D, Wang SJY & Speakman JR 2006 Some mathematical and technical issues in the measurement and interpretation of open-circuit indirect calorimetry in small animals. International Journal of Obesity 30 1322–1331.[CrossRef][Web of Science][Medline]

Balaban RS, Nemoto S & Finkel T 2005 Mitochondria, oxidants, and aging. Cell 120 483–495.[CrossRef][Web of Science][Medline]

Bale LK & Conover CA 2005 Disruption of insulin-like growth factor-II imprinting during embryonic development rescues the dwarf phenotype of mice null for pregnancy-associated plasma protein-A. Journal of Endocrinology 186 325–331.[Abstract/Free Full Text]

Barbieri M, Bonafe M, Franceschi C & Paolisso G 2003a Insulin/IGF-I-signaling pathway: an evolutionarily conserved mechanism of longevity from yeast to humans. American Journal of Physiology. Endocrinology and Metabolism 285 E1064–E1071.[Abstract/Free Full Text]

Barbosa MT, Soares SM, Novak CM, Sinclair D, Levine JA, Aksoy P & Chini EN 2007 The enzyme CD38 (a NAD glycohydrolase, EC 3.2.2.5) is necessary for the development of diet-induced obesity. FASEB Journal 21 3629–3639.[Abstract/Free Full Text]

Bartke A, Brown-Borg H, Mattison J, Kinney B, Hauck S & Wright C 2001 Prolonged longevity of hypopituitary dwarf mice. Experimental Gerontology 36 21–28.[CrossRef][Web of Science][Medline]

Bluher M, Kahn BB & Kahn CR 2003 Extended longevity in mice lacking the insulin receptor in adipose tissue. Science 299 572–574.[Abstract/Free Full Text]

Boldt HB & Conover CA 2007 Pregnancy-associated plasma protein-A (PAPP-A): a local regulator of IGF bioavailability through cleavage of IGFBPs. Growth Hormone and IGF Research 17 10–18.[CrossRef]

Bonkowski MS, Rocha JS, Masternak MM, Al Regaiey KA & Bartke A 2006 Targeted disruption of growth hormone receptor interferes with the beneficial actions of calorie restriction. PNAS 103 7901–7905.[Abstract/Free Full Text]

Broughton SJ, Piper MDW, Ikeya T, Bass TM, Jacobson J, Driege Y, Martinez P, Hafen E, Withers DJ, Leevers SJ et al. 2005 Longer lifespan, altered metabolism, and stress resistance in Drosophila from ablation of cells making insulin-like ligands. PNAS 102 3105–3110.[Abstract/Free Full Text]

Conover CA & Bale LK 2007 Loss of pregnancy-associated plasma protein A extends lifespan in mice. Aging Cell 6 727–729.[CrossRef][Web of Science][Medline]

Conover CA, Bale LK, Overgaard MT, Johnstone EW, Laursen UH, Fuchtbauer E-M, Oxvig C & van Deursen J 2003 Metalloproteinase pregnancy-associated plasma protein A is a critical growth regulatory factor during fetal development. Development 131 1187–1194.[CrossRef][Web of Science]

Coschigano KT, Holland AN, Riders ME, List EO, Flyvbjerg A & Kopchick JJ 2003 Deletion, but not antagonism, of the mouse growth hormone receptor results in severely decreased body weights, insulin, and insulin-like growth factor I levels and increased life span. Endocrinology 144 3799–3810.[Abstract/Free Full Text]

Devedjian JC, George M, Casellas A, Pujol A, Visa J, Pelegrin M, Gros L & Bosch F 2000 Transgenic mice overexpressing insulin-like growth factor II in β cells develop type 2 diabetes. Journal of Clinical Investigation 105 731–740.[Web of Science][Medline]

Dominici FP, Hauck S, Argentino DP, Bartke A & Turyn D 2002 Increased insulin sensitivity and upregulation of insulin receptor, insulin receptor substrate (IRS)-1 and IRS-2 in liver of Ames dwarf mice. Journal of Endocrinology 173 81–94.[Abstract]

Facchini FS, Hua N, Abbasi F & Reaven GM 2001 Insulin resistance as a predictor of age-related diseases. Journal of Clinical Endocrinology and Metabolism 86 3574–3578.[Abstract/Free Full Text]

Faulks SC, Turner N, Else PL & Hulbert AJ 2006 Calorie restriction in mice: effects on body composition, daily activity, metabolic rate, mitochondrial reactive oxygen species production, and membrane fatty acid composition. Journal of Gerontology: Biological Sciences61A781–794.[Abstract/Free Full Text]

Ferrannini E, Vichi S, Beck-Nielsen H, Laasko M, Paolisso G & Smith U 1996 For European Group for the Study of Insulin Resistance (EGIR). Insulin action and age. Diabetes 45 947–953.[Abstract]

Herndon LA, Schmeissner PJ, Dudaronek JM, Brown PA, Listner KM, Sakano Y, Paupard MC, Hall DH & Driscoll M 2002 Stochastic and genetic factors influence tissue-specific decline in ageing C. elegans. Nature 419 808–814.[CrossRef][Web of Science][Medline]

Holzenberger M, Dupont J, Ducos B, Leneuve P, Geloen A, Even PC, Cervera P & Le Bouc Y 2003 IGF-1 receptor regulates lifespan and resistance to oxidative stress in mice. Nature 421 182–187.[CrossRef][Web of Science][Medline]

Holzenberger M, Kappeler L & De Magalhaes Filho C 2004 IGF-1 signaling and aging. Experimental Gerontology 39 1761–1764.[CrossRef][Web of Science][Medline]

Hulbert AJ, Clancy DJ, Mair W, Braeckman BP, Gems D & Partridge L 2004 Metabolic rate is not reduced by dietary-restriction or by lowered insulin/IGF-1 signalling and is not correlated with individual lifespan in Drosophila melanogaster. Experimental Gerontology 39 1137–1143.[CrossRef][Web of Science][Medline]

Kenyon C 2001 A conserved regulatory system for aging. Cell 105 165–168.[CrossRef][Web of Science][Medline]

Krystal BS & Yu BP 1994 Aging and its modulation by dietary restriction. In Modulation of Aging Process by Dietary Restriction , pp 1–36. Eds BP Yu. London: CRC Press.

Kulkarni RN, Holzenberger M, Shih DQ, Ozcan U, Stoffel M, Magnuson MA & Kahn R 2002 β-Cell-specific deletion of the Igf1 receptor leads to hyperinsulinemia and glucose intolerance but does not alter β-cell mass. Nature Genetics 31 111–115.[Web of Science][Medline]

Kurosu H, Yamamoto M, Clark JD, Pastor JV, Nandi A, Gurnani P, McGuinness OP, Chikuda H, Yamaguchi M, Kawaguchi H et al. 2005 Suppression of aging in mice by the hormone klotho. Science 309 1829–1833.[Abstract/Free Full Text]

Lane MA, Baer DJ, Rumpler WV, Weindruch R, Ingram DK, Tilmont EM, Cutler RG & Roth GS 1996 Calorie restriction lowers body temperature in rhesus monkeys, consistent with a postulated anti-aging mechanism in rodents. PNAS 93 4159–4164.[Abstract/Free Full Text]

Liu J-L, Coschigano KT, Robertson K, Lipsett M, Guo Y, Kopchick JJ, Kumar U & Liu YL 2004 Disruption of growth hormone receptor gene causes diminished pancreatic islet size and increased insulin sensitivity in mice. American Journal of Physiology. Endocrinology and Metabolism 287 E405–E413.[Abstract/Free Full Text]

Masoro EJ 2005 Overview of caloric restriction and ageing. Mechanisms of Ageing and Development 126 913–922.[CrossRef][Web of Science][Medline]

Masternak MM, Al-Regaiey KA, Del Rosario Lim MM, Jimenez-Ortega V, Panici JA, Bonkowski MS & Bartke A 2005 Effects of caloric restriction on insulin pathway gene expression in the skeletal muscle and liver of normal and long-lived GHR-KO mice. Experimental Gerontology 40 679–684.[CrossRef][Web of Science][Medline]

Novak CM, Kotz CM & Levine JA 2006 Central orexin sensitivity, physical activity, and obesity in diet-induced obese and diet-resistant rats. American Journal of Physiology. Endocrinology and Metabolism 290 E396–E403.[Abstract/Free Full Text]

Packard GC & Boardman TJ 1999 The use of percentages and size-specific indices to normalize physiological data for variation in body size: wasted time, wasted effort? Comparative Biochemistry and Physiology, Part A 122 37–44.[CrossRef]

Papaconstantinou J, DeFord JH, Gerstner A, Hsieh C-C, Boylston WH, Guigneaux MM, Flurkey K & Harrison DE 2005 Hepatic gene and protein expression of primary components of the IGF-I axis in long lived Snell dwarf mice. Mechanisms of Ageing and Development 126 692–704.[CrossRef][Web of Science][Medline]

Parsons JA, Bartke A & Sorenson RL 1995 Number and size of islets of Langerhans in pregnant, human growth hormone-expressing transgenic, and pituitary dwarf mice: effect of lactogenic hormones. Endocrinology 136 2013–2021.[Abstract]

Richardson A, Liu F, Adamo ML, Van Remmen H & Nelson JF 2004 The role of insulin and insulin-like growth factor-I in mammalian ageing. Best Practice and Research. Clinical Endocrinology and Metabolism 18 393–406.[CrossRef]

Rincon M, Muzumdar R, Atzmon G & Barzilai N 2004 The paradox of the insulin/IGF-1 signaling pathway in longevity. Mechanisms of Ageing and Development 125 397–403.[CrossRef][Web of Science][Medline]

Tang Z, Yu R, Lu Y, Parlow AF & Liu J-L 2005 Age-dependent onset of liver-specific IGF-I gene deficiency and its persistence in old age: implications for postnatal growth and insulin resistance in LID mice. American Journal of Physiology. Endocrinology and Metabolism 289 E288–E295.[Abstract/Free Full Text]

Yakar S, Liu J-L, Fernandez AM, Wu Y, Schally AV, Frystyk J, Chernausek SD, Mejia W & Le Roith D 2001 Liver-specific igf-1 gene deletion leads to muscle insulin insensitivity. Diabetes 50 1110–1118.[Abstract/Free Full Text]

Received in final form 10 June 2008
Accepted 17 June 2008
Made available online as an Accepted Preprint 19 June 2008

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This Article Abstract Full Text (PDF) All Versions of this Article: JOE-08-0179v1 198/3/599    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (2) Citing Articles via Google Scholar Google Scholar Articles by Conover, C. A Articles by Novak, C. M Search for Related Content PubMed PubMed Citation Articles by Conover, C. A Articles by Novak, C. M