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. 2008 Oct 23;150(3):1353–1360. doi: 10.1210/en.2008-1199

Age-Related Changes in Body Composition of Bovine Growth Hormone Transgenic Mice

Amanda J Palmer 1, Min-Yu Chung 1, Edward O List 1, Jennifer Walker 1, Shigeru Okada 1, John J Kopchick 1, Darlene E Berryman 1
PMCID: PMC2654748  PMID: 18948397

Abstract

GH has a significant impact on body composition due to distinct anabolic and catabolic effects on lean and fat mass, respectively. Several studies have assessed body composition in mice expressing a GH transgene. Whereas all studies report enhanced growth of transgenic mice as compared with littermate controls, there are inconsistencies in terms of the relative proportion of lean mass to fat mass in these animals. The purpose of this study was to characterize the accumulation of adipose and lean mass with age and according to gender in a bovine (b) GH transgenic mouse line. Weight and body composition measurements were assessed in male and female bGH mice with corresponding littermate controls in the C57BL/6J genetic background. Body composition measurements began at 6 wk and continued through 1 yr of age. At the conclusion of the study, tissue weights were determined and triglyceride content was quantified in liver and kidney. Although body weights for bGH mice were significantly greater than their corresponding littermate controls at all time points, body composition measurements revealed an unexpected transition midway through analyses. That is, younger bGH mice had relatively more fat mass than nontransgenic littermates, whereas bGH mice became significantly leaner than controls by 4 months in males and 6 months in females. These results reveal the importance in timing and gender when conducting studies related to body composition or lean and fat tissue in GH transgenic mice or in other genetically manipulated mouse strains in which body composition may be impacted.

Longitudinal assessment of body composition in GH transgenic mice reveals novel age dependent and gender-specific effects of GH on adiopse tissue.

In studies addressing endocrine function in rodents, weight is often used as a quick, easy measurement to assess the efficacy of experimental or genetic manipulations. However, changes in body weight do not provide information about the contribution of lean vs. fat tissue to the animal’s total weight, possibly leading to incorrect assumptions about adiposity or lean mass. Thus, measurement of body composition, or the relative percentage of body weight that is fat and fat-free tissue, is invaluable in studies that manipulate both endocrine function and adiposity.

A number of different measurement techniques are available for assessing body composition in small animals, which vary in accuracy, acceptance, cost, invasiveness, and efficiency (1,2,3,4). Ideally, a method that is a rapid, safe, and accurate would be preferred for longitudinal assessment of body composition. Nuclear magnetic resonance (NMR) technology provides such a method for determining in vivo body composition in live, awake animals (2,5) and allows for the repeated measurement of body composition without invasive procedures.

GH is a key regulator of body composition, possessing both anabolic and catabolic activities. Increased protein accretion in muscle and lipolysis in adipose tissue are considered to be major contributors as to how GH promotes a lean phenotype (as reviewed in Ref. 6). Not surprisingly, excess or deficiency in GH function leads to significant alterations of body composition in humans. That is, excess GH in humans, as occurs with acromegaly, results in increased muscle mass and decreased body fat mass (7). At the other extreme, deficiency or absence of GH function is associated with increased fat mass and decreased lean body mass (8,9,10). To further support this notion, various GH dosing regimens in GH-deficient adult populations have been documented to improve body composition (11,12,13,14,15,16,17). Fat and fat-free tissue are altered in an analogous manner in many animal models with altered GH function (18,19,20,21).

Mice that express a GH transgene have been generated by multiple independent laboratories. These mice have a fairly uniform phenotype regardless of genetic background or the species of GH used. Bovine (b) GH transgenic mice exhibit accelerated growth and have a greater adult body weight compared with control mice (21,22,23). Elevated circulating GH also results in elevated plasma levels of IGF-I and hyperinsulinemia despite euglycemia (24). Importantly, the life span of these GH transgenic mice is drastically reduced (∼50%) compared with nontransgenic controls (25). Specific pathological organ changes also have been noted as a result of high GH levels including severe glomerulosclerosis and lipid accumulation in the kidney and enlargement of most GH sensitive tissues, with liver and spleen experiencing the greatest increase in size (26,27,28,29,30,31,32).

Results of body composition studies in GH transgenic mice have been less consistent. By 6 months of age, adult male bGH mice are lean with reductions in all adipose depots and have reduced leptin and adiponectin levels compared with control mice (18,33). These mice also have been reported to be somewhat resistant to the effects of diet-induced obesity (19,34). Thus, GH has clear and poignant impact on fat mass in adult transgenic mice. Data in mice younger than 3 months of age have been somewhat conflicting, with some showing increases in fat mass (23,35,36) and others that use zinc-inducible GH expression showing decreases (35). Because timing of data collection appears critical, it is important to note that no data are currently available measuring fat mass over the life span of these animals or, for that matter, even for wild-type C57BL/6J mice. In addition, data are lacking for body composition in adult female bGH mice. Finally, because these mice have significantly reduced life spans, it is of interest to evaluate the longitudinal changes in body composition.

In this study, NMR technology was used to assess age-related changes in body composition in male and female bGH mice compared with littermate controls over 1 yr. Weights of dissected tissues were assessed including that of four distinct fat depots. Triglyceride content of kidney and liver from bGH and control mice was assessed to further understand the effect of GH on fat accumulation in these tissues. Collectively, these data show the importance of age in studies of bGH mice when assessing fat or lean tissue and reveal a significant gender difference in body composition between transgenic and control animals. Also, data from wild-type C57BL/6J mice show dramatic increases in fat mass and a corresponding decrease in lean tissue with advancing age.

Materials and Methods

Animals

Male and female bGH transgenic and nontransgenic littermate controls (NT) were generated and identified as previously described (18). Briefly, animals were generated by a pronuclear injection into a C57BL/6J oocyte with the bGH cDNA fused to the mouse metallothionein transcriptional regulatory element. These animals were screened for the bGH gene using PCR, and bGH males were bred to NT females to propagate the line. Successive generations were bred and screened in this same manner. Initially, a total of 10 mice for each sex and genotype (40 mice total) were used; however, due to the death of several animals, seven male bGH, eight male NT, 10 female bGH, and 10 female NT mice were used in the final analyses. Male and female mice were housed separately, with a maximum of four mice per cage. They were maintained on a 14-h light, 10-h dark cycle and supplied food and water ad libitum. Mice were bred within the animal facility at Ohio University and weaned at 28 d of age on a standard rodent diet (ProLab RMH 3000 PMI Nutrition International, Brentwood, MO; 14% of kilocalories from fat, 26% from protein, and 60% from carbohydrates). All procedures were approved by the Ohio University Institutional Care and Use Committee and fully complied with federal, state, and local policies.

Weight and body composition measurements

Beginning at 6 wk of age, weight and body composition measurements of all mice were taken every other week for 10 wk (to 16 wk of age). After 16 wk of age, weight and body composition measurements were taken monthly up to 1 yr of age. Body weights were measured in duplicate at each time point; mean body weight was used for analysis. Body composition measurements were performed in duplicate using the Bruker Minispec (The Woodlands, TX), which uses NMR technology to estimate the fat, lean, and fluid mass of the animals. Percent fat and lean and fluid mass were calculated at each time point using body weight and fat and lean mass, respectively.

Plasma GH and IGF-I measurements

For serum collection, male animals were fasted for 8 h before collection of whole-blood samples. Mice were placed under a heat lamp and blood was collected from the tail vein using heparinized capillary tubes. Blood samples were centrifuged at 7000 × g for 10 min at 4 C to collect plasma. Plasma was stored at −80 C until further analysis.

Plasma measurements of mouse (m) IGF-I and mGH were taken at 7 months of age. Plasma mIGF-I was measured using the mouse/rat IGF-I ELISA kit (Diagnostic Systems Laboratories, Webster, TX). For NT mice, plasma mGH was measured using the mouse/rat GH ELISA kit (Linco Research, St. Charles, MO). In these assays, the minimum detectable concentration of mGH and mIGF-I was 0.07 and 25 ng/ml, respectively. The intraassay coefficient of variation was less than 10% for both assays. For bGH mice, plasma samples were sent to Dr. A. F. Parlow (National Hormone and Peptide Program, Torrance, CA) where levels of bGH were measured using a bGH RIA.

Organ weights

Mice were killed at 54 wk of age by cervical dislocation. Several tissues including liver, heart, spleen, kidney, and four adipose depots (inguinal representing a sc depot; epididymal/parametrial, retroperitoneal and mesenteric representing three distinct intraabdominal depots) were collected, weighed, and immediately placed in liquid nitrogen. Tissues were stored at −80 C for future analysis.

Organ triglyceride measurements

Liver and kidney samples were used for the extraction and measurement of triglyceride levels as described previously (37). Briefly, samples were digested in a 3 m KOH in 65% ethanol solution overnight. Triglycerides-GPO (Pointe Scientific, Canton, MI) was used to measure the glycerol content of the samples. Calculations were performed to estimate triglyceride levels assuming the average molecular weight of triglyceride is 885 g/mol.

Statistical analysis

Data were analyzed using the SPSS version 14.0 software (Chicago, IL) and are presented as mean ± sem. For longitudinal data of body weights and body composition, repeated-measures two-way ANOVA was used. When the treatment by time interactions (gendertime, genotypetime or gendergenotypetime) were significant (P < 0.5, 2 tailed), X pairwise contrasts were conducted for specific weeks using a Bonferroni procedure. Data for other measurements were analyzed using the appropriate Student’s t test or two-way ANOVA. Differences were considered significant at P < 0.05.

Results

Body weight

The mean body mass of both male and female bGH mice was significantly greater than their NT controls at all time points measured (Fig. 1). Specifically, at 6 wk of age, the weight of the male bGH mice was 1.3 times greater than the male NT mice, and female bGH mice were 1.5 times greater than the female NT mice. By the last measurement, at 52 wk of age, weights of male and female bGH mice were 1.3 times greater in both genders than their littermate controls. The greatest difference in weight occurred at 20 wk in males and 24 wk in females. Besides genotype, there was a significant impact of gender on weight with male bGH having significantly greater body weight at all time points than female bGH mice; likewise, male NT mice weighed significantly more than female NT mice.

Figure 1.

Figure 1

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Weight gain over time in male and female bGH and NT littermate control mice. Weight measurements were taken every other week from 6 to 16 wk of age and then monthly to 1 yr of age. Data are expressed as mean ± sem, n = 7 (male bGH), n = 8 (male NT), and n = 10 (female bGH and NT). Two-way, repeated-measures ANOVA test revealed a significant impact of gender [F(1,31) = 279.8, P < 0.001] and genotype [F(1,31) = 76.4, P < 0.001] on body weight. Effect of gender on body weight was independent of genotype.

Fat mass

The absolute fat mass of both male and female bGH mice was greater than their NT counterparts at the beginning of the study and remained higher until approximately 16 wk of age (Fig. 2A). In males, there was no significant difference in absolute fat mass from 20 to 32 wk of age between bGH and NT mice. In females, there was no significant difference in absolute fat mass from 20 to 48 wk of age. However, from 36 to 52 wk of age in male bGH mice and at 52 wk of age in female bGH mice, the absolute fat mass was significantly less than their NT counterparts. Whereas both male and female bGH mice had significantly more fat mass in early weeks, their absolute fat mass remained relatively consistent throughout 52 wk. In fact, there was no significant difference in fat mass between the first body composition measurement made at 6 wk compared with final measurement at 52 wk. Conversely, NT male and female mice continued to gain additional fat mass throughout the 52-wk period. For example, male and female NT mice at wk 6 had 0.94 ± 0.15 and 0.94 ± 0.07 g fat mass, respectively, and by 52 wk had 10.56 ± 1.15 and 7.87 ± 0.70 g fat mass, respectively. In contrast, male and female bGH mice at wk 6 had 2.23 ± 0.16 and 2.23 ± 0.17 g fat mass, respectively, and by 52 wk had 2.64 ± 0.33 and 3.57 ± 0.55 g, respectively. Thus, as the mice aged, the fat mass of both male and female NT mice eventually surpassed that of the male and female bGH mice. This crossover occurred later in females than males. Significant differences in absolute fat mass were also found when comparing male and female mice of the same genotype. Furthermore, the effect of gender on fat mass was dependent on genotype.

Figure 2.

Figure 2

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Absolute fat mass (A) and percent fat mass (B) for male and female bGH and NT mice. Data are expressed as mean ± sem, n = 7 (male bGH), n = 8 (male NT), and n = 10 (female bGH and NT). For absolute fat mass, repeated-measures ANOVA test revealed a significant effect of gender [F(1,31) = 19.1, P < 0.001] and genotype [F(1,31) = 6.2, P = 0.02] as well as a significant effect of interaction between genotype and gender [F(1,31) = 5.8, P = 0.02]. For percent fat mass, a repeated-measures ANOVA test revealed a significant effect of gender [F(1,31) = 6.1, P = 0.02] and genotype [F(1,31) = 36.8, P < 0.001] as well as a significant effect of interaction between genotype and gender [F(1,31) = 6.2, P = 0.0018].

Because the body weights of the bGH and NT mice differ, it is also valuable to compare the relative proportion of fat mass per total body weight. As expected, percent fat mass had a trend similar to absolute fat mass in male and female mice (Fig. 2B). Specifically, male bGH mice had significantly greater percent fat mass at earlier time points and then significantly less percent fat mass at later time points when compared with male NT mice. A similar trend was observed in female bGH mice, except the time point at which this transition occurred was delayed with a greater percent fat mass observed for female NT mice by wk 36.

Lean mass

Unlike fat mass, male and female bGH mice had significantly greater absolute lean mass than their NT counterparts at all time points measured (Fig. 3B). Specifically, at 6 wk of age, male and female bGH mice had 1.4 and 1.5 times more lean mass, respectively, than their NT counterparts. By 52 wk of age, male bGH mice had 1.6 times more lean mass than male NT mice and female bGH mice had 1.5 times more lean mass than female NT mice. Additionally, the greatest difference in lean mass was measured at 36 wk of age in the male bGH vs. male NT mice and at 28 wk of age in the female bGH vs. female NT mice. It appears that in both genotypes and both sexes, lean mass is gained earlier in life and then plateaus by 24–32 wk of age. As expected based on changes in percent fat mass, percent lean mass decreased for both male and female NT mice with age (Fig. 3B). When absolute lean mass was compared between male and female mice of the same genotype, male bGH and NT mice had significantly greater absolute lean mass than female bGH and NT mice, respectively, at all time points measured.

Figure 3.

Figure 3

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Absolute lean mass (A) and percent lean mass (B) for male and female bGH and NT mice. Data are expressed as mean ± sem, n = 7 (male bGH), n = 8 (male NT), and n = 10 (female bGH and NT). For absolute lean mass, a repeated-measures ANOVA test revealed a significant effect of genotype [F(1,31) = 531.9, P < 0.001] and gender [F(1,31) = 102.2, P < 0.001] as well as a significant effect of interaction between genotype and gender [F(1,31) = 4.2, P = 0.049]. For percent lean mass, a repeated-measures ANOVA test revealed a significant effect of genotype [F(1,31) = 33.2, P < 0.001] but not gender. There was a significant effect of interaction between the two [F(1,31) = 6.7, P = 0.015].

Fluid mass

Both male and female bGH mice had significantly greater absolute fluid mass than their littermate controls at most time points (data not shown). However, when calculated as a percentage of total body weight, there was no significant difference in percent fluid mass between male bGH and male NT mice, although female bGH mice had significantly greater percent fluid mass at a few early time points (wk 6–28). Compared with their female counterparts, males of both genotypes had significantly less percent fluid than females at most time points.

Plasma GH and IGF-I

As would be expected, plasma GH levels were severely elevated in bGH transgenic mice as levels of bGH were more than 400-fold higher than mGH levels in NT mice at the 7-month time point (Table 1). Plasma IGF-I levels were significantly higher in bGH transgenic mice compared with controls, as would be expected with consistent expression of a bGH transgene.

Table 1.

Fasting plasma levels of GH and IGF-I (nanograms per milliliter) in 7-month-old male NT mice and male bGH mice

NT bGH
GH (ng/ml) 1.6 ± 0.5 636.3 ± 136a
IGF-I (ng/ml) 492.5 ± 20.1 868.4 ± 25.1a
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Data are shown as mean ± sem. 

a

Significant differences as determined by Student’s t test (P > 0.05). 

Organ weights

The absolute (Fig. 4) and normalized weight (data not shown) of all adipose depots measured were significantly less in both male and female bGH animals compared with their respective littermate controls. Using a two-way ANOVA to compare the organ weights for each genotype and gender, there was an expected main effect of genotype for all organ weights (with F values ranging from 32 to 131, P < 0.001) but no significant effect of gender except for spleen weights [F (1,31) = 33.6, P < 0.001]. In addition to the absolute values shown in Fig. 4, the normalized weight of the sc, epididymal, retroperitoneal, and mesenteric adipose depots of the male NT mice were 11.3, 4.2, 8.6, and 3.7 times greater, respectively, compared with values found in male bGH mice. In the female NT mice, the normalized weight of sc, parametrial, retroperitoneal, and mesenteric adipose depots were 4.8, 4.8, 5.2, and 4.9 times greater, respectively, in female bGH mice. Unlike adipose tissue, the absolute (Fig. 4) and normalized (data not shown) weight of the spleen, liver, heart, and kidneys were greater in the male and female bGH animals compared with their respective littermate controls. The normalized weights were 1.8 and 1.7 times greater in the heart; 1.5 and 1.9 times greater in the liver; 2.6 and 1.9 times greater in the spleen; and 1.4 and 1.2 times greater in the kidney of the male and female bGH mice, respectively, compared with their sex-matched littermate controls.

Figure 4.

Figure 4

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Absolute organ weights of male and female bGH and NT mice at 1 yr of age. Weights of inguinal (SubQ), epididymal (Epi) in males, parametrial (PM) in females, retroperitoneal (Retro), mesenteric (Mes), spleen, liver, kidney, and heart were taken at time of dissection. Data are expressed as means ± sem, n = 7 (male bGH), n = 8 (male NT), and n = 10 (female bGH and NT).

Liver and kidney triglycerides

Because bGH animals had decreased adipose depot mass and increased liver and kidney mass at dissection, we hypothesized that the bGH animals may represent a model of lipodystrophy with increased ectopic fat accumulation. Therefore, several nonadipose tissues collected were tested for accumulation of triglycerides. The data showed that the liver triglyceride levels in bGH male and female mice were significantly lower than their NT counterparts (Table 2). Livers of male NT mice were 3.2 times higher in triglyceride levels than male bGH mice. Female NT mice had 1.8 times higher liver triglyceride levels than female bGH mice. There were no significant differences between male and female mice of the same genotype. Kidney triglycerides, although lower than the levels found in the liver, showed a similar trend as the liver (i.e. highest levels in the NT mice and lower levels in the bGH mice of both sexes).

Table 2.

Liver and kidney tissue triglyceride (TAG) levels at conclusion of the study, represented as milligrams TAG per gram tissue

Sex Genotype Liver TAG (mg/g) Kidney TAG (mg/g)
Male NT 52.9 ± 16.9a 11.26 ± 0.6a
Male bGH 16.6 ± 0.8b 6.39 ± 0.3b
Female NT 24.9 ± 1.6a,b 10.49 ± 0.4a
Female bGH 13.6 ± 0.7b 6.5 ± 0.4b
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Data are shown as mean ± sem. Means within a column of the same sex or genotype without a common letter differ significantly (P < 0.05). 

Discussion

Transgenic mice that express various species of GH have been the source of extensive investigation. In addition to a reduced longevity, most studies using GH transgenic mice have suggested that excess GH results in a lean phenotype, although data reveal differences, depending on the age of the mice (18,19,23,34,35,36). The goal of this study was to track body composition of bGH mice over the course of their shortened life span as well as document the typical body composition changes in NT mice of both genders.

Wild-type C57BL/6J mice have a reported life span of approximately 2.5 yr (38), whereas bGH mice from the line used in this study have a mean life span of approximately 405 d or a little over a year (18). As expected, both male and female bGH mice have significantly greater body weights compared with gender-matched NT mice throughout 1 yr of age. Whereas body weights were consistently increased in transgenic mice, body composition profiles for these mice show markedly different trends compared with NT controls. Transgenic mice of both genders show an increase in lean mass up to approximately 24 wk of age, at a time point that corresponded to the plateau observed for body weight. Fat mass of transgenic mice also increased but to a lesser extent than lean mass through the first 20 wk but decreased throughout the remainder of the measurement period. Taking into account the extreme differences in body size, percent fat mass was relatively stable throughout the first year of the study for both male and female bGH mice. Thus, the increased size of bGH mice was due to a relatively proportional increase in lean and fat mass. Furthermore, the bGH transgene resulted in protection against the accumulation of fat mass in later weeks as was observed in littermate controls. Although free fluid levels were elevated in the bGH mice of both genders, the increased fluid was not significantly different from control animals when normalized to body weight, suggesting edema is not significantly influencing the body composition measurements.

Collectively, these data expose the importance of timing when doing body composition measurements or studies of lean and fat tissue in GH transgenic mice. Data collected in the early weeks of this study show that bGH mice have greater fat mass and percent fat mass for both genders. Similarly, bGH mice, both male and female, were reported previously to have slightly more fat mass using electrical conductivity to assess body composition in 84-d-old mice in a CF1 outbred genetic background (23). Similar results were reported using dual-energy x-ray absorptiometry in 12-wk-old NMRI outbred mice (36). Potentially the high circulating levels of GH and IGF-I in early life may contribute to proliferation and differentiation of preadipoyctes into adipocytes (for reviews see Refs. 39 and 40), establishing a larger pool of adipocyte precursor cells in adipose tissue and an associated increase in adipose tissue mass. Body composition analyses in later weeks show that males have significantly less percent fat mass than littermate controls by 24 wk of age and females by 36 wk of age. Thus, both male and female bGH mice become leaner over the course of the year, albeit the timing is different between genders. A lean phenotype of bGH mice is in accordance with other studies of older bGH mice (18,19,34). Presumably the well-documented role of GH on lipolysis contributes to this lean phenotype in later months. Regardless of the mechanism, studies done in young transgenic and control mice vs. older mice would provide a very different conclusion in terms of mass of lean and fat tissue, suggesting the importance of age in these transgenic mice when evaluating parameters relevant to body composition.

To the best of our knowledge, no study has assessed body composition longitudinally in C57BL/6J mice, a common genetic background strain for diet-induced obesity studies (41). In NT mice, there is a marked increase in fat mass with age, an increase that is more dramatic in male vs. female mice. In fact, ongoing studies in our laboratory on a separate cohort of mice show that fat mass continues to increase late into the second year in this strain of mice (data not shown). Increases in lean mass is observed for both genders, but the increase is relatively modest compared with lean mass in bGH or to fat mass in C57 mice. Thus, most of the weight gain observed for C57 mice is due to increasing fat mass.

In the present study, male and female mice responded differently to excess GH, with females showing some delay in body composition changes. This increased response by male mice is a phenomenon reported previously (23). This difference may not be surprising because estrogen has been shown to affect GH responsiveness and is postulated to act either through the regulation of the release, clearance, or action of GH (42). For example, studies by Leung et al. (43) reported that estrogen increases the amount of suppressor of cytokine signaling 2 mRNA, which was related to a decrease in Janus kinase 2 phosphorylation and GH signaling. Furthermore, during different stages of the menstrual cycle, women show variable IGF-I levels, with low levels of IGF-I occurring during times of higher estrogen (44). Additional studies have reported sexually dimorphic responses to GH, including differences in drug metabolism (45) and GH release during exercise (46). In rodents, other differences in the effects of GH have been reported previously. For example, in female bGH mice, trabecular bone volume and femoral bone density are both increased, with no change reported for male mice (36). Also, suppression in the Janus kinase 2/signal transducer and activator of transcription 5b pathway and elevation in Cis mRNA have been shown specifically in female rat hepatocytes in response to GH in culture (47), and female human GH transgenic mice have decreased skin thickness compared with controls, whereas males have increased skin thickness (48). These data support gender differences relevant to body composition as observed in the present study. The pulsatility of GH has also been shown to differ between males and females (49). However, the bGH mice represent a model in which the pulsatility of GH should be muted as expression of the transgene is controlled by a constitutively active metallothionein transcriptional regulatory element. Yet the sexually dimorphic response to GH is still present, suggesting that it is another mechanism, not GH pulsatility, that is responsible for the gender difference.

By the end of our longitudinal study, male and female bGH mice had dramatic losses in adipose tissue mass, indicative of a lipodystrophic state (for a recent review, see Ref. 50). In lipodystrophy, the severity of adipose tissue loss is often associated with the degree of metabolic abnormality, such as insulin resistance, with less adipose tissue loss being more of a cosmetic issue and extreme loss resulting in severe metabolic complications. Interestingly, obesity, or excess fat mass, is at the opposite end of the clinical spectrum than lipodystrophy, yet the metabolic complications are somewhat similar. That is, severe generalized lipodystrophy and obesity result in severe insulin resistance and an ectopic deposition of fat. At dissection, the bGH mice had a dramatic decrease in absolute and normalized fat pad mass in all depots collected compared with controls, with some depots, such as inguinal, having at times no dissectible fat tissue. The combination of decreased or absent fat pad mass and the larger proportional weights of other nonadipose tissues observed in this study suggested that older bGH mice may represent a model of acquired lipodystrophy. Whereas bGH mice have repeatedly been shown to be hyperinsulinemic and insulin resistant by multiple groups (27,34,51,52), assessment of ectopic deposition of fat with advancing age has been insufficiently evaluated and inconsistent. Wang et al. (33), which used a bGH transgenic line under the control of the rat phosphoenolpyruvate carboxykinase promoter, reported that triglyceride levels in the liver were decreased in male bGH mice at 28 wk of age and showed no significant difference in triglyceride content of skeletal muscle; these data would be inconsistent with many lipodystrophic models. On the other hand, neutral lipid content in kidney cortex was shown to be elevated in bGH mice (29), which is more consistent with lipodystrophy. In this study, triglyceride content, as determined by enzymatic assay, was decreased in both the liver and kidneys of bGH male and female mice, suggesting the ectopic deposition of fat, at least in these tissues, is not significant. Additional studies are warranted to determine the fate of the lipid loss in adipose tissue and the resultant impact on endocrine function.

In conclusion, control C57BL/6J mice show significant increases in fat mass during the first year, whereas bGH mice of both genders show resistance to this midlife fat mass gain. The bGH mice do not appear to have excessive edema because fluid weight remains proportional to body size. In early life, the proportional increase in fat mass of bGH mice might suggest a role of the GH/IGF-I axis in establishing the number of adipocyte-competent cells within the tissue, which then are protected later against significant accumulation of triglycerides due to the potent lipolytic effect of GH. Future studies assessing adipocytes cell number and cell size over the course of the year will help confirm this possibility. Regardless, these data show the importance of timing and gender when conducting body composition studies in GH transgenic animals and are probably applicable to other animal models.

Acknowledgments

We thank Al Parlow (National Hormone and Peptide Program, Torrance, CA) for performing the bGH measurements for this study.

Footnotes

D.E.B. is supported in part by funds from the National Institute of Diabetes and Digestive and Kidney Diseases (Grant DK064905) and the Diabetes Research Initiative at Ohio University. J.J.K. is supported by funds from National Institute on Aging (Grant AG19899), National Institute of Diabetes and Digestive and Kidney Diseases (Grant DK075436), the State of Ohio’s Eminent Scholar Program that includes a gift from Milton and Lawrence Goll, the Diabetes Research Initiative at Ohio University, a grant from DiAthegen LLC, and AMVETS.

Disclosure Statement: The authors have nothing to disclose.

First Published Online October 23, 2008

Abbreviations: b, Bovine; m, mouse; NMR, nuclear magnetic resonance; NT, nontransgenic.

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