Growth Hormone Receptor Gene-disrupted Mice Research Articles

SAT-181 Characterization of Adult Growth Hormone Receptor Gene-Disrupted Mice

Reduced growth hormone (GH) action can extend life- and health-span. Mice with a disrupted GHR gene since birth are dwarf and obese with low levels of IGF-1 and insulin; high levels of GH; and hold the record for the longest-lived laboratory mouse. They are also somewhat resistant to several age-related diseases including cancer and diabetes. In order to determine if disruption of GHR after birth can also extend lifespan, we previously disrupted the GHR gene at 6 weeks of age and found that females have extended maximal lifespans. In the current study, we wanted to evaluate if disrupting GH action at an adult age (6 months) will lead to the same lifelong benefits seen mice with no GH action since birth, but without dwarfism. In this study, we used the Cre-lox system to ablate the GHR in mice of 6 months (a6mGHRKO mice). Following this, we determined body weights, composition and glucose homeostatic criteria including glucose and insulin tolerance testing. We then dissected the mice 6 months after GHR disruption (1 year of age) to measure circulatory GH, Insulin growth factor 1 (IGF-1), inflammatory factors, as well as the GHR expression levels in different tissues including liver, skeletal muscle and adipose tissue. We found that, the a6mGHRKO mice were not dwarf and have decreased IGF-1 and increased circulating GH levels. We also found that the GHR gene expression was decreased in all the tissues tested, and IGF-1 mRNA levels were reduced in adipose tissue liver and heart, but not in muscle or kidneys when compared to WT mice. Also, circulatory levels of interleukin-6, tumor necrosis alpha and monocyte chemoattractant protein 1 were unchanged between a6mGHRKO mice and controls. Moreover, differences in glucose metabolism can be seen in male and female a6mGHRKO mice. While glucose tolerance was not changed between experimental mice and controls, male a6mGHRKO mice showed increased insulin sensitivity, whereas female a6mGHRKO mice showed no difference in insulin sensitivity with respect to littermate controls. Studies that assess the status of age-related mechanisms such as mechanistic target of rapamycin (mTOR) activity and oxidative stress are being performed. Finally, a separate cohort of the a6mGHRKO mice have been generated and are being evaluated for the life span of these mice. In conclusion, mice that have the GHR gene ablated at 6 months of age have a normal body length, improved insulin sensitivity in males and normal inflammatory status when compared to controls. Molecular mechanisms to determine age-related effects of GH including longevity studies are ongoing.

MECHANISMS IN ENDOCRINOLOGY: Lessons from growth hormone receptor gene-disrupted mice: are there benefits of endocrine defects?

Growth hormone (GH) is produced primarily by anterior pituitary somatotroph cells. Numerous acute human (h) GH treatment and long-term follow-up studies and extensive use of animal models of GH action have shaped the body of GH research over the past 70 years. Work on the GH receptor (R)-knockout (GHRKO) mice and results of studies on GH-resistant Laron Syndrome (LS) patients have helped define many physiological actions of GH including those dealing with metabolism, obesity, cancer, diabetes, cognition and aging/longevity. In this review, we have discussed several issues dealing with these biological effects of GH and attempt to answer the question of whether decreased GH action may be beneficial.

Expression of Apoptosis-Related Genes in Liver-Specific Growth Hormone Receptor Gene-Disrupted Mice Is Sex Dependent

Apoptosis is a process that affects life span and health. Mice with liver-specific disruption of the growth hormone receptor (GHR) gene (ie, Ghr gene) liver-specific growth hormone receptor knockout [LiGHRKO] mice), as opposed to mice with global deletion of the Ghr gene (GHRKO; Ghr-/-), are characterized by severe hepatic steatosis and lack of improved insulin sensitivity. We have previously shown that levels of proapoptotic factors are decreased in long-lived and insulin-sensitive GHRKO mice. In the current study, expression of specific apoptosis-related genes was assessed in brains, kidneys, and livers of male and female LiGHRKO and wild-type mice using real-time PCR. In the brain, expression of Caspase 3, Caspase 9, Smac/DIABLO, and p53 was decreased in females compared with males. Renal expression of Caspase 3 and Noxa also decreased in female mice. In the liver, no differences were seen between males and females. Also, no significant genotype effects were detected in the examined organs. Lack of significant genotype effect in kidneys contrasts with previous observations in GHRKO mice. Apparently, global GHR deletion induces beneficial changes in apoptotic factors, whereas liver-specific GHR disruption does not. Furthermore, sexual dimorphism may play an important role in regulating apoptosis during liver-specific suppression of the somatotrophic signaling.

Decreased levels of proapoptotic factors and increased key regulators of mitochondrial biogenesis constitute new potential beneficial features of long-lived growth hormone receptor gene-disrupted mice.

Decreased somatotrophic signaling is among the most important mechanisms associated with extended longevity. Mice homozygous for the targeted disruption of the growth hormone (GH) receptor gene (GH receptor knockout; GHRKO) are obese and dwarf, are characterized by a reduced weight and body size, undetectable levels of GH receptor, high concentration of serum GH, and greatly reduced plasma levels of insulin and insulin-like growth factor-I, and are remarkably long lived. Recent results suggest new features of GHRKO mice that may positively affect longevity-decreased levels of proapoptotic factors and increased levels of key regulators of mitochondrial biogenesis. The alterations in levels of the proapoptotic factors and key regulators of mitochondrial biogenesis were not further improved by two other potential life-extending interventions-calorie restriction and visceral fat removal. This may attribute the primary role to GH resistance in the regulation of apoptosis and mitochondrial biogenesis in GHRKO mice in terms of increased life span.

Age-Related Alterations in Pituitary and Testicular Functions in Long-Lived Growth Hormone Receptor Gene-Disrupted Mice

The somatotropic axis, GH, and IGF-I interact with the hypothalamic-pituitary-gonadal axis in health and disease. GH-resistant GH receptor-disrupted knockout (GHRKO) male mice are fertile but exhibit delayed puberty and decreases in plasma FSH levels, testicular content of LH, and prolactin (PRL) receptors, whereas PRL levels are elevated. Because the lifespan of GHRKO mice is much greater than the lifespan of their normal siblings, it was of interest to compare age-related changes in the hypothalamic-pituitary-gonadal axis in GHRKO and normal animals. Plasma IGF-I, insulin, PRL, LH, FSH, androstenedione and testosterone levels, and acute responses to GnRH and LH were measured in young (2-4 and 5-6 months of age) and old (18-19 and 23-26 months of age) male GHRKO mice and their normal siblings. Plasma IGF-I was not detectable in GHRKO mice. Plasma PRL levels increased with age in normal mice but declined in GHRKO males, and did not differ in old GHRKO and normal animals. Plasma LH responses to acute GnRH stimulation were attenuated in GHRKO mice but increased with age only in normal mice. Plasma FSH levels were decreased in GHRKO mice regardless of age. Plasma testosterone responses to LH stimulation were attenuated in old mice regardless of genotype, whereas plasma androstenedione responses were reduced with age only in GHRKO mice. Testicular IGF-I mRNA levels were normal in young and increased in old GHRKO mice, whereas testicular concentrations and total IGF-I levels were decreased in these animals. These findings indicate that GH resistance due to targeted disruption of the GH receptor gene in mice leads to suppression of testicular IGF-I levels, and modifies the effects of aging on plasma PRL levels and responses of the pituitary and testes to GnRH and LH stimulation. Plasma testosterone levels declined during aging in normal but not in GHRKO mice, and the age-related increase in the LH responses to exogenous GnRH was absent in GHRKO mice, perhaps reflecting a delay of aging in these remarkably long-lived animals.

Puberty is delayed in male growth hormone receptor gene-disrupted mice.

The role of insulin-like growth factor-I (IGF-I) in the initiation of puberty and testicular function is poorly understood. Growth hormone (GH) receptor (R) gene-disrupted mice or GHR gene "knockouts" (GHR-KO) are GH resistant and IGF-I deficient. To assess whether the age of sexual maturation is affected by the absence of IGF-I, various parameters of sexual development including testicular and accessory reproductive organ weights, balanopreputial separation, germ cell development, and intratesticular testosterone levels were determined in normal and GHR-KO mice between the ages of 25 and 60 days. In addition, at 36 days of age, the testosterone response to luteinizing hormone (LH) treatment was assessed in these mice. The results indicate that the balanopreputial separation was delayed 5 days, and a significant increase in the weights of the seminal vesicles (SV) occurred later in GHR-KO mice than in normal animals (between 30 and 35 days and between 35 and 40 days, respectively). Also, the weights of testes and epididymii were significantly reduced in GHR-KO mice. The intratesticular testosterone levels and the testosterone response to LH treatment were attenuated in GHR gene-disrupted mice. Furthermore, elongated spermatids appeared later in the testes of GHR-KO mice than in the testes of normal mice. These results suggest that the absence of IGF-I secretion delays the normal course of sexual maturation in male GHR-KO mice, indicating that IGF-I plays an important role in the initiation of puberty in male mice.