Regulation of insulin-like growth factor-I and binding protein-3 expression in oMtla-oGH transgenic mice

Biological markers of neonatal calf performance: the relationship of insulin-like growth factor-I, zinc, and copper to poor neonatal growth

Raising a heifer calf to reproductive age represents an enormous cost to the producer. Poor neonatal growth exacerbates the costs incurred for rearing, and use of blood variables that may be associated with poorly growing calves may offer predictive value for growth and performance. Thus, the principal objective of the present study was to describe changes in serum IGF-I, zinc, and copper from birth to 90 d in Holstein calves, while accounting for sex and twin status, in poorly growing calves and calves growing well. A second objective was to test the hypothesis that an association exists between these serum variables and morphometric indicators of growth. Measurements of BW, length, and height were recorded at birth and at 30, 60, and 90 d of age. Jugular blood (12 mL) was collected from each calf on d 1 to determine serum total protein, serum IgG, packed cell volume, serum zinc, serum copper, serum IGF-I, and CD18 genotype for bovine leukocyte adhesion deficiency; serum zinc, serum copper, and serum IGF-I (predictor variables) were also determined for each calf on d 2 through 10 and on d 30, 60, and 90. Stepwise multiple regression and logistic regression analyses were used to examine the relationships between the predictor variables and the dependent variables (BW, height, and length at d 30, 60, and 90 of life). Birth weight, sex, serum IGF-I (at all ages), serum copper, and the serum copper-to-zinc ratio were associated, to varying degrees, with the dependent growth variables. Birth weight was consistently the dominant predictor. In conclusion, these results suggest that lighter birth weight, reduced serum IGF-I, and inflammation may be important causes of poor growth in neonatal Holstein dairy calves.

Transgene transmission to progeny by oMt1a-oGH transgenic mice

Most studies utilizing transgenic technology focus on the impact to traits of interest, rather than propagation of the transgene to offspring. In animals containing growth hormone constructs, transgene transmission to progeny follows a Mendelian pattern of inheritance in the first few generations following generation of a founder animal, but decreases in subsequent generations. In the present study, the ovine metallothionein 1a-ovine growth hormone (oMt1a-oGH) transgenic mouse was used to determine whether transgene transmission rate to progeny was affected by overexpression of ovine growth hormone in the transgenic parent. The oMt1a-oGH mouse is a useful model for assessing transgene transmission, as the construct is easily regulatable and transgene inactivation results in a return of plasma GH to basal levels. Male and female hemizygous oMt1a-oGH mice were assigned to 1 of 3 treatment groups: (1) mice never actively expressing the transgene, (2) mice actively expressing the transgene from 3 weeks of age, and (3) mice actively expressing the transgene from 3 to 11 (males) or 3 to 8 (females) weeks of age. Transgenic mice were mated to wild type animals and the resulting progeny were genotyped. Males never actively expressing the transgene passed on the transgene to progeny in a Mendelian fashion, while males actively expressing the transgene transmitted the transgene to a smaller than expected number of progeny. However, following inactivation of the oMt1a-oGH construct in transgenic males, subsequent offspring demonstrated Mendelian inheritance of the transgene. In contrast, females expressing the transgene from 3 to 8 weeks of age were able to pass on the oMt1a-oGH construct in a Mendelian fashion, but females from other treatment groups were not. In oMt1a-oGH males, reduced transgene transmission appears to be due to selection against transgenic gametes. In females, however, selection against the transgenic genotype likely occurs at the embryonic level.

Growth hormone has no direct effect on human adrenal steroid and insulin-like growth factor-binding protein secretion

Although it is known that growth hormone (GH) exerts its growth-promoting effects mainly via Insulin-like growth factor-I (IGF-I), an increasing number of direct effects of GH has been described in many tissues. In vivo, mice transgenic for human growth hormone (hGH) show significantly elevated levels of corticosterone, enlarged adrenal glands, and altered levels of insulin-like growth factor binding proteins (IGF-BPs). Recently, we have shown that IGF's induce the secretion of cortisol and IGF-BP's in adult human adrenocortical cells. However, since human adrenal glands express the intact GH-receptor, the objective of this study was to investigate whether GH exerts a direct effect on the steroidogenesis and IGF-BP synthesis in adult human adrenocortical cells. Primary cell cultures in monolayer were incubated under serum-free conditions with human growth hormone and/or ACTH for up to 72 hours. Cortisol was measured by specific RIA and the secretion of insulin-like growth factor binding proteins was analyzed by Western ligand blotting. hGH alone was unable to stimulate basal or ACTH-induced cortisol secretion. Additionally, neither hGH alone or in combination with ACTH did significantly alter the secretion of IGF-BP's. Therefore we conclude that hGH is unable to directly stimulate cortisol secretion and IGF-BP secretion in cultured human adrenocortical cells.

Overexpression of growth hormone affects alternatively spliced IGF-I mRNA expression in oMt1a-oGH transgenic mice

Restorative growth hormone (GH) treatment of hypophysectomized rats differentially enhances the transcription of alternative IGF-I mRNA classes in liver. The goal of the present study was to determine the effects of GH overexpression on various classes of hepatic IGF-I mRNA in GH transgenic mice. Unstimulated oMt1a-oGH transgenic mice had low levels of transgene expression, and therefore were used to determine the effects of long-term, slightly elevated GH levels on the abundance on each alternative IGF-I mRNA class. The acute effects of high GH levels on the expression of alternative IGF-I mRNA were studied by gavaging transgenic mice with 25 mM zinc sulfate to activate oMt1a-oGH transgene expression. Long-term, low levels of oGH transgene expression in unstimulated transgenic mice resulted in a 73% down regulation of IGF-I 2Ea mRNA but not 1Ea and 2Eb mRNA. Acute stimulation of transgene expression triggered a rapid, 240% increase in 1Ea mRNA levels within 4 hours of transgene expression while 2Ea mRNA was down regulated to nearly non-detectable levels by 6 hours. IGF-I 2Eb mRNA was not affected by the short-term GH elevation. Our results showed that IGF-I 1Ea and 2Ea mRNA were differentially regulated by chronic low or acute high levels of GH. These results suggest that the regulation of IGF-I 1Ea and 2Ea mRNA transcription involve different postreceptor molecules and/or feedback mechanisms.

Development of obesity following inactivation of a growth hormone transgene in mice

Mice with a temporally regulatable ovine metallothionein 1a--ovine growth hormone transgene (oMT1a-oGH) were utilized to study the effects of withdrawal of elevated circulating levels of growth hormone (GH) on growth and body composition. The transgene was activated from 21-42 days of age by provision of zinc sulfate in the drinking water. At 42 days, mice were allocated to either activated transgenic (remain on zinc sulfate) or inactivated transgenic (removal of zinc sulfate) groups, and to receive either ad libitum or restricted (80-90% of ad libitum) access to feed. Non-transgenic control mice were treated similarly. Body weights and intakes were recorded weekly. Mice were killed at 70 d and epididymal and subcutaneous fat pads, trimmed hind carcass and various organs were weighed. The main findings of this study are: (1) food-restricted mice possessing an activated oMT1a-oGH transgene fail to demonstrate increased growth, but exhibit significantly reduced levels of fat (P < 0.05) relative to all other genotype x feed level combinations; and (2) inactivation of the oMT1a-oGH transgene, following a period of elevated GH levels, leads to development of obesity as evidenced by two to three fold increases in epididymal and subcutaneous fat pad weights (P < 0.01) relative to both activated transgenic and non-transgenic control mice. These large increases in fat deposition also occurred when intake was restricted to 80-90% of ad libitum levels, indicating that metabolic changes independent of intake occur in these inactivated transgenic mice. It is possible that highly elevated production of GH in activated oMT1a-oGH transgenic mice leads to (1) enhanced promotion of preadipocyte differentiation, leading to increased numbers of adipocytes that, upon cessation of oGH production, are available for lipid deposition resulting in obesity, or (2) alterations in production of or responsiveness to insulin, leading to increased fat deposition upon removal of the chronic anti-lipogenic actions of GH. The oMT1a-oGH transgenic mouse line should provide a new genetic model with which to investigate the mechanisms by which growth hormone affects obesity.

Growth hormone and IGF-I stimulate cell function in distinct zones of the rat epiphyseal growth plate

Proliferation and maturation of growth plate chondrocytes are primarily responsible for linear bone elongation, although the exact mechanisms involved have not been fully characterized. We have used discrete chondrocyte populations to address the mode of growth hormone (GH) action on the growth plate. Low doses of GH, and insulin-like growth factor-I (IGF-I) preferentially enhanced cell proliferation in proliferative zone chondrocytes; the mitogenic response of immature proliferative and resting zone cells was minimal. Proliferation was not enhanced by combining the effects of GH and IGF-I. Exposure to IGF-I increased IGF-I mRNA in resting zone cells. Both GH and IGF-I stimulated the accumulation of IGF-I receptor mRNA in the most immature proliferative zone cells but did not alter the accumulation of IGF-binding protein 4 mRNA in any fraction. These results confirm a direct effect of GH on growth plate chondrocytes and suggest that GH preferentially acts on the actively proliferating chondrocytes.

Expression of an ovine growth hormone transgene in mice increases arachidonic acid in cellular membranes

The degree of unsaturation of membrane lipids has been implicated in a number of physiological disorders, yet its regulation remains poorly understood, especially the regulation of the synthesis and distribution of arachidonic acid levels, the most abundant long chain polyunsaturated fatty acid in membranes. Transgenic mice expressing the ovine metallothionein 1a-ovine growth hormone (oMt1a-oGH) fusion gene exhibited significantly elevated levels of a number of long chain polyunsaturated fatty acids in serum, including arachidonic acid. In oMt1a-oGH transgenic mice the products of all three desaturation pathways are affected by the expression of the ovine growth hormone transgene. The essential precursors of membrane long chain polyunsaturated fatty acids, 18:2n-6 and 18:3n-3, were reduced in transgenics relative to controls, and their desaturation and elongation products, arachidonic acid (20:4n-6) and docosahexaenoic acid (22:6n-3), were elevated. As rare intermediate long chain polyunsaturated fatty acids such as eicosatrienoic acid (20:3n-9) were also significantly elevated, we conclude that these observations reflect increased activity of the delta-5 and delta-6 desaturase enzymes. In contrast, the products of the stearoyl CoA or delta-9 desaturase, were significantly reduced in oMt1a-oGH expressing transgenics relative to their levels in control mice.