Growth hormone regulates the abundance of insulin-like growth factor I RNA in adult rat liver

Role of insulin-like growth factors in embryonic and postnatal growth

A developmental analysis of growth kinetics in mouse embryos carrying null mutations of the genes encoding insulin-like growth factor I (IGF-I), IGF-II, and the type 1 IGF receptor (IGF1R), alone or in combination, defined the onset of mutational effects leading to growth deficiency and indicated that between embryonic days 11.0 and 12.5, IGF1R serves only the in vivo mitogenic signaling of IGF-II. From E13.5 onward, IGF1R interacts with both IGF-I and IGF-II, while IGF-II recognizes an additional unknown receptor (XR). In contrast with the embryo proper, placental growth is served exclusively by an IGF-II-XR interaction. Additional genetic data suggested that the type 2IGF/mannose 6-phosphate receptor is an unlikely candidate for XR. Postnatal growth curves indicated that surviving Igf-1(-/-) mutants, which are infertile and exhibit delayed bone development, continue to grow with a retarded rate after birth in comparison with wild-type littermates and become 30% of normal weight as adults.

Insulin-like growth factor-I induces hypertrophy with enhanced expression of muscle specific genes in cultured rat cardiomyocytes

BACKGROUND

Cardiac hypertrophy is commonly observed in acromegalic patients, in whom serum insulin-like growth factor-I (IGF-I) levels are elevated. In the present study, we examined whether IGF-I induces hypertrophy in cultured neonatal rat cardiomyocytes through its specific receptor and whether IGF binding protein-3 (IGFBP-3), which is a major circulating carrier protein for IGF-I, inhibits IGF-I-induced cardiac hypertrophy in vitro.

METHODS AND RESULTS

Because the response of cardiac hypertrophy is characterized by the induction of expression for muscle-specific genes, the effect of IGF-I on steady-state levels of mRNA for myosin light chain-2 (MLC-2) and troponin I and for skeletal and cardiac alpha-actin isoforms was evaluated by Northern blot analysis. IGF-I (10(-7) M) increased mRNA levels for MLC-2 and troponin I as early as 60 minutes with maximum levels by 6 hours, which were maintained for as long as 24 hours. IGF-I (10(-7) M) also increased transcripts for skeletal alpha-actin but not for cardiac alpha-actin. The cell size as evaluated morphometrically was almost doubled after 48-hour treatment with IGF-I. IGF-I induction of protein synthesis was dose dependent (10(-10) to 10(-7) M) with a maximal 2.2-fold increase seen at 10(-8) M. In contrast to the hypertrophic effect of IGF-I, growth hormone affected neither protein synthesis nor expression for muscle-specific genes. Binding study using 125I-IGF-I revealed the presence of specific binding sites for IGF-I in rat cardiomyocytes. IGFBP-3 induced a dose-dependent inhibition of protein synthesis stimulated by IGF-I; IGFBP-3 (10(-7) M) completely inhibited the [3H]leucine uptake stimulated by IGF-I (10(-8) M). IGFBP-3 similarly inhibited the IGF-I-stimulated gene expressions for MLC-2 and troponin I.

CONCLUSIONS

These results suggest that IGF-I directly causes cardiac hypertrophy and that its effect can be blocked by IGFBP-3.

Interactive effects of nutrients and hormones on the expression of insulin-like growth factor binding protein-1 (IGFBP-1) mRNA and peptide, and IGF I release from isolated adult rat hepatocytes

Isolated adult rat hepatocytes were used to investigate and compare the actions of glucose or amino acids and insulin, glucagon, growth hormone, and dexamethasone on the expression of insulin-like growth factor binding protein (IGFBP) mRNA, or the release of IGFBP and IGF peptides in vitro. Ligand blot analysis of culture medium conditioned for 24 h by monolayers of hepatocytes in the presence of 6.5 mM glucose revealed two species of IGFBPs, an abundant form of 30-32 kDa and a minor species of 22-24 kDa. Western blotting showed that two IGFBPs of 29-30 and 32 kDa were recognized by antiserum against hIGFBP-1, whereas hepatocytes contained a 1.6 kb transcript on Northern blot with a rat IGFBP-1 cDNA. Insulin-like growth factor BP-2 mRNA was not detected in hepatocytes and IGFBP-2 immunoreactive peptide not present in conditioned medium. The release of IGFBP-1, determined by ligand blot, was independent of glucose concentration over the range of 2.7 mM-11.1 mM, but IGFBP-1 mRNA was decreased following incubation with 6.5 mM glucose compared with 2.7 mM glucose. The release of IGFBP-1 by hepatocytes was inhibited by insulin (10 nM-1 microM), as was mRNA abundance. However, these effects of insulin on IGFBP-1 diminished with increasing glucose concentration. Increasing concentrations of total amino acids increased IGFBP-1 release as did dexamethasone (100 pM-100 nM), whereas growth hormone and glucagon were without effect. The release of IGF I was increased by insulin, growth hormone and dexamethasone but was decreased by glucagon and amino acids, whereas changes in glucose concentration had no effect. The results show that isolated adult rat hepatocytes release IGF I and IGFBP-1 under the interactive control of nutrients and hormones involved in metabolic homeostasis.

Hyposomatomedinemia in quadriplegic men

Many studies have shown that vigorous exercise acutely stimulates growth hormone (GH) release but the relative contribution of daily physical activity to maintaining the GH/somatomedin C (SmC) axis is not known. It has been reported that basal and post-exercise plasma SmC values are higher in physically conditioned young men than in sedentary men of similar age. To assess the effect of severe inactivity on the plasma SmC level, basal concentrations of this hormone were measured in patients with quadriplegia (QP) resulting from spinal cord injury (SCI). Venous blood samples were obtained after overnight fast in 41 QP men, ages 24-66, and compared with 119 healthy men of similar ages. Nonparametric analysis of variance showed SmC to be significantly lower in QP than in healthy men (p < .007). Plasma SmC below 0.35 U/ml in adults usually indicates little or no GH secretion by the pituitary gland. In QP, 46% of plasma SmC values were < 0.35 U/ml compared to 24% in the healthy group (p < .02). In both groups, an inverse relationship of SmC and increasing age was observed (p < .01). The data suggest that severe inactivity or SCI tend to cause hyposomatomedinemia. The latter endocrine alteration may contribute to the decrease in lean body mass and muscle atrophy of QP patients, and add further functional impairment to the original neurologic deficit. In addition, hyposomatomedinemia could increase the tendency for pressure sore formation and osteoporosis in SCI patients.

Developmental expression of hepatic growth hormone receptor and insulin-like growth factor-I mRNA in the chicken

We have examined the ontogeny of expression of growth hormone (GH) receptor (GHR) and insulin-like growth factor-I (IGF-I) mRNA in chicken liver from day 13 of incubation until 31 weeks of age. The profiles of GHR and IGF-I mRNA levels were compared to developmental changes in body weight and plasma levels of GH and IGF-I. In the embryo, hepatic GHR mRNA was not detectable until day 15, highest on days 17 and 19, and then declined at hatching (day 21). Following an initial 2-week delay after hatching, there was a progressive increase in hepatic GHR mRNA which continued after the birds reached mature body weight. Plasma GH reached peak levels at 3-4 weeks of age and then fell sharply until maintenance of a low basal level after 10 weeks of age. Thus, there appears to be a strong inverse relationship between expression of the GHR and basal plasma GH levels in the prepubertal chicken. Although IGF-I mRNA was undetectable in embryonic liver by Northern blot analysis, there is a good correlation between expression of hepatic IGF-I mRNA and the plasma IGF-I profile during post-hatching development in the chicken. The highest levels of IGF-I mRNA were reached at 4 weeks of age which was followed by a slow decline to the basal levels maintained after 10 weeks of age. It appears that the decline in plasma IGF-I lags considerably behind the sharp fall in plasma GH levels and expression of hepatic IGF-I mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation by growth hormone and glucocorticoid of testosterone metabolism in long-term cultures of hepatocytes from male and female rats

The activities of 2-, 6 beta-, 7 alpha- and 16 alpha-testosterone hydroxylase and 5 alpha-testosterone reductase were measured in intact hepatocytes from male and female rats cultured for 8 days in a modified Waymouth medium supplemented with 0.1 or 1.0 microM dexamethasone with or without addition of 1 microgram/mL growth hormone. During culture of hepatocytes from female rats the activity of the male-specific 16 alpha-testosterone hydroxylase increased. This increase was significantly inhibited at day 8 by 1 microM dexamethasone as well as by growth hormone. Furthermore, in cultures of hepatocytes from male rats, the activity of the constitutive 16 alpha-testosterone hydroxylase was decreased by 1 microM dexamethasone as well as by growth hormone. The induction of 6 beta-testosterone hydroxylase by dexamethasone was suppressed by growth hormone in hepatocytes from both male and female rats, while the 7 alpha-testosterone hydroxylase activity was unaffected by culture time, hormone additions and gender. The decrease in female-specific 5 alpha-reductase activity with culture time in hepatocytes from female rats was significantly attenuated by growth hormone at 0.1 microM dexamethasone. The effects of growth hormone on testosterone hydroxylase activities in hepatocyte cultures from male and female rats are in accordance with the concept of growth hormone as a "feminization signal". The results suggest that the glucocorticoid-dependent expression of the male constitutive 16 alpha-hydroxylase requires periods of low levels of growth hormone.

Growth failure in renal disease

Children with congenital CRF lose height potential mainly during two distinct growth periods; infancy and puberty. The onset of puberty is late, the pubertal growth spurt starts from a very low rate of growth velocity, and peak height velocity is lower than normal although the absolute increment of height velocity is comparable to the increment in normal children. Furthermore, the duration of pubertal growth spurt is reduced in CRF. During infancy and early childhood, malnutrition, electrolyte disturbances and metabolic acidosis are the main contributing factors for reduced growth, whereas hormonal disturbances are responsible for growth impairment during puberty. There is evidence for resistance to growth hormone in CRF, which starts in early childhood and persists until the end of puberty. Growth hormone secretion is normal in CRF, but GH half-life is prolonged. The binding activity of the stable growth hormone binding protein is reduced, which points to a low receptor expression in the liver. Hepatic IGF-I production is diminished. However, the serum concentration of IGF binding proteins (IGFBP) is increased due to reduced renal filtration of low molecular weight subunits of IGFBP. Mainly, the accumulation of IGFBP-3 leads to increased IGF-binding capacity of the uraemic serum. Both, reduced IGF-I production and increased binding of IGF to IGFBP-3 result in decreased IGF bioactivity. During infancy, loss of growth potential can be prevented by adequate nutrition. Later in life, catch-up growth cannot be induced by nutritional intervention or dialysis. Renal transplantation allows catch-up growth in only a small percentage of patients. Treatment with one IU rhGH/kg/week improves growth velocity and growth in all stages of renal disease. The mean increment of height in prepubertal children is +1.5 SDS within two treatment years. The effect of rhGH during puberty as well as the effect on final height remain to be determined.

Discoordinate hormonal and ontogenetic regulation of four rat serpin genes

To understand the roles of four highly homologous rat hepatic serine protease inhibitor genes (Spi 2.1, Spi 2.2, Spi 2.3, and alpha 1-antitrypsin), we measured the hepatic content of their specific mRNAs under several physiological conditions. Spi 2.1 and 2.3 mRNAs, which are regulated by growth hormone, paralleled serum growth hormone levels developmentally. Only Spi 2.1 mRNA decreased with starvation, while Spi 2.2, 2.3, and alpha 1-antitrypsin mRNAs did not change. Despite the close homology of the Spi genes to mouse contrapsin, which is regulated by testosterone, none of the serine protease inhibitor mRNAs examined here was dependent on androgens for expression. Spi 2.2 mRNA displayed a unique ontogenetic regulation, with a rise in hepatic content at day 19 to levels five times that of any other age group. These studies confirm the importance of growth hormone in the regulation of Spi 2.1 and 2.3 mRNAs and suggest that Spi 2.2 mRNA may be regulated by metabolic alterations occurring in the weaning period.

Effect of epidermal growth factor on insulin-like growth factor-I (IGF-I) and IGF-binding protein synthesis by adult rat hepatocytes

Growth hormone has been established as a primary regulator of IGF-I gene expression in adults, not only in liver but also in many extrahepatic tissues. We considered the possibility that IGF-I production by adult rat liver could also be stimulated by epidermal growth factor (EGF), a peptide known to be involved in liver regeneration. Chromatographic analysis performed after acid treatment of conditioned media revealed the presence of both immunoreactive (IR) IGF-I and IGF binding protein (IGFBP). Both IR IGF-I and IGFBP were present in the conditioned medium of adult rat hepatocytes in basal conditions. The stimulation of IGF-I and IGFBP secretion by EGF appears to be dose-dependent with a significant increment already evident at 5 nM. That EGF stimulates secretion is supported by the finding that IGF-I and IGFBP-1 mRNA levels are increased after EGF supplementation. We conclude that adult rat hepatocytes spontaneously produce IGF-I and IGFBP, and that EGF is able to increase their synthesis and secretion. This non-growth hormone-dependent regulation of IGF-I and IGFBP-1 production by adult rat hepatocytes in culture indicates an important autocrine/paracrine role for IGF-I, particularly during liver regeneration after extensive organ mass loss.

Effect of dietary proteins on insulin-like growth factor-1 (IGF-1) messenger ribonucleic acid content in rat liver

Effects of quantity and quality of dietary proteins on plasma immunoreactive insulin-like growth factor-1 (IGF-1) concentration, and content of IGF-1 mRNA in rat liver were investigated in rats. Plasma immunoreactive IGF-1 concentration in rats given a casein diet was higher than that in rats given a soya-bean-protein or protein-free diet. The IGF-1 mRNA content in liver was estimated by the Northern blot hybridization technique employing 32P-labelled rat IGF-1 complementary DNA (cDNA). At least four molecular species of IGF-1 mRNA of different molecular weight were found in rat liver. The sizes were 0.8-1.2, 2.0, 3.6-4.0 and 7.4 kb. Most of the mRNA species decreased in the livers of rats given a gluten diet (120 g gluten/kg diet) compared with rats given the casein diet. In particular, mRNA of 7.4 kb decreased markedly. When rats were fed on the protein-free diet, mRNA of all species decreased significantly. The estimated IGF-1 mRNA in the livers of rats fed on the gluten or protein-free diet was almost 0.4 of that of the rats given the casein diet. Feeding the soya-bean-protein diet did not result in a marked effect on the hepatic content of mRNA species of IGF-1. The results showed that liver IGF-1 mRNA content is sensitively regulated by quantity and nutritional quality of dietary proteins.

Tamoxifen reduces serum insulin-like growth factor I (IGF-I)

Antiestrogens are widely used in the management of hormonally responsive breast cancer in both adjuvant and palliative settings, and are currently being evaluated as chemopreventive agents. The classical mechanism of action of these drugs involves inhibition of estrogen-stimulated neoplastic cell proliferation by blockade of estrogen receptors present on breast cancer cells. This paper reviews recent clinical and laboratory data that suggest that the commonly used antiestrogen tamoxifen also acts to reduce serum IGF-I levels. Estrogens appear to play a permissive role in growth hormone (GH) release by the pituitary gland and GH is known to stimulate IGF-I expression by hepatocytes. It is therefore possible that blockade of estrogen receptors in the hypothalamic-pituitary axis by tamoxifen interferes with GH release, leading to reduced hepatic IGF-I expression. In view of results suggesting that IGF-I is a more potent mitogen than estradiol for breast cancer cells and data demonstrating a positive correlation between estrogen receptor level and IGF-I receptor level of breast cancer cells, the IGF-I lowering effect of tamoxifen may contribute to the cytostatic activity of the drug. The interrelationships between steroid hormone physiology and IGF-I physiology may have relevance to a variety of commonly used treatments for hormonally responsive cancers.

Insulin regulates insulin-like growth factor I mRNA in rat hepatocytes

To evaluate the regulatory role of growth hormone (GH) and insulin on insulin-like growth factor I (IGF-I) mRNA levels, we employed primary rat hepatocytes. Cells were incubated for 16 h with 10 nM insulin, 10 nM GH, or a combination thereof, and IGF-I mRNA levels were analyzed by Northern blotting. Insulin results in 2.5-fold and GH in 3.8-fold higher IGF-I mRNA levels than hormone-free controls, and a combination of insulin and GH had an additive effect (6.7-fold). The effect of 10 nM insulin was constant at variable GH concentrations. Therefore, GH and insulin affect IGF-I mRNA levels independently of each other. The half-maximal effective dose of insulin was 4.7 X 10(-10) M, and, in kinetic experiments, insulin was effective within 2 h. These findings demonstrate that insulin modulates hepatic IGF-I production by a direct regulation of the transcript levels of IGF-I.

Insulin-like growth factor I gene expression in vascular cells

Insulin-like growth factor I (IGF I), a potent growth factor in vitro, is present in blood and in multiple tissues and is a major mediator of the effects of growth hormone on postnatal growth. IGF I is internalized and retained largely intact in cultured vascular endothelial cells. Neovasculature transiently expresses IGF I immunoreactivity, but it is not known whether this represents internalization of the circulating growth factor or vascular cell synthesis of IGF I. As an initial approach to defining the role of endogenous production of IGF I in the growth program of the vessel wall, Northern hybridizations were performed with RNA from cultured rat aortic smooth muscle cells and bovine aortic endothelial cells. Rat aortic smooth muscle cells expressed three primary IGF I messenger RNA transcripts sized 8.2, 1.7, and 0.9-1.2 kb. Bovine aortic endothelial cells expressed one major and one minor IGF I transcript of 2.1 and 1.6 kb, respectively. IGF I gene expression in smooth muscle cells was also demonstrated by ribonuclease protection assays using a rat exon 3 riboprobe. Both endothelial and vascular smooth muscle cells secreted IGF I, as detected by radioimmunoassay of conditioned medium after separation of IGF I from its binding proteins by gel filtration chromatography. Because IGF I stimulates growth of vascular cells, characterization of IGF I gene expression in blood vessels may be key to understanding developmental as well as abnormal growth in the cardiovascular system.

Effect of cell density on induction of growth hormone receptors by dexamethasone in primary cultured rat hepatocytes

The effect of cell density on the regulation of growth hormone (GH) receptors was studied by measuring specific binding of [125I]hGH to primary cultured hepatocytes with or without dexamethasone, which induces GH receptors. In cell cultures without dexamethasone, the cell density did not affect the level of binding of labeled GH appreciably. On the other hand, in the presence of dexamethasone, which induced an increase in the level of GH receptors on the cells, GH-binding by cultured cells at low cell density (3.3 x 10(4) cells/cm2) was about one-third of that of cells at high cell density (10(5) cells/cm2). Scatchard plot analysis showed that the cell-density dependent change in induction of GH binding, by dexamethasone was due to change in the number of binding sites without significant change in their affinity. The binding capacity of glucocorticoid receptors, measured as specific binding of [3H]dexamethasone to the hepatocytes, was not significantly affected by cell density. These results suggest that cell density modulates GH receptor induction by dexamethasone via events after glucocorticoid receptor binding.

Structure, evolution, expression and regulation of insulin-like growth factors I and II

Insulin-like growth factors (IGF) I and II are chemically-related single-chain peptides with diverse actions on cellular growth and metabolism. This review will focus on recent information pertinent to the biochemical and molecular biological aspects of these peptides. Three areas will be examined: The structure of the two IGF molecules and their precursors will be analyzed; the complicated anatomy of the IGF genes and their mRNAs will be described; and the multiple ways in which the expression of IGF-I and IGF-II can be regulated will be discussed. Gaps in our understanding of these peptides will be highlighted in the context of opportunities for further investigation in this field.

Regulation of insulin-like growth factor-I messenger ribonucleic acid expression in Leydig cells

In the present study, we evaluated insulin-like growth factor-I (IGF-I) messenger RNA expression in the rat testis. Crude interstitial cells were separated into three distinct bands on 15-60% Percoll density gradients. IGF-I mRNA was mainly localized in the Leydig cell-enriched fraction (band 3), while band 1 and band 2 cells did not contain significant amounts of IGF-I mRNA. Leydig cell IGF-I mRNA consisted of multiple species varying from 0.8 to 7.5 kb and was present in rat Leydig cells all ages examined, from 25 to 55 days old. To further document that IGF-I mRNAs are present in Leydig cells, the method of Klinefelter et al. (Biol. Reprod. (1987) 36, 769-783) was used to isolate highly purified (greater than 98% pure) Leydig cells. Most of the IGF-I mRNA was localized in these Leydig cells, while there was no detectable IGF-I mRNA in the whole testis or other interstitial cells. Furthermore, IGF-I mRNA in Leydig cells was increased more than 2-fold by growth hormone (GH) administration in vivo. This suggests that IGF-I mRNA in Leydig cells is also GH dependent. Interstitial IGF-I produced in Leydig cells may have both autocrine and paracrine effects in the testis.

Primary structure and assignment to chromosome 6 of three related rat genes encoding liver serine protease inhibitors

Three closely related SPI genes which encode highly homologous proteins of the serine protease inhibitor family secreted by rat liver (SPI-1, SPI-2 and SPI-3), were isolated from genomic libraries and sequenced, totally (SPI-2) or partially (SPI-1 and SPI-3). These genes all map on rat chromosome 6. Each of them spans about 10 kb and contains five exons separated by four introns, located at equivalent positions. S1 mapping analysis indicated that initiation of transcription occurs at the same position (tsp) in each of the three genes. In vitro transcription experiments demonstrated the presence of promoter elements upstream from the putative tsp. Detailed analysis of 5'-flanking sequences in the three SPI revealed major differences. A high degree of identity (70%) was found within a 350-bp region preceding the 'cap' site, with the exception of a 42-bp spacer, which was only found in SPI-3. Upstream from that point, SPI-1 and SPI-2 sequences remain largely homologous over at least 1 kb but completely diverge from the corresponding sequence in SPI-3. This may, at least partly, account for the differential regulation of the three SPI observed during acute inflammation and upon hypophysectomy.

Stimulation of insulin-like growth factor I production in primary cultures of chicken hepatocytes by chicken growth hormone

Extracts of conditioned media from primary cultures of chicken hepatocytes stimulated [3H]thymidine incorporation into chick embryo fibroblasts demonstrating that the cells released mitogen(s) into the medium. There was also a simultaneous release of insulin-like growth factor I (IGF-I) immunoreactivity into the medium. Acid chromatography of freeze-dried extracts of conditioned media by high performance gel permeation chromatography demonstrated that IGF-I immunoreactivity eluted in a major peak with a molecular weight of 7500 Da and a minor peak with a molecular weight of 55,000 Da. The release of IGF-I immunoreactivity was increased by pituitary-derived chicken growth hormone (cGH) in a dose-dependent manner with half-maximum stimulation occurring at a cGH concentration of 40 ng/ml and maximum stimulation at cGH concentrations greater than 800 ng/ml. These results demonstrate that cultured chicken hepatocytes produce IGF-I and that this can be stimulated by cGH in vitro.

Effects of streptozotocin-induced diabetes mellitus on growth and hepatic insulin-like growth factor I gene expression in the rat

Poorly controlled diabetes mellitus in humans and animals is often accompanied by impaired growth. We undertook this study in young rats to determine whether the reduction in growth rate associated with streptozotocin (STZ)-induced diabetes might be related to changes in both serum insulin-like growth factor I (IGF-I) and IGF-II levels, and, if so, whether these changes reflect alterations in serum growth hormone (GH) and in hepatic IGF-I and IGF-II gene expression. Serum rat GH (rGH) levels were variable during the first 4 days after STZ administration, but during the subsequent 5- to 11-day period the mean (+/- SEM) levels in insulin-treated (DI) (21.4 +/- 4.9 ng/mL) and untreated (D) (8.5 +/- 1.5 ng/ml) diabetic rats were significantly (P less than .001) lower than in controls (C) (117.8 +/- 22.9 ng/mL). Multiple transcripts of IGF-I (7.0, 4.0, 1.9, 1.0 kb), but barely detectable amounts of IGF-II mRNA, were found in the livers of normal and diabetic rats by Northern blot analysis. Using dot blot analysis, we have shown that the abundance of total hepatic IGF-I mRNA in untreated, growth-retarded diabetic animals decreases rapidly over a period of 3 days after STZ administration. Both serum IGF-I and IGF-II levels are also diminished during this interval in these markedly hyperglycemic rats. Insulin treatment for 3 to 4 days, started either immediately (6 hours) or within 3 days after administering STZ, blunts diabetes-induced impairment of growth and restores mean hepatic IGF-I mRNA abundance to control levels, but does not normalize serum IGF-I and IGF-II concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of ovarian function on insulin-like growth factor I plasma levels and hepatic IGF-I mRNA levels in diabetic rats treated with insulin

When insulin was administered to streptozotocin-induced diabetic female rats, the percentage of glycohemoglobin, growth rate, ovulatory cycle, uterus to body weight ratio, and insulin-like growth factor (IGF-I) level returned to near normal. In untreated diabetic rats there were no normal estrous cycles, and hepatic IGF-I mRNA (7.94 +/- 1.02 O.D. units per micrograms total RNA) levels were significantly lower than the control or insulin-treated groups in proestrus (16.47 +/- 0.91 and 17.15 +/- 1.84, respectively). Insulin therapy restored the hypothalamic-pituitary-ovarian axis with the reinstitution of normal estrous cycles. Plasma IGF-I levels were highest in non-diabetic proestrous animals (277 +/- 36.9 ng/ml), significantly higher than IGF-I levels in insulin-treated diabetic rats in diestrus (174 +/- 23.1 ng/ml), non-diabetic diestrus rats (165 +/- 18.4 ng/ml) and untreated diabetic rats (135 +/- 19.7 ng/ml). Plasma IGF-I levels were elevated in insulin-treated diabetic rats in proestrus (221 +/- 78.3 ng/ml), however this was not significantly different from any other group. The increases observed in plasma IGF-I and hepatic IGF-I mRNA after insulin therapy correlate with the normalization of sex hormone secretion. Though this study does not prove a causal relationship between restoration of ovarian function and normalization of circulating IGF-I levels, a relationship has been established, as evidenced by higher levels of IGF-I in both the control and insulin-treated diabetic proestrous groups when compared to the diestrus groups.

Decreased expression of the growth hormone receptor and growth hormone binding protein in rat liver nodules

The growth hormone receptor (GHR) and growth hormone-binding protein (GH-BP) expression were characterized in liver nodules and hepatomas from male Wistar rats. The mRNA levels of GHR and GH-BP, studied by northern blot analysis and solution hybridization, were 35-50% (in nodules) and 2-6% (in hepatomas) of the level found in liver from untreated, age-matched rats. The binding of 125I-labeled human growth hormone to a low-density membrane fraction (LDMF) containing Golgi and endosome membranes in the presence of excess ovine prolactin was 75-80% lower in nodules than in liver. When endogenous ligand was removed, the binding increased in nodules but not in liver. Affinity cross-linking experiments revealed identical specific receptor-binding protein complexes at Mr of 95,000, 55,000, and 43,000 in both nodules and liver, assuming stochiometric binding of ligand. The in vivo endocytosis of 125I-labeled bovine growth hormone correlated with the level of GH binding and was thus reduced 75-80% in nodules, compared with liver. The level of insulin-like growth factor-I (IGF-I) mRNA was reduced by 50% in nodules; however, GH administration resulted in a twofold induction of IGF-I mRNA in both nodules and liver. It is concluded that the greater proportion of occupied GHR in nodules could result from an impaired dissociation of endogenous GH, which might explain the reduced GHR mRNA expression. Furthermore the low GHR mRNA expression may reflect a de-differentiated phenotype in nodules.

Characterization of transgenic livestock production

The objective of transgenic livestock improvement projects is to develop and bring to market superior breeding stock, as well as germplasm for the artificial insemination and embryo transfer industries. Livestock animal biotechnology programs hold the promise of achieving, in a single generation, improvements in commercially important livestock species previously possible only through long-term traditional selective breeding practices or by chance mutation. Transgenic farm animals harboring growth hormone or metabolically related structural genes have been created. Studies of these animals demonstrate the effects of inadequate regulation of transgene expression. Research continues to explore the intricacies of developmental regulation of such genes and phenotypic consequences of mammalian gene transfer. Ultimately, genetically engineered livestock will provide producers with the benefit of increased production efficiencies while the consumer will have healthier animal food products. Conceivably, products will be produced with lower levels of fat, cholesterol, feed additives and pharmaceutical residues from animals with altered carcass composition that will result in greater nutritional benefit for the consumer.

Cloning of ovine insulin-like growth factor-I cDNAs: heterogeneity in the mRNA population

We have isolated and characterized lamb liver cDNAs encoding ovine insulin-like growth factor-I (oIGF-I) precursor polypeptide to study IGF-I gene expression in ruminants. Four cDNA clones were sequenced revealing two different exon 1 sequences (designated 1A and 1B) and four different putative poly(A) adenylation sites. cDNAs containing exon 1A or exon 1B encode precursor polypeptides of 138 or 154 amino acids, respectively. A 130-amino-acid peptide is encoded by all cDNAs examined. These precursors include a hydrophobic leader peptide of varying lengths, the 70-amino-acid oIGF-I, and a 35-amino-acid carboxyl terminal extension peptide. The predicted amino acid sequence of the oIGF-I peptide differs from the human, bovine, and porcine IGF-Is at a single amino acid (at position 66, alanine is substituted for proline) and differs from rat and mouse IGF-Is at 4 and 5 positions, respectively. Both the amino- and carboxy-terminal extension peptides showed regions of extensive sequence homology. Ovine IGF-I amino-terminal peptides are 1 amino acid longer than other mammalian IGFs due to the presence of an extra amino acid (glutamine) present at the proposed boundary of exon 1 and exon 2. Northern blot analysis revealed multiple oIGF-I transcripts in a broad band at 800-1,100 nucleotides and other transcripts of higher molecular weight in liver. There was no detectable expression in either spleen or brain.

Growth factors and the regulation of pre- and postnatal growth

Peptide growth factors represent a largely paracrine level of intercellular communication that is basic to the process of life. Growth factors are present in the ovum and are amongst the first products expressed by the embryonic genome. They function as both signals and progression factors for embryonic tissue growth, induction, differentiation, maturation and function. While a widespread tissue expression is demonstrable during fetal development, and in certain postnatal tissues such as the epiphyseal growth plate, growth factor presence in the adult is restricted to tissues sharing rapid cellular turnover such as ovary. However, a transient re-expression of peptide growth factors occurs during adult tissue repair. In addition to mitogenic peptides such as IGFs or EGF, the family of growth factors also includes physiological growth inhibitors such as TGF beta and certain neuropeptides. Insulin is mitogenic in the early embryo and evidence is presented to support a continuation of this role, under defined nutritional conditions, in late gestation. The importance of insulin to pre- and postnatal growth has prompted an expanding literature dealing with the interactions of nutrients, hormones and growth factors during the growth and functional maturation of the islets of Langerhans. While the expression of growth factors in the early embryo is apparently autonomous, some, such as IGFs, become increasingly dependent on nutrient, insulin and GH availability during fetal development and in childhood growth. This has resulted in circulating IGF I and II determinations becoming useful diagnostic markers of endocrine-based growth disorder and nitrogen balance.

Homologous rat hepatic protease inhibitor genes show divergent functional responses to inflammation

The genes encoding three distinct serine protease inhibitors (Spi) have been cloned from rat liver. These inhibitors are highly homologous with each other and are similar to alpha 1-antitrypsin at the nucleic and amino acid sequence level. Although previous investigators have examined the regulation of the Spi 2 locus by inflammation, the use of various techniques and the complexity of this genetic locus have led to incomplete and somewhat confusing results. Oligonucleotide probes specific for Spi 2.1, Spi 2.2, Spi 2.3, and a 3' mouse cDNA probe for alpha 1-antitrypsin mRNA were used to measure these mRNA after induction of inflammation with subcutaneous turpentine in Fischer rats. alpha 1-Antitrypsin mRNA increased 1.8-fold, and Spi 2.2 increased 7-fold. In contrast, Spi 2.1 and 2.3 mRNA sequences decreased fourfold. The maximal changes occurred between 24 and 48 h after inflammation, with a gradual return toward normal over the next 4 days. Since Spi 2.1, Spi 2.3, and alpha 1-antitrypsin mRNA sequences are responsive to growth hormone, two other growth hormone-responsive mRNA sequences, alpha 2u-globulin and insulin-like growth factor I, were measured, and they also decreased after induction of inflammation. The results of this study show that, despite a marked similarity of nucleotide sequence, Spi 2.1 and 2.3 genes respond very differently from Spi 2.2 and alpha 1-antitrypsin to both growth hormone and inflammation. We speculate that the functions of Spi 2.1 and 2.3 products are different from those of Spi 2.2 and alpha 1-antitrypsin and may involve the regulation of growth.

The gene structure of the insulin-like growth factor family

The insulin-like growth factors (IGF) constitute a family of proteins with insulin-like and growth-stimulating properties. The best characterized members of this family are IGF-I, a protein of 70 amino acids which plays an important role in post-natal growth, and IGF-II, a 67 amino acid protein which is most likely involved in fetal development. The gene structure of IGF-II has been elucidated for the human and the rat and shows extensive interspecies homologies. The gene structure of IGF-I has only partially been established. A striking feature of the IGF genes is that they are controlled by multiple promoters which are expressed in a tissue-specific and development-dependent way.

Comparison between constant-protein, calorie-restricted and protein-restricted, calorie-restricted diets on growth, in vitro lipogenesis and plasma growth hormone, thyroxine, triiodothyronine and somatomedin-C (Sm-C) of young chickens

1. We studied the effects of calorie-restricted, constant-protein and calorie-restricted, protein-restricted diets on growth and in vitro metabolism of male chickens from select (Cobb Line 500) and byproduct (Cobb female line) lines of broiler chickens. 2. Chickens consumed 40, 60, 80 or 100% of a prescribed formula for dietary energy (body weight in g0.70 x 16.7 kJ) in the presence of set (CEP) or varied dietary protein (VEP). 3. Chickens fed VEP were heavier (P less than 0.05) at all energy intakes than chickens fed CEP. Slope analysis of data for in vitro lipogenesis showed a significant difference between the two treatment series. 4. Plasma growth hormone was inversely related (P less than 0.05) to Sm-C. Growth hormone levels were greater in chickens on a low plane of energy nutrition (40%) than on the maximum plane (100%). 5. Plasma Sm-C levels (pooled across energy series) were greater in the select than in the byproduct line. There were no differences in plasma T3 between the two lines. There was a significant increase (P less than 0.05) in T3 and a decrease in the T4/T3 ratio accompanying an increase in dietary energy. 6. Restricting dietary carbohydrate and protein compromises anabolic processes more than restricting carbohydrate alone.

Lipid synthesis and lipoprotein secretion by chick liver cells in culture: influence of growth hormone and insulin-like growth factor-I

1. Chick liver cells were incubated in unsupplemented medium (control), or medium supplanted with either 1 microgram/ml pituitary derived chicken growth hormone (GH), 50 ng/ml recombinant human insulin like growth factor-I (IGF-I), or 1 microgram growth hormone/ml and 50 ng insulin like growth factor-I/ml (GH + IGF-I). 2. GH supplementation stimulated acetate incorporation into liver cell lipid. Low density lipoprotein (LDL) lipid secretion was increased quantitatively by GH. 3. Cells incubated with IGF-I incorporated more acetate into lipid and secreted more lipid as VLDL and HDL than controls. 4. A metabolic antagonism between GH and IGF-I was evident with respect to lipogenesis. 5. Neither GH nor IGF-I altered, quantitatively, cell protein synthesis or apoprotein secretion.

Salt stimulation of serum insulin-like growth factor binding protein activity

Insulin-like growth factors (IGF)-I and -II are bound to carrier or binding proteins in serum. There are at least two classes of binding protein: a high molecular weight complex and a low molecular weight species that is relatively unsaturated. Total binding capacity in serum generally is determined by incubating [125I]IGF with protein that has been stripped of IGF by acid gel filtration. We found that addition of NaCl to the assay increased binding to stripped guinea pig binding protein to about two to four times the level measured in the absence of salt. Stimulation by NaCl was optimal between concentrations of 0.6 and 1.4 M and also was observed when fetal calf or human sera were used as sources of stripped binding protein or when IGF-II was the ligand. Using chloride salts, the order of activity with respect to cations was Na+ greater than K+ greater than Li+. Na2HPO4 at 0.6 M was as stimulatory as 1.2 M NaCl but 0.6 M Na2SO4 was less effective. NH4HCO3 was as effective as NaCl at 0.6 M. Scatchard plots of data from competitive dilution experiments with [125I]IGF-I and unlabeled IGF-I showed that binding was heterogeneous in the absence of 0.6 M NaCl but linear in its presence. NaCl did not stimulate binding when whole serum was used, but after gel filtration of serum on Sephacryl 200 at pH 8, which does not dissociate IGFs from binding protein, binding to individual fractions was stimulated three- to fourfold by NaCl. Fractions stimulated included those containing the large complex or the unsaturated binding protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of mRNA for IGF-I and -II during growth hormone-stimulated muscle hypertrophy

The expression of insulin-like growth factor (IGF) genes during skeletal and cardiac muscle hypertrophy was examined using skeletal and cardiac muscle hypertrophy was examined using adult 5-mo-old female Wistar-Furth rats implanted with growth hormone-secreting GH3 cells. Control and treated animals were killed at 40, 60, and 80 days after initiation of the experiment. From the time of injection to day 80, body, heart, skeletal muscle, and liver weights increased 112, 93, 55, and 314%, respectively. RNA was extracted and steady-state levels of IGF-I and IGF-II mRNAs were quantitated using a solution-hybridization nuclease-protection assay. Low levels of mRNA for both growth factors were detected in control tissues. By day 80 IGF-I mRNA had increased eightfold and IGF-II mRNA sixfold in skeletal muscle from treated rats. In cardiac muscle the levels of mRNA for both growth factors rose three- to fourfold. Although growth hormone induced an increase in hepatic IGF-I mRNA, IGF-II mRNA remained nearly undetectable. This study shows that during growth hormone-stimulated muscle growth mRNAs for both IGF-I and IGF-II accumulate, supporting other observations implicating the IGFs as paracrine or autocrine factors involved in skeletal muscle growth.

Dwarf mice produced by genetic ablation of growth hormone-expressing cells

Fusion of the 310 bp located 5' of the rat growth hormone (GH) gene to the human GH structural gene resulted in somatotrope-specific expression in transgenic mice. Human GH transcripts were detected only in pituitaries of these mice, and immunocytochemical analyses revealed that this expression was limited to GH-expressing cell types. The rat GH 5' sequences were then used to direct the expression of diphtheria toxin to the GH-expressing cells of transgenic mice. A line of mice was established which lacks detectable levels of circulating GH. This deficiency resulted in dwarfism; transgenic animals grew only to half the size of nontransgenic littermates. Nearly all somatotropes were absent, as shown by GH immunostaining in the transgenic pituitaries. Prolactin (PRL)-producing lactotropes, thought to share a common cellular origin with somatotropes, were also reduced in numbers. A model for the lineal relationships between GH- and PRL-synthesizing cells is proposed.

The insulin-like growth factors and their binding proteins

1. This review provides a brief overview of the structure of the insulin-like growth factors (IGFs or somatomedins), their mRNA and genes; the regulation and sites of production of these peptides; their binding and actions in target tissues; and the structure and biological role of their binding proteins. 2. Molecular cloning techniques have allowed the prediction of precursor forms of IGF-I and IGF-II, have provided tools to study the regulation of the synthesis and translation of IGF mRNAs, and have recently yielded the primary sequence of the IGF-I receptor, supplementing other rapidly-accumulating structural data. 3. Several of the IGF binding proteins have also been purified, and initial structural studies performed. 4. The increased knowledge of the structures of the IGFs, their receptors and binding proteins should now permit rapid progress in understanding the physiology and functions of these proteins.

Developmental and hormonal regulation of mRNAs for insulin-like growth factor II and steroidogenic enzymes in human fetal adrenals and gonads

Insulin-like growth factor II (IGF-II) is regulated developmentally and hormonally in human fetal gonads and adrenals. The abundance of IGF-II mRNA is greatest in RNA from human fetal adrenals, followed by fetal liver, testis, placenta, and ovaries. Fetal testicular IGF-II mRNA decreases significantly with increasing gestational age, in parallel with our previous measurements of the mRNAs for the steroidogenic enzymes P450scc (cholesterol side-chain cleavage enzyme) and P450c17 (17 alpha-hydroxylase/17,20 lyase) (J. Clin. Endocrinol. Metab. 63, 1145, 1986). The abundances of P450scc and P450c17 mRNAs in cultured fetal testis cells rose 2.5-fold (p less than 0.01) and 9.2-fold (p less than 0.001), respectively, in response to 0.5 mM cAMP, but the abundance of IGF-II mRNA was not affected. This suggests that the IGF-II gene is regulated differently in fetal testes than it is in fetal adrenals, placenta, or adult granulosa cells, where we have previously shown that ACTH, cAMP, and gonadotropins, respectively, increase IGF-II mRNA accumulation (Proc. Natl. Acad. Sci. USA 84, 1590, 1987). Exogenously added IGF-I and IGF-II had no effect on mRNAs for P450c17 or P450c21 (21-hydroxylase), but decreased IGF-II mRNA in ACTH-stimulated fetal adrenal cells. Thus, the IGFs appear to exert short-loop feedback inhibition on accumulation of IGF-II mRNA.

The molecular basis of growth hormone deficiency

A well-known law states that 'if a thing can go wrong it will go wrong'. This clearly applies to the hypothalamic-pituitary-somatic axis as to many other physiological and biochemical systems. Defects of this axis, giving rise to stunted growth, can occur at several different points, as has been discussed in detail in this review. Defects at the level of the brain can lead to inadequate production or secretion of the factors that control growth hormone secretion. Defects at the level of the pituitary can lead to failure to produce or secrete adequate quantities of growth hormone, or to production of inactive hormone. Defects at the level of target organs can lead to inability to respond to growth hormone or somatomedins. The axis involved in the production and effects of growth hormone is a complex one, and defects have been identified at most of the points that 'could go wrong', although in many cases the molecular details are far from fully understood. Increased understanding of the biochemistry and physiology of the hormonal control of growth, and of the impairments to which it is subject, should provide an improved basis for treatment of growth defects. Nevertheless, there remain many points at which our knowledge is very incomplete. The field is a rapidly moving one and further developments in both basic understanding and clinical treatment are to be expected during the next few years.

Presence of somatomedin receptors on primary human breast and colon carcinomas

Competitive binding techniques were used to study the interaction of insulin-like growth factor I (IGF-I) with a plasma membrane-enriched subcellular fraction purified from primary breast and colon carcinoma specimens obtained at surgery. The presence of specific binding sites for IGF-I was detected in all tumour specimens studied. Scatchard analysis and competition studies with insulin and insulin-like growth factor-II (IGF-II) revealed the presence of specific IGF-I receptors, showing a Kd-value of approximately 2 nM. These results are consistent with the hypothesis that somatomedins play a role in determining the proliferative behaviour of human breast and colon tumors, and suggest that recent laboratory studies showing dependence of neoplastic cells on somatomedins for optimum proliferation may have clinical relevance.

Rat IGF-I cDNA's contain multiple 5'-untranslated regions

DNA sequencing of several independent rat IGF-I cDNA clones has revealed three different 5'-untranslated region sequences which contain multiple, upstream, in-frame initiation codons. Use of these codons could generate N-terminal heterogeneity in IGF-I precursor proteins. One of these 5'-untranslated region sequences contains a 40-bp segment which is an inverted repeat of a region in the common 3'-untranslated region. The ends of the IGF-I mRNA corresponding to this cDNA could form a stable duplex structure. Such a complex could prevent ribosomal access to the AUG codons preceding the coding region for the pre-pro-IGF-I peptide, suggesting the possibility of translational regulation of this form of IGF-I mRNA. The 3'-untranslated region inverted repeat sequence also is present in human and mouse IGF-I cDNA's, and, intriguingly, is more highly conserved than the rest of the 3'-untranslated region.