Regulation of insulin-like growth factor-I and growth hormone receptor gene expression by diabetes and nutritional state in rat tissues

The insulin-like growth factor (IGF) system and gonadotropin regulation: actions and interactions

Insulin-like growth factors (IGF) are polypeptides that regulate growth, differentiation and survival in a multitude of cells and tissues. The IGF system consists of ligands, receptors, binding proteins and binding protein proteases. The influence of the IGF system on reproductive parameters, specifically gonadotropin release and interactions between the IGF system and other effectors of gonadotropin release will be examined in this review.

A quantitative PCR measurement of messenger RNA expression of IGF-I, IGF-II and IGFBP-5 in human skeletal muscle

Insulin-like growth factor-I and -II (IGF-I and IGF-II) and their binding proteins are important components in growth promotion and tissue maintenance. We determined the presence of IGF-I, -II, and binding protein 5 (IGFBP-5) gene expression in human skeletal muscle and that mRNA abundance is not altered by nutrients and insulin. In the first protocol, (control) subjects were given water. In the second protocol, half of these subjects drank Polycose (carbohydrate) and the remaining subjects drank equal calories as a mixed meal. Quadriceps muscle biopsies were taken at 10 h. A semi-quantitative polymerase chain reaction was designed to measure gene expression. IGF-I, IGF-II and IGFBP-5 mRNA are present in adult human skeletal muscle, but no significant changes between meal groups were observed for IGF-I, IGF-II or IGFBP-5 mRNA levels, indicating that the expression of these genes are not altered acutely by nutrients and insulin.

Insulin inhibits growth hormone signaling via the growth hormone receptor/JAK2/STAT5B pathway

Insulin is important for maintaining the responsiveness of the liver to growth hormone (GH). Insulin deficiency results in a decrease in liver GH receptor (GHR) expression, which can be reversed by insulin administration. In osteoblasts, continuous insulin treatment decreases the fraction of cellular GHR localized to the plasma membrane. Thus, it is not clear whether hyperinsulinemia results in an enhancement or inhibition of GH action. We asked whether continuous insulin stimulation, similar to what occurs in hyperinsulinemic states, results in GH resistance. Our present studies suggest that insulin treatment of hepatoma cells results in a time-dependent inhibition of acute GH-induced phosphorylation of STAT5B. Whereas total protein levels of JAK2 were not reduced after insulin pretreatment for 16 h, GH-induced JAK2 phosphorylation was inhibited. There was a concomitant decrease in GH binding and a reduction in immunoreactive GHR levels following pretreatment with insulin for 8-24 h. In summary, continuous insulin treatment in rat H4 hepatoma cells reduces GH binding, immunoreactive GHR, GH-induced phosphorylation of JAK2, and GH-induced tyrosine phosphorylation of STAT5B. These findings suggest that hepatic GH resistance may develop when a patient exhibits chronic hyperinsulinemia, a condition often observed in patients with obesity and in the early stage of Type 2 diabetes.

Diazoxide-induced insulin deficiency greatly reduced muscle protein synthesis in rats: involvement of eIF4E

We have investigated the effect of a postprandial acute insulin deficiency induced by diazoxide injection on rat skeletal muscle protein synthesis. Diazoxide administration lowered plasma insulin >85% within 3 h after injection, whereas other hormones (insulin-like growth factor I, glucagon, corticosterone) involved in the regulation of muscle protein synthesis were not altered significantly compared with control animals. The fractional rate of muscle protein synthesis, measured in vivo, was reduced significantly (P < 0.05) in epitrochlearis (-46%), gastrocnemius (-41%), and soleus (-35%). The reduction in protein synthesis did not result from a reduced total RNA content but was associated with diminished translation efficiency. Analysis of ribosomal subunits revealed that the decreased translation efficiency resulted from an impairment in the initiation phase of protein synthesis. Diazoxide-induced insulin deficiency was associated with a dramatic decrease in eukaryotic initiation factor (eIF) 4G bound to eIF4E and a 2.5-fold increase in the amount of the eIF4E. 4E-binding protein 1 (BP1) complex. In contrast, diazoxide injection did not change either the relative amount of eIF4E present in gastrocnemius or its phosphorylation state. These results indicate that an acute insulin deficiency significantly decreases postprandial muscle protein synthesis by modulating the interaction between 4E-BP1, eIF4G, and eIF4E to control translation initiation.

Acute treatment with corticosteroids decreases IGF-1 and IGF-2 expression in the rat diaphragm and gastrocnemius

Massive doses of methylprednisolone (M) or triamcinolone (T) induced diaphragmatic type IIx/b atrophy, resulting in a leftward shift of the force-frequency curve in rats (). To examine the role of insulin-like growth factors (IGFs) in these changes, IGF mRNA content was measured in costal diaphragm, gastrocnemius, and liver removed from 32 rats treated daily during 5 d either with saline (control, C and pair-fed, PF), M, or T (80 mg/kg). Blood samples were taken to measure IGF-1 serum levels. RNA levels were measured by Northern and dot-blots after hybridization with rat IGF-1 or IGF-2 cDNA probes labeled with alpha-32P. Compared with C (845 +/- 128 ng/ml), IGF-1 serum levels were significantly decreased in M (699 +/- 90 ng/ml, p < 0.001 versus C) and PF animals (505 +/- 33 ng/ml, p < 0.001 versus others) and even more so, in T-treated animals (273 +/- 134 ng/ml, p < 0.001 versus others). Along the same lines, IGF-1 expression in the liver was depressed after corticosteroid treatment and in PF, whereas IGF-2 mRNA content remained unchanged. Compared with C, the relative expression of IGF-1 mRNA in the diaphragm was depressed by 44% and 69% in the M and T groups, respectively (p < 0.0001 versus C), while it was unchanged in PF animals. In the gastrocnemius, IGF-1 expression was reduced after M and T (-51% and -59%, respectively, p < 0.0001 versus C) as well as in PF animals (-40%, p < 0.001 versus C). For IGF-2, a similar pattern of expression was found in the diaphragm and the gastrocnemius. Indeed, IGF-2 mRNA tended to decrease in corticosteroid-treated rats (NS) whereas it was unchanged in PF rats. We conclude that decreased IGF expression after corticosteroid treatment was similar in diaphragm and gastrocnemius and may be responsible for the diaphragmatic changes observed after steroid treatment.

Thyroid hormones are involved in insulin-like growth factor-I (IGF-I) production by stimulating hepatic growth hormone receptor (GHR) gene expression in the chicken

Effect of thyroid status on IGF-I production in growing chickens was studied. Serum concentrations of GH were not affected by propylthiouracil (PTU) or thyroxine (T4) treatments, whereas serum IGF-I levels were significantly decreased in PTU-treated chickens. The lowered serum IGF-I levels in the PTU-treated group were completely restored to the control levels by T4 injections. In the liver, the messenger RNA (mRNA) expressions both for GH receptor (GHR) and IGF-I were significantly repressed by PTU treatment, and were restored again by T4 replacement. In addition, the results of analysis on radiolabelled GH binding to the liver membrane were consistent with the levels of hepatic GHR mRNA expression. Serum concentrations of IGF-I were positively correlated with hepatic IGF-I mRNA and GHR mRNA expressions. The correlation coefficient between serum T3 levels and hepatic IGF-I mRNA expressions was also significant. These results indicate that thyroid hormones regulate IGF-I production in the chicken by affecting hepatic GHR expression.

Regulation of growth hormone receptor gene expression

Pituitary growth hormone (GH) is essential for postnatal growth in animals. GH exerts its actions by direct effect on target organs and by stimulating the production of insulin-like growth factor I (IGF-I). At the tissue level, the pleiotropic actions of GH result from the interaction of GH with a specific cell surface receptor, the GH receptor (GHR). The GHR belongs to the hematopoietic receptor superfamily. The human GHR is the product of a single gene located on chromosome 5p13.1-p12 and spans at least 87 kb. Transcripts from this gene are characterized by the presence of disparate 5' untranslated exons. In the liver at least eight different GHR 5' untranslated regions (UTRs) have been described. This heterogeneity in the 5' UTR most likely results from the splicing of the various exon 1 fragments to a common splice site located 11 bp upstream of the initiating ATG. Heterogeneity in the 5' UTR sequences of the GHR transcripts indicates that transcriptional control of the locus is complex. GHR gene expression is minimal to absent in the fetus, with the postnatal increase in expression in the liver being maximal during pregnancy. GHR gene expression is also regulated by factors such as nutritional intake, GH, steroid hormones, and diabetes mellitus. Available information about the molecular mechanisms regulating expression of the GHR gene is discussed. Thus the GHR gene presents a picture of multiple 5' untranslated exons under the control of multiple promoters. The use of alternate promoters for initiation of transcription in conjunction with differential splicing allows for exquisite regulation of gene expression. This schema is appropriate for a protein that is essential to many of the physiological processes that are crucial for the survival and well-being of the organism.

Autoregulation of growth hormone receptor and growth hormone binding protein transcripts in brain and peripheral tissues of the rat

Growth hormone (GH) differs from other pituitary hormones in that it can affect a wide spectrum of cellular activities in many different tissues. These disparate actions are, however, mediated by a common receptor, suggesting tissue-specific differences in the post-receptor mechanisms and/or tissue sensitivities to GH stimulation may confer specificity. Tissue sensitivity depends upon the abundance of GH receptors (GHRs) and may be modulated by the amplitude and pulsatility of GH secretion. It may also be dependent upon the presence of non-signal transducing GH-binding proteins (GHBPs), which result from the alternate splicing of GHR gene transcripts. Tissue-specific autoregulation of GHRs and GHBPs could, therefore, contribute to differential tissue responsiveness to GH action. The autoregulation of GHR and GHBP gene transcription in novel central (hypothalamus, brainstem, and cortex/neocortex) and peripheral (spleen) tissues was therefore examined in adult, male Sprague-Dawley rats. For comparative purposes, GHR/GHBP gene expression was also examined in the liver, which has traditionally been considered the major GH-target site. Chronic hyposomatotropism, induced by hypophysectomy, exerted tissue-specific effects on the abundance of GHR gene products 10 days post-hypophysectomy. Both GHR and GHBP transcripts were reduced in the hypothalamus of hypophysectomized rats by 20% (P < 0.001), although neither transcript was affected in the liver, spleen, cortex/neocortex or brainstem. In contrast, 2 h after a single bolus GH injection that was designed to simulate a pulsatile increase in circulating GH concentrations, GHR and GHBP mRNA content was significantly increased by 25-30% (P < 0.001) in all brain regions and in the spleen of hypophysectomized or sham-hypophysectomized rats. Production of the two transcripts was differentially regulated, however, as GHBP, but not GHR, transcripts were increased in the liver (P < 0.001), whereas the GHR:GHBP ratio was decreased in the hypothalamus of GH-treated rats (P < 0.001). These results suggest that GHR gene transcription and splicing are acutely autoregulated in a tissue-specific way.

Effects of dietary protein and tumor necrosis factor on components of the insulin-like growth factor-I pathway in the colon and small intestine in protein-depleted rats

Intestinal cell growth is markedly affected by nutrient intake and the presence of cytokines. Since insulin-like growth factor-I (IGF-I) is an important hormone regulator of intestinal proliferation, this study examined the effects of dietary protein content and tumor necrosis factor (TNF) on mRNA levels of IGF-I, IGF-I receptor, IGF binding protein-3 (IGFBP-3), and IGFBP-4 and on the histology of the colon, jejunum, and ileum in protein-malnourished rats. After 2 weeks of feeding a 2% casein diet, rats continued on the 2% casein diet or were refed with a 20% casein diet and received daily intraperitoneal injections of either TNF (50 microg/kg) or saline for 4 days. The abundance of mRNA in the intestine was determined by RNA dot-blot analysis, and morphology measurements were performed by light microscopy. Simultaneous refeeding with the 20% casein diet and administration of TNF led to a modest increase in IGF-I and IGFBP-4 mRNA abundance in the colon. However, in the jejunum and ileum, refeeding had no effect but TNF caused a decrease in IGF-I and IGFBP-3 mRNA levels in malnourished rats. Refeeding with the 20% casein diet resulted in relatively modest histologic changes, which were greater in the colon versus the small intestine. The decreased magnitude of histologic changes in the order of the colon, ileum, and jejunum may reflect a response to a gradient of amino acid availability from intraluminal nutrients. These data demonstrate that TNF has distinct effects on colon and small intestine mRNA, but these mild changes had only a slight impact in the colon and did not translate into identifiable histologic changes in the small intestine. Combined protein restriction and TNF administration had only a modest effect on intestinal mRNA levels and mucosal histology.

Differential expression of renal growth hormone receptor and its binding protein in experimental diabetes mellitus

Growth hormone (GH) may have a role in the development of diabetic nephropathy. The effect of experimental diabetes on renal expression of the growth hormone receptor gene products, including the receptor itself (GHR) and its binding protein (GHBP) was examined. Adult female rats received i.v. streptozotocin and were killed at 7, 30, 90 and 180 days after the induction of diabetes. Diabetic animals had a pronounced increase in kidney weight and progressive albuminuria. In renal cortex, no change was seen in GHR mRNA levels throughout the observation period of 6 months, while a significant increase in cortical GHBP mRNA levels was observed after 1 month of diabetes and sustained for the rest of the study period. Immunohistochemical analysis of kidney sections revealed a stronger staining for GHBP at the cortical and inner medullary areas in the diabetic animals. These data indicate that although the GHR and GHBP mRNAs originate from the same gene, their renal levels are differentially regulated during the development of experimental diabetic kidney disease, suggesting a functional role for GHBP.

Moderate food restriction affects skeletal muscle and liver growth hormone receptors differently in pigs

The present study was conducted to determine the influence of food restriction on growth hormone receptor (GHR) in porcine skeletal muscle (longissimus dorsi and trapezius) and liver in relationship to plasma growth hormone binding protein (GHBP). At 76 d of age, pigs were allotted to one of three groups. In Group R100kg-196d, pigs were fed 70% of control food intake and slaughtered at 100 kg. Control pigs had free access to food and were slaughtered either at the same weight (Group C100kg-161d) or at the same age (Group C130kg-196d) as R100kg-196d pigs. Plasma insulin-like growth factor-I concentrations tended to be lower in food-restricted pigs than in control pigs at 40 kg (P < 0.1) and 70 kg (P < 0.05). At slaughter, there were no significant differences among the three groups. In liver, 125I-labeled bovine GH specific binding was higher (P < 0.05) in food-restricted pigs than in control pigs, and GHR mRNA level was higher (P < 0.05) in food-restricted pigs than in C100kg-161d pigs. In trapezius, 125I-labeled bovine GH specific binding was lower (P < 0.05) in food-restricted pigs than in C130kg-196d pigs, and the level of GHR mRNA was higher (P < 0.01) in food-restricted pigs than in control pigs. The levels of GHR in longissimus dorsi were not affected by food restriction. The level of plasma GHBP was lower (P < 0.05) in food-restricted pigs than in C130kg-196d pigs. These data indicate that nutritional status regulates GHR in a tissue-specific manner and that there is no simple relationship between plasma GHBP and hepatic GHR in pigs.

Nutrition and the insulin-like growth factor system

Nutritional status is a key regulator of the circulating and tissue insulin-like growth factor (IGF) system. IGF-I mRNA and protein levels decrease in tissues such as liver and intestine with fasting and are restored with refeeding. Additional studies suggest that the level of protein and calorie intake independently regulate plasma IGF-I concentrations in man. The level of nutrition effects the biological actions of recombinant growth hormone (GH) and IGF-I administration in humans. Limited data demonstrate that plasma and tissue levels of the insulin-like growth factor binding proteins (IGFBPs) are also sensitive to nutrient intake. Specific micronutrients, such as potassium, magnesium and zinc also appear to be important for optimal IGF-I synthesis and anabolic effects in animal models. Malnutrition is common in elderly patients, however, the interaction between specific nutrients, general nutritional status and the aging process on the IGF system is incompletely understood. Mechanisms of nutrient-IGF system interactions which may affect the biological actions of IGF-I, IGF-II, and the IGFBPs are increasingly being determined in basic studies. The effects of underlying nutritional status and responses to dietary intake will be important to evaluate in clinical studies of the IGF system and exogenous growth factor therapy.

Somatostatin, growth hormone, insulin-like growth factor-1, and diabetes: friends or foes?

Major findings with regard to the somatostatin-growth hormone (GH)-insulin-like growth factor (IGF-1) axis and diabetes are summarized. GH hypersecretion and reduced circulating IGF-1 levels are prevalent in insulin-dependent diabetes. Somatostatin improves metabolism in insulin-dependent diabetics. Insulin resistance and poor metabolic regulation, which may partly be due to hypersecretion of GH, are believed to accelerate the development of diabetic angiopathy. Diabetic hypersomatotrophinemia may be due to hepatic resistance to GH and increased hepatic production of IGF-1-binding protein-1 (IGFBP-1), leading to reduced levels of circulating IGF-1 and further stimulation of GH production. Studies in vitro and in diabetics suggest a causal link between diabetic hypersomatotrophinemia and diabetic angiopathy. In vitro evidence for the involvement of IGF-1 in diabetic angiopathy is reviewed. Also reviewed is evidence, from rat and human studies, of the possible involvement of GH and IGF-1 in diabetic nephropathy. The role of somatostatin in late diabetic vascular complications remains to be elucidated.

Serum levels of insulin-like growth factor-I and insulin-like growth factor binding protein-3 in children with insulin-dependent diabetes mellitus

Pubertal development has recently been evaluated from the standpoint of changes in insulin-like growth factor (IGF)-I and IGF-binding protein-3 (IGFBP-3) levels in healthy children. We studied puberty related changes in serum IGF-I and IGFBP-3 levels in 24 patients (11 prepubertal) with insulin dependent diabetes mellitus (IDDM) and 26 healthy subjects (14 prepubertal). Serum IGF-I and IGFBP-3 levels were assayed using immunoradiometric assays and radioimmunoassays, respectively. Serum IGF-I and IGFBP-3 levels in diabetics did not increase during puberty, as opposed to those in healthy children. Serum IGF-I and IGFBP-3 levels of diabetic patients were found to be lower than those of control subjects during puberty (p < 0.0001 and p < 0.05, respectively). Proteolysis is believed to be a general mechanism to increase IGF bioavailability in the presence of IGFBPs. Increased IGFBP-3 protease activity has been shown in sera of children with IDDM as well as a decrease in this activity in response to insulin therapy. Our data displaying low IGFBP-3 levels in diabetic children may be due to increased proteolysis, which also causes a shift in IGF-I to its lower molecular weight forms. Higher rate of clearance of the latter may be the reason for the low IGF-I levels we observed in children with IDDM. The moderate correlation between insulin dose and IGFBP-3 levels (r = 0.5, p < 0.01) may suggest insulin to be a contributing factor in the regulation of IGFBP-3 levels. We conclude that regulation of IGF-I and IGFBP-3 concentrations is disturbed in children with IDDM, in particular during adolescence.

Nutritional regulation of insulin-like growth factor-I

Several lines of evidence indicate that in the human, insulin-like growth factor-I (IGF-I) is nutritionally regulated. Both energy and protein availability are required for maintenance of IGF-I. Measurements of serum IGF-I constitute a sensitive means for monitoring the response of acutely ill patients to nutritional intervention. Serum IGF-I may also serve as a marker for evaluation of nutritional status. Our findings and those of others in animal models suggest that nutrients influence synthesis and action of IGF-I and its binding proteins (IGFBPs) at multiple levels. In fasting, liver growth hormone (GH) binding is decreased, providing one explanation for decreased IGF-I. In protein restriction, GH receptors are maintained, but there is evidence for a postreceptor defects. The latter results from pretranslational and translational defects. Amino acid availability to the hepatocytes is essential for IGF-I gene expression. Protein malnutrition not only decreases IGF-I production rate, but also enhances its serum clearance and degradation. Finally, there is evidence for selective organ resistance to the growth-promoting effects of IGF-I in protein-restricted rats.

The insulin-like growth factor system in vascular smooth muscle: interaction with insulin and growth factors

Vascular smooth muscle cells (SMCs) occur throughout the vascular tree and have important physiological functions. They are also involved in pathological processes such as development and progression of atherosclerotic lesions, restenosis following angioplasty, and in hypertension. This review is focused on the role of the insulin-like growth factor (IGF) system in proliferation, migration, and hypertrophy of vascular SMCs and its interaction with insulin and other growth factors. The IGF-I receptor is highly expressed in SMCs in intact arteries and in cultured SMCs and is activated by binding of IGF-I to the two alpha-subunits. Insulin and IGF-II from the circulation can interact with the IGF-I receptor at higher concentrations. Insulin receptors are few or absent in SMCs and circulating insulin concentrations in vivo are probably too low for a direct action of insulin on the IGF-I receptor in SMCs. Receptor activation initiates a number of signal transduction pathways. Increased phosphatidylinositol turnover and calcium mobilization correlates with actin filament reorganization and stimulation of directed migration of the SMC in a gradient of IGF-I. The effects of IGF-I receptor activation on signal transduction pathways (eg, the MAP kinase cascade) implicated in DNA synthesis and proliferation are weak and this correlates with the meager mitogenic activity of IGF-I in SMC. Several components of the IGF-system in SMC are regulated by growth factors such as platelet-derived growth factor (PDGF)-BB and basic fibroblast growth factor (bFGF).(ABSTRACT TRUNCATED AT 250 WORDS)

The role of growth hormone, insulin-like growth factors (IGFs), and IGF-binding proteins in experimental diabetic kidney disease

Early renal changes in type I diabetes are characterized by an increase in renal size, glomerular volume, and kidney function, and later by development of mesangial proliferation, accumulation of glomerular extracellular matrix, and increased urinary albumin excretion (UAE). Growth hormone (GH) and insulin-like growth factors (IGFs) have a long and distinguished history in diabetes mellitus, with possible participation in the development of long-term complications. In experimental diabetes in dwarf rats with isolated GH and IGF-I deficiency, a slower and lesser renal and glomerular hypertrophy is observed as compared with diabetic control animals with intact pituitary. Furthermore, diabetic dwarf rats with a diabetes duration of 6 months display a smaller increase in UAE, indicating that GH and IGF-I may be involved in the development of diabetic kidney changes. In line with this, administration of octreotide to streptozotocin (STZ)-diabetic animals with normal pituitary inhibits initial renal growth without affecting blood glucose levels, and 6 months' administration of octreotide to diabetic rats reduces long-term renal/glomerular hypertrophy and UAE. In addition, the initial increase in renal size and function in experimental diabetes is preceded by an increase in renal IGF-I, IGF-binding proteins (IGFBPs), and IGF-II/mannose-6-phosphate receptor (IGF-II/Man-6-P receptor) concentration. Finally, specific changes occur in renal GH-binding protein (GHBP) mRNA, IGF-I receptor mRNA, and IGFBP mRNA expression in long-term diabetes. In conclusion, the knowledge we have today indicates that GH and IGFs, through a complex system consisting of GHBP, IGFs, IGF receptors, and IGFBPs, may be responsible for both early and late renal changes in experimental diabetes.

Effect of fasting and growth hormone (GH) administration on GH receptor (GHR) messenger ribonucleic acid (mRNA) and GH-binding protein (GHBP) mRNA levels in male rats

To elucidate whether GHR and GHBP are coordinately regulated or not, we studied the effect of fasting with or without GH administration on the GHR and GHBP mRNAs in the liver as well as in extrahepatic tissues in rats. Tissues were collected from 7-week-old male rats by decapitation 1,3, and 7 days after the start of fasting. Liver GHR mRNA levels were not affected 1 day after the start of fasting but progressively decreased for the subsequent 3 and 7 days of fasting as compared with those in control rats fed ad libitum. In contrast, liver GHBP mRNA levels significantly rose after 1 day fasting, returned to the control level after 3 days and further reduced after 7 days of fasting. Changes in GHBP mRNA level after fasting were different among the tissues. A transient increase in GHBP mRNA levels was observed in muscle and heart as well as liver, while the GHBP mRNA levels in fat tissues did not change throughout 7 days of fasting. Next, bovine GH(bGH) was administered ip to the fasted rats and control fed rats for either 1 day(100 micrograms [corrected], tid) or 5 days(150 micrograms [corrected], daily). In fed rats, liver GHR mRNA level was significantly increased by 1 day bGH treatment, but after 5 days treatment with bGH it was not different from the level in saline-injected control. Accordingly, net increment of plasma IGF-I was 296.0 ng/ml with 1 day bGH treatment and 234.2 ng/ml with bGH administration for 5 days. In fasted rats, liver GHR mRNA level did not changed after 1 day treatment with bGH, but markedly decreased 5 days after bGH administration. Net increment of plasma IGF-I was slightly reduced to 284 ng/ml with 1 day treatment with bGH, and markedly decreased to 37.0 with bGH administration for 5 days. The effect of GH administration on liver GHBP mRNA level was virtually absent in either fasting or fed state. These findings suggest that GHR and GHBP mRNAs in the liver are expressed in different ways and that expression of GHBP mRNA is differently regulated among tissues.

Regulation of growth factor mRNA levels in the eyes of diabetic rats

The underlying etiology of diabetic microvascular disease remains unknown. To examine the potential contribution of basic fibroblast growth factor (bFGF), which is an angiogenic factor, and insulin-like growth factor-I (IGF-I) to the development of diabetic microvascular disease, bFGF and IGF-I mRNA levels were measured in tissues of control, diabetic, and insulin-treated diabetic rats. Diabetes was induced in rats by intravenous injection of streptozotocin (STZ) 65 mg/kg, and the rats were maintained for 21 days. bFGF mRNA levels increased threefold in the eyes of diabetic versus control rats, whereas a consistent change in bFGF mRNA levels was not observed in other tissues. In contrast, IGF-I mRNA levels decreased in the eyes and other tissues, including kidney, lung, and skeletal muscle, of diabetic as compared with control rats. Insulin treatment prevented the diabetes-induced increase in bFGF and decrease in IGF-I mRNA levels. Acidic FGF (aFGF) mRNA levels were unchanged in eyes from diabetic versus control rats. In partially purified retinas, diabetes increased bFGF mRNA levels twofold as compared with levels in control retinas, whereas IGF-I mRNA levels decreased to 58% of control levels in retinas from diabetic rats. Insulin treatment again prevented the diabetes-induced increase in IGF-I mRNA levels in the retina but had no effect on the diabetes-induced increase in bFGF mRNA levels. bFGF peptide levels were minimally increased in diabetic versus control retinas.(ABSTRACT TRUNCATED AT 250 WORDS)

The aetiology of diabetic neuropathy: the combined roles of metabolic and vascular defects

A combination of metabolic and vascular defects have been implicated in the pathogenesis of diabetic neuropathy. Animal studies have demonstrated that a reduction in nerve blood flow may be an important early defect and that vasodilators can prevent or ameliorate nerve dysfunction. The potential factors contributing to nerve ischaemia include structural defects in the endoneurial microvasculature together with rheological abnormalities, abnormalities in vasoactive agents which regulate nerve blood flow including nitric oxide and the eicosanoids, and alterations in tone of the autonomic innervation of the nerve vasculature. The principle metabolic defects which have been implicated include disruption of the polyol pathway, altered lipid metabolism, advanced glycosylated end-product formation, increased oxidative stress, and diabetes-induced defects in growth factors. The demonstration that activation of the polyol pathway in experimental diabetes may affect nerve blood flow, and conversely that vasoactive agents appear to be important in regulating some aspects of nerve metabolism, has highlighted the interdependence of the metabolic and vascular defects in the pathogenesis of this condition. Thus, selective intervention aimed at a key defect early in this cascade may subsequently correct a number of later abnormalities offering therapeutic hope in this chronic debilitating complication.

In situ gene expression of growth hormone (GH) receptor and GH binding protein in adult male rat tissues

Pituitary growth hormone (GH) acts as a growth promoter in a wide range of tissues after binding to its specific GH receptor (GHR) or to a cytosolic circulating GH binding protein (GHBP). To further characterize GH target cells in the rat, in situ hybridization was used to investigate the tissue and cell distribution of mRNAs encoding GHR and GHBP, and their hepatic developmental expression was examined. Cryostat sections of adult male rat tissue were hybridized with [35S]dATP-labeled oligonucleotide antisense probes, one directed against a specific sequence of the intracellular domain of rat GHR mRNA, the other against the hydrophilic tail of rat GHBP mRNA. Several tests were carried out to validate the in situ detection of mRNA. Co-expression of the two transcripts in liver, spleen, thymus, kidney, adrenal, skin, muscle, heart, and pituitary was autoradiographically detected. However, relative expression levels, as demonstrated by computer-assisted microdensitometry, appeared to be variable. Both transcripts showed higher levels of expression in the liver, anterior and intermediate pituitary lobes, outer kidney medulla, adrenal cortex, skin epidermis, heart and muscle, but lower levels in spleen, thymus, hypodermis, adrenal medulla and posterior pituitary lobe. As a physiological control, hepatic levels of expression were examined during development, and the two forms of mRNA were found to be present at low levels in fetal liver, increasing considerably after birth. These results permit the identification in the adult male rat of cells that might be directly responsive to GH, and demonstrate the differential expression of rGHR and rGHBP transcripts.

Insulin-like growth factors protect against diabetic neuropathy: effects on sensory nerve regeneration in rats

Neuropathy is an enigmatic and debilitating complication of diabetes. A consensus as to the pathogenesis of this disorder has yet to emerge. Recently, it has been found that the insulin-like growth factors (IGFs) regulate peripheral nerve regeneration, and IGF content is reduced in various diabetic tissues. We tested herein the hypothesis that IGF administration can prevent or ameliorate the impairment of sensory nerve regeneration in streptozotocin diabetic rats. Miniosmotic pumps released small local doses of IGF-I from a catheter routed near a site of sciatic nerve crush or larger systemic doses of IGF-I or IGF-II from a distant subcutaneous site. Whether administered locally or systemically, IGFs protected against the impairment of sensory nerve regeneration. Surprisingly, this protection was obtained despite unabated hyperglycemia. Therefore, the neuropathy involving sensory nerve regeneration in diabetes can be ameliorated or prevented by IGF treatment, independently of hyperglycemia.

Reduced insulin-like growth factor-I mRNA content in liver, adrenal glands and spinal cord of diabetic rats

Circulating insulin-like growth factor I (IGF-I) concentrations are known to be reduced in experimental and clinical diabetes mellitus. The IGF-I mRNA content was measured in several tissues of rats treated with streptozotocin to determine whether a correlation with neuropathy could be found. IGF-I mRNA content was sharply reduced relative to total and poly(A)+ RNA in diabetic liver and adrenal glands. In contrast, histone 3.3 mRNA content was not significantly reduced relative to poly(A)+ RNA in liver, and alpha-tubulin mRNA content instead was increased in adrenal glands, showing that the decline in IGF-I mRNAs in these tissues was selective. In addition, spinal cord IGF-I mRNA content was significantly reduced per tissue, total RNA, and poly(A)+ RNA after 1 and 2 weeks of diabetes. This was correlated with a concurrent and significant decrease in conduction velocity in both spinal cord and peripheral nerves in a separate study. The decline in liver and spinal cord IGF-I mRNA was not due to streptozotocin toxicity, because it was significantly opposed by insulin which was continuously infused beginning the day after diabetes induction. These results, when taken together with those of others, indicate that the reduction in IGF-I mRNA content may be widespread among diabetic tissues, and might contribute in part to certain syndromes of diabetes, such as neuropathy.

Insulin-like growth factor mRNA expression in tissues of lean and obese male SHFF/Mcc-fa(cp) rats

No differences were detected in serum IGF-I levels between lean and obese male SHFF/Mcc-fa(cp) rats expressing non-insulin-dependent diabetes mellitus (NIDDM). In contrast, serum insulin levels, and blood glucose levels were significantly elevated in obese as compared to lean littermates (P < 0.05), indicating that the obese animals were diabetic. Northern blot analysis of total tissue RNA using labeled cDNAs for IGF-I and IGF-II revealed a decrease in liver and adipose IGF-I mRNA expression in the obese littermates while IGF-II mRNA expression was decreased only in adipose tissue of obese animals as compared to lean littermates.

IGF-I and IGF-binding protein gene expressions in spontaneous dwarf rat

Effects of growth hormone (GH) and fasting on hepatic expressions of insulin-like growth factor I (IGF-I) and IGF-I-binding protein (IGFBP)-1, -2, -3, and -4 were examined in spontaneous dwarf rats (SDR), which completely and specifically lack GH among pituitary hormones. The hepatic expressions of mRNA encoding IGF-I and IGFBP-3 were reduced and IGFBP-1 mRNA was elevated in the SDR. Both chronic and acute administration of GH restored these changes, indicating the association of GH but not other pituitary hormones with hepatic expressions of these genes. In addition, the present examination revealed that mRNA level of IGFBP-2 was elevated in SDR, which could not be attenuated by exogenous GH, and that GH may not be directly relevant to the regulation of hepatic IGFBP-4 expression. Fasting for 2 days reduced IGF-I mRNA level and increased IGFBP-2 mRNA level in the SDR, as well as in the normal rat, suggesting the presence of factors other than reduced serum GH responsible for fasting-induced alteration in the expression of these mRNAs. On the other hand, fasting resulted in little change or even a reduction of IGFBP-1 mRNA level in the SDR.

Estradiol inhibits growth hormone receptor gene expression in rabbit liver

We studied the ontogeny of GH receptor mRNA levels and the effect of exogenous estradiol administration on GH receptor mRNA levels in rabbit liver. A solution hybridization-RNase protection assay revealed a predominant 370-base long protected band corresponding to the mRNA encoding the transmembrane GH receptor, and a 241-base long protected band, representing about 9.0%, with the predicted size for the truncated form of the GH receptor. To study the developmental profile of GH receptor expression, we studied 12 female rabbits, at ages 1, 3, 5 and 7 months. Maximal GH receptor mRNA levels were observed in 3-month-old animals and decreased in 7-month-old animals. To investigate the effect of estradiol, 8-week-old immature female rabbits were randomly divided into five groups, and received subcutaneous pellets containing either placebo or estradiol at doses of 0.1, 0.5, 1.5 and 5.0 mg for 3 weeks. Exogenous administration of estradiol, at doses that resulted in physiological circulating levels, induced a reduction in GH receptor expression, measured both by GH binding (36 and 46%), and GH receptor mRNA levels (38 and 87%), in animals receiving pellets containing 1.5 and 5.0 mg of estradiol, respectively. We conclude that estradiol decreases GH receptor expression in rabbit liver. The results of our study suggest that there is an inverse relationship between circulating estrogen concentrations and liver GH receptor expression.

Diminished levels of insulin-like growth factor-I in lungs in streptozotocin-induced diabetes: relation to nutritional status and growth

It is yet unknown whether the impaired nutritional status of streptozotocin-induced diabetic rats influences changes in levels of insulin-like growth factor-I (IGF-I) in this experimental model of diabetes. To explore this possibility, simultaneous studies were undertaken of rats made diabetic by streptozotocin (75 mg/kg body wt, intraperitoneally) and undernourished control rats with similar somatic growth rate (determined by body weight gain), in comparison with normal controls. Serum IGF-I levels were diminished in the untreated diabetic and undernourished control animals, but more so in the diabetic group. Lung IGF-I levels (per lung and per lung DNA) and DNA contents were diminished to similar degrees in the untreated diabetic animals and the undernourished control group. Lung dry weights of the diabetic rats were greater than those of the undernourished control group, such that lung IGF-I/100 mg tissue dry wt in the former was significantly lower than in the latter group. Insulin treatment of the diabetic rats restored their body weights, serum and lung IGF-I levels, and DNA contents to normal control values. Lung IGF-I levels in the diabetic rats correlated strongly with serum glucose (r = .75) and body weight (r = .79), and moderately with lung weight (r = .43) and lung DNA (r = .58). These findings suggest that the diminished lung IGF-I levels in streptozotocin-induced diabetes may be related to the impaired nutritional status and/or somatic growth of the experimental animals, and that this relationship may be responsible, at least in part, for the diminished lung cellular proliferation observed in experimental diabetic animals.

Characterization of type I IGF receptor and IGF-I mRNA expression in cultured human and bovine glomerular cells

Glomerular hypertrophy is reported in several endocrine disorders such as acromegaly and diabetes mellitus, where abnormalities of growth hormone and insulin-like growth factor (IGF-I) have been reported. In the present report, we have cultured bovine and human glomerular endothelial cells, and bovine glomerular epithelial and mesangial cells, and characterized the expression of IGF-I mRNA and its receptor in these cells. High affinity, specific receptors for IGF-I were identified in all three types of cells by radioreceptor assays. Receptor number (Ro) derived by Scatchard analysis revealed an unusually high number of Type I IGF receptors, approx. 1.2 x 10(5) receptors/cell in glomerular endothelial cells. Affinity crosslinking studies and immunoprecipitation with antibodies against the Type I IGF receptor identified the alpha-subunit of the IGF-I receptor as having a molecular mass of 140 kDa. Biologically, IGF-I was more potent than insulin or IGF-II in stimulating DNA synthesis in glomerular endothelial cells. Northern blot analysis showed that glomerular and aortic endothelial cells expressed IGF-1 mRNA of 1.7 kb. In contrast, renal glomeruli showed several IGF-1 mRNAs of 7.5, 1.7 and 1.2 kb. Thus, the demonstration of both a prepondence of Type I IGF receptors coupled with the growth promoting effects of IGF-I in glomerular endothelial and epithelial cells, as well as the local production of IGF-I mRNA suggests that IGF-I serves an important role as an autocrine or paracrine regulator of the growth of renal glomeruli.

Relationships between the thyroid and somatotropic axes in steers. II: Effects of thyroid status on plasma concentrations of insulin-like growth factor I (IGF-I) and the IGF-I response to growth hormone

Three studies assessed the effect of thyroid status on regulation of plasma IGF-I in cattle. First, four Angus-Hereford steers (av wt 345 kg) were fed 4 mg/d propylthiouracil daily for 35 d. With continued feeding of PTU steers were sequentially injected with thyroxine (T4, 5 mg/d, IM for 5 d) followed by triiodothyronine (T3, 2 mg/d, IM for 5 d). An injection of bovine pituitary growth hormone (GH, 0.1 mg/kg, IM) was given to each steer on day 35 of PTU, day 5 of T4 and again on day 5 of T3. PTU alone increased plasma thyroid stimulating hormone (TSH), decreased plasma T4 and T3 but had no influence on IGF-I. T3, but not T4, lowered plasma TSH, IGF-I and the IGF-I response to GH (P < .05). Next, twelve bull calves (av wht 167 kg) were divided equally into two groups. A control group was injected daily for five d with buffered saline; the experimental group was concurrently treated with T3 (5 mg/d, sc) for five d. Beginning the sixth day, all calves were injected with GH (0.1 mg/kg, IM daily) for three d with the respective buffer or T3 treatments continuing. Plasma IGF-I was depressed 29% by T3. The incremental area under the three-d response curve was less (P < .03) in T3 cattle. A growth trial was conducted in which twenty-four Angus x Hereford steers were injected daily with T3 (2 mg/kg, bi-daily x 56 d) or implanted with Synovex-S (S) in a 2 x 2 factorial arrangement. Synovex increased empty body protein gain (EBPG) and plasma IGF-I 15.5 and 27.9% (P < .01), respectively; T3 decreased EBPG and plasma IGF-I 13.9 and 15.1% (P < .07), respectively, in steers which maintained suppression in plasma TSH. The data support the conclusion that elevated T3 decreases plasma IGF-I, in part, through a diminished GH-responsiveness and anabolic treatments such as S can reverse the effects of excess T3.

Serum levels of growth hormone-binding protein and insulin-like growth factor I in children and adolescents with type 1 (insulin-dependent) diabetes mellitus

Serum levels of insulin-like growth factor I are reduced in patients with Type 1 (insulin-dependent) diabetes mellitus. To evaluate the role of the hepatic growth hormone receptor in the decreased serum concentrations of insulin-like growth factor I, serum levels of the high affinity growth hormone-binding protein, which is qualitatively and quantitatively related to the hepatic growth hormone receptor, and of insulin-like growth factor I were measured in 70 children and adolescents with Type 1 diabetes and 105 healthy control children. Analysis of variance revealed a significant negative effect of Type 1 diabetes on serum levels of the growth hormone-binding protein and of insulin-like growth factor I. In the diabetic patients, serum levels of the growth hormone-binding protein were positively related to body mass index and to insulin dose per kg body weight, and were not influenced by pubertal stage, gender, or plasma levels of haemoglobin A1c. Serum levels of insulin-like growth factor I increased during early puberty reaching peak levels at mid-puberty and decreasing thereafter. No relationship was found between serum levels of growth hormone-binding protein and of insulin-like growth factor I. Our data suggest that decreased liver somatogenic receptor levels, as reflected by the concentrations of circulating growth hormone-binding protein, play a minor role in the suppressed concentrations of circulating insulin-like growth factor I. Post-growth hormone receptor defects or changes in the insulin-like growth factor binding proteins probably contribute more to the lower serum levels of insulin-like growth factor I.

Characterization of a rat insulin-like growth factor I gene promoter

Rat IGF-I mRNAs contain one of two alternative 5'-untranslated regions which are encoded by alternative exons (exons 1 and 2) and whose expression is controlled by alternative promoter elements. We investigated the ability of fragments of DNA which contain exon 1 and its 5'-flanking region to regulate transcription of a luciferase reporter gene in transient transfection assays. Maximal promoter activity was obtained with a construct which contained 412 bp of 5'-flanking region, while constructs which contained 1120 and 1690 bp of 5'-flanking region induced approximately 50% less enzymatic activity. Mapping of transcription start sites by RNase protection assay demonstrated that native start sites were used by these constructs, although the relative use of different start sites was different from start site usage by the endogenous gene. These data demonstrate that the 5'-flanking region of exon 1 is capable of regulating transcription of IGF-I mRNAs.

The role of insulin-like growth factors and IGF-binding proteins in the physiological and pathological processes of the kidney

Insulin-like growth factors (IGFs) and their binding proteins are implicated in the growth regulation of the kidney during embryogenesis and differentiation. Recent evidence also suggests that IGFs play a role in kidney physiology (glomerular filtration rate, renal plasma flow) and pathology (diabetic renal hypertrophy, nephritis, glomerulosclerosis, kidney tumours, chronic renal failure). This review focuses on the biology of IGFs at the molecular, protein and receptor levels and considers their importance in renal physiology and pathology. The current data demonstrate a central role for the IGFs in the mediation of a wide variety of effects on renal growth, function and malignancy.

Stimulation of coho salmon growth by insulin-like growth factor I

The effect of insulin-like growth factor I on growth rate of coho salmon (Oncorhynchus kisutch) was examined. Juvenile coho salmon received implants of osmotic minipumps containing recombinant bovine insulin-like growth factor I (rbIGF-I) or saline for a period of 3 to 4 weeks. High doses of rbIGF-I (greater than 0.13 microgram.g-1.d-1) resulted in hypoglycemia and death. In 2-year-old coho salmon, 0.09 microgram.g-1.d-1 rbIGF-I administered for 25 days doubled linear growth rate and increased growth rate in weight by 40%. In rapidly growing, 1-year-old coho salmon, growth rate was not altered by rbIGF-I at 0.01 or 0.05 micrograms.g-1.d-1 for 31 days. In ration-limited fish exhibiting slow growth in the control group, rbIGF-I (0.02 microgram.g-1.d-1) increased linear growth rate by up to threefold and growth rate in weight by up to fourfold. The results indicate that exogenous treatment with mammalian IGF-I can stimulate coho salmon growth under some conditions, and that endogenous IGF-I may be an important factor in regulating growth of teleosts.

Increased serum clearance and degradation of 125I-labeled IGF-I in protein-restricted rats

Dietary protein restriction in young rats stunts growth and decreases serum insulin-like growth factor I (IGF-I) concentrations. To investigate the possibility of a diet-induced accelerated clearance of IGF-I, the clearance of 125I-labeled IGF-I was determined by measuring the radioactivity in serum obtained at different intervals after bolus injection in 5-wk-old rats that had received either a normal 15% protein diet (P15; n = 6) or a 5% protein isocaloric diet (P5; n = 7) for 1 wk. The clearance and volume of distribution of 125I-IGF-I were increased in P5 rats by 58 and 75%, respectively, whereas the terminal elimination half-life was not changed. The calculated IGF-I production rate was decreased by 40% in P5 rats. The intermediate distribution phase (alpha) was twofold faster in P5 than in P15 animals, and 125I-IGF-I was preferentially bound to IGF binding proteins in the 40-kDa complex in P5 rats. These observations might explain the increased clearance and degradation of IGF-I in the P5 rats. Dietary protein restriction appears to lower serum IGF-I concentrations by both decreasing its production rate and increasing its clearance.

In vivo proliferation of rat vascular smooth muscle in relation to diabetes mellitus insulin-like growth factor I and insulin

The roles of diabetes mellitus, insulin-like growth factor I and insulin in vascular smooth muscle proliferation in vivo were studied. Proliferation was induced by endothelial injury (balloon catheterization) of rat aorta, and was measured as 3H-thymidine incorporation into DNA. Levels of insulin-like growth factor I mRNA and insulin-like growth factor I receptor mRNA were measured with a solution hybridization assay. The increase in DNA synthesis was most pronounced 2 days after injury in both normal and diabetic rats and declined thereafter, but DNA synthesis in aortas from diabetic rats was lower throughout the time period studied. Levels of insulin-like growth factor I mRNA and the receptor mRNA were both increased in balloon catheterized aortas, and time-course studies showed an increase in receptor mRNA prior to the increase in insulin-like growth factor I mRNA. Diabetic rats were treated with equimolar concentrations of insulin (35 nmol/day) or insulin-like growth factor I (31 nmol/day) for 5 days. Insulin-like growth factor I increased DNA synthesis in injured aortas 2 days after injury without improving blood glucose, whereas the effect of insulin was associated with a decrease in blood glucose levels. In conclusion, vascular smooth muscle proliferation is impaired by diabetes and stimulated by insulin treatment. Insulin-like growth factor I infusion stimulates vascular smooth muscle proliferation without affecting blood glucose, and gene expressions of insulin-like growth factor I and its receptor are increased in proliferating vascular smooth muscle, indicating that insulin-like growth factor I and involved in vascular smooth muscle proliferation in vivo.

Effect of insulin-like growth factor I infusion on renal hypertrophy in experimental diabetes mellitus in rats

Initial diabetic renal hypertrophy is preceded by a transient increase in kidney insulin-like growth factor I suggesting that insulin-like growth factor I may be implicated in diabetic kidney growth. The present study was undertaken to examine the effects of exogenous insulin-like growth factor I infusion on diabetic renal hypertrophy at a time when renal insulin-like growth factor I concentration had returned to normal and the initial steep kidney growth rate had diminished to a much slower rate. Groups of rats with diabetes duration of 5 days were infused s.c. for 4 subsequent days with equimolar concentrations of insulin-like growth factor I (36 nmol/day) or insulin (35 nmol/day). Insulin infusion lowered blood glucose to a normal level within 2 days and induced an average body-weight gain of 9.3 +/- 0.6 g/day. Insulin-like growth factor I infused diabetic rats maintained the original diabetic state with blood glucose levels comparable to those of 0.154 mol/l NaCl-infused diabetic rats, but had nevertheless an average body-weight gain of 6.8 +/- 1.0 g/day while untreated diabetic rats had a lower body-weight gain amounting to 3.3 +/- 0.8 g/day (p less than 0.01). The kidney weight at day 9 in untreated diabetic animals was about 25% greater than that of non-diabetic control animals, while in insulin-like growth factor I treated diabetic rats a further increase (p less than 0.05) was seen, amounting to 36% above control level. No increase was seen in the insulin-treated diabetic group.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of gene expression by insulin

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Renal insulin-like growth factor I and growth hormone receptor binding in experimental diabetes and after unilateral nephrectomy in the rat

We have measured specific binding of insulin-like growth factor I and growth hormone to renal plasma membranes from control, streptozotocin-diabetic, insulin-treated diabetic, uninephrectomised and combined diabetic-uninephrectomised male Wistar rats. Control, insulin-treated and uninephrectomised rats had similar body weights after 7 days (243 +/- 2 g), whereas diabetic and diabetic-uninephrectomised animals were significantly lighter (219 +/- 4 and 203 +/- 4 g, p less than 0.05). Blood glucose concentrations were similar in the diabetic and diabetic-uninephrectomised animals (around 26 mmol/l) but significantly lower in the insulin-treated group. Right kidney weight increased by 14% in the control, insulin-treated and sham-nephrectomised animals, by 33% in the diabetic group, 38% in thenephrectomised animals and 60% in the diabetic-nephrectomised group. The renal content of insulin-like growth factor I was similar and stable in the control, insulin-treated and sham-nephrectomised animals (208 +/- 14 ng/g wet weight) but rose to a peak of 669 +/- 35 ng/g in the diabetic group (p less than 0.001), 871 +/- 34 ng/g in the nephrectomised animals (p less than 0.001) and 1012 +/- 43 ng/g in the diabetic-uninephrectomised group (p less than 0.001). Maximum binding of insulin-like growth factor I fell on day 1 in the diabetic group (8.3 +/- 1.4 vs 5.2 +/- 0.71 x 10(-11) mol/l; p less than 0.01) but thereafter was identical to control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Contributions of growth hormone receptor and postreceptor defects to growth hormone resistance in malnutrition

Malnutrition results in poor growth and is associated with resistance to growth hormone (GH) action. The mechanisms involved in the GH resistance depend on the severity and the timing of the nutritional insult. Stringent dietary restrictions such as fasting may produce GH resistance by reducing the number of GH receptors. Less severe nutritional deprivation such a short-term protein restriction may cause GH insensitivity mainly through postreceptor mechanisms.

Growth hormone and insulin-like growth factor I: nutritional pathophysiology and therapeutic potential

The growth hormone--insulin-like growth factor I axis has been appreciated for more than 30 years and the effects of malnutrition on this axis for more than 20 years. Over the last decade, advances in molecular biology have permitted enhanced understanding of feedback regulation between growth hormone and IGF-I at the gene level, including limited information on nutritional influences. Similarly, the availability of recombinant human growth hormone has allowed controlled clinical studies demonstrating its net anabolic actions at hypocaloric dietary energy intake levels and its ability to enhance height velocity in children with various causes of diminished growth. Although investigational use of recombinant IGF-I in humans has been limited, its actions are likely to complement those of growth hormone during periods of profound dietary energy deficit. From the information presented, two hypotheses are developed. First, recombinant IGF-I administration will enhance substrate anabolic events during the acutely malnourished state when dietary intake is severely limited. Second, administration of recombinant human growth hormone will accelerate protein anabolism and catch-up growth during the period of recovery from protein-energy malnutrition. Given current clinical investigational tools and the availability of both recombinantly-produced hormones, these are testable hypotheses.

Kidney IGF-I mRNA in initial renal hypertrophy in experimental diabetes in rats

It has recently been demonstrated that immunoassayable kidney insulin-like growth factor I concentration increases 24-48 h after induction of diabetes, preceding the initial renal hypertrophy. To elucidate whether this increase is due to increased local production we studied rat kidney insulin-like growth factor I gene expression during the first four days after induction of streptozotocin diabetes. Eighteen hours after injection with streptozotocin the diabetic animals were divided into two groups, one of which was treated with insulin, and daily for four days animals from each group were taken out for investigation. After four days the wet kidney weight had increased from baseline by 20% (from 687 +/- 23 to 827 +/- 6 mg (mean +/- SEM), p less than 0.01) in the untreated diabetic group, while no significant increase occurred in the insulin-treated group (687 +/- 23 vs 732 +/- 21 mg, NS). Kidney insulin-like growth factor I increased rapidly from baseline, the rise amounting to 52% after 48 h (from 271 +/- 11 to 411 +/- 32 ng/g, p less than 0.01) with a decline to control level on day four in the untreated diabetic group. Kidney insulin-like growth factor I remained unchanged in the insulin-treated diabetic group. Insulin-like growth factor I mRNA was measured by solution-hybridization assay. No differences were found in kidney insulin-like growth factor I mRNA between the two diabetic groups over the study period, while in liver, insulin-like growth factor I mRNA tended to be lower on day four in diabetic rats when compared to insulin-treated rats (p = 0.07).(ABSTRACT TRUNCATED AT 250 WORDS)