Hepatic Insulin-like Growth factor-I Expression Research Articles

Abomasal Nitrogen Flow Affects the Relationship between Dietary Nitrogen and Insulin-Like Growth Factor-I in Growing Lambs

Twelve abomasally cannulated wether lambs were fed isocaloric diets containing 9, 12 or 15% crude protein to determine insulin-like growth factor-I (IGF-I) responses to altered abomasal nitrogen flow and nitrogen status. Lambs were offered 1100 g/d of their respective diets. Voluntary feed consumption was not affected by nitrogen intake. Ruminal and total tract digestibilities of dry matter, organic matter and nitrogen increased linearly (P < 0.05) with increased dietary nitrogen. Abomasal flows of total, bacterial and rumen escape nitrogen increased (linear, P < 0.01), whereas dry matter and organic matter flows decreased (linear, P < 0.01). Total amino acid flow was greater (linear, P < 0.01) in lambs fed additional nitrogen due to increased (linear, P < 0.01) flows of essential and nonessential amino acids. Nitrogen retention and blood urea nitrogen increased linearly (P < 0.01). Serum IGF-I concentrations and relative hybridization intensity of hepatic IGF-I mRNA increased (linear, P < 0.05) as lambs consumed more nitrogen. Serum IGF-I and hepatic IGF-I mRNA were correlated positively (P < 0.05) with nitrogen intake and abomasal flows of nitrogen and various amino acids. These data provide evidence of a relationship between abomasal amino acid flow, as influenced by nitrogen intake, and hepatic gene expression and serum concentrations of IGF-I in growing lambs.

Dietary vitamin B12 requirement of fingerling blunt snout bream Megalobrama amblycephala determined by growth performance, digestive and absorptive capability and status of the GH-IGF-I axis

This study aimed to investigate the optimal dietary vitamin B12 requirement of fingerling blunt snout bream Megalobrama amblycephala based on the results of growth performance, intestinal digestive and absorptive capability and responses of the growth hormone (GH)-insulin-like growth factor-I (IGF-I) axis. Fish (initial body weight: 0.71±0.03g) were randomly assigned to six purified diets containing graded levels of vitamin B12 (0, 0.028, 0.051, 0.12, 0.23 and 0.41mg/kg) for 12weeks. Both weight gain (WG) and specific growth rate increased significantly (P<0.05) as dietary vitamin B12 levels increased from 0 to 0.051mg/kg, then plateaued with further increasing levels. Feed conversion ratio decreased as dietary vitamin B12 levels increased from 0 to 0.12mg/kg, but no statistical difference was observed. Whole-body composition and intestinal brush border enzymes activities also exerted little difference within the vitamin B12 range tested, whereas hepatic vitamin B12 contents increased remarkably (P<0.01) with increasing dietary vitamin B12 levels. Intestinal protease, amylase and lipase activities all increased significantly (P<0.05) as dietary vitamin B12 levels increased from 0 to 0.12mg/kg, but decreased remarkably (P<0.05) with further increasing vitamin B12 levels. Similar results were also observed in hepatic IGF-I expressions. Furthermore, the mRNA expressions of GH in the brain decreased significantly (P<0.05) with increasing dietary vitamin B12 levels. Based on the broken-line regression analysis of WG, the optimal dietary vitamin B12 requirement of fingerling blunt snout bream was estimated to be 0.06mg/kg. In addition, a supplementation of 0.12mg/kg vitamin B12 boosted intestinal digestive capability and hepatic IGF-I expression. Statement of relevanceThis study investigated the optimal dietary vitamin B12 requirement of fingerling blunt snout bream based on growth performance. The underlying mechanisms were also investigated with emphasis on intestinal digestive and absorptive capability and modulation of the GH-IGF-I axis. Data obtained here might facilitate the development of low-cost formulated feed for aquatic species.

Divergence between hepatic insulin-like growth factor (IGF)-I mRNA expression and IGF-I serum levels in Leishmania (Leishmania) infantum chagasi-infected dogs

Visceral leishmaniasis (VL) caused in the New World by Leishmania (Leishmania) infantum chagasi has dog as important peridomestic reservoir in its transmission cycle. Since VL in infected animals is similar to human VL, its study is interesting for human pathology. During Leishmania infection, insulin-like growth factor-I (IGF-I) plays a role in the host-parasite interaction, favoring parasite growth, particularly acting directly on Leishmania. We evaluated IGF-I mRNA expression in different organs/tissues, which was differently modulated in dogs naturally infected by L. (L.) infantum chagasi. We also evaluated the hepatic IGF-I mRNA and serum IGF-I levels in infected dogs. Hepatic mRNA IGF-I expression was higher in the infected dogs than in control animals. However, the serum levels of IGF-I, which are related to the production of this factor in the liver, were reduced in the infected dogs compared with the non-infected controls. Thus, we suggest interference in post-transcriptional processing in IGF-I production in active visceral leishmaniasis.

Effects of dietary rapeseed meal on growth performance, digestion and protein metabolism in relation to gene expression of juvenile cobia (Rachycentron canadum)

A 60-day feeding trial in seawater floating cages (1.5×1.5×2.5m) was conducted to investigate the effects of dietary rapeseed meal (RM) levels on feed intake, growth, survival, digestion and protein metabolism in relation to gene expression of juvenile cobia (initial body weight 94.6g). Five isonitrogenous (crude protein 450gkg−1 of dry matter) and isoenergetic (20kJg−1) practical diets were formulated by replacing 0 (the control), 125, 250, 375 and 500gkg−1 fish meal protein with RM protein. Each diet was randomly fed to triplicate groups of fish, and each cage was stocked with 20 fish. Fish were fed twice daily (06:00 and 18:00) to apparent satiation. The survival ranged from 96.7 to 98.3%, and no significant difference was observed among dietary treatments (P>0.05). With increasing dietary RM levels, feed intake (FI), specific growth rate (SGR) and feed efficiency (FE) decreased. Fish fed the diet with 250gkg−1 or more protein from RM had significantly lower SGR and FE than the control group (P<0.05), but there was no significant difference in FI at this level compared with the control group (P>0.05). Apparent digestibility coefficients (ADCs) of dry matter (DM), crude protein and energy significantly decreased with increasing dietary RM levels (P<0.05). Fish fed the diet with 250gkg−1 or more protein from RM had significantly lower ADC values of crude protein and energy compared with the control group (P<0.05). Whole-body crude protein and crude lipid decreased with increasing dietary RM levels. Fish fed the diet with 500gkg−1 protein from RM had significantly lower whole-body crude protein and crude lipid compared with the control group (P<0.05). However, whole-body moisture and ash showed opposite trends with crude protein and crude lipid. Moisture, crude protein and crude lipid contents in cobia muscle showed similar trends with those in whole body. There were no significant differences in plasma ammonia, urea, cholesterol and amino acids among fish fed the experimental diets (P>0.05). Fish fed the diet with 500gkg−1 protein from RM had significantly lower aspartate aminotransferase (AST) activity in liver than the control group (P<0.05). Hepatic insulin-like growth factor I (IGF-I) gene expression level was significantly decreased in fish fed the diet with 500gkg−1 protein from RM compared with the control group (P<0.05). However, IGF-I gene expression level in dorsal muscle was significantly increased in fish fed this diet compared with the control group (P<0.05). No significant differences were observed in target of rapamycin (TOR) expression levels in cobia liver and dorsal muscle at different RM levels (P>0.05). Results of the present study indicated that protein from RM could substitute 125gkg−1 fish meal protein without influencing the growth, feed utilization and protein metabolism in cobia. The higher substitution levels of RM induced negative influences on feed intake, growth and hepatic IGF-I expression level.

Insulin-like growth factor and growth hormone receptor in postpartum lactating beef cows

The objective of this study was to evaluate the plasma concentrations of insulin-like growth factor-I (IGF-I), and the mRNA hepatic expression of IGF-I and of the growth hormone receptors GHR and GHR 1A, in postpartum beef cows. Four Angus and four crossbred (Angus x Nelore) postpartum suckled beef cows were used. Liver and blood samples were collected every 10 days, from calving to 40 days postpartum, for gene expression and for β-hydroxybutyrate and IGF-I assays, respectively. Samples for progesterone assay were collected every other day, from day 10 to 40 postpartum. Three cows ovulated before 40 days postpartum. IGF-I concentration was higher in Angus x Nelore than in Angus cows. There was no difference in the expression of GHR, GHR 1A and IGF-I according to breed or ovulatory status. IGF-I concentrations were higher in crossbred cows, but have not changed according to postpartum ovulatory status. Moreover, changes in postpartum IGF-I concentrations are not associated with changes in liver GHR, GHR 1A and IGF-I mRNA expression in either breed.

Hepatic gene expression in multiparous Holstein cows treated with bovine somatotropin and fed n-3 fatty acids in early lactation

Multiparous cows were fed supplemental dietary fat and treated with bST to assess effects of n-3 fatty acid supply, bovine somatotropin (bST), and stage of lactation on hepatic gene expression. Cows were blocked by expected calving date and previous milk yield and assigned randomly to treatment. Supplemental dietary fat was provided from calving as either whole high-oil sunflower seeds (SS; 10% of dietary dry matter; n-6/n-3 ratio of 4.6) as a source of linoleic acid or a mixture of Alifet-High Energy and Alifet-Repro (AF; 3.5 and 1.5% of dietary dry matter, respectively; n-6/n-3 ratio of 2.6) as a source of protected n-3 fatty acids. Cows were treated with 0 (SSN, AFN) or 500 (SSY, AFY) mg of bST every 10 d from 12 to 70 d in milk (DIM) and at 14-d intervals thereafter. Liver biopsies were collected on −12, 10, 24, and 136 DIM for gene expression analysis. Growth hormone receptor (GHR), insulin-like growth factor-I (IGF-I), IGF-binding protein-3 (IGFBP3), hepatic nuclear factor 4α (HNF4α), fibroblast growth factor-21 (FGF-21), and peroxisome proliferator-activated receptor α (PPARα) were the target genes and hypoxanthine phosphoribosyltransferase (HPRT) was used as an endogenous control gene. Expression was measured by quantitative real-time reverse transcription-PCR analyses of 4 samples from each of 32 cows (8 complete blocks). Amounts of hepatic HPRT mRNA were not affected by bST or diet but were increased by approximately 3.8% in early lactation (3.42, 3.52, 3.54, and 3.41×104 message copies for −12, 10, 24, and 136 DIM, respectively). This small change had little detectable impact on the ability of HPRT to serve as an internal control gene. Amounts of hepatic GHR, IGF-I, and IGFBP3 mRNA were reduced by 1.5 to 2-fold after calving. Expression of GHR and IGF-I increased and IGFBP3 tended to increase within 12 d (by 24 DIM) of bST administration. These effects of bST persisted through 136 DIM. Hepatic HNF4α mRNA was not altered by DIM or any of the treatments. Abundance of PPARα mRNA was unchanged through 24 DIM but increased by 136 DIM. There was a trend for an interaction of bST, diet, and DIM on PPARα mRNA abundance from 24 to 136 DIM because the amount of PPARα mRNA increased in SSN, SSY, and AFN cows but was not altered in AFY cows. The amount of FGF-21 mRNA increased markedly in early lactation but, like HNF4α mRNA, was not affected by bST, diet, or their interactions. These results indicate 1) that bST induced increases in hepatic expression of GHR, IGF-I, and IGFBP3 when cows were in negative energy balance in early lactation, 2) there was no effect of reduced dietary n-6/n-3 content on hepatic gene expression, and 3) there was support for a potential homeorhetic role of hepatic FGF-21 via uncoupling the somatotropin-IGF-axis in early lactation.

Environmentally Relevant Concentrations of 17α-Ethinylestradiol (EE2) Interfere With the Growth Hormone (GH)/Insulin-Like Growth Factor (IGF)-I System in Developing Bony Fish

The aim of this study was to evaluate whether effects of environmental estrogens on fish growth and reproduction may be mediated via modulating the growth hormone (GH)/insulin-like growth factor I (IGF-I) system. To this end, developing male and female monosex populations of tilapia were exposed to 17alpha-ethinylestradiol (EE2) at 5 and 25 ng EE2/l water from 10-day postfertilization (DPF) until 100 DPF. Under exposure to both EE2 concentrations, sex ratio shifted toward more females and body length, and weight were significantly reduced in males. The growth-reducing effect was associated with significant changes in hepatic IGF-I expression, both in males and females and with significant alterations of IGF-I mRNA and GH mRNA in the brain. The changes in IGF-I and GH mRNA were accompanied by altered estrogen receptor alpha (ERalpha) expression in brain and liver. These findings point to an influence of estrogenic exposure on the endocrine GH/IGF-I axis. In addition, the EE2 treatment resulted in significant changes of ERalpha and IGF-I expression in ovaries and testis, suggesting that the estrogens interact not only with the endocrine but also with the autocrine/paracrine part of the IGF-I system. Overall, our results provide evidence that EE2 at environmentally relevant concentrations is able to interfere with the GH/IGF-I system in bony fish and that the impairing effects of estrogens reported on fish growth and reproductive functions may rather result from a cross talk between the sex steroid and the IGF-I system than be toxicological.

Combined replacement of fish meal and oil in practical diets for fast growing juveniles of gilthead sea bream ( Sparus aurata L.): Networking of systemic and local components of GH/IGF axis

Growth performance and growth regulatory pathways were examined in juvenile gilthead sea bream fed diets containing largely plant-based ingredients. Four isonitrogenous and isolipidic extruded diets with a low level (20%) of fish meal inclusion were formulated with graded levels of a vegetable oil mixture (17:58:25 of rapeseed: linseed: palm oils) replacing fish oil at 33, 66 and 100% (33VO, 66VO and VO diets). All diets were supplemented with lysine (0.55%) and contained soy lecithin (1%). Daily growth coefficients and feed efficiency over the course of an 11-week trial were almost identical in fish fed the FO, 33VO and 66VO diets. The VO diet reduced feed intake and growth without significant effects in proximate whole body composition, nitrogen or energy retentions. The highest concentration of plasma levels of insulin-like growth factor-I (IGF-I) was found in fish fed the 33VO diet. The lowest concentration was attained in fish fed the VO diet, whereas intermediate values were found in fish fed FO and 66VO diets. An opposite trend was found for circulating levels of growth hormone (GH), probably as a result of a reduced negative feedback inhibition from circulating IGF-I. Hepatic expression of IGF-I and GH receptor type I (GHR-I) was regulated in concert and mRNA levels paralleled plasma levels of IGF-I. Hepatic IGF-II and GHR-II were expressed in a more constitutive manner and no changes at the mRNA level were detected. In the skeletal muscle, IGF-I and GHR-I mRNAs did not vary significantly among groups. By contrast, IGF-II mRNA was up-regulated in fish fed the control diet, whereas the highest amount of GHR-II mRNA was attained in fish fed the 66VO diet. All together, these results suggest different growth compensatory mechanisms mediated by IGF-II and GHR-II at the local tissue level. These new insights prompted us to propose that practical diets low in marine ingredients can be used over the productive cycle of gilthead sea bream when essential fatty acids are supplied above the requirement levels.

Alcohol-Reduced Plasma IGF-I Levels and Hepatic IGF-I Expression Can Be Partially Restored by Retinoic Acid Supplementation in Rats

Chronic and excessive ethanol intake in rats results in low levels of hepatic retinoic acid (RA) either by inhibiting the biosynthesis of RA or by enhancing its catabolism of RA. Chronic ethanol intake also decreases both hepatic expression of insulin-like growth factor-I (IGF-I) and plasma IGF-I concentration in rats. It is not known whether RA supplementation in alcohol-fed rats can restore plasma IGF-I concentrations and hepatic IGF-I expression. In the present study, we examined both plasma IGF-I level and hepatic IGF-I mRNA expression in alcohol-fed rats with or without RA (100 microg/kg body weight) supplementation for 6 mo. Hepatic IGF-I mRNA levels and plasma IGF-I concentration were decreased (84 and 29%, respectively) significantly in alcohol-fed rats compared with the control. In contrast, RA supplementation in ethanol-fed rats partially restored both hepatic IGF-I mRNA levels and plasma IGF-I concentration compared with rats fed ethanol alone. These data suggest that alcohol-impaired hepatic RA status contributes to the decreased plasma IGF-I level and hepatic IGF-I expression in alcoholics.

Molecular cloning and expression of insulin-like growth factor-I (IGF-I) in Eurasian perch (Perca fluviatilis): lack of responsiveness to growth hormone treatment

The cloning of Eurasian perch (Perca fluviatilis) insulin-like growth factor-I (IGF-I) liver cDNA resulted in the identification of two transcripts of 794 and 713 nucleotides. Whereas the long IGF-Ib isoform consisted of 186 residues, the short IGF-Ia isoform was lacking 27 residues of the E domain due to alternative exon splicing. The long isoform was more abundant than the short isoform in liver, while the expression of both isoforms was rather low in non-hepatic tissues. In order to elucidate the specific endocrine regulation of growth in perch, effects of growth hormone (GH) treatment on weight gain and hepatic IGF-I expression were investigated in juvenile Eurasian perch. Fish injected with Southern black bream (Acanthopagrus butcheri, Perciformes) GH (0.5 μg/g body weight) grew significantly faster than the saline injected control group, whereas treatment with bovine GH (1.0 μg/g body weight) gave no growth enhancement. No effects of the GH treatments on hepatic IGF-I mRNA expression were shown by Northern blot analysis. Thus, the impact of GH-IGF-I axis on growth regulation in Eurasian perch remains unclear, but might be affected by IGF-binding proteins (IGFBPs).

Hepatic mRNA expression and plasma levels of insulin-like growth factor-I (IGF-I) in broiler chickens selected for different growth rates

The hepatic expression and plasma concentrations of IGF-I were investigated in three broiler chicken strains selected for different growth rates (HP-Hubbard-Pettersen, a fast growing strain; NN-Naked-neck, a strain with an intermediate growth rate and a heterozygous genotype, and C-Caipira, a slow growing crossbred strain). The chickens were studied at 1, 21 and 42 days of age and had free access to food throughout the study. Hepatic IGF-I mRNA expression was assessed by dot blot analysis using a randomly labeled chicken IGF-I cDNA as the probe and plasma IGF-I concentrations were assayed by radioimmunoassay. The hepatic levels of IGF-I mRNA increased from 1 to 21 days of age in all strains, with NN chickens showing a higher (p < 0.05) IGF-I expression than the other strains. Plasma IGF-I concentrations increased (p < 0.05) with broiler chicken age, but there were no significant differences among the strains. These results indicate that despite differences in the growth rates among the strains, the changes in the expression of IGF-I mRNA in liver and in the plasma levels of IGF-I were independent of broiler chicken strain, but varied with chicken age.

Age-dependent adaptation of the liver thyroid status and recovery of serum levels and hepatic insulin-like growth factor-I expression in neonatal and adult diabetic rats

The effect of treatment with thyroxine (T4) on the hepatic deiodinase (5′D-I) activity and triiodothyronine (T3) content and on insulin-like growth factor-I (IGF-I) secretion and mRNA hepatic expression were studied in neonatal and adult diabetic (D) rats and compared with 4 thyroidectomized (Tx) groups: neonatal and adult Tx rats treated or not with T4. Serum T3 and T4 decreased by 92% in both Tx populations and by 80% to 70% in D adults according to the severity of diabetes: −70 mg/kg body weight (BW) (D70) or 50 mg/kg BW (D50) of streptozotocin (STZ) injected, whereas only a 30% to 33% decrease was found in D neonates. A similar decrease of liver 5′D-I activity and T3 concentrations was found in neonatal and adult Tx rats, whereas a significant reduction in those parameters was observed only in adult diabetics, either D70 or D50, but not in D neonates. Serum levels and liver mRNA expression of IGF-I determined by ribonuclease protection assay, plasma and pituitary growth hormone (GH), plasma insulin, and glycemia were also measured in both D populations. A decrease in circulating IGF-I, previously reported for Tx adult rats, was also found in both D populations. T4 treatment recovered IGF-I and liver T3 in both Tx groups and D neonates, but not in D adults. These results show an age-dependent adaptation of the liver thyroid economy in diabetes, as hepatic 5′D-I does not respond to diabetes in neonates and IGF-I is insensitive to T4 treatment in adult diabetics and suggest a positive correlation between hepatic T3 content and IGF-I expression in conditions of diabetes and Tx.

Chronic salinity adaptation modulates hepatic heat shock protein and insulin-like growth factor I expression in black sea bream.

Black sea bream ( Mylio macrocephalus) hepatic heat shock proteins hsp90, hsp70, and hsp60 were found to be thermally and reversibly inducible as they were elevated 2.0, 3.2, and 2.1 fold, respectively, on acute heat shock and returned to pre-heat-shock levels after a 40-hour recovery period. To establish whether salinity plays a role in regulating heat shock protein (hsp) and insulin-like growth factor-I (IGF-I) expression in a euryhaline marine fish, we adapted groups of juvenile black sea bream to salinities of 50 ppt (hypersaline), 33 ppt (seawater), 12 ppt (isoosmotic), and 6 ppt (hypoosmotic) for 8 months. The lowest levels of hsps were found in fish reared in an isoosmotic salinity and the highest in those adapted to hypersaline and hypoosmotic salinities. Hepatic beta-actin messenger RNA abundance remained unchanged in all groups during salinity adaptation, whereas IGF-I mRNA abundance was highest in isoosmotic adapted black sea bream. This study is the first report of an effect of salinity ranging from hypersaline to hypoosmotic on the expression of different hsp forms and IGF-I in fish, and the possible relationship between environmental salinity, hepatic IGF-I expression, and hsp regulation is discussed.

Increased Expression of Insulin-like Growth Factor-I in Serum and Liver after Recombinant Human Growth Hormone Administration in Thermally Injured Rats

Background. Recombinant human growth hormone (rhGH) has been shown to modulate the hypermetabolic response and the hepatic acute-phase response after thermal injury. In vitro studies, however, demonstrated that rhGH activates insulin-like growth factor-I (IGF-I) gene transcription and production, suggesting that rhGH may exert some of its effects indirectly through IGF-I stimulation. The purpose of this study was to determine the effects of rhGH on serum and hepatic IGF-I in thermally injured rats.Methods. Sprague–Dawley rats (56 males) receiving a 60% TBSA third-degree scald burn were randomly divided to receive either rhGH (2.5 mg/kg/day im) or saline (control). Rats were sacrificed on postburn days 1, 2, 5, and 7 and serum IGF-I, hepatic IGF-I mRNA, and IGF-I protein concentration were measured. The physiologic response to changes in IGF-I levels was evaluated by measuring hepatocyte proliferation, total liver protein concentration, and muscle dry/wet weights.Results. Serum IGF-I was increased from postburn day 1 through day 7 in rats receiving rhGH compared to controls (P < 0.05). Hepatic IGF-I mRNA and IGF-I protein expression were increased from day 1 to 7 after burn in animals receiving rhGH when compared to controls (P < 0.05). Recombinant hGH increased hepatocyte proliferation at 5 days and total liver protein concentration at 5 and 7 days postburn compared to controls (P < 0.05). Muscle dry/wet weights increased in rats receiving rhGH at 7 days after burn compared to controls (P < 0.05).Summary. Liver and serum IGF-I levels decreased after a thermal injury. Recombinant hGH attenuated this decrease by stimulating hepatic IGF-I expression. Increases in IGF-I were associated with increases in hepatocyte proliferation and protein concentration in liver and muscle.Conclusion. We suggest that rhGH modulates the hypermetabolic response through IGF-I stimulation in the hepatic parenchyma.

Metabolic Regulation of IGF-I Gene Expression

Insulin-like growth factor-I (IGF-I) is structurally similar to proinsulin and has wide-ranging actions in the promotion of cellular growth, proliferation, and differentiation. Circulating IGF-I is produced mainly in the liver and acts in an endocrine manner at many sites throughout the body, but IGF-I is also produced in a wide variety of organs and tissues, where it has local paracrine and autocrine effects. Consistent with its role as an important regulator of cell growth, IGF-I is tightly modulated by hormonal and nutritional status. In intact animals, reduction in levels of growth hormone or insulin, reduced food intake, and reduction in dietary protein all lead to a fall in circulating IGF-I levels and a parallel fall in hepatic IGF-I expression. Much of this regulation has been confirmed in cultured cell models showing responses to growth hormone, insulin, and availability of amino acids. Regulation takes place at the level of gene transcription since growth hormone, insulin, and dietary protein all increase both hepatic IGF-I expression and rates of transcript elongation in nuclear runon assays. Although the IGF-I gene is complex, extending over 80 kb and including six exons, most transcripts are initiated at several sites in exon 1. We have used the model of streptozotocin-induced diabetes in rats to study mechanisms of metabolic regulation. Diabetic rats exhibit dose-dependent decreases in serum IGF-I levels and hepatic IGF-I expression, preceded by a reduction in IGF-I gene transcription. A similar impact of diabetes is seen with in vitro transcription systems driven by hepatic nuclear extracts from normal and diabetic rats. In such systems, the study of deletion mutants reveals that a region downstream from the major initiation sites in exon 1 is necessary for the diabetes-associated fall in IGF-I gene transcription. We have localized a metabolism-responsive element to a footprinted region ∼ 140 bp downstream and found that mutation of this element blunts the decrease in IGF-I transcription that occurs with diabetic nuclear extracts. Gel mobility shift analysis with oligonucleotides containing this element reveals reduced DNA-protein interactions with diabetic nuclear extracts. Fasting or protein restriction also lead to diminished hepatic IGF-I expression and a fall in IGF-I gene transcription. Although the downstream region is also involved in nutrition-regulated transcription, the sequence elements that confer this control have not yet been identified. Work now in progress is directed toward identification of the transcription factors that mediate IGF-I regulation by alterations in insulin levels and nutritional status. Such understanding should provide further insight into the role and regulation of IGF-I.

Basal and growth hormone-induced hepatic messenger ribonucleic acid expression of insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 is independent of hyperinsulinemia and increased energy status in the genetically obese Zucker rat.

Genetically obese Zucker rats, like obese humans, have normal or elevated circulating insulin-like growth factor-I (IGF-I) levels in the presence of low GH secretion. Hyperinsulinemia, increased energy status, or other nutritional factors associated with obesity could be responsible for these findings directly by increasing hepatic IGF-I production at the transcriptional or posttranscriptional level. Alternatively, circulating IGF-I could be modulated indirectly by affecting its binding proteins. To further elucidate this point, we quantitated hepatic IGF-I, IGF-binding protein-3 (IGFBP-3), and GH receptor messenger RNAs (mRNAs) expression in obese Zucker rats under different serum GH and insulin conditions using lean rats as controls. Eleven-week-old male rats were studied basally (intact) or after hypophysectomy (hx) at 9 weeks. In each condition, animals were killed before or 6 h after one dose of recombinant human GH (1.5 micrograms/g body weight ip). At this time, in addition to the mRNA expression of the above-mentioned genes, body weight, glycemia, insulinemia, serum GH (rat and human), and serum IGF-I levels were determined. Obese Zucker rats were significantly heavier than controls in all the conditions studied and did not show differences in glycemia. Severely hyperinsulinemic intact obese rats (146.9 +/- 14 vs. 46.3 +/- 3 microU/ml, P < 0.001) showed compared with intact lean rats significantly lower serum GH (2.39 +/- 0.9 vs. 4.98 +/- 0.68 ng/ml, P < 0.01), decreased hepatic IGF-I mRNA and IGFBP-3 mRNA accumulation (IGF-Ia: 79 +/- 5.9% vs. 100 +/- 0.9%, P < 0.05; IGF-Ib: 67 +/- 5.5% vs. 100.1 +/- 1.9%, P < 0.001; IGFBP-3: 54.7 +/- 2.75% vs. 100.5 +/- 1.55%, P < 0.001), and similar circulating IGF-I levels (1439 +/- 182 vs. 1516 +/- 121 ng/ml). Under comparable serum GH levels in GH-treated intact, hx, and GH-treated hx animals, hyperinsulinemia and/or increased body weight present in obese rats were not associated with increased hepatic IGF-I and IGFBP-3 mRNA amount. No differences in GH receptor/GH-binding protein mRNAs were found in any experimental condition. These results suggest that in vivo the imbalance of the serum GH/IGF-I axis present in obesity is primarily due to events distal to the hepatic IGF-I and IGFBP-3 mRNAs expression, which is tightly correlated to GH levels.

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.

Stimulation of Hepatic Insulin-Like Growth Factor-I Gene Expression by Ovine Prolactin: Evidence for Intrinsic Somatogenic Activity in the Rat*

The effect of ovine PRL (oPRL) on hepatic insulin-like growth factor I (IGF-I) gene expression was examined in the hypophysectomized (hypox) rat and compared with that seen in hypox rats treated with ovine GH (oGH) and human GH (hGH). A single injection of oPRL (100 micrograms/100 g BW) resulted in a 15-fold increase in hepatic IGF-I messenger RNA abundance 12 h after administration. Serum IGF-I increased from 82.9 +/- (SE) 9.0 to 137.0 +/- 15.4 ng/ml. In acute studies, where the slopes of 4-point dose-response curves were compared, oPRL was approximately 50% as potent as oGH in stimulating serum IGF-I and hepatic IGF-I messenger RNA accumulation. In chronic experiments, where a single daily injection of oPRL (0, 10, 30, or 90 micrograms/day) was administered for 10 days, a significant increase in body wt was observed at each dose. However, in contrast to hypox rats which received oGH, no dose-response relationship was apparent, and oPRL was less than 10% as potent as oGH in this regard. A small but significant increase in serum IGF-I and hepatic IGF-I expression was observed in hypox rats chronically injected with oPRL when killed 6-7 h after the final injection. However, by extrapolation from the hGH dose-response curves, oPRL was 6.8% and 22.7% as potent as hGH in stimulating serum IGF-I and hepatic IGF-I accumulation respectively. The observations reported here demonstrate that oPRL has significant somatogenic activity in the rat as determined by the three variables measured. Whereas oPRL had significant effects on body weight gain, serum IGF-I and hepatic IGF-I expression, the potency of oPRL by comparison with the two other somatogenic hormones was more marked in acute experiments compared to the effects seen after chronic administration. These results suggest that oPRL, like human GH, has both somatogenic and lactogenic activity in the rat and therefore may be an inappropriate control in studies of the effects of somatogenic hormones in the rat.

Differential effects of estrogen and growth hormone on uterine and hepatic insulin-like growth factor I gene expression in the ovariectomized hypophysectomized rat.

The acute and chronic effects of 17 beta-estradiol (E2) and GH on uterine and hepatic insulin-like growth factor I (IGF-I) gene expression in ovariectomized hypophysectomized (ovx-hypox) rats were examined. Six hours after a single injection of E2 (5 micrograms/100 g BW), uterine IGF-I gene expression was increased 22.5 +/- 5.4-fold (P less than 0.005) above that in untreated rats. In the same experiment E2 alone had no significant effect on hepatic IGF-I gene expression. Similarly, in chronic experiments uterine IGF-I in ovx-hypox rats receiving 0.1 or 1 microgram/rat.day E2 for 10 days was significantly increased compared to that in ovx-hypox rats that did not receive E2 [5.38 +/- 0.79 vs. 1.10 +/- 0.15 (P less than 0.005) and 6.64 +/- 0.28 vs. 0.93 +/- 0.06 (P less than 0.005), respectively]. While administration of human GH alone significantly increased uterine IGF-I expression (3.76 +/- 1.61-fold compared to that in untreated rats; P less than 0.05), a significant and reproducible attenuation of E2-induced IGF-I expression was seen in the two acute experiments where GH reduced the E2-induced response by 36 +/- 3.7% and 53 +/- 19.4%. While chronic administration of E2 to ovx-hypox rats resulted in uterine growth, a significant decrease in body weight was seen in rats treated with 1 microgram/day E2 compared to that in untreated ovx-hypox controls (-4.3 +/- 1.5 vs. 2.5 +/- 0.6 g; P less than 0.0005). E2 treatment also significantly decreased the GH-induced increase in weight gain at each GH dose by approximately 40%. GH-induced hepatic IGF-I gene expression and serum IGF-I concentration were similarly reduced by chronic E2 administration. In contrast, in acute experiments where E2 alone had no effect on hepatic IGF-I expression, it acted in a synergistic fashion with GH and resulted in significantly greater accumulation of IGF-I mRNA. From these observations we conclude that 1) both E2 and human GH are potent stimulators of IGF-I gene expression in appropriate target tissues; and 2) in addition to any effects E2 has on GH secretion and IGF-I action, the growth-retarding effect of estrogen in the rat involves inhibition of GH-dependent hepatic IGF-I expression.