Effect of protein deprivation on insulin-like growth factor-binding proteins in rats

Endogenous testosterone reduces hepatic lipid accumulation in protein-restricted male rats

OBJECTIVES

Protein deficiency is known to cause ectopic fat accumulation in the liver. The aim of this study was to analyse the mechanism of suppression of hepatic fat accumulation by testosterone and to clarify the mechanism behind the gender difference in fatty liver formation due to protein deficiency.

METHODS

Hepatic fat accumulation due to protein deficiency was evaluated in male and female rats before and after sexual maturation. Then, the effects of testosterone on liver lipid, muscle protein metabolism and energy expenditure in adipose tissue were investigated in castrated or testosterone-injected male rats fed control or protein-restricted diet.

RESULTS

Hepatic triglyceride accumulation diminished with sex maturation in male but not in female protein-restricted rats. Protein restriction resulted in a significant increase in hepatic triglyceride content in castrated rats but not in sham-operated rats demonstrating that endogenous testosterone reduces hepatic lipid accumulation in male rats. Protein restriction reduced plasma IGF-I and muscle protein synthesis measured using the SUnSET method. Castration increased the plasma corticosterone level and muscle autophagic activity. Muscle weight was reduced and energy expenditure in adipose tissue was increased only when both factors were combined.

CONCLUSIONS

Muscle protein synthesis downregulation owing to protein restriction and activation of autophagy following castration reduced muscle mass thereby releasing surplus energy and promoting steatosis in protein-restricted castrated rats despite increased energy expenditure in adipose tissue. We hypothesize that endogenous testosterone reduces hepatic lipid accumulation in protein-deficient male rats and provide novel findings on the gender-specific differences in hepatic steatosis.

Effects of age, nursing, and oral IGF1 supplementation on neonatal porcine cervical development

Nursing supports neonatal porcine uterine and testicular development, however, lactocrine effects on cervical development are undefined. Studies were conducted to determine the effects of i) age and the imposition of the lactocrine-null state from birth (postnatal day 0 (PND0)) by milk replacer feeding on cervical histology; ii) imposition of the lactocrine-null state for 2 days from birth on cervical cell proliferation, as reflected by proliferating cell nuclear antigen immunostaining; and iii) a single feeding of colostrum or milk replacer, administered at birth, with or without oral IGF1, on cervical cell proliferation and phosphorylated AKT (pAKT) and B-cell lymphoma 2 (BCL2) protein levels at 12 h postnatal. Cervical crypt depth and height of luminal epithelium (LE) increased with age by PND14, when both responses were reduced in replacer-fed gilts. Cell proliferation was reduced in LE at PND2, and in crypt epithelium and stroma by PND14 in replacer-fed gilts. Returning replacer-fed gilts to nursing on PND2 did not rescue the cervical phenotype by PND14. A single feeding of colostrum, but not milk replacer, was sufficient to support cervical cell proliferation at 12 h postnatal. IGF1 supplementation induced cell proliferation in replacer-fed gilts, and increased cervical pAKT and BCL2 levels in colostrum-fed gilts and replacer-fed gilts at 12 h postnatal. Results indicate that age and nursing support porcine cervical development, support is initiated at first ingestion of colostrum, IGF1 may be lactocrine-active, and identification of lactocrine-active factors can be accomplished by 12 h postnatal using this bioassay system.

Cadmium Cation Increases the Production and mRNA Levels of Insulin-Like Growth Factor-Binding Protein-1 in HepG2

The production of insulin-like growth factor-binding protein-1 (IGFBP-1) in HepG2 was increased by cadmium cation (Cd2+) at 3 microM, but not by other divalent cations. The mRNA level of IGFBP-1 was also increased by the administration of 3 microM of Cd(2+). These results suggest that Cd(2+) impacts the gene expression of IGFBP-1, which leads to production of IGFBP-1.

Dietary restriction of single essential amino acids reduces plasma insulin-like growth factor-I (IGF-I) but does not affect plasma IGF-binding protein-1 in rats

The effects of dietary restriction of a single essential amino acid (EAA) on insulin-like growth factor-I (IGF-I) and IGF-binding protein (IGFBP)-1 were investigated in rats. Rats were fed experimental diets containing amino acid (AA) mixtures in which the concentrations of all EAA were at levels recommended by the National Research Council (control), in which a single EAA was restricted to 20% of that of the control diets (Leu(-), Lys(-), Met(-) or Thr(-)), or in which the diet was devoid of amino acids (AA(-)). To eliminate the effect of differences in energy intake, rats were fed the mean amount of food as consumed by the AA(-) group on the previous day. Growth was significantly retarded in rats fed diets restricted in just one EAA compared with that of rats fed the control diet, and further growth retardation was observed in rats fed the AA(-) diet. On the other hand, the plasma IGF-I concentrations in the groups with a single EAA restriction or in the AA(-) group were 66% (P: < 0. 05) and 50% (P: < 0.05) of that of the control group, respectively. The effect of any single EAA restriction was not significantly different from that of total AA deprivation. The plasma IGFBP-1 concentration in the control group did not differ from that of rats fed diets with the single EAA restrictions except for methionine restriction, but it was approximately 6-fold greater in the AA(-) group. Differences in plasma IGFBP-1 concentration under these conditions could be explained by differences in hepatic IGFBP-1 mRNA contents. Based on these results, we conclude that restriction of single EAA does not affect IGFBP-1 synthesis in vivo, although the deprivation of a single EAA has been reported to increase IGFBP-1 production in hepatocyte cultures. Our results also indicated that a single EAA restriction decreased IGF-I production but did not affect IGFBP-1 production. The present study suggests that not only plasma IGF-I, but also IGFBP-1, affects the magnitude of growth retardation in vivo.

Influence of dietary protein on insulin-like growth factor binding proteins in the chicken

We determined the effect of dietary protein on the distribution of insulin-like growth factor (IGF) binding proteins in chicken plasma. Three groups of male broilers (n = 6 per group) were fed (ad libitum) isocaloric diets containing 12, 21 or 30% dietary protein. Birds were fed respective diets beginning at 7 days of age and killed at 28 days. No differences were observed between adequate (21%) and high (30%) protein intakes for any of the parameters investigated (growth criteria, plasma levels of IGF-I, growth hormone or IGF-binding proteins). Feeding protein deficient diets (12%) resulted in a 34% decrease in body weight, 17% decrease in feed intake and a 39% increase in feed/gain ratio. IGF-binding proteins in plasma samples were separated by SDS-PAGE and transferred to nitrocellulose sheets. Nitrocellulose blots were probed with [125I]chicken IGF-II. Four regions of binding activity corresponding to 70, 43, 30 and 24 kDa were observed in all samples. Birds consuming 12% dietary group protein had less than 50% of the 43-kDa binding activity of birds consuming 21 or 30% dietary protein. The 30-kDa binding activity was 42% lower in the 12% dietary protein group compared to birds consuming adequate protein. In contrast, 70- and 24-kDa binding activities were not influenced by dietary protein. Chickens consuming 12% dietary protein had higher levels of growth hormone and lower levels of IGF-I than those consuming 21 or 30% dietary protein. These data indicate that in chickens, the circulating levels of at least two independent IGF-binding proteins are influenced by dietary protein.

Effect of protein restriction on messenger RNA of insulin-like growth factor-I and insulin-like growth factor-binding proteins in liver of ovariectomized rats

Effects of dietary protein restriction and ovariectomy on plasma concentrations and hepatic messenger RNA (mRNA) of insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding proteins (IGFBP) were investigated in young female rats. Ovariectomy increased plasma IGF-I concentration in rats fed on either a 50 g casein/kg diet (protein-restricted diet) or a 200 g casein/kg diet (control diet), but it increased IGF-I mRNA in liver only in the rats fed on the control diet. On the other hand, by Western ligand blot analysis, we observed that ovariectomy increased plasma IGFBP-3 concentration, and decreased plasma IGFBP-4 concentration. Ovariectomy did not affect IGFBP-1 and IGFBP-2 mRNA in liver, but dietary protein restriction significantly increased them, which may correspond to their plasma concentrations. The present results show that ovarian hormones and dietary protein content affect the plasma concentrations of IGF-I, IGFBP-3 and IGFBP-4 and hepatic mRNA of IGF-I, IGFBP-1, IGFBP-2, IGFBP-3 and IGFBP-4 in different manners.

Long-term effect of recombinant human insulin-like growth factor I on metabolic and growth control in a patient with leprechaunism

Leprechaunism is the most severe form of insulin resistance, manifesting with abnormal glucose metabolism and retarded growth. In the present study, we investigated the biological actions of recombinant human insulin-like growth factor I (rhIGF-I) in fibroblasts derived from a patient with leprechaunism. In the same patient, we also investigated the pharmacokinetics of IGF-I and the long-term effect of rhIGF-I treatment on metabolic control and physical growth. The patient's fibroblasts showed normal binding of IGF-I, normal phosphorylation of the beta-subunit of the IGF-I receptor, and normal [3H]thymidine incorporation in response to IGF-I. The fibroblast studies suggested that the patient would respond to IGF-I therapy, but certainly did not exclude the possibility of IGF-I resistance in vivo. Administration of recombinant human GH at the dose of 2.0 IU/kg for 3 consecutive days induced a minimal response of serum total IGF-I and IGF-binding protein-3 (IGFBP-3), suggesting partial GH resistance. To increase the serum total IGF-I level, we administered rhIGF-I with combination therapy of intermittent and continuous s.c. injection. This sustained the serum total IGF-I level, but not the serum IGFBP-3 level, within the normal range. The patient was treated with combination therapy of rhIGF-I by both s.c. injection and continuous s.c. infusion for 6 yr and 10 months. Administration of rhIGF-I at total daily dose of 1.6 mg/kg maintained her growth rate and hemoglobin A1c level nearly within the normal range. These findings suggest 1) that this leprechaun patient has an IGF-I-deficient state and partial GH resistance, as reflected by impaired production of IGF-I and IGFBP-3; 2) that rhIGF-I treatment works effectively for preventing postnatal growth retardation and normalizing glucose metabolism in patients with extreme insulin resistance; 3) that this treatment requires relatively higher dose of rhIGF-I; and 4) that treatment appears to be safe and devoid of adverse effects.

Effect of protein restriction on the messenger RNA contents of bone-matrix proteins, insulin-like growth factors and insulin-like growth factor binding proteins in femur of ovariectomized rats

It has been reported that loss of ovarian oestrogen after menopause or by ovariectomy causes osteoporosis. In order to elucidate the effect of dietary protein restriction on bone metabolism after ovariectomy, we fed ovariectomized young female rats on a casein-based diet (50 g/kg diet (protein restriction) or 200 g/kg diet (control)) for 3 weeks and measured mRNA contents of bone-matrix proteins such as osteocalcin, osteopontin and alpha 1 type I collagen, insulin-like growth factors (IGF) and IGF-binding proteins (IGFBP) in femur. Ovariectomy decreased the weight of fat-free dry bone and increased urinary excretion of pyridinium cross-links significantly, although dietary protein restriction did not affect them. Neither ovariectomy nor protein restriction affected the content of mRNA of osteopontin and osteocalcin; however, ovariectomy increased and protein restriction extensively decreased the alpha 1 type I collagen mRNA content in bone tissues. Ovariectomy increased IGF-I mRNA only in the rats fed on the control diet. Conversely, protein restriction increased and ovariectomy decreased the IGF-II mRNA content in femur. Furthermore, the contents of IGFBP-2, IGFBP-4 and IGFBP-5 mRNA increased, but the content of IGFBP-3 mRNA decreased in femur of the rats fed on the protein-restricted diet. In particular, ovariectomy decreased the IGFBP-2 mRNA content in the protein-restricted rats and the IGFBP-6 mRNA content in the rats fed on the control diet. These results clearly show that the mRNA for some of the proteins which have been shown to be involved in bone formation are regulated by both quantity of dietary proteins and ovarian hormones.

Plasma levels of insulin-like growth factor binding protein-3 in acute trauma patients

Insulin-like growth factors (IGFs) are a family of polypeptides that regulate cell growth. Their action and bioavailability are modified by binding proteins such as IGF binding protein-3 (IGFBP-3). Plasma IGFBP-3 level was found to be growth hormone (GH)-dependent, which makes detection of IGFBP-3 useful in the evaluation of GH secretion. In the early catabolic flow phase of severe injury, when plasma levels of GH and IGF-1 are low versus uninjured levels, the role of IGFBP-3 has not been investigated. We have measured basal levels of these polypeptide hormones in 16 adult (13 men and three women aged 47 +/- 7 years) severely injured (Injury Severity Score, 32 +/- 2), hypermetabolic resting energy expenditure [REE] to basal energy expenditure [BEE] ratio, 1.30 +/- 0.05), ventilator-dependent, multiple-trauma patients within 48 to 60 hours of injury when the patients were receiving maintenance fluids without calories or nitrogen. These basal values were compared with those of 16 age-matched postabsorptive normals. In the catabolic flow phase of injury, plasma levels of GH, IGF-1, and IGFBP-3 were significantly reduced by 50%, 46%, and 45%, respectively. There was a significant linear inverse relationship between IGFBP-3 and age and also a positive correlation between IGFBP-3 and IGF-1 in both control and injured subjects. The ratio of IGFBP-3 to IGF-1 was not changed in trauma victims. Measurement of plasma IGFBP-3 levels has potential as a marker for monitoring GH therapeutic efficacy.

Effect of protein nutrition on the mRNA content of insulin-like growth factor-binding protein-1 in liver and kidney of rats

Effect of quantity and nutritional quality of dietary proteins on the content of mRNA of insulin-like growth factor-binding protein-1 (IGFBP-1) was studied in rat liver and kidney. IGFBP-1 mRNA content per unit RNA increased in liver and kidney of rats fed on a protein-free diet and in those of fasted rats compared with that in the rats fed on a casein diet. When rats were given a gluten diet for 7 d, IGFBP-1 mRNA content in liver did not change significantly but that in kidney increased considerably compared with that in those organs of the rats fed on the casein diet. Because IGFBP-1 mRNA has been demonstrated both in liver parenchymal and non-parenchymal cells (Takenaka et al. 1991), the effect of the protein-free diet on these two types of cells has been studied. An increase in IGFBP-1 mRNA content under protein deprivation was observed in both liver parenchymal and non-parenchymal cells, suggesting that these two types of cells are regulated in a similar mode as far as IGFBP-1 mRNA content is concerned. The physiological and nutritional significance of the previously stated results on protein anabolism are discussed when considered together with our previous observations on the plasma concentrations of IGF-1 (Takahashi et al. 1990) and IGFBP (Umezawa et al. 1991) and insulin-like growth factor-1 mRNA content in liver (Miura et al. 1991).