IGF-I has important roles in regulating growth and metabolism. Circulating IGF-I is bound to specific binding proteins (IGFBP-1 to -6), with hepatocytes containing IGF-I, IGFBP-1 and -2 mRNA. Although many hepatic proteins are regionally expressed in the liver acinus, no studies have reported zonation of IGF protein expression. In this study we investigated the pattern of hepatic mRNA for the IGF proteins, vs the previously reported pepriportal gradient of phosphoenolpyruvate carboxykinase (PEPCK) expression. In situ hybridisation was used to analyse IGF-I, IGFBP-1, -2 and PEPCK mRNA in female Sprague-Dawley rats fed diets containing low (6%), normal (21%) or high (35%) protein. We report for the first time that IGFBP-1 and -2 and IGF-I are differentially expressed in the liver acinus. In the normal- and high-protein groups, levels of IGFBP-1 mRNA were higher in the perivenous region, i.e. the opposite gradient to PEPCK, with a higher gradient of IGFBP-1 expression in the high-protein group. In contrast, IGFBP-2 had a similar pattern to PEPCK, and a periportal gradient of IGF-I mRNA was also seen in the low-protein group. Using computerised image analysis, levels of IGFBP-1 and -2 mRNA were elevated 2- and 10-fold respectively, in the low- vs normal-protein groups. The level of IGF-I mRNA was reduced to 65% of normal, with circulating IGF-I levels at 30% and insulin levels 39% of normal. These results demonstrate that hepatocytes are a heterogeneous population with respect to regulation of IGF proteins, having specific expression patterns dependent on the position of the hepatocyte within the liver acinus.
SJ Hazel, AC Nordqvist, K Hall, M Nilsson and M Schalling
JC Divino Filho, SJ Hazel, P Furst, J Bergstrom and K Hall
Elevated insulin-like growth factor binding protein (IGFBP) levels, including IGFBP-1, occur in renal failure, and may contribute towards reduced IGF bioactivity in uraemia. The reduced IGF bioactivity may, in turn, contribute towards the disturbances in protein metabolism present in renal failure. In this study, the relationships between intra- and extracellular amino acid (AA) levels and IGF-I and/or IGFBP-1 levels were studied in 30 adult patients (aged 24-70 years) on haemodialysis who had no clinical signs of malnutrition. Blood samples (n = 30) and muscle biopsies (n = 13) were collected for determination of free AA in erythrocytes (RBC), plasma and muscle by reverse-phase HPLC while IGFBP-1, IGF-I and insulin plasma levels were determined by radioimmunoassay The patients on haemodialysis had elevated glutamate concentrations in RBC and plasma compared with healthy controls (524 +/- 26 vs 448 +/- 17 mumol/l, P < 0.05 and 45 +/- 4 vs 32 +/- 4 mumol/l, P < 0.01 respectively), although glutamate levels in muscle were within the normal range. The mean IGF-I level was slightly increased (s.d. score +0.74 +/- 0.30) but insulin levels were within the normal range. IGFBP-1 levels, which were inversely correlated to insulin (r = -0.40, P < 0.02), were elevated threefold compared with controls. No plasma AA level displayed a significant correlation with IGF-I, IGFBP-1 or insulin levels. However, glutamate concentrations in RBC were positively correlated to IGFBP-1 (r = 0.51, P < 0.01) and inversely correlated to IGF-I (r = -0.46. P < 0.01), although unrelated to insulin. Muscle glutamate, which was inversely related to RBC glutamate, displayed an opposite pattern with an inverse relation to IGFBP-1 levels (r = - 0.73, P < 0.01) and a positive correlation to IGF-I levels (r = 0.64, P < 0.02). Glutamate was the only AA to display an inverse correlation between RBC and muscle (r = -0.65, P < 0.02, n = 12). These findings lead us to propose that, in uraemia, the elevated IGFBP-I levels, which reduce the bioavailability of IGFs, are linked to glutamate uptake in muscle, resulting in accumulation of RBC glutamate. Whether there is a causal relationship or the correlation is due to some common regulator is not clarified in the present study.
MS Lewitt, SJ Hazel, DB Church, AD Watson, SE Powell and K Tan
The 140 kDa ternary complex of insulin-like growth factor-binding protein-3 (IGFBP-3), IGFs and an acid-labile subunit (ALS) has previously been shown to be decreased in diabetes mellitus in humans and rats. We have studied IGF-I levels and ternary complex formation in normal and diabetic cats. Total IGF-I concentrations, measured by RIA using des(1-3)-IGF-I as tracer were (+/-s.e.m.) 54+/-13 nmol/l in eight normal and 227+/-57 nmol/l in eight diabetic cats (P<0.01). The size-distribution of IGFBPs in the cat circulation was determined by incubation with (125)I-IGF-II and Superose 12 chromatography. In normal animals 26+/-2% of the (125)I-IGF-II were in a 140 kDa form compared with 48+/-5% in diabetic cats (P<0.01). When samples from normal and diabetic animals were co-incubated 52+/-3% were at 140 kDa. A similar shift was seen when normal cat and normal human serum were co-incubated. A 2-fold increase in the 140 kDa form in diabetic cats was confirmed first by size-fractionating samples and then performing a ligand-binding assay with (125)I-IGF-I or -II and charcoal separation. SDS-PAGE and Western ligand blotting demonstrated a 45 kDa doublet (presumably IGFBP-3) and 30-35 kDa forms. There were no apparent differences between normal and diabetic profiles on SDS-PAGE, suggesting that a proportion of IGFBP-3 which circulates 'free' in normal cats forms a ternary complex in the diabetic circulation. We conclude that (i) in contrast to humans and rats, ALS is the limiting factor for ternary complex formation in normal cats, (ii) ALS concentrations increase in feline diabetes mellitus and, by promoting ternary complex formation, this leads to an increase in total IGF-I concentrations, and (iii) total IGF-I concentrations may not be reliable in the diagnosis of acromegaly in diabetic cats.