Tissue-specific response of IGF-I mRNA expression to obesity-associated GH decline in the male Zucker fatty rat.

ABSTRACT

Diminished GH secretion is a well known association of obesity. As in obese humans, Zucker fatty rats develop a progressive GH deficiency, present at 6 weeks of age and maximal at 10 to 12 weeks. The aim of this study was to investigate the GH dependence of IGF-I gene expression in liver and extrahepatic tissues of the obese Zucker rat as a model of progressive GH reduction during adult life. Six- and 11-week-old obese Zucker rats and their lean littermates were used to compare body weight, glycemia, insulinemia, serum GH and IGF-I levels and IGF-I mRNA expression in liver, heart, aorta, kidney and skeletal muscle. In comparison with lean controls, obese Zucker rats showed at both ages comparable glycemia, severe hyperinsulinemia (mU/ml, mean+/-s.e.m.; 6 weeks 138+/-10 vs 45+/-6 P<0.001; 11 weeks 147+/-14 vs 46+/-3, P<0.001) and lower GH (ng/ml; 6 weeks 1.7+/-0.9 vs 2.7+/-1.1; 11 weeks 1.5+/-0.9 vs 4.2+/-1.2) in the presence of similar circulating IGF-I levels (ng/ml; 6 weeks 774+/-26 vs 694+/-28; 11 weeks 1439+/-182 vs 1516+/-121). Hepatic IGF-I mRNA expression was already reduced at 6 weeks of age due to a significant decrease in the IGF-Ib transcript compared with lean controls (relative units; IGF-Ia 99+/-2% vs 100+/-5%; IGF-Ib 69+/-10% vs 100+/-2%, P<0.05) and this reduction was more marked in 11-week-old animals when both IGF-I transcripts were significantly diminished (relative units; IGF-Ia 80+/-6% vs 100+/-1%, P<0.05; IGF-Ib 65+/-5% vs 100+/-2%, P<0.01). Extrahepatic tissues expressed almost exclusively the IGF-Ia transcript, the amount of which relative to controls was (1) similar at 6 weeks and decreased at 11 weeks in kidney and skeletal muscle extracts (relative units; kidney 6 weeks 88+/-10% vs 100+/-2%; 11 weeks 76+/-3% vs 100+/-4%, P<0.05; vastus lateralis 6 weeks 95+/-7% vs 100+/-10%; 11 weeks 59+/-4% vs 100+/-2%, P<0.001); (2) similar at both ages in thoracic aorta (relative units; 6 weeks 121+/-6% vs 105+/-5%; 11 weeks 91+/-14% vs 100+/-4%); and (3) increased at both ages in left ventricle extracts (relative units; 6 weeks 114+/-2% vs 99+/-9%, P<0. 05; 11 weeks 119+/-7% vs 95+/-3%, P<0.05). -specific dependence of IGF-I mRNA on GH levels during adulthood, reflected by the different behavior of IGF-I expression for each tissue in conditions of progressive decrease of GH levels.