Mosaic analysis of insulin receptor function

Abstract

Insulin promotes both metabolism and growth. However, it is unclear whether insulin-dependent growth is merely a result of its metabolic actions. Targeted ablation of insulin receptor (Insr) has not clarified this issue, because of early postnatal lethality. To examine this question, we generated mice with variable cellular mosaicism for null Insr alleles. Insr ablation in approximately 80% of cells caused extreme growth retardation, lipoatrophy, and hypoglycemia, a clinical constellation that resembles the human syndrome of leprechaunism. Insr ablation in 98% of cells, while resulting in similar growth retardation and lipoatrophy, caused diabetes without β-cell hyperplasia. The growth retardation was associated with a greater than 60-fold increase in the expression of hepatic insulin-like growth factor binding protein-1. These findings indicate that insulin regulates growth independently of metabolism and that the number of insulin receptors is an important determinant of the specificity of insulin action. Conflict of interest:The authors have declared that no conflict of interest exists. Nonstandard abbreviations used: insulin receptor (Insr); forkhead transcription factor o1 (Foxo1); insulin-like growth factor binding protein-1 (Igfbp1); phosphatidylinositol-tris-phosphate (PIP3); pancreatic and duodenal homeobox protein-1 (Pdx1); peroxisome proliferative activated receptor, γ, coactivator 1α (Pgc1α); heat shock promoter (HSP); HSP 70-1 Cre transgenic (Hs-Cre6); growth hormone (GH); triglyceride (TG); 125 I-labeled insulin ( 125 I-insulin); 125 I-labeled IGF2 ( 125 I-IGF2); brown adipose tissue (BAT); phosphoenol-pyruvate carboxykinase (Pck1); glucose-6-phosphatase (G6pc); hepatic glucose transporter Glut2 (Slc2a2); glucokinase (Gck); sterol regulatory element binding transcription factor-1 (Srebf1); glycogen synthase (Gys1); glycogen phosphorylase (Pygl); phosphofructokinase, liver (Pfkl); forkhead transcription factor a2 (Foxa2); uncoupling protein-1 (Ucp1); peroxisome proliferator activator receptor γ (Pparγ); adrenergic β3 receptor (Adrb3).

Glucose Regulates Foxo1 Through Insulin Receptor Signaling in the Pancreatic Islet β-cell

“…Our studies of Foxo1 activity used a promoter-luciferase construct containing an insulin response element and conserved Foxo1 binding site in the IGFBP-1 promoter (30). In the liver, IGFBP-1 transcription has been shown to be dependent on Foxo1 activity (31). We first observed that in starved cells at 5 mmol/l glucose, endogenous Foxo1 activity was increased by twofold (P Ͻ 0.01) compared with that in fed cells at 25 mmol/l glucose (Fig.…”

Hepatic insulin signaling regulates VLDL secretion and atherogenesis in mice

“…Our studies used mice with low levels of IRs in liver (~10% of controls in L1 B6 Ldlr -/-mice and 20% of controls in IR shRNA-treated Ldlr -/-mice) versus absent IRs in the LIRKO mice. The different outcomes of complete and partial knockdown of hepatic IRs could be partly related to developmentally determined structural changes in the liver, including 36%-50% decreases in liver weight and mitochondrial and functional defects in LIRKO mice (16), with no comparable changes in mice with up to 95% IR deficiency including the L1 strain (48,49) and no comparable change in liver weight in L1 B6 Ldlr -/-mice (1.2 ± 0.07 g vs. 1.3 ± 0.08 g; P = 0.2). It is notable that LIRKO mice had increased hepatic-free and hepatic-esterified cholesterol accumulation on chow or high-cholesterol diets, despite markedly reduced expression of cholesterol biosynthetic genes.…”

Disruption of the Insulin-like Growth Factor Type 1 Receptor in Osteoblasts Enhances Insulin Signaling and Action

“…Assessment of insulin actions in the mammalian skeleton has been difficult to approach experimentally because mice globally lacking IR die shortly after birth. Mice created with mosaic deletion of IR alleles survive for several weeks and exhibit extreme postnatal growth retardation, lipoatrophy, and hypoglycemia, a clinical constellation that resembles the human syndrome of Leprechaunism because of functional mutations of IR (48). However, these changes were accompanied with 60-fold increase in IGF-binding protein 1, which is known to bind circulating IGF-1 and limit its bioavailability.…”