High circulating retinol-binding protein 4 is associated with elevated liver fat but not with total, subcutaneous, visceral, or intramyocellular fat in humans.
OBJECTIVE
Retinol-binding protein 4 (RBP4) is an adipokine that induced insulin resistance in mice, and high plasma RBP4 levels were associated with insulin-resistant states in humans. To determine which fat compartments are associated with elevated RBP4 levels in humans, we measured circulating RBP4 in 75 healthy subjects and used state-of-the-art measurements of body fat distribution.


METHODS
Total body, visceral, and subcutaneous abdominal fat were determined by magnetic resonance tomography and liver fat and intramyocellular fat by localized proton magnetic resonance spectroscopy. Insulin sensitivity was measured by the euglycemic-hyperinsulinemic clamp and, together with insulin clearance, estimated from the oral glucose tolerance test (OGTT).


RESULTS
Adjusted circulating RBP4 correlated negatively with insulin sensitivity (clamp: r = -0.33, P = 0.005; OGTT: r = -0.36, P = 0.002) and positively with parameters in the fasting state as insulin levels (r = 0.35, P = 0.003) and homeostasis model assessment of insulin resistance (r = 0.34, P = 0.004). In addition, circulating RBP4 correlated negatively with hepatic insulin clearance (r = -0.25, P = 0.04). Circulating RBP4 was not associated with total body, visceral, or subcutaneous abdominal fat (all P > or = 0.29). Plasma RBP4 levels were also not associated with intramyocellular fat or circulating adiponectin or leptin. In contrast, plasma RBP4 levels correlated positively with liver fat in cross-sectional (r = 0.27, P = 0.03) and longitudinal (r = 0.37, P = 0.04) analyses.


CONCLUSIONS
Circulating RBP4 is not associated with the amount of fat in the classical depots or in the ectopic depots in muscle. However, it correlates positively with liver fat. Furthermore, metabolic parameters support the close relationship between circulating RBP4 with liver fat and, presumably, hepatic insulin resistance.
Stefan
Norbert
N
Division of Endocrinology, Diabetology, Nephrology, Vascular Disease, and Clinical Chemistry, Department of Internal Medicine, University of Tübingen, D-72076 Tübingen, Germany. norbert.stefan@med.uni-tuebingen.de
Hennige
Anita M
AM
Staiger
Harald
H
Machann
Jürgen
J
Schick
Fritz
F
Schleicher
Erwin
E
Fritsche
Andreas
A
Häring
Hans-Ulrich
HU
D000273:Adipose Tissue / Q000033:anatomy & histology*; D000328:Adult; D003924:Diabetes Mellitus, Type 2 / Q000097:blood; D005215:Fasting / Q000502:physiology; D005230:Fatty Acids, Nonesterified / Q000097:blood; D005260:Female; D006801:Humans; D007328:Insulin / Q000097:blood / Q000502:physiology; D007333:Insulin Resistance; D008099:Liver / Q000033:anatomy & histology*; D008297:Male; D008875:Middle Aged; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D012016:Reference Values; D012177:Retinol-Binding Proteins / Q000378:metabolism*; D054839:Retinol-Binding Proteins, Plasma; D012867:Skin; D014781:Viscera
D005230:Fatty Acids, Nonesterified; D007328:Insulin; C101326:RBP4 protein, human; D012177:Retinol-Binding Proteins; D054839:Retinol-Binding Proteins, Plasma