Shortcomings in methodology complicate measurements of serum retinol binding protein (RBP4) in insulin-resistant human subjects |
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Authors: | T E Graham C J Wason M Blüher B B Kahn |
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Institution: | (1) Division of Endocrinology, Diabetes and Metabolism, Beth Israel Deaconess Medical Center, 99 Brookline Avenue/Research North, Rm. 380C, Boston, MA 02215, USA;(2) Department of Medicine, University of Leipzig Medical Center, Leipzig, Germany |
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Abstract: | Aims/hypothesis Levels of retinol binding protein (RBP4) are increased in the serum of insulin-resistant human subjects even before overt
diabetes develops. RBP4 levels correlate with insulin resistance, BMI, WHR, dyslipidaemia and hypertension. Improvement of
insulin sensitivity with exercise training is associated with reduction in serum RBP4 levels. Therefore serum RBP4 may be
useful for early diagnosis of insulin resistance and for monitoring improvements in insulin sensitivity. We sought to determine
the performance of assays for this application.
Subjects and methods We compared quantitative western blotting and three commercially available multiwell immunoassays in parallel measurements
of RBP4 concentrations in serum from insulin-sensitive subjects and from insulin-resistant subjects with impaired glucose
tolerance or type 2 diabetes.
Results The assays yielded different absolute values and magnitudes of elevation of serum RBP4. Western blotting and a sandwich ELISA
reported RBP4 concentrations that highly inversely correlated with insulin sensitivity measured by euglycaemic–hyperinsulinaemic
clamp. However, western blotting yielded concentrations with a greater dynamic range and less overlap between control and
insulin-resistant subjects. Two competitive enzyme-linked immunoassays undervalued serum RBP4 concentrations in insulin-resistant
subjects, possibly due to assay saturation. Poor linearity of dilution also limited assay utility. All assays tested exhibited
greater immunoreactivity with urinary (C-terminal proteolysed) RBP4 than with full-length RBP4, the predominant form in serum.
Conclusions/interpretations These findings support the use of quantitative western blotting standardised to full-length RBP4 protein as a ‘gold standard’
method for measuring serum RBP4 in insulin-resistant states. Other assays should use full-length RBP4 and be extensively cross-validated
using other methods. |
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Keywords: | Adipokine Insulin-resistance Metabolic syndrome Obesity Type 2 diabetes Vitamin A |
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