ABSTRACT
Retinol-binding protein 4 (RBP4) is synthesized in the liver as the carrier for Vitamin A. RBP4 was identifi ed as an adipokine when adipose tissue expression in adipose-tissue-specifi c GLUT4 knockout mice was increased and transgenic overexpression of RBP4 induced muscle and liver insulin resistance (IR). In humans, its expression was found increased in obese visceral adipose tissue along with IR. Clinical investigations have yielded confl icting results on the relationship between RBP4 and IR. Renal function as assessed by glomular fi ltration rate seems to be an important variable infl uencing RBP4 serum concentrations. Likewise, interactions between RBP4 and steroid hormone abnormalities need to be taken into account as potential confounders when studying RBP4 as an adipokine. Clinically, measurement of RBP4 serum concentrations has been used to assess nutritional status, and calorie excess as well as defi cit may infl uence serum and adipose tissue concentrations independently of IR. RBP4 gene transcription is regulated in a complex manner where the involved transcription factors also infl uence glucose metabolism genes and steroid hormones. In summary, although there is evidence that RBP4 serves as an adipokine in mice, its role in human health or disease is not clear.
