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1 Division of Endocrinology, Diabetes & Nutrition, University of Maryland, Baltmore, MD, USA
2 Division of Gerontology, University of Maryland, Baltmore, MD, USA
3 Obesity and Diabetes Research Center, University of Maryland, Baltmore, MD, USA
4 Division of Endocrinology, Diabetes & Nutrition, University of Maryland, Baltmore, MD, USA; Geriatrics Research and Education Clinical Center, Baltimore VA Medical Center, Baltimre, MD, USA
5 Division of Gerontology, University of Maryland, Baltmore, MD, USA; Geriatrics Research and Education Clinical Center, Baltimore VA Medical Center, Baltimre, MD, USA
* To whom correspondence should be addressed. E-mail: dgong{at}medicine.umaryland.edu.
Central (visceral) obesity is more closely associated with insulin resistance, type 2 diabetes and cardiovascular disease than is peripheral (subcutaneous) obesity, but the underlying mechanism for this pathophysiologic difference is largely unknown. To understand the molecular basis of this difference, we sequenced 10,437 expressed sequence tags (ESTs) from a human omental fat cDNA library and discovered a novel visceral fat depot-specific secretory protein, which we have named omentin. Omentin ESTs were more abundant than many known adipose genes, such as perilipin, adiponectin and leptin in the cDNA library. Protein sequence analysis indicates that omentin mRNA encodes a peptide of 313 amino acids, containing a secretory signal sequence and a fibrinogen-related domain. Northern analysis demonstrated that omentin mRNA was predominantly expressed in visceral adipose tissue and was barely detectable in subcutaneous (sc) fat depots in humans and rhesus monkeys. Quantative real-time PCR showed that omentin mRNA was expressed in stromal vascular cells, but not fat cells, isolated from omental adipose tissue, with > 150-fold less in sc cell fractions. Accordingly, omentin protein was secreted into the culture media of omental, but not sc, fat explants. Omentin was detectable in human serum by Western blotting analysis. Addition of recombinant omentin in vitro did not affect basal but enhanced insulin-stimulated glucose uptake in both sc (+47%, n=9, p =0.003) and omental (n=3, +~30%, p<0.05) human adipocytes. Omentin increased AKT phosphorylation in the absence and presence of insulin. In conclusion, omentin is a new adipokine that is expressed in omental adipose tissue in humans and may regulate insulin action.
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