Omentin

Omentin was originally identified as a human lectin binding to galactofuranosyl residues and named intelectin.

Omentin Structural and Functional Relations

The mature omentin is a secretory glycoprotein consisting of 295 amino acids and N-linked oligosaccharides, and its basic structural unit is a 120– kDa homotrimer in which 40-kDa polypeptides are bridged by disulfide bonds.

Recombinant omentin is a trimer, disulfide-linked through Cys-31 and Cys- 48, and N-glycosylated at Asn-163. Despite 84.9% amino acid identity to omentin, recombinant and intestinal murine mITLN-1 are unglycosylated 30– kDa monomers.

The omentin gene is split into 8 exons on chromosome 1q21.3, and omentin mRNA is expressed in the heart, small intestine, colon, and thymus. Omentin showed high levels of homology with mouse intelectin, Xenopus laevis cortical granule lectin/oocyte lectin, lamprey serum lectin, and ascidian galactose-specific lectin. These homologues commonly contained no carbohydrate recognition domain, which is a characteristic of C-type lectins, although some of them have been reported as Ca(2+)-dependent lectins. Recombinant omentin revealed affinities to d-pentoses and a d-galactofuranosyl residue in the presence of Ca(2+), and recognized the bacterial arabinogalactan of Nocardia containing d-galactofuranosyl residues. These results suggested that omentin is a new type lectin recognizing galactofuranose, and that omentin plays a role in the recognition of bacteria-specific components in the host.

Later on, it was found that it is expressed in Paneth and goblet cells of the small intestine and proposed to serve a protective role in the innate immune response to parasite infection. In addition, it is structurally identical to the intestinal lactoferrin receptor known to reside in the enterocyte brush border.

Omentin and Adipose Tissue

An independent study reported a novel visceral fat depot-specific secretory protein, which was named omentin. Protein sequence analysis indicated 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 subcutaneous cell fractions. Accordingly, omentin protein was secreted into the culture medium of omental, but not subcutaneous, fat explants. Omentin was detectable in human serum by Western blot analysis.

Omentin Action

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 (approximately 30%, n = 3, P < 0.05) human adipocytes. Omentin increased Akt phosphorylation in the absence and presence of insulin. These data suggenst that omentin is a new adipokine that is expressed in omental adipose tissue in humans and may regulate insulin action.

Omentin in Circulation

Lean subjects had significantly higher plasma omentin levels than obese and overweight subjects. In addition, higher plasma omentin levels were detected in women compared with men. Plasma omentin levels were inversely correlated with BMI, waist circumference, leptin levels, and insulin resistance as measured by homeostasis model assessment and positively correlated with adiponectin and HDL levels. Omentin gene expression was decreased with obesity. In summary, decreased omentin levels are associated with increasing obesity and insulin resistance. Therefore, omentin levels may be predictive of the metabolic consequences or co-morbidities associated with obesity.

Immunoblot analysis revealed no alterations when plasma levels before and 2 h after glucose intake were compared.

Omentin and Polycystic Ovary Syndrom

In polycystic ovary syndrom (PCOS) women decreased plasma and adipose tissue omentin was foound. Furthermore, in omental adipose tissue explants, insulin and glucose significantly dose-dependently decreased omentin mRNA expression, protein levels and secretion into conditioned media (P < 0.05, P < 0.01). Also, hyperinsulinemic induction in healthy subjects significantly reduced plasma omentin levels (P < 0.01). These findings reveal that omentin is down regulated by insulin and glucose. These may in part explain the decreased omentin levels observed in overweight PCOS women.

Omentin and The Respiratory Tract

In man, omentin/intelectin expression has been shown in respiratory tract epithelium, with upregulation occurring in bronchoalveolar lavage fluid of asthmatic individuals.

It was shown that when sheep tracheal explants were cultured for 48h+/- recombinant sheep IL-4, sheep ITLN transcripts were upregulated compared with controls. Putative roles for omentin/intelectin have included an antibacterial role and an alteration of the character of mucus. These data suggest that omentin/intelectin may play an important role in the mucosal response in allergy and parasitic infections.

Omentin and Cancer

Omentin/Intelectin gene overexpression (>139-fold) was found in malignant pleural mesothelioma (MPM). Online SAGE datasets revealed omentin/intelectin to be consistently present in mesothelioma(s), ovarial cancer, and colon cancer. Omentin mRNA expression was found by RT-PCR in 4 of 5 resected MPM tumors, and Omentin/Intelectin protein expression was confirmed by immunohistoche­mistry in 28 of 53 MPM tumors, and in 4 of 4 mesothelioma cell lines studied by Western blot. A marked induction in omentin/intelectin gene expression was observed among human primary mesothelial cells as a consequence of crocidolite asbestos exposure and simian virus 40 infection. Omentin/Intelectin overexpression in mesothelioma could have potential screening, and therapeutic implications.

Catalog Number	Protein	Source	Size
RD172100100	Omentin (Intelectin-1, Intestinal lactoferrin receptor, Galactofuranose-binding lectin, Endothelial lectin HL-1) Human (E. coli)	E. coli	0.1 mg
RD172100100+	Omentin (Intelectin-1, Intestinal lactoferrin receptor, Galactofuranose-binding lectin, Endothelial lectin HL-1) Human (E. coli)	E. coli	10 x 0.1 mg