Immnohistochemical and ultrastructural localization of leptin and leptin receptor in human white adipose tissue and differentiating human adipose cells in primary culture

Leptin is mainly produced in white adipose tissue and acts both at distant sites and locally at the tissue from which it originates. The cellular and subcellular localization of leptin and its receptor (Ob-receptor [Ob-R]) an d their relationship to various stages of fat cell maturation have not been characterized as Set. Therefore, we analyzed leptin and Ob-R by using reve rse transcriptase-polymerase chain reaction, immunohistochemistry, and ultr astructural immunogold labeling in human white adipose tissue and in human adipocyte cell cultures at early and late stages of differentiation. Both l eptin and its receptor were present in mature unilocular fat cells. The thi n cytoplasmic rim of the adipocytes exhibited the strongest expression of b oth leptin and Ob-R. At early stages of differentiating human adipocytes, l eptin was mainly expressed in multilocular preadipocytes, whereas the Ob-R was found predominantly on fibroblast-like cells. Other cellular components of human white adipose tissue were characterized by anti-CD31 for endothel ial cells, anti-CD68 for macrophages, and antibodies specifically labeling B-cells and T-cells. In addition to fat cells, endothelial cells were immun opositive for the full-length leptin receptor. On the ultrastructural level , leptin was mainly found attached to cellular membranes and in small alveo late vesicle-like structures in the cytoplasm of adipocytes. Leptin was als o present on the cell membranes of endothelial cells and macrophages. We co nclude that the expression of the Ob-R in human white adipose tissue is not restricted to adipocytes but is present in resident endothelial and immune cells. Ultrastructural localization studies revealed an association of lep tin with cell membranes and small vesicles. The cellular and subcellular di stribution of leptin and its receptor suggests an important autocrine and p aracrine role for leptin in human adipose tissue.