Scavenger receptor class B, type I (SR-BI) mediates the selective uptake of lipids from high density lipoproteins and is expressed in several types of tissues. However, to date little is known about its role in adipocytes. In this study, we investigated the cellular distribution of SR-BI in 3T3-L1 adipocytes and its regulation by hormones known to increase lipid storage such as angiotensin II (Ang II) and insulin. SR-BI was mainly distributed in the cytoplasm as determined by laser-scanning confocal analysis of the immunofluorescence labeling of SR-BI or the study of an enhanced green fluorescent protein-tagged SR-BI fusion protein. Exposure of cells to either insulin or Ang II (1-2 h) induced the mobilization of SR-BI from intracellular pools to the plasma membrane. This was further confirmed by Western blotting on purified plasma membrane and by fluorescence-activated cell sorter analysis of the SR-BI receptor. Similar results were also observed in primary adipocytes. We also demonstrated that, in the presence of either insulin or Ang II, SR-BI translocation to the cell membrane is functional, because insulin and Ang II induced a significant increase in the high density lipoprotein-delivered 22-(N-7-nitrobenz-2-oxa-1,3-diazo-4-yl)-amino-23,24-bisnor-5-cholen-3-ol uptake and in total cholesterol content. These data demonstrate that SR-BI can be acutely mobilized from intracellular stores to the cell surface by insulin or Ang II, two hormones that exert lipogenic effects in adipocytes. This suggests that SR-BI might participate in the storage of lipids in the adipose tissue. Scavenger receptor class B, type I (SR-BI)2 is a membrane protein that can bind various classes of modified and unmodified lipoproteins. High density lipoproteins (HDLs), to which SR-BI binds with high affinity, are now considered as its physiological ligands (1, 2). As a consequence of HDL binding, it has been shown that SR-BI mediates the selective uptake of cholesteryl esters, a process that clearly differs from the low density lipoprotein receptor endocytic pathway (3, 4). SR-BI is particularly abundant in several tissues, including the liver and steroidogenic tissue (4). Several studies have reported a determinant role for hepatic SR-BI in the control of HDL cholesterol. Thus, mice lacking a functional SR-BI gene have increased levels of plasma cholesterol, whereas overexpression of SR-BI in mouse liver by adenovirus injection lowers HDL levels and increases biliary cholesterol (5-8). Modulation of SR-BI expression in vivo has also revealed that in steroidogenic tissues this receptor plays a major role in delivering cholesterol to be used for the synthesis of steroid hormones (4). These studies have left unresolved the role of this receptor in adipose tissue known to express SR-BI (9). SR-BI is also expressed in adipocyte cell lines and is strongly induced during the course of adipose differentiation, suggesting a role for SR-BI in the fully differentiated adipocyte (9).To get some insight into the function of SR-BI in adipocytes, we st...