Fellowship Bing Sun: Class B Scavenger Receptor: Defining Receptor and Ligand Trafficking

Macrophage accumulation of lipid is a key event in the initiation of atherosclerosis. This proposal focuses on lipid accumulation mediated by the class B scavenger receptors SR-BI and C036. SR-BI and C036 bind HOl and oxlOl with high affinity. The metabolic fate of HOl and oxlOl after SR-BI or C036 binding appears to be distinct. SR-BI mediates selective uptake of cholesterol ester (CE) from HOl, a process in which cellular uptake of CE exceeds that of protein. The mechanism remains controversial. One model suggests that HOl particles undergo SR-BI-dependent internalization into an intracellular compartment containing transferrin. To date it is. not clear to what extent SR-BI and HOl internalization playa role in selective CE uptake. Scavenger receptor class B type II (SR-BII), which has a distinct cytoplasmic C-terminal tail from SR-BI and mediates less efficient selective uptake, traffics differently than SR-BI. C036 is another member of the scavenger receptor type B family that does not mediate efficient CE uptake, despite high affinity HOl binding. C036 mediates uptake and degradation of oxlOL. SR-BI binds oxlOl; it does not mediate degradation of oxlOL. We plan to quantify SR-BI, SR-BII and C036-dependent HOl and oxlOl internalization and resecretion, and define intracellular trafficking of SR-BI, SR-BII and C036 following HOl or oxlOl binding. Studies will be performed in transfected cas and macrophage cells using biotinylated 125/ labeled ligand or ligands or receptors tagged with fluorescent probes. We also propose to define the role of the cytoplasmic C-terminal tail in determining the intracellular trafficking of class B scavenger receptors and bound ligand by using modified SR-BIIC036 receptors with deleted or swapped C-terminal cytoplasmic tails. Our proposed studies will illuminate how non-polarized cells such as macrophages take up lipid from HOl and oxlOl via class B scavenger receptors and provide valuable information for further studies in polarized cells.