uPA binding increases UPAR localization to lipid rafts and modifies the receptor microdomain composition

Area of Science:

• Cell Biology
• Biochemistry
• Membrane Biology

Background:

• Urokinase plasminogen activator receptor (uPAR) is a GPI-anchored protein found in plasma membrane microdomains.
• The precise localization and lipid environment of uPAR influence its function and downstream signaling.

Purpose of the Study:

• To investigate the role of the urokinase plasminogen activator (uPA) ligand on uPAR localization within lipid rafts.
• To analyze the composition of the lipid membrane microdomains occupied by uPAR.
• To determine the requirement of uPAR's lipid environment for uPA-induced intracellular signaling.

Main Methods:

• Preparation and fractionation of detergent-resistant membrane (DRM) fractions from HEK293-uPAR cells using Triton X-100 and sucrose gradient centrifugation.
• Isolation of uPAR-specific lipid microdomains via immunoprecipitation with an anti-uPAR antibody.
• Assessment of uPA-induced ERK phosphorylation in the presence of methyl-beta-cyclodextrin to disrupt lipid rafts.

Main Results:

• uPAR partially associates with DRMs, and this association is significantly increased upon ligand (uPA) binding, independent of uPA's catalytic activity.
• In the absence of ligands, uPAR resides in a lipid environment similar to total DRMs, rich in sphingomyelin and glycosphingolipids.
• Ligand binding (uPA or ATF) leads to a lipid environment for uPAR that is markedly depleted of neutral glycosphingolipids.

Conclusions:

• uPAR's association with lipid rafts is ligand-dependent and crucial for initiating intracellular signaling pathways.
• Ligand binding alters the specific lipid microenvironment surrounding uPAR, impacting its functional interactions.
• Disruption of lipid rafts using methyl-beta-cyclodextrin affects uPA-induced signaling, highlighting the importance of membrane microdomains.