SR-BI-directed HDL-cholesteryl ester hydrolysis

We have examined the metabolic fate of HDL cholesteryl ester (CE) delivered to cells expressing scavenger receptor class B type I (SR-BI). Comparison of SR-BI with a related class B scavenger receptor, CD36, showed a greater uptake and a more rapid and extensive hydrolysis of HDL-CE when delivered by SR-BI. In addition, hydrolysis of HDL-CE delivered by both receptors was via a neutral CE hydrolase. These data indicate that SR-BI, but not CD36, can efficiently direct HDL-CE to a neutral CE hydrolytic pathway. In contrast, LDL-CE was delivered and hydrolyzed equally well by SR-BI and CD36. Hydrolysis of LDL-CE delivered by SR-BI was via a neutral CE hydrolase but that delivered by CD36 occurred via an acidic CE hydrolase, indicating that SR-BI and CD36 deliver LDL-CE to different metabolic pathways. Comparison of inhibitor sensitivities in Y1-BS1 adrenal, Fu5AH hepatoma, and transfected cells suggests that hydrolysis of HDL-CE delivered by SR-BI occurs via cell type-specific neutral CE hydrolases. Furthermore, HDL-CE hydrolytic activity was recovered in a membrane fraction of Y1-BS1 cells. These findings suggest that SR-BI efficiently delivers HDL-CE to a metabolically active membrane compartment where CE is hydrolyzed by a neutral CE hydrolase.