Cholesterol oxidase and the hydroxymethylglutaryl coenzyme A reductase inhibitor mevinolin perturb endocytic trafficking in cultured vascular smooth muscle cells.

Cholesterol is a component of cellular membranes and especially abundant in caveolae (50-80 nm flask-shaped invaginations of the plasma membrane). Caveolae are highly numerous in vascular endothelial and smooth muscle cells and have been implicated in a variety of functions, including signal transduction, lipid transport and uptake of macromolecules. Here, the effects of cholesterol oxidase (an enzyme that oxidizes cholesterol in caveolae of living cells) and mevinolin (an inhibitor of cholesterol synthesis) on fine structure and internalization of exogenous markers were studied in rat aortic smooth muscle cells grown on a substrate of fibronectin in serum-free primary cultures. Cholesterol oxidase caused a growth in size of the endocytic compartment with accumulation of enlarged endosomes and lysosomes containing tracer molecules. In parallel, the number of caveolae was reduced by about one fifth. Moreover, the morphology of the Golgi complex was altered with swollen cisternae surrounded by empty-looking vacuoles. Mevinolin suppressed transition of the cells from a differentiated or contractile to a dedifferentiated or synthetic phenotype. In addition, contractile cells were found to ingest horseradish peroxidase (HRP) not only into endosomes and lysosomes but also into Golgi cisternae, especially on the convex/cis side of the stacks, and the endoplasmic reticulum. A similar pathway was noted in contractile cells exposed to cholera toxin B subunit (CTB)-HRP conjugates, a ligand that binds to ganglioside GM1 and at least in part is ingested via caveolae. Mevinolin did not prevent the transport of CTB-HRP to the Golgi complex, but the conjugates were in this case concentrated to the concave/trans side of the cisternal stacks. However, no clear effect on the number of caveolae was noted. The observations indicate an important role of cholesterol and caveolae in the control of endocytic traffic in smooth muscle cells. This function appears most significant when the cells are in a differentiated state.