Cholesterol-Dependent and -Independent CD40 Internalization and Signaling Activation in Cardiovascular Endothelial Cells

Abstract

Objective— It remains elusive how CD40 endocytosis or clustering on the cell surface is induced by different forms of CD40 agonist. This study aims to investigate whether lipid rafts differentially regulate CD40 traffic and signaling in proinflammatory activation of cardiovascular endothelial cells (ECs). Methods and Results— Using fluorescent microscopy and flow cytometry, we demonstrated that soluble CD40L and agonistic antibody G28.5 induced CD40 internalization via clathrin-independent pathway. Furthermore, depletion of cholesterol by methyl-β-cyclodextrin (MCD) or siRNA knockdown of caveolin-1 efficiently blocked CD40 internalization, suggesting that caveolae-rafts pathway regulates CD40 internalization. In contrast, a membrane-bound CD40L mimic (megamer) triggered aggregation of CD40 rafts outside of the conventional cholera toxin B subunit-positive lipid rafts resistant to cholesterol depletion. Finally, both G28.5 and megamer induced CD40 translocation to Brij58-insoluble, low buoyant density rafts, a movement insensitive to cholesterol depletion. However, MCD effectively inhibited G28.5 but not megamer-induced CD40 activation, and such inhibition could be alleviated by cholesterol reconstitution, suggesting that 2 different raft structures of CD40 induced by G28.5 or megamer possess differential sensitivity to cellular cholesterol levels in downstream signaling. Conclusions— Depending on different forms of agonist, CD40 uses either a cholesterol-dependent or -independent mode for trafficking and signaling in ECs. Although activated CD40 can translocate to lipid rafts despite cholesterol depletion, different forms of CD40L, either soluble or membrane-bound, required distinct membrane constituents and microdomains for CD40 internalization and signaling activation. In particular, stimulation with G28.5, but not membrane-bound CD40L, required cellular cholesterol for CD40 internalization and downstream signaling.