Cyclosporin A (CsA) is an
immunosuppressant that inhibits
protein phosphatase 2B (PP2B/
calcineurin) and is associated with
hyperlipidemia, decreased
cholesterol efflux via
ATP-binding cassette transporter A1 (ABCA1), and increased risk of
atherosclerosis.
Apolipoprotein E (
apoE) is an important regulator of lipid metabolism and
atherosclerosis, the secretion of which from human macrophages is regulated by the
serine/threonine protein kinase A (PKA) and intracellular
calcium (Ca(2+)) (Kockx, M., Guo, D. L., Huby, T., Lesnik, P., Kay, J., Sabaretnam, T., Jary, E., Hill, M., Gaus, K., Chapman, J., Stow, J. L., Jessup, W., and Kritharides, L. (2007) Circ. Res. 101, 607-616). As PP2B is Ca(2+)-dependent and has been linked to PKA-dependent processes, we investigated whether CsA modulated
apoE secretion. CsA dose- and time-dependently inhibited secretion of
apoE from primary human macrophages and from Chinese hamster ovary cells stably transfected with human
apoE and increased cellular
apoE levels without affecting
apoE mRNA. [(35)S]Met kinetic modeling studies showed that CsA inhibited both secretion and degradation of
apoE, increasing the half-life of cellular
apoE 2-fold. CsA also inhibited secretion from primary human
Tangier disease macrophages and from mouse macrophages deficient in ABCA1, indicating that the effect is independent of the known inhibition of ABCA1 by CsA. The role of PP2B in mediating
apoE secretion was confirmed using additional
peptide and chemical inhibitors of PP2B. Importantly, kinetic modeling, live-cell imaging, and confocal microscopy all indicated that CsA inhibited
apoE secretion by mechanisms quite distinct from those of PKA inhibition, most likely inducing accumulation of
apoE in the endoplasmic reticulum compartment. Taken together, these results establish a novel mechanism for the pro-atherosclerotic effects of CsA, and establish for the first time a role for PP2B in regulating the intracellular transport and secretion of
apoE.