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Vol. 299, Issue 1, 204-212, October 2001
Department of Pharmaceutical Sciences, University of Toronto,
Toronto, Ontario, Canada (G.L., R.B.); Division of Cellular and
Molecular Biology, Toronto Western Research Institute, University
Health Network, and Department of Physiology, University of Toronto,
Toronto, Ontario, Canada (L.S.); and Département de Pathologie et
Biologie Cellulaire, Université de Montréal,
Montréal, Québec, Canada (M.B.)
In the central nervous system, the primary targets of the human
immunodeficiency virus-1 (HIV-1) are microglia, resulting in a disorder
called HIV-1 dementia. P-glycoprotein (P-gp), a membrane-associated
ATP-dependent efflux transporter, limits entry into the brain of
numerous xenobiotics, including anti-HIV drugs (i.e., protease
inhibitors). This project investigates the functional expression of
P-gp in the endogenous immune cells of the brain, a parenchymal
compartment not previously studied. We used a cell line (MLS-9) derived
from rat microglia to study the transport of digoxin, a known P-gp
substrate. Reverse transcriptase-polymerase chain reaction
analysis detected mRNA for only mdr1b in MLS-9 cells, whereas both
mdr1a and mdr1b mRNA were expressed in primary cultured microglia from
which they were derived. Western blot analysis with the C219 antibody
detected a single band at ~170 to 180 kDa in MLS-9 cells, which is
the size previously reported for P-gp. Immunocytochemical analysis with
the monoclonal antibodies C219, MRK16, and MAB-448 labeled P-gp protein
along the plasma membrane and nuclear envelope of MLS-9 cells.
[3H]Digoxin accumulation by monolayers of MLS-9 cells was
significantly enhanced in the presence of any of several P-gp
inhibitors (verapamil, cyclosporin A, quinidine, PSC 833), protease
inhibitors (i.e., saquinavir, indinavir, and ritonavir), and sodium
azide, an ATPase inhibitor. These results provide the first evidence
for the functional expression of P-gp in microglia and imply that entry
of pharmacological agents, including protease inhibitors, may be
prevented within the brain parenchyma, as well as at the blood-brain barrier.
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