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Functional expression of the multidrug resistance protein 1 in microglia
Shannon Dallas1, Xiaoping Zhu, Sylvain Baruchel
1Department of Pharmaceutical Sciences, Faculty of Pharmacy, University of Toronto, Ontario, Canada.
Abstract:
Brain expression of the multidrug resistance proteins (MRPs), a collection of membrane-associated ATP-dependent efflux transporters, is poorly understood. Although several studies have examined the expression of these proteins within the brain barriers (i.e., the blood-brain barrier and choroid plexus), little information is available with respect to brain parenchyma cells such as microglia and astrocytes. Because microglia are the primary brain cells infected by the human immunodeficiency virus type 1 (HIV-1), MRP1 expression within microglia may contribute to lower brain accumulation of anti-HIV drugs. To examine the expression pattern of MRP1 within microglia, we performed reverse transcriptase-polymerase chain reaction analysis and Western blotting on a rat brain microglia cell line MLS-9, and in primary cultures of rat microglia. Both rat MRP1 (rMPR1) mRNA and protein were expressed in the cell line, as well as the primary cultures. We then characterized rMRP1-mediated transport properties in MLS-9 cells using [3H]vincristine, a known MRP1 substrate. Vincristine accumulation by monolayers of MLS-9 cells increased significantly in the presence of several well established MRP1 inhibitors (MK571, genistein, sulfinpyrazone, probenecid, and indomethacin), protease inhibitors, or the ATPase inhibitor sodium azide. In addition, vincristine accumulation was significantly modulated by altering the intracellular concentration of the reduced form of glutathione, further suggesting the involvement of rMRP1-mediated transport. These results provide strong evidence that the MRP1 protein is both expressed and functional in microglia cells. They also suggest that brain parenchyma can act as a "second" barrier to drug permeability and regulate brain distribution/accumulation of various xenobiotics, including protease inhibitors.
Insights
Multidrug resistance-associated protein 1 (MRP1) is expressed and functional in microglia. This finding suggests microglia may limit anti-HIV drug penetration into the brain parenchyma, impacting treatment efficacy.
Area of Science:
- Neuroscience
- Pharmacology
- Cell Biology
Background:
- Brain expression of multidrug resistance-associated proteins (MRPs) is not well understood.
- Microglia, key cells in the brain parenchyma, are infected by HIV-1.
- MRP1 expression in microglia may affect anti-HIV drug accumulation in the brain.
Purpose of the Study:
- To investigate the expression and function of MRP1 in microglia.
- To determine if MRP1 in microglia influences drug transport.
Main Methods:
- Reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting were used to detect MRP1 in rat microglia (MLS-9 cell line and primary cultures).
- Transport assays with [3H]vincristine were performed to assess MRP1 function.
- Inhibitors of MRP1, protease inhibitors, and ATPase inhibitors were used to characterize transport.
Main Results:
- Rat MRP1 (rMRP1) mRNA and protein were detected in both microglia cell line and primary cultures.
- Vincristine accumulation in microglia was significantly increased by MRP1 inhibitors, protease inhibitors, and sodium azide.
- Glutathione levels modulated vincristine accumulation, confirming MRP1-mediated transport.
Conclusions:
- The multidrug resistance-associated protein 1 (MRP1) is expressed and functionally active in microglia.
- Microglia-expressed MRP1 may act as a barrier, regulating xenobiotic and drug distribution within the brain parenchyma.
- This highlights the potential role of microglia in limiting the efficacy of certain medications, including anti-HIV drugs.