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P-glycoprotein (P-gp), an adenosine triphosphate (ATP)-binding cassette transporter which acts as a drug efflux pump, is highly expressed at the blood-brain barrier (BBB) where it plays an important role in brain protection. Recently, P-gp has been reported to be located in the caveolae of multidrug-resistant cells. In this study, we investigated the localization and the activity of P-gp in the caveolae of endothelial cells of the BBB. We used an in vitro model of the BBB which is formed by co-culture of bovine brain capillary endothelial cells (BBCEC) with astrocytes. Caveolar microdomains isolated from BBCEC are enriched in P-gp, cholesterol, caveolin-1, and caveolin-2. Moreover, P-gp interacts with caveolin-1 and caveolin-2; together, they form a high molecular mass complex. P-gp in isolated caveolae is able to bind its substrates, and the caveolae-disrupting agents filipin III and nystatin decrease P-gp transport activity. In addition, mutations in the caveolin-binding motif present in P-gp reduced the interaction of P-gp with caveolin-1 and increased the transport activity of P-gp. Thus, P-gp expressed at the BBB is mainly localized in caveolae and its activity may be modulated by interaction with caveolin-1.

The blood-brain barrier (BBB) performs a neuroprotective function by tightly controlling access to the brain; consequently it also impedes access of proteins as well as pharmacological agents to cerebral tissues. We demonstrate here that recombinant human melanotransferrin (P97) is highly accumulated into the mouse brain following intravenous injection and in situ brain perfusion. Moreover, P97 transcytosis across bovine brain capillary endothelial cell (BBCEC) monolayers is at least 14-fold higher than that of holo-transferrin, with no apparent intra-endothelial degradation. This high transcytosis of P97 was not related to changes in the BBCEC monolayer integrity. In addition, the transendothelial transport of P97 was sensitive to temperature and was both concentration- and conformation-dependent, suggesting that the transport of P97 is due to receptor-mediated endocytosis. In spite of the high degree of sequence identity between P97 and transferrin, a different receptor than the one for transferrin is involved in P97 transendothelial transport. A member of the low-density lipoprotein receptor protein family, likely LRP, seems to be involved in P97 transendothelial transport. The brain accumulation, high rate of P97 transcytosis and its very low level in the blood suggest that P97 could be advantageously employed as a new delivery system to target drugs directly to the brain.

1. The blood-brain barrier (BBB) contributes to brain homeostastis and fulfills a protective function by controlling the access of solutes and toxic substances to the central nervous system (CNS). The efflux transporter P-glycoprotein (P-gp) is a key element of the molecular machinery that confers special permeability properties to the BBB. 2. P-gp, which was initially recognized for its ability to expel anticancer drugs from multidrug-resistant cancer cells, is strongly expressed in brain capillaries. Its expression in the BBB limits the accumulation of many hydrophobic molecules and potentially toxic substances in the brain. 3. The purpose of this review is to summarize the current state of knowledge about the expression of P-gp, its cellular localization as well as its possible functions in the BBB.

Malignant brain tumors and brain metastases present a formidable clinical challenge against which no significant advances have been made over the last decade. Multidrug resistance (MDR) is one of the main factors in the failure of chemotherapy against central nervous system tumors. The MDR1 gene encoding P-glycoprotein (P-gp), a drug efflux pump which plays a significant role in modulating MDR in a wide variety of human cancers, is highly expressed in the blood-brain barrier (BBB). The BBB controls central nervous system exposure to many endogenous and exogenous substances. The exact molecular mechanisms by which the BBB is involved in the resistance of brain tumors to chemotherapy remain to be identified. The purpose of this review is to summarize reports demonstrating that P-gp, one of the most phenotypically important markers of the BBB, is present in primary brain tumors and thus plays a crucial role in their clinical resistance to chemotherapy.