Abstract
P-glycoprotein, an active efflux pump of antitumor agents in multidrug-resistant tumor cells, exists in various normal tissues, including brain capillaries. To study the physiological function of P-glycoprotein expressed in brain capillary endothelium, we established nine mouse brain capillary endothelial cell (MBEC) lines and examined the transport of antitumor agents across the monolayer of MBEC epithelia. In the MBECs, the activities of alkaline phosphatase and gamma-glutamyl transpeptidase, specific markers for brain capillary endothelial cells, were about three times higher than those in other cells including human umbilical vein endothelial cells. By immunoblot analysis, P-glycoprotein was detected in all of the nine MBEC clones. The P-glycoprotein expressed in MBECs specifically bound [125I]iodoaryl azidoprazosin as that in multidrug-resistant cells, and efflux of vincristine was observed in the MBECs. When MBECs were grown on a porous filter membrane, they formed a monolayer of epithelium. By immunoelectron microscopic analysis, P-glycoprotein in MBEC epithelia was shown to be localized to the apical surface of the cells. Moreover, the unidirectional transepithelial transport of vincristine from basal side to apical side was demonstrated in vitro. These observations indicate that P-glycoprotein in brain capillary endothelium prevents vincristine from entering the central nervous system and thus may be one of the functional components of the blood-brain barrier.
Highlights
P-glycoprotein, an active efflux pump of antitumor the bile [17]
T o study the physiological function of P-glycoprotein ex- vitro culture of the cells did not seem to induce the expression pressed in brain capillary endothelial cells, we isolated capil- of P-glycoprotein because neither in vitro culture of human
We observed the expression of a 4.8- and 4.3-kb RNA species, which perhaps corresponds to the mdrlu and mdrlb genes in mouse brain, as did Croop et al [39]
Summary
Characterization of MBECs-We established eight clones of mouse brain capillary endothelial cells from BALB/c mice (MBECOM, BEC1M, BEC2M, BEC3M, BEC4M, BEC5, MBEC6, and MBEC7) and one clone from C57BL/6 mice (MBEC C-1) as described under “Materials and Methods.” Respectively,but most of the mouse peritoneal adherent cells expressed the Mac-1 antigen (Fig. 28’). Retained significantly higher enzymaectivities of brain capil- Three different sizes (4.8,4.3,and 4.0 kb) of mdr RNA species lary endothelial cells than HUEC and NL-17cells. These were observed as reported for normal mouse tissues [39].
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