Expression and functional characterization of ABCG2 in brain endothelial cells and vessels

Abstract

Delivery of drugs to the brain is impeded by the activity of efflux pumps expressed by endothelial cells of brain vasculature. The ATP binding cassette (ABC) transporters, among which ABCB1/MDR1 P-glycoprotein and ABCC1/multidrug resistance-associated protein 1 are expressed in brain endothelial cells, participate in drug efflux properties of the blood-brain barrier (BBB). Searches of the EST (expressed sequence tags) database with the conserved ABC domain, conducted to identify other ABC transporters expressed in the BBB, recovered 15 ABC transporter sequences expressed in human brain cDNA libraries. One of these sequences, identical to ABCG2, was highly expressed in cultured human cerebromicrovascular endothelial cells and human brain tissue at both mRNA and protein levels. Overexpression of human ABCG2 in immortalized rat brain endothelial cells resulted in enhanced polarized abluminal to luminal transport of various substrates tested in the in vitro BBB model. Brain vessels extracted from tissue sections of nonmalignant human brain and glioblastoma tumors by laser capture microdissection microscopy and analyzed by real-time polymerase chain reaction showed higher expression of ABCG2 relative to ABCB1/MDR1 and ABCC1/MRP1. ABCG2 was up-regulated in both glioblastoma vessels and parenchymal tissue. These studies suggest a role for brain endothelial ABCG2 transporter in modulating drug delivery to the brain and in conferring drug resistance to glioblastomas.

Polarized P-glycoprotein expression by the immortalised human brain endothelial cell line, hCMEC/D3, restricts apical-to-basolateral permeability to rhodamine 123

“…However, in contrast to the relatively modest increase in the efflux t 1/2 caused by another competitive inhibitor, verapamil, in that study (t 1/2 ~ 20 min), in the present study vinblastine caused a much more potent inhibition (t 1/2 ~ 110 min). Several factors may contribute to the differences in P-gp inhibition between this study and the one by Ji et al: 1) differences in P-gp expression levels between rat and hCMEC/D3 cells; 2) levels of ATP in the culture media; 3) differences in the method In agreement with other studies, FTC, a BCRP inhibitor, did not inhibit rh123 efflux (Weksler et al, 2005;Zhang et al, 2003). BCRP has been demonstrated to use rh123 as a substrate only when there is a mutation at position 482, characteristic of tumour cells (Alqawi et al, 2004;Sarkadi et al, 2004).…”

ATP-binding cassette, subfamily G (ABCG family)

“…As far as the blood-brain barrier is concerned, the importance of BCRP remains controversial. In contrast to in vitro data supporting the functional importance of ABCG2 in the blood-brain barrier [88], imatinib is the only compound exhibiting an increased brain concentration in Abcg2 knockout mice [11]. No change has been reported for other BCRP substrates, such as mitoxantrone [44], dehydroepiandrosterone sulfate (DHEAS) [44], and PhIP [75].…”

D2-40 antibody immunoreactivity in developing human brain, brain tumors and cultured neural cells

“…30 And more, it is reported nestin-positive human neural stem/progenitor cells highly express ATPbinding cassette (ABC) transporters, like ABCB1 31 or ABCG2 (breast cancer resistance protein), 32 that are generally expressed in endothelial cells in normal brains. 33,34 It is also reported nestin is expressed in proliferative endothelial cells. 35,36 Taken together, these previous findings and our present study suggest that both neural stem/progenitor cells and vascular and/or lymphatic endothelial cells share the same antigens and there might be some developmental analogies between them.…”