Breast cancer resistance protein (Bcrp1/Abcg2) is expressed in the harderian gland and mediates transport of conjugated protoporphyrin IX.

Proper regulation of intracellular levels of protoporphyrin IX (PPIX), the direct precursor of heme, is important for cell survival. A deficiency in ferrochelatase, which mediates the final step in heme biosynthesis, leads to erythropoietic protoporphyria (EPP), a photosensitivity syndrome caused by the accumulation of PPIX in the skin. We have previously shown that mice with a deficiency in the ABC transporter Bcrp1/Abcg2 display a novel type of protoporphyria. This protoporphyria is mild compared with ferrochelatase-dependent EPP, and in itself not sufficient to cause phototoxicity, but it might exacerbate the consequences of other porphyrias. In this study, we identified the mouse harderian gland as a novel expression site of Bcrp1. Because of its pronounced role in porphyrin secretion, the harderian gland presents a useful tool to study the mechanism of Bcrp1-related protoporphyria and transport of porphyrins. Bcrp1(-/-) harderian gland displayed a highly increased accumulation of PPIX glycoconjugates, and a similar shift was seen in Bcrp1(-/-) liver. Tear- and hepatobiliary excretion data suggest that Bcrp1 controls intracellular levels of PPIX by mediating high affinity transport of its glycoconjugates and possibly low-affinity transport of unconjugated PPIX. This mechanism may allow cells to prevent or reduce cytotoxicity of PPIX under excess conditions, without spillage under physiological conditions where PPIX is needed.

Contribution of the ABC transporters Bcrp1 and Mdr1a/1b to the side population phenotype in mammary gland and bone marrow of mice.

The ability of cells to export Hoechst 33342 can be used to identify a subpopulation of cells (side population [SP]) with characteristics of stem cells in many tissues. The ATP-binding cassette transporters Bcrp1 (Abcg2) and Mdr1a/1b (Abcb1a/1b) have been implicated as being responsible for this phenotype. To further explore the involvement of these transporters in the SP phenotype, we have generated Bcrp1/Mdr1a/1b triple knockout mice and studied the effect of their absence on the SP in bone marrow and mammary gland. Whereas in bone marrow Bcrp1 was almost exclusively responsible for the SP, both transporters contributed to the SP phenotype in the mammary gland, where their combined absence resulted in a nearly complete loss of SP. Interestingly, bone marrow of Mdr1a/1b-/- mice frequently displayed an elevated SP, which was reversible by the Bcrp1 inhibitor Ko143, suggesting that Bcrp1 can compensate for the loss of Mdr1a/1b in bone marrow.

The breast cancer resistance protein BCRP (ABCG2) concentrates drugs and carcinogenic xenotoxins into milk.

Contamination of milk with drugs, pesticides and other xenotoxins can pose a major health risk to breast-fed infants and dairy consumers. Here we show that the multidrug transporter BCRP (encoded by ABCG2) is strongly induced in the mammary gland of mice, cows and humans during lactation and that it is responsible for the active secretion of clinically and toxicologically important substrates such as the dietary carcinogen PhIP, the anticancer drug topotecan and the antiulcerative cimetidine into mouse milk.