The breast cancer resistance protein (BCRP/ABCG2) is a member of the ATP-binding cassette family of drug transporters and confers resistance to various anticancer drugs. To study the physiological and pharmacological functions of BCRP in vivo, we have previously generated mice with a deficiency in Bcrp1, the mouse homologue of BCRP. Although Bcrp1 knockout mice are viable and healthy, they display a pronounced increase in the oral availability and fetal penetration of substrate drugs such as the anticancer drug topotecan and the dietary carcinogen PhIP. Moreover, Bcrp1 knockout mice also exhibit several intriguing physiological phenotypes. First, they can show extreme photosensitivy as a result of increased exposure to the dietary chlorophyll breakdown product pheophorbide a. Secondly, Bcrp1 knockout mice display a novel type of protoporphyria, a group of metabolic disorders frequently associated with skin photosensitivity in patients. The protoporphyria in the Bcrp1 knockout mice is characterized by increased erythrocyte levels of the heme precursor and phototoxin protoporphyrin IX (PPIX). The protoporphyria appears to be bone marrow autonomous as transplantation of Bcrp1 knockout mice with wild-type bone marrow cured the protoporphyria. This indicates that the increased erythrocyte levels of PPIX primarily originate in the bone marrow during hematopoiesis and that they are sustained in the mature red blood cell. In line with this, it has been shown by others that Bcrp1 is expressed in hematopoietic stem cells and in erythroid precursor cells in the bone marrow where it is responsible for the “side-population” (SP) phenotype, associated with primitive stem cells and caused by the active extrusion of the fluorescent dye Hoechst 33342. Using Bcrp1/Mdr1a/1b triple-knockout (TKO) mice, we now show that in addition to Bcrp1 also Mdr1a/1b P-glycoprotein contributes to this phenotype. In the Bcrp1 knockout mice we observed a clearly reduced but still present SP, as compared to the wild-type mice, whereas in the TKO mice the SP was virtually absent. Although the physiological function of Bcrp1 and Mdr1a/1b in the hematopoietic (stem) cells is still unknown, our data indicate that Bcrp1 might be involved in the regulation of the porphyrin hemostasis, possibly via transport of PPIX. However, despite the high structural similarity between PPIX and pheophorbide a, which is an excellent substrate of Bcrp1/BCRP, PPIX appears not to be transported by Bcrp1. Recent findings in our lab now suggest that not PPIX itself but rather a conjugation product is transported. In conclusion, our data show that BCRP is important in processes involving handling of porphyrins and they indicate that humans or animals with low or absent BCRP activity may be at increased risk for developing protoporphyria.

[Proc Amer Assoc Cancer Res, Volume 45, 2004]