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

Abstract

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.

Structural Determinants of Imidazoacridinones Facilitating Antitumor Activity Are Crucial for Substrate Recognition by ABCG2

“…This negative sidechain effect on ABCG2 efflux activity may resemble the negative impact of folate and anti-folate poly-␥-glutamylation via folylpolyglutamate synthetase activity on ABCG2-mediated efflux (Shafran et al, 2005), demonstrating a general constraint on ABCG2 substrate size and bulk. This presumption is of interest because of the broad spectrum of large substrates that are efficiently extruded by ABCG2 including the chlorophyll derivative PhA and the hemoglobin component porphyrin (Jonker et al, 2002(Jonker et al, , 2007, both of which seem to exceed the typical size of IAs. The lack of significant ABCG2-mediated efflux competition between group A IAs and PhA observed in our current study supports a recent report suggesting multiple allosteric substrate-binding sites for ABCG2, based upon differential substrate interaction kinetics with this transporter (Clark et al, 2006).…”

Conductive carbon tape used for support and mounting of both whole animal and fragile heat-treated tissue sections for MALDI MS imaging and quantitation

“…1A.5 shows that m/z 563.538 is only detected within the spinal cord (distribution overlaid onto optical image), and in Fig. 1A.6 m/ z 563.288 (Protoporphyrin IX, identified based on accurate mass and spatial distribution) [27] localizes exclusively to the rat tear duct. Such closely associated masses would be hard to differentiate using lower spectral resolution instruments.…”

Functional expression and characterization in Xenopus laevis oocytes of the ABCG2 transporter derived from A549 human lung adenocarcinoma cells

“…These functions indicate a primary biological role for ABCG2 in the protection of these cells from xenobiotics. However, several other physiological functions have been observed, including the extrusion of porphyrins and/or porphyrin conjugates from hematopoietic cells, liver and harderian glands (26) as well as the secretion of vitamin B 2 and other vitamins into breast milk (27). Various compounds have been tested as inhibitors of the ABCG2 transporter, and estrogenic compounds, including estrone, several tamoxifen derivatives, phytoestrogens and flavonoids, have been shown to reverse ABCG2-mediated drug resistance (28).…”