BREAST CANCER RESISTANCE PROTEIN - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 324

were identified at this position, and methotrexate, for example, was found to be
transported only by the wild-type protein. 45 The latter findings support a hypothesis
that an arginine at this position is required to transport a yet unidentified physiologic
substrate of ABCG2, giving the arginine a selective advantage, despite the gain-of-
function effect observed when replacing it with multiple other residues.
The nucleotide-binding domain of ABCG2 shows greater sequence similarity to
other ABC transporters than does TM domain. This segment of the protein contains the
characteristic Walker A and Walker B motifs and the C signature sequence that defines
ABC transporters. Mutating a conserved lysine to methionine (K86M) in the Walker
A motif, as in other ABC transporters, 46 renders the protein catalytically inactive
with an intact dimerization and trafficking pattern. 23 , 47 This mutant, when cotrans-
fected with the wild-type protein, reduced its activity in a dominant negative manner,
providing further evidence that ABCG2 functions as a homodimer. Similarly, a muta-
tion (D210N) introduced in the Walker B motif, supposedly involved in magnesium
binding, abrogates transport activity despite intact trafficking to the cell surface. 22
12.4. FACTORS CONTROLLING ABCG2 EXPRESSION
Beyond recent studies suggesting that ABCG2 expression may be regulated by sex
hormones or hypoxia, little is known about the molecular mechanisms controlling
ABCG2 expression. An estrogen response element was identified in the ABCG2 gene
by electrophoretic mobility shift assay and luciferase reporter gene assay. 48 Contra-
dictory results have been obtained regarding the regulation of ABCG2 by steroid
hormones. Up-regulation by physiologically relevant quantities of estradiol was ob-
served in one study, while in another, estrogen was shown to induce posttranscriptional
down-regulation of ABCG2 in estrogen receptor-positive cell lines. 48 , 49 Wang and
colleagues demonstrated that progesterone increased and estradiol decreased ABCG2
expression and function in placental BeWo cells 50 ; however, Yasuda et al. found that
progesterone decreased ABCG2 expression in the same cells. 51 A study of ABCG2
expression in rat and mouse tissues attributed high ABCG2 expression in the male
rat kidney to the suppressive effects of estradiol, while high ABCG2 expression in
the mouse liver was due to the inducive effects of testosterone. 52 Merino et al. re-
ported that Abcg2 expression in the liver was higher in male mice than in female
mice by a factor of 2 to 3, and the biliary extrusion of the Abcg2 substrates nitro-
furantoin and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) were nine-
and twofold higher in males. 53 In a small series of human liver tissues examined
for ABCG2 expression by Western blot analysis, expression levels were found to be
higher in men than in women, although the levels were not quantified. 53 However,
since a greater sensitivity in women to toxic agents has not been demonstrated, it
remains to be determined whether women have other mechanisms of protection to
endogenous and exogenous toxins that compensate for their relatively lower levels of
ABCG2 in the liver. Despite the conflicting results, all of the studies mentioned above
do point to the involvement of sex hormones in the regulation of ABCG2 expression.