BREAST CANCER RESISTANCE PROTEIN - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 325

Krishnamurthy and colleagues were the first to demonstrate that hypoxia regulates
ABCG2 expression. They observed a hypoxia responsive element in the ABCG2
promoter and found expression to be regulated by the HIF-1 pathway, suggesting that
this mechanism minimized the detrimental effects of excess porphyrins such as heme
during hypoxia. 54 Up-regulation of ABCG2 expression was demonstrated in a number
of cell lines exposed to hypoxic conditions, and increased transport of Hoechst 33342
was also observed. 54 Stem cells or tumor cells in hypoxic environments may therefore
be protected by high levels of ABCG2. Elucidation of the genomic structure of the
ABCG2 gene and a better characterization of the mechanisms controlling ABCG2
gene expression are warranted.
Interestingly, Akt has been shown to be involved in controlling ABCG2 expression
at the cell membrane. Mogi and colleagues first noted that Akt1-null mice displayed
a reduced number of SP cells in their bone marrow. 55 When they treated bone marrow
cells with the phosphatidylinositol 3-kinase inhibitor LY294002, in effect an Akt in-
hibitor, they also noted fewer SP cells. Treatment with LY294002 was found to cause
a shift of Abcg2 from the membrane to the cytoplasm, resulting in fewer SP cells. 55
Takada and colleagues confirmed the role of Akt in regulating surface expression of
ABCG2, reporting decreased surface expression of the protein in ABCG2-transfected
LLC-PK1 cells treated with LY294002 compared to untreated cells. 56 The exact mech-
anism by which Akt controls cell surface ABCG2 localization has yet to be elucidated.
The mechanisms described to date are most likely to be involved in the regulation of
ABCG2 expression in normal tissues. In cancer cells, much work remains to be done
to determine the mechanism of activation. Using Pgp as a model, gene amplification
is unlikely to occur in the clinical setting. However, gene rearrangement and gene
capture by aberrant promoters have been described for Pgp and could also be true for
ABCG2. 57

12.5. SUBSTRATES OF ABCG2

As ABCG2 was cloned originally from drug-resistant cancer cells, chemotherapeutic
agents were among the first compounds identified as ABCG2 substrates. A high level
of resistance to mitoxantrone is a hallmark of cells that overexpress ABCG2, but
the range of substrates now reported may well be comparable to that observed for
Pgp. 9 , 12 The mitoxantrone-related fluorescent compound BBR3390 was also one of
the earlier identified substrates of the protein. 58 Cross-resistance to anthracyclines,
etoposide, teniposide, and the camptothecin derivatives topotecan, irinotecan, and
SN-38 (the active metabolite of irinotecan) is observed in ABCG2-overexpressing,
drug-selected cancer cells. 3 , 4 , 6 , 7 , 11 , 59 The topoisomerase active drugs J-107088 and
NB-506 are readily transported by ABCG2. 60 Cancer cells selected in topotecan 61 , 62
or SN-38 63 have been shown to up-regulate ABCG2 as a mechanism of resistance.
Even selection with camptothecins that are relatively poor ABCG2 substrates, such
as DX-8951f and BNP-1350, results in ABCG2 overexpression. 64 − 66
Overexpression of ABCG2 has also been shown to confer resistance to

methotrexate, 67 although no data exist showing up-regulation of ABCG2 when