MRP9, an unusual truncated member of the ABC transporter superfamily, is highly expressed in breast cancer

Abstract

Combining a computer-based screening strategy and functional genomics, we previously identified MRP9 (ABCC12), a member of the ATP-binding cassette (ABC) superfamily. We now show that the gene has two major transcripts of 4.5 and 1.3 kb. In breast cancer, normal breast, and testis, the MRP9 gene transcript is 4.5 kb in size and encodes a 100-kDa protein. The protein is predicted to have 8 instead of 12 membrane-spanning regions. When compared with closely related ABC family members, it lacks transmembrane domains 3, 4, 11, and 12 and the second nucleotide-binding domain. In other tissues including brain, skeletal muscle, and ovary, the transcript size is 1.3 kb. This smaller transcript encodes a nucleotide-binding protein of Ϸ25 kDa in size. An in situ hybridization study shows that the 4.5-kb transcript is expressed in the epithelial cells of breast cancer. An antipeptide antibody designed to react with the amino terminus of the protein detects a 100-kDa protein in testis and the membrane fraction of a breast cancer cell line. Because the 4.5-kb RNA is highly expressed in breast cancer and not expressed at detectable levels in essential normal tissues, MRP9 could be a useful target for the immunotherapy of breast cancer. Because of the unusual topology of the two variants of MRP9, we speculate that they may have a different function from other family members.

The MRP family of drug efflux pumps

“…The predominant working model is that agents such as vinca alkaloids and anthracyclines are cotransported with glutathione ( Figure 3b). This model is supported by studies showing that: (i) transport of vincristine and anthracyclines in membrane vesicle uptake assays is dependent upon glutathione (Loe et al, 1996b); (ii) etoposide stimulates glutathione extrusion in wild-type but not mrp1-deficient ES cells ; (iii) MRP1 can be photolabeled with glutathione analogs (Ciaccio et al, 1996); (iv) glutathione stimulates vanadate-induced trapping of 8-azido-ATP (Taguchi et al, 1997);and (v) glutathione derivatives lacking a free sulfhydryl group can support MRP1-mediated transport . Alternative possibilities involving allosteric interactions are also under consideration.…”

“…Functional studies on MRP9 have yet to be reported, but its structural resemblance to MRP4, MRP5 and MRP8 (Figure 1) raises the possibility that it may share some of the properties of the latter pumps. Two reports indicate that MRP9 transcript expression is relatively restricted, whereas a third study found more generalized expression (Tammur et al, 2001;Yabuuchi et al, 2001;Bera et al, 2002). MRP9 appears to be subject to an usual degree of alternative splicing, and this may account for differences in the results of RT/PCR analyses of expression.…”

“…MRP9 appears to be subject to an usual degree of alternative splicing, and this may account for differences in the results of RT/PCR analyses of expression. In addition, MRP9 transcript was also reported for nine of 12 breast cancer samples (Bera et al, 2002).…”

NGEP, a gene encoding a membrane protein detected only in prostate cancer and normal prostate

“…The in vitro translation of the NGEP cDNA was examined in an in vitro transcriptioncoupled translation system (TNT system, Promega) following the supplier's instruction. Analysis of the reaction mixture was performed as described (19).…”

Targeting TARP, a novel breast and prostate tumor‐associated antigen, with T cell receptor‐like human recombinant antibodies

“…Identification of new tumor-specific antigens is an essential step for the successful development of immunotherapy ap- [12][13][14]. One of these, T cell receptor gamma alternate reading frame protein (TARP), is expressed on breast and prostate cancer cells [15,16].…”