Gene: maps to 16p13.1. The gene spans at least 200 kb. It contains 31 exons and a high proportion of class 0 introns, alternative splicing of which results in significant levels of variant transcripts that maintain the original open reading frame of MRP-1 mRNA. (A class 0 splice occurs between 2 codons, e.g., ATG/ATG = Met.Met. A class 1 splice would be ATGA/TG and a class 2 splice would be ATGAT/G).
mRNA: size: 7.8 to 8.2 kb. Compared to
P-glycoprotein, MRP-1 displays a different tissue profile of expression. MRP-1 mRNA was found to be readily detectable in lung, testis, and peripheral blood mononuclear cells; inversely, MRP-1 transcripts were below the level of detection by Northern blot analysis in placenta, brain, kidney, salivary gland, uterus, liver, and spleen.
Protein:a 1522-amino acids, 190-kDa plasma membrane drug-efflux pump (Zaman G.J.R. et al., 1994) glycoprotein, member of the
ATP-binding cassette (ABC) superfamily of transporters. Inhibited by probenecid, not by verapamil (see
P-glycoprotein).
Cell lines:
- A MCF-7 subline was selected for resistance to etoposide (VP-16) by stepwise exposure to 2-fold increasing concentrations of this agent. This subline was 28-fold resistant to VP-16 and its expression of the MRP gene was increased at least 10-fold as compared to sensitive MCF-7 cells (Schneider E. et al., 1994).
- Three breast cancer cell (BCC) sublines were initially isolated as single clones from parental MCF-7, ZR-75, and MDA-MB-231 BCC lines cells by exposure to the chemotherapeutic drug VP-16 (etoposide). Subsequently, a population of cells from each subline was exposed to 3-fold higher drug concentrations, allowing stable sublines to be established at higher extracellular drug concentrations. With advancing resistance, MRP-1 expression increased and VP-16 accumulation decreased. In all three sublines, high levels of resistance were attained as a result of synergism between the reduced
topoisomerase II alpha levels and MRP overexpression (Matsumoto Y. et al., 1997).
Tumors:
- MRP-1 expression was studied in breast tumor biopsies using a quantitative RNase protection assay and immunohistochemistry. The tumors generally expressed low levels of MRP-1 mRNA, comparable to the levels found in normal tissues (Nooter K. et al., 1995).
- The frequency and intensity of MRP-1 protein expression was investigated by monoclonal antibody immunohistochemistry in a series of 259 resected invasive primary breast carcinomas. Overall, 34% of the tumours were positive for anti-MRP-1 antibody: 19% showed weak cytoplasmic staining, 14% had clear cytoplasmic staining and only 1% of the tumours had a strong cytoplasmic as well as membranous staining. MRP-1 expression was not related to patient's age, menopausal status, tumour size, differentiation grade, oestrogen and progesterone receptor level or lymph node involvement. However, MRP-1 was found to be of possible prognostic significance in the subgroups of patients with the more favourable prognosis, i.e. patients with small tumours and node-negative patients (Nooter K. et al., 1997).
- In a study (by RT-PCR) of 74 breast cancer surgical biopsies, obtained before any treatment, a clear relationship between the expression of MRP-1 and
glutathione-S-transferase p (GSTp) was found (Lacave R. et al., 1998).
- Quantitative RT-PCR analysis was performed to evaluate the expression of the MRP-1 and KAI-1 genes in 109 breast cancer patients. The results were confirmed with immunohistochemistry. 36 tumors were MRP-1-negative. The disease-free survival rate and the 5-year survival rate of patients with MRP-1-negative tumors were both significantly lower than that in patients with MRP-1-positive tumors. 65 tumors were
KAI1-negative. The disease-free survival rate of patients with
KAI1-negative tumors was significantly lower than that of patients with
KAI1-positive tumors. The disease-free survival rate and 5-year survival rate of patients with either MRP-1-negative or
KAI1-negative tumors were both significantly lower than patients who were positive for both genes (Huang C.I. et al., 1998).
- Using quantitative RT-PCR, MRP-1 expression was investigated in breast tumors (n = 74) and normal adjacent tissue (n = 55). MRP-1 expression did not exceed control cell line levels, and immunohistochemistry detected only moderate levels of expression (Dexter D.W. et al., 1998).
- The long-term prognostic value of tumoural
MDR1 and MRP-1, along with p53 and other classical parameters, was analysed on 85 node-positive breast cancer patients receiving anthracycline-based adjuvant therapy. All patients underwent tumour resection plus irradiation and adjuvant chemotherapy (the majority receiving fluorouracil-epirubicin-cyclophosphamide). Median follow-up for the 54 alive patients was 7.8 years. Mean age was 53.7 years (range 28-79) and 54 patients were post-menopausal.
MDR1 and MRP-1 expression were quantified according to an original reverse transcription polymerase chain reaction multiplex assay with colourimetric enzyme-linked immunosorbent assay detection (beta2-microglobulin as control). P53 protein was analysed using an immunoluminometric assay.
MDR1 expression varied within an 11-fold range (mean 94, median 83), MRP-1 within a 45-fold range (mean 315, median 242) and p53 protein from the limit of detection (0.002 ng mg
-1) up to 35.71 ng mg
-1 (mean 1.18, median 0.13 ng mg
-1). P53 protein was significantly higher in
oestrogen receptor (ER)-negative than in
ER-positive tumours. The higher the p53, the lower the
MDR1 expression. P53 was not linked to
progesterone receptor (PgR) status, S phase fraction, or MRP-1. Significantly greater
MDR1 expression was observed in grade I tumours. No relationship was observed between
MDR1 and MRP-1. Neither
MDR1 nor MRP-1 was linked to
ER or
PgR status. Unlike
MDR1, MRP-1 was correlated with the S phase: the greater the MRP-1, the lower the S phase. Univariate Cox analyses revealed that
MDR1, MRP-1, p53 and S phase had no significant influence on progression-free or specific survival (Ferrero J.M. et al., 2000).
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Cole S.P.C. et al., (1992) Overexpression of a transporter gene in a multidrug resistant human lung cancer cell line. Science 258, 1650-1654.
Dexter D.W. et al. (1998) Quantitative reverse transcriptase-polymerase chain reaction measured expression of MDR1 and MRP in primary breast carcinoma. Clin. Cancer Res. 4, 1533-1542.
Ferrero J.M. et al. (2000) Application of an original RT-PCR-ELISA multiplex assay for
MDR1 and MRP-1, along with p53 determination in node-positive breast cancer patients. Br. J. Cancer 82, 171-177.
Grant C.E. et al. (1997) Analysis of the intron-exon organization of the human multidrug-resistance protein gene (MRP) and alternative splicing of its mRNA. Genomics 45, 368-378.
Huang C.I. et al. (1998) Correlation of reduction in MRP-1/CD9 and KAI1/CD82 expression with recurrences in breast cancer patients. Am. J. Pathol. 153, 973-983.
Lacave R. et al.(1998) Comparative evaluation by semiquantitative reverse transcriptase polymerase chain reaction of MDR1, MRP and GSTp gene expression in breast carcinomas. Br. J. Cancer 77, 694-702.
Matsumoto Y. et al. (1997) Cellular adaptation to drug exposure: evolution of the drug-resistant phenotype. Cancer Res. 57, 5086-5092.
Nooter K. et al. (1995) Expression of the multidrug resistance-associated protein (MRP) gene in human cancers. Clin. Cancer Res. 1, 1301-1310.
Nooter K. et al. (1997) The prognostic significance of expression of the multidrug resistance-associated protein (MRP) in primary breast cancer. Br. J. Cancer 76, 486-493.
Schneider E. et al. (1994) Multidrug resistance-associated protein gene overexpression and reduced drug sensitivity of topoisomerase II in a human breast carcinoma MCF7 cell line selected for etoposide resistance. Cancer Res. 54, 152-158.
Zaman G.J.R. et al. (1994) The human multidrug resistance-associated protein MRP is a plasma membrane drug-efflux pump. Proc. Natl. Acad. Sci. 91, 8822-8826.
