We have isolated the full-length cDNA for human ATP-binding cassette, sub-family A, member 2 transporter (ABCA2). The ORF of this cDNA encodes a protein consisting of 2436 amino acids with apparent ...molecular weight of M(r) 270,000. Accordingly, ABCA2 is the largest known mammalian ABC transporter described thus far. Analysis of mRNA expression levels indicated that ABCA2 is highest in human brain and has a broad expression pattern in a panel of tumor cell lines. Using specific antibodies to ABCA2 and various organelle marker proteins, ABCA2 was found to colocalize with the lysosomal/endosomal marker LAMP1, forming discrete, punctate intracellular vesicles. In ABCA2-transfected cells, the transporter also colocalized with a fluorescently labeled steroid analogue, estramustine. The sequestration of the steroid into the lysosomal/endosomal compartment indicates a potential substrate specificity for ABCA2. Furthermore, the presence of a lipocalin signature motif in the ABCA2 sequence suggests a possible broad role for this protein in the transport of steroids, lipids, and related molecules.
: Pseudoxanthoma elasticum (PXE) is a heritable disorder characterized by ectopic mineralization of connective tissues, with considerable intra‐ and interfamiliar phenotypic variability. PXE is ...caused by mutations in the ABCC6 gene, which encodes a transporter protein, MRP6, and targeted ablation of Abcc6 in mice recapitulates the manifestations of PXE. In this study, we examined the hypothesis that the expression of other members of the Abcc family may be altered in Abcc6 null mice, possibly explaining the phenotypic variability because of the functional overlap of these transporters. Analysis of the transcript levels of Abcc1–10 and 12 in the liver of Abcc6 −/− mice by quantitative RT‐PCR indicated that the levels of other C family mRNAs were not significantly different from wild‐type mice. Next, we developed Abcc6/1−/− and Abcc6/3−/− double null mice and examined them for tissue mineralization. Histopathologic examination, coupled with computerized morphometric analysis, and chemical assay of calcium × phosphate product in the muzzle skin of Abcc1−/− and Abcc3−/− mice did not reveal evidence of mineralization. Abcc6/1−/− and Abcc6/3−/− double knock‐out mice exhibited connective tissue mineralization similar to that in Abcc6 −/− mice. These results emphasize the importance of the Abcc6 gene in the ectopic mineralization process and further suggest that other members of the Abcc family, particularly Abcc1 and Abcc3, do not modulate the effects of Abcc6 in this mouse model.
An estramustine-resistant human ovarian carcinoma cell line, SKEM, was generated to explore resistance mechanisms associated with this agent. Cytogenetic analysis revealed that SKEM cells have a ...homogeneously staining region (hsr) at chromosome 9q34. Microdissection of the hsr, followed by fluorescence in situ hybridization to SKEM and normal metaphase spreads, confirmed that the amplified region was derived from sequences from 9q34. In situ hybridization with a probe specific for ABC2, a gene located at 9q34 that encodes an ATP-binding cassette 2 (ABC2) transporter, indicated that this gene is amplified approximately 6-fold in the estramustine-resistant cells. Southern analysis confirmed that ABC2 was amplified in SKEM, and Northern analysis indicated that the ABC2 transcript was overexpressed approximately 5-fold. The ABC1 gene located at 9q22-31 was not amplified in the resistant cells, and mRNA levels of several other ABC transporter genes were unaltered. Consistent with the concept that increased ABC2 expression contributes to the resistant phenotype, we observed that the rate of efflux of dansylated estramustine was increased in SKEM compared with control cells. In addition, antisense treatment directed toward ABC2 mRNA sensitized the resistant cells to estramustine. Together, these results suggest that amplification and overexpression of ABC2 contributes to estramustine resistance and provides the first indication of a potential cellular function for this product.
Multidrug resistance-associated protein 1 and P-glycoprotein are major ATP-binding cassette transporters that function as efflux pumps and confer resistance to a variety of structurally unrelated ...anticancer agents. To evaluate the comparative importance of these transporters with respect to anticancer agents, we established and characterized SV40-immortalized mrp1(-/-) (KO), mdr1a/1b(-/-) (DKO), and combined mrp1 (-/-), mdr1a/1b(-/-) (TKO) deficient fibroblast lines derived from primary embryonic fibroblasts of knockout mice. Western blot analyses demonstrated that KO and DKO fibroblasts exhibited similar levels of P-glycoprotein and mrp1, respectively, to that of wild-type (WT) fibroblasts. In addition, semiquantitative reverse transcription-PCR measurements of other multidrug resistance-associated protein (mrp) family members demonstrated that TKO fibroblasts displayed expression profiles of mrps 2-7 comparable to that of WT fibroblasts. These results indicate that loss of mrp1, P-glycoprotein, or both transporters does not cause overt compensatory changes in the expression of the other determined transporters. Using cell viability and calcein accumulation assays, we demonstrated that KO and DKO fibroblasts exhibited a low to moderate increase in sensitivity to vincristine and etoposide and in calcein accumulation compared to WT fibroblasts, whereas TKO fibroblasts displayed a markedly enhanced sensitivity to these agents and further elevated calcein accumulation. Furthermore, verapamil, an inhibitor of both mrp1 and P-glycoprotein, significantly sensitized WT fibroblasts to both vincristine and etoposide while having no effect on the sensitivity of TKO cells to these agents. Collectively, these findings indicate that mrp1 and P-glycoprotein are major determinants of drug sensitivity in immortalized mouse embryonic fibroblasts. They also suggest the existence of a compensatory mechanism by which the loss of one transporter can be functionally offset by the other in the transport of common drug substrates.
MRP is a recently described ATP-binding cassette transporter that confers cellular resistance to natural product cytotoxic drugs. To examine the biochemical activity and cellular physiology of this ...transporter, we isolated the murine MRP homologue and analyzed its in vitro substrate specificity. Murine MRP transcript is widely expressed in tissues and encodes a protein of 1528 amino acids that is 88% identical to its human homologue. Hydropathy analysis indicated that murine and human MRP, the yeast cadmium resistance transporter and the sulfonylurea receptor share a conserved topology distinguished from P-glycoprotein and the cystic fibrosis conductance regulator by an N-terminal hydrophobic region that contains several potential transmembrane domains. Drug uptake assays performed with membrane vesicles prepared from NIH3T3 cells transfected with a murine MRP expression vector revealed ATP-dependent transport for the natural product cytotoxic drugs daunorubicin and vincristine, as well as for the glutathione S-conjugates leukotriene C4 and azidophenacyl-S-glutathione. Drug transport was osmotically sensitive and saturable with regard to drug and ATP concentrations, with K m values of 19 μM, 19 μM, 26 nM, 17 μM, and 77 μM for daunorubicin, vincristine, leukotriene C4, APA-SG, and ATP, respectively. Consistent with broad substrate specificity, the drug glutathione conjugate APA-SG, oxidized glutathione, the LTD4 antagonist MK571, arsenate, and genistein were competitive inhibitors of daunorubicin transport, with K i values of 32 μM, 25 μM, 1.9 μM, 108 μM, and 23 μM, respectively. This study demonstrates that the substrate specificity of murine MRP is quite broad and includes both the neutral or mildly cationic natural product cytotoxic drugs and the anionic products of glutathione conjugation. The widespread expression pattern of murine MRP in tissues, combined with its ability to transport both lipophilic xenobiotics and the products of phase II detoxification, indicates that it represents a widespread and versatile cellular defense mechanism.
Approximately 15% of gastrointestinal stromal tumors (GISTs) in adults and 85% in children lack mutations in
KIT
and
PDGFRA
and are known as wild type GISTs. Wild type GISTs from adults and children ...express high levels of insulin-like growth factor 1 receptor (IGF1R) and exhibit stable genomes compared to mutant GISTs. Pediatric wild type GISTs, GISTs from the multi-tumor Carney-Stratakis syndrome and the Carney triad share other clinico-pathological properties (e.g. early-onset, multifocal GISTs with epitheliod cell morphology) suggesting a common etiology. Carney-Stratakis is an inherited association of GIST and paragangliomas caused by germline mutations in succinate dehydrogenase (SDH) genes. The connection between defective cellular respiration and GIST pathology has been strengthened by the utilization of SDHB immunohistochemistry to identify SDH deficiency in pediatric GISTs, syndromic GISTs, and some adult wild type GISTs. SDHB and IGF1R expression was examined in 12 wild type and 12 mutant GIST cases. Wild type GISTs were screened for coding-region alterations in
SDH
genes and for chromosomal aberrations using genome-wide SNP and MIP arrays. SDHB-deficiency, identified in 11/12 wild type GIST cases, was tightly associated with over-expression of IGF1R protein and transcript. Biallelic inactivation of the
SDHA
gene was a surprisingly frequent event, identified in 5/11 SDHB-negative cases, generally due to germline point mutations accompanied by somatic
SDHA
allelic losses. As a novel finding, inactivation of the
SDHC
gene from a combination of a heterozygous coding-region mutation and hyper-methylation of the wild type allele was found in one SDHB-negative case.
Multidrug resistance-associated protein (MRP) and canalicular multispecific organic anion transporter (cMOAT) are closely related mammalian ATP-binding cassette transporters that export organic ...anions from cells. Transfection studies have established that MRP confers resistance to natural product cytotoxic agents, and recent evidence suggests the possibility that cMOAT may contribute to cytotoxic drug resistance as well. Based upon the potential importance of these transporters in clinical drug resistance and their important physiological roles in the export of the amphiphilic products of phase I and phase II metabolism, we sought to identify other MRP-related transporters. Using a degenerate PCR approach, we isolated a cDNA that encodes a novel ATP-binding cassette transporter, which we designated MOAT-B. The MOAT-B gene was mapped using fluorescence in situ hybridization to chromosome band 13q32. Comparison of the MOAT-B predicted protein with other transporters revealed that it is most closely related to MRP, cMOAT, and the yeast organic anion transporter YCF1. Although MOAT-B is closely related to these transporters, it is distinguished by the absence of a approximately 200 amino acid NH2-terminal hydrophobic extension that is present in MRP and cMOAT and which is predicted to encode several transmembrane spanning segments. In addition, the MOAT-B tissue distribution is distinct from MRP and cMOAT. In contrast to MRP, which is widely expressed in tissues, including liver, and cMOAT, the expression of which is largely restricted to liver, the MOAT-B transcript is widely expressed, with particularly high levels in prostate, but is barely detectable in liver. These data indicate that MOAT-B is a ubiquitously expressed transporter that is closely related to MRP and cMOAT and raise the possibility that it may be an organic anion pump relevant to cellular detoxification.
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