MRP subfamily transporters and resistance to anticancer agents

The MRP subfamily of ABC transporters from mammals consists of at least seven members, six of which have been implicated in the transport of amphipathic anions. MRP1, MRP2, and MRP3 bear a close structural resemblance, confer resistance to a variety of natural products as well as methotrexate, and have the facility for transporting glutathione and glucuronate conjugates. MRP1 is a ubiquitously expressed efflux pump for the products of phase II of xenobiotic detoxification, while MRP2, whose hereditary deficiency results in Dubin-Johnson syndrome, functions to extrude organic anions into the bile. MRP3 is distinguished by its capacity to transport the monoanionic bile constituent glycocholate, and may function as a basolateral back-up system for the detoxification of hepatocytes when the usual canalicular route is impaired by cholestatic conditions. MRP4 and MRP5 resemble each other more closely than they resemble MRPs 1-3 and confer resistance to purine and nucleotide analogs which are either inherently anionic, as in the case of the anti-AIDS drug PMEA, or are phosphorylated and converted to anionic amphiphiles in the cell, as in the case of 6-MP. Given their capacity for transporting cyclic nucleotides, MRP4 and MRP5 have also been implicated in a broad range of cellular signaling processes. The drug resistance activity and physiological substrates of MRP6 are unknown. However, its hereditary deficiency results in pseudoxanthoma elasticum, a multisystem disorder affecting skin, eyes, and blood vessels. It is hoped that elucidation of the resistance profiles and physiological functions of the different members of the MRP subfamily will provide new insights into the molecular basis of clinical drug resistance and spawn new strategies for combating this phenomenon.

Transport of methotrexate (MTX) and folates by multidrug resistance protein (MRP) 3 and MRP1: effect of polyglutamylation on MTX transport

We have recently determined that human multidrug resistance protein (MRP) 3, which confers resistance to certain natural product agents and methotrexate (MTX), is competent in the MgATP-energized transport of MTX and the monoanionic bile constituent glycocholate as well as several glutathione and glucuronate conjugates. Of these capabilities, the facility of MRP3 in conferring resistance to and mediating the transport of MTX is of particular interest because it raises the possibility that this pump is a component of the previously described cellular efflux system for this antimetabolite. However, if this is to be the case, a critical property of cellular MTX efflux that must be addressed is its ability to mediate the export of MTX but not that of its intracellular polyglutamylated derivatives. Here we examine the role of MRP3 in these and related processes by determining the selectivity of this transporter for MTX, MTX polyglutamates, and physiological folates. In so doing, we show that MRP3 is not only active in the transport of MTX but is also active in the transport the physiological folates folic acid (FA) and N(5)-formyltetrahydrofolic acid (leucovorin) and that polyglutamylation of MTX abolishes transport. Both FA and leucovorin are subject to high-capacity (V(max(FA)), 1.71 +/- 0.05 nmol/mg/min; V(max(leucovorin)), 3.63 +/- 1.20 nmol/mg/min), low-affinity (K(m(FA)), 1.96 +/- 0.13 mM; K(m(leucovorin)), 1.74 +/- 0.65 mM) transport by MRP3. Addition of a single glutamyl residue to MTX is sufficient to diminish transport by >95%. We also show that polyglutamylation similarly affects the capacity of MRP1 to transport MTX and that physiological folates are also subject to MgATP-stimulated transport by MRP1. On the basis of the capacity to transport MTX but not MTX-Glu(2), it is concluded that MRP3 and MRP1 represent components of the previously described cellular efflux system for MTX. The capacity of MRP3 to transport folates indicates that it may reduce intracellular levels of these compounds and thereby indirectly influence antifolate cytotoxicity, and it also implies that this pump may play a role in the response to chemotherapeutic regimens in which leucovorin is a component.

Transport of cyclic nucleotides and estradiol 17-beta-D-glucuronide by multidrug resistance protein 4. Resistance to 6-mercaptopurine and 6-thioguanine

Human multidrug resistance protein 4 (MRP4) has recently been determined to confer resistance to the antiviral purine analog 9-(2-phosphonylmethoxyethyl)adenine and methotrexate. However, neither its substrate selectivity nor physiological functions have been determined. Here we report the results of investigations of the in vitro transport properties of MRP4 using membrane vesicles prepared from insect cells infected with MRP4 baculovirus. It is shown that expression of MRP4 is specifically associated with the MgATP-dependent transport of cGMP, cAMP, and estradiol 17-beta-D-glucuronide (E(2)17 beta G). cGMP, cAMP, and E(2)17 beta G are transported with K(m) and V(max) values of 9.7 +/- 2.3 microm and 2.0 +/- 0.3 pmol/mg/min, 44.5 +/- 5.8 microm and 4.1 +/- 0.4 pmol/mg/min, and 30.3 +/- 6.2 microm and 102 +/- 16 pmol/mg/min, respectively. Consistent with its ability to transport cyclic nucleotides, it is demonstrated that the MRP4 drug resistance profile extends to 6-mercaptopurine and 6-thioguanine, two anticancer purine analogs that are converted in the cell to nucleotide analogs. On the basis of its capacity to transport cyclic nucleotides and E(2)17 beta G, it is concluded that MRP4 may influence diverse cellular processes regulated by cAMP and cGMP and that its substrate range is distinct from that of any other characterized MRP family member.

Reversal of drug resistance mediated by multidrug resistance protein (MRP) 1 by dual effects of agosterol A on MRP1 function

We previously isolated agosterol A (AG-A) from a marine Spongia sp. and found that it completely reversed colchicine resistance in P-glycoprotein (Pgp)-over-expressing KB-C2 cells and vincristine resistance in multidrug-resistance protein (MRP)1-over-expressing CV60 cells. However, a tri-deacetylated derivative of AG-A (IAG-A) showed almost no activity in reversing Pgp- or MRP1-mediated drug resistance. In this study, we examined the mechanisms by which AG-A reverses MRP1-mediated drug resistance by investigating the interaction between agosterols and MRP1 in MRP1-over-expressing human KB carcinoma (KB/MRP) cells. [3H]-Leukotriene C4 (LTC4), [3H]-2,4-dinitrophenyl-S-glutathione uptake into membrane vesicles prepared from KB/MRP cells and intracellular [3H]-vincristine accumulation and efflux in KB/MRP cells were measured with or without AG-A and/or inactive IAG-A. AG-A reduced MRP1-mediated [3H]-LTC4 transport in a dose-dependent manner, but IAG-A did not. Inhibition by AG-A was competitive, with a K(i) value of 31 microM. AG-A at 10 microM enhanced the accumulation of [3H]-vincristine in KB/MRP cells to the level of that in control cells in the absence of the agent. Likewise, ATP-dependent efflux of [3H]-vincristine from KB/MRP cells was enhanced compared with KB-3-1 cells and inhibited by AG-A. In addition, AG-A reduced intracellular levels of glutathione, a compound required for MRP1-mediated transport of some anti-cancer drugs. These findings suggest that AG-A reverses MRP1-mediated drug resistance by directly inhibiting the capacity of MRP1 to transport drugs. In addition, the capacity of AG-A to reduce cellular glutathione levels may contribute to the modulating activity of MRP1.

Analysis of the structure and expression pattern of MRP7 (ABCC10), a new member of the MRP subfamily

The MRP subfamily of ABC transporters currently consists of at least six members, several of which have been demonstrated to transport amphipathic anions and to confer in vitro resistance to chemotherapeutic agents. In searching the data bases we identified the product of a cDNA sequencing project that bears significant similarity to MRP subfamily transporters. In this report the predicted coding sequence, protein product and expression pattern of this cDNA, termed MRP7, are analyzed. The MRP7 cDNA sequence encodes a 1492 amino acid ABC transporter whose structural architecture resembles that of MRP1, MRP2, MRP3, and MRP6, in that its transmembrane helices are arranged in three membrane spanning domains. However, in contrast to the latter transporters, a conserved N-linked glycosylation site is not found at the N-terminus of MRP7. Comparisons of the MRP7 amino acid sequence indicated that while it is most closely related to other MRP subfamily members, its degree of relatedness is the lowest of any of the known MRP-related transporters. The integrity of the predicted MRP7 coding sequence was confirmed by the synthesis of an approximately 158 kDa protein in reticulocyte lysates programmed with the MRP7 cDNA. While MRP7 transcript was detected in a variety of tissues by RT/PCR, it was not readily detectable by RNA blot analysis, suggesting that it is expressed at low levels in these tissues. Fluorescence in situ hybridization indicated that MRP7 maps to chromosome 6p12-21, in proximity to several genes associated with glutathione conjugation and synthesis. On the basis of these findings and evolutionary cluster analysis, we conclude that MRP7 is a member of the MRP subfamily of amphipathic anion transporters.

Analysis of the MRP4 drug resistance profile in transfected NIH3T3 cells

BACKGROUND

Multidrug resistance-associated protein (MRP) 1 and canalicular multispecific organic anion transporter (cMOAT or MRP2) are adenosine triphosphate-binding cassette transporters that confer resistance to anticancer agents. In addition to these two transporters, there are at least four other human MRP subfamily members (MRP3 through MRP6). We and others reported previously that MRP3 is capable of conferring resistance to certain anticancer agents. In this study, we investigated whether MRP4 (MOAT-B), whose transcript accumulates to the highest levels in prostate tissue, has the capacity to confer drug resistance.

METHODS

MRP4-transfected NIH3T3 cells were generated, and their drug sensitivity was analyzed. The subcellular localization of MRP4 was assessed by immunohistochemical analysis in transfected cells and in prostate tissue. Statistical tests were two-sided.

RESULTS

MRP4 was detected as a 170-kd protein that was localized in the plasma membrane and cytoplasm of transfected cells. The MRP4 transfectants displayed 5.5-fold increased resistance to methotrexate in short-term drug-exposure assays (P=.022) and exhibited decreased cellular accumulation of this agent at 4 hours (P=.006) and 24 hours (P<.001). In continuous-exposure assays, however, the MRP4 transfectants did not display increased resistance for either methotrexate or natural product cytotoxic agents (anthracyclines, etoposide, vinca alkaloids, and paclitaxel [Taxol]). However, the transfectants did show increased resistance (2.3-fold) for the anti-acquired immunodeficiency syndrome nucleoside analogue 9-(2-phosphonylmethoxyethyl)adenine (PMEA) (P=.022) in continuous-exposure assays. Consistent with MRP4's plasma membrane localization in transfected cells, analysis of prostate tissue showed that MRP4 protein was localized primarily in the basolateral plasma membranes of tubuloacinar cells.

CONCLUSIONS

These results indicate that MRP4 confers resistance to short-term methotrexate and continuous PMEA treatment. Given its structure, drug resistance profile and subcellular localization, MRP4 probably functions as an amphipathic anion efflux pump whose substrate range includes glutamate and phosphate conjugates.

Transport of amphipathic anions by human multidrug resistance protein 3

The multidrug resistance-associated protein 1 (MRP1) and the canalicular multispecific organic anion transporter (cMOAT or MRP2) are ATP-binding cassette transporters that confer resistance to some anticancer drugs and efflux glutathione and glucuronate conjugates from the cell. The MRP subfamily of ABC transporters, however, contains at least four other members of which MRP3 (MOAT-D) bears the closest structural resemblance to MRP1. Although transfection studies have established that human MRP3 confers increased resistance to several anticancer agents, neither the substrate selectivity nor physiological functions of this transporter have been determined. Here we report the results of investigations of the in vitro transport properties of cloned human MRP3 using membrane vesicles prepared from MRP3-transfected HEK293 cells. It is shown that the expression of MRP3 is specifically associated with enhancement of the MgATP-dependent transport into membrane vesicles of the glucuronide estradiol 17-beta-D-glucuronide (E(2)17betaG), the glutathione conjugates 2,4-dinitrophenyl S-glutathione (DNP-SG) and leukotriene C4 (LTC4), the antimetabolite methotrexate, and the bile acid glycocholate. DNP-SG, LTC4, and E(2)17betaG are transported at moderate affinity and low capacity with Km and Vmax values of 5.7 +/- 1.7 microM and 3.8 +/- 0.1 pmol/mg/min, 5.3 +/- 2.6 microM and 20.2 +/- 5.9 pmol/mg/min, and 25.6 +/- 5.4 microM and 75.6 +/- 5.9 pmol/mg/min, respectively. Methotrexate and glycocholate are transported at low affinity and high capacity with Km and Vmax values of 776 +/- 319 microM and 288 +/- 54 pmol/mg/min and 248 +/- 113 microM and 183 +/- 34 pmol/mg/min, respectively. On the basis of these findings, the osmotic dependence of the transport measured and its inability to transport taurocholate, MRP3, like MRP1 and cMOAT, is concluded to be competent in the transport of glutathione S-conjugates, glucuronides, and methotrexate, albeit at low to moderate affinity. In contrast to MRP1, cMOAT, and all other characterized mammalian ABC transporters, however, MRP3 is active in the transport of the monoanionic human bile constituent glycocholate.

The influence of coordinate overexpression of glutathione phase II detoxification gene products on drug resistance

Glutathione (GSH), glutathione S-transferases (GSTs), and the multidrug resistance-associated protein 1 (MRP1) have been independently studied for their contributions to drug resistance. Single cDNA transfection experiments have provided inconsistent and disparate conclusions with respect to the importance of GSH and GST in conferring a resistant phenotype. Because these three proteins can act as a concerted coordinated pathway, we reasoned that equivalent increases may be required for enhanced resistance to be expressed. We have assembled these proteins together, or in various combinations, to determine whether they show cooperativity in determining drug response. Increased expression through single cDNA transfection of GSTpi, gamma-glutamylcysteine synthetase (gamma-GCS) (regulatory plus catalytic subunits), or MRP1 enhanced resistance to a number of anticancer drugs. Cotransfection of GSTpi and GCS, gave higher resistance to doxorubicin, etoposide, and vincristine than with either alone. Resistance toward chlorambucil and ethacrynic acid was similar in cells overexpressing either component or overexpressing GST alone. Coexpression of GSTpi with MRP1 conferred significant resistance above that seen for MRP1 alone to chlorambucil, etoposide, ethacrynic acid, and vincristine. The combination of GCS and MRP1 did not afford additional resistance above MRP1 alone. When all three were transfected, significantly higher levels of resistance were found for doxorubicin and etoposide. These results support the concept that coordinate enhancement of focal thiol elements of detoxification pathways provides a more efficient protective phenotype than do single components alone.

Expression of multidrug resistance protein-3 (multispecific organic anion transporter-D) in human embryonic kidney 293 cells confers resistance to anticancer agents

Multidrug resistance-associated protein (MRP)1 and canalicular multispecific organic anion transporter (cMOAT)/MRP2 are ATP-binding cassette (ABC) transporters that confer resistance to natural product cytotoxic drugs. We recently described the complete coding sequences of four human MRP/cMOAT subfamily members and found that, among these proteins, MRP3/MOAT-D is most closely related to MRP1 (58% identity; M. G. Belinsky and G. D. Kruh, Br. J. Cancer, 80: 1342-1349, 1999). In the present study, we sought to determine whether MRP3 is capable of conferring resistance to cytotoxic drugs. To address this question, human embryonic kidney 293 cells were transfected with an MRP3 expression vector, and the drug resistance phenotype of the transfected cells was analyzed. The MRP3-transfected cells displayed approximately 4-fold resistance to etoposide and approximately 2-fold resistance to vincristine, compared with control transfected cells. In addition, approximately 1.7-fold resistance was observed for the antimetabolite methotrexate. Increased resistance was not observed for several other natural product agents, including anthracyclines and Taxol. The MRP-transfected cells exhibited reduced accumulation of radiolabeled etoposide, consistent with the operation of a plasma membrane efflux pump. These results indicate that MRP3 confers resistance to some anticancer agents but that its resistance pattern is distinct from the resistance patterns of other ABC transporters involved in resistance to natural product chemotherapeutic agents.

MOAT-E (ARA) is a full-length MRP/cMOAT subfamily transporter expressed in kidney and liver

Multidrug resistance-associated protein (MRP) and the canalicular multispecific organic anion transporter (cMOAT) are organic anion pumps that have been linked to cytotoxic drug resistance. We previously reported the isolation of three human MRP/cMOAT-related transporters, MOAT-B (MRP4), MOAT-C (MRP5) and MOAT-D (MRP3). In the present study we describe the fourth MRP/cMOAT-related transporter. We analysed ARA, a human cDNA reported to encode a 453 residue MRP-related transporter, and found that it represents a fused transcript composed of MRP sequences and partial sequences of a novel transporter. The complete coding sequence of this novel transporter, which we designated MOAT-E, was isolated. MOAT-E encodes a 1503 residue transporter that is most closely related to MRP (45%), MOAT-D (44%) and cMOAT (39%), both in terms of amino acid identity and sharing a common topology in which approximately 17 transmembrane spanning helices are distributed within three membrane spanning domains. RNA blot analysis indicated that MOAT-E expression is restricted to kidney and liver. These observations suggest that MOAT-E may function as an organic anion transporter involved in cellular detoxification and possibly in the hepatobiliary and renal excretion of xenobiotics and/or endogenous metabolites. Isolation of MOAT-E helps to define the MRP/cMOAT subfamily of transporters.

Characterization of MOAT-C and MOAT-D, new members of the MRP/cMOAT subfamily of transporter proteins

BACKGROUND

Multidrug resistance-associated protein (MRP) and canalicular multispecific organic anion transporter (cMOAT) are transporter proteins that pump organic anions across cellular membranes and have been linked to resistance to cytotoxic drugs. We previously identified MOAT-B, an MRP/cMOAT-related transporter, by use of a polymerase chain reaction approach. However, analysis of expressed sequence tag (EST) databases indicated that there might be additional MRP/cMOAT-related transporters. To further define the MRP/cMOAT subfamily of transporters, we used EST probes to isolate complementary DNAs for two related transporter proteins, MOAT-C and MOAT-D.

METHODS

MOAT-C and MOAT-D expression patterns in human tissues were determined by RNA blot analysis, and chromosomal localization of the genes was determined by fluorescence in situ hybridization.

RESULTS

MOAT-C is predicted to encode a 1437-amino-acid protein that, among eukaryotic transporters, is most closely related to MRP, cMOAT, and MOAT-B (about 36% identity). However, MOAT-C is less related to MRP and cMOAT than MRP and cMOAT are to each other (about 48% identity). Like MOAT-B, MOAT-C lacks an N-terminal membrane-spanning domain, indicating that the topology of this protein is similarly distinct from that of MRP and cMOAT. MOAT-D is predicted to encode a 1527-amino-acid protein that is the closest known relative of MRP (about 58% identity). MOAT-D is also highly related to cMOAT (about 47% identity). The presence of an N-terminal membrane-spanning domain indicates that the topology of MOAT-D is quite similar to that of MRP and cMOAT. MOAT-C transcripts are widely expressed in human tissues; however, MOAT-D transcript expression is more restricted. The MOAT-C and MOAT-D genes are located at chromosomes 3q27 and 17q21.3, respectively.

CONCLUSIONS

On the basis of amino acid identity and protein topology, the MRP/cMOAT transporter subfamily falls into two groups; the first group consists of MRP, cMOAT, and MOAT-D, and the second group consists of MOAT-B and MOAT-C.

Increased expression of DNA-dependent protein kinase confers resistance to adriamycin

Acquired resistance to adriamycin (ADR) in an HL60 cell line is shown to be accompanied by an increase in DNA-dependent protein kinase catalytic subunit (DNA-PKcs) at both the protein and mRNA levels (15-20-fold) and an overall 3-fold increase in DNA-PK enzyme activity. The other components of the DNA-PK Ku autoantigen complex, Ku70 and Ku80, were 3-fold increased and unchanged, respectively. Time dependent repair of ADR-induced DNA damage was measured by the neutral comet assay and found to be more efficient in the drug resistant cell line (HL60/ADR). Antisense RNA transfection reduced the protein expression of DNA-PKcs to 50% in HL60/ADR and partially reversed drug resistance. A fibroblast cell line from a severe combined immunodeficient (SCID) mouse was deficient in functional DNA-PKcs and showed increased sensitivity to ADR and other DNA damaging agents compared to wild type. These studies demonstrate that alteration in DNA-PK can contribute to chronic stress response leading to acquired drug resistance. The overexpression of DNA-PK is thus shown to be a novel cellular adaptation mechanistically contributing to the resistance of cancer cells to the anthracycline drug adriamycin, and as such, may have implications for its therapeutic use.

Isolation of MOAT-B, a widely expressed multidrug resistance-associated protein/canalicular multispecific organic anion transporter-related transporter

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.

Development of highly selective SH3 binding peptides for Crk and CRKL which disrupt Crk-complexes with DOCK180, SoS and C3G

Many Src Homology 3 (SH3) domains function as molecular adhesives in intracellular signal transduction. Based on previous ultrastructural studies, short motifs which bind to the first SH3 domains of the adapters Crk and CRKL were selectively mutagenised to generate Crk/CRKL SH3-binding peptides of very high affinity and selectivity. Affinities were increased up to 20-fold compared to the best wildtype sequences, while the selectivity against a similar SH3 domain [Grb2SH3(N)] was not only retained, but sometimes increased. Blot techniques with GST-fusion peptides and in solution precipitation assays with biotinylated high affinity Crk binding peptides (HACBPs) were subsequently used to analyse the binding of these sequences to a large panel of SH3 domain-containing fusion proteins. Only those proteins which contained the CrkSH3(1) or CRKLSH3(1) domains bound efficiently to the HACBPs. A GST-HACBP fusion protein precipitated Crk and CRKL proteins out of 35S-labelled and unlabelled cell lysates. Very little binding of other cellular proteins to HACBP was detectable, indicative of a great preference for Crk and CRKL when compared to the wide variety of other endogenous cellular proteins. Moreover, HACBP disrupted in vitro preexisting Crk-complexes with DOCK180 and the exchange factors SoS and C3G, which are known targets of Crk adapters, in a concentration dependent manner. HACBP-based molecules should therefore be useful as highly selective inhibitors of intracellular signalling processes involving Crk and CRKL.

Amplification of the ATP-binding cassette 2 transporter gene is functionally linked with enhanced efflux of estramustine in ovarian carcinoma cells

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.

Overexpression of the multidrug resistance genes mdr1, mdr3, and mrp in L1210 leukemia cells resistant to inhibitors of ribonucleotide reductase

L1210 MQ-580 is a murine leukemia cell line resistant to the cytotoxic activity of the alpha-(N)-heterocyclic carboxaldehyde thiosemicarbazone class of inhibitors of ribonucleotide reductase. The line is cross-resistant to etoposide, daunomycin, and vinblastine. L1210 MQ-580 cells expressed 8-fold resistance to 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP), a relatively newly developed inhibitor of ribonucleotide reductase. The accumulation of [14C]3-AP by L1210 MQ-580 cells was 5- to 6-fold less than by parental L1210 cells. An increased rate of efflux of 3-AP was responsible for the lower steady-state concentration of 3-AP in resistant cells. In reverse transcription-polymerase chain reaction assays, L1210 MQ-580 cells were found to overexpress the multidrug resistance genes mdr1, mdr3, and mrp, but not the mdr2 gene, compared with parental L1210 cells. Measurement of the steady-state concentration of doxorubicin, a potential substrate for both the mdr and mrp gene products, demonstrated that L1210 MQ-580 cells accumulated 4-fold less anthracycline than parental cells. These findings indicate that drug efflux is a major determinant of the pattern of cross-resistance of L1210 MQ-580 cells. To extrapolate these observations to the human homologues of the mdr1, mdr3, and mrp murine genes, the effects of 3-AP were measured in L1210/VMDRC0.06 and NIH3T3 36-8-32 cells transfected with human MDR1 and MRP cDNAs, respectively. The transfectants were 2- to 3-fold resistant to the cytotoxic effects of 3-AP and accumulated less [14C]3-AP than their parental mock-transfected counterparts. Moreover, the cytotoxic activity of 3-AP was significantly greater in two double mrp gene knockout cell lines than in parental W 9.5 embryonic stem cells. Thus, the results suggest that 3-AP is a substrate for both the P-glycoprotein and MRP and that baseline MRP expression has the capacity to exert a protective role against the toxicity of this agent.

Tyrosine phosphorylation of RNA polymerase II carboxyl-terminal domain by the Abl-related gene product

The largest subunit of RNA polymerase II contains a C-terminal repeated domain (CTD) that is the site of phosphorylation by serine (threonine) and tyrosine kinases. Phosphorylation of the CTD is correlated with transcription elongation. A number of different kinases have previously been shown to phosphorylate the CTD; among them is a nuclear tyrosine kinase encoded by the c-abl proto-oncogene. The processive and high stoichiometric phosphorylation of RNA polymerase II by c-Abl requires the tyrosine kinase, the SH2 domain, and a CTD-interacting domain (CTD-ID) in the Abl protein. The physiological tyrosine phosphorylation of RNA polymerase II by c-Abl in DNA damage response has previously been demonstrated. Basal tyrosine phosphorylation of RNA polymerase II, however, is observed in cells derived from abl-deficient mice, indicating the existence of other CTD tyrosine kinases. In this report, we show that the tyrosine kinase encoded by an Abl-related gene (Arg) also phosphorylates the CTD in vitro and in transfected cells. The SH2 and kinase domain of Arg are 95% identical to that of c-Abl. However, these two proteins share only 29% identity in the large C-terminal region. Interestingly, a CTD-ID is also found in the C-terminal region of Arg. Mapping studies and sequence analysis have led to the identification of the CTD-ID that is highly conserved among the divergent C-terminal regions of Abl and Arg. These results indicate that tyrosine phosphorylation of RNA polymerase II CTD could be catalyzed by either c-Abl or Arg kinase.

ArgBP2, a multiple Src homology 3 domain-containing, Arg/Abl-interacting protein, is phosphorylated in v-Abl-transformed cells and localized in stress fibers and cardiocyte Z-disks

Arg and c-Abl represent the mammalian members of the Abelson family of protein-tyrosine kinases. A novel Arg/Abl-binding protein, ArgBP2, was isolated using a segment of the Arg COOH-terminal domain as bait in the yeast two-hybrid system. ArgBP2 contains three COOH-terminal Src homology 3 domains, a serine/threonine-rich domain, and several potential Abl phosphorylation sites. ArgBP2 associates with and is a substrate of Arg and v-Abl, and is phosphorylated on tyrosine in v-Abl-transformed cells. ArgBP2 is widely expressed in human tissues and extremely abundant in heart. In epithelial cells ArgBP2 is located in stress fibers and the nucleus, similar to the reported localization of c-Abl. In cardiac muscle cells ArgBP2 is located in the Z-disks of sarcomeres. These observations suggest that ArgBP2 functions as an adapter protein to assemble signaling complexes in stress fibers, and that ArgBP2 is a potential link between Abl family kinases and the actin cytoskeleton. In addition, the localization of ArgBP2 to Z-disks suggests that ArgBP2 may influence the contractile or elastic properties of cardiac sarcomeres and that the Z-disk is a target of signal transduction cascades.

Transforming activity and mitosis-related expression of the AKT2 oncogene: evidence suggesting a link between cell cycle regulation and oncogenesis

The AKT2 oncogene encodes a protein-serine/threonine kinase containing a pleckstrin homology domain characteristic of many signaling proteins. Recently, it was shown that AKT2 kinase activity can be induced by platelet-derived growth factor through phosphatidylinositol-3-OH kinase, suggesting that AKT2 may be an important signal mediator that contributes to the control of cell proliferation. We previously reported amplification and overexpression of AKT2 in human cancers. To investigate the transforming activity of AKT2, we used a retrovirus-based construct to express AKT2 in NIH3T3 cells. Overexpression of AKT2 was found to transform NIH3T3 cells, as determined by growth in soft agar and tumor formation in nude mice. The oncogenic activity of AKT2 was diminished by truncation of a 70-amino acid proline-rich region at the carboxyl-terminus. To facilitate the characterization of AKT2, we generated monoclonal and polyclonal antibodies against this protein. AKT2 was localized to the cytoplasm by cell fractionation experiments, immunocytochemistry, and immunofluorescence. Protein levels were more abundant in mitotic cells than in interphase cells. Western blot analysis of synchronized pancreatic cancer cells demonstrated that the expression level of AKT2 protein in mitotic cells is three to fivefold higher than in their interphase counterparts. A time-course study of phytohemagglutinin-stimulated lymphocytes revealed that AKT2 mRNA and AKT2 protein levels are highest 48-72 h after addition of mitogen, when cells are actively dividing. These data suggest that AKT2 could play a significant role in cell cycle progression and that the oncogenic activity of overexpressed AKT2 may be mediated by aberrant regulation of cellular proliferation.

ATP-dependent transport of lipophilic cytotoxic drugs by membrane vesicles prepared from MRP-overexpressing HL60/ADR cells

MRP is an ATP-binding cassette family transporter that confers cellular resistance to a variety of natural product cytotoxic agents. However, the biochemical mechanism by which MRP confers resistance has not been established. To gain insight into its mechanism of action, the in vitro substrate specificity of MRP was examined by analyzing drug uptake into membrane vesicles prepared from MRP-overexpressing HL60/ADR cells. Compared to control HL60 membrane vesicles, HL60/ADR membrane vesicles exhibited markedly enhanced ATP-dependent transport of daunorubicin, etoposide, and vincristine. In contrast, little or no increased uptake was observed for vinblastine and Taxol. This pattern of in vitro substrate specificity was consistent with the analysis of the HL60/ADR drug resistance phenotype, which revealed substantial levels of resistance to anthracyclines, etoposide, and vincristine, but only slightly increased resistance to vinblastine and Taxol. Drug transport into HL60/ADR membrane vesicles was osmotically sensitive and dependent on ATP concentration, with a K(m) value of 45 microM for ATP. Lineweaver-Burk analysis indicated that substrate transport was concentration-dependent, with apparent K(m) values of 6.1, 5.7, and 5.5 microM for daunorubicin, etoposide, and vincristine, respectively. The P-glycoprotein-modulating agents cyclosporin A, PSC833, and verapamil, which have modest reversing effects on MRP-overexpressing cell lines, were weak competitive inhibitors of daunorubicin transport, with Ki values of 35, 84, and 95 microM, respectively. In addition, the glutathione analog azidophenacyl-glutathione, oxidized glutathione, and the LTD4 antagonist MK571 were competitive inhibitors of daunorubicin transport with Ki values of 69, 31, and 3.0 microM, respectively. Genistein, an MRP-specific modulating agent, and arsenate, a compound for which MRP has previously been reported to confer resistance, were also competitive inhibitors, with Ki values of 17 and 29 microM, respectively. These results are consistent with a previous report in which we demonstrated that HL60/ADR membrane vesicles transport azidophenacylglutathione and that transport of this agent is competitively inhibited by daunorubicin, vincristine, and etoposide [Shen et al., (1966) Biochemistry 35, 5719-5725]. Together, these uptake studies performed with HL60/ADR membrane vesicles constitute a consistent body of evidence that indicates that MRP transports both glutathione S conjugates and unaltered natural product drugs and support the idea that the direct transport of unaltered lipophilic cytotoxic drugs is the predominant biochemical mechanism whereby MRP confers multidrug resistance.

Structure and in vitro substrate specificity of the murine multidrug resistance-associated protein

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 microM, 19 microM, 26 nM, 17 microM, and 77 microM 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 Ki values of 32 microM, 25 microM, 1.9 microM, 108 microM, and 23 microM, 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.

Proline-rich sequences mediate the interaction of the Arg protein tyrosine kinase with Crk

Arg is a ubiquitously expressed member of the Abelson family of nonreceptor protein-tyrosine kinases. Defining the Arg sequences that mediate its interaction with other proteins is essential to elucidating its role in cellular signaling. In this report we demonstrate that Arg associates with c-Crk, an adaptor protein composed of an SH2 domain and two SH3 domains, and examine the molecular mechanism of the interaction. In vitro experiments revealed that three proline-rich sequences with distinct specificities for SH3 domains are located in the Arg C-terminal domain, just C-terminal to the kinase domain, and that two of these sequences bind to the Crk N-terminal SH3 domain. These two sequences conform to the PxLPxK/R motif that has been observed in other proteins that bind the Crk N-terminal SH3 domain. The interaction of Arg with c-Crk in living cells was confirmed by the detection of coimmunoprecipitation in coinfected Sf9 cells. In addition, increased phosphorylation of c-Crk was observed in cotransfected COS cells, indicating that Crk is an Arg substrate. The site of c-Crk phosphorylation by Arg was identified as tyrosine 221, a residue whose modification has been shown to result in an intramolecular SH2 interaction and a folded conformation. These experiments extend the known Arg protein interacting motifs to include SH3 binding sites and suggest that Arg may function as an effector as well as a regulator of Crk activity.

ATP-dependent uptake of natural product cytotoxic drugs by membrane vesicles establishes MRP as a broad specificity transporter

MRP is a recently isolated ATP-binding cassette family transporter. We previously reported transfection studies that established that MRP confers multidrug resistance [Kruh, G. D., Chan, A., Myers, K., Gaughan, K., Miki, T. & Aaronson, S. A. (1994) Cancer Res. 54, 1649-1652] and that expression of MRP is associated with enhanced cellular efflux of lipophilic cytotoxic agents [Breuninger, L. M., Paul, S., Gaughan, K., Miki, T., Chan, A., Aaronson, S. A. & Kruh, G. D. (1995) Cancer Res. 55, 5342-5347]. To examine the biochemical mechanism by which MRP confers multidrug resistance, drug uptake experiments were performed using inside-out membrane vesicles prepared from NIH 3T3 cells transfected with an MRP expression vector. ATP-dependent transport was observed for several lipophilic cytotoxic agents including daunorubicin, etoposide, and vincristine, as well as for the glutathione conjugate leukotriene C4 (LTC4). However, only marginally increased uptake was observed for vinblastine and Taxol. Drug uptake was osmotically sensitive and saturable with regard to substrate concentration, with Km values of 6.3 microM, 4.4 microM, 4.2 microM, 35 nM, and 38 microM, for daunorubicin, etoposide, vincristine, LTC4, and ATP, respectively. The broad substrate specificity of MRP was confirmed by the observation that daunorubicin transport was competitively inhibited by reduced and oxidized glutathione, the glutathione conjugates S-(p-azidophenacyl)-glutathione (APA-SG) and S-(2,4-dinitrophenyl)glutathione (DNP-SG), arsenate, and the LTD4 antagonist MK571. This study establishes that MRP pumps unaltered lipophilic cytotoxic drugs, and suggests that this activity is an important mechanism by which the transporter confers multidrug resistance. The present study also indicates that the substrate specificity of MRP is overlapping but distinct from that of P-glycoprotein, and includes both the neutral or mildly cationic natural product cytotoxic drugs and the anionic products of glutathione conjugation. The widespread expression of MRP in tissues, combined with its ability to transport both lipophilic xenobiotics and the products of phase II detoxification, indicates that the transporter represents a widespread and remarkably versatile cellular defense mechanism.

Subcellular localization of the Arg protein tyrosine kinase

Arg and c-Abl are ubiquitously expressed proteins which represent the mammalian members of the Abelson family of nonreceptor protein tyrosine kinases (PTKs). In contrast to most nonreceptor PTKs, c-Abl is located in the nucleus and cytoplasm. In this study the subcellular localization of Arg was examined to gain insight into its possible functions. Using indirect immunofluorescence, Arg was detected in the cytoplasm but not the nucleus of both transiently transfected COS cells and stable NIH3T3 transfectants. Arg is expressed as IA and IB isoforms which bear alternative amino termini, with the IB isoform containing a potential myristic acid acceptor sequence. Modification of ArgIB by myristic acid was confirmed by the detection of [3H]myristic acid labeling and the observation that labeling was abolished by mutating Gly-2. These results indicate that the subcellular localization of Arg is distinct from that of c-Abl and thus suggest that the two proteins have different functions in intracellular signaling. In addition, Arg, like c-Abl, is expressed as myristoylated and nonmyristoylated isoforms, suggesting that it may have dual cytoplasmic subcellular localizations, and possibly participate in diverse signaling pathways.

Cellular and in vitro transport of glutathione conjugates by MRP

MRP is a recently identified ATP-binding cassette transporter. We previously established that MRP confers resistance to a spectrum of natural product cytotoxic drugs [Kruh, G.D., (1994) Cancer Res. 54, 1649-1652], that expression of MRP is associated with enhanced drug efflux [Breuninger, L.M., (1995) Cancer Res. 55, 5342-5347], and that MRP transcript is widely expressed in human tissues and solid tumor cell lines [Kruh, G.D., (1995) J. Natl. Cancer Inst. 87, 1256-1258]. In the present study the relationship between MRP and drug glutathione S-conjugates was examined. We observed that MRP was labeled by azidophenacylglutathione (APA-SG), a photoaffinity analog of glutathione, and that inside-out membrane vesicles prepared from MRP-overexpressing HL60/ADR cells transported this compound. Transport into membrane vesicles was ATP-dependent, sensitive to osmolarity, and saturable with regard to APA-SG and ATP concentrations, with Km values of 15 and 61 microM, respectively. APA-SG transport was competitively inhibited by the natural product cytotoxic drugs daunorubicin, vincristine, and etoposide, with Ki values of 4.8, 3.8, and 5.5 microM, respectively. Oxidized glutathione, the drug-glutathione S-conjugate DNP-SG, the LTD4 antagonist MK571 and arsenate were also competitive inhibitors, with Ki values of 9.0, 23.4, 1.1, and 15.0 microM, respectively. Analysis of the fate of monochlorobimane in MRP transfectants revealed reduced intracellular concentrations of drug-glutathione S-conjugates associated with enhanced efflux and altered intracellular distribution. These results indicate that MRP can transport glutathione conjugates in vitro and in living cells and suggest the possibility that the transporter may represent a link between cellular resistance to some classes of cytotoxic drugs and glutathione-mediated mechanisms of resistance. In addition, the observation that both mildly cationic or neutral natural product cytotoxic drugs and anionic compounds such as DNP-SG, MK571, and arsenate are competitive inhibitors of MRP action suggests that the substrate specificity of the transporter is quite broad.

Effects of chronic ethacrynic acid exposure on glutathione conjugation and MRP expression in human colon tumor cells

Chronic exposure to ethacrynic acid of a subcloned HT29 human colon cancer cell line produces a 3- to 4-fold increase in the level of resistance to this agent. The resistant cells (HT6-8) have an enhanced capacity to metabolize the parent drug and efflux it from the cell. This is reflected in a 5-fold enhanced decompositioning rate constant for ethacrynic acid in HT6-8 (3.47 x 10-3 min-1) versus wild type cells (1.58 x 10-2 min-1). We observed that the glutathione conjugate of ethacrynic acid is an effective competitive inhibitor for binding to the multidrug resistance-associated protein by [35S]azidophenacyl-glutathione, a photoaffinity analog of glutathione. In addition, the HT6-8 cells overexpressed multidrug resistance-associated transcript 2- to 3-fold. These results suggest that resistance to ethacrynic acid results from a concerted, coordinate increase in defense mechanisms which detoxify the drug and remove its conjugate via plasma membrane efflux.

Identification of ArgBP1, an Arg protein tyrosine kinase binding protein that is the human homologue of a CNS-specific Xenopus gene

Arg and c-Abl represent the mammalian members of the Abelson family of nonreceptor protein-tyrosine kinases. To gain insight into the biological role of Arg we used the two-hybrid approach to identify interacting proteins. Using a C-terminal segment of Arg we identified a novel protein, ArgBP1 (Arg binding protein 1). ArgBP1 contains a C-terminal SH3 domain, several PEST sequences, a serine rich domain and an SH3 binding site. ArgBP1 is ubiquitously expressed as two transcripts of approximately 2.2 kb and approximately 8 kb with highest levels in brain, heart and testis. The association of ArgBP1 with Arg in living cells was confirmed by coimmunoprecipitation in cotransfected COS cells. Analysis of the mechanism of association indicated that the ArgBP1 SH3 domain binds to a C-terminal Arg SH3-binding site, and that an N-terminal ArgBP1 proline-rich sequence binds to the Arg SH3 domain. Immunostaining indicated that the subcellular localization of ArgBP1 is cytoplasmic. The similarity of the ArgBP1 expression pattern and subcellular localization to those of Arg and the potential for a highly specific and potentially strong association mediated by two pairs of SH3 domain/proline-rich motif interactions, suggest that ArgBP1 is likely to be a regulator and/or effector of Arg function.

Analysis of chimeric Gag-Arg/Abl molecules indicates a distinct negative regulatory role for the Arg C-terminal domain

Arg and c-Abl represent the mammalian member of the Abelson family of nonreceptor protein tyrosine kinases. The two proteins are composed of SH2, SH3, kinase and C-terminal domains. To examine Arg structure-function relationships we analysed a Gag-Arg fusion protein, analogous to the oncogenic Gag-Abl fusion protein of Abelson Murine Leukaemia Virus and found that in contrast to Gag-Abl, it lacked transforming activity. Three observations indicated that the difference in the transforming activity was mediated by the distinct Arg and Abl C-terminal domains. (1) The analysis of chimeric Gag-Arg/Abl molecules revealed that the Arg C-terminal domain completely abrogated Gag-Abl transforming activity and that the Abl C-terminus conferred transforming activity to Gag-Arg. Substitutions of SH2 and kinase domains did not affect activity. (2) Alterations in the Arg C-terminus were observed in spontaneous foci that developed in transfections of two nontransforming chimera. (3) An engineered Gag-Arg molecule containing a truncation of almost the entire C-terminal domain, including three SH3 domain-binding sites, was oncogenic, whereas a slightly smaller truncation that deleted two of three SH3 domain-binding sites, lacked transforming activity. These observations indicate that the C-terminal domain regulates Arg biological activity in a manner distinct from c-Abl and suggest that this effect may be mediated in part by SH3 domain-binding sites.

Expression of multidrug resistance-associated protein in NIH/3T3 cells confers multidrug resistance associated with increased drug efflux and altered intracellular drug distribution

Multidrug resistance is a major obstacle to cancer treatment. Using an expression cDNA library transfer approach to elucidating the molecular basis of non-P-glycoprotein-mediated multidrug resistance, we previously established that expression of multidrug resistance protein (MRP), an ATP-binding cassette superfamily transporter, confers multidrug resistance (G. D. Kruh et al., Cancer Res., 54: 1649-1652, 1994). In the present study, we generated NIH/3T3 MRP transfectants without using chemotherapeutic drugs to facilitate the pharmacological analysis of the MRP phenotype. MRP transfectants displayed increased resistance to several lipophilic drugs, including doxorubicin, daunorubicin, etoposide, actinomycin D, vincristine, and vinblastine. However, increased resistance was not observed for Taxol, a drug for which transfection of MDR1 confers high levels of resistance. Verapamil increased the sensitivity of MRP transfectants relative to control transfectants, but reversal was incomplete for doxorubicin and etoposide, the drugs for which MRP conferred the highest resistance levels. For the latter two drugs, MRP transfectants, which were approximately 8- and approximately 10-fold more sensitive than control cells in the absence of verapamil, exhibited 3.8- and 3.3-fold relative sensitization with 10 microM verapamil, respectively, but remained approximately 2 and approximately 3-fold more resistant than control cells. Analysis of drug kinetics using radiolabeled daunorubicin revealed decreased accumulation and increased efflux in MRP transfectants. Confocal microscopic analysis of intracellular daunorubicin in MRP transfectants was consistent with reduced intracellular drug concentrations, and also revealed an altered pattern of intracellular drug distribution characterized by the initial accumulation of drug in a perinuclear location, followed by the development of a punctate pattern of drug scattered throughout the cytoplasm. This pattern was suggestive of a process of drug sequestration, possibly followed by vesicle transport. Both increased drug efflux and perinuclear drug accumulation are consistent with the reported localization of MRP in plasma and cytosolic membranes (N. Krishnamachary and M. S. Center, Cancer Res., 53: 3658-3663, 1993; M. J. Flens et al., Cancer Res., 54: 4557-4563, 1994). These results thus indicate that the drug specificity of MRP is quite similar to that of MDR1, but also suggest potential differences in Taxol specificity and the level of verapamil sensitivity. In addition, these results indicate that MRP functions to extrude drug from the cell, but additionally suggest the intriguing possibility that drug sequestration contributes to drug resistance by protecting cellular targets and/or contributing to drug efflux.

Expression complementary DNA library transfer establishes mrp as a multidrug resistance gene

The emergence of drug-resistant cancer cells is a major obstacle to cancer treatment. Resistant cells often display a multidrug-resistant phenotype that reduces the promise of combination chemotherapy, the classic approach to the prevention of drug resistance. mdr1, a member of the ABC cassette superfamily of transporters which encodes an energy-dependent drug efflux pump, is the only gene known to confer the multidrug-resistant phenotype. Other multidrug resistance mechanisms must exist, since cell lines which have this phenotype in the absence of mdr1 overexpression have been described. We report here the application of a novel approach involving expression complementary DNA library transfer to the identification of drug-resistant genes. Using this approach we establish that mrp, a member of the ABC cassette superfamily of transporters, is capable of conferring a multidrug-resistant phenotype. This approach should be useful in the identification of other novel resistance genes.

Doxorubicin and multidrug resistance

The circumvention of P-glycoprotein function by pharmacologic agents is a major focus of clinical trials aimed at increasing the cytotoxicity of multidrug resistance-associated drugs, including doxorubicin. The success of this approach will likely depend on the clinical significance of P-glycoprotein expression, which has not yet been elucidated for the common solid tumors, and the ability to achieve effective levels of circumventing agents without dose-limiting toxicities. Initial clinical studies suggested that biologically relevant concentrations of multidrug resistance modulators, eg, cyclosporine, can be achieved. Because pharmacologic agents that inhibit drug efflux by P-glycoprotein are themselves pumped out of cells by the transporter, this approach may have an inherent barrier to success. Laboratory studies have suggested alternative strategies for circumventing P-glycoprotein action, eg, the use of monoclonal antibodies directed against P-glycoprotein and liposome-encapsulated drugs.

Arg encodes a widely expressed 145 kDa protein-tyrosine kinase

Arg encodes a protein highly related to the c-abl gene product with regard to overall structural architecture as well as the amino acid sequences of their tyrosine kinase, and src-homologous 2 and 3 domains. The two genes form a distinct subfamily of non-receptor tyrosine kinases and share a common homolog in Drosophila. In this study we characterized the arg protein product by expression of its coding sequence in bacteria. The recombinant arg protein was detected in bacterial lysates by immunoblotting and exhibited a molecular mass of 145 kDa. Phosphoamino acid analysis of the arg gene product following an immune complex autokinase reaction revealed tyrosine phosphorylation and established that it possesses tyrosine kinase activity. High-titer antibody capable of detecting the cellular arg gene product was generated by expressing a carboxy-terminal segment of arg in bacteria and using the recombinant protein as an immunogen. The arg gene product was identified in cultured human cells as a 145 kDa protein that exhibited autokinase activity. Analysis of arg expression in murine tissues revealed that arg, like c-abl, is widely expressed, further extending the similarities between the two genes, and suggesting that arg probably functions in signaling pathways fundamental to many cell types.

The complete coding sequence of arg defines the Abelson subfamily of cytoplasmic tyrosine kinases

We have previously described partial genomic sequences of arg, a human gene related to c-abl, and shown that it is expressed as a 12-kilobase transcript and is located at chromosome position 1q24-25. In this study we elucidate the complete coding sequence of arg by characterization of cDNA clones. Analysis of the predicted amino acid sequence of arg revealed that it is indeed closely related to that of c-abl. The two proteins are strikingly similar with regard to overall structural architecture as well as the amino acid sequences of their tyrosine kinase and src homologous 2 and 3 domains. In addition, arg, like c-abl, is expressed as two transcripts that result from a process of alternative splicing and encode alternative protein forms that differ only in their amino termini. The two genes define the Abelson subfamily of cytoplasmic tyrosine kinases and share a common homolog in Drosophila.

Mapping of Abll within a conserved linkage group on distal mouse chromosome 1 syntenic with human chromosome 1 using an interspecific cross

A human Abelson related gene (ABLL) cDNA clone was used to detect restriction fragment length polymorphisms (RFLPs) on mouse Southern blots. Abll was mapped to mouse chromosome 1 by analysis of segregation with other distal chromosome 1 genetic polymorphisms by using a panel of DNAs from [(C3H/HeJ-gld/gld x Mus spretus) F1 x C3H/HeJ-gld/gld] interspecific backcross mice. The data indicate the following gene order: (centromere)-CD45-6.5 cM-Lamb-2-1 cM-Abll-2 cM-At-3. The results extend the analysis of a large conserved linkage group spanning nearly 30 cM on distal mouse chromosome 1 syntenic with human chromosome 1q21-32. Within this linkage group similar relative positions have been characterized in both species for C4BP, REN, CD45, LAMB2, ABLL, AT3, APOA2, and SPTA.

A novel human gene closely related to the abl proto-oncogene

A DNA sequence related to the abl proto-oncogene was identified in human placenta. Molecular cloning and nucleotide sequence analysis revealed two putative exons whose predicted amino acid sequence was most homologous to the corresponding sequences of c-abl and v-abl but was related to other tyrosine kinase genes as well. The new sequence was localized by in situ hybridization and somatic cell genetic analysis to human chromosome 1q24-25, which differs from the location of any previously identified tyrosine kinase gene. The detection of a novel 12-kb transcript by this gene in human normal and tumor cells establishes it as a new member of the tyrosine kinase family that is closely related to but distinct from c-abl.

Forward and reverse mutations affecting the kinetics and apparent molecular weight of mammalian HGPRT

Chinese hamster cells selected for resistance to 8-azaguanine following mutagenesis have hypoxanthine-guanine phosphoribosyltransferase (HGPRT; E.C. 2.4.2.8) with characteristics compatible with different mutations in the structural gene for that enzyme. Using immunopurification and SDS-polyacrylamide electrophoresis, mutants producing antigenically active forms of the enzyme can be analyzed for changes in the molecular weight of HGPRT. Enzyme subunits from mutants RJK3 and RJK39 are reduced in molecular weight by an estimated 4 and 2%, respectively. HGPRT activity is not detectable in RJK39. The enzyme from RJK3 is active but has altered substrate binding properties. Enzymes from two other mutants with altered kinetic properties, RJK44 and RJK47, have normal molecular weights. The genetic alterations of RJK44 and 47 are probably missense mutations, while RJK3 and 39 might contain either deletions or mutations causing premature peptide chain termination. Somatic cell hybridization between RJK39 and a revertant of that strain with HGPRT of normal molecular weight revealed that the revertant probably arose by intragenic mutation rather than extragenic mutation or suppression.