Drug Binding Domains of MRP1 (ABCC1) as Revealed by Photoaffinity Labeling

Drug resistance is a major impediment in the treatment of cancer patients receiving single or multiple drug treatment. Efforts to reverse drug resistance of tumor cells have not been successful. In recent years, considerable emphasis has been placed on understanding the underlying mechanisms that confer drug resistance. The expression of the multidrug resistance protein 1 (MRP1 or ABCC1) in cancer cells has been shown to confer resistance to diverse classes of anti-cancer drugs. MRP1 is a member of the ATP-binding cassette (ABC) family whose function, in tumor cells, is to reduce drug accumulation through energized drug efflux. To learn more about the functions of MRP1 in tumor drug resistance, knowledge of the protein binding characteristics and the location of its binding sites are essential. Photoaffinity labeling (PAL) has emerged as a leading technique that can rapidly shed light on a protein's drug binding characteristics and ultimately drug binding domains. Several MRP1-specific photoreactive probes have been developed. PAL of MRP1 was first demonstrated with the quinoline-based drug, IAAQ. Other studies showed that the high affinity endogenous substrate of MRP1, LTC(4), has intrinsic photoreactive properties and binds within both N- and C-terminal domains of MRP1. LTC(4) is conjugated to glutathione (GSH), a property common to several MRP1 substrates. In addition, several unconjugated drugs have been identified that interact with MRP1: [(3)H]VF-13,159, IAAQ, IACI and IAARh123. Mapping studies showed that IACI and IAARh123 bind two sites within transmembrane (TM) regions 10-11 and 16-17 of MRP1. Interestingly, the GSH-dependent PAL of [(125)I]azidoAG-A and [(125)I]LY475776 occurs within, or proximal to TM 16-17. The PAL with several analogs of GSH, IAAGSH and azidophenacyl-[(35)S]GSH found to interact specifically with MRP1 within TM 10-11 and TM 16-17 in addition to binding two cytoplasmic regions in MRP1, L0 and L1. This review focuses on the use of PAL for studying MRP1 interactions with various drugs and cell metabolites. Furthermore, knowledge of MRP1 drug binding domains, as identified by PAL with various photoreactive drug analogs, provides an important first step towards more detailed analyses of MRP1 binding domains.

Functional expression of multidrug resistance protein 1 in Pichia pastoris

Overexpression of the multidrug resistance-associated protein (MRP1) causes multidrug resistance in cultured cells. MRP1 transports a large number of glutathione, glucuronide, and sulfate-conjugated organic anions by an ATP-dependent efflux mechanism. Six other MRP proteins exist (MRP2-7), and mutations in some of these genes cause major pathological conditions in humans. A detailed characterization of the structure and mechanism of action of these proteins requires an efficient expression system from which large amounts of active protein can be obtained. We report the expression of a recombinant MRP1 in the methylotrophic yeast Pichia pastoris. The protein is expressed in the membrane fraction of these cells, as a stable and underglycosylated 165 kDa peptide. Expression levels are very high, and 30 times superior to those seen in multidrug-resistant HeLa/MRP1 transfectants. MRP1 expressed in P. pastoris binds 8-azido[alpha-(32)P]ATP in a Mg(2+)-dependent and EDTA-sensitive fashion, which can be competed by a molar excess of ADP and ATP. Under hydrolysis conditions (at 37 degrees C), orthovanadate induces trapping of the 8-azido[alpha-(32)P]nucleotide in MRP1, which can be further modulated by known MRP1 ligands. MRP1 is also labeled by a photoactive analogue of rhodamine 123 (IAARh123) in P. pastoris/MRP1 membranes, and this can be competed by known MRP1 ligands. Finally, MRP1-positive membrane vesicles show ATP-dependent uptake of LTC(4). Thus, MRP1 expressed in P. pastoris is active and shows characteristics of MRP1 expressed in mammalian cells, including drug binding, ligand-modulated formation of the MRP1-MgADP-P(i) intermediate (ATPase activity), and ATP-dependent substrate transport. The successful expression of catalytically active and transport-competent MRP1 in P. pastoris should greatly facilitate the efficient production and isolation of the wild type or inactive mutants of MRP1, or of other MRP proteins for structural and functional characterization.

Major photoaffinity drug binding sites in multidrug resistance protein 1 (MRP1) are within transmembrane domains 10-11 and 16-17

MRP1 is an ABC (or ATP binding cassette) membrane transport protein shown to confer resistance to structurally dissimilar drugs. Studies of MRP1 topology suggested the presence of a hydrophobic N-domain with five potential membrane-spanning domains linked to an MDR1-like core (MSD1-NBD1-L1-MSD2-NBD2) by an intracellular linker domain (L0). MRP1-mediated multidrug resistance is thought to be due to enhanced drug efflux. However, little is known about MRP1-drug interaction and its drug binding site(s). We previously developed several photoreactive probes to study MRP1-drug interactions. In this report, we have used eight MRP1-HA variants that were modified to have hemagglutinin A (HA) epitopes inserted at different sites in MRP1 sequence. Exhaustive in-gel digestion of all IAARh123 photoaffinity-labeled MRP1-HA variants revealed the same profile of photolabeled peptides as seen for wild type MRP1. Photolabeling of the different MRP1-HA variants followed by digestion with increasing concentrations of trypsin or Staphylococcus aureus V8 protease (1:800 to 1:5 w/w) and immunoprecipitation with anti-HA mAb identified two small photolabeled peptides ( approximately 6-7 kDa) from MRP1-HA(574) and MRP1-HA(1222). Based on the location of the HA epitopes in the latter variants together with molecular masses of the two peptides, the photolabeled amino acid residues were localized to MRP1 sequences encoding transmembranes 10 and 11 of MSD1 (Ser(542)-Arg(593)) and transmembranes 16 and 17 of MSD2 (Cys(1205)-Glu(1253)). Interestingly, the same sequences in MRP1 were also photolabeled with a structurally different photoreactive drug, IACI, confirming the significance of transmembranes 10, 11, 16 and 17 in MRP1 drug binding. Taken together, the results in this study provide the first delineation of the drug binding site(s) of MRP1. Furthermore, our findings suggest the presence of common drug binding site(s) for structurally dissimilar drugs.

Rhodamine 123 binds to multiple sites in the multidrug resistance protein (MRP1)

The mechanisms of MRP1-drug binding and transport are not clear. In this study, we have characterized the interaction between MRP1 and rhodamine 123 (Rh123) using the photoreactive-iodinated analogue, [(125)I]iodoaryl azido-rhodamine 123 (or IAARh123). Photoaffinity labeling of plasma membranes from HeLa cells transfected with MRP1 cDNA (HeLa-MRP1) with IAARh123 shows the photolabeling of a 190 kDa polypeptide not labeled in HeLa cells transfected with the vector alone. Immunoprecipitation of a 190 kDa photolabeled protein with MRP1-sepcific monoclonal antibodies (QCRL-1, MRPr1, and MRPm6) confirmed the identity of this protein as MRP1. Analysis of MRP1-IAARh123 interactions showed that photolabeling of membranes from HeLa-MRP1 with increasing concentrations of IAARh123 was saturable, and was inhibited with excess of IAARh123. Furthermore, the photoaffinity labeling of MRP1 with IAARh123 was greatly reduced in the presence of excess Leukotreine C(4) or MK571, but to a lesser extent with excess doxorubicin, colchicine or chloroquine. Cell growth assays showed 5-fold and 14-fold increase in the IC(50) of HeLa-MRP1 to Rh123 and the Etoposide VP16 relative to HeLa cells, respectively. Analysis of Rh123 fluorescence in HeLa and HeLa-MRP1 cells with or without ATP suggests that cross-resistance to Rh123 is in part due to reduced drug accumulation in the cytosol of HeLa-MRP1 cells. Mild digestion of purified IAARh123-photolabeled MRP1 with trypsin showed two large polypeptides (approximately 111 and approximately 85 kDa) resulting from cleavage in the linker domain (L1) connecting the multiple-spanning domains MSD0 and MSD1 to MSD2. Exhaustive proteolysis of purified IAARh123-labeled 85 and 111 kDa polypeptides revealed one (6 kDa) and two (approximately 6 plus 4 kDa) photolabeled peptides, respectively. Resolution of total tryptic digest of IAARh123-labeled MRP1 by HPLC showed three radiolabeled peaks consistent with the three Staphylococcus aureus V8 cleaved peptides from the Cleveland maps. Together, the results of this study show direct binding of IAARh123 to three sites that localize to the N- and C-domains of MRP1. Moreover, IAARh123 provides a sensitive and specific probe to study MRP1-drug interactions.

The multidrug resistance protein is photoaffinity labeled by a quinoline-based drug at multiple sites

Tumor cells overcome cytotoxic drug pressure by the overexpression of either or both transmembrane proteins, the P-glycoprotein (P-gp) and the multidrug resistance protein (MRP). The MRP has been shown to mediate the transport of cytotoxic natural products, in addition to glutathione-, glucuronidate-, and sulfate-conjugated cell metabolites. However, the mechanism of MRP drug binding and transport is at present not clear. In this study, we have used a photoreactive quinoline-based drug, N-(hydrocinchonidin-8'-yl)-4-azido-2-hydroxybenzamide (IACI), to show the photoaffinity labeling of the 190 kDa protein in membranes from the drug resistant SCLC H69/AR cells. The photoaffinity labeling of the 190 kDa protein by IACI was saturable and specific. The identity of the IACI-photolabeled protein as the MRP was confirmed by immunoprecipitation with the monoclonal antibody QCRL-1. Furthermore, a molar excess of leukotriene C(4), doxorubicin, colchicine, and other quinoline-based drugs, including MK571, inhibited the photoaffinity labeling of the MRP. Drug transport studies showed lower IACI accumulation in MRP-expressing cells which was reversed by depleting ATP levels in H69/AR cells. Mild digestion of the purified IACI-photolabeled MRP with trypsin showed two large polypeptides ( approximately 111 and approximately 85 kDa). The 85 kDa polypeptide which contains the QCRL-1 and MRPm6 monoclonal antibody epitopes corresponds to the C-terminal half of the MRP (amino acids approximately 900-1531) containing the third multiple spanning domain (MSD3) and the second nucleotide binding site. The 111 kDa polypeptide which contains the epitope sequence of the MRPr1 monoclonal antibody encodes the remainder of the MRP sequence (amino acids 1-900) containing the MSD1 and MSD2 plus the first nucleotide binding domain. Cleveland maps of purified IACI-labeled 85 and 111 kDa polypeptides revealed 6 kDa and approximately 6 plus 4 kDa photolabeled peptides, respectively. In addition, resolution of the exhaustively digested IACI-photolabeled MRP by HPLC showed two major and one minor radiolabeled peaks that eluted late in the gradient (60 to 72% acetonitrile). Taken together, the results of this study show direct binding of IACI to the MRP at physiologically relevant sites. Moreover, IACI photolabels three small peptides which localize to the N- and C-halves of the MRP. Finally, IACI provides a sensitive and specific probe for studying MRP-drug interactions.

Reversal of MRP-mediated doxorubicin resistance with quinoline-based drugs

The overexpression of P-glycoprotein (P-gp) and the multidrug resistance-associated protein (MRP) have been shown to confer broad drug resistance in tumor cells. We have demonstrated previously direct binding between MRP and a quinoline-based photoreactive drug (iodo-azido-amino quinoline, IAAQ) (Vezmar et al., Biochem Biophys Res Commun 241: 104-111, 1997). In this report, we show the reversal of multidrug resistance in two MRP-overexpressing cell lines, HL60/AR and H69/AR, with four quinoline-based drugs. Non-toxic concentrations (5-20 microM) of chloroquine, quinine, quinidine, and primaquine potentiated the toxicity of doxorubicin in a concentration-dependent manner. These quinoline-based drugs showed a 5- to 10-fold decrease in the IC(50) of doxorubicin in H69/AR and HL60/AR cells. Primaquine was the most active, with modulation ratios of 10- and 5-fold versus 8- and 3-fold with MK-571 for H69/AR and HL60/AR, respectively. Moreover, using IAAQ, we showed that molar excesses of chloroquine, quinine, quinidine, and MK-571 inhibit the photoaffinity labeling of MRP. Primaquine and vinblastine showed lesser inhibition of MRP photoaffinity labeling by IAAQ. Taken together, the results of this study demonstrated the reversal of doxorubicin resistance with several quinoline-based drugs. Moreover, these drugs have been shown to reverse P-gp-mediated MDR and are clinically well tolerated.

Pleiotropic resistance to diverse antimalarials in actinomycin D-resistant Plasmodium falciparum

The development and spread of multidrug-resistant Plasmodium falciparum are major health concerns. The molecular mechanisms of multidrug resistance, including resistance to many quinoline-based antimalarials, are largely unknown. In this study, we report on the isolation and partial characterization of actinomycin D (actD)-resistant P. falciparum (3D7(R)/actD2.3) from a chloroquine-susceptible strain, 3D7. The stepwise selection of an actD-resistant clone (3D7(R)/actD2.3) led to the isolation and cloning of P. falciparum that grew in the presence of 2 ng/mL of actD. The parental isolate (3D7) did not grow in the presence of a 10-fold lower drug concentration (0.2 ng/mL). The latter estimate of parasite growth was determined by direct counting of parasites in infected red blood cells. Estimates of drug resistance levels to actD, using a [(3)H]hypoxanthine uptake and incorporation method, showed a 3-fold difference in the IC(50) between 3D7 and 3D7(R)/actD2.3. Interestingly, 3D7(R)/actD2.3 P. falciparum parasites were less sensitive to several antimalarials (chloroquine, mefloquine, quinidine, and artemisinin) and to the mitochondrial specific dye Rhodamine 123. Drug transport studies using [(3)H]actD showed that 3D7(R)/actD2.3 accumulated less drug than 3D7. Moreover, the accumulation of [(3)H]actD was energy dependent. To determine if Pfmdr1 expression, previously implicated in drug resistance to certain antimalarials, mediated the resistance phenotype of 3D7(R)/actD2.3, Pfmdr1 levels in 3D7 and 3D7(R)/actD2.3 were compared by Southern and northern blot analyses. Our results revealed no differences in Pfmdr1 copy number or mRNA levels between 3D7 and 3D7(R)/actD2.3. Furthermore, comparison of Pfmdr1 sequences between 3D7 and 3D7(R)/actD2.3 showed no differences. In addition, verapamil, which reverses P-glycoprotein-mediated drug resistance in mammalian cells, did not reverse the resistance of 3D7(R)/actD2.3 to actD or chloroquine. Taken together, the findings of this study demonstrated that in vitro selection of P. falciparum for resistance to actD leads to decreased sensitivity to diverse drugs and that this pleiotropic drug resistance is associated with reduced drug accumulation not mediated by Pfmdr1.

Cloning and partial characterization of the proteasome S4 ATPase from Plasmodium falciparum

Certad, G., Abrahem, A., and Georges, E. 1999. Cloning and Partial characterization of the proteasome S4 ATPase from Plasmodium falciparum. Experimental Parasitology 93, 123-131. The ATP-ubiquitin-proteasome pathway mediates the nonlysosomal degradation of cytosolic proteins in eukaryotic cells. The activities of this pathway have been shown to regulate cell growth and differentiation through modulation of regulatory proteins. The proteasome is a large complex consisting of two multisubunit structures, the 20S and 19S(PA700) or P28 complexes, that combine to form the 26S particles. In this study, we describe the cloning of a cDNA encoding the proteasome subunit 4 ATPase homologue from Plasmodium falciparum (PFS4). Analysis of the PFS4 cDNA sequence shows an open reading frame encoding a deduced protein of 455 amino acids. Moreover, comparison of PFS4 cDNA sequence to that of genomic fragments encoding PFS4 showed identical sequences with no detectable introns. Database searches revealed a high sequence identity to those of rice, yeast, mouse, Drosophila, and human S4 ATPases. However, PFS4 contains two unique inserts of nine and seven amino acid residues in the N-terminal domain. Interestingly, only the rice S4 contains the latter (seven amino acids) insert with four identical amino acids. In vitro expression of the full-length cDNA encoding the PFS4, using a transcription-translation-coupled reticulocyte lysate, shows a 50-kDa [(35)S]methionine-labeled protein which was immunoprecipitated with PFS4 anti-peptide antiserum. Southern blot analysis of genomic DNA digests shows a single gene copy of PFS4 in P. falciparum. Of interest was the effect of the proteasome-specific natural product, lactacystin, on the growth of the parasite, with IC(50) values of 0.6-0.92 microM. The latter IC(50) values of lactacystin for different clones of P. falciparum are comparable to those obtained for mammalian cell lines (0.65 microM), suggesting the presence of a conserved proteasome complex. Moreover, lactacystin was equally toxic to drug-sensitive and resistant parasites.

[Disseminated infestation of Enterocytozooon bieneusi a an HIV-infected patient]

An HIV-positive patient developed disseminated Enterocytozoon bieneusi infection. The parasite was identified in stool, duodenal biopsy, nasal discharge, and sputum specimens using transmission electron microscopy. Albendazole therapy failed to improve the symptoms or eradicate the parasite. The patient survived for nine months after the diagnosis of E. bieneusi infection.

Direct binding of chloroquine to the multidrug resistance protein (MRP): possible role for MRP in chloroquine drug transport and resistance in tumor cells

Multidrug resistance protein (MRP) transports a range of compounds that include glutathione S-conjugates, amphiphilic anionic drugs, and natural-product toxins. However, the mechanism of MRP drug binding and transport is presently unclear. We recently demonstrated the direct binding of a quinoline-based photoactive drug, N-[4-[1-hydroxy-2-(dibutylamino)ethyl]quinolin-8-yl]-4-az idosalicylamide (IAAQ), to MRP at a biologically relevant site [Vezmar et al., Biochem Biophys Res Commun 241: 104-111, 1997]. In the present report, we demonstrated that the lysosomotropic or antimalarial drug chloroquine is a substrate for MRP. Specifically, our results showed that chloroquine, similar to leukotriene C4 (LTC4) and 3-(3-(2-(7-chloro-2-quinolinyl)ethenyl-phenyl)((3-(dimethyl amino-3-oxo propyl)thio)methyl)thio) propanoic acid (MK 571), inhibits the photoaffinity labeling of MRP by IAAQ. Furthermore, cell growth assays showed MRP-expressing multidrug-resistant cells (H69/AR and HL60/AR) to be more resistant to chloroquine than their parental cells (i.e., IC50 of 121 microM versus 28 microM chloroquine for H69/AR and H69, respectively). Moreover, MK 571, an LTD4 receptor antagonist, reversed the resistance of H69/AR cells to chloroquine. Drug transport studies using [14C]chloroquine demonstrated that MRP-expressing cells accumulate less drug than the parental drug-sensitive cells. The reduced accumulation of [14C]chloroquine in resistant cells was ATP dependent and was due to enhanced drug efflux. Taken together, the results of this study show that MRP modulates the transport of chloroquine by direct binding.

Enterocytozoon bieneusi multiorgan microsporidiosis in a HIV-infected patient

Multiorgan microsporidiosis due to Enterocytozoon bieneusi was diagnosed in an HIV-infected patient. The parasite was found and identified as E. bieneusi by transmission electron microscopy in stools, duodenal biopsy, nasal discharge and sputum. No clinical improvement or parasite eradication was obtained after albendazole therapy, but the patient remained alive 9 months after diagnosis.

The quinoline-based drug, N-[4-[1-hydroxy-2-(dibutylamino)ethyl] quinolin-8-yl]-4-azidosalicylamide, photoaffinity labels the multidrug resistance protein (MRP) at a biologically relevant site

MRP is a member of the ABC trafficking proteins thought to mediate the transport of glutathione S-conjugates and amphiphilic natural products. However, unlike P-glycoprotein, the biochemical mechanism by which MRP mediates the resistance to cytotoxic drugs is not clear. In this report, we describe the interactions of a quinoline-based drug, N-{4-[1-hydroxy-2-(dibutylamino)ethyl] quinolin-8-yl}-4-azidosalicylamide (IAAQ), with MRP. Our results demonstrate the ability of IAAQ to photoaffinity label a 190 kDa protein in resistant Small Cell Lung Cancer cells (H69/AR) but not in the parental H69 cells. The photoaffinity labeling of the 190 kDa protein with IAAQ was both saturable and specific. The identity of the 190 kDa protein, as MRP, was confirmed by immunoprecipitation with the monoclonal antibody, QCRL-1. Furthermore, a molar excess of LTC4, MK 571 or vinblastine inhibited the photoaffinity labeling of MRP with IAAQ in intact cells and plasma membranes. Cell growth and drug transport studies showed H69/AR cells to be less sensitive to and to accumulate less IAAQ than the parental H69 cells. In addition, MK 571 and doxorubicin increased the sensitivity to and the accumulation of IAAQ in H69/AR cells. Together, the results of this study show for the first time the direct binding of unaltered cytotoxic drug to MRP. Moreover, given the structural similarities between IAAQ and MK 571, we suggest that MK 571 modulates MRP-mediated resistance by direct binding to MRP.

Overexpression of a 40-kDa protein in human multidrug resistant cells

The use of anticancer drugs in the chemotherapeutic treatment of cancer patients frequently results in the emergence of drug resistant tumors. Selection of tumor cell lines in vitro has led to the identification of several proteins that mediate drug resistance to anticancer drugs. In this study, an immuno-dot blot method was used to isolate a monoclonal antibody (IPM96) which recognized a 40 kDa protein (or P-40) co-expressed with P-glycoprotein and MRP in several multidrug resistant cell lines (MCF-7/Adr, SKOV/VLB1.0, H69/Adr, and HL60/AR). Furthermore, P-40 levels dropped significantly in one revertant cell line (H69/PR) derived from H69/AR cells. Interestingly, the expression of P-40 was also higher in two tumor cell lines (SKTax6a and A2780CP) that were selected with paclitaxel or cisplatin but do not express P-gp or MRP. Immuno-fluorescence staining of cells with IPM96 showed both membrane and cytoplasmic staining. These results were confirmed by Western blot analysis of different subcellular fractions from MCF-7/Adr cells. The membrane bound P-40 was resistant to extraction with high salt, chelating agents, and denaturing agents, but was solubilized with 10 mM CHAPS. Taken together, the overexpression of P-40 in multidrug resistant cells has not been previously determined and therefore could be important in the expression of the drug resistance phenotype.

Toxoplasmosis in kidney transplant recipients: report of six cases and review

Six patients with toxoplasmosis complicating renal transplantation are described, and 25 other reported cases are reviewed. The mean age of the 31 patients was 35.16 years. Most of the recipients (25 of 29) showed signs of toxoplasmosis within 3 months post-transplantation, with fever, neurological disturbances, and pneumonia as the main clinical features. Diagnosis was established at autopsy in 15 cases, by serology in 13 cases, and by direct examination, culture, or polymerase chain reaction of biological samples in 5 cases. Seventeen patients also had concomitant infections. The donor was the likely source of transmission to 10 recipients; reactivation was suspected in two cases. The source of transmission could not be determined for the remaining 19 patients. The mortality rate was 64.5%. Ten of the 11 patients given specific treatment survived, indicating that early diagnosis and therapy are essential.

Reversal of P-glycoprotein-associated multidrug resistance by ivermectin

P-Glycoprotein (P-gp) causes a multidrug resistance (MDR) phenotype in tumour cells. In some cancers, the expression of P-gp has been correlated with low clinical response to chemotherapy and survival of patients. Previous studies have shown that certain lipophilic drugs bind to P-gp and reverse the MDR phenotype of tumour cells. In this study, we extend that list of compounds and present evidence for the capacity of a potent and clinically safe anthelmintic, ivermectin (IVM), as an MDR-reversing drug. Using a highly drug-resistant human cell line, we compared IVM with other MDR-reversing agents and showed that IVM is 4- and 9-fold more potent than cyclosporin A and verapamil, respectively. The capacity of IVM to inhibit iodoaryl-azidoprazosin photolabeling of P-gp is consistent with direct binding to P-gp. Studies showed that [3H]IVM binding to membranes from resistant cells is specific and saturable with KD and Bmax values of 10.6 nM and 19.8 pmol/mg, respectively. However, while cyclosporin A or vinblastine inhibited [3H]IVM binding to membranes from drug-resistant but not drug-sensitive cells, neither verapamil nor colchicine had any effect. Furthermore, both IVM and cyclosporin A and, to a lesser extent, verapamil also inhibited [3H]vinblastine binding to membranes from drug-resistant cells. Drug transport studies showed that [3H]IVM is a substrate for the P-gp drug efflux pump. However, it was transported less efficiently by P-gp than [3H]vinblastine. Moreover, only cyclosporin A was effective in potentiating the accumulation of [3H]IVM in drug-resistant cells. Taken together, the high efficiency of MDR reversal by IVM combined with its low toxicity are consistent with the properties of an ideal MDR-reversing agent.

N-ethylmaleimide increases P-glycoprotein photoaffinity labeling with iodoaryl-azidoprazosin in multidrug resistant cells

P-glycoprotein (P-gp) mediates a multidrug resistance (MDR) phenotype in tumor cell lines selected with lipophilic cytotoxic drugs. Transport studies using purified P-glycoprotein reconstituted into defined liposomes have shown energy-dependent drug efflux of structurally dissimilar drugs. In this report, we have examined the effects of N-ethylmaleimide, a potent inhibitor of the P-gp ATPase, on P-gp drug binding in intact MDR cells and in plasma membranes. Our results show that short term treatment of MDR cells with 1-50 microM N-ethylmaleimide led to a concentration dependent increase in P-gp photoaffinity labeling with iodoaryl-azidoparazosin (IAAP). In addition, N-ethylmaleimide increases [3H] vinblastine accumu-lation in drug-resistant but not in sensitive cells. Comparison of IAAP photolabeled P-gp from intact cells with or without N-ethylmaleimide treatment did not show differences in the pattern of IAAP photolabeled peptides. Thus, the observed increase in P-gp photolabeling with IAAP in N-ethylmaleimide treated cells is not due to photolabeling at different sites. Incubation of MDR cells with [14C] N-ethylmaleimide showed that P-gp is directly modified at several Cysteine residues, as found from a complete proteolytic digestion of [14C] Nethylmaleimide labeled P-gp. The comparison of V8 staphylococcus aureas peptides from [14C] Nethylmaleimide or IAAP modified P-gp showed some peptides to co-migrate on SDS PAGE. However, modification of plasma membranes from drug resistant cells treated with N-ethylmaleimide did not show a dose-dependent increase in P-gp photolabeling with IAAP as seen with intact MDR cells. Interestingly, N-ethylmaleimide increases P-gp phosphorylation by inhibiting the turnover of Pgp phosphates. However, inhibition of P-gp phosphorylation with calyculin A did not show an increase in P-gp photolabeling in MDR cells. Taken together, the results of this study suggest that N-ethylmaleimide potentiates P-gp photolabeling with IAAP by inhibiting P-gp ATPase thereby increasing the local concentration of IAAP in intact MDR cells. Furthermore, inhibition of P-gp ATPase by N-ethylmaleimide does not lead to conformational changes that affects P-gp drug binding.

Sensitization to doxorubicin resistance in breast cancer cell lines by tamoxifen and megestrol acetate

Acquired drug resistance is a major factor in the failure of doxorubicin-based chemotherapy in breast cancer. We determined the ability of megestrol acetate and/or tamoxifen to reverse doxorubicin drug resistance in a doxorubicin-resistant breast cancer line (the human MCF-7/ADR). The cytotoxicity of doxorubicin, megestrol acetate, and/or tamoxifen was determined in the sensitive and resistant cell lines utilizing the sulphorhodamine B assay. Tamoxifen alone produced an IC50 (concentration resulting in 50% inhibition of control growth) of 10.6 microM, whereas megestrol acetate alone resulted in an IC50 of 48.7 microM in the MCF-7/ADR cell line. The IC50 of doxorubicin in MCF-7/ADR was 1.9 microM. Neither megestrol acetate alone nor tamoxifen alone at 1 or 5 microM altered the IC50 of doxorubicin. However, the combination of tamoxifen (1 or 5 microM) and megestrol acetate (1 or 5 microM) synergistically sensitized MCF-7/ADR cells. Additionally, megestrol acetate and tamoxifen inhibited iodoarylazidoprazosin binding to P-glycoprotein, and, in their presence, there was an increased doxorubicin accumulation in the MCF-7/ADR cells. Furthermore, the combination of tamoxifen and megestrol acetate had much less effect on the cytotoxicity of doxorubicin in MCF-7 wild-type cells. Clinically achievable concentrations of tamoxifen and megestrol acetate can largely sensitize MCF-7/ADR to doxorubicin. The combination of these three drugs in a clinical trial may be informative.

BIBW22 BS, potent multidrug resistance-reversing agent, binds directly to P-glycoprotein and accumulates in drug-resistant cells

The expression of P-glycoprotein (P-gp) in tumor cells causes a multidrug resistance (MDR) phenotype. P-gp has been shown to mediate the transport of structurally dissimilar drugs across the cell membrane in an energy-dependent manner. In this report, we show that BIBW22 BS, a phenylpteridine analog, reverses the MDR phenotype of CEM human lymphoma cells in a dose-dependent fashion. Using a photoactive analog of BIBW22 BS {[3H]azido-4-[N-(2-hydroxy-2-methylpropyl)-ethanolamino]-2, 7-bis(cis-2,6-dimethyl-morpholino)-6-phenylpteridine}, we show the photoaffinity labeling of a 170-kDa protein in drug-resistant cells immunoprecipitated with P-gp-specific monoclonal antibodies. The photolabeling of P-gp by [3H]azido-BIBW22 BS was specific and saturable. Furthermore, BIBW22 BS, vinblastine, and verapamil, but not colchicine, inhibited the photolabeling of P-gp by [3H]azido-BIBW22 BS. Drug binding studies showed that membranes from MDR cells bound more BIBW22 BS than parental drug-sensitive cells, and this binding was inhibited with vinblastine and, to a lesser extent, with uridine. However, drug transport studies demonstrated that BIBW22 BS is not a substrate for P-gp efflux pump. Interestingly, BIBW22 BS was shown to accumulate more in resistant cells. Also, BIBW22 BS accumulation in drug-sensitive and -resistant cells was not energy dependent. These results are in contrast with the observed decrease in accumulation or enhanced efflux of [3H]vinblastine seen in the same MDR cells. A comparison of [3H]azido-BIBW22 BS or [3H]azidopine photolabeled P-gp by Cleveland mapping with Staphylococcus aureus V8 protease showed differences in the photolabeled peptides. Taken together, the results of this study show that BIBW22 BS is a potent MDR-reversing agent that binds directly to P-gp but is not effluxed from drug-resistant cells.

Fetal ultrasound imaging and the production of authoritative knowledge in Greece

In Greece repetitive and intensive fetal scanning is now a universal feature of prenatal care. This article examines some of the ways in which pregnant women and obstetricians experience the intensive use of fetal ultrasound in a small city in eastern Greece. Based on observations and interviews conducted in a public hospital, it is argued that fetal imaging plays a privileged role in the production of authoritative knowledge around pregnancy for both doctors and women. The authority of the technology rests primarily on its ability to create a straightforward sense of reality and visual pleasure. These qualities lead women to actively demand fetal scanning, which helps them to feel the reality of their pregnancies, reassures them of fetal health, and provides a pleasurable sense of contact with, and knowledge about, the fetus. Doctors freely offer multiple scans to attract women to the public hospital, to practice "modern" obstetrics, and to negotiate among themselves for control over the management of pregnant women.

p-Azidosalicyl-5-amino-6-phenoxybenzimidazole photolabels the N-terminal 63-103 amino acids of Haemonchus contortus beta-tubulin 1

Benzimidazoles (BZ) are broad spectrum anthelmintics thought to exert their effects by interacting with and disrupting the functions of microtubules. However, direct biochemical evidence for binding between BZ and tubulin has not been shown nor is it known what sequences in tubulin interact with BZ. In this study, a photoactive analogue of 2-acetamido-5-(3-aminophenoxy)benzimidaz ole that has biological activity similar to other benzimidazoles was synthesized and used to photoaffinity label cell lysates from the parasitic nematode of sheep Haemonchus contortus. The photoactive analogue, 2-acetamido-5-[3-(4-azido-3-125I-salicyl amido)phenoxy]benzimida zol e or 125I-ASA-BZ, was shown to photolabel a 54-kDa protein that was specifically immunoprecipitated with anti-tubulin monoclonal antibodies. Tubulin photoaffinity labeling by 125I-ASA-BZ was also inhibited with molar excess of various BZ analogues and colchicine. Interestingly, 125I-ASA-BZ photoaffinity-labeled the beta- and not the alpha-subunits of tubulin. Proteolytic digestion of 125I-ASA-BZ-labeled tubulin with Staphylococcus aureus V8 proteinase revealed one major peptide with an apparent molecular mass of 3.5 kDa. Exhaustive digestion of 125I-ASA-BZ-labeled beta-tubulin with trypsin resulted in two fractions containing radioactive peptides. Protein sequencing of the high performance liquid chromatography-purified tryptic ASA-BZ-photolabeled peptides identified the N-terminal 63-77 and 78-103 sequences as the BZ binding domain.

Abortion policy and practice in Greece

Despite its illegality until recently, abortion is estimated to have been responsible for almost half of the sharp postwar decline in the Greek birth rate. This article examines abortion as a part of a Greek contraceptive culture which has taken shape during the postwar period both in response, and in resistance to, a variety of macro- and micropolitical institutions and forces. During much of this period, pronatalist policies and discourses of both state and church combined to foreclose most medical contraceptive alternatives. In contrast, illegal abortion was a relatively safe, medicalized procedure widely practiced by doctors. Even after being legalized in 1980, female medical contraceptive methods continue to be rejected by the great majority of Greek women, and abortion and male methods of birth control remain the principal means of controlling fertility. The article focuses on the specific abortion practices and meanings of three generations of married women living in the city of Rhodes, capital of the Dodecanese Province of Greece's Eastern Aegean, and explores the ways in which they have been shaped by, and reflect, local cultural understandings of the body, health, sexuality, morality, motherhood and childhood, as well as micropolitical relations within the family.

Identification of a sister gene to P-glycoprotein

The P-glycoproteins (Pgps) are a small family of transport proteins associated with the multidrug resistance phenotype of cell lines selected for growth in cytotoxic drugs. Utilizing low stringency screening, we have identified a novel gene closely related to the Pgps expressed in the pig and other mammalian liver which we have called Sister of P-glycoprotein (spgp). Sequence of this gene shows it to be a member of the ATP-binding cassette family of transporters and the gene most closely related to Pgp identified to date. The function of spgp is not known, but it can be recognized by at least one Pgp mAb, C219. This cross-reactivity has implications for expression studies in tissues and tumors utilizing this and other Pgp antibodies.

Benzimidazoles, potent anti-mitotic drugs: substrates for the P-glycoprotein transporter in multidrug-resistant cells

P-glycoprotein is though to mediate the energy-dependent efflux of many structurally and functionally unrelated lipophilic compounds. Presently, the molecular mechanism underlying the binding and efflux of drugs by P-glycoprotein is not well understood. However, it has been suggested that two planar benzene ring structures and a cationic charge are commonly found in many drugs that interact with P-glycoprotein. The benzimidazoles (BZs) are potent anti-tumour, anti-fungal and anti-parasitic agents, whose mode of action is thought to result from their inhibition of microtubule functions. Although other classes of microtubule inhibitors, such as colchicine and vinblastine, have been studied extensively with respect to their interaction and efflux by P-glycoprotein, the BZ group of drugs has not been characterized. In this study, we have characterized the interaction of BZ with multidrug-resistant cells and found that resistant cells accumulated substantially less BZ compared with drug-sensitive cells. Furthermore, BZ was more toxic to sensitive than to drug-resistant cells, suggesting that BZ is likely to be a substrate for the P-glycoprotein drug efflux pump. In addition, we used a photoactive analogue of BZ ([125I]ASA-BZ) to demonstrate a direct binding between BZ and P-glycoprotein. Results showing that a molar excess of vinblastine, unmodified BZ, verapamil and rhodamine 123, but not colchicine, inhibited the photoaffinity labelling of P-glycoprotein by [125I]ASA-BZ confirmed the binding specificity of BZ to P-glycoprotein. Protease digestion of [125I]ASA-BZ photoaffinity labelled P-glycoprotein yielded two peptides that were similar to those obtained with other P-glycoprotein-associated drugs, e.g. azidopine and iodoaryl azidoprazosin. Taken together, these results demonstrate a direct and specific interaction between P-glycoprotein and BZ in a manner that is probably similar to other previously characterized P-glycoprotein-associated drugs.

Characterization of rhodamine 123 binding to P-glycoprotein in human multidrug-resistant cells

The overexpression of P-glycoprotein is currently believed to be responsible for the enhanced efflux or decreased influx of cytotoxic drugs across the cell membrane in drug-resistant cells. P-glycoprotein has been proposed to mediate the efflux of a large number of structurally and functionally unrelated drugs. Although it has been suggested that P-glycoprotein binds directly to many lipophilic cations, it remains unclear whether one or more sites in P-glycoprotein mediate its broad substrate specificity. In this report, a photoactive derivative of rhodamine 123 (Rh123) [125I-azidosalicylic acid (ASA)-Rh123] was synthesized and used in a photoaffinity labeling assay to demonstrate, for the first time, direct and specific binding to P-glycoprotein. The photoaffinity labeling of P-glycoprotein by ASA-Rh123 was specifically inhibited in the presence of vinblastine and verapamil but not in the presence of colchicine. Surprisingly, ASA-Rh123 photoaffinity labeled a 6-kDa V8 peptide in P-glycoprotein that was previously shown to be photoaffinity labeled by another multidrug resistance-associated drug, [125I]iodoarylazidoprazosin. Photoaffinity labeling of mitochondria from drug-sensitive or -resistant cells with 125I-ASA-Rh123 did not reveal significant differences in the mitochondrial proteins from sensitive or resistant cells. Interestingly, however, 125I-ASA-Rh123 did photolabel a 66-kDa protein in mitochondria that was not detected in plasma membrane preparations with this assay. Taken together, our results demonstrate for the first time that Rh123 binds specifically to P-glycoprotein and that its binding site may be shared by other multidrug resistance-associated drugs.

Effects of nonionic detergents on P-glycoprotein drug binding and reversal of multidrug resistance

Multidrug-resistant cells are thought to maintain low intracellular cytotoxic drug concentration though the active efflux of drugs across the cell membrane. It is presently believed that P-glycoprotein mediates this energy-dependent drug efflux by interacting directly with various lipophilic compounds. In this report, we have used [3H]azidopine in a photoaffinity labeling assay to study the effect of detergents and denaturing agents on P-glycoprotein drug binding in intact cells. Nonionic detergents such as Triton X-100 or Nonidet P-40 at very low concentrations were found to completely abolish azidopine photolabeling to P-glycoprotein and are able to reverse the multidrug resistance phenotype. In contrast, high concentrations of the denaturing agent urea or the zwitterionic detergent 1-[(3-cholamidopropyl)dimethylamino]-1-propanesulfonate did not inhibit azidopine photolabeling to P-glycoprotein. A comparison between verapamil and Triton X-100 revealed that the latter was more effective in inhibiting azidopine photolabeling to P-glycoprotein while verapamil was more effective in potentiating [3H]vinblastine accumulation in drug-resistant cells. Drug transport studies showed that [3H]Triton X-100 accumulated in both drug-sensitive and -resistant cells, and its accumulation was not modulated by excess vinblastine, verapamil, or colchicine. Taken together, these findings suggest that low concentrations of Triton X-100 reverse the multidrug resistance phenotype by inhibiting P-glycoprotein drug binding. In addition, it is also suggested that the site(s) of P-glycoprotein drug binding is localized to sequences found within the lipid bilayer.

Body fat distribution in men and women of the Hispanic health and nutrition examination survey of the United States: associations with behavioural variables

Body fat distribution is a biological risk factor for cardiovascular disease and diabetes. There are known genetic factors influencing body fat distribution, but variation in this characteristic is also attributable to human behavioural and socioeconomic variables such as social class. Björntorp has proposed that these associations may be due to a series of physiological responses to psychosocial stress, most prominently chronic stimulation of the adrenal-cortical system. This system is known to affect body fat distribution. Elsewhere we have shown that general socioeconomic status is related to body fat distribution in men and women of the Hispanic Health and Nutrition Examination Survey (HHANES) of the United States. In this paper we explore the relationship with those behavioural variables available from the HHANES which could hypothetically serve as indicators of psychosocial stress: smoking, drinking and depression. For both sexes in all Hispanic ethnic groups except Puerto Rican men, as socioeconomic status declined, subcutaneous fat became more centrally distributed. This relationship continued to be significant after controlling for the behavioural variables. A positive relationship was also found between smoking and central body fat distribution which was independent of socioeconomic status. This relationship was statistically significant for all subsamples except Cuban-American women. No consistent relationships were found between body fat distribution, drinking and depression. The data support the hypothesis that body fat distribution may be linked to the social stress of low socioeconomic status, independent of the behavioural factors tested.

Topology of P-glycoprotein as determined by epitope mapping of MRK-16 monoclonal antibody

There is growing evidence for the direct role of P-glycoprotein mediating multidrug resistance in tumor cells. P-glycoprotein is thought to function as an energy-dependent drug efflux pump. The monoclonal antibody MRK-16 binds to an external domain of P-glycoprotein and partially inhibits drug efflux in multidrug-resistant cells. As an approach toward elucidating the mechanism by which MRK-16 affects drug transport, we undertook the definition of the precise binding site of this antibody. In this study we have mapped the epitope of MRK-16 monoclonal antibody to a resolution of a single amino acid using a series of overlapping synthetic peptides. We demonstrate that MRK-16 recognizes only the class I isoform (MDR1) of human P-glycoprotein and that its epitope encompasses at least two (first and fourth) of the six predicted extracellular peptide loops. These results suggest that the epitope of MRK-16 is discontinuous and that the sequences involved which are separated by about 625 amino acids in the linear sequence must be spatially situated in close proximity in the native protein. Based on these results, we present a model for transmembrane alpha-helical packing of P-glycoprotein in the lipid bilayer. This may have implications for understanding the function of P-glycoprotein in drug transport.

Toremifene resistance in a rat mammary tumour model

The effect of Toremifene on cell growth in vitro was tested on the R3230AC rat mammary adenocarcinoma as model. Toremifene differed from Tamoxifen in that it did not induce resistance; some cross-resistance was observed in Tamoxifen tolerant tumour cell lines. Toremifene does not influence P-glycoprotein expression in this model and at the levels studied.

Molecular and biological characterization of cottontail rabbit papillomavirus variant DNA sequences integrated in the VX7 carcinoma

The transplantable VX7 carcinoma was derived from a tumor induced by a recoverable strain of cottontail rabbit papillomavirus (CRPV) able to replicate in domestic rabbits. Low levels of late viral gene expression have been retained through serial propagation in rabbits. We have cloned and characterized the three major types of CRPV sequences integrated in this tumor, a genome-length 8-kb DNA molecule and two rearranged 9- and 3.8-kb molecules. The VX7 8-kb DNA displays only a few differences in its restriction map, when compared to the wild-type (wt) CRPV DNA. The VX7 9- and 3.8-kb DNAs derive from the VX7 8-kb DNA since they share the same restriction site polymorphism. The VX7 9-kb DNA contains a duplication of the E6 open reading frame. The VX7 3.8-kb DNA results from the deletion of most of the E region and the insertion, between the borders of the deletion, of 174-nucleotide-long segment of the long control region potentially driving the expression of a truncated L2 protein. Both VX7 9- and 3.8-kb species potentially allow the expression of abnormal E6 fusion proteins. Nineteen point mutations were detected in the 3.8-kb DNA, compared to the wt CRPV DNA. None of these molecules were able to induce warts in domestic rabbits, in contrast to wt CRPV DNA. Furthermore, when cloned VX7 DNAs were inoculated together with wt CRPV DNA, none of the VX7 CRPV sequences, as identifiable by their specific restriction enzyme cleavage patterns, could be detected in the resulting warts. This suggests that CRPV sequences integrated in the VX7 carcinoma are no longer able to replicate as episomes, which might be a prerequisite for the production of warts.

Modulation of ATP and drug binding by monoclonal antibodies against P-glycoprotein

The role of P-glycoprotein in mediating the drug-resistance phenotype in multidrug resistant cells is now well documented. It is thought to function as an energy-dependent drug-efflux pump of broad specificity. Structurally, P-glycoprotein is an internally duplicated molecule containing two large multi-spanning transmembrane domains and two cytoplasmic ATP binding domains. In this report we demonstrate that monoclonal antibodies C219, C494, and C32 directed against short linear regions of the P-glycoprotein molecule inhibit ATP binding to P-glycoprotein in vitro. We also provide direct evidence that both predicted ATP-binding domains bind ATP and that there is co-operativity between the two sites. In addition, the capacity of P-glycoprotein to bind the calcium channel blocker, azidopine, is inhibited differentially by the antibodies. These observations are the first evidence linking specific perturbations of the P-glycoprotein molecule with ATP and drug binding.

R-verapamil decreases anti-estrogen resistance in a breast cancer model

Drug resistance eventually limits the effectiveness of antiestrogens in breast cancer treatment. Pharmacological reversal of this refractoriness has been attempted with R-Verapamil, a well tolerated calcium channel blocker. This drug significantly decreased the incidence of lung foci after intravenous seeding of the R3230AC rat adenocarcinoma; this effect was correlated with reduction in the expression of P-glycoprotein. The simultaneous administration of antiestrogens with a non-toxic enantiomer of Verapamil was beneficial in the tumour model investigated.

Sex-dependent and independent expression of the P-glycoprotein isoforms in Chinese hamster

P-glycoprotein (Pgp) is a small family of membrane proteins which belongs to a superfamily of energy-dependent membrane transport proteins identified in phylogenetically distant species, from bacteria to man. Among mammalian species, some of the Pgp isoforms can mediate multidrug resistance by acting as an energy-dependent drug efflux pump. However, the physiologic functions of the Pgp isoforms have not been defined. In this study we examined the expression of the three hamster Pgp isoforms in normal hamster tissues, by using isoform-specific monoclonal antibodies in a competitive immunohistochemical assay. We showed that each Pgp isoform is predominantly expressed in a small, distinct group of differentiated cells, where it is likely to function in specific secretory pathways. The expression of the Pgp isoforms appears to be tightly regulated and, at least in some cells, under complex hormonal control. Furthermore, there is a striking sex difference in Pgp content of the adrenal cortex. These findings are important for the ultimate understanding of the normal physiologic roles of the Pgp gene family members.

Identification of a membrane glycoprotein overexpressed in murine lymphoma sublines resistant to cis-diamminedichloroplatinum(II)

A series of murine thymic lymphoma cell sublines was selected in vitro for resistance to cis-diamminedichloroplatinum(II) (CDDP). The level of CDDP resistance correlated with reduced drug accumulation in these cells. A rabbit antiserum was raised against the plasma membrane of a CDDP-resistant subline and used in Western blot analyses. Increased expression of a surface antigen of approximately 200 kDa was observed and found to correlate with the degree of resistance. Further biochemical and immunological studies demonstrated that this is a plasma membrane glycoprotein. However, it is different from the multidrug resistance-associated P-glycoprotein with a molecular weight of about 170,000. We have called this unique CDDP resistance-associated membrane protein CPR-200.

Detection of P-glycoprotein isoforms by gene-specific monoclonal antibodies

P-glycoprotein is a highly conserved membrane protein shown to be overexpressed in many multidrug-resistant tumor cell lines. P-glycoprotein is encoded by a small gene family in mammalian cells. Class I and II isoforms cause multidrug resistance, whereas class III does not. In this report, we have characterized three P-glycoprotein-specific monoclonal antibodies (mAbs) by high-resolution epitope mapping with a series of hexapeptides. mAb C494 is gene specific, binding to a sequence present only in the class I isoform of hamster and human. The mAb C32 recognizes a sequence conserved in hamster class I and II isoforms but not in class III isoforms. In contrast, the mAb C219 recognizes a highly conserved amino acid sequence found in all P-glycoprotein isoforms characterized to date. These mAbs were used to reveal differential expression and specific localization of the three P-glycoprotein isoforms in hamster tissues by immunohistochemical staining and competition with epitope-specific peptides. Colonic epithelial cells expressed predominantly the class I isoform in a polarized manner, adrenal cortical cells expressed predominantly the class II isoform, whereas a small percentage of skeletal muscle fibers expressed the class III isoform of P-glycoprotein. These findings suggest that the P-glycoprotein isoforms have distinct physiological roles associated with specialized cell functions.

Neurofilament phosphorylation in cultured bovine adrenal chromaffin cells is stimulated by phorbol ester

Primary cultures of bovine adrenal chromaffin cells contain neurofilament proteins that are hypophosphorylated. When the cells were grown in medium containing 32Pi and 0.1 microM 12-O-tetradecanoyl-phorbol 13-acetate (TPA), 32P-labelling of the three neurofilament subunits was increased 6- to 20-fold relative to controls, the highest level of stimulation occurring for the mid-sized subunit. Addition of the protease inhibitor leupeptin to the growth medium had no effect on TPA-stimulated phosphorylation. The increased 32P incorporation was accompanied by a marked reduction in the gel electrophoretic mobilities of the two largest subunits. The augmented phosphorylation was observed 10 min after addition of TPA to a concentration of 0.1 microM or after 1 h of incubation in the presence of 0.01 microM TPA. One-dimensional peptide mapping and phosphoamino acid analysis indicated that TPA stimulated the phosphorylation of seryl residues at new sites in the mid-sized subunit. All of the latter subunit contained in the cytoskeletal fraction of chromaffin cells was converted to a more highly phosphorylated state after the cells were grown in the presence of TPA for 1 h.

[Cleft lip and palate in Campeche Mayas]

It has been suggested that among American Indians, as in some genetically-related Asiatic ethnic groups, incidence of cleft lip and/or cleft palate is higher than among people of Caucasian extraction. Such hypothesis, plus growing demand for services observed at a center for the surgery of cleft lip and cleft palate in Campeche state, led the authors to undertake research among the Maya residents of that region. However, neither careful review of case histories nor field research performed in several Indian communities could confirm the hypothesis of a higher incidence among this ethnic community.

Electrophoretic analysis of P-glycoproteins produced by mouse J774.2 and Chinese hamster ovary multidrug-resistant cells

P-glycoprotein (P-gp) plays a fundamental role in multidrug resistance. The quantity of P-gp relates to the degree of drug resistance. A comparison was made between P-gps in mouse and hamster cell lines in both Laemmli and modified Fairbanks gel systems. Both proteins are derived from precursors of similar size that undergo differential N-linked glycosylation. The electrophoretic mobility and the amount of P-gp are remarkably dependent on the conditions of analysis. Notably, boiling P-gp before Laemmli gel electrophoresis decreases its mobility by an amount that is equivalent to approximately equal to 15 kDa and results in an apparent diminution in the amount of protein. The latter effect can give a false impression concerning the quantity of P-gp in cells.

Hypophosphorylated neurofilament subunits in the cytoskeletal and soluble fractions of cultured bovine adrenal chromaffin cells

The neurofilament proteins in cultured bovine adrenal chromaffin cells are in a hypophosphorylated state, as determined by the co-migration of the 160,000 and 210,000 molecular weight subunits with in vitro dephosphorylated bovine brain subunits on sodium dodecyl sulfate polyacrylamide gels. In addition, chromaffin cells were not stained by anti-heavy neurofilament subunit that binds only to phosphorylated epitopes. Pulse-labeling with 32Pi in the presence and absence of the protein synthesis inhibitor emetine indicated that some neurofilament protein phosphorylation occurred co-translationally and/or immediately after synthesis of the proteins. Pulse-chase experiments showed that the three neurofilament proteins rapidly attained their maximal phosphorylation levels, as multiple forms of either of the respective subunits were not seen after a one hour chase. We found that Triton X-100-soluble forms of high molecular weight neurofilament and middle molecular weight neurofilament subunits were present in chromaffin cells, and they also co-migrated with standard neurofilament proteins dephosphorylated in vitro. However, there were differences between the phosphopeptide maps of cytoskeleton-associated and soluble middle molecular weight neurofilament subunit, suggesting that the localization of phosphate moieties rather than extent of phosphorylation influences the association of the subunit with neurofilaments. Double immunofluorescence staining of cell cultures with antibody to the 70,000 molecular weight subunit and with anti-vimentin showed that chromaffin cells do not express vimentin.

Chemical modification of charged amino acid moieties alters the electrophoretic mobilities of neurofilament subunits on SDS/polyacrylamide gels

The increase in the mobilities of neurofilament subunits on SDS-PAGE after dephosphorylation was reversed upon boiling in urea or trifluoroacetylation of lysine epsilon-amino groups. Trifluoroacetylation of native and dephosphorylated neurofilaments also resulted in an overall increase in the phosphorylation of the three subunits by the catalytic subunit of cyclic-AMP-dependent protein kinase. The gel-electrophoretic mobility of neurofilament subunits was also shown to be influenced by carboxylic amino acid residues, as neutralization of these moieties by glycinamidation increased the mobilities of all three subunits on SDS-PAGE. Neurofilament subunits that were both glycinamidated and dephosphorylated had apparent molecular masses of approximately 60 kDa, 112 kDa and 138 kDa. The major sites of these changes in the two largest subunits were shown to be the carboxy-terminal tail domains, which are known to contain high percentages of glutamate. Since interspecies differences in the apparent molecular masses of neurofilament subunits were shown to persist after glycinamidation and dephosphorylation, they appear to be due to differences in polypeptide chain length, rather than glutamate content.

Dephosphorylation of neurofilaments by exogenous phosphatases has no effect on reassembly of subunits

Exhaustive in vitro dephosphorylation of porcine neurofilaments (NFs) by alkaline or acid phosphatase did not cause a dissociation of the 210-kD (NF-H), 160-kD (NF-M), or 70-kD (NF-L) subunits and had no effect on the reassembly of NFs from urea or guanidine solution. Electron microscopy revealed that the NFs reassembled from isolated or dephosphorylated subunits had similar morphologies. Phosphatase treatment caused significant increases in the mobilities of NF-M and NF-H on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, suggesting that the subunits underwent marked conformational changes after dephosphorylation. Chemical phosphate analysis showed that as isolated NF-H, NF-M, and NF-L contained about 22, 11, and 3 mol phosphate/mol polypeptide, respectively. The corresponding values for the three subunits from alkaline phosphatase-treated NFs were about 8, 6, and 2 mol phosphate/mol polypeptide, respectively. These results indicate the occurrence of a class of phosphate moieties that is not accessible to exogenous phosphatases.

Neurofilament proteins in cultured chromaffin cells

Antibodies were raised against the 200-kd, 145-kd, and 68-kd subunits of a rat neurofilament preparation. Immunoblots showed that each antibody was specific for its antigen and that it did not cross-react with any of the two other neurofilament polypeptides. Use of the three antibody preparations to stain bovine chromaffin cells in culture by the indirect immunofluorescence technique indicated that the three neurofilament polypeptides are present in chromaffin cells maintained in culture for 3 or 7 days. The three anti-neurofilament antibodies labelled the cells in a similar pattern: very thin filaments specifically localized around the nucleus were observed whereas neurites and growth cones, developed by cultured chromaffin cells, were generally not stained. Some fibroblasts were present in our cultures but they were never stained by any of the neurofilament antibodies. This indicated that the antibodies used do not react with vimentin, the major intermediate filament protein found in fibroblasts. The three neurofilament antibodies were also used to immunoprecipitate specifically three proteins of molecular weights 210 kd, 160 kd, 70 kd from solubilized extracts of cultured chromaffin cells that were radiolabelled with [35S]methionine. These proteins correspond in molecular weight to the neurofilament triplet found in bovine brain. Finally, the presence of neurofilaments in freshly isolated chromaffin cells was tested by immunoblotting using the 68-kd antibody. A 70-kd protein was specifically stained by this antibody, suggesting that neurofilaments are not only present in cultured chromaffin cells but also in the adrenal gland in vivo. It is concluded from these results that chromaffin cells contain completely assembled neurofilaments. This additional neuronal property again illustrates that chromaffin cells are closely related to neurons and therefore represent an attractive model system for the study of functional aspects of adrenergic neurons.