Resistance of colorectal cancer cells to 5-FUdR and 5-FU caused by Mycoplasma infection

BACKGROUND

5-Fluorouracil (5-FU) is an antineoplastic drug that targets thymidylate synthase (TS). Tumour cells can develop resistance to anti-TS drugs by a variety of mechanisms including up-regulation of TS protein and alterations in drug uptake and degradation. The possible mechanisms of the observed rapid development of resistance to the pyrimidine analogs 5-FUdR and 5-FU in cultured HCT116 colon cancer cells were investigated.

MATERIALS AND METHODS

Cell survival was determined in resistant and control HCT116 cells treated with 5-FUdR and 5-FU for 7 days. The ability of the cells to take up and metabolize these drugs was determined by Western blotting and [3H]thymidine incorporation.

RESULTS AND CONCLUSION

Resistant HCT116 cells were 5- and 100-fold more resistant to killing by 5-FU and 5-FUdR, respectively, than the parental cells and exhibited impaired uptake. Although the HCT116R cells were initially Mycoplasma free, a low level of Mycoplasma contamination was found in these cells after several weeks in culture. Sensitivity to 5-FUdR was restored by treatment with an anti-Mycoplasma antibiotic. Our observations emphasize the need for frequent testing for Mycoplasma contamination in any cell line under investigation for resistance to anti-TS drugs.

Human growth hormone-releasing factor (hGRF)1-29-albumin bioconjugates activate the GRF receptor on the anterior pituitary in rats: identification of CJC-1295 as a long-lasting GRF analog

In vivo bioconjugation to the free thiol on Cys34 of serum albumin by a strategically placed reactive group on a bioactive peptide is a useful tool to extend plasma half-life. Three maleimido derivates of human GH-releasing factor (hGRF)(1-29) were synthesized and bioconjugated to human serum albumin ex vivo. All three human serum albumin conjugates showed enhanced in vitro stability against dipeptidylpeptidase-IV and were bioactive in a GH secretion assay in cultured rat anterior pituitary cells. When the maleimido derivatives were individually administered sc to normal male Sprague Dawley rats, an acute secretion of GH was measured in plasma. The best compound, CJC-1295, showed a 4-fold increase in GH area under the curve over a 2-h period compared with hGRF(1-29). CJC-1295, a tetrasubstituted form of hGRF(1-29) with an added N epsilon-3-maleimidopropionamide derivative of lysine at the C terminus, was selected for further pharmacokinetic evaluation, where it was found to be present in plasma beyond 72 h. A Western blot analysis of the plasma of a rat injected with CJC-1295 showed the presence of a CJC-1295 immunoreactive species on the band corresponding to serum albumin, appearing after 15 min and remaining in circulation beyond 24 h. These results led to the identification of CJC-1295 as a stable and active hGRF(1-29) analog with an extended plasma half-life.

Development and characterization of a glucagon-like peptide 1-albumin conjugate: the ability to activate the glucagon-like peptide 1 receptor in vivo

The rapid degradation of native glucagon-like peptide 1 (GLP-1) by dipeptidyl peptidase-IV (DPP-IV) has fostered new approaches for generation of degradation-resistant GLP-1 analogues. We examined the biological activity of CJC-1131, a DPP-IV-resistant drug affinity complex (DAC) GLP-1 compound that conjugates to albumin in vivo. The CJC-1131 albumin conjugate bound to the GLP-1 receptor (GLP-1R) and activated cAMP formation in heterologous fibroblasts expressing a GLP-1R. CJC-1131 lowered glucose in wild-type mice, but not in GLP-1R-/- mice. Basal glucose and glycemic excursion following glucose challenge remained significantly reduced 10-12 h following a single injection of CJC-1131. Twice daily administration of CJC-1131 to db/db mice significantly reduced glycemic excursion following oral and IP glucose challenge (P < 0.01 to 0.05) but did not significantly lower body weight during the 4-week study period. Levels of random fed glucose were significantly lower in CJC-1131-treated +/+ and db/db mice and remained significantly lower even 1 week following discontinuation of CJC-1131 administration. CJC-1131 increased levels of pancreatic proinsulin mRNA transcripts, percent islet area, and the number of bromodeoxyuridine-positive islet cells. These findings demonstrate that an albumin-conjugated DAC:GLP-1 mimics the action of native GLP-1 and represents a new approach for prolonged activation of GLP-1R signaling.

Role of stem B, loop B, and nucleotides next to the primer binding site and the kissing-loop domain in human immunodeficiency virus type 1 replication and genomic-RNA dimerization

Stem-loop B is a 12-nucleotide [nt]-long completely conserved sequence postulated to form a 4-bp stem and a 4-nt internal loop under the kissing-loop hairpin (klh) (nt 248 to 270) of human immunodeficiency virus type 1 (HIV-1) genomic RNA. We investigated its role in viral replication, genomic RNA dimerization, and dimerization of partial HIV-1 RNA transcripts. The putative CUCG246-CGAG277 duplex was replaced by nine alternative complementary sequences, five likely to base pair only in short RNAs and four likely to base pair in long (approximately 500-nt) RNAs, as assessed by the algorithm mfold. Among the five former sequences, none preserved genome dimerization and all reduced viral replication by 98 to 99.9%. Among the four latter sequences, three (MB6, -9, and -10) preserved genome dimerization, one (MB7) did not significantly inhibit it, and two (MB9 and -10) preserved viral replication. We conclude that duplex formation by stem B nucleotides is necessary for viral infectivity and complete genome dimerization. Deleting the 5' or 3' side of loop B or of stem B had little impact on dimerization of partial RNA transcript and no impact on klh folding (and, for loop B mutations, on stem B folding), but each deletion inhibited genome dimerization almost as much as klh destruction. This suggests that loop B is required for complete genome dimerization and that loop B and stem B stimulate dimerization only in very long RNAs and/or in the presence of unidentified viral and cellular factors. Finally, we asked if nine deletions or nucleotide substitutions within nt 200 to 242 and/or nt 282 to 335 could influence genome dimerization. These mutations had intermediate inhibitory impacts consistent with their predicted influence on stem B, loop B, and klh formation. Two exceptions were Delta200-226 and Delta236-242 genomic RNAs, which dimerized relatively poorly despite having neutral or positive influences on stem B, loop B, and klh folding.

Role of distal zinc finger of nucleocapsid protein in genomic RNA dimerization of human immunodeficiency virus type 1; no role for the palindrome crowning the R-U5 hairpin

Genomic RNA isolated from HIV-1 variously mutated in nucleocapsid protein (NC) was characterized by nondenaturing gel electrophoresis. Mutations in the C-terminal, the N-terminal, and the linker regions had no effect on genomic RNA dimerization [they are R7R10K11S, P31L, R32G, S3(32-34), and K59L], while a C36S/C39S mutation in the distal zinc knuckle (Cys-His box or zinc finger) inhibited genome dimerization as much as disrupting the kissing-loop domain. The four mutations which inhibited tRNA(Lys3) genomic placement (i.e., the in vivo placement of tRNA(Lys3) on the primer binding site) had no effect on genome dimerization. Among five mutations which inhibited genome packaging, four had no effect on genome dimerization. Thus the N-terminal and linker regions of NC control genome packaging/tRNA(Lys3) placement (two processes which do not require mature NC) but have little influence on genome dimerization and 2-base extension of tRNA(Lys3) (two processes which are likely to require mature NC). It has been suggested, based on electron microscopy, that the AAGCUU82 palindrome crowning the R-U5 hairpin stimulates genomic RNA dimerization. To test this hypothesis, we deleted AGCU81 from wild-type viruses and from viruses bearing a disrupted kissing-loop hairpin or kissing-loop domain; in another mutant, we duplicated AGCU81. The loss of AGCU81 reduced dimerization by 2.5 +/- 4%; its duplication increased it by 3 +/- 6%. Dissociation temperature was left unchanged. We reach two conclusions. First, the palindrome crowning the R-U5 hairpin has no impact on HIV-1 genome dimerization. Second, genomic RNA dimerization is differentially influenced by NC sequence: it is Zn finger dependent but independent of the basic nature of the N-terminal and linker subdomains. We propose that the NC regions implicated in 2-base extension of tRNA(Lys3) are required for a second (maturation) step of tRNA placement. Genome dimerization and mature tRNA placement would then become two RNA-RNA interactions sharing similar NC sequence requirements.

Impact of human immunodeficiency virus type 1 RNA dimerization on viral infectivity and of stem-loop B on RNA dimerization and reverse transcription and dissociation of dimerization from packaging

The kissing-loop domain (KLD) encompasses a stem-loop, named kissing-loop or dimerization initiation site (DIS) hairpin (nucleotides [nt] 248 to 270 in the human immunodeficiency virus type 1 strains HIV-1(Lai) and HIV-1(Hxb2)), seated on top of a 12-nt stem-internal loop called stem-loop B (nt 243 to 247 and 271 to 277). Destroying stem-loop B reduced genome dimerization by approximately 50% and proviral DNA synthesis by approximately 85% and left unchanged the dissociation temperature of dimeric genomic RNA. The most affected step of reverse transcription was plus-strand DNA transfer, which was reduced by approximately 80%. Deleting nt 241 to 256 or 200 to 256 did not reduce genome dimerization significantly more than the destruction of stem-loop B or the DIS hairpin. We conclude that the KLD is nonmodular: mutations in stem-loop B and in the DIS hairpin have similar effects on genome dimerization, reverse transcription, and encapsidation and are also "nonadditive"; i.e., a larger deletion spanning both of these structures has the same effects on genome dimerization and encapsidation as if stem-loop B strongly impacted DIS hairpin function and vice versa. A C258G transversion in the palindrome of the kissing-loop reduced genome dimerization by approximately 50% and viral infectivity by approximately 1.4 log. Two mutations, CGCG261-->UUAA261 (creating a weaker palindrome) and a Delta241-256 suppressor mutation, were each able to reduce genome dimerization but leave genome packaging unaffected.

Variant effects of non-native kissing-loop hairpin palindromes on HIV replication and HIV RNA dimerization: role of stem-loop B in HIV replication and HIV RNA dimerization

The genome of all retroviruses consists of two identical RNAs noncovalently linked near their 5' end. In vitro synthesized RNAs from human immunodeficiency virus type 1 (HIV-1) can form loose or tight dimers depending on whether their respective kissing-loop hairpins (nts 248-270 in HIV-1Lai) bond via their hexameric autocomplementary sequences (ACS), also called palindromes, or via the ACS and stem sequences [Laughrea, M., and Jetté, L. (1996) Biochemistry 35, 1589-1598]. To understand the role of the ACS in HIV-1 replication and in the formation and stability of HIV-1 RNA dimers, we replaced the central CGCG261(or tetramer) of the HIV-1Lai ACS by two other HIV-1 tetramers (UGCA/UGCG), four non-HIV-1 tetramers [GUAC, UUAA (respectively found in HIV-2Rod and SIVmnd), GGCC and AGCU (absent from HIV and SIV viruses)], or GGCG, a nonpalindromic tetramer. The infectivity of GGCC, GUAC, and UGCA viruses was unchanged or insignificantly decreased; the infectivity of AGCU and UGCG viruses was decreased by 80%; the infectivity of UUAA and GGCG viruses was decreased by 92-98%. Thus, the four non-HIV-1 palindromes yielded phenotypes ranging from wild-type to as defective as a virus bearing a nonpalindrome. Studies of in vitro synthesized HIV-1 RNAs were generally consistent with in vivo results, specifically: (i) loose dimerization of GGCC and GUAC RNAs, but not of UUAA and AGCU RNAs, was influenced by the 3' DLS (a sequence located downstream of the 5' splice junction) in a way expected for a wild-type ACS; (ii) the 3' DLS strongly reduced tight dimerization of UUAA and AGCU RNAs, but not of GGCC and GUAC RNAs. We conclude that HIV-1 is sensitive to the ACS sequence without discriminating against all nonnative ACS: GGCC/GUAC, but not AGCU/UUAA, are good substitutes for the prevalent CGCG/UGCA native tetramers and better substitutes than the very rare UGCG native tetramer. The correlation between in vivo and in vitro results suggests that in vitro assays measure parameters of in vivo relevance. Deletion of CUCGG247 (the 5' strand of stem-loop B) decreased the replicative capacity by more than 99.9% and metamorphosed the 3' DLS into an inhibitor of the loose dimerization of HIV-1 RNA.

Drug binding to P-glycoprotein is inhibited in normal tissues following SDZ-PSC 833 treatment

Rats were treated with daily injections of SDZ-PSC 833 (PSC) to study the interaction of this potent modulator of multidrug resistance (MDR) with P-glycoprotein (P-gp) expressed in normal tissues. After 2 days of treatment, the level of P-gp expression, detected by Western blot analysis, was not modified in renal brush border membranes (BBMs) and brain capillaries. However, the amount of P-gp detected with the photoaffinity probe [125I]-arylazidoprazosin (IAAP) was decreased in both tissues, suggesting that the drug binding properties of P-gp were altered by PSC treatment. This effect was further characterized by treating rats with PSC for 10 days. Following these treatments, the amount of immunodetected P-gp was increased in renal BBMs and brain capillaries. However, no increase in P-gp expression was observed in photolabeling experiments, suggesting that induced P-gp was not functional. In vitro experiments performed with renal BBMs showed that the inhibition of P-gp photolabeling by cyclosporin A (CsA), verapamil and vinblastine could be reversed by performing washing steps to remove these drugs before incubating the samples with IAAP. However, the inhibition mediated by PSC was less reversible since photolabeling of P-gp remained inhibited following the washing steps. Pre-incubation of intact CHRC5 cells with PSC, CsA and verapamil also inhibited P-gp photolabeling and increased rhodamine 123 accumulation. For PSC pre-treated samples, these effects were not completely reversed following washing, but were abolished for CsA and Ver pre-treated samples. Our results suggest that PSC could block P-gp function by a different mechanism from that of CsA and verapamil, involving modification of the drug binding sites.

P-glycoprotein is a dimer in the kidney and brain capillary membranes: effect of cyclosporin A and SDZ-PSC 833

Radiation-inactivation studies were performed in order to elucidate the oligomeric nature of P-glycoprotein (P-gp) expressed in brain capillaries and renal brush border membranes (BBMs). Irradiation of renal BBMs resulted in a dose-dependent loss of P-gp, which corresponded to a target size (TS) of 255 and 211 kDa, as detected by Western blot and [125I]arylazidoprazosin labeling, respectively. Similar TSs were determined for P-gp expressed in brain capillaries. These TSs correspond to approximately twice the size (120 kDa) of deglycosylated P-gp. Furthermore, the estimated TS for P-gp was not significantly different when renal BBMs were incubated with SDZ-PSC 833 (PSC) prior and during exposure to ionizing radiation. To confirm these results, the size of P-gp was evaluated from its mobility on blue-native polyacrylamide gels followed by Western blot analysis. Using this method, an apparent molecular size of 334 and 264 kDa was determined for P-gp in brain capillaries and renal BBMs, respectively. This corresponds to approximately twice the size of the glycosylated monomeric subunit of P-gp in brain capillaries (162 kDa) or renal BBMs (140 kDa). P-gp expressed in renal BBMs isolated from rats which had been treated daily with cyclosporin A (CsA) or PSC also migrated as a 264 kDa protein. These results suggest that P-gp exists mainly as a dimer in normal tissues and that resistance modulators such as CsA and PSC do not alter its oligomeric state.

HIV-1 genome dimerization: kissing-loop hairpin dictates whether nucleotides downstream of the 5' splice junction contribute to loose and tight dimerization of human immunodeficiency virus RNA

The genome of all retroviruses consists of two identical RNAs noncovalently linked near their 5' end. Adjacent genomic RNAs from human immunodeficiency virus type 1 (HIV-1) can form loose or tight dimers depending on whether their respective kissing-loop hairpins (nts 248-270 in HIV-1Lai) bond via their autocomplementary sequences (ACS) or via the ACS and stem sequences [Laughrea, M., & Jetté, L. (1996a) Biochemistry 35, 1589-1598]. Loose dimers from HIV-1Mal, but not HIV-1Lai, are stabilized by a sequence (3'DLS) located downstream of the 5' splice junction [Laughrea, M., & Jetté, L. (1996b) Biochemistry 35; 9366-9374]. To understand the ACS-3'DLS interplay in the formation and stability of loose and tight HIV-1 RNA dimers, we replaced the ACS of HIV-1Lai (GCGCGC262) by GUGCAC, GUGCGC (two alternative HIV-1 ACS), or GAGCUC (a non-HIV ACS). For each mutant, RNAs truncated immediately upstream or downstream of the 3'DLS were prepared; their ability to dimerize and their thermal stabilities were compared. The results suggest that the ACS determines whether the 3'DLS participates in RNA dimerization: (1) GAGCUC262 led to poorly stable loose dimers due to the inability of the 3'DLS to stabilize them (the 3'DLS stabilized the GUGCAC and GUGCGC RNAs); (2) GAGCUC262 led to poor formation of tight dimers, due to an inhibitory effect of the 3'DLS (the 3'DLS had little effect on the tight dimerization of the GUGCAC, GUGCGC and GCGCGC RNAs). The results indicate that communication exists between HIV-1 RNA sequences respectively located upstream and downstream of the 5' splice junction; they are consistent with the idea that the 3'DLS plays two ACS-dependent roles in the dimerization process: loose dimer stabilization in HIV-1 RNAs bearing an HIV ACS (unless the ACS already conferred a thermostability equal or superior to that offered by the 3'DLS), and inhibition of tight dimer formation in an HIV-1 RNA bearing a non-HIV ACS.

Mutations in the kissing-loop hairpin of human immunodeficiency virus type 1 reduce viral infectivity as well as genomic RNA packaging and dimerization

A stem-loop termed the kissing-loop hairpin is one of the most highly conserved structures within the leader of human immunodeficiency virus type 1 (HIV-1) and chimpanzee immunodeficiency virus genomic RNA. Because it plays a key role in the in vitro dimerization of short HIV-1 RNA transcripts (M. Laughrea and L. Jette, Biochemistry 35:1589-1598, 1996, and references therein; M. Laughrea and L. Jette, Biochemistry 35:9366-9374, 1996, and references therein) and because dimeric RNAs may be preferably encapsidated into the HIV-1 virus, alterations of the kissing-loop hairpin might affect the in vivo dimerization and encapsidation processes. Accordingly, substitution and deletion mutations were introduced into the kissing-loop hairpin of an infectious HIV-1 molecular clone in order to produce viruses by transfection methods. The infectivity of the resulting viruses was decreased by at least 99%, the amount of genomic RNA packaged per virus was decreased by 50 to 75%, and the proportion of dimeric genomic RNA was reduced from >80 to 40 to 50%, but the dissociation temperature of the genomic RNA was unchanged. There is evidence suggesting that the deletion mutations moderately inhibited CAp24 production but had no significant effect on RNA splicing. These results are consistent with the kissing-loop model of HIV-1 RNA dimerization. In fact, because intracellular viral RNAs are probably more concentrated in transfected cells than in cells infected by one virus and because the dimerization and encapsidation processes are concentration dependent, it is likely that much larger dimerization and encapsidation defects would have been manifested within cells infected by no more than one virus.

HIV-1 genome dimerization: formation kinetics and thermal stability of dimeric HIV-1Lai RNAs are not improved by the 1-232 and 296-790 regions flanking the kissing-loop domain

The genome of all retroviruses consists of two identical RNAs noncovalently linked near their 5' end. Dimerization of genomic RNA is thought to modulate several steps in the retroviral life cycle, such as recombination, translation, and encapsidation. The kissing-loop model of HIV-1 genome dimerization posits that the 233-285 region of the HIV-1 genome, by forming a hairpin and initiating dimerization through a loop-loop interaction, is at least the core dimerization domain of HIV-1 RNA. This region is called the kissing-loop domain. In addition, it can be argued that sequences within the 296-401 region [Paillart et al. (1994) J. Biol. Chem. 269, 27486-27493] or 5' of the primer binding site [Laughrea & Jetté (1996) Biochemistry 35, 1589-1598] might play some role in the dimerization process. Accordingly, we have studied the effect of regions 1-232 and 296-790 on the dimerization kinetics and thermal stability of HIV-1Lai RNAs containing the kissing-loop domain (HIV-1Lai is a typical representative of North American and European HIV-1 viruses). Experiments conducted at high and low ionic strength indicate that these regions have no strongly positive effect on the dimerization process. Our experiments also indicate that the kissing-loop domain of HIV-1Lai has an apparent dissociation temperature 13 degrees C higher than that of the HIV-1Mal kissing-loop domain (HIV-1Mal is a Central African virus whose kissing-loop domain has a "weak" GUGCAC autocomplementary sequence). Because the 296-401 region of HIV-1Mal RNA stabilizes dimeric RNAs by < or = 12 degrees C (Paillart et al., 1994), we infer that the contributions of sequences downstream of U295 are (at best) concealed in HIV-1Lai and in most American and European HIV-1 viruses, i.e., in viruses whose kissing-loop domain is characterized by a "strong" GCGCGC autocomplementary sequence.

Cyclosporin A treatment induces overexpression of P-glycoprotein in the kidney and other tissues

To see whether P-glycoprotein (PGP) expressed in renal brush-border membranes (BBM) could interact with compounds known as modulators of multidrug resistance (MDR), photoaffinity-labeling experiments were performed. A 145k-Da protein was photolabeled with [125I] iodoarylazidoprazosin, and this labeling was reduced in the presence of cyclosporin A (CsA) and PSC-833 (PSC). Interaction of CsA with PGP was further investigated by treating rats with daily subcutaneous injections of CsA (10 mg.kg-1.day-1). After this treatment, PGP expression levels were dramatically increased in renal BBM, intestine, liver, and many other tissues except the brain. This induction was a reversible process, since after cessation of CsA administration PGP levels declined to reach values similar to those of the control groups. The increase in PGP expression in the kidney was also detected in photolabeling experiments, suggesting the induction of a functional PGP. A higher dose of CsA (50 mg/kg) given as a bolus injection did not modify PGP expression] in renal BBM. These results demonstrate that CsA induces reversible overexpression of PGP in the rat. This may present significant relevance in the design of clinical trials using CsA as a chemosensitizing agent.

Kissing-loop model of HIV-1 genome dimerization: HIV-1 RNAs can assume alternative dimeric forms, and all sequences upstream or downstream of hairpin 248-271 are dispensable for dimer formation

The genome of all retroviruses consists of two identical RNAs noncovalently linked near their 5' end. Dimerization of genomic RNA is thought to modulate several steps in the retroviral life cycle, such as recombination, translation, and encapsidation. The kissing-loop model of HIV-1 genome dimerization [Laughrea, M., & Jetté, L. (1994) Biochemistry 33, 13464-13474; Skripkin et al. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 4945-4949] posits that the 248-270 region of the HIV-1 genome, by forming a hairpin and initiating dimerization through a loop-loop interaction, is the full or at least the core dimerization domain of HIV-1 RNA. Here, we show by nested deletion analysis that the 3' boundary of the HIV-1 dimerization domain is immediately downstream of hairpin 248-270 and that the isolated region 248-271 dimerizes at least as readily as longer RNAs. Among various HIV-1Lai RNA transcripts containing hairpin 248-270, all form two types of dimer, as is implicit in the kissing-loop model. The high-stability dimer resists semidenaturing conditions and the low-stability dimer cannot, which is consistent with the model. At physiological temperatures, low-stability dimers are usually formed, as if dimerization without nucleocapsid proteins corresponded to loop-loop interaction without switching from intra- to interstrand hydrogen bonding. Our results show that the 3' DLS (a sequence immediately 3' from the 5' splice junction and originally thought to be the dimerization domain of the HIV-1 genome) and adjacent nucleotides are not necessary for efficient dimerization of HIV-1Lai RNA at low and high ionic strength. Upstream of hairpin 248-270 exists another "DLS-like" sequence that we name 5' DLS: like the isolated 3' DLS, the isolated 5' DLS forms an apparently nonphysiological structure that can become substantially dimeric at high ionic strength.

Interaction of drugs with P-glycoprotein in brain capillaries

P-glycoprotein (P-gp) is expressed at high levels in a variety of non-cancerous tissues such as the endothelial cells of the blood-brain barrier (BBB) capillaries. These thin capillaries tightly regulate the movement of substrates from the circulating blood into the brain. P-gp may be involved in the exclusion of various drugs from the capillary endothelial cells, blocking their entry into the brain. However, interactions of drugs with P-gp expressed in brain capillaries remain to be characterized. We have performed photoaffinity labeling studies using [125I]arylazidoprazosin (IAAP) to evaluate the inhibitory efficiency of various compounds. Cyclosporin A (CsA) and its derivative PSC 833 (PSC) were the most effective inhibitors of IAAP binding among the drugs tested. The magnitude of inhibition was: PSC > CsA > quinidine > vinblastine > verapamil < actinomycin D > colchicine > reserpine > bilirubin > doxorubicin > progesterone. Cremophor El, the vehicle used to administer CsA and PSC intravenously, was also able to inhibit IAAP photolabeling of P-gp. Labeling experiments were also performed using a photoactivatable [3H]CsA derivative. Photolabeling of P-gp with this compound was abolished almost completely by CsA and PSC. In vivo studies were also performed by treating rats with CsA [10 mg/(kg.day) for 10 days]. Following this treatment, no alteration in the level of P-gp expression in brain capillaries was observed. These results suggest that, at the proper dosage, administration of CsA to cancer patients could help to enhance the response of brain tumors to chemotherapeutic agents without modifying the intrinsic level of P-gp expression in this tissue.

Isoform I (mdr3) is the major form of P-glycoprotein expressed in mouse brain capillaries. Evidence for cross-reactivity of antibody C219 with an unrelated protein

P-glycoprotein (P-gp) is expressed in various non-cancerous tissues such as the endothelial cells of the blood-brain barrier. We used several monoclonal antibodies (mAbs) and isoform-specific polyclonal antibodies to establish which P-gp isoforms are expressed in isolated mouse brain capillaries. P-gp class I isoform was detected in capillaries with a Western immunoblotting procedure using a specific antiserum. No immunoreactivity was observed with either class II- or class III-specific antisera. Immunoreactivity was observed with mAb C219. However, this antibody detected two distinct immunoreactive proteins (155 and 190 kDa) in the isolated brain capillaries. These two proteins comigrated as a broad band when the samples were submitted to heat prior to gel electrophoresis. The glycoprotein nature of these two antigens was evaluated by their sensitivity to N-glycanase treatment. Following this treatment, the size of the proteins was reduced from 190 and 155 kDa to 180 and 120 kDa, respectively. Triton X-114 phase-partitioning studies showed that the 190 kDa immunoreactive protein was poorly solubilized by Triton X-114, while the 155 kDa protein was partitioned in the detergent-rich phase. In labelling experiments, only the 155 kDa protein was photolabelled with [125I]iodoarylazidoprazosin. These results show that a 190 kDa protein detected by antibody C219 is an antigen unrelated to the three P-gp isoforms presently known. Cross-reactivity of C219 with an unrelated protein emphasizes the fact that more than one antibody should be used in the assessment of P-gp expression in cell lines and tissues.

A 19-nucleotide sequence upstream of the 5' major splice donor is part of the dimerization domain of human immunodeficiency virus 1 genomic RNA

The genome of all retroviruses, including human immunodeficiency virus type 1 (HIV-1), consists of two identical RNAs noncovalently linked near their 5' end. Dimerization of genomic RNA is thought to modulate several steps in the retroviral life cycle, such as recombination, translation, and encapsidation. We report the results of experiments designed to identify the 5' and 3' boundaries of the dimerization domain of the HIV-1 genome: (1) An HIV-1 RNA starting at nucleotide 252 or at other downstream positions (four tested) does not dimerize despite the inclusion of the whole of a previously proposed dimerization domain (nucleotides 295-401); (2) an RNA starting between nucleotides 242 and 249 (five positions tested) dimerizes to a variable extent depending on the starting position; (3) an RNA starting at nucleotide 233 or at other upstream positions (five tested) is fully or > 80% dimeric; (4) an RNA starting at nucleotide 1 but lacking the 233-251 or the 242-251 region is, respectively, fully monomeric or about 50% monomeric; (5) the 343-401 region contains two strings of G's (GGGGG367 and GGG384) that had been postulated to promote genome dimerization through the formation of guanine quartets. We have deleted the 379-401, 358-401, and 343-401 regions from otherwise dimeric RNAs without changing their ability to dimerize. We reach three conclusions: (1) a dimerization signal exists upstream of the major 5' splice donor (nucleotide 290); (2) the previously proposed downstream dimerization domain is insufficient to promote dimerization and has a 3' half that is not necessary to obtain fully dimeric RNAs; (3) the 5' boundary of the HIV-1 dimerization domain is located somewhere between nucleotides 233 and 242, and the 3' boundary is located no farther than at nucleotide 342, making it possible that the 5' and 3' boundaries of the HIV-1 dimerization domain are both located within the leader sequence. We speculate that the 248-270 or 233-285 region forms a hairpin that is the core dimerization domain of HIV-1 RNA.

Intracoronary infusion of bradykinin: effects on noradrenaline overflow following reperfusion of ischemic myocardium in the anesthetized dog

OBJECTIVE

The aim of this study was to investigate the effects of intracoronary bradykinin (BK) infusion on noradrenaline release and ventricular arrhythmias induced by coronary occlusion and reperfusion in the anesthetized dog.

METHODS

14 anesthetized adult mongrel dogs of either sex underwent a 60 min occlusion of the left anterior descending coronary artery (LAD) followed by a 30-min reperfusion period. BK (1 ng.kg-1.min-1, n = 7), or its vehicle (Lactate Ringer, n = 7), infusions just distal to the left coronary ostium started 15 min before the LAD occlusion and were maintained throughout the experimental period. An epicardial vein, running parallel to the LAD was cannulated to enable the biochemical determinations. The effects of BK on ventricular arrhythmias, cardiac noradrenaline and lactate releases and creatine kinase activity were assessed.

RESULTS

BK significantly reduced the amount of noradrenaline released at reperfusion by ischemic myocardium (from 82.1 +/- 31.7 to 11.9 +/- 9.6 ng.min-1), as well as plasma creatine kinase activity at 30 min of reperfusion. This is accompanied by a significant reduction in the incidence of reperfusion-induced sustained ventricular tachycardia.

CONCLUSION

This suggests that the protective effect of bradykinin against reperfusion-induced sustained ventricular tachycardia could be associated with a reduction in cardiac noradrenaline release.

High levels of P-glycoprotein detected in isolated brain capillaries

P-glycoprotein (P-gp) is a highly-conserved membrane protein expressed in various multidrug-resistant cell lines. P-glycoprotein was detected in capillaries isolated from human, beef and rat brains with a Western immunoblotting procedure using the monoclonal antibody C219 (mAb C219) specific for P-gp. The mAb C219 detected a 180 kDa protein in brain capillaries isolated from all three species. The largest amount of antigen was detected in capillaries isolated from human brain. Specific binding was assessed by competitive inhibition of mAb C219 binding by the synthetic epitope VQEALD. The glycoprotein nature of the brain capillary proteins was confirmed by its sensitivity to N-glycanase treatment, which reduced their apparent molecular mass by 5 to 10 kDa. In addition, immunohistochemical studies using the antibodies C219, JSB-1 and C494 were performed. Bovine and rat capillaries showed reactivity only with the mAb C219. Heavy staining of human brain capillaries was observed with both antibodies C219 and JSB-1, while only weak staining was observed with antibody C494. These results clearly show that P-glycoprotein is strongly expressed at the blood-brain barrier (BBB) site and suggest that this protein may play a physiological role in regulating the access of certain molecules to the central nervous system, or in the secretory functions of the BBB.

Intracoronary administration of saralasin: effects on cardiac arrhythmias induced by ischaemia and reperfusion in the anaesthetised dog

OBJECTIVE

The aim was to study (1) the effects of intracoronary saralasin, an angiotensin II receptor antagonist, on ischaemia induced and reperfusion induced regional cardiac noradrenaline release and ventricular arrhythmias; and (2) the implication of angiotensin II in coronary constriction during myocardial ischaemia.

METHODS

Eighteen adult mongrel dogs, weight 22.6(SD 1.1) kg, anaesthetised with sodium pentobarbitone, were used for the study. The left anterior descending coronary artery was ligated for 60 min and then reperfused for 30 min. Saralasin (60 micrograms.kg-1, n = 9) or its vehicle (Ringer lactate, n = 9) was injected into the artery at the beginning of the occlusion period. Two epicardial veins, one running parallel to the left anterior descending coronary artery and the other parallel to the circumflex coronary artery, were cannulated for the measurement of their respective blood flows and of noradrenaline, lactate, and creatine kinase release.

RESULTS

Saralasin decreased the incidence of ventricular fibrillation during coronary occlusion (from 44% in the vehicle treated group to 0% in the saralasin treated group, p = 0.0412). This effect was accompanied by significant vasodilatation in both epicardial veins during myocardial ischaemia. Neither the increases in noradrenaline, lactate, and creatine kinase release nor the incidence and duration of the ventricular arrhythmias following reperfusion were modified by the administration of saralasin.

CONCLUSIONS

Intracoronary saralasin in the early phase of myocardial ischaemia increases the epicardial venous blood flow significantly, suggesting that angiotensin II is implicated in coronary constriction during ischaemia. This haemodynamic effect is accompanied by a significant decrease in the incidence of ventricular fibrillation. However, the renin-angiotensin system does not appear to be implicated in the reperfusion induced noradrenaline release nor in the incidence of the ventricular arrhythmias.