Search

The distribution and level of 5-fluorouracil (FUra) incorporation into RNA were studied in S-180 murine tumor cells for two different fluoropyrimidine nucleosides: 5-fluorouridine (FUrd) and 5'-deoxy-5-fluorouridine (5'-dFUrd) under both cytotoxic (thymidine nonreversible) and nontoxic conditions. Exposure of cells to 200 microM 5'-dFUrd for 6 hr resulted in minor thymidine-nonreversible toxicity (11% cell kill) and produced an FUra incorporation level of 12.5 pmol of FUra/microgram of RNA. Exposure of cells to 1 microM FUrd for 6 hr resulted in considerably more toxicity (74% cell kill) but was found to produce less FUra incorporation (7.8 pmol of FUra/microgram of RNA) than the nontoxic concentration of 5'-dFUrd. Subcellular fractionation (i.e., nucleolar, nucleoplasmic, cytoplasmic) was completed, and the density of FUra incorporation from 5'-dFUrd was found to be approximately equally distributed between nucleoplasmic RNA and nucleolar RNA, independent of toxicity or concentration levels (average nucleolar/nucleoplasmic FUra ratio = 0.75). FUrd, however, was preferentially incorporated into nucleolar RNA (average nucleolar/nucleoplasmic FUra ratio = 22). The nontoxic accumulation of FUra in RNA of cells treated with 200 microM 5'-dFUrd was also associated with the mature cytoplasmic 28 S and 18 S rRNA, the levels of which increased considerably with exposure time. In contrast, in cells exposed to 1 microM FUrd, there was very little FUra measured in the mature 28 S and 18 S rRNA, consistent with an inhibition of rRNA processing. It is concluded that 5'-dFUrd can produce large nontoxic levels of FUra accumulation in cellular RNA compared with FUrd because total FUra incorporation in nuclear RNA was almost equally dispersed between nucleoplasmic and nucleolar RNA, whereas nuclear FUra incorporation from FUrd was preferential for nucleolar RNA. Also, for nontoxic doses of 5'-dFUrd, unaltered rRNA processing over time can result in the large accumulation of FUra-containing rRNA in the cytoplasm.

Studies were completed in C3-L5178Y cells, in which the DNA-coding region for dihydrofolate reductase messenger RNA (DHFR-mRNA) is amplified, to determine the acute effect of 5-fluorouracil (FUra) on DHFR-mRNA metabolism. There was minimal to no effect of 100 microM FUra on total cytoplasmic DHFR-mRNA levels by 6 and 12 h and only a 25% reduction by 24 h. These results contrasted with the nuclear DHFR-mRNA levels which by 6 h following exposure to FUra increased by 80% in a dose-dependent manner. Furthermore, some of the increased nuclear DHFR-mRNA was found to be in a non-polyadenylated form. Under conditions to examine only RNA synthesized during the drug exposure, FUra was found to markedly enhance the level of newly synthesized nuclear DHFR-mRNA in a dose-dependent manner, while also producing an apparent dose-dependent reduction in the cytoplasmic DHFR-mRNA. RNA fractionated by 1.5% agarose-urea gel electrophoresis revealed two major cytoplasmic DHFR-mRNA species approximately 1.8 and 0.8 kilobases in size. Following a 24-h FUra exposure, a dose-dependent loss of the 0.8-kilobase DHFR-mRNA was observed. The combined results of these experiments indicate that FUra treatment reduces the ability of nascent DHFR-mRNA to relocate to the cytoplasm, suggesting either an inhibition of mRNA processing or nuclear-cytoplasmic transport.

Activation of muscarinic cholinergic receptors (mAChRs) in the central nervous system reduces the catalytic activity of membrane-bound adenylate cyclase and attenuates depolarization-dependent release of acetylcholine (ACh). Inasmuch as reports have indicated that these mAChR-mediated responses exhibit pharmacological profiles similar to the M2 subclass of mAChR, the present studies were undertaken to ascertain whether attenuation of presynaptic adenylate cyclase activity [and concurrent reduction of intraneuronal cyclic AMP (cAMP) levels] underlies mAChR-mediated autoinhibition of electrically evoked ACh release. In [3H]choline-prelabeled rat hippocampal slices, the mAChR agonists oxotremorine (EC50 = 15 microM) and carbachol (EC50 = 80 microM) caused atropine-reversible inhibition of [3H]ACh release up to a maximum of 80% reduction. The rank order of potency for antagonist reversal of this inhibitory action (N-methylatropine = atropine greater than scopolamine much greater than pirenzepine) was generally consistent with an M2 mAChR-mediated response although pirenzepine was ineffective up to 1 mM. Under these assay conditions, forskolin (1-10 microM) and 8-bromo-cAMP (30-300 microM) enhanced electrically evoked [3H]ACh release maximally by 50 to 60%; however, neither agent significantly reversed mAChR agonist-induced inhibition of [3H]ACh release. Additional studies were undertaken to determine the consequences of chemically uncoupling mAChR from their G protein-adenylate cyclase effector system in this tissue. Whereas brief pretreatment with the sulfhydryl alkylating agent N-ethylmaleimide (30 microM) or pertussis toxin (1 microgram/ml) markedly attenuated carbachol inhibition of adenylate cyclase activity in hippocampal tissue, there was no concurrent reduction of carbachol-inhibited [3H] ACh release.(ABSTRACT TRUNCATED AT 250 WORDS)

The potassium permeability of the human red blood cell increases with the free intracellular calcium concentration. The efflux of potassium can be inhibited by iodoacetic acid. This inhibitory effect correlates directly with the carboxymethylation of a protein band found in both the hemolysate and membrane fractions. The present study provides two additional lines of evidence that this protein is involved directly with the calcium-dependent changes in potassium permeability: its association with the membrane is calcium dependent; and calcium-dependent potassium efflux from resealed ghost is inhibited by the incorporation of antibodies raised against this cytoplasmic protein.

A diversity of nicotinic acetylcholine receptor (nAChR) subtypes has been identified in mammalian brain using recombinant DNA technology. Alterations in the activity of these acetylcholinegated ion channels have been implicated in a number of central nervous system disorders including Alzheimer's disease (AD). The potential therapeutic usefulness of (-)-nicotine [(S)-3-(1-methyl-2-pyrrolidinyl) pyridine], the prototypic agonist at nAChRs, is severely limited by side effects that are the result of activation of both cholinergic and noncholinergic pathways in the central and peripheral nervous systems. This study sought to determine the in vitro selectivity of (S)-3-methyl-5-(1methyl-2-pyrrolidinyl)isoxazole (ABT 418), a novel analog of (-)-nicotine in which the pyridine ring was replaced with an isoxazole bioisotere, to activate nAChRs. ABT 418 was a potent inhibitor of [3H]-cytisine binding to nAChR in rat brain (Ki = 3 nM) but was inactive (Ki > 10,000 nM) in 37 other receptor/neurotransmitter-uptake/enzyme/transduction system binding assays, including those for alpha-bungarotoxin, muscarinic and 5-hydroxytryptamine3 receptors. In PC12 cells, patch-clamp studies indicated that ABT 418 was an agonist with an EC50 value of 209 microM, a potency to activate cholinergic channel currents some 4-fold less than that of (-)-nicotine (52 microM). Channel current responses elicited by ABT 418 were prevented by the cholinergic channel blocker, mecamylamine. ABT 418 was also approximately 10-fold less potent (EC50 value = 380 nM) than (-)-nicotine (40 nM) in increasing [3H]-dopamine release from rat striatal slices, an effect that was blocked by the nAChR antagonist, dihydro-beta-erythroidine (10 microM).2+ In contrast, ABT 418 appeared equipotent with (-)-nicotine in enhancing 86Rb+ flux from mouse thalamic synaptosomes. ABT 418 demonstrated an in vitro pharmacological profile of cholinergic channel activation that was robust at some nAChR, but not others. The reasons for this are unclear. However, a nAChR subtype selectivity may account for the in vitro potency differences of ABT 418 on various neurotransmitter systems, and the substantial separation between the cognitive enhancement/anxiolytic benefits, and the reduced central nervous system side-effect liabilities seen in vivo. ABT 418 represents the first neuronal nAChR ligand that differentiates the toxicities/liabilities and other negative aspects normally associated with liabilities and other negative aspects normally associated with (-)-nicotine from the potential pharmacological benefits of selective cholinergic channel activation.

Studies were completed to examine the effects of the antineoplastic agent 5-Fluorouridine (FUrd) on the metabolism of the small molecular weight nuclear RNA (snRNA). Cultured Sarcoma-180 murine tumor cells were exposed to FUrd concurrent with [3H]cytidine for 6 h, the drug was removed, and the RNA was isolated at 0, 24, or 48 h following the drug treatment. The results of these studies demonstrated that FUrd produced three dose-dependent changes in snRNA metabolism. The electrophoretic migration of the U4 and U6 snRNA was altered in nondenaturing 10% polyacrylamide slab gels. These results were not observed in denaturing gels or when RNA was extracted at temperatures exceeding 25 degrees C, suggesting that the incorporation of 5-fluorouracil induced secondary structural changes in these RNA. A dose- and time-dependent selective reduction in the turnover of the U1 snRNA synthesized in the presence of FUrd was observed as well, with levels over 100% higher than control cells at 48 h after exposure to 10 microM FUrd. These changes in snRNA metabolism may contribute to the reported alterations in large molecular weight RNA metabolism that also result in fluoropyrimidine-treated cells, or to cytotoxicity.