Inhibition of virus-induced cell transformation by synthetic analogues of S-adenosyl homocysteine

Cap ribose methylation of c-mos mRNA stimulates translation and oocyte maturation in Xenopus laevis

In Xenopus oocytes, progesterone stimulates the cytoplasmic polyadenylation and resulting translational activation of c-mos mRNA, which is necessary for the induction of oocyte maturation. Although details of the biochemistry of polyadenylation are beginning to emerge, the mechanism by which 3' poly(A) addition stimulates translation initiation is enigmatic. A previous report showed that polyadenylation induced cap-specific 2'-O-methylation, and suggested that this 5' end modification was important for translational activation. Here, we demonstrate that injected c-mos RNA undergoes polyadenylation and cap ribose methylation. Inhibition of this methylation by S-isobutylthioadenosine (SIBA), a methyltransferase inhibitor, has little effect on progesterone-induced c-mos mRNA polyadenylation or general protein synthesis, but prevents the synthesis of Mos protein as well as oocyte maturation. Maturation can be rescued, however, by the injection of factors that act downstream of Mos, such as cyclin A and B mRNAs. Most importantly, we show that the translational efficiency of injected mRNAs containing cap-specific 2'-O-methylation (cap I) is significantly enhanced compared to RNAs that do not contain the methylated ribose (cap 0). These results suggest that cap ribose methylation of c-mos mRNA is important for translational recruitment and for the progression of oocytes through meiosis.

Stimulatory effects of antiviral adenosine analogs on steroidogenesis in Leydig cells

We studied the effects of 12 adenosine analogs which are active as antiviral agents on basal and LH-stimulated steroidogenesis in Leydig cells. It is shown that several of these analogs markedly stimulate the production of androgens and androgen precursors in the absence of LH. These effects are observed in interstitial cell cultures derived from immature rats as well as in freshly prepared Percoll-purified Leydig cells derived from adult mice. Some compounds (neplanocin A, S-isobutyladenosine) are active from a concentration of 10(-6) M on. In the presence of maximally effective concentrations of LH or dbcAMP the stimulatory effects disappear and some compounds even become inhibitory. Only within the neplanocin series of derivatives did we observe a correlation between antiviral and steroidogenic activity. Four representative test compounds were studied in more detail: neplanocin A, 7-deazaadenosine, 4'-thioadenosine and S-isobutyladenosine. The first three significantly inhibit phospholipid N-methyltransferase activity in intact Leydig cells. However, our data do not suggest a close link between phospholipid methylation and the stimulatory or inhibitory effects of these test compounds on steroidogenesis. In cultured rat interstitial cells neplanocin A, S-isobutyladenosine and in particular 4'-thioadenosine markedly stimulate the production of cAMP. This effect is probably mediated via adenosine (A2) receptors which are known to appear in such cultures. Comparable effects are not observed in freshly prepared mouse Leydig cells. Again, however, there is no obvious correlation between the ability of the test compounds to stimulate cAMP production and their effects on steroidogenesis. It is concluded that compounds to stimulate cAMP production and their effects on steroidogenesis. It is concluded that antiviral adenosine analogs have complex effects on Leydig cell steroidogenesis. There may not be a unifying mechanism of action underlying the various biological effects of these agonists.

Association of SIBA treatment and a Met-depleted diet inhibits in vitro growth and in vivo metastatic spread of experimental tumor cell lines

We have used 5'-deoxy-5'-S isobutyl-thioadenosine (SIBA), an analog of S-adenosylhomocysteine, alone or in association with a methionine-depleted diet in order to obtain an antitumoral effect in two different tumor models: a transplantable rat rhabdomyosarcoma (RMS-J1) induced by i.m. injection of nickel and the well-known Lewis lung carcinoma (3LL) of C57BL/6 mice. Since SIBA has been reported to inhibit the methyl group transfer from methionine to S-adenosylhomocysteine, among other activities, its association with a reduction of methyl donors, achieved by methionine depletion of the diet (in vivo) or the culture medium (in vitro), should logically lead to an additive effect. In vitro, 3LL and RMS-J1 were sensitive to the cytotoxic effect of SIBA and were methionine-dependent for their proliferation. Fibroblast proliferation was not affected by these two treatments alone or in association. In vivo, either SIBA treatment or a low methionine diet led to a significant decrease in the metastatic character of these two tumors; however, local tumor growth was not significantly affected. The median number of 3LL metastases counted in the lungs was reduced from 100 to 18 by SIBA treatment, and to 27 by the low methionine diet. No additive effect could be detected when the treatments were given simultaneously. RMS-J1-bearing rats treated with SIBA and fed a low Met diet underwent primary tumor excision. The median numbers of lung metastatic nodules were 27, 26, 14 and 8 for the control, SIBA-treated rats, methionine-deprived rats and rats receiving the combined therapy. Expressed as percentages 20 per cent were cured, 23 per cent showed a low number of lung metastases (P less than 10), whereas all the rats in the control group developed more than 10 pulmonary nodules. No cytotoxic effect could be observed on the treated rats. The role of SIBA and methionine depletion, as agents interfering with transmethylation processes, in regard to the control of tumor development, namely metastatic invasiveness, is discussed.

5'-Deoxy-5'-methylthioadenosine phosphorylase--III. Role of the enzyme in the metabolism and action of 5'-halogenated adenosine analogs

5'-Deoxy-5'-halogenated adenosines are alternative substrates for 5'-deoxy-5'-methylthioadenosine phosphorylase (MTAPase), an enzyme responsible for the metabolism of 5'-deoxy-5'-methylthioadenosine (MTA), a by-product of polyamine biosynthesis. The relative reactivity of these nucleosides with MTAPase from HL-60 human promyelocytic leukemia cells is MTA greater than 5'-deoxy-5'-fluoroadenosine (5'-FlAdo) greater than 5'-chloro-5'-deoxyadenosine (5'-ClAdo) greter than 5'-bromo-5'-deoxyadenosine (5'-BrAdo) greater than 5'-deoxy-5'-iodoadenosine (5'-IAdo). In MTAPase-containing cells, the adenine released from the 5'-halogenated adenosine was incorporated into adenine nucleotide pools; cleavage by (MTAPase appeared to be the rate-limiting step in this process. 5'-BrAdo and 5'-IAdo were growth inhibitors (EC50 values less than 10 microM) of MTAPase-containing cell lines (HL-60 human promyelocytic leukemia and the L5178Y murine lymphoblastic leukemia) but were much less active (EC50 values greater than 65 microM) against MTAPase-deficient cell lines (the CCRF-CEM human T cell leukemia and the L1210 murine leukemia). The full cytotoxicity of these compounds, therefore, appeared to be related to their phosphorolysis by MTAPase. Indirect evidence suggests that 5-halogenated ribose-1-phosphate derivatives of 5'-BrAdo or 5'-IAdo produced by the MTAPase reaction were the active metabolites of these 5'-halogenated adenosines.

Broad-spectrum antiviral activity of adenosine analogues

In recent years certain aliphatic and carbocyclic adenosine analogues have been developed which are of potential clinical importance as antiviral agents. This includes (S)-9-(2,3-dihydroxypropyl)adenine [(S)-DHPA] and carbocyclic 3-deazaadenosine (C-c3Ado). (S)-DHPA and C-c3Ado are remarkably similar in their antiviral spectrum in that they are particularly active against (-) RNA viruses such as measles, para-influenza, respiratory syncytial virus, rabies virus, vesicular stomatitis virus and (+/-)RNA viruses such as reo- and rotavirus, whereas (+)RNA viruses such as polio, coxsackie and (+/-)DNA viruses such as herpes simplex are only minimally affected or not inhibited at all. In contrast with (S)-DHPA and C-c3Ado which are quite selective in their antiviral action, other adenosine analogues, i.e., 3-deazaadenosine and 7-deazaadenosine (tubercidin), exhibit little, if any, selectivity as antiviral agents. Also, tubercidin has a broader activity spectrum, encompassing (+)RNA viruses as well as herpes simplex in addition to the (-)RNA viruses. Considering the high antiviral potency of tubercidin, attempts have been undertaken to increase its selectivity, i.e., by chemical substitutions at C-5 of the pyrrolo(2,3-d)pyrimidine ring. These attempts have been partially successful.

Modified nucleosides in body fluids of tumor-bearing patients

The catabolism of nucleic acids, particularly tRNA, produces a variety of modified nucleosides which are not reutilized by mammalian cells. Investigation of these compounds in body fluids, mainly urine, has recently provided evidence of altered metabolic situations in tumor-bearing patients. The factors involved in the alterations of modified nucleosides formation are connected with altered tRNA-modifying enzymes and/or altered turnover of subpopulations of tRNA. A common pattern in tumor cells or tissues is the presence of isoaccepting tRNA species containing aberrant nucleoside modifications. Several modified nucleosides have been detected and quantitated by HPLC analysis of the urine of normal subjects and cancer patients. Results obtained, in the authors' laboratory, among others, indicate a possible correlation between urinary excretion of these compounds and the course of the disease, with implications for the follow-up of therapeutic treatment. Particular reference should be made to psi, which appears to be a suitable marker for monitoring these subjects. The data from the authors' laboratory also show that the analysis of modified nucleosides in blood may be considered a useful tool in the search for proper markers associated with the cancer status. In this respect psi is suggested as a biochemical indicator for cancer patients.

Inhibition of phospholipid methylation in isolated rat hepatocytes by analogues of adenosine and S-adenosylhomocysteine

Phospholipid methylation in isolated hepatocytes was inhibited in the presence of 3-deazaadenosine (ID50=1.7 microM) 9-beta-D-arabinofuranosyladenine (ID50=6.0 microM), S-tubercidinylhomocysteine (ID50=30 microM), and 5'-deoxy-5'-isobutylthioadenosine (ID50=177 microM). A transient inhibitory effect was observed with adenosine, whereas S-adenosyl-L-homocysteine and Sinefungin were essentially without effect. The inhibition of phospholipid methylation by S-tubercidinylhomocysteine and 9-beta-D-arabinofuranosyladenine showed a lag-phase, whereas the effect of the other inhibitors was apparent within a few minutes. Cells exposed to 9-beta-D-arabinofuranosyladenine or 3-deazaadenosine accumulated large amounts of AdoHcy, and adenosine induced a transient increase in the AdoHcy level. In addition, 3-deazaadenosine served as a precursor for the formation of S-3-deazaadenosylhomocysteine, which accumulated rapidly in cells exposed to this agent. The inhibitory effects of 3-deazaadenosine, 9-beta-arabinofuranosyladenine and adenosine could be explained by the increase in total nucleosidylhomocysteine induced by these agents. In contrast, only a slight (less than 2-fold) increase in S-adenosyl-L-homocysteine content was observed in hepatocytes treated with 5'-deoxy-5'-isobutylthioadenosine, and this metabolic effect could not explain the inhibition of phospholipid methylation induced by this agent. None of the compounds tested reduced the amount nor the specific radioactivity of S-adenosylmethionine. Biological processes determining the inhibitory effects of adenosine, S-adenosyl-L-homocysteine and their analogues on phospholipid methylation in intact cells are discussed.

Effect of 5'-deoxy-5'-isobutylthioadenosine on putrescine uptake and polyamine biosynthesis by chick embryo fibroblasts

The effect of 5'-deoxy-5'-S-isobutylthioadenosine (SIBA) on polyamine biosynthesis has been studied by using cultured chick embryo fibroblasts. It has been shown that the drug inhibits the uptake of [14C]putrescine and its conversion into labelled spermidine or spermine. The inhibitory effect is reversed by removing the inhibitor after exposing the cells to the drug for 24 h. SIBA also caused a significant decrease in cellular spermine levels and an accumulation of putrescine. These changes are reversed by removing the inhibitor. SIBA had the same effect on chick embryo fibroblasts transformed by Rous sarcoma virus; a decrease in cellular spermine levels in SIBA-treated cells was observed. In all the experiments SIBA caused a reduction in the spermine/putrescine and spermidine/putrescine ratios. It is suggested that SIBA is not only an inhibitor of transmethylation but also interferes with polyamine biosynthesis, probably by blocking aminopropyltransferase.

Structural analogs of 5'-methylthioadenosine as substrates and inhibitors of 5'-methylthioadenosine phosphorylase and as inhibitors of human lymphocyte transformation

5'-Deoxy-5'-methylthioadenosine (MTA) phosphorylase was purified 13.4-fold from human peripheral lymphocytes. The enzyme demonstrated normal Michaelis-Menten kinetics with Km values of 26 microM and 7.5 mM for the two substrates, MTA and phosphate, respectively. The rate of MTA degradation was temperature dependent, 47 degrees being the optimum temperature. Five structural analogs served as alternative substrates with Km values ranging from 31 to 53 microM while two compounds, 5'-deoxy-5'-methylthiotubercidin (MTT) (Ki = 31 microM) and adenine (Ki = 172 microM), were inhibitory. These same analogs were examined as inhibitors of mitogen-induced human lymphocyte blastogenesis. MTT was found to be the most effective inhibitor of lymphocyte transformation with an I50 of 80 microM.

The effect of adenosine analogues on the in vitro growth of Trypanosoma cruzi

Trypanosoma cruzi (Brazil strain) was maintained in liver infusion tryptose and medium supplemented with 5 or 10% foetal calf serum at 27 degrees C on a rotating shaker platform. 85 to 95% of the organisms under these conditions are epimastigotes and the medium supported logarithmic growth for up to 24 hours. The effect of S-isobutyl adenosine and Sinefungin against cultured T. cruzi epimastigotes was studied: growth rate was slowed by both in a dose-dependent fashion; 500 micrometer Sinefungin caused complete inhibition which was irreversible after 24-hour exposure but the effect of S-isobutyl adenosine (100 micrometer) was reversible. Motility and morphology appeared to be unaffected.

Decreased transmethylation of biogenic amines after in vivo elevation of brain S-adenosyl-l-homocysteine

The ability of S-adenosyl-L-homocysteine (AdoHcy) to inhibit biologic transmethylation reactions in vitro has led us to explore the possibility of pharmacologically manipulating AdoHcy levels in vivo and examining the consequences of these alterations on the transmethylation of some biogenic amines. Swiss-Webster mice were injected intraperitoneally with different doses of adenosine (Ado) and D, L-homocysteine thiolactone (Hcy) and were killed at various times thereafter. S-Adenosyl-methionine (AdoMet) and AdoHcy concentrations were determined by using a modified isotope dilution-ion exchange chromatography-high pressure liquid chromatography technique sensitive to less than 10 pmol. Increasing doses of Ado + Hcy (50-1000 mg/kg of each) produced a dose-related increase in blood, liver, and brain AdoHcy levels. At a dose level of 200 mg/kg Ado + Hcy, AdoHcy levels were markedly elevated, with minimal concomitant perturbations of AdoMet. This elevation was maximal 40 min after giving Ado + Hcy, returning to control values within 6 h. Ado + Hcy treatment resulted in decreased activities of catechol-O-methyltransferase, histamine-N-methyltransferase, and AdoHcy hydrolase in vitro. The cerebral catabolism of intraventricularly administered [(3)H]histamine (HA) was decreased in a dose-related manner by Ado + Hcy treatment as evidenced by higher amounts of nonutilized [(3)H]HA in brain, concurrent decreases in [(3)H]methylhistamine formation, and decreases in the transmethylation conversion index. Steady state levels of HA also showed dose-related increases after Ado + Hcy treatment. It is concluded that injections of Ado + Hcy can markedly elevate AdoHcy levels in vivo, which can, in turn, decrease the rate of transmethylation reactions.

Inhibition of sterol transmethylation by S-adenosylhomocysteine analogs

Structural analogs of S-adenosylhomocysteine were tested in vitro for inhibition of the yeast S-adenosylmethionine:delta 24-sterol-C-methyltransferase enzyme. A wide inhibitory range by these compounds was observed, suggesting which structural features of the parent compound are important for binding to the enzyme. No analog tested had inhibitory activity specific only for this enzyme. The most active compound was sinefungin, a metabolite of Streptomyces griseolus, which was also able to inhibit growth of yeast cultures. Sterol extracts of cells grown in the presence of sinefungin revealed a dramatic increase in the levels of zymosterol, the sterol substrate in the transmethylation under study, and a concomitant decrease in the levels of ergosterol. Evidence is presented that sinefungin is transported inside the cell by the same permease as S-adenosylmethionine. We conclude that sinefungin is blocking the in vivo methylation of sterols in yeast. The implications of this finding are discussed.

Simultaneous decrease in deamination of 5'-adenylic acid and 5'-deoxy-5'-S-isobutylthioadenosine in chick-embryo fibroblasts infected by Rous sarcoma virus

The rate of deamination of 5'-deoxy-5'-S-isobutylthioadenosine [(iBuS5'Ado] in chick embryo fibroblasts was substantially reduced after their infection and morphological transformation by Rous sarcoma virus. Concomitant with the reduction in rate of (iBuS)5'Ado deamination there was a decrease in adenosine deaminase and 5'-adenylic acid deaminase activities. The drop of these activities was related to infection and not to the expression of the src gene. (iBuS)5'Ado was deaminated by at least three enzymes or isoenzymes whose apparent molecular weights have been estimated to be 295000, 121000 and 37000 respectively. Two of these enzymes have been characterized as 5'-adenylic acid deaminase and the heavy form of adenosine deaminase, respectively.

Binding capacities of various analogues of S-adenosyl-L-homocysteine to protein methyltransferase II from human erythrocytes

A series of analogues of S-adenosyl-L-homocysteine, modified mainly in the amino acid portion of the molecule, have been synthesized. All were found to be competitive inhibitors of protein methyltransferase II from human erythrocytes. S-adenosyl-L-homocysteine remains however by far the most effective inhibitor of the methylase.

[Effect of S-adenosyl-L-homocysteine of dopamine catabolism]

1.--The administration of SAH to rats, at physiologically active dose on the sleep, does not change the urinary level of MD and NM. On the other hand, the excretion of DA and NA decreases. 2.--In the brain, SAH does not modify neither the concentration of NA and NM in hypothalamus and thalamus, nor the concentration of DA and MD in corpus striatum. 3.--After intracisternally injection of [14C]DA or [3H]NA, SAH increases the level of [14C]MD and [3H]NM. 4.--Contrary to the studies in vitro, where SAH is an inhibitor of COMT, on the rat it does not seem prevent the methylation of DA and NA.

Protein methylation in animal cells. II. Inhibition of S-adenosyl-L-methionine:protein(arginine) N-methyltransferase by analogs of S-adenosyl-L-homocysteine

1. Protein methylase I (S-adenosyl-L-methionine: protein (arginine) N-methyltransferase, EC 2.1.1.23) has recently been purified in our laboratory from Krebs II ascites cells (Casellas, P. and Jeanteur, P. (1978) Biochim. Biophys. Acta 519, 243--254). In order to probe its binding site for S-adenosyl-L-methionine, three series of compounds deriving from the most potent competitive inhibitor, S-adenosyl-L-homocysteine, by specific alterations in each of the three regions of the molecule (amino acid side chain, ribose and adenine) have been tested for inhibitor activity. A competitive type of inhibition was assumed for all of them and demonstrated for five representative ones. The contribution of each of these regions to the binding could therefore be established as follows: (i) Any modification of the side chain results in a drop in affinity of about two orders of magnitude. Adenosine itself remained significantly inhibitory thereby demonstrating that the presence of a side chain was not critical, although important. (ii) The ribose moiety appears to be an essential part of the molecule as the loss of either 2'- or 3'-hydroxyls or their change to arabino configuration resulted in a nearly complete loss of activity. (iii) The amino group at position 6 and the nitrogen atom at position 7 of the adenine ring also play a crucial role although some substitutions can be tolerated. 2. S-Isobutyladenosine was shown to specifically inhibit the methylation of arginine residues as compared to lysine.

Protein methylation in animal cells. I. Purification and properties of S-adenosyl-L-methionine:protein (arginine) N-methyltransferase from Krebs II ascites cells

1. A protein methylase which specifically transfers methyl groups from S-adenosyl-L-methionine to arginine residues of histones has been substantially purified from Krebs II ascites cells. The purified enzyme was obtained free of contamination by other protein methyl transferases specific for carboxyl and lysine residues. This latter activity copurified with the present enzyme until advanced stages of purification. 2. The purified enzyme does not require any divalent cation for maximum activity. It is inhibited by ionic strength, N-ethylmaleimide and S-adenosyl-L-homocysteine. It has an apparent molecular weight on gel filtration of approx. 5 . 10(5). A Km value for S-adenosyl-L-methionine of 2.5 . 10(-6) M was determined, while the dissociation constant Ki for S-adenosyl-L-homocysteine, which acts as a competitor, was 1.4 . 10(-6) M.

Identification of some metabolic products of 5' -deoxy-5' -S-isobutylthioadenosine, an inhibitor of virus-induced cell transformation

5' -Deoxy-5' -S-isobutylthioadenosine (iBuS)5' Ado has been shown to be rapidly degraded to 5-deoxy-5-S-isobutylthioribose and adenine in procaryotes. In chick embryo fibroblasts there are two metabolic pathways for (iBuS)5' Ado degradation: (a) oxidative deamination into 5' -deoxy-5'-S-isobutylthioinosine (the main product) and (b) hydrolysis into 5-deoxy-5-S-isobutylthioribose plus adenine. The latter reaction is not due to bacterial contamination, since the same results were obtained under sterile conditions and in chick embryo fibroblasts in culture. The inhibition of the virus-induced cell transformation reported by us previously was due to (iBuS)5' Ado rather than to the main metabolic product of this molecule in chick embryo fibroblasts.