Inhibition of phosphoinositide metabolism in human polymorphonuclear leukocytes by S-adenosylhomocysteine

Homocysteine metabolism, growth performance, and immune responses in suckling and weanling piglets

Homocysteine (Hcy), an intermediary sulfur AA, is recognized as a powerful prooxidant with deleterious effects on physiological and immune functions. In piglets, there is an acute 10-fold increase of plasma concentrations of homocysteine (pHcy) during the first 2 wk of life. This project aimed to maximize pHcy variations within physiological ranges using typical supplies of folates and vitamin B12 (B12) to sows and piglets. Growth, immune response, and Hcy metabolism of piglets were studied until piglets reached 56 d of age. Third-parity sows were randomly assigned to a 2 × 2 split-plot design with 2 dietary treatments during gestation and lactation, S(-) (1 mg/kg folates and 20 µg/kg B12, n = 15) and S(+) (10-fold S(-) levels, n = 16), and 2 treatments to piglets within each half litter, intramuscular injections (150 µg) of B12 (P(+)) at d 1 and 21 (weaning) and saline (P(-)). Within each litter of 12 piglets, 3 P(+) and 3 P(-) piglets were studied for growth and Hcy metabolism, and the others were studied for immune responses. During lactation, plasma B12 decreased and was transiently greater in S(+) vs. S(-) piglets on d 1 and P(+) vs. P(-) piglets on d 7 (sow treatment × age and piglet treatment × age; P < 0.05). From 14 to 21 d of age, pHcy was 33% lower in S(+)P(+) vs. S(-)P(-) piglets (sow treatment × piglet treatment interaction; P < 0.05). At 56 d of age, hepatic B12 was greater and pHcy was lower for P(+) vs. P(-) piglets (P < 0.05). No treatment effect was observed on growth except for a lower postweaning G:F in S(+)P(-) piglets than in others (sow treatment × piglet treatment interaction; P < 0.05). Positive correlations were observed between pHcy and growth (r > 0.29, P < 0.05) before and after weaning. Antibody responses to ovalbumin and serum tumor necrosis factor-α were not affected by treatments, but postweaning serum IL-8 peaked earlier in S(-)P(-) vs. S(+)P(+) piglets (piglet treatment × age; sow treatment × piglet treatment interaction, P < 0.05). Proliferation of lymphocytes in response to the mitogen concanavalin A tended to be lower in culture media supplemented with sera from S(-) vs. S(+) piglets (P = 0.081) and P(-) vs. P(+) piglets (P = 0.098), and the reduction of response was more marked (P < 0.05) with high (>21 µM) compared to medium (17 to 21 µM) or low (<17 µM) pHcy. In conclusion, the present vitamin supplements to sows and/or piglets produced variations of pHcy that were not apparently harmful for growth performance of piglets. The greater pHcy, particularly prevalent in S(-) and/or P(-) piglets, had negative effects on some indicators of immune responses, suggesting that these young animals may be immunologically more fragile.

Critical role of transmethylation in TLR signaling and systemic lupus erythematosus

Post-translational protein modifications can play a significant role in immune cell signaling. Recently, we showed that inhibition of transmethylation curtails experimental autoimmune encephalomyelitis, notably by reducing T cell receptor (TCR)-induced activation of CD4(+) T cells. Here, we demonstrate that transmethylation inhibition by a reversible S-adenosyl-l-homocysteine hydrolase inhibitor (DZ2002) led to immunosuppression by reducing TLR-, B cell receptor (BCR)- and TCR-induced activation of immune cells, most likely by blocking NF-κB activity. Moreover, prophylactic treatment with DZ2002 prevented lupus-like disease from developing in both BXSB and MRL-Fas(lpr) mouse models. DZ2002 treatment initiated during active disease significantly improved outcomes in both in vivo models, suggesting methylation inhibition as a novel approach for the treatment of autoimmune/inflammatory diseases.

Transmethylation in immunity and autoimmunity

The activation of immune cells is mediated by a network of signaling proteins that can undergo post-translational modifications critical for their activity. Methylation of nucleic acids or proteins can have major effects on gene expression as well as protein repertoire diversity and function. Emerging data indicate that indeed many immunologic functions, particularly those of T cells, including thymic education, differentiation and effector function are highly dependent on methylation events. The critical role of methylation in immunocyte biology is further documented by evidence that autoimmune phenomena may be curtailed by methylation inhibitors. Additionally, epigenetic alterations imprinted by methylation can also exert effects on normal and abnormal immune responses. Further work in defining methylation effects in the immune system is likely to lead to a more detailed understanding of the immune system and may point to the development of novel therapeutic approaches.

S-adenosyl-L-homocysteine hydrolase inactivation curtails ovalbumin-induced immune responses

The reversible S-adenosyl-L-homocysteine (AdoHcy) hydrolase inhibitor methyl 4-(adenin-9-yl)-2-hydroxybutanoate (DZ2002) suppresses macrophage activation and function. The effects of DZ2002 on T cell function, however, are still unclear. Here, we examined whether DZ2002 alters type 1 helper T cell (Th1) and/or type 2 helper T cell (Th2) immune responses, and whether these effects are associated with both the inhibition of AdoHcy hydrolase and intracellular elevation of endogenous AdoHcy. Male C57BL/6 mice immunized with ovalbumin (OVA) were treated with DZ2002 (1, 5, and 25 mg/kg/day) after which lymphocyte proliferation, cytokine production, and IgG responses to OVA were monitored. Administration of DZ2002 dose dependently suppressed OVA-specific lymphocyte proliferation and anti-OVA IgG production compared with controls. Interleukin (IL)-2 and interferon (IFN)-gamma as well as anti-OVA IgG2a and IgG3, indicators of Th1 immune responses, were markedly decreased in mice treated with DZ2002, whereas IL-4 and anti-OVA IgG1, indicators of Th2 immune responses, were only mildly suppressed. AdoHcy hydrolase activity in spleens of DZ2002-treated mice was substantially blocked, and not surprisingly, AdoHcy levels were significantly elevated compared with controls. Finally, similar immunosuppressive effects were also observed in mice treated with AdoHcy. These data strongly indicate that DZ2002 suppresses antigen-induced specific immune responses, particularly Th1 responses, through inhibition of AdoHcy hydrolase and elevation of endogenous AdoHcy.

Inhibition ofS-Adenosyl-l-homocysteine Hydrolase Induces Immunosuppression

Lymphocytes depend on transmethylation reactions for efficient activation and function. These reactions are primarily catalyzed by S-adenosylmethionine-dependent methyltransferases, which convert S-adenosylmethionine to S-adenosyl-L-homocysteine. S-adenosyl-L-homocysteine is then hydrolyzed by S-adenosyl-L-homocysteine hydrolase to prevent feedback inhibition of transmethylation reactions. By impeding S-adenosyl-L-homocysteine hydrolase, a build-up of S-adenosyl-L-homocysteine occurs, and most intracellular transmethylation reactions cease. Thus, a nontoxic inhibitor of this enzyme might be a useful immunosuppressive therapeutic agent. We identified a potent reversible type III inhibitor of S-adenosyl-L-homocysteine hydrolase, DZ2002 [methyl 4-(adenin-9-yl)-2-hydroxybutanoate], and determined its cytotoxic and immunologic effects. We demonstrated that DZ2002 blocked S-adenosyl-L-homocysteine hydrolase more effectively than a type I inhibitor, but cytotoxicity from DZ2002 was greatly reduced. Although DZ2002 did not prevent concanavalin A-induced T cell proliferation or interleukin (IL)-2 production, it significantly reduced both a mixed lymphocyte reaction and IL-12 production from in vitro-stimulated splenocytes. In addition, levels of CD80 and CD86 on human monocytic THP-1 cells were decreased in a dose-dependent manner in the presence of 0.1 to 10 microM DZ2002, and decreases were also seen in IL-12 and tumor necrosis factor-alpha production from both mouse thioglycollate-stimulated peritoneal macrophages and THP-1 cells. In vivo, DZ2002 significantly suppressed a delayed-type hypersensitivity reaction as well as antibody secretion. We conclude that DZ2002's immunosuppressive effects are likely not solely attributed to T cell inhibition but also to the obstruction of macrophage activation and function through reductions in cytokine output and/or T cell costimulation. These data suggest an important dual role for the S-adenosyl-l-homocysteine hydrolase in both macrophage and T cell function.

Induction of apoptosis in leukemia U937 cells by 5'-deoxy-5'-methylthioadenosine, a potent inhibitor of protein carboxylmethyltransferase

We found dramatic changes in leukemia U937 cells treated with 5'-deoxy-5'-methylthioadenosine (MTA), a potent inhibitor of protein carboxylmethyltransferase (protein methylase II). Initiation of cell death was observed by 1 day after MTA treatment, and it was induced in a dose- and time-dependent manner. However, cell viability measured by trypan blue exclusion was not consistent with the actual percentage of cell death. These results indirectly indicated that the type of cell death is apoptosis rather than necrosis. Nuclear fragmentation and DNA condensation of MTA-treated U937 cells were analyzed by both fluorescent and electron microscopy. MTA-treated cells first began to arrest in the M phase of the cell cycle, and they then exhibited a mitotic-like nuclear fragmentation process with partially membraneless chromatin. Furthermore, agarose gel electrophoresis of DNA extracted from cells treated with MTA showed DNA laddering with production of fragments of approximately 200 bp multiples. These studies indicated that cell death induced by MTA has the characteristics of apoptosis, although nuclear fragmentation is atypical. It seems likely that the process of apoptosis in U937 cells induced by MTA correlates with incomplete assembly of the nuclear envelope, since MTA itself could inhibit the carboxylmethylation of nuclear lamin B and delayed incorporation of lamin B into the nuclear envelope.

Mechanism of erythrocyte accumulation of methylation inhibitor S-adenosylhomocysteine in uremia

We have recently demonstrated that methyl esterification of erythrocyte membrane proteins, a reaction involved in recognition and repair of specifically damaged proteins, is impaired in uremia. This is accompanied by a significant increase in intracellular S-adenosylhomocysteine (AdoHcy), a potent inhibitor of methyltransferases. AdoHcy accumulation is normally prevented by its enzymatic hydrolysis to homocysteine (Hcy) and adenosine, a reversible reaction catalyzed by AdoHcy hydrolase. To assess the contribution that Hcy offers in the elevation of AdoHcy, we measured plasma and red blood cell Hcy, AdoHcy, adenosine, and S-adenosylmethionine (AdoMet) intracellular concentrations, as well as RBC AdoHcy hydrolase specific activity, in standard hemodialysis patients and normal subjects. Plasma and red blood cell Hcy levels are significantly higher in the dialysis group, and are positively correlated to AdoHcy levels. Adenosine and AdoMet levels, and AdoHcy hydrolase specific activity are not significantly different between the two groups. The enzymatic formation of labeled AdoHcy from Hcy and tracer adenosine appears to be significantly increased, in vitro, in erythrocytes from both control and uremic patients, when 50 microM Hcy (concentration comparable to plasma levels actually found in vivo in uremic patients) is added to the incubation medium. When erythrocytes from uremic patients are incubated in vitro in absence of Hcy, a significant reduction of intracellular AdoHcy is observed with time compared to identical samples incubated in presence of 50 microM Hcy, with a T1/2 of approximately 270 minutes. The results allow us to conclude that plasma and red cell Hcy levels actually found in uremia can be effectively responsible for the intracellular accumulation of the toxic compound AdoHcy.

Effect of prenylcysteine analogues on chemoattractant receptor-mediated G protein activation

The hypothesis that carboxylmethylation of gamma subunits plays a role in G protein activation was tested by examining the ability of N-acetyl-S-farnesyl-L-cysteine (AFC) and its methyl ester (AFC-ME) to inhibit G protein-mediated signalling in intact HL-60 granulocytes and isolated HL-60 plasma membranes. Incubation of HL-60 granulocytes with AFC or AFC-ME inhibited superoxide release stimulated by fMet-Leu-Phe, but not by opsonized bacteria. AFC-ME, but not AFC, inhibited NaF- and PMA-stimulated superoxide release. Addition of AFC to HL-60 membranes inhibited fMet-Leu-Phe-, leukotriene B4- (LTB4) and C5a-stimulated GTP gamma S binding and GTP hydrolysis more potently than it inhibited basal guanine nucleotide exchange. AFC-ME inhibited basal- and ligand-stimulated G protein activation with equal potency, but less potently than AFC. AFC also inhibited mastoparan-stimulated GTP gamma S binding. Binding of fMet-Leu-Phe and LTB4 to HL-60 membranes was completely inhibited by AFC, while AFC-ME inhibited ligand binding by less than 50%. Neither AFC nor AFC-ME inhibited pertussis toxin or cholera toxin-catalysed ADP-ribosylation of alpha i. It was concluded that AFC interrupts signal propagation in G protein-dependent pathways by multiple mechanisms, including inhibition of ligand-receptor interactions, of receptor-G protein coupling and of guanine nucleotide binding to G proteins. Carboxylmethylation alters the specificity of AFC interruption of signal propagation in intact cells and isolated membranes.

S-adenosyl-l-methionine inhibits phosphoinositide metabolism in the rat brain synaptosomal suspensions

S-adenosyl-l-methionine (AdoMet) has been reported to affect events linked to noradrenergic neurotransmission. In the present work, we studied the effect of AdoMet on norepinephrine (NE)-stimulated inositol phosphate production in 3H-inositol-labelled crude synaptosomal suspensions of rat brain. AdoMet (50-1000 microM) decreased both the synthesis of labelled polyphosphoinositide (30-50%) and the release of inositol mono- and bisphosphate (40-50%). The AdoMet effect was not dependent on NE concentration (10-1000 microM), suggesting that the inhibition of inositol phosphate release was not the result of a modification of the norepinephrine binding to its receptor sites. S-adenosyl-L-homocysteine (AdoHcy) (1 mM) an inhibitor of methyltransferase activities, partially inhibited (70%) the AdoMet (0.1 mM) effect, indicating that the methylation processes cannot explain all the effects observed. We conclude that, in addition to previously reported effects of AdoMet on NE transport, AdoMet may reduce NE-linked intracellular signalling.

Inhibitors of membrane transmethylation reactions prevent the lymphocyte chemokinetic response

The methylation of membrane phospholipids has been shown to occur following receptor-mediated activation of leukocytes. The present studies show that the human lymphocyte response to two positive chemokinetic signals (bradykinin and lymphocyte chemoattractant factor) can be interrupted by inhibitors of S-adenosyl-L-methionine-mediated transmethylation reactions. The chemokinetic response to the nonphysiologic stimulant colchicine is not affected. We speculate that phospholipid methylation accompanies receptor-mediated lymphocyte migration, and may facilitate activation of second messenger systems.

MAC-1 mediates adherence of human monocytes to endothelium via a protein kinase C dependent mechanism

Leukocyte adherence is mediated by a superfamily of glycoproteins denoted LFA-1 (the lymphocyte function-associated antigen-1), Mac-1 (macrophage antigen-1) and p150,95. The relative importance of these in mediating human monocyte adherence to endothelium, and the biochemical mechanisms which modulate these events, are not understood. In this report, the role of protein kinase C (pkC) in regulating human monocyte adherence to endothelial cells has been investigated. Addition of phorbol 12,13-dibutyrate (PDBu), which specifically stimulates pkC, caused a dose-dependent increase in their adherence to monolayers of bovine aortic endothelial cells. 4 alpha-phorbol didecanoate (4 alpha-PDD), a structural analogue of PDBu which does not stimulate pkC, failed to increase monocyte adhesion. PDBu also produced a dose-dependent increase in the expression of both Mac-1 and p150,95. The pkC-stimulated adherence of monocytes to endothelium was inhibited by the presence of a monoclonal antibody to Mac-1, while monoclonal antibodies to p150,95 and LFA-1 did not influence adherence. It is concluded that monocyte adherence to endothelial cells is regulated through a pkC-dependent mechanism; moreover, this process is mediated primarily via the Mac-1 adhesion glycoprotein.

Increased phosphatidylinositol kinase activity in psoriatic epidermis

Phosphatidylinositol (PI) kinase is activated by growth factors, such as epidermal growth factor (EGF), and is thought to be involved in cellular proliferation. Psoriasis is a hyperproliferative epidermal disease in which EGF receptor expression is altered and phospholipase C activity is increased. Considering the potential importance of growth factor stimulated phosphoinositide metabolism in the genesis of abnormal growth, we measured PI kinase activity in epidermal keratome biopsies from normal skin and the lesional and nonlesional skin of psoriatic patients. The PI kinase activity in 10 psoriatic involved plaques was increased 6.7-fold (Vmax = 67.1 +/- 23.9 pmol formed/min/mg protein +/- SE) when compared with 11 normal epidermal biopsies (Vmax = 10.0 +/- 1.3 pmol/min/mg protein, p less than 0.025). Similar results were noted when enzyme activity was standardized using DNA content. The apparent Km of PI kinase for ATP in involved psoriatic biopsies (0.45 +/- 0.14 mM) was also significantly (p less than 0.025) increased compared with normals (0.11 +/- 0.02 mM). The PI kinase activity in 11 biopsies of nonlesional psoriatic epidermis was not statistically different from normal epidermis. Both psoriatic and normal PI kinases required Mg++ and were inhibited by Ca++. The polyamine, spermine, a known activator of PI kinase in other tissues, stimulated normal but not psoriatic epidermal PI kinase. Both normal and psoriatic PI kinase activities had an apparent mol wt of 85,000. Increased synthesis of phosphoinositides by PI kinase in psoriatic tissue may provide more substrate for phospholipase C; a key enzyme in growth factor-mediated signal transduction.