Rapid increase of phosphoribosyl pyrophosphate concentration after mitogenic stimulation of lymphocytes

Metabolic pathway compartmentalization: an underappreciated opportunity?

For eukaryotic cells to function properly, they divide their intracellular space in subcellular compartments, each harboring specific metabolic activities. In recent years, it has become increasingly clear that compartmentalization of metabolic pathways is a prerequisite for certain cellular functions. This has for instance been documented for cellular migration, which relies on subcellular localization of glycolysis or mitochondrial respiration in a cell type-dependent manner. Although exciting, this field is still in its infancy, partly due to the limited availability of methods to study the directionality of metabolic pathways and to visualize metabolic processes in distinct cellular compartments. Nonetheless, advances in this field may offer opportunities for innovative strategies to target deregulated compartmentalized metabolism in disease.

Aerobic glycolysis: meeting the metabolic requirements of cell proliferation

Warburg's observation that cancer cells exhibit a high rate of glycolysis even in the presence of oxygen (aerobic glycolysis) sparked debate over the role of glycolysis in normal and cancer cells. Although it has been established that defects in mitochondrial respiration are not the cause of cancer or aerobic glycolysis, the advantages of enhanced glycolysis in cancer remain controversial. Many cells ranging from microbes to lymphocytes use aerobic glycolysis during rapid proliferation, which suggests it may play a fundamental role in supporting cell growth. Here, we review how glycolysis contributes to the metabolic processes of dividing cells. We provide a detailed accounting of the biosynthetic requirements to construct a new cell and illustrate the importance of glycolysis in providing carbons to generate biomass. We argue that the major function of aerobic glycolysis is to maintain high levels of glycolytic intermediates to support anabolic reactions in cells, thus providing an explanation for why increased glucose metabolism is selected for in proliferating cells throughout nature.

Purine metabolism and control of cell proliferation

Exposure of normal lymphocytes to phytohaemagglutinin or other lectin mitogens results in increased concentrations of 5-phosphoribosyl-1-pyrophosphate (PP-ribose-P) within minutes. Subsequently, synthesis of purine nucleotides by both the de novo and the salvage pathways is facilitated. This change is prevented by proliferation-inhibiting concentrations of exogenous adenosine. The capacity of lymphocytes to metabolize both adenine and adenosine is increased several-fold by incubation with phytohaemagglutinin but the specific activities of the respective first-step enzymes are not significantly altered. These results suggest that the relatively low quantity of PP-ribose-P available in normal lymphocytes is a major factor limiting the synthesis of purine nucleotides and may be important for the maintenance of the quiescent state. Increased availability of PP-ribose-P may also be associated with proliferative activation of fibroblast-like cells: chick embryo fibroblast cultures released from density-dependent inhibition of growth by insulin, trypsin or serum rapidly increase the rate of adenine incorporation into nucleotides. Chick embryo fibroblasts transformed by Rous sarcoma virus, but not cells infected with the respective non-transforming leukosis virus, show PP-ribose-P concentrations higher than those observed in normal cells.

Prophylaxis and treatment of acute graft-versus-host disease

Acute graft-versus-host disease (GVHD) remains a major obstacle to successful allogeneic hematopoietic stem cell transplantation (HSCT). The ability to prevent GVHD--the application of successful prophylaxis--is crucial as treatment when prophylaxis fails or remains suboptimal. A calcineurin inhibitor in combination with methotrexate is still the mainstream regimen for prophylaxis of GVHD. Despite a steady increase in the repertoire of available drugs, corticosteroids remain the first-line therapy for patients who fail prevention and develop GVHD. Pan T-cell depletion studies suggest that success in prophylaxis and treatment of GVHD will depend on whether GVHD can be prevented without losing anti-malignancy and anti-infectious effects. Better understanding of the allogeneic response that is responsible for GVHD will facilitate the development of such an approach.

Mycophenolate mofetil and its mechanisms of action

Mycophenolate mofetil (MMF, CellCept(R)) is a prodrug of mycophenolic acid (MPA), an inhibitor of inosine monophosphate dehydrogenase (IMPDH). This is the rate-limiting enzyme in de novo synthesis of guanosine nucleotides. T- and B-lymphocytes are more dependent on this pathway than other cell types are. Moreover, MPA is a fivefold more potent inhibitor of the type II isoform of IMPDH, which is expressed in activated lymphocytes, than of the type I isoform of IMPDH, which is expressed in most cell types. MPA has therefore a more potent cytostatic effect on lymphocytes than on other cell types. This is the principal mechanism by which MPA exerts immunosuppressive effects. Three other mechanisms may also contribute to the efficacy of MPA in preventing allograft rejection and other applications. First, MPA can induce apoptosis of activated T-lymphocytes, which may eliminate clones of cells responding to antigenic stimulation. Second, by depleting guanosine nucleotides, MPA suppresses glycosylation and the expression of some adhesion molecules, thereby decreasing the recruitment of lymphocytes and monocytes into sites of inflammation and graft rejection. Third, by depleting guanosine nucleotides MPA also depletes tetrahydrobiopterin, a co-factor for the inducible form of nitric oxide synthase (iNOS). MPA therefore suppresses the production by iNOS of NO, and consequent tissue damage mediated by peroxynitrite. CellCept(R) suppresses T-lymphocytic responses to allogeneic cells and other antigens. The drug also suppresses primary, but not secondary, antibody responses. The efficacy of regimes including CellCept(R) in preventing allograft rejection, and in the treatment of rejection, is now firmly established. CellCept(R) is also efficacious in several experimental animal models of chronic rejection, and it is hoped that the drug will have the same effect in humans.

Inosine-5'-monophosphate dehydrogenase: regulation of expression and role in cellular proliferation and T lymphocyte activation

Guanine nucleotide synthesis is essential for the maintenance of normal cell growth and function, as well as for cellular transformation and immune responses. The expression of two genes encoding human inosine-5'-monophosphate dehyrogenase (IMPDH) type I and type II results in the translation of catalytically indistinguishable enzymes that control the rate-limiting step in the de novo synthesis of guanine nucleotides. Cellular IMPDH activity is increased more than 10-fold in activated peripheral blood T lymphocytes and is attributable to the increased expression of both the type I and type II enzymes. In contrast, abrogation of cellular IMPDH activity by selective inhibitors prevents T lymphocyte activation and establishes a requirement for elevated IMPDH activity in T lymphocytic responses. In order to assess the molecular mechanisms governing the expression of the IMPDH type I and type II genes in resting and activated peripheral blood T lymphocytes, we have cloned the human IMPDH type I and type II genes and characterized their genomic organization and their respective 5'-flanking regions. Both genes contain 14 highly conserved exons that vary in size from 49 to 207 base pairs. However, the intron structures are completely divergent, resulting in disparities in gene length (18 kilobases for type I and 5.8 kilobases for type II). In addition, the 5'-regulatory sequences are highly divergent; expression of the IMPDH type I gene is controlled by three distinct promoters in a tissue specific manner while the type II gene is regulated by a single promoter and closely flanked in the 5' region by a gene of unknown function. The conservation of the IMPDH type I and type II coding sequence in the presence of highly divergent 5'-regulatory sequences points to a multifactorial control of enzyme expression and suggests that tissue-specific and/or developmentally specific regulation of expression may be important. Delineation of these regulatory mechanisms will aid in the elucidation of the signaling events that ultimately lead to the synthesis of guanine nucleotides required for cellular entry into S phase and the initiation of DNA replication.

Lymphocyte-selective cytostatic and immunosuppressive effects of mycophenolic acid in vitro: role of deoxyguanosine nucleotide depletion

Mycophenolic acid (MPA), an inhibitor of inosine monophosphate dehydrogenase, in nanomolar concentrations blocks proliferative responses of cultured human, mouse and rat T lymphocytes and B lymphocytes to mitogens or in mixed lymphocyte reactions. The inhibitory effect of MPA on lymphocyte proliferation is reversed by addition to culture media of deoxyguanosine or guanosine but not by addition of deoxyadenosine or adenosine. The findings suggest that the principal mechanism of action of low concentrations of MPA is depletion of deoxyguanosine triphosphate which is required for DNA synthesis. In immunosuppressive doses, MPA does not affect the formation of IL-1 by LPS-activated human peripheral blood monocytes. Unlike cyclosporin A and FK-506, MPA does not inhibit the formation of IL-2 and the expression of the IL-2 receptor in mitogen-activated human T lymphocytes. MPA suppresses mixed lymphocyte reactions when added 3 days after their initiation. These findings suggest that MPA does not inhibit early responses of T and B lymphocytes to mitogenic or antigenic stimulation but blocks the cells at the time of DNA synthesis. The cytostatic effect of MPA is more potent on lymphocytes than on other cell types, such as fibroblasts and endothelial cells. MPA also inhibits antibody formation by polyclonally activated human B lymphocytes. MPA is an immunosuppressive agent reversibly inhibiting proliferation of T and B lymphocytes and antibody formation, with a profile of activity different from that of other immunosuppressive drugs. Human T and B lymphocytic and promonocytic cell lines are highly sensitive to the antiproliferative effects of MPA, whereas the erythroid precursor cell line K562 is less susceptible. The effect of MPA on cells of the monocyte-macrophage lineage could exert long-acting anti-inflammatory activity. MPA or analogues may have therapeutic utility in diseases such as rheumatoid arthritis, for prevention of allograft rejection and in lymphocytic or monocytic leukaemias and lymphomas.

Purine nucleotide metabolism in phytohemagglutinin-induced human T lymphocytes

The comprehensive studies of purine nucleotide metabolism were done in nonstimulated and phytohemagglutinin (PHA)-stimulated human peripheral blood T lymphocytes. Nonstimulated lymphocytes synthesize nucleotides in two alternative pathways: via biosynthesis de novo and salvage pathways. Although synthesis of triphosphonucleosides in unstimulated lymphocytes was the predominant pathway, interconversion of monophosphonucleosides was also active. Exposure of cells to PHA affects differently various pathways of nucleotide metabolism. The most marked changes observed were rapid activation of purine salvage within minutes after exposure to PHA, and significant increase of 5-phosphoribosyl-1-pyrophosphate levels. In addition, significant increases were found in de novo purine biosynthesis, nucleotide interconversions, and RNA and DNA synthesis, whereas catabolism of nucleotides remained unchanged. These results indicate that PHA activation of T lymphocytes causes a rapid synthesis of nucleotides which may be required immediately for increases in energy metabolism and later as the precursors of nucleic acid synthesis.

Mitogenic enhancement of purine base phosphoribosylation in Swiss mouse 3T3 cells

Increased uptake of guanosine and the purine bases, adenine, hypoxanthine, and guanine, occurs within minutes after addition of fresh serum or purified growth-promoting agents to density-arrested cultures of Swiss 3T3, Swiss 3T6, and tertiary mouse embryo fibroblasts. Enhancement of uptake is maximal by about 50 min, is potentiated by combinations of growth promoters, and involves a process distinguishable from that of the enhanced uridine uptake on the basis of time course, pattern of growth-factor responsiveness and the failure of uridine to inhibit purine uptake. Enhanced purine uptake in 3T3 cells results from stimulation of the phosphoribosylation of purine bases and consequent trapping of nucleotide derivatives rather than from increased rates of membrane transport of the purine compounds. Increased availability of 5-phosphoribosyl 1-pyrophosphate (PRPP), identifiable within 30 min of serum addition, provides a likely explanation for increased purine base phosphoribosylation. Mitogenic stimulation of purine uptake is unaffected by cycloheximide but is markedly reduced in sodium-free medium. Enhanced purine uptake does not clearly depend upon changes in concentrations of effectors of intracellular PRPP synthesis. Nevertheless, when the possibility of direct mitogenic activation of PRPP synthase was studied, no differences were found in activities of this enzyme in extracts from stimulated and unstimulated 3T3 cells and mouse embryo fibroblasts.

Purine enzymes and immune function

The term immune function is taken to mean the process by which lymphocytes respond to an antigenic challenge. This response involves cell division and differentiation, which depend on purine nucleotides. The present state of knowledge relating events at the cell surface to the necessarily increased rate of purine de novo synthesis, and the modulation of purine nucleotide interconversions through the mediation of putative intracellular messengers, is reviewed. These physiological processes are related, where possible, to some genetic and pharmacologically induced perturbations of the systems involved.

Some regulatory and integrative aspects of purine nucleotide biosynthesis and its control: an overview

The regulation and integration of purine nucleotide biosynthesis is considered from the viewpoint of the main groups of reaction sequences involved and with respect to some specific organs and tissues. Inhibiting either IMP dehydrogenase or adenylosuccinate synthetase in rat liver in vitro reduced the rate of purine do novo synthesis with respect to the purine remaining in the tissue and did not materially affect the rate with respect to the purines extruded into the incubation medium. These results are considered in contrast to the results of previous studies in cultured lymphoblasts. The relative activities of purine de novo synthesis and of purine salvage have been assessed in different tissues by the activities of amidophosphoribosyltransferase and hypoxanthine phosphoribosyltransferase (HPRT), respectively. Changes in purine de novo synthesis as measured by [14C]formate incorporation into cellular purines were reflected in the amidophosphoribosyltransferase activities. The capacity of different tissues to synthesize purines de novo is widespread and the role of the liver as the main site of purine de novo synthesis in vivo and exporting purines to other tissues appears questionable. Regulatory mechanisms may well be tissue specific. The age-related changes in the activity of the purine de novo synthesis and purine salvage pathways, respectively, in the brain suggest that it is physiological or neuropharmacological functions of the developed brain rather than cell division and organogenesis which require a high level of purine salvage relative to purine de novo synthesis. This is compatible with the observation that purine de novo synthesis alone can meet the needs for additional purine nucleotides which lectin induced lymphocyte transformation involves. The mechanism whereby purine de novo synthesis is initiated during lectin induced lymphoblast transformation remains obscure.

Adenine and hypoxanthine metabolism in phythohemagglutinin-stimulated and unstimulated human lymphocytes

The uptake and subsequent metabolism of adenine and hypoxanthine in phytohemagglutinin-stimulated and unstimulated peripheral human blood lymphocytes, freshly prepared or cultured, were studied. To investigate the initial step of nucleic acid metabolism the incorporation of 14C-purines into the acid soluble material was examined. No preferential uptake of adenine or hypoxanthine was observed in freshly prepared and cultured lymphocytes during an incubation of 1 h. However, cultured cells utilized approximately 1/3 of the purines compared to freshly drawn cells. Within the cells 2/3 of adenine and 1/2 of hypoxanthine were metabolized to nucleotides (mainly AMP and ADP). Incubation of lymphocytes with PHA for 1 h produced in the freshly prepared cells an increase of adenine- and hypoxanthine-uptake to 191% and 153%, in 48 h stimulated cells to 158% and 132%. There was, however, no change in the relative rates of the metabolic routes though the intracellular concentrations of nucleotides formed increased with adenine as substrate to 152% and with hypoxanthine to 161% during a 1 h stimulation. In contrast no enhanced formation of acid soluble nucleotide formation could be observed with PHA stimulation during 48 h. The increased rates of purine uptake and metabolism apparent 1 h after addition of mitogen may be due to an altered transport mechanism at the beginning of the transformation as an adaptive response to the increased requirements for the synthetic processes soon to follow. Once the lymphocytes are transformed no demand of purines is necessary and the uptake and metabolism is switched off.

The effects of added purines on urate and purine synthesis de novo by isolated chick liver, kidney and lymphoid cells

1. Isolated chick lymphoid cells, together with isolated chick liver and kidney cells, incorporate [1-14C]glycine or [14C]formate into urate. 2. Of the cell types used, bursal cells incorporate 14C into urate at the fastest rate, although the output of total urate by bursal cells is only 10% that of liver cells. 3. When suspended in Eagle's medium the incorporation of 14C into urate is inhibited by adenine and guanine up to 1 mM. In contrast, the addition of 1 mM-AMP or -GMP results in a relatively large stimulation of this incorporation. 4. Added adenine is rapidly taken up by liver cells and then released in an unmetabolized form; AMP is taken up more slowly and is rapidly metabolized. The metabolites (possibly including adenine) are then released. 5. Intracellular liver 5-phosphoribosyl 1-pyrophosphate is approx. 0.7mM and remains constant or falls slightly during a 3 h incubation of the cells. 6. The addition of adenine or guanine, AMP or GMP, does not alter liver intracellular 5-phosphoribosyl 1-pyrophosphate concentrations. Added 5-phosphoribosyl 1-pyrophosphate is not taken up by liver cells. 7. The results are discussed in the context of the control of urate and purine synthesis de novo in the chick.

Immunological observations on patients with Lesch-Nyhan syndrome, and on the role of de-novo purine synthesis in lymphocyte transformation

Three patients with the Lesch-Nyhan syndrome were found to have normal delayed hypersensitivity, peripheral-blood T-lymphocyte counts, lymphocyte responses to P.H.A., and serum IgM, IgA, and IgE levels. However, the percentages of B-lymphocytes, IgG levels, serum-isohaemagglutinin titres, and lymphocyte responses to pokeweed mitogen (P.W.M.) were subnormal. These observations suggest that activity of the salvage pathway of purine synthesis catalysed by hypoxanthine-guanine phosphoribosyl transferase (H.G.P.R.T.) is not required for the responses of T-lymphocytes to mitogenic or antigenic stimulation, but may contribute to the proliferation and function of B lymphocytes. The major role of the de-novo pathway of purine synthesis in human lymphocyte responses to mitogenic or antigenic stimulation is shown by the effects of inhibitors of this pathway, including immunosuppressive agents, and by the effects of congenital deficiency or inhibition of adenosine deaminase.