Carbohydrate metabolism in transforming lymphocytes from the aged

Glucose metabolism in lymphoid and inflammatory cells and tissues

PURPOSE OF REVIEW

To examine the role of glucose as a fuel for immune cells and the influence of glucose supply on immune-cell functional responses.

RECENT FINDINGS

Immune cells express the insulin receptor and a range of glucose-transporter isoforms. Glucose transporters are responsive to both immune stimulation and insulin. The pattern of glucose-transporter upregulation differs among different types of immune cell. In-vitro studies reveal that both hypo- and hyperglycaemia impair immune-cell functions and promote inflammatory responses. Clamp studies have revealed proinflammatory effects of hyperglycaemia and antiinflammatory and immune-promoting effects of insulin.

SUMMARY

Glucose is readily utilized by cells of the immune system and is used to generate energy and biosynthetic precursors. Activation of immune cells is associated with increased glucose utilization and this is facilitated, in part, by increased expression of glucose transporters. Immune cells express the insulin receptor and respond to insulin. Both hypo- and hyperglycaemia impair immune-cell functions and promote inflammatory responses. Insulin therapy in hyperglycaemic subjects may be of benefit through effects of both insulin itself and lowered glucose concentration. Excessive lowering of blood glucose concentration may also be harmful to the immune response.

Telomeres, telomerase, and telomerase inhibition: clinical implications for cancer

Telomeres are located at the ends of eukaryotic chromosomes. The enzyme telomerase synthesized them, and they are responsible for maintaining the lengths of chromosomes. Absence of telomerase is associated with telomere shortening and aging of somatic cells, but high telomerase activity is observed in over 90% of human cancer cells. Although the disappearance of telomerase with aging is considered a natural defense against development of cancer, it is not known what triggers the reappearance of telomerase in cancer cells. Telomerase activity is directly correlated with the expression of its active catalytic component, the human telomerase reverse transcriptase (hTERT), which is controlled primarily at the level of transcription. An earlier paper discussed the relationship of telomerase with aging. In this article, the contemporary literature is reviewed to explore the associations between telomerase, telomerase inhibition, and cancer. Because most cancers occur in old age, with the aging of the population, the number of people suffering from cancer is expected to increase in the coming decades. It is not known what roles telomerase and hTERT play in the complex relationship between aging and cancer. Data from experimental studies suggest that telomerase assay could potentially play a role in the diagnosis and prognosis of cancers. There is also evidence that telomerase inhibitors might be used as anticancer agents. As the knowledge of the relationships between telomerase and cancer and between telomerase and aging advances, it is hoped that more about the interacting relationships between telomerase, aging, and cancer will be learned.

Increased formation of reactive oxygen species due to glucose depletion in primary cultures of rat thymocytes inhibits proliferation

Glycolytic glucose degradation to lactate is the only way of energy production in mammalian cells that does not use oxygen, and is thus a means of reducing oxidative stress. The aim of this work was to study the effect on proliferation of progressive glucose depletion during the cell cycle of stimulated cultured rat thymocytes. It has been shown that under conditions of limited glucose supply, these cells increase the generation of reactive oxygen species. Because of the sensitivity of the stimulating transcription factor Sp1 to redox changes with respect to its DNA binding, gene expression of glycolytic enzymes is markedly suppressed. As a consequence, cell division does not occur in glucose depleted thymocytes.

Aerobic glycolysis by proliferating cells: protection against oxidative stress at the expense of energy yield

Primary cultures of mitogen-activated rat thymocytes were used to study energy metabolism, gene expression of glycolytic enzymes, and production of reactive oxygen species during cell cycle progression. During transition from the resting to the proliferating state a 7- to 10-fold increase of glycolytic enzyme induction occurs which enables the cells to meet the enhanced energy demand by increased aerobic glycolysis. Cellular redox changes have been found to regulate gene expression of glycolytic enzymes by reversible oxidative inactivation of Sp1-binding to the cognate DNA-binding sites in the promoter region. In contrast to nonproliferating cells, production of phorbol 12-myristate 13-acetate (PMA)-primed reactive oxygen species (ROS) in proliferating rat thymocytes and HL-60 cells is nearly abolished. Pyruvate, a product of aerobic glycolysis, is an effective scavenger of ROS, which could be shown to be generated mainly at the site of complex III of the mitochondrial respiratory chain. Aerobic glycolysis by proliferating cells is discussed as a means to minimize oxidative stress during the phases of the cell cycle when maximally enhanced biosynthesis and cell division do occur.

A qualitative and quantitative protein database approach identifies individual and groups of functionally related proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes: an overview of the functional changes associated with the transformed phenotype

A qualitative and quantitative two-dimensional (2-D) gel database approach has been used to identify individual and groups of proteins that are differentially regulated in simian virus 40 (SV40) transformed human keratinocytes (K14). Five hundred and sixty [35S]methionine-labeled proteins (462 isoelectric focusing, IEF; 98 nonequilibrium pH gradient electrophoresis, NEPHGE), out of the 3038 recorded in the master keratinocyte database, were excised from dry, silver-stained gels of normal proliferating primary keratinocytes and K14 cells and the radioactivity was determined by liquid scintillation counting. Two hundred and thirty five proteins were found to be either up- (177) or down-regulated (58) in the transformed cells by 50% or more, and of these, 115 corresponded to known proteins in the keratinocyte database (J.E. Celis et al., Electrophoresis 1993, 14, 1091-1198). The lowest abundance acidic protein quantitated was present in about 60,000 molecules per cell, assuming a value of 10(8) molecules per cell for total actin. The results identified individual, and groups of functionally related proteins that are differentially regulated in K14 keratinocytes and that play a role in a variety of cellular activities that include general metabolism, the cytoskeleton, DNA replication and cell proliferation, transcription and translation, protein folding, assembly, repair and turnover, membrane traffic, signal transduction, and differentiation. In addition, the results revealed several transformation sensitive proteins of unknown identity in the database as well as known proteins of yet undefined functions. Within the latter group, members of the S100 protein family--whose genes are clustered on human chromosome 1q21--were among the highest down-regulated proteins in K14 keratinocytes. Visual inspection of films exposed for different periods of time revealed only one new protein in the transformed K14 keratinocytes and this corresponded to keratin 18, a cytokeratin expressed mainly by simple epithelia. Besides providing with the first global overview of the functional changes associated with the transformed phenotype of human keratinocytes, the data strengthened previous evidence indicating that transformation results in the abnormal expression of normal genes rather than in the expression of new ones.

Age-related studies of SIg, Leu-4 and concanavalin A receptor densities and capping in human lymphocytes

We compared the cell surface antigen density and capping of three antigens in lymphocytes obtained from healthy, young (mean age 27 years) and elderly (mean age 76), population. There were no differences in the expression of surface immunoglobulin (SIg), concanavalin A (con A) receptors and Leu-4 antigen between the two groups. Kinetic analysis of these molecules revealed a slight decrease in capping in the older population, but the differences were not statistically significant. In order to test the possibility that subjecting the cells to metabolic stress might bring out the differences, we performed a kinetic analysis of SIg and con A capping in the presence of various concentrations of the metabolic inhibitor sodium azide. Although the capping in cells from elderly subjects was slightly more sensitive to azide, no statistical difference was found. Analysis of con A capping by a flow cytometric method yielded similar results, confirming the data obtained by visual capping experiments. We conclude that although a trend toward decreased capping was observed, there is little alteration in the surface molecule capping phenomenon in the age-groups studied.

Expression of glycolytic isoenzymes in activated human peripheral lymphocytes: cell cycle analysis using flow cytometry

Human peripheral lymphocytes activated with concanavalin A and phorbol myristate ester exhibit an increase in glycolysis on a time-course similar to that for DNA synthesis. Elevated glycolysis is accompanied by increased specific activities of the glycolytic enzymes. Increased enzyme activities are accounted for by the appearance of specific isoenzyme forms (muscle forms) normally expressed in rapidly growing tumor cells or in growth-stimulated cells. In the present study we analyzed the expression of the glycolytic isoenzymes during cell cycle progression of activated human lymphocytes using two-parameter (DNA and protein) flow cytometry. Time-course studies and analysis of subpopulations prepared by elutriation centrifugation showed that the inducible isoenzymes are expressed at low levels or not at all in G0 cells. They are expressed first during the G0 to G1 transition or in early G1. However, expression increases throughout G1, reaching a maximum in S-phase. Thus, induction of glycolytic isoenzymes provides an excellent marker of T-cell activation and progression toward DNA synthesis.

Expression of a new set of glycolytic isozymes in activated human peripheral lymphocytes

The combination of phorbol 12-myristate 13-acetate (PMA) and concanavalin A induces the expression of a new set of glycolytic isozymes in human peripheral lymphocytes. The induced isozyme for each enzyme tested (hexokinase, phosphofructokinase, enolase, pyruvate kinase and lactate dehydrogenase) is usually the muscle form, which is often associated with rapidly dividing tumor cells. Increases in a specific isozyme can account for the 2-5-fold increase in specific activities of the enzyme induced by Con A plus PMA. Increased specific activities and the appearance of the new isozyme forms both occur relatively late, and are probably associated with the G1 or S phase of the cell cycle.

An in vitro analogue of immune dysfunction with altered immunoglobulin production in the aged

The consequences of aging of the immune system include impaired T-lymphocyte responsiveness and aberrant immunoglobulin production. Although T cells from elderly individuals have a well-described defect in lymphoblastic transformation in response to some polyclonal mitogens, immunoglobulin abnormalities have lacked a clear in vitro model. Peripheral blood mononuclear cells from 13 young and 13 old healthy donors were cultured with phytohemagglutinin (PHA) or pokeweed mitogen (PWM). Old-donor-cell phytohemagglutinin (PHA), but not PWM, cultures had significantly lower lymphoblastic transformation compared with young donor cultures. IgG, IgA, and IgM production tended to be lower in old- versus young-donor PWM cell cultures. By contrast, despite lower lymphoblastic transformation in old-donor PHA cell cultures, immunoglobulin production was higher for old- versus young-donor cell cultures. No significant age differences were present in initial lymphocyte counts, percent B cells, T cells or monocytes, or helper/suppressor ratios to explain this enhancement in immunoglobulin production. PHA-stimulated mononuclear cell cultures in the aged demonstrate not only a defect in proliferation but also increased immunoglobulin production. This in vitro system may be useful to characterize further the pathogenesis of altered immunoglobulin production in the elderly.

Membrane protein organization of peripheral blood lymphocytes from healthy young and aged adults

Membrane-mediated events are important in the function of lymphocytes and age-dependent changes in the integrity of lymphocytic membranes could account, in part, for the loss of immune responsiveness observed in the aged. To test the hypothesis that membranes of lymphocytes undergo age-dependent change in their biophysical parameters, a method utilizing electron spin resonance spectroscopy was developed as a means to assess the dynamic character of lymphocytic membrane protein organization. This methodology allowed identification of weakly (W) and strongly (S) immobilized membrane proteins. By taking the ratio of these two protein classes (W/S ratio), lymphocytes from elderly were found to have a slightly increased W/S ratio when compared to young adults, but this difference was not statistically significant. This study contrasts with previous reports which indicated that the lipid microenvironment of lymphocytes undergoes age-dependent change. Thus, despite previously reported age-related changes in the biophysical parameters of lipids in resting lymphocytes, this alteration does not appear to affect the dynamic character of membrane-bound proteins.

Effect of phorbol myristate acetate and concanavalin A on the glycolytic enzymes of human peripheral lymphocytes

The effects of Concanavalin A and the tumor promoting agent, phorbol 12-myristate 13-acetate (PMA), on glycolytic enzymes in human peripheral lymphocytes have been studied. A combination of Concanavalin A plus PMA stimulates DNA and protein synthesis to a significantly greater extent than when each are added individually. PMA and concanavalin A together, but not individually, also increase the levels of the activity of the glycolytic enzymes in peripheral lymphocytes treated for 48 h. The increase in hexokinase activity induced by PMA plus concanavalin A appeared to be due to the expression of the isoenzyme form, hexokinase II. The results suggest that the expression of glycolytic enzymes in stimulated lymphocytes is a late event (perhaps associated with the S phase) which is regulated by a cellular signal system controlled by the combined action of PMA plus concanavalin A.

Cell-cycle-related metabolic and enzymatic events in proliferating rat thymocytes

Cell-cycle progression of rat thymocytes stimulated with concanavalin A and interleukin 2 was monitored at 12-h intervals by pulse labeling aliquots of the cell culture with [3H]thymidine, by measuring cellular DNA and protein content and by counting the number of cells in the cultures. The cell cycle was completed after 96 h of culture with the S phase peaking at 48 h. Early events in thymocyte activation were enhanced phosphatidylinositol turnover and the induction of ornithine decarboxylase. Concomitant changes were observed in the rates of DNA synthesis and glycolysis accompanied by a 20-fold increase in glucose uptake 48 h after stimulation. However, the maximal increment in the glycolytic rate preceded that of DNA synthesis by 12 h. Apart from the quantitative changes which occurred during the cell-cycle progression, there was also a change from partial aerobic glucose degradation to CO2 (26%) to almost complete anaerobic conversion of glucose to lactate (85%) and less than 3% to CO2. Glycolytic enzyme levels increased fourfold to tenfold and reached their maxima 48 h after mitogenic stimulation. Maximal increments of glycolytic enzyme activities preceded or coincided with the maximal increments of the glycolytic rate. Actinomycin D (1.5 ng/ml) completely inhibited DNA and RNA synthesis but did not show any inhibitory effect either on glycolytic enzyme induction or on enhanced glycolysis. During mitosis and return of the cells to the non-proliferative state, all of the enhanced metabolic rates returned to their initial levels and the elevated enzyme activities were decreased also. The marked changes of metabolic rates and enzyme activities observed at the various phases of the cell cycle suggest that these biochemical events may also serve as suitable parameters for evaluating the response of lymphocytes towards mitogens and lymphokines.

Expression of intracellular biochemical defects of lymphocytes in aging: proposal of a general aging mechanism which is not cell-specific

There is a decline in immune capacity with age which is expressed on the organismic level by association with numerous immune-related diseases, on the cellular level by impaired mitogenesis, on the biochemical level by impaired metabolic pathways, and on the molecular level by decreased protein synthesis and degradation. Defects in various cofactors such as calcium and several nucleotides also occur and may be related to the impaired enzyme function during mitogenesis in the aged. The central cause for decreased mitogenesis in the aged could be a decrease in protein synthesis which appears to cause impaired enzyme induction. This impaired enzyme induction accounts in part for the decreased glycolytic flux and DNA synthesis in these cells. Decreased protein synthesis also has been associated with a decreased synthesis of lymphokines which help these cells to proliferate. Numerous other intracellular age-related defects of lymphocytes also occur which may collectively play important interdependent roles in the impaired lymphocyte function of the aged. A potential general underlying mechanism of cellular senescence is proposed based on a genetic "slowing-cycle" effect of transcription, translation, and enzyme induction with immunosenescence presented as an example of an expression of these basic defects.