Failure of T lymphocytes from elderly humans to enter the cell cycle is associated with low Cdk6 activity and impaired phosphorylation of Rb protein

Cell cycle analyses of activated T lymphocytes from elderly humans generally show that the proportion of noncycling cells increases with age. T cells that are not definitively blocked in G0 usually strain to traverse the G1 phase and may still be arrested at the G1/S boundary. The molecular mechanisms underlying these cell cycle arrests are unknown. Because G0/G1 and G1/S transitions are regulated in part by cyclin-dependent kinase Cdk6, we investigated the possibility that a loss of activity of this kinase is implicated in the age-related dysfunction of the cell cycle in its initial phases. G0/G1 and G1/S blocks were first confirmed by [(3)H]uridine and [(3)H]thymidine incorporation studies in anti-CD3 activated T lymphocytes derived from elderly donors. In the same cell preparations, in vitro phosphorylation of recombinant truncated Rb protein by immunoprecipitated Cdk6 was significantly decreased. The reduced Cdk6 activity was not attributable to a low level of the protein since a 24-h activation resulted in a comparable expression of the kinase in T cells from young and old individuals. However, at least two other mechanisms might be incriminated in the loss of Cdk6 activity: (1) a poor induction of the associated cyclin D2 upon anti-CD3 stimulation and (2) a delayed downregulation of the Cdk inhibitor p27 following cell activation. The low Cdk6 activity observed in T lymphocytes from the elderly was associated with a defective phosphorylation of the endogenous Rb protein and an increased sequestration of the E(2)F-1 transcription factor, possibly resulting in early cell cycle arrest.

Age-associated decline in cdk1 activity delays cell cycle progression of human T lymphocytes

Despite the repeatedly observed impaired proliferative response of T lymphocytes from aged donors, the precise molecular basis underlying such a defect is still poorly understood. The aim of this study was to determine whether cyclin-dependent kinase 1 (cdk1), a serine-threonine kinase required for entry into mitosis, is implicated in this age-associated dysregulation of the cell cycle. T lymphocytes derived from young and elderly donors were blocked in S phase by hydroxyurea after a 48-h activation by anti-CD3 Abs. Under these experimental conditions, only the cells that were already located beyond the S phase were able to complete the cell cycle, decreasing their DNA content from 4n to 2n chromosomes. Using this procedure, a delay in the accomplishment of mitosis could be observed in cells from elderly individuals, as evidenced by propidium iodide staining. In this age group, only a minimal cdk1 activity could be immunoprecipitated from cells sorted in G2/M after nocodazole block. The decrease in cdk1 activity observed in T lymphocytes from aged donors could be accounted for by at least three mechanisms: 1) a failure of these cells to express a sufficient amount of cdk1, 2) a reduced level of the associated cyclin B1, and 3) an incomplete dephosphorylation of the kinase on tyrosine. This low cdk1 activity is likely to postpone the progression through the G2/M transition and participates in the dysfunction of the cell cycle during the process of aging.

Susceptibility to apoptosis of T lymphocytes from elderly humans is associated with increased in vivo expression of functional Fas receptors

We recently showed that mature T lymphocytes derived from elderly humans were more susceptible to activation-induced cell death than similar cells from young individuals. Because this excessive apoptosis is unrelated to either the age-associated decrease in IL-2 production, a differential Bcl-2 expression or to a modification of the antioxidant pathway, we examined the possibility that the Fas receptor (FasR) is directly implicated in the generation of the unwarranted death signal. We investigated the expression and the function of FasR on T lymphocyte populations from healthy young and elderly individuals. We found that the frequency of FasR+ T cells increases as a function of age. The FasR expressed at the surface of freshly isolated T lymphocytes from elderly donors appear to be fully functional since their ligation by a cytocidal IgM anti-Fas mAb leads to a significant increase in DNA fragmentation in this cell population. Conversely, exposure of T cells derived from aged individuals to an antagonistic anti-FasR mAb partially prevents the age-related increase in apoptotic cell death. The population of FasR+ T lymphocytes is essentially constituted of previously activated CD45RO+ cells and also includes recently activated lymphocytes bearing the CD25 and CD69 activation markers. The accumulation of chronically and recently in vivo activated T-cells with age probably contributes to the amplification of the process of Fas-mediated cell death in T lymphocytes isolated from senescent organisms.

[Molecular mechanisms of age-related lymphocyte dysfunction]

Aging is classically accompanied by a dysregulation of the immunologic machinery. As a consequence, the immune response developed in senescent organisms is usually inappropriate, often inefficient, sometimes aberrant, and potentially detrimental. The age-associated immune dysfunction may be implicated to some degree in the extreme susceptibility of the elderly to infection and neoplasia and may even participate in various aspects of senescence. The current understanding of the molecular mechanisms underlying immunosenescence is still fragmentary. The most extensively studied phenomenon is the progressive decline in the proliferative capacities of T lymphocytes with aging. The loss of proliferative potential in response to antigenic challenge is a characteristic feature of immune senescence. It is directly implicated in the emergence of the age-related immune deficiency. The purpose of this review is to show how the accumulation of various biochemical lesions with advancing age leads to the failure of a critical cell function, namely the activation-induced lymphocyte proliferation. The biochemical modifications responsible for the defect in transduction and execution of the proliferative signal are analyzed as a function of age. The multiple alterations observed on the various biochemical pathways may appear as a consequence of a unique deleterious mechanism more fundamentally related to the process of senescence such as the inability to cope with oxidative stress.

Age-related tyrosine-specific protein phosphorylation defect in human T lymphocytes activated through CD3, CD4, CD8 or the IL-2 receptor

Although transmembrane signaling defect has been recognized as one of the major functional alterations involved in immune senescence, its biochemical nature as well as its precise molecular localization are still unknown. The available data indicate that an early step in the signaling cascade may be affected during the aging process. Because protein tyrosine kinases (PTK) are ubiquitously implicated in the initiation of physiological signals, they appear as prime candidates for age-related changes. The present investigation examined the effect of age on the activity of PTK associated with CD3, CD4, CD8 or the IL-2 receptor (IL-2R) in human T lymphocytes. By comparison with cells derived from young individuals, anti-CD3-activated T lymphocytes from elderly donors were more susceptible to herbimycin A, a PTK inhibitor known to prevent signal transduction by the T cell antigen receptor. This increased sensitivity of cells from senescent organisms to PTK inhibitors is most likely related to a lesser PTK activity since a significant decrease in the tyrosine phosphorylation of particular endogenous substrates was observed as a consequence of either CD3, CD4, CD8 or IL-2R activation. However, no age-related difference in tyrosine phosphorylation could be demonstrated when T cells were activated by pervanadate, a pharmacological activator of PTK. These results suggest that the intrinsic activity of the enzymes is preserved and that the age-associated defect in PTK activation occurs as a consequence of an upstream biochemical alteration. The defect in PTK activation could be the primary cause for the dysfunction of various components of the signaling cascade observed during the course of aging.

Excessive apoptosis of mature T lymphocytes is a characteristic feature of human immune senescence

Recent evidence suggests that apoptotic deletion of activated mature lymphocytes is an essential physiological process implicated in both the regulation of the immune response and the control of the overall number of immunocompetent cells. Tightly interrelated signaling mechanisms convey either activation or death messages, achieving the necessary equilibrium between cell proliferation and cell deletion. During the course of aging, numerous alterations of these signaling pathways may shift the balance toward cell death. In the present investigation, the reduced DNA synthesis of anti-CD3 activated T lymphocytes isolated from elderly individuals is associated with an important and early cell deletion from the cultures. Visualization of DNA fragmentation in the remaining activated cells argues in favour of the apoptotic nature of the cell deletion. Quantification of histone-associated DNA fragments shows that the apoptotic process is greatly amplified in activated lymphocytes derived from senescent organisms. Further analysis reveals that IL-2 deprivation does not play a significant role in the age-related increase in apoptosis. Partial correction of this excessive apoptosis by products that bypass the early steps of the signaling cascade suggests that transmembrane signaling defects are involved in this process. Exploration of the antioxidant pathway reveals that the increased susceptibility of lymphocytes from senescent organisms to apoptosis is not explained by a decreased Bcl-2 expression and is not influenced by a modification of the intracellular concentration of glutathione (GSH).

Sinefungin shares AdoMet-uptake system to enter Leishmania donovani promastigotes

The involvement of a carrier for sinefungin (SF) uptake in Leishmania donovani promastigotes is indicated by saturation kinetics, competition studies and SF accumulation against a 270-fold concentration gradient across the cell membrane. Whether SF uptake occurs via nucleoside- or AdoMet-carrier systems was investigated by competition experiments and comparison of the uptake of various molecules in wild-type and SF-resistant cells. Results show that SF did not inhibit purine or pyrimidine uptake whereas it competitively inhibited AdoMet uptake. Furthermore, the uptake of nucleosides in SF-resistant cells is similar to that in wild-type cells, whereas uptake of SF and AdoMet is lower.

Expression of the major surface glycoprotein of Leishmania, gp63, in wild-type and sinefungin-resistant promastigotes

In this study, we have surveyed gp63 expression in sinefungin-(SF)-resistant and wild-type Leishmania promastigotes. Documentation of gp63 expression in Leishmania promastigotes was carried out by Western blotting, purification of the protein and assessment of gp63 protease activity. We demonstrated a 3-4-fold and 1.5-2-fold increase of gp63 protein in SF-resistant Leishmania donovani and Leishmania tropica promastigotes compared to wild-type, respectively. Northern blot analysis showed that the increase in the amount of gp63 protein in SF-resistant compared to wild-type parasites was concomitant with an increase in gp63 mRNA. No extrachromosomal DNA was identified by alkaline lysis of isolated DNA samples and Southern blot analysis. Treatment of SF-resistant and wild-type L. donovani promastigotes with cycloheximide resulted in an increase of the steady state levels of gp63 mRNA in the SF-resistant parasites to approximately fivefold that of the wild type. After treating parasites with actinomycin D, estimated gp63 mRNA t1/2 in the wild type was 40 min and increased to 83 min in SF-resistant promastigotes. Therefore, the overexpression of gp63 may be mediated, at least in part, by post-transcriptional stabilization of a gp63 transcript by a protein factor. Down regulation of the latter factor may account for the observed increase in gp63 expression in SF-resistant promastigotes. Attempts to correlate gp63 expression with promastigote virulence suggested that the observed increase in gp63 expression did not result in a significant change in the virulence of SF-resistant compared to wild-type L. donovani promastigotes.

Characterization of sinefungin-resistant Leishmania donovani promastigotes

Promastigotes resistant to sinefungin (SF), a nucleoside antibiotic that is structurally related to S-adenosylmethionine (AdoMet), were obtained starting from two cloned strains of Leishmania donovani. The resistance was induced by increasing the drug pressure gradually until promastigotes capable of growing in the presence of concentrations 10,000 times higher than the 50% growth-inhibitory (IC50) values for the control cells were obtained. The resistance to SF of both clones was specific and stable in the absence of drug pressure. High-performance liquid chromatographic (HPLC) analyses indicated highly reduced levels of SF in the two resistant clones. However, the intracellular SF concentration in these resistant cells was much higher than the IC50 values for wild-type cells. In one clone, the decreased drug uptake was coupled to a decrease in the affinity of two protein methylases for SF, whereas in the other clone the biosynthesis of polyamine precursors was modified. This study demonstrates that resistance to a drug molecule with pleiotropic targets can be developed through various mechanisms by different strains.

Leishmania donovani: antagonistic effect of S-adenosyl methionine on ultrastructural changes and growth inhibition induced by sinefungin

Sinefungin, an antifungal and antiparasitic nucleoside antibiotic, is a very potent antileishmanial agent in vitro and in vivo (Bachrach et al. 1980, FEBS Letters 121, 287-291; Neal et al. 1985, Transactions of the Royal Society of Tropical Medicine and Hygiene 79, 85-122). It was previously shown that this molecule is a competitive inhibitor of AdoMet for transmethylases (Paolantonacci et al. 1986, Molecular and Biochemical Parasitology 21, 47-54; Avila et al. 1987, Molecular and Biochemical Parasitology 26, 69-76) and that it induces shape changes of Leishmania donovani promastigotes as observed by light microscopy (Lawrence and Robert-Gero 1990; Bulletin de la Societé Française de Parasitologie 8, 13-18). In the present work the effect of the antibiotic on the ultrastructure was analyzed by electron microscopy. The main changes induced at sublethal concentrations (0.26 microM sinefungin for 16 hr) were progressive rounding, decreased motility, enlargement of the flagellar pocket, and shortening and loss of the external part of the flagellum. The comparison with control cells showed shorter Golgi saccules and fragmentation of the trans-Golgi network into vesicles, indicating a stimulated Golgi apparatus activity. This result, associated with the enlarged flagellar pocket, suggests an unbalanced cytoplasmic exchange between exocytosis and endocytosis. These effects are quite different from those induced by tunicamycin (Dagger et al. 1984, Biology of the Cell 50; 173-180) or paromomycin. In addition, other nucleoside and nonnucleoside growth inhibitors failed to induce similar changes. AdoMet antagonized the sinefungin-induced shape changes and ultrastructural modifications but had no effect with respect to other growth inhibitors. This suggests that the sinefungin activity at the cellular level is specifically related to competition with AdoMet. A comparative study of N-methylation and carboxylmethylation of proteins in sinefungin-treated promastigotes showed that the antibiotic preferentially inhibits the latter, catalyzed by protein-O-methyltransferases. These enzymes are known to regulate the function of various proteins involved in secretion. Overall the results suggest that one of the main targets of sinefungin in exponentially growing cells is the protein carboxylmethylation involved in membrane transport.

Cutaneous leishmaniasis in south-western Ethiopia: Ocholo revisited

The borough of Ocholo, on the western side of the Ethiopian Rift Valley, is an endemic focus for Leishmania aethiopica infection and has been surveyed thrice between 1987 and 1990. In 1989, 3022 inhabitants (> 95% of the population) were interviewed and examined. The overall prevalence of localized cutaneous leishmaniasis (LCL) was 3.6-4.0%, with a peak value of 8.5% in the 0-10 years old age group. In half of the patients the active disease was estimated to last for 9.6 +/- 6 months; in 10%, it exceeded 3 years. Scars of LCL were present in 34.3% of the residents. Leishmanin skin tests were positive in 54% of 120 school-children without signs of the disease. Therefore, in Ocholo a minimum of 71.6% of the population has been exposed to L. aethiopica infection. Two cases of the diffuse form of cutaneous leishmaniasis were observed. In this highland biotope, Phlebotomus pedifer was found to be the major, and possibly the only, vector for L. aethiopica.