Retinoids enhance glucocorticoid-induced apoptosis of T cells by facilitating glucocorticoid receptor-mediated transcription

Glucocorticoid-induced apoptosis of thymocytes is one of the first recognized forms of programmed cell death. It was shown to require gene activation induced by the glucocorticoid receptor (GR) translocated into the nucleus following ligand binding. In addition, the necessity of the glucocorticoid-induced, but transcription-independent phosphorylation of phosphatidylinositol-specific phospholipase C (PI-PLC) has also been shown. Here we report that retinoic acids, physiological ligands for the nuclear retinoid receptors, enhance glucocorticoid-induced death of mouse thymocytes both in vitro and in vivo. The effect is mediated by retinoic acid receptor (RAR) alpha/retinoid X receptor (RXR) heterodimers, and occurs when both RARα and RXR are ligated by retinoic acids. We show that the ligated RARα/RXR interacts with the ligated GR, resulting in an enhanced transcriptional activity of the GR. The mechanism through which this interaction promotes GR-mediated transcription does not require DNA binding of the retinoid receptors and does not alter the phosphorylation status of Ser232, known to regulate the transcriptional activity of GR. Phosphorylation of PI-PLC was not affected. Besides thymocytes, retinoids also promoted glucocorticoid-induced apoptosis of various T-cell lines, suggesting that they could be used in the therapy of glucocorticoid-sensitive T-cell malignancies.

Missing endomysial and reticulin binding of coeliac antibodies in transglutaminase 2 knockout tissues

BACKGROUND

Autoantibodies against transglutaminase 2 (TG2) are thought to be responsible for the endomysial (EMA), reticulin (ARA), and jejunal antibody (JEA) tissue binding of serum samples from coeliac patients but the exclusive role of TG2 in these staining patterns has not yet been established.

AIMS

To evaluate whether antigens other than TG2 contribute to EMA/ARA/JEA reactions.

PATIENTS

Serum samples from 61 EMA/ARA/JEA positive untreated patients with coeliac disease, 40 dermatitis herpetiformis patients, and 34 EMA/ARA/JEA negative non-coeliac controls were tested.

METHODS

TG2 knockout (TG2-/-) and wild-type mouse oesophagus, jejunum, liver, and kidney sections, and TG2-/- sections coated with human recombinant TG2 were used as substrates in single and double immunofluorescent studies for patient IgA binding and tissue localisation of TG2, fibronectin, actin, and calreticulin.

RESULTS

None of the patient serum samples elicited EMA, ARA, or JEA binding in TG2-/- morphologically normal tissues. In contrast, 96 of 101 gluten sensitive patient samples (95%) reacted with wild-type mouse tissues and all 101 reacted in EMA/ARA/JEA patterns with TG2-/- mouse tissues coated with human TG2. Serum IgA binding to TG2-/- smooth muscle cells was observed in low titres in 31.1%, 27.5%, and 20.5%, and to TG2-/- epithelium in 26.3%, 5.0%, and 8.8% of coeliac, dermatitis herpetiformis, and control samples, respectively. These positivities partly colocalised with actin and calreticulin but not with TG2 or fibronectin.

CONCLUSIONS

EMA/ARA/JEA antibody binding patterns are exclusively TG2 dependent both in coeliac and dermatitis herpetiformis patients. Actin antibodies are responsible for some positivities which are not part of the EMA/ARA/JEA reactions.

Cell death in HIV pathogenesis and its modulation by retinoids

Patients infected with the human immunodeficiency virus exhibit a progressive decline in the CD4 T-cell number, resulting in immunodeficiency and increased susceptibility to opportunistic infections and malignancies. Although CD4 T cell production is impaired in patients infected with HIV, there is now increasing evidence that the primary basis of T cell depletion is accelerated apoptosis of CD4 and CD8 T cells. The rate of lymphocyte apoptosis in HIV infection correlates inversely with the progression of the disease: it is low in long-term progressors and in patients undergoing highly active antiretroviral therapy. Interestingly, only a minor fraction of apoptotic lymphocytes are infected by HIV, indicating that the enhanced apoptosis does not necessarily always serve to remove the HIV+ cells and results from mechanisms other than direct infection. Thus, understanding and influencing the mechanisms of HIV-associated lymphocyte apoptosis may lead to new therapies for HIV disease. In this paper the potential effects of retinoids on CD4 T cell apoptosis is discussed.

Activation-induced apoptosis and cell surface expression of Fas (CD95) ligand are reciprocally regulated by retinoic acid receptor alpha and gamma and involve nur77 in T cells

It has been previously shown that CD4+ T cells enter the apoptotic suicide program via the Fas ligand (FasL)/Fas-mediated pathway upon T cell receptor (TCR) stimulation. In Jurkat cells TCR stimulation regulates the de novo synthesis of FasL, while in the influenza hemagglutinin-specific CD4+ murine T cell hybridoma (IP-12-7) the cell surface appearance of a preformed FasL is initiated. Both processes are dependent on new mRNA and protein synthesis, involve up-regulation of nur77, and can be inhibited by retinoic acids (RA). Two groups of nuclear receptors for RA have been identified: retinoic acid receptors (RAR) and retinoid X receptors (RXR). In this study various synthetic retinoids were used to define which receptors regulate TCR-mediated apoptosis. It is demonstrated that the inhibition is mediated via RARalpha, while RARgamma enhances TCR-mediated apoptosis, and when both receptors are stimulated, the costimulation by RXR will promote the effect of RARalpha. Evidence is presented that these receptors affect the transcriptional activity of nur77 and consequently the expression of FasL. Our data suggest a complex interaction between the various isoforms of retinoid receptors in regulating T cell death and demonstrate that the target through which retinoids regulate TCR-mediated apoptosis is nur77.

Activation-induced apoptosis and cell surface expression of Fas (CD95) ligand are reciprocally regulated by retinoic acid receptor alpha and gamma and involve nur77 in T cells

It has been previously shown that CD4+ T cells enter the apoptotic suicide program via the Fas ligand (FasL)/Fas-mediated pathway upon T cell receptor (TCR) stimulation. In Jurkat cells TCR stimulation regulates the de novo synthesis of FasL, while in the influenza hemagglutinin-specific CD4+ murine T cell hybridoma (IP-12-7) the cell surface appearance of a preformed FasL is initiated. Both processes are dependent on new mRNA and protein synthesis, involve up-regulation of nur77, and can be inhibited by retinoic acids (RA). Two groups of nuclear receptors for RA have been identified: retinoic acid receptors (RAR) and retinoid X receptors (RXR). In this study various synthetic retinoids were used to define which receptors regulate TCR-mediated apoptosis. It is demonstrated that the inhibition is mediated via RARalpha, while RARgamma enhances TCR-mediated apoptosis, and when both receptors are stimulated, the costimulation by RXR will promote the effect of RARalpha. Evidence is presented that these receptors affect the transcriptional activity of nur77 and consequently the expression of FasL. Our data suggest a complex interaction between the various isoforms of retinoid receptors in regulating T cell death and demonstrate that the target through which retinoids regulate TCR-mediated apoptosis is nur77.

Activation-induced apoptosis and cell surface expression of Fas (CD95) ligand are reciprocally regulated by retinoic acid receptor alpha and gamma and involve nur77 in T cells

It has been previously shown that CD4+ T cells enter the apoptotic suicide program via the Fas ligand (FasL)/Fas-mediated pathway upon T cell receptor (TCR) stimulation. In Jurkat cells TCR stimulation regulates the de novo synthesis of FasL, while in the influenza hemagglutinin-specific CD4+ murine T cell hybridoma (IP-12-7) the cell surface appearance of a preformed FasL is initiated. Both processes are dependent on new mRNA and protein synthesis, involve up-regulation of nur77, and can be inhibited by retinoic acids (RA). Two groups of nuclear receptors for RA have been identified: retinoic acid receptors (RAR) and retinoid X receptors (RXR). In this study various synthetic retinoids were used to define which receptors regulate TCR-mediated apoptosis. It is demonstrated that the inhibition is mediated via RARalpha, while RARgamma enhances TCR-mediated apoptosis, and when both receptors are stimulated, the costimulation by RXR will promote the effect of RARalpha. Evidence is presented that these receptors affect the transcriptional activity of nur77 and consequently the expression of FasL. Our data suggest a complex interaction between the various isoforms of retinoid receptors in regulating T cell death and demonstrate that the target through which retinoids regulate TCR-mediated apoptosis is nur77.

Apoptosis-linked in vivo regulation of the tissue transglutaminase gene promoter

Tissue transglutaminase (tTG) is upregulated in various cells undergoing apoptosis. To investigate the transcriptional regulation of tTG a mouse strain carrying a beta-galactosidase reporter gene under the control of a 3.8 kilobase fragment of the tTG promoter was characterised. The transgene construct was shown to be expressed in the apoptotic regions of the mouse embryo. Here we report that the regulation of the transgene is also apoptosis-linked in adult animals. The transgene is induced in endocrine apoptosis involving mammary gland involution and corpus luteum regression. Induction of the reporter gene is detectable during in vivo but not in vitro apoptosis of thymocytes induced by the glucocorticoid receptor, the nur77, p53 and the retinoid receptor gamma mediated pathways. Additionally, the lacZ expression mimics the activation of the endogenous promoter in tissues characterised by high apoptotic turnover. These results suggest that the apoptosis-specific transcriptional regulation of tTG is mediated through elements of a 3.8 kb promoter and may require cosignals available only in tissue environment. Cell Death and Differentiation (2000) 7, 1225 - 1233.

Correlation of increased susceptibility to apoptosis of CD4+ T cells with lymphocyte activation and activity of disease in patients with primary Sjögren's syndrome

OBJECTIVE

To investigate whether a change in the CD95-related apoptosis of T lymphocytes might have a share in the development of the disease in patients with primary Sjögren's syndrome (SS).

METHODS

Two-color cytometric analysis was used to study the phenotype of freshly separated mononuclear cells, while Western blotting was used to detect CD95 ligand (CD95L) expression in total homogenates of isolated CD4+ T lymphocytes. The ability of various subpopulations of T cells to undergo apoptosis was investigated in 1-day cultures in medium alone or following various (anti-CD3, anti-CD95 monoclonal antibody, calcium ionophore) treatments. Apoptosis was detected using 7-aminoactinomycin D dye.

RESULTS

Compared with the findings in healthy controls, the number of CD4+ T lymphocytes was decreased, while their expression of CD95, HLA-DR, and CD45RO was significantly increased in patients with primary SS. A positive correlation was found between the activity of disease, the decrease in the CD4+ T cell number, and the increase in the expression of CD95, CD95L, HLA-DR, and CD45RO molecules within the CD4+ T cell subset. An increased rate of spontaneous, anti-CD3-, or anti-CD95-induced apoptosis was found in the T cells of SS patients, and this was more pronounced in the CD4+ T cell population, correlated with the decreased CD4+ T cell number, increased CD45RO expression, and activity of disease, and concerned mainly the CD95+ cells.

CONCLUSION

These observations indicate that the increased susceptibility to apoptosis of peripheral CD4+ T cells from SS patients correlates with disease activity. These findings support the hypothesis that the chronic activation of the immune system that occurs in this autoimmune disease is the primary mechanism responsible for this cell-deletion process.

Regulation of cell surface expression of Fas (CD95) ligand and susceptibility to Fas (CD95)-mediated apoptosis in activation-induced T cell death involves calcineurin and protein kinase C, respectively

We show that an influenza hemagglutinin-specific CD4+ murine T cell hybridoma (IP-12-7) enters the apoptotic suicide program via the Fas ligand (FasL)/Fas-mediated pathway upon T cell receptor (TCR) stimulation. These cells express Fas and FasL mRNA, cell surface Fas and intracellular FasL, but do not enter apoptosis upon Fas ligation prior to TCR stimulation. TCR stimulation additionally results in protein synthesis-dependent cell surface expression of the preformed FasL. Addition of phorbol dibutyrate (PBu2) alone was sufficient to induce susceptibility to Fas ligation induced apoptosis, while addition of both PBu2 and calcium ionophore A23187 were required to induce FasL cell surface expression. Addition of cyclosporin A completely inhibited TCR-mediated death and FasL cell surface up-regulation, but had no effect on apoptosis induced directly by Fas ligation following TCR stimulation. Inhibitors of protein kinase C (PKC) (Gö 6976 and GF 109203X) completely inhibited TCR-induced susceptibility to Fas ligation, but only partially inhibited TCR-induced cell surface expression of FasL. PKC isoenzymes alpha, beta, delta and zeta were expressed by this cell line and only the alpha and betaI isoforms translocated to the membrane fraction upon TCR stimulation. Our data suggest that in activation-induced T cell apoptosis PKC is involved in pathways that mediate the acquisition of Fas susceptibility, while calcineurin is required for cell surface expression of the preformed FasL.

All-trans retinoic acid inhibition of anti-CD3-induced T cell apoptosis in human immunodeficiency virus infection mostly concerns CD4 T lymphocytes and is mediated via regulation of CD95 ligand expression

This study analyzes the influence of all-trans retinoid acid (tRA) on apoptosis of peripheral lymphocytes from human immunodeficiency virus (HIV)-positive patients. tRA inhibits the ex vivo apoptosis in T cells; a more potent effect was observed on activation-induced apoptosis. Phenotypic characterization of T cell subsets prevented from anti-CD3-induced apoptosis by tRA revealed a more potent effect on CD4 T cells. A central regulatory system for apoptosis is the CD95 system, and inappropriate induction of this pathway is thought to contribute to AIDS pathogenesis. In investigation of CD95-based apoptosis, tRA had no effect on activation-dependent induction of CD95 on T lymphocytes, but it inhibited the induction of CD95 ligand expression on anti-CD3-activated T cells. The previously reported in vivo effect of tRA inhibiting HIV-associated apoptosis and the present observations suggest that tRA could be considered to down-regulate apoptosis associated with AIDS pathogenesis.

Inhibition of activation-induced apoptosis of thymocytes by all-trans- and 9-cis-retinoic acid is mediated via retinoic acid receptor alpha

Thymocytes can be induced to undergo apoptotic cell death by activation through the T-cell receptor (TCR). This process requires macromolecular synthesis and has been shown to be inhibited by retinoic acids (RAs). Two groups of nuclear receptors for RAs have been identified: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). All-trans-RA is the high-affinity ligand for RARs, and 9-cis-RA additionally binds to RXRs with high affinity. Because 9-cis-RA is much more potent in inhibiting TCR-mediated death than all-trans-RA, it was suggested that RXRs participate in the process. In the present study various synthetic retinoid analogues were used to address this question further. The results presented suggest that the inhibitory effect of RAs on activation-induced death of thymocytes is mediated via RARalpha, because (1) it can be reproduced by various RARalpha analogues both in vitro and in vivo, (2) the effect of RAs can be inhibited by the addition of an RARalpha antagonist, (3) CD4+CD8+thymocytes, which die on TCR stimulation, express RARalpha. Stimulation of RARgamma, in contrast, enhances the activation-induced death of thymocytes and inhibits its prevention by RARalpha stimulation. RXR co-stimulation suspends this inhibitory effect of RARgamma and permits the preventive function of RARalpha on activation-induced death. Our results suggest a complex interaction between the various isoforms of retinoid receptors and demonstrate that low (physiological) concentrations of all-trans-RA do not affect the activation-induced death of thymocytes because the RARalpha-mediated inhibitory and the RARgamma-mediated enhancing pathways are in balance, whereas if 9-cis-RA is formed, additional stimulation of RXRs permits the inhibitory action of RARalpha.

Retinoic acids regulate apoptosis of T lymphocytes through an interplay between RAR and RXR receptors

Vitamin A deficiency has been known for a long time to be accompanied with immune deficiency and susceptibility to a wide range of infectious diseases. Increasing evidence suggests that retinoic acids derived from vitamin A are involved in the functional regulation of the immune system. Of the two groups of retinoid receptors, retinoic acid receptors (RARs) and retinoid X receptors (RXRs) all-trans and 9-cis retinoic acids are high affinity ligands for RARs and 9-cis retinoic acid additionally binds to RXRs. In cells, at high concentrations, all-trans retinoic acid can be converted to 9-cis retinoic acid by unknown mechanisms. Apoptosis plays a major role in shaping the T cell repertoire and one way in which retinoids may affect immune functions is to influence the various apoptosis pathways. Indeed, it has been shown that retinoic acids can induce apoptosis, increase the rate of dexamethasone-induced death and inhibit activation-induced death of thymocytes and T lymphocytes. Therefore, retinoids together with glucocorticoids may be involved in regulating positive and negative selection of T lymphocytes. Here we demonstrate that retinoids can induce apoptosis of T cells through the stimulation of RARgamma. Specific stimulation of RARalpha, on the other hand, prevents both RARgamma-dependent and TCR-mediated cell death. In all these functions 9-cis retinoic acid proved to be more effective than all-trans retinoic acid suggesting the involvement of RXRs. Based on these results a possible mechanism through which costimulation of RARs and RXRs might affect spontaneous and activation-induced death of T lymphocytes is proposed.

Methylprednisolone and 2-chloroadenosine induce DNA fragmentation at different stages of human T-lymphocyte development

Both methylprednisolone (MPS) and 2-chloroadenosine (2-CA) were shown previously to induce DNA fragmentation and cell death in human thymocytes at an optimum concentration of 1 and 40 microM, respectively. Though both compounds affected the CD4+CD8+ population, 2-CA depleted primarily thymocytes expressing medium or high levels of CD3-T-cell receptor molecule, while the glucocorticoid treatment affected cells expressing a lower level of CD3-T-cell receptor. Their effect on thymocyte viability and DNA fragmentation was observed already at day 1 of culture and involved the bcl-2 negative thymocytes. Incubation of peripheral T-lymphocytes (which express bcl-2) with the same concentration of MPS did not affect the viability for up to 5 days, while 2-CA induced 100% cell death and DNA fragmentation by day 5. If T-cells were stimulated with concanavalin A in the presence of MPS or 2-CA the cell proliferation was inhibited and a decrease in cell viability with a concomittant increase in DNA fragmentation was observed. If MPS was added at 24 h or later after mitogenic stimulation, it was not able to induce apoptosis and the inhibition of proliferation was less pronounced. 2-CA, on the other hand, inhibited proliferation and induced cell death whenever it was added to the culture. The decreased sensitivity towards the apoptosis induction effects of glucocorticoids at later phases of mitogenic stimulation can not be explained by an increased bcl-2 expression, since its expression level remained constant up to 48 h after mitogenic stimulation. Our data presented in this paper suggest: (1) that T-cells may show different sensitivity towards the same apoptosis inducer signals at different stages of the T-cell development; (2) the apoptotic sensitivity towards various signals may be different at the same stage of T-cell differentiation; and (3) their apoptotic sensitivity does not always correlate with the bcl-2 expression alone.

Induction of apoptosis by retinoids and retinoic acid receptor gamma-selective compounds in mouse thymocytes through a novel apoptosis pathway

Retinoic acids are morphogenic signaling molecules that are derived from vitamin A and involved in a variety of tissue functions. Two groups of their nuclear receptors have been identified: retinoic acid receptors (RARs) and retinoic acid X receptors (RXRs). All-trans retinoic acid is the high affinity ligand for RARs, and 9-cis retinoic acid also binds to RXRs with high affinity. In cells at high concentrations, all-trans retinoic acid can be converted to 9-cis retinoic acid via unknown mechanisms. It was previously shown that retinoic acids prevents activation-induced death of thymocytes. Here, we report that both all-trans and 9-cis retinoic acid induce apoptosis of mouse thymocytes and purified CD4+CD8+ cells in ex vivo cultures, with 9-cis retinoic acid being 50 times more effective. The induction of apoptosis by retinoic acids is mediated by RARgamma because (a) the phenomenon can be reproduced only by RARgamma-selective retinoic acid analogs, (b) the cell death induced by either retinoic acids or RARgamma analogs can be inhibited by RARgamma-specific antagonists, and (c) CD4+CD8+ thymocytes express RARgamma. In vivo administration of an RARgamma analog resulted in thymus involution with the concomitant activation of the apoptosis-related endonuclease and induction of tissue transglutaminase. The RARgamma pathway of apoptosis is RNA and protein synthesis dependent, affects the CD4+CD8+ double positive thymocytes, and can be inhibited by the addition of either Ca2+ chelators or protease inhibitors. Using various RAR- and RXR-specific analogs and antagonists, it was demonstrated that stimulation of RAR alpha inhibits the RARgamma-specific death pathway (which explains the lack of apoptosis stimulatory effects of all-trans retinoic acid at physiological concentrations) and that costimulation of the RXR receptors (in the case of 9-cis retinoic acid) can neutralize this inhibitory effect. It is suggested that formation of 9-cis retinoic acid may be a critical element in regulating both the positive selection and the "default cell death pathway" of thymocytes.

Differential expression of tissue transglutaminase during in vivo apoptosis of thymocytes induced via distinct signalling pathways

A significant increase in the expression and activity of tissue transglutaminase (tTG), one of the effector elements of apoptosis, was observed during involution of thymus elicited by treatment with either anti-CD3 antibody or dexamethasone or by irradiation. The blood plasma concentration of epsilon(gamma-glutamyl)lysine isodipeptide, the end-product of the digestion of transglutaminase cross-linked proteins, was also elevated in each of these cases. tTG was localized in cells of the cortical layer of the thymus and immunofluorescence double staining revealed that the enzyme appeared in the apoptotic cells. None of these observations could be made when apoptosis was induced by fas-receptor stimulation. The lack of tTG activity in fas-stimulated cells was accompanied with a less organized apoptotic morphology. Our data suggest that distinct signalling pathways, which induce apoptosis within the same cell type, can differentially regulate the expression of tTG, and this enzyme may be involved in structural stabilization of the apoptotic cells.

Transglutaminase induction by various cell death and apoptosis pathways

Clarification of the molecular details of forms of natural cell death, including apoptosis, has become one of the most challenging issues of contemporary biomedical sciences. One of the effector elements of various cell death pathways is the covalent cross-linking of cellular proteins by transglutaminases. This review will discuss the accumulating data related to the induction and regulation of these enzymes, particularly of tissue type transglutaminase, in the molecular program of cell death. A wide range of signalling pathways can lead to the parallel induction of apoptosis and transglutaminase, providing a handle for better understanding the exact molecular interactions responsible for the mechanism of regulated cell death.

The 2-chlorodeoxyadenosine-induced cell death signalling pathway in human thymocytes is different from that induced by 2-chloroadenosine

2-chloroadenosine induced DNA fragmentation and cell death in human thymocytes primarily by Ca(2+)-dependent mechanisms. Incubation of human thymocytes with 2-chlorodeoxyadenosine (5-1000 nM) also induced cell death (apoptosis) which was dependent on macromolecule synthesis and involved activation of an endonuclease which was inhibited by Zn2+. The effect of 2-chlorodeoxyadenosine was prevented by addition of dipyridamole, a strong nucleoside transport inhibitor, or of deoxycytidine, previously shown to compete for uptake by deoxycytidine kinase. 2-Chlorodeoxyadenosine-induced apoptosis did not involve increases in the cytosolic Ca2+ concentration, but required the presence of intracellular Ca2+. It was not inhibited by activators of protein kinase C previously shown to inhibit Ca(2+)-dependent cell death. Addition of 2-chlorodeoxyadenosine induced an increase in the amount of p53 in human thymocytes, while 2-chloroadenosine had no effect. These data suggest that 2-chloroadenosine and 2-chlorodeoxyadenosine induce cell death in human thymocytes via different signalling pathways.

The effects of cell number, concentrations of mitogen and glutamine and time of culture on [3H]thymidine incorporation into cervical lymph node lymphocytes stimulated by concanavalin-A

The amount of [3H]thymidine incorporated into DNA in lymphocytes stimulated with Concanavalin-A increases exponentially with time at different concentrations of glutamine, reaches a peak value, then gradually decreases. When the value (log10 thymidine incorporation glutamine present -log10 thymidine incorporation glutamine absent) obtained from the exponential phase is plotted against time, a linear plot is obtained for each glutamine concentration. When these linear rates of incorporation are plotted against glutamine concentration, hyperbolic curves are obtained for different times of culture. The peak value of incorporation (which reflects the final number of cells which entered the cell cycle) is determined by the concentration of mitogen and occurs at an earlier time as the number of cells in culture is increased and as the concentration of glutamine is increased. These findings suggest that increasing the plasma glutamine concentration above the normal physiological level may be of value in increasing the proliferation of lymphocytes in conditions of lymphopenia. Adenosine, a fuel of purine nucleotide synthesis, which may affect the lymphoproliferative response also via specific adenosine receptors, increases the rate of incorporation of [3H]thymidine but this effect depends upon the concentration of glutamine; at low concentrations of glutamine, the stimulation by adenosine is apparent whereas at high concentrations of glutamine adenosine appeared to inhibit proliferation.

Probing the molecular program of apoptosis by cancer chemopreventive agents

This paper provides a rational molecular basis for studies intended to clarify the interactions between cancer chemopreventive agents and apoptosis, one of the natural forms of cell death that overlaps molecular mechanisms with other forms such as programmed cell death and specialized forms of physiological cell death. Molecular details of the process show the existence of distinct molecular pathways leading to the activation of critical effector elements (apoptosis gene products) functioning under the control of a network of negative regulatory elements. Dysregulation of either apoptosis or anti-apoptosis genes has a significant role in multistage carcinogenesis. Inhibition of apoptosis is one of the underlying mechanisms of the action of tumor promoters. The network of apoptosis and anti-apoptosis gene products provides multiple targets for compounds with cancer chemopreventive potential. Many data in the literature show initiating, potentiating or inhibitory effects of such compounds on apoptosis. However, the molecular mechanism of these effects is largely unknown. We initiated a series of studies using mouse thymocytes which undergo apoptosis through distinct molecular mechanisms after T-cell receptor activation (TCR pathway), following the addition of glucocorticoids (DEX pathway) or DNA damaging agents (p53 pathway). All trans-and 9-cis-retinoic acid induced apoptosis, elicited through the DEX pathway, inhibited the TCR pathway, and did not affect p53- initiated apoptosis. N-acetylcysteine can inhibit all forms. Sodium salicylate enhanced spontaneous cell death, decreased p53-dependent apoptosis, and did not affect the DEX and TCR pathways. These preliminary results, which show differential effects of the studied compounds on distinct molecular pathways of apoptosis, warrant further investigations in the effort to utilize the molecular elements of apoptosis in proper cancer chemoprevention, and find biochemical targets for apoptosis-related surrogate endpoint biomarker assays of chemoprevention.

Adenosine stimulates DNA fragmentation in human thymocytes by Ca(2+)-mediated mechanisms

Incubation of human thymocytes with an optimum concentration of adenosine and its receptor site agonist, 2-chloroadenosine, induced increases in intracellular cyclic AMP (cAMP) (from a resting 0.6 +/- 0.1 to 4.1 +/- 0.2 pmol/10(7) cells within 5 min) and Ca2+ (from the resting 85 +/- 7 nM to a peak of 210 +/- 25 nM) levels and resulted in internucleosomal DNA fragmentation and cell death (apoptosis). Other adenosine analogues were also effective at inducing DNA fragmentation, the order of potency being 2-p-(carboxyethylphenylethylamino)-5'-carboxyamidoadenosine < 5'-(N-ethylcarboxamide)adenosine < or = cyclopentyladenosine < 2-chloroadenosine (2-CA). 2-CA treatment (with an optimum concentration of 40 microM) selectively depleted a thymocyte subpopulation (15-20% of the total cells) which expressed higher levels of the CD3 molecule and which was found mainly in the CD4+CD8+ double positive immature thymocyte population. DNA fragmentation was prevented by the addition of actinomycin D or cycloheximide to the thymocyte suspension, indicating that this process required both mRNA and protein synthesis. Endonuclease activation and cell killing were dependent on an early, sustained increase in cytosolic Ca2+ concentration, most of which was of extracellular origin and was a result of an adenosine-induced inositol trisphosphate release. Other agents known to elevate intracellular cAMP levels by different mechanisms failed to induce similar DNA fragmentation, but enhanced the effect of adenosine. This suggested a supporting role for cAMP in adenosine-induced DNA fragmentation. Phorbol dibutyrate, a protein kinase. C activator, previously shown to inhibit Ca(2+)-dependent DNA fragmentation and cell killing in human thymocytes [McConkey, Hartzell, Jondal and Orrenius (1989) J. Biol. Chem. 264, 13399-13402], at 60 ng/ml concentration also prevented adenosine-induced DNA fragmentation when added prior to adenosine. This suggested a complex cross-talk between the adenosine-triggered signal transduction cascade and the activation state of protein kinase C in regulating apoptosis of human thymocytes.

The effect of time of addition of glutamine or nucleosides on proliferation of rat cervical lymph-node T-lymphocytes after stimulation by concanavalin A

1. Addition of concanavalin A to T-cell lymphocytes from rat cervical lymph nodes increases the activity of glutaminase within 1 h and those of carbamoyl-phosphate synthase II and aspartate transcarbamoylase within 3 h. There was a similar time course for the effects of concanavalin A on rates of glutamine utilization, which was increased within 1 h, and on pyrimidine nucleotide synthesis, which was increased by 40% at 2 h and by 100% at 3 h. 2. A delay in the addition of glutamine to the culture medium after addition of concanavalin A caused a decrease in [3H]thymidine incorporation only after 4-6 h. In the absence of glutamine, delay in addition of guanosine or inosine caused a decrease in [3H]thymidine incorporation only after 6-8 h after the addition of concanavalin A. 3. In contrast, a delay in addition of adenosine or uridine to the culture medium had an immediate effect (i.e. within 2 h) on the rate of incorporation of [3H]thymidine. It is suggested that adenosine and uridine have specific effects on proliferation via specific receptors for these nucleosides in the membrane.

The effect of glutamine concentration on the activity of carbamoyl-phosphate synthase II and on the incorporation of [3H]thymidine into DNA in rat mesenteric lymphocytes stimulated by phytohaemagglutinin

The maximum catalytic activities of carbamoyl-phosphate synthase II, a limiting enzyme for pyrimidine nucleotide synthesis, are very much less than those of glutaminase, a limiting enzyme for glutamine utilization, in lymphocytes and macrophages; and the flux through the pathway for pyrimidine formation de novo is only about 0.4% of the rate of glutamine utilization by lymphocytes. The Km of synthase II for glutamine is about 16 microM and the concentration of glutamine necessary to stimulate lymphocyte proliferation half-maximally is about 21 microM. This agreement suggests that the importance of glutamine for these cells is provision of nitrogen for biosynthesis of pyrimidine nucleotides (and probably purine nucleotides). However, the glutamine concentration necessary for half-maximal stimulation of glutamine utilization (glutaminolysis) by the lymphocytes is 2.5 mM. The fact that the rate of glutamine utilization by lymphocytes is markedly in excess of the rate of the pathway for pyrimidine nucleotide synthesis de novo and that the Km and 'half-maximal concentration' values are so different, suggests that the glutaminolytic pathway is independent of the use of glutamine nitrogen for pyrimidine synthesis.

The rate of the AMP/adenosine substrate cycle in concanavalin-A-stimulated rat lymphocytes

The effect of adenosine on the metabolism of prelabelled adenine nucleotides was investigated in concanavalin-A-stimulated rat lymphocytes. Adenosine in the presence of the adenosine deaminase inhibitor, deoxycoformycin, caused a 2-fold increase in the ATP concentration. This effect was, in part, countereacted by an increased rate of adenine nucleotide catabolism, which could be explained by a stimulation of AMP deaminase (EC 3.5.4.6). At the same time a continuous rate of labelled adenosine production was found, which was not affected by the increased ATP concentration and which could only be detected by the trapping effect of a high concentration of added unlabelled adenosine. It is concluded that the rate of the substrate cycle between AMP and adenosine is low (1.9 +/- 0.2 nmol/h per 10(7) cells) in comparison to the rate of AMP deamination.

Effect of dimethylnitrosamine upon carbamoyl phosphate and ornithine utilization in rat liver

Newborn CFY rats were intraperitoneally treated with a single dose of dimethylnitrosamine. Changes in the activity of carbamoyl-phosphate synthase I and II, ornithine carbamoyltransferase, aspartate carbamoyltransferase and dihydroorotase, i.e. utilization of ornithine and carbamoyl-phosphate pools were followed for 300 days. One or two months after treatment the activity of enzymes which utilize ornithine and carbamoyl-phosphate was enhanced in the cytosol, whereas the activity of the intramitochondrial enzymes in urea synthesis was decreased. Changes in the utilization of ornithine and carbamoyl-phosphate pools, characteristic of proliferating tissues, appeared to be irreversible.