Natural killer cells and cytotoxic T cells induce DNA fragmentation in both human and murine target cells in vitro

Modulation of natural killer cell-mediated cytotoxicity by tamoxifen and estradiol

BACKGROUND

The nonsteroidal antiestrogenic drug, tamoxifen, inhibits the growth of estrogen receptor-positive tumors by interfering with the growth-stimulatory effect of estradiol. However, there is compelling evidence that tamoxifen treatment also is beneficial for patients with estrogen receptor-negative tumors. The hypothesis that tamoxifen is capable of enhancing the immunologic defense of tumor-bearing hosts was been investigated as a possible method for targeting receptor-negative neoplasms.

METHODS

Natural killer (NK) cells in the spleen of female Fisher and Wistar-Furth rats were used against the YAC-1 murine lymphoma target in 51Cr-release assays. The effect of various concentrations of estradiol and tamoxifen (1 nM-1 microM) and of the metabolic inhibitors actinomycin D and cyclohexamide on target-cell killing was investigated.

RESULTS

Tamoxifen enhanced and estradiol inhibited killing if applied for the entire 5 hours of the cytotoxic reaction. When applied jointly in this experimental setup, estradiol interfered with the enhancing effect of tamoxifen. Pretreatment of target cells with tamoxifen led to highly significant enhancement of cytotoxicity; estradiol also enhanced target cell killing, but to a lesser extent. After joint treatment, the level of cytotoxicity was comparable with that obtained with tamoxifen alone. Both pharmacologic (100 nM and 1 microM) and physiologic (1 or 10 nM) concentrations of estradiol and equimolar tamoxifen enhanced target cell lysis. However, pharmacological levels of estradiol inhibited effector cells when applied alone or in combination with tamoxifen. Highly significant enhancement of target-cell destruction occurred if both target and effector cells were pretreated with tamoxifen, whereas estradiol treatment of both cell types led to slight enhancement or no effect on cytotoxicity. Treatment of the target cells with actinomycin D or cycloheximide inhibited the lysis of untreated and tamoxifen- or estradiol-exposed cells. Treatment of YAC-1 target cells with tamoxifen or estradiol also enhanced the NK cell-mediated release of the nuclear label, 3H-thymidine, indicating DNA degradation. Similarly treated P815 cells resisted lysis by NK cells, but showed sensitization when the NK cells were stimulated by interleukin-2 for 48 hours before the lytic reaction. Estradiol and tamoxifen changed the kinetics of 3H-thymidine incorporation by YAC-1 cells, but the cells were capable of growing with the highest drug concentrations (1 microM) used in the cytotoxicity experiments. YAC-1 cells have no cytosolic estradiol receptors and are weakly positive for cytosolic progesterone receptors.

CONCLUSIONS

These experiments indicate that NK cell-mediated target-cell destruction can be enhanced by tamoxifen primarily through sensitizing the target for lysis. Estradiol also sensitizes the target but inhibits the effector cells simultaneously so that little or no change results in cytolysis. Target-cell sensitization is not mediated by classical estrogen receptors and requires the active metabolic participation of the cells treated. A likely mechanism of this phenomenon is priming the target cell for apoptosis.

Inhibition of programmed cell death by catalase and phenylalanine methyl ester

1. Programmed cell death proceeds by an unknown mechanism which results in characteristic morphological changes known as apoptosis. 2. We have proposed that, in hormone-induced apoptosis, cell death may be the result of an attack of cells destined to die by cytotoxic macrophages. 3. We have investigated the effects of superoxide dismutase, catalase and the macrophage toxin, phenylalanine methyl ester, on the regression of tadpole tail slices in culture. 4. Our findings, that regression of bullfrog tadpole tails is blocked by catalase and phenylalanine methyl ester, support the working hypothesis.

The role of apoptosis in antibody-dependent cellular cytotoxicity

Apoptosis in three lymphoma cell lines has been studied following cytotoxicity induced in vitro by normal human blood lymphocytes utilizing either natural killer (NK) or antibody-dependent cellular cytotoxic (ADCC) mechanisms. Guinea-pig L2C leukaemic lymphocytes, but not the human cell lines Daudi and Jurkat, revealed a degree of time- and temperature-dependent apoptotic death upon simple culture in vitro. NK cytotoxicity at low effector:target ratios (E:T) induced both release of 51Cr and apoptosis. However NK cytotoxicity at higher E:T, and ADCC at all E:T, increased the level of 51Cr release while reducing the level of apoptosis. The findings were consistent with the apoptotic process being cut short by intervention of necrotic death. The same characteristics accompanied ADCC whether the effectors were recruited by Fc gamma regions of antibody coating the targets, or by bispecific antibodies attaching one arm to the targets and the other to Fc gamma receptors type III on effectors. This finding, and the high level of cytotoxicity elicited by the bispecific method, confirm the belief that NK cells, in addition to exerting NK cytotoxicity, represent the principal effectors for ADCC among blood mononuclear cells. Our results suggest that NK cells have both apoptotic and necrotic mechanisms available for killing their targets, but use only the latter for ADCC.

Programmed cell death: concept, mechanism and control

Programmed cell death or apoptosis occurs under physiological conditions as a result of physiological effectors. It is a relatively slower process and requires active participation of the cell in the suicidal mechanism. Apoptosis is controlled by precise intrinsic genetic programme and may be induced by almost all those stimuli causing necrosis. The role played by the intensity in determining the death process and the underlying mechanism is imperfectly understood. Morphologically apoptotic cells appear as small condensed body. The chromatin is dense and fragmented, packed into compact membrane-bound bodies together with randomly distributed cell organelles. The plasma membrane loses its characteristic architecture and shows extensive blebbing. It buds off projections so that the whole cell may split into several membrane-bound apoptotic bodies. Significant chemical changes take place in the plasma membrane. This helps in recognition of the apoptotic bodies by phagocytes. At this moment it is unclear if all cells can undergo apoptosis or it is a characteristic of only some tissues which are predisposed to apoptotic death being directly under the control of hormones or growth factors. Experimental studies aimed at comparison of induction of apoptosis in cells of different origin are warranted to elucidate this point. Biochemically a pre-commitment step for induction of death programmation through macromolecular synthesis is essential for most systems. The double-stranded linker DNA between nucleosomes is cleaved at regular inter-nucleosomal sites through the action of a Ca2+, Mg(2+)-sensitive neutral endonuclease. Zinc is a potent inhibitor of the enzyme. Calcium probably plays a key controlling role in activation of the enzyme since prevention of Ca2+ increase prevents endonuclease activation. It is becoming evident that signal transduction through appropriate receptors control the Ca2+ flux in the cells. Most apoptotic cells require synthesis of RNA and proteins. Delay or abrogation of apoptosis by inhibition of macromolecular synthesis is well known. The dying cells show high mRNA levels for several enzymes. Several degradative enzymes become active. Regulatory proteins maintain control over the apoptotic cascade. At the molecular level, search has been initiated for the mammalian equivalents of the cell death (ced) gene. Activation of several specific genes is indicated. Specific expression of cell death-associated gene products (e.g. TRPM-2/SGP-2) has been reported in several unrelated apoptotic cell systems. Sequential induction of c-fos, c-myc and 70 kDa heat shock protein is reported. Studies demonstrate that certain genes must remain in a transcriptionally active demethylated state during programmed cell death. Recent evidences clearly indicate that apoptosis may be positively or negatively modulated by certain genes.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction of apoptosis in chronic lymphocytic leukemia cells and its prevention by phorbol ester

Chronic lymphocytic leukemia lymphocytes were used to study mechanisms involved in apoptosis (programmed cell death). Apoptosis, which was determined by morphological changes including cell death and by internucleosomal DNA fragmentation, occurred during culture for 1 to 2 days in a portion of the cells from three of the four patients tested. Most of the cells underwent apoptosis and DNA fragmentation was greatly enhanced when cells were cultured in the presence of the microtubule inhibitor colchicine, the topoisomerase II inhibitor etoposide, or the glucocorticoid methylprednisolone. Tumor-promoting phorbol esters inhibited spontaneous DNA fragmentation and cell death including that induced by colchicine, etoposide, and methylprednisolone, indicating that they act on an event common to apoptosis caused by diverse stimuli. Phorbol esters probably act through protein phosphorylation, since they were effective at concentrations which modulated protein kinase C (PKC) and their action was prevented by H-7, which binds to and inactivates the catalytic site of PKC. In the absence of phorbol ester, H-7 itself induced some apoptosis. These findings implicate PKC in the suppression of apoptosis, but its precise role requires systematic investigation.

Apoptosis: a potential role for cytosolic transglutaminase and its importance in tumour progression

The relationship between transglutaminase activity, apoptosis and the propensity of a tumour to metastasise, was investigated in a series of metastatic variants of an HSV-2 induced hamster fibrosarcoma and two metastatic variants of the B16 mouse melanoma. The data suggest an inverse relationship between metastatic potential and cytosolic transglutaminase activity. A direct relationship was found between measured cytosolic activity and the levels of the endogenous product of transglutaminase, the protein crosslink epsilon(gamma-glutamyl)lysine. Increasing metastatic potential and decreasing cytosolic transglutaminase activity was accompanied by a corresponding decrease in the number of detergent-insoluble apoptotic envelopes isolated from variant cell lines. These apoptotic envelopes were found to be highly crosslinked structures, containing more than 85% of the cells content of epsilon(gamma-glutamyl)lysine. These data are in keeping with the idea that a major role for the cytosolic transglutaminase is in the formation of the highly crosslinked apoptotic envelope during programmed cell death and that perturbation of this function may be an important determinant in the development of the metastatic phenotype.

Apoptosis induced in macrophages and T blasts by the mycotoxin sporidesmin and protection by Zn2+ salts

Incubation of 48 h concanavalin A stimulated spleen cells (T blasts) and murine peritoneal macrophages with the mycotoxin sporidesmin results in DNA fragmentation characteristic of apoptosis. Morphological changes, particularly condensed chromatin, observed following incubation of these cells with sporidesmin and the immunotoxin gliotoxin and related epipolythiodioxopiperazines (ETP) also show changes characteristic of apoptosis. The presence of Zn2+ salts in the culture medium at concentrations non toxic to the cells over the time period studied protects against DNA damage and morphological change. Interaction between Zn2+ and the reduced form of a simple ETP compound assessed by spectral changes demonstrated the formation of a weak complex between the two molecules. Complex formation between zinc and thiol however was insufficient to prevent oxidative damage to plasmid DNA in vitro by inhibiting auto-oxidation of the reduced ETP compound because of the looseness of the interaction. Cd2+, which appears to form a tighter complex with the dithiol does inhibit cleavage of plasmid DNA. These results establish that the toxicity of sporidesmin may be due in part to its ability to induce apoptosis or programmed cell death in sensitive cells. In addition the immunotoxin gliotoxin and related compounds have now been shown to induce the same characteristic morphological changes in cells of haemopoietic origin. The inhibition of apoptosis induced by ETP compounds by Zn2+ appears to be due to direct inhibition of apoptosis rather than Zn2+ acting as an antioxidant. These results demonstrate the inhibition of apoptosis induced by ETP compounds by Zn2+ and suggest an alternate explanation for the known prophylactic effect of Zn2+ on sporidesmin induced tissue damage.