Activation of phosphatidylinositol 3'-kinase by insulin-like growth factor-I rescues promyeloid cells from apoptosis and permits their differentiation into granulocytes

Insulin-like growth factor-I (IGF-I) promotes cell division and prevents programmed cell death in hemopoietic progenitors. Human HL-60 promyeloid cells differentiate toward the granulocytic lineage when stimulated with retinoic acid (RA) in serum-containing medium. When deprived of serum, however, we found that these cells differentiate poorly in the presence of RA, as assessed by expression of the alpha subunit of the beta2 integrin heterodimer, CD11b/CD18. However, when IGF-I is added to RA-treated cells, the proportion of CD11b-positive cells increases to a level similar to that in RA-treated cells cultured in serum-containing medium. Cells treated with RA alone not only differentiate poorly but also undergo apoptosis, as assessed by flow cytometry using propidium iodide and HO33342. In serum-free medium, one-third of RA-treated cells become apoptotic compared with only 5% apoptotic cells in the absence of RA. However, addition of IGF-I to RA-treated cells prevents the appearance of this apoptotic population and increases phosphatidylinositol 3'-kinase (PI 3-kinase) activity by fivefold. Wortmannin, a PI 3-kinase inhibitor, potently decreases this IGF-I-induced lipid kinase activity, blocks the ability of IGF-I to prevent apoptosis, and inhibits IGF-I-enhanced CD11b expression. These data demonstrate that IGF-I acts on RA-treated progenitors to promote their differentiation along the granulocytic lineage. IGF-I acts by rescuing these cells from apoptotic cell death via a downstream pathway that is dependent upon PI 3-kinase.     

Effects of pyrethroid insecticides on gap junctional intercellular communications in Balb/c3T3 cells by dye-transfer assay

The effects of fenvalerate, esfenvalerate, permethrin, cypermethrin, deltamethrin, p-chlorophenylisovaleric acid (CPIA, major metabolite of fenvalerate) and DDT, a liver tumor promoter, on gap junctional intercellular communication (GJIC) were examined in Balb/c3T3 cells by dye-transfer assay. Separate groups of Balb/c3T3 cells were exposed to the chemicals for 1 day. On the following day, GJIC was measured by counting the number of dye-transferring cells per injection of Lucifer Yellow under a fluorescent microscope. Fenvalerate, esfenvalerate, permethrin, cypermethrin, deltamethrin and DDT inhibited GJIC at noncytotoxic concentrations, while CPIA did not inhibit GJIC even at a cytotoxic concentration. It is concluded that the examined pyrethyroid insecticides, but not a metabolite, have inhibitory effects on GJIC in Balb/c3T3 cells.     

Activation of protein kinase C-zeta and phosphatidylinositol 3'-kinase and promotion of macrophage differentiation by insulin-like growth factor-I

Phosphoinositides that are phosphorylated at the D3 position have been reported to activate an atypical, Ca2-independent protein kinase C (PKC) isoform designated PKC-zeta, and overexpression of this enzyme leads to monocytic differentiation. In this study, we cultured human HL-60 promyeloid cells with vitamin D3 and insulin-like growth factor-I (IGF-I), a 70-amino-acid peptide that activates phosphatidylinositol 3'-kinase (PI 3-kinase) in murine promyeloid cells. Two days later, the proportion of cells differentiating into macrophages in serum-free medium, as assessed by expression of the alpha-subunit of the beta2 integrin CD11b, increased from 5 +/- 1% to 25 +/- 3%. Addition of IGF-I increased the proportion of cells differentiating into CD11b-positive macrophages to 78 +/- 5%. In the absence of vitamin D3, IGF-I did not induce expression of CD11b (6 +/- 1%). The IGF-I-promoted macrophage differentiation was blocked specifically by preincubation of HL-60 cells with a mAb (alphaIR3) directed against the IGF type I receptor. Similarly, pretreatment of cells with either alphaIR3 or an IGF-binding protein, IGFBP-3, led to a 75% inhibition of CD11b expression when cells were cultured with vitamin D3 in serum-containing medium. IGF-I, but not vitamin D3, caused a sevenfold increase in the enzymatic activity of both PI 3-kinase and atypical PKC-zeta. Inhibition of IGF-I-inducible PI 3-kinase with either wortmannin or LY294002 abrogated the IGF-I-induced activation of PKC-zeta and totally blocked the enhancement in macrophage differentiation caused by IGF-I. These data establish that PKC-zeta is a putative downstream target of PI 3-kinase that is activated during IGF-I-promoted macrophage differentiation.     

Relaxin-induced deoxyribonucleic acid synthesis in porcine granulosa cells is mediated by insulin-like growth factor-I

Relaxin stimulates in vitro DNA synthesis and cell proliferation of porcine granulosa cells (GC) and theca cells. The objective of the study reported here was to determine whether components of the ovarian insulin-like growth factor (IGF) system mediate relaxin's growth-promoting effects on porcine GC in vitro. In small follicle GC, relaxin (1-100 ng/ml) significantly (p < 0.05) increased IGF-I secretion to 25-34% above control. Hormonal responsiveness of GC was shown by incubation with FSH (200 ng/ml), which resulted in 125% stimulation of IGF-I secretion relative to that in cells incubated alone. When IGF-I activity in the GC cultures was neutralized with a specific IGF-I antibody, relaxin (10 and 100 ng/ml)-induced [3H]thymidine incorporation was inhibited (p < 0.05). Coincubation with IGF-I antibody also suppressed basal and IGF-I (10 ng/ml)-induced [3H]thymidine incorporation into GC DNA, but had no effect on insulin (1 microgram/ml)-induced DNA synthesis, demonstrating the specificity and lack of toxicity of the IGF-I antibody. Ligand blot analysis showed no change in secretion of GC IGF binding protein (IGFBP) in response to relaxin (1, 10, and 100 ng/ml). In contrast, IGF-I (10 ng/ml) increased secretion of IGFBP-3 and -5, whereas FSH (200 ng/ml) decreased IGFBP-3 secretion and increased IGFBP-4 secretion (p < 0.05). In IGF-I receptor competition studies, IGF-I, but not relaxin, displaced [125I]IGF-I from the GC IGF-I receptor. These studies provide direct evidence for an interaction of relaxin and the ovarian IGF system. They are the first to show 1) a stimulatory effect of relaxin on IGF-I secretion; 2) the necessity of IGF-I activity for relaxin-induced GC DNA synthesis; and 3) the absence of an effect of relaxin on GC IGFBPs or IGF-I receptor. These findings support a paracrine role for relaxin in the porcine follicle and show that relaxin acts indirectly to promote follicle growth by stimulating GC IGF-I secretion.     

The regulation of L-proline transport by insulin-like growth factor-I in human osteoblast-like SaOS-2 cells

OBJECTIVE: To examine the usefulness of clinical stage, tumour differentiation and prostate-specific antigen (PSA) level, alone and in combination, to predict regional nodal metastases in individual patients with localized prostate cancer. PATIENTS AND METHODS: The usefulness of digital rectal examination (DRE), biopsy Gleason sum and PSA, alone and in combination, to predict nodal metastases in an individual patient was examined. The study included 689 patients who had laparoscopic or open pelvic lymph node dissection for clinical stage T1-3 prostate cancer. The Kruskal-Wallis test, Mantel-Haenszel test, chi-squared test and logistic regression were used for continuous, ordinal, categorical, and multivariate analysis, respectively. RESULTS: Of the 689 patients who underwent radical prostatectomy, 52 (8%) had nodal metastases. Although clinical stage, DRE, pre-operative PSA level and biopsy Gleason sum were significantly related in the univariate analysis, only pre-operative PSA level and biopsy Gleason sum were significant predictors of lymph node status in a multivariate analysis. However, based on a receiver operating characteristic curve, a model with satisfactory sensitivity and specificity could not be obtained. CONCLUSION: Current estimations of primary prostate cancer biology using pre-operative PSA level, clinical stage and biopsy Gleason sum are not sufficiently sensitive to predict nodal metastases, and pelvic lymphadenectomy remains the definitive method of detection.     

Insulin and insulin-like growth factor-I receptor mediated differentiation of 3T3-F442A cells into adipocytes: effect of PI 3-kinase inhibition

Sperm-zona pellucida binding, a crucial step in the process of fertilization, takes place in vivo in the upper portion of the fallopian tube. The presence of GnRH-like peptides in the female and the male genital tract has been described. In this work, the effect of GnRH and related peptides upon sperm-zona pellucida binding ability was studied. Sperm aliquots, capacitated for 4.5 h, were incubated for 5 min with saline (control) or 20 nM of GnRH, C-terminal (1-5) or N-terminal (5-10) fragments of GnRH, Substance P, dynorphin, bombesin, or mixed GnRH (a synthetic peptide with the same amino acids as GnRH but in different order). Sperm were also incubated with the GnRH antagonist Ac-3,4-dehydro-Pro1, -p-fluoro-D-Phe2, D-Trp3,6-LHRH, alone or before adding GnRH. The sperm were then tested using the hemizona assay. After 10 min, the number of zona-bound sperm was determined. In addition, the effect of GnRH upon the acrosome reactions, sperm movement characteristics, and sperm-zona collisions was evaluated. Sperm treated with GnRH bound in higher numbers to the zona than did control sperm (p < 0.005). The GnRH fragments, the GnRH antagonist, and related peptides did not have any effect on sperm-zona interaction; however, the GnRH antagonist totally blocked the stimulatory effect of GnRH. GnRH did not affect the percentage of acrosome-reacted sperm, pattern of sperm movement, or frequency of sperm-zona collisions. I suggest that the increased ability of the sperm to bind to the zona may be due to exposure and/or change of affinity of zona receptors on the sperm plasma membrane.     

Repression of p27kip1 synthesis by platelet-derived growth factor in BALB/c 3T3 cells

We have investigated the regulation of p27kip1, a cyclin-dependent kinase inhibitor, in BALB/c 3T3 cells during growth factor-stimulated transition from quiescence (G0) to a proliferative (G1) state. The level of p27kip1 protein falls dramatically after mitogenic stimulation and is accompanied by a decrease in cyclin E associated p27kip1, as well as a transient increase in cyclin D1-associated p27kip1 that later declines concomitantly with the loss of total p27kip1. Analysis of metabolically labelled cells revealed that cyclin D2, cyclin D3, and cdk4 were also partnered with p27kip1 in quiescent BALB/c 3T3 cells and that this association decreased after platelet-derived growth factor (PDGF) treatment. Furthermore, the decline in p27kip1 and reduced association with cyclin D3, initiated by the addition of PDGF but not plasma-derived factors, suggested that these changes are involved in competence, the first step in the exit from G0. Synthesis of p27kip1 as determined by incorporation of [35S]methionine was repressed upon mitogenic stimulation, and PDGF was sufficient to elicit this repression within 2 to 3 h. Pulse-chase experiments demonstrated the reduced rate of synthesis was not the result of an increased rate of degradation. Full repression of p27kip1 synthesis required the continued presence of PDGF and failed to occur in the presence of the RNA polymerase inhibitor 5,6-dichlorobenzimidazole riboside. These characteristics demonstrate that repression was a late effect of PDGF and was consistent with our finding that conditional expression of activated H-ras did not affect synthesis of p27kip1. Northern (RNA) analysis of p27kip1 mRNA revealed that the repression was not accompanied by a corresponding decrease in p27kip1 mRNA, suggesting that the PDGF-regulated decrease in p27kip1 expression occurred through a translational mechanism.     

Follicular fluid insulin-like growth factor-I and insulin-like growth factor-binding protein-1 and -3 vary as a function of ovarian reserve and ovarian stimulation

PURPOSE: Follicular fluid concentrations of insulin-like growth factor (IGF)-I, IGF-II, IGF-binding protein (BP)-1, and IGFBP-3 in 57 women undergoing in vitro fertilization and embryo transfer were examined to determine whether levels reflected differences in patients' exposure to gonadotropin stimulation and a diminished ovarian reserve. METHODS: Preovulatory follicular fluid was obtained from both gonadotropin-stimulated and unstimulated cycles. Subjects were grouped according to normal or decreased ovarian reserve and whether or not they received gonadotropin stimulation. RESULTS: The mean follicular fluid concentrations of IGF-I and IGFBP-1 were significantly lower in the "decreased" ovarian reserve group compared with the "normal" ovarian reserve group, with no change in estradiol or IGF-II levels. This resulted in a decreased molar IGF-I: BP ratio and an increased molar IGF-II:IGFBP-1 ratio. In unstimulated cycles, mean follicular fluid concentrations of IGFs did not differ significantly compared with those in stimulated cycles, whereas concentrations of IGFBP-1 and IGFBP-3 were significantly lower, leading to higher molar ratios of the IGFs to the binding proteins. CONCLUSIONS: Follicular fluid IGF and binding proteins vary as a function of ovarian reserve and gonadotropin stimulation. This may reflect either differences in oocyte quality or a suboptimal follicular fluid environment.     

CD44 protein levels and its biological activity are regulated in Balb/c 3T3 fibroblasts by serum factors and by transformation with the ras but not with the sis oncogene

CD44s (standard isoform) levels and hyaluronan-binding activity were investigated in Balb/c 3T3 cells and their derivatives transformed with ras or sis oncogenes as a function of serum concentration in the medium. 3T3 cells contained low levels of CD44 and did not bind hyaluronan when grown in medium containing 0.5 or 10% serum. In 5% serum, however, the cells had much higher levels of CD44 and were able to bind hyaluronan. CD44 levels also increased in 3T3 cells restimulated with either 5 or 10% serum after prior maintenance in low serum. In cells restimulated with 5% serum, high levels of CD44 were sustained for at least 72 hr. In cells restimulated with 10% serum, however, the increase in CD44 levels reverted by 48 hr. Transformation of 3T3 cells with ras (but not with sis) oncogene rendered CD44 levels insensitive to serum modulation: ras-transformed cells contained high levels of CD44 and bound hyaluronan at all serum concentrations and at all time points tested. Sis-transformed cells behaved like 3T3 cells in these modulatory changes. Platelet-derived growth factor (PDGF), when supplementing 0.5% serum, mimicked the effects of serum on the levels and hyaluronan-binding capacity of CD44 in 3T3 cells and the CD44-upregulating activity of serum was neutralized by incubation with anti-PDGF antibodies. These data demonstrate that serum factors, specifically PDGF, mediate regulation of CD44 levels in BAlb/c 3T3 cells and that transformation of 3T3 cells by ras renders CD44 expression insensitive to the modulating effects of serum in vitro. These results correlate with the metastatic capacity of these cells in vivo.     

Mitogenic activity of neu differentiation factor/heregulin mimics that of epidermal growth factor and insulin-like growth factor-I in human mammary epithelial cells

BACKGROUND: Twin registers provide a valuable source for research into disease causation. The existing population-based registers comprise mostly old twins. In order to be able to study diseases which occur in childhood and youth a new Danish twin register has been established. METHODS: The register is based on the Danish Civil Registration, with information on number of twin births from the Danish Vital Statistics Office as the source of validation. All twins resident in Denmark at 1 March 1991 were sent a one-side questionnaire asking about diabetes, willingness to participate in other research projects and similarity in the twins. RESULTS: The register, comprising 20,888 twin pairs, covers 74.4% of all twin pairs born 1953-1967 (incl.) and 97.4% of those born 1968-1982 (incl.). The response rate to the questionnaire study was 92.3%. The responders represented 19,180 twin pairs distributed as 5304 monozygotic pairs, 6861 same-sex dizygotic pairs, 6244 opposite-sex dizygotic pairs and 771 pairs of unknown zygosity. Of the respondent twins, 96% declared their willingness to participate in additional studies. An analysis of trends in the twinning rates for the years 1968-1982 showed that the rate of monozygotic twinning is increasing and the twinning rate of opposite-sex twin pairs is decreasing. CONCLUSIONS: Earlier estimated trends in twinning rates have been confirmed. Due to the high response rate and opportunities for linkage with other Danish registers, the present material provides a valuable resource for twin studies in diseases and human traits.     

Up-regulation of insulin/insulin-like growth factor-I hybrid receptors during differentiation of HT29-D4 human colonic carcinoma cells

To assess the autocrine function of insulin-like growth factor II (IGF-II) in the balance of proliferation and differentiation in HT29-D4 human colonic cancer cells, we studied the expression of IGF-I receptors (IGF-IR) and insulin receptors (IR) in relation to the state of cell differentiation. IGF-IR and IR were expressed in both undifferentiated and enterocyte-like differentiated HT29-D4 cells. IGF-IR had two isoforms with a 97-kDa and a 102-kDa beta-subunit. In addition, HT29-D4 cells expressed hybrid receptors (HR) formed by the association of two alphabeta heterodimers from both IR and IGF-IR. HR were evidenced through 1) inhibition of IGF-I binding by the B6 anti-IR antibody and 2) immunoprecipitation with the alpha-IR3 anti-IGF-IR antibody, which revealed an additional 95-kDa IR beta-subunit that disappeared when the heterotetrameric receptor was dissociated by disulfide reduction into alphabeta heterodimers before immunoprecipitation. Like IGF-IR, HR had a high affinity for IGF-I (Kd, approximately 1.5 nM), but did not bind insulin significantly; the latter interacted with the native IR only (Kd, approximately 4 nM). In the differentiated HT29-D4 cell monolayer, all receptor species were strongly polarized (>97%) toward the basolateral membrane. Moreover, HT29-D4 cell differentiation was accompanied by an approximately 2-fold increase in the number of IR, whereas the number of IGF-I-binding sites was unaltered. However, in differentiated HT29-D4 cells, approximately 55% of the latter were involved in HR vs. approximately 20% in undifferentiated HT29-D4 cells. Thus, HT29-D4 cell differentiation is characterized by an up-regulation (approximately 3-fold) of the level of HR coupled to a down-regulation (approximately 40%) of the level of native tetrameric IGF-IR. Alterations were induced early during the cell differentiation process, i.e. 5 days postconfluence, and remained unchanged for at least 21 days. Taken together, these results suggest that the IGF-II autocrine loop in HT29-D4 cells may trigger distinct signaling pathways if it activates native IGF-IR, which predominate in undifferentiated cells, or if it activates HR, which are up-regulated in differentiated cells.     

Divergent effects of short-term, very-low-calorie diet on insulin-like growth factor-I and insulin-like growth factor binding protein-3 serum concentrations in premenopausal women with obesity

Patients who have sustained alcohol-related injuries are frequently treated in departments of oral and maxillofacial surgery. Often, an alcohol intervention will not be possible in accident and emergency departments due to intoxication but, when attending out-patient clinics for follow-up, patients are usually sober. This presents a unique opportunity for encouraging patients to review their alcohol consumption at a time when their facial injury may make them more receptive to advice. This article reviews the convincing evidence of the effectiveness of advice and brief interventions designed to be incorporated into standard out-patient consultations and describes practical screening of patients for harmful drinking, the Stages of Change Model of behaviour change and motivational interviewing for facilitating behavioural change.     

Neurotransmitter-induced novel modulation of a nonselective cation channel by a cAMP-dependent mechanism in rat pineal cells

In the rat, circadian rhythm in melatonin is regulated by noradrenergic and neuropeptide inputs to the pineal via adenosine 3',5'-cyclic monophosphate (cAMP)- and Ca2+-dependent mechanisms. We have identified a large conductance (170 pS), voltage-dependent, nonselective cation channel on rat pineal cells in culture that shows a novel mode of modulation by cAMP. Pituitary adenylate cyclase activating peptide (PACAP), norepinephrine, or 8-Br-cAMP increase channel open probability (Po) with a hyperpolarizing shift in voltage dependence such that the channel becomes active at resting membrane potentials. The increase in Po was accompanied by a change in current rectification properties such that the channel was transformed from being inactive at rest to an inwardly rectifying cation conductance in the presence of agonist, which depolarizes the cell. This channel is calcium insensitive, is blocked by Cs+, and shows a permeability sequence: K+ > Na+ >/= NH+4 > Li+. The data suggest that PACAP and norepinephrine acting through a cAMP-dependent mechanism modulate this nonselective cation channel, resulting in a slow onset depolarization that may be important in regulation of pineal cell excitability.     

Activation of two types of Ca2+-permeable nonselective cation channel by endothelin-1 in A7r5 cells

1. In A7r5 cells loaded with the Ca2+ indicator fura-2, we examined the effect of a Ca2+ channel blocker SK&F 96365 on increases in intracellular free Ca2+ concentrations ([Ca2+]i) and Mn2+ quenching of fura-2 fluorescence by endothelin-1 (ET-1). Whole-cell patch-clamp was also performed. 2. Higher concentrations (> or = 10 nM) of ET-1 (higher [ET-1]) evoked a transient peak and a subsequent sustained elevation in [Ca2+]i: removal of extracellular Ca2+ abolished only the latter. A blocker of L-type voltage-operated Ca2+ channel (VOC) nifedipine at 1 microM reduced the sustained phase to about 50%, which was partially sensitive to SK&F 96365 (30 microM). 3. Lower [ET-1] (< or = 1 nM) evoked only a sustained elevation in [Ca2+]i which depends on extracellular Ca2+. The elevation was partly sensitive to nifedipine but not SK&F 96365. 4. In the presence of 1 microM nifedipine, higher [ET-1] increased the rate of Mn2+ quenching but lower [ET-1] had little effect. 5. In whole-cell recordings, both lower and higher [ET-1] induced inward currents at a holding potential of -60 mV with linear I-V relationships and reversal potentials close to 0 mV. The current at lower [ET-1] was resistant to SK&F 96365 but was abolished by replacement of Ca2+ in the bath solution with Mn2+. The current at higher [ET-1] was abolished by the replacement plus SK&F 96365. 6. In a bath solution containing only Ca2+ as a movable cation, ET-1 evoked currents: the current at lower [ET-1] was sensitive to Mn2+, whereas that at higher [ET-1] was partly sensitive to SK&F 96365. 7. These results indicate that in addition to VOC, ET-1 activates two types of Ca2+-permeable nonselective cation channel depending on its concentrations which differ in terms of sensitivity to SK&F 96365 and permeability to Mn2+.     

Growth suppression of transformed BALB/c 3T3 cells by transforming growth factor beta 1 occurs only in the presence of their normal counterparts

Transforming growth factor beta 1 (TGF-beta 1) enhances the yield of transformed foci of BALB/c 3T3 cells, but the continuous presence of TGF-beta 1 after foci formation inhibits the growth of transformed foci. The focus-forming ability of Ha-ras-, v-src- and PyMT-transformed cells growing on a monolayer of non-transformed cells was completely suppressed by TGF-beta 1, whereas growth of the transformed cells was little inhibited by TGF-beta 1 in the absence of their normal counterparts. The inhibition by TGF-beta 1 of focus formation by transformed BALB/c 3T3 cells on a normal cell monolayer remained when TGF-beta 1 was removed from the culture medium after 2 weeks. However, the transformed cells were not killed, since they grew in culture conditions under which only transformed cells are able to grow (soft agar). These results suggest that TGF-beta 1 suppresses growth of transformed cells in the presence of normal cells. Furthermore, when non-transformed cells were treated with TGF-beta 1 before co-culture with Ha-ras-transformed cells, formation of transformed foci was inhibited. When normal and transformed cells were cultured in the same dish but separated physically, focus formation was still inhibited. On the other hand, TGF-beta 1 enhanced the growth and changed the morphology of non-transformed cells only in the presence of transformed counterparts. The growth inhibitory effect of TGF-beta 1 on transformed cells and its growth stimulatory effect on non-transformed cells in co-culture conditions suggest the induction of reciprocal paracrine growth regulatory factors. As TGF-beta 1 inhibits the growth of transformed BALB/c 3T3 cells only in the presence of their normal counterparts, a paracrine negative growth control mechanism appears to be operating.     

Activation of caspase-3-like enzymes in non-apoptotic T cells

The activation of the caspase family of cysteine proteases is a key step in the implementation of apoptotic cell death leading to further downstream effects such as DNA fragmentation. In cultured tumor cells, caspase activity appears only when cells are undergoing apoptosis. Here we show that human and murine T lymphocytes acquire high intracellular activities of cell death-specific caspases upon activation by mitogens and IL-2 without evidence that apoptosis is proceeding. The highest activity is seen when cells are mitogen activated for 3 days. On a per cell basis, caspase activity in activated T cells is much higher than in tumor cells induced to undergo apoptosis. In the presence of exogenously added IL-2 cells stay alive and maintain a high level of caspase activity while IL-2 withdrawal results in cell death and decline of caspase activity. Caspase activity can also be measured in extracts from spleen and lymph nodes from mice injected with superantigen. While in tumor cell lines caspase activity correlates with cleavage of poly(ADP)-ribose polymerase (PARP) and DNA fragmentation, in activated T cells cleavage products of cellular PARP can be detected whereas DNA fragmenting activity appears only upon IL-2 withdrawal which coincides with cell death. These data show that caspase activation in intact cells does not necessarily lead to cell death and argue for a checkpoint in the apoptotic pathway downstream of caspases. Furthermore, they provide a molecular correlate for the high susceptibility of activated T cells for apoptosis.