Platelet-derived growth factor isoforms prevent cell death during starvation of AKR-2B fibroblasts

Effects of ADMA on gene expression and metabolism in serum-starved LoVo cells

Serum starvation is a typical way for inducing tumor cell apoptosis and stress. Asymmetric dimethylarginine (ADMA) is an endogenous metabolite. Our previous study reveals the plasma ADMA level is elevated in colon cancer patients, which can attenuate serum starvation-induced apoptosis in LoVo cells. In current study, we evaluated the effects of ADMA on gene expression and metabolism in serum-starved LoVo cells with gene microarray and metabolomic approaches. Our results indicated that 96 h serum starvation induced comprehensive alterations at transcriptional level, and most of them were restored by ADMA. The main signaling pathways induced by serum starvation included cancers-related pathways, pathways in cell death, apoptosis, and cell cycle etc. Meanwhile, the metabolomic data showed serum-starved cells were clearly separated with control cells, but not with ADMA-treated cells in PCA model. The identified differential metabolites indicated serum starvation significantly suppressed TCA cycle, altered glucose and fatty acids metabolism, as well as nucleic acids metabolism. However, very few differential metabolites were identified between ADMA and serum-starved cells. In summary, our current results indicated serum starvation profoundly altered the gene expression and metabolism of LoVo cells, whereas ADMA could restore most of the changes at transcriptional level, but not at metabolic level.

Involvement of Src in the Adaptation of Cancer Cells under Microenvironmental Stresses

Protein-tyrosine phosphorylation, which is catalyzed by protein-tyrosine kinase (PTK), plays a pivotal role in a variety of cellular functions related to health and disease. The discovery of the viral oncogene Src (v-Src) and its cellular nontransforming counterpart (c-Src), as the first example of PTK, has opened a window to study the relationship between protein-tyrosine phosphorylation and the biology and medicine of cancer. In this paper, we focus on the roles played by Src and other PTKs in cancer cell-specific behavior, that is, evasion of apoptosis or cell death under stressful extracellular and/or intracellular microenvironments (i.e., hypoxia, anoikis, hypoglycemia, and serum deprivation).

Caspase-9 plays a marginal role in serum starvation-induced apoptosis

Serum withdrawal represents a potent trigger to induce caspase-dependent apoptosis in a series of cell culture models. In rat 423-cells, caspase-8 and caspase-3 were apparently sufficient to initiate and proceed apoptosis without involving the intrinsic amplification loop via caspase-9. To assess the reasons for this inactivity of an otherwise crucial initiator caspase, we examined the ability for apoptosome assembly in 423-cells. Caspase-9 and Apaf-1 were expressed and cytochrome c released from mitochondria upon serum withdrawal. Although functional apoptosomes were assembled successfully in vitro, caspase-9 processing was found essentially refrained during apoptosis in 423-cells. Cell fractionation experiments revealed that sequestration of caspase-9 to cytoskeletal structures in 423-cells contributed to the observed impairment of apoptosome formation. Altogether, these findings provide evidence that serum starvation-induced apoptosis may occur independently of the intrinsic pathway and that caspase-9 sequestration potentially represents a novel biological antiapoptotic strategy.

Mutations dislocate caspase-12 from the endoplasmatic reticulum to the cytosol

Mouse AKR-2B cells express two forms of caspase-12: the full-length form coding for a protein of 47.8 kDa and a new splice variant of 40.2 kDa which is devoid of the CARD domain. In addition, three point mutations were disclosed: I/L-15, E/D-46 and P/L-105. A major portion of the two protein variants was found in the cytosol. Immunofluorescence studies showed an even distribution of caspase-12 within the cell, indicative for a cytoplasmatic localization. Transfection of AKR-2B cells with wild-type caspase-12 showed a colocalization of this protein with the endoplasmic reticulum (ER). Unlike mouse embryonal fibroblasts (MEF) which contain wild-type caspase-12, AKR-2B cells were largely resistant against treatment with the endoplasmatic reticulum stressing reagents brefeldin and tunicamycin. In AKR-2B cells, cytoplasmatic caspase-12 is bound to high molecular weight complexes of >1000 kDa [Cell Death Differ. 9 (2001) 125] and serum depletion leads to cleavage and detachment of caspase-12 from this high molecular weight complex. Cleavage of caspase-12 and -3 occurred almost simultaneously reaching a maximum 3-5 h after serum deprivation at which time also maximum apoptosis is found. Analysis of caspase-12 cleavage in vitro in comparison with fragmentation in vivo suggests that during death in AKR-2B cells induced by starvation, cleavage was brought about by caspase-3 at positions D24 and D94. Thus, mutated caspase-12 is differently integrated in signaling pathways of cell death and has lost its function as initiator caspase upon ER-stress. Instead, it is turned into a substrate of effector caspases. The implication of these findings in the pathological phenotype of ARK-2B mice is discussed.

Formation of caspase-3 complexes and fragmentation of caspase-12 during anisomycin-induced apoptosis in AKR-2B cells without aggregation of Apaf-1

Treatment of AKR-2B fibroblasts with anisomycin (10 microM) led to a rapid disintegration of the cells (t1/2 = 5 h) which was complete after 24 h. Cell death was associated with typical hallmarks of apoptosis like membrane blebbing, exposure of phophatidylserine on the cell surface, nuclear condensation and specific cleavage of rRNA. However, there was no dissipation of the mitochondrial potential and no intranucleosomal fragmentation. By affinity labeling with YVK(-bio)D.aomk in combination with immunostaining against activated caspase-3 analyzed by 2-D gel electrophoresis it was shown that caspase-3 is the dominant executioner caspase. Gel filtration experiments of cytosolic extract analyzed by Western blotting revealed the formation of high-molecular-weight complexes of caspase-3 (600 kDa and 250 kDa, respectively), but there was no complex formation of Apaf-1. Anisomycin treatment led to a strong activation of the stress kinases p38 kinases and the jun kinases, that was not sufficient for the activation of caspase-3 which required much higher concentrations. By using the selective inhibitors SB 203580 for p38 kinases and SP 600125 for c-jun kinases, respectively, it is shown that activation of these kinases is not necessary for cell death induced by anisomycin in AKR-2B cells. Furthermore, we disclose the activation of caspase-12 in AKR-2B cells following the addition of anisomycin. Caspase-12 zymogen present as a cytosolic complex (> 600 kDa) is activated by anisomycin leading to an uncomplexed cleaved enzyme. Since anisomycin treatment did neither lead to stress of the endoplasmic reticulum nor to a breakdown of intracellular Ca(2+)-stores, alternative pathways involved in the activation of caspases are discussed.

Formation of noncanonical high molecular weight caspase-3 and -6 complexes and activation of caspase-12 during serum starvation induced apoptosis in AKR-2B mouse fibroblasts

Apoptosis is mainly brought about by the activation of caspases, a protease family with unique substrate selectivity. In mammals, different complexes like the DISC complex or the apoptosome complexes have been delineated leading to the cleavage and thus activation of the executioner caspases. Although caspase-3 is the main executioner caspase in apoptosis induced by serum starvation in AKR-2B fibroblasts as demonstrated by affinity labeling with YVK(-bio)D.aomk and partial purification of cytosolic extracts by high performance ion exchange chromatography, its activation is apparently caused by a noncanonical pathway: (1) Expression of CrmA, an inhibitor of caspase-8, failed to suppress apoptosis; (2) There was no formation of high molecular weight complexes of Apaf-1 indicative for its activation. Furthermore no cleavage of caspase-9 was observed. But surprisingly, gelfiltration experiments revealed the distribution of caspase-3 and -6 into differently sized high molecular weight complexes during apoptosis. Though the apparent molecular weights of the complexes containing caspase-3 (600 kD for apoptosome and 250 kD for microapoptosome) are in accordance with recently published data, the activity profiles differ strikingly. In AKR-2B cells caspase-3 is mainly recovered as uncomplexed enzyme and in much lower levels in the apoptosomes. Remarkably, the 600 kD and 250 kD complexes containing activated caspase-3 were devoid of Apaf-1 and cytochrome c. In addition a new 450 kD complex containing activated caspase-6 was found that is clearly separated from the caspase-3 containing complexes. Furthermore, we disclose for the first time the activation of caspase-12 in response to serum starvation. Activated caspase-12 is detectable as non-complexed free enzyme in the cytosol.

Selective degradation of the PKC-epsilon isoform during cell death in AKR-2B fibroblasts

The protein kinase C (PKC) family of serine/threonine protein kinases is involved in intracellular signals that regulate growth, differentiation, and apoptosis. AKR-2B cells express the PKC isoforms alpha, gamma, epsilon, lambda, mu, und zeta (J. Hoppe, R. Schäfer, V. Hoppe, and A. Sachinidis, Cell Death Differ. 6, 546-556). Here we show that during serum starvation only PKC-epsilon was cleaved. An N-terminal fragment of 42 kDa remained associated with subcellular components, presumably the Golgi apparatus. The C-terminal part (catalytic domain) was further degraded and was no longer detectable in vivo. As published before, the activation of the DEVDase in AKR-2B cells is prevented by numerous agents like PDGF, TPA, and DEVD.cmk (R. Schäfer, D. Karbach, and J. Hoppe, Exp. Cell Res. 240, 28--39). All these agents completely prevented PKC-epsilon cleavage, indicating a tight correlation between DEVDase activity and PKC-epsilon cleavage. By using recombinant caspase-3 or highly purified DEVDase from cytosolic extracts we localized by Edman degradation the cleavage site in recombinant PKC-epsilon to asp383 in the hinge region between regulatory and catalytic domains. The corresponding tetrapeptide sequences SSPD and SATD for human and mouse PKC-epsilon, respectively, are unusual for caspase-3. Expression of the catalytic domain or of the cleavage-resistant mutant D383A had no effect on cell death in AKR-2B cells.

Transforming growth factor-beta 1 suppresses serum deprivation-induced death of A549 cells through differential effects on c-Jun and JNK activities

Transforming growth factor (TGF)-beta1, a pleiotropic cytokine involved in regulating growth and differentiation, can exert both pro-apoptotic and anti-apoptotic effects depending on the cell type or circumstances. We observed that TGF-beta1 blocked apoptosis resulting from serum withdrawal in A549 human lung carcinoma cells. This was associated with suppression of JNK activation that occurs concomitant with the onset of apoptosis in the absence of TGF-beta1, suggesting that JNK plays an active role in the death process and that TGF-beta1 exerts its protective influence by altering JNK activity. Overexpression of a dominant negative mutant form of SEK1, an upstream activator of JNK, likewise suppressed JNK activation and inhibited apoptosis. Investigation of early events following TGF-beta1 treatment revealed an early induction and phosphorylation of c-Jun that was absent in cells subjected to serum withdrawal alone. That TGF-beta1-induced expression of c-Jun is important for survival was supported by the finding that overexpression of non-phosphosphorylatable dominant negative mutant c-Jun, c-Jun(S73A), attenuated the protective influence of TGF-beta1. Our findings suggest that JNK activation is a late but essential event in serum deprivation-induced apoptosis in A549 cells. TGF-beta1 prevents apoptosis, in part, through the early induction and phosphorylation of c-Jun, which in turn results in attenuated JNK activation.

Macrophage colony-stimulating factor promotes cell survival through Akt/protein kinase B

The signaling pathways activated by the macrophage colony-stimulating factor (M-CSF) to promote survival of monocyte and macrophage lineage cells are not well established. In an effort to elucidate these pathways, we have used two cell types responsive to M-CSF: NIH 3T3 fibroblasts genetically engineered to express human M-CSF receptors (3T3-FMS cells) and human monocytes. M-CSF treatment induced M-CSF receptor tyrosine phosphorylation and recruitment of the p85 subunit of phosphatidylinositol 3-kinase (PI3K) to these receptors. These M-CSF receptor events correlated with activation of the serine/threonine kinase Akt. To clarify that PI3K products activate Akt in response to M-CSF, NIH 3T3 fibroblasts expressing mutant human M-CSF receptors (3T3-FMS(Y809F)) that fail to activate Ras in response to M-CSF also exhibit increased Akt kinase activity in response to M-CSF challenge. Furthermore, Akt appears to be the primary regulator of survival in 3T3-FMS cells, as transfection of genes encoding dominant-negative Akt isoforms into these fibroblasts blocked M-CSF-induced survival. In normal human monocytes, M-CSF increased the levels of tyrosine-phosphorylated proteins and induced Akt activation in a PI3K-dependent manner. The PI3K inhibitor LY294002 blocked M-CSF-mediated monocyte survival, an effect that was partially restored by caspase-9 inhibitors. These data suggest that M-CSF may induce cell survival through Akt-induced suppression of caspase-9 activation.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

Multiple intracellular pathways interfere with the activation of a CPP32-like protease induced by serum deprivation of AKR-2B cells

As previously described, confluent AKR-2B fibroblasts rapidly disintegrate upon removal of serum. Platelet-derived growth factor isoforms AB or BB (PDGF-AB, -BB) added immediately after serum deprivation caused complete survival of the cells without initiating proliferation (Simm et al., 1994, J. Cell. Physiol. 160, 295). Here the role of cAMP as a protective agent was investigated by using forskolin or 8-Br-cAMP. Both reagents afforded high cellular protection. The phorbolester TPA, an activator of protein kinase C isoforms, also exerted a high protection against cell death (ED50 = 7 nM). Unexpectedly colchicine (ED50 = 1.5 microM) an inhibitor of tubulin polymerization also protected cells from death. The protective effects of PDGF-BB and TPA were dependent on protein synthesis as indicated by their complete suppression by cycloheximide (CHx). Surprisingly, forskolin and 8-Br-cAMP remained effective even in the presence of CHx. Detailed studies of several signalling pathways were performed. These investigations showed no interference between PDGF-BB and cAMP-dependent pathways at the early stage of signal transduction. As previously described, the ICE-like protease inhibitor tyr-val-ala-asp-chloromethylketone (YVAD-cmk) protected cells from death (Simm et al., 1997, J. Cell Sci. 110, 819-828). As shown here, a substantial protection was also achieved by the addition of two other caspase inhibitors: asp-glu-val-asp-aldehyde (DEVD-cho; ED50 = 100 microM) and benzoylcarbonyl-asp-glu-val-asp-chloromethylketone (Z-DEVD-cmk; ED50 = 100 microM). The activity of caspases was studied using either tyr-val-ala-asp-aminomethylcoumarine (YVAD-amc) or asp-glu-val-asp-aminomethylcoumarine (DEVD-amc) as substrates. There was no activation of a YVADase, whereas as pronounced increase in DEVDase activity was found with a maximum 3 h after serum removal. Cross competition experiments in vitro showed that the latter activity is inhibited also by low concentrations of YVAD-cmk (300-600 nM), suggesting that both inhibitors inactivated the same target protease. Remarkably all tested protective reagents lead to an inhibition of the DEVDase activity in intact cells. Since these reagents act via distinct intracellular pathways, the existence of a regulatory element upstream of the DEVDase is proposed which integrates signals from a variety of pathways.

Cell death of AKR-2B fibroblasts after serum removal: a process between apoptosis and necrosis

AKR-2B cells disintegrate after serum removal. After a delay of approximately 90 minutes, cell death began and reached after six hours a plateau of 40–50% remaining living cells. We used time-lapse video microscopy to monitor dynamic structural changes and to measure the time span of individual cells to die. The first change was the rapid appearance of membrane blebs. Membrane vesicles were rapidly extruded and reintegrated by the cell. This highly dynamic process of an affected cell stopped after 80+/−20 minutes with its death. Conductivity measurements showed that at that time the membrane was electrically permeable. By using fluorescence double staining with propidium iodide and Hoechst 33258, we show that membrane leakage leading to disintegration is accompanied, and for some cells preceded, by nuclear condensation. The energy state of the intact cells was monitored by measuring the intracellular ATP content which remained high (6 mM) throughout the entire time of investigation. Mitochondrial potential was determined by rhodamine 123 fluorescence in parallel to the measurement of membrane permeability via uptake of propidium iodide and lead to the detection of a cell population that exhibits a high mitochondrial potential and an uptake of propidium iodide indicating a membrane disruption of cells which still have a high energy charge. It is shown by electron microscopy that mitochondria were swollen and damaged in parallel to nuclear condensation. There was no DNA fragmentation as shown by two independent methods. Addition of the ICE-like protease inhibitor tyr-val-ala-asp-chloromethylketone immediately after serum starvation lead to an almost complete survival of the cells up to 6 hours. A pronounced protection was still observed after 24 hours, suggesting an involvement of this type of protease in the onset of cell death after serum removal. Apparently, serum withdrawal activates a succession of initial events that are similar to those defined as ‘apoptosis’, i.e. nuclear condensation and membrane blebbing. These steps are, however, accompanied or rapidly followed by cell lysis and disruption of mitochondria, both of which are characteristic of necrosis.