A functional role for death proteases in s-Myc- and c-Myc-mediated apoptosis

MYC Oncogene: A Druggable Target for Treating Cancers with Natural Products

Various diseases, including cancers, age-associated disorders, and acute liver failure, have been linked to the oncogene, MYC. Animal testing and clinical trials have shown that sustained tumor volume reduction can be achieved when MYC is inactivated, and different combinations of therapeutic agents including MYC inhibitors are currently being developed. In this review, we first provide a summary of the multiple biological functions of the MYC oncoprotein in cancer treatment, highlighting that the equilibrium points of the MYC/MAX, MIZ1/MYC/MAX, and MAD (MNT)/MAX complexes have further potential in cancer treatment that could be used to restrain MYC oncogene expression and its functions in tumorigenesis. We also discuss the multifunctional capacity of MYC in various cellular cancer processes, including its influences on immune response, metabolism, cell cycle, apoptosis, autophagy, pyroptosis, metastasis, angiogenesis, multidrug resistance, and intestinal flora. Moreover, we summarize the MYC therapy patent landscape and emphasize the potential of MYC as a druggable target, using herbal medicine modulators. Finally, we describe pending challenges and future perspectives in biomedical research, involving the development of therapeutic approaches to modulate MYC or its targeted genes. Patients with cancers driven by MYC signaling may benefit from therapies targeting these pathways, which could delay cancerous growth and recover antitumor immune responses.

DOT1L stimulates MYC/Mondo transcription factor activity by promoting its degradation cycle on chromatin

The proto-oncogene c-MYC is a key representative of the MYC transcription factor network regulating growth and metabolism. MML-1 (Myc- and Mondo-like) is its homolog in C. elegans. The functional and molecular cooperation between c-MYC and H3 lysine 79 methyltransferase DOT1L was demonstrated in several human cancer types, and we have earlier discovered the connection between C. elegans MML-1 and DOT-1.1. Here, we demonstrate the critical role of DOT1L/DOT-1.1 in regulating c-MYC/MML-1 target genes genome-wide by ensuring the removal of "spent" transcription factors from chromatin by the nuclear proteasome. Moreover, we uncover a previously unrecognized proteolytic activity of DOT1L, which may facilitate c-MYC turnover. This new mechanism of c-MYC regulation by DOT1L may lead to the development of new approaches for cancer treatment.

MYC: a multipurpose oncogene with prognostic and therapeutic implications in blood malignancies

MYC oncogene is a transcription factor with a wide array of functions affecting cellular activities such as cell cycle, apoptosis, DNA damage response, and hematopoiesis. Due to the multi-functionality of MYC, its expression is regulated at multiple levels. Deregulation of this oncogene can give rise to a variety of cancers. In this review, MYC regulation and the mechanisms by which MYC adjusts cellular functions and its implication in hematologic malignancies are summarized. Further, we also discuss potential inhibitors of MYC that could be beneficial for treating hematologic malignancies.

Molecular cloning and functional characterization of MYC transcription factor in pathogen-challenged Apostichopus japonicus

Myelocytomatosis viral oncogene (MYC), a transcription factor in the MYC family, plays vital roles in vertebrate innate immunity by regulating related immune gene expressions. In this study, we cloned and characterized an MYC gene from sea cucumber Apostichopus japonicus via RNA-seq and RACE approaches (designated as AjMYC). A 2074 bp fragment representing the full-length cDNA of AjMYC was obtained. This gene includes an open reading frame (ORF) of 1296 bp encoding a polypeptide of 432 amino acid residues with the molecular weight of 48.85 kDa and theoretical pI of 7.22. SMART analysis indicated that AjMYC shares an MYC common HLH motif (354-406 aa) at the C-terminal. Spatial expression analysis revealed that AjMYC is constitutively expressed in all detected tissues with peak expression in the tentacle. Vibrio splendidus-challenged sea cucumber could significantly boost the expression of AjMYC transcripts by a 5.58-fold increase in the first stage. Similarly, 2.75- and 3.23-fold increases were detected in LPS-exposed coelomocytes at 1 and 24 h, respectively. In this condition, coelomocyte apoptotic rate increased from 11.98% to 56.23% at 1 h and to 59.08% at 24 h. MYC inhibitor treatment could not only inhibit the expression of AjMYC and Ajcaspase3, but also depress the coelomocyte apoptosis. Furthermore, AjMYC overexpression in EPC cells for 24 h also promoted the cell apoptosis rate from 21.31% to 45.85%. Collectively, all these results suggested that AjMYC is an important immune factor in coelomocyte apoptosis toward pathogen-challenged sea cucumber.

Camptothecin enhances c-Myc-mediated endoplasmic reticulum stress and leads to autophagy by activating Ca2+-mediated AMPK

Camptothecin (CPT) from Camptotheca acuminate was discovered for anticancer drugs, which targets topoisomease I. However, whether CPT regulates c-Myc expression has not been understood in endoplasmic reticulum (ER) stress and autophagy. In this study, we found that CPT enhanced c-Myc expression and that the transient knockdown of c-Myc abrogated reactive oxygen species (ROS) generation, which resulted in the accumulation of ER stress-regulating proteins, such as PERK, eIF2α, ATF4, and CHOP. Moreover, the transfection of eIF2α-targeted siRNA attenuated CPT-induced autophagy and decreased the levels of Beclin-1 and Atg7, which indicated that CPT upregulated ER stress-mediated autophagy. In addition, CPT phosphorylated AMPK in response to intracellular Ca²⁺ release. Ca²⁺ chelators, ethylene glycol tetraacetic acid and a CaMKII inhibitor, K252a, decreased CPT-induced Beclin-1 and Atg7, and downregulated AMPK phosphorylation, which suggested that CPT-induced Ca²⁺ release leads to the activation of autophagy through CaMKII-mediated AMPK phosphorylation. CPT also phosphorylated JNK and activated the DNA-binding activity of AP-1; furthermore, knockdown of JNK abolished the expression level of Beclin-1 and Atg7, which implied that the JNK-AP-1 pathway was a potent mediator of CPT-induced autophagy. Our findings indicated that CPT promoted c-Myc-mediated ER stress and ROS generation, which enhances autophagy via the Ca²⁺-AMPK and JNK-AP-1 pathways.

Preventive and protective effects of silymarin on doxorubicin-induced testicular damages correlate with changes in c-myc gene expression

This study aimed to investigate the preventive and protective effects of silymarin (SMN) on doxorubicin (DOX)-induced damages in the testis. Wistar rats were divided into six groups (n=8), including: control (C), DOX-treated (DOX, 15 mg/kg, i.p.), DOX- and SMN-treated and SMN-treated animals (SMN, 50 mg/kg, orally). Those groups, which received either compounds, were sub-grouped based on the preventive (PVT), protective (PTT) and/or therapeutic regimens (TPT) of SMN administration. The antioxidant status analyses, hormonal assay, and histopathological examinations in the testis were conducted. The expression of c-myc at mRNA level also was analyzed. SMN in preventive and protective forms significantly (p<0.05) improved the DOX-induced weight loss and lowered the alkaline phosphatase level. Pretreatment and co-treatment with SMN attenuated the DOX-induced carbonyl stress. The DOX-induced histopathological damages including negative TDI and IR were significantly (p<0.05) improved with SMN pretreatment and co-administration. SMN in preventive and protective forms prevented from DOX-induced DNA fragmentation in the testis. SMN ameliorated the DOX-reduced serum level of sexual hormones including testosterone, inhibin B, LH and FSH in PVT and PTT groups. The c-myc expression at mRNA level was completely and relatively down regulated in the testis of animals that received SMN as pretreatment and concurrent administration, respectively. Our data suggests that the DOX-induced biochemical and histopathological alterations could be prevented and/or protected by SMN. Moreover, the SMN protective and preventive effects attribute to its capacity in the reduction of DOX-induced carbonyl stress and DNA damage, which may be mediated by c-myc expression.

Mitochondrial mitophagic mechanisms of myocardial matrix metabolism and remodelling

High levels of homocysteine (Hcy), known as hyperhomocysteinmia (HHcy), are correlated with an increase in extracellular matrix remodelling (ECM) via the matrix metalloproteinases (MMPs) and plasminogen/plasmin system. This results in an increase deposition of collagen that leads to endothelial-myocyte (EM) and myocyte-myocyte (MM) uncoupling; the physiological consequences are a plethora of cardiovascular pathologies. Homocysteine-induced increase in intracellular and mitochondrial Ca(2+) plays an important role in increasing reactive oxygen species (ROS) within mitochondria and instigating mitophagy within the cell. This occurs via several Hcy-mitigated processes: agonizing N-methyl-d-aspartate receptor-1 (NMDA-R1), decreasing expression of peroxisome proliferator activator receptor (PPAR) [thereby increasing oxidation], impairing Ca(2+) handling via Na(+)/Ca(2+) exchanger (NCX1) and Sarco endoplasmic reticulum Ca(2+) ATPase (SERCA-2a). The end result is an increase in ROS that directly or indirectly lead to MMP activation within mitochondria or the cytoplasm. Hcy induces a mitochondrial permeability transition that allows MMPs to be released from mitochondria thereby metabolizing matrix and impairing cardiac function. Further work remains to be elucidated concerning the specific mitochondrial mitophagic mechanisms under which matrix metabolism and remodelling occurs. Moreover, the therapeutic implications of NMDA and PPAR ligands are some promise to patient.

c-MYC suppresses BIN1 to release poly(ADP-ribose) polymerase 1: a mechanism by which cancer cells acquire cisplatin resistance

Cancer cells acquire resistance to DNA-damaging therapeutic agents, such as cisplatin, but the genetic mechanisms through which this occurs remain unclear. We show that the c-MYC oncoprotein increases cisplatin resistance by decreasing production of the c-MYC inhibitor BIN1 (bridging integrator 1). The sensitivity of cancer cells to cisplatin depended on BIN1 abundance, regardless of the p53 gene status. BIN1 bound to the automodification domain of and suppressed the catalytic activity of poly(ADP-ribose) polymerase 1 (PARP1, EC 2.4.2.30), an enzyme essential for DNA repair, thereby reducing the stability of the genome. The inhibition of PARP1 activity was sufficient for BIN1 to suppress c-MYC-mediated transactivation, the G(2)-M transition, and cisplatin resistance. Conversely, overexpressed c-MYC repressed BIN1 expression by blocking its activation by the MYC-interacting zinc finger transcription factor 1 (MIZ1) and thereby released PARP1 activity. Thus, a c-MYC-mediated positive feedback loop may contribute to cancer cell resistance to cisplatin.

Islet isolation for clinical transplantation

Islet transplantation is emerging as a viable treatment option for selected patients with type 1 diabetes. Following the initial report in 2000 from Edmonton of insulin independence in seven out of seven consecutive recipients, there has been a huge expansion in clinical islet transplantation. The challenge we now face is the apparent decline in graft function over time. Isolating high-quality human islets which survive and function for a longer period will no doubt contribute to further improvement in long-term clinical outcome. This chapter reviews the selection of appropriate donors for islet isolation and transplantation, describes each step during islet isolation, and discusses the scope for further improvements.

Myc down-regulation affects cyclin D1/cdk4 activity and induces apoptosis via Smac/Diablo pathway in an astrocytoma cell line

OBJECTIVES

We investigated the antiproliferative effect of Myc down-regulation via cell proliferation inhibition, cell cycle perturbation and apoptosis in two human astrocytoma models (T98G and ADF) steadily expressing an inducible c-myc Anti-sense RNA.

MATERIALS AND METHODS

Cell growth experiments were performed using the trypan blue dye exclusion test and cell cycle analysis was evaluated by flow cytometry. Cell cycle molecules were detected by Western blot analysis, co-immunoprecipitation and reverse transcription-polymerase chain reaction assays.

RESULTS

We showed that Myc down-regulation in astrocytoma cells led to G1 accumulation and an inhibition of cell proliferation characterized by S phase delay. Co-immunoprecipitation experiments detected formation of inactive cyclin D1/cdk4 complexes as evaluated by presence of an active unphosphorylated form of retinoblastoma protein, the best characterized target substrate for cyclin D1/cdk4 complex, in ADF pINDc-myc anti-sense 7 cells. We also found that either p57Kip2 "apice" or p27Kip1 "apice" inhibitors bound to cyclin D1/cdk4 complex, thus, suggesting that they cooperated to inhibit the activity of cyclin D1/cdk4. Moreover, c-Myc down-regulation led to activation of the apoptotic mitochondrial pathway, characterized by release of cytochrome c and Smac/Diablo proteins and by reduction of c-IAP levels through activation of proteasome-mediated protein degradation system.

CONCLUSIONS

Our results suggest that c-Myc could be considered as a good target for the study of new approaches in anticancer astrocytoma treatment.

A Bax/Bak-independent Mechanism of Cytochrome c Release

Bax and Bak are multidomain pro-apoptotic members of the Bcl-2 family of proteins that regulate mitochondria-mediated apoptosis by direct modulation of mitochondrial membrane permeability. Since double-knock-out mouse embryonic fibroblasts with deficiency of Bax and Bak are resistant to multiple apoptotic stimuli, Bax and Bak are considered to be an essential gateway for various apoptotic signals. Here we showed that the combination of calcium ionophore A23187 and arachidonic acid induced cytochrome c release and caspase-dependent death of double-knock-out mouse embryonic fibroblasts, indicating that other mechanisms of cytochrome c release exist. Furthermore, A23187/arachidonic acid (ArA)-induced caspase-dependent death was significantly suppressed by the treatment of several serine protease inhibitors including 4-(2-aminoethyl)benzenesulfonylfluoride and l-1-chloro-3-(4-tosylamido)-4-phenyl-2-butanone but not the overexpression of anti-apoptotic Bcl-2 family of proteins or the inhibition of mitochondrial membrane permeability transition. These results indicate that there are at least two mechanisms of cytochrome c release leading to caspase activation, a Bax/Bak-dependent mechanism and a Bax/Bak-independent, but serine protease(s)-dependent, mechanism.

Cleavage of epidermal growth factor receptor by caspase during apoptosis is independent of its internalization

Epidermal growth factor receptor (EGFR) plays a critical role in cell proliferation, differentiation, and transformation. EGFR downregulation attenuates its signaling intensity and duration to maintain cellular homeostasis. Here, we report that during apoptosis EGFR is cleaved by activated caspase-3 or related proteases at its C-terminus domain. EGFR downregulation by activation of caspases is neither stimulus- nor cell type-specific. EGFR internalization during apoptosis required dynamin and cholesterol since dominant-negative dynamin (K44A) or cholesterol depletion by methyl-beta-cyclodextrin prevented EGFR internalization. However, EGFR downregulation did not require its internalization. The EGFR cleavage fragment was detected in the membrane blebs in addition to the cell pellets. Mutations at the consensus sequence (DXXD) at the C-terminus domain revealed that DVVD1012 and to a lesser extent DNPD1172 may be target sites for active recombinant caspase-3 in vitro and activated caspase-3 or related proteases in vivo. We have detected the N-terminus and C-terminus fragments in vitro and in vivo. A cleavage-deficient EGFR mutant delayed apoptosis process. We conclude that the evolutionarily conserved C-terminus domain of EGFR is the target of caspases and subjected to degradation during apoptosis to shut down its signaling.

Membrane localization of all class I PI 3-kinase isoforms suppresses c-Myc-induced apoptosis in Rat1 fibroblasts via Akt

Phosphoinositide 3'-kinases (PI3Ks) constitute a family of lipid kinases implicated in signal transduction through tyrosine kinase receptors and heterotrimeric G protein-linked receptors. PI3Ks are heterodimers made up of four different 110-kDa catalytic subunits (p110alpha, p110beta, p110gamma, and p110delta) and a smaller regulatory subunit. Despite a clear implication of PI3Ks in survival signaling, the contribution of the individual PI3K isoforms has not been elucidated. To address this issue, we generated Rat1 fibroblasts that co-express c-Myc and membrane targeted derivates of the different p110 isoforms. Here we present data for the first time showing that activation of PI3-kinase signaling through membrane localization of p110beta, p110gamma, and p110delta protects c-Myc overexpressing Rat1 fibroblasts from apoptosis caused by serum deprivation like it has been described for p110alpha. Expression of each p110 isoform reduces significantly caspase-3 like activity in this apoptosis model. Decreased caspase-3 activity correlates with the increase in Akt phosphorylation in cells that contain one of the myristoylated p110 isoforms. p110 isoform-mediated protection from cell death was abrogated upon expression of a kinase-negative version of Akt.

Okadaic acid stimulates caspase-like activities and induces apoptosis of cultured rat mesangial cells

Glomerular mesangial cells play an important role in the development of glomerulosclerosis. Mesangial cell apoptosis has been shown to be involved in different stages of development of glomerulonephritis. The aim of the present study was to evaluate the effect of inhibition of serine/threonine phosphatases by okadaic acid, a shell fish toxin, on rat mesangial cell apoptosis and to examine the molecular mechanisms particularly the role of caspases. Okadaic acid significantly induced mesangial cell apoptosis, as measured by an increase in cytoplasmic nucleosome-associated DNA fragmentation. The induction of apoptosis was dependent on protein synthesis, because cyclohexamide, a protein synthesis inhibitor, blocked okadaic acid-induced apoptosis. In addition, okadaic acid stimulated caspase activities (as measured by caspase substrate peptide hydrolysis) in cultured rat mesangial cells at different time points. After 12 h treatment, okadaic acid caused a modest increase in caspase-8 (IETD-pNAse) (159.3 +/- 6.7%) activity, while after 18 h treatment, okadaic acid caused a significant increase in caspase-3 (DEVD-pNAse) (906 +/- 245%) activity. Okadaic acid-stimulated caspase-3 activity was inhibited by Z-IETD-FMK (caspase-8 inhibitor) suggesting that the caspase-3 activity is downstream of caspase-8 activity. Both caspase-3 and caspase-8 inhibitors blocked okadaic acid-stimulated apoptosis. These data suggest that inhibition of protein phosphatases by okadaic acid induces apoptosis in rat mesangial cells by activating caspase-3- and -8-like activities and that caspase-3-like activity is downstream of caspase-8-like activity.

Serine proteases mediate apoptosis-like cell death and phagocytosis under caspase-inhibiting conditions

Effective execution of apoptosis requires the activation of caspases. However, in many cases, broad-range caspase inhibitors such as Z-VAD.fmk do not inhibit cell death because death signaling continues via basal caspase activities or caspase-independent processes. Although death mediators acting under caspase-inhibiting conditions have been identified, it remains unknown whether they trigger a physiologically relevant cell death that shows typical signs of apoptosis, including phosphatidylserine (PS) exposure and the removal of apoptotic cells by phagocytosis. Here we show that cells treated with ER stress drugs or deprived of IL-3 still show hallmarks of apoptosis such as cell shrinkage, membrane blebbing, mitochondrial release of cytochrome c, PS exposure and phagocytosis in the presence of Z-VAD.fmk. Cotreatment of the stressed cells with Z-VAD.fmk and the serine protease inhibitor Pefabloc (AEBSF) inhibited all these events, indicating that serine proteases mediated the apoptosis-like cell death and phagocytosis under these conditions. The serine proteases were found to act upstream of an increase in mitochondrial membrane permeability as opposed to the serine protease Omi/HtrA2 which is released from mitochondria at a later stage. Thus, despite caspase inhibition or basal caspase activities, cells can still be phagocytosed and killed in an apoptosis-like fashion by a serine protease-mediated mechanism that damages the mitochondrial membrane.

A serine protease is involved in the initiation of DNA damage-induced apoptosis

Caspases are considered to be the key effector proteases of apoptosis. Initiator caspases cleave and activate downstream executioner caspases, which are responsible for the degradation of numerous cellular substrates. We studied the role of caspases in apoptotic cell death of a human melanoma cell line. Surprisingly, the pancaspase inhibitor zVAD-fmk was unable to block cleavage of poly(ADP-ribose) polymerase (PARP) after treatment with etoposide, while it did prevent DEVDase activity. It is highly unlikely that caspase-2, which is a relatively zVAD-fmk-resistant caspase, is mediating etoposide-induced PARP cleavage, as a preferred inhibitor of this caspase could not prevent cleavage. In contrast, caspase activation and PARP degradation were blocked by pretreatment of the cells with the serine protease inhibitor 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF). We therefore conclude that a serine protease regulates an alternative initiation mechanism that leads to caspase activation and PARP cleavage. More importantly, while zVAD-fmk could not rescue melanoma cells from etoposide-induced death, the combination with AEBSF resulted in substantial protection. This indicates that this novel pathway fulfills a critical role in the execution of etoposide-induced programmed cell death.

Loss of MYC Confers Resistance to Doxorubicin-induced Apoptosis by Preventing the Activation of Multiple Serine Protease- and Caspase-mediated Pathways

c-Myc plays an essential role in proliferation, differentiation, and apoptosis. Because of its relevance to cancer, most studies have focused on the cellular consequences of c-Myc overexpression. Here, we address the role of physiological levels of c-Myc in drug-induced apoptosis. By using c-MYC null cells we confirm and extend recent reports showing a c-Myc requirement for the induction of apoptosis by a number of anticancer agents. In particular, we show that c-Myc is required for the induction of apoptosis by doxorubicin and etoposide, whereas it is not required for camptothecin-induced cell death. We have investigated the molecular mechanisms involved in executing doxorubicin-induced apoptosis and show caspase-3 activation by both mitochondria-dependent and -independent pathways. Moreover, serine proteases participate in doxorubicin-induced apoptosis partly by contributing to caspase-3 activation. Finally, a complete rescue from doxorubicin-induced apoptosis is obtained only when serine proteases, caspase-3, and mitochondrial activation are inhibited simultaneously. Interestingly, doxorubicin requires c-Myc for the activation of all of these pathways. Our findings therefore support a model in which doxorubicin simultaneously triggers multiple c-Myc-dependent apoptosis pathways.

A serine protease inhibitor prevents endoplasmic reticulum stress-induced cleavage but not transport of the membrane-bound transcription factor ATF6

Mammalian cells express several transcription factors embedded in the endoplasmic reticulum (ER) as transmembrane proteins that are activated by proteolysis, and two types of these proteins have been extensively investigated. One type comprises the sterol regulatory element-binding proteins (SREBP-1 and SREBP-2). The other type comprises the activating transcription factors 6 (ATF6alpha and ATF6beta), which are activated in response to ER stress. It was shown previously that both SREBP and ATF6 are cleaved sequentially first by the Site-1 protease (serine protease) and then by the Site-2 protease (metalloprotease) (Ye, J., Rawson, R. B., Komuro, R., Chen, X., Dave, U. P., Prywes, R., Brown, M. S., and Goldstein, J. L. (2000) Mol. Cell 6, 1355-1364). In this study, we examined various protease inhibitors and found that 4-(2-aminoethyl)benzenesulfonyl fluoride (AEBSF), a serine protease inhibitor, prevented ER stress-induced cleavage of ATF6alpha and ATF6beta, resulting in inhibition of transcriptional induction of ATF6-target genes. AEBSF also inhibited production of the mature form of SREBP-2 that was induced in response to sterol depletion, and appeared to directly prevent cleavage of ATF6alpha and ATF6beta by inhibiting Site-1 protease. As the Site-1 protease is localized in the Golgi apparatus, both SREBP and ATF6 must relocate to the Golgi apparatus to be cleaved. We showed here that AEBSF treatment had little effect on ER stress-induced translocation of ATF6 from the ER to the Golgi apparatus, but blocked nuclear localization of ATF6. These results indicate that the transport of ATF6 from the ER to the Golgi apparatus and that from the Golgi apparatus to the nucleus are distinct steps that can be distinguished by treatment with AEBSF.

c-myc induces autophagy in rat 3Y1 fibroblast cells

The proto-oncogene c-myc is a multifunctional gene that regulates cell division, cell growth, and apoptosis. Here we report a new function of c-myc: induction of autophagy. Autophagy is a bulk degradation system for intracellular proteins. Autophagy proceeds with characteristic morphologies, which begins with the formation of a double-membrane structure called the autophagosome surrounding a portion of the cytoplasm, after which its outer membrane then fuses with the lysosomal membrane to become an autolysosome. Autophagosomes and autolysosomes are generally called autophagic vacuoles. When c-Myc protein was overexpressed in rat 3Y1 fibroblasts or when the chimeric protein c-MycER was activated by estrogen, the number of autophagic vacuoles in cells increased significantly. The formation of autophagic vacuoles induced by c-Myc was completely blocked by a specific inhibitor of autophagosome formation, 3-methyladenine. A c-Myc mutant lacking Myc Box II induced neither apoptosis nor oncogenic transformation, but still stimulated autophagy. An inhibitor of caspases suppressed apoptosis but not autophagy. These results suggest that the autophagy caused by c-myc is not due to the apoptosis or tumorigenesis induced by c-myc. Taken together, our results suggest that the induction of autophagy is a novel function of c-myc.

Degradation of alpha-actin filaments in venous smooth muscle cells in response to mechanical stretch

Mechanical stretch has been shown to induce the degradation of alpha-actin filaments in smooth muscle cells (SMC) of experimental vein grafts. Here, we investigate the possible role of ERK1/2 and p38 MAPK in regulating this process using an ex vivo venous culture model that simulates an experimental vein graft. An exposure of a vein to arterial pressure induced a significant increase in the medial circumferential strain, which induced rapid alpha-actin filament disruption, followed by degradation. The percentage of SMC alpha-actin filament coverage was reduced significantly under arterial pressure (91 +/- 1%, 43 +/- 13%, 51 +/- 5%, 28 +/- 3%, and 19 +/- 5% at 1, 6, 12, 24, and 48 h, respectively), whereas it did not change significantly in specimens under venous pressure at theses times. The degradation of SMC alpha-actin filaments paralleled an increase in the relative activity of caspase 3 (3.0 +/- 0.7- and 1.7 +/- 0.4-fold increase relative to the control level at 6 and 12 h, respectively) and a decrease in SMC density (from the control level of 1,368 +/- 66 cells/mm(2) at time 0 to 1,205 +/- 90, 783 +/- 129, 845 +/- 61, 637 +/- 55, and 432 +/- 125 cells/mm(2) at 1, 6, 12, 24, and 48 h of exposure to arterial pressure, respectively). Treatment with a p38 MAPK inhibitor (SB-203580) significantly reduced the stretch-induced activation of caspase 3 at 6 h (from 3.0 +/- 0.7- to 2.2 +/- 0.3-fold) in conjunction with a significant rescue of alpha-actin filament degradation (from 43 +/- 13% to 69 +/- 15%) at the same time. Treatment with an inhibitor for the ERK1/2 activator (PD-98059), however, did not induce a significant change in the activity of caspase 3 or the percentage of SMC alpha-actin filament coverage. These results suggest that p38 MAPK and caspase 3 may mediate stretch-dependent degradation of alpha-actin filaments in vascular SMCs.

Macrophage recognition of externalized phosphatidylserine and phagocytosis of apoptotic Jurkat cells--existence of a threshold

Phosphatidylserine (PS) is predominantly confined to the inner leaflet of plasma membrane in cells, but it is externalized on the cell surface during apoptosis. This externalized PS is required for effective phagocytosis of apoptotic cells by macrophages. Because PS trans-bilayer asymmetry is not absolute in different types of nonapoptotic cells, we hypothesized that the amounts of externalized PS may be critical for macrophage discrimination between apoptotic and nonapoptotic cells. We developed a sensitive electron paramagnetic resonance method to quantify the amounts of externalized PS based on specific binding of paramagnetic annexin V-microbead conjugates with PS on cell surfaces. Using this technique, we found that nonapoptotic Jurkat cells externalize 0.9 pmol of endogenous PS/10(6) Jurkat cells. For cells with different amounts of integrated exogenous PS on their surface, no phagocytic response was observed at PS levels <5 pmol/10(6) Jurkat cells; at higher PS concentrations, phagocytosis increased in a concentration-dependent manner. Apoptosis in Jurkat cells caused externalization of approximately 240 pmol PS/10(6) Jurkat cells; these amounts of externalized PS are manyfold higher than the threshold amounts of PS required for phagocytosis. Thus, macrophages have a sensitivity threshold for PS externalized on the cell surface that provides for reliable recognition and distinction between normal cells with low contents of externalized PS and apoptotic cells with remarkably elevated PS levels.

Guanine nucleotide exchange factor, Tiam1, directly binds to c-Myc and interferes with c-Myc-mediated apoptosis in rat-1 fibroblasts

The transcription factor c-Myc is important for the control of cell growth, cell cycle progression, neoplasia, and apoptotic cell death. Recently, c-Myc-binding proteins, which bind either to the N-terminal domain or the C-terminal domain of c-Myc, have been proposed as the key molecules to realize the mechanisms of these multiple c-Myc functions. We report in the present study on another protein, Tiam1, which is a specific guanine nucleotide exchange factor of Rac1 and which binds to c-Myc and modulates several of its biological functions. We were able to detect the direct binding and in vivo association between c-Myc and Tiam1. The necessary role in this interaction of the Myc box II of c-Myc was revealed in the cell extracts. The additional discovery of the intranuclear localization of Tiam1 in Rat1 cells and in neuronal cells of the mouse brain suggests this interaction may occur in the nucleus. Overexpression of Tiam1 repressed the luciferase activity of c-Myc and also inhibited the c-Myc apoptotic activity through this protein-protein interaction. Taken together, we concluded that Tiam1 is another c-Myc regulator, working in the nuclei to control c-Myc-related apoptosis.

c-MYC apoptotic function is mediated by NRF-1 target genes

A detailed understanding of the signaling pathways by which c-Myc elicits apoptosis has proven elusive. In the current study, we have evaluated whether the activation of the mitochondrial apoptotic signaling pathway is linked to c-Myc induction of a subset of genes involved in mitochondrial biogenesis. Cytochrome c and other nuclear-encoded mitochondrial genes are regulated by the transcription factor nuclear respiratory factor-1 (NRF-1). The consensus binding sequence (T/C)GCGCA(C/T)GCGC(A/G) of NRF-1 includes a noncanonical CA(C/T)GCG Myc:MAX binding site. In this study, we establish a link between the induction of NRF-1 target genes and sensitization to apoptosis on serum depletion. We demonstrate, by using Northern analysis, transactivation assays, and in vitro and in vivo promoter binding assays that cytochrome c is a direct target of c-Myc. Like c-Myc, NRF-1 overexpression sensitizes cells to apoptosis on serum depletion. We also demonstrate that selective interference with c-Myc induction of NRF-1 target genes by using a dominant-negative NRF-1 prevented c-Myc-induced apoptosis, without affecting c-Myc-dependent proliferation. These results suggest that c-myc expression leads to mitochondrial dysfunction and apoptosis by deregulating genes involved in mitochondrial function.

NADPH oxidase-dependent oxidation and externalization of phosphatidylserine during apoptosis in Me2SO-differentiated HL-60 cells. Role in phagocytic clearance

Resolution of inflammation requires clearance of activated neutrophils by phagocytes in a manner that protects adjacent tissues from injury. Mechanisms governing apoptosis and clearance of activated neutrophils from inflamed areas are still poorly understood. We used dimethylsulfoxide-differentiated HL-60 cells showing inducible oxidase activity to study NADPH oxidase-induced apoptosis pathways typical of neutrophils. Activation of the NADPH oxidase by phorbol myristate acetate caused oxidative stress as shown by production of superoxide and hydrogen peroxide, depletion of intracellular glutathione, and peroxidation of all three major classes of membrane phospholipids, phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine. In addition, phorbol myristate acetate stimulation of the NADPH oxidase caused apoptosis, as evidenced by apoptosis-specific phosphatidylserine externalization, increased caspase-3 activity, chromatin condensation, and nuclear fragmentation. Furthermore, phorbol myristate acetate stimulation of the NADPH oxidase caused recognition and ingestion of dimethylsulfoxide-differentiated HL-60 cells by J774A.1 macrophages. To reveal the apoptosis-related component of oxidative stress in the phorbol myristate acetate-induced response, we pretreated cells with a pancaspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (z-VAD-fmk), and found that it caused partial inhibition of hydrogen peroxide formation as well as selective protection of only phosphatidylserine, whereas more abundant phospholipids, phosphatidylcholine and phosphatidylethanolamine, were oxidized to the same extent in the absence or presence of z-VAD-fmk. In contrast, inhibitors of NADPH oxidase activity, diphenylene iodonium and staurosporine, as well as antioxidant enzymes, superoxide dismutase/catalase, completely protected all phospholipids against peroxidation, inhibited expression of apoptotic biomarkers and externalization of phosphatidylserine, and reduced phagocytosis of differentiated HL-60 cells by J774A.1 macrophages. Similarly, zymosan-induced activation of the NADPH oxidase resulted in the production of superoxide and oxidation of different classes of phospholipids of which only phosphatidylserine was protected by z-VAD-fmk. Accordingly, zymosan caused apoptosis in differentiated HL-60 cells, as evidenced by caspase-3 activation and phosphatidylserine externalization. Finally, zymosan triggered caspase-3 activation and extensive SOD/catalase-inhibitable phosphatidylserine exposure in human neutrophils. Overall, our results indicate that NADPH oxidase-induced oxidative stress in neutrophil-like cells triggers apoptosis and subsequent recognition and removal of these cells through pathways dependent on oxidation and externalization of phosphatidylserine.

E2F activity is essential for survival of Myc-overexpressing human cancer cells

Effective cell cycle completion requires both Myc and E2F activities. However, whether these two activities interact to regulate cell survival remains to be tested. Here we have analysed survival of inducible c-Myc-overexpressing cell lines derived from U2OS human osteosarcoma cells, which carry wild-type pRb and p53 and are deficient for p16 and ARF expression. Induced U2OS-Myc cells neither underwent apoptosis spontaneously nor upon reconstitution of the ARF-p53 axis and/or serum-starvation. However, they died massively when concomitantly exposed to inhibitors of E2F activity, including a constitutively active pRb (RbDeltacdk) mutant, p16, a stable p27 (p27T187A) mutant, a dominant-negative (dn) CDK2, or dnDP-1. Similar apoptotic effect was observed upon down-modulation of endogenous E2Fs through overexpression of E2F binding site oligonucleotides in U2OS-Myc cells, upon expression of RbDeltacdk or dnDP-1 in the Myc-amplified HL-60 (ARF-; p53-) human leukemia cells, and upon co-transfection of Myc and RbDeltacdk in SAOS-2 (ARF+; p53-) human osteosarcoma cells but not in human primary fibroblasts. Consistent with these results, a dnp53 mutant did not abrogate the Myc-induced apoptotic phenotype, which instead strictly depended on caspase-3-like proteases and on Myc transcriptional activity. Our data indicate that in contrast to normal cells, Myc-overexpressing human cancer cells need E2F activity for their survival, regardless of their ARF and p53 status, a notion that may have important implications for antineoplastic treatment strategies.

Phosphatidylserine peroxidation/externalization during staurosporine-induced apoptosis in HL-60 cells

Although oxidative stress is commonly associated with apoptosis, its specific role in the execution of the apoptotic program has yet to be described. We hypothesized that catalytic redox interactions between negatively charged phosphatidylserine (PS) and positively charged cytochrome c released into the cytosol, along with the production of reactive oxygen species (ROS), results in pronounced oxidation and externalization of PS, and subsequent recognition of apoptotic cells by macrophages. By using staurosporine, a protein kinase inhibitor that does not act as a prooxidant, we were able to induce apoptosis in HL-60 cells without triggering the confounding effects of non-specific oxidation reactions. Through this approach, we demonstrated for the first time that PS underwent a statistically significant and pronounced oxidation at an early stage (2 h) of non-oxidant-induced apoptosis while the most abundant phospholipid, phosphatidylcholine, did not. Glutathione (GSH), the most abundant cytosolic thiol, also remained unoxidized at this time point. Furthermore, PS oxidation and the appearance of cytochrome c in the cytosol were concurrent; PS externalization was followed by phagocytosis of apoptotic cells. These findings are compatible with our proposed roles for oxidative PS-dependent signaling during apoptosis and phagocytosis.

A cellular mechanism that reversibly inactivates pancaspase inhibitor zAsp-CH(2)-DCB: a potential pitfall causing discrepancy between in vitro and in vivo caspase assays

Cell-permeable pancaspase inhibitors such as zAsp-CH2-DCB and zVAD-fmk are widely used to examine the involvement of caspases in cell death models. While examining the caspase-dependence of staurosporine (STS)-induced neuroblastoma cell death, we found that zVAD-fmk but not zAsp-CH2-DCB inhibits apoptosis. Time course analysis revealed that, in contrast to zVAD-fmk which constantly inhibited the processing of endogenous caspase substrates, zAsp-CH2-DCB inhibited substrate processing only for the first few hours after its addition to the culture medium. However, when the caspase activity in lysates prepared from cells treated with STS and zAsp-CH2-DCB was measured in vitro, quite unexpectedly, it was found that zAsp-CH2-DCB completely inhibits the STS-mediated activation of caspases throughout the observation period even when it apparently failed to inhibit the processing of caspase substrates within intact cells. These findings together suggest that there exists a cellular mechanism that inactivates zAsp-CH2-DCB in a reversible manner. This reversible inactivation was an active, intracellular process requiring de novo protein synthesis and was observed in another cell line HeLa and with different apoptotic stimuli such as ultraviolet irradiation. Our results have important implications that require consideration when designing experiments involving the use of caspase inhibitors as well as interpreting their results.

Apoptosis: letting slip the dogs of war

Recent studies have shown that, during cell death, the protein Omi is released from the mitochondrial intermembrane space into the cytosol, where it augments caspase-dependent apoptosis by blocking inhibitors and may induce caspase-independent cell death via its serine protease activity.

Deregulated c-Myc prematurely recruits both Type I and II CD95/Fas apoptotic pathways associated with terminal myeloid differentiation

Previously we have reported that deregulated expression of c-myc in normal and leukemic myeloid cells blocked differentiation and, concomitantly, induced p53-independent apoptosis. Here, we show that this morbidity was due to premature recruitment of the Fas/CD95 cell death pathway which normally operates to induce apoptosis at the end of the terminal myeloid differentiation program. Analysis of the regulated components of this pathway revealed that IL6-mediated induction of differentiation resulted in rapid cell surface expression of CD95 receptor. Deregulated c-myc prevented the downregulation of CD95 ligand by maintaining its transcription, but caused premature downregulation of c-FLIP. First, the Type II (mitochondria-dependent, bcl-2-sensitive) and, then, the Type I (mitochondria-independent, bcl-2-insensitive) pathway were activated. Stable exogenous c-FLIP expression completely rescued the apoptotic phenotype. Furthermore, when the deregulated c-myc transgene was stably transduced into bone marrow cells from Fas(lpr/lpr) (CD95 receptor mutant) and FasL(gld/gld) (CD95 ligand mutant) mice, cell death was significantly suppressed relative to c-myc-transduced wild type bone marrow cells upon induction of differentiation. These data indicate that c-myc-mediated apoptosis associated with blocks in myeloid differentiation is dependent on the Fas/CD95 pathway. Our findings offer important new insights into understanding how deregulated c-myc alters normal blood cell homeostasis, and how additional mutations might promote leukemogenesis.

Cellular lipid peroxidation end-products induce apoptosis in human lens epithelial cells

Hydrogen peroxide (H(2)O(2)), an oxidant present in high concentrations in the aqueous humor of the elderly eyes, is known to impart toxicity to the lens---apoptosis being one of the toxic events. Since H(2)O(2) causes lipid peroxidation leading to the formation of reactive end-products, it is important to investigate whether the end-products of lipid peroxidation are involved in the oxidation-induced apoptosis in the lens. 4-Hydroxynonenal (HNE), a major cytotoxic end product of lipid peroxidation, has been shown to mediate oxidative stress-induced cell death in many cell types. It has been shown that HNE is cataractogenic in micromolar concentrations in vitro, however, the underlying mechanism is not yet clearly understood. In the present study we have demonstrated that H(2)O(2) and the lipid derived aldehydes, HNE and 4-hydroxyhexenal (HHE), can induce dose- and time-dependent loss of cell viability and a simultaneous increase in apoptosis involving activation of caspases such as caspase-1, -2, -3, and -8 in the cultured human lens epithelial cells. Interestingly, we observed that Z-VAD, a broad range inhibitor of caspases, conferred protection against H(2)O(2)- and HNE-induced apoptosis, suggesting the involvement of caspases in this apoptotic system. Using the cationic dye JC-1, early apoptotic changes were assessed following 5 h of HNE and H(2)O(2) insult. Though HNE exposure resulted in approximately 50% cells to undergo early apoptotic changes, no such changes were observed in H(2)O(2) treated cells during this period. Furthermore, apoptosis, as determined by quantifying the DNA fragmentation, was apparent at a much earlier time period by HNE as opposed to H(2)O(2). Taken together, the results demonstrate the apoptotic potential of the lipid peroxidation end-products and suggest that H(2)O(2)-induced apoptosis may be mediated by these end-products in the lens epithelium.

Akt protein kinase inhibits non-apoptotic programmed cell death induced by ceramide

A growing body of evidence now suggests that programmed cell death (PCD) occurs via non-apoptotic mechanisms as well as by apoptosis. In contrast to apoptosis, however, the molecular mechanisms involved in the regulation of non-apoptotic PCD remain only poorly understood. Here we show that ceramide induces a non-apoptotic PCD with a necrotic-like morphology in human glioma cells. Characteristically, the cell death was not accompanied by loss of the mitochondrial transmembrane potential, cytosolic release of cytochrome c from mitochondria, or the activation of the caspase cascade. Consistent with these characteristics, this ceramide-induced cell death was inhibited neither by the overexpression of Bcl-xL nor by the pan-caspase inhibitor zVAD-fmk. However, strikingly, the ceramide-induced non-apoptotic cell death was inhibited by the activation of the Akt/protein kinase B pathway through the expression of a constitutively active version of Akt. The results for the first time indicate that the Akt kinase, known to play an essential role in survival factor-mediated inhibition of apoptotic cell death, is also involved in the regulation of non-apoptotic PCD.

Inhibitors of trypsin-like serine proteases prevent DNA damage-induced neuronal death by acting upstream of the mitochondrial checkpoint and of p53 induction

We have previously shown that the pharmacological agents 4-(2-aminoethyl)=benzenesulfonylfluoride hydrochloride (AEBSF) and Na-p-tosyl-L-lysine chloromethylketone (TLCK), inhibitors of trypsin-like serine proteases, prevent the death of trophic factor-deprived PC12 cells and sympathetic neurons. Both AEBSF and TLCK inhibit caspase activation in this model, but it is unclear whether they do so indirectly or through a direct effect at the level of the caspases. In the current study, we have used these agents in another model of neuronal death that is induced by DNA damage. We find that both agents delay the death of DNA-damaged PC12 cells, neonatal rat sympathetic neurons and embryonic rat cortical neurons. As in the trophic deprivation model, they act upstream of the caspases. In addition, they prevent mitochondrial alterations, such as cytochrome c release or loss of transmembrane potential. In contrast, the general caspase inhibitor bok-asp-fmk does not prevent cytochrome c release and has only a partial and transient effect on loss of transmembrane potential. Interestingly, both AEBSF and TLCK prevent the induction and nuclear accumulation of p53 that is induced by DNA damage in cortical neurons. Therefore, these serine protease inhibitors act at a point upstream in the apoptotic pathway, prior to p53 induction and the mitochondrial checkpoint, to delay neuronal death in this model, and do not act at the level of the caspases. We conclude that therapeutic strategies based on serine protease inhibition may be useful in preventing neuronal cell death.

The serine protease Omi/HtrA2 is released from mitochondria during apoptosis. Omi interacts with caspase-inhibitor XIAP and induces enhanced caspase activity

Proteome analysis of supernatant of isolated mitochondria exposed to recombinant tBid, a proapoptotic Bcl-2 member, revealed the presence of the serine protease Omi, also called HtrA2. This release was prevented in mitochondria derived from Bcl-2-transgenic mice. Release of Omi under apoptotic conditions was confirmed in vivo in livers from mice injected with agonistic anti-Fas antibodies and was prevented in livers from Bcl-2 transgenic mice. Omi release also occurs in apoptotic dying but not in necrotic dying fibrosarcoma L929 cells, treated with anti-Fas antibodies and TNF, respectively. The amino acid sequence reveals the presence of an XIAP interaction motif at the N-terminus of mature Omi. We demonstrate an interaction between endogeneous Omi and recombinant XIAP. Furthermore we show that endogenous Omi is involved in enhanced activation of caspases in cytosolic extracts.

Proteolytic loss of bcl-x(L) in FL5.12 Cells undergoing apoptosis induced by MK886

Apoptosis induced in the IL3-dependent murine pro-B lymphocytic (FL5.12) cell line by the 5-lipoxygenase activating protein inhibitor MK886 is accompanied by the rapid loss of the anti-apoptotic bcl-x(L) and bcl-2, but not the proapoptotic bax proteins (Datta et al., J. Biol. Chem. 273, 28163-28169, 1998). Since several reports indicate important roles for noncaspase proteases in apoptosis, the participation of lysosomes, as well as serine, cysteine, or aspartic acid proteases, in the effects of MK886 were investigated. Consistent with the involvement of various proteases, lysosomal degranulation was evident, as observed by a decrease in acridine orange fluorescence at 2 h and an increase in cytosolic beta-hexosaminidase activity at 4 h after treating FL5.12 cells with 10 microM MK886. The disappearance of bcl-x(L) from FL5.12 cells upon MK886 treatment was prevented in a dose-dependent manner by pretreatment with leupeptin, pepstatin, phenylmethylsulfonyl fluoride, or the broad-spectrum caspase inhibitor Boc-D-FMK. Each of the noncaspase protease inhibitors partially inhibited MK886-induced apoptosis as measured by phosphatidylserine externalization and DNA fragmentation. The noncaspase inhibitors also blocked about half of the increase in caspase-3-like activity. Boc-D-FMK completely inhibited this enzyme and prevented apoptosis. None of the inhibitors were able to directly inhibit activated caspase-3 in cell lysates, suggesting their effects were upstream of caspase activation. These observations suggest the involvement of various proteases, possibly originating from lysosomes, upstream of active caspase-3, in the loss of bcl-x(L) protein and in the signaling pathway of MK886-induced apoptosis in FL5.12 cells. This pathway may be unique to MK886 since these same protease inhibitors had only minimal effects on etoposide-induced apoptosis and the accompanying moderate loss of bcl-x(L) in FL5.12 cells.

Involvement of p38 in apoptosis-associated membrane blebbing and nuclear condensation

The stress-activated protein kinase p38 is often induced by cytotoxic agents, but its contribution to cell death is ill defined. In Rat-1 cells, we found a strong correlation between activation of p38 and induction of c-Myc-dependent apoptosis. In cells with deregulated c-Myc expression but not in control cells, cis-diamminedichloroplatinum induced p38 activity and typical features of apoptosis, including internucleosomal DNA degradation, induction of caspase activities, and both nuclear (nuclear condensation and fragmentation) and extranuclear (cell blebbing) morphological alterations. The pan-caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone did not block p38 activation and the p38 inhibitor SB203580 had no detectable effect on the activation of caspases or the in vivo cleavage of several caspase substrates, suggesting that p38 and caspase activation can contribute distinct features of apoptosis. Accordingly, we found that cell blebbing was independent of caspase activity and, rather, depended on p38-sensitive changes in microfilament dynamics likely mediated by heat shock protein 27 phosphorylation. Furthermore, p38 activity contributed to both caspase-dependent and caspase-independent nuclear condensation and fragmentation, suggesting a role in an early event triggering both mechanisms of apoptosis or sensitizing the cells to the action of both types of apoptosis executioners. Inhibiting p38 also resulted in a significant enhancement in cell survival estimated by colony formation. This capacity to modulate the sensitivity to apoptosis in cells with deregulated c-Myc expression suggests an important role for p38 in tumor cell killing by chemotherapeutic agents.

Association between bcl-x(L) and 5-lipoxygenase activating protein (FLAP) levels in IL-3-dependent FL5.12 cells

The expression of 5-lipoxygenase activating protein (FLAP) in murine hematopoietic FL5.12 cells that are transfected to overexpress bcl-x(L) is less than in control cells. In addition, the withdrawal of IL-3 from the bcl-x(L) overexpressing cells, but not control cells, leads to the rapid loss of FLAP even though these cells, in contrast to control cells, do not undergo apoptosis (Datta et al., J. Biol. Chem. 273, 28163-28169 [1998]). The mechanism(s) underlying these observations is not known. Basal FLAP mRNA levels were actually 2.8-fold higher in bcl-x(L) than control cells indicating that this difference does not have a transcription basis. In addition, an examination of FLAP mRNA levels in response to withdrawal of IL-3 revealed a 2-3-fold increase after 4 and 8 h relative to time-matched samples in both control and bcl-x(L) overexpressing cells. This further indicates that the decrease in FLAP levels in bcl-x(L) overexpressing cells is not related to transcription and suggests an attempt at compensation perhaps in response to increased FLAP degradation/turnover. A proteolytic mechanism was explored by examining the effect of the general caspase inhibitor Boc-D-FMK, and the non-caspase protease inhibitors phenylmethylsulfonyl fluoride (PMSF), pepstatin and leupeptin, on the loss of FLAP in bcl-x(L) overexpressing cells subsequent to IL-3 withdrawal. All inhibitors provided some protection from the loss of FLAP, with PMSF being the most effective, actually increasing FLAP levels above those seen in untreated cells. Given the absence of apoptosis in bcl-x(L) cells, it appears that protease activation is an effect that can accompany a variety of cellular perturbations. The functional consequences of a loss of FLAP in growth-factor deprived cells overexpressing bcl-x(L) is not known. However, these data continue to suggest some link between bcl-x(L) and FLAP.

Role of tensile stress and strain in the induction of cell death in experimental vein grafts

Tensile stress and strain are known to induce vascular cell proliferation, a process that is physiologically counterbalanced by cell death. Here we investigate whether tensile stress and strain regulate vascular-cell death by using an end-to-end anastomosed rat vein graft model. In such a model, the circumferential tensile stress in the graft wall was increased by approximately 140 times immediately after surgery compared with that in the venous wall. This change was associated with an increase in the percentage of TUNEL-positive cells at 1, 6, 24, 120, 240, and 720h with two distinct peaks at 1 and 24h (10.1+/-3.5 and 14.4+/-3.2%, respectively) compared with that in control jugular veins (0.4+/-0.5 and 0.5+/-0.5% at 1 and 24h, respectively). When tensile stress and strain in the vein graft wall were reduced by using a biomechanical engineering approach, the rate of cell death was reduced significantly (3.6+/-1.1 and 1.6+/-0.5% at 1 and 24h, respectively). Furthermore, DEVD-CHO, a tetrapeptide aldehyde that inhibits the activity of caspase 3, significantly suppressed this event. These results suggest that a step increase in tensile stress and strain in experimental vein grafts induces rapid cell death, which is possibly mediated by cell death signaling mechanisms.

Nerve growth factor prevents 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced cell death via the Akt pathway by suppressing caspase-3-like activity using PC12 cells: relevance to therapeutical application for Parkinson's disease

Nerve growth factor (NGF) mediates a variety of nerve cell actions through receptor tyrosine kinase TrkA. It has been revealed that the Akt pathway contributes to the prevention of apoptosis. It is thought that Parkinson's disease involves apoptosis, and NGF prevents apoptosis in an in vivo model system. However, there is no evidence that the Akt pathway helps to prevent parkinsonism. Here, we report that NGF prevents apoptosis induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in PC12 cells as an in vitro model system of parkinsonism and that this survival effect diminishes on addition of LY294002, a specific inhibitor of phosphatidylinositol 3-kinase. Immunocytochemical analysis revealed that 1 mM MPTP-treated cells or dominant negative Akt-expressing cells, to which were added NGF and MPTP, undergo apoptosis. Moreover, the caspase-3-like activity is increased by addition of MPTP or MPTP with NGF and LY294002. The importance of another signal pathway is shown by PD98059, a specific inhibitor of MAP kinase (MAPK) kinase, but PD98059 does not alter the survival effect in this model system. These results indicate that the Akt pathway helps to prevent parkinsonism by suppressing caspase-3-like activity, but the MAPK pathway is not involved in the NGF-dependent survival enhancing effect in this model system.

The c-Myc-interacting adaptor protein Bin1 activates a caspase-independent cell death program

Cell death processes are progressively inactivated during malignant development, in part by loss of tumor suppressors that can promote cell death. The Bin1 gene encodes a nucleocytosolic adaptor protein with tumor suppressor properties, initially identified through its ability to interact with and inhibit malignant transformation by c-Myc and other oncogenes. Bin1 is frequently missing or functionally inactivated in breast and prostate cancers and in melanoma. In this study, we show that Bin1 engages a caspase-independent cell death process similar to type II apoptosis, characterized by cell shrinkage, substratum detachment, vacuolated cytoplasm, and DNA degradation. Cell death induction was relieved by mutation of the BAR domain, a putative effector domain, or by a missplicing event that occurs in melanoma and inactivates suppressor activity. Cells in all phases of the cell cycle were susceptible to death and p53 and Rb were dispensable. Notably, Bin1 did not activate caspases and the broad spectrum caspase inhibitor ZVAD.fmk did not block cell death. Consistent with the lack of caspase involvement, dying cells lacked nucleosomal DNA cleavage and nuclear lamina degradation. Moreover, neither Bcl-2 or dominant inhibition of the Fas pathway had any effect. In previous work, we showed that Bin1 could not suppress cell transformation by SV40 large T antigen. Consistent with this finding, we observed that T antigen suppressed the death program engaged by Bin1. This observation was interesting in light of emerging evidence that T antigen has roles in cell immortalization and human cell transformation beyond Rb and p53 inactivation. In support of a link to c-Myc-induced death processes, AEBSF, a serine protease inhibitor that inhibits apoptosis by c-Myc, potently suppressed DNA degradation by Bin1. Our findings suggest that the tumor suppressor activity of Bin1 reflects engagement of a unique cell death program. We propose that loss of Bin1 may promote malignancy by blunting death penalties associated with oncogene activation.

Dual mechanisms of action of the retinoid CD437: nuclear retinoic acid receptor-mediated suppression of squamous differentiation and receptor-independent induction of apoptosis in UMSCC22B human head and neck squamous cell carcinoma cells

The synthetic retinoid 6-[3-(adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437), which can bind to and activate the nuclear retinoic acid receptors beta and gamma (RARbeta/gamma), is a potent inducer of apoptosis in various cancer cell lines. However, this effect was reported to be independent of RARs. In this study, we compared and contrasted the potencies and mechanisms of action of CD437 and several other receptor-selective retinoids in induction of apoptosis and modulation of squamous differentiation in UMSCC22B human head and neck squamous cell carcinoma cell line. CD437 and the structurally related retinoid CD2325 exhibited almost equal potency in inducing apoptosis, whereas several other retinoids failed to induce apoptosis. The RAR-specific pan antagonist AGN193109 failed to suppress CD437-induced apoptosis, indicating that the induction of apoptosis by CD437 was RAR-independent. c-Fos expression was induced by CD437 and CD2325 that induced apoptosis in the cell line but not by other retinoids that failed to induce apoptosis, suggesting a role for c-Fos in CD437-induced apoptosis. At low concentration (0.01 microM), CD437 shared with several other receptor-selective retinoids the ability to suppress the mRNA levels of the squamous differentiation markers Spr1, involucrin, and cytokeratin 1. This effect of CD437 could be blocked by AGN193109. We conclude that CD437 can exert its effects in UMSCC22B human human head and neck squamous cell carcinoma cells by at least two mechanisms: RAR-mediated suppression of squamous differentiation and RAR-independent induction of apoptosis.

p53-independent apoptosis associated with c-Myc-mediated block in myeloid cell differentiation

Previously we have shown that deregulated expression of c-myc in M1 myeloid leukemic cells blocked IL-6-induced differentiation and its associated growth arrest; however, the cells proliferated at a significantly reduced rate compared to untreated cells. The basis for the increased doubling time of IL-6-treated M1myc cells was found to be due to the induction of a p53-independent apoptotic pathway. The apoptotic response was not completely penetrant; in the same population of cells both proliferation and apoptosis were continuously ongoing. Down-regulation of Bcl-2 was insufficient to account for the apoptotic response, since deregulated expression of Bcl-2 delayed, but did not block, the onset of apoptosis. Furthermore, our results indicated that the IL-6-induced partial hypophosphorylation of the retinoblastoma gene product (Rb), observed in M1myc cells, was not responsible for the apoptotic response. Finally, the findings in M1 cells were extended to myeloid cells derived from the bone marrow of wild type and p53-deficient mice, where the deregulated expression of c-myc was also shown to block terminal differentiation and induce apoptosis independent of p53. These findings provide new insights into how myc participates in the neoplastic process, and how additional mutations can promote more aggressive tumors. Oncogene (2000) 19, 2967 - 2977

Caspases and mitochondria in c-Myc-induced apoptosis: identification of ATM as a new target of caspases

The mechanism(s) of c-Myc transcription factor-induced apoptosis is still obscure. The activation of c-Myc has been found to lead into the processing/activation of caspases (caspase-3), but the significance of this for the cell demise is debatable. Here we report that several targets of caspases (PKCdelta, MDM2, PARP, replication factor C, 70 kDa U1snRNP, fodrin and lamins) are cleaved during c-Myc-induced apoptosis in Rat-1 MycER cells, indicating an important role for caspases in the apoptotic process. We further found that the ATM (ataxia telangiectasia mutated)--protein is a novel key substrate of caspases. In in vitro assays, purified recombinant ATM protein was found to be cleaved by the effector caspases 3 and 7. The functional significance of the ATM cleavage is supported by the finding that ectopic expression of ATM protected in part against apoptosis. We also show that c-Myc-induced apoptosis involves loss of mitochondrial transmembrane potential, release of cytochrome c from mitochondria into the cytosol and subsequent processing of caspase-9. The cleavage of caspase-9 is, however, minimal and a much later event than the processing/activation of caspase-3, suggesting that it is not the apical caspase. Evidence is provided that there is, nevertheless, an upstream caspase(s) regulating the functions of caspase-3 and mitochondria. Additionally, it was found that p53 becomes upregulated, together with its transcriptional targets MDM2 and p21, upon c-Myc induction, but this occurs also at a later time than the activation of caspase-3.

Elevated levels of Ca(II) modulate the activity and inhibition of serine proteases: implication in the mechanism of apoptosis

Elevated levels of intracellular Ca(II) are a prominent feature of apoptosis, a natural form of cell death involved in many physiological and pathological processes. Serine proteases play crucial roles in apoptosis and have been implicated in the genomic DNA degradation and the massive protein degradation that occur during apoptosis. In this study, the effects of the elevated level of Ca(II) on the activity and inhibition of serine proteases were examined by spectrophotometric methods. The effects of the elevated levels of Ca(II), Mg(II), K(I), and Na(I) on the activity and inactivation of three representative members of serine proteases were determined. The level of serine protease activity in CEM-C7-14 leukemic cells was also evaluated in the presence and absence of dexamethasone-induced apoptosis, and also in the presence of A23187, a Ca(II)-ionophore. Among the four metal-ions studied, only Ca(II) was found to significantly enhance the activity of mammalian serine proteases. Ca(II) was also found to significantly protect the enzymes from inhibition, while the other three metal-ions showed no significant effect on the inactivation of the enzymes. Compared to the control sample, the enzymic activity was found to be higher during apoptosis, and in the presence of the Ca(II)-ionophore. Results of this study indicate that Ca(II) can significantly enhance the catalytic efficiency of serine proteases during apoptosis.

Differential role of the JNK and p38 MAPK pathway in c-Myc- and s-Myc-mediated apoptosis

The s-Myc is similar to c-Myc in its ability to induce apoptosis requiring caspase activation. However, s-Myc is distinct from c-Myc in that it has activity to suppress tumor growth and does not require wild-type p53 to induce apoptosis. These facts suggest differential regulation between s-Myc and c-Myc. Here we showed that s-Myc-mediated apoptosis triggered by UV was not inhibited by the inactive form mutant JNK (APF), though c-Myc-mediated apoptosis was. Moreover, we found that JNK did not affect the transactivation activity of s-Myc, but stimulated that of c-Myc. In contrast, both Myc-mediated apoptosis and caspase-3-like protease activation were suppressed by kinase-negative MKK6 and an inactive form mutant p38(AGF). Our results indicate that s-Myc does not require the JNK signaling unlike c-Myc during UV-triggered apoptosis, but the MKK6/p38MAPK pathway might regulate common apoptotic machinery for both s-Myc and c-Myc upstream of caspase.

Synergistic roles for Pim-1 and c-Myc in STAT3-mediated cell cycle progression and antiapoptosis

The activation of STAT3 by the cytokine receptor gp130 is required for both the G1 to S cell cycle transition and antiapoptosis. We found that Pim-1 and Pim-2 are targets for the gp130-mediated STAT3 signal. Expression of a kinase-defective Pim-1 mutant attenuated gp130-mediated cell proliferation. Constitutive expression of Pim-1 together with c-myc, another STAT3 target, fully compensated for loss of the STAT3-mediated cell cycle progression, antiapoptosis, and bcl-2 expression. We also identified valosine-containing protein (VCP) as a target gene for the Pim-1-mediated signal. Expression of a mutant VCP led cells to undergo apoptosis. These results indicate that Pim-family proteins play crucial roles in gp130-mediated cell proliferation and explain the synergy between Pim and c-Myc proteins in cell proliferation and lymphomagenesis.

Regulation of c-Myc through phosphorylation at Ser-62 and Ser-71 by c-Jun N-terminal kinase

The expression of c-myc promotes cell proliferation and also sensitizes cells to various extracellular apoptotic stimuli. However, signal pathways regulating the function of Myc proteins during apoptosis are unknown. c-Jun N-terminal kinase (JNK) is activated by various apoptotic stimuli, but neither the target molecule(s) or the action of JNK has been identified in Myc-mediated apoptosis. Here, we found that JNK selectively interacted with, and phosphorylated, c-Myc at Ser-62 and Ser-71 as confirmed with phospho-c-Myc-specific antibodies. Interestingly, dominant negative mutant JNK(APF) impaired the c-Myc-dependent apoptosis, but not mutated c-Myc (S62A/S71A)-dependent apoptosis triggered by UV irradiation. Furthermore, c-Myc (S62A/S71A)-expressing NIH3T3 cells were not sensitized like wild type c-Myc-expressing NIH3T3 cells to JNK-activating apoptotic stimuli, such as UV and Taxol. These results indicate that the JNK pathway is selectively involved in the c-Myc-mediated apoptosis and that the apoptotic function of c-Myc is directly regulated by JNK pathway through phosphorylation at Ser-62 and Ser-71.

Oligodendrocyte programmed cell death and central myelination deficiency induced in transgenic mice by synergism between c-Myc and Oct-6

The basic helix-loop-helix transcription factor c-Myc is a potent trigger of programmed cell death when overexpressed during late oligodendrocyte development in transgenic mice. Here we provide evidence that c-Myc can act synergistically with the Pit, Oct, Unc homeodomain transcription factor Oct-6 to produce myelin disease pathogenesis in transgenic mice. More than 70% of c-myc/Oct-6 bitransgenic mice, obtained from crosses between phenotypically normal heterozygous mice of various My (c-Myc) and Oc (Oct-6) transgenic strains that express c-myc and oct-6 transgenes under transcriptional control of the myelin basic protein gene, developed severe neurological disturbances characterized by action tremors, recurrent seizures, and premature death. Affected bitransgenic mice exhibited multiple hypomyelinated lesions in the white matter that did not stain with myelin-specific antibodies against myelin basic protein, proteolipid protein, CNPase, and myelin-associated glycoprotein. The mice also exhibited a larger number of terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end-labeling positive cells in the white matter as well as ultrastructural evidence of glial cell death and astrogliosis. These observations indicate that the myelin lesions observed in the c-myc/oct-6 bitransgenic mice result from the untimely programmed cell death of oligodendroglia and that the c-myc and oct-6 transgenes act synergistically in producing the lesions.

Implication of c-Myc in apoptosis induced by the retinoid CD437 in human lung carcinoma cells

The novel synthetic retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) has been recently identified to be a potent inducer of apoptosis in human non-small cell lung carcinoma (NSCLC) cells through a nuclear retinoic acid receptor independent mechanism. To approach the mechanism by which CD437 induces apoptosis in NSCLC cells, we investigated the involvement of c-Myc in CD437-induced apoptosis. CD437 (1 microM) up-regulated the expression of c-Myc and of its downstream target genes ornithine decarboxylase (ODC) and cdc25A in all three NSCLC cell lines (i.e., H460, SK-MES-1 and H1792) used. These effects were correlated with cellular susceptibilities to induction of apoptosis by CD437. Furthermore, CD437-induced apoptosis could be blocked by the ODC inhibitor difluoromethylornithine, the caspase inhibitors Z-VAD FMK and Z-DEVD FMK, and c-Myc antisense oligodeoxynucleotide, respectively. These data indicate that c-Myc gene plays an important role in mediating CD437-induced apoptosis in human NSCLC cells.

Physical and functional interactions between Pim-1 kinase and Cdc25A phosphatase. Implications for the Pim-1-mediated activation of the c-Myc signaling pathway

The pim-1 oncogene encodes a serine/threonine kinase (Pim-1) involved in the transduction of cytokine-triggered mitogenic signals. Pim-1 is unique in that it closely cooperates with c-Myc not only in oncogenesis, but also in apoptosis induction. However, the molecular basis of Pim-1 function remains poorly understood, largely because the downstream effector molecule(s) for Pim-1 kinase has not been identified. Here we provide several lines of evidence that Cdc25A cell cycle phosphatase, a direct transcriptional target for c-Myc, is a substrate for Pim-1 kinase and functions as an effector for Pim-1. We found that Pim-1 physically interacts with Cdc25A both in vitro and in vivo and phosphorylates Cdc25A. We also observed that Pim-1-mediated phosphorylation of Cdc25A increases its phosphatase activity. In addition, wild-type Pim-1, but not kinase-inactive Pim-1, enhanced Cdc25A-mediated cellular transformation and apoptosis. Our results indicate that Cdc25A might be a key molecule that links Pim-1 and c-Myc and that also ties Pim-1-mediated mitogenic signals to cell cycle machinery.