Biomedical subjects

Gerard Evan

Publications and source records attributed to Gerard Evan.

12 recordsLinked to original sources

PubMed2006

Genetic analysis of myc and telomerase interactions in vivo.

Myc is a transcription factor with pleiotropic effects on tumorigenesis which are likely to be mediated by its target genes. A known Myc transcriptional target is the catalytic subunit of telomerase, Tert. However, the contribution of Tert activation to Myc-induced tumorigenesis in vivo remains unknown. In this study, we addressed the role of telomerase in Myc-induced skin papillomatosis by using compound mice with a switchable Myc gene, Inv-MycERTAM mice, in combination with either telomerase deficiency (Terc-/-) or telomerase overexpression (K5-mTert) in the skin. We first demonstrated that Myc activates telomerase in the skin. With Inv-MycERTAM x Terc-/- mice, we further showed that this telomerase activation is partially required to elicit a full hyperplastic Myc-induced response. The presence of critically short telomeres in late-generation Inv-MycERTAM x Terc-/- mice further reduced the skin lesion induced by Myc. On the other hand, telomerase overexpression in the skin of K5-mTert mice augments Myc-induced hyperplasia in the absence of changes in telomere length, suggesting a direct role of telomerase in the Myc protumorigenic response. Taken together, these results highlight telomerase as a mediator of Myc-induced papillomatosis and suggest telomerase as a putative therapeutic target for Myc-dependent lesions.

Animals↗

PubMed2004

Intrinsic tumour suppression.

Mutations that drive uncontrolled cell-cycle progression are requisite events in tumorigenesis. But evolution has installed in the proliferative programmes of mammalian cells a variety of innate tumour-suppressive mechanisms that trigger apoptosis or senescence, should proliferation become aberrant. These contingent processes rely on a series of sensors and transducers that act in a coordinated network to target the machinery responsible for apoptosis and cell-cycle arrest at different points. Although oncogenic mutations that disable such networks can have profound and varied effects on tumour evolution, they may leave intact latent tumour-suppressive potential that can be harnessed therapeutically.

Animals↗

PubMed2004

Differentiation-dependent sensitivity to apoptogenic factors in PC12 cells.

We have investigated the role of the mitochondrial pathway during cell death following serum and nerve growth factor (NGF)/dibutyryl cyclic AMP (Bt(2)cAMP) withdrawal in undifferentiated or NGF/Bt(2)cAMP-differentiated PC12 cells, respectively. Holocytochrome c, Smac/DIABLO, and Omi/HtrA2 are released rapidly following trophic factor deprivation in PC12 cells. Bcl-2 and Akt inhibited this release. The protection, however, persisted longer in differentiated PC12 cells. In differentiated, but not undifferentiated cells, Bcl-2 and Akt also inhibited apoptosis downstream of holocytochrome c release. Thus, undifferentiated PC12 cells showed marked sensitivity to induction of apoptosis by microinjected cytochrome c even in the presence of NGF, Bcl-2, or Akt. In contrast, in differentiated cells these factors suppressed cell death. Consistent with these observations, in vitro processing of procaspase 9 in response to cytochrome c was observed in extracts from undifferentiated but not differentiated cells expressing Akt or Bcl-2. Endogenous caspase 9 was cleaved during cell death, whereas dominant negative caspase 9 inhibited cell death. The results from determining the role of inhibitors of apoptosis (IAPs) suggest that acquisition of inhibition by IAPs is part of the differentiation program. Ubiquitin-DeltaN-AVPI Smac/DIABLO induced cell death in differentiated cells only. c-IAP-2 is unregulated in differentiated cells, whereas X-linked IAP levels decreased in these cells coincident with cell death. Moreover, expressing X-linked IAP rendered undifferentiated cells resistant to microinjected cytochrome c. Overall, the inhibitory regulation, of cell death at the level of release of mitochondrial apoptogenic factors and at post-mitochondrial activation of caspase 9 observed in differentiated PC12 cells, is reduced or absent in the undifferentiated counterparts.

Animals↗

PubMed2003

c-Myc recruits P-TEFb for transcription, cellular proliferation and apoptosis.

c-Myc promotes cellular proliferation, sensitizes cells to apoptosis and prevents differentiation. It binds cyclin T1 structurally and functionally from the positive transcription elongation factor b (P-TEFb). The cyclin-dependent kinase 9 (Cdk9) in P-TEFb then phosporylates the C-terminal domain of RNA polymerase II, which is required for the transition from initiation to elongation of eukaryotic transcription. Inhibiting P-TEFb blocks the transcription of its target genes as well as cellular proliferation and apoptosis induced by c-Myc.

Animals↗

PubMed2003

Minimal BH3 peptides promote cell death by antagonizing anti-apoptotic proteins.

The pro-apoptotic "BH3 domain-only" proteins of the Bcl-2 family (e.g. Bid and Bad) transduce multiple death signals to the mitochondrion. They interact with the anti-apoptotic Bcl-2 family members and induce apoptosis by a mechanism that requires the presence of at least one of the multidomain pro-apoptotic proteins Bax or Bak. Although the BH3 domain of Bid can promote the pro-apoptotic assembly and function of Bax/Bak by itself, other BH3 domains do not function as such. The latter point raises the question of whether, and how, these BH3 domains induce apoptosis. We show here that a peptide comprising the minimal BH3 domain from Bax induces apoptosis but is unable to stimulate the apoptotic activity of microinjected recombinant Bax. This relies on the inability of the peptide to directly induce Bax translocation to mitochondria or a change in its conformation. This peptide nevertheless interferes with Bax/Bcl-xL interactions in vitro and stimulates the apoptotic activity of Bax when combined with Bcl-xL. Similarly, a peptide derived from the BH3 domain of Bad stimulates Bax activity only in the presence of Bcl-xL. Thus, BH3 domains do not necessarily activate multidomain pro-apoptotic proteins directly but promote apoptosis by releasing active multidomain pro-apoptotic proteins from their anti-apoptotic counterparts.

Amino Acid Sequence↗

PubMed2003

Oxidised lipoproteins may promote inflammation through the selective delay of engulfment but not binding of apoptotic cells by macrophages.

During normal tissue homeostasis apoptotic cells (AC) are rapidly recognised and engulfed by neighbouring cells or macrophages (Mphi), thus preventing an inflammatory response. Conversely, in chronically inflamed tissues, including the atherosclerotic artery and rheumatoid joint, removal of AC is defective despite the co-localisation of seemingly adequate numbers of Mphi. Mechanisms preventing removal of AC in vivo remain obscure, but might include oxidised low-density lipoprotein (ox-LDL), which is abundant in chronic inflammatory lesions. Although implicated in their pathogenesis, defining the role of ox-LDL on inflammatory processes has proved complicated. In fact, seemingly contradictory results have previously been described, though these may in part reflect the heterogeneous nature of ox-LDL applied in these studies. We wished to investigate the effect of physiologically representative ox-LDL on the binding and engulfment of apoptotic vascular smooth muscle cells (VSMC) and fibroblasts, as these have previously been shown to co-localise with Mphi in chronically inflamed tissues in vivo. We show that Mphi recognition of AC in vitro is not affected at physiological levels of ox-LDL. However, engulfment of intact AC is dramatically reduced/delayed. Importantly, in the absence of ox-LDL rapid phagocytosis of intact AC suppresses Mphi inflammatory cytokine release. In striking contrast, in the presence of ox-LDL, despite binding of AC to Mphi, release of IL-6 and MCP-1 is no longer suppressed. We propose that ox-LDL could maintain an inflammatory response by inhibiting the engulfment of AC, required for Mphi de-activation. This mechanism may contribute to chronic persisting inflammation in the atherosclerotic artery and rheumatoid joint.

Apoptosis↗

PubMed2002

c-Myc augments the apoptotic activity of cytosolic death receptor signaling proteins by engaging the mitochondrial apoptotic pathway.

Activation of c-Myc sensitizes cells to apoptosis induction by ligand-activated death receptors. Such sensitization to death receptors by oncogenes may well be the mechanism underlying tumor cell sensitivity to tumor necrosis factor (TNF) or TNF-related apoptosis-inducing ligand (TRAIL). The mechanism by which this c-Myc-induced sensitization occurs is unclear but could involve modulation of expression of death receptors or their ligands or potentiation of the sensitivity of mitochondria to release pro-apoptotic effectors such as holocytochrome c. Here, we show that ectopic expression of the death receptor signaling protein RIP (receptor-interactive protein) triggers apoptosis via a FAS-associated death domain protein (FADD) and caspase 8-dependent pathway. Induction of apoptosis by this intracellular activation of the death receptor signaling pathway is significantly augmented by c-Myc expression. Moreover, c-Myc expression strongly promotes the potential of RIP to induce cytochrome c release from mitochondria. This implicates the mitochondrial apoptotic pathway in this synergy, a notion confirmed by the inability of c-Myc to sensitize to RIP killing in cells lacking the obligate mitochondrial apoptotic effectors Bax and Bak. We conclude that the lethality of the RIP-activated cytosolic caspase 8 pathway is augmented by c-Myc priming mitochondria to release cytochrome c. This places the intersection of apoptotic synergy between c-Myc and death receptor signaling downstream of the death receptors.

Adaptor Proteins, Signal Transducing↗

PubMed2002

Wnt signaling promotes oncogenic transformation by inhibiting c-Myc-induced apoptosis.

Aberrant activation of the Wnt/beta-catenin signaling pathway is associated with numerous human cancers and often correlates with the overexpression or amplification of the c-myc oncogene. Paradoxical to the cellular transformation potential of c-Myc is its ability to also induce apoptosis. Using an inducible c-MycER expression system, we found that Wnt/beta-catenin signaling suppressed apoptosis by inhibiting c-Myc-induced release of cytochrome c and caspase activation. Both cyclooxygenase 2 and WISP-1 were identified as effectors of the Wnt-mediated antiapoptotic signal. Soft agar assays showed that neither c-Myc nor Wnt-1 alone was sufficient to induce cellular transformation, but that Wnt and c-Myc coordinated in inducing transformation. Furthermore, coexpression of Wnt-1 and c-Myc induced high-frequency and rapid tumor growth in nude mice. Extensive apoptotic bodies were characteristic of c-Myc-induced tumors, but not tumors induced by coactivation of c-Myc and Wnt-1, indicating that the antiapoptotic function of Wnt-1 plays a critical role in the synergetic action between c-Myc and Wnt-1. These results elucidate the molecular mechanisms by which Wnt/beta-catenin inhibits apoptosis and provide new insight into Wnt signaling-mediated oncogenesis.

Animals↗

PubMed2002

Myc-Is this the oncogene from Hell?

A new paper implicates the Myc oncoprotein in the direct induction of DNA damage and consequent genome instability in cultured cells. However, it is less clear whether Myc induces the same genetic pandemonium in vivo.

DNA Damage↗

PubMed2002

c-MYC: more than just a matter of life and death.

Deregulated expression of c-MYC occurs in a broad range of human cancers and is often associated with poor prognosis, indicating a key role for this oncogene in tumour progression. However, as established human tumours often bear multiple genetic lesions, it is difficult to determine whether c-MYC is instrumental in the initiation/progression of the tumour, or indeed whether inactivating c-MYC would lead to tumour regression. Regulatable transgenic mouse models of oncogenesis have shed light on these issues and provide hope for effective cancer therapies.

Animals↗

PubMed2002

c-Myc functionally cooperates with Bax to induce apoptosis.

c-Myc promotes apoptosis by destabilizing mitochondrial integrity, leading to the release of proapoptotic effectors including holocytochrome c. Candidate mediators of c-Myc in this process are the proapoptotic members of the Bcl-2 family. We show here that fibroblasts lacking Bak remain susceptible to c-Myc-induced apoptosis whereas bax-deficient fibroblasts are resistant. However, despite this requirement for Bax, c-Myc activation exerts no detectable effects on Bax expression, localization, or conformation. Moreover, susceptibility to c-Myc-induced apoptosis can be restored in bax-deficient cells by ectopic expression of Bax or by microinjection of a peptide comprising a minimal BH3 domain. Microinjection of BH3 peptide also restores sensitivity to c-Myc-induced apoptosis in p53-deficient primary fibroblasts that are otherwise resistant. By contrast, there is no synergy between BH3 peptide and c-Myc in fibroblasts deficient in both Bax and Bak. We conclude that c-Myc triggers a proapoptotic mitochondrial destabilizing activity that cooperates with proapoptotic members of the Bcl-2 family.

Amino Acid Sequence↗

PubMed1994

Effects of Kelatorphan and morphine before and after noxious stimulation on immediate-early gene expression in rat spinal cord neurons.

Expression of the immediate-early genes (IEG) c-FOS, NGF1-A and c-JUN was induced by noxious thermal stimulation in neurons of the rat spinal cord dorsal horn. Intravenous injection of Kelatorphan (5, 10 and 20 mg/kg), an inhibitor of multiple enkephalin-degrading enzymes, 20 min before noxious stimulation reduced the overall number of dorsal horn neurons expressing c-FOS and NGF1-A by up to 20-30%. While c-FOS expression was suppressed in superficial and deep laminae of the spinal cord, NGF1-A and c-JUN was only suppressed in superficial laminae. Morphine (5, 7.5 and 10 mg/kg) produced a dose-dependent reduction of c-FOS expression by up to 70% only when injected before noxious stimulation. Morphine injected 10 min after the noxious treatment was virtually ineffective. The depressant effect of Kelatorphan and morphine could be prevented by prior application of the opioid antagonist naloxone. Naloxone itself slightly increased the overall number of c-FOS-positive neurons in all laminae of the spinal cord. The present data support the existence of a tonic release of endogenous opioid peptides at the spinal level and show that inhibition of their peptidase-induced degradation modulates IEG expression in dorsal horn neurons of the rat. The finding that opioid agonists were ineffective when applied after stimulation underline the necessity of pre-emptive analgesia to prevent long-term activity-dependent changes in spinal cord neurons.

Analgesics↗