c-Myc-induced sensitization to apoptosis is mediated through cytochrome c release

Apoptosis in cancer

In the last decade, basic cancer research has produced remarkable advances in our understanding of cancer biology and cancer genetics. Among the most important of these advances is the realization that apoptosis and the genes that control it have a profound effect on the malignant phenotype. For example, it is now clear that some oncogenic mutations disrupt apoptosis, leading to tumor initiation, progression or metastasis. Conversely, compelling evidence indicates that other oncogenic changes promote apoptosis, thereby producing selective pressure to override apoptosis during multistage carcinogenesis. Finally, it is now well documented that most cytotoxic anticancer agents induce apoptosis, raising the intriguing possibility that defects in apoptotic programs contribute to treatment failure. Because the same mutations that suppress apoptosis during tumor development also reduce treatment sensitivity, apoptosis provides a conceptual framework to link cancer genetics with cancer therapy. An intense research effort is uncovering the underlying mechanisms of apoptosis such that, in the next decade, one envisions that this information will produce new strategies to exploit apoptosis for therapeutic benefit.

Sensitivity to myc-induced apoptosis is retained in spontaneous and transplanted lymphomas of CD2-mycER mice

To study the effects of the Myc oncoprotein in a regulatable in vivo system, we generated lines of transgenic mice in which a tamoxifen inducible Myc fusion protein (c-mycER) is expressed under the control of the CD2 locus control region. Activation of the Myc oncoprotein resulted in both proliferation and apoptosis in vivo. Lines with a high transgene copy number developed spontaneous lymphomas at low frequency, but the tumour incidence was significantly increased with tamoxifen treatment. Surprisingly, we found that cellular sensitivity to Myc-induced apoptosis was retained in tumours from these mice and in most lymphoma cell lines, even when null for p53. Resistance to Myc-induced apoptosis could be conferred on these cells by co-expression of Bcl-2. However, acquired resistance is clearly not an obligatory progression event as sensitivity to apoptosis was retained in transplanted tumours in athymic mice. In conclusion, lymphomas arising in CD2-mycER mice retain the capacity to undergo apoptosis in response to Myc activation and show no phenotypic evidence of the presence of an active dominant inhibitor.

Tumor suppressor genes

Although tumor suppressor genes continue to be discovered, the most recent advances have been made in attributing new and exciting functions to existing ones - such as the apparent role of VHL as a regulator of proteolysis. Great insights have also come from piecing genes together into pathways and networks. For instance the discovery that cyclin D1 is regulated by beta-catenin/Tcf-4 allows us to tie the APC pathway to the RB pathway and cell cycle control. Similarly, tumor suppressor genes have been fitted together with oncogenes into the various pathways that regulate apoptosis such that tumor suppressor function is now attributed to some of the basic components of the apoptotic machinery, such as caspases and Apaf-1. The great pace at which mouse models of tumorigenesis continue to advance our knowledge of tumor suppressor gene function has led us to look anew at the role of genes such as TCF-1 and SMAD-3 in human cancer. Finally, the realisation that different growth regulatory pathways give rise to generic signals suggests that future work may lie in integrating the signals from different pathways and in understanding the importance of protein levels to cellular function.

Action of Myc in vivo - proliferation and apoptosis

The protein products of many dominant oncogenes are capable of inducing both cell proliferation and apoptosis. Recent experiments employing transgenic mice that express an ectopically regulatable myc gene or protein have begun to elucidate the role of the balance between proliferation and apoptosis in Myc-induced carcinogenesis. An outstanding feature of these experiments is the demonstration that the balance between oncogene-induced proliferation and apoptosis in a given tissue can be a critical determinant in the initiation and maintenance of the tumor.

c-Myc protein synthesis is initiated from the internal ribosome entry segment during apoptosis

Recent studies have shown that during apoptosis protein synthesis is inhibited and that this is in part due to the proteolytic cleavage of eukaryotic initiation factor 4G (eIF4G). Initiation of translation can occur either by a cap-dependent mechanism or by internal ribosome entry. The latter mechanism is dependent on a complex structural element located in the 5' untranslated region of the mRNA which is termed an internal ribosome entry segment (IRES). In general, IRES-mediated translation does not require eIF4E or full-length eIF4G. In order to investigate whether cap-dependent and cap-independent translation are reduced during apoptosis, we examined the expression of c-Myc during this process, since we have shown previously that the 5' untranslated region of the c-myc proto-oncogene contains an IRES. c-Myc expression was determined in HeLa cells during apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand. We have demonstrated that the c-Myc protein is still expressed when more than 90% of the cells are apoptotic. The presence of the protein in apoptotic cells does not result from either an increase in protein stability or an increase in expression of c-myc mRNA. Furthermore, we show that during apoptosis initiation of c-myc translation occurs by internal ribosome entry. We have investigated the signaling pathways that are involved in this response, and cotransfection with plasmids which harbor either wild-type or constitutively active MKK6, a specific immediate upstream activator of p38 mitogen-activated protein kinase (MAPK), increases IRES-mediated translation. In addition, the c-myc IRES is inhibited by SB203580, a specific inhibitor of p38 MAPK. Our data, therefore, strongly suggest that the initiation of translation via the c-myc IRES during apoptosis is mediated by the p38 MAPK pathway.

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.

Apoptosis in human disease: a new skin for the old ceremony?

Naturally occurring cell death or apoptosis is essential for the maintenance of tissue homeostasis and serves to remove extraneous or dangerous cells in a swift and unobtrusive manner. Recent studies have indicated a role for apoptosis in a plethora of human diseases. Hence, dysregulation of apoptosis has been implicated in autoimmune disease, acquired immune deficiency syndrome, and other viral (and bacterial) infections, as well as in neurodegenerative disorders and cancer. Furthermore, dysregulated apoptosis signaling may impinge on other age-related disorders such as osteoporosis and atherosclerosis and perhaps on the process of aging itself. The present review provides an overview of human diseases, which are associated with defective or inadvertent apoptosis, with examples of pathological conditions in which putative apoptosis defects have been elucidated at the molecular level. Novel apoptosis-modulating therapeutic strategies are also discussed.

Caspase-9 and APAF-1 form an active holoenzyme

Autocatalytic activation of initiator caspases is the link between pro-apoptotic signals and the destruction machinery of apoptosis. Activation of caspase-9, which mediates oncogene and drug-induced apoptosis, requires binding to the protein APAF-1. We found that the proteolytic activity of caspase-9 in a complex with APAF-1 is several orders of magnitude higher than that of the free enzyme. Thus, this complex functions as a holoenzyme in which caspase-9 is the catalytic subunit and APAF-1 its allosteric regulator. We argue that caspase-9 is activated by allosteric regulation and suggest that this mechanism is common for other initiator caspases.

Cell proliferation and apoptosis

Cell proliferation and cell death are essential yet opposing cellular processes. Crosstalk between these processes promotes a balance between proliferation and death, and it limits the growth and survival of cells with oncogenic mutations. New insights into the mechanisms by which strong signals to proliferate and activation of cyclin-dependent kinases promote apoptosis have recently been published, and a novel cell cycle regulated caspase inhibitor, Survivin, has been described.

The pro-apoptotic proteins, Bid and Bax, cause a limited permeabilization of the mitochondrial outer membrane that is enhanced by cytosol

During apoptosis, an important pathway leading to caspase activation involves the release of cytochrome c from the intermembrane space of mitochondria. Using a cell-free system based on Xenopus egg extracts, we examined changes in the outer mitochondrial membrane accompanying cytochrome c efflux. The pro-apoptotic proteins, Bid and Bax, as well as factors present in Xenopus egg cytosol, each induced cytochrome c release when incubated with isolated mitochondria. These factors caused a permeabilization of the outer membrane that allowed the corelease of multiple intermembrane space proteins: cytochrome c, adenylate kinase and sulfite oxidase. The efflux process is thus nonspecific. None of the cytochrome c-releasing factors caused detectable mitochondrial swelling, arguing that matrix swelling is not required for outer membrane permeability in this system. Bid and Bax caused complete release of cytochrome c but only a limited permeabilization of the outer membrane, as measured by the accessibility of inner membrane-associated respiratory complexes III and IV to exogenously added cytochrome c. However, outer membrane permeability was strikingly increased by a macromolecular cytosolic factor, termed PEF (permeability enhancing factor). We hypothesize that PEF activity could help determine whether cells can recover from mitochondrial cytochrome c release.

Disruption of the ARF-Mdm2-p53 tumor suppressor pathway in Myc-induced lymphomagenesis

Transgenic mice expressing the c-Myc oncogene driven by the immunoglobulin heavy chain enhancer (Emu) develop B-cell lymphoma and exhibit a mean survival time of approximately 6 months. The protracted latent period before the onset of frank disease likely reflects the ability of c-Myc to induce a p53-dependent apoptotic program that initially protects animals against tumor formation but is disabled when overtly malignant cells emerge. In cultured primary mouse embryo fibroblasts, c-Myc activates the p19(ARF)-Mdm2-p53 tumor suppressor pathway, enhancing p53-dependent apoptosis but ultimately selecting for surviving immortalized cells that have sustained either p53 mutation or biallelic ARF deletion. Here we report that p53 and ARF also potentiate Myc-induced apoptosis in primary pre-B-cell cultures, and that spontaneous inactivation of the ARF-Mdm2-p53 pathway occurs frequently in tumors arising in Emu-myc transgenic mice. Many Emu-myc lymphomas sustained either p53 (28%) or ARF (24%) loss of function, whereas Mdm2 levels were elevated in others. Its overexpression in some tumors lacking p53 function raises the possibility that Mdm2 can contribute to lymphomagenesis by interacting with other targets. Emu-myc transgenic mice hemizygous for ARF displayed accelerated disease (11-week mean survival), and 80% of these tumors lost the wild-type ARF allele. All ARF-null Emu-myc mice died of lymphoma within a few weeks of birth. About half of the tumors arising in ARF hemizygous or ARF nullizygous Emu-myc transgenic mice also overexpressed Mdm2. Therefore, Myc activation strongly selects for spontaneous inactivation of the ARF-Mdm2-p53 pathway in vivo, cancelling its protective checkpoint function and accelerating progression to malignancy.

Disruption of the ARF-Mdm2-p53 tumor suppressor pathway in Myc-induced lymphomagenesis

Transgenic mice expressing the c-Myc oncogene driven by the immunoglobulin heavy chain enhancer (Emu) develop B-cell lymphoma and exhibit a mean survival time of approximately 6 months. The protracted latent period before the onset of frank disease likely reflects the ability of c-Myc to induce a p53-dependent apoptotic program that initially protects animals against tumor formation but is disabled when overtly malignant cells emerge. In cultured primary mouse embryo fibroblasts, c-Myc activates the p19(ARF)-Mdm2-p53 tumor suppressor pathway, enhancing p53-dependent apoptosis but ultimately selecting for surviving immortalized cells that have sustained either p53 mutation or biallelic ARF deletion. Here we report that p53 and ARF also potentiate Myc-induced apoptosis in primary pre-B-cell cultures, and that spontaneous inactivation of the ARF-Mdm2-p53 pathway occurs frequently in tumors arising in Emu-myc transgenic mice. Many Emu-myc lymphomas sustained either p53 (28%) or ARF (24%) loss of function, whereas Mdm2 levels were elevated in others. Its overexpression in some tumors lacking p53 function raises the possibility that Mdm2 can contribute to lymphomagenesis by interacting with other targets. Emu-myc transgenic mice hemizygous for ARF displayed accelerated disease (11-week mean survival), and 80% of these tumors lost the wild-type ARF allele. All ARF-null Emu-myc mice died of lymphoma within a few weeks of birth. About half of the tumors arising in ARF hemizygous or ARF nullizygous Emu-myc transgenic mice also overexpressed Mdm2. Therefore, Myc activation strongly selects for spontaneous inactivation of the ARF-Mdm2-p53 pathway in vivo, cancelling its protective checkpoint function and accelerating progression to malignancy.