Oncogene-dependent tumor suppression: using the dark side of the force for cancer therapy

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Role of ARK5 in cancer and other diseases (Review)

Malignant tumors are often exposed to hypoxic and glucose-starved microenvironments. AMP-activated protein kinase (AMPK) is an energy sensor that is stimulated during energy-deficient conditions and protects cells from hypoxic injury by regulating metabolism. AMPK-related protein kinase 5 (ARK5) is a member of the catalytic sub-unit of the AMPK family and has an important role in energy regulation and hypoxia. ARK5 is regulated by Akt and liver kinase B1 and is associated with numerous tumor-related molecules to exert the negative effects of tumors. Studies have revealed ARK5 overexpression in cases of tumor invasion and metastasis and a positive association with the degree of cancer cell malignancy, which is regarded as a key element in determining cancer prognosis. Furthermore, ARK5 downregulation improves drug sensitivity through the epithelial-mesenchymal transition pathway, indicating that it may be a potential therapeutic target. In other non-cancer conditions, ARK5 has various roles in neurodegenerative diseases (Alzheimer's and Huntington's disease), renal disorders (diabetic nephropathy and renal fibrosis) and physiological processes (striated muscle generation). In the present review, the upstream and downstream molecular pathways of ARK5 in cancer and other diseases are described and potential therapeutic strategies are discussed.

Calcium signalling links MYC to NUAK1

NUAK1 is a member of the AMPK-related family of kinases. Recent evidence suggests that NUAK1 is an important regulator of cell adhesion and migration, cellular and organismal metabolism, and regulation of TAU stability. As such, NUAK1 may play key roles in multiple diseases ranging from neurodegeneration to diabetes and metastatic cancer. Previous work revealed a crucial role for NUAK1 in supporting viability of tumour cells specifically when MYC is overexpressed. This role is surprising, given that NUAK1 is activated by the tumour suppressor LKB1. Here we show that, in tumour cells lacking LKB1, NUAK1 activity is maintained by an alternative pathway involving calcium-dependent activation of PKCα. Calcium/PKCα-dependent activation of NUAK1 supports engagement of the AMPK-TORC1 metabolic checkpoint, thereby protecting tumour cells from MYC-driven cell death, and indeed, MYC selects for this pathway in part via transcriptional regulation of PKCα and ITPR. Our data point to a novel role for calcium in supporting tumour cell viability and clarify the synthetic lethal interaction between NUAK1 and MYC.

BIM is the primary mediator of MYC-induced apoptosis in multiple solid tissues

MYC is one of the most frequently overexpressed oncogenes in human cancer, and even modestly deregulated MYC can initiate ectopic proliferation in many postmitotic cell types in vivo. Sensitization of cells to apoptosis limits MYC's oncogenic potential. However, the mechanism through which MYC induces apoptosis is controversial. Some studies implicate p19ARF-mediated stabilization of p53, followed by induction of proapoptotic BH3 proteins NOXA and PUMA, whereas others argue for direct regulation of BH3 proteins, especially BIM. Here, we use a single experimental system to systematically evaluate the roles of p19ARF and BIM during MYC-induced apoptosis, in vitro, in vivo, and in combination with a widely used chemotherapeutic, doxorubicin. We find a common specific requirement for BIM during MYC-induced apoptosis in multiple settings, which does not extend to the p53-responsive BH3 family member PUMA, and find no evidence of a role for p19ARF during MYC-induced apoptosis in the tissues examined.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

Genetically engineered murine models--contribution to our understanding of the genetics, molecular pathology and therapeutic targeting of neuroblastoma

Genetically engineered mouse models (GEMM) have made major contributions to a molecular understanding of several adult cancers and these results are increasingly being translated into the pre-clinical setting where GEMM will very likely make a major impact on the development of targeted therapeutics in the near future. The relationship of pediatric cancers to altered developmental programs, and their genetic simplicity relative to adult cancers provides unique opportunities for the application of new advances in GEMM technology. In neuroblastoma the well-characterized TH-MYCN GEMM is increasingly used for a variety of molecular-genetic, developmental and pre-clinical therapeutics applications. We discuss: the present and historical application of GEMM to neuroblastoma research, future opportunities, and relevant targets suitable for new GEMM strategies in neuroblastoma. We review the potential of these models to contribute both to an understanding of the developmental nature of neuroblastoma and to improved therapy for this disease.

Apoptosis-stimulating protein of p53 (ASPP2) heterozygous mice are tumor-prone and have attenuated cellular damage-response thresholds

The expression of ASPP2 (53BP2L), a proapoptotic member of a family of p53-binding proteins, is frequently suppressed in many human cancers. Accumulating evidence suggests that ASPP2 inhibits tumor growth; however, the mechanisms by which ASPP2 suppresses tumor formation remain to be clarified. To study this, we targeted the ASPP2 allele in a mouse by replacing exons 10-17 with a neoR gene. ASPP2(-/-) mice were not viable because of an early embryonic lethal event. Although ASPP2(+/-) mice appeared developmentally normal, they displayed an increased incidence of a variety of spontaneous tumors as they aged. Moreover, gamma-irradiated 6-week-old ASPP2(+/-) mice developed an increased incidence of high-grade T cell lymphomas of thymic origin compared with ASPP2(+/+) mice. Primary thymocytes derived from ASPP2(+/-) mice exhibited an attenuated apoptotic response to gamma-irradiation compared with ASPP2(+/+) thymocytes. Additionally, ASPP2(+/-) primary mouse embryonic fibroblasts demonstrated a defective G(0)/G(1) cell cycle checkpoint after gamma-irradiation. Our results demonstrate that ASPP2 is a haploinsufficient tumor suppressor and, importantly, open new avenues for investigation into the mechanisms by which disruption of ASPP2 pathways could play a role in tumorigenesis and response to therapy.

Characterization of the p53 response to oncogene-induced senescence

BACKGROUND

P53 activation can trigger various outcomes, among them reversible growth arrest or cellular senescence. It is a live debate whether these outcomes are influenced by quantitative or qualitative mechanisms. Furthermore, the relative contribution of p53 to Ras-induced senescence is also matter of controversy.

METHODOLOGY/PRINCIPAL FINDINGS

This study compared situations in which different signals drove senescence with increasing levels of p53 activation. The study revealed that the levels of p53 activation do not determine the outcome of the response. This is further confirmed by the clustering of transcriptional patterns into two broad groups: p53-activated or p53-inactivated, i.e., growth and cellular arrest/senescence. Furthermore, while p53-dependent transcription decreases after 24 hrs in the presence of active p53, senescence continues. Maintaining cells in the arrested state for long periods does not switch reversible arrest to cellular senescence. Together, these data suggest that a Ras-dependent, p53-independent, second signal is necessary to induce senescence. This study tested whether PPP1CA (the catalytic subunit of PP1alpha), recently identified as contributing to Ras-induced senescence, might be this second signal. PPP1CA is induced by Ras; its inactivation inhibits Ras-induced senescence, presumably by inhibiting pRb dephosphorylation. Finally, PPP1CA seems to strongly co-localize with pRb only during senescence.

CONCLUSIONS

The levels of p53 activation do not determine the outcome of the response. Rather, p53 activity seems to act as a necessary but not sufficient condition for senescence to arise. Maintaining cells in the arrested state for long periods does not switch reversible arrest to cellular senescence. PPP1CA is induced by Ras; its inactivation inhibits Ras-induced senescence, presumably by inhibiting pRb dephosphorylation. Finally, PPP1CA seems to strongly co-localize with pRb only during senescence, suggesting that PP1alpha activation during senescence may be the second signal contributing to the irreversibility of the senescent phenotype.

c-myc in the hematopoietic lineage is crucial for its angiogenic function in the mouse embryo

The c-myc proto-oncogene, which is crucial for the progression of many human cancers, has been implicated in key cellular processes in diverse cell types, including endothelial cells that line the blood vessels and are critical for angiogenesis. The de novo differentiation of endothelial cells is known as vasculogenesis, whereas the growth of new blood vessels from pre-existing vessels is known as angiogenesis. To ascertain the function of c-myc in vascular development, we deleted c-myc in selected cell lineages. Embryos lacking c-myc in endothelial and hematopoietic lineages phenocopied those lacking c-myc in the entire embryo proper. At embryonic day (E) 10.5, both mutant embryos were grossly normal, had initiated primitive hematopoiesis, and both survived until E11.5-12.5, longer than the complete null. However, they progressively developed defective hematopoiesis and angiogenesis. The majority of embryos lacking c-myc specifically in hematopoietic cells phenocopied those lacking c-myc in endothelial and hematopoietic lineages, with impaired definitive hematopoiesis as well as angiogenic remodeling. c-myc is required for embryonic hematopoietic stem cell differentiation, through a cell-autonomous mechanism. Surprisingly, c-myc is not required for vasculogenesis in the embryo. c-myc deletion in endothelial cells does not abrogate endothelial proliferation, survival, migration or capillary formation. Embryos lacking c-myc in a majority of endothelial cells can survive beyond E12.5. Our findings reveal that hematopoiesis is a major function of c-myc in embryos and support the notion that c-myc functions in selected cell lineages rather than in a ubiquitous manner in mammalian development.

Placental rescue reveals a sole requirement for c-Myc in embryonic erythroblast survival and hematopoietic stem cell function

The c-Myc protein has been implicated in playing a pivotal role in regulating the expression of a large number of genes involved in many aspects of cellular function. Consistent with this view, embryos lacking the c-myc gene exhibit severe developmental defects and die before midgestation. Here, we show that Sox2Cre-mediated deletion of the conditional c-myc(flox) allele specifically in the epiblast (hence trophoectoderm and primitive endoderm structures are wild type) rescues the majority of developmental abnormalities previously characterized in c-myc knockout embryos, indicating that they are secondary defects and arise as a result of placental insufficiency. Epiblast-restricted c-Myc-null embryos appear morphologically normal and do not exhibit any obvious proliferation defects. Nonetheless, these embryos are severely anemic and die before E12. c-Myc-deficient embryos exhibit fetal liver hypoplasia, apoptosis of erythrocyte precursors and functionally defective definitive hematopoietic stem/progenitor cells. Specific deletion of c-myc(flox) in hemogenic or hepatocytic lineages validate the hematopoietic-specific requirement of c-Myc in the embryo proper and provide in vivo evidence to support a synergism between hematopoietic and liver development. Our results reveal for the first time that physiological levels of c-Myc are essential for cell survival and demonstrate that, in contrast to most other embryonic lineages, erythroblasts and hematopoietic stem/progenitor cells are particularly dependent on c-Myc function.

miRNAs: Little known mediators of oncogenesis

Cancer progression is mediated by overexpression of oncogenes and downregulation or loss of tumor suppressors. Proteins, which were traditionally categorized into these groups, have been recently joined by a species of RNA molecules known as microRNAs (miRNAs). miRNAs belong to a class of approximately 22-nt-long non-coding RNAs found in eukaryotes that hinder gene expression by inducing degradation or inhibiting translation of select mRNAs. A growing number of miRNAs have been implicated in promoting or suppressing tumorigenesis in a variety of tissues. The supporting evidence ranges from suggestive expression profiling data to direct functional validation using methods of forward and reverse genetics. We discuss the nature of published results, as well as the merits and pitfalls of various approaches aimed at identification of cancer-related miRNAs and their mRNA targets.

The BCL-2 protein family: opposing activities that mediate cell death

BCL-2 family proteins, which have either pro- or anti-apoptotic activities, have been studied intensively for the past decade owing to their importance in the regulation of apoptosis, tumorigenesis and cellular responses to anti-cancer therapy. They control the point of no return for clonogenic cell survival and thereby affect tumorigenesis and host-pathogen interactions and regulate animal development. Recent structural, phylogenetic and biological analyses, however, suggest the need for some reconsideration of the accepted organizational principles of the family and how the family members interact with one another during programmed cell death. Although these insights into interactions among BCL-2 family proteins reveal how these proteins are regulated, a unifying hypothesis for the mechanisms they use to activate caspases remains elusive.

Apoptosis inhibition in cancer cells: A novel molecular pathway that involves BAG3 protein

Stress-induced apoptosis regulates neoplasia pathogenesis and response to therapy. Indeed, cell transformation induces a stress response, that is overcome, in neoplastic cells, by alterations in apoptosis modulators; on the other hand, antineoplastic therapies largely trigger the apoptosis stress pathway, whose impairment results in resistance. Therefore, the study of the roles of apoptosis-modulating molecules in neoplasia development and response to therapy is of key relevance for our understanding of these processes. Among molecules that regulate apoptosis, a role is emerging for BAG3, a member of the BAG co-chaperone protein family. Proteins that share the BAG domain are characterized by their interaction with a variety of partners (heat shock proteins, steroid hormone receptors, Raf-1 and others), involved in regulating a number of cellular processes, including proliferation and apoptosis. BAG3, also known as CAIR-1 or Bis, forms a complex with the heat shock protein (Hsp) 70. This assists polypeptide folding, can mediate protein delivery to proteasome and is able to modulate apoptosis by interfering with cytochrome c release, apoptosome assembly and other events in the death process. It has been recently shown that, in human primary lymphoid and myeloblastic leukemias and other neoplastic cell types, BAG3 expression sustains cell survival and underlies resistance to therapy, through downmodulation of apoptosis. This review summarizes findings that assign an apoptotic role to BAG3 in some neoplastic cell types and identify the protein as a candidate target of therapy.