Apoptosis: biochemical aspects and clinical implications

Regulation of apoptosis proteins in cancer cells by ubiquitin

Ubiquitin inhibitors act at many levels to enhance apoptosis signaling. For TNF-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis signaling, there are at least five mechanisms by which apoptosis are regulated by the ubiquitin-proteasome pathway. First, proteasome inhibitors can decrease Fas-like inhibitor protein (FLIP) protein levels in tumors, resulting in increased apoptosis signaling due to increased caspase-8 activation. This appears to involve the ubiquitin ligase TNF receptor activation factor-2 (TRAF2) and acts indirectly by causing cell-cycle arrest at a stage where there is high degradation of the FLIP-TRAF2 complex. Second, the regulation of the proapoptotic Bcl-2 family member BAX occurs indirectly. Apoptosis signaling and caspase activation results in a confirmation change in the normally monomeric BAX, which exposes the BH3 domain of BAX, leading to dimerization and resistance to ubiquitin degradation. BAX then translocates into the mitochondria, resulting in the release of proapoptotic mitochondrial factors such as cytochrome c and second mitochondria-derived activator of caspase (SMAC). This results in the activation of caspase-9 and formation of the apoptosome and efficient apoptosis signaling. A third mechanism of the regulation of TRAIL signaling in the ubiquitin-proteasome pathway is mediated by the inhibitor of apoptosis proteins (IAP) E3 ligases. These IAPs can directly bind to caspases but also can act as ubiquitin ligases for caspases, resulting in the degradation of these caspases. IAP binding to caspases can be inhibited by SMAC, which exhibits a caspase-9 homology domain. The fourth mechanism for apoptosis activation by proteasome inhibitors is through the stabilization of the inhibitor of the kappaB (IkappaB)/NF-kappaB complex and prevention of nuclear translocation of the antiapoptosis transcription factor NF-kappaB. During TRAIL-DR4, DR5 signaling, this pathway is activated by interactions of activated Fas-associated death domain with activated receptor-interacting protein (RIP), which in turn activates NF-kappaB-inducing kinase and phosphorylates IkappaB. Therefore, the inhibition of IkappaB degradation blocks this RIP-mediated antiapoptosis signaling event. Last, p53 protein levels, and susceptibility to apoptosis, can be deregulated by the human homolog Hdm2 (Mdm2) E3 ligase. This process is inhibited by p53 phosphorylation and by sequestration of Mdm2 by ARF. Better mechanisms to inhibit the ubiquitin-proteasome pathway targeted at the ubiquitin-proteasome degradation process itself, or more specifically at the E3 ligases known to modulate and downregulate proapoptosis pathways will lead to the enhancement of TRAIL apoptosis signaling and better cancer therapeutic outcomes act through this pathway.

Molecular phenotype of airway side population cells

Lung epithelial-specific stem cells have been localized to discrete microenvironments throughout the adult conducting airway. Properties of these cells include pollutant resistance, multipotent differentiation, and infrequent proliferation. Goals of the present study were to use Hoechst 33342 efflux, a property of stem cells in other tissues, to purify and further characterize airway stem cells. Hoechst 33342 effluxing lung cells were identified as a verapamil-sensitive side population by flow cytometry. Lung side population cells were further subdivided on the basis of hematopoietic (CD45 positive) or nonhematopoietic (CD45 negative) origin. Nonhematopoietic side population cells were enriched for stem cell antigen-1 reactivity and expressed molecular markers specific to both airway and mesenchymal lineages. Analysis of the molecular phenotype of airway-derived side population cells indicates that they are similar to neuroepithelial body-associated variant Clara cells. Taken together, these data suggest that the nonhematopoietic side population isolated from lung is enriched for previously identified airway stem cells.

Poncirus trifoliata fruit induces apoptosis in human promyelocytic leukemia cells

BACKGROUND

Substances inducing apoptosis have shown efficacy in the treatment of cancers. Poncirus trifoliata (L.) Raf. (Rutaceae) fruits (PTF) has been used for the treatment of various cancers among Korean Oriental Medical doctors.

METHODS

PTF-induced cytotoxicity of human leukemia HL-60 cells was monitored by the MTT assay. The apoptosis was determined by (a) apoptotic morphology in microscopy; (b) DNA fragmentation in electrophoresis and FACS analysis; and (c) activation of caspase-3 and poly-ADP-ribose polymerase (PARP) cleavage assay.

RESULTS

The cytotoxic activity of PTF in HL-60 cells was increased in a concentration- and time-dependent manner. PTF caused the cell shrinkage, cell membrane blebbing, apoptotic body and DNA fragmentation. PTF-induced apoptosis is accompanied by the activation of caspase-3 and the specific proteolytic cleavage of PARP. However, PTF did not show cytotoxicity in normal peripheral blood mononuclear cells.

CONCLUSIONS

Our novel finding provides evidence that PTF could be a candidate as an anti-leukemic agent through apoptosis of cancer cells.

Molecular genetics of human cervical cancer: role of papillomavirus and the apoptotic cascade

Cervical cancer is rated the second most common malignant tumour globally, and is aetiologically linked to human papillomavirus (HPV) infection. Here the cellular pathology under consideration of stem/progenitor cell carcinogenesis is reviewed. Of the three causative molecular mechanisms of cervical cancer, two are associated with HPV: firstly, the effect of the viral oncogenes, E6 and E7; and secondly, integration of the viral DNA into chromosomal regions of tumour phenotype. The third process involved is the repetitive loss of heterozygosity in some chromosomal regions. HPV can be classified into high- and low-risk types; the high-risk types encode two oncoproteins, E6 and E7, which interact with tumour suppressor proteins. The association results in the inactivation of tumour suppressor proteins and the abrogation of apoptosis. Apoptosis is referred to as programmed cell death, whereby a cell deliberately commits suicide, and thus regulates cell numbers during development and maintenance of cellular homeostasis. This review attempts to elucidate the role of apoptotic genes, and considers external factors that interact with HPV in the development and progression of cervical cancer. Therefore, an in-depth understanding of the apoptotic genes that control molecular mechanisms in cervical cancer are of critical importance. Useful targets for therapeutic strategies would be those that alter apoptotic pathways in a manner where the escape of HPV from surveillance by the host immune system is prevented. Such an approach directed at the apoptotic genes maybe useful in the treatment of cervical cancer.

Generation of an oxidative stress precedes caspase activation during 7?-hydroxycholesterol-induced apoptosis in U937 cells

The oxysterol 7beta-hydroxycholesterol (7beta-OH) has been shown to induce apoptosis in a number of cell lines. Though not fully elucidated, the mechanism through which this oxysterol induces cell death is thought to involve the generation of an oxidative stress leading to perturbation of the mitochondrion and release of cytochrome c into the cytosol. Cytochrome c together with Apaf-1 causes activation of the initiator caspase, caspase-9, which in turn activates caspase-3 ultimately leading to the degradation of poly(ADP-ribose) polymerase (PARP). The objective of the present study was to investigate the signalling pathway in 7beta-OH-induced apoptosis in U937 cells, a human monocytic blood cell line known to undergo apoptosis upon treatment with 7beta-OH, over a time course of 48 h. Apoptosis was evident after 24 h incubation. Glutathione levels were decreased after 6 h and this was coupled with an increase in SOD activity. Through western blot analysis we examined expression of caspase-3, -8, and -9 and cleavage of the caspase-3 substrate PARP. The sequence proceeded with activation of caspase-9 after 9 h, caspase-3 at the 12 h timepoint, and cleavage of PARP after 24 h treatment with 7beta-OH. Caspase-8 did not appear to play a major role in this particular apoptotic pathway.

Celecoxib derivatives induce apoptosisvia the disruption of mitochondrial membrane potential and activation of caspase 9

Celecoxib is a potent nonsteroid antiinflammatory drug (NSAID) that has shown great promise in cancer chemoprevention and treatment. The tumor suppression activity of celecoxib and other NSAIDs have been related to the induction of apoptosis in many cancer cell lines and animal models. While celecoxib is a specific inhibitor of cyclooxygenase (COX)-2, recent data indicate that its apoptotic properties may also be mediated through COX-independent pathways. In our study, we evaluated second generation celecoxib derivatives, lacking COX-2 inhibitory activity, in a premalignant and malignant human oral cell culture model to determine their potential anticancer effect and mechanisms responsible for the COX-independent apoptotic activity. Celecoxib and its derivatives delayed the progression of cells through the G(2)/M phase and induced apoptosis. The derivatives with apolar substituents at the terminal phenyl moiety of celecoxib greatly enhanced apoptosis and cell cycle delay. Apoptosis and cell cycle arrest appeared to be independent of derivative induced inhibition of PDK1 and phosphorylation of Akt and Erk1/2. Derivatives induced apoptosis was mediated by the cleavage and activation of caspase-9 and caspase-3, but not caspase 8, implicating the mitochondrial pathway for apoptosis induction. Inhibitors of caspase-3 and caspase-9 and cyclosporin A, a mitochondrial membrane potential stabilizer, attenuated derivative induced apoptosis. Inhibition of caspase-3 prevented the activation of caspase 8, while the inhibition of caspase-9 inhibitor blocked activation of both caspase 3 and 8 by the derivatives. Apoptosis was independent of Bcl-2. These results indicate that the second generation celecoxib derivatives induce apoptosis in human oral cancer lines by the disruption of mitochondrial membrane potential activating caspase 9 and downstream caspase 3 and 8. This suggests that the modification of the celecoxib structure can lead to highly effective COX-independent growth inhibitory and apoptotic agents in chemoprevention and therapy.

Molecular imaging of proteolytic activity in cancer

The early detection of both primary tumors and metastatic disease continue to be significant challenges in the diagnosis and staging of cancer. The growing recognition of the role of proteinases and proteolytic cascades in both the growth and metastasis of tumors has led to the development not only of therapeutic strategies using proteinase inhibitors, but also of methods to detect and image tumors in vivo via tumor-associated proteolytic activities. These imaging strategies derive from the enhanced sensitivity afforded by amplification that can be obtained by enzymatic processing to increase the efficacy of imaging "contrast agents" coupled with the inherent substrate specificity and selectivity of proteinases. This review describes key proteinases important in cancer progression, the strategies that have been devised to detect and image proteolytic activity in vivo, and the potential for this kind of functional imaging to serve as a marker for targeted therapy. The intent is to draw attention to the developing methods of molecular imaging to facilitate not only cancer diagnosis, but also for devising strategies for individualized targeted therapy and non-invasive monitoring of therapeutic efficacy.

Hoechst 33342 alters luciferase gene expression in transfected BC3H-1 myocytes

BACKGROUND

Hoechst 33342 and Hoechst 33258 bind to the minor groove of DNA. Hoechst 33342 induces apoptosis in a variety of cell types by a mechanism that is associated with disruption of the formation of the TATA box-binding protein/DNA complex.

OBJECTIVE

To further investigate the role of Hoechst 33342 in gene regulation using BC3H-1 myocytes transfected with 4 different pGL3 luciferase reporter vectors constructed with or without the SV40 promoter and/or enhancer regions or with 2 synthetic Renilla luciferase vectors (phRL-null and phRL-TK).

METHODS

Luciferase messenger RNA content was measured by reverse transcriptase-polymerase chain reaction, and luciferase activity was measured by luminometry. The ability of transcription factors in nuclei prepared from BC3H-1 myocytes to bind to a [32P]-labeled 24-base pair oligonucleotide containing the TATA box-binding element was determined by a gel mobility shift assay.

RESULTS

In vivo, 4.4 and 8.9 microM of Hoechst 33342 (sublethal doses) increased luciferase enzyme activity in cells transfected with each of the 4 pGL3 luciferase reporter vectors and both of the Renilla luciferase vectors. Hoechst 33258 had no effect on luciferase enzyme activity. In vitro, Hoechst 33342 increased transcription factor binding to the 24-mer oligonucleotide containing the TATA box-binding element, which would be favorable to increased RNA polymerase II efficiency.

CONCLUSION

Hoechst 33342 stimulates luciferase activity by a pathway that is independent of the integrity of the promoters in the luciferase gene expression vectors used (pGL3 basic, pGL3 control, pGL3 enhancer, and pGL3 promoter vectors, phRL-null, or phRL-TK).

Genomics, transcriptomics, proteomics, and numbers

OBJECTIVE

To review the advances in clinically useful molecular biologic techniques and to identify their applications in clinical practice, as presented at the 11th Annual William Beaumont Hospital DNA Symposium.

DATA SOURCES

The 8 manuscripts submitted were reviewed, and their major findings were compared with literature on the same or related topics.

STUDY SELECTION

Manuscripts address the use of molecular techniques in microbiology to evaluate infectious disease and epidemiology; molecular microbiology methods, including rapid-cycle real-time polymerase chain reaction; peroxisome proliferator-activated receptor gamma as a potential therapeutic target in inflammatory bowel disease or colon cancer; the effect of nonapoptotic doses of the bisbenizamide dye Hoechst 33342 on luciferase expression in plasmid-transfected BC3H-1 myocytes; the routine use of cystic fibrosis screening and its challenges; and the use of flow cytometry and/or chromosomal translocation in the diagnostic evaluation of hematopoietic malignancies.

DATA SYNTHESIS

Three current issues related to the use of molecular tests in clinical laboratories are (1) the restriction on introducing new tests secondary to existing patents or licenses; (2) the preanalytic variables for the different specimen types currently in use, including whole blood, plasma, serum, fresh or frozen tissues, and free-circulating DNA; and (3) the interpretation of studies evaluating the association of complex diseases with a single mutation or single-nucleotide polymorphism. Molecular methods have had a major impact on infectious disease through the rapid identification of organisms, the evaluation of outbreaks, and the characterization of drug resistance when compared with standard culture techniques. The activation of peroxisome proliferator-activated receptor gamma stimulated by thiazolidinedione is useful in the treatment of type II diabetes mellitus and may have value in preventing inflammatory bowel disease or colon cancer. Hoechst 33342 binding to adenine-thymine-rich regions in the minor groove of DNA is a fluorescent stain for DNA and initiates apoptosis at >10 microg/mL. Lower doses of Hoechst 33342 promote luciferase expression by a mechanism that may involve binding to cryptic promoters facilitated by dye-associated misalignment of the tertiary structure of DNA. The routine use of cystic fibrosis screening is complicated by the more than 1000 mutations associated with the disease. The use of 4-color flow cytometry and the detection of chromosomal translocation are both invaluable aids in establishing the diagnosis of lymphoid or myeloid hematopoietic malignancies.

CONCLUSIONS

The current postgenomic era will continue to emphasize the use of microarrays and database software for genomic, transcriptomic, and proteomic screening in the search for useful clinical assays. The number of molecular pathologic techniques will expand as additional disease-associated mutations are defined.