Bcl-2 accelerates multistep prostate carcinogenesis in vivo

Current mouse and cell models in prostate cancer research

Mouse models of prostate cancer (PCa) are critical for understanding the biology of PCa initiation, progression, and treatment modalities. Here, we summarize recent advances in PCa mouse models that led to new insights into specific gene functions in PCa. For example, the study of transgenic mice with TMPRSS2/ERG, an androgen-regulated fusion protein, revealed its role in developing PCa precursor lesions, prostate intraepithelial neoplasia; however, it is not sufficient for PCa development. Double deficiency of Pten and Smad4 leads to a high incidence of metastatic PCa. Targeted deletion of Pten in castration-resistant Nkx3-1-expressing cells results in rapid carcinoma formation after androgen-mediated regeneration, indicating that progenitor cells with luminal characteristics can play a role in initiation of PCa. Transgenic mice with activated oncogenes, growth factors, and steroid hormone receptors or inactivated tumor suppressors continue to provide insights into disease progression from initiation to metastasis. Further development of new PCa models with spatial and temporal regulation of candidate gene expression will probably enhance our understanding of the complex events that lead to PCa initiation and progression, thereby invoking novel strategies to combat this common disease in men.

Modeling prostate cancer in mice: something old, something new, something premalignant, something metastatic

More than 15 years ago, the first generation of genetically engineered mouse (GEM) models of prostate cancer was introduced. These transgenic models utilized prostate-specific promoters to express SV40 oncogenes specifically in prostate epithelium. Since the description of these initial models, there have been a plethora of GEM models of prostate cancer representing various perturbations of oncogenes or tumor suppressors, either alone or in combination. This review describes these GEM models, focusing on their relevance for human prostate cancer and highlighting their strengths and limitations, as well as opportunities for the future.

Genetically engineered mouse models of prostate cancer

Despite major improvement in treatment of early stage localised prostate cancer, the distinction between indolent tumors and those that will become aggressive, as well as the lack of efficient therapies of advanced prostate cancer, remain major health problems. Genetically engineered mice (GEM) have been extensively used to investigate the molecular and cellular mechanisms underlying prostate tumor initiation and progression, and to evaluate new therapies. Moreover, the recent development of conditional somatic mutagenesis in the mouse prostate offers the possibility to generate new models that more faithfully reproduce the human disease, and thus should contribute to improve diagnosis and treatments. The strengths and weaknesses of various models will be discussed, as well as future opportunities.

Androgen action during prostate carcinogenesis

Androgens are critical for normal prostate development and function, as well as prostate cancer initiation and progression. Androgens function mainly by regulating target gene expression through the androgen receptor (AR). Many studies have shown that androgen-AR signaling exerts actions on key events during prostate carcinogenesis. In this review, androgen action in distinct aspects of prostate carcinogenesis, including (i) cell proliferation, (ii) cell apoptosis, and (iii) prostate cancer metastasis will be discussed.

Modeling prostate cancer: a perspective on transgenic mouse models

Despite considerable success in treatment of early stage localized prostate cancer (PC), acute inadequacy of late stage PC treatment and its inherent heterogeneity poses a formidable challenge. Clearly, an improved understanding of PC genesis and progression along with the development of new targeted therapies are warranted. Animal models, especially, transgenic immunocompetent mouse models, have proven to be the best ally in this respect. A series of models have been developed by modulation of expression of genes implicated in cancer-genesis and progression; mainly, modulation of expression of oncogenes, steroid hormone receptors, growth factors and their receptors, cell cycle and apoptosis regulators, and tumor suppressor genes have been used. Such models have contributed significantly to our understanding of the molecular and pathological aspects of PC initiation and progression. In particular, the transgenic mouse models based on multiple genetic alterations can more accurately address the inherent complexity of PC, not only in revealing the mechanisms of tumorigenesis and progression but also for clinically relevant evaluation of new therapies. Further, with advances in conditional knockout technologies, otherwise embryonically lethal gene changes can be incorporated leading to the development of new generation transgenics, thus adding significantly to our existing knowledge base. Different models and their relevance to PC research are discussed.

Bcl-2 modulation to activate apoptosis in prostate cancer

Apoptosis resistance is a hallmark of cancer linked to disease progression and treatment resistance, which has led to the development of anticancer therapeutics that restore apoptotic function. Antiapoptotic Bcl-2 is frequently overexpressed in refractory prostate cancer and increased following standard hormonal therapy and chemotherapy; however, the rationally designed Bcl-2 antagonist, ABT-737, has not shown single agent apoptosis-promoting activity against human prostate cancer cell lines. This is likely due to the coordinate expression of antiapoptotic, Bcl-2-related Mcl-1 that is not targeted by ABT-737. We developed a mouse model for prostate cancer in which apoptosis resistance and tumorigenesis were conferred by Bcl-2 expression. Combining ABT-737 with agents that target Mcl-1 sensitized prostate cancer cell lines with an apoptotic block to cell death in vitro. In mice in vivo, ABT-737 showed single agent efficacy in prostate tumor allografts in which tumor cells are under hypoxic stress. In human prostate cancer tissue, examined using a novel tumor explant system designated Tumor Tissue Assessment for Response to Chemotherapy, combination chemotherapy promoted efficient apoptosis. Thus, rational targeting of both the Bcl-2 and Mcl-1 mechanisms of apoptosis resistance may be therapeutically advantageous for advanced prostate cancer.

Restoration of tumor suppressor miR-34 inhibits human p53-mutant gastric cancer tumorspheres

BACKGROUND

MicroRNAs (miRNAs), some of which function as oncogenes or tumor suppressor genes, are involved in carcinogenesis via regulating cell proliferation and/or cell death. MicroRNA miR-34 was recently found to be a direct target of p53, functioning downstream of the p53 pathway as a tumor suppressor. miR-34 targets Notch, HMGA2, and Bcl-2, genes involved in the self-renewal and survival of cancer stem cells. The role of miR-34 in gastric cancer has not been reported previously. In this study, we examined the effects of miR-34 restoration on p53-mutant human gastric cancer cells and potential target gene expression.

METHODS

Human gastric cancer cells were transfected with miR-34 mimics or infected with the lentiviral miR-34-MIF expression system, and validated by miR-34 reporter assay using Bcl-2 3'UTR reporter. Potential target gene expression was assessed by Western blot for proteins, and by quantitative real-time RT-PCR for mRNAs. The effects of miR-34 restoration were assessed by cell growth assay, cell cycle analysis, caspase-3 activation, and cytotoxicity assay, as well as by tumorsphere formation and growth.

RESULTS

Human gastric cancer Kato III cells with miR-34 restoration reduced the expression of target genes Bcl-2, Notch, and HMGA2. Bcl-2 3'UTR reporter assay showed that the transfected miR-34s were functional and confirmed that Bcl-2 is a direct target of miR-34. Restoration of miR-34 chemosensitized Kato III cells with a high level of Bcl-2, but not MKN-45 cells with a low level of Bcl-2. miR-34 impaired cell growth, accumulated the cells in G1 phase, increased caspase-3 activation, and, more significantly, inhibited tumorsphere formation and growth.

CONCLUSION

Our results demonstrate that in p53-deficient human gastric cancer cells, restoration of functional miR-34 inhibits cell growth and induces chemosensitization and apoptosis, indicating that miR-34 may restore p53 function. Restoration of miR-34 inhibits tumorsphere formation and growth, which is reported to be correlated to the self-renewal of cancer stem cells. The mechanism of miR-34-mediated suppression of self-renewal appears to be related to the direct modulation of downstream targets Bcl-2, Notch, and HMGA2, indicating that miR-34 may be involved in gastric cancer stem cell self-renewal/differentiation decision-making. Our study suggests that restoration of the tumor suppressor miR-34 may provide a novel molecular therapy for p53-mutant gastric cancer.

X-linked inhibitor of apoptosis deficiency in the TRAMP mouse prostate cancer model

Deregulation of apoptotic pathways plays a central role in cancer pathogenesis. X-linked inhibitor of apoptosis protein (XIAP), is an antiapoptotic molecule, whose elevated expression has been observed in tumor specimens from patients with prostate carcinoma. Studies in human cancer cell culture models and xenograft tumor models have demonstrated that loss of XIAP sensitizes cancer cells to apoptotic stimuli and abrogates tumor growth. In view of these findings, XIAP represents an attractive antiapoptotic therapeutic target for prostate cancer. To examine the role of XIAP in an immunocompetent mouse cancer model, we have generated transgenic adenocarcinoma of the mouse prostate (TRAMP) mice that lack XIAP. We did not observe a protective effect of Xiap deficiency in TRAMP mice as measured by tumor onset and overall survival. In fact, there was an unexpected trend toward more aggressive disease in the Xiap-deficient mice. These findings suggest that alternative mechanisms of apoptosis resistance are playing a significant oncogenic role in the setting of Xiap deficiency. Our study has implications for XIAP-targeting therapies currently in development. Greater understanding of these mechanisms will aid in combating resistance to XIAP-targeting treatment, in addition to optimizing selection of patients who are most likely to respond to such treatment.

Molecular markers in the diagnosis of prostate cancer

The genetic alterations leading to prostate cancer are gradually being discovered. A wide variety of genes have been associated with prostate cancer development as well as tumor progression. Knowledge of gene polymorphisms associated with disease aid in the understanding of important pathways involved in this process and may result in the near future in clinical applications. Urinary molecular markers will soon be available to aid in the decision of repeat prostate biopsies. Recent findings suggest the importance of androgen signaling in disease development and progression. The further understanding of interaction of inflammation, diet, and genetic predisposition will improve risk stratification in the near future.

Molecular mediators of cell death in multistep carcinogenesis: a path to targeted therapy

A consistent, if not invariant, feature of cancer cells is the acquired ability to evade apoptosis. The pioneering work of Dr. Stan Korsmeyer was invaluable in characterizing the molecular foundations of cell death signaling mechanisms during normal development and during multistep carcinogenesis. This foundation now forms the basis for the rational design of therapeutic strategies to selectively activate cell death in cancer cell populations. These strategies are currently being evaluated in an increasing number of clinical trials targeting diverse tumor types.

Constitutive activation of P I3 K-Akt and NF-kappaB during prostate cancer progression in autochthonous transgenic mouse model

BACKGROUND

Cancer progression is usually facilitated by independent growth signals that may lead to increased cell survival and evasion of apoptosis. Phosphatidylinositol 3'-OH kinase (P I3 K)-Akt and transcription factor NF-kappaB are important signaling molecules and key survival factors involved in the control of cell proliferation, apoptosis, and oncogenesis. Although P I3 K-Akt and NF-kappaB have been implicated in the development and progression of prostate cancer, expression of these molecules during progression of autochthonous disease has not been elucidated.

METHODS

Prostate cancer growth and progression in autochthonous transgenic adenocarcinoma of the mouse prostate (TRAMP) mice and male non-transgenic littermates were observed by magnetic resonance imaging (MRI). Expression patterns of P I3 K-Akt, NF-kappaB, I kappaB, and associated signaling molecules during different stages of cancer progression in these mice were examined by Western blot analysis, electrophoretic mobility shift assay (EMSA), enzyme-linked immunoabsorbent assay (ELISA), kinase assay, and immunohistochemistry.

RESULTS

Sequential MRI and gross analysis of prostate gland exhibited increasing prostate volume associated with the development and progression of prostatic adenocarcinoma in TRAMP mice, compared to male non-transgenic littermates. Differential protein expression of P I3 K, phosphorylated-Akt (Ser 473), I kappa Balpha and its phosphorylation, IKK kinase activity, NF-kappaB/p65, p50, DNA binding, and transcriptional-regulated genes, viz., Bc l2, cyclin D1, MMP-9, and VEGF were observed during prostate cancer progression in TRAMP mice, compared to male non-transgenic littermates. Expressions of these molecules were significantly increased during cancer progression observed at 24 and 32 weeks of age.

CONCLUSIONS

Differential expression pattern of P I3 K-Akt, NF-kappaB and I kappaB during prostate cancer progression in TRAMP mice suggest that these molecules represent potential molecular targets for prevention and/or therapeutic intervention.

The androgen receptor directly targets the cellular Fas/FasL-associated death domain protein-like inhibitory protein gene to promote the androgen-independent growth of prostate cancer cells

Androgens provide survival signals to prostate epithelial cells, and androgen ablation induces apoptosis in the prostate gland. However, the molecular mechanisms of actions of the androgen-signaling pathway in these processes are not fully understood. Here, we report that androgens induced expression of the cellular Fas/FasL-associated death domain protein-like inhibitory protein (c-FLIP) gene, which is a potent inhibitor of Fas/FasL-mediated apoptosis. The androgen receptor was recruited to the promoter of the c-FLIP gene in the presence of androgens. We found that c-FLIP promoter contained multiple functional androgen response elements. In addition, we show that c-FLIP overexpression accelerated progression to androgen independence by inhibiting apoptosis in LNCaP prostate tumors implanted in nude mice. Our results suggest that the androgen receptor affects survival and apoptosis of prostate cells through regulation of the c-FLIP gene in response to androgens.

The role of Bcl-2 family members in tumorigenesis

The Bcl-2 family consists of about 20 homologues of important pro- and anti-apoptotic regulators of programmed cell death. The established mode of function of the individual members is to either preserve or disturb mitochondrial integrity, thereby inducing or preventing release of apoptogenic factors like Cytochrome c (Cyt c) from mitochondria. Recent findings also indicate further Bcl-2-controlled mitochondria-independent apoptosis pathways. Bcl-2 represents the founding member of the new and growing class of cell death inhibiting oncoproteins. In this review, we try to briefly summarize current models of Bcl-2 family function and to outline the work demonstrating the influence of deregulated Bcl-2 family member expression on tumorigenesis and cancer therapy. Since several Bcl-2 homologues, in addition to influencing apoptotic behaviour, also impinge on cell cycle progression, we discuss possible implications of this additional role for the expression of Bcl-2 family members in tumor cells.

Evidence that transfer of functional p53 protein results in increased apoptosis in prostate cancer

PURPOSE

INGN 201 (Ad-p53) is a replication-defective adenoviral vector that encodes a wild-type p53 gene driven by the cytomegalovirus promoter. INGN 201 has been shown to have antitumoral activity against human prostate cancer cell lines. This study was undertaken to determine the safety of INGN 201 in patients with locally advanced prostate cancer, to assess transgene expression, and to evaluate antitumoral activity.

EXPERIMENTAL DESIGN

Our study included patients with clinical stage T3, T1c-T2a with Gleason score 8-10 disease, or T2a-T2b with Gleason score 7 disease and a prostate-specific antigen level >10 ng/ml. INGN 201 was administered by intraprostatic injection under ultrasonographic guidance. One course of INGN 201 was defined as three separate INGN 201 administrations 2 weeks apart. Biopsies at baseline and 24 h after the first administration were assessed for p53 protein by immunohistochemical staining and for apoptosis by terminal deoxynucleotidyl transferase-mediated nick end labeling assay.

RESULTS

A total of 38 courses of INGN 201 gene therapy were administered to 30 patients, of whom 26 underwent radical prostatectomy. There were no grade 3 or 4 adverse events related to INGN 201 administration. Of the 11 patients with negative baseline immunostaining for p53 protein, 10 had positive p53 immunostaining after the first administration of INGN 201, and 8 had an increase in apoptotic cells by terminal deoxynucleotidyl transferase-mediated nick end labeling staining. All 26 of the patients who underwent radical prostatectomy had significant residual viable prostate cancer, and 12 have experienced biochemical failure (median follow-up, 42 months).

CONCLUSION

Intraprostatic INGN 201 gene therapy is safe and can reliably result in p53 protein production and apoptosis.

Early onset of neoplasia in the prostate and skin of mice with tissue-specific deletion of Pten

PTEN is a tumor suppressor gene mutated in various advanced human neoplasias, including glioblastomas and prostate, breast, endometrial, and kidney cancers. This tumor suppressor is a lipid phosphatase that negatively regulates cell survival and proliferation mediated by phosphatidylinositol 3-kinase/protein kinase B signaling. Using the Cre-loxP system, we selectively inactivated Pten in murine tissues in which the MMTV-LTR promoter is active, resulting in hyperproliferation and neoplastic changes in Pten-null skin and prostate. These phenotypes had early onset and were completely penetrant. Abnormalities in Pten mutant skin consisted of mild epidermal hyperplasia, whereas prostates from these mice exhibited high-grade prostatic intraepithelial neoplasia (HGPIN) that frequently progressed to focally invasive cancer. These data demonstrate that Pten is an important physiological regulator of growth in the skin and prostate. Further, the early onset of HGPIN in Pten mutant males is unique to this animal model and implicates PTEN mutations in the initiation of prostate cancer. Consistent with high PTEN mutation rates in human prostate tumors, these data indicate that PTEN is a critical tumor suppressor in this organ.

Human prostate cancer risk factors

Prostate cancer has the highest prevalence of any nonskin cancer in the human body, with similar likelihood of neoplastic foci found within the prostates of men around the world regardless of diet, occupation, lifestyle, or other factors. Essentially all men with circulating androgens will develop microscopic prostate cancer if they live long enough. This review is a contemporary and comprehensive, literature-based analysis of the putative risk factors for human prostate cancer, and the results were presented at a multidisciplinary consensus conference held in Crystal City, Virginia, in the fall of 2002. The objectives were to evaluate known environmental factors and mechanisms of prostatic carcinogenesis and to identify existing data gaps and future research needs. The review is divided into four sections, including 1) epidemiology (endogenous factors [family history, hormones, race, aging and oxidative stress] and exogenous factors [diet, environmental agents, occupation and other factors, including lifestyle factors]); 2) animal and cell culture models for prediction of human risk (rodent models, transgenic models, mouse reconstitution models, severe combined immunodeficiency syndrome mouse models, canine models, xenograft models, and cell culture models); 3) biomarkers in prostate cancer, most of which have been tested only as predictive factors for patient outcome after treatment rather than as risk factors; and 4) genotoxic and nongenotoxic mechanisms of carcinogenesis. The authors conclude that most of the data regarding risk relies, of necessity, on epidemiologic studies, but animal and cell culture models offer promise in confirming some important findings. The current understanding of biomarkers of disease and risk factors is limited. An understanding of the risk factors for prostate cancer has practical importance for public health research and policy, genetic and nutritional education and chemoprevention, and prevention strategies.

A comprehensive assessment of p53-responsive genes following adenoviral-p53 gene transfer in Bcl-2-expressing prostate cancer cells

The p53 protein can induce cell cycle arrest or apoptosis following activation in response to DNA damage. The function of p53 is largely mediated by regulating the expression of downstream target genes. Adenoviral-p53 gene transfer (Ad-p53) is currently being evaluated in clinical trials as a therapeutic intervention. Tumor response is likely to be influenced by context-dependent variables, such as expression of bcl-2. Bcl-2 is upregulated in a variety of neoplasms, and can inhibit p53-dependent apoptosis. It was therefore of interest to use a global genomic strategy to assess gene expression following Ad-p53 gene transfer and to determine if the expression of specific Ad-p53-responsive genes could be modulated in the context of bcl-2 gene deregulation. cDNA arrays were used to identify p53-responsive genes following Ad-p53 gene transfer in control and bcl-2-overexpressing PC3 prostate cancer cells. A total of 40 transcripts were significantly upregulated by Ad-p53 in both control and bcl-2-transfectant PC3 cells. Conversely, 19 transcripts were significantly repressed in both cell lines. These Ad-p53-responsive transcripts included previously identified p53 targets, known genes representing candidate p53 targets, and transcripts identified as expressed sequence tags. A subset of 15 transcripts was differentially modulated by Ad-p53 in the context of bcl-2. Some of these genes were also differentially modulated in LNCaP (wt p53) cells following DNA damage. These results document a number of potential p53 targets and mediators of therapeutically relevant genotoxic stress. The findings further suggest that bcl-2 may inhibit cell death at multiple points downstream of p53 activation.

Role of inherited defects decreasing Fas function in autoimmunity

Fas is a death receptor belonging to the TNFR superfamily and induces cell apoptosis by both activating a caspase cascade and altering mitochondria. In the immune system, Fas is involved in the switching-off of the immune responses and cell mediated cytotoxicity. In humans, genetic defects decreasing Fas function cause the Autoimmune Lymphoproliferative Syndrome (ALPS) where autoimmunities are associated with accumulation of polyclonal lymphocytes in the secondary lymphoid tissues and expansion of T cells lacking both CD4 and CD8 (DN cells). Expansion of DN cells is absent in an ALPS variant, named Dianzani's Autoimmune Lymphoproliferative Disease (DALD). The observation that DALD patients' families display increased frequency of autoimmune diseases different from ALPS suggests that defects of Fas function may also play a role in development of "common" autoimmune diseases. This possibility is supported by detection of defective Fas function in substantial proportions of patients with the multiple autoimmune syndrome or aggressive forms of type 1 diabetes or multiple sclerosis. This article reviews data suggesting that development of autoimmune/lymphoproliferative patterns may involve several alterations hitting the Fas system, but might also involve alterations in other systems contributing to the switching-off or proliferation of lymphocytes.

Bcl-2 promotes premature senescence induced by oncogenic Ras

The expression of the apoptosis inhibitory protein, Bcl-2, is increased in naturally senescing human fibroblasts and upon induction of their senescence-like growth arrest by oxidative stress, implying its role in maintaining their extended viability. Oncogenic Ras(V12) protein induces signaling cascades that result in the premature senescence of primary fibroblast cells, which are insensitive to oncogene-dependent apoptosis. Here we show that constitutive expression of Bcl-2 accelerates selected features of the Ras-induced senescence program in primary human fibroblasts. Yet, Bcl-2 also inhibits fibroblast apoptosis induced by exogenous H(2)O(2), while both signals induce an increased endogenous Bcl-2 expression in these cells. Together, these data suggest a context-dependent phenotypic function of Bcl-2 in the regulation of overlapping cell fate specification programs, with potential implications for both physiology and multistep tumorigenesis.

Molecular markers of outcome after radiotherapy in patients with prostate carcinoma: Ki-67, bcl-2, bax, and bcl-x

BACKGROUND

Abnormal expression of key proteins of the apoptotic pathway has been associated with poor prognosis, although there have been few studies of these correlations in patients with prostate carcinoma who are treated with radiotherapy. The current study examined the association between expression levels of Ki-67, bcl-2, bax, and bcl-x in pretreatment biopsy specimens and patient outcome after definitive radiotherapy alone.

METHODS

Archival pretreatment prostate biopsy tumor tissue was retrieved from 106 patients with Stage T1-T3 prostate carcinoma who were treated at the University of Texas M. D. Anderson Cancer Center with external beam radiotherapy between 1987 and 1993. Expression levels of Ki-67 (MIB-1 staining; n = 106 patients), bcl-2 (n = 77 patients), bax (n = 70 patients), and bcl-x (both long and short splice variants; n = 72 patients) were determined by immunohistochemical staining. The Ki-67 labeling index (Ki67-LI) was available for all patients and was derived from the percentage of Ki-67 positive cells. Biochemical failure after radiotherapy was defined as three consecutive rises in prostate specific antigen level on follow-up. The median follow-up was 62 months.

RESULTS

High Ki67-LI (> 3.5%) expression was observed in 33% of patients, overexpression of bcl-2 was observed in 16% of patients, altered bax expression was observed in 23% of patients, and altered bcl-x expression was observed in 53% of patients. There was no correlation found between the biomarkers. Kaplan-Meier survival estimates of freedom from biochemical failure (bNED) and the log-rank test revealed significantly lower rates in association with high Ki67-LI, positive bcl-2, and altered bax staining. No correlation was observed between bcl-x staining and bNED. Cox proportional hazards multivariate analysis confirmed that bcl-2 and bax were independent of pretreatment PSA level, Gleason score, disease stage, and Ki67-LI in predicting bNED.

CONCLUSIONS

Abnormalities in the expression levels of bcl-2 and bax were associated with increased failure after patients were treated for prostate carcinoma with external beam radiotherapy. These biomarkers appeared to be useful in categorizing patient risk further, beyond Ki-67 staining and conventional clinical prognostic factors.

Hormone-refractory prostate cancer: a multi-step and multi-event process

Since the pioneering studies of Huggins in 1941, it has been known that prostate cancer cells, like certain normal epithelial cells, can chronically depend on a critical level of androgenic stimulation for their continuous growth and survival. The entire issue of the development of resistance to androgen ablation therapy for metastatic prostate cancer is based on the fact that a portion of cells can survive without androgen stimulation. The cell mechanism of androgen independent status is unclear. For some authors, a portion of the cells present within a patient with a prostate cancer before therapy is naturally androgen independent (selection hypothesis). However, this hypothesis does not consider gene alteration during prostate cancer natural history and probably hormone-refractory prostate cancer (HRPC) is due to a multi-step and multi-event process. In this literature review, different cell pathways that lead to HRPC are described.Prostate Cancer and Prostatic Diseases (2001) 4, 204-212.

Effect of Bcl-2 expression on hepatic preneoplasia in mice

In a transgenic model of hepatocellular carcinoma induced by the expression of SV40 early sequences (TAg mice), deregulation of hepatocyte proliferation induces an apoptotic process whose decrease coincides with the appearance of neoplastic foci. Mating these mice with transgenic mice overexpressing Bcl-2 led to a dramatic reduction in the number of apoptotic hepatocytes during preneoplasia, resulting in an enlargement of the liver. This decrease in apoptosis was followed, 2 weeks later, by a reduction in hepatocellular proliferation. Sequential reduction in apoptosis and proliferation rate suggests that the anti-apoptotic and the anti-mitotic activities of Bcl-2 might be operative in distinct stages of preneoplasia.

Diacylglycerol (DAG)-lactones, a new class of protein kinase C (PKC) agonists, induce apoptosis in LNCaP prostate cancer cells by selective activation of PKCalpha

Phorbol esters, the archetypical (PKC) activators, induce apoptosis in androgen-sensitive LNCaP prostate cancer cells. In this study we evaluate the effect of a novel class of PKC ligands, the diacylglycerol (DAG)-lactones, as inducers of apoptosis in LNCaP cells. These unique ligands were designed using novel pharmacophore- and receptor-guided approaches to achieve highly potent DAG surrogates. Two of these compounds, HK434 and HK654, induced apoptosis in LNCaP cells with much higher potency than oleoyl-acetyl-glycerol or phorbol 12,13-dibutyrate. Moreover, different PKC isozymes were found to mediate the apoptotic effect of phorbol 12-myristate 13-acetate (PMA) and HK654 in LNCaP cells. Using PKC inhibitors and dominant negative PKC isoforms, we found that both PKCalpha and PKCdelta mediated the apoptotic effect of PMA, whereas only PKCalpha was involved in the effect of the DAG-lactone. The PKCalpha selectivity of HK654 in LNCaP cells contrasts with similar potencies in vitro for binding and activation of PKCalpha and PKCdelta. Consistent with the differences in isoform dependence in intact cells, PMA and HK654 show marked differences in their abilities to translocate PKC isozymes. Both PMA and HK654 induce a marked redistribution of PKCalpha to the plasma membrane. On the other hand, unlike PMA, HK654 translocates PKCdelta predominantly to the nuclear membrane. Thus, DAG-lactones have a unique profile of activation of PKC isozymes for inducing apoptosis in LNCaP cells and represent the first example of a selective activator of a classical PKC in cellular models. An attractive hypothesis is that selective activation of PKC isozymes by pharmacological agents in cells can be achieved by differential intracellular targeting of each PKC.

Apoptosis regulators and their role in tumorigenesis

It has become clear that, together with deregulated growth, inhibition of programmed cell death (PCD) plays a pivotal role in tumorigenesis. In this review, we present an overview of the genes and mechanisms involved in PCD. We then summarize the evidence that impaired PCD is a prerequisite for tumorigenesis, as indicated by the fact that more and more neoplastic mutations appear to act by interfering with PCD. This has made the idea of restoration of corrupted 'death programs' an intriguing new area for potential cancer therapy.

Paradox of Bcl-2 (and p53): why may apoptosis-regulating proteins be irrelevant to cell death?

Although the Bcl-2 family members and p53 are involved in the regulation of apoptosis, the status of apoptotic machinery (eg caspases) plays a major role in determining the mode and timing of cell death. If the apoptotic machinery is lost, inhibited, or intrinsically inactivated, the "death stars", Bcl-2 and p53, may become irrelevant to cell death. In this light, high levels of Bcl-2 may indicate that downstream apoptotic pathways are still functional. This explains why Bcl-2 overexpression can be a marker of chemosensitivity and favorable prognosis in certain cancers and why retention of wild-type p53 may manifest inactivation of caspases in aggressive cancers.