Apoptosis and nuclear levels of p53 protein and proliferating cell nuclear antigen in human hepatoma cells cultured with tumor promoters

Endoplasmic reticulum stress induces p53 cytoplasmic localization and prevents p53-dependent apoptosis by a pathway involving glycogen synthase kinase-3beta

The tumor suppressor p53, a sensor of multiple forms of cellular stress, is regulated by post-translational mechanisms to induce cell-cycle arrest, senescence, or apoptosis. We demonstrate that endoplasmic reticulum (ER) stress inhibits p53-mediated apoptosis. The mechanism of inhibition involves the increased cytoplasmic localization of p53 due to phosphorylation at serine 315 and serine 376, which is mediated by glycogen synthase kinase-3 beta (GSK-3beta). ER stress induces GSK-3beta binding to p53 in the nucleus and enhances the cytoplasmic localization of the tumor suppressor. Inhibition of apoptosis caused by ER stress requires GSK-3beta and does not occur in cells expressing p53 with mutation(s) of serine 315 and/or serine 376 to alanine(s). As a result of the increased cytoplasmic localization, ER stress prevents p53 stabilization and p53-mediated apoptosis upon DNA damage. It is concluded that inactivation of p53 is a protective mechanism utilized by cells to adapt to ER stress.

Structural and functional involvement of p53 in BER in vitro and in vivo

p53 is involved in several DNA repair pathways. Some of these require the specific transactivation of p53-dependent genes and others involve direct interactions between the p53 protein and DNA repair associated proteins. Previously, we have shown that p53 acts directly in Base Excision Repair (BER) when assayed under in vitro conditions. Our present data indicate that this involvement is independent of the transcriptional activity of the p53 molecule. We found that under both in vitro and in vivo conditions, a p53 transactivation-deficient molecule, p53-22-23 was more efficient in BER activity than was wild type p53. However, mutations in the core domain or C-terminal alterations strongly reduced p53-mediated BER activity. These results are consistent with the hypothesis that the involvement of p53 in BER activity, a housekeeping DNA repair pathway, is a prompt and immediate one that does not involve the activation of p53 transactivation-dependent mechanisms, but rather concerns with the p53 protein itself. In an endogenous DNA damage status p53 is active in BER pathways as a protein and not as a transcription factor.

Induction of apoptosis in HL-60 cells by eicosapentaenoic acid (EPA) is associated with downregulation of bcl-2 expression

Dietary polyunsaturated fatty acids (PUFAs) have been reported as a potential group of natural products which modulate tumor cell growth. In present study, eicosapentaenoic acid (EPA) was found to inhibit proliferation of human leukemic HL-60 and K-562 cells in vitro. EPA arrested cell cycle progression at G0/G1 phase, and induced necrosis in both HL-60 and K-562 cells. However, EPA induced apoptosis only in HL-60 but not K-562 cells. Also, bcl-2 protein expression was downregulated in much greater extent than that of bax showing that depression of bcl-2 might be an important step during the EPA-induced apoptosis in HL-60 cells.

Abrogation of cyclin D1 expression predisposes lung cancer cells to serum deprivation-induced apoptosis

Cyclin D1 antisense (D1AS)-transfected lung epithelial cell lines were serum deprived and then analyzed for three hallmarks of apoptosis: appearance of single-strand DNA breaks, alteration of apoptosis-related protein expression, and induction of chromatin condensation. Single-strand DNA breaks appeared at significant levels 24 h after serum deprivation, whereas induction of chromatin condensation was observed after 72 h. The antioxidants dimethyl sulfoxide, ascorbate, and glutathione, as well as insulin-like growth factor-I, inhibited induction of DNA damage in this assay. Additionally, proliferating cell nuclear antigen expression is completely suppressed in the D1AS cells, indicating a mechanism to explain the reduced capacity for DNA repair. Increased expression of cyclin D1, which is a common lesion in lung cancer, may thus prevent induction of apoptosis in an oxidizing and growth factor-poor environment. Reducing cyclin D1 expression in lung cancer cells by expression of D1AS RNA disrupted these protective pathways.

Effect of the chlorinated hydrocarbons heptachlor, chlordane, and toxaphene on retinoblastoma tumor suppressor in human lymphocytes

Organochlorine use over the past 50 years has resulted in the contamination of soil, water, plant and animal species. This contamination has created a long-lasting environmental problem, as the members of the organochlorine class of pesticides are resistant to degradation and have been labeled as persistent bioaccumulators. Studies have shown certain organochlorines to be tumor promoters, liver toxicants and to induce immune cell dysfunction in rats and mice. Our laboratory has shown that the organochlorines heptachlor and chlordane affect leukocytic gene expression and differentiation. In this study, experiments with CEM x 174 cells, a hybrid of human T and B cells, were performed to investigate the effects of the tumor promoter heptachlor and its congeners chlordane and toxaphene on retinoblastoma (Rb) gene expression. The results indicated that heptachlor, chlordane or toxaphene, in the range of 10-50 microM, were able to reduce Rb protein levels in a concentration-dependent manner. In the case of heptachlor, the reduction could be seen as early as 12 h and was time-dependent. Analysis of Rb mRNA levels revealed no detectable difference over the same concentration range. These results suggest that members of the organochlorine class are able to downregulate Rb expression at the post-transcriptional level, an effect similar to that on p53 tumor suppressor previously reported by our laboratory.

The effect of heptachlor, a chlorinated hydrocarbon insecticide, on p53 tumor suppressor in human lymphocytes

Previous studies have shown that heptachlor, a chlorinated hydrocarbon insecticide, is a liver tumor promoter in rats and mice and induces tumor promoting-like alterations in human myeloblastic leukemia cells. The nature of tumor promotion is multifaceted and has recently been shown to include suppression of programmed cell death (apoptosis) as a mechanism by which a tumor promoter can prolong cell viability. The ability of tumor promoters to suppress apoptosis prompted us to address the question of whether heptachlor is capable of effecting the expression of genes involved in lymphocyte apoptosis, in particular, the p53 tumor suppressor gene. Experiments with a CEM x 174 cell line, a hybrid of human T and B cells, revealed that heptachlor downregulated p53 gene expression at the post-transcriptional level without changing levels of mRNA in the cells. The heptachlor-induced reduction in the basal levels of expression of this gene was both in a concentration and time-dependent manner.

Apoptotic cell death in neuronal differentiation of P19 EC cells: cell death follows reentry into S phase

Apoptotic cell death was observed during aggregate culture of the mouse embryonal carcinoma cell line P19 exposed to all-trans retinoic acid (tRA). This finding was confirmed by genomic DNA agarose gel electrophoresis and transmission electron microscopy. Apoptosis was associated with P19 cell neuronal differentiation; alternative causes of cell death, i.e., cavitation-related, cytotoxicity of tRA, or spontaneous cell death were excluded. Analysis by flow cytometry revealed that the apoptosis was likely to occur in multiplying cells that underwent to reentering into S phase. We therefore examined 5-bromo-2'-deoxyuridine (BrdU) incorporation and proliferating cell nuclear antigen (PCNA) expression and localization in the aggregates by immunofluorescent staining. Although the P19 cells in the aggregates exposed to tRA incorporated BrdU at an equivalent level to those not exposed to tRA, the cells showed diminished PCNA expression and nuclear accumulation. We propose that P19 apoptosis during neuronal differentiation is a model system in which programmed cell death occurs simultaneously with cell division leading to differentiation.

Apoptosis in human hepatocellular carcinoma and in liver cell dysplasia is correlated with p53 protein immunoreactivity

AIMS

To investigate the prevalence of apoptosis in human hepatocellular carcinomas (HCC) of different types and grades and in liver cell dysplasia, and to test whether the apoptotic rate is correlated with the p53 protein status.

METHODS

37 HCC and 66 six liver samples with liver cell dysplasia were analysed for apoptosis using in situ DNA end labelling (ISEL), and for p53 protein expression by immunohistochemistry. In HCCs, proliferative activity was quantitatively assessed using proliferating cell nuclear antigen labelling.

RESULTS

The apoptotic index in HCC as based on ISEL ranged from 0.1 to 13.5 per 1000 cells analysed and was not related to type or grade. No nuclear staining was observed in multinuclear tumour cells. There was a significant correlation between the apoptotic rate and both the proliferative activity and p53 protein reactivity. In liver samples containing p53 protein positive liver cell dysplasia cells, there was a significantly higher apoptotic rate of these cells.

CONCLUSIONS

Apoptosis is detectable in HCC, and is not related to type and grade. There is a highly significant positive correlation between the apoptotic rate in HCC and both the proliferative activity and p53 protein expression. A similar phenomenon occurs for putative cancer precursors. The findings support the role of p53 in regulating apoptosis in preneoplastic and neoplastic liver lesions.

Ubiquitination of p53 and p21 is differentially affected by ionizing and UV radiation

Levels of the tumor suppressor protein p53 are normally quite low due in part to its short half-life. p53 levels increase in cells exposed to DNA-damaging agents, such as radiation, and this increase is thought to be responsible for the radiation-induced G1 cell cycle arrest or delay. The mechanisms by which radiation causes an increase in p53 are currently unknown. The purpose of this study was to compare the effects of gamma and UV radiation on the stability and ubiquitination of p53 in vivo. Ubiquitin-p53 conjugates could be detected in nonirradiated and gamma-irradiated cells but not in cells which were UV treated, despite the fact that both treatments resulted in the stabilization of the p53 protein. These results demonstrate that UV and gamma radiation have different effects on ubiquitinated p53 and suggest that the UV-induced stabilization of p53 results from a loss of p53 ubiquitination. Ubiquitinated forms of p21, an inhibitor of cyclin-dependent kinases, were detected in vivo, demonstrating that p21 is also a target for degradation by the ubiquitin-dependent proteolytic pathway. However, UV and gamma radiation had no effect on the stability or in vivo ubiquitination of p21, indicating that the radiation effects on p53 are specific.

Apoptosis and cancer risk assessment

Apoptosis is one form of physiological or active cell death. The balance between cell proliferation and cell death or apoptosis not only effects organ growth but also has a profound impact on the net increase and growth of initiated cells and preneoplastic and tumor cell populations. With respect to cancer development apoptosis is becoming widely recognized as being an innate tissue defense against carcinogens by inhibiting survival and controlling growth of precancerous cell populations and tumors at different stages of carcinogenesis. Experimental data on cell birth and cell death rates help identify the mode of action of a chemical and can be incorporated into biologically based cancer models. This article describes the quantitation and regulation of apoptosis in rodent liver and how loss of regulation can have a role in hepatocarcinogenesis. A biologically-based mouse liver cancer model is presented and utilized to describe how treatment related growth effects affect the process of carcinogenesis. Advantages and limitations of biologically based cancer models in cancer research and risk assessment are discussed.

Cationic liposomes enhance retrovirus-mediated multinucleated cell formation and retroviral transduction

A retroviral vector carrying herpes simplex virus thymidine kinase (HSV-TK) gene was introduced into psi 2 packaging cells (psi 2tkn) and XC tumor cells (XCtkn). psi 2tkn, XCtkn and XC cells co-cultured with TK-carrying cells were killed by ganciclovir (GCV) treatment. The growth of XC cells transplanted into nude mice was also suppressed by intratumoral injection of psi 2tkn or XCtkn cells and subsequent GCV administration. In addition, the XC cells cultured with either psi 2tkn cells or cell-free retrovirus suspension formed giant multinucleated cells. The multinucleated cell formation was specific to the combination of XC cells and the retrovirus produced by the psi 2tkn cells. Cationic liposomes enhanced the retrovirus-induced multinucleated cell formation and retroviral transduction. The correlation between the two actions of liposomes suggests that liposomes which enhance multinucleated cell formation are potent enhancers of retroviral transduction.

Gene therapy of hepatoma: bystander effects and non-apoptotic cell death induced by thymidine kinase and ganciclovir

A retroviral vector carrying herpes simplex virus thymidine kinase gene was constructed, and transfected into the psi 2 packaging cells. The replication-defective retrovirus produced by this cell line (psi 2 tkn cells) was transduced into XC rat hepatoma cells, from which a cell line (XCtkn2) highly sensitive to ganciclovir was cloned. Ganciclovir suppressed the growth of XCtkn2 hepatoma and psi 2tkn cells. Both of these HSV-tk-carrying cells treated with ganciclovir showed potent 'bystander effect' on co-culturing with genetically unmodified XC hepatoma cells. In addition, intratumoral injection of XCtkn2 and psi 2tkn cells into the XC hepatomas transplanted in nude mice and subsequent ganciclovir administration suppressed in vivo growth of the hepatomas. Flow cytometry disclosed that the ganciclovir-treatment increased the relative number of XCtkn2 hepatoma and psi 2tkn cells at the G2 phase of the cell cycle. However, the nuclear fragmentation and internucleosomal DNA cleavage were not observed, indicating that the death of XCtkn2 hepatoma and psi 2tkn cells treated with ganciclovir was not apoptotic.