Induction of apoptosis by wild-type p53 in a human colon tumor-derived cell line

Epstein-Barr virus latent membrane protein 1 blocks p53-mediated apoptosis through the induction of the A20 gene

Two Epstein-Barr virus (EBV)-associated malignancies, nasopharyngeal carcinoma and posttransplant lymphoma, rarely have mutations in the p53 tumor suppressor gene, suggesting that a viral protein interferes with p53 function. The EBV oncogene, LMP1, induces expression of the cellular antiapoptotic genes bcl-2 and A20 and could in this way interfere with p53-mediated apoptosis. Two derivatives of the p53-null epithelial cell line H1299 were prepared, one of which (H1299-p53) stably expressed a temperature-sensitive (ts) p53 protein, and another (H1299-p53+LMP1) which stably expressed both ts-p53 and latent membrane protein 1 (LMP1). At the permissive temperature, the p53 protein in the H1299-p53 cell line transcriptionally activated two of its target genes, the cyclin-dependent kinase inhibitor p21 and the mdm2 gene product, in an LMP1-independent manner. Upon serum withdrawal at the permissive temperature, p53-mediated apoptosis was induced in 50 to 60% of the cells. In the H1299-p53 cell line which stably expressed LMP1, however, only 20 to 25% of the cells underwent apoptosis. While stable expression of LMP1 did not affect levels of bcl-2 family members in these cells, it did induce expression of A20. Stable expression of A20 in the H1299-p53 cell line inhibited p53-mediated apoptosis equivalent to inhibition by LMP1. The induction of A20 may underlie the ability of LMP1 to protect EBV-infected epithelial cells from p53-mediated apoptosis.

Apoptosis in ocular disease: a molecular overview

Apoptosis is a form of genetically programmed cell death that can be induced by a variety of different stimuli. It is often referred to as a form of cellular suicide. Typically, apoptosis is characterized by the condensation and shrinkage of the cellular nucleus and cytoplasm, followed by the complete fragmentation of the cell and subsequent phagocytosis of the debris by surrounding cells. Although important during development, and also for maintaining homeostasis in some adult tissues, apoptosis can also be associated with disease processes. Recent laboratory studies indicate that apoptosis is a mechanism of cell death in several important ocular diseases including glaucoma, retinitis pigmentosa, cataract formation, retinoblastoma, retinal ischemia, and diabetic retinopathy. This review summarizes the results of these studies and provides a brief description of some of the key molecules that are involved in the genetic regulation of apoptosis. It is possible that a complete understanding of how these molecules function may someday lead to new treatment options aimed at blocking the death of cells in a variety of ocular diseases.

Apoptosis in the terminal endbud of the murine mammary gland: a mechanism of ductal morphogenesis

Ductal morphogenesis in the rodent mammary gland is characterized by the rapid penetration of the stromal fat pad by the highly proliferative terminal endbud and subsequent formation of an arborized pattern of ducts. The role of apoptosis in ductal morphogenesis of the murine mammary gland and its potential regulatory mechanisms was investigated in this study. Significant apoptosis was observed in the body cells of the terminal endbud during the early stage of mammary ductal development. Apoptosis occurred predominately in defined zones of the terminal endbud; 14.5% of the cells within three cell layers of the lumen were undergoing apoptosis compared to 7.9% outside this boundary. Interestingly, DNA synthesis in the terminal endbud demonstrated a reciprocal pattern; 21.1% outside three cell layers and 13.8% within. Apoptosis was very low in the highly proliferative cap cell laver and in regions of active proliferation within the terminal endbud. In comparison to other stages of murine mammary gland development, the terminal endbud possesses the highest level of programmed cell death observed to date. These data suggest that apoptosis is an important mechanism in ductal morphogenesis. In p53-deficient mice, the level of apoptosis was reduced, but did not manifest a detectable change in ductal morphology, suggesting that p53-dependent apoptosis is not primarily involved in formation of the duct. Immunohistochemical examination of the expression of the apoptotic checkpoint proteins, Bcl-x, Bax and Bcl-2, demonstrated that they are expressed in the terminal endbud. Bcl-x and Bcl-2 expression is highest in the body cells and lowest in the nonapoptotic cap cells, implying that their expression is associated with increased apoptotic potential. Bax expression was distributed throughout the terminal endbud independent of the observed pattern of apoptosis. A functional role for Bcl-2 family members in regulating endbud apoptosis was demonstrated by the significantly reduced level of apoptosis observed in WAP-Bcl-2 transgenic mice. The pattern of apoptosis and ductal structure of endbuds in these mice was also disrupted. These data demonstrate that p53-independent apoptosis may play a critical role in the early development of the mammary gland.

Wild-type p53 negatively regulates the expression of a microtubule-associated protein

The product of the p53 tumor suppressor gene has a well-documented activity as a transcriptional activator, and several studies indicate that this function is at least in part essential for the ability of p53 to suppress cellular proliferation. However, there is growing evidence that some activities of wild-type p53 may be independent of its trans-activation function; in fact, recent investigations have indicated that the transcriptional repression function of p53, rather than its trans-activation function, may be influential in p53-mediated apoptosis. The focus of this study has been on the identification of genes that exhibit decreased expression during p53-dependent apoptosis, and therefore represent potential p53-repressed genes influential in programmed cell death. This report identifies the gene encoding the microtubule-associated protein MAP4 as one whose mRNA and protein expression decrease in cells following induction of wild-type p53. Importantly, decreased MAP4 expression following p53 induction can be inhibited by molecules that prevent p53-mediated transcriptional repression and apoptosis, such as the adenovirus E1B-19K protein and the Wilms tumor gene product WT1. Additionally, overexpression of MAP4 in cells induced to undergo p53-dependent apoptosis significantly delays this process, indicating that the negative regulation of this gene by p53 may be influential in the rapid progression of apoptosis.

Identification of BTG2, an antiproliferative p53-dependent component of the DNA damage cellular response pathway

Cell cycle regulation is critical for maintenance of genome integrity. A prominent factor that guarantees genomic stability of cells is p53 (ref. 1). The P53 gene encodes a transcription factor that has a role as a tumour suppressor. Identification of p53-target genes should provide greater insight into the molecular mechanisms that mediate the tumour suppressor activities of p53. The rodent Pc3/Tis21 gene was initially described as an immediate early gene induced by tumour promoters and growth factors in PC12 and Swiss 3T3 cells. It is expressed in a variety of cell and tissue types and encodes a remarkably labile protein. Pc3/Tis21 has a strong sequence similarity to the human antiproliferative BTG1 gene cloned from a chromosomal translocation of a B-cell chronic lymphocytic leukaemia. This similarity led us to speculate that BTG1 and the putative human homologue of Pc3/Tis21 (named BTG2) were members of a new family of genes involved in growth control and/or differentiation. This hypothesis was recently strengthened by the identification of a new antiproliferative protein, named TOB, which shares sequence similarity with BTG1 and PC3/TIS21 (ref. 7). Here, we cloned and localized the human BTG2 gene. We show that BTG2 expression is induced through a p53-dependent mechanism and that BTG2 function may be relevant to cell cycle control and cellular response to DNA damage.

The cellular and molecular pathogenesis of colorectal cancer

The development of colorectal neoplasia originates from normal colonic mucosa, progresses to the adenomatous polyp, and later may evolve into carcinoma. This procession of histologic change can be defined by a series of successive waves of clonal expansion that contain certain genetic alterations. These genetic alterations include mutations in the K-ras oncogene and mutation in the one allele coupled with loss of the second allele for the tumor suppressor genes APC, DCC, and p53. The normal forms of these genes encode for proteins that regulate cell growth, cell-to-cell adhesion, and cell cycle checkpoints. Information on the function of these genes, as well as a proposed model of sequential mutation and loss of these regulatory genes during colorectal tumorigenesis are presented.

Abrogation of apoptosis induced by DNA-damaging agents in human bladder-cancer cell lines with p21/WAF1/CIP1 and/or p53 gene alterations

The p53-inducible cyclin-dependent kinase inhibitor, p21/WAF1/CIP1 (p21), plays a pivotal role in the G1 arrest or apoptosis of cells exposed to genotoxic stimuli. To determine whether p21 is a putative tumor-suppressor gene, p21 status was investigated in 4 human bladder-cancer cell lines of known p53 status. A p21-gene mutation, one base-pair insertion at codon 20 resulting in a chain-termination change at codon 35, was observed in one cell line, HT1376, suggesting structural or functional alteration of the p21 protein. When exposed to DNA-damaging agents, cisplatin or mitomycin C, apoptosis was induced in RT4 with the wild-type (wt) p53/wt p21, whereas T24 with the p53 non-sense mutation/wt p21 was resistant. Of the other 2 cell lines with the p53 mis-sense mutation, apoptosis was induced in SCaBER with the wt p21, but HT1376 with the p21 frame-shift mutation was fairly resistant. These findings suggest that not only p53 alteration, but also p21 alteration, is important to prevent apoptosis induced by DNA-damaging agents. When exposed to these agents, p53 and p21 expression was increased in RT4, and not induced in T24. p53 was not induced, but p21 expression was increased in SCaBER, whereas p53 expression was increased but p21 expression was absent in HT1376. Thus, p21 expression itself may have an important role in the induction of apoptosis by DNA-damaging agents.

Analysis of phosphorylated isoforms of the p53 tumor suppressor protein in human lung carcinoma cells undergoing apoptosis

The expression of p53 tumor suppressor protein isoforms in H460a cells induced to undergo apoptosis by 2-methoxyestradiol was investigated by two-dimensional gel electrophoresis. Whole-cell proteins from control and apoptotic H460a cells were separated by two-dimensional electrophoresis and were transferred to nitrocellulose. The p53 isoforms were detected by immunoblotting using p53 monoclonal antibody Bp53-12. Four isoforms of p53 (2, 3, 5, and 6) differing in phosphorylation state were detected in control cells. Three additional isoforms (1, 4, and 7) were observed to be expressed at significant levels only in apoptotic cells. The differential expression of isoforms 1, 4, and 7 in apoptotic cells suggests that one or more specific phosphorylation events generating these forms of p53 could play a role in regulating the function of p53 in apoptosis.

The p53 tumour suppressor gene: a mediator of a G1 growth arrest and of apoptosis

The tumour suppressor gene p53 plays a major role in the protection of cells from DNA damage. Activation of the protein in response to irradiation or genotoxic agents, and possibly by other signals, results in growth arrest at the G1 phase of the cell cycle or in apoptosis. While it has been shown that the ability of p53 to function as a sequence-specific transcriptional activator is necessary for the induction of growth arrest, the mechanism of p53-mediated apoptosis is not yet clear. It appears that under some conditions activation of the G1 checkpoint will prevent apoptosis, but the cellular environment may alter the result of p53 activation towards cell death. p53 may also directly induce apoptosis through several pathways, which may be transcriptionally dependent or independent. The outcome-a G1 arrest or apoptosis-will depend on a complex network of regulatory signals.

Apoptosis -- the story so far

The process of programmed cell death, or apoptosis, has become one of the most intensively studied topics in biological sciences in the last two decades. Apoptosis as a common and universal mechanism of cell death, distinguishable from necrosis, is now a widely accepted concept after the landmark paper by Kerr, Wyllie and Currie in the early seventies [1]. Different components of the death machinery in eukaryotes are discussed in this issue.

bcl-2 overexpression combined with p53 protein accumulation correlates with hormone-refractory prostate cancer

Seventy-seven men with histologically proven and newly diagnosed prostate cancer we investigated for the presence of bcl-2 protein overexpression and p53 protein accumulation 1 immunohistochemistry. Forty-five men had evidence of locally advanced and metastatic disease and we treated by means of hormone manipulation. Twenty-eight patients either failed to respond to initial hormone manipulation or relapsed within 37 months from diagnosis (median 20 months). Of the 77 cancers, 37 (48% showed bcl-2 overexpression at diagnosis. Twenty-seven of those were treated with androgen ablation and 2 (74%) had hormone-refractory disease (P = 0.0128). Twenty-three of 77 men (29.8%) had nuclear staining for p53 protein. Twenty-one of those were treated with hormone manipulation and 14 (66.6%) showed hormone resistance (P = 0.0012). Seventeen patients had both bcl-2 overexpression and p53 protein accumulation, 16 of whom were hormonally treated, with 13 (81.2%) having hormone-refractory disease (P < 0.0001). These findings suggest that the combined detection of p53 protein accumulation and bcl-2 overexpression may be useful in predicting hormone resistance in prostate cancer. By deregulating programmed cell death, alteration in these genes may prevent patients from responding to androgen ablation, or allow them to escape hormonal control of the disease.

Expression of apoptosis, proliferating cell nuclear antigen, and apoptosis-related antigens (bcl-2, c-myc, Fas, Lewis(y) and p53) in human cholangiocarcinomas and hepatocellular carcinomas

In situ expression of apoptosis and its related antigens has rarely been evaluated in human liver tumors. Therefore, investigation using in situ nick end-labeling and immunohistochemical methods of the in situ expression of apoptosis, proliferating cells, and apoptosis-related antigens in 7 normal livers, 20 cholangiocarcinomas (CC) and 17 hepatocellular carcinomas (HCC) was done. Apoptotic cells as determined by the nick end-labeling method and proliferating cell nuclear antigen-positive cells were present in all specimens, and the percentage of them was significantly higher in CC than in HCC. Bcl-2 protein was present only in one CC and one HCC, but was occasionally noted in bile ducts in non-cancerous livers. C-myc and Fas antigens were not found in any of the cases. Lewisy antigen was expressed in 8 CC, but was absent in the other cases although bile ducts in non-cancerous livers frequently expressed Lewisy. p53 protein was present in 8 CC, but was absent in the other cases. Serial section observations showed that apoptotic cancer cells were consistently negative for proliferating cell nuclear antigen; bcl-2-positive cells did not show apoptosis; p53-positive cancer cells showed apoptosis. Some Lewisy-positive cancer cells showed apoptosis, while others did not. These data suggest that apoptosis and cell proliferation are involved in CC and HCC, and their degree is more severe in CC than in HCC. p53 protein (stimulative) may regulate apoptosis in some cases, whereas c-myc, Fas and Lewisy are not related to apoptosis in CC and HCC in vivo. Many other factors may regulate apoptosis in CC and HCC in vivo.

A topoisomerase II inhibitor, NK109, induces DNA single- and double-strand breaks and apoptosis

2,3-(Methylenedioxy)-5-methyl-7-hydroxy-8-methoxybenzo[c]phenanthr idinium hydrogensulfate dihydrate, called NK109, is a benzo[c]phenanthridine derivative, which inhibits DNA topoisomerase II activity by stabilizing the DNA-enzyme-drug complex, and shows strong growth-inhibitory effects on several human cancer cells. In the present study, NK109 treatment induced DNA fragmentation and a rise in the level of cytoplasmic nucleosomes, which are markers of apoptosis, in human small-cell lung carcinoma SBC-3 cells. These effects were inhibited by zinc ions and enhanced by cycloheximide or actinomycin D. Dose-dependent single- and double-strand DNA breaks were observed, using alkaline and neutral elution assays, in SBC-3 cells treated with more than 0.2 microM NK109 for 4 h. Treatment with NK109 caused more DNA single- and double-strand breaks than treatment with an equimolar amount of VP-16. These results suggest that NK109 induces DNA strand breaks and apoptosis. In addition, it appears that this process does not require protein or RNA synthesis, but involves a specific endonuclease which is inhibited by zinc ions.

Adenovirus-mediated p53 gene transfer suppresses growth of human glioblastoma cells in vitro and in vivo

Alterations in the p53 tumor-suppressor gene occur in 35-60% of human glioblastomas, and re-introduction of p53 can suppress neoplastic growth. To evaluate the potential for p53 gene therapy of glioblastoma, we have analyzed the response of human glioblastoma cell lines in vitro and in vivo to experimental therapy with replication-deficient recombinant adenoviruses encoding wild-type p53 (rAd-p53). Western blot analyses showed high-level expression of p53 protein after treatment with rAd-p53, and transgene expression was dependent on promoter strength. A p53-specific dose-dependent inhibition of in vitro cellular proliferation was observed in 5 of 6 cell lines, and growth inhibition corresponded to adenovirus-mediated gene transfer and expression. p53-specific cell death was quantitated by release of the lactate dehydrogenase enzyme. Fragmentation of DNA into nucleosomal oligomers and the occurrence of a hypodiploid cell population detected by flow cytometry provided evidence for apoptosis. Studies in nude mice demonstrated that ex vivo infection with rAd-p53 suppressed the tumorigenic potential of human glioblastoma cells. Furthermore, direct injection of rAd-p53 into established s.c. xenografts inhibited tumor growth. Our observations suggest that re-introduction of wild-type p53 may have potential clinical utility for gene therapy of glioblastoma.

Anti-apoptotic activity of low levels of wild-type p53

Induction of apoptosis is a function of both an external stimulus and the physiology of the cell, which includes the expression of multiple oncogenes and tumor suppressors. Here we have studied the apoptotic response of immortalized mouse fibroblasts to serum withdrawal. We show that, in addition to the p53-independent apoptosis observed in p53- cells, overexpression of wild-type p53 tumor suppressor results in a high rate of programmed cell death. However, physiological range, low levels of the p53 protein protect fibroblasts from induction of apoptosis. Our results indicate that, as a function of its dose, the wild-type p53 can either protect from death or promote apoptosis. This new, anti-apoptotic, activity of p53 may have implications for the understanding of the role played by p53 in embryonic development as well as in initial stages of oncogenesis.

Involvement of p53 in DNA strand break-induced apoptosis in postmitotic CNS neurons

The tumour suppressor p53 gene serves as a critical regulator of the cell cycle and of apoptosis following the exposure of normal cells to DNA damage. To examine the role of p53 in postmitotic CNS neurons, we cultured cerebellar neurons from normal wild-type mice and mutant p53-null mice under various conditions inducing neuronal death. When cerebellar neurons from 15- to 16-day postnatal wild-type mice were treated with ionizing radiation or DNA-damaging agents, massive neuron death occurred after 24-72 h. In contrast, neurons from p53-/- mice evidently resisted gamma-irradiation and some DNA-damaging agents, such as etoposide and bleomycin. On the other hand, low-K+ medium-induced apoptosis of cerebellar neurons was not affected by p53 status. Neither cell cycle progression nor DNA synthesis occurred during cell death induced by gamma-irradiation and low-K+ medium, as well as in normal cultures of p53+/+ and p53-/- neurons. These results suggest that p53 is required for the apoptotic death of postmitotic cerebellar neurons induced by DNA strand breaks.

Retrovirus-mediated wild-type p53 gene transfer to tumors of patients with lung cancer

A retroviral vector containing the wild-type p53 gene under control of a beta-actin promoter was produced to mediate transfer of wild-type p53 into human non-small cell lung cancers by direct injection. Nine patients whose conventional treatments failed were entered into the study. No clinically significant vector-related toxic effects were noted up to five months after treatment. In situ hybridization and DNA polymerase chain reaction showed vector-p53 sequences in posttreatment biopsies. Apoptosis (programmed cell death) was more frequent in posttreatment biopsies than in pretreatment biopsies. Tumor regression was noted in three patients, and tumor growth stabilized in three other patients.

Presence of human papillomavirus DNA and abnormal p53 protein accumulation in lung carcinoma

BACKGROUND

In some carcinomas inactivation of the tumour suppressor gene product p53, either by point mutation or indirectly by the human papillomavirus (HPV), has been suggested as two alternative routes to malignant transformation. To test this hypothesis in lung tumours, 43 lung carcinomas were analysed by in situ hybridisation and polymerase chain reaction (PCR) for the presence of HPV DNA, and the results were compared with p53 protein immunohistochemical analysis.

METHODS

The presence of HPV DNA in lung carcinoma was detected by nucleic acid in situ hybridisation for HPV types 6, 11, 16, 18, 31, and 33 using nonradioactively labelled DNA probes. Polymerase chain reaction (PCR) analysis was performed on all cases showing positive HPV DNA labelling by in situ hybridisation and in an additional 13 negative cases. Abnormal nuclear accumulation of the p53 protein was revealed by immunohistochemistry using the avidin-biotin-peroxidase complex method and a CM-1 polyclonal anti-human p53 antibody and a monoclonal mutation-specific Pab 240 p53 antibody.

RESULTS

HPV DNA was found by in situ hybridisation in 13 lung carcinomas (30%). In all these cases subtype-specific HPV DNA could also be detected by PCR. Abnormal p53 protein accumulation was seen in 21 of the 43 carcinomas (49%), of which 18 were HPV negative. Twelve (57%) of the CM-1 positive cases were also positive for the mutation-specific antibody Pab 240. There was an obvious inverse relationship between the presence of papilloma viral DNA and abnormal p53 protein accumulation.

CONCLUSIONS

p53 plays an important part in the development of lung carcinomas and, in some cases, HPV may contribute to it by binding and inactivating the p53 protein.

p53 plays a regulatory role in differentiation and apoptosis of central nervous system-associated cells

This study demonstrated the involvement of the tumor suppressor protein p53 in differentiation and programmed cell death of neurons and oligodendrocytes, two cell types that leave the mitotic cycle early in development and undergo massive-scale cell death as the nervous system matures. We found that primary cultures of rat oligodendrocytes and neurons, as well as of the neuronal PC12 pheochromocytoma cell line, constitutively express the p53 protein. At critical points in the maturation of these cells in vitro, the subcellular localization of p53 changes: during differentiation it appears mainly in the nucleus, whereas in mature differentiated cells it is present mainly in the cytoplasm. These subcellular changes were correlated with changes in levels of immunoprecipitated p53. Infection of cells with a recombinant retrovirus encoding a C-terminal p53 miniprotein (p53 DD), previously shown to act as a dominant negative inhibitor of endogenous wild-type p53 activity, inhibited the differentiation of oligodendrocytes and of PC12 cells and protected neurons from spontaneous apoptotic death. These findings suggest that p53, upon receiving appropriate signals, is recruited into the nucleus, where it plays a regulatory role in directing primary neurons', oligodendrocytes, and PC12 cells toward either differentiation or apoptosis in vitro.

Cellular oxidative stress and the control of apoptosis by wild-type p53, cytotoxic compounds, and cytokines

Apoptosis induced by wild-type p53 or cytotoxic compounds in myeloid leukemic cells can be inhibited by the cytokines interleukin 6, interleukin 3, granulocyte-macrophage colony-stimulating factor, and interferon gamma and by antioxidants. The antioxidants and cytokines showed a cooperative protective effect against induction of apoptosis. Cells with a higher intrinsic level of peroxide production showed a higher sensitivity to induction of apoptosis and required a higher cytokine concentration to inhibit apoptosis. Decreasing the intrinsic oxidative stress in cells by antioxidants thus inhibited apoptosis, whereas increasing this intrinsic stress by adding H2O2 enhanced apoptosis. Induction of apoptosis by wild-type p53 was not preceded by increased peroxide production or lipid peroxidation and the protective effect of cytokines was not associated with a decrease in these properties. The results indicate that the intrinsic degree of oxidative stress can regulate cell susceptibility to wild-type p53-dependent and p53-independent induction of apoptosis and the ability of cytokines to protect cells against apoptosis.

High frequency of apoptosis in infantile capillary haemangioma

Infantile capillary haemangioma (ICH) is a well-established clinicopathological entity which often regresses spontaneously. To elucidate the cause of spontaneous involution of ICH, the apoptotic and proliferative activities in seven cases of ICH were compared with those in five cases of lobular capillary haemangioma (LCH), using formalin-fixed paraffin-embedded tissue sections. The number of apoptotic cells detected by the modified in situ end-labelling method was significantly higher in ICH than in LCH, while the proliferative activities evaluated with mitosis and Ki-67 antigen expression did not differ significantly. Lewisy (Ley) antigen, an apoptosis-associated marker, was expressed in all cases of ICH but in none of LCH, while labelling for p53 protein and bcl-2 protein was almost completely negative in both tumours. These findings clearly demonstrate a much higher apoptotic activity in ICH than in LCH and suggest that apoptosis might be a cause of the spontaneous involution of ICH.

Genetic determinants of p53-induced apoptosis and growth arrest

Previous studies have suggested that expression of p53 in cancer cells can result in either growth arrest or apoptosis. Accordingly, expression of p53 in a series of colorectal cancer cell lines yielded growth arrest in some lines (A-lines) and apoptosis in others (D-lines). To investigate the basis of this difference, we evaluated the role of p21WAF1/Cip1, a known mediator of p53-induced growth arrest. Inactivation of p21 by homologous recombination converted an A-line to a D-line, suggesting that p21 could protect cells from apoptosis. However, examination of p53-induced p21 expression in naturally occurring D-lines and A-lines demonstrated that the induction of p21 could not account for the differential response to p53. Moreover, when a D-line was fused to an A-line, the resulting hybrid cells underwent apoptosis in response to p53, indicating that the apoptosis pathway was dominant over the growth arrest pathway. Therefore, the apoptotic response to p53 in colorectal cancer cells is modulated by at least two factors: p21-mediated growth arrest that can protect cells from apoptosis in A-cells, and trans-acting factors in D-cells that can overcome this protection, resulting in cell death.

Comparative alterations in p53 expression and apoptosis in the irradiated rat small and large intestine

Temporal and spatial relationships between radiation-induced apoptosis and expression of p53 mRNA and protein were compared in rat small and large intestine. Apoptosis was quantified using morphological criteria, and p53 expression determined by immunohistochemistry or whole-tissue Northern analysis. In the small intestine, peak levels of apoptosis appeared earlier (4 h) than in the large intestine (6 h). p53 mRNA transcript levels in small and large intestine were not significantly altered from control levels at any time after treatment. However, in treated small and large intestine, cells showed increased positivity for p53 protein, increasing 10-fold over control levels 4-5 h after irradiation. A strong spatial relationship was found between high incidence apoptosis and p53 protein positivity. We compared published data of stem cell population positions for small and large intestine with our results. Target cells for apoptosis and p53 expression occurred at approximately fifth position from the crypt base of the small intestine, a zone coincident with stem cell population. Target cell position for apoptosis and p53 expression in the large intestine was again at fifth or sixth position from the base, but this zone is not the reported stem cell position (first or second position) for large intestine. Results from our model of radiation-induced intestinal apoptosis indicate that p53 protein is closely associated both temporally and spatially with the induction of apoptosis, and support the work of others in suggesting that p53 expression is modulated post-transcriptionally. Furthermore, our results support a hypothesis that apoptotic targeting of damaged stem cell populations, early response for apoptotic removal of DNA-damaged cells and/or early repair of these damage cells are all important parameters that determine differences in levels of tumorigenesis in the small and large intestine.

The p53-binding protein 53BP2 also interacts with Bc12 and impedes cell cycle progression at G2/M

Using the yeast two-hybrid system, we have isolated a cDNA (designated BBP, for Bcl2-binding protein) for a protein (Bbp) that interacts with Bcl2. Bbp is identical to 53BP2, a partial clone of which was previously isolated in a two-hybrid screen for proteins that interact with p53. In this study, we show that specific interactions of Bbp/53BP2 with either Bcl2 or p53 require its ankyrin repeats and SH3 domain. These interactions can be reproduced in vitro with bacterially expressed fusion proteins, and competition experiments indicate that Bcl2 prevents p53 from binding to Bbp/53BP2. BBP/53BP2 mRNA is abundant in most cell lines examined, but the protein cannot be stably expressed in a variety of cell types by transfection. In transiently transfected cells, Bbp partially colocalizes with Bcl2 in the cytoplasm and results in an increased number of cells at G2/M, possibly accounting for the inability to obtain stable transfectants expressing the protein. These results demonstrate that a single protein can interact with either Bcl2 or p53 both in yeast cells and in vitro. The in vivo significance of these interactions and their potential consequences for cell cycle progression and cell death remain to be determined.

The role of p53 in cell cycle regulation

The tumor suppressor gene p53, implicated in diverse types of human tumors, functions both as a gene-specific transcription factor as well as a specific inhibitor of the transcription of certain genes. The two physiological outcomes of re-expression of wild type p53 in tumor cells, not expressing wild type p53, are G1 arrest and apoptosis. The mechanism of G1 arrest by p53 is much better documented than its ability to trigger apoptosis. P53 as a transcription factor induces the expression of p21WAF1/CIP1/Sdi1, an inhibitor of the cyclin dependent kinases (CDKs)2, 3, 4 and 6. Thus, a G1 arrest can result simply by the p53 induced expression of p21WAF1/CIP1/Sdi1. Amongst the other genes presently characterized to be regulated by p53 are BAX, a homologue of the BCL-2 gene. Bax does not trigger apoptosis, but simply accelerates the rate at which apoptosis proceeds54. P53 also down regulates the expression of cyclin A, providing a secondary break on cell cycle progression into the through the S phase.

A functionally inactive p53 protein in teratocarcinoma cells is activated by either DNA damage or cellular differentiation

Testicular teratocarcinomas never contain p53 gene mutations even though these tumors express high levels of nuclear p53 protein. We have characterized two murine teratocarcinoma cell lines and find no evidence that endogenous p53-regulated genes are correspondingly upregulated. Differentiation of these teratocarcinoma cells with retinoic acid results in a marked decrease in p53 protein levels but is accompanied by a marked increase in p53-mediated transcriptional activity. Together these results support the hypothesis that the p53 protein in undifferentiated teratocarcinoma cells is transcriptionally inactive and accounts for the lack of selection for p53 gene mutations in this tumor type. These teratocarcinoma cells undergo p53-mediated apoptosis in response to DNA damage, which may explain the routine cures of human testicular tumors with combination chemotherapy.

Significance of spontaneous apoptosis during colorectal tumorigenesis

To determine if apoptosis is involved in colorectal tumorigenesis and its progression, colorectal adenomas (n = 63), carcinomas (n = 49), and normal mucosa were investigated by using in situ end-labeling (TUNEL) method. The expression of Ki-67 was also analyzed immunohistochemically. TUNEL labeling index (TLI) and Ki-67 labeling index (KLI) were determined. TLI/KLI was significantly higher in the adenomas of small size and/or of low and middle grade atypia than those of large size and/or of high grade atypia. No difference was observed in the indices between adenomas and carcinomas and among the cancer groups classified on the basis of their clinicopathological features. The results indicate that the reduction of susceptibility to apoptosis plays an important role in the early stage of the adenoma-carcinoma sequence. Apoptosis can explain the enormous cell loss thought to exist in normal colorectal mucosa and in the tumor growth process.

Expression of bcl-2 and p53 in Merkel cell carcinoma. An immunohistochemical study

Bcl-2 is a protooncogene thought to play a role in oncogenesis by inhibiting programmed cell death. It may interact with p53, a tumor-suppressor gene which induces apoptosis in certain circumstances. We have studied these gene products by immunohistochemistry in 15 cases of Merkel cell carcinoma, a tumor characterised by prominent apoptosis. Five cases showed moderate/strong staining for p53, with moderate/strong bcl-2 staining in 10 patients. In seven cases abundance of p53 and bcl-2 expression was mutually exclusive. Two patients died within 1 year of diagnosis and six had nodal recurrences. Gene expression and survival appear unrelated. The role of Bcl-2 and p53 in tumorigenesis is complicated and may be inter-related with other genes known to be involved in programmed cell death.

The XPB and XPD DNA helicases are components of the p53-mediated apoptosis pathway

The molecular pathway of p53-dependent apoptosis (programmed cell death) is poorly understood. Because p53 binds to the basal transcription-repair complex TFIIH and modulates its DNA helicase activities, we hypothesized that TFIIH DNA helicases XPB and XPD are members of the p53-mediated apoptotic pathway. Whereas transfer of a wild-type p53 expression vector by microinjection or retroviral infection into primary normal human fibroblasts resulted in apoptosis, primary fibroblasts from individuals with xeroderma pigmentosum (XP), who are deficient in DNA repair and have germ-line mutations in the XPB or XPD gene, but not in the XPA or XPC gene, have a deficiency in the apoptotic response. This deficiency can be rescued by transferring the wild-type XPB or XPD gene into the corresponding mutant cells. XP-D lymphocytes also have a decreased apoptotic response to DNA damage by adriamycin, indicating a physiologically relevant deficiency. The XP-B or XP-D mutant cells undergo a normal apoptotic response when microinjected with the Ich-L, and ICE genes. Analyses of p53 mutants and the effects of microinjected anti-p53 antibody, Pab421, indicate that the carboxyl terminus of p53 may be required for apoptosis. Direct microinjection of the p53 carboxy-terminal-derived peptide (amino acid residues 319-393) resulted in apoptosis of primary normal human fibroblasts. These results disclose a novel pathway of p53-induced apoptosis.

Frequent expression of Bcl-2 in renal-cell carcinomas carrying wild-type p53

The p53 tumor-suppressor gene is the most commonly mutated gene in cancer. However, p53 gene alterations are infrequent in renal-cell cancer (RCC). Bcl-2 has been shown to inhibit apoptosis triggered by wild-type p53 and an inverse correlation between Bcl-2 expression and p53 mutation has been observed in breast cancer and glioma. To characterize the expression of bcl-2 in RCC and its relationship to the p53 status, we analyzed 25 RCCs by immunohistochemistry for Bcl-2 and p53, Southern hybridization for bcl-2, and PCR-SSCP and sequencing for p53. Positive Bcl-2 staining was detected in 17 of 25 RCCs, whereas positive p53 staining was seen in only 1. Amplification of bcl-2 or p53 mutation was not detected in any of the tumors. Bcl-2 protein was expressed in all 7 RCC cell lines examined. Only one of the 7 lines had p53 mutation. These results suggest that overexpression of bcl-2, rather than p53 mutation, may prevent apoptosis during RCC development.

In situ analysis of tissue dynamics and p53 expression in human gastric mucosa

In situ tissue dynamics were studied in 12 cases of human gastric mucosa, including normal gastric body mucosa and gastric glands with intestinal metaplasia, obtained from gastrectomy specimens of adenocarcinoma. Cell proliferation was determined by Ki67 immunoreactivity. DNA fragmentation was studied in situ by TdT-mediated dUTP-biotin nick end labelling (TUNEL). In addition, p53 expression was examined by both immunohistochemistry and mRNA in situ hybridization. In the oxyntic gastric glands, Ki67 immunoreactivity was observed exclusively in the proliferative zone and TUNEL-positive cells were present predominantly in the surface foveolar epithelium. In the gastric glands with complete intestinal metaplasia, Ki67-positive cells were present in the lower portion of the glands and TUNEL-positive cells in the superficial epithelium. In the gastric glands with incomplete intestinal metaplasia, TUNEL-positive cells were detected in the lower gastric glands adjacent to cells immunoreactive for Ki67; the proportion of these gastric glands with TUNEL-positive cells (40 out of 108 glands) was significantly higher than for oxyntic glands (94 out of 620 glands) or for glands with complete metaplasia (31 out of 254 glands). Relatively strong p53 immunoreactivity and mRNA hybridization were also observed in the proliferative and apoptotic areas of gastric glands with incomplete intestinal metaplasia. These results indicate that incomplete intestinal metaplasia is associated with increased cell turnover and p53 overexpression, possibly in response to various noxious or DNA-damaging stimuli.

Apoptotic cell death and its relationship to carcinogenesis in colorectal carcinoma

BACKGROUND

Apoptotic cell death plays an important role in the proliferation and turnover of cells in various tumors. The relationship between apoptosis and cell proliferation was studied to determine each of their roles in colorectal carcinogenesis.

METHODS

Apoptotic cells were identified by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) method. The occurrence of apoptosis was examined in colorectal cancer that had invaded the submucosa. Specimens were obtained from 38 cases of cancer with adenoma and 29 cases of cancer de novo. Apoptotic indices (AIs) as percentages of TUNEL-positive cells relative to the number of tumor cells and Ki-67 labeling indices (PI) were investigated. The relationship between the frequency of apoptosis and the expression of p53 and c-myc proteins was also investigated.

RESULTS

In cancer with adenoma, the ratio of AI/PI in adenoma cells was significantly higher than that of cancer cells (P < 0.0001). Mean AIs of cancer with adenoma were significantly higher than those of cancer de novo particularly in the flat-type cancers (P < 0.05). Among p53 negative tumors, the ratio of AI/PI for cancer de novo was significantly lower than that for cancer with adenoma (P < 0.05). AI of cancer de novo was lower than that of cancer with adenoma in cases with overexpression of c-myc protein (P < 0.05), whereas there was no significant difference in the ratio of AI/PI between cancer de novo and cancer with adenoma.

CONCLUSIONS

Colorectal carcinogenesis is related to the inhibition of apoptosis and to the augmentation of proliferative activity both in cancer with adenoma and in cancer do novo. A reduction of the rate of apoptosis as compared with that of cell proliferation might explain the rapid-growing nature of cancer de novo particularly in cases with the flat-type appearance.

Functional interactions between p53 and the TFIIH complex are affected by tumour-associated mutations

The p53 tumour suppressor is mutated in the majority of human tumours. p53's proposed role as the guardian of the genome is reflected in its multiple effects on transcription genome stability, cell growth and survival. We show that p53 interacts both physically and functionally with the TFIIH complex. There are multiple protein-protein contacts, involving two regions of p53 and three subunits of TFIIH, ERCC2 (XPD), ERCC3 (XPB) and p62. p53 and its C-terminus (amino acids 320-393) inhibit both of the TFIIH helicases and in vitro transcription in the absence of TFIIH. Transcription inhibition is overcome by TFIIH. The N-terminal region of p53 (1-320), lacking the C-terminus, is inactive on its own, yet apparently affects the activity of the C-terminus in the native protein. Interestingly, mutant p53s that are frequently found in tumours are less efficient inhibitors of the helicases and transcription. We hypothesize that the interactions provide an immediate and direct link for p53 to the multiple functions of TFIIH in transcription, DNA repair and possibly the cell cycle.

Antisense oncogene and tumor suppressor gene therapy of cancer

Rapid advances in cancer gene therapy are driven by an explosive development of gene transfer technology and a strong demand for seeking alternatives to unsatisfactory conventional cancer therapies. Discovery of the genetic basis of cancer has indicated that cancer is a disease of genes. Among a variety of approaches to gene therapy of cancer, antisense oncogene and tumor suppressor gene therapy of cancer are the two strategies that aim at correcting genetic disorders of cancer through suppression of the abnormal expression of the proliferative genes. The potential effectiveness of these approaches is promised by their precise targeting at the mechanisms of the disease. Examples of several preclinical studies of these types of approaches that led to the approval of clinical trials are reviewed. Limitation and future development of these approaches are also discussed.

Tumour cells surviving in vivo cisplatin chemotherapy display elevated c-myc expression

The c-myc oncogene has been extensively implicated in cell proliferation, cell differentiation and programmed cell death. Aberrant expression of the c-myc gene product has been observed in a range of tumours and has also been implicated in cisplatin (cis-dichlorodiammineplatinum)-mediated chemoresistance. A solid transplantable tumour model in syngeneic DA rats was subjected to treatment with cisplatin to determine the impact of such therapy on endogenous c-myc gene expression. Serially transplanted tumours were intravenously treated with a single cisplatin dose (1 mg/kg) and c-myc expression analysed 2 and 7 days after treatment. The surviving tumour cells display a significant 2-fold elevation in c-myc expression at 48 h and 7 days after treatment. Primary cell cultures have been derived from untreated in vivo tumours of the same model and subjected to treatment with a c-myc phosphorothioate antisense oligomer. Administration of 5 microM c-myc antisense oligomer directed at the initiation codon and first four codons of c-myc mRNA results in total inhibition of c-myc expression and coincident suspension of cell growth for a period of 4 days in culture. Antisense therapies directed at the c-myc gene may well prove an effective tool for treating tumours in conjunction with cisplatin as these findings show that tumour cells surviving cisplatin chemotherapy display elevated c-myc expression.

p53 protein in sympathetic neurons: cytoplasmic localization and no apparent function in apoptosis

The p53 tumour suppressor gene plays a major role in controlling cell cycle and apoptosis in many different cell types. Here we have examined the status and the potential apoptosis inducing activity of p53 in sympathetic neurons. The p53 protein is expressed in rat sympathetic neurons cultured in the presence of NGF. The protein is not upregulated when these neurons are induced to die upon NGF deprivation. Over-expression of wild-type human p53 in neurons cultured in the presence of NGF does not trigger apoptosis nor does it accelerate apoptosis when the neurons are deprived of NGF. Finally endogenous p53 expression is not necessary for neuronal cell death triggered by NGF deprivation since neurons prepared from p53 knockout mice undergo normal cell death upon NGF deprivation. Our results suggest that p53 may have an unknown function in post-mitotic neurons which is distinct from its well described roles in apoptosis or cell cycle control.

Apoptosis of colon cancer: comparison with Ki-67 proliferative activity and expression of p53

The role of apoptosis in colon cancer was investigated in terms of control of growth and expression on p53, using the nick-ended-DNA labelling method and immunohistochemistry. The apoptotic labeling index was highest in the T1 stage (24 cases), as was the proliferative activity, assessed in terms of the Ki-67 labeling index. Both labeling indices demonstrated similar overall incidence curves for the total 95 colon cancer cases, and examination of individual cases revealed a statistically significant correlation (P=0.01). However, neither index had any relation to p53. The results thus suggest that apoptosis in colon cancers has a linkage with proliferative activity that can be assessed by Ki-67 labeling, but is not regulated by the p53 system. This might contribute to the diversity of colon cancer growth.

Programmed cell death and radioresistance

Whereas apoptosis is a critical mode of cell deletion in normal organism development, apoptotic cells are also observed in tumors, especially following cytotoxic treatments, leading to questions about their role in tumor response to therapy. We have conducted a series of studies using murine tumor models and found that the ability of the tumor cells to undergo apoptosis correlates with tumor response to radiation. The best correlation was with the pretreatment apoptotic index, suggesting that apoptosis in some tumors may govern radiocurability by regulating the number of tumor clonogens. However, other roles for apoptosis in tumor response to radiation have not been ruled out. One of the important observations that has come from this work has been the heterogeneity in apoptosis propensity both within the cell population of a given tumor and among different types of tumors. Such findings underscore the fact that apoptosis is under complex genetic control and that some of the same oncogenes and tumor suppressor genes that are responsible for tumor initiation and progression to malignancy also dictate the apoptotic response to treatment. Understanding the biochemical and molecular pathways that govern this process may ultimately allow the development of strategies for modulating apoptosis for therapeutic benefit.

Specific loss of apoptotic but not cell-cycle arrest function in a human tumor derived p53 mutant

The p53 tumor-suppressor gene product is frequently inactivated in malignancies by point mutation. Although most tumor-derived p53 mutants show loss of sequence specific transcriptional activation, some mutants have been identified which retain this activity. One such mutant, p53175P, is defective for the suppression of transformation in rodent cells, despite retaining the ability to suppress the growth of p53-null human cells. We now demonstrate that p53175P can induce a cell-cycle arrest in appropriate cell types but shows loss of apoptotic function. Our results therefore support a direct role of p53 transcriptional activation in mediating a cell-cycle arrest and demonstrate that such activity is not sufficient for the full apoptotic response. These data suggest that either p53 can induce apoptosis through a transcriptionally independent mechanism, a function lost by p53175P, or that this mutant has specifically lost the ability to activate genes which contribute to cell death, despite activation of genes responsible for the G1 arrest. This dissociation of the cell-cycle arrest and apoptotic activities of p53 indicates that inactivation of p53 apoptotic function without concomitant loss of growth inhibition can suffice to relieve p53-dependent tumor-suppression in vivo and thereby contribute to tumor development.

Targeted in vivo expression of the cyclin-dependent kinase inhibitor p21 halts hepatocyte cell-cycle progression, postnatal liver development and regeneration

The CDK inhibitor p21 (WAF-1/CIP-1/SDI-1) has been implicated in DNA damage-induced p53-mediated G1 arrest, as well as in physiological processes, such as cell differentiation and senescence, that do not involve p53 function. To determine the impact of p21 on normal development and cell-cycle regulation in vivo, we have generated transgenic mice that abundantly express p21 specifically in hepatocytes. During postnatal liver development, when transgenic p-21 protein becomes detectable, hepatocyte proliferation is inhibited dramatically. This disturbance causes a reduction in the overall number of adult hepatocytes, resulting in aberrant tissue organization, runted liver and body growth, and increased mortality. The transgenic p21 protein is associated with most, if not all, of the cyclin D1-CDK4 in liver but not significantly with other cyclin/CDK proteins, indicating the importance of cyclin D1-CDK4 function in normal liver development. The appearance of large polyploid nuclei in some hepatocytes indicates that p21 may also cause arrest during the G2 phase of the cell cycle. Significantly, partial hepatectomy failed to stimulate hepatocytes to proliferate in p21 transgenic animals. These results provide the first in vivo evidence that appropriate p21 levels are critical in normal development and further implicate p21 in the control of multiple cell-cycle phases.

Apoptosis: inhibitor or instigator of carcinogenesis?

Carcinogenesis is considered to require an initiating event that results in an irreversible genetic change in a subpopulation of cells. Based on the available evidence, it seems likely that apoptosis may act to attenuate this process by causing the deletion of genetically damaged cells from the host organism. Nevertheless, the existence of an active pathway leading to apoptotic cell death may be a double-edged sword, simply because it can be overcome. Some cells may exhibit preexisting genetic or epigenetic insensitivity to induction of apoptosis. Surviving cells may contain sub- lethal levels of DNA damage and be induced to proliferate as an indirect result of the carcinogen-induced apoptotic cell death of surrounding tissue. This process would facilitate the acquisition mutations in the genome, possibly resulting in further insensitivity to apoptosis through activation of the bcl-2 oncogene or inactivation of the p53 tumor suppressor gene. In this context, the propensity of a cell to undergo apoptosis could be viewed as a selection pressure that a tumor cell must overcome. For neoplastic growth to occur, an imbalance between proliferation and apoptosis must be established such that cell growth predominates. Genetic mutations or epigenetic factors that diminish the propensity of a cell to undergo apoptosis may therefore confer on that cell a growth advantage.

Induction of apoptosis by c-Fos protein

The role of c-Fos in apoptosis was examined in two Syrian hamster embryo cell lines (sup+I and sup-II) and a human colorectal carcinoma cell line (RKO), using the chimeric Fos-estrogen receptor fusion protein c-FosER. As previously reported, contrasting responses were observed when these two cell lines were placed under growth factor deprivation conditions; sup+I cells were highly susceptible to apoptosis, whereas sup-II cells were resistant. In this report, we show that the activated c-FosER protein induces apoptosis in sup-II preneoplastic cells in serum-free medium, indicating that c-Fos protein can induce apoptotic cell death in these cells. c-Fos-induced apoptosis was not blocked by the protein synthesis inhibitor cycloheximide, suggesting that the c-Fos transcriptional activation activity is not involved. This conclusion was further supported by the observation that overexpression of v-Fos, which is highly proficient in transcriptional activation but deficient in the transcriptional repression activity associated with c-Fos, did not induce apoptosis. Constitutively expressed Bcl-2 delayed the onset of low-serum-induced apoptosis in sup+I cells and enhanced survival in sup-II cells. Further, coexpression of Bcl-2 and c-FosER in sup+I or sup-II cells protected the cells from c-FosER-induced apoptosis. The possibility that c-FosER-induced apoptosis requires a p53 function was examined. Colorectal carcinoma RKOp53+/+ cells, which do not normally undergo apoptosis in serum-free medium, showed apoptotic DNA fragmentation upon expression and activation of c-FosER. Further, when the wild-type p53 protein was diminished in the RKO cells by infection with the papillomavirus E6 gene, subsequent c-FosER-induced apoptosis was blocked. The data suggest that c-Fos protein plays a causal role in the activation of apoptosis in a p53-dependent manner. This activity does not require new protein synthesis and is blocked by overexpression of Bcl-2 protein.

The association between cell proliferation and apoptosis: studies using the cell cycle-associated proteins Ki67 and DNA polymerase alpha

The process of apoptosis is associated with the inappropriate expression of cell cycle regulatory proteins, which has led to the proposal that the apoptotic pathway represents an abortive attempt to pass through the cell proliferation cycle. To investigate this hypothesis, we examined the expression of two proliferation-associated antigens in apoptotic cells. Apoptotic bodies seen in a range of normal and pathological tissues are often positive for the Ki67 antigen, indicating that these cells were in the cell cycle during the period that they died. In contrast, spontaneous apoptosis of human polymorphonuclear leukocytes maintained in culture was not associated with the expression of either Ki67 or DNA polymerase a. In addition, apoptotic bodies in the pre-menstrual endometrium did not express the Ki67 antigen. These results indicate that, contrary to previous suggestions, apoptosis does not always depend on cell cycle entry. The use of antibodies to Ki67 should be valuable in defining the association of apoptosis with proliferation in a wide range of cells and tissues.

Apoptosis and Expression of the Proto-Oncogenes bcl-2 and p53 and the Proliferation Factor Ki-67 in Human Medullary Thyroid Carcinoma

Apoptosis and Immunoreactivity for bcl-2, p53, and Ki-67 were studied in 21 patients with meduilary thyroid carcinoma (MTC). The DNA nick end labeling method was used to assess apoptosis. The relationships between the different factors were analyzed, as were their relations to clinicopathologic data, including survival. More than 80% of the tumors harbored apoptotic cells. Tumors in individuals who had died of the MTC disease had a higher percentage of apoptosis. All cases demonstrated immunoreactivity to bcl-2 disease-free individuals had a higher rate than those with recurrent disease. No obvious pattern could be discerned in the relation of p53 or Ki-67 to the outcome of the disease. An inverse correlation between bcl-2 and apoptosis (r=-0.81; p < 0.01) was demonstrated. bcl-2 was significantly (p = 0.014) associated with apoptosis even after taking both p53 and Ki-67 into consideration, but these two factors were unrelated to apoptosis. None of the factors studied were correlated to crude survival, either in univarlate or in muiltivariate analyses. This study established that bcl-2 immunoreactivity is closely associated with apoptosis in MTC, suggesting that a down-regulation of the bcl-2 protein is related to a more aggressive growth rate and might be a useful marker for the evaluation of MTC.