Induction of the TRAIL receptor KILLER/DR5 in p53-dependent apoptosis but not growth arrest.

The TRAIL death receptor KILLER/DR5 is induced by DNA damaging agents in wild-type p53-expressing cells. Here we show that, unlike the p53-target CDK-inhibitor p21WAF1/CIP1, the TRAIL death receptor KILLER/DR5 is only induced in cells undergoing p53-dependent apoptosis and not cell cycle arrest. Thus GM glioblastoma cells carrying an inducible MMTV-driven p53 gene undergo cell cycle arrest and upregulate p21 but not KILLER/DR5 expression upon dexamethasone exposure. WI38 normal lung fibroblasts undergoing cell cycle arrest in response to ionizing irradiation also induce p21 but not KILLER/DR5 gene expression. KILLER/DR5 upregulation is also deficient in irradiated lymphoblastoid cells derived from patients with Ataxia Teleangiectasia suggesting a role for the ATM-p53 pathway in regulating KILLER/DR5 expression after DNA damage. Inhibition of transcription by Actinomycin D blocks both KILLER/DR5 and p21 induction in cells undergoing p53-dependent apoptosis. Our results suggest that the p53-dependent transcriptional induction of KILLER/DR5 death receptor is restricted to cells undergoing apoptosis and not cells undergoing exclusively p53-dependent G1 arrest.

Bypass of abnormal MDM2 inhibition of p53-dependent growth suppression.

Oncoprotein MDM2 inhibits p53-dependent cell cycle arrest and apoptosis. MDM2-overexpressing human cancer cell lines (n = 3) were found to be resistant to growth inhibition after infection by p53-expressing adenovirus (Ad-p53), as compared to low MDM2-expressing tumors (n = 3), in vitro. The growth of MDM2-overexpressing tumors, however, was inhibited by p21-expressing adenovirus (Ad-p21) infection, and the cyclin-dependent kinase-inhibitory region of p21 was sufficient to bypass the MDM2-p53 feedback loop. The phosphorylation state of Rb correlated with the response to either p53 or p21 gene therapy. MDM2-overexpressing cancer cells infected by Ad-p21 also developed a quiescent large-cell morphology. The results suggest that MDM2-mediated resistance to p53 may be bypassed by p21 and that the Rb phosphorylation state may predict the effects on growth after Ad-p53 or Ad-p21 infection.

p53-independent increase in E2F-1 expression enhances the cytotoxic effects of etoposide and of adriamycin.

The transcription factor E2F-1 drives cell cycle progression at the G1- to S-phase boundary; however, overexpression of E2F-1 can induce apoptosis. We show here that E2F-1 protein levels increase in human medulloblastoma, glioma, lung, colon, and bladder cancer cell lines (n=7) following treatment with the DNA damaging agents adriamycin or etoposide. This induction of E2F-1 occurs independently of Rb or p53 status and involves new protein synthesis. Although E2F-1 protein levels increase following DNA damage, several genes transcriptionally targeted by E2F-1 are not similarly induced. Rather, induction of E2F-1 in the tumor cells correlates with their sensitivity to adriamycin or to etoposide. Correspondingly, fibroblasts from E2F-1 knockout mice are more resistant to DNA damage than cells from normal mice. Overexpression of E2F-1 protein in tumor cell lines infected with an adenovirus encoding wild-type E2F-1 leads to enhanced cytotoxicity following exposure to DNA damaging agents, which results from enhanced apoptosis. The results of this study implicate a role for E2F-1 in p53-independent cytotoxicity of chemotherapy and provide a pharmacological tool for increasing levels of the apoptosis-inducing E2F-1 protein.

Tumor suppressor gene therapy for cancer: from the bench to the clinic.

There is a growing list of possible tumor suppressors that can potentially be used to control cancer cell growth in the clinic. These include p53, Rb, p21, p16, p27, BRCAI and APC, some of which are already in clinical trials, p53 induces apoptosis and suppresses cancer cells containing multiple genetic alterations as well as multidrug-resistant cells, making it a promising and popular target. Other agents such as CDK-inhibitors are generally cytostatic with little evidence for apoptosis. The genetic make-up may help guide a rational therapy of particular tumors. Preclinical studies are exploring combinations of gene therapy and chemotherapy. Some early results are beginning to emerge from clinical trials including those using the E1b-deleted adenovirus that is unique in being a tumor-specific cytotoxic agent for the most common types of cancer.

p53-independent upregulation of KILLER/DR5 TRAIL receptor expression by glucocorticoids and interferon-gamma.

KILLER/DR5 is a death-domain-containing proapoptotic receptor that binds to the cytotoxic ligand TRAIL. It was originally reported that induction of KILLER/DR5 mRNA following DNA damage was p53-dependent, but some drugs that induce apoptosis can upregulate KILLER/DR5 mRNA expression in cell lines with mutated p53. We further extend those findings by classifying the capability of various apoptosis-inducing drugs to increase the expression of KILLER/DR5 mRNA in a p53-independent manner. beta-Lapachone, a topoisomerase inhibitor, increased KILLER/DR5 mRNA in colon cancer cell lines with wild-type p53 but not with mutant p53. In contrast, betulinic acid, a novel chemotherapeutic compound, induced apoptosis and KILLER/DR5 mRNA in melanoma and glioblastoma cells through a p53-independent mechanism. The synthetic glucocorticoid dexamethasone elevated KILLER/DR5 mRNA in glioblastoma, ovarian cancer, and colon cancer cell lines with mutant p53 undergoing apoptosis, and this induction was inhibited by the transcriptional inhibitor actinomycin D. Although another glucocorticoid, prednisolone, also induced apoptosis, it did not increase KILLER/DR5 mRNA. Finally, the cytokine interferon-gamma (IFN-gamma) induced apoptosis and KILLER/DR5 in cell lines with mutant p53, and the induction of KILLER/DR5 mRNA by IFN-gamma was delayed in cells lacking wild-type STAT1, a transcription factor implicated in IFN-gamma signaling. Similarly, the induction of KILLER/DR5 mRNA by the cytokine TNF-alpha was also delayed in cell lines with mutated STAT1. These findings suggest that KILLER/DR5 may play a role in p53-independent apoptosis induced by specific drugs and warrants further investigation as a novel target for chemotherapy of tumors lacking wild-type p53.

"p53: Twenty Years On" in Trieste, Italy, May 20-22, 1999.

Study of the p53 tumor suppressor has blossomed into a field of its own. From analysis of its mutations in several different cancers to deciphering the function of the protein in human cells, publications describing research on p53 annually number in the thousands. In the interest of presenting the most up-to-date information on the progress of cancer research involving p53, the workshop "p53: Twenty Years On" was organized in Trieste, Italy over May 20-22, 1999. The meeting highlighted some of the most exciting basic and clinical scientific findings on p53 within the last year.

Growth inhibitory effect of p21 and p53 containing adenoviruses on transitional cell carcinoma cell lines in vitro and in vivo.

Altered p53 expression has been demonstrated in the majority of advanced transitional cell carcinoma (TCC) of the bladder tumors. The objective of this investigation was to examine the effect of the introduction of a p53 or p21((WAF1/CIP1)) adenovirus on the proliferation and apoptosis of various human TCC cell lines in vitro and in vivo. Proliferation was measured by 3H-thymidine incorporation. Apoptosis was measured by DNA fragmentation and bax expression. We also examined the effect of ex vivo introduction of the p21((WAF1/CIP1)) or the p53 gene on growth of the T24 TCC cells and UMUC-3 TCC cells introduced subcutaneously into athymic nude mice. We found that although the effect of the p21-adenovirus on the proliferation of various TCC lines varied with each individual cell line, there was a substantial growth inhibition observed (greater than 80% growth inhibition) in seven of the eight TCC cell lines at the highest viral dosage. In contrast, after 24 h, the highest dosage of the p53-adenovirus produced only a heterogeneous decrease in proliferation compared to the highest dose of the p21((WAF1/CIP1))-adenovirus (40-90%). In ex vivo experiments, no tumors were found in nude mice injected subcutaneously with either TCC cell line exposed in vitro to the AdSCMV-p21((WAF1/CIP1)) or AdSCMV-p53 viruses before three weeks. There was a threefold decrease in tumor square area at week 5 in the Ad5CMV-p21((WAF1/CIP1)) or Ad5CMV-p53 TCC cells injected mice (p<0.001, p<0.009) compared to either mock or Ad5CMVLacZ TCC bladder tumor cells. These data suggest that significant portion of the effect of altered p53 on TCC phenotype may be mediated through the p21((WAF1/CIP1)) pathway. Thus, the restoration of p21((WAF1/CIP1)) function in this tumor system may be a beneficial therapeutic strategy.

Comparative gene expression profiling in response to p53 in a human lung cancer cell line.

The p53 tumor suppressor gene functions through the p53-mediated transcriptional activation and regulation of critical downstream target genes. The mechanism behind such regulation, however, remains unclear. In this study, we compared the expression of 30 genes that are involved in cell cycle checkpoint control and/or apoptosis in a human lung cancer cell line, which contains endogenous wild-type p53, in response to ectopic p53 expression. Of the 30 genes studied, 22 genes have shown an increase in expression. The increase in gene expression of 2 genes-Gadd45 and PIG2-was more than 10-fold. These results suggest that the genes with the highest expression level in a p53-dependent pathway may play a dominant role in determining the pathway that the cell follows: cell cycle arrest or apoptosis. Our screen illustrates the development of a simple and inexpensive p53-specific cDNA array to begin to analyze downstream events in the p53 pathway under physiological, pathological, and stress-induced states.

The TRAIL decoy receptor TRUNDD (DcR2, TRAIL-R4) is induced by adenovirus-p53 overexpression and can delay TRAIL-, p53-, and KILLER/DR5-dependent colon cancer apoptosis.

The cell surface decoy receptor proteins TRID (also known as DcR1 or TRAIL-R3) and TRUNDD (DcR2, TRAIL-R4) inhibit caspase-dependent cell death induced by the cytotoxic ligand TRAIL in part because of their absent or truncated cytoplasmic death domains, respectively. We previously identified the death domain containing proapoptotic TRAIL death receptor KILLER/DR5 (TRAIL-R2) as an upregulated transcript following exposure of cancer cells, with wild-type but not with mutant or degraded p53 proteins, to a cytotoxic dose of adriamycin. In the present studies we provide evidence that expression of the TRAIL decoy receptors TRUNDD and TRID increases following infection of cancer cells with p53-expressing adenovirus (Ad-p53), in a manner similar to other p53 target genes such as KILLER/DR5 and p21WAF1/CIP1. Subsequent overexpression of TRUNDD in colon cancer cell lines caused a significant delay in killing induced by TRAIL. Furthermore, cotransfection of TRUNDD with either p53 or KILLER/DR5 (at a 4:1 DNA ratio) in colon cancer cells decreased cell death caused by either gene. This protective effect of TRUNDD was not dependent on the presence of TRAIL, and overexpression of TRUNDD did not alter the protein levels of either p53 or KILLER/ DR5. Further deletion studies showed that whereas protection by TRUNDD against TRAIL-mediated apoptosis did not require an intact intracellular domain (ICD), the first 43 amino acids of the ICD of TRUNDD were needed for protection against cell death induced by p53 or KILLER/DR5. Our results suggest a model in which the TRAIL decoy receptors may be induced by p53, thereby attenuating an apoptotic response that appears to involve KILLER/DR5. Therefore, the p53-dependent induction of TRUNDD may provide a mechanism to transiently favor cell survival over cell death, and overexpression of TRUNDD may be another mechanism of escape from p53-mediated apoptosis in gene therapy experiments.