Reversal of p53-induced cell-cycle arrest

Inactivation of p53 rescues the maintenance of high risk HPV DNA genomes deficient in expression of E6

The human papillomavirus DNA genome undergoes three distinct stages of replication: establishment, maintenance and amplification. We show that the HPV16 E6 protein is required for the maintenance of the HPV16 DNA genome as an extrachromosomal, nuclear plasmid in its natural host cell, the human keratinocyte. Based upon mutational analyses, inactivation of p53 by E6, but not necessarily E6-mediated degradation of p53, was found to correlate with the ability of E6 to support maintenance of the HPV16 genome as a nuclear plasmid. Inactivation of p53 with dominant negative p53 rescued the ability of HPV16 E6STOP and E6SAT mutant genomes to replicate as extrachromosomal genomes, though not to the same degree as observed for the HPV16 E6 wild-type (WT) genome. Inactivation of p53 also rescued the ability of HPV18 and HPV31 E6-deficient genomes to be maintained at copy numbers comparable to that of HPV18 and HPV31 E6WT genomes at early passages, though upon further passaging copy numbers for the HPV18 and 31 E6-deficient genomes lessened compared to that of the WT genomes. We conclude that inactivation of p53 is necessary for maintenance of HPV16 and for HPV18 and 31 to replicate at WT copy number, but that additional functions of E6 independent of inactivating p53 must also contribute to the maintenance of these genomes. Together these results suggest that re-activation of p53 may be a possible means for eradicating extrachromosomal HPV16, 18 or 31 genomes in the context of persistent infections.

Human papillomaviruses (HPVs) infect epithelial tissues. HPVs that infect mucosal epithelia cause infectious lesions in the anogenital tract and oral cavity. HPV infections are normally cleared by the immune system; however, in rare cases, infections can persist for years. Persistent infections by certain HPVs place one at a high risk of developing carcinomas of the cervix, other anogenital tissues, and the head/neck region. These HPVs are responsible for over 5% of all human cancers. For an HPV infection to persist, the viral circular genome must be maintained, i.e. replicated and inherited during cell division. In this study we define the mechanism by which the viral gene E6 contributes to the maintenance of the HPV genome. We demonstrate that E6 must inactivate the cellular factor, p53, for the viral genome to be maintained. Significantly, p53, is inactivated in many types of human cancers and because much research has been done on p53, promising new drugs have been identified that can re-activate p53. If such drugs can re-activate the p53 that has been inactivated by E6, then we hypothesize that these drugs could be used to cure patients with persistent HPV infections and thereby reduce their risk of developing HPV associated cancers.

The role of mRNA decay in p53-induced gene expression

The p53 tumor suppressor is a DNA-damage-responsive sequence-specific transcriptional activator. The sustained activation of the p53 response is incompatible with cell growth and viability. To circumvent this issue, a variety of negative feedback loops exist to limit the duration of p53 activation. Despite our understanding of p53 regulation, very little is known about the effect of transient p53 activation on the long-term expression of p53 target genes. Here we used a temperature-sensitive variant of p53 and oligonucleotide microarrays to monitor gene expression during and following reversible p53 activation. The expression of most p53-induced transcripts was rapidly reversible, consistent with active mRNA decay. Representative 3' UTRs derived from short-lived transcripts (i.e., DDB2 and GDF15) conferred instability on a heterologous mRNA, while 3' UTRs derived from more stable transcripts (i.e., CRYAB and TP53I3) did not. The 3' UTRs derived from unstable p53-induced mRNAs were significantly longer than those derived from stable mRNAs. These 3' UTRs had high uridine and low cytosine content, leading to a higher density of U-, AU-, and GU-rich sequences. Remarkably, short-lived p53 targets were induced faster, reaching maximum transcript levels earlier than the stable p53 targets. Taken together, the evidence indicates that the p53 transcriptional response has evolved with primarily short-lived target mRNAs and that post-transcription processes play a prominent role in the p53 response.

Human bone marrow stromal cells display variable anatomic site-dependent response and recovery from irradiation

OBJECTIVES

Orofacial bone is commonly affected by osteoradionecrosis (ORN) during head and neck cancer radiotherapy possibly due to interactions of several factors including radiation damage to resident bone marrow stromal cells (BMSCs). Irradiation causes DNA damage, triggers p53-dependent signalling resulting in either cell-cycle arrest or apoptosis. In same individuals, disproportionately higher rapid growth of orofacial BMSCs relative to those of axial/appendicular bones suggests their response to radiation is skeletally site-specific. We hypothesised that survival and osteogenic recovery capacity of irradiated human BMSCs is site-dependent based on anatomic skeletal site of origin.

METHODS

Early passage BMSCs from maxilla, mandible and iliac crest of four normal volunteers were exposed to 2.5 to 10 Gy gamma radiation to evaluate clonogenic survival, effects on cell cycle, DNA damage, p53-related response and in vivo osteogenic regenerative capacity.

RESULTS

Orofacial bone marrow stromal cells (OF-MSCs) survived higher radiation doses and recovered quicker than iliac crest (IC-MSCs) based on clonogenic survival, proliferation and accumulation in G0G1 phase. Post-irradiation p53 level was relatively unchanged but expression of p21, a downstream effector was moderately increased in OF-MSCs. Re-establishment of in vivo bone regeneration was delayed more in irradiated IC-MSCs relative to OF-MSCs.

CONCLUSIONS

Effect of irradiation on human BMSCs was skeletal site-specific with OF-MSCs displaying higher radio-resistance and quicker recovery than IC-MSCs.

The tumor suppressor protein p53 is required for neurite outgrowth and axon regeneration

Axon regeneration is substantially regulated by gene expression and cytoskeleton remodeling. Here we show that the tumor suppressor protein p53 is required for neurite outgrowth in cultured cells including primary neurons as well as for axonal regeneration in mice. These effects are mediated by two newly identified p53 transcriptional targets, the actin-binding protein Coronin 1b and the GTPase Rab13, both of which associate with the cytoskeleton and regulate neurite outgrowth. We also demonstrate that acetylation of lysine 320 (K320) of p53 is specifically involved in the promotion of neurite outgrowth and in the regulation of the expression of Coronin 1b and Rab13. Thus, in addition to its recognized role in neuronal apoptosis, surprisingly, p53 is required for neurite outgrowth and axonal regeneration, likely through a different post-translational pathway. These observations may suggest a novel therapeutic target for promoting regenerative responses following peripheral or central nervous system injuries.

Distinct p53 acetylation cassettes differentially influence gene-expression patterns and cell fate

The activity of the p53 gene product is regulated by a plethora of posttranslational modifications. An open question is whether such posttranslational changes act redundantly or dependently upon one another. We show that a functional interference between specific acetylated and phosphorylated residues of p53 influences cell fate. Acetylation of lysine 320 (K320) prevents phosphorylation of crucial serines in the NH₂-terminal region of p53; only allows activation of genes containing high-affinity p53 binding sites, such as p21/WAF; and promotes cell survival after DNA damage. In contrast, acetylation of K373 leads to hyperphosphorylation of p53 NH₂-terminal residues and enhances the interaction with promoters for which p53 possesses low DNA binding affinity, such as those contained in proapoptotic genes, leading to cell death. Further, acetylation of each of these two lysine clusters differentially regulates the interaction of p53 with coactivators and corepressors and produces distinct gene-expression profiles. By analogy with the “histone code” hypothesis, we propose that the multiple biological activities of p53 are orchestrated and deciphered by different “p53 cassettes,” each containing combination patterns of posttranslational modifications and protein–protein interactions.

p14ARF Protein Expression Is a Predictor of Both Relapse and Survival in Squamous Cell Carcinoma of the Anterior Tongue

PURPOSE

The INK4A-ARF locus at chromosome 9p21 is frequently altered in head and neck squamous cell carcinoma (SCC) and encodes two distinct tumor suppressors, p16(INK4A) and p14(ARF). This study addressed the role of p14(ARF) as a potential prognostic marker in this disease.

EXPERIMENTAL DESIGN

p14(ARF) protein expression was assessed by immunohistochemistry in a cohort of 140 patients with SCC of the anterior tongue. Using univariate and multivariate Cox's proportional hazards models, the outcomes examined were time to disease recurrence or death, with or without clinicopathologic covariates, including nodal status, disease stage, treatment status, Ki-67 staining, and molecular markers with known functional or genetic relationships with p14(ARF) (p16(INK4A), p53, pRb, p21(WAF1/CIP1), E2F-1).

RESULTS

On multivariate analysis, p14(ARF) positivity (nucleolar p14(ARF) staining and/or nuclear p14(ARF) staining in >/=30% of tumor cells) was an independent predictor of improved disease-free survival (DFS; P = 0.002) and overall survival (OS; P = 0.002). This was further enhanced when p14(ARF) positivity was cosegregated with positive (>/=1%) p16(INK4A) staining (DFS, P < 0.001; OS, P < 0.001). Patients whose cancers were p14(ARF) negative and p53 positive (>50%) had the poorest outcome (DFS, P < 0.001; OS, P < 0.001) of any patient subgroup analyzed.

CONCLUSIONS

These data show that in patients with SCC of the tongue, combined nuclear and nucleolar expression of p14(ARF) protein predicts for improved DFS and OS independent of established prognostic markers.

HDM2 phosphorylation by MAPKAP kinase 2

p53 stability is regulated by HDM2, a RING domain protein that acts as an E3 ligase to ubiquitinate p53 and target its degradation. Phosphorylation of HDM2 on serine 166 by AKT has been shown to enhance HDM2 activity and promote the degradation of p53. Here, we show that MAPKAP kinase 2 (MK2) can phosphorylate HDM2 on serine 157 and 166 in vitro. Treatment of cells with anisomycin, which activates MK2, also results in phosphorylation of HDM2 on serine 157 and 166 in vivo. Mutation of the MK2 phosphorylation sites in HDM2 to aspartic acid renders HDM2 slightly more active in the degradation of p53, and mouse cells deficient for MK2 show reduced Mdm2 phosphorylation and elevated levels of p53 protein. Together, our results suggest that MK2 may act to dampen the extent and duration of the p53 response.

Loss of heterozygosity: An independent prognostic factor of colorectal cancer

AIM: Colorectal cancers result from the accumulation of several distinct genetic alterations. This study was to investigate the frequency and prognostic value of loss of heterozygosity (LOH) and microsatellite instability (MSI) at 14 genetic loci located near or within regions containing important genes implicated in colorectal tumorigenesis.

METHODS: We studied colorectal cancers with corresponding normal mucosae in 207 patients (139 males and 68 females, mean age at the time of tumor resection 66.2±12.4 years, range 22-88 years). There were 37 right-sided colonic tumors, 85 left-sided colonic tumors and 85 rectal tumors. The distribution of tumor staging was stage I in 25, stage II in 73, stage III in 68, and stage IV in 41. We analyzed the LOH and MSI of HPC1, hMSH2, hMLH1, APC, MET, P53, NH23-H1, DCC, BAT25, BAT26, D17S250, MYCL1 and D8S254 with fluorescent polymerase chain reaction and denatured gel electrophoresis. High-frequency LOH was determined to be greater than three, or more than 50% of the informative marker with LOH. High-frequency MSI (MSI-H) was determined as more than four markers with instability (>30%). Correlations of LOH and MSI with clinical outcomes and pathological features were analyzed and compared.

RESULTS: The occurrence of MSI-H was 7.25%, located predominantly in the right colons (7/15) and had a higher frequency of poor differentiation (6/15) and mucin production (7/15). LOH in at least one genetic locus occurred in 78.7% of the tumors and was significantly associated with disease progression. Of the 166 potentially cured patients, 45 developed tumor recurrence within 36 mo of follow-up. Clinicopathological factors affecting 3-year disease-free survival (DFS) were TNM staging, grade of differentiation, preoperative CEA level, and high LOH status. Patients with high LOH tumors had a significantly lower DFS (50%) compared with patients with low LOH tumors (84%). Of the patients developing subsequent tumor recurrence, the number and percentage of LOH were 2.97 and 46.8% respectively, similar to the stage IV disease patients. TNM staging had the most significant impact on DFS, followed by high LOH status.

CONCLUSION: Clinical manifestations of LOH and MSI are different in colorectal cancer patients. High-frequency LOH is associated with high metastatic potential of colorectal cancers.

Difference in the centrosome duplication regulatory activity among p53 'hot spot' mutants: potential role of Ser 315 phosphorylation-dependent centrosome binding of p53

The p53 tumor suppressor protein regulates centrosome duplication through multiple pathways, and p21(Waf1/Cip1) (Waf1), a major target of p53's transactivation function, has been shown to be one of the effectors. However, it had been unclear whether the p53's Waf1-independent centrosome duplication regulatory pathways require its transactivation function. In human cancers, specific residues of p53 are mutated at a high frequency. These 'hot spot' mutations abrogate p53's transactivation function. If p53 regulates centrosome duplication in a transactivation-independent manner, different 'hot spot' mutants may regulate centrosome duplication differently. To test this, we examined the effect of two 'hot spot' mutants (R175H and R249S) for their centrosome duplication regulatory activities. We found that R175H lost the ability to regulate centrosome duplication, while R249S partially retained it. Moreover, R249S associates with both unduplicated and duplicated centrosomes similar to wild-type p53, while R175H only associates with duplicated, but not unduplicated centrosomes. Since cyclin-dependent kinase 2 (CDK2) triggers initiation of centrosome duplication, and p53 is phosphorylated on Ser 315 by CDK2, we examined the p53 mutants with a replacement of Ser 315 to Ala (A) and Asp (D), both of which retain the transactivation function. We found that S315D retained a complete centrosome duplication activity, while S315A only partially retained it. Moreover, S315D associates with both unduplicated and duplicated centrosomes, while S315A associates with only duplicated, but not unduplicated centrosomes. Thus, p53 controls the centrosome duplication cycle both in transactivation-dependent and transactivation-independent manners, and the ability to bind to unduplicated centrosomes, which is controlled by phosphorylation on Ser 315, may be important for the overall p53-mediated regulation of centrosome duplication.

Inhibition of p53 tumor suppressor by viral interferon regulatory factor

The irreversible cell cycle arrest and apoptosis induced by p53 are part of the host surveillance mechanisms for viral infection and tumor induction. Kaposi's sarcoma-associated herpesvirus (KSHV), the most recently discovered human tumor virus, is associated with the pathogenesis of Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The K9 open reading frame of KSHV encodes a viral interferon (IFN) regulatory factor (vIRF) which functions as a repressor for cellular IFN-mediated signal transduction and as an oncoprotein to induce cell growth transformation. Here, we demonstrate that KSHV vIRF interacts with the cellular p53 tumor suppressor through the putative DNA binding region of vIRF and the central region of p53. This interaction suppresses the level of phosphorylation and acetylation of p53 and inhibits transcriptional activation of p53. As a consequence, vIRF efficiently prevents p53-mediated apoptosis. These results suggest that KSHV vIRF interacts with and inhibits the p53 tumor suppressor to circumvent host growth surveillance and to facilitate uncontrolled cell proliferation.

Chromosomal effects of newly identified water pollutants PBTA-1 and PBTA-2 and their possible mother compounds (azo dyes) and intermediates (non-ClPBTAs) in two Chinese hamster cell lines

We performed the in vitro micronucleus (MN) test on 2-[2-(acetylamino)-4-[bis(2-methoxyethyl)amino]-5-methoxyphenyl]-5-amino-7-bromo-4-chloro-2H-benzotriazole (PBTA-1) and 2-[2-(acetylamino)-4-[N-(2-cyanoethyl)-ethylamino]-5-methoxyphenyl]-5-amino-7-bromo-4-chloro-2H-benzotriazole (PBTA-2), which are newly identified water pollutants from the Nishitakase river in Kyoto, Japan, and on their possible mother compounds (AZO DYE) and intermediates (non-ClPBTAs). We tested these compounds in the absence and presence of S9 mix in two Chinese hamster cell lines CHL and V79-MZ and scored MN, polynuclear and karyorrhectic (PN), and mitotic (M) cells. PBTA-2 in the absence of S9 mix induced the strongest responses in both cell lines. It was also a strong inducer of binucleate cells in PN cells in both cell lines, which suggested that it induced polyploidy. PBTA-1 showed clear positive results only in the absence of S9 mix and only in V79-MZ cells, inducing aneuploidy. In CHL cells AZO DYE-1 significantly induced MN cells in the presence of S9 mix, and AZO DYE-2 induced MN and PN cells, including binucleate cells and cells with a multilobed nucleus, in the absence of S9 mix. In V79-MZ cells, AZO DYE-1 and -2 induced primarily M cells in the presence of S9 mix. 9% of the M cells treated with 50 microg/ml AZO DYE-1 showed endoreduplication. AZO DYE-2 at 200 microg/ml condensed the chromatin in 100% of the cells. The non-ClPBTAs were a bit more cytotoxic than the other compounds and induced a slight increase in MN cells in both cell lines. Some of the chemicals tested induced a characteristic karyomorphology that might reflect abnormal cell division. Abnormalities of cell division could be detected in PN and M cells as well as in MN cells. Structure-activity relationships have also been discussed.

Sequence of lethal events in HeLa cells exposed to the G2 blocking cytolethal distending toxin

The bacterial cytolethal distending toxin (CDT) was previously shown to block the cell cycle of several cell lines at stage G2 through inactivation of the cyclin-dependent kinase Cdkl and without induction of DNA strand breaks. In the present study, we have analyzed, using various methods of analytical cytometry, the progressive transformation and delayed lethal events in the tumor-derived HeLa cell line temporarily exposed to CDT. The cell proliferation arrest induced by CDT was irreversible but, starting about two days after exposure, the G2 block released partially, concomitantly with a decline in the level of Cdkl phosphorylation. This partial release resulted in endoreduplication, leading to the emergence of a significant subpopulation of cells with a 8C DNA content, and by multipolar abortive mitosis which accounted for the mortality recorded 2 and 3 days after exposure. The other major lethal event was a micronucleation process which started to be significant about 3 days after exposure and amplified later on. Both multipolar abortive mitosis and micronucleation appeared topologically related to centrosomal amplification.