P63 and P73: P53 mimics, menaces and more

Inactivation of the tumour suppressor p53 is the most common defect in cancer cells. The discovery of its two close relatives, p63 and p73, was therefore both provocative and confounding. Were these new genes tumour suppressors, p53 regulators, or evolutionary spin-offs? Both oncogenic and tumour-suppressor properties have now been attributed to the p53 homologues, perhaps reflecting the complex, often contradictory, protein products encoded by these genes. p63 and p73 are further implicated in many p53-independent pathways, including stem-cell regeneration, neurogenesis and sensory processes.     

Interrelations between p73 and p53: a model, neuroblastoma

Homologies in sequence and gene organization of p53 and their relatives, p73 and p63, suggest similar biological functions. However differences exist between the p53 family members. Indeed in human tumors p53 is often mutated while p63 and p73 are very rarely mutated. In addition, in contrast to p53 which is transcribed in a unique mRNA species spanning all gene exons, each homologue expresses two types of isoforms: some with transactivation domain (TAD) showing tumor suppressive properties, the others deprived of TAD, with oncogenic properties. If p53 responds to immediate genotoxic stress, its homologues participate to the cell homeostasis of specific tissues along their development and differentiation, neuronal tissue for p73, epithelial for p63. However a collaboration between the three p53 family members has been shown to occur in response to cell genotoxic damages. Neuroblastic tumors characterized by a large spectrum of neuronal differentiation constitute a good model to study relationship between p73 and p53 as well as the regulation of their respective expression.     

p73 and p63: Why Do We Still Need Them?

When p73 and p63 were initially described as homologues of the tumor suppressor p53, the three family members seemed almost exchangeable, raising the question why all three were retained during evolution. It later turned out that the corresponding genes, TP63 and TP73, appear phylogenetically older than TP53, and that their targeted deletion causes severe developmental defects, in contrast to a deletion of TP53. Hence, p63 and p73 are responsible for biological effects that cannot be elicited by p53 alone. Here, we provide an overview of properties ascribed to p63 and p73 that distinguish them from p53. Differences occur at the following levels: i) protein structure, especially with regard to the aminoterminal transactivation domains and the carboxyterminal portions unique to p63 and p73; ii) regulation, affecting mRNA levels, posttranslational modifications and interaction with other cellular proteins; iii) activities, resulting in the regulation of gene expression, the programming of development, and the emergence of tumors. We speculate that, during the course of evolution, p63 and p73 have first pursued a broader range of activities, whereas p53 later specialized on genome maintenance.     

p73 and p63 are homotetramers capable of weak heterotypic interactions with each other but not with p53.

Mutations in the p53 tumor suppressor gene are the most frequent genetic alterations found in human cancers. Recent identification of two human homologues of p53 has raised the prospect of functional interactions between family members via a conserved oligomerization domain. Here we report in vitro and in vivo analysis of homo- and hetero-oligomerization of p53 and its homologues, p63 and p73. The oligomerization domains of p63 and p73 can independently fold into stable homotetramers, as previously observed for p53. However, the oligomerization domain of p53 does not associate with that of either p73 or p63, even when p53 is in 15-fold excess. On the other hand, the oligomerization domains of p63 and p73 are able to weakly associate with one another in vitro. In vivo co-transfection assays of the ability of p53 and its homologues to activate reporter genes showed that a DNA-binding mutant of p53 was not able to act in a dominant negative manner over wild-type p73 or p63 but that a p73 mutant could inhibit the activity of wild-type p63. These data suggest that mutant p53 in cancer cells will not interact with endogenous or exogenous p63 or p73 via their respective oligomerization domains. It also establishes that the multiple isoforms of p63 as well as those of p73 are capable of interacting via their common oligomerization domain.     

Nucleotide sequence of the p53 cDNA of beluga whale (Delphinapterus leucas)

The cDNA (DNA complementary to RNA) of the p53 gene of the beluga whale (Delphinapterus leucas) was sequenced by the method of 5'- and 3'-rapid amplification of cDNA ends (RACE) with the cDNA made for the RNA obtained from fresh peripheral blood leukocytes isolated from two animals. Primers for the RACE method were synthesized based on the sequence of the DNA of beluga whale corresponding to exon 5 of the human p53 gene, which was determined after amplification of the DNA isolated from the liver from a beluga whale by using a pair of primers for the human sequence. The sequenced cDNA had a 2150-nucleotide length and contained the whole region corresponding to human exons 1 through 11. The reading frame was 1164 bp (base pair) long and began in exon 2 and ended in exon 11, coding for a 387-amino acid protein. The nucleotide sequence of the reading frame showed high similarity over 85% with pig, sheep, cow, and human genes. The similarities with the former two animals at the amino acid level were also more than 85%. Lower similarity of the beluga whale p53 gene was also found with those of lower tetrapods, fish and invertebrates.     

The 5'' region of the p53 gene: evolutionary conservation and evidence for a negative regulatory element.

The 5' regions of the mouse, rat and human functional p53 genes were isolated and analysed. All three genes possess a non-coding exon, comprising exclusively 5' untranslated sequences. This exon contains extensive diad symmetry near the 5' end of p53 mRNA, possibly allowing for the formation of a stable hairpin structure in this mRNA. The nucleotide sequence within this hairpin element is highly conserved among the species. A DNA stretch of 225 bp preceding the p53 mRNA cap site possesses distinct promoter activity when assayed in the CAT system. However, this activity is practically abolished when further upstream p53 sequences (approximately 120 bp) are included in front of the CAT gene. This suggests that the control of p53 gene expression is complex and involves a negative regulatory element.     

Inactivation of retinoblastoma (RB) tumor suppressor by oncogenic isoforms of the p53 family member p73

The p53 family includes three members that share significant sequence homology, yet exhibit fundamentally different functions in tumorigenesis. Whereas p53 displays all characteristics of a classical tumor suppressor, its homologues p63 and p73 do not. We have previously shown, that NH(2)-terminally truncated isoforms of p73 (Delta TA-p73), which act as dominant-negative inhibitors of p53 are frequently overexpressed in cancer cells. Here we provide evidence that Delta TA-p73 isoforms also affect the retinoblastoma protein (RB) tumor suppressor pathway independent of p53. Delta TA-p73 isoforms inactivate RB by increased phosphorylation, resulting in enhanced E2F activity and proliferation of fibroblasts. By inactivating the two major tumor suppressor pathways in human cells they act functionally analogous to several viral oncoproteins. These findings provide an explanation for the fundamentally different functions of p53 and p73 in tumorigenesis.     

Development and changes at settlement in the barbel structure of the reef fish,Upeneus tragula (Mullidae)

Synopsis The development of the sensory barbels of the tropical goatfish, Upeneus tragula (Mullidae), was examined from their first appearance early in planktonic life through to the reef-associated juvenile period. The structure of the barbel was examined histologically and found to represent an outgrowth of the gustatory (taste) system, composed of at least 50% sensory tissue at settlement. Abrupt changes in morphology were found to be coincident with the 6–12 h settlement period: barbels rapidly moved forward along the hyoid arch to abut the dentary; the length of the barbels increased by up to 52%; the epidermal layer increased to comprise 75% of the cross-sectional area; and the mean size of the taste bud cells increased by up to 100%. A strong relationship was found between barbel length and mean taste-bud size. This relationship was used to predict the mean taste-bud sizes for 237 newly-settled fish, collected as 12 samples over two recruitment seasons. Mean taste-bud size varied significantly among samples. Experiments examined whether food availability or temperature of the water within the pelagic phase influenced the size of the barbels at settlement. Food availability influenced the relationship between barbel length and fish size. Slower growing fish had larger barbels relative to fish length than those that grew faster. Temperature did not influence the relationship between barbel length and fish size. Variability in sensory development at settlement, and the factors which influence it, may have important ramifications for the potential success of the fish once on the reef.     

p53 gene expression is modulated by endocrine disrupting chemicals in the hermaphroditic fish, Kryptolebias marmoratus

The full-length of cDNA of tumour suppressor gene p53 from the self-fertilizing fish Kryptolebias marmoratus (Km-p53) was determined using molecular cloning and rapid amplification of cDNA ends (RACE). The Complete cDNA sequences of K. marmoratus (Km-p53) gene was 1.8 kb in length. K. marmoratus p53 amino acid sequence showed a high degree of homology with the sequences from fishes, amphibians, and mammals. Although basal level of expression of Km-p53 mRNA was low, all the studied tissues showed some level of expression. After exposure of K. marmoratus to endocrine disrupting chemicals (EDCs) such as bisphenol A, 4-nonylphenol, and 4-tert-octylphenol, Km-p53 expression was significantly increased within 3 h of exposure in juveniles. However, expression was down-regulated by exposure to most of the EDCs when measured at 96 h in adult fish. In adult fish, suppressive effect of EDCs was more pronounced in liver as compared to other tissues. These findings suggest that Km-p53 gene would be involved in cellular defense mechanism in early stage of exposure to EDCs and long-term exposure may suppress its expression. It may be possible that the suppression of p53 by EDCs may predispose the host to environmental chemical carcinogenesis.