From p63 to p53 across p73

Most genes are members of a family. It is generally believed that a gene family derives from an ancestral gene by duplication and divergence. The tumor suppressor p53 was a striking exception to this established rule. However, two new p53 homologs, p63 and p73, have recently been described [1, 2, 3, 4, 5 and 6]. At the sequence level, p63 and p73 are more similar to each other than each is to p53, suggesting the possibility that the ancestral gene is a gene resembling p63/p73, while p53 is phylogenetically younger [1 and 2].

The complexity of the family has also been enriched by the alternatively spliced forms of p63 and p73, which give rise to a complex network of proteins involved in the control of cell proliferation, apoptosis and development [1, 2, 4, 7, 8 and 9].

In this review we will mainly focus on similarities and differences as well as relationships among p63, p73 and p53.     

Expression of p53, p63 and p73 in the orofacial region of human embryos

We studied p53, p63, p73 protein expression in the orofacial region of five human embryos aged 7-18 weeks of intrauterine development using a three-step immunohistochemical method. Expression of proteins in various locations was evaluated semiquantitatively. A decrease in p53, p63 and p73 proteins occurred in the 13-week-old material with the exception of the tooth germ where a drop in p73 appeared in the ninth week.     

Evaluation of p63 and p73 antibodies for cross-reactivity

The tumor suppressor p53 is commonly mutated in human cancers. However, two homologs of p53, p63 and p73, are frequently over-expressed in tumors and are associated with tumor subtypes, clinical outcomes, and responses to therapy. There are many isoforms of p53, p63, and p73 (the p53 family). Proper detection of and discrimination between the members of this tumor suppressor family in human tissues is of critical importance to cancer research and clinical care. In this study, we assessed the specificity of several commercially available and newly generated p73 antibodies, focusing on antibodies that distinguish between the TAp73 and ?Np73 isoforms by Western analysis, immunohistochemistry, and immunofluorescence. In addition, we found that the pan-p63 and pan-p73 antibodies tested cross-react with p73 and p63 respectively. The results of this study have important implications for analysis of p63 and p73 expression and co-expression in human tumors, and for potential use of these reagents in molecular diagnostics and therapeutic decision-making.     

p63 and p73: roles in development and tumor formation

The tumor suppressor p53 is critically important in the cellular damage response and is the founding member of a family of proteins. All three genes regulate cell cycle and apoptosis after DNA damage. However, despite a remarkable structural and partly functional similarity among p53, p63, and p73, mouse knockout studies revealed an unexpected functional diversity among them. p63 and p73 knockouts exhibit severe developmental abnormalities but no increased cancer susceptibility, whereas this picture is reversed for p53 knockouts. Neither p63 nor p73 is the target of inactivating mutations in human cancers. Genomic organization is more complex in p63 and p73, largely the result of an alternative internal promoter generating NH2-terminally deleted dominant-negative proteins that engage in inhibitory circuits within the family. Deregulated dominant-negative p73 isoforms might play an active oncogenic role in some human cancers. Moreover, COOH-terminal extensions specific for p63 and p73 enable further unique protein-protein interactions with regulatory pathways involved in development, differentiation, proliferation, and damage response. Thus, p53 family proteins take on functions within a wide biological spectrum stretching from development (p63 and p73), DNA damage response via apoptosis and cell cycle arrest (p53, TAp63, and TAp73), chemosensitivity of tumors (p53 and TAp73), and immortalization and oncogenesis (DeltaNp73).     

p63 and p73: Life and Death in Squamous Cell Carcinoma

The p53 family member p63 plays an essential role in the developing epithelium, and overexpression of the ΔNp63a isoform is frequently observed in human squamous cell carcinomas (SCCs). These findings have suggested that ΔNp63a might function as an oncogene within squamous epithelial cells. Nevertheless, the mechanism by which ΔNp63a might promote tumorigenesis remains poorly understood, and data from mouse models implies that the p63 locus might in fact function as a tumor suppressor in these same tissues. A recent study using RNA interference in human SCC-derived cell lines shows that ΔNp63a mediates an essential survival function in human SCC cells by virtue of its ability to suppress the pro-apoptotic function of the related p53 family member p73. These findings support an oncogenic role for ΔNp63a and they demonstrate the existence of critical physical and functional interactions between endogenous p53 family members in human cancer. Specific chemotherapeutic agents and future targeted approaches may be able to exploit this pathway to therapeutic advantage.     

p63/p73 in the control of cell cycle and cell death

The p53 family apparently derives from a common ancient ancestor that dates back over a billion years, whose function was protecting the germ line from DNA damage. p63 and p73 would maintain this function through evolution while acquiring novel roles in controlling proliferation and differentiation of various tissues. p53 on the other hand would appear in early vertebrates to protect somatic cells from DNA damage with similar mechanism used by its siblings to protect germ line cells. For the predominant role played by p53 mutations in cancer this was the first family member to be identified and soon became one of the most studied genes. Its siblings were identified almost 20 years later and interestingly enough their ancestral function as guardians of the germ-line was one of the last to be identified. In this review we shortly summarize the current knowledge on the structure and function of p63 and p73.     

p63 and p73 expression in extrahepatic bile duct carcinoma and their clinical significance

p53 plays a pivotal role in the prevention of human tumor formation. p73 and p63 are new members of the p53 tumor suppressor family, which are becoming increasingly recognized as important players in human tumorigenesis. However, the roles of these proteins are not well elucidated in extrahepatic bile duct (EBD) carcinoma. We examined expressions of the p63 and p73 genes and proteins in normal biliary epithelia, biliary dysplasias, and EBD carcinomas using immunohistochemistry and RT-PCR analysis. p63 and p73 proteins were overexpressed in 26.3 and 41.0% of EBD carcinomas, respectively. p63 protein expression was more frequent in tumors with vascular invasion (P = 0.002) and distal location (P = 0.04), while p73 expression was more common in cancers with deeper tumor invasion (P = 0.04). Patients with tumors co-expressing both p63 and p73 were found to have a significantly worse overall survival rate compared to those with either p63 or p73 expression (P < 0.05) as determined in univariate and multivariate analyses. Our results strongly imply that the p53 family members have different functions in EBD carcinomas. Our data also indicate that interactions between p63 and p73 play an important role in tumorigenesis of EBD carcinoma.     

Scotin: A new p63 target gene expressed during epidermal differentiation

Accumulating evidence indicates that bacteria and bacterial products promote hepatic fibrogenesis. The activation of hepatic stellate cells (HSC) plays a central role in hepatic fibrosis. Here, we demonstrate that HSC express toll-like receptor 9 (TLR9), a pattern recognition receptor that is activated by CpG motifs present specifically in bacterial DNA. Upon CpG stimulation human as well as murine HSC isolated from wild-type (TLR9+/+) mice express increased levels of the profibrogenic chemokine monocyte chemotactic protein 1 (MCP-1). In contrast, HSC isolated from TLR9 deficient (TLR9−/−) mice lacked CpG motif induced MCP-1 expression indicating the functionality of TLR9 in HSC. Bile duct ligation revealed significantly lower hepatic MCP-1 and collagen expression and less hepatic fibrosis in TLR9−/− compared to TLR9+/+ mice. In addition, the expression of hepatic α-smooth-muscle actin, a known marker for HSC activation, was reduced in TLR9−/− mice indicating that bacterial DNA induces the activation of HSC and therefore promotes hepatic fibrosis.     

p53, p63 and p73--solos, alliances and feuds among family members

p53 controls crucial stress responses that play a major role in preventing malignant transformation. Hence, inactivation of p53 is the single most common genetic defect in human cancer. With the recent discovery of two close structural homologs, p63 en p73, we are getting a broader view of a fascinating gene family that links developmental biology with tumor biology. While unique roles are apparent for each of these genes, intimate biochemical cross-talk among family members suggests a functional network that might influence many different aspects of individual gene action. The most interesting part of this family network derives from the fact that the p63 and p73 genes are based on the "two-genes-in-one" idea, encoding both agonist and antagonist in the same open reading frame. In this review, we attempt to present an overview of the current status of this fast moving field.