Interaction of regulators Mdm2 and Mdmx with transcription factors p53, p63 and p73

The negative regulation of p53, a major human tumor suppressor, by Mdm2 and Mdmx is crucial for the survival of a cell, whereas its aberrant function is a common feature of cancer. Both Mdm proteins act through the spatial occlusion of the p53 transactivation (TA) domain and by the ubiquitination of p53, resulting in its degradation. Two p53 homologues, p63 and p73, have been described in humans. Unlike p53, these proteins regulate developmental processes rather than genome stability. Both p63 and p73 contain TA domains homologous to that of p53, but relatively little is known about their regulation by Mdm2 or Mdmx. Here, we present a detailed characterization of the interaction of Mdm2 and Mdmx with the TA domains of p63 and p73. Earlier reports of Mdm2 and Mdmx interactions with p73 are substantiated by the detailed quantitative characterization reported in this study. Most importantly, earlier contradictions concerning the presumed interaction of the Mdm proteins with p63 are convincingly resolved and for the first time, the affinities of these interactions are determined. Finally, the contribution of these findings to our understanding of the physiological role of these interactions is discussed.     

The p53 family: same response, different signals?

TP53, the gene that encodes p53, is a well-defined tumor suppressor gene that is frequently mutated in human cancers. Recently, two proteins homologous to p53, termed p73 and p63, were identified. Current data indicate that both p73 and p63, like p53, can induce cell-cycle arrest and apoptosis, suggesting that they might also be tumor suppressors. However, the physiological signals that can regulate p53, for example, DNA damage, have no effect on p73, as tested in several cell lines. Furthermore, the signaling pathways by which p73 (and possibly p63) induces cell-cycle arrest and apoptosis appear to be similar to those of p53, but also have important differences. Thus, the p53 family proteins are closely related but might have distinct physiological functions.     

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).     

The coordinated interaction of multifunctional members of p53 family determines many key processes in multicellular organisms

First time p53 was found in the complex with viral large T-antigene in the cells transformed by small DNA virus SV40. The cloning of p53 cDNA was done in the beginning of eighties and soon after that the whole p53 gene was cloned. The p53 family is comprised of three genes: TP53,TP63 and TP73, each of which is expressed as a set of structurally and functionally different isoforms. All of them intensively interact with each other forming a united functional network of proteins. In this review we discuss evolution of the p53 family and significance of all its members in embryonic development, reproduction, regeneration, regulation of aging and life span, as well as in the body's defense against cancer. With special attention we review the role of less studied members of the p53 family: p63 and p73, in oncogenesis and tumor progression and show that different isoforms of these proteins might exert a contrary effect on these processes.     

p53家族及其通路相关蛋白调节母性生殖的新功能

p53是一种重要的抗癌基因,同时它也是机体感受环境压力并进行相应调节的关键基因之一。最近的研究发现东亚人群p53 Arg72Pro受到冬季温度自然选择,表明p53可能在生殖中发挥作用。同时,p53及其通路中的癌基因鼠双微体2(Murinedoubleminute2,Mdm2)、MdmX和Hausp(Herpesvirus-associated ubiquitin-specific protease)基因的单核苷酸多态性(Single nucleotide polymorphisms,SNP)与女性生殖疾病易感性相关。P53蛋白通过其DNA结合区(DNA-binding domain,DBD)调控白血病抑制因子(Leukaemiainhibitory factor,LIF)表达,从而影响胚胎植入过程,实现其在母性生殖中的作用。p53通路中Mdm2、MdmX和Hausp可以调控P53蛋白的表达水平和活性,同时还可以在胚胎植入时准确的调控p53的表达水平,促进胚胎植入;P53家族成员P63、P73具有P53相似的DBD区,但P63和P73是通过别的途径影响到母性生殖;在卵母细胞受到射线或者化学损伤后,P63能促进其凋亡,减少畸型的产生;P73能影响纺锤体复合物的组装,而纺锤体复合物缺失将导致胚泡质量低下,微管结合的动粒缺失和细胞非整倍性的增加。文章主要综述了p53家族、p53通路中的相关蛋白对母性生殖的影响,为提高IVF-ET成功指出了新方法,同时也为不明原因的不孕患者提供了新的诊断思路,将有助于制定合理的个性化治疗不孕方案。     

Coordinated interaction of multifunctional members of the p53 family determines many key processes in multicellular organisms

For the first time, p53 was found in complex with the viral large T-antigen in cells transformed with the small DNA virus SV40. p53 cDNA was cloned in the early 1980s, and the full-length p53 gene was cloned soon afterwards. The p53 family is comprised of three genes—TP53, TP63, and TP73—each of which is expressed as a set of structurally and functionally different isoforms. All of them intensely interact with each other, forming a united functional network of proteins. The review discusses the evolution of the p53 family and the significance of all its members in embryo development, reproduction, regeneration, regulation of aging and lifespan, and defense against cancer. Special attention is paid to the role of poorly studied members of the p53 family, p63 and p73, in carcinogenesis and tumor progression. Different isoforms of these proteins might exert opposite effects on these processes.     

4-Hydroxynonenal modulation of p53 family gene expression in the SK-N-BE neuroblastoma cell line

4-Hydroxynonenal (HNE), a product of lipid peroxidation, inhibits proliferation of several tumor cells. The p53 tumor suppressor protein plays a critical role in cell cycle control, by inducing p21 expression, and in apoptosis, by inducing bax expression. Recently, two other proteins with many p53-like properties, TAp73 (p73) and TAp63 (p63), have been discovered. SK-N-BE human neuroblastoma cells express the three p53 family proteins and can be used for the study of their induction. We investigated HNE action in the control of proliferation, differentiation, and apoptosis in SK-N-BE cells and the HNE effect on the expression of p53, p63, p73, p21, bax, and G1 cyclins. Retinoic acid (RA) was used as a positive control. HNE inhibited cell proliferation without inducing differentiation; it decreased S-phase cells and increased the number of apoptotic cells. RA reduced the proportion of S-phase cells and did not induce apoptosis. HNE increased p53, p73, p63, p21, and bax expression at different time points. HNE reduced cyclin D2 expression and the phosphorylation of pRb protein. Our results demonstrated that HNE inhibits SK-N-BE cell proliferation by increasing the expression of p53 family proteins and p53 target proteins which modulate cell cycle progression and apoptosis.     

Regulating the genome surveillance system: miRNAs and the p53 super family

The p53 gene super family consists of three members; TP53, TP63 and TP73, encoding proteins p53, p63 and p73. Whilst p63 appears to have an essential role in embryonic development with a less clear role in carcinogenesis, irregularities in p53 and p73 signalling are implicated in tumour formation. As such, p53 is a tumour suppressor which is mutated in over 50% cancers and p73 was recently formally classified as a tumour suppressor based on data showing p73 deficient mice generate spontaneous tumours similar to those observed in p53 null mice. Dysregulation of both p53 and p73 has been correlated with cancer progression in many cell types and although mutation of these genes is often observed, some form of p53/p73 deregulation likely occurs in all tumour cells. The discovery that complementary micro RNAs (miRNAs) are able to target both of these genes provides a potential new means of perturbing p53/p73 signalling networks in cancer cells. Here we summarise the current literature regarding the involvement of miRNAs in the modulation of p53 family proteins and cancer development and detail the use of in silico methods to reveal key miRNA targets.