Mechanisms of TP53 Pathway Inactivation in Embryonic and Somatic Cells-Relevance for Understanding (Germ Cell) Tumorigenesis

Abstract

The P53 pathway is the most important cellular pathway to maintain genomic and cellular integrity, both in embryonic and non-embryonic cells. Stress signals induce its activation, initiating autophagy or cell cycle arrest to enable DNA repair. The persistence of these signals causes either senescence or apoptosis. Over 50% of all solid tumors harbor mutations in TP53 that inactivate the pathway. The remaining cancers are suggested to harbor mutations in genes that regulate the P53 pathway such as its inhibitors Mouse Double Minute 2 and 4 (MDM2 and MDM4, respectively). Many reviews have already been dedicated to P53, MDM2, and MDM4, while this review additionally focuses on the other factors that can deregulate P53 signaling. We discuss that P14^ARF (ARF) functions as a negative regulator of MDM2, explaining the frequent loss of ARF detected in cancers. The long non-coding RNA Antisense Non-coding RNA in the INK4 Locus (ANRIL) is encoded on the same locus as ARF, inhibiting ARF expression, thus contributing to the process of tumorigenesis. Mutations in tripartite motif (TRIM) proteins deregulate P53 signaling through their ubiquitin ligase activity. Several microRNAs (miRNAs) inactivate the P53 pathway through inhibition of translation. CCCTC-binding factor (CTCF) maintains an open chromatin structure at the TP53 locus, explaining its inactivation of CTCF during tumorigenesis. P21, a downstream effector of P53, has been found to be deregulated in different tumor types. This review provides a comprehensive overview of these factors that are known to deregulate the P53 pathway in both somatic and embryonic cells, as well as their malignant counterparts (i.e., somatic and germ cell tumors). It provides insights into which aspects still need to be unraveled to grasp their contribution to tumorigenesis, putatively leading to novel targets for effective cancer therapies.

Keywords: P53 pathway; cancers; embryonic and somatic cells; mutations.

Figure 1

Phenotypic differences between embryonic and somatic cells. Fertilization of an oocyte by a spermatozoon forms a zygote, which subsequently develops the blastocyst. The blastocyst contains embryonic stem (ES) cells that eventually give rise to all somatic cell types. ES and somatic cells can be distinguished according to multiple aspects: their potency, telomerase activity, self-renewal capacity, cell cycle regulation, and preferred DNA repair mechanism. Both germ cells and germ cells tumors are derived from ES derived PGCs and therefore harbor the same embryonal characteristics.

Figure 2

Timeline depicting key discoveries in P53 signaling and germ cell tumors. A timeline of the discoveries regarding P53 as a guardian of the genome (green) and studies regarding TP53 in GCT development and resistance (orange). References key discoveries P53 (green): 49–58, references key discoveries TP53 in GCT development (orange): 25–41, 59–62.

Figure 3

Schematic overview of the P53 protein. Indicated are (from N- to T-terminus) TAD: Transactivation domain, PRD: Proline-rich domain, DBD: DNA-binding domain, TD: Tetramerization domain and REG: Regulatory domain. Numbers indicate the amino acids included in each domain.

Figure 4

A schematic overview representing the favored mechanism of P53 activation in embryonic cells and somatic cells. DNA damage, both in embryonic (blue) and somatic (black) cells, will induce P53 activation. To protect genomic integrity, embryonic cells favor apoptosis over DNA repair. Conversely, somatic cells preferentially engage in cell cycle arrest and DNA repair; however, if DNA damage persists, these cells will enter apoptosis or become senescent. Furthermore, telomere dysfunction and oncogene activation are other means to activate P53 signaling (mostly in somatic cells). In addition to apoptosis, P53 activation and its downstream pathways can result in autophagy, senescence, and cell cycle arrest.

Figure 5

An overview of the crosstalk between P53 and autophagy. Autophagy can repress P53 through the inhibition of upstream activating proteins as well as P53 itself. This forms a negative feedback loop (red) as P53 induces autophagy through the inhibition of the IGF–AKT–mTOR pathway and subsequent increase in BECLIN-1 levels, which plays a role in autophagosome formation. P53 also induces the expression of autophagy-related genes (ARG). Autophagy induces cell survival, which is effectuated by the binding of ATG7 to P53, ultimately leading to P53-dependent P21 expression and subsequent cell cycle arrest (green). The loss of ATG-7 results in apoptosis (blue), as P53 induces the expression and activation of pro-apoptotic proteins. Apoptosis and autophagy have mutually inhibitory effects on one another.

Figure 6

The different post-translational modifications of P53 and Mouse Double Minute 2 and 4. P53 can be activated (left) through phosphorylation (P), acetylation (Ac), and methylation (CH3) on lysine and arginine residues. Mouse Double Minute 2 (MDM2) can be phosphorylated and ubiquitinated (Ub) and Mouse Double Minute 4 (MDM4) can be phosphorylated. Post-translational modifications (PTMs) can also inhibit these proteins, which also decreases the regulation of each other (right). Phosphorylation, ubiquitination, and lysine methylation inactivate P53. Phosphorylation, acetylation, and ubiquitination inhibit MDM2, and phosphorylation and ubiquitination obstruct MDM4 activity. Of note, human genes and proteins are annotated in all capital letters (e.g., MDM2 and MDM2, respectively), whereas the murine homologs are annotated with a single capital letter (e.g., Mdm2 and Mdm2, respectively).

Figure 7

The regulatory mechanisms of the P53 pathway. P53 is regulated by MDM2 and MDM4, which are all affected by the PTMs and are also determining each other’s activity. MDM2 is indirectly activated by miR-cluster 371a-3 and ANRIL, while it is inhibited by ARF and miR-339. MEG3, CTCF, TRIM proteins, and miR-125 can regulate P53 activity directly. All these regulatory mechanisms combined will lead to the inhibition or activation of P21 expression, leading to apoptosis, cell cycle arrest, or senescence. MDM2 and MDM4 are both able to inhibit DNA repair mechanisms, resulting in genomic instability.