p53/p73 Protein Network in Colorectal Cancer and Other Human Malignancies

Abstract

The p53 tumor suppressor protein is crucial for cell growth control and the maintenance of genomic stability. Later discovered, p63 and p73 share structural and functional similarity with p53. To understand the p53 pathways more profoundly, all family members should be considered. Each family member possesses two promoters and alternative translation initiation sites, and they undergo alternative splicing, generating multiple isoforms. The resulting isoforms have important roles in carcinogenesis, while their expression is dysregulated in several human tumors including colorectal carcinoma, which makes them potential targets in cancer treatment. Their activities arise, at least in part, from the ability to form tetramers that bind to specific DNA sequences and activate the transcription of target genes. In this review, we summarize the current understanding of the biological activities and regulation of the p53/p73 isoforms, highlighting their role in colorectal tumorigenesis. The analysis of the expression patterns of the p53/p73 isoforms in human cancers provides an important step in the improvement of cancer therapy. Furthermore, the interactions among the p53 family members which could modulate normal functions of the canonical p53 in tumor tissue are described. Lastly, we emphasize the importance of clinical studies to assess the significance of combining the deregulation of different members of the p53 family to define the outcome of the disease.

Keywords: colorectal cancer; isoform crosstalk; p53 family; p53 isoforms; p73 isoforms.

Figure 1

The TP53 gene architecture and generation of the p53 isoforms. (A) The scheme of the TP53 gene structure. The human TP53 gene consists of 11 exons and two alternative exons 9β and 9γ. The exons are shown as boxes of different colors and noncoding sequences in black. Introns 2 (i2) and 4 (i4) are shown as boxes with a striped pattern. The TP53 gene can be transcribed from two different promoters, the canonical promoter P1 upstream of exon 1 (giving rise to the long p53/Δ40p53 isoforms) and the alternative P2 located in intron 4 (giving rise to the short Δ133/Δ160p53 isoforms). The TP53 mRNAs can be alternatively spliced at intron 2 or intron 9, producing isoforms with different N- and/or C-termini. There are four possible different start codons for mRNA translation (ATG1, ATG40, ATG133, and ATG160) resulting in protein isoforms of varying length. (B) Modular structure of the p53 protein isoforms. Functional protein domains are shown in different colors matching those used for the encoding exons of the TP53 gene in (A). The full-length p53α protein consists of two transactivation domains (TAD1 and TAD2), a proline-rich domain (PRD), a DNA-binding domain (DBD), a hinge domain (HD), an oligomerization domain (OD), and a C-terminal domain (CTD). There are 12 distinct p53 protein isoforms differing in the composition of the structural domains. Model adapted from [8,9,10].

Figure 2

The TP73 gene architecture and generation of the p73 isoforms. (A) The scheme of the human TP73 gene structure. The human TP73 gene consists of 14 exons and additional alternative exon 3′ (shown as gray box). The exons are shown as boxes of different colors and noncoding sequences in black. There are two different promoters, the canonical promoter P1 upstream of exon 1 (giving rise to TAp73 isoforms) and the alternative P2 located in intron 3 (giving rise to ΔNp73 isoforms). The P1 transcript can be alternatively spliced leading to the expression of several N-terminally truncated isoforms (ΔEx2p73, ΔEx2/3p73, and ΔN′p73). Alternative splicing is also possible at the C-terminus, giving rise to seven potential isoforms (α, β, γ, δ, ε, ζ, and η). (B) Modular structure of the p73 protein isoforms. Functional protein domains are shown in different colors matching those used for the encoding exons of the TP73 gene in (A). The full-length TAp73α protein consists of a transactivation domain (TAD), a proline-rich domain (PRD), a DNA-binding domain (DBD), a nuclear-localization signal (NLS), an oligomerization domain (OD), a sterile α-motif (SAM), and an inhibitory domain (ID). There are 28 possible distinct p73 protein isoforms differing in the composition of the structural domains (as Δ’Np73 and ΔNp73 mRNAs translate into identical proteins).

Figure 3

A model representing regulation of the p53 isoforms’ expression and stability. The positive regulators are shown in green, while the negative regulators are shown in red. On the transcriptional level, expression of the p53 isoforms is regulated by usage of two different promoters P1 or P2, producing the long (p53, Δ40p53) or the short (Δ133p53, Δ160p53) isoforms, respectively. Several regulators can influence the P2 activity. The canonical p53 and p53 family members (p63 and p73 and their isoforms) are shown to transactivate P2 with different efficiency. The transcription from P2 can be activated by the AP-1 transcription factor that mediates the expression of Δ133p53 in H. pylori-infected cells. The single-nucleotide polymorphisms (SNP) and their haplotypes in the internal promoter region (shown as box with a striped pattern) that comprises intron 3, exon 4, and intron 4 can affect the P2 activities. Furthermore, the G4 structures, PIN3 and PIN2 polymorphisms, can decrease the level of the p53I2 mRNA that encodes the Δ40p53 isoforms. The p53 isoforms are regulated on the posttranscriptional level by different splicing factors. SRSF1 and SRSF3, activated by Clk, promote complete exclusion of intron 9 and, thus, negatively regulate the level of p53β and p53γ isoforms. However, SRSF1 upregulates the Δ133p53α expression in human aortic smooth muscle cells. In addition, the binding of RPL26 to the TP53 pre-mRNA allows the recruitment of SRSF7 that prompts alternative splicing and, thus, generates p53β isoforms. Due to IRES, the level of p53 and Δ40p53 is regulated by ITAFs (PTB, Annexin A2, PSF, DAP5, TCP80, RHA) or proteins such as RPL26 or nucleolin. Interestingly, Δ40p53 can be generated by the 20S proteasome that degrades the full-length p53 protein. The level of the full-length p53 protein is regulated by MDM2 that was shown to promote the degradation of p53β. In addition, the level of p53β is also regulated by the MDM2-dependent neddylation, proteasome, and deneddylating enzyme Nedp1. The level of the Δ133p53α isoform is regulated by the proteasome, as well as via autophagic degradation, upon replicative senescence, where the proautophagic proteins (ATG5, ATG7, Beclin-1) act as positive regulators, while the STUB1/CHIP acts as a negative regulator of Δ133p53α degradation and senescence.

Figure 4

A model representing regulation of the p73 isoforms’ expression and stability. The positive regulators are shown in green, and the negative regulators are shown in red. On the transcriptional level, expression of the p73 isoforms is regulated by usage of two different promoters (P1 and P2) producing the TAp73 or ΔNp73 isoforms. Black bars represent the TP73 gene promoters. Several regulators specifically activate the P1 promoter, with transcription factor E2F-1 being the most important. In contrast, transcription from P1 is repressed by ZEB, C-EBPα, and hSirT1. The full-length p53 and the TAp73 proteins, as well as Nrf-2, have been found to induce both promoters. On the posttranscriptional level, the alternative splicing of P1 pre-mRNA, leading to increased expression of the ΔEx2p73 isoform, is induced by the activation of EGFR by its ligand amphiregulin (AR) in hepatocellular carcinoma cells. The expression of the ΔEx2p73 isoform is enabled by the inhibition of the RNA splicing factor SLU7 through JNK1 signaling. The stability of the p73 mRNA is increased by the RNA-binding protein RNPC1. Ribosomal protein RPL26 has been found to regulate the p73 translation and protein stability. The p73 protein isoforms are also extensively regulated on a posttranslational level. Here are shown different regulators of the TAp73 and ΔNp73 isoforms’ stability and degradation. Some of them target both TAp73 and ΔNp73 isoforms including Itch, NGFR, calpains, and NQO1 (through 20S proteasome). On the contrary, antizyme (Az) pathway and ligase PIR2 specifically target the ΔNp73 isoforms for degradation after DNA damage.

Figure 5

Schematic overview of the p53/p73 isoform interactions mediating transactivation. (A) The p53/p73 isoforms form homotetramers to transactivate target genes; (B) certain p53/p73 isoforms can form heterotetramers and consequently antagonize p53/TAp73 transactivation ability or (C) compete with p53/TAp73 for the same promoter sequences, thus exerting a dominant-negative effect; (D) p53β can modulate the promoter activity of target genes (e.g., BAX) only in the presence of p53α; (E) the p53 isoforms can independently mediate transactivation; (F) the p53 isoforms cooperate with the other family members, e.g., p73, to mediate transactivation. Adapted from [14,120,277].

Figure 6

Biological functions of the p53/p73 family isoforms in colorectal cancer. The p53 and p73 isoforms have been implicated in many biological roles in colorectal cancer, namely, drug resistance, proliferation, autophagy, inflammation, stemness, metabolism, DNA repair, angiogenesis, and invasion. Here, the functions are organized in a circular fashion with isoforms associated with a certain biological role listed next to the roles. See Table 1 for details on molecular pathways and references.