PIRH2 ja p53 säätely

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5343976/

4. Pirh2 (p53-Induced Protein with a RING-H2 Domain) and p53 Regulation

Pirh2, also called ring finger and CHY (conserved cysteine and histidine involved in the binding of one zinc atom) zinc finger domain-containing 1 (Rchy1), is an E3 ubiquitin ligase that has three distinct zinc fingers: the CHY-type, the CTCHY-type (located at the C-terminus of the CHY-type), and a RING finger domain [102].

 Pirh2 physically binds to p53 (residues 82–292) [102], which is distinct from Mdm2’s binding site (residues 1–51 as well as the C-terminus of p53) [103,104,105]. By binding to p53, Pirh2 promotes ubiquitination and decreases the level of p53 protein in cells.

 In contrast, silencing of endogenous Pirh2 expression leads to elevation of p53. The Pirh2-dependent ubiquitination and degradation of p53 suppress p53 tumor suppressor function, including transactivation and growth inhibition [102]. A set of studies using NMR spectroscopy revealed that p53 binds to both the N- and C-terminal domains of Pirh2. The C-terminal domain of Pirh2 binds to the tetramerization domain (TET) of p53, which can be enhanced by a weak interaction between the N-terminus domain of Pirh2 and the p53 DNA binding domain. By binding to the TET domain, Pirh2 preferably ubiquitinates the tetrameric form of p53 in vitro and in vivo, suggesting that Pirh2 can effectively downregulate the transcriptional active form of p53 in the cell [106].
 

While Pirh2 targets and ubiquitinates p53 independently of Mdm2, current evidence indicates that Pirh2 and Mdm2 could simultaneously bind to a single p53 protein and efficiently enhance its ubiquitination [46,107]. Like the Mdm2–p53 feedback mechanism, Pirh2 gene expression is regulated by p53, indicating the presence of a feedback mechanism that regulates p53 protein levels and functions [102]. It has been shown that Tip60 (Tat-interactive protein of 60 kDa) binds to Pirh2 and increases the half-life of Pirh2 protein in a COS-7 (CV-1 in Origin with SV40 genes) fibroblast-like cell line. Further studies will determine whether Tip60 can stabilize Pirh2 in cancer cells and if Tip60 alteration can change the development of tumors both in vitro and in vivo [108].

A set of in vivo experiments showed the level of p53 proteins is mildly affected in Pirh2-deficient mice. However, Pirh2 deficiency leads to higher p53 levels in response to DNA damage in several tissues. Whole-body irradiation of Pirh2^−/− mice or irradiation of Pirh2 knockout cells leads to elevation of p53, p53’s downstream target proteins, and apoptosis, in comparison to WT mice [109]. In addition to p53, Hakem et al. showed Pirh2 binds and mediates the ubiquitination and proteasome degradation of c-Myc, an oncoprotein frequently overexpressed in various human cancers, including lung, breast, and ovarian cancer [110]. Development of solid tumors such as sarcoma in Pirh2^+/− and Pirh2^−/− mice as well as double knockout p53^−/− and Pirh2^−/− mice indicates that Pirh2 can function as a tumor suppressor protein (summarized in Figure 2B).

While Pirh2 directly targets and inhibits p53, it also binds to the Axin–HIPK2 complex, which is involved in p53 activation through phosphorylation of p53 at Ser 46. The inhibitory effect of Pirh2 on the Axin–HIPK2 complex depends on the level of DNA damage. In cells treated with sublethal doses of doxorubicin or ultra-violet (UV) radiation, Pirh2 blocks Axin-induced p53 activation by competing with HIPK2 for binding to Axin. With a lethal dose of UV or doxorubicin, cells overexpress another regulatory protein called Tip60. Tip60 binds to Axin and suppresses formation of the Pirh2–Axin complex. A generated Axin–Tip60–HIPK2–p53 complex in the presence of a lethal dose of genotoxic factors allows full activation of p53, resulting in p53-dependent apoptosis [111]. Interestingly, Sho et al. reported that overexpression of TRIM29 enhances degradation and alters subcellular localization of Tip60, resulting in the abrogation of p53 acetylation mediated by Tip60. By suppressing p53, TRIM29 promotes cell growth, suppresses apoptosis, and triggers transforming activity. Upregulation of TRIM29 by UV irradiation suggests that TRIM29 can function as an oncogene by reversing Tip60-dependent activation of p53 [112].

A recent study completed by Yang et al. reported that a microRNA named miR-100 can enhance ubiquitination and proteasomal degradation of p53 protein in poorly differentiated gastric cancer cells while non-cancerous gastric cells remain intact. Their results indicate that knocking down miR-100 reduces the expression of Pirh2, a key E3 ubiquitin ligase for p53 ubiquitination in gastric cancer cells, resulting in stabilization and upregulation of p53 in gastric cancer cells and activation of p53’s downstream apoptosis pathway. Further studies by Yang et al showed Pirh2 is not a direct target for miR-100. In fact, miR-100 suppresses expression of RNF114B, which functions as an E3 ubiquitin ligase for Pirh2. RNF144B binds and ubiquitinates Pirh2 for proteasomal degradation. Taken together, the above studies indicate that miR-100 can indirectly trigger ubiquitination and proteasomal degradation of the p53 tumor suppressor protein in poorly differentiated gastric cancer via the miR-100–RNF144B–Pirh2–p53 pathway in both in vitro and in vivo models [113].

The ability of Pirh2 to negatively regulate the protein levels of p53 and c-Myc plus upregulation [114,115] and downregulation [109] of Pirh2 and its regulators in different types of solid tumors (summarized in Figure 2B) suggests Pirh2 may have a dual function during tumorigenesis as an oncoprotein and tumor suppressor protein in tissue-, stress-, and grade-dependent manners. The subcellular localization of Pirh2 could be another factor for its dual function, as reported for other proteins such as p27^Kip1 [116].