Targeting the p53-MDM2 pathway for neuroblastoma therapy: Rays of hope

Abstract

Despite being the subject of extensive research and clinical trials, neuroblastoma remains a major therapeutic challenge in pediatric oncology. The p53 protein is a central safeguard that protects cells against genome instability and malignant transformation. Mutated TP53 (the gene encoding p53) is implicated in many human cancers, but the majority of neuroblastomas have wild type p53 with intact transcriptional function. In fact, the TP53 mutation rate does not exceed 1-2% in neuroblastomas. However, overexpression of the murine double minute 2 (MDM2) gene in neuroblastoma is relatively common, and leads to inhibition of p53. It is also associated with other non-canonical p53-independent functions, including drug resistance and increased translation of MYCN and VEGF mRNA. The p53-MDM2 pathway in neuroblastoma is also modulated at several different molecular levels, including via interactions with other proteins (MYCN, p14^ARF). In addition, the overexpression of MDM2 in tumors is linked to a poorer prognosis for cancer patients. Thus, restoring p53 function by inhibiting its interaction with MDM2 is a potential therapeutic strategy for neuroblastoma. A number of p53-MDM2 antagonists have been designed and studied for this purpose. This review summarizes the current understanding of p53 biology and the p53-dependent and -independent oncogenic functions of MDM2 in neuroblastoma, and also the regulation of the p53-MDM2 axis in neuroblastoma. This review also highlights the use of MDM2 as a molecular target for the disease, and describes the MDM2 inhibitors currently being investigated in preclinical and clinical studies. We also briefly explain the various strategies that have been used and future directions to take in the development of effective MDM2 inhibitors for neuroblastoma.

Keywords: Inhibitors; MDM2; Neuroblastoma; Targeted therapy; p53.

Figure 1.. Molecular mechanisms responsible for the cytoplasmic sequestration of p53 in neuroblastoma cells.

(1) hyperactive nuclear export; (2) resistance to p53 degradation; (3) p53 hyperubiquitylation; (4) binding of a glucocorticoid receptor complex with p53 protein. Abbreviations: herpesvirus-associated ubiquitin-specific protease (HAUSP); ubiquitin (Ub), and glucocorticoid receptor (GR).

Figure 2.. p53-dependent and –independent oncogenic roles of MDM2 in neuroblastoma.

(1) The MDM2 protein attenuates the transcriptional activity of p53 and contributes to tumor formation. MDM2 also catalyzes the ubiquitylation of p53 in neuroblastoma cells. (2) MDM2 increases the translation of VEGF (thereby increasing angiogenesis) and MYCN, leading to neuroblastoma growth. MDM2 expression leads to the development of multidrug resistance and also inhibits the transcriptional activity of p73 in neuroblastoma cells.

Figure 3.. p53-MDM2 pathway regulation in neuroblastoma cells.

The p53-MDM2 pathway is regulated by several proteins including MYCN, focal adhesion kinase (FAK), ARF, B cell-specific Moloney murine leukemia virus integration site 1 (BMI-1), twist family BHLH transcription factor 1 (TWIST-1), and wild-type p53-induced phosphatase 1 (WIP-1).

Figure 4.. Molecular mechanisms of action of Nutlins in neuroblastoma cells.

(a) The p53-dependent activity of Nutlins 3 and 3a. Nutlins bind to the p53 binding pocket in MDM2 and inhibit the p53-MDM2 interaction. In addition, Nutlin-3a also inhibits p53 hyperubiquitylation, allowing the re-import of p53 into the nucleus to activate apoptosis. In neuroblastoma cells, Nutlin-3 also activates HIPK2 to increase the phosphorylation of p53 at serine 46, resulting in apoptosis. (b) The p53-independent activity of Nutlins. Nutlin-3 inhibits P-glycoprotein (P-gp) and multidrug resistance protein 1 (MRP1) and increases the sensitivity of cancer cells to chemotherapeutic drugs. Nutlin treatment also decreases VEGF production and angiogenesis in neuroblastoma cells, and blocks the interaction of MDM2 with p73, resulting in increased expression of p73 and PUMA/Noxa to induce apoptosis in neuroblastoma cells.