Nutlin-3

The tumor suppressor p53 activates transcription of the IER5 gene, which encodes an adapter protein of protein phosphatase PP2A. IER5 binds to both the B55 regulatory subunit of PP2A and PP2A's target proteins, facilitating PP2A/B55-catalyzed dephosphorylation of these proteins. Here, we show that IER5 functions as a positive regulator of p53 by inhibiting its ubiquitination, thereby increasing cellular p53 levels. Mechanistically, this effect of IER5 requires its nuclear localization and binding to both PP2A/B55 and the p53 ubiquitin E3 ligase MDM2. Importantly, IER5 fails to inhibit p53 ubiquitination in cells treated with the MDM2 inhibitor Nutlin-3. The IER5-PP2A/B55 complex dephosphorylates MDM2 at Ser166, leading to MDM2 ubiquitination and a reduction in nuclear MDM2. Altogether, our data provide evidence that IER5-PP2A/B55 regulates the nuclear balance between MDM2 and p53 via MDM2 dephosphorylation.

Despite missense mutation accounts for over 60% of p53 alterations while homozygous deletion (HOM) for only 5% or less in advanced bladder cancer cases, most of the previously reported mouse models are deficient of p53. Accordingly, few studies have addressed the mechanisms of missense mutation occurrence and its functional advantage over HOM in bladder cancer development. Organoids derived from Krt5-expressing mouse urothelium (K5-mUrorganoid) demonstrated the crucial role of Pten loss in driving loss of wild-type allele of Trp53 (Trp53R172H/LOH), which conferred tumorigenic ability to K5-mUrorganoid in athymic mice. These tumors recapitulated the histological and genetic characteristics of the human basal-squamous subtype bladder cancer. Both Trp53R172H/Δ; PtenΔ/Δ and Trp53Δ/Δ; PtenΔ/Δ K5-mUrorganoids formed tumors in athymic mice, whereas only Trp53R172H/Δ; PtenΔ/Δ K5-mUrorganoid formed tumors even when directly inoculated in immunocompetent syngeneic mice. The absence of wild-type Trp53 was associated with upregulation of proliferative signaling, and the presence of a mutant Trp53 allele was associated with immune-excluded microenvironment. This study highlights the functional significance of p53 mutant LOH in bladder carcinogenesis conferring several hallmarks of cancer such as sustaining proliferative signaling and avoiding immune destruction, thus provides a novel immunocompetent mouse model of urothelial carcinoma harboring p53 mutations as a novel tool for cancer immunology research.