Abstract 1785: Chemical inhibition or transient knockdown of wild-type p53 induced phosphatase 1 (WIP1/PPM1D) potentiates the response to MDM2 inhibitors in a p53-dependent manner
Authors: Esfandiari Arman , Curtin Nicola J. , Lunec John 
Published: August 2015

Abstract
The mechanisms that determine p53 cell fate decision after its non-genotoxic activation by MDM2 inhibitors are poorly understood. Wild-type p53-inducible phosphatase-1 (WIP1/PPM1D) forms an autoregulatory loop with p53 and is involved in the homeostasis of the p53 stress response network. WIP1 is activated, gained/amplified and overexpressed in a range of p53 wild-type malignancies.
We investigated cellular growth/proliferation of p53 wild-type (wt) and matched p53 mutant (mut)/null cell line pairs, differing in WIP1 genetic status, in response to Nutlin-3 + GSK2830371 (WIP1i)/WIP1 siRNA-mediated knockdown. We also assessed the effects of WIP1i or knockdown in combination with Nutlin-3 on p53 post-translational modifications, p53 downstream transcriptional targets and apoptosis.
Phosphorylation of p53S15 (pp53S15) was used as a marker of WIP1 catalytic activity. WIP1-amplified MCF-7p53wt cells were very sensitive to the WIP1i, with a 50% growth inhibitory concentration (GI50) of 2.65μM + 0.54 (SEM). The three p53 wild-type and mutant/null cell line pairs showed relatively little sensitivity to single agent WIP1i (GI50&gt;10μM). However the WIP1i, at a non-growth inhibitory dose (2.5μM) in vitro, potentiated their response to Nutlin-3 in a p53-dependent manner. HCT116p53+/+; WIP1L450fs*1/+ (L450fs*1 is a WIP1 activating mutation), NGPp53wt and SJSA-1p53wt cell lines showed a 2.4-fold (p = 0.007), 2.1-fold (p = 0.04) and 1.3-fold (p = 0.02) decrease in their Nutlin-3 GI50 values respectively in the presence of WIP1i at 2.5μM. In contrast Nutlin-3 GI50 did not change in HCT116p53-/−, N20R1p53Mut and SN40R2p53Mut, NGP and SJSA-1 Nutlin-3 resistant daughter cell lines respectively. WIP1 transient siRNA knockdown also resulted in potentiation of cells to Nutlin-3. In WIP1-non-amplified (WNA) cells treated with either Nutlin-3 at the GI50 dose or 2.5μM WIP1i, pp53S15 was not detected, however in combination, these drugs resulted in dramatic levels of pp53S15. Interestingly protein expression of p53 targets MDM2 and p21 were not affected in WNA cells by the increased p53S15 phosphorylation; whereas BAX expression was elevated. The Nutlin-3 + WIP1i/siRNA knockdown combination also resulted in a significant increase in cell death markers between 24-48Hrs post-treatment in a p53-dependent manner.
Although the strongest WIP1i potentiation of Nutlin-3 was observed in HCT116p53+/+; WIP1L450fs*1/+ cells which possess an activating WIP1 mutation, the NGPp53wt and SJSA-1p53wt cell lines with wild-type WIP1 also showed potentiation. These findings highlight the mechanistic importance of the p53-WIP1 autoregulatory loop in cell fate determination after non-genotoxic p53 activation by Nutlin-3. Potent inhibitors of WIP1 may potentiate the response to other therapeutics aiming to activate p53 mediated tumour suppression.
Citation Format: Arman Esfandiari, Nicola J. Curtin, John Lunec. Chemical inhibition or transient knockdown of wild-type p53 induced phosphatase 1 (WIP1/PPM1D) potentiates the response to MDM2 inhibitors in a p53-dependent manner. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1785. doi:10.1158/1538-7445.AM2015-1785