Abstract 4913: Invasive melanoma cells commandeer p53 activity to promote the survival of a therapy resistant subpopulation
Authors: Webster Marie R. , Kaur Amanpreet , Ndoye Abibatou , Kugel Curtis , Basu Subhasree , Valiga Alexander , Appleton Jessica , Wang Ying-Jie , Murphy Maureen , Weeraratna Ashani T. 
Published: July 2017

Abstract
Metastatic melanoma is highly aggressively and often therapy resistant. Signaling pathways which promote invasion of metastatic melanoma, also promote therapy resistance. Therapy resistant melanomas are characterized by high levels of the pro-invasive non-canonical Wnt molecule, Wnt5A. Previously, we described an adaptive stress response in highly invasive melanoma cells, which is characterized by a growth arrest and an increase in senescence markers, yet these cells retain the ability to invade and form colonies. These highly invasive cells do not undergo apoptosis following treatment with DNA damaging agents such as doxorubicin and are resistant to BRAFV600E targeted therapy. Recently, Lukin et al. have shown that p53 expression promotes survival of colorectal cancer cells via p21 expression and a reversible cell cycle arrest, allowing for repair of damaged DNA.
Here, we show that highly invasive and therapy resistant melanoma cells express Wnt5A, p53 and p21, which regulate proliferation and slow cycling in invasive melanoma cells. The expression of p53 is promoted by Wnt5A, knock down of Wnt5A decreases p53 and p21 expression in these cells and decreases the number of cells arrested in G2/M following DNA damage. The cell cycle and apoptotic functions of p53 are highly regulated. iASPP, MDMX, and MDM2 have been shown to regulate the function of wild type p53 in melanoma cells. MDM2, an E4 ubiquitin ligase, regulates p53 by shuttling it out of the nucleus and targeting it for proteasomal degradation. We found that MDM2 expression increases in invasive melanoma cells following DNA damage, however, in these cells it is phosphorylated at serine 395. Phosphorylation of MDM2 at ser395 blocks its ability to export p53 from the nucleus, leading to increased p53 expression. Even with increased p53 expression, invasive melanoma cells undergo a cell cycle arrest following stress instead of apoptosis. The apoptotic function of p53 has been shown to be inhibited by nuclear iASPP, which is enriched in metastatic melanoma. We found that knocking down Wnt5A in invasive melanoma cells decreases the expression of iASPP. These data suggest that Wnt5A promotes MDM2 Ser395 phosphorylation and iASPP expression, blocking down regulation of p53 and its apoptotic function, while promoting cell cycle arrest and survival following stress. These data may reveal a mechanism by which highly invasive melanoma cells evade therapy to form therapy resistant sub-colonies at distant sites.
Citation Format: Marie R. Webster, Amanpreet Kaur, Abibatou Ndoye, Curtis Kugel, Subhasree Basu, Alexander Valiga, Jessica Appleton, Ying-Jie Wang, Maureen Murphy, Ashani T. Weeraratna. Invasive melanoma cells commandeer p53 activity to promote the survival of a therapy resistant subpopulation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4913. doi:10.1158/1538-7445.AM2017-4913