Abstract 4091: Interfering with interferon: An axis of ATF2-mediated chemoresistance
Authors: Lau Eric , Claps Giuseppina , Hoon David S. , Ronai Ze'ev A. 
Published: May 2015

Abstract
Malignant melanoma is currently the most lethal skin cancer and comprises approximately 8% of total U.S. cancers cases. Despite the advent of even the most effective targeted monotherapies to date (e.g., mutant B-RAF inhibitors), clinical responses have largely been transient, as melanomas rapidly develop therapeutic resistance, highlighting the need to target other critical signaling pathways. In melanoma, Activating Transcription Factor 2 (ATF2) elicits an oncogenic function when nuclear but a tumor suppressor function when it is cytosolic/mitochondrial. In progressively malignant melanoma, increased PKCϵ-mediated phosphorylation of ATF2 on threonine 52 (T52) promotes its nuclear localization and transcriptional activity, and renders the cells more resistant to genotoxic stress.
To determine how PKCϵ-phosphorylated ATF2 contributes to melanoma biology, we performed gene expression profiling of melanoma cells that were reconstituted with wild-type, T52 phosphomutant or phosphomimic ATF2, in the presence or absence of genotoxic stress (which attenuates the T52 phosphorylation of ATF2). We report that in melanoma cells, genotoxic stress induces the expression of a cluster of Interferon β1 (IFNβ1) and related genes, which is suppressed by the expression of ATF2T52E. ATF2 binds to the 5′-promoter region of IFNβ1, and we identified the ATF2/AP-1 binding site required for transcriptional repression. The stress-induced expression of IFNβ1 mediates S-phase accumulation during genotoxic stress with a concomitant induction of a senescence-like phenotype. In contrast, blockade of type-1 interferon signaling by shRNA-mediated depletion of the IFNAR receptor abrogates the S-phase accumulation, blocking cells in G1 phase and reducing the extent of stress-induced cell death. The expression of or treatment with exogenous IFNβ1 blunts the proliferation of melanoma cells in both 2- and 3-dimensional spheroid cultures. Notably, the treatment of melanoma cells with mutant BRAF inhibitor (Vemurafenib/PLX4720) also induces the expression of IFNβ1 and senescence markers, and the co-treatment of melanoma cells with IFNβ1 and etoposide or PLX4720 significantly enhances the cell death induced by either etoposide or PLX4720 alone. Lastly, we found that the stress-induced IFNβ1 expression by melanoma cells elicits paracrine effects, enhancing the melanoma cell clearance by lymphocytes in an in vitro co-culture system.
Our data demonstrate a novel transcriptional regulatory role exhibited by ATF2 on IFNβ1, which influences cell cycle dynamics and stress/treatment resistance of melanoma cells. The therapeutic and prognostic implications of these findings will be discussed.
Citation Format: Eric Lau, Giuseppina Claps, David S. Hoon, Ze'ev A. Ronai. Interfering with interferon: An axis of ATF2-mediated chemoresistance. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4091. doi:10.1158/1538-7445.AM2014-4091