Abstract LB186: MAPK pathway activation represents a therapeutic vulnerability in ASCL1-driven SCLC
Authors: Caeser Rebecca , Hulton Christopher , Durani Vidushi , Costa Emily , Little Megan , Shah Nisargbhai S. , de Stanchina Elisa , Poirier John T. , Rudin Charles M. , Sen Triparna 
Published: July 2021

Abstract
Background Lung cancer is the leading cause of cancer death, killing more people than colon, breast, and prostate cancers combined (Siegel et al. 2013). Small cell lung cancer (SCLC) is a high grade neuroendocrine tumor accounting for ~15% of all lung cancers (Hann et al. 2019). Metastasis is often found at first diagnosis, making SCLC exceptionally lethal (2-year survival &lt5%) (Byers et al. 2015). Whilst MAPK mutations can be found in roughly 30% of human cancers (Schubbert et al. 2007) including non-small cell lung cancer (NSCLC), genomic and proteomic analyses have indicated suppression of MAPK pathway activity in SCLC (Cerami et al. 2012; Gao et al. 2013; Wagle et al. 2018). This striking difference is not well understood and previous attempts to determine whether this might be therapeutically important (Ravi et al, 1998; Cristea et al. 2020) have had conflicting conclusions. SCLC has recently been defined by the relative expression of four major transcriptional regulators (ASCL1, NeuroD1, POU2F3, YAP1) (Rudin et al., 2019). In this study we aimed to elucidate the effect of MAPK activation in these different SCLC subtypes and explore its therapeutic vulnerability.
Results We used a doxycycline-inducible vector for expression of MEKDDS217D/S221D (MEK1) in a cohort of ASCL1-, NEUROD1, and POU2F3- driven cell lines. Activation through MEK1 in ASCL1-driven SCLC cell lines resulted in a significant decrease in cell growth over 9 days. This was associated with a decrease in neuroendocrine markers ASCL1 and INSM1, a G2 cell cycle arrest and no significant increase in apoptotic cells. Expression of MEK1 in other SCLC subtypes and NSCLC failed to show any appreciable changes in cell growth. Remarkably, athymic mice injected with a MEK1 expressing ASCL1-driven cell line showed significantly slower tumor formation and longer survival than the ASCL1-driven cell line not expressing MEK1. Previous work established that hyperactivation of BRAFV600E, RAS and MYC can result in oncogene-induced senescence (Serrano et al. 1997) which is caused by upregulation of negative feedback such as SPRY2, DUSP6, ETV5 rather than MAPK pathway activation in some solid tumors and pre-B ALL (Courtois-Cox et al 2006; Shojaee et al. 2015). Similarly, we also observed strong upregulation of DUSP6, SPRY2, but not ETV5 upon MAPK activation. This was especially prominent in ASCL1-driven cell lines that changed from the normal phenotype of being in suspension to a more adherent morphology as a result of MAPK activation. Interestingly, phosphokinase array in the major subtype cell lines after MEK1 activation, demonstrated that, almost exclusively, the STAT pathways, in particular STAT3 through phosphorylation at S727 was strongly upregulated in the ASCL1-driven subtype. This prompted us to examine whether these cells were sensitive to STAT3 inhibition. Upon treatment with a STAT3 inhibitor, Stattic (1μM), ASCL1-driven SCLC cells reached their IC50 after 3-5 days in comparison to 9 days for other SCLC subtypes. NSCLC cells were resistant to STAT3 inhibition.
Summary These findings suggest that ASCL1-driven SCLC in vitro and in vivo is sensitive to activation of MAPK signaling in comparison to other SCLC subtypes. Whilst activation of the MAPK pathway might seem counterintuitive to current treatment strategies that aim to inhibit oncogenic signaling, we propose the use of a STAT3 inhibitor that has shown to be effective in vitro.
Citation Format: Rebecca Caeser, Christopher Hulton, Vidushi Durani, Emily Costa, Megan Little, Nisargbhai S. Shah, Elisa de Stanchina, John T. Poirier, Charles M. Rudin, Triparna Sen. MAPK pathway activation represents a therapeutic vulnerability in ASCL1-driven SCLC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB186.