Abstract 1997: The small GTPase ARF6 augments BRAF and PI3K expression to promote melanoma survival
Authors: Wang Junhua , Rich Coulson , Rogers Aaron , Holmen Sheri , Wolff Roger , Grossmann Allie 
Published: March 2024

RAF and MEK inhibitors greatly benefit BRAF-mutant melanoma patients, but eventually, nearly 80% of patients relapse. Reactivation of MAPK signaling and activation of PI3K/AKT signaling play a major role in the development of targeted therapy resistance. Therefore, understanding how tumor cells regulate these signaling pathways may yield new insights into drug resistance. The small GTPase ARF6, a member of the RAS superfamily, influences oncogenic signaling by controlling cytoskeletal rearrangements and through endomembrane trafficking. We previously reported that ARF6 is aberrantly activated in metastatic melanoma and that activation of ARF6 is sufficient to promote invasion and accelerate spontaneous metastasis in vivo through WNT5A/beta-catenin and PI3K/AKT signaling. In contrast to ARF6 activation, herein we show that tumor-specific deletion of Arf6 significantly limits development and progression of BRAFV600E melanoma in vivo. Mechanistically, we discovered these ARF6-dependent phenotypes may be attributed to a previously unknown role for ARF6 in augmenting BRAFV600E and PI3K protein levels. Specifically, pharmacologic or genetic activation ARF6 (ARF6-GTP) is sufficient to increase BRAFV600E and PI3K protein levels in both murine and human melanoma. As a result, ARF6-GTP is also sufficient to enhance activation of MEK, ERK and AKT. Importantly, murine tumors expressing constitutively active ARF6 (ARF6Q67L) show increased expression of anti-apoptotic proteins compared to ARF6WT tumors. Together these data suggest that ARF6 promotes tumor survival through augmentation of the BRAF/MAPK and PI3K/AKT pathways. To test this, we pharmacologically activated ARF6 during serum withdrawal or vemurafenib treatment. In both conditions, ARF6 activation is sufficient to prevent apoptosis. Surprisingly, we found that vemurafenib treatment leads to ARF6 activation, suggesting that ARF6 may be an important component of the cellular machinery that reactivates MAPK signaling after RAF inhibition. Indeed, pharmacologic activation of ARF6 maintains MAPK signaling (pERK levels) in the presence of vemurafenib and enhances pERK and pAKT-mediated suppression of apoptotic signaling. In summary, our data suggest that in addition to well-established roles in tumor cell invasion, ARF6 is a crucial mediator of survival during tumor progression and in the context of BRAF targeted therapy. More work is needed to understand how ARF6 activation increases BRAF and PI3K expression and if ARF6 activation contributes to the development of persister cells in melanoma.

Citation Format: Junhua Wang, Coulson Rich, Aaron Rogers, Sheri Holmen, Roger Wolff, Allie Grossmann. The small GTPase ARF6 augments BRAF and PI3K expression to promote melanoma survival [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1997.