Abstract LB261: BRAF inhibitor reprograms cancer-associated fibroblasts to elicit matrix remodeling and therapeutic escape in melanoma

Abstract

Abstract The tumor stroma and its cellular components are known to play important roles in tumor resilience towards treatment but remain to be urgently characterized in melanoma targeted therapy. By performing a paired-analysis of melanoma tissue samples collected from the patients before and after combined BRAF inhibitors (BRAFi)/MEK inhibitors (MEKi) treatment, we discovered that the number of intratumoral cancer-associated fibroblasts (CAFs) increased significantly in BRAFi/MEKi-treated melanoma stroma comparing with the samples before the therapy. Those intratumoral CAFs exhibit increased amounts of nuclear β-catenin under BRAFi/MEKi therapy. Further studies show that BRAFi not only leads to increased β-catenin nuclear accumulation in CAFs and enhances their biological properties, suggesting BRAFi induces a new resistant phenotype in CAFs. In both in vitro and in vivo models, depleting β-catenin in CAFs abolishes their ability to elicit melanoma cell growth and drug resistance. BRAFi induces BRAF and CRAF dimerization and subsequently the activation of MEK/ERK signaling in CAFs, leading to the inactivation of the β-catenin destruction complex. RNA-Seq data reveals that nuclear β-catenin is a major regulator of CAF phenotype. We identify matricellular protein periostin (POSTN) as a downstream effector of β-catenin. In melanoma, POSTN is uniquely expressed in CAFs, and BRAFi can specifically upregulate POSTN production in CAFs but not in melanoma cells. Recombinant POSTN contributes to melanoma cell BRAFi/MEKi resistance and can compensate for the loss of β-catenin in CAFs. By reactivating ERK pathway in melanoma cells through PI3K/AKT signaling, POSTN compensates for the loss of β-catenin in CAFs in conferring melanoma cell BRAFi/MEKi resistance. In summary, our data reveal a novel resistance mechanism in melanoma that is elicited by BRAFi-induced non-canonical nuclear β-catenin signaling in CAFs and reveal POSTN as an important matrix target to eliminate BRAFi/MEKi resistance in melanoma. Citation Format: Tianyi Liu, Linli Zhou, Yuhang Zhang. BRAF inhibitor reprograms cancer-associated fibroblasts to elicit matrix remodeling and therapeutic escape in melanoma [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 LB261.