The tumor stroma and its cellular components are known to play an important role in tumor response to treatment. Here, we report a novel resistance mechanism in melanoma that is elicited by BRAF inhibitor (BRAFi)–induced noncanonical activation of nuclear β-catenin signaling in cancer-associated fibroblasts (CAF). Treatment with BRAFi leads to an expanded CAF population with increased β-catenin nuclear accumulation and enhanced biological properties. This CAF subpopulation is essential for melanoma cells to proliferate and acquire resistance to BRAFi/MEK inhibitors (MEKi). Mechanistically, BRAFi induces BRAF-CRAF heterodimerization and subsequent activation of ERK signaling in CAFs, leading to inactivation of the β-catenin destruction complex. RNA-seq identified periostin (POSTN) as a major downstream effector of β-catenin in CAFs. POSTN compensates for the loss of β-catenin in CAFs and mediates melanoma cell BRAFi/MEKi resistance. In melanoma cells, POSTN activates phosphoinositide 3-kinase (PI3K)/AKT signaling and subsequently reactivates the ERK pathway that was inhibited by BRAFi/MEKi. Collectively, these data underscore the role of BRAFi-induced CAF reprogramming in matrix remodeling and therapeutic escape of melanoma cells.

Significance: β-Catenin activation in cancer-associated fibroblasts in response to BRAF inhibitors stimulates POSTN secretion to promote resistance in cancer cells, revealing POSTN as a potential matrix target in cancer therapy.