Although the approval of novel targeted therapy drugs, such as BRAF inhibitors (BRAFi), MEK inhibitors (MEKi), and immune checkpoint inhibitors, has revolutionized melanoma treatment, long-term outcomes are still disappointing for many patients because of the development of drug resistance. A major contributing factor to tumor resilience and relapse is the presence of a “plastic” microenvironment, which is comprised of heterogeneous stromal cell populations embedded in a dense and stiff extracellular matrix (ECM). Particularly, the ECM not only functions as a barrier to drug penetration and distribution and also provides structural and adaptive signals, which can induce therapeutic escape pathways in melanoma cells. Genetically stable cancer- associated fibroblasts (CAFs) are known to be a notorious ECM-remodeling machine in the tumor stroma. We have discovered that the number of CAFs with nuclear β-catenin in the melanoma stroma increases significantly after the patients are treated with BRAFi/MEKi. We have established that increased nuclear β-catenin in CAFs is induced by BRAFi but not MEKi. Nevertheless, how BRAFi stimulates CAFs to reprogram their biological functions via hyperactivated nuclear β-catenin activity remains to be understood. We have obtained compelling data demonstrating that targeted depletion of β-catenin in CAFs ablates their ability to remodel the tumor microenvironment, downregulates abnormal BRAF/MAPK/ERK signaling in melanoma cells, and suppresses melanoma cell drug resistance in vitro and in vivo. RNA-Seq data show that β-catenin is essential for CAF to remodel the ECM by coining the CAF transcriptome. We have identified the β-catenin/TCF4 target gene periostin (POSTN) as an important matricellular protein secreted by CAFs to promote BRAFi resistance. The central hypothesis is that decoding and targeting the ECM-remodeling CAFs has the potential to create a drug-sensitive microenvironment that sensitizes melanoma cells to therapeutic agents and increase their response rate. In Aim 1, we will elucidate the pro-activation pathway(s) by which BRAFi stimulates CAFs to mediate melanoma drug resistance phenotypes. We will assess the role of hyperactivated nuclear β-catenin in the function of CAFs in melanoma. In Aim 2, we will determine whether the β-catenin-TCF4 transcriptional complex is the signaling hub that controls CAF-driven ECM remodeling and BRAFi/MEKi resistance. We will evaluate whether disrupting the β-catenin-TCF4 interaction in CAFs will sensitize BRAF-mutant melanoma cells to BRAFi/MEKi in vivo. In Aim 3, we will determine the roles of CAF-derived POSTN in melanoma cell growth and resistance to BRAFi/MEKi. We will investigate signaling pathways that are activated by POSTN in melanoma cells to promote their proliferation and resistance to BRAFi. The expected outcomes are to be an in-depth mechanistic characterization of the complex interactions among CAFs, BRAFi, and the ECM microenvironment that promote the grow...