SUMMARY Despite the emergence novel therapeutic modalities, including BRAF inhibitors and immunotherapies, over 7,100 people are expected to die each year from malignant melanoma, primarily from metastatic dissemination and therapy resistance. The proposed studies leverage the expertise of the two co-PIs in tumor microenvironment, the Integrated Stress Response (ISR) and melanoma progression to test the overall hypothesis that as part of the adaptive response to tumor microenvironmental (TME) stress, bi-directional interactions between melanoma cells and fibroblasts, endothelial cells and adipocytes promote survival, metastatic dissemination, and therapy resistance. Preliminary results and published reports from the two PIs indicate that the ISR is activated in human melanomas and that genetic or pharmacological disruption of the ISR severely impairs primary tumor growth and metastasis in multiple experimental tumors, including melanomas. The studies will delineate the salient roles of the transcription factor ATF4, a major transcriptional effector of the Integrated Stress Response, in a pro-survival and pro-metastatic program mediated by the non- canonical tumor suppressor BRN2, a transcription factor effector of multiple melanoma-associated signaling pathways. To test the central hypothesis, we will pursue three specific aims: In Aim 1 we will use a novel, conditional global ATF4 knockout mouse model, as well as Fibroblast (FB)-specific and Endothelial (EC)- specific ATF4 knockout mice and cells to determine the role of ATF4 expression in each TME component on melanoma progression and metastasis. Preliminary results with global or FB-specific deletion of ATF4 results in a severe deficiency in tumor growth of flank melanoma tumors. In Aim 2 we will study how SRC promotes melanoma progression via ATF4-BRN2 cooperativity. Both BRN2 and ATF4 can repress anoikis/apoptosis, and in preliminary studies we reveal that BRN2 interacts with ATF4. Therefore, we will test the hypothesis that Extracellular Matrix (ECM)-driven integrin signaling, and monounsaturated fatty acid (MUFA) uptake from adipocytes and lymph converge on SRC to impose a cooperative BRN2-ATF4 anti-apoptotic and pro- metastasis gene expression program, driven in part by hippo signaling. Finally, under Aim 3, we will determine how MUFAs dictate melanoma phenotype. Here we will dissect a novel mechanism underpinning MUFA- mediated nuclear localization of -catenin and test the hypothesis that the resulting SRC-driven nuclear CAV1- -catenin complex promotes a pro-metastasis gene expression program via BRN2-ATF4, and the contribution of ATF4 and BRN2 to melanoma phenotypic heterogeneity and tumor immune infiltration. By delineating how the ISR uses ATF4 to coordinate the output of the TME to shape melanoma progression, we will identify therapeutically exploitable pathways for anti-melanoma approaches.