Thioredoxin interacting protein (TXNIP), a redox regulator, mediates the EPAC-RAP1 signaling dependency of primary melanoma

Progression of cutaneous primary melanoma that arises from melanocytes leads to lethal metastatic disease. Molecular mechanisms that control the growth of primary melanoma in the skin and promote progression are not fully understood. Previously we showed that RAP guanine exchange factors EPAC1/2 (Exchange Proteins Activated by cyclic AMP) promote the growth of primary melanoma and loss of dependency on EPACs is associated with metastatic progression. In this study, we show that EPACs are activated during malignant transformation of melanocytes, and chemical inhibition or genetic deletion of EPAC inhibits melanomagenesis in Braf/Pten mice. Low expression of EPAC mRNA and its effector RAP1-GTP protein in primary melanoma correlate with better recurrence-free survival. RNAseq analysis of matched primary and metastatic melanoma cells treated with an EPAC inhibitor showed that TXNIP, an important regulator of redox homeostasis, is a downstream effector of EPAC signaling. We also show that EPACs promote melanoma growth by regulating redox homeostasis and mitochondrial ROS through activation of mechanistic target of rapamycin complex 1 (mTORC1) that stabilizes hypoxia-inducible factor 1-alpha (HIF-1α), a transcriptional activator of redox regulator TXNIP and glycolytic enzymes. Our data suggest that targeting mechanisms that melanoma cells employ to bypass EPAC dependency is a potential therapeutic approach.