Thioredoxin interacting protein (TXNIP), a redox regulator, mediates the RAPGEF3/4 signaling dependency in primary...

RAP guanine exchange factors (RAPGEF3/4) also known as EPAC1/2 (Exchange Protein Activated by cyclic AMP) are important signaling proteins. In cutaneous melanoma, we reported that loss of dependency on RAPGEF3/4 is associated with metastatic progression. Here, we investigated the molecular mechanisms underlying EPAC1/2 signaling in melanoma. Using transformed human melanocytes, chemical inhibition and genetic deletion of EPAC in Braf/Pten mice, we show that EPAC activation is an early event in melanomagenesis and is required for the growth of transformed melanocytes in vitro and melanomagenesis in vivo . Query of the Cancer Genome Atlas (TCGA) and immunohistochemical analysis of melanoma tumors showed that low EPAC mRNA and RAP1-GTP protein correlate with better diseases free survival of patients with primary melanoma. RNAseq analysis of patient-matched primary and metastatic melanoma cells treated with EPAC inhibitor ESI-09 revealed that TXNIP, an important regulator of redox homeostasis, is a downstream effector of EPAC-RAP1 signaling. Our data also show that EPACs promote melanoma growth by regulation of redox homeostasis and mitochondrial reactive oxygen species through activation of mechanistic target of rapamycin complex 1 (mTORC1) that stabilizes hypoxia-inducible factor 1-alpha (HIF-1α), a transcriptional activator of TXNIP and glycolytic enzymes. Our data suggest that targeting mechanisms that metastatic melanoma cells employ to bypass EPAC dependency as a potential therapeutic approach for melanoma.