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

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-1), 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. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=134 SRC="FIGDIR/small/696903v1_ufig1.gif" ALT="Figure 1"> View larger version (30K): org.highwire.dtl.DTLVardef@1b0bc1eorg.highwire.dtl.DTLVardef@e8499org.highwire.dtl.DTLVardef@1237175org.highwire.dtl.DTLVardef@1edbd02_HPS_FORMAT_FIGEXP M_FIG C_FIG