The transcription factor NRF2 enhances melanoma malignancy by blocking differentiation and inducing COX2 expression

Abstract

The transcription factor NRF2 is the major mediator of oxidative stress responses and is closely connected to therapy resistance in tumors harboring activating mutations in the NRF2 pathway. In melanoma, such mutations are rare, and it is unclear to what extent melanomas rely on NRF2. Here we show that NRF2 suppresses the activity of the melanocyte lineage marker MITF in melanoma, thereby reducing the expression of pigmentation markers. Intriguingly, we furthermore identified NRF2 as key regulator of immune-modulating genes, linking oxidative stress with the induction of cyclooxygenase 2 (COX2) in an ATF4-dependent manner. COX2 is critical for the secretion of prostaglandin E2 and was strongly induced by H2O2 or TNFα only in presence of NRF2. Induction of MITF and depletion of COX2 and PGE2 were also observed in NRF2-deleted melanoma cells in vivo. Furthermore, genes corresponding to the innate immune response such as RSAD2 and IFIH1 were strongly elevated in absence of NRF2 and coincided with immune evasion parameters in human melanoma datasets. Even in vitro, NRF2 activation or prostaglandin E2 supplementation blunted the induction of the innate immune response in melanoma cells. Transcriptome analyses from lung adenocarcinomas indicate that the observed link between NRF2 and the innate immune response is not restricted to melanoma.

NRF2‐dependent stress defense in tumor antioxidant control and immune evasion

“…Many melanomas express oncogenic BRAF V600E/K or NRAS Q61K/R , which are present in approximately 50% and 20% of cutaneous melanomas, respectively (Appenzeller et al., 2019; Akbani et al, 2015). Oncogenic BRAF V619E , the murine counterpart of BRAF V600E , can elevate NRF2‐dependent transcription of target genes, an observation that was confirmed for inducible BRAF V600E in melanocytes (DeNicola et al., 2011; Jessen et al., 2020). As oncogenic KRAS G12D has a similar effect (DeNicola et al., 2011), it is plausible to assume that an activation of the MAPK pathway by RAF or RAS isoforms could be generally regarded as bona fide NRF2 activator.…”

“…In addition, the cytosolic Cu/Zn superoxide dismutase (SOD1), mitochondrial Mn SOD (SOD2), and catalase are further enzymes that detoxify superoxide to H 2 O 2 (SOD) or H 2 O 2 to water and O 2 (catalase; reviewed in Hayes et al., 2020). Many of these antioxidant systems were shown to be required for melanoma maintenance (Cassidy et al., 2015; Jessen et al., 2020; Khamari et al., 2018; Leikam et al., 2014; Lokaj et al., 2009; Sato et al., 2020; Schmitt et al., 2015). Stress‐induced transcription factors are major contributors for triggering these antioxidant pathways in response to ROS stress.…”

“…An influence of NRF2 on pigmentation was also shown for melanoma. In a recent study comparing the transcriptome of human melanoma cells transfected with control and NRF2‐specific siRNA, the process “pigmentation” was found to be upregulated in NRF2 silenced cells (Jessen et al., 2020). While differentiation genes TYR , DCT, and MLANA were increased, expression levels of MITF were unaltered, which was consistent with the observation that NRF2 does not bind to the MITF promoter.…”

“…While differentiation genes TYR , DCT, and MLANA were increased, expression levels of MITF were unaltered, which was consistent with the observation that NRF2 does not bind to the MITF promoter. However, NRF2 inhibited MITF transcriptional activity in a TYR promoter‐driven luciferase assay, indicating that NRF2 counteracts MITF by a yet to be defined mechanism (Jessen et al., 2020). This underscores the role of NRF2 for melanoma malignancy, as melanomas with dedifferentiated features have a less favorable outcome (Takeuchi et al., 2003), and fits the observation that high nuclear NRF2 correlates with worse overall survival in melanoma (Hintsala et al., 2016).…”

“…As ATF4 responds to these various stress sources, it becomes activated under many conditions. Interestingly, TNFα is not only a trigger for ATF4, but also serves as potent activator of NRF2, which furthermore contributes to the dedifferentiation effect (Jessen et al., 2020). ATF4 and NRF2 are reported to physically interact and activate a subset of downstream genes interdependently (DeNicola et al., 2015; He et al., 2001), thus both transcription factors seem to jointly coordinate the TNFα‐initiated stress response in melanoma.…”