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KLF9-TXNRD2 axis in melanoma progression and metastasis

Mikhail Nikiforov

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National Institutes of Health (NIH)
Malignant melanoma is one of the most aggressive types of human cancer. Its ability to metastasize makes melanoma extremely difficult to cure, and consequently, the median survival of patients with metastatic melanoma is only 8.5 months. The molecular mechanisms underlying melanoma development are not well understood, and effective treatment is not currently available. Melanomas often originate from nevi, benign aggregates of senescent melanocytes harboring activating mutations in BRAF, NRAS genes. The causal role of BRAFV600E or NRASQ61R in induction of senescence (termed oncogene-induced senescence (OIS)) has been demonstrated in normal human melanocytes (NHM) and mouse models. Ectopic expression of these oncogenes in NHM leads to accumulation of reactive oxygen species (ROS), whereas chemical suppression of ROS abrogates senescence. Endogenous mechanisms suppressing OIS in NHM via down-regulation of excessive ROS have not been investigated. Metastasis is the most detrimental feature of melanoma. Detachment of cancer cells from the extracellular matrix often causes a specific form of apoptosis, termed anoikis. Invasion, anchorage-independent growth and abrogation of anoikis are critical steps of metastasis. Several independent studies have suggested that increase in ROS levels promotes anoikis and suppresses other transformed phenotypes. Mechanisms controlling these processes via regulation of intracellular ROS in melanoma cells are largely unknown. Tumors often develop adaptive mechanisms to suppress high levels of ROS. The most general one detected in multiple malignancies is an increase in the amounts of NRF2, a universal regulator of antioxidant response genes. Increase in NRF2 has also been shown in melanomas. Recently, we have discovered that paradoxically under conditions of high oxidative stress NRF2 amplifies ROS. It is well known that increase in ROS causes accumulation of NRF2 in the nucleus and activation of anti-oxidant genes. However, if ROS continue to elevate and exceed a critical threshold, NRF2 induces expression of a transcription factor KLF9 (Kruppel-like factor 9).KLF9 represses several anti-oxidant genes, which are not NRF2 targets, including mitochondrial thioredoxin reductase (TXNRD2), thus resulting in ROS amplification. TXNRD2 protein is critical for maintenance of intracellular red-ox status and ROS detoxification. We demonstrated that TXNRD2 depletion generates ROS, whereas its overexpression lowers KLF9-generated ROS. Accordingly, depletion of KLF9 inhibits oxidative stress in all tested cell lines. Thus, KLF9 downregulation or TXNRD2 upregulation could be beneficial for the malignancies where oxidative stress (that hinders tumor progression) overwhelms NRF2- dependent anti-oxidant defense. According to the TCGA database, high KLF9 or low TXNRD2 mRNA levels correlate with the overall survival of patients with lymph node metastases (major determinants of outcome for melanoma patients), suggesting the importance of both genes for melanoma progression. Furthermore, we confirmed that KLF9 directly regulates TXNRD2 in melanocytic cells. Combinational treatment is critical for overcoming resistance to BRAFV600E inhibitor vemurafenib (VEMU). Several independent reports characterized activation of oxidative phosphorylation (which is accompanied by oxidative stress) as an adaptive response to BRAFV600E inhibition in melanoma cells. Our preliminary data demonstrate that oxidative stress-inducing agent AUR (a FDA-approved antirheumatic drug) synergizes with VEMU in inducing cell death in BRAFV600E-melanoma cells. Additionally, AUR is equally potent in causing cell death as a single agent in wildtype and mutant BRAF melanoma cells. This finding is important since no effective treatment exists for wildtype BRAF melanoma patients. The proposed research will establish the mechanisms by which KLF9-TXNRD2 axis regulates OIS in normal melanocytic cells and multiple transformed phenotypes in melanoma cells. Our findings will have broad scientific appeal since the suppression of OIS is a prerequisite for tumorigenesis, and suppression of anoikis, increase in invasion and anchorage-independent growth are prerequisites for metastasis, the most detrimental feature of cancer. Additionally, repurposing of anti-rheumatic agent auranofin for the treatment of malignant melanomas may significantly improve management of this very aggressive disease.

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