The role of thioredoxin reductase 1 in melanoma metabolism and metastasis

Melanoma redox biology and the emergence of drug resistance

Melanoma is the deadliest form of skin cancer, with the loss of approximately 60,000 lives world-wide each year. Despite the development of targeted therapeutics, including compounds that have selectivity for mutant oncoproteins expressed only in cancer cells, many patients are either unresponsive to initial therapy or their tumors acquire resistance. This results in five-year survival rates of below 25%. New strategies that either kill drug-resistant melanoma cells or prevent their emergence would be extremely valuable. Melanoma, like other cancers, has long been described as being under increased oxidative stress, resulting in an increased reliance on antioxidant defense systems. Changes in redox homeostasis are most apparent during metastasis and during the metabolic reprogramming associated with the development of treatment resistance. This review discusses oxidative stress in melanoma, with a particular focus on targeting antioxidant pathways to limit the emergence of drug resistant cells.

Redox pathways in melanoma

Cases of melanoma are doubling every 12 years, and in stages III and IV, the disease is associated with high mortality rates concomitant with unresectable metastases and therapeutic drug resistance. Despite some advances in treatment success, there is a marked need to understand more about the pathology of the disease. The present review provides an overview of how melanoma cells use and modulate redox pathways to facilitate thiol homeostasis and melanin biosynthesis and describes plausible redox targets that may improve therapeutic approaches in managing malignant disease and metastasis. Melanotic melanoma has some unique characteristics. Making melanin requires a considerable dedication of cellular energy resources and utilizes glutathione and glutathione transferases in certain steps in the biosynthetic pathway. Melanin is an antioxidant but is also functionally important in hematopoiesis and influential in various aspects of host immune responses, giving it unique characteristics. Together with other redox traits that are specific to melanoma, a discussion of possible therapeutic approaches is also provided.

MITF Is Regulated by Redox Signals Controlled by the Selenoprotein Thioredoxin Reductase 1

TR1 and other selenoproteins have paradoxical effects in melanocytes and melanomas. Increasing selenoprotein activity with supplemental selenium in a mouse model of UV-induced melanoma prevents oxidative damage to melanocytes and delays melanoma tumor formation. However, TR1 itself is positively associated with progression in human melanomas and facilitates metastasis in melanoma xenografts. Here, we report that melanocytes expressing a microRNA directed against TR1 (TR1low) grow more slowly than control cell lines and contain significantly less melanin. This phenotype is associated with lower tyrosinase (TYR) activity and reduced transcription of tyrosinase-like protein-1 (TYRP1). Melanoma cells in which the TR1 gene (TXNRD1) was disrupted using Crispr/Cas9 showed more dramatic effects including the complete loss of the melanocyte-specific isoform of MITF; other MITF isoforms were unaffected. We provide evidence that TR1 depletion results in oxidation of MITF itself. This newly discovered mechanism for redox modification of MITF has profound implications for controlling both pigmentation and tumorigenesis in cells of the melanocyte lineage.

Thioredoxin Reductase 1 Modulates Pigmentation and Photobiology of Murine Melanocytes in vivo

Pigment-producing melanocytes overcome frequent oxidative stress in their physiological role of protecting the skin against the deleterious effects of solar UV irradiation. This is accomplished by the activity of several endogenous antioxidant systems, including the thioredoxin antioxidant system, in which thioredoxin reductase 1 (TR1) plays an important part. To determine whether TR1 contributes to the redox regulation of melanocyte homeostasis, we have generated a selective melanocytic Txnrd1-knockout mouse model (Txnrd1mel‒/‒), which exhibits a depigmentation phenotype consisting of variable amelanotic ventral spotting and reduced pigmentation on the extremities (tail tip, ears, and paws). The antioxidant role of TR1 was further probed in the presence of acute neonatal UVB irradiation, which stimulates melanocyte activation and introduces a spike in oxidative stress in the skin microenvironment. Interestingly, we observed a significant reduction in overall melanocyte count and proliferation in the absence of TR1. Furthermore, melanocytes exhibited an elevated level of UV-induced DNA damage in the form of 8-oxo-2'-deoxyguanosine after acute UVB treatment. We also saw an engagement of compensatory antioxidant mechanisms through increased nuclear localization of transcription factor NRF2. Altogether, these data indicate that melanocytic TR1 positively regulates melanocyte homeostasis and pigmentation during development and protects against UVB-induced DNA damage and oxidative stress.

NRF2 and Key Transcriptional Targets in Melanoma Redox Manipulation

Melanocytes are dendritic, pigment-producing cells located in the skin and are responsible for its protection against the deleterious effects of solar ultraviolet radiation (UVR), which include DNA damage and elevated reactive oxygen species (ROS). They do so by synthesizing photoprotective melanin pigments and distributing them to adjacent skin cells (e.g., keratinocytes). However, melanocytes encounter a large burden of oxidative stress during this process, due to both exogenous and endogenous sources. Therefore, melanocytes employ numerous antioxidant defenses to protect themselves; these are largely regulated by the master stress response transcription factor, nuclear factor erythroid 2-related factor 2 (NRF2). Key effector transcriptional targets of NRF2 include the components of the glutathione and thioredoxin antioxidant systems. Despite these defenses, melanocyte DNA often is subject to mutations that result in the dysregulation of the proliferative mitogen-activated protein kinase (MAPK) pathway and the cell cycle. Following tumor initiation, endogenous antioxidant systems are co-opted, a consequence of elevated oxidative stress caused by metabolic reprogramming, to establish an altered redox homeostasis. This altered redox homeostasis contributes to tumor progression and metastasis, while also complicating the application of exogenous antioxidant treatments. Further understanding of melanocyte redox homeostasis, in the presence or absence of disease, would contribute to the development of novel therapies to aid in the prevention and treatment of melanomas and other skin diseases.

Redox-Related Proteins in Melanoma Progression

Melanoma is the most aggressive type of skin cancer. Despite the available therapies, the minimum residual disease is still refractory. Reactive oxygen and nitrogen species (ROS and RNS) play a dual role in melanoma, where redox imbalance is involved from initiation to metastasis and resistance. Redox proteins modulate the disease by controlling ROS/RNS levels in immune response, proliferation, invasion, and relapse. Chemotherapeutics such as BRAF and MEK inhibitors promote oxidative stress, but high ROS/RNS amounts with a robust antioxidant system allow cells to be adaptive and cooperate to non-toxic levels. These proteins could act as biomarkers and possible targets. By understanding the complex mechanisms involved in adaptation and searching for new targets to make cells more susceptible to treatment, the disease might be overcome. Therefore, exploring the role of redox-sensitive proteins and the modulation of redox homeostasis may provide clues to new therapies. This study analyzes information obtained from a public cohort of melanoma patients about the expression of redox-generating and detoxifying proteins in melanoma during the disease stages, genetic alterations, and overall patient survival status. According to our analysis, 66% of the isoforms presented differential expression on melanoma progression: NOS2, SOD1, NOX4, PRX3, PXDN and GPX1 are increased during melanoma progression, while CAT, GPX3, TXNIP, and PRX2 are decreased. Besides, the stage of the disease could influence the result as well. The levels of PRX1, PRX5 and PRX6 can be increased or decreased depending on the stage. We showed that all analyzed isoforms presented some genetic alteration on the gene, most of them (78%) for increased mRNA expression. Interestingly, 34% of all melanoma patients showed genetic alterations on TRX1, most for decreased mRNA expression. Additionally, 15% of the isoforms showed a significant reduction in overall patient survival status for an altered group (PRX3, PRX5, TR2, and GR) and the unaltered group (NOX4). Although no such specific antioxidant therapy is approved for melanoma yet, inhibitors or mimetics of these redox-sensitive proteins have achieved very promising results. We foresee that forthcoming investigations on the modulation of these proteins will bring significant advances for cancer therapy.

Thioredoxin reductase is a major regulator of metabolism in leukemia cells

Despite the fact that AML is the most common acute leukemia in adults, patient outcomes are poor necessitating the development of novel therapies. We identified that inhibition of Thioredoxin Reductase (TrxR) is a promising strategy for AML and report a highly potent and specific inhibitor of TrxR, S-250. Both pharmacologic and genetic inhibition of TrxR impairs the growth of human AML in mouse models. We found that TrxR inhibition leads to a rapid and marked impairment of metabolism in leukemic cells subsequently leading to cell death. TrxR was found to be a major and direct regulator of metabolism in AML cells through impacts on both glycolysis and the TCA cycle. Studies revealed that TrxR directly regulates GAPDH leading to a disruption of glycolysis and an increase in flux through the pentose phosphate pathway (PPP). The combined inhibition of TrxR and the PPP led to enhanced leukemia growth inhibition. Overall, TrxR abrogation, particularly with S-250, was identified as a promising strategy to disrupt AML metabolism.

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

The transcription factor NRF2 is known as the master regulator of the oxidative stress response. Tumor entities presenting oncogenic activation of NRF2, such as lung adenocarcinoma, are associated with drug resistance, and accumulating evidence demonstrates its involvement in immune evasion. In other cancer types, the KEAP1/NRF2 pathway is not commonly mutated, but NRF2 is activated by other means such as radiation, oncogenic activity, cytokines, or other pro-oxidant triggers characteristic of the tumor niche. The obvious effect of stress-activated NRF2 is the protection from oxidative or electrophilic damage and the adaptation of the tumor metabolism to changing conditions. However, data from melanoma also reveal a role of NRF2 in modulating differentiation and suppressing anti-tumor immunity. This review summarizes the function of NRF2 in this tumor entity and discusses the implications for current tumor therapies.

Adaptive redox homeostasis in cutaneous melanoma

Cutaneous melanoma is the most aggressive type of skin cancer. Although cutaneous melanoma accounts for a minority of all types of skin cancer, it causes the greatest number of skin cancer related deaths worldwide. Oxidative stress and redox homeostasis have been shown to be involved at each stage of a malignant melanocyte transformation, called melanomagenesis, as well as during drug resistance. Reactive oxygen species (ROS) play an important and diverse role that regulate many aspects of skin cell behaviors ranging from proliferation and stemness, to oxidative damage and cell death. On the other hand, antioxidants are associated with melanoma spread and metastasis. Overall, the contribution of redox homeostasis to melanoma development and progression is controversial and highly complex. The aim of this study is to examine the association between redox homeostasis and the melanomagenic process. To this purpose we are presenting what is currently known about the role of ROS in melanoma initiation and progression. In addition, we are discussing the role of antioxidant mechanisms during the spread of the disease and in cases of melanoma drug resistance. Although challenging, targeting redox homeostasis in melanoma progression remains to be a promising therapeutic approach, especially valid during melanoma drug resistance.

10H-3,6-Diazaphenothiazines triggered the mitochondrial-dependent and cell death receptor-dependent apoptosis pathways and further increased the chemosensitivity of MCF-7 breast cancer cells via inhibition of AKT1 pathways

Breast cancer is one of the leading causes of death in cancer categories, followed by lung, colorectal, and ovarian among the female gender across the world. 10H-3,6-diazaphenothiazine (PTZ) is a thiazine derivative compound that exhibits many pharmacological activities. Herein, we proceed to investigate the pharmacological activities of PTZ toward breast cancer MCF-7 cells as a representative in vitro breast cancer cell model. The PTZ exhibited a proliferation inhibition (IC50  = 0.895 µM) toward MCF-7 cells. Further, cell cycle analysis illustrated that the S-phase checkpoint was activated to achieve proliferation inhibition. In vitro cytotoxicity test on three normal cell lines (HEK293 normal kidney cells, MCF-10A normal breast cells, and H9C2 normal heart cells) demonstrated that PTZ was more potent toward cancer cells. Increase in the levels of reactive oxygen species results in polarization of mitochondrial membrane potential (ΔΨm), together with suppression of mitochondrial thioredoxin reductase enzymatic activity suggested that PTZ induced oxidative damages toward mitochondria and contributed to improved drug efficacy toward treatment. The RT2 PCR Profiler Array (human apoptosis pathways) proved that PTZ induced cell death via mitochondria-dependent and cell death receptor-dependent pathways, through a series of modulation of caspases, and the respective morphology of apoptosis was observed. Mechanistic studies of apoptosis suggested that PTZ inhibited AKT1 pathways resulting in enhanced drug efficacy despite it preventing invasion of cancer cells. These results showed the effectiveness of PTZ in initiation of apoptosis, programmed cell death, toward highly chemoresistant MCF-7 cells, thus suggesting its potential as a chemotherapeutic drug.

Accessing the transcriptional status of selenoproteins in skin cancer-derived cell lines

BACKGROUND

Selenoproteins are selenocysteine (Sec)-containing proteins that exhibit numerous physiological functions, mainly antioxidative activities. Studies have suggested that several human selenoproteins play an important role in tumor initiation and progression, including melanoma.

METHODS

Using RNA-seq data set from Sequence Reads Archive (SRA) experiments published at the National Center for Biotechnology Information (NCBI), we determined and compared the transcriptional levels of the 25 selenoproteins-coding sequences found in 16 human-derived melanoma cell lines and compared to four melanocyte controls.

RESULTS

15 selenoprotein-coding genes were found to be expressed in melanoma and normal melanocyte cells, and their mRNA levels varied among the cell lines. All melanoma cells analyzed with BRAF or NRAS mutations presented upregulated levels of SELENOI, TXNRD1, and SELENOT transcripts and downregulated levels of SELENOW and SELENON transcripts in comparison with melanocytes controls. Moreover, SELENOW, SELENON, SELENOI, TXNRD1, and SELENOT-coding transcripts were affected when BRAF-mutated A375 cells were treated with CPI203, A771726 or Vorinostat drugs.

CONCLUSION

Our results indicate that melanoma cells can modify, in a different manner, the selenoprotein transcript levels, as a possible mechanism to control tumor progression. We suggest that the usage of diet and supplements containing selenium should be carefully used for patients with melanoma.

A Diselenide Turn-On Fluorescent Probe for the Detection of Thioredoxin Reductase

We report the first diselenide-based probe for the selective detection of thioredoxin reductase (TrxR), an enzyme commonly overexpressed in melanomas. The probe design involves conjugation of a seminaphthorhodafluor dye with a diselenide moiety. TrxR reduces the diselenide bond, triggering a fluorescence turn-on response of the probe. Kinetic studies reveal favorable binding of the probe with TrxR with a Michaelis-Menten constant (K_m ) of 15.89 μm. Computational docking simulations predict a greater binding affinity to the TrxR active site in comparison to its disulfide analogue. In vitro imaging studies further confirmed the diselenide probe exhibited improved signaling of TrxR activity compared to the disulfide analogue.

Control of tumor angiogenesis and metastasis through modulation of cell redox state

Redox reactions pervade all biology. The control of cellular redox state is essential for bioenergetics and for the proper functioning of many biological functions. This review traces a timeline of findings regarding the connections between redox and cancer. There is ample evidence of the involvement of cellular redox state on the different hallmarks of cancer. Evidence of the control of tumor angiogenesis and metastasis through modulation of cell redox state is reviewed and highlighted.

Thioredoxin Reductase as a Novel and Efficient Plasma Biomarker for the Detection of Non-Small Cell Lung Cancer: a Large-scale, Multicenter study

There is an increased demand for efficient biomarkers for the diagnosis of non-small cell lung cancer (NSCLC). This study aimed to evaluate plasma levels of TrxR activity in a large population to confirm its validity and efficacy in NSCLC diagnosis. Blood samples were obtained from 1922 participants (638 cases of NSCLC, 555 cases of benign lung diseases (BLDs) and 729 sex- and age-matched healthy controls). The plasma levels of TrxR activity in patients with NSCLC (15.66 ± 11.44 U/ml) were significantly higher (P < 0.01) than in patients with BLDs (6.27 ± 3.78 U/ml) or healthy controls (2.05 ± 1.86 U/ml). The critical value of plasma TrxR activity levels for diagnosis of NSCLC was set at 10.18 U/ml, with a sensitivity of 71.6% and a specificity of 91.9%. The combination of NSE, CEA, CA19-9, Cyfra21-1, and TrxR was more effective for NSCLC diagnosis (sensitivity and specificity in the training set: 85.6%, 90.2%; validation set: 86.2%, 92.4%) than was each biomarker individually (P < 0.001). TrxR can also efficiently distinguish the metastatic status of the tumor, and it can further differentiate between various histological differentiations. Together, plasma TrxR activity was identified as a convenient, non-invasive, and efficient biomarker for the diagnosis of NSCLCs, particularly for discriminating between metastatic and non-metastatic tumors, or for histologic differentiation.

The antimetastatic effect and underlying mechanisms of thioredoxin reductase inhibitor ethaselen

Treating cancer metastasis is of vital importance to prolong patients' survival. Thioredoxin reductase (TrxR) is overexpressed in many cancer types and has been recognized as an anti-cancer target. The organoselenium compound ethaselen (BBSKE) has been proved to be a TrxR inhibitor and inhibit various types of tumor growth. However, whether BBSKE could inhibit tumor metastasis remains unclear. In this study, we aim to explore the antimetastatic effect of BBSKE and underlying mechanisms. BBSKE was found to dose-dependently suppress migration and invasion of MCF-7 and LoVo cells in vitro. The underlying mechanisms may include inhibition of TrxR activity, elevation of reactive oxygen species (ROS), decrease of EGFR activation and HER2 expression. Besides, the epithelial to mesenchymal transition process and expression of CD44, MMP-9, VEGFR2 and PD-L1 were also abrogated. Decreased migration and invasion, lower expression levels of EGFR, HER2, N-cadherin, CD44, MMP-9, VEGFR2 and PD-L1 were also observed in TrxR1-knockdown MCF-7 and LoVo cells. In the mouse breast cancer 4T1 model, BBSKE not only inhibited progression of primary tumor, but also suppressed formation of metastatic lung nodules and liver micro-metastases, indicating that BBSKE could effectively abolish tumor metastasis. In conclusion, our findings show that BBSKE is able to inhibit migration and invasion of cancer cells in vitro and in vivo, and may be used to prevent and treat metastasis.

Mitochondrial ROS control of cancer

Mitochondria serves a primary role in energy maintenance but also function to govern levels of mitochondria-derived reactive oxygen species (mROS). ROS have long been established to play a critical role in tumorigenesis and are now considered to be integral to the regulation of diverse signaling networks that drive proliferation, tumor cell survival and malignant progression. mROS can damage DNA, activate oncogenes, block the function of tumor suppressors and drive migratory signaling. The mitochondrion's oxidant scavenging systems including SOD2, Grx2, GPrx, Trx and TrxR are key of the cellular redox tone. These mitochondrial antioxidant systems serve to tightly control the levels of the primary ROS signaling species, H2O2. The coordinated control of mROS levels is also coupled to the activity of the primary H2O2 consuming enzymes of the mitochondria which are reliant on the epitranscriptomic control of selenocysteine incorporation. This review highlights the interplay between these many oncogenic signaling networks, mROS and the H2O2 emitting and consuming capacity of the mitochondria.

A Phase II Randomized Placebo-Controlled Trial of Oral N-acetylcysteine for Protection of Melanocytic Nevi against UV-Induced Oxidative Stress In Vivo

Oxidative stress plays a role in UV-induced melanoma, which may arise from melanocytic nevi. We investigated whether oral administration of the antioxidant N-acetylcysteine (NAC) could protect nevi from oxidative stress in vivo in the setting of acute UV exposure. The minimal erythemal dose (MED) was determined for 100 patients at increased risk for melanoma. Patients were randomized to receive a single dose (1,200 mg) of NAC or placebo, in double-blind fashion, and then one nevus was irradiated (1-2 MED) using a solar simulator. One day later, the MED was redetermined and the irradiated nevus and a control unirradiated nevus were removed for histologic analysis and examination of biomarkers of NAC metabolism and UV-induced oxidative stress. Increased expression of 8-oxoguanine, thioredoxin reductase-1, and γ-glutamylcysteine synthase modifier subunit were consistently seen in UV-treated compared with unirradiated nevi. However, no significant differences were observed in these UV-induced changes or in the pre- and postintervention MED between those patients receiving NAC versus placebo. Similarly, no significant differences were observed in UV-induced changes between subjects with germline wild-type versus loss-of-function mutations in the melanocortin-1 receptor. Nevi showed similar changes of UV-induced oxidative stress in an open-label post-trial study in 10 patients who received NAC 3 hours before nevus irradiation. Thus, a single oral dose of NAC did not effectively protect nevi from UV-induced oxidative stress under the conditions examined. Cancer Prev Res; 10(1); 36-44. ©2016 AACR.