Melanoma is the deadliest type of skin cancer. Human melanomas often show hyperactivity of nitric oxide synthase (NOS) and NADPH oxidase (NOX), which, respectively, generate nitric oxide (NO^·) and superoxide (O₂^·−). The NO^· and O₂ ⁻ react instantly with each other to generate peroxynitrite (ONOO⁻) which is the driver of melanin chemiexcitation. Melanoma precursors, the melanocytes, are specialized skin cells that synthesize melanin, a potent shield against sunlight's ultraviolet (UV) radiation. However, melanin chemiexcitation paradoxically demonstrates the melanomagenic properties of melanin. In a loop, the NOS activity regulates melanin synthesis, and melanin is utilized by the chemiexcitation pathway to generate carcinogenic melanin-carbonyls in an excited triplet state. These carbonyl compounds induce UV-specific DNA damage without UV. Additionally, the carbonyl compounds are highly reactive and can make melanomagenic adducts with proteins, DNA and other biomolecules. Here we review the role of the melanin chemiexcitation pathway in melanoma initiation, progression, and drug resistance. We conclude by hypothesizing a non-classical, positive loop in melanoma where melanin chemiexcitation generates carcinogenic reactive carbonyl species (RCS) and DNA damage in normal melanocytes. In parallel, NOS and NOX regulate melanin synthesis generating raw material for chemiexcitation, and the resulting RCS and reactive nitrogen species (RNS) regulate cellular proteome and transcriptome in favor of melanoma progression, metastasis, and resistance against targeted therapies.

UV-like DNA damage is created in the dark by chemiexcitation, in which UV-activated enzymes generate reactive oxygen and nitrogen species that create a dioxetane on melanin. Thermal cleavage creates an electronically excited triplet-state carbonyl whose high energy transfers to DNA. Screening natural compounds for the ability to quench this energy identified polyenes, polyphenols, mycosporine-like amino acids, and related compounds better known as antioxidants. To eliminate false positives such as ROS and RNS scavengers, we then used the generator of triplet-state acetone, tetramethyl-1,2-dioxetane (TMD), to excite the triplet-energy reporter 9,10-dibromoanthracene-2-sulfonate (DBAS). Quenching measured as reduction in DBAS luminescence revealed three clusters of 50% inhibitory concentration, ~50 μM, 200–500 μM, and >600 μM, with the former including sorbate, ferulic acid, and resveratrol. Representative triplet-state quenchers prevented chemiexcitation-induced “dark” cyclobutane pyrimidine dimers (dCPD) in DNA and in UVA-irradiated melanocytes. We conclude that (i) the delocalized pi electron cloud that stabilizes the electron-donating activity of many common antioxidants allows the same molecule to prevent an electronically excited species from transferring its triplet-state energy to targets such as DNA and (ii) the most effective class of triplet-state quenchers appear to operate by energy diversion instead of electron donation and dissipate that energy by isomerization.

Activating NRAS mutations occur in 15% to 25% of all melanomas. However, this subtype remains refractory to existing therapeutics, including immunotherapy and RAS inhibitors; therefore, identifying innovative treatment strategies is of utmost importance. We investigated the role of nitrosylation, a nitric oxide–induced posttranslational modification, in melanoma progression and therapeutic resistance. Inhibiting nitrosylation sensitized NRAS-mutant melanomas to targeted MEK inhibitors (MEKi), leading to sustained downregulation of the ERK-MAPK pathway, along with concomitant denitrosylation of NRAS, MEK, ERK, RSK1, and DUSPs. Global nitrosylome profiling using mass spectrometry revealed nitrosylation of multiple ERK regulators. Gain- and loss-of-function studies confirmed a positive association between nitrosylation and ERK activation. ERK and MEK proteins harbored potential nitrosylation sites, mutation of which abrogated their phosphorylation and inhibited cell growth. The nitrosylome also contained damage-associated molecular patterns (DAMP), factors known to induce immunogenic cell death (ICD). Notably, nitrosylation inhibition combined with MEKi markedly inhibited NRAS-mutant melanoma growth in an immunocompetent mouse model. This was accompanied by downregulated MEK-ERK signaling and extracellular release of DAMPs such as calreticulin, phospho-eIF2α, and HMGB1, confirming ICD induction. Furthermore, the combination significantly increased the repertoire of CD8+ T cells, dendritic cells, and macrophages in the tumor microenvironment, which was validated in cocultures of dendritic cells and T lymphocytes. In conclusion, the current study demonstrates that nitrosylation inhibition sensitizes NRAS-mutant melanomas to targeted MEKi-induced cell death and causes the release of non-nitrosylated (active) DAMPs that induce a potent antimelanoma immune response via ICD. These findings highlight potential therapeutic vulnerabilities in the currently untreatable NRAS-mutant melanoma subtype. Significance: Nitrosylation-regulated molecular mechanisms present a vulnerability that can be exploited to sensitize NRAS-mutant melanomas to existing targeted therapies while enhancing antitumor immunity.

Cyclobutane pyrimidine dimers (CPDs) are ultraviolet radiation (UV)-induced carcinogenic DNA photoproducts that lead to UV signature mutations in melanoma. Previously, we discovered that, in addition to their incident formation (iCPDs), UV exposure induces melanin chemiexcitation (MeCh), where UV generates peroxynitrite (ONOO⁻), which oxidizes melanin into melanin-carbonyls (MCs) in their excited triplet state. Chronic MeCh and energy transfer by MCs to DNA generates CPDs for several hours after UV exposure ends (dark CPD, dCPDs). We hypothesized that MeCh and the resulting dCPDs can be inhibited using MeCh inhibitors, and MC and ONOO⁻ scavengers. Here, we investigated the efficacy of Acetyl Zingerone (AZ), a plant-based phenolic alkanone, and its chemical analogs in inhibiting iCPDs and dCPDs in skin fibroblasts, keratinocytes, and isogenic pigmented and albino melanocytes. While AZ and its methoxy analog, 3-(4-Methoxy-benzyl)-Pentane-2,4-dione (MBPD) completely inhibited the dCPDs, MBPD also inhibited ~50% of iCPDs. This suggests the inhibition of ~80% of total CPDs at any time point post UV exposure by MBPD, which is markedly significant. MBPD downregulated melanin synthesis, which is indispensable for dCPD generation, but this did not occur with AZ. Meanwhile, AZ and MBPD both upregulated the expression of nucleotide excision repair (NER) pathways genes including Xpa, Xpc, and Mitf. AZ and its analogs were non-toxic to the skin cells and did not act as photosensitizers. We propose that AZ and MBPD represent “next-generation skin care additives” that are safe and effective for use not only in sunscreens but also in other specialized clinical applications owing to their extremely high efficacy in blocking both iCPDs and dCPDs.

Cyclobutane pyrimidine dimers (CPDs) are the major form of DNA damage induced by UVR. Although CPDs form instantaneously during UV exposure, our prior studies demonstrated that melanin chemiexcitation, in which UV-induced peroxynitrite oxidizes melanin into excited-state intermediates, continues to generate CPDs for hours after UV exposure ends. These "delayed" CPDs contribute up to 50% of the total CPD burden. Although melanin's involvement is well-established, it remains unclear whether active melanin biosynthesis is required or whether the mere presence of pigment is sufficient. To address this, we developed a depigmentation-repigmentation system using cultured melanocytes. Cells were depigmented in tyrosine-deficient medium and repigmented either by reintroducing tyrosine or by supplementing with 5,6-dihydroxyindole-2-carboxylic acid, a melanin monomer that polymerizes nonenzymatically. In both conditions, delayed CPD formation was restored. Notably, 5,6-dihydroxyindole-2-carboxylic acid treatment alone was sufficient to induce pigmentation and delayed CPD formation even in tyrosinase-deficient isogenic albino melanocytes. Pigment accumulation was also associated with nitric oxide synthase activity, a key contributor to chemiexcitation, although the mechanisms linking melanin synthesis to nitric oxide synthase activity remain to be elucidated. These findings demonstrated pigment presence, not biosynthesis, as the key determinant of delayed CPD formation, with broad implications for photobiology, pigmentation, and skin cancer risk.

Editorial: Mechanisms of resistance to the targeted therapy and immunotherapy in cutaneous melanoma