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Mitochondrial DNA (mtDNA) mutations are frequent in cancer, yet their precise role in cancer progression remains debated. To functionally evaluate the impact of mtDNA variants on tumor growth and metastasis, we developed an enhanced cytoplasmic hybrid (cybrid) generation protocol and established isogenic human melanoma cybrid lines with wild-type mtDNA or pathogenic mtDNA mutations with partial or complete loss of mitochondrial oxidative function. Cybrids with homoplasmic levels of pathogenic mtDNA reliably established tumors despite dysfunctional oxidative phosphorylation. However, these mtDNA variants disrupted spontaneous metastasis from primary tumors and reduced the abundance of circulating tumor cells. Migration and invasion of tumor cells were reduced, indicating that entry into circulation is a bottleneck for metastasis amid mtDNA dysfunction. Pathogenic mtDNA did not inhibit organ colonization following intravenous injection. In heteroplasmic cybrid tumors, single-cell analyses revealed selection against pathogenic mtDNA during melanoma growth. Collectively, these findings experimentally demonstrate that functional mtDNA is favored during melanoma growth and supports metastatic entry into the blood.

Melanin synthesis involves the successful coordination of metabolic pathways across multiple intracellular compartments including the melanosome, mitochondria, ER/Golgi, and cytoplasm. While pigment production offers a communal protection from UV damage, the process also requires anabolic and redox demands that must be carefully managed by melanocytes. In this report we provide an updated review on melanin metabolism, including recent data leveraging new techniques, and technologies in the field of metabolism. We also discuss the many aspects of melanin synthesis that intersect with metabolic pathways known to impact melanoma phenotypes and behavior. By reviewing the metabolism of melanin synthesis, we hope to highlight outstanding questions and opportunities for future research that could improve patient outcomes in pigmentary and oncologic disease settings., (© 2024 The Authors. Pigment Cell & Melanoma Research published by John Wiley & Sons Ltd.)

Importance: Melanoma in Black individuals has an annual incidence of approximately 1 in 100 000 people. Most studies of melanoma in Black patients have used population databases, which lack important, precise clinical details., Objective: To identify patient-level and tumor-level characteristics of melanoma in Black patients., Design, Setting, and Participants: This case series included Black patients with melanoma at 2 tertiary care centers (University of Texas Southwestern [UTSW] Medical Center and Parkland Health), affiliated with a single institution, UTSW in Dallas, Texas. Self-reported Black patients with a histopathologic diagnosis of melanoma were identified between January 2006 and October 2022., Main Outcomes and Measures: The main variables were demographics, clinical characteristics, personal and family medical history, immunosuppression history, comorbidities, histopathology reports, molecular/genetic studies, imaging reports, melanoma treatments and responses, time to progression, metastatic sites, and survival rates., Results: A total of 48 Black patients with melanoma (median [range] age at diagnosis, 62 [23-86] years; 30 [63%] female) were included in the study. Of 40 primary cutaneous melanomas, 30 (75%) were located on acral skin, despite only 10 of 30 (33%) being histologically classified as acral lentiginous melanomas. Compared with those with acral disease, patients with nonacral cutaneous melanomas were more likely to be immunocompromised (4 of 10 [40%] vs 2 of 30 [7%]) or have a personal history of cancer (6 of 10 [60%] vs 5 of 30 [17%]), with all 3 patients with superficial spreading melanoma having a history of both. No patients had more than 1 confirmed primary melanoma. Overall, 13 Black patients (27%) with melanoma developed stage IV disease, of whom 12 died because of disease progression. Those diagnosed with advanced acral melanoma, mucosal/ocular melanoma, or melanoma of unknown primary lacked actionable sequence variations, were nonresponsive to immunotherapy, and had the poorest outcomes. No patients with nonacral cutaneous melanomas developed distant metastases or died of melanoma., Conclusions and Relevance: This single-institution case series highlights several features of melanoma in Black patients that have not been captured in existing population-level registries, including precise anatomic sites, immune status, family and personal cancer history, and genetics. Multi-institutional registries would improve understanding of melanoma in Black patients.

Despite being the leading cause of cancer deaths, metastasis remains a poorly understood process. To identify novel regulators of metastasis in melanoma, we performed a large-scale RNA sequencing screen of 48 samples from patient-derived xenograft (PDX) subcutaneous melanomas and their associated metastases. In comparison with primary tumors, expression of glycolytic genes was frequently decreased in metastases, whereas expression of some tricarboxylic acid (TCA) cycle genes was increased in metastases. Consistent with these transcriptional changes, melanoma metastases underwent a metabolic switch characterized by decreased levels of glycolytic metabolites and increased abundance of TCA cycle metabolites. A short isoform of glyceraldehyde-3-phosphate dehydrogenase, spermatogenic (GAPDHS) lacking the N-terminal domain suppressed metastasis and regulated this metabolic switch. GAPDHS was downregulated in metastatic nodules from PDX models as well as in human patients. Overexpression of GAPDHS was sufficient to block melanoma metastasis, whereas its inhibition promoted metastasis, decreased glycolysis, and increased levels of certain TCA cycle metabolites and their derivatives including citrate, fumarate, malate, and aspartate. Isotope tracing studies indicated that GAPDHS mediates this shift through changes in pyruvate carboxylase activity and aspartate synthesis, both metabolic pathways critical for cancer survival and metastasis. Together, these data identify a short isoform of GAPDHS that limits melanoma metastasis and regulates central carbon metabolism., Significance: This study characterizes metabolic changes during cancer metastasis and identifies GAPDHS as a novel regulator of these processes in melanoma cells., (©2022 American Association for Cancer Research.)

The pentose phosphate pathway is a major source of NADPH for oxidative stress resistance in cancer cells but there is limited insight into its role in metastasis, when some cancer cells experience high levels of oxidative stress. To address this, we mutated the substrate binding site of glucose 6-phosphate dehydrogenase (G6PD), which catalyzes the first step of the pentose phosphate pathway, in patient-derived melanomas. G6PD mutant melanomas had significantly decreased G6PD enzymatic activity and depletion of intermediates in the oxidative pentose phosphate pathway. Reduced G6PD function had little effect on the formation of primary subcutaneous tumors, but when these tumors spontaneously metastasized, the frequency of circulating melanoma cells in the blood and metastatic disease burden were significantly reduced. G6PD mutant melanomas exhibited increased levels of reactive oxygen species, decreased NADPH levels, and depleted glutathione as compared to control melanomas. G6PD mutant melanomas compensated for this increase in oxidative stress by increasing malic enzyme activity and glutamine consumption. This generated a new metabolic vulnerability as G6PD mutant melanomas were more dependent upon glutaminase than control melanomas, both for oxidative stress management and anaplerosis. The oxidative pentose phosphate pathway, malic enzyme, and glutaminolysis thus confer layered protection against oxidative stress during metastasis., Competing Interests: The authors declare no competing interest., (Copyright © 2022 the Author(s). Published by PNAS.)

This is the first prospective study of a combination therapy involving a cardenolide and a MEK inhibitor for metastatic melanoma. Whereas BRAF mutant melanomas can exhibit profound responses to treatment with BRAF and MEK inhibitors, there are fewer options for BRAF wild-type melanomas. In preclinical studies, we discovered that cardenolides synergize with MEK inhibitor to promote the regression of patient-derived xenografts irrespective of BRAF mutation status. We therefore conducted a phase 1B study of digoxin 0.25 mg and trametinib 2 mg given orally once daily in 20 patients with advanced, refractory, BRAF wild-type melanomas. The most common adverse events were rash, diarrhea, nausea, and fatigue. The response rate was 4/20 or 20% with response durations of 2, 4, 6, and 8 months. The disease control rate (including partial responses and stable disease) was 13/20 or 65% of patients, including 5/6 or 83% of patients with NRAS mutant melanomas and 8/14 or 57% of NRAS wild-type melanomas. Patients with stable disease had disease control for 2, 2, 2, 4, 5, 6, 7, 10, and 10 months. Xenografts from four patients recapitulated the treatment responses observed in patients. Based on these pilot results, an expansion arm of digoxin plus MEK inhibitor is warranted for NRAS mutant metastatic melanoma patients who are refractory or intolerant of immunotherapy., Key Points: Digoxin plus trametinib is well tolerated and achieves a high rate of disease control in BRAF wild-type metastatic melanoma patients., (Copyright © 2017 The Authors. Published by Elsevier Inc. All rights reserved.)

New therapies are required for melanoma. Here, we report that multiple cardiac glycosides, including digitoxin and digoxin, are significantly more toxic to human melanoma cells than normal human cells. This reflects on-target inhibition of the ATP1A1 Na(+)/K(+) pump, which is highly expressed by melanoma. MEK inhibitor and/or BRAF inhibitor additively or synergistically combined with digitoxin to induce cell death, inhibiting growth of patient-derived melanomas in NSG mice and synergistically extending survival. MEK inhibitor and digitoxin do not induce cell death in human melanocytes or haematopoietic cells in NSG mice. In melanoma, MEK inhibitor reduces ERK phosphorylation, while digitoxin disrupts ion gradients, altering plasma membrane and mitochondrial membrane potentials. MEK inhibitor and digitoxin together cause intracellular acidification, mitochondrial calcium dysregulation and ATP depletion in melanoma cells but not in normal cells. The disruption of ion homoeostasis in cancer cells can thus synergize with targeted agents to promote tumour regression in vivo.