Oxidative Stress and Immunomarkers in Equine Dermal Melanomatosis: Single-Case Comparison with Healthy Individual.

Oxidative stress and antioxidant enzyme activity were compared between melanomatosis lesions in a grey horse and healthy skin from a black horse, revealing increased oxidative damage and decreased antioxidant defenses in the tumor tissue. This single-case study suggests oxidative stress plays a role in equine dermal melanomatosis.

Background and Purpose

• Dermal melanomatosis is a frequent tumor affecting grey horses, involving growth of melanocytic cells in skin and subcutaneous tissues.
• Though the clinical presentation and progression (including metastasis) have been well studied, the role of oxidative stress in this tumor type is not well understood.
• This study aimed to investigate oxidative stress and antioxidant responses in melanomatosis lesions compared with normal skin to understand how oxidative imbalance might relate to tumor behavior.

Methods

• A single grey horse presenting with dermal melanomatosis was studied, with lesion tissue sampled.
• Control samples were taken from healthy skin of a black horse without tumors.
• Biochemical assays assessed levels of oxidative damage markers in protein extracts:
• Malondialdehyde (MDA): marker of lipid peroxidation.
• Advanced oxidation protein products (AOPP): indicator of protein oxidation.
• Antioxidant defense was evaluated by measuring activities of key antioxidant enzymes:
• Catalase (CAT)
• Superoxide dismutase (SOD)
• Levels of glutathione (GSH), a crucial antioxidant molecule, were also measured.
• Histopathological examination confirmed the presence of melanocytic proliferation in lesion tissue.
• Immunohistochemistry used markers:
• S100 protein: sensitive marker for melanocytes.
• Ki-67: marker for cell proliferation to assess tumor growth rate.

Key Findings

• Oxidative damage markers were significantly elevated in melanomatosis lesions:
• MDA increased by 73%, indicating higher lipid peroxidation in tumor tissue.
• AOPP increased by 56%, reflecting enhanced protein oxidation.
• Antioxidant enzyme activities were reduced in the melanomatosis tissue:
• Catalase and superoxide dismutase activities were decreased, suggesting impaired enzymatic antioxidant defense.
• Glutathione levels were 29% lower, indicating depletion of this non-enzymatic antioxidant in tumor tissue.
• Histology confirmed melanocytic tumor growth consistent with dermal melanomatosis.
• Immunohistochemistry results:
• S100 showed moderate positive staining, confirming melanocytic origin.
• Ki-67 showed low proliferation rate (~8%), indicating relatively slow tumor growth in this case.

Interpretation and Significance

• The study demonstrated an oxidative imbalance in equine dermal melanomatosis within the one case examined, characterized by higher oxidative damage and diminished antioxidant capacity in tumor tissue versus healthy skin.
• These findings support the hypothesis that oxidative stress contributes to the pathophysiology of melanomatosis in grey horses, potentially influencing tumor development and progression.
• Moderate S100 positivity and low Ki-67 suggest the tumor had melanocytic characteristics with limited proliferation at the time of sampling.
• The results highlight the potential use of oxidative stress markers and antioxidant enzymes as targets for future research or therapeutic monitoring in equine melanocytic tumors.
• Given the single-case design, further larger studies are needed to validate these findings and explore oxidative stress involvement across a broader population of affected horses.

Summary

• This research provides novel evidence linking oxidative stress to equine dermal melanomatosis by comparing tumor and healthy tissues in a grey versus black horse.
• Decreased antioxidant defenses and increased oxidative damage in the tumor tissue suggest oxidative stress is an important factor in the disease’s underlying biology.
• The study emphasizes the importance of exploring oxidative and immunohistochemical markers to better understand and potentially manage this common equine tumor.