# Cyclooxygenase-2 (COX-2) Expression in Equine Melanocytic Tumors

## TL;DR

This study examines COX-2 levels in equine melanocytic tumors and finds that low COX-2 may explain their less aggressive growth compared to tumors in other species.

## Contribution

The study provides new insights into the molecular behavior of equine melanocytic tumors through COX-2 immunohistochemical analysis.

## Key findings

- Melanomas showed significantly higher COX-2 expression than melanocytomas.
- Low COX-2 levels may contribute to the non-invasive growth pattern of equine melanocytic tumors.
- Only 15.5% of tumors had high COX-2 expression, while 42.9% were negative.

## Abstract

Cyclooxygenase-2 (COX-2) has been associated with melanoma progression in humans and dogs. Its overexpression is related to tumor aggressiveness. Equine melanoma has a different pattern of evolution compared to that in dogs and humans since it is characterized by a slow, expansive growth rather than a high degree of invasiveness and frequent metastasis. The aim of this study was to evaluate the immunohistochemical expression of COX-2 in equine melanocytic tumors. Immunohistochemistry was used to evaluate 39 melanocytomas and 38 melanomas. The findings indicated that 42.9% of melanocytic tumors were negative, 41.6% had low COX-2 expression, and 15.5% had high COX-2 expression. Meanwhile, 13.2% of malignant tumors were negative and 63.2% presented low COX-2 expression. COX-2 was significantly higher in melanomas than in melanocytomas. Overall, low COX-2 levels may be one of the molecular differences that may contribute to the different clinical behavior of equine melanocytic tumors compared with those of other species.

Equine melanocytic tumors are common and have an unusual benign behavior with low invasiveness and metastatic rates. However, tumoral mass growth is usually a concern that can have life-threatening consequences. COX-2 is related to oncogenesis, promoting neoplastic cell proliferation, invasion, and metastasis. The aim of this study was to evaluate the immunohistochemical expression of COX-2 in equine melanocytic tumors. Through extension and intensity of labeling, 39 melanocytomas and 38 melanomas were evaluated. Of the malignant tumors, 13.2% were negative and 63.2% presented a low COX-2 expression. Only 6 malignant tumors presented >50% of labeled cells, 18 malignant and 8 benign had an expression between 21 and 50%, 8 malignant and 3 benign tumors had an expression between 6 and 20%, 1 malignant tumor had an expression between 1 and 5%, and 5 malignant and 28 benign tumors had no expression. Malignant tumors showed higher COX-2 expression than did benign tumors, with statistically significant differences. The low levels of COX-2 may be one of the molecular reasons for the presence of expansive mass growth instead of the invasive pattern of other species, which is related to high COX-2 levels.