Expression of aquaporin 3 in squamous cell carcinoma in dogs and cats

Aquaporins (AQPs) are membrane proteins that facilitate the transport of small molecules such as water, glycerol, and urea. In mammals, 13 AQP isoforms (AQP0 to AQP12) exhibit tissue-specific expression. Over the past two decades, aquaporins (AQPs) have received considerable attention for their role in tumor biology. Squamous cell carcinoma (SCC) originates from stratified squamous cells and commonly affects the skin and oral cavity. SCC is prevalent in dogs and cats; however, no studies have reported an association between SCC and AQPs in these species. This study investigated the immunohistochemical expression of AQP1, AQP3, AQP5, E-cadherin, and Ki-67 in cutaneous and oral SCCs of dogs and cats. AQP1 and AQP5 were not expressed in tumor cells. In contrast, AQP3 and E-cadherin localized to the plasma membrane, primarily within island-like structures in both cutaneous and oral SCCs. Ki-67-positive cells appeared in small group and cord structures within tumor regions exhibiting reduced AQP3 expression. Therefore, AQP3 expression may be down-regulated in malignant tumor cells in SCCs of dogs and cats. These findings indicate that AQP3 may serve as a novel biomarker for assessing malignancy in SCCs of dogs and cats.