Reduction of phosphorylated signal transducer and activator of transcription-5 expression in feline mammary carcinoma

Signal transducers and activators of transcription (STATs) are a family of transcription factors involved in various normal physiological cellular processes. Moreover, STATs have been recently identified as novel therapeutic targets for various human tumors. STAT3, STAT5a, and STAT6 have been suggested to be involved in tumorigenesis in human breast cancer. Owing to the similarity between feline mammary carcinomas (FMCs) and human breast cancers, these factors may play an important role in FMCs. However, studies on the expression of STATs in animal tumors are limited. Therefore, in this study, we aimed to characterize the expression of total STAT5 (tSTAT5) and phosphorylated STAT5 (pSTAT5) in FMCs, feline mammary adenomas, non-neoplastic proliferative mammary gland lesions, and normal feline mammary glands using immunohistochemistry. High expression of tSTAT5 was observed in the cytoplasm of all the samples assessed in this study. Moreover, high expression of tSTAT5 was observed in the nucleus; however, its levels varied depending on the lesion. The percentage of pSTAT5-nuclear positive cells varied among normal feline mammary glands (40.1 +/- 25.1%), and non-neoplastic lesions, including mammary hyperplasia (43.2 +/- 28.6%) and fibroadenomatous changes (18.0 +/- 13.6%). Moreover, the percentage of pSTAT5nuclear-positive cells in feline mammary adenomas was 24.5 +/- 19.2%, which was significantly reduced in feline mammary carcinomas (2.4 +/- 5.6%), regardless of histopathological subtype. This study suggests that decreased STAT5 activity may be involved in the development and malignant progression of feline mammary carcinomas.