Comparison of steroid receptor expression in normal, dysplastic, and neoplastic canine and feline mammary tissues

Steroid receptor expression was assessed by immunohistochemistry in neoplastic, hyperplastic/dysplastic, and normal mammary tissue samples removed from 68 queens and 47 bitches, using monoclonal antibodies against human oestrogen-alpha (ER) and progesterone receptors (PR). Mammary lesions were classified according to World Health Organization (WHO) criteria, and all animals with invasive carcinomas were clinically followed for 2 years. Stromal and/or lymphatic invasion and histological grading were also recorded. In both species, ER expression was significantly higher in healthy tissues, hyperplastic/dysplastic lesions, and benign tumours than in carcinomas. The loss of ER expression was more marked in feline than in canine carcinomas. In queens, PR expression increased in dysplastic lesions and "in situ" carcinomas and decreased in invasive carcinomas, even if parts of these tumours were still PR-positive. In bitches no significant variation in PR expression was observed between normal tissue, dysplasias, and benign neoplasms, but was significantly lower in carcinomas. In both species ER and PR expression in invasive carcinomas did not correlate either with histological parameters or overall survival time. This study demonstrates several differences in steroid hormone dependency between the two species. The percentage of PR-positive feline carcinomas suggests a possible role of progesterone in promoting early tumour cell growth in queens. The low percentage of ER-positive invasive carcinomas further demonstrated the aggressive phenotype and behaviour of feline mammary tumours.