Expression of claudin-1, -2, -3, -4, -5 and -7 proteins in benign and malignant canine mammary gland epithelial tumours.

Claudins are tight junction proteins expressed by epithelial and endothelial cells. The present study has evaluated the expression of claudin-1, -2, -3, -4, -5 and -7 in 115 hyperplastic and neoplastic lesions of the canine mammary gland and compared this expression with that of normal mammary epithelium. The lesions studied included lobular hyperplasia (n=15), simple adenoma (n=20), non-infiltrating carcinoma in situ (n=20) and infiltrating carcinomas of histological grades I, II and III (n=20 of each). There was strong expression of claudin-1, -3, -4, -5 and -7 by epithelia within examples of lobular hyperplasia and simple adenoma, and strong expression of claudin-3 and -4 by non-infiltrating carcinomas and all three grades of infiltrating carcinoma. By contrast, there was reduced expression of claudin-5 and -7 by non-infiltrating and infiltrating carcinomas and the expression of these two molecules was inversely correlated with histological grade. Claudin-1 was expressed focally within carcinoma in situ, but this molecule was not detected in any invasive carcinoma. Claudin-2 was weakly expressed within areas of lobular hyperplasia and by simple adenomas, but was not expressed by any carcinoma cells. These results suggest that loss or reduction of expression of claudin-1, -2, -5 and -7 may lead to cellular disorientation, detachment and invasion in canine mammary neoplasia.