Expression of c-kit protein in proliferative lesions of human breast: sexual difference and close association with phosphotyrosine status.

PURPOSE: The c-kit gene which codes transmembrane tyrosine kinase receptor protein plays an important role in several types of normal and/or neoplastic human tissues. We examined the expression patterns of c-kit protein in proliferative lesions of human breast tissues in both sexes.
METHODS: The localization of c-kit protein was examined immunohistochemically in human breast, consisting of 366 normal tissue, 156 benign lesions (fibroadenoma, fibrocystic change, intraductal papilloma, benign phyllodes tumor, and gynecomastia), 13 borderline diseases (atypical ductal hyperplasia, atypical lobular hyperplasia, and borderline malignant phyllodes tumor), and 197 malignant lesions (non-invasive and/or invasive ductal carcinoma and malignant phyllodes tumor).
RESULTS: In normal tissues and benign proliferative lesions, c-kit product was consistently detected on epithelial cell membranes and/or cytoplasms regardless of gender difference. In contrast, we failed to find c-kit product in female borderline epithelial lesions, including atypical lobular hyperplasia, or in female malignant lesions, except for two carcinomas. In situ hybridization analysis of c-kit mRNA in female tissues gave results comparable to those obtained by immunohistochemistry. On the other hand, c-kit product was consistently detected in male benign and malignant proliferative lesions. Apart from the female breast carcinomas which lacked c-kit, c-kit expression was almost always accompanied by positivity for phosphotyrosine in the breast tissues examined, suggesting possible phosphorylation of tyrosine residues of the c-kit receptor protein.
CONCLUSIONS: Loss of c-kit product was related to malignant transformation in female breast, but not in the case of male breast. We suggest that the oncogenesis pathway of breast epithelium is different between males and females in terms of c-kit expression.