Hormone receptors and Japanese breast cancer.

In Japan, the incidence of breast cancer is remarkably lower than in Western countries. We examined tumor estrogen (ER) and progesterone receptors (PgR) in 1060 Japanese breast cancer patients. In Japan, 58% and 36% of breast cancers revealed measurable amounts of cytosol ER and PgR, respectively. These rates are slightly lower than those reported in Western countries, which were found to be 60-80% and 40-60%, respectively, though the difference is mainly confined to postmenopausal non-overweight (less than 60 kg) patients. The objective remission rates to endocrine therapy were 48% for ER+ tumors, 10% for ER-tumors, 66% for ER+, PgR+ tumors, 30% for ER+, PgR- tumors and 10% for ER-, PgR- tumors. These response rates are similar to those in Western breast cancer patients. ER and/or PgR in the breast cancer cytosol are useful markers for predicting response to endocrine therapy. The present findings also indicate that ER- tumors are estrogen independent but ER+ tumors include both estrogen-dependent and -independent tumors and that some of ER+ but endocrine-resistant tumors have postreceptor defect. The presence of ER+ but endocrine-resistant tumors may be due in part to the loss of hormone-dependency of estrogen-dependent breast cancer during endocrine therapy. The transformation of hormone-dependent mouse tumor cells into independent tumor cells was demonstrated as one of the mechanisms involved in the loss of hormone-dependency.