Expression of estrogen receptors (α, β) and insulin-like growth factor-I in breast tissue from surgically postmenopausal cynomolgus macaques after long-term treatment with HRT and tamoxifen

The novel estrogen receptor ERβ could be a key factor for proliferation and breast cancer risk. In a primate model for long-term HRT, surgically postmenopausal cynomolgus macaques were treated for 35 months with conjugated equine estrogens (CEE), medroxyprogesterone acetate (MPA), CEE+MPA and tamoxifen (n = 5 in all groups). The immunohistochemical expression of ERα, ERβ and IGF-I in breast tissue was quantified by image analysis. Overall the levels of ERβ were higher than for ERα. In untreated animals, the median area of positive cells was 58% and 21%. The lowest levels for ERβ were seen during treatment with CEE/MPA (3%) and in this group the expression of ERβ was lower than for ERα. Tamoxifen had effects similar to estrogen. ERβ may have a role to modulate the proliferative response following activation of ERα. The results suggest that hormonal treatments have a different influence on the balance ERβ/ERα in breast tissue.