Increased estrogen receptor βcx expression during mammary carcinogenesis

Identification of proteins that markedly vary during early steps of mammary carcinogenesis may help to understand its pathophysiology and to develop a prevention strategy. The expression of total estrogen receptor β (ERβ) protein and of its COOH-terminally spliced variant ERβcx (or ERβ2) was compared in 43 invasive breast cancers and in 39 adjacent normal mammary glands and 26 ductal carcinoma in situ (DCIS). Thirty-six breast cancers were ER positive by radioligand binding assay. The analysis was done by immunohistochemistry on adjacent sections of formalin-fixed, paraffin-embedded tumors using polyclonal anti-ERβ 503 IgY and sheep polyclonal ERβcx antibodies that were previously validated. Nuclear staining was quantified using a computerized image analyzer in selected areas of normal and cancer epithelial cells. Total ERβ expression was high in normal glands, decreased in DCIS (P = 0.0004), and increased from DCIS to invasive tumor (P = 0.029). In contrast, the ERβcx expression was low in normal glands, increased significantly in DCIS (P = 0.0014), and continued to increase in invasive carcinomas (P = 0.0027) in both ERα-positive and ERα-negative tumors. This is the first study showing significant increase of the ERβcx variant protein in DCIS and invasive breast cancer compared with adjacent normal glands. This contrasts with the decrease of the total ERβ level in the same patients and indicates different mechanisms to explain these variations during mammary carcinogenesis. It also suggests a role of the ERβcx variant in carcinogenesis opposite to the protective effect of the wild-type ERβ1.