The Predominant Proteins that React to the MC-20 Estrogen Receptor Alpha Antibody Differ in Molecular Weight between the Mammary Gland and Uterus in the Mouse and Rat

摘要

There are many estrogen receptor a (ERα) antibodies available but few of them target a rodent ERα. Using the MC-20 antibody raised against the C-terminus of mouse ERα, we show in this communication that in the mammary gland of female mice and rats, the wild type (wt) ERα was detected on immunoblots as a dominant protein only during lactation, and the protein was lactating specific as it migrated slightly faster than the 67-kD wt ERα in the uterus, likely due to a different phosphorylation status. In contrast, in the nulliparous, pregnant, involuting and involuted mammary glands, the dominant protein recognized by MC-20 was about 61-kD, which is dubbed herein as "MC-20 reactive protein" or MC2ORP in abbreviation as its identity is unknown. Our results showed that it was not derived from proteolysis or de-phosphorylation of the 67-kD ERα and was unlikely to be translated from an ERα mRNA variant. Ovariectomy decreased the lactating specific wt ERα but increased the 61-kD MC2ORP in the mammary tumors from MMTV-c-,nre transgenic mice but these two proteins in the uterus were unaffected. The 61-k D MC2ORP was decreased in the mammary tumors, compared with proliferating mammary glands, in estrogen-treated ACI rats. These results suggest that while the lactating specific wt ERa alone or together with the MC2ORP may sustain lactation, the MC2ORP may support proliferation of the mammary gland and some mammary tumors.