Cell Membrane and Nuclear Estrogen Receptors (ERs) Originate from a Single Transcript: Studies of ERα and ERβ Expressed in Chinese Hamster Ovary Cells

Abstract The existence of a putative membrane estrogen receptor (ER) has been supported by studies accomplished over the past 20 yr. However, the origin and functions of this receptor are not well defined. To study the membrane receptor, we transiently transfected cDNAs for ERα or ERβ into Chinese hamster ovary (CHO) cells. Transfection of ERα resulted in a single transcript by Northern blot, specific binding of labeled 17β-estradiol (E2), and expression of ER in both nuclear and membrane cell fractions. Competitive binding studies in both compartments revealed near identical dissociation constants (Kds) of 0.283 and 0.287 nm, respectively, but the membrane receptor number was only 3% as great as the nuclear receptor density. Transfection of ERβ also yielded a single transcript and nuclear and membrane receptors with respective Kd values of 1.23 and 1.14 nm; the membrane receptor number was only 2% compared with expressed nuclear receptors. Estradiol binding to CHO-ERα or CHO-ERβ activated Gαq and Gαs proteins in the membrane and rapidly stimulated corresponding inositol phosphate production and adenylate cyclase activity. Binding by 17-β-E2 to either expressed receptor comparably enhanced the nuclear incorporation of thymidine, critically dependent upon the activation of the mitogen-activated protein kinase, ERK (extracellular regulated kinase). In contrast, c-Jun N-terminal kinase activity was stimulated by 17-β-E2 in ERβ-expressing CHO, but was inhibited in CHO-ERα cells. In summary, membrane and nuclear ER can be derived from a single transcript and have near-identical affinities for 17-β-E2, but there are considerably more nuclear than membrane receptors. This is also the first report that cells can express a membrane ERβ. Both membrane ERs activate G proteins, ERK, and cell proliferation, but there is novel differential regulation of c-Jun kinase activity by ERβ and ERα.