Bone marrow stromal cells express two distinct splice variants of ER-α that are regulated by estrogen

Estrogen plays a critical role in bone metabolism in both sexes. While the major action of estrogen is to inhibit bone resorption, it is now clear that early osteoblastic (or stromal) cells are a target for estrogen action, mediating the effects of estrogen on bone formation as well as resorption. However, little is known about the expression or regulation of the estrogen receptor (ER)-α in these cells. The expression of ER-α is regulated by a complex set of promoters and ER-α splice variants are present in different tissues. Thus, we sought to define the ER-α splice variants and their regulation by estrogen in the mouse bone marrow stromal cell line, ST-2, which can be induced to differentiate into mature osteoblasts. ST-2 cells expressed the mRNAs and proteins for both the 66 and 46 kDa forms of ER-α; the latter lacks the AF-1 domain and can transduce estrogen signaling in some tissues, while serving as a dominant negative receptor in others. Using primers specific for each of the five 5′-untranslated exons of ER-α, we found that ST-2 cells utilized only the promoters upstream of exons F and C (in contrast to most reproductive tissues, which utilize promoters upstream of virtually all the five exons). Moreover, 17β-estradiol (10^-8 M) treatment of ST-2 cells markedly diminished levels of the 66 kDa as well as the 46 kDa ER-α proteins, largely through suppression of the transcript arising from the F1 promoter. These data thus indicate that: (1) bone marrow stromal cells express at least two variants of ER-α and (2) estrogen down regulates the ER-α mRNA and protein in these cells.