Expression of the vitamin D receptor, of estrogen and thyroid hormone receptor α- and β-isoforms, and of the androgen receptor in cultures of native mouse bone marrow and of stromal/osteoblastic cells

Marrow stromal cells mediate the effect of 1α,25-dihydroxyvitamin D3 on formation of osteoclast-like cells from undifferentiated hematopoetic precursors in bone marrow. Induction by the vitamin D hormone of multinucleated, calcitonin receptor- and tartrate-resistant acid phosphatase-positive cells in primary mouse bone marrow culture can be modulated by other members of the steroid/thyroid hormone family, such as triiodothyronine, which has a positive effect, as well as 17β-estradiol and 5α-dihydrotestosterone, which both act as inhibitors of osteoclastogenesis. In an attempt to relate these effects of the steroid/thyroid hormones to the presence of their respective nuclear receptors, we studied expression of the vitamin D receptor (VDR), estrogen receptor (ER)-α and -β, thyroid hormone receptor (TR)-α and -β, and androgen receptor (AR) in total bone marrow as well as primary marrow stromal cell cultures. By using reverse-transcriptase–polymerase chain reaction, in both cases amplification products were obtained, which were identified by multiple restriction fragment length analysis as transcripts from mRNA specific for the ligand-binding domains of the VDR, ER-α, ER-β, TR-α, TR-β, and AR. Specific immunostaining by indirect peroxidase labeling revealed that among the various cell types present in bone marrow, the steroid/thyroid hormone receptors are abundant particularly in marrow stromal cells. In another series of experiments, we extended our survey on receptor expression also to stromal/osteoblastic cell lines. At the mRNA level, the complete repertoire of steroid/thyroid hormone receptors was present in preadipocytic ST2 cells as well as in osteoblastic MC3T3-E1 cells. By immunocytochemical staining of the latter, it became apparent that single cells exhibit wide variations in intensity of specific signals for all the receptors investigated, so that, notably in contrast to primary stromal cells and ST2 cells, MC3T3-E1 display a mosaic pattern of receptor protein expression.