The receptor, metabolism and effects of androgen in osteoblastic MC3T3-E1 cells

We investigated the androgen receptor (AR), metabolism and effects of androgens in osteoblastic MC3T3-E1 cells. AR was proved as a transcript of a 10-kb mRNA and as a 110-kDa protein. An immunocytochemical study showed that AR was located mainly in the nuclei. Specific binding of [³H]DHT was observed in both the nuclear and cytosol fractions. MC3T3-E1 cells possessed approximately 1190 binding sites per cell and most of the sites (1150 sites) situated in the nucleus. The apparent K_d value in the nuclear fraction was 1.35 nM for [³H]DHT binding, and it was similar to that for [³H]testosterone. In the competition analysis, there was not much difference in the displacement of the [³H]DHT binding from AR between the addition of radioinert DHT and testosterone. In studies of 5α-reductase activity and aromatase activity of the cells, both activities were lower than the respective values in classical androgen target tissues. Androgens stimulated the incorporation of [³H]thymidine into the cell, and DHT and testosterone had a similar potency on the cell proliferation. Thus, these results suggest testosterone itself acts mainly on the osteoblasts without conversion to DHT.