Nongenomic effects of an anti-idiotypic antibody as an estrogen mimetic in female human and rat osteoblasts

We investigated the early effects of the anti-idiotypic antibody (clone 1D₅), which recognized the estrogen receptor (ER), on cytosolic free calcium concentration ([Ca²⁺]i) and its long term effects on creatine kinase (CK) specific activity in female human and rat osteoblasts. These actions were compared to the known membrane and genomic effects of 17β estradiol (E₂). Like E₂, clone 1D₅ increased within 5 s [Ca²⁺]i in both cell types by two mechanisms: 1) Ca²⁺ influx through voltage-gated Ca²⁺ channels as shown by using EGTA, a chelator of extracellular Ca²⁺, and nifedipine, a Ca²⁺ channel blocker; 2) Ca²⁺ mobilization from the endoplasmic reticulum as shown by using phospholipase C inhibitors, such as neomycin and U-73122, which involved a Pertussis toxin-sensitive G-protein. Clone 1D₅ and [E₂ stimulated CK specific activity in human and rat osteoblasts with ten fold higher concentrations than those needed for the membrane effects (0.1 μg/ml and 10 pM, respectively). Both effects were gender-specific since testosterone and 5α-dihydotesterone were uneffective. Tamoxifen and Raloxifene, two estrogen nuclear antagonists, inhibited CK response to 1 D₅ and E₂ and Ca²⁺ response to 1D₅, but not Ca²⁺ response to E₂. By contrast, (Fab')₂ dimer, a proteolytic fragment of 1 D₅ with antagonist properties, inhibited both membrane and genomic effects of 1 D₅ and E₂. In conclusion, these results imply that clone 1 D₅ has an estrogen like activity both at the membrane and nuclear levels in female human and rat osteoblasts. 1 D₅ must therefore interact with membrane binding sites, penetrate the cells, and reach the nuclear receptors by an as yet uncharacterized mechanism.