Divergent responsiveness of the dentary and vertebral bone to a selective estrogen-receptor modulator (SERM) in the teleost Sparus auratus

In teleosts the regulation of skeletal homeostasis and turnover by estrogen is poorly understood. For this reason raloxifene, a selective estrogen-receptor modulator (SERM), was administered to sea bream (Sparus auratus) and its effect on plasma calcium balance and transcript expression in dentary (dermal bone) and vertebra (perichondral bone) was studied. The concentration of total calcium or phosphorus in plasma was unchanged by raloxifene treatment for 6 days. The activity of alkaline phosphatase (ALP) in dentary bone of raloxifene treated fish was significantly (p < 0.05) higher than control fish but it was not changed in vertebral bone. Transcripts for estrogen receptor (ER) a were in very low abundance in the sea bream dentary and vertebra and were unchanged by raloxifene treatment. In contrast, raloxifene caused significant (p < 0.05) up-regulation of the duplicate ERb transcripts in the dentary but did not affect specific transcripts for osteoclast (TRAP), osteoblast (ALP, Runx2, osteonectin) or cartilage (IGF1, CILP2, FN1a). In the vertebra ERbb was not changed by raloxifene but ERba was significantly (p < 0.05) down-regulated as was the skeletal specific transcripts, TRAP, ALP, CILP2, FN1a. In summary, ERbs regulate estrogen sensitivity of the skeleton in sea bream, which responds in a non uniform manner. In common with mammals raloxifene appears to have an anti-resorptive role (in sea bream vertebra), but also an osteoblast stimulatory role, inducing ALP activity in the dentary of sea bream. Overall, the results indicate bone specific responsiveness to raloxifene in the sea bream. Further work will be required to understand the basis of bone responsiveness and the role of E2 and ERs in teleost bone homeostasis.The authors would like to acknowledge E. Caldas for her help with the experiment and plasma measurements. This work was funded by the Portuguese National Science Foundation (FCT) grant (POCI/CVT/61052/2004) and a European collective research project (COLL-CT-2005-012451, FINE FISH). F. A. Vieira was funded by FCT (SFRH/BD/17630/2004) and P.I. Pinto was funded by FCT (SFRH/ BPD/25247/2005)