Estrogen receptor mRNA expression in callus during fracture healing in the rat

Estrogen has profound effects on bone metabolism, yet its precise mechanism of action remains unknown. Several recent investigations have located the estrogen receptor (ER) on osteoblast-like cells, suggesting a potential direct action of estrogen via its own receptor on bone cells. The protective role of estrogen on bone in osteoporosis is well known; however, neither the existence nor the mechanism of an estrogenic role in fracture healing has been well studied. In this investigation we used a modification of polymerase chain reaction (PCR) amplification to detect and quantify ER messenger RNA (mRNA) levels in rat fracture callus previously undetectable by northern analysis. Verification of correctly amplified cDNA fragments was provided by restriction digestion. PCR failed to detect ER mRNA in total RNA extracts from tendon and fibrocartilage. The ER message was present in fracture callus at levels up to 70% of that found in rat uterus, and demonstrated a transient 13-fold increase in transcription by day 14 with return to baseline by day 31 of fracture healing. These data demonstrate that the rat estrogen receptor gene is expressed in fracture callus and suggest that estrogen may play an important role in the normal fracture healing process. In addition, this work adds to the growing body of evidence which supports a possible direct action of estrogen via its receptor on osteogenic cells.