OSTEOCLASTS FROM HUMAN GIANT-CELL TUMORS OF BONE LACK ESTROGEN-RECEPTORS

Although estrogen is important in human skeletal homeostasis, the majo r target cell in bone is unknown. Estrogen receptors (ER) have been de monstrated in osteoblasts and bone marrow stromal cells, but their pre sence in osteoclasts remains controversial because completely pure pre parations have not been available. We have examined expression of ER-a lpha and ER-beta messenger RNA (mRNA) by RT-PCR in samples from human giant cell tumor of bone (GCT), including: whole tumor, cultured monon uclear cells, and a pure osteoclast population obtained by microisolat ion. Whole tumor expressed both ER-alpha and calcitonin receptor (CTR) mRNA and apparently lower levels of ER-beta mRNA. Passaged cultures o f tumor mononuclear stromal cells also expressed ER-alpha and low ER-b eta but not CTR mRNA. In pure preparations of microisolated osteoclast s, expression of ER-alpha or ER-beta mRNA was not detected, whereas ex pression of CTR mRNA was readily identified. Microisolated GCT mononuc lear cells expressed ER-alpha, but no detectable CTR mRNA. Fluorescenc e in situ hybridization (FISH) using an ER-alpha riboprobe demonstrate d strong signal in the mononuclear cells but multinucleated osteoclast s showed no detectable signal. In contrast, CTR mRNA was detected in m ultinucleated osteoclasts but not in stromal-like tumor cells by FISH. 17 beta-estradiol consistently showed no effect on bone resorbing act ivity of osteoclasts from GCT cultured on cortical bone, although calc itonin was a potent inhibitor. These findings indicate that significan t expression of ER does not occur in osteoclasts derived from human GC T and suggest that estrogen effects are mediated by other cells of the bone environment.