Evidence of a Correlation of Estrogen Receptor Level and Avian Osteoclast Estrogen Responsiveness

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Abstract
Isolated osteoclasts from 5-week-old chickens respond to estradiol treatment in vitro with decreased resorption activity, increased nuclear proto-oncogene expression, and decreased lysosomal enzyme secretion. This study examines osteoclasts from embryonic chickens and egg-laying hens for evidence of estrogen responsiveness. Although osteoclasts from both of these sources express estrogen receptor mRNA and protein, estradiol treatment had no effect on resorption activity. In contrast to the lack of effect on resorption, estradiol treatment for 30 minutes resulted in steady-state mRNA levels of c-fos and c-jun increasing in osteoclasts from embryonic chickens and decreasing in osteoclasts from egg-laying hens. These data suggest that a nuclear proto-oncogene response may not be involved in estradiol-mediated decreased osteoclast resorption activity. To examine the influence of circulating estrogen on osteoclast estrogen responsiveness, 5-week-old chickens were injected with estrogen for 4 days prior to sacrifice. Estradiol treatment of osteoclasts from these chickens did not decrease resorption activity in vitro. Transfection of an estrogen receptor expression vector into osteoclasts from the estradiol-injected chickens and egg-laying hens restored estrogen responsiveness. Osteoclasts from 5-week-old chickens and estradiol treated 5-week-old chickens transfected with the estrogen receptor expression vector contained significantly higher levels of estrogen receptor protein and responded to estradiol treatment by decreasing secretion of cathepsins B and L and tartrate-resistant acid phosphatase. In contrast, osteoclasts from embryonic chickens, egg-laying hens, and estradiol-treated 5-week-old chickens either untransfected or transfected with an empty expression vector did not respond similarly. These data suggest that modulation of osteoclast estrogen responsiveness may be controlled by changes in the osteoclast estrogen receptor levels.