ESTROGEN ENHANCES ALPHA(V)BETA(3) INTEGRIN EXPRESSION BY AVIAN OSTEOCLAST PRECURSORS VIA STABILIZATION OF BETA(3) INTEGRIN MESSENGER-RNA

Although bone resorption is accelerated with menopause, the means by which diminished circulating 17 beta-estradiol (E(2)) promotes osteoclastic activity are unknown. We hypothesized that since the integrin alpha(v) beta(3) is essential to the resorptive process, reduced E(2) levels may increase the integrin's expression by osteoclast precursors. Thus, avian osteoclast precursors (known to contain E(2) receptors) were exposed +/- E(2), surface iodinated, and lysed. The lysate was immunoprecipitated with an antibody recognizing the intact alpha(v) beta(3) heterodimer. We find E(2) alone fails to impact on alpha(v) beta(3) expression. Most importantly, however, picomolar (i.e. postmenopausal), but not nanomolar (i.e. premenopausal) concentrations of E(2), when added in conjunction with 1,25-dihydroxyvitamin D-3 [1,25-(OH)(2)D-3], enhance alpha(v) beta(3) expression on the plasma membrane of avian osteoclast precursors relative to 1,25-(OH)(2)D-3 alone. Induction of alpha(v) beta(3) by picomolar levels of E(2) is dose-dependent, maximizing at 10(-11)-10(-12) M, wherein the sex steroid enhances 1,25-(OH)(2)-D-3-stimulated integrin expression approximately 2.5-fold. Northern analysis reveals that beta(3) mRNA levels parallel those of alpha(v) beta(3). E(2) (10(-12) M) increases expression of beta(3) mRNA induced by a range of 1,25-(OH)(2)D-3 concentrations extending from 10(-10) M-10(-8) M. The E(2) + 1,25-(OH)(2)D-3 additive effect on beta(3) mRNA appears as early as 1 day of treatment and progresses for at least 3 days. Consistent with evidence that the beta(3) subunit regulates heterodimer expression, the sex steroid does not impact alpha(v) mRNA. Attesting to the specificity of E(2) on beta(3) mRNA expression, the steroid does not impact on beta(5) mRNA, and its stereoisomer, 17 alpha E(2), is inactive in these experiments. Likewise, E(2) has no effect on retinoic acid-induced stimulation of beta(3) mRNA levels. While 1,25-(OH)(2)D-3 induction of beta(3) mRNA reflects transcriptional activation, nuclear run-on studies indicate that, despite its inductive effect on beta(3) mRNA levels, E(2) does not alter beta(3) gene transcription. Transcriptional arrest experiments demonstrate the t1/2 of beta(3) mRNA derived from 1,25(OH)(2)D-3-treated cells cultured in the presence or absence of 10(-8) M E(2) is approximately 4 h. On the other hand, 10(-12) M E(2), in conjunction with 1,25(OH)(2)D-3, more than triples stability of beta(3) message. Thus, in conjunction with 1,25-(OH)(2)D-3, E(2), at levels circulating in postmenopausal (but not premenopausal) females, in whom the rate of bone resorption is accelerated, up-regulates, post-transcriptionally, in osteoclast precursors, alpha(v) beta(3), an integrin heterodimer pivotal to the resorptive process.