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 w hich diminished circulating 17 beta-estradiol (E(2)) promotes osteocla stic activity are unknown. We hypothesized that since the integrin alp ha(v) beta(3) is essential to the resorptive process, reduced E(2) lev els may increase the integrin's expression by osteoclast precursors. T hus, avian osteoclast precursors (known to contain E(2) receptors) wer e exposed +/- E(2), surface iodinated, and lysed. The lysate was immun oprecipitated with an antibody recognizing the intact alpha(v) beta(3) heterodimer. We find E(2) alone fails to impact on alpha(v) beta(3) e xpression. Most importantly, however, picomolar (i.e. postmenopausal), but not nanomolar (i.e. premenopausal) concentrations of E(2), when a dded 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 os teoclast precursors relative to 1,25-(OH)(2)D-3 alone. Induction of al pha(v) beta(3) by picomolar levels of E(2) is dose-dependent, maximizi ng 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 a nalysis reveals that beta(3) mRNA levels parallel those of alpha(v) be ta(3). E(2) (10(-12) M) increases expression of beta(3) mRNA induced b y a range of 1,25-(OH)(2)D-3 concentrations extending from 10(-10) M-1 0(-8) M. The E(2) + 1,25-(OH)(2)D-3 additive effect on beta(3) mRNA ap pears as early as 1 day of treatment and progresses for at least 3 day s. Consistent with evidence that the beta(3) subunit regulates heterod imer expression, the sex steroid does not impact alpha(v) mRNA. Attest ing 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 ret inoic 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. T ranscriptional 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 sta bility 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-regula tes, post-transcriptionally, in osteoclast precursors, alpha(v) beta(3 ), an integrin heterodimer pivotal to the resorptive process.