Estrogen Enhances CY,& lntegrin Expression by Avian Osteoclast Precursors via Stabilization of p3 lntegrin mRNA INTRODUCTION Cheng-fu Li, F. Patrick Ross, Xu Cao, and Steven L. Teitelbaum Department of Pathology Washington University School of Medicine St. Louis, Missouri 63110 Although bone resorption is accelerated with menopause, the means by which diminished circu- lating 17/S-estradiol (E,) promotes osteoclastic ac- tivity are unknown. We hypothesized that since the integrin %p3 is essential to the resorptive process, reduced E, levels may increase the integrin’s ex- pression by osteoclast precursors. Thus, avian os- teoclast precursors (known to contain E, recep- tors) were exposed f E,, surface iodinated, and lysed. The lysate was immunoprecipitated with an antibody recognizing the intact aQ.& heterodimer. We find E, alone fails to impact on a;ps expression. Most importantly, however, picomolar u.e. post- menopausal), but not nanomolar (i-e. premeno- pausal) concentrations of E,, when added in con- junction with 1,25dihydroxyvitamin D, [I ,25- (OH),DJ, enhance q,& expression on the plasma membrane of avian osteoclast precursors relative to I,25(OH),D, alone. Induction of q,& by pico- molar levels of E, is dose-dependent, maximizing at 10-“-10-‘2 M, wherein the sex steroid enhances I,25(OH),-D,-stimulated integrin expression ap- proximately 2.5-fold. Northern analysis reveals that p3 mRNA levels parallel those of (Y&,. E, (IO-l2 M) increases expression of p3 mFlNA induced by a range of 1,25-(OH),D, concentrations extending from IO-” M-IO-’ M. The E, + 1,25-(OH),D, addi- tive effect on p3 mRNA appears as early as 1 day of treatment and progresses for at least 3 days. Con- sistent with evidence that the & subunit regulates heterodimer expression, the sex steroid does not impact a; mRNA. Attesting to the specificity of E, on &, mRNA expression, the steroid does not im- pact on ps mRNA, and its stereoisomer, 17cuE,, is inactive in these experiments. Likewise, E, has no effect on retinoic acid-induced stimulation of p3 mRNA levels. While I,25(OH),D, induction of p3 mRNA reflects transcriptional activation, nuclear run-on studies indicate that, despite its inductive effect on &, mRNA levels, E, does not alter p3 gene transcription. Transcriptional arrest experiments demonstrate the V/2 of p3 mRNA derived from 1,25- (OH),D,-treated cells cultured in the presence or absence of 1Om8 M E, is approximately 4 h. On the other hand, IO-l2 M E,, in conjunction with 1,25- (OH),D,, more than triples stability of & message. Thus, in conjunction with I,25(OH),D,, E,, at levels circulating in postmenopausal (but not premeno- pausal) females, in whom the rate of bone resorp- tion is accelerated, up-regulates, post-transcrip- tionally, in osteoclast precursors, (yv&, an integrin heterodimer pivotal to the resorptive process. (Molecular Endocrinology 9: 5tXi-313, 1995) Among those agents used to arrest osteoporosis, 17p- estradiol (E2) is clearly most effective. This steroid, when administered at the time of menopause, pre- vents the accelerated bone loss typically associated with termination of ovarian function (1). While many of the skeletal properties of E, may not yet be in hand, the principal effect of the steroid appears to be medi- ated through the osteoclast, whose activity it dampens (2, 3). 0888-8809/95/$8.00/O Molecular Endocrinology Copyright 0 1995 by The Endocrine Society Osteoclastic resorption is initiated by attachment of the cell to bone (4) and thus, molecules mediating osteoclast-matrix recognition are of particular interest. Among the most important attachment proteins are integrins, surface-residing, transmembrane het- erodimers consisting of individual, noncovalently linked, (Y- and P-chains. These proteins serve not only to bind cells to matrix, but upon occupancy, are ca- pable of transmitting signals to the cell’s interior (5). We and others find the integrin ol,& is expressed by mature osteoclasts and plays an essential role in the resorptive process (5). Thus, regulation of (Y,& pre- sents itself as a potential means of modulating bone resorption and perhaps, preventing osteoporosis. The fact that E, exerts its bone-sparing effects via oste- oclast inhibition (2, 3) raises the possibility that the 805 Downloaded from https://academic.oup.com/mend/article-abstract/9/7/805/2715177 by guest on 05 June 2020