1,25 dihydroxyvitamin D3 and dexamethasone induce the cyclooxygenase 1 gene in osteoclast-supporting stromal cells

Commitment of members of the monocyte/macrophage family to the bone resorpt ive phenotype, in vitro, requires contact, of these osteoclast precursors, with osteoblasts or related stromal cells. The osteoclast-inductive propert ies of these stromal cells are typically expressed, however, only in the pr esence of steroid hormones such as 1,25-dihydroxyvitamin D (1,25D3) and dex amethasone (DEX). To gain insight into the means by which steroid treated a ccessory cells induce osteoclast differentiation we asked, using differenti al RNA display (DRD), if gene expression by this stromal cell population di ffers from that of their untreated, non-osteoclastogenic counterpart. We id entified four known genes specifically expressed by 1,25D3/DEX-treated ST2 stromal cells: 1) a family of rat organic anion transporters, 2) Na/K ATPas e beta-subunit, 3) tazarotene-induced gene 2 (TIG2), and 4) prostaglandin G /H synthase I, or cyclooxygenase 1 (Cox-1). The regulation of these genes i n 1,25D3/DEX-treated ST2 cells was demonstrated by Northern blot analysis o f treated (osteoclast-supporting) and untreated (non-osleoclast-supporting) ST2 cells; the genes have a limited and specific tissue mRNA expression pa ttern. Northern blot analysis of treated and untreated ST2 cell total RNA u sing either a DRD-derived Cox-1 cDNA or a Cox-1 specific oligonucleotide co nfirmed the steroid regulation of Cox-1 mRNA. Surprisingly, there is no det ectable expression by untreated or steroid exposed ST2 cells, of Cox-2, the classical regulated cyclooxygenase isoform. In contrast to 1,25D3/DEX, ser um treatment rapidly induces Cox-2 mRNA, substantiating the capacity of ST2 cells to express the gene. These data establish that steroid induction of the osteoclastogenic properties of stromal cells is attended by Cox gene ex pression, a phenomenon consistent with the capacity of eicosinoids to impac t the resorptive process. The response of osteoclast-supporting ST2 cells t o 1,25D3/DEX treatment may be one prostaglandin-mediated event which specif ically involves Cox-1 regulation. (C) 1999 Wiley-Liss, Inc.