Estrogen prevents bone loss through transforming growth factor β signaling in T cells

Abstract

Estrogen (E) deficiency leads to an expansion of the pool of tumor necrosis factor (TNF)-producing T cells through an IFN-␥-dependent pathway that results in increased levels of the osteoclastogenic cytokine TNF in the bone marrow. Disregulated IFN-␥ production is instrumental for the bone loss induced by ovariectomy (ovx), but the responsible mechanism is unknown. We now show that mice with T cell-specific blockade of type ␤ transforming growth factor (TGF␤) signaling are completely insensitive to the bone-sparing effect of E. This phenotype results from a failure of E to repress IFN-␥ production, which, in turn, leads to increased T cell activation and T cell TNF production. Furthermore, ovx blunts TGF␤ levels in the bone marrow, and overexpression of TGF␤ in vivo prevents ovx-induced bone loss. These findings demonstrate that E prevents bone loss through a TGF␤-dependent mechanism, and that TGF␤ signaling in T cells preserves bone homeostasis by blunting T cell activation. Thus, stimulation of TGF␤ production in the bone marrow is a critical ''upstream'' mechanism by which E prevents bone loss, and enhancement of TGF␤ levels in vivo may constitute a previously undescribed therapeutic approach for preventing bone loss.A lthough multiple genotropic and nongenotropic effects contribute to explain how estrogen (E) controls bone remodeling (1, 2), most of the bone-sparing activity exerted by E occurs through modulation of bone cell life span and decreased cytokine-driven osteoclastogenesis (3, 4). Among the factors that up-regulate osteoclast (OC) formation and lead to bone loss in estroprevic humans and rodents is tumor necrosis factor (TNF) ␣ (5). This E-regulated cytokine promotes osteoclastogenesis by augmenting the production of receptor activator of nuclear factor-B ligand (RANKL) (1), the nonredundant cytokine responsible for OC development (6) and by increasing the responsiveness of maturing OCs to this factor (7-9). Furthermore, TNF stimulates the production of other cytokines known to be implicated in the pathogenesis of ovariectomy (ovx)-induced bone loss, such as IL-1, IL-6, IL-7, and macrophage colony-stimulated factor (1, 10).ovx increases TNF levels in the bone marrow (BM) via an expansion of the pool of TNF-producing T cells (7, 11) induced by a complex mechanism driven by IFN-␥ (12). This cytokine augments antigen (Ag) presentation by enhancing MHCII expression on BM macrophages (BMMs), through induction of class II transactivator (CIITA) expression (13). Up-regulation of Ag presentation results, in turn, in increased T cell activation. Thus, upregulation of IFN-␥ production induced by ovx leads to increased T cell proliferation and life span, a phenomenon that results in an increase in both the total number of T cells and the pool of TNF-producing T cells (12). T cell-produced TNF plays a pivotal role in the mechanism of ovx-induced bone loss, as demonstrated by the failure of ovx to induce bone loss in T cell-deficient nude mice and by the ability to reconstitute with WT T cells, but not TNFϪ͞Ϫ ...

Ovariectomy disregulates osteoblast and osteoclast formation through the T-cell receptor CD40 ligand

“…In support of a role of T cells in the bone loss induced by E deprivation are reports from our laboratory that ovx fails to induce trabecular and cortical bone loss in nude mice (17,24,25,44) and in WT mice treated with abatacept (43), an agent that blocks T-cell costimulation and induces T-cell anergy and apo- ptosis (46,47). An independent confirmation of the role of T cells was provided by Yamaza et al (26), who reported that ovx fails to induce bone loss in nude mice and WT mice in which T-cell activation was blocked by aspirin.…”

“…This phenomenon results from enhanced thymic-dependent differentiation of BM-derived progenitors and peripheral expansion of mature T cells (41,42). Ovx causes the peripheral expansion of T cells by enhancing antigen presentation through increased expression of MHCII (41,43), which, in turn, is driven by a complex mechanism that involves increased production of IFN-γ and IL-7, blunted generation of TGF-β in the BM (22,25,41,44), and up-regulation of the costimulatory molecule CD80 on dendritic cells secondary to increased oxidative stress (43,45).…”

“…Attesting to the relevance of T cell-produced TNF, T cell-deficient nude mice are protected against the loss of cortical and trabecular bone induced by ovx (24)(25)(26). Furthermore, the capacity of ovx to induce bone loss is restored by adoptive transfer into nude mice of T cells from WT mice but not of those from TNF −/− mice (17).…”

Estrogen‐TGFβ cross‐talk in bone and other cell types: Role of TIEG, Runx2, and other transcription factors

“…There are a significant number of reports demonstrating the interactions/cross-talk between estrogens and TGFβ signaling pathways in bone [Oursler et al, 1991b;Hughes et al, 1996;Oursler, 1998;Gao et al, 2004;Janssens et al, 2005]. Cross-talk between E 2 and TGFβ occurs at several levels, as depicted in Figure 1A and as described in detail below.…”

“…This increase in TGFβ production then acts on the surrounding OB cells to activate the TGFβ signaling pathway. The E 2 activation of the TGFβ signaling pathway via induction of TGFβ gene expression has been shown in numerous tissues, including trophoblasts [Rama et al, 2004], bone marrow and prostatic stromal cells [Hong et al, 2004], mouse T cells [Gao et al, 2004], mouse mesenchymal cells [Eger et al, 2004], and mouse adipocytes [Okazaki et al, 2002].…”