BoneKEy-Osteovision | Commentary
Estrogen loss, cytokines, macrophages, lymphocytes, osteoclasts and bone loss: Six characters in search of an author or an endocrine-immune system relay causing osteoporosis?
DOI:10.1138/2002012
Estrogens and androgens slow the rate of bone remodeling and protect against bone loss. Conversely, loss of sex steroids leads to increased rate of remodeling and tilts the balance between bone resorption and formation in favor of the former. During the last ten years it has been elucidated that estrogens and androgens decrease the number of remodeling cycles by attenuating the birth rate of both osteoclasts and osteoblasts from their respective progenitors (). The restraining effects on osteoclastogenesis result from actions of the hormones on cells of the bone marrow stromal/osteoblastic lineage, and in particular transcriptional downregulation of genes encoding cytokines and/or their receptors (e.g. IL-6, TNF, MCSF, osteoprotegerin, gp130, IL-1RI/IL-1RII) which promote osteoclastogenesis (). By and large, these effects are exerted through interactions of the ligand-activated receptors for estrogens or androgens with other transcription factors. The restraining effect of sex steroids on osteoblastogenesis results from the suppression of mesenchymal cell replication and/or differentiation, but its precise molecular mechanism is still unclear ().
Increased remodeling alone cannot explain why loss of sex steroids tilts the balance of resorption and formation in favor of the former. Estrogens and androgens also exert effects on the life span of mature bone cells: pro-apoptotic effects on osteoclasts, but anti-apoptotic effects on osteoblasts and osteocytes (). These latter effects stem from a heretofore unexpected function of the classical “nuclear” sex steroid receptors outside the nucleus and result from activation of cytosolic signal transduction pathways probably within in preassembled scaffolds (). Strikingly, ERα or β or the AR can transmit anti-apoptotic signals with similar efficiency irrespective of whether the ligand is an estrogen or an androgen. More importantly, these nongenotropic, sex-nonspecific actions are mediated by the ligand binding domain of the receptor and can be functionally dissociated from transcriptional activity with synthetic ligands. Taken together, these lines of evidence strongly suggest that in sex steroid deficiency, loss of transcriptional effects may be responsible for the increased osteoclastogenesis and osteoblastogenesis and thereby the increased rate of bone remodeling. Loss of nongenotropic anti-apoptotic effects on mature osteoblasts and osteocytes, in combination with an opposite effect on the lifespan of mature osteoclasts, may be responsible for the imbalance between formation and resorption and the progressive loss of bone mass and strength.
The balanced production of osteoblasts and osteoclasts that is required for the generation of new BMUs and the progression of existing ones is accomplished through a tight linkage between osteoblast and osteoclast formation. Thus, osteoclast formation from hematopoietic precursors strictly depends on bone marrow stromal-derived M-CSF and results from the binding of RANKL to its cognate receptor RANK in hematopoietic osteoclast precursors (). RANKL expression is largely restricted to stromal/osteoblastic cells and T-lymphocytes. In stromal/osteoblastic cells, RANKL expression is greatly upregulated after treatment with cytokines or hormones that stimulate osteoclast formation. RANKL is undetectable in resting T-cells, but greatly upregulated following antigen receptor engagement and activation. Mutant mice lacking T and B lymphocytes exhibit normal bone morphology and BMD indicating that lymphocytes are not required for normal bone homeostasis. However, when T-lymphocytes become activated in the synovium of rats with adjuvant arthritis, T-lymphocytes can directly trigger osteoclastogenesis through RANKL and thereby become responsible for the bone loss and joint distraction associated with this condition ().
Provocative new results by Roggia et al. () challenge an extensive body of earlier evidence which indicates that the targets of sex steroid action, and consequently the cells responsible for triggering the increased osteoclastogenesis that ensues upon loss of sex steroids, are mesenchyme-derived cells of the bone marrow stromal/osteoblastic lineage (). Instead, they suggest that the cells responsible for the increased osteoclastogenesis that follows estrogen deficiency are T-lymphocytes. Specifically, these authors show that ovariectomy in mice causes a 5-fold increase in the number of total T-cells as well as the number of T-cells that produce TNF, both in the bone marrow and the spleen. In a series of elegant studies they go on to show that TNF- or TNF receptor (p55)- deficient mice are protected against the ovariectomy induced bone loss. However, transfer of wild-type T-cells from ovariectomized mice into T-cell deficient (nude) mice restores the capacity of ovariectomy to induce bone loss; but, transfer of T-cells unable to produce TNF does not. These findings are consistent with the authors’ interpretation that ovariectomy increases the total number of T-cells without altering the fraction of T-cells that produce TNF and without upregulating the amount TNF produced by each active T-cell. And, enhanced T-cell production of TNF resulting from an increase in the number of T-lymphocytes in the bone marrow is a key mechanism by which estrogen deficiency induces bone loss.
As is often the case, the intriguing findings of this paper provide more questions than answers. First, how does estrogen deficiency increase the number of T-cells (5-fold) both in the bone marrow and the spleen? Is it because estrogens control the differentiation of progenitors in the bone marrow, T-cell commitment in the thymus or in extrathymic sites, or because estrogens control the proliferative response of mature T-cells to antigenic activation? The markers used by Roggia et al. to enumerate T-cells are characteristic of immature T-cell progenitors (CD90 [Thy-1]) committed to the T-cell lineage (indicated by the presence of CD3, a component of the T-cell receptor complex). Therefore, the observed effect may be secondary to hormonal actions on the production of lymphopoiesis- regulating cytokines by bone marrow stromal cells. In line with this scenario, others have shown earlier that ovariectomy, as well as orchidectomy, stimulate B-lymphopoiesis, and for this matter the entire myeloid lineage, in the bone marrow of mice and rats () via actions on bone marrow stromal cells. The stromal cell requirement is clearly evidenced by the requirement of stromal cell expression of the sex steroid receptors as well as stromal cell derived mediators such as IL-7 and TGF-β for B-lymphopoiesis (). Interestingly, immature pre-B cells (B220+) may retain bi-potentiality for differentiation along both B lymphoid and osteoclast lineages; and may play a role in the accelerated osteoclastogenesis due to estrogen deficiency ().
Roggia and co-workers have not determined directly the state of activation of T-lymphocytes, but the fact that these cells produce TNF, a seminal indicator of T-cell activation, strongly suggests that estrogen deficiency influences T-cell activation. This finding has potentially important implications both for the pathophysiology of osteoporosis and for the well-known influence of estrogens on the incidence and intensity of several autoimmune disorders. For example, if estrogen deficiency causes activation of T-lymphocytes, these cells should express RANKL and then directly trigger osteoclastogenesis in the marrow as well as in the spleen. This may be consistent with the idea of circulating osteoclast precursors in blood, but it is hard to see how in states of estrogen deficiency, such as menopause, a systemic activation of lymphocytes would be contained to osteoclastogenesis without a noticeable immune reaction. Or is there an immune reaction in postmenopausal women that is so limited as to lie beneath clinical detection? If estrogen deficiency upregulates osteoclastogenesis through T-cells, what mechanism is responsible for the simultaneous upregulation of osteoblastogenesis?
These issues aside, what could be the mechanistic basis of the immunosuppressive effects of estrogens? Direct suppressive actions on T-lymphocytes or interference with the activity of the antigen presenting macrophages? An inevitable consequence of T-cell activation would be not just RANKL upregulation but also production of cytokines that down-regulate osteoclastogenesis, e.g. IFNγ, IL-12, or IL-18. Do all these changes happen? And if they do, why are the pro-osteoclastogenic effects of TNF not counter-balanced by the anti-osteoclastogenic cytokines?
To complicate matters further, the requirement for TNFα and its p55 receptor for the bone loss induced by estrogen deficiency in mice is evidently not universal, as in a different strain of TNFα or p55 null mice from the ones used by Roggia et al., ovariectomy produces as much bone loss as in wild type controls (Abstract submitted by Hofstetter and co-workers to the Frontiers of Skeletal Biology in Davos 2002). This very intriguing set of seemingly conflicting observations suggests strongly that the mechanism indicated by the results of Roggia et al. is strictly dependent on the genetic background and perhaps the specificity of the T-sell immune response to particular antigens
These and many more questions must be answered before the findings of Roggia et al. can be placed in a proper perspective and be fully appreciated. But irrespective of the answers to the questions raised by these observations, Roggia et al. deserve credit for alerting the field to the possibility that estrogen deficiency modifies the signaling threshold required for T-lymphopoiesis (and for T-cell activation) and that there might be a role of the immune system in the pathogenesis of osteoporosis.
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