Bone Marrow Cytokines in the Pathophysiology of Osteoporosis
by Stavros Manolagas, M.D., Ph.D., Professor of Medicine and Director, Division of Endocrinology and Metabolism, University of Arkansas for Medical Sciences, at the Inter-Institute Interest Group Lecture Series, July 13, 1994
Osteoclasts and osteoblasts, originating in the bone marrow from hematopoietic progenitors and mesenchymal stromal cells, respectively, are responsible for remodeling the skeleton throughout adult life. A series of in vitro and in vivo studies in animal models suggests that altered production of, and responsiveness to, cytokines in the bone marrow are key pathogenic events in diseases associated with abnormal skeletal remodeling, such as osteoporosis. Indeed, upon loss of sex steroids, the production of osteoclasts in the bone marrow is increased. This is mediated by an increase in production of interleukin-6 (IL-6) and increased sensitivity of the osteoclastic precursors to cytokines such as IL-6, due to an upregulation of the gp130 signal-transduction pathway. Consistent with this, estrogens as well as androgens inhibit the expression of the gp130 gene and inhibit IL-6 production through an indirect effect of its specific receptors on the transcriptional activity of the human IL-6 gene promoter. With advancing age, the high rate of bone remodeling and the loss of bone caused by loss of gonadal function slows, probably due to a relative decrease of the ability of the marrow to maintain the high rate of osteoclastogenesis caused by the acute loss of sex steroids. This appears to be the result of a negative effect of senescence on the ability of the marrow to produce stromal or osteoblastic cells, which provide the essential support for osteoclastogenesis; hence, inappropriate production of osteoclasts or inadequate production of osteoblasts in the bone marrow may represent fundamental cellular changes in the pathogenesis of postmenopausal and senescence-associated osteoporosis, respectively.
Q: What was your starting point in this research, and how have your questions evolved?
A: The starting point for this work was the idea that the critical cellular changes leading to osteoporosis occur at the early stages of development of osteoclasts and osteoblasts in bone marrow. Specifically, we postulated that loss of sex steroids, a primary cause of the imbalance between resorption and formation of bone mass that characterizes osteoporosis, must somehow interfere with normal, orderly replenishment of cellular constituents of skeleton, rather than affecting the function of fully differentiated cells. This general concept was initially tested in in vitro studies examining the effect of sex steroids on the production of cytokines that influence osteoclast development in the bone marrow. The results of these studies demonstrated that estrogens and androgens are potent inhibitors of interleukin-6 production. We then went on to examine in vivo whether loss of sex steroids upregulates IL-6 production. Using ovariectomized mice and specific antibodies that neutralize IL-6, we were able to demonstrate that, indeed, loss of ovarian function leads to an IL-6-mediated upregulation of osteoclast production in the marrow and increased numbers of osteoclasts in bone. These findings, as well as the demonstration of the essential role of IL-6 in the bone loss associated with loss of estrogens, were confirmed later by another group using IL-6 - knockout mice. They observed that IL-6 - deficient mice were protected against loss of bone following loss of ovarian function.
Knowing that loss of sex steroids increases bone remodeling, we subsequently hypothesized and found that loss of ovarian function upregulates not only osteoclast but also osteoblast precursors in the bone marrow. Adding to this the fact that the action of osteoclastogenic cytokines (such as IL-6) and osteoblastogenic cytokines (such as LIF) are mediated by the gp130 signal-transduction pathway, we went on to examine the effects of sex steroids on this particular pathway in cells of the bone marrow stromal-osteoblastic lineage. We found that the gp130 signal transduction pathway is, indeed, regulated by sex steroids; hence, changes in the status of gonadal hormones could affect not only the production of cytokines, but also the responsiveness of osteogenic precursors to cytokines. Our findings that sex steroids inhibit -- whereas parathyroid hormone and vitamin D stimulate -- the expression of the signal transducer gp130, raise the possibility that the high rate of remodeling following loss of gonadal function (and perhaps other states such as hyperparathyroidism) may be due to increased sensitivity of bone marrow progenitors to both osteoclastogenic and osteoblastogenic signals, resulting from upregulation of gp130.
Finally, in an attempt to explain why bone loss associated with menopause slows with advancing age, we attempted to dissect loss of gonadal function from the effects of senescence on bone cell progenitors of the marrow. These studies revealed that aging may interfere with the ability of bone marrow to maintain a normal rate of development of osteoblast precursors.
Q: Which findings have been most surprising to you or to other scientists?
A: That IL-6 was redundant in terms of osteoclast development in the physiologic state, and that nonetheless, it could play such a critical role in the pathologic bone resorption caused by loss of sex steroids. Equally surprising was the observation that gp130 is regulated by systemic hormones. Assuming that gp130 is regulated by sex steroids in tissues other than bone and bone marrow cells, the significance of these observations may extend to the mechanism(s) underlying the protective effect of estrogens on the cardiovascular system, and brain, as suggested by the lower incidence of Alzheimer's disease in postmenopausal women receiving estrogen replacement therapy.
Q: What were the greatest stumbling blocks, and what new observations, techniques, reagents, or insights helped you get past them?
A: Once we were convinced that estrogens do indeed regulate cytokine production in the bone marrow, I don't think that we experienced major stumbling blocks. In fact, we were surprised to see how easily the pieces of the puzzle fit together. The availability of large quantities of specific neutralizing antibodies against IL-6 was critical in our ability to demonstrate the role of this cytokine in the bone marrow changes caused by loss of estrogen.
Q: Do you see any potential areas where this research might provide insight to clinical scientists?
A: This work is of direct and immediate relevance to clinicians. Our observations provide insight into the cellular and biochemical basis of osteoporosis -- knowledge that is essential for clinical scientists attempting to either treat or develop rational and specific therapies for the management of this widespread disease.
Q: How are you following up on this work, and what questions would you ultimately like to answer?
A: We have just initiated an extensive NIH-funded study to assess the relevance of animal-findings to humans. Specifically, we are investigating the effects of the loss of ovarian function in women on the behavior of the bone marrow. In addition, we are trying to determine whether production of specific cytokines by bone marrow cells -- or the sensitivity of these cells to cytokines -- is altered by the aging process. Finally, we are attempting to control bone remodeling by manipulating the development of bone-cell progenitors in the marrow by antagonizing the cytokines that have adverse effects on bone balance, or by enhancing the effectiveness and targeting of cytokines with beneficial effects. Theoretically, by such means, one could tilt the balance between bone resorption and bone formation in favor of the latter and thereby restore skeletal mass. The best of our current therapeutic options can only slow or stop bone deterioration.