Journal Facts

Journal
BoneKEy Knowledge Environment
ISSN
1533-4368
Volume
Year
2003

Article Facts

Title
The importance of STAT-smad cross-talk in bone
DOI
10.1138/2003070
Author

Online Date
02/27/2003

Article Links

BoneKEy-Osteovision | Commentary

The importance of STAT-smad cross-talk in bone


DOI:10.1138/2003070

Commentary on: Takeuchi Y, Watanabe S, Ishii G, Takeda S, Nakayama K, Fukumoto S, Kaneta Y, Inoue D, Matsumoto T, Harigaya K, Fujita T. Interleukin-11 as a stimulatory factor for bone formation prevents bone loss with advancing age in mice. J Biol Chem. 2002 Dec 13;277(50):49011-8.

Much of the data on cytokines of the leukemia inhibitory factor (LIF)/interleukin-6 (IL-6) family, which signal through gp130 (), points toward a role for at least some family members, including IL-6 and IL-11, in osteoclastogenesis. However, some of the earliest evidence for an important bone activity of LIF/IL-6 cytokines came from studies that showed that mice engrafted with cells overexpressing LIF exhibited excessive new bone formation and ectopic calcifications in skeletal muscles and heart (). Since then, a plethora of often contradictory data has been reported on LIF/IL-6 cytokine effects on stromal and osteoblast lineage cells in vitro (). Thus, a recent report from Takeuchi et al. (), in which a clear anabolic effect of IL-11 was seen in transgenic mice, is particularly notable.

An Mx promoter-human IL-11 (hIL-11) transgene was used to generate founder mice with high constitutive expression of hIL-11 in bone, bone marrow, and other tissues. The IL-11 transgenic mice were indistinguishable from wild-type littermates at birth; however, by 10 weeks of age, they exhibited thicker cortical bones, higher bone mineral density (BMD), and greater bone mechanical strength. Histomorphometry indicated that they had no change in bone resorption, but that they had an increase in bone formation parameters compared to their wild-type littermates. Consistent with this finding, comparable osteoclastogenesis (and comparable receptor activator of NF-kB ligand [RANKL] expression) was seen in ex vivo bone marrow cell cultures from transgenic and wild-type mice. On the other hand, transgenic bone marrow stromal cell cultures expressed higher levels of alkaline phosphatase (ALP), generated more ALP-positive and mineralized colonies, and expressed higher levels of osteocalcin than wild-type cells, whose activity could, however, be increased by treatment with IL-11. The observations suggest that IL-11 is an anabolic factor for bone in vivo through local (IL-11 levels were not increased in circulating blood of the transgenic mice) stimulatory effects on osteoblast lineage cells. The observations also support the view that increases in IL-11 do not increase bone resorption in vivo, although numerous studies have found that IL-11 is stimulatory to osteoclastogenesis in vitro through upregulation of RANKL. See also ().

What is the relationship between the IL-11 transgenic mice results and bone loss in aging? A histomorphometric feature of age-related osteopenia is reduced mean trabecular wall thickness and concomitant reduced osteoblast number and activity (). This feature, along with other kinds of data, such as reduced number, size, and/or responsiveness of colony-forming unit fibroblasts (CFU-Fs) in bone marrow stromal cultures from older versus younger mice (), has led to the hypothesis that age-related bone loss is caused by a decline in the supply of osteoblasts. Both IL-11 transgenic and wild-type mice lost trabecular bone with age; however, whereas cortical bone mass and BMD decreased with time in wild-type mice, they increased in transgenic animals up to 52 weeks of age (the latest age reported) because of growth and maintenance of the cortical bone. These new data are interesting in relation to other studies from this and other groups, in which bone marrow cells from the P6 strain of senescence-accelerated mice (SAMP6) show decreased osteoprogenitors and osteoblastogenesis and suppressed osteoclastogenesis secondary to the decreased osteoblastogenesis (). In terms of the IL-11 transgenics, most notable is that IL-11 expression was decreased in SAMP6 (compared to control SAMR1) bone marrow cells and that increasing IL-11 rescued the impairment of both osteoblastogenesis and osteoclastogenesis in the cultures (). IL-11 is also known to be inhibitory for adipogenesis (), a result confirmed in marrow cells of SAMP6 and unrelated mouse strains. Thus, the authors speculated that decreased IL-11 expression with aging might account for and provide a molecular pathway for the hypothesized change in commitment of mesenchymal stem cells (or bipotential adipocyte-osteoblast precursors) to the adipocyte lineage in osteoporosis. Strikingly, however, the authors present no evidence for decreased marrow adipocytes in the IL-11 transgenic mice, which suggests that enhanced osteogenesis does not occur at the expense of adipogenesis in this model, as it does in certain other models.

Signal transducer and activator of transcription 3 (STAT3) is known to be critical downstream of gp130, activated by IL-11 in at least some cell types. Takeuchi et al. () found that IL-11 stimulated transcription of BMP-2 target genes (e.g., smad6), an effect abrogated in the presence of dominant-negative STAT3. The fact that STAT3 overexpression also enhanced the transcriptional activity of the BMP-specific transcription factor (Smad1) suggests that cross-talk between BMP-Smad and gp130-STAT3 signaling pathways plays a role in the anabolic effects of IL-11. Such cross-talk has been observed previously with either enhanced signaling (e.g., LIF-LIFR can synergize with BMP2-Smad signaling through a p300 coactivator bridge) or reduced signaling (e.g., when competition occurs between p300 and CREB-binding protein), depending on cell context and coactivator status ().

How do the IL-11 mice compare to other transgenic or mutant mice with increased bone density, in particular, increased bone density with age as a consequence of changes in osteoblasts and bone formation? Transgenic mice overexpressing the AP-1 family members, Fra-1 () or DFosB (), are two such examples (). However, these mice become osteosclerotic with age, a phenotype not observed in the IL-11 transgenic mice. This raises interesting questions about the mechanisms underlying coupling or uncoupling of compensatory osteoclastogenesis in mice with enhanced bone formation. More similar to the IL-11 transgenics are the recently described tob-knockout mice that also show a progressively greater bone mass without osteosclerosis with aging (). Tob is a negative regulator of BMP-Smad signaling, suggesting that stimulation of BMP-Smad signaling in bone may underlie the increased bone formation in both IL-11 transgenic and tob-deficient mice, which points toward an important paradigm in relation to looking for new anabolic agents.

It is also interesting to consider what regulatory pathways cause cortical bone to respond differently than trabecular bone in the IL-11 transgenics, a feature apparently not seen in the Tob-deficient mice. Note that the authors do not discuss differences in cortical versus trabecular bone compartments; however, see Figure 1 of (), which shows no apparent cortical bone differences, and trabecular bone is maintained in nine-month Tob-deficient mice, but bone loss, as expected, is seen in wild-type mice. The site-specificity of IL-11 effects is consistent with a growing number of examples of site-specific regulation of bone cells in transgenic and knockout animals, as is also true in humans (). Takeuchi et al. () present no data on the uniformity of IL-11 expression in different bone compartments, so it is possible that only periosteal osteoblasts expressed high levels of the transgene and participated in the increased formation in the cortical bone. However, this seems inconsistent with the high IL-11 transgene expression seen in at least some bone marrow compartments and with the increased osteogenic capacity of the stromal cells in vitro, although whether the increased osteogenesis was stromal cell autonomous was not explored. A second possibility is that IL-11R expression is different in different osteoblast populations, which would be consistent with reports that subsets of osteoblasts express different gene repertoires, including different receptor profiles (). However, there are already examples in which different responses in different bone compartments cannot be ascribed to receptor expression differences—for example, transgenics expressing a constitutively active parathyroid hormone receptor (PTH1R) (). This implies exquisite control of signaling thresholds, other intrinsic differences downstream of IL-11R in different osteoblast populations, or that microenvironments in different bone sites modulate osteoblast response to IL-11.

Should we now expect other LIF/IL-6 ligands to be bone anabolic agents? The LIF/IL-6 family consists of LIF, IL-6, IL-11, oncostatin M (OSM), ciliary neurotrophic factor, cardiotrophin 1, and cardiotrophin-like cytokine/novel neurotrophin-1; their receptors are oligomeric complexes composed of one or two gp130 signal transducing receptors, with or without other cognate receptors (). Diverse biological functions, with both redundant and unique activities, have been ascribed to each cytokine. Understanding the basis for many of these biological roles requires a better understanding of the kinds of receptor complexes formed, their tissue distribution and changes during development and adulthood, and the redundancy versus specificity of ligand binding and affinities to particular complexes, which affects downstream signaling; these issues are complicated further by species issues. In the case of IL-11, not only does controversy remain about whether IL-11 binds both gp130 homodimers and heterodimers with IL-11R, but hIL-11 was used in the transgenic mice, raising questions about which receptors and pathways mediate the anabolic effects. As mentioned above, increased circulating levels of LIF in mice are accompanied by new bone formation and ectopic calcification (). Transgenic mice overexpressing bovine OSM exhibit osteopetrosis with stimulation of bone formation and inhibition of bone resorption (), whereas mice overexpressing IL-6 display decreased osteoblasts and bone formation and decreased osteoclast number-decreased resorption (). New mutant mouse models for various family members, as well as further analysis of mice in which bone phenotypes have not been analyzed in detail, should be informative. However, genome sequencing and homology searches, as well as unexpected phenotypes in mice with loss-of-function of some family members, imply yet unknown ligands and receptors in the family, making predictions of bone activities precarious.

How generalizable is the observed decrease in IL-11 in SAMP6 marrow cultures and how relevant are the IL-11 transgenic mice results to humans? The authors have already found that IL-11 is decreased with aging in bone marrow cells of normal mice (E. Tohjima, D. Inoue, N. Yamamoto, S. Kido, Y. Ito, S. Kato, Y. Takeuchi, S. Fukumoto, and T. Matsumoto, submitted for publication). IL-11 mRNA has also been found to be decreased with age in human femoral head autopsy samples (). Such tantalizing observations beg the question of what regulatory molecules lie upstream of IL-11 and contribute to its diminished levels in aging.

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