Increased Osteoclastogenesis in Mice Lacking the Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1
Increased Osteoclastogenesis in Mice Lacking the Carcinoembryonic Antigen- Related Cell Adhesion Molecule 1
Timo Heckt 0 5
Thomas Bickert 1 5
Anke Jeschke 0 5
Sebastian Seitz 0 5
Jochen Schulze 0 5
Wulf D. Ito 2 5
Wolfgang Zimmermann 3 5
Michael Amling 0 5
Thorsten Schinke 0 5
Andrea Kristina Horst 1 4 5
Johannes Keller * 0 5
0 Department of Osteology and Biomechanics, University Medical Center Hamburg-Eppendorf , Hamburg 20246, Germany,
1 Institute of Clinical Chemistry, University Medical Center Hamburg-Eppendorf , Hamburg 20246, Germany,
2 Cardiovascular Center Oberallga u-Kempten , Im Stillen 3, Immenstadt 87509, Germany,
3 Tumor Immunology Laboratory, LIFE-Center, Klinikum Grosshadern, Ludwig-Maximilians-University Munich , Marchionistrae 15, Munich 81377, Germany,
4 Institute of Experimental Immunology and Hepatology, University Medical Center Hamburg-Eppendorf , Hamburg 20246 , Germany
5 Editor: Chi Zhang, University of Texas Southwestern Medical Center , United States of America
Alterations in bone remodeling are a major public health issue, as therapeutic options for widespread bone disorders such as osteoporosis and tumor-induced osteolysis are still limited. Therefore, a detailed understanding of the regulatory mechanism governing bone cell differentiation in health and disease are of utmost clinical importance. Here we report a novel function of carcinoembryonic antigenrelated cell adhesion molecule 1 (CEACAM1), a member of the immunoglobulin superfamily involved in inflammation and tumorigenesis, in the physiologic regulation of bone remodeling. Assessing the expression of all members of the murine Ceacam family in bone tissue and marrow, we found CEACAM1 and CEACAM10 to be differentially expressed in both bone-forming osteoblasts and bone-resorbing osteoclasts. While Ceacam10-deficient mice displayed no alteration in structural bone parameters, static histomorphometry demonstrated a reduced trabecular bone mass in mice lacking CEACAM1. Furthermore, cellular and dynamic histomorphometry revealed an increased osteoclast formation in Ceacam1-deficient mice, while osteoblast parameters and the bone formation rate remained unchanged. In line with these findings, we detected accelerated osteoclastogenesis in Ceacam1-deficient bone marrow cells, while osteoblast differentiation, as determined by mineralization and alkaline phosphatase assays,
was not affected. Therefore, our results provide in vivo and in vitro evidence for a
physiologic role of CEACAM1 in the regulation of osteoclastogenesis.
In the healthy organism, bone remodeling is performed by the balanced activity of
bone-forming osteoblasts and bone-resorbing osteoclasts, assuring the constant
renewal of bone tissue and maintenance of adequate bone stability [1, 2]. In
osteoporosis, the most prevalent bone disease worldwide, a relative increase of
bone resorption over bone formation occurs, thereby resulting in bone loss and a
subsequent increase in fracture risk . As excessive osteoclastogenesis is
detrimental not only in osteoporosis, but also tumor-induced osteolysis and
Pagets disease of bone [4, 5], the molecular understanding of the processes
regulating osteoclast formation and function is of paramount clinical importance.
Osteoclasts represent highly specialized, multinuclear giant cells, which are
formed by the fusion of hematopoietic precursor cells from the monocyte/
macrophages lineage. The process of osteoclast formation (osteoclastogenesis)
depends on two essential cytokines, macrophage colony-stimulating factor
(MCSF) [6, 7] and receptor activator of nuclear factor kappa-B ligand (RANKL)
[8, 9], which are produced by bone marrow cells and osteoblasts, respectively.
While M-CSF is required for the early differentiation of monocytes and
macrophages, RANKL is essential for the subsequent cellular fusion to yield
mature and functional osteoclasts. This is best demonstrated by mice lacking
RANKL which display osteopetrosis, a condition characterized by the absence of
functional osteoclasts and resulting in a marked increase in bone mass with
consecutive displacement of bone marrow [10, 11]. Through binding to the
receptor activator of nuclear factor kB (RANK), expressed on osteoclasts and their
precursors, RANKL activates two key transcription factors, nuclear factor
kappalight-chain-enhancer of activated B-cells (NF-kB) and cytoplasmic calcineurin/
nuclear factor of activated t cells (NFATC1), which have been demonstrated to be
of crucial importance for osteoclastogenesis [12, 13] Once fully differentiated,
osteoclasts express Acp5 (Tartrate-resistant acid phosphatase) and Calcr
(Calcitonin receptor) and attach to the bone matrix, which is subsequently
resorbed by the secretion of hydrochloric acid and matrix-degrading peptidases
While many systemic and local factors, including endocrine organs, the central
nervous system, and mechanical load bearing, have been identified as pivotal
regulators of bone turnover [15, 16], recent research has unraveled an
unanticipated role of cell adhesion molecules in the regulation of bone cell
differentiation and function. For example, vascular cellular adhesion molecule 1,
which is expressed on myeloma cells and interacts with integrins mediating
osteoclast attachment to bone surface, was shown to tether osteoclast progenitors
to accelerate their maturation, thus facilitating tumor-induced osteolysis [17, 18].
Furthermore, it could be demonstrated that the intercellular adhesion molecule-1
provides a high affinity adhesion between osteoblast and osteoclast precursors,
thereby enhancing the binding of Rank to membrane-bound Rankl on osteoblasts
. Another group of cell-to-cell adhesion molecules that has raised great
scientific and clinical interest in recent years are carcinoembryonic antigen-related
cell adhesion molecules (CEACAMs), representing a subdivision of the
immunoglobulin-related glycoproteins. Apart from functioning as receptors for
host-specific bacteria and viruses, CEACAMs have been shown to regulate tissue
architecture, cell-to-cell recognition, tumor proliferation, neovascularization and
metastasis . However, despite the extensive characterization of CEACAMs in
pathologic conditions such as inflammation and cancer, their role in bone
remodeling remained unclear to date.
In the present study, we found Ceacam1 and Ceacam10 to be expressed in bone
marrow and tissue, including osteoblasts and osteoclast precursors. While no
alterations in bone remodeling were detected in Ceacam10-deficient mice, an
osteoporotic bone phenotype due to increased osteoclastogenesis was observed in
mice lacking Ceacam1. Ex vivo assays demonstrated an increased osteoclast
formation in bone marrow cultures derived from Ceacam1-deficient mice, which
was accompanied by an elevated expression of Nfatc1, the master transcription
factor governing osteoclastogenesis. Taken together, these findings not only
provide in vivo and in vitro evidence for a role of CEACAM1 in the regulation of
bone remodeling, they also raise the possibility that pharmacologic targeting of
CEACAM1 may be an alternative approach to treat skeletal disorders caused by
excessive bone resorption.
Ceacam1- and Ceacam10-deficient animals were generated and genotyped as
described previously [21, 22, 23]. All animal experiments were approved by the
local animal care committee.
2. Skeletal analysis
All mice received dual calcein injections for the determination of the bone
formation rate at 9 and 7 days before sacrifice. The lumbar vertebrae were
dehydrated and embedded non-decalcified into methyl methacrylate for
sectioning. 4 mm-thin sections were stained with toluidine blue or von Kossa/van
Gieson procedure as described . Static and cellular histomorphometry was
carried out using the OsteoMeasure system (Osteometrics, Decatur, USA)
following the guidelines of the American Society of Bone and Mineral Research.
Dynamic histomorphometry for the determination of the bone formation rate was
performed on non-stained 12 mm-sections. The cortical thickness and the mean
3. Primary osteoblasts
Primary osteoblasts were isolated from bone marrow cells derived from 12 to
18week-old mice as described . At 80% confluency, cells were differentiated by
adding b-glycerophosphate and ascorbic acid. For staining with alizarin red, cells
were fixed with 90% ethanol, washed twice with water and incubated with 40 mM
alizarin red staining solution (pH 4.2). After washing, the stained cultures were
incubated with 10% acetic acid to quantify matrix mineralization. After removing
the cellular remnants by centrifugation, the supernatant was neutralized with
ammonium hydroxide, and absorption was determined at 405 nm.
4. Primary osteoclasts
For the assessment of osteoclastogenesis, the bone marrow from 12 to
18-weekold mice was flushed out of the femora as described previously . At day 2 of
differentiation, M-CSF (Peprotech) was added, followed by RANKL (Peprotech)
at day 4.
For TRAP-activity staining, cells were washed with PBS and then fixed in cold
methanol. After two washing steps with distilled water, the fixed cells were dried
before the TRAP-specific substrate naphthol AS-MX phosphate (Sigma-Aldrich)
was added. For the quantification of osteoclast formation, the number of
TRAPpositive, multinucleated (.3 nuclei/cell) cells per well was determined.
5. RAW264.7 cell line
RAW264.7 cells were obtained from ATCC (Wesel, Germany) and cultured in
DMEM containing 10% fetal calf serum. For induction of osteoclastogenesis cells
were cultured with RANKL for 5 days.
6. Expression analysis
Total RNA was isolated from indicated tissues, whole and marrow-flushed bones,
or primary bone cells using the RNeasy Minikit (QIAGEN). Subsequently, DNase
digestion was performed according to the manufacturers instructions.
Concentration and quality of extracted RNA was measured using an ND-1000
system (NanoDrop Technology). For cDNA synthesis, 0,51 mg of RNA was
reverse transcribed using cAMV First-Strand Synthesis Kit (Invitrogen) according
to the manufacturers instructions. For semi-quantitative RT-PCR analysis, the
following primers were used to amplify fragments of Ceacam1a
(59-TCAGCACATCTCCACAAAGG-39 and 59-CTCTCTGCCGCTGTATGCTT-39), Ceacam1/2
59-AGAAGAAGGGGCTGAAGTTGGC-39), Ceacam2 (59-GCTATGAAAAGCAGGGCAGA-39 and
(59-CTTAACCTGCTGGAATGCACCCGCCG-39 and 59-CAGCTTCTGTTACCGCGGTGCTGTCT-39),
Ceacam10 (59-GCTAGATCAAAACTTTGAAATTACTCC-39 and
(59-CACAGGAGTTAAACCACTCAAGAA-39 and 59-AAACCTGCAGGAGAATATTGTCA-39), Ceacam12
59-AGTTGAGAAGTAGGATGCTTTC-39), Ceacam13 (59-GGAGCTGCACCGTTCAAGT-39 and
(59-CCTGGTTCACAGGAGCTAGAGT39 and 59-GGCATCTGAAAGACCCACAA-39), Ceacam15
(59-CCTCTAAAGAAATGCGCTTCTC-39 and 59-GACACGCAGGTGAGAATTGA-39), Ceacam16
(59TCCTGGTGGCCAGTTACATT-39 and 59-GCTGCTACAGACGAGACGAA-39),
Ceacam17 (59AAACGGCCGATAGACAACGA-39 and
59-GAACGGGTCACTATGGAAGG-39), Ceacam18 (59-TGAAGTGGACACTAGCAACG-39 and
(59CACATCGAGATGATCCCAGA-39 and 59-TCCCAATGATGATGGCTACC-39), Ceacam20 A1-B1-domain
(59CAAGCTCACCCTCACAGTCA-39 and 59-AAGTTCACGGTGTTGCCTTC-39),
Ceacam20 B2-cytopl. domain 5 (59-GATCTGCCTCTGTCCTGGTC-39 and
59TGGGGTGATCTTGCAGTAAA-39), Psg17 (59-GGTACAAAGGGGTGGCAA-39
and 59- CAAGCTTGTTAAACACAACTGCT-39), Psg30
(59-CTGCACAAATAACCATTGAATTAGA-39 and 59-CTTGACTTGCAAAGGGTGATAA-39), Psg31
59-GCTCAGATTTCTCCTCTGCAATT-39) and bactin (59-ATG GAT GAC GAT ATC GCT-39 and
59ATGAGGTAGTCTGTCAGGT-39) according to the study by Zebauser et al. .
Due to a considerable homology Ceacam1a and Ceacam2 were specifically
amplified in addition to Ceacam1/2 in order to detect expression of Ceacam1.
Quantitative RT-PCR expression analysis was performed using StepOnePlus
predesigned TaqMan gene expression assays (Applied Biosystems). bactin or
Gapdh expression was used as an internal control, as indicated. For western
blotting CEACAM1 protein was detected using the polyclonal rabbit
antiCEACAM1 antiserum P1 after transfer of proteins from an SDS-page onto a
PVDF membrane .
7. Serum analysis
Serum concentrations of bone-specific collagen degradation products (Crosslaps)
and OPG were quantified using antibody-based detection kits (#AC-06F1,
Immunodiagnostic Systems; #MOP00, R&D Systems; #MTR00, R&D Systems).
Mice were fasted for 4 h prior to blood sampling.
For visualization of the actin cytoskeleton, cells were fixed in Formafix
(Pathomed, Germany), followed by permeabilization with ice-cold acetone and
rinses in phosphate buffered saline. Subsequently, cells were stained with
fluorescein-labelled phalloidin and 49,6-diamidin-2-phenylindol (DAPI; both
Molecular Probes, Germany) at room temperature in the dark (1:300 in
phosphate buffered saline). After washes in phosphate buffered saline, slides were
mounted with Vectashield mounting media (Vector Labs, United Kingdom).
Pictures were taken on a Leica DM5000B fluorescent microscope (Leica,
9. Statistical analysis
All data were analyzed by two-tailed Students t test using Excel software. All data
are reported as mean SD. p,0.05 was considered statistically significant.
To elucidate a potential role of CEACAMs in bone remodeling, we monitored the
expression of all genes of the CEACAM family in whole bone, bone marrow and
marrow-flushed bone. While the expression of most members was not detectable
by semi-quantitative PCR, we exclusively found Ceacam1 and Ceacam10
expression in bone and bone marrow (S1 Figure). Comparing the expression in
various tissues, Ceacam1 and Ceacam10 were expressed at comparable levels in
bone samples (Fig. 1A). Interestingly, while the expression of Ceacam1 was
highest in liver and femur, Ceacam10 was expressed at much higher levels in all
bone specimens compared to non-skeletal tissues (Fig. 1A). To further
characterize the expression of Ceacam1 and Ceacam10 in the course of osteoblast
and osteoclast formation, we differentiated bone marrow cells into osteoblasts and
osteoclasts and performed quantitative RT-PCR. Here we found that the
expression of both Ceacam1 and Ceacam10 increased during the early stages of
osteoblast differentiation, and decreased towards terminal osteoblast
differentiation (Fig. 1B). In addition, although Ceacam1 and Ceacam10 were both expressed
at high levels in undifferentiated bone marrow cultures, their expression markedly
decreased during the course of osteoclastogenesis (Fig. 1C). This observation was
confirmed in experiments using the macrophage cell line RAW264.7 where we
found Ceacam1 to be differentially expressed during the course of
osteoclastogenesis on mRNA (Fig. 1D) and protein level (Fig. 1E).
As this observation pointed towards a specific role of CEACAM1 and
CEACAM10 in the regulation of bone remodeling, we next applied
nondecalcified bone histology in mice lacking the respective genes. Von Kossa staining
of spine sections from 6-month-old mice demonstrated a reduced bone mass in
Ceacam1-deficient mice compared to WT controls, whereas no alteration could be
detected in mice lacking CEACAM10 (Fig. 2A). These findings were confirmed by
static histomorphometry, which demonstrated Ceacam1-deficient mice exhibit
decreased trabecular bone volume accompanied by a reduction in trabecular
number and an increase in trabecular separation (Fig. 2B). In contrast, none of
these parameters were altered in mice lacking CEACAM10. In order to analyze
whether the observed phenotype is also detectable in younger mice, we
additionally performed static histomorphometry of 3-month-old mice. Spine
Fig. 1. Expression of Ceacam1 and Ceacam10 in various tissues and differentiated bone cells. (A, B) qRT-PCR of Ceacam1 and Ceacam10 in various
tissues. nd 5 not detectable. (C) qRT-PCR of the same genes during osteoblast (Obl) differentiation at the indicated days (d) of differentiation, using Bglap
expression (OSTEOCALCIN) as a control. (D) qRT-PCR of Ceacam1 and Ceacam10 during osteoclast (Ocl) differentiation at the indicated days (d) of
differentiation, using Calcr expression (CALCITONIN RECEPTOR) as a control. (E) qRT-PCR of Ceacam1 and Ceacam10 in undifferentiated (undiff.) and
differentiated (diff.) RAW cells, using Acp5 expression (TRAP) as a control. (F) Western blot showing CEACAM1 expression in cell lysates from
undifferentiated and differentiated RAW cells. Arrows indicate size of the nearest marker. All bars represent mean + SD (n$3 independent experiments).
Asterics indicate statistically significant differences compared to day 0 in the case of osteoblasts or day 1 in the case of osteoclasts, respectively (p,0.05).
sections of Ceacam1-deficient mice were characterized by a significantly reduced
trabecular bone mass accompanied by reduced trabecular numbers (S2 Figure),
similar to what was observed in 6-month-old animals. In addition, 3-month-old
mutant animals displayed a reduced trabecular thickness and increased trabecular
separation (S2 Figure). To address the question whether the lack of Ceacam1 not
only affects skeletal architecture in vertebrae, but also long bones, we finally
performed histomorphometry of non-decalcified tibia sections and mCT-scanning
of femora derived from Ceacam1-deficient mice. While we found a decreased
trabecular bone volume accompanied by a decrease in trabecular numbers in the
tibiae of mutant animals (Fig. 3A), cortical thickness and mean diameter were
unaltered in the femora of Ceacam1-deficient mice (Fig. 3B).
These findings suggested an important role of CEACAM1 in the regulation of
trabecular bone remodeling, while CEACAM10 was found to have no overt effect.
Therefore, we focused our further analyses on CEACAM1 and performed a full
histomorphometric characterization of the spine sections derived from
6-monthold Ceacam1-deficient mice. Cellular histomorphometry using toluidine-blue
stained spine sections revealed no alteration in the number of osteoblasts or
osteoblast surface, indicating normal osteoblastogenesis in Ceacam1-deficient
mice (Fig. 4A). Likewise, dynamic histomorphometry following dual calcein
labeling demonstrated a normal bone formation rate, ruling out an impaired
osteoblast function as the underlying cause of the observed phenotype. In
contrast, Ceacam1-deficient mice were found to display an increased number of
osteoclasts and osteoclast surface, suggesting increased bone resorption (Fig. 4B).
Although an absolute increase in collagen degradation products was undetectable
(Fig. 4C), normalization of serum crosslaps to the reduced bone mass revealed an
elevated net bone resorption in mice lacking CEACAM1 (data not shown).
In order to analyze, whether the observed increase in osteoclastogenesis could
be explained by gross alterations in OPG levels, we measured this cytokine in the
serum of Ceacam1-deficient mice and controls by ELISA. While no changes were
detectable in 3-month-old Ceacam1-deficient mice, we measured an increased
concentration of OPG in 6-month-old mice, pointing towards an age-dependent
compensatory regulation in vivo in the light of enhanced osteoclastogenesis. This
assumption was confirmed in vitro, were we detected normal expression of Tnfsf11
and Tnfrsf11b, encoding RANKL and OPG, in primary osteoblasts at day 10 of
differentiation (S3 Figure).
Investigating the possibility of cell-autonomous defects in these mice, we next
differentiated WT and Ceacam1-deficient bone marrow cells into osteoblasts and
Fig. 2. Decreased trabecular bone mass in 6-month-old mice lacking Ceacam1. (A) Von Kossa staining of non-decalcified spine sections from controls
(Ceacam1+/+, Ceacam10+/+) and Ceacam1- or Ceacam10-deficient mice (Ceacam1-/-, Ceacam10-/-). Histomorphometric quantification of the trabecular
bone volume (BV/TV, bone volume per tissue volume), trabecular number (Tb.N.), trabecular thickness (Tb.Th.) and trabecular separation (Tb.Sp.). All bars
represent mean SD (n55 mice per group). Asterisks indicate statistically significant differences (p,0.05).
osteoclasts ex vivo. Von Kossa staining at day 10 of differentiation revealed a
normal formation of mineralized nodules in Ceacam1-deficient osteoblast
cultures (Fig. 5A). Likewise, the assessment of alizarin red staining demonstrated
regular extracellular matrix mineralization in osteoblasts derived from
Ceacam1deficient mice. In addition, normal levels of intracellular alkaline phosphatase
Fig. 3. Analysis of long bones derived from 6-month-old mice lacking Ceacam1. (A) Von Kossa staining of non-decalcified tibia sections from controls
(Ceacam1+/+) and Ceacam1-deficient mice (Ceacam1-/-) and histomorphometric quantification of the trabecular bone volume (BV/TV) and trabecular
number (Tb.N.) below. (B) mCT scanning of femora derived from the same mice. Mean femoral diameter and cortical thickness (C.Th.) are indicated below.
All bars represent mean SD (n55 mice per group). Asterisks indicate statistically significant differences (p,0.05).
Fig. 4. Increased osteoclastogenesis in mice lacking Ceacam1. (A) Histomorphometric quantification of the osteoblast number per bone perimeter
(ObN/BPm), osteoblast surface per bone surface (ObS/BS) and the bone formation rate per bone surface (BFR/BS). (B) Quantification of the osteoclast
number per bone perimeter (OcN/BPm), osteoclast surface per bone surface (OcS/BS). (C) Quantification of serum crosslaps. All bars represent mean
SD (n55 mice per group). Asterisks indicate statistically significant differences (p,0.05).
Fig. 5. Accelerated osteoclast formation in bone marrow cells lacking Ceacam1. (A) Alizarin red and von Kossa staining of mineralized matrix and
nodules, respectively, in osteoblast cultures at day 10 of differentiation. The quantification of Alizarin red staining extracellular matrix mineralization is
indicated on the right. (B) Quantification of intracellular alkaline phosphatase activity in bone marrow-derived osteoblast cultures at day 10 of differentiation.
(C) TRAP activity staining of terminally differentiated osteoclasts. The quantification of TRAP-positive, multinuclear osteoclasts is given on the right. (D)
Immunofluorescence using DAPI (nucleus) and Phalloidin (actin) staining of osteoclast cultures at day 7 of differentiation. The quantification of the number of
nuclei per osteoclast (Nuclei/Oc) is indicated on the right. (E) qRT-PCR expression analysis of the indicated genes (Acp5, Tartrate-resistant acid
phosphatase; Tmf7sf4, DC-STAMP, dendrocyte-expressed seven transmembrane protein; NF-kB, nuclear factor kappa-light-chain-enhancer of activated
Bcells; Nfat1c, nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1, Tnfrsf11a, receptor activator of nuclear factor kB, Rank; c-Fos, FBJ
osteosarcoma oncogene; and Calcr, Calcitonin receptor) at day 0, 3, 4 and 7 of osteoclast differentiation. All bars represent mean SD (n53 cultures per
group). Asterisks indicate statistically significant differences (p,0.05).
activity were measured, ruling out a functional osteoblast defect caused by
Ceacam1-deficiency (Fig. 5B). In contrast, TRAP-activity staining of
Ceacam1deficient bone marrow cells cultured in the presence of M-CSF and RANKL
displayed a marked increase in osteoclastogenesis at day 7 of differentiation
(Fig. 5C), while no alteration in the number of nuclei per osteoclast could be
detected by immunofluorescence (Fig. 5D). To understand this effect on the
molecular level, we finally screened for differentially expressed genes essential for
osteoclast differentiation and function. On day 0 of differentiation, no significant
differences could be measured among the tested genes (Fig. 5E). On day 3 of
differentiation, only a tendency towards a reduced expression of Tm7sf4, encoding
the fusion protein DC-STAMP, could be detected in Ceacam1-deficient cultures,
whereas the expression of the late osteoclast marker gene Calcr was still
undetectable in mutant and control cells. In contrast, on day 4 of differentiation
we observed a trend towards enhanced expression of Acp5 and significantly
increased expression of Nfatc1 (Fig. 5E). However, while the increased expression
of Acp5 was only temporarily detectable, we found Nfatc1 to be overexpressed in
Ceacam1-deficient osteoclasts again at day 7, pointing towards a crucial role of
this key transcription factor in mediating the effects of CEACAM1 on
The molecular understanding of bone remodeling represents an ongoing clinical
challenge, as therapeutic options for skeletal disorders such as osteoporosis and
tumor-induced osteolysis caused by excessive osteoclast function remain limited.
While the biological actions of CEACAMs have been studied extensively in the
context of pathological conditions including inflammation, stroke and cancer
[20, 22] a potential role in the regulation of bone turnover has not been
sufficiently addressed to date. This is indeed surprising, given the growing body of
evidence establishing a link between skeletal integrity and immune or cancer cells
not only in basic, but also clinical research. For example, rheumatoid arthritis, a
chronic autoimmune disease characterized by joint destruction, is considered to
be driven by the secretion of pro-inflammatory cytokines including TNFa and
IL17 from lymphocytes, resulting in excessive activation of osteoclasts [28, 29].
Likewise, expression of IL-1b and inhibitors of Wnt signaling, such as DKK-1, in
tumor cells have been reported to be involved in malignant osteolysis through
both an activation of osteoclastogenesis and an inhibition of bone formation
[30, 31]. Based on their broad array of biological effects in the immune system and
tumorigenesis, it was thus important to analyze the role of CEACAMs in the
regulation of bone turnover.
Monitoring the expression of all genes encoding the members of the CEACAM
family, we could exclusively detect the expression of Ceacam1 and Ceacam10 in
bone tissue. Although this observation does not necessarily rule out a potential
role of other Ceacams in bone remodeling, they pointed towards a specific
function of CEACAM1 and CEACAM10 in regulating bone cell function. This
notion was further supported by the high cDNA levels of the two encoding genes
in bone tissue compared to other organs. Based on the specific expression
dynamics of both Ceacam1 and Ceacam10 during osteoblast and osteoclast
differentiation, we went on to analyze the bone phenotype of the respective animal
models. While Ceacam10-deficiency was not associated with any alterations in
structural bone parameters, static histomorphometry demonstrated a decreased
trabecular bone volume in 3- and 6-month old Ceacam1-deficient mice. This was
indeed an interesting finding, as Ceacam1-deficient mice, under basal conditions,
have been reported to display no gross phenotypical abnormalities . As
mentioned above, while previous studies have primarily focused on the
pathophysiologic functions of CEACAM1 in various in vivo and in vitro disease
models, this observation pointed towards a physiologic role of CEACAM1 in the
regulation of bone cell activity.
Although Ceacam1 was differentially expressed during osteoblastogenesis, the
cellular and dynamic histomorphometry failed to detect defective
osteoblastogenesis or osteoblast function. Surprisingly, increased osteoclast parameters were
found in trabecular bone, indicating accelerated osteoclastogenesis in
Ceacam1deficient mice. To further characterize this effect at the cellular level, we
differentiated bone marrow cells into osteoblasts and osteoclasts. In line with our
in vivo observations, primary osteoblasts derived from Ceacam1-deficient mice
displayed normal matrix mineralization and alkaline phosphatase activity in vitro.
In contrast, although no alteration in the numbers of nuclei per osteoclasts could
be detected, Ceacam1-deficient bone marrow cells demonstrated an increased
osteoclastogenesis when cultured with M-CSF and RANKL. Therefore, it is now
possible to conclude that CEACAM1 functions as a negative regulator of
osteoclastogenesis in vivo and in vitro. The fact that we could detect increased
levels of serum OPG in 6-month old animals is interesting, however does not
explain the observed bone phenotype and increased osteoclastogenesis associated
with the lack of CEACAM1. This is supported by the finding that 3-month-old
mutant animals displayed a low bone mass phenotype despite normal OPG levels.
Furthermore, as we failed to detect differences in the expression of Tnfsf11 and
Tnfrsf11b in primary osteoblasts derived from Ceacam1-deficient mice, this
particular phenomenon is most likely explained by an age-dependent counter
regulatory mechanism rather than an intrinsic osteoblast defect.
On the molecular level, we could detect differential expression of Ceacam1 not
only in bone marrow derived osteoclast progenitors, but also in the pure
macrophage cell line RAW264.7, providing a potential explanation for the
increased osteoclast formation in Ceacam1-deficient mice. Since the formation of
mature osteoclasts primarily depends on RANKL-induced activation of key
transcription factors and the subsequent expression of several osteoclast marker
genes, we monitored the expression of NF-kb, Nfatc1, Acp5, Tmf7sf4, Tnfrsf11a,
cFos, and Calcr during osteoclastogenesis in bone marrow cells derived from WT
and Ceacam1-deficient mice. While indicators of mature osteoclasts, including
Calcr and Acp5, were found to be expressed at similar or only temporarily elevated
levels compared to WT controls, respectively, increased expression of Nfatc1 in
Ceacam1-deficient cells was found at day 4 and 7 of osteoclast differentiation. As
this coincided with the induction of monocyte/macrophage fusion into mature
osteoclasts by the addition of RANKL to these cultures, it appears tempting to
speculate that CEACAM1 may modulate osteoclastogenesis through the
regulation of NFATC1, the master transcription factor required for osteoclast
differentiation. Although a previous study reported a stimulatory effect of a short
splice variant of CEACAM1 on the expression of Nfatc1 in CD4+ T cells ,
further research is necessary to elucidate the influence of CEACAM1 on the
regulation of this transcription factor specifically in osteoclasts and
myeloidderived cells. This is of particular importance, as the liver-specific inactivation of
Ceacam1, characterized by hyperinsulinemia and glucose intolerance, results in
decreased osteoclastogenesis . Thus, it appears that the impact of global
Ceacam1 deletion, including cells of the osteoclast lineage, is dominant over the
effects of hepatic Ceacam1 expression on bone metabolism.
Taken together, our study reports a novel function of CEACAM1 in bone
remodeling. Using in vivo and in vitro assays, we demonstrate that deficiency of
CEACAM1 is associated with a reduced bone mass due to increased
osteoclastogenesis, at least in mice. Given its previously reported function in regulating tumor
cell differentiation and the immune system, future studies investigating the role of
CEACAM1 in pathologic bone conditions, such as tumor-induced osteolysis and
inflammation-induced bone loss, will be of crucial importance.
S1 Figure. Expression of Ceacam genes in bone tissue and differentiated bone
cells. RT-PCR of the indicated genes in the spine (S), femur (F), flushed femur
(FF) and bone marrow (BM) using the same primers as described previously (26).
S2 Figure. Decreased trabecular bone mass in 3-month-old mice lacking
Ceacam1. Von Kossa staining of non-decalcified spine sections from controls
(Ceacam1+/+) and Ceacam1-deficient mice (Ceacam1-/-). Histomorphometric
quantification of the trabecular bone volume (BV/TV, bone volume per tissue
volume), trabecular number (Tb.N.), trabecular thickness (Tb.Th.) and trabecular
separation (Tb.Sp.). All bars represent mean SD (n55 mice per group).
Asterisks indicate statistically significant differences (p,0.05).
S3 Figure. OPG in Ceacam1-deficient mice. (A) Serum concentrations of OPG in
3- and 6-month old Ceacam1-deficient mice. (B) qRT-PCR of Tnfsf11 and
Tnfrsf11b encoding RANKL and OPG, respectively, in primary osteoblasts at day
10 of differentiation. All bars represent mean SD (n55 mice and n53 cultures
per group, respectively). Asterisks indicate statistically significant differences
The authors would like to thank Gudrun Arndt for managing the mouse colonies.
They are grateful to Mona Neven for preparing bone sections for
histomorphometric assessment. This work was supported by the Deutsche
Forschungsgemeinschaft (SPP1468 IMMUNOBONE Project SCHI 504/7-2 to T.
Schinke and IT13-3 to W.D. Ito and A.K. Horst).
Conceived and designed the experiments: TH TB AJ JS MA TS AKH JK.
Performed the experiments: TH TB AJ JS SS AKH JK. Analyzed the data: TH TB
AJ JS SS AKH JK. Contributed reagents/materials/analysis tools: WZ WDI MA.
Wrote the paper: TH TS AKH JK.
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