Inhibitory function of parathyroid hormone-related protein on chondrocyte hypertrophy: the implication for articular cartilage repair
Arthritis Research & Therapy
Inhibitory function of parathyroid hormone- related protein on chondrocyte hypertrophy: the implication for articular cartilage repair
Wei Zhang 0 2 3
Jialin Chen 0 2 3
Shufang Zhang 0 2 3
Hong Wei Ouyang 0 3
0 Center for Stem Cell and Tissue Engineering, School of Medicine, Zhejiang University , Hangzhou, China 310058
1 Department of Sports Medicine, School of Medicine, Zhejiang University , Hangzhou, China 310058
2 Zhejiang Provincial Key Laboratory of Tissue Engineering and Regenerative Medicine , Hangzhou, China 310000
3 Center for for Stem Cell and Tissue Engineering, School of Medicine, Zhejiang University , Hangzhou, China 310058
Cartilage repair tissue is usually accompanied by chondrocyte hypertrophy and osseous overgrowths, and a role for parathyroid hormone-related protein (PTHrP) in inhibiting chondrocytes from hypertrophic differentiation during the process of endochondral ossification has been demonstrated. However, application of PTHrP in cartilage repair has not been extensively considered. This review systemically summarizes for the first time the inhibitory function of PTHrP on chondrocyte hypertrophy in articular cartilage and during the process of endochondral ossification, as well as the process of mesenchymal stem cell chondrogenic differentiation. Based on the literature review, the strategy of using PTHrP for articular cartilage repair is suggested, which is instructive for clinical treatment of cartilage injuries as well as osteoarthritis.
Articular cartilage injuries and osteoarthritis (OA) are
commonly encountered in joint diseases. Current
treatments aim at generating hyaline-like repair tissue with a
stable, permanent chondrocyte phenotype. However, the
repair tissue is often accompanied by chondrocyte
hypertrophy and bony outgrowths, in particular with
respect to bone marrow-eroding techniques [1,2]. The
progressive abnormal hypertrophy may result in
degradation of the matrix, impairing the function of the repair
tissue. A role for parathyroid hormone-related protein
(PTHrP) in regulating endochondral ossification by
inhibiting chondrocytes from hypertrophy has been
Chondrocyte hypertrophy is commonly found during
both endochondral ossification and the process of
articular cartilage repair. The former is a physiological
process of bone formation, during which chondrocytes
become larger and produce collagen type X (Figure 1A,B).
These cells are called hypertrophic chondrocytes (quite
different from normal chondrocytes, which secrete and
maintain the cartilaginous matrix rich in collagen type II
and aggrecan). Hypertrophic chondrocytes mineralize
surrounding matrix, secrete vascular endothelial growth
factor to induce blood vessel formation, and finally
undergo apoptosis. Then osteoblasts fill up the vacancy
left by hypertrophic chondrocytes and synthesize bone
matrix, a process that results in new bone formation .
Because chondrocytes go through a series of orderly
changes during endochondral ossification, the cartilage is
referred to as ‘transient’ cartilage. In articular cartilage,
hypertrophic chondrocytes usually exist quiescently in
calcified layers under the tidemark  (Figure 1C,D). The
upper healthy articular chondrocytes maintain a stable
phenotype and are resistant to hypertrophic
differentiation . So articular cartilage is called ‘permanent’
cartilage, which maintains the structure of functional
hyaline cartilage throughout life. Under pathological
conditions, however, such as cartilage injuries and OA,
chondrocyte hypertrophy can be reactivated in the repair
process (Figure 2). The normal articular chondrocytes
thus enter hypertrophic differentiation, resembling the
process of endochondral bone formation [11,12], which
can cause the upper cartilage to become calcified and a
relatively thin repair tissue to be formed [1,9].
Hypertrophic chondrocytes in endochondral ossification and
cartilage repair tissue show similar characteristics, like
the production of collagen type X  and matrix
metalloproteinase 13 (MMP-13) , promoting
degradation of the cartilage matrix. The close link between
aberrant hypertrophy and the inferior quality of cartilage
repair tissue indicates that chondrocyte hypertrophy
might be a potential therapeutic target to improve
Biology of PTHrP and its correlation with chondrocyte hypertrophy
Biology of PTHrP
PTHrP is a protein member of the parathyroid hormone
(PTH) family. It was first identified as a factor required
for humoral hypercalcemia of malignancy . PTHrP
has been found in many tissues - not only in
highabundance sites such as breast, hair follicles and cartilage,
but also in previously unrecognized sites, including
enthesis and periosteum . PTHrP is a 141 amino acid
polypeptide, and most of its biological functions are
mediated by its amino terminus, such as the effect on
cartilage . Its homolog, parathyroid hormone (PTH),
has the same amino-terminal 34-residue peptide
fragments as PTHrP, and they thus share a common receptor,
parathyroid hormone 1 receptor (PTH1R) . The high
similarity in the functional domain makes PTH and
PTHrP equally potent at inhibiting chondrocyte
hypertrophy, which has been proved by some studies [19-21].
Thus, information obtained from PTH research on
hypertrophy inhibition [3,22-24] is also applicable for PTHrP.
The role of PTHrP in endochondral ossification
During endochondral ossification, chondrocytes move
through an orderly differentiation program: periarticular
proliferating chondrocytes, columnar proliferating
chondrocytes, prehypertrophic chondrocytes and
hypertrophic chondrocytes. The role of PTHrP in endochondral
ossification was initially studied about 20 years ago.
PTHrP is secreted by periarticular proliferating
chondrocytes, while its receptor, PTH1R, is mainly expressed in
prehypertrophic chondrocytes . When the PTHrP
gene was knocked out via homologous recombination in
murine embryonic stem cells, the mice died shortly after
birth and showed abnormal endochondral bone
development . The absence of PTHrP caused diminished
chondrocytes and accelerated hypertrophic
differentiation leading to premature mineralization of extracellular
matrix and apoptosis . Targeted overexpression of
PTHrP under the control of the cartilage-specific
collagen type II promoter resulted in the opposite effect
of chondrodysplasia through delay of the terminal
differentiation of chondrocytes, inhibition of apoptosis
and disruption of endochondral ossification . Because
Bcl-2, an anti-apoptotic protein, lies downstream of
PTHrP , overexpression of PTHrP increased the
expression of Bcl-2, inhibiting apoptotic cell death and
disrupting growth plate architecture as well. Similar
results were obtained when the PTHrP receptor was
manipulated genetically [30,31].
Another important factor involved in endochondral
ossification is Indian hedgehog (Ihh), which is expressed
by prehypertrophic chondrocytes  and acts in
conjunction with PTHrP to modulate endochondral
ossification. The foundational discovery of the
IhhPTHrP regulatory axis was made by Vortkamp and
colleagues in 1996 . They found that Ihh stimulated
proliferating chondrocytes to produce PTHrP, which in
turn accelerated the proliferation of periarticular cells
and prevented the onset of chondrocyte hypertrophy,
finally keeping chondrocytes in a proliferating state. This
negative feedback loop regulates the balance between
proliferation and maturation of chondrocytes, ensuring
orderly bone formation .
The prevailing paradigm - Ihh is mediated by PTHrP to
regulate chondrocyte hypertrophy indirectly (it is also
the best characterized regulatory function of PTHrP)
gained wide acceptance for more than a decade until
Kobayashi and colleagues  and Mak and colleagues
 made a breakthrough. Using PTHrP and Ihh mutant
mice, Kobayashi and colleagues found that decreasing or
enhancing Ihh caused delayed or accelerated periarticular
chondrocyte differentiation and reduced or increased the
length of the columnar region, respectively, with these
effects remaining unchanged regardless of whether
PTHrP signaling was maintained or disrupted. Therefore,
they concluded that Ihh could stimulate periarticular
chondrocyte differentiation and cause elongation of the
columnar region independent of PTHrP . Based on
their results, Mak and colleagues further revealed a novel
role of Ihh in regulating chondrocyte hypertrophy
without the influence of PTHrP. During endochondral
bone formation, PTHrP-dependent Ihh signaling
inhibiting chondrocyte hypertrophy is dominant, thereby
obscuring the promoting effect of PTHrP-independent Ihh
signaling  (Figure 3A). Moreover, they speculated
that canonical Wnt and bone morphogenetic protein
(BMP) signaling may contribute to this non-canonical
pathway as well .
Besides Ihh, PTHrP can be regulated by other factors
as well. Amano and colleagues  reported that Sox9
family members inhibited the late stages of endochondral
ossification by up-regulating the expression of PTHrP.
Human MSCs transfected with the SOX9 gene exhibited
enhanced PTHrP expression together with reduction of
hypertrophic markers . However, the regulatory
effect of SOX9 on expression of PTHrP differed
depending on the target organs in vivo . It has also been
welldemonstrated by in vitro and in vivo studies that
insulinlike growth factor 1 (IGF-1) signaling suppresses PTHrP
expression, and thus modulates growth plate
development . Besides, the canonical Wnt pathway is known
to promote chondrocyte hypertrophy via inhibition of
PTHrP signaling activity instead of PTHrP expression
So how does PTHrP function to inhibit premature
hypertrophy of chondrocytes? The intracellular pathways
have not been completely clarified yet. The PTHrP
receptor PTH1R is a classical G-protein-coupled
receptor, which transduces signals via the Gs or Gq/11 family
and then activates the cAMP/protein kinase A (PKA) or
phospholipase C (PLC)/protein kinase C (PKC) pathway
. It has been shown that the inhibitory effects of
PTHrP preventing precocious chondrocyte hypertrophy
are primarily facilitated by the activation of cAMP/PKA
signaling downstream . Chondrocyte hypertrophy
could be delayed in mice with a PTH1R mutation that
specifically interrupted the PLC/PKC pathway but did
not affect cAMP/PKA activation . In chimeric mice
with disrupted Gnas exon 2 (encoding Gsа), chondrocytes
prematurely undergo hypertrophy, resulting in a
phenotype similar to that of the PTH1R-/- cells . Taken
together, PTHrP favors the Gsа/cAMP/PKA-dependent
signaling pathway to inhibit hypertrophic differentiation.
Several studies have focused on the downstream
signaling pathways of PKA. Kozhemyakina and colleagues 
found that protein phosphatase 2A (PP2A), which is
activated by PKA, promoted dephosphorylation of
histone deacetylase 4 (HDAC4), which was translocated
into the nucleus and repressed the activity of MEF2
transcription factors, ultimately attenuating the rate of
chondrocyte hypertrophy. Also, Correa and colleagues
 showed that Zfp521, the zinc finger transcriptional
coregulator, interacted with HDAC4 in the nucleus and
this complex repressed expression of runx2-mediated
target genes. They generated mice with Zfp521
conditionally deleted in chondrocytes, resulting in early
hypertrophic transition and reduced growth plate thickness.
This phenomenon is similar to that in the PTHrP-/- and
PTH1R-/- mice as well. Their recent research indicated
that deletion of Zfp521 from chondrocytes rescued
Jansen metaphyseal chondrodysplasia, a disorder caused
by a constitutively activating mutation of PTH1R .
Furthermore, PTHrP inhibits runx2 expression in
chondrocytes via Nkx3.2/Bapx1-mediated repression  or
cyclin-D1-CDK4-induced phosphorylation and
degradation of runx2 and runx3 . All the pathways mentioned
above are transduced via the cAMP/PKA pathway.
Several studies have also implicated the PLC/PKC
pathway in PTHrP’s inhibitory function. Chen and colleagues
 reported that p38 mitogen-activated protein kinase
(MAPK) induced the expression of Bcl-2 as well as
collagen type X, and PTH1R activation could block p38
MAPK activity, which was mediated by PKC instead of
PKA. The above-mentioned pathways are summarized in
Figure 3B. Further investigations are needed to improve
our understanding of the mechanisms by which PTHrP
inhibits chondrocyte hypertrophy.
The role of PTHrP in articular cartilage maintenance
Based on the well established theory of PTHrP’s role in
endochondral ossification, researchers have also tried to
apply it to articular cartilage to better understand its
pathogenetic mechanisms. A few publications
demonstrated that PTHrP could regulate articular chondrocytes.
In 2008, Chen and colleagues found that PTHrP
participated in the maintenance of articular cartilage as it did in
growth plate cartilage , using a PTHrP-LacZ
knockin mouse that can be used to characterize PTHrP gene
expression . The PTHrP-producing cells, in both
growth plate cartilage and articular cartilage, are derived
from the same source, the chondroepiphysis, which is
divided by the secondary ossification center into two
distinct PTHrP-expressing subpopulations - the
chondrocytes of the articular surface and periarticular
proliferating chondrocytes of the growth plate  - while
PTH1R is expressed in the underlying prehypertrophic
Jiang and colleagues  used a co-culture model to
evaluate the interaction of chondrocytes derived from
different layers of articular cartilage. They demonstrated
that PTHrP produced by the superficial or middle layer
could block the alkaline phosphatase activity and related
mineralization of chondrocytes from the deep layer. This
study provides in vitro evidence that, in healthy articular
cartilage, PTHrP secreted by chondrocytes from surface
layers inhibits the hypertrophic potential of chondrocytes
residing in the deep layer so as to maintain the
homeostasis of articular cartilage, but the effect was not
confirmed in vivo. More convincing evidence was
provided by conditional deletion of PTHrP in midregion
articular chondrocytes via growth and differentiation
factor 5 (Gdf5) control sequences. After destabilizing
medial meniscus, the knock-out mice exhibited severe
cartilage degeneration, indicating that PTHrP plays an
important role in the physiological regulation of articular
cartilage maintenance .
Since PTHrP shows inhibitory effects on articular
chondrocyte hypertrophy physiologically, the role of
PTHrP under pathological conditions was explored as
well. It was reported that the expression of PTHrP in
articular cartilage was 5.42-fold higher than that of
osteophytic chondrocytes, which represents a prototype of
cartilage repair tissue . It is probable that the
decreased expression of PTHrP is insufficient to inhibit
abnormal hypertrophy in repair tissue; therefore,
supplement of exogenous PTHrP to diseased cartilage is of
great significance. Wang and colleagues  transfected
bovine articular chondrocytes with human PTHrP
(hPTHrP) constructs and applied cyclic tensile strain to
induce arthritic changes simultaneously. Overexpression
of hPTHrP inhibited cyclic tensile strain-induced collagen
type X expression, suggesting the involvement of PTHrP
in resisting mechanical strain-induced hypertrophic-like
changes. In another study, human articular chondrocytes
were treated with azacytidine to induce terminal
differentiation, mimicking the situation in OA .
Treatment with exogenous PTH (
aminoterminal fragment of PTH) significantly eliminated the
increased expression of collagen type X, alkaline
phosphatase and Ihh induced by azacytidine. Moreover,
when papain-induced OA rats were injected
intraarticularly with PTH (
) for 5 weeks, the OA cartilage
was almost restored to a normal state. It should be noted
that, in this research, PTH did not exert any adverse
effects on normal chondrocytes or healthy joints, only on
OA-affected cartilage. However, studies from Kudo and
colleagues  showed that bone marrow-derived MSCs
transformed from a chondrogenic to a fibroblastic
phenotype and osteochondral defects were never covered with
cartilage after 4-week PTH (
) treatment. This
contradiction probably results from the differences in the animal
models, injury types and PTH fragments in these two
studies. Our group found that the time window for PTHrP
administration is of great importance, which could
influence chondrogenesis and chondrocyte hypertrophy of
repair tissue. More details are given in the next section.
The canonical Ihh-PTHrP pathway has been commonly
invoked when discussing results in most publications
investigating the mechanisms by which PTHrP regulates
articular cartilage [3,4]. Chen and colleagues, however,
raised a different and challenging possibility [51,52]: in
articular cartilage, the master regulatory factor is
mechanical loading, rather than Ihh. Such loading would
induce expression of PTHrP in the articular chondrocytes
from the superficial and middle layers, and then
transduction of the PTHrP to inhibit terminal differentiation
and promote proliferation, a downstream signaling
pathway similar to that of endochondral bone formation.
It is surprising that the Ihh-PTHrP regulatory axis is
uncoupled in articular chondrocytes. In injured and OA
cartilage, tensile stiffness in the superficial layer of
articular cartilage decreased substantially . It is very
likely that the alteration of mechanical loading influences
the primary responder, PTHrP, and finally impacts the
downstream regulatory system. Although this system has
not been fully confirmed, it provides a new idea to study
the mechanisms of PTHrP and the pathogenesis of
chondrocyte hypertrophy in cartilage repair.
The role of PTHrP in MSC chondrogenic differentiation
Large and unconfined cartilage injuries are nearly
impossible for the eroded articular cartilage to repair by
itself. Autologous chondrocyte implantation may be a
practical approach to solve this problem and has been
applied to the clinical treatment of cartilage defects.
However, during the in vitro expansion period for
collection of sufficient cells, chondrocytes often undergo
rapid dedifferentiation, which limits the clinical
application of autologous chondrocyte implantation .
MSCs are considered an alternative cell source for
cellbased therapies of cartilage injuries. They can be isolated
from different tissues, expand rapidly and stably, and
differentiate into chondrocytes effectively. It has been
reported, however, that induction of chondrogenesis of
MSCs in vitro is generally accompanied by unwanted
hypertrophic differentiation . Furthermore, ectopic
transplantation of MSC pellets in nude mice was followed
by calcification and vascular invasion, leading to cartilage
phenotypic instability . Because PTHrP exerts
inhibitory effects on terminal differentiation of articular
cartilage, some researchers proposed that PTHrP may be
a candidate to inhibit hypertrophy during MSC
chondrogenic dfferentiation as well.
It was reported that co-culture of human MSCs with
human articular chondrocytes under chondrogenic
induction could promote chondrogenesis and inhibit
hypertrophy of the engineered cartilage . Similarly,
Fischer and colleagues  described that when MSC
pellets were induced to differentiate in
chondrocyteconditioned medium, hypertrophy was significantly
inhibited both in vitro and in the following ectopic
transplantation study. Furthermore, when MSCs were
cocultured with chondrocytes directly, in vivo calcification
was completely inhibited. Further investigations were
carried out to identify the chondrocyte-derived soluble
factors involved in these effects and PTHrP was found to
be the regulator; however, in situ repair studies are
needed to confirm this conclusion. In the studies using
PTHrP as a supplement of the chondrogenic medium,
MSC pellets differentiated with suppressed hypertrophy
in vitro [5-7]. Nevertheless, the effect of PTHrP on
chondrogenesis was discrepant. Weiss and colleagues 
reported an inhibitory effect on collagen type II
expression with 1 or 10 ng/ml PTHrP after 21 days of
chondrogenesis, while Kim and colleagues  showed
collagen type II and SOX9 gene expression increased up
to 4-fold in bone marrow-derived MSCs and adipose
tissue-derived MSCs when cells were treated with 10 or
100 ng/ml PTHrP from the 14th day of culture. This
contradiction is probably due to the diverse treatment
time or the different fragments and concentrations of
PTHrP they used. The above phenomenon was reported
in normal cells, whereas Kafienah and colleagues 
engineered cartilage with MSCs from OA patients.
Hypertrophy was remarkably inhibited with PTHrP treatment,
though the OA cells were more inclined to terminal
differentiation. Moreover, the process of chondrogenesis
was not affected by PTHrP. Overall, PTHrP serves as a
predominant factor in MSC-based approaches to
promote cartilage repair; however, further work is still
needed to minimize the undesirable effect on
chondrogenesis before clinical use. According to what Weiss and
colleagues  found, adding 0.1 ng/ml of PTHrP from
day 21 could suppress collagen type X deposition without
any negative effects on chondrogenic differentiation,
while higher doses (10 or 100 ng/ml) or earlier treatment
(from day 0) would lead to the suppression of
chondrogenesis, valuable information when considering clinical
Application of PTHrP for articular cartilage repair
As described above, it is likely that PTHrP can be used to
restrain abnormal hypertrophy in the cartilage repair
process. Treatment with PTHrP would consist of two
main methods: administration of recombinant protein or
gene therapy using genetic manipulation (Figure 4).
Recombinant protein treatment
Recombinant PTHrP protein used in most studies
includes only its functional domains, such as PTHrP
) [3,5,6], to which the PTHrP receptor binds.
Experiments have been conducted where MSC pellets
have been pre-treated with recombinant human PTHrP
) and then the pellets transplanted subcutaneously
into SCID mice . Hypertrophy was inhibited during in
vitro chondrogenesis but subsequent in vivo calcification
was not repressed. This inefficacy is possibly due to the
absence of PTHrP in ectopic transplantation sites, which
implies the importance of sustained and sufficient PTHrP
for complete inhibition of hypertrophy.
Injection of PTHrP in sites can maintain the working
concentration to some extent. Direct delivery of
recombinant PTHrP without scaffolds, however, requires
frequent injections to compensate for its rapid
degradation and removal. Chang and colleagues  reported
that injection of 10 nM PTH (
) intra-articularly every
3 days delayed progression of OA in rats. Kudo and
colleagues  and Mizuta and colleagues 
demonstrated that constant PTH administration into the joint
cavity by an osmotic pump placed subcutaneously
avoided the complexities of frequent injection.
Tissue engineering approaches are potent methods for
cartilage repair. Biodegradable injectable scaffolds are
utilized as a delivery system for the controlled release of
PTHrP. Various kinds of materials have been developed
to improve local delivery in cartilage tissue engineering,
such as collagen-alginate . Release rate and
degradation time change with the scaffold materials and
structures and an appropriate release system should be chosen
according to the release profile of PTHrP, the specificity
of the target region, and the actual target effect.
On the other hand, the treatment time must be
optimized to avoid the side effects of early or prolonged
PTHrP treatment. Mizuta and colleagues 
demonstrated that a 2-week treatment with PTHrP for
fullthickness articular cartilage defects resulted in successful
regeneration, while a 4-week treatment resulted in an
inferior repair, although the effect on hypertrophy
inhibition in repair tissue was not assessed in this study.
As mentioned above, our unpublished data indicate that
it is quite important to choose a suitable time window for
PTHrP administration post-injury. We injected 50 μg/ml
) into rabbit joint cavity once a week at
three different time windows: 4 to 6 weeks, 7 to 9 weeks
and 10 to 12 weeks after osteochondral defect
construction. All rabbits in the three groups were sacrificed
16 weeks post-injury. The results showed that joint
cartilage injected at 4 to 6 weeks after osteochondral
defect construction exhibited a better morphological and
histological appearance than the other groups. The repair
tissue with PTHrP injection at 4 to 6 weeks displayed less
expression of collagen type X and other hypertrophic
markers than the groups without PTHrP injection or
treated over different time windows (unpublished data).
These results suggest that 4 to 6 weeks post-injury is a
suitable time window to administer PTHrP for
osteochondral defect repair in rabbit, but further tests are
needed to determine whether this is the case for other
animal models or PTHrP concentrations. Kudo and 
colleagues administrated hPTH (
) continuously or
intermittently for 2 weeks after creation of full-thickness
defects. The repair tissue was found to be fibrous or
fibrocartilaginous with continuous treatment after 8
weeks post-injury, while intermittent treatment resulted
in restoration of hyaline-like cartilage. Based on the
above-mentioned studies, we conclude that short-term
and intermittent treatment using PTHrP in the early
phase (but not the initiation) after injury may lead to
superior cartilage repair. However, those pilot studies
were done by different groups under different situations,
and systematic research must be carried out to determine
the conditions leading to the best therapeutic effect.
Gene therapy represents another new approach,
providing persistent synthesis of required proteins at target sites
in vivo. Wang and colleagues  generated bovine
articular chondrocytes that were transfected with human
PTHrP constructs, demonstrating that
PTHrP-transfected chondrocytes resisted mechanical strain-induced
hypertrophy. This effect was not confirmed by in vivo
experiments. Though few studies on PTHrP gene therapy
have been conducted, studies focusing on cartilage repair
with other genes are numerous, making PTHrP gene
transfer feasible and applicable.
PTHrP gene therapy could be carried out in two ways.
The PTHrP gene (delivered by adenovirus, lentivirus,
adeno-associated virus or other vectors) could be directly
introduced to the joint space to affect surrounding cells,
such as synovial lining cells, articular chondrocytes and
MSCs , so that they secrete PTHrP. Alternatively, the
PTHrP gene could be transfected into cultured cells, such
as chondrocytes, MSCs and other cells, following
transplantation of those cells to the target locations  for
sustained production of PTHrP. In both methods, gene
constructs or gene-transfected cells can be pre-mixed
with tissue engineered scaffolds before transplantation to
guarantee more durable delivery.
However, the problem of unrestrained PTHrP
production, similar to excessive administration of recombinant
PTHrP mentioned above, could occur with gene therapy,
so the level of PTHrP would need to be precisely
controlled. According to previous research, 1 ng/ml
PTHrP was suitable for MSC chondrogenesis while
higher doses suppressed collagen type II expression .
Thus, PTHrP expression should be confined to a certain
level for better therapy.
The time window for PTHrP expression also requires
consideration. According to our results from protein
treatment, PTHrP administration at 4 to 6 weeks post-injury is
the optimum time window. Therefore, it would be better to
switch the PTHrP gene on during this period. Various
kinds of regulatory strategies have been explored to
achieve temporal control of transgene expression. Besides
the conventional exogenous stimuli, such as tetracycline
, that have been utilized for years, new types of stimuli,
such as light  and ultrasound , have emerged and
can be applied to control of PTHrP expression.
Conclusions and perspectives
Chondrocyte hypertrophy is found in the process of
cartilage repair and endochondral bone formation.
PTHrP has long been recognized as a potent inhibitor of
hypertrophic differentiation during endochondral
ossification. Pilot studies demonstrated PTHrP’s capacity to
suppress abnormal hypertrophy in both the articular
cartilage and MSC chondrogenic differentiation
processes. So, it could be reasoned that PTHrP may
potentially inhibit chondrocyte hypertrophy in cartilage
repair tissue as well. Like other bioactive factors, therapy
with PTHrP could be achieved using recombinant
protein or gene manipulation. However, to develop an
efficient and applicable way for functional cartilage repair
with PTHrP, several key issues need to be further
First, a suitable PTHrP concentration should be
determined. This should be sufficient to inhibit hypertrophy
but have no adverse effects on chondrogenesis. The
concentration changes depending on the animal model,
PTHrP fragments used, administration time, and so on.
Second, the time over which PTHrP functions best
should be determined. This should be early, short and
intermittent to avoid side effects of earlier or prolonged
PTHrP treatment. The release frequency and duration
need to be optimized according to practical requirements.
Third, the PTHrP delivery system should be optimized,
including the delivery material, methods of recombinant
protein injection, as well as methods of PTHrP gene
Finally, more in vivo studies are necessary to gain more
direct evidence on the efficacy of PTHrP on articular
cartilage repair and to elucidate the underlying
mechanisms more clearly in a variety of animal models.
hPTHrP, human parathyroid hormone-related protein; Ihh, Indian hedgehog;
MSC, mesenchymal stem cell; OA, osteoarthritis; PK, protein kinase; PLC,
phospholipase C; PTH, parathyroid hormone; PTH1R, parathyroid hormone 1
receptor; PTHrP, parathyroid hormone-related protein.
The authors declare that they have no competing interests.
We appreciate Dr Yangzi Jiang and Dr Hua Liu’s comments on the manuscript.
This work was supported by National Key Basic Research Program
(2012CB966600), National Natural Science Foundation of China (81125014,
81071461, J1103603, 31000436), International Science & Technology
Cooperation Program of China (2011DFA32190), Zhejiang Province Grants
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