Increased expression of nerve growth factor (NGF) and high affinity NGF receptor (p140 TrkA) in human osteoarthritic chondrocytes
F. Iannone,
C. De Bari,
F. Dell'Accio,
M. Covelli,
V. Patella1,
G. Lo Bianco1 and
G. Lapadula
Rheumatology Unit, DIMIMP and
1 Orthopaedics Clinic II, CISO (Centro Interdipartimentale di Studi sull'Osteoartrosi), University of Bari, Italy
 |
Abstract
|
---|
Objective. We aimed to investigate the expression of nerve growth factor (NGF) and high affinity NGF receptor (p140 TrkA) on chondrocytes from human healthy and osteoarthritic cartilage.
Methods. We recruited 12 patients with osteoarthritis (OA) undergoing surgical knee replacement. Articular cartilage was split into two zones showing macroscopically and histologically the lowest (MIN) and highest (MAX) degree of osteoarthritic damage. Additional specimens of cartilage were obtained from three healthy donors. Chondrocytes were isolated by enzymatic digestion and freshly processed for NGF protein, Trk A detection and mRNA extraction. NGF-ß mRNA was determined by a reverse transcriptasepolymerase chain reaction (RTPCR). NGF-ß and TrkA expression was evaluated by immunofluorescence and flow cytometry analysis.
Results. NGF-ß-specific mRNA was detected in normal and osteoarthritic chondrocytes. NGF-ß protein levels were low in normal chondrocytes, increased in MIN osteoarthritic cartilage and further enhanced in MAX osteoarthritic cartilage. Likewise, TrkA was scarcely expressed on normal chondrocytes and progressively increased on osteoarthritic chondrocytes based on the extent of anatomic damage.
Conclusions. This is the first study showing that human chondrocytes synthesize NFG-ß and express on their surface the high affinity NGFR (p140 TrkA). Of note, NGF-ß and TrkA were upregulated in osteoarthritic chondrocytes suggesting a role of NGF in the pathophysiology of OA. We can speculate that NGF, like other growth factors, stimulates chondrocyte metabolism in the osteoarthritic process.
KEY WORDS: Cartilage, NGF-ß, NGFR, Trk A.
 |
Introduction
|
---|
Nerve growth factor (NGF) is a metabolically active peptide, which was first isolated in nerve tissue [1]. NGF plays a key role in differentiation, development and survival of sympathetic and sensory nerve cells and cholinergic neurons of the central nervous system (CNS) [1]. To exert its biological effects, NGF needs to interact with a specific receptor system expressed on target cells. So far, two classes of NGF receptors, with different binding avidity, have been identified. The low affinity receptor (p75 NGFR) has some sequence similarity (three or four cysteine-rich motifs in the extracellular domain) to the tumour necrosis factor receptors, the Fas/Apo-1 antigen and the B cell antigen CD40, which all together have been named the NGF receptor superfamily [2]. Expression of unbound p75 NGFR on transfected neurons induces apoptosis, while cell death is prevented by adding NGF [3]. The high affinity receptor is the trk protooncogene product, a transmembrane tyrosine kinase, (p140 TrkA) [4] that is required for functional NGF signal transduction. Whether TrkA is the high affinity receptor by itself [5] or whether both TrkA and p75 NGFR are essential in forming a functional receptor complex [6] is a matter of debate.
It has also been shown that NGF has immune and haematological effects, and that it is even synthesized by extraneural sources. NGF increases T- and B-cell proliferation in vitro [7, 8], enhances IL-2 receptor expression on the cell surface of T lymphocytes [9], and stimulates haemopoietic colony growth and differentiation [10]. NGF-specific mRNA has been detected in mouse T lymphocytes, which also secrete NGF protein [11]. These findings suggest that NGF, in addition to the effects as a neuronal survival factor, has other functional properties.
Recently, it has been also shown that human lung and skin fibroblasts synthesize NGF and express NGF receptors [12]. Fibroblasts share with chondrocytes a mesenchymal origin, and it is well known that articular chondrocytes during in vitro passaging undergo a de-differentiation process towards a fibroblast-like phenotype [13]. NGF has never been studied in human chondrocytes. We reasoned that NGF might modulate chondrocyte metabolism and that its expression could be regulated during osteoarthritis. Nonetheless, we have recently shown that human chondrocytes express some neuropeptides, such as substance P and met-enkephalin [14], which were initially believed to be exclusively released by neural cells. This prompted us to investigate the NGF/NGF-receptor system in human articular cartilage and its possible modulation in osteoarthritic chondrocytes.
 |
Patients and methods
|
---|
Cartilage specimens
Articular cartilage was obtained, after written informed consent, from 12 OA patients with genu varum (nine females, three males; aged 5472 yr) undergoing knee replacement surgery. The joint surface was subdivided into two zones showing the lowest (MIN) and highest (MAX) degree of osteoarthritic damage, and samples were taken from each zone. The articular cartilage in the MIN zone resembled normal cartilage with a translucent, smooth, integer surface; in the MAX zone, the cartilage surface was yellowish, softened and fibrillated.
Macroscopic findings were validated by the histological study performed on full-thickness specimens biopsied from each zone and stained with safranin-O (Fig. 1
). The degree of microscopic cartilage damage was evaluated using the Mankin grading scheme [15].

View larger version (139K):
[in this window]
[in a new window]
|
FIG. 1. Photomicrographs of safranin-O-stained histological sections of zones with macroscopically defined minumum (left) and maximum (right) damage in typical osteoarthritic cartilage (original magnification x60). A diffuse staining of the cartilage is present in the minimum zone, while in the maximum zone there is an evident hypocellularity together with some clusters of chondrocytes and severe reduction of the safranin-O-staining.
|
|
Healthy knee cartilage from three human donors (all males, aged 1833 yr), obtained post-mortem following approval by an ethical commitee, was also studied.
Chondrocyte isolation
Chondrocytes were isolated as described previously [16]. Briefly, chondrocytes were released from the cartilage matrix by hyaluronidase (0.2%, 30 min, 37°C; Sigma, Milan, Italy), pronase (0.25%, 90 min, 37°C; Sigma) and collagenase (0.2%, 3 h, 37°C; Sigma) enzymatic digestion. More than 95% of the chondrocytes were viable (Trypan Blue exclusion test) after their isolation.
NGF RTPCR
Human articular chondrocytes were obtained post-mortem from the knee joints of two healthy and two osteoarthritic individuals, and released as described above. Total RNA was extracted and purified using TRIzol reagent (Life Technologies, Merelbeke, Belgium) according to the manufacturer's protocol. Complementary DNA (cDNA) was obtained by reverse transcription of 1 µg of total RNA (Thermoscript; Life Technologies) with oligo(dT)20 as primer.
PCR was performed in a 10-µl volume. cDNA was added to the following PCR mixture: 0.5 U Taq polymerase (Eurogentec, Seraing, Belgium), 0.2 mM dNTPs, 0.5 µM specific primers and 1.5 mM MgCl2. Negative control was RT without enzyme. PCR reactions were carried out in a Perkin Elmer Thermal Cycler 9600 (Applied Biosystems, Lennik, Belgium). After 1 min of denaturation at 95°C, cycles (20 for ß-actin and 32 for NGF) were 10 s at 94°C, 10 s at 60°C and 30 s at 72°C. Cycling was followed by 10 min of elongation at 72°C.
Primers for ß-actin were: sense primer 5'-TGACGGGGTCACCCACACTGTGCCCATCTA-3'; reverse primer 5'-CTAGAAGCATTTGCGGTGGACGATGGAGGG-3'. Primers for NGF were: sense primer 5'-CACACTGAGGTGCATAGCGT-3'; reverse primer 5'-TGATGACCGCTTGCTCCTGT-3'.
cDNA was equalized for the expression of the housekeeping gene ß-actin. PCR products were electrophoresed in 1.5% agarose gel in Tris-borate/EDTA electrophoresis buffer, stained with ethidium bromide, visualized by UV transillumination and analysed by densitometry. Expected sizes of amplification products were 352 bp for NGF and 661 bp for ß-actin (Fig. 2
).

View larger version (7K):
[in this window]
[in a new window]
|
FIG. 2. Nerve growth factor (NGF) and ß-actin mRNA expression in normal (lanes 1 and 2) and osteoarthritic (OA) (lanes 3 and 4) human articular chondrocytes. The negative control is in lane 5. Amplification products were 352 bp for NGF and 661 bp for ß-actin. NGF mRNA expression by chondrocytes is clearly evident.
|
|
Intracellular NGF-ß detection
Freshly isolated chondrocytes were resuspended in PBS containing 0.1% sodium azide and 0.2% bovine serum albumin, and blocked by incubating with 2% normal human serum (Advanced Protein Products, UK). After fixation with paraformaldehyde and permeabilization with saponin (Fix & Perm Cell Permeabilization Kit; Caltag Laboratory, Burlingame, CA), chondrocytes were first incubated (20 min at 4°C) with 5 µl of unconjugated mouse anti-human NGF-ß mAb IgG1 (Sigma), and then washed and incubated with 5 µl of fluorescein-conjugated affinity goat anti-mouse immunoglobulin F(ab')2 fragments (GAM-FITC; Becton Dickinson, Erembodegem-Aalst, Belgium) for 20 min at 4°C. Control samples were incubated with mouse IgG1-FITC/IgG2-PE (DAKO, Glostrup, Denmark) and GAM-FITC alone. Stained cells were analysed on a FACScan (Lysis 2; Becton Dickinson). The FACS setting was identical throughout all the study.
NGF receptor p140 TrkA expression
High-affinity NGFR expression on cell surface was assessed by flow cytometry on non-permeabilized chondrocytes. Cell suspension was first reacted (20 min at 4°C) with 10 µl of unconjugated mouse anti-human TrkA mAb IgG1 (Oncogene Research Products, Cambridge, MA, USA), and then incubated with 5 µl of fluorescein-conjugated affinity goat anti-mouse immunoglobulin F(ab')2 fragments (GAM-FITC; Becton Dickinson) for 20 min at 4°C.
Statistical analysis
Results are shown as mean±1 standard deviation (S.D.). The statistical difference among the distinct specimens of cartilage was assessed using a two-way analysis of variance (ANOVA) with a least square differences (LSD) range test. The significance level was set at P < 0.05.
 |
Results
|
---|
NGF mRNA expression
As shown in detail in Fig. 2
, RTPCR analysis demonstrated that human articular chondrocytes express NGF mRNA. The experiment was performed on chondrocytes from two healthy donors and two osteoarthritic patients.
NGF and TrkA expression
A representative flow cytometry experiment demonstrating the intracellular expression of NGF and surface expression of TrkA by normal chondrocytes and chondrocytes isolated from minimum and maximum damaged zones of the same osteoarthritic cartilage is shown in Fig. 3
. It is clearly evident as NGF and its cell surface receptor expression was low on healthy chondrocytes and progressively increased on osteoarthritic chondrocytes depending on the degree of cartilage injury.

View larger version (21K):
[in this window]
[in a new window]
|
FIG. 3. Fluorescence intensity for intracellular nerve growth factor (NGF) and cell membrane high affinity NGF receptor (trkA) from a healthy cartilage (normal), and from an osteoarthritic cartilage with minimal (MIN) and maximum (MAX) anatomic damage. Negative control histograms (a non-binding monoclonal antibody) are shown on the left. The longitudinal axis shows the percentage of chondrocytes positive for intracellular NGF and trkA, and the horizontal axis the mean channel fluorescence. NGF protein expression is definitely low in normal adult chondrocytes and progressively increases in MIN and MAX osteoarthritic chondrocytes.
|
|
The results of all the experiments and statistical analyses are summarized in Figs 4 and 5
. The percentage of NGF-positive chondrocytes (Fig. 4
) was significantly higher in MAX (41.3±25%) than in either MIN osteoarthritic articular cartilage (21.3±12%; P < 0.01) or in healthy articular cartilage (3.8±2.4%; P < 0.01). A significant difference (P < 0.01) between MIN osteoarthritic chondrocytes and healthy chondrocytes was also found. High affinity NGF receptor (TrkA) (Fig. 4
) was more expressed on MAX osteoarthritic chondrocytes (41.4±19%) than MIN osteoarthritic chondrocytes (31±11%; P < 0.05) or normal chondrocytes (3.4%±1; P < 0.01). Again, TrkA expression was significantly higher on MIN chondrocytes than on normal chondrocytes (P < 0.01).

View larger version (37K):
[in this window]
[in a new window]
|
FIG. 4. Percentage (%) of chondrocytes expressing intracellular nerve growth factor (NGF) and cell membrane high affinity NGF receptor (trkA) in healthy cartilage (HD), in the lowest (MIN) and highest (MAX) damaged zones of osteoarthritic cartilage. Values are expressed as means±1 S.D. (*P < 0.05; ** P < 0.01).
|
|

View larger version (38K):
[in this window]
[in a new window]
|
FIG. 5. Mean intensity of fluorescence (mif) of chondrocytes expressing intracellular nerve growth factor (NGF) and cell membrane high affinity NGF receptor (trkA) in healthy cartilage (HD), in the lowest (MIN) and highest (MAX) damaged zones of osteoarthritic cartilage. Values are expressed as mean±1 S.D. (*P < 0.05; ** P < 0.01).
|
|
An identical pattern of expression was found when NGF and TrkA were evaluated as mif, which is a measure of the density of the antigen per cell (Fig. 5
). Chondrocyte NGF mif was significantly higher in MAX (56±30) than in MIN osteoarthritic cartilage (27.1±14; P<0.01) and healthy chondrocytes (12.8±2.7; P<0.01). A significant difference (P<0.01) between MIN osteoarthritic chondrocytes and healthy chondrocytes was detected. Likewise, TrkA mif was significantly higher on MAX osteoarthritic chondrocytes (59.4±31) than MIN osteoarthritic chondrocytes (41±19, P < 0.01) or normal chondrocytes (10.7±0.4, P < 0.01). Finally, TrkA mif was significantly higher on MIN chondrocytes than on normal chondrocytes (P < 0.01).
 |
Discussion
|
---|
This study demonstrates for the first time that NGF and its high affinity receptor TrkA are expressed by human articular chondrocytes and that their expression increases in osteoarthritic cartilage according to the degree of tissue injury.
In recent years, an increasing body of evidence has shown that articular cartilage is not just a tissue that passively undergoes structural derangement under the pressure of loading [17]. Chondrocytes, the sole cells resident in cartilage, regulate growth and differentiation of cartilage during development, manage the tissue homeostasis by controlling the balance between synthesis and degradation of extracellular matrix (ECM), and direct the osteoarthritic process by producing an array of substances involved in the breakdown of ECM [18]. During OA, chondrocytes undergo a metabolic boost, maybe in an attempt to restore the cartilage homeostasis, and produce high amounts of ECM components, proteases, cytokines and inflammatory mediators [18]. This prompted many researchers to explore the cellular mechanisms modulating chondrocyte metabolism and activating the repair process. The most studied factors that appear to be involved in promoting cartilage repair during OA are IGF-1, bFGF, TGF-ß and BMP family members [19, 20], while NGF has never been studied in these settings.
NGF has been widely investigated as a growth factor for neural cells, especially sympathetic and sensory nerve cells and the cholinergic neurons of CNS [1]. However, NGF has been also detected in cells outside the nervous system. Murine lymphocytes synthesize and secrete NGF [11]. NGF is produced by human keratinocytes and it seems to act as a survival factor by increasing the levels of bcl-2 [21]. Recently, it has been shown that endothelial cells of vessels growing into intervertebral discs of patients with low back pain [22] synthesize NGF and, more interestingly, cells with a phenotype closely related to chondrocytes, such as human fibroblasts, produce NGF [12]. In this paper it has been also shown that human skin and lung fibroblasts express the high affinity receptor p140 TrkA, and that NGF stimulates fibroblast repair functions. The capability of NGF to promote the growth of keratinocyte, fibroblast and endothelial cells has been tested in vivo on chronic vasculitic ulcers [22].
NGF-mediated cellular effects are modulated by at least two receptors with different binding affinity: the low affinity receptor p75 NGFR [2], and the high affinity receptor, a transmembrane tyrosine kinase, p140 TrkA [4]. The exact functions of each receptor and whether they work together is still under investigation; however, it seems to be established that TrkA is essential for functional NGF signal transduction [5].
NGF receptors have also been detected on cells other than neural cells. High affinity TrkA receptor has been detected on activated murine CD4 T cells, while its expression was unremarkable on resting T cells [23]. Low affinity receptor was undetectable on resting and stimulated CD4 T cells, suggesting that only TrkA receptor is involved in NGF signal transduction on activated T cells [23]. Functional NGF receptors have also been detected on human B lymphocytes [24] and monocytes [25], and more recently on human keratinocytes [26] and fibroblasts [12]. In all these conditions, NGF has been always regarded as a possible mediator of the cross-talk between the nervous system and immune system, which has led to the development of the concept of neuro-inflammation. Moreover, NGF protein levels have been found to be increased in some immune diseases such as chronic arthritides [10, 27] and systemic sclerosis [28].
Our discovery of NGF and NGFR on human chondrocytes creates a new perspective on the physiological functions of NGF. Adult articular cartilage is an aneural tissue and chondrocytes are not targets for neurons, therefore chondrocyte autocrine NGF cannot be related to perception of pain or neuro-inflammation, or other nervous system-related functions. It is likely that NGF, as has been reported to happen in other cells [7, 10, 21, 29, 30], can regulate chondrocyte metabolism and maintain cell survival. Like other chondrocyte growth factors [3134], NGF and high affinity NGF receptor increase in osteoarthritic cartilage according to anatomic damage. It is conceivable that NFG is also involved in promoting the cartilage repair process during OA, and possibly interacts with other growth factors as already shown in neural cells [35, 36].
Further studies are warranted to explore the role of NGF in the pathophysiology of human articular cartilage, to examine its capability to regulate chondrocyte metabolism, to modulate cartilage matrix components and to influence collagenase production.
 |
Acknowledgments
|
---|
This work was partially supported by Centro Nazionale Ricerche ITALY (grant no. 97.4164.04).
 |
Notes
|
---|
Correspondence to: G. Lapadula, Cattedra di Reumatologia, Piazza G. Cesare 11, 70124 Policlinico, Bari, ltaly. 
 |
References
|
---|
- Levi-Montalcini R. The nerve growth factor 35 years later. Science1987;237:115462.[ISI][Medline]
- Mallett S, Barclay AN. A new superfamily of cell surface proteins related to the nerve growth factor receptor. Immunol Today1991;12:2203.[ISI][Medline]
- Rabizadeh S, Oh J, Zhong LT et al. Induction of apoptosis by the low-affinity NGF receptor. Science1993;261:3458.[ISI][Medline]
- Kaplan DR, Hempstead BL, Martin-Zanca D, Chao MV, Parada LF. The trk proto-oncogene product: a signal transducing receptor for nerve growth factor. Science1991;252:5548.[ISI][Medline]
- Cordon-Cardo C, Tapley P, Jing SQ et al. The trk tyrosine protein kinase mediates the mitogenic properties of nerve growth factor and neurotrophin-3. Cell1991;66:17383.[ISI][Medline]
- Hempstead BL, Martin-Zanca D, Kaplan DR, Parada LF, Chao MV. High-affinity NGF binding requires coexpression of the trk proto-oncogene and the low-affinity NGF receptor. Nature1991;350:67883.[ISI][Medline]
- Otten U, Ehrhard P, Peck R. Nerve growth factor induces growth and differentiation of human B lymphocytes. Proc Natl Acad Sci USA1989;86:1005963.[Abstract]
- Thorpe LW, Perez-Polo JR. The influence of nerve growth factor on the in vitro proliferative response of rat spleen lymphocytes. J Neurosci Res1987;18:1349.[ISI][Medline]
- Thorpe LW, Werrbach-Perez K, Perez-Polo JR. Effects of nerve growth factor on the expression of interleukin-2 receptors on cultured human lymphocytes. Ann NY Acad Sci1987;496:3101.[ISI][Medline]
- Falcini F, Matucci CM, Lombardi A et al. Increased circulating nerve growth factor is directly correlated with disease activity in juvenile chronic arthritis. Ann Rheum Dis1996;55:7458.[Abstract]
- Santambrogio L, Benedetti M, Chao MV et al. Nerve growth factor production by lymphocytes. J Immunol1994;153:448895.[Abstract/Free Full Text]
- Micera A, Vigneti E, Pickholtz D et al. Nerve growth factor displays stimulatory effects on human skin and lung fibroblasts, demonstrating a direct role for this factor in tissue repair. Proc Natl Acad Sci USA2001;98:61627.[Abstract/Free Full Text]
- Benya PD, Shaffer JD. Dedifferentiated chondrocytes reexpress the differentiated collagen phenotype when cultured in agarose gels. Cell1982;30:21524.[ISI][Medline]
- Iannone F, Lapadula G. Neuropeptides and human osteoarthritis. J Rheumatol1998;25:3868.[ISI][Medline]
- Mankin HJ, Dorfman H, Lippiello L, Zarins A. Biochemical and metabolic abnormalities in articular cartilage from osteo-arthritic human hips. II. Correlation of morphology with biochemical and metabolic data. J Bone Joint Surg Am1971;53:52337.[Medline]
- Lapadula G, Iannone F, Zuccaro C et al. Expression of membrane-bound peptidases (CD10 and CD26) on human articular chondrocytes. Possible role of neuropeptidases in the pathogenesis of osteoarthritis. Clin Exp Rheumatol1995;13:1438.[ISI][Medline]
- Henrotin Y, Reginster JY. Anabolic events in osteoarthritis. Osteoarthritis Cartilage1999;7:3102.[ISI][Medline]
- Goldring MB. The role of the chondrocyte in osteoarthritis. Arthritis Rheum2000;43:191626.[ISI][Medline]
- Erlacher L, Ng CK, Ullrich R, Krieger S, Luyten FP. Presence of cartilage-derived morphogenetic proteins in articular cartilage and enhancement of matrix replacement in vitro. Arthritis Rheum1998;41:26373.[ISI][Medline]
- Trippel SB. Growth factor actions on articular cartilage. J Rheumatol Suppl1995;43:12932.[Medline]
- Pincelli C, Haake AR, Benassi L et al. Autocrine nerve growth factor protects human keratinocytes from apoptosis through its high affinity receptor (TRK): a role for BCL-2. J Invest Dermatol1997;109:75764.[Abstract]
- Tuveri M, Generini S, Matucci-Cerinic M, Aloe L. NGF, a useful tool in the treatment of chronic vasculitic ulcers in rheumatoid arthritis. Lancet2000;356:173940.[ISI][Medline]
- Ehrhard PB, Erb P, Graumann U, Otten U. Expression of nerve growth factor and nerve growth factor receptor tyrosine kinase Trk in activated CD4-positive T-cell clones. Proc Natl Acad Sci USA1993;90:109848.[Abstract]
- Brodie C, Gelfand EW. Functional nerve growth factor receptors on human B lymphocytes. Interaction with IL-2. J Immunol1992;148:34927.[Abstract/Free Full Text]
- Caroleo MC, Costa N, Bracci-Laudiero L, Aloe L. Human monocyte/macrophages activate by exposure to LPS overexpress NGF and NGF receptors. J Neuroimmunol2001;113:193201.[ISI][Medline]
- Di Marco E, Mathor M, Bondanza S et al. Nerve growth factor binds to normal human keratinocytes through high and low affinity receptors and stimulates their growth by a novel autocrine loop. J Biol Chem1993;268:2283846.[Abstract/Free Full Text]
- Aloe L, Manni L, Sebastiani G, Tuveri MA. Nerve growth factor in the synovia of patients with rheumatoid arthritis: correlation with TNF-
and IL-1 ß and possible functional significance. Clin Exp Rheumatol1999;17:6323.[ISI][Medline]
- Matucci-Cerinic M, Giacomelli R, Pignone A et al. Nerve growth factor and neuropeptides circulating levels in systemic sclerosis (scleroderma). Ann Rheum Dis2001;60:48794.[Abstract/Free Full Text]
- Pitchford S, De Moor K, Glaeser BS. Nerve growth factor stimulates rapid metabolic responses in PC12 cells. Am J Physiol1995;268:C93643.[Abstract/Free Full Text]
- Thorpe LW, Jerrells TR, Perez-Polo JR. Mechanisms of lymphocyte activation by nerve growth factor. Ann NY Acad Sci1990;594:7884.[ISI][Medline]
- Pfander D, Cramer T, Weseloh G et al. Hepatocyte growth factor in human osteoarthritic cartilage. Osteoarthritis Cartilage1999;7:54859.[ISI][Medline]
- Lafeber FP, van Roy HL, van der Kraan PM, van den Berg WB, Bijlsma JW. Transforming growth factor-beta predominantly stimulates phenotypically changed chondrocytes in osteoarthritic human cartilage. J Rheumatol1997;24:53642.[ISI][Medline]
- Middleton J, Manthey A, Tyler J. Insulin-like growth factor (IGF) receptor, IGF-I, interleukin-1 beta (IL-1 ß), and IL-6 mRNA expression in osteoarthritic and normal human cartilage. J Histochem Cytochem1996;44:13341.[Abstract/Free Full Text]
- Iannone F, De Bari C, Dell'Accio F et al. Interleukin-10 and interleukin-10 receptor in human osteoarthritic and healthy chondrocytes. Clin Exp Rheumatol2001;19:13945.[ISI][Medline]
- Cosgaya JM, Aranda A. Nerve growth factor regulates transforming growth factor-beta 1 gene expression by both transcriptional and posttranscriptional mechanisms in PC12 cells. J Neurochem1995;65:248490.[ISI][Medline]
- Lindholm D, Hengerer B, Zafra F, Thoenen H. Transforming growth factor-beta 1 stimulates expression of nerve growth factor in the rat CNS. Neuroreport1990;1:912.[Medline]
Submitted 22 October 2001;
Accepted 20 May 2002