The present invention relates to a method for treating or preventing osteoarthritis (OA).
Osteoarthritis (OA) is a multifactorial disease characterized by degradation of the extracellular matrix and destruction of articular cartilage. Because chondrocytes are the only resident cells in human articular cartilage, on which cells matrix turn over in cartilage is solely dependent. Thus, the death of articular chondrocytes is generally considered to plays a central role in OA cartilage destruction. To date, stimuli involved in chondrocyte death and their signaling pathways have been in the spotlight as pathogenetic factors leading to joint cartilage degradation.
OA is now considered more complex disease with different clinical subtypes. Among these subtypes, metabolic OA is distinguished from different subtypes by the presence of obesity or metabolic syndrome and low-grade systemic inflammation and the earlier onset and a faster progression. The concept of “wear and tear disease”, which is traditionally accepted for the pathophysiology of OA does not seem to account for the cartilage destruction in metabolic OA. Not only joint overload is unable to explain strong epidemiological data which have shown the association between obesity and hand OA, but obese patients with metabolic syndrome have increased risk of knee OA compared to obese patients without metabolic syndrome. Thus, systemic factors must be involved in the pathogenesis of OA. Recent studies led to the discovery of proinflammatory cytokines and adipokines produced by the adipose tissue as central contributors of metabolic OA of the hand but potentially other locations.
Lipid imbalance is a key metabolic alteration associated with metabolic syndrome and obesity. In hyperlipidemic states, lipids abnormally accumulate in no-adipose tissues. Articular chondrocytes unlike most other cells is characterized as having substantial stores of lipid deposits. A previous study demonstrated the existence of a marked and graded increase in total fatty acids in articular cartilage from osteoarthritic joints. In hyperlipidemic states, accumulation of excess lipid in non-adipose tissue, exerts lipotoxicity, leading to cell dysfunction and/or cell death. Free fatty acids (FFAs), which are elevated associated with metabolic syndrome or obesity, have been considering the principal offender exerting lipotoxicity, inducing the apoptosis, the insulin resistance and inflammation. Thus, it would have been readily presumable that the accumulation of FFA contributes to OA pathogenesis. However, the causal relationship between FFA and OA pathogenesis was recently demonstrated. A pioneering study demonstrated that palmitate but not oleate has proapoptotic effect on interleukin 1 beta (IL-1-β)-stimulated articular chondrocytes, suggesting that elevated levels of saturated FFA may contribute to OA pathogenesis.
The present invention is directed to providing a method for treating or preventing osteoarthritis in a subject, comprising promoting the expression of STAMP2 (Six-transmembrane protein of prostate 2) in chondrocyte in chondrocyte in the subject.
Also, the present invention is directed to providing a method for diagnosing osteoarthritis measuring the expression level of STAMP2 or the amount of STAMP2 protein.
Also, the present invention is directed to providing a method for screening a therapeutic agent for osteoarthritis.
However, the problems to be solved according to the present invention are not limited to the above-described problems, and other problems which are not disclosed herein may be made apparent to those skilled in the art by the detailed description provided below.
This invention discloses a method for treating or preventing osteoarthritis in a subject, comprising promoting the expression of STAMP2 (Six-transmembrane protein of prostate 2) in chondrocyte in chondrocyte in the subject.
In some embodiments of the present invention, the promoting the expression of STAMP2 is by administering cilostazol or TNF-α, to a subject.
In some embodiments of the present invention, the administration is an oral or intravenous administration.
In some embodiments of the present invention, the promoting the expression of STAMP2 is by administering a vehicle, into which a gene encoding STAMP2 is introduced, to a subject.
In some embodiments of the present invention, the vehicle is an adenovirus.
In some embodiments of the present invention, the administration is intravenous administration.
This invention discloses a method for diagnosing osteoarthritis, comprising following steps: a) measuring an expression level of STAMP2 or an amount of STAMP2 protein in chondrocyte from the subject; and b) comparing the measured results in step a) with that in a control sample.
In some embodiments of the present invention, the expression level of STAMP2 is measured by any one selected from the group consisting of RT-PCR, Competitive RT-PCR, Realtime RT-PCR, RPA (RNaseprotection assay), Northern blotting and DNA chip.
In some embodiments of the present invention, the amount of STAMP2 protein is measured by any one selected from the group consisting of Western blot, ELISA (enzyme linked immunosorbent assay), RIA (radioimmunoassay), radioimmunodiffusion, Ouchterlony immunodiffusion, rocket immunoelectrophoresis, INC. (immunohistochemistry), Immunoprecipitation assay, complement fixation assay, FACS (fluorescence activated cell sorter) and protein chip.
This invention discloses a method for screening a therapeutic agent for osteoarthritis, comprising following steps: a) treating chondrocyte with a candidate agent; b) measuring an expression level of STAMP2 or an amount of STAMP2 protein in the chondrocyte treated with the candidate agent; and c) identifying a candidate agent as the therapeutic agent when the expression level of STAMP2 or the amount of STAMP2 protein measured in step b) is increased in the chondrocyte as compared to the chondrocyte before treatment with the candidate agent.
Exemplary embodiments of the present invention will be described in detail below with reference to the accompanying drawings. While the present invention is shown and described in connection with exemplary embodiments thereof, it will be apparent to those skilled in the art that various modifications can be made without departing from the scope of the invention.
Unless specifically stated otherwise, all the technical and scientific terms used in this specification have the same meanings as what are generally understood by a person skilled in the related art to which the present invention belongs. In general, the nomenclatures used in this specification and the experimental methods described below are widely known and generally used in the related art.
The present invention provides a method for treating or preventing osteoarthritis in a subject, comprising promoting the expression of STAMP2 (Six-transmembrane protein of prostate 2) in chondrocyte.
By promoting the expression of STAMP2 in the chondrocyte, the method of the present invention may be used to protect articular chondrocytes against FFAs-induced lipoapotosis, thereby preventing or treating osteoarthritis. And PKCK2/STAMP2/FSP27-mediated LD accumulation in the articular chondrocytes protects chondrocyte against lipotoxicity.
In the present invention, PKCK2 participates in a series of complex cellular functions, including cell growth and proliferation, by catalyzing the phosphorylation of a large number of proteins. CK2 involves in FFAs-induced lipotoxicity or LD accumulation and that cilostazol prevents FFAs-induced lipotoxicity.
In the present invention, STAMP2 plays a pivotal role in lipid homeostasis and dysregulation of STAMP2 was implicated in metabolic and inflammatory diseases. STAMP2 protect articular chondrocytes against FFAs-induced lipoapotosis in metabolic OA.
In the present invention, FSP27, also known as CIDEC, belongs to cell death-inducing DNA fragmentation factor 45 (DFF45)-like effector (CIDE) family of proteins. FSP27 is involved in protecting articular chondrocytes against FFAs-induced lipoapotosis in metabolic OA through LD accumulation.
In some embodiments of the present invention, the method of the present invention includes promoting the expression of STAMP2 by administering cilostazol, TNF-α, or a vehicle into which a gene encoding STAMP2 is introduced, to a subject.
In the present invention, the vehicle is an adeno-associated virus, a retrovirus, a lentivirus, a herpes simplex virus, an alpha virus, etc., and preferably an adenovirus.
In the present invention, “administering” may be used without limitation as long as the composition according to one exemplary embodiment of the present invention can reach target tissues. For example, the administration method encompasses oral administration, intraarterial injection, intravenous injection, percutaneous injection, intranasal administration, transbronchial administration, or intramuscular administration.
Also, the present invention provides a method for treating a subject having osteoarthritis by administering an effective amount of a vehicle, which a gene encoding STAMP2 is introduced.
In yet another embodiment of the present invention, the pharmaceutical composition of the present invention may include the vehicle into which a gene encoding STAMP2 is introduced.
The term “subject” refers to an animal, preferably a mammal, and most preferably a human, who is the object of treatment, observation or experiment. The mammal may be selected from the group consisting of mice, rats, hamsters, gerbils, rabbits, guinea pigs, dogs, cats, sheep, goats, cows, horses, giraffes, platypuses, primates, such as monkeys, chimpanzees, and apes. In some embodiments, the subject is a human.
The term “effective amount” of a compound refers to a sufficient amount of the compound that provides a desired effect but with no- or acceptable-toxicity. This amount may vary from subject to subject, depending on the species, age, and physical condition of the subject, the severity of the disease that is being treated, the particular compound used, its mode of administration, and the like. A suitable effective amount may be determined by one of ordinary skill in the art.
The gene encoding STAMP2 is a gene encoding STAMP2 derived from the subject. In this case, genes known in the related art may be used as the gene encoding STAMP2.
Vehicles known in the related art may be used without limitation as the vehicle delivering the gene encoding STAMP2, and may, for example, include a liposome, a plasmid vector, a cosmid vector, a bacteriophage vector, a viral vector, etc., and preferably a viral vector.
Specific examples of the viral vector may include an adenovirus, an adeno-associated virus, a retrovirus, a lentivirus, a herpes simplex virus, an alpha virus, etc., and preferably an adenovirus.
In some embodiments, the vehicle into which the gene encoding STAMP2 is introduced described herein is administered systemically. As used herein, “systemic administration” refers to any means by which the compounds described herein can be made systemically available. In some embodiments, systemic administration encompasses intravenous administration, intraperitoneal administration, intramuscular administration, intracoronary administration, intraarterial administration (e.g., into a carotid artery), intradermal administration, subcutaneous administration, transdermal delivery, intratracheal administration, subcutaneous administration, intraarticular administration, intraventricular administration, inhalation (e. g., aerosol), intracerebral, nasal, naval, oral, intraocular, pulmonary administration, impregnation of a catheter, by suppository and direct injection into a tissue, or systemically absorbed topical or mucosal administration. Mucosal administration includes administration to the respiratory tissue, e.g., by inhalation, nasal drops, ocular drop, etc.; anal or vaginal routes of administration, e.g., by suppositories; and the like. In some embodiments, the compounds described herein are administered intravenously.
The dosage may be adjusted according to factors such as a formulation method, a mode of administration, the age, weight and sex of a subject, a degree of severity of a disease, a diet, an administration time, a route of administration, a secretion rate, and the susceptibility to response. According to one embodiment in which the viral vector is used, the viral vector may be intravenously administered at a dose of 1×108 to 1×1011 plaque-forming units (pfus).
Pharmaceutical formulations suitable for use with the present invention may also include excipients, preservatives, pharmaceutically acceptable carriers and combinations thereof the term “pharmaceutically acceptable carrier or excipient” means a carrier or excipient that is useful in preparing a pharmaceutical composition that is generally safe, non-toxic and neither biologically nor otherwise undesirable, and includes a carrier or excipient that is acceptable for veterinary use as well as human pharmaceutical use. A “pharmaceutically acceptable carrier or excipient” as used in the specification and claims includes both one and more than one such carrier or excipient.
Examples of suitable excipients include, but are not limited to, lactose, dextrose, sucrose, sorbitol, mannitol, starches, gum acacia, calcium phosphate, alginates, tragacanth, gelatin, calcium silicate, microcrystalline cellulose, polyvinylpyrrolidone, cellulose, water, saline, syrup, methylcellulose, ethylcellulose, hydroxypropylmethylcellulose, and polyacrylic acids such as Carbopols. The compositions can additionally include lubricating agents such as talc, magnesium stearate, and mineral oil; wetting agents; emulsifying agents; suspending agents; preserving agents such as methyl-, ethyl-, and propyl-hydroxy-benzoates; pH adjusting agents such as inorganic and organic acids and bases; sweetening agents; and flavoring agents.
Also, the present invention provides a method for diagnosing osteoarthritis, comprising following steps: a) measuring an expression level of STAMP2 or an amount of STAMP2 protein in chondrocyte from the subject; and b) comparing the measured results in step a) with that in a control sample.
In the present invention, the expression level of STAMP2 is measured using any one selected from the group consisting of RT-PCR, Competitive RT-PCR, Realtime RT-PCR, RPA (RNaseprotection assay), Northern blotting and DNA chip.
In the present invention, the amount of STAMP2 protein is measured using any one selected from the group consisting of Western blot, ELISA (enzyme linked immunosorbent assay), RIA (radioimmunoassay), radioimmunodiffusion, Ouchterlony immunodiffusion, rocket immunoelectrophoresis, IHC (immunohistochemistry), Immunoprecipitation assay, Complement fixation assay, FACS (fluorescence activated cell sorter) and protein chip.
Also, the present invention provides a method for screening a therapeutic agent for osteoarthritis, comprising following steps: a) treating chondrocyte with a candidate agent; b) measuring an expression level of STAMP2 or an amount of STAMP2 protein in the chondrocyte treated with the candidate agent; and c) identifying a candidate agent as the therapeutic agent when the expression level of STAMP2 or the amount of STAMP2 protein measured in step b) is increased in the chondrocyte as compared to the chondrocyte before treatment with the candidate agent.
Hereinafter, preferred Examples are provided to aid in understanding the present invention. However, it should be understood that detailed description provided herein is merely intended to provide a better understanding of the present invention and is not intended to limit the scope of the present invention.
1: Preparation and Method
1-1. Animals
All procedures for animal study were approved by the Committee on Animal Investigations at Dong-A University (DIACUC-15-12). Seven-week-old male C57BL/6 mice were obtained from Samtoko (Osan, Korea). Mice were maintained in a temperature-controlled room (22° C.) on a 12:12 h light-dark cycle and free access to water and food.
1-2. Reagents
All The following reagents were obtained commercially: goat polyclonal anti-human CKIIα and HRP-conjugated donkey anti-goat IgGs antibodies from Santa Cruz Biotechnology (Santa Cruz, Calif., USA); rabbit polyclonal anti-human caspase-3 and -7 antibodies from Cell Signaling (Danvers, Mass., USA); rabbit polyclonal anti-human STAMP2 antibody from Proteintech (Rosemont, Ill., USA); mouse monoclonal anti-FSP27 antibody from Abcam (Cambridge, Mass., USA); HRP-conjugated donkey anti-rabbit and sheep anti-mouse IgGs from Amersham Pharmacia Biotech (Piscataway, N.J., USA); Ketamine hydrochloride from Sanofi-Ceva (Dusseldorf, Germany); Rompun from Bayer (Leverkusen, Germany); Dulbecco's modified Eagle's medium (DMEM) and fetal bovine serum (FBS) from Gibco BRL (Gaithersburg, Md., USA); TNF-α and ApopTag FITC In Situ Apoptosis Detection Kit from Millipore (Temecular, CA); mouse monoclonal anti-human actin antibody, Hoechst 33342, dimethylsulfoxide (DMSO), RNase A, proteinase K, aprotinin, leupeptin, propidium iodide (PI), phenylmethylsulfonyl fluoride (PMSF), protein-A agarose and 5,6-dichlorobenzimidazol riboside (DRB), fatty acid-free bovine serum albumin (BSA), Oil red 0, palmitate, oleate, stearic acid (SA), mono-iodoacetate (MIA), 3,3′-diaminobenzidine (DAB), cilostazol and type II collagenase from Sigma (St. Louis, Mo., USA); caspase inhibitor I (zVAD-fmk) from Calbiochem (San Diego, Calif., USA); 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethylbenzimidazol carbocyanine iodide (JC-1), Nile red and BODIPY 493/503 from Molecular Probes (Eugene, Oreg., USA); SuperSignal WestPico enhanced chemiluminescence western blotting detection reagent from Pierce (Rockford, Ill., USA).
1-3. Osteoarthritis (OA) Models
Mice subjected to surgery for experimental OA. Sixty mice were used. After a normal chow diet for one week, half of mice were fed HFD and the other half SD. Mouse fed an HFD or a SD for 8 weeks was subjected to surgery for experimental OA. Mice were anesthetized with ketamine hydrochloride (15 mg/kg) and Rompun (3.45 mg/kg) and knees were prepared for aseptic surgery. The skin incision for cutting of anterior cruciate ligament was a 5 mm longitudinal incision over the distal patella to proximal tibial plateau. The joint capsule, which site was located medial to the patellar tendon, was incised with a #15 blade and the joint capsule opened with micro-iris scissors. Blunt dissection of the fat pad over the intercondylar area was performed to expose either the intercondylar region, providing visualization of the anterior cruciate ligament. The anterior cruciate ligament was transected with a micro-surgical knife under direct visualization and complete transection confirmed by the presence of anterior drawer. After anesthesia was released, mice had excellent mobility within 2 h after either surgery. At 4, 6 and 8 weeks after surgery, ten mice from each diet were used for histological evaluation.
HFD-induced OA mouse model. Sixty mice were fed a normal chow diet for one week and then mice were fed an HFD or a SD for 25 weeks. Ten mice from each diet were used histologically examine the effect of high fat diet. Forty mice were used to examine the effect of cilostazol. Mice fed a SD or HFD for 15 weeks were orally administered cilostazol at concentrations of 30 mg/kg/day for additional 10 weeks. SD+vehicle mice (n=10) were fed a SD and received DMSO. SD+cilostazol mice (n=10) were fed a SD and received cilostazol. HFD+vehicle mice (n=10) were fed a HFD and received DMSO. HFD+cilostazol mice (n=10) were fed a HFD and received cilostazol.
1-4. Tissue Preparation and Histologic Examination
Tissue preparation and histologic examination. Animals were killed by ether inhalation. Whole knee joints were removed by dissection, fixed in in PBS (pH 7.4) containing 4% paraformaldehyde, decalcified in 12.5% EDTA, dehydrated, and embedded in paraffin blocks. Five-micrometer microsections were prepared and stained with hematoxylin and eosin and with Safranin O-fast green.
Immunohistochemical observation and analysis. Tissue sections were incubated in 1:70-diluted goat serum solution for 30 minutes at room temperature, and then for 2 hours with 1:100-diluted primary antibody at room temperature. Next, sections were incubated with secondary antibody for 1 hour at 37° C. and developed using ABC complex. Peroxidase was revealed by DAB and examined by light microscopy. Histological images were observed and analyzed using a Aperio ScanScope® CS system. Total numbers of positive for PKCK2, FSP27 and STAMP2 in four fields per animal were counted by an observer blinded to the experiment and the percentage of positive cells were calculated.
1-5. Cell Culture of Articular Chondrocytes
Rat articular chondrocytes for primary culture were isolated from knee joint cartilage slices by enzymatic digestion for 1 h with 0.2% type II collagenase (381 units/mg) in DMEM. After the isolated cells were collected by brief centrifugation, they were resuspended in DMEM supplemented with 10% (v/v) FBS, 50 mg/ml streptomycin and 50 units/ml penicillin (Gibco). The cells were plated on culture dishes at a density of 5×104 cells/cm2. The medium was replaced every 2 days, and they reached confluence after approximately 5 days in culture. In each experiment, the cells from three animals were pooled and analyzed three times.
1-6. Treatment of Free Fatty Acids or Combination Treatment with Other Chemicals.
FFA were dissolved in absolute ethanol at a concentration of 500 mM and diluted to final concentrations of FFAs with the appropriate concentration of 1% (w/v) FFA-free BSA. Controls were incubated with equal concentrations of FFA-free BSA containing ethanol. To examine the effect of several chemicals, cells were pretreated with 150 μg/mL DRB or 30 μM cilostazol for 24 h or 25 ng/ml TNF-α for 3 h before FFA treatment.
1-7. siRNA
Rat STAMP2 and FSP27 siRNA. Rat STAMP2 siRNA (SMART pool; L-105419-02-0050) and FSP27 siRNA (SMART pool; L-105647-02-0050) were purchased from Thermo Scientific (Hudson, N.H., USA). As a negative control, the same nucleotides were scrambled to form nongenomic combinations.
siRNA transfection or combination treatment with FFAs. Transfection of siRNA was performed with the use of siPORT Amine and Opti-MEM media. Cells grown to a confluence of 40% to 50% in six-well plates were transfected with 100 nM final siRNA concentration per well. Transfection mixture was added to each well, and incubation occurred for 4 h. Then 2 ml of growth medium was added, and cells were incubated for another 20 h. After siRNA transfection medium was removed, each well was washed in PBS solution. Cells were treated with FFAs for 24 hours.
1-8. Recombinant Adenoviral STAMP2 Infection
Recombinant adenoviral STAMP2 was prepared as described previously34. Cells (1×107) were infected with recombinant adenoviral STAMP2 at multiplicities of infection (MOI) of 500, 1000 and 1500 to the medium.
1-9. Staining of Lipid Droplets, Confocal Microscopy and Quantification
Cells cultured on a coverslip were incubated with diluted Nile red or BODIPY 493/503. Some sections were double-labelled with TUNEL and BODIPY 493/503 and counterstained with Hoechst 33342. Fluorescence images were observed and analyzed using a Zeiss LSM 510 laser-scanning confocal microscope (Goettingen, Germany). Twenty cells from each experiment were observed and auantification of fluorescence intensity of the confocal images were obtained with the use of ImageJ software. Fluorescence intensity was expressed as AU.
1-10. Oil Red O Staining
Cells were washed twice in PBS and fixed for 1 h with 10% (w/v) formaldehyde in PBS. After two washes in 60% isopropyl alcohol, the cells were stained for 30 min in freshly diluted Oil Red O solution. Then, the stain was removed, and the cells were washed four times in water.
1-11. Total Cytosol FFA Content Measurement
Cell were collected after treatment with trypsin (0.2% trypsin, 0.02% EDTA and 0.2% glucose in PBS) and pelleted by centrifugation (200×g for 5 min at 4° C.). Cell pellet was resuspended in ice-cold hypotonic lysis medium containing 20 mM Tris-HCl, pH 7.4 and 1 mM EDTA. Cells were homogenized with a Dounce homogenizer and then centrifuged (800×g for 5 min at 4° C.). The post-nuclear supernatant fraction was ultra-centrifuged (800×g for 5 min at 4° C.) using Beckman Table-top ultracentrifuge. LD fraction with the distinct white band on the preparation was removed with a pipetman and LD-free cytosol was used for FFA measurement. Cytosol FFA levels were measured using a commercial free fatty acid quantification kit from Abcam (ab65341).
1-12. Cell Viability Assay
An automated trypan blue exclusion assay was undertaken. Total cells and trypan bluestained (i.e., nonviable) cells were counted, and the percentage of nonviable cells was calculated using the Vi-Cell cell counter (Beckman Coulter, Miami, Fla., USA).
1-13. Nuclear Morphology Study for Apoptosis
Cells were harvested and then washed with PBS. They were fixed in 4% paraformaldehyde for 20 min at room temperature. The cells were washed with PBS twice, and stained in 4 μg/ml Hoechst 33342 for 1 h at 37° C. Stained cells were coated onto clean, lipid-free glass slides and mounted with a cover glass. The samples were observed and photographed under an epifluorescence microscope (Axiophot, Zeiss, Germany).
1-14. Quantification of DNA Hypoploidy and Cell Cycle Phase Analysis by Flow Cytometry
Cells were washed twice with PBS, and fixed with cold 70% ethanol at 4° C. overnight. The fixed cells were pelleted and ethanol was removed by washing twice with PBS containing 1% bovine serum albumin (BSA). The cells were resuspended in 1 ml of PBS containing 11 Kunitz U/ml RNase A, incubated at 4° C. for 30 min and washed once with BSAPBS. Cells were resuspended in PI solution (50 μg/ml) and incubated at 37° C. for 30 min in dark. Cells were washed with PBS, the DNA content of 10,000 cells was used for the generation of simultaneous estimation of the cell cycle parameters and apoptosis using an Epics XL (Beckman Coulter, FL).
1-15. Western Blot Analysis
Cell (2×106) were washed twice with ice-cold PBS. Cells were resuspended in lysis buffer [200 ll of icecold solubilizing buffer (300 mM NaCl, 50 mM Tris-Cl (pH 7.6), 0.5% Triton X-100, protease inhibitor cocktail)] and incubated at 4° C. for 30 min. The lysates were centrifuged at 14,000 rpm for 20 min at 4° C. The protein concentrations of the cell lysates were measured with the Bradford protein assay reagent (Bio-Rad). Then, 50 μg of proteins was loaded onto 15 SDS-PAGE. The separated proteins were transferred to nitrocellulose membranes (Amersham Pharmacia Biotech, Piscataway, N.J., USA) and probed with each antibody. Immunostaining with the antibodies was carried out using the Super Signal West Pico enhanced chemiluminescence substrate and detected with LAS-3000PLUS.
1-16. Assay of Mitochondrial Membrane Potential (MMP)
Disruption of mitochondrial membrane potential (MMP) was measured using a specific fluorescent probe, JC-1, that was added directly to the cell culture medium (5 μg/mL final concentration) and incubated for 15 minutes at 37° C. Cells were stained with JC-1, and flow cytometry to measure MMP was performed (an Epics XL; Beckman Coulter). Data were acquired and analyzed using EXP032 ADC XL 4 color software.
1-17. TUNEL Staining of Cell Suspensions
Cell suspensions were cytospun onto clean fat-free glass slides in a cytocentrifuge. After being fixed with 4% paraformaldehyde, the cells were incubated with terminal deoxynucleotidyl transferase (TdT) enzyme for 1 h at 37° C., and antidigoxigenin-FITC was applied for 30 min at room temperature. Afterward the cells were incubated with a PI primary antibody for 1 h at 37° C. Nuclei were counterstained with DRAQ5™. Fluorescent images were observed and analyzed using a Zeiss LSM 510 laser-scanning confocal microscope.
1-18. Statistics
Four independent experiments were carried out in vitro. The results are expressed as means S.D. from four experiments, each performed in triplicate. The results of the experimental and control groups were tested for statistical significance by the Kruskal-Wallis nonparametric test. To test a statistical significance for a difference in the occurrences of the onset of OA between HFD and SD, we first made two-by-three contingency table where HFD/SD is in the row and 4/6/8 weeks are in the column and Pearson's χ̂2 test was conducted to analyze an association between diet and time. In order to test the significance of an individual variable, we next fitted a Poisson regression model where the occurrences of the onset of OA among 10 mice is on the response and both two different types of diet (HFD/SD) and three different weeks (4/6/8) are on the predictors. The likelihood ratio test for a diet variable was conducted. For the comparison between HFD and SD for 25 weeks without surgery, the Chi-squared test for the same probabilities of two groups was conducted (P<0.01).
2: Results
2-1. HFD Accelerates the Onset of OA.
Using two experimental mouse OA models, we examined whether a high-fat diet (HFD) accelerated the onset of OA. The onset of OA was determined by histological findings characteristic of OA, such as an irregular surface, the disappearance of surface layer cells, and reduced Safranin O staining. First, mice fed a HFD or a standard diet (SD) for 12 weeks were subjected to surgery for experimental OA, and after the indicated time, cartilages were histologically observed. We could observe the onset of OA 4 weeks (6/10) or at least 6 weeks (10/10) after surgery in mice fed a HFD. In contrast, these findings were not observed in any mouse fed a SD 4 weeks after surgery, while these findings were observed at 6 weeks (3/10) and 8 weeks (10/10) after surgery in mice fed a SD (
2-2. Palmitate but not Oleate at Usual Clinical Ranges Exerts Lipotoxicity, while Oleate at High Pathological Ranges Exerts Lipotoxicity in Rat Articular Chondrocytes Through Apoptosis.
We next examined whether FFAs exert lipotoxicity in rat articular chondrocytes. Although several studies have suggested that there is an increased FFA concentration in OA synovial fluid, no documented studies have reported the FFA levels in the synovial fluid of OA subjects. Although FFAs have been extensively used at concentrations of 50 to 750 μM in experimental studies, the FFA levels in plasma are usually only 0.2 to 2 mM. Thus, in the present study, we screened the effect of FFAs at 0.1 to 2 mM on rat articular chondrocyte viability. We observed that palmitate or stearate at 0.5 to 2 mM reduced the viability of rat articular chondrocytes. While treatment at 0.5 to 1.25 mM did not reduce cell viability, oleate at 1.5 to 2.0 mM significantly reduced the viability of rat articular chondrocytes (
2-3. LD Accumulation Through FSP27 is Associated with the Resistance of Articular Chondrocytes to Oleate-Caused Lipotoxicity.
We analysed cells stained with BODIPY 493/503 using flow cytometry and observed an increase in size and granularity after 1 to 1.25 mM oleate treatment, which indicate the accumulation of neutral lipids. Noticeably, the size and granularity were decreased after 1.5 mM oleate treatment. Because these findings suggest that LD accumulation is associated with resistance to lipotoxicity, further studies focused on LD accumulation. Confocal microscopy demonstrated that giant LDs formed in chondrocytes treated with nontoxic concentrations of oleate. Importantly, toxic concentrations of oleate reduced the size of LDs and the total LD volume (
2-4. PKCK2 Inhibition Prohibits Oleate-Induced LD Accumulation, Resulting in Chondrocytes Death.
Elevated FFAs should act synergistically with destructive stimuli in the pathogenesis of OA. Because a previous study reported that FFAs augment chondrocyte death through IL-1-β, we further examined whether FFAs augment chondrocyte death by another representative stimulus known to induce articular chondrocytes, protein kinase casein kinase 2 (PKCK2). Our viability assay revealed that all types of FFA tested sensitized chondrocytes to cell death caused by 5,6-dichlorobenzimidazol riboside (DRB), a PKCK2 inhibitor (
2-5. STAMP2 Confers Articular Chondrocytes the Resistance to Oleate-Caused Lipotoxicity Through Maintaining LD Accumulation.
We observed for the first time that six transmembrane protein of prostate 2 (STAMP2), which plays a pivotal role in lipid homeostasis, is substantially expressed in rat articular chondrocytes in vitro and in vivo (
2-6. PKCK2/STAMP2/FSP27 Axis Confers Articular Chondrocytes the Resistance to Lipotoxicity.
We next examined the hierarchical regulation of the resistance to lipotoxicity. We first observed that PKCK2 inhibition decreased the expression levels of FSP27 and STAMP2 in vitro (
2-7. Articular Chondrocytes Co-Incubated with Palmitate and Oleate Also Survive Through PKCK2/STAMP2/FSP27-Mediated LD Accumulation.
Under physiological conditions, the FFA pool contains different saturated and unsaturated species that influence each other. Palmitate and oleate are two of the most common fatty acids in articular chondrocytes. Thus, using several combinations of palmitate and oleate, we examined whether articular chondrocytes gain resistance to lipotoxicity through LD accumulation. We observed that 0.2 and 0.4 mM oleate supplementation markedly suppressed 0.4-1.8 mM palmitate-induced lipotoxicity (
2-8. Increase of Cytosolic FFAs is Correlated with Exertion of Lipotoxicity in Articular Chondrocytes.
We next examined whether cytosolic FFAs, which is not incorporated in LD, is increased in articular chondrocytes which succumb to lipotoxicity. We measured total cytosolic FFA content in articular chondrocytes after various combination treatments. Notably, we observed that concentrations of cellular FFAs were significantly higher in articular chondrocytes which succumb to lipotoxicity, compared to articular chondrocytes surviving FFAs-induced lipotoxicity (
It will be apparent to those skilled in the art that various modifications can be made to the above-described exemplary embodiments of the present invention without departing from the scope of the invention. Thus, it is intended that the present invention covers all such modifications provided they come within the scope of the appended claims and their equivalents.