Expression of functional mu-opioid receptors in human osteoarthritic cartilage and chondrocytes

Reproductive and developmental toxicity assessments of LPM3480392, a novel mu opioid receptor biased agonist in rats

Opioids remain the most powerful analgesics for moderate to severe pain but their clinical use, misuse and abuse has been an alarming medical problem, especially for those users at child-bearing age. Mu-opioid receptor (MOR) biased agonists have been suggested as superior alternatives with better therapeutic ratios. We recently discovered and characterized a novel MOR biased agonist, LPM3480392, which demonstrates robust analgesic effect, favorable pharmacokinetic performance, and mild respiratory suppression in vivo. To understand the safety profile of LPM3480392 on the reproductive system and embryonic development, this study evaluated the effects of LPM3480392 on the fertility and early embryonic development, embryo-fetal development, and pre- and postnatal development in rats. Results showed that LPM3480392 had mild effects on parental male and female animals, accompanied by subtle early embryonic loss and delayed ossification of fetal development during organogenesis period. In addition, although minor effects were found on normal developmental milestones and behaviors in the pups, there was no evidence of malformed effect. In conclusion, these results suggest that LPM3480392 has a favorable safety profile with only minor effects on the reproductive and developmental outcomes in animals, which support the development of LPM3480392 as a novel analgesic.

Analgesic effects and arthritic changes following tramadol administration in a rat hip osteoarthritis model

We investigated the analgesic effects of tramadol and the arthritic changes following tramadol administration in the rat hip osteoarthritis (OA) model using mono-iodoacetate (MIA). The right hip joints of male Sprague-Dawley rats (n = 5 rats/group) in the Sham group were injected with 25 μl of sterile saline and 1% of fluorogold (FG) retrograde neurotracer. In the MIA + Vehicle and MIA + Tramadol groups, FG and 25 μl of sterile saline with 0.5 mg of MIA were injected into the right hip joint. The MIA + Vehicle and MIA + Tramadol groups were administered daily for 4 weeks, either sterile saline (10 mg/kg, intraperitoneal [i.p.]) or tramadol (10 mg/kg, i.p.). We assessed hyperalgesia every week after MIA administration. Histopathological changes and immunoreactive neurons for calcitonin gene-related peptide (CGRP) in dorsal root ganglia (DRG) were evaluated after 4 weeks of treatment. MIA injection into the hip joint led to mechanical hyperalgesia (p < 0.01), which was significantly reduced by tramadol administration (p < 0.01). Furthermore, daily i.p injection of tramadol significantly suppressed CGRP expression in DRG (p < 0.0001). MIA + Vehicle and MIA + Tramadol groups showed significant cartilage reduction and degeneration compared to the Sham group (p < 0.0001). Interestingly, OA changes significantly progressed in the MIA + Tramadol group compared to the MIA + Vehicle group (p < 0.0001).

Osteoarthritis-Induced Metabolic Alterations of Human Hip Chondrocytes

Osteoarthritis (OA) alters chondrocyte metabolism and mitochondrial biology. We explored whether OA and non-OA chondrocytes show persistent differences in metabolism and mitochondrial function and different responsiveness to cytokines and cAMP modulators. Hip chondrocytes from patients with OA or femoral neck fracture (non-OA) were stimulated with IL-1β, TNF, forskolin and opioid peptides. Mediators released from chondrocytes were measured, and mitochondrial functions and glycolysis were determined (Seahorse Analyzer). Unstimulated OA chondrocytes exhibited significantly higher release of IL-6, PGE₂ and MMP1 and lower production of glycosaminoglycan than non-OA chondrocytes. Oxygen consumption rates (OCR) and mitochondrial ATP production were comparable in unstimulated non-OA and OA chondrocytes, although the non-mitochondrial OCR was higher in OA chondrocytes. Compared to OA chondrocytes, non-OA chondrocytes showed stronger responses to IL-1β/TNF stimulation, consisting of a larger decrease in mitochondrial ATP production and larger increases in non-mitochondrial OCR and NO production. Enhancement of cAMP by forskolin prevented IL-1β-induced mitochondrial dysfunction in OA chondrocytes but not in non-OA chondrocytes. Endogenous opioids, present in OA joints, influenced neither cytokine-induced mitochondrial dysfunction nor NO upregulation. Glycolysis was not different in non-OA and OA chondrocytes, independent of stimulation. OA induces persistent metabolic alterations, but the results suggest upregulation of cellular mechanisms protecting mitochondrial function in OA.

Effects of maternal consumption of morphine on rat skeletal system development

Background

Opioid abuse is among the most ubiquitous issues world-wide, and when it happens in mothers, it puts them at risk of diseases that can be transferred to the next generation. Previous studies have indicated that morphine addiction during pregnancy could inhibit development in rat embryos and infants.

The present study focused on the effects of maternal consumption of morphine on rat skeletal system development and also investigate the molecular pathway of chondrogenesis and osteogenesis of infants from control and addicted rat groups.

Methods

Thirty-two female rats were randomly assigned to four groups. The groups consisted of one- and seven-day-old female infants which were born of morphine-dependent mothers and a control group for each of them. Experimental groups received oral morphine at the final dose of 0.4 mg/ml/day. Withdrawal signs were confirmation of morphine dependency. Female rats were crossed with male rats and coupling time was recorded. Fixed bones of all groups were processed and then stained by hematoxyline-eosin method. Thickness and cell number of proximal and distal growth plate of bones were measured. The cartilage and bone cells were stained by alcian blue/alizarin red method. Additionally, the gene expression of alkaline phosphatase, osteocalcin, and COLL2 and SOX9 gene expression were studied immuno-histochemically.

Results

Unfavorable effects of morphine on histological measurements were observed in one-day and seven-day infants, with more effects on seven-day infants. The thickness and cell number of the proximal and distal growth plate of morphine-dependent rat offsprings were reduced significantly. Furthermore, morphine reduced growth of primary and secondary ossification centers, and thus, longitudinal bone growth was reduced. Moreover, a decrease in the alkaline phosphatase, osteocalcin, COLL2 and SOX9 gene expression, and the number of stained cells was observed. More adverse effects of morphine in seven-day infants compared to one-day infants which showed the time dependent of morphine to the time length of administration.

Conclusion

Histochemistry and immunohistochemistry findings on cartilage and bone matrix formation, as well as protein expression of chondrogenic and osteogenic markers suggest that morphine dependence in pregnant mothers may impair intra-cartilaginous osteogenesis in post-natal rats.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04321-6.

Analgesic Effect of Morphine Added to Bupivacaine in Serratus Anterior Plane Block Following Modified Radical Mastectomy. Only a Local Effect? Randomized Clinical Trial

Background

Serratus anterior plane (SAP) block, a novel regional anesthetic procedure, involves the anterolateral chest wall. Opioid receptors have been found on peripheral nerve terminals, so morphine may have a local action.

Objective

This work aimed at exploring the analgesic efficacy of morphine added to bupivacaine in SAPB in patients for whom modified radical mastectomy was conducted and whether it is a mere local effect.

Methods

Forty female patients were planned to have modified radical mastectomy participated in the study. Patients were randomly divided into two groups; Control group (C): received ultrasound-guided serratus anterior plane block with 20 mL of bupivacaine hydrochloride 0.25%; Morphine group (M): received the same in addition to 10 mg morphine sulfate. Intra- and post-operative blood samples were taken for the assessment of morphine serum levels. All patients were assessed for VAS scores during rest and movement (VAS-R and VAS-M). Time to the first request and the total amount of the rescue analgesia were recorded.

Results

In group M, Morphine was not detected in the plasma of all patients. Both VAS-R and VAS-M were significantly higher in group C than in group M (P<0.001) and (P≤0.003), respectively. Time to the first request of rescue analgesia was 8.5 h in group C compared to 20 h in group M (P=0.005) with a median dose of acetaminophen consumption of 2 g in group C compared to 1 g in group M (P=0.006).

Conclusion

Ten mg of morphine, when added to bupivacaine in SAPB, improved postoperative analgesia in patients to whom modified radical mastectomy was conducted. This effect seems to be attributed merely to local mechanisms.

Registration

The registration number of this study is NCT02962024 at www.clinicaltrial.gov.

Serotonin-evoked cytosolic Ca2+ release and opioid receptor expression are upregulated in articular cartilage chondrocytes from osteoarthritic joints in horses

Osteoarthritis is a pain-associated progressive disease and pain mediators, such as opioid receptors, expressed in articular cartilage could represent novel therapeutic targets. Acute and chronic stages of OA indicate different metabolic abilities of the chondrocytes depending on inflammatory state. This study aimed to investigate the response of healthy and osteoarthritic chondrocytes and their expression and release of pain mediators in response to acute inflammation. Interleukin-1 beta (IL-1β) and lipopolysaccharide (LPS) were used to induce an acute inflammatory response in cultured equine chondrocytes harvested from healthy joints (HC) and osteoarthritic joints (OAC), the latter representing acute exacerbation of a chronic inflammatory state. Intracellular Ca2+ release was determined after exposure to serotonin (5-hydroxytryptamine (5-HT), glutamate or ATP. Protein expression levels of F- and G-actin, representing actin rearrangement, and opioid receptors were investigated. Glutamate concentrations in culture media were measured. Cartilage was immunohistochemically stained for µ (MOR), κ (KOR), and δ (DOR) opioid receptors. Upon exposure to acute inflammatory stimuli, OAC showed increased intracellular Ca2+ release after 5-HT stimulation and increased expression of MOR and KOR. When cells were stimulated by inflammatory mediators, glutamate release was increased in both HC and OAC. Immunostaining for MOR was strong in OA cartilage, whereas KOR was less strongly expressed. DOR was not expressed by cultured HC and OAC and immunostaining of OA cartilage equivocal. We show that chondrocytes in different inflammatory stages react differently to the neurotransmitter 5-HT with respect to intracellular Ca2+ release and expression of peripheral pain mediators. Our findings suggest that opioids and neurotransmitters are important in the progression of equine OA. The inflammatory stage of OA (acute versus chronic) should be taken into consideration when therapeutic strategies are being developed.

Assessment of Intraoperative Intra-articular Morphine and Clonidine Injection in the Acute Postoperative Period After Hip Arthroscopy

BACKGROUND

Previous authors have suggested that intra-articular morphine and clonidine injections after knee arthroscopy have demonstrated equivocal analgesic effect in comparison with bupivacaine while circumventing the issue of chondrotoxicity. There have been no studies evaluating the effect of intra-articular morphine after hip arthroscopy.

PURPOSE

To evaluate the efficacy of intra-articular morphine in combination with clonidine on postoperative pain and narcotic consumption after hip arthroscopy surgery for femoroacetabular impingement.

STUDY DESIGN

Cohort study; Level of evidence, 3.

METHODS

A retrospective chart review was performed on 43 patients that underwent hip arthroscopy for femoroacetabular impingement at a single institution between September 2014 and May 2015. All patients received preoperative celecoxib and acetaminophen, and 22 patients received an additional intra-articular injection of 10 mg morphine and 100 μg of clonidine at the conclusion of the procedure. Narcotic consumption, duration of anesthesia recovery, and perioperative pain scores were compared between the 2 groups.

RESULTS

Patients who received intra-articular morphine and clonidine used significantly less opioid analgesic (mEq) in the postanesthesia recovery (median difference, 17 mEq [95% CI, -32 to -2 mEq]; P = .02) compared with the control group. There were no differences in time spent in recovery before hospital discharge or in visual analog pain scores recorded immediately postoperatively and at 1 hour after surgery.

CONCLUSION

Intraoperative intra-articular injection of morphine and clonidine significantly reduced the narcotic requirement during the postsurgical recovery period after hip arthroscopy. The reduction in postsurgical opioids may decrease adverse effects, improve overall pain management, and lead to better quality of recovery and improved patient satisfaction.

Comparison of efficacy of intra-articular morphine and steroid in patients with knee osteoarthritis

Introduction:

Primary therapeutic aim in treatment of osteoarthritis of the knee is to relieve the pain of osteoarthritis. The aim of this study was to compare the efficacy of intra-articular triamcinolone with intra-articular morphine in pain relief due to osteoarthritis of the knee in the elderly population.

Materials and Methods:

Patients between 50 and 80 years of age were randomized into three groups. Group M received morphine plus bupivacaine intra-articularly, Group T received triamcinolone plus bupivacaine intra-articularly, and Group C received saline plus bupivacaine intra-articularly. Patients were evaluated before injection and in 2nd, 4th, 6th, and 12th weeks after injection. First-line supplementary analgesic was oral paracetamol 1500 mg/day. If analgesia was insufficient with paracetamol, oral dexketoprofen trometamol 50 mg/day was recommended to patients.

Results:

After the intra-articular injection, there was statistically significant decrease in visual analog scale (VAS) scores in Groups M and T, when compared to Group C. The decrease of VAS scores seen at the first 2 weeks continued steadily up to the end of 12th week. There was a significant decrease in Groups M and T in the WOMAC scores, when compared to Group C. There was no significant difference in the WOMAC scores between morphine and steroid groups. Significantly less supplementary analgesics was used in the morphine and steroid groups.

Conclusion:

Intra-articular morphine was as effective as intra-articular triamcinolone for analgesia in patients with osteoarthritis knee. Intra-articular morphine is possibly a better option than intra-articular steroid as it has lesser side effects.

Role of proopiomelanocortin-derived peptides and their receptors in the osteoarticular system: from basic to translational research

Proopiomelanocortin (POMC)-derived peptides such as melanocortins and β-endorphin (β-ED) exert their pleiotropic effects via binding to melanocortin receptors (MCR) and opioid receptors (OR). There is now compelling evidence for the existence of a functional POMC system within the osteoarticular system. Accordingly, distinct cell types of the synovial tissue and bone have been identified to generate POMC-derived peptides like β-ED, ACTH, or α-MSH. MCR subtypes, especially MC1R, MC2R (the ACTH receptor), MC3R, and MC4R, but also the μ-OR and δ-OR, have been detected in various cells of the synovium, cartilage, and bone. The respective ligands of these POMC-derived peptide receptors mediate an increasing number of newly recognized biological effects in the osteoarticular system. These include bone mineralization and longitudinal growth, cell proliferation and differentiation, extracellular matrix synthesis, osteoprotection, and immunomodulation. Importantly, bone formation is also regulated by the central melanocortin system via a complex hormonal interplay with other organs and tissues involved in energy metabolism. Among the POMC-derived peptides examined in cell culture systems from osteoarticular tissue and in animal models of experimentally induced arthritis, α-MSH, ACTH, and MC3R-specific agonists appear to have the most promising antiinflammatory actions. The effects of these melanocortin peptides may be exploited in future for the treatment of patients with inflammatory and degenerative joint diseases.

Pharmacokinetics of intra-articular morphine in horses with lipopolysaccharide-induced synovitis

OBJECTIVE

To describe the pharmacokinetics of intra-articularly (IA) administered morphine.

STUDY DESIGN

Experimental randomized, cross-over study.

ANIMALS

Eight adult healthy mixed breed horses aged 6.5 +/- 2.3 (mean +/- SD) years and weighing 535 +/- 86 kg.

METHODS

Unilateral radiocarpal synovitis was induced by IA injection of 3 microg lipopolysaccharide (LPS) on two occasions (right and left radiocarpal joint, respectively) separated by a 3-week wash-out period. Treatments were administered 4 hours post-LPS-injection: Treatment IA; preservative free morphine IA (0.05 mg kg(-1)) plus saline intravenous (IV) and treatment IV; saline IA plus preservative free morphine IV (0.05 mg kg(-1)). Concentrations of morphine, morphine-3-glucuronide and morphine-6-glucuronide (M6G) were determined repeatedly in serum and synovial fluid (SF) by high-performance liquid chromatography mass spectrometry, at 2 and 4 hours and then at 4 hours intervals until 28 hours post-treatment.

RESULTS

Injection of LPS elicited a marked and comparable synovitis in all LPS-injected radiocarpal joints. IA administered morphine was detectable in SF of all eight joints 24 hours post-treatment and in 6/8 joints 28 hours post-treatment. The terminal half-life of morphine in SF was estimated to be 2.6 hours. IA administration of morphine resulted in mean serum concentrations of morphine below 5 ng mL(-1) from 2 to 28 hours after treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

Intra-articularly administered morphine remained within the joint for at least 24 hours. At the same time only very low serum concentrations of morphine and M6G were detected. The present results suggest that IA morphine at 0.05 mg kg(-1) may be used for IA analgesia lasting at least 24 hours and give strong support to the theory that previously observed analgesic and anti-inflammatory effects of IA morphine in horses are most likely to be mediated peripherally.

Beta-endorphin regulation of MAPKs in cultured human articular chondrocytes: MAPK inhibitors prevent the increase of IL-1 beta protein levels during beta-endorphin stimulation

We investigated the effect of beta-endorphin on the activities of mitogen-activated protein kinases in cultured human articular chondrocytes in order to elucidate its effect on cartilage. Monolayer cultures of chondrocytes obtained from patients undergoing total knee arthroplasty were treated with 60, 600, or 6000 ng/ml beta-endorphin, or 100 ng/ml naltrexone combined with 600 ng/ml beta-endorphin. The regulation of three major mitogen-activated protein kinases phosphorylation, ERKp44/p42, p38, and JNK, was determined by Western blotting. We also examined the influence of specific mitogen-activated protein kinase inhibitors on IL-1 beta protein levels during beta-endorphin stimulation. The results demonstrate that beta-endorphin, dependent on concentration and duration of stimulation, significantly affected the activation of the three mitogen-activated protein kinases in cultured human articular chondrocytes. Naltrexone in some cases significantly regulated the mitogen-activated protein kinases in different ways when added to beta-endorphin 600 ng/ml. Furthermore, specific mitogen-activated protein kinase inhibitors hindered the increase of IL-1 beta during beta-endorphin incubation. The effect of beta-endorphin seen in this study is considered critical for the production of several mediators of cartilage damage in an arthritic joint.

Amelioration of pain and histopathologic joint abnormalities in the Col1-IL-1beta(XAT) mouse model of arthritis by intraarticular induction of mu-opioid receptor into the temporomandibular joint

OBJECTIVE

To evaluate opioid receptor function as a basis for novel antinociceptive therapy in arthritis.

METHODS

We induced human mu-opioid receptor (HuMOR) expression in arthritic joints of mice, using the feline immunodeficiency virus (FIV) vector, which is capable of stably transducing dividing, growth-arrested, and terminally differentiated cells. Male and female Col1-IL-1beta(XAT)-transgenic mice developed on a C57BL/6J background and wild-type littermates were studied.

RESULTS

A single injection of FIV(HuMOR) into the temporomandibular joints of Col1-IL-1beta(XAT)-transgenic mice 1 week prior to induction of arthritis prevented the development of orofacial pain and joint dysfunction, and reduced the degree of histopathologic abnormality in the joint. In addition, FIV(HuMOR) prevented the attendant sensitization of trigeminal sensory neurons and activation of astroglia in brainstem trigeminal sensory nuclei. These effects were mediated by the transduction of primary sensory neurons via transport of FIV vectors from peripheral nerve endings to sensory ganglia, as evidenced by HuMOR expression in neuronal cell bodies located in the trigeminal ganglia, as well as in their proximal and distal nerve branches located in the main sensory and subnucleus caudalis of the brainstem and joints, respectively. The presence of MOR ligands predominantly in the descending trigeminal nucleus suggested that the observed antinociception occurred at the subnucleus caudalis. Articular chondrocytes and meniscal tissue were also infected by FIV(HuMOR), which presumably exerted an antiinflammatory effect on cartilage.

CONCLUSION

Our results indicate that prophylactic therapy with MOR overexpression in joints can successfully prevent the development of pain, dysfunction, and histopathologic abnormalities in the joints in arthritis. These findings may provide a basis for the future development of spatiotemporally controlled antinociceptive and antiinflammatory therapy for arthritis.

Opioid peptides and receptors in joint tissues: study in the rat

Using immunohistochemical and biochemical techniques, the occurrence of endogenous opioid peptides and their receptors in normal rat bone and joint tissues was investigated. Opioid receptors were detected, quantified, and characterized in homogenates from capsule/synovium and periosteum using radioligand binding assays. Receptor binding of the nonselective opioid [3H]naloxone to tissue homogenates was stereospecific and saturable, showing similar characteristics to that of brain tissue, although with lower binding capacities. By immunohistochemistry, the neuronal occurrence of four different enkephalins was demonstrated in synovium, bone marrow, periosteum, and juxta-articular bone, whereas no neuronal dynorphin immunoreactivity was detected. Double-staining studies disclosed that enkephalins coexisted with substance P in primary afferent fibers. The applied techniques can be used to assess changes in the distribution of endogenous opioids and their receptors in joint tissues in conditions associated with pain and inflammation. The endogenous opioid system now demonstrated might be targeted and exploited therapeutically to obtain peripheral control of symptoms in joint disorders.

Analgesic action of acetaminophen in symptomatic osteoarthritis of the knee

OBJECTIVES

The study was designed to investigate the analgesic effects and mechanisms of acetaminophen (paracetamol) in symptomatic osteoarthritis (OA) of the knee.

METHODS

Twenty patients with symptomatic OA were randomly allocated to two groups treated with either acetaminophen or rofecoxib for 3 months. Visits and measurements were scheduled upon entry (T0), at month 1 (T1) and at month 3 (T3). The intensity of joint pain was evaluated with a 100-mm visual analogue scale (VAS). The physical function of the affected knee was evaluated with a questionnaire comparable to the Western Ontario McMaster Universities Osteoarthritis Index (WOMAC). Levels of serotonin, substance P (SP) and beta-endorphin (BEND) were determined with commercial enzyme-linked immunoassay kits. The expression of kappa opioid receptor (KOR) in peripheral mononuclear blood cells (PBMCs) was quantified by real-time PCR.

RESULTS

Both acetaminophen and rofecoxib relieved pain considerably but with different kinetics, and affected different biomarkers. Rofecoxib appeared to be more efficient, reducing pain intensity by 56% at T1 (P<0.01), whereas acetaminophen reduced it by only 29%. Physical function improved in both groups by T3. Correlated with the pain relief, acetaminophen significantly reduced plasma BEND levels, whereas rofecoxib did not do so. In both groups plasma SP levels were elevated compared with T0. A reduction in serum serotonin was detected in the rofecoxib group at T1 (P=0.004) but had recovered at T3. No changes in KOR mRNA in PBMCs were observed in either group.

CONCLUSIONS

There is a correlation between reduction in circulating BEND and OA pain relief in patients treated with acetaminophen.

Detection of mRNA transcripts of truncated opiate precursor (POMC) in human cartilage

In the present study, we have investigated the presence of pro-opiomelanocortin C-terminal fragment derived-peptides in human articular cartilage and cultured chondrocytes. beta-Lipotropin and beta-endorphin were monitored in different cell cultures and biopsies using different techniques. Biopsies were taken from patients undergoing total knee arthroplasty due to osteoarthritis. Both fresh tissue sections and chondrocytes cultured in monolayer were used in the study. Immunohistochemistry, immunocytochemistry, reverse transcriptase-polymerase chain reaction and qualitative Western blots were carried out. The results of the reverse transcriptase-polymerase chain reaction showed transcription of a truncated-form of mRNA for pro-opiomelanocortin in native cartilage and cultured chondrocytes. There was no detection of endogenous production of beta-lipotropin or beta-endorphin in human articular chondrocytes, either in situ or in vitro. Whether pro-opiomelanocortin-derived peptides of non-cartilaginous origin are present in articular cartilage itself still remains unclear.

Opiate regulation of IL-1β and TNF-α in cultured human articular chondrocytes

In order to investigate if beta-endorphins anti-inflammatory effect in cartilage-damaging states is mediated via tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta), we examined its influence on these two cytokines in vitro. Human articular chondrocytes were obtained from patients undergoing total knee arthroplasty and stimulated with beta-endorphin (60-6000 ng/ml). Protein levels of TNF-alpha and IL-1 beta were measured by ELISA in supernatants from articular chondrocyte cultures. beta-Endorphin significantly increased the levels of IL-1 beta for all concentrations used after 15 min incubation, and when stimulated with 600 and 6000 ng/ml after 24 h incubation. The opioid-induced increase in IL-1 beta was blocked by naltrexone in the group tested. TNF-alpha expression was also significantly stimulated by 60 and 600 ng/ml beta-endorphin after 15 min, an effect blocked by naltrexone in the group tested. These findings indicate that the mechanism of beta-endorphins anti-inflammatory influence in cartilage-damaging states is not apparently mediated via these two cytokines modulation.

Signaling transduction: target in osteoarthritis

PURPOSE OF REVIEW

The pathophysiology of osteoarthritis is the result of an imbalance between anabolic and catabolic pathways. This imbalance is the result of the activation of joint cells by inflammatory mediators, matrix components, and mechanical stress. All these mediators act through specific receptors that transmit the signals to the nucleus to activate the transcription of matrix metalloproteinases and inflammatory genes. Targeting these signaling pathways in osteoarthritis is considered a novel approach to modulate this imbalance.

RECENT FINDINGS

Although many signaling pathways are necessary for physiologic cell life, it is now well established that a few are more specifically induced in an inflammatory environment. In osteoarthritis, the nuclear factor-kappaB and mitogen-activated protein kinase pathways have been shown to play a predominant role in the expression of metalloproteinases and inflammatory genes and proteins. Also involved in the activation of osteoarthritic cells are other molecules interacting with one or several signaling pathways, such as nitric oxide, peroxisome proliferator-activated receptor-gamma ligands, or C/EBP transcriptional factors. Based on this knowledge, specific inhibitors for some of these signaling pathways have been designed and include p38 mitogen-activated protein kinase or nuclear factor-kappaB inhibitors. Experimental studies evaluating cartilage degradation in arthritis models are promising, although fewer have been done specifically in osteoarthritis models.

SUMMARY

Targeting signaling pathways in osteoarthritis did not seem feasible a few years ago because of the complexity of the multiple intracellular pathways, mainly physiologic, defined by a high degree of redundancy and cross-talk. However, important advances in the knowledge of chondrocyte and synoviocyte signaling in osteoarthritis have been achieved in recent years and suggest that inhibitors of specific signaling pathways could shortly provide effective treatments for this disease.