Requirement for protein kinase R in interleukin-1alpha-stimulated effects in cartilage.

Interleukin-1 (IL-1) has pleiotropic effects in cartilage. The interferon-induced, double stranded RNA-activated protein kinase PKR that phosphorylates eukaryotic initiation factor 2 (eIF2) alpha has been implicated in cytokine effects in chondrocytes. A compound was recently identified that potently suppresses PKR autophosphorylation (IC50 approximately 200 etaM) and partially restores PKR-inhibited translation in a cell-free system with significant effect in the nanomolar range. The objectives of this study were to exploit this potent PKR inhibitor to assess whether PKR kinase activity is required for catabolic and proinflammatory effects of IL-1alpha in cartilage and to determine whether IL-1alpha causes an increase in eIF2alpha phosphorylation that is antagonized by the PKR inhibitor. Cartilage explants were incubated with the PKR inhibitor and IL-1alpha. Culture media were assessed for sulfated glycosaminoglycan as an indicator of proteoglycan degradation and for prostaglandin E(2). Cartilage extracts were analyzed by Western blot for cyclooxygenase-2 and phosphorylated signaling molecules. Nanomolar concentrations of the PKR inhibitor suppressed proteoglycan degradation and cyclooxygenase-2 accumulation in IL-1alpha-activated cartilage. The PKR inhibitor stimulated or inhibited PGE(2) production with a biphasic dose response relationship. IL-1alpha increased the phosphorylation of both PKR and eIF2alpha, and nanomolar concentrations of PKR inhibitor suppressed the IL-1alpha-induced changes in phosphorylation. The results strongly support PKR involvement in pathways activated by IL-1alpha in chondrocytes.

Differential effects of hyperosmotic challenge on interleukin-1-activated pathways in bovine articular cartilage.

Chondrocytes in situ experience fluctuations in extracellular osmolarity resulting from mechanical loading. The objective of this study was to determine whether hyperosmotic stress causes or exacerbates interleukin-1 (IL-1)-mediated effects in bovine articular cartilage. Disks of cartilage cut from the articular surface of calf radiocarpal joints were incubated for 24h in the presence or absence of IL-1 in Dulbecco's modified Eagle's medium adjusted to various osmolalities with sucrose or NaCl. Cyclooxygenase (COX)-2 levels in the cartilage were examined by Western blot. Culture media were assayed for prostaglandin E(2) (PGE(2)), nitrite as an indicator of nitric oxide (NO) production, and sulfated glycosaminoglycan as an indicator of proteoglycan degradation. We report the osmolality-dependent potentiation of COX-2 and PGE(2) production, and the osmolality-dependent inhibition of NO production and proteoglycan degradation in IL-1-activated cartilage. The data demonstrate that osmotic and cytokine signaling interact to differentially modulate IL-1-stimulated effects in calf articular cartilage.

Cyclooxygenase-2 expression in chondrosarcoma.

OBJECTIVE: In recent years it has become evident that tissue cyclooxygenase-2 (COX-2) may play a role in carcinogenesis and tumor malignancy. There is now a mounting body of information that strongly implies that COX-2 inhibitors may be of some value in the management of patients with carcinomas, and most recently several similar reports have appeared relating to sarcomas. METHODS: The authors studied 32 samples of cartilage tumors from our tumor tissue bank for the presence of COX-2 by a Western blot technique. There were 29 patients from whom the samples were obtained, including 8 with enchondromas and 21 with chondrosarcomas. RESULTS: Thirteen of the 24 chondrosarcoma samples and none of the 8 enchondromas were positive for COX-2. An attempt was made to correlate these results with clinical data including age, gender, staging according to the Musculoskeletal Tumor Society, anatomical site, ploidic pattern, presence of metastases and death rate but no statistically valid correlation could be found. CONCLUSION: It is evident that COX-2 may play some role in chondrosarcoma but not in the benign enchondroma and that further studies with COX-2 inhibitors are warranted.

A role for the interleukin-1 receptor in the pathway linking static mechanical compression to decreased proteoglycan synthesis in surface articular cartilage.

Loading of articular cartilage during weight bearing is essential for the maintenance of cartilage function. Although certain cyclic loading protocols stimulate extracellular matrix synthesis, constant or static compression decreases proteoglycan and collagen synthesis in cartilage explants. The goal of this study was to determine whether the compression-induced decrease in proteoglycan synthesis involves an interleukin-1 (IL-1) signaling pathway. Cartilage explants were compressed 50% in the presence of IL-1 receptor antagonist (IL-1ra), and the incorporation of [35S]sulfate into macromolecules was measured. IL-1ra increased sulfate incorporation in compressed cartilage but not in cartilage maintained at the in situ thickness (0% compression). IL-1alpha and IL-1beta mRNAs were detected in cartilage compressed 50% for at least 3h, while nitric oxide synthase II mRNA was only detected in cartilage compressed 50% for 6h. The data support a role for the IL-1 receptor in the pathway linking static compression to reduced proteoglycan synthesis.

Intracellular interleukin-1 receptor antagonist in osteoarthritis chondrocytes.

The objective of the current study was to determine whether the balance of interleukin-1 and intracellular interleukin-1 receptor antagonist in chondrocytes in osteoarthritic human joints favors agonist action. Chondrocytes were isolated from cartilage specimens taken at the time of joint arthroplasty. Interleukin-1alpha, interleukin-1beta, and intracellular interleukin-1 receptor antagonist messenger ribonucleic acids were assessed by reverse transcriptase-polymerase chain reaction, and chondrocyte lysates were analyzed by enzyme-linked immunosorbent assay for the respective proteins. Type I intracellular interleukin receptor antagonist transcripts were the only intracellular variant detected in osteoarthritis chondrocytes. In cartilage graded as advanced osteoarthritis both interleukin proteins in chondrocyte lysates decreased, correlating with decreased interleukin-1alpha and beta messenger ribonucleic acids. Interleukin-1 receptor antagonist exceeded interleukin-1alpha in chondrocyte lysates by one order of magnitude except that in moderate osteoarthritis, antagonist was only two- to fourfold in excess. Interleukin-1alpha and interleukin-1beta proteins were correlated closely in individual lysates, with interleukin-1beta exceeding interleukin-1beta by one order of magnitude. In moderately degenerated cartilage, intracellular antagonist may not be sufficiently abundant to block postulated intracellular functions of precursor interleukin-1alpha. Furthermore, if stored interleukin-1alpha, interleukin-1beta, and interleukin receptor antagonist are released from chondrocytes, the localized antagonist would be insufficient to prevent signaling through cell surface receptors. Chondrocyte-derived interleukin-1alpha and interleukin-1beta may locally overwhelm inhibition by interleukin receptor antagonist to promote the early degenerative changes in osteoarthritis.

Photodynamic treatment has chondroprotective effects on articular cartilage.

Osteoarthritis (OA) is a disabling joint disease for which there is currently no cure. It is characterized by the destruction of articular cartilage. One strategy that is being explored for protecting the cartilage in OA is the administration of transforming growth factor-beta, which in vitro antagonizes cartilage degradation initiated by catabolic stimulants such as interleukin-1 (IL-1). The problems associated with selective delivery of the growth factor to chondrocytes, undesirable side-effects on joint tissues, and short biological half-life have led us to explore modalities aimed at activating transforming growth factor-beta that is stored in the cartilage as latent complexes. Photodynamic therapy is a two-step protocol of tissue sensitization with a light-activatable chemical called a photosensitizer followed at some interval by irradiation with the appropriate wavelength visible light. Biological effects are typically elicited through oxygen-dependent photochemistry without heat generation. Transforming growth factor-beta1 can be activated by oxidative mechanism(s), prompting us to explore whether photodynamic technology can be harnessed to modulate cartilage metabolism. Disks of bovine articular cartilage were photosensitized by incubation with a chlorin(e6)-succinylated polylysine conjugate and irradiated with 1-2 J/cm2 red light (lambdamax = 671 nm). This two-step regimen dramatically inhibited IL-1-stimulated proteoglycan degradation and concomitantly increased latent and active transforming growth factor-beta1 in culture medium. This research may lead to the development of minimally invasive photodynamic therapy in which light is delivered to locally activate a chondroprotective program in photosensitized cartilage in the context of OA.