Modulation of endogenous osteogenic protein-1 (OP-1) by interleukin-1 in adult human articular cartilage

Background: Osteogenic protein-1 (OP-1, BMP-7) induces bone formation and cartilage growth. Since OP-1 is an anabolic factor expressed by human articular chondrocytes, we examined the response of endogenous OP-1 to interleukin-1β (IL-1β) in human articular cartilage. Methods: Normal adult human articular cartilage explants were cultured for twenty-five days in the presence of medium only or were treated with a low dose (0.1 ng/mL) or high dose (1.0 ng/mL) of IL-1β for forty-eight or ninety-six hours. Alternately, cartilage explants were cultured forty-eight hours with IL-1β, followed by forty-eight hours in standard medium (recovery). Tissue was analyzed for OP-1 message (by means of the reverse transcriptase-polymerase chain reaction), protein (by means of enzyme-linked immunosorbent assay and Western blot analysis) and proteoglycan content. Medium was analyzed for released proteoglycans and OP-1. Results: In the presence of medium, OP-1 maintained its steady state of mRNA and protein expression for as long as twenty-five days in culture. A low dose of IL-1β led to some upregulation in message and a twofold (p < 0.02) increase in OP-1 protein characterized by enhanced processing and activation of OP-1. Removal of IL-1β (recovery experiments) did not reverse its effect on OP-1 synthesis. A high dose of IL-1β caused stronger upregulation of message and a two-fold decrease in OP-1 protein content (p < 0.007) in the cartilage matrix. However, this decrease in the matrix was primarily due to a release of active OP-1 into the medium. After removal of the 1.0-ng/mL IL-1β, the levels of OP-1 protein did not recover. Conclusion: The results of the present study indicate that human adult chondrocytes have an ability to respond anabolically to initial or early catabolic events through an upregulation of endogenous OP-1. Clinical Relevance: A balance between anabolism and catabolism is perturbed and not fully synchronized in the degenerative processes seen in osteoarthritis. The aim of the current study was to investigate the function of a cartilage endogenous anabolic factor, i.e., OP-1, in a model of early degeneration induced by a catabolic mediator IL-1. The findings of the present study contribute to our understanding of the mechanisms involved in articular cartilage regeneration and repair.