Pathophysiological Mechanisms in Osteoarthritis Lead to Novel Therapeutic Strategies

Abstract

Osteoarthritis (OA) is a debilitating, progressive disease of diarthrodial joints associated with aging. At the molecular level, OA is characterized by an imbalance between anabolic (i.e. extracellular matrix biosynthesis) and catabolic (i.e. extracellular matrix degradation) pathways in which articular cartilage is the principal site of tissue injury responses. The pathophysiology of OA also involves the synovium in that ‘nonclassical’ inflammatory synovial processes contribute to OA progression. Chondrocytes are critical to the OA process in that the progression of OA can be judged by the vitality of chondrocytes and their ability to resist apoptosis. Growth factors exemplified by insulin-like growth factor-1, its binding proteins and transforming growth factor-β contribute to anabolic pathways including compensatory biosynthesis of extracellular matrix proteins. Catabolic pathways are altered by cytokine genes such as interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α) which are upregulated in OA. In addition, IL-1 and TNF-α downregulate extracellular matrix protein biosynthesis while concomitantly upregulating matrix metalloproteinase (MMP) gene expression. When MMPs are activated, cartilage extracellular matrix degradation ensues apparently because levels of endogenous cartilage MMP inhibitors cannot regulate MMP activity. Therapeutic strategies designed to modulate the imbalance between anabolic and catabolic pathways in OA may include neutralizing cytokine activity or MMP gene expression or inhibiting signaling pathways which result in apoptosis dependent on mature caspase activity or mitogen-activated protein kinase (MAPK) activity. MAPK activity appears critical for regulating chondrocyte and synoviocyte apoptosis and MMP genes.

Neutrophil gelatinase–associated lipocalin is expressed in osteoarthritis and forms a complex with matrix metalloproteinase 9

“…MMPs are the primary enzymes involved in cartilage collagen digestion (1), and expression of MMP-9 in OA SF has previously been reported (20). The important role of MMP-9 in long bone development is well established (42), and it is known that multiple Figure 6.…”

“…Although OA is not considered an inflammatory disease by traditional standards, elevated levels of proinflammatory cytokines, such as interleukin-1␤ (IL-1␤) and tumor necrosis factor ␣, have been observed in OA synovial fluid (SF), supporting the notion that there is an inflammatory component associated with the pathogenesis of OA (for review, see refs. [1][2][3].…”

Characterization of metalloprotease cleavage products of human articular cartilage

“…In OA, the excessive degradation is primarily due to the action of proteases released by chondrocytes, synovial cells, and macrophages (4). Numerous serine proteases and metalloproteases are overexpressed by these cells during the disease process, but specific metalloproteases, including the collagenase matrix metalloproteinase 13 (MMP-13) and the aggrecanase ADAMTS-5, are thought to be especially important for initiating and promoting cartilage matrix degradation in OA (5,6).…”

Interleukin‐1β impairment of transforming growth factor β1 signaling by DOWN‐REGULATION of transforming growth factor β receptor type II and up‐regulation of Smad7 in human articular chondrocytes

“…During osteoarthritis (OA), IL-1␤ is found in large amounts in synovial fluids and may actively participate in cartilage breakdown (12). It induces the activation of protein kinase cascades involving the MAPKs and/or the NF-B pathway.…”