T1 - Proteoglycan synthesis in normal and Lowe syndrome fibroblasts
T1 - Proteoglycan synthesis in vitamin d-deficient cartilage
One of the earliest features of the development of osteoarthritis is degeneration of the articulating surfaces of the joint. This is characterized by fibrillation of the articular cartilage, in which the mesh of collagen fibers is disrupted. Degeneration of type II collagen is seen, as well as a decrease in the extracellular matrix.22 Loss of proteoglycan from the matrix is characteristic. The loss of aggrecan, the predominant PRG in articular cartilage imposes an increasing load on the collagen fibrils, causing further breakdown.23 Early in the course of OA, the tissue mounts an attempt at repair. Chondrocytes proliferate and there is an increase in matrix synthesis.24 However, if this repair process is disrupted for any reason including the use of NSAIDs, degradative enzymes overwhelm the synthetic capability and the repair fails. Particular compositional, molecular, and structural changes will continue to occur within the articular cartilage including decreased proteoglycan and increased water content, collagen fibril network disorganization, and proteoglycan separation, as long as the inciting issue (NSAID use) continues. (See Figure 5.)
Heat treatment alone did not increase proteoglycan synthesis.
Articular cartilage functions as a wear-resistant, smooth, nearly frictionless, load-bearing surface. The composition and physiochemical properties of articular cartilage, the fundamental organization of the collagen network, and the molecular organization of collagen and proteoglycans all have profound effects on the intrinsic mechanical properties of the extracellular matrix.18 Cartilage is composed of a complex extracellular matrix of collagen and elastic fibers within a hydrated gel of glycosaminoglycans and proteoglycans. This extracellular matrix, which makes up 98% of the articular cartilage volume, is synthesized by the chondrocytes which comprise the other 2% of the cartilage tissue. It is well known that chondrocytes can synthesize the extracellular matrix such as proteoglycans, collagen, fibronectin, integrins, and other adhesive proteins which are needed to maintain the high tensile strength and low compressibility under load of the articular cartilage.19, 20 Type II collagen is the predominant collagen type in the extracellular matrix with proteoglycan (PRG) macromolecules dispersed throughout. They contain highly negatively charged carboxyl and sulfate groups (keratin and chondroitin sulfate) on the glycosaminoglycans, giving them a high affinity for water. (See Figure 4.)