T1 - Proteoglycan synthesis in the human intervertebral disc

The IGF-I (insulin-like growth factor-I) signalling pathway responsible for regulation of proteoglycan synthesis in chondrocytes has not been defined and is the focus of the present study. Chondrocytes isolated from normal human articular cartilage were stimulated with IGF-I in monolayer culture or in suspension in alginate. IGF-I activated members of both the PI3K (phosphoinositide 3-kinase) pathway and the ERK (extracellular-signal-regulated kinase)/MAPK (mitogen-activated protein kinase) pathway. The PI3K inhibitors LY294002 and wortmannin blocked IGF-I-stimulated Akt phosphorylation without blocking ERK phosphorylation and this was associated with complete inhibition of proteoglycan synthesis. A decrease in IGF-I-stimulated proteoglycan synthesis was also observed upon inhibition of mTOR (mammalian target of rapamycin) and p70S6 kinase, both of which are downstream of Akt. The MEK (MAPK/ERK kinase) inhibitors PD98059 and U0126 blocked IGF-I-stimulated ERK phosphorylation but did not block the phosphorylation of Akt and did not decrease proteoglycan synthesis. Instead, in alginate-cultured chondrocytes, the MEK inhibitors increased IGF-I-stimulated proteoglycan synthesis when compared with cells treated with IGF-I alone. This is the first study to demonstrate that IGF-I stimulation of the PI3K signalling pathway is responsible for the ability of IGF-I to increase proteoglycan synthesis. Although IGF-I also activates the ERK/MAPK pathway, ERK activity is not required for proteoglycan synthesis and may serve as a negative regulator.

T1 - Proteoglycan synthesis in normal and Lowe syndrome fibroblasts

T1 - Regulation of vascular proteoglycan synthesis by metabolic factors associated with diabetes

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.)

T1 - Modulation of chondrocyte proteoglycan synthesis by endogeneously produced nitric oxide
T1 - Proteoglycan synthesis in increased in cells with impaired clathrin-dependent endocytosis

Proteoglycan Synthesis by Bovine Keratocytes and …

AB - Slices of human annulus fibrosus were cultured under conditions that controlled their hydration and prevented loss of proteoglycans from the extracellular matrix. A quantitative analysis of proteoglycan synthesis was carried out. Both the absolute rate of synthesis and the topographicaf variation in chondrocyte activity changed with age; the most active cells in the adult were found In the mid-annuius region, whereas in the fetal disc the cells in the inner annujus were the most active. The conditions under which the tissue was stored, and changes in hydration during culture, had considerable effects on synthesis. Pathological discs had a wide range of biological activity that reflected the heterogeneous properties of these specimens. It Is suggested that this culture method provides a means of investigating the way In which the synthesis of the macromolecular components of the intervertebral disc are coordinated and subsequently incorporated into the extracellular matrix.

T1 - Regulation of vascular proteoglycan synthesis by angiotensin II type 1 and type 2 receptors

proteoglycan and glycosaminoglycan synthesis ..

One way in which NSAIDs stop the chondrocytes from repairing themselves is by the inhibition of the synthesis of Prostaglandin E2 (PGE2). Prostaglandins (PG) are produced by most human cell types (including chondrocytes) and have a variety of physiologic functions. PG synthesis is initiated by the mobilization of arachidonic acid from cell membrane phospholipids as a result of the enzyme phospholipase A2. The enzyme cyclooxygenase along with other enzymes converts arachidonic acid to five primary prostaglandins: PGD2, PGE2, PGI2 (Prostacyclin), PGF2a, and TXA2 (thomboxane). (See Figure 13.) These PGs have a variety of functions including the mediation of inflammation, calcium movement, sensitization of spinal neurons to pain, blood clotting, blood pressure, circulation, control of blood flow in kidneys, hormone regulation, protection of gastrointestinal lining, and the control of cell growth.80, 81 Chondrocytes and synovial fibroblasts produce PGE2. PGE2 levels are increased to an impact load on articular cartilage or during cartilage degeneration.82, 83 PGE2 is reported to have anabolic effects on cartilage: increasing proteoglycan and DNA and collagen synthesis,84, 85 stimulating proliferation and proteoglycan aggrecan synthesis,86, 87 and, at low concentrations, stimulating type II collagen synthesis.88

KW - Proteoglycan synthesis

Proteoglycan and Hyaluronic Acid Synthesis by …

At present no quantitative non-invasive method for determining the anabolic (building up) and catabolic (breaking down) activity of NSAIDs on human cartilage in vivo exists. Most information on the effects of NSAIDs on the turnover of extra-cellular matrix macromolecules comes from short-term organ culture studies. Initial evaluations into the pathophysiology of osteoarthritis concentrated on the effects of NSAIDs on glycosaminoglycan synthesis. It was established that in all but the most severe cases of osteoarthritis, the chondrocyte response to proteoglycan depletion was an increase in glycosaminoglycan synthesis.72, 73 One of the first to show that NSAIDs diminished glycosaminoglycan synthesis in aged human cartilage cells (taken during hip surgery) in vitro was a research group from the University of Sydney in 1976.74 J.T. Dingle, led several of the follow-up studies on the effects of NSAIDs on human cartilage metabolism. The initial studies revealed significant declines in glycosaminoglycan synthesis in both normal and osteoarthritic human cartilages.75 (See Figure 11.)