Augmentation of inorganic pyrophosphate elaboration in cartilage by serum factors

Stimulation of cartilage inorganic pyrophosphate elaboration by ascorbate

Ascorbate (0-500 microM) stimulates synthesis and secretion of collagen by cartilage explants in a dose-dependent fashion as estimated by [3H]proline incorporation. Concurrent elaboration of inorganic pyrophosphate (PPi) parallels [3H]proline incorporation (less than 0.001, Wilcoxon rank sum). The effect on proline and PPi is abolished by ascorbate oxidase. Another reducing agent, Vitamin E, did not promote PPi accumulation about cartilage. Inhibitors of collagen synthesis, including monensin, tumor necrosis factor alpha, and 2',2'dipyridyl also inhibited PPi elaboration. Cycloheximide (1 micrograms/ml) inhibited the ascorbate stimulated PPi elaboration 54% but did not attenuate the secretion of PPi by unstimulated cartilage. Cosecretion of collagen and PPi by chondrocytes may explain these results. Moreover, at least two pathways exist for PPi elaboration, a cycloheximide sensitive path and another independent of new protein synthesis.

Transforming growth factor beta 1 stimulates inorganic pyrophosphate elaboration by porcine cartilage

The overproduction of inorganic pyrophosphate (PPi) by cartilage is thought to be a key element in the formation of calcium pyrophosphate dihydrate (CPPD) crystals in joints, and the subsequent development of pseudogout or chondrocalcinosis. We report herein that transforming growth factor beta 1 (TGF beta 1), alone and in synergy with epidermal growth factor (EGF) or TGF alpha, markedly stimulates PPi elaboration by porcine articular cartilage in organ culture and monolayer culture. This effect is not seen with platelet-derived growth factor, basic fibroblast growth factor, or insulin-like growth factor types 1 and 2, substances which also affect chondrocyte metabolism or are mitogenic. TGF beta 1 produces only a modest increase in nucleoside triphosphate pyrophosphohydrolase (NTPPPH), a chondrocyte ectoenzyme that produces PPi; this implies the existence of other pathways for PPi elaboration. TGF beta 1 is present in joint fluid and cartilage. TGF beta 1, TGF alpha, and EGF are the first known physiologic modifiers of cartilage PPi production. They provide a novel model for the study of CPPD crystal formation in cartilage, as well as new insights into the pathogenesis of this common affliction of aging.