Cartilage inorganic pyrophosphate elaboration is independent of sulfated glycosaminoglycan synthesis

Hyperglycemia reduces proteoglycan levels in tendons

RATIONALE

Diabetic tendinopathy is characterized by increased stiffness, thickness, and excess calcification of affected tendons. We investigated the hypothesis that proteoglycans (PGs), as key regulators of tendon structure and calcification, are altered in diabetic tendons.

METHODS

Adult porcine patellar tendons were incubated in iso-osmolar media with high or normal glucose levels for 2 weeks. The PG fraction was isolated and analyzed. Protein and mRNA levels of five PGs were measured. PG production was assessed in primary tenocyte monolayers by (35)S-sulfate labeling in high and normal glucose conditions with and without exposure to advanced glycation end-products (AGEs). Levels of transforming growth factor β, which commonly mediates some effects of hyperglycemia, were also measured and the effects of free radical scavengers on (35)S-sulfate incorporation were determined.

RESULTS

PG levels were significantly decreased in tendons exposed to high glucose media compared with tendons in iso-osmolar control media. Relative quantities of individual PGs were unchanged by exposure to hyperglycemia and mRNAs for PGs were variably affected. High glucose media decreased PG production by tenocytes as measured by (35)S-sulfate labeling, whereas AGE-modified type I collagen and free radical scavengers had no effects. Hyperglycemic conditions increased levels of transforming growth factor β1 in an AGE-independent manner.

CONCLUSIONS

Hyperglycemia produces a reduction in PG levels related to decreased synthesis or sulfation of glycosaminoglycans, which may contribute to the tendon pathology observed clinically in diabetes.

Interleukin 1 beta suppresses transforming growth factor-induced inorganic pyrophosphate (PPi) production and expression of the PPi-generating enzyme PC-1 in human chondrocytes

Articular cartilage chondrocytes have the unique ability to elaborate large amounts of extracellular pyrophosphate (PPi), and transforming growth factor beta (TGF beta) appears singular among cartilage regulatory factors in stimulating PPi production. TGF beta caused a time and dose-dependent increase in intracellular and extracellular PPi in human articular chondrocyte cultures. TGF beta and interleukin 1 beta (IL-1 beta) antagonistically regulate certain chondrocyte functions. IL-1 beta profoundly inhibited basal and TGF beta-induced PPi elaboration. To address mechanisms involved with the regulation of PPi synthesis by IL-1 beta and TGF beta, we analyzed the activity of the PPi-generating enzyme NTP pyrophosphohydrolase (NTPPPH) and the PPi-hydrolyzing enzyme alkaline phosphatase. Human chondrocyte NTPPPH activity was largely attributable to plasma cell membrane glycoprotein 1, PC-1. Furthermore, TGF beta induced comparable increases in the activity of extracellular PPi, intracellular PPi, and cellular NTPPPH and in the levels of PC-1 protein and mRNA in chondrocytes as well as a decrease in alkaline phosphatase. All of these TGF beta-induced responses were completely blocked by IL-1 beta. Thus, IL-1 beta may be an important regulator of mineralization in chondrocytes by inhibiting TGF beta-induced PPi production and PC-1 expression.

Calcium pyrophosphate dihydrate crystal deposition disease: a review of the literature and a light and electron microscopic study of a case of the temporomandibular joint with numerous intracellular crystals in the chondrocytes

The pathogenesis of calcium pyrophosphate dihydrate (CPPD) crystal deposition disease of synovial joints is still unclear, although overproduction of extracellular pyrophosphate (PPi) is thought to play a key role. We studied the light and electron microscopic appearances of a case of CPPD crystal deposition disease of the temporomandibular joint (TMJ) in search of new clues for its pathogenesis. Light microscopic examination of CPPD-containing material from the joint space revealed cartilaginous nodules with various degrees of crystallization. Transmission electron microscopic examination revealed numerous extra- as well as intracellular crystals and crystal shaped spaces in the chondrocytes. Other striking ultrastructural features of the chondrocytes included the presence of many mitochondria, frequently containing crystalline material, and the presence of highly dilated rough endoplasmic reticulum and large glycogen islands. The presence of intramitochondrial crystals may hypothetically imply a derangement in mitochondrial adenosine triphosphate or PPi metabolism. The finding of intracellular CPPD crystals in chondrocytes points to the existence of an intracellular pathway of CPPD crystal formation in CPPD crystal deposition disease of the TMJ and possibly in CPPD crystal deposition disease in general.

Causal link between nucleotide pyrophosphohydrolase overactivity and increased intracellular inorganic pyrophosphate generation demonstrated by transfection of cultured fibroblasts and osteoblasts with plasma cell membrane glycoprotein-1. Relevance to calcium pyrophosphate dihydrate deposition disease

OBJECTIVE

In subjects with idiopathic calcium pyrophosphate dihydrate (CPPD) deposition disease, cartilage chondrocytes elaborate increased amounts of PPi. The mechanism of the intracellular PPi elevation is not known. Plasma membrane 5'-nucleotide phosphodiesterase I/nucleotide pyrophosphohydrolase (NTPPPH) activity also is elevated in chondrocytes and dermal fibroblasts of patients with idiopathic CPPD deposition disease. NTPPPH, as an ecto-enzyme, could act within certain intracellular compartments. Thus, we hypothesized a potential causal link between increased NTPPPH activity and increased intracellular PPi.

METHODS

Transformed simian fibroblasts (COS cells) and human osteoblasts (U2OS cells) were transfected with the 5'-nucleotide phosphodiesterase I ecto-enzyme plasma cell membrane glycoprotein-1 (PC-1), recently shown to be expressed in cartilage, osteoblasts, and fibroblasts.

RESULTS

Transfection with PC-1 markedly up-regulated 5'-nucleotode phosphodiesterase I activity and increased intracellular PPi concentrations by increasing the capacity of cells to generate PPi. Importantly, this did not require supplementation with exogenous nucleotides.

CONCLUSION

Cellular overexpression of PC-1 produces NTPPPH overactivity and increased intracellular PPi generation in vitro. These findings support the potential importance of NTPPPH overactivity in PPi generation, both inside and outside the cell, in some subjects with CPPD deposition disease.

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.