Influence of extracellular pyrophosphate on the synthesis of collagen by chondrocytes

Effects of thapsigargin, an intracellular calcium-mobilizing agent, on synthesis and secretion of cartilage collagen and proteoglycan

The calcium-mobilizing agents thapsigargin and 2,5-di-(tert-butyl)-1,4- benzohydroquinone were shown to markedly elevate the intracellular calcium concentration of chick embryo chondrocytes in a dose-dependent manner. Under these conditions, the metabolism of macromolecules was variably affected. The synthesis and secretion of protein in general, and of collagen in particular, were significantly inhibited; in contrast, proteoglycan synthesis (but not glycosaminoglycan synthesis) was inhibited, whereas secretion was unaffected. Flunarizine, which prevented the thapsigargin-induced intracellular calcium elevation, and EGTA, which caused only a transient thapsigargin-induced intracellular calcium elevation, did not reverse these alterations. It was concluded, therefore, that the observed effects of thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone on chondrocyte macromolecule metabolism were not related to the ability of these drugs to increase the cytosolic free calcium concentration but may have been due to the specific depletion of the calcium sequestered in the endoplasmic reticulum. The differential effect of these drugs on protein and proteoglycan secretion suggests that the intracellular trafficking of these two classes of macromolecules may be controlled independently.

Effects of calcium deficiency on chondrocyte hypertrophy and type X collagen expression in chick embryonic sternum

Maintenance of chick embryos in long-term culture without their calcareous egg-shell is a useful method for studying the relationship between calcium homeostasis and cell differentiation during skeletogenesis. Previously, we have shown that in shell-less (SL) embryos, calcium deficiency induces a cartilage-like phenotype in osteogenic tissues, such as calvaria (Jacenko and Tuan [1986] Dev. Biol. 115:215). In this investigation, we have studied the relationship between cartilage calcification and hypertrophy, and the expression of type X collagen, a specific product of hypertrophic chondrocytes. For this study, the cephalic (calcifying) and caudal (permanently cartilaginous) regions of sterna from day 18 and day 20 normal (NL) and SL embryos were metabolically labeled with [14C]-proline. Analysis of the biosynthetic products revealed significant differences in type X collagen expression in the cephalic region of sternal cartilage. In NL tissues, type X collagen production increased from 13.1% of total collagen at day 18 to 43.7% at day 20. In contrast, in SL embryos, type X collagen was not detectable until day 20, when it represented only 1% of total collagen. Comparison of the NL and SL embryos with respect to their serum calcium level and sternal calcium content and histology revealed a direct relationship between low systemic calcium and limited cartilage hypertrophy, undermineralization, and decreased type X collagen production in the sternal cephalic cartilage. Supplementation of CaCO3 to SL embryos increased their serum and sternal calcium, and restored cartilage hypertrophy, mineralization, and type X collagen synthesis in the cephalic portion of the sterna. These findings confirm that a critical relationship exists between calcium homeostasis, chondrocyte hypertrophy, mineralization, and type X collagen synthesis in the cephalic region of sternal cartilage. These results further demonstrate the importance of calcium in the morphogenetic events of endochondral ossification, in particular the transition from hyaline cartilage to hypertrophic cartilage, and eventually to bone.

Measurement of soluble pyrophosphate in plasma and synovial fluid of patients with various rheumatic diseases

Soluble pyrophosphate was measured in the plasma and synovial fluid of various groups of patients and in the plasma of two control groups. The two control groups consisted of 13 healthy subjects and 19 patients suffering from benign lumbar back pain. The other group of patients had rheumatoid arthritis (RA) (14 plasma and 19 synovial fluid examinations), osteoarthrosis (OA) (19 plasma and 26 synovial fluids) and articular chondrocalcinosis (ACC) (27 plasma and 43 synovial fluids). The level of soluble pyrophosphate in the plasma was 3.5 mumol/l in healthy subjects, 4.0 mumol/l in patients with lumbar back pain, 4.1 mumol/l in individuals having OA and 3.5 mumol/l in the group suffering from RA as well as for those with ACC. The differences between these values are not significant statistically. In the synovial fluid the values were 4.6 mumol/l for the group with RA, 12.7 mumol/l for those with OA and 34.2 mumol/l in the group having ACC. If a normal distribution of these values is assumed and the average values and standard deviations recalculated for each group after elimination of cases more than 3 standard deviations above the mean, then we obtain 9.8 mumol/l for the group with OA and 23.8 mumol/l for those with ACC. The difference between the group with RA and that with OA is highly significant (p greater than 0.0001). Even more significant is the difference between the group with RA and ACC (p less than 0.0005). The difference between the OA and the ACC is also highly significant (p less than 0.001). On the basis of these observations various mechanisms leading to the pyrophosphage crystal deposition disease are discussed.