α₂-HS-glycoprotein: Expression in chondrocytes and augmentation of alkaline phosphatase and phospholipase A₂ activity

The α₂-HS-glycoprotein is a plasma protein synthesized in liver and enriched in bone. The concentration of α₂-HS-glycoprotein dynamically changes in various physiological conditions and is highest in bone during growth, suggesting that it is involved in regulation of endochondral ossification. Northern blot analysis demonstrated that mRNA transcripts from growth zone and resting zone costochondral chondrocyte cultures hybridized with α₂-HS-glycoprotein cDNA. However, a difference of mRNA transcript size was observed, with chondrocyte mRNA transcripts being 2.2 kb, while mRNA isolated from liver was 1.6 kb. Presence of α₂-HS-glycoprotein in cartilage cells was found by immuno-histochemical staining of human fetal epiphyses using anti-human α₂-HS-glycoprotein antibody. To understand the role of α₂-HS-glycoprotein in cartilage growth, the effects of exogenous α₂-HS-glycoprotein were correlated with alkaline phosphatase (ALPase) and phospholipase A₂ (PA₂) activity in the chondrocyte cultures. Alkaline phosphatase specific activity was stimulated by α₂-HS-glycoprotein at concentrations between 0.25 and 1.25μ/mL in the growth zone and resting zone cultures 2.7 and 2.0-fold, respectively. Matrix vesicle PA₂ activity was increased only in the growth zone chondrocyte cultures. These results suggested that α₂-HS-glycoprotein may contribute to the regulation of the expression of the chondrocyte phenotype. Steady state mRNA levels of ALPase were analyzed in chondrocytes after additions of α₂-HS-glycoprotein. The ALPase mRNA levels remained stationary during the stimulation of enzymatic activity, indicating that the effect of α₂-HS-glycoprotein upon alkaline phosphatase activity is not at the transcriptional level.