α2-HS-glycoprotein: Expression in chondrocytes and augmentation of alkaline phosphatase and phospholipase A 2 activity

Abstract

The α 2-HS-glycoprotein is a plasma protein synthesized in liver and enriched in bone. The concentration of α 2-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 α 2-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 α 2-HS-glycoprotein in cartilage cells was found by immuno-histochemical staining of human fetal epiphyses using anti-human α 2-HS-glycoprotein antibody. To understand the role of α 2-HS-glycoprotein in cartilage growth, the effects of exogenous α 2-HS-glycoprotein were correlated with alkaline phosphatase (ALPase) and phospholipase A 2 (PA 2) activity in the chondrocyte cultures. Alkaline phosphatase specific activity was stimulated by α 2-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 PA 2 activity was increased only in the growth zone chondrocyte cultures. These results suggested that α 2-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 α 2-HS-glycoprotein. The ALPase mRNA levels remained stationary during the stimulation of enzymatic activity, indicating that the effect of α 2-HS-glycoprotein upon alkaline phosphatase activity is not at the transcriptional level.