Alpha 2-HS-glycoprotein: expression in chondrocytes and augmentation of alkaline phosphatase and phospholipase A2...

The alpha 2-HS-glycoprotein is a plasma protein synthesized in liver and enriched in bone. The concentration of alpha 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 alpha 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 alpha 2-HS-glycoprotein in cartilage cells was found by immunohistochemical staining of human fetal epiphyses using anti-human alpha 2-HS-glycoprotein antibody. To understand the role of alpha 2-HS-glycoprotein in cartilage growth, the effects of exogenous alpha 2-HS-glycoprotein were correlated with alkaline phosphatase (ALPase) and phospholipase A2 (PA2) activity in the chondrocyte cultures. Alkaline phosphatase specific activity was stimulated by alpha 2-HS-glycoprotein at concentrations between 0.25 and 1.25 micrograms/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 alpha 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 alpha 2-HS-glycoprotein. The ALPase mRNA levels remained stationary during the stimulation of enzymatic activity, indicating that the effect of alpha 2-HS-glycoprotein upon alkaline phosphatase activity is not at the transcriptional level.