Regulation of type II collagen synthesis during osteoarthritis by prolyl-4-hydroxylases -: Possible influence of low oxygen levels

Osteoarthritic (OA) chondrocytes are metabolically active, displaying increased synthesis of type H collagen. Here, we show by immunohistochemistry and polymerase chain reaction that in comparison with healthy cartilage, OA articular chondrocytes; exhibit increased in vivo synthesis of collagen prolyl-4-hydroxylase type II, a pivotal enzyme in collagen triple helix formation. Exposure of primary human articular chondrocytes to 1% oxygen enhanced accumulation of native type H collagen and stabilized hypoxia-inducible factor-la (HEF-1 alpha). This effect was abolished by addition of the HIF-1 inhibitor 2-methoxyestradiol. Real-time polymerase chain reaction analyses of mRNAs from these cultures revealed increased transcript levels of both alpha-subunits of prolyl-4-hydroxylase (P4HA1, similar to 2-fold; P4HA2, similar to 2.3-fold) and of classical HIF-1 target genes (glucosetransporter-1, similar to 2.1-fold; phosphoglyceratekinase-1, similar to 2.2-fold). Treatment of hypoxic chondrocytes; with 2-methoxyestradiol reduced transcriptional activity of HIT-1 and synthesis of a(H), and to a lesser extent alpha(I), subunits of collagen prolyl-4-hydroxylases. mRNA levels of type H collagen (Col2A1) and the beta-subunit (P4HB) of prolyl-4-hydroxylase, however, displayed only modest changes at 1% oxygen. From these results and our in vivo data, we inferred that besides increased Col2A1 mRNA expression by OA chondrocytes, accelerated posttranslational modification processes might contribute to the increased synthesis and accumulation of type H collagen during OA and experimental hypoxia.