Osteoarthritic (OA) chondrocytes are metabolically active, displaying increased synthesis of type II 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 II collagen and stabilized hypoxia-inducible factor-1alpha (HIF-1alpha). 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, approximately 2-fold; P4HA2, approximately 2.3-fold) and of classical HIF-1 target genes (glucosetransporter-1, approximately 2.1-fold; phosphoglyceratekinase-1, approximately 2.2-fold). Treatment of hypoxic chondrocytes with 2-methoxyestradiol reduced transcriptional activity of HIF-1 and synthesis of alpha(II), and to a lesser extent alpha(I), subunits of collagen prolyl-4-hydroxylases. mRNA levels of type II 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 II collagen during OA and experimental hypoxia.