To investigate crosslinks between catabolic and anabolic pathways in articular cartilage by examining the synthesis and distribution pattern of microsomal prostaglandin E synthase 1 (mPGES-1) in healthy and osteoarthritic (OA) cartilage and analyzing its functional relationship to hypoxia-inducible factor 1alpha (HIF-1alpha) in primary articular chondrocytes.
Normal cartilage and OA cartilage were subjected to immunohistochemical staining for mPGES-1 and HIF-1alpha. Isolated chondrocytes were cultivated under 21% or 1% O(2). Microarray analysis and quantitative reverse transcriptase-polymerase chain reaction were used to detect genes differentially expressed in chondrocytes cultured under normoxic compared with hypoxic conditions. Immunoblotting was conducted to evaluate intracellular protein levels of mPGES and nuclear accumulation of HIF-1alpha under different oxygen tension levels and with different stimulatory or inhibitory chemical agents.
We found enhanced levels of expression of the mPGES-1 gene and an increased number of OA chondrocytes showing staining for mPGES-1 in OA cartilage. Microarray analysis demonstrated that mPGES-1 was among the genes that were up-regulated to the greatest degree in primary chondrocytes exposed to 1% O(2). In vitro, hypoxia led to an enhanced synthesis of mPGES-1, coinciding with a nuclear accumulation of the transcription factor HIF-1alpha. In chondrocyte culture, stimulation with dimethyloxaloylglycine promoted the expression of mPGES-1, phosphoglycerate kinase 1, and cyclooxygenase 2 (COX-2) by stabilizing HIF-1alpha protein levels. A reduction of mPGES-1 synthesis was detected after treatment with 2-methoxyestradiol, correlating with lower HIF-1alpha activity. In contrast, synthesis of mPGES-1 was not influenced by treatment with the specific COX-2 inhibitor NS398.
These findings suggest that the transcription factor HIF-1alpha is involved in the up-regulation of mPGES-1 and may therefore play an important role in the metabolism of OA cartilage.