Abstract
Transcription factor hypoxia-inducible factor (HIF)-1 protein accumulates and activates the transcription of genes that are of fundamental importance for oxygen homeostasis - including genes involved in energy metabolism, angiogenesis, vasomotor control, apoptosis, proliferation, and matrix production - under hypoxic conditions. We speculated that HIF-1alpha may have an important role in chondrocyte viability as a cell survival factor during the progression of osteoarthritis (OA). The expression of HIF-1alpha mRNA in human OA cartilage samples was analyzed by real-time PCR. We analyzed whether or not the catabolic factors IL-1beta and H2O2 induce the expression of HIF-1alpha in OA chondrocytes under normoxic and hypoxic conditions (O2 <6%). We investigated the levels of energy generation, cartilage matrix production, and apoptosis induction in HIF-1alpha-deficient chondrocytes under normoxic and hypoxic conditions. In articular cartilages from human OA patients, the expression of HIF-1alpha mRNA was higher in the degenerated regions than in the intact regions. Both IL-1beta and H2O2 accelerated mRNA and protein levels of HIF-1alpha in cultured chondrocytes. Inhibitors for phosphatidylinositol 3-kinase and p38 kinase caused a significant decrease in catabolic-factor-induced HIF-1alpha expression. HIF-1alpha-deficient chondrocytes did not maintain energy generation and cartilage matrix production under both normoxic and hypoxic conditions. Also, HIF-1alpha-deficient chondrocytes showed an acceleration of catabolic stress-induced apoptosis in vitro. Our findings in human OA cartilage show that HIF-1alpha expression in OA cartilage is associated with the progression of articular cartilage degeneration. Catabolic-stresses, IL-1beta, and oxidative stress induce the expression of HIF-1alpha in chondrocytes. Our results suggest an important role of stress-induced HIF-1alpha in the maintenance of chondrocyte viability in OA articular cartilage.