Differential regulation and expression of hyaluronan synthases in human articular chondrocytes, synovial cells and osteosarcoma cells

Abstract

Recently three isoforms of hyaluronan synthase (HAS), the enzyme responsible for hyaluronate/hyaluronan (HA) biosynthesis, have been cloned, allowing us to study their expression pattern. Our objective was to determine which of the HAS isoenzymes were expressed in human articular chondrocytes, synovial fibroblasts and osteosarcoma cells, whether their expression could be modulated by growth factors (insulin-like growth factor-1, basic fibroblast growth factor and transforming growth factor (TGF-β1) and cytokines [interleukin 1β1 (IL-1β)], and whether changes in the rate of HA synthesis by the cells correlated with changes in mRNA levels for one or more of the HAS isoforms. All three HAS isoforms were found to be expressed in the cultured cells analysed in this study, although the relative proportions varied for each cell type. HAS2 mRNA was usually predominant in chondrocytes, whereas synovial cells contained increased amounts of HAS1. HAS3 was always the least abundant message. The rapidly growing osteosarcoma cells contained almost exclusively HAS2 message. HAS usage in uncultured cartilage and synovial tissues was similar to that in the cultured cells, with HAS2 message being the predominant species in cartilage and HAS1 usually being the predominant species in synovium. HA synthesis was stimulated by the growth factors, but the extent of the response was cell-type specific. Synovial cells responded particularly well to IL-1β, and showed a unique synergistic response when IL-1β was used in combination with TGF-β1. This response was much reduced in articular chondrocytes and absent in the osteosarcoma cells. Analysis of changes in HAS message levels indicated that there was often no correlation with the changes in HA secretion following exposure to growth factors. Although HAS-1 mRNA was increased in synovial cells after exposure to TGF-β1/IL-1β, the magnitude of the change was far less than the effect on HA synthesis. Our data thus suggest that HAS gene usage is tissue specific, and the regulation by growth factors is unique for each HAS gene and is further modulated by cell-specific factors. In addition, regulation of HA biosynthesis appears to be multi-faceted, with control of HAS gene expression and mRNA levels being only one aspect of this process.