Small hyaluronan (HA) oligosaccharides displace HA from the cell surface and induce cell signaling events. In articular chondrocytes this cell signaling is mediated by the HA receptor CD44 and includes stimulation of genes involved in matrix degradation such as matrix metalloproteinases (MMPs) as well as matrix repair genes including collagen type II, aggrecan and HA synthase-2 (HAS-2). The objective of this study was to determine whether stimulation of HAS-2 and MMP-3 by HA oligosaccharides is due to the activation of a single, cascading pathway or multiple signaling pathways.
Bovine articular chondrocytes were pre-treated with a variety of inhibitors of major signaling pathways prior to the addition of HA oligosaccharides. Changes in HA were monitored by real time reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of HAS-2 mRNA, HA ELISA and HA accumulation at the cell surface. A 1900 base pair sequence containing the proximal promoter of HAS-2 was inserted into a luciferase reporter construct, transfected into human immortalized chondrocytes and assayed in a similar fashion.
While our previous studies demonstrated that HA oligosaccharides stimulate MMP-13 activity via activation of p38 MAP kinase and NF-kappaB, inhibitors of these pathways did not affect the stimulation of HAS-2 mRNA expression. However, inhibiting the phosphatidylinositol-3-kinase pathway blocked HA oligosaccharide-mediated stimulation of HAS-2 yet had no effect on MMP-3. Wortmannin and LY294002 also blocked HA oligosaccharide-induced serine and threonine Akt phosphorylation. Treatment of transfected immortalized chondrocytes with HA oligosaccharides resulted in stimulation of HAS-2 mRNA, activation of Akt and enhanced luciferase activity-activity that was blocked by inhibitors of Akt phosphorylation.
Changes in chondrocyte-matrix interactions by HA oligosaccharides induce altered matrix metabolism by the activation of least two distinct signaling pathways.