CD44-mediated uptake and degradation of hyaluronan.

Hyaluronan turnover occurs systemically from the lymph and serum as well as locally by the same cells responsible for its synthesis. Local turnover involves receptor-mediated uptake and delivery to lysosomes. Of the many hyaluronan binding proteins/receptors known, the participation of CD44 in the internalization of hyaluronan has been best characterized. Some fraction of the hyaluronan bound to CD44 becomes internalized and delivered to lysosomes by a mechanism that is not dependent on clatherin, caveolae or pinocytosis. In cells such as chondrocytes, anabolic and catabolic cytokines can alter the activity of CD44 toward hyaluronan internalization. However, the mechanism of cellular regulation remains unclear. Regulation may involve the participation of alternatively spliced isoforms of CD44, changes in CD44 phosphorylation, changes in cytoskeletal binding proteins or, the activity or extracellular proteolytic activity.