CD44 ISOFORMS CONTAINING EXON V3 ARE RESPONSIBLE FOR THE PRESENTATIONOF HEPARIN-BINDING GROWTH-FACTOR

Glycosaminoglycan-modified isoforms of CD44 have been implicated in gr owth factor presentation at sites of inflammation, In the present stud y we show that COS cell transfectants expressing CD44 isoforms contain ing the alternatively spliced exon V3 are modified with heparan sulfat e (HS), Binding studies with three HS-binding growth factors, basic-fi broblast growth factor (b-FGF), heparin binding-epidermal growth facto r (HB-EGF), and amphiregulin, showed that the HS-modified CD44 isoform s are able to bind to b-FGF and HB-EGF, but not AR, b-FGF and HB-EGF b inding to HS-modified CD44 was eliminated by pretreating the protein w ith heparitinase or by blocking with free heparin. HS-modified CD44 im munoprecipitated from keratinocytes, which express a CD44 isoform cont aining V3, also bound to b-FGF, We examined whether HS-modified CD44 i soforms were expressed by activated endothelial cells where they might present HS-binding growth factors to leukocytes during an inflammator y response. PCR and antibody-binding studies showed that activated cul tured endothelial cells only express the CD44H isoform which does not contain any of the variably spliced exons including V3. Immunohistolog ical studies with antibodies directed to CD44 extracellular domains en coded by the variably spliced exons showed that vascular endothelial c ells in inflamed skin tissue sections do not express CD44 spliced vari ants. Keratinocytes, monocytes, and dendritic cells in the same specim ens were found to express variably spliced CD44. (SO4-2)-S-35-labeling experiments demonstrated that activated cultured endothelial cells do not express detectable levels of chondroitin sulfate or HS-modified C D44, Our results suggest that one of the functions of CD44 isoforms ex pressing V3 is to bind and present a subset of HS-binding proteins. Fu rthermore, it is probable that HS-modified CD44 is involved in the pre sentation of HS-binding proteins by keratinocytes in inflamed skin, Ho wever, our data suggests that CD44 is not likely to be the proteoglyca n principally involved in presenting HS-binding growth factors to leuk ocytes on the vascular cell wall.