The CD44 receptor of lymphoma cells: Structure-function relationships and mechanism of activation

Migration of some tumor cells, and their lodgment in target organs, is depe ndent on the activation of cell surface CD44 receptor, usually detected by its ability to bind hyaluronic acid (HA) or other ligands. In an attempt to reveal the mechanism of tumor cell CD44 activation, we compared the physic al and chemical properties of CD44 in nonactivated LB cell lymphoma with th ose in phorbol 12-myristate 13-acetate (PMA)-activated LB cells and of an L B cell subline (designated HA9) expressing constitutively-active CD44. In c ontrast to nonactivated LB cells, PMA-activated LB cells and HA9 cells disp layed a CD44-dependent ability to bind HA. The ability of activated cell CD 44 to bind HA was not dependent on microfilament or microtubule integrity o r on changes in CD44 mobility on the membrane plane, indicating that the CD 44 activation status is not associated with cytoskeleton function. Aside fr om the increased expression of CD44 on the surface of PMA-activated LB cell s and HA9 cells, qualitative differences between the CD44 of nonactivated a nd activated LB cells were also detected: the CD44 of the activated lymphom a was (i) larger in molecular size, (ii) displayed a broader CD44 isoform r epertoire, including a CD44 variant that binds HA, and (iii) its glycoprote in contained less sialic acid. Indeed, after removal of sialic acid from th eir cell surface by neuraminidase, LB cells acquired the ability to bind HA . However, a reduced dose of neuraminidase did not confer HA binding on LB cells, unless they were also activated by a low concentration of PMA, which by itself was ineffective. Similarly, under suboptimal conditions, a syner gistic effect was obtained with tunicamycin and PMA: each one alone was ine ffective but in combination they induced the acquisition of HA binding by t he lymphoma cells, while their CD44 expression was not enhanced. Unveiling of the activation mechanism of CD44, by exposing the cells to PMA stimulati on or to deglycosylation, is not only academically important, but it also h as practical implications, as activated CD44 may be involved in the support of tumor progression.