UP-REGULATION OF CD9 EXPRESSION DURING TPA TREATMENT OF K562 CELLS

The CD9 antigen, a major platelet glycoprotein, is a member of the tet raspan superfamily. We show that treatment of K562 cells with 12-O-tet radecanoylphorbol-13-acetate (TPA) which induces megakaryocytic differ entiation, leads to a seven-fold increase in CD9 expression, which bec omes associated with the integrin beta 1, suggesting that it is functi onally relevant. The upregulation of CD9 expression precedes the appea rance of the megakaryocytic-specific marker GPllb (CD41) as well as in tegrins beta 3 (GPllla/CD61), cuv (CD51) and VLA-5 (CD49b). Both GPllb /llla expression and CD9 upregulation are dependent on protein kinase C (PKC) activation since they are blocked by the specific inhibitor GF 109203X. Steady-state levels of CD9 and GPllb mRNA were also measured by quantitative RT-PCR. Both messengers were detected on resting cells and were shown to accumulate during TPA treatment. However, the incre ase of the CD9 mRNA was detected much earlier than the increase of GPl lb mRNA (1-2 h vs 24-48 h). Using different constructs of the 5'-flank ing domain of the CD9 gene cloned ahead of the CAT reporter gene, we c ould demonstrate that a responsive element was located in a 52 bp frag ment of the promoter of the CD9 gene. Altogether, these data suggest t hat CD9 upregulation in the megakaryocytic lineage could occur at earl y stages of differentiation.