The CD9 antigen, a major platelet glycoprotein, is a member of the tetraspan superfamily. We show that treatment of K562 cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) which induces megakaryocytic differentiation, leads to a seven-fold increase in CD9 expression, which becomes associated with the integrin beta1, suggesting that it is functionally relevant. The upregulation of CD9 expression precedes the appearance of the megakaryocytic-specific marker GPIIb (CD41) as well as integrins beta3 (GPIIIa/CD61), alpha v (CD51) and VLA-2 (CD49b). Both GPIIb/IIIa expression and CD9 upregulation are dependent on protein kinase C (PKC) activation since they are blocked by the specific inhibitor GF109203X. Steady-state levels of CD9 and GPIIb 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 increase of the CD9 mRNA was detected much earlier than the increase of GPIIb mRNA (1-2 h vs 24-48 h). Using different constructs of the 5'-flanking domain of the CD9 gene cloned ahead of the CAT reporter gene, we could demonstrate that a responsive element was located in a 52 bp fragment of the promoter of the CD9 gene. Altogether, these data suggest that CD9 upregulation in the megakaryocytic lineage could occur at early stages of differentiation.