REGULATION OF CD9 EXPRESSION DURING 12-O-TETRADECANOYLPHORBOL-13-ACETATE-INDUCED DIFFERENTIATION OF HUMAN MYELOID-LEUKEMIA (HL-60) CELLS

CD9 antigen is a member of the tetra spans superfamily of proteins whi ch are expressed on the surface of a variety of hematopoietic and epit helial cell types. CD9 appears to play a role in platelet activation a nd to enhance sensitivity of cells to diphtheria toxin through its ass ociation with the diphtheria toxin receptor. Although several studies indicate that treatment of specific hematopoietic cells with the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA), induces CD9 exp ression, the mechanisms by which CD9 expression is regulated have not been elucidated. Here, we provide evidence that, in HL-60 cells, incre ases in the level of CD9 protein occur in parallel with TPA-induced di fferentiation. More than 80% of HL-60 cells exposed to 17 nM TPA becom e CD9 positive within 24 h. CD9 mRNA levels increase within 8-10 h aft er starting TPA treatment. Activation of CD9 transcription occurs duri ng the same time period. Both transcriptional activation and accumulat ion of CD9 mRNA require protein synthesis. However, once CD9 mRNA has accumulated, inhibition of protein synthesis has no effect on its leve l or rate of turnover. These results suggest that CD9 expression in TP A-treated HL-60 cells is regulated at the transcriptional level and th at activation of transcription occurs subsequent to the production of proteins induced as an immediate-early response to TPA. Since CD9 expr ession is not induced in HL-60TR cells, which respond to TPA but are r esistant to its differentiating effects, the results also indicate tha t CD9 expression may serve as a marker for monocyte/macrophage differe ntiation of HL-60 cells.