We studied the role of the immediate early gene c-jun in cell proliferation and phorbol 12-myristate 13-acetate (PMA)-induced differentiation in U937 human monoblastic cells, using c-jun-specific antisense (AS) phosphorothioate oligonucleotides. In selecting the most specific and potent oligonucleotide sequence, we performed extensive analyses for the binding specificity between all candidates of c-jun AS oligonucleotides and the whole sequences in GenBank database, using a computer program. Among the 20 selected oligonucleotides, two potent 15-mer AS oligonucleotides (C-JUN AS oligonucleotides) exhibited significant inhibition of cell growth in a dose-dependent manner between 2 and 10 microM. Reverse transcription-PCR and Western blot analysis demonstrated that 10 microM of C-JUN AS oligonucleotides reduced c-jun expression at both the mRNA and protein levels. More importantly, C-JUN AS oligonucleotides showed distinct effects on two markers of PMA-induced differentiation; the C-JUN AS oligonucleotides inhibited cell adhesion, whereas they did not affect another marker of differentiation, respiratory burst (measured by nitro blue tetrazolium reduction assay). These results suggest a critical role of c-jun in both cell proliferation and PMA-induced cell adhesion but not in PMA-induced respiratory burst in U937 cells.