Mechanism of lipopolysaccharide-triggered junB activation in a mouse macrophage-like cell line (J774).

The effect of lipopolysaccharide (LPS) on the activation of junB in a mouse macrophage cell line (J774) was investigated. J774 cells responded to either phorbol 12-myristate 13-acetate (PMA) or LPS by the transient increase in the expression levels of c-jun and junB mRNA, but not of junD mRNA. The prior depletion of protein kinase C from J774 cells blocked the action of PMA, but not of LPS, to activate junB. Pretreatment of cells with H-89 or H-7, but not with HA1004, W-7, ML-7, or tyrphostin 47, inhibited LPS-triggered junB activation. Treatment with forskolin also activated junB of J774 cells through an H-89- or H-7-sensitive pathway. Since cAMP-dependent protein kinase activity of J774 cells was inhibited by H-89, but not by H-7, LPS appears to activate junB through a cascade involving two steps, the one sensitive to H-89 and the other to H-7. Western blot analysis showed that LPS-triggered junB activation is accompanied by the increased expression of JunB proteins in the cell lysate as well as in the nuclear extract. JunB in nuclear fraction appears to specifically bind to 12-O-tetradecanoylphorbol-13-acetate-response element (TRE), since preincubation of nuclear extracts with anti-JunB serum reduced the amount of TRE-binding proteins and since the amount of JunB, but not of c-Jun or JunD, immunoprecipitated from TRE-cross-linked nuclear proteins increased in response to LPS. Thus, JunB may play an important role in LPS-triggered gene activation.