CD40-mediated signaling in monocytic cells: up-regulation of tumor necrosis factor receptor-associated factor mRNAs and activation of mitogen-activated protein kinase signaling pathways

The biochemical pathways involved in CD40 signaling have been extensively s tudied in B cells and B cell lines, and appear to be primarily initiated by recruitment of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) signaling proteins to the CD40 cytoplasmic domain, Signaling pathway s activated through CD40 in monocytes/macrophages have not been characteriz ed as well as in a cells. Using human monocytes and the human monocytic cel l line THP1, we examined signal transduction events induced by CD40 engagem ent with its ligand, CD154. In human monocytes, all TRAF mRNAs were express ed constitutively and CD40 ligation resulted in a strong up-regulation of T RAF1 mRNA. In THP1 cells, CD40 ligation induced expression of TRAF1 and TRA F5 mRNAs, Engagement of CD40 in both monocytes and THP1 cells led to the ra pid and transient activation of the extracellular signal-regulated kinases (ERK) 1 and 2, and to low levels of JNK activation, No CD-40-dependent acti vation of p38 mitogen-activated protein kinase (MAPK) was found. In CD154-s timulated monocytes and THP1 cells the upstream ERK1/2 activator, MAPK kina se (MEK) 1/2, and downstream substrate, c-Myc, were activated, By blocking activation of ERK1/2 with a MEK-specific inhibitor, PD98059, CD40-dependent secretion of the pro-inflammatory cytokines, TNF-alpha, IL-6 and IL-8, was demonstrated to be linked to the ERK1/2 pathway. The ERK1/2 pathway did no t appear to be involved in up-regulating TRAF1 and TRAF5 mRNAs in THP1 cell s, Collectively, these results suggest distinct differences between B cells and monocytic cells in CD40-dependent activation of MAPK pathways.