Dual regulation of soluble tumor necrosis factor-α induced activation of human monocytic cells via modulating transmembrane TNF-α-mediated ‘reverse signaling’

Transmembrane tumor necrosis factor-α (mTNF-α) is known to be the precursor of soluble TNF-α (sTNF-α). mTNF-α can act as a ligand on the TNF receptor- (TNFR)- bearing cell through ‘forward signaling’ or as a receptor on the TNF producing cell through ‘reverse signaling’. In the current study, we investigated the role of mTNF-α-mediated reverse signaling in regulating sTNF-α-induced activation of human monocytic U937 cells. We demonstrated that pretreatment with sTNFRI, for inducing reverse signaling through mTNF-α, sensitized U937 cells to sTNF-α stimulation, as evidenced by an increase in reactive oxygen production and mRNA levels of proinflammatory cytokines (TNF-α, IL-1β, and IL-8) in these cells. Further experiments revealed that IκB-α degradation was increased in the monocytic cells primed with sTNFRI, implying that reverse signaling of mTNF-α sensitizes U937 cells via an NF-κB-dependent mechanism. On the other hand, binding of sTNFRI to mTNF-α after sTNF-α-induced activation of U937 cells reduced mRNA stability (half-life) of IL-1β and IL-8. The involvement of reverse signaling in the process was verified by using a mutated form of mTNF-α lacking the majority of the cytoplasmic domain. Our results clearly showed that enhanced mRNA degradation of the cytokines occurred only in U937 cells transfected with a wild-type mTNF-α, but not in those cells transfected with the mutant mTNF-α. Taken together, these data suggest that reverse signaling through mTNF-α may exert a double role in modulating sTNF-α bioactivity. It is positive when reverse signaling occurs prior to sTNF-α stimulation, while it is negative when reverse signaling occurs after the sTNF-α signal. Thus, our findings strengthen a role of mTNF-α-mediated reverse signaling in the regulation of immune-inflammatory response and control of inflammatory reaction.