Implication of Gα_i proteins and Src tyrosine kinases in edotoxin-induced signal transduction events and mediator production

Previous studies have suggested that heterotrimeric G proteins and tyrosine kinases may be involved in lipopolysaccharide (LPS) signaling events. Signal transduction pathways activated by LPS were examined in human promonocytic THP-1 cells. We hypothesized that G_i proteins and Src tyrosine kinase differentially affect mitogen-activated protein (MAP) kinases (MAPK) and nuclear factor kappa (NF-κB) activation. Post-receptor coupling to Gα_i, proteins were examined using pertussis toxin (PTx), which inhibits Gα_i receptor-coupling. The involvement of the Src family of tyrosine kinases was examined using the selective Src tyrosine kinase inhibitor pyrazolopyrimidine-2 (PP2). Pretreatment of THP-1 cells with PTx attenuated LPS-induced activation of c-Jun-N-terminal kinase (JNK) and p38 kinase, and production of tumor necrosis factor-alpha (TNF-α) and thromboxane B₂ (TxB₂). Pretreatment with PP2 inhibited TNF-a and TxB₂ production, but had no effect on p38 kinase or JNK signaling. Therefore, the Gα_i-coupled signaling pathways and Src tyrosine kinase-coupled signaling pathways are necessary for LPS-induced TNF-α and TxB₂ production, but differ in their effects on MAPK activation. Neither PTx nor PP2 inhibited LPS-induced activation of interleukin receptor activated kinase (IRAK) or inhibited trmslocation of NF-κB. However, PP2 inhibited LPS-induced NF-κB transactivation of a luciferase reporter gene construct in a concentration-dependent manner. Thus, LPS induction of Src tyrosine kinases may be essential in downstream NF-κB transactivation of genes following DNA binding. PTx had no effect on NF-κB activation of the reporter construct. These data suggest upstream divergence in signaling through Gα_i pathways leading to MAPK activation and other signaling events leading to IκBα degradation and NF-κB DNA binding.