Pure lipopolysaccharide or synthetic lipid A induces activation of p21Ras in primary macrophages through a pathway dependent on Src family kinases and PI3K.

Recognition of bacterial LPS by macrophages plays a critical role in host defense against infection by Gram-negative bacteria. However, when not tightly regulated, the macrophage's response to LPS can induce severe disease and septic shock. Although LPS triggers the activation of multiple signaling pathways in macrophages, it was unclear whether these include activation of the p21Ras GTPases. We report that p21Ras is rapidly and transiently activated in murine primary macrophages stimulated with an ultra-pure preparation of LPS or with synthetic lipid A. The molecular basis of this activation was investigated using a pharmacological approach. LPS-induced activation of p21Ras was inhibited in the presence of PP2, LY294002, or wortmannin, suggesting that it depends on the activity of one or more members of the Src kinase family and the subsequent activation of PI3K. In that pharmacological inhibitors of PI3K inhibited LPS-induced activation of p21Ras, but not activation of ERK, we concluded that LPS-induced activation of ERK occurs through a pathway that is not dependent on the activation of p21Ras.