Mature macrophage cell lines exhibit variable responses to LPS

Abstract

Bacterial lipopolysaccharide (LPS) is a potent activator of cells of the macrophage/monocyte lineage. Two mature macrophage cell lines, P388D1 and RAW264.7, exhibit very different biological responses to LPS. Although RAW264.7 cells release arachidonic acid from phospholipid in response to LPS stimulation, P388D1 cells do not respond in this manner. However, LPS primes P388D1 cells to release arachidonic acid in response to other stimuli. The goal of this work is to contrast the biochemical events that occur in LPS-treated P388D1 and RAW264.7 macrophages. Enzyme assays indicate that LPS treatment induces the activation of cytosolic PLA2 in RAW264.7, but not in P388D1 cells. Phorbol ester (PMA), a receptor-independent stimulus, also fails to induce arachidonic acid release from P388D1 cells, suggesting that these cells may have a defect in the signal transduction machinery that is common to LPS and PMA. This hypothesis is supported by the observation that the expression of the LPS receptors CD14 and CD11b/CD18 is similar on P388D1 and RAW264.7 cells. Western blot analyses indicate that the erk kinases are activated upon LPS treatment of RAW264.7 but not P388D1 cells. LPS-induced arachidonic acid release is reduced in cells treated with the MEK inhibitor PD98059, suggesting that activated erk kinases mediate the phosphorylation and activation of cPLA2 in this system. Interestingly, the p42 isoform of erk (erk2) appears to be activated in resting P388D1 cells. This observation indicates that the MAP kinase cascade may be constitutively activated in P388D1 cells which may in turn limit their ability to respond to LPS. Together, these data provide evidence that mature macrophages from different sources can exhibit variable responses to LPS and highlight the danger of making generalizations regarding the effects of LPS on macrophages.