Mature macrophage cell lines exhibit variable responses to LPS

Bacterial lipopolysaccharide (LPS) is a potent activator of cells of the macrophage/monocyte lineage. Two mature macrophage cell lines, P388D(1) and RAW264.7, exhibit very different biological responses to LPS. Although RAW264.7 cells release arachidonic acid from phospholipid in response to LPS stimulation, P388D(1) cells do not respond in this manner. However, LPS primes P388D(1) 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 P388D(1) and RAW264.7 macrophages. Enzyme assays indicate that LPS treatment induces the activation of cytosolic PLA(2) in RAW264.7, but not in P388D(1) cells. Phorbol ester (PMA), a receptor-independent stimulus, also fails to induce arachidonic acid release from P388D(1) 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 P388D(1) and RAW264.7 cells. Western blot analyses indicate that the erk kinases are activated upon LPS treatment of RAW264.7 but not P388D(1) 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 cPLA(2) in this system. Interestingly, the p42 isoform of erk (erk2) appears to be activated in resting P388D(1) cells. This observation indicates that the MAP kinase cascade may be constitutively activated in P388D(1) 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. (C) 1998 Elsevier Science Ltd. All rights reserved.