Mature macrophage cell lines exhibit variable responses to LPS

Bacterial lipopolysaccharide (LPS) is a potent activator of cells of the ma crophage/monocyte lineage. Two mature macrophage cell lines, P388D(1) and R AW264.7, exhibit very different biological responses to LPS. Although RAW26 4.7 cells release arachidonic acid from phospholipid in response to LPS sti mulation, P388D(1) cells do not respond in this manner. However, LPS primes P388D(1) cells to release arachidonic acid in response to other stimuli. T he goal of this work is to contrast the biochemical events that occur in LP S-treated P388D(1) and RAW264.7 macrophages. Enzyme assays indicate that LP S treatment induces the activation of cytosolic PLA(2) in RAW264.7, but not in P388D(1) cells. Phorbol ester (PMA), a receptor-independent stimulus, a lso fails to induce arachidonic acid release from P388D(1) cells; suggestin g that these cells may have a defect in the signal transduction machinery t hat is common to LPS and PMA. This hypothesis is supported by the observati on 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) cell s. LPS-induced arachidonic acid release is reduced in cells treated with th e MEK inhibitor PD98059, suggesting that activated erk kinases mediate the phosphorylation and activation of cPLA(2) in this system. Interestingly, th e p42 isoform of erk (erk2) appears to be activated in resting P388D(1) cel ls. This observation indicates that the MAP kinase cascade may be constitut ively activated in P388D(1) cells which may in turn limit their ability to respond to LPS. Together, these data provide evidence that mature macrophag es from different sources can exhibit variable responses to LPS and highlig ht the danger of making generalizations regarding the effects of LPS on mac rophages. (C) 1998 Elsevier Science Ltd. All rights reserved.