Adherence regulates macrophage signal transduction and primes tumor necrosis factor production

While monocyte/macrophage (MΦ) adherence to a matrix is necessary for differentiation and prolonged survival, the effect of adherence on the signaling mechanisms responsible for MΦ activation is unknown. Lipopolysaccharide (LPS) activates MΦ by signaling through members of the mitogen activated protein kinase (MAPK) family thereby inducing transcription of proinflammatory cytokines, such as TNF-α. Since adherence has been shown to affect different activities of various myeloid phagocytes, we investigated whether adherence affects intracellular signaling and modulates activation of the MΦ proinflammatory phenotype. We assessed the effect of adherence on activation of rabbit alveolar MΦ by measuring LPS-induced TNF-α mRNA and TNF-α secreted product in adherent versus nonadherent cells, in vitro. The effect of adherence on LPS-induced activation of MAPK was assessed by western analysis using a dual phosphospecific antibody against p38MAPK, p42,44ERK, and p54SAPK. LPS is known to induce activation of NF-κB and AP-1. Modulation of these two transcription factors by LPS under adherent versus nonadherent conditions was evaluated by gel-shift analyses. The results were that adherent cells treated with LPS, 10 ng/mL or 1 μg/ml, elicited a 26- and 132-fold increase, respectively, in TNF-α production. Nonadherent cells did not elicit significant TNF-α in response to LPS. Adherence alone induced significant ERK and AP-1 activation, but did not stimulate a significant TNF-α response and no further activation of ERK and AP-1 was observed with LPS stimulation. Adherence alone did not activate p38MAPK or NF-κB, but primed MΦ for an augmented response to LPS in activation of p38, NF-κB and in production of TNF-α. We conclude that adherence primes MΦ for activation and regulates MAPK signal transduction pathways.