Insulin inhibits LPS-induced signaling pathways in alveolar macrophages

The systemic inflammatory response syndrome (SIRS) is triggered by lipopolysaccharide (LPS) from Gram-negative bacteria. Insulin was shown to have a protective role in SIRS related to sepsis. Lungs are particularly affected in this condition and provide a second wave of mediators/cytokines which amplifies SIRS. The aim of the present study was to investigate the effect of insulin on the signaling pathways elicited by LPS in alveolar macrophages (AMs) and its consequence in cellular response to LPS measured as production of tumor necrosis factor (TNF). To this purpose, resident AMs from male Wistar rats were obtained by lung lavage and stimulated by LPS (100 ng/mL). Insulin (1 mU/mL) was added 10 min before LPS. Activation (phosphorylation) of signaling molecules by LPS was analyzed by western blot, 30 min after LPS stimulation. TNF was measured in the AMs culture supernatants by bioassay using L-929 tumor cells. Relative to controls, LPS induced a significant increase in the activation of ERK (3.6-fold), p38 (4.4-fold), Tyr-326 Akt (4.7-fold), Ser-473 Akt (6.9-fold), PKCalpha (4.7-fold) and PKCdelta (2.3-fold). Treatment of AMs with insulin before LPS stimulation, significantly reduced the activation of ERK (54%), p38 (48%), Tyr-326 Akt (64%), Ser-473 Akt (41%), PKCalpha (62%) and PKCdelta (39%). LPS induced TNF production in AMs which was also inhibited by insulin (60%). These results show that insulin down-regulates MAPK, PI3K and PKCs and inhibits a downstream effect of LPS, TNF production, in rat AMs stimulated with LPS and suggest that the protective effect of insulin in sepsis could be through modulation of signal transduction pathways elicited by LPS in lung macrophages.