| Liver macrophages and endothelial cells have been implicated in the pathophysiology of bacterially derived endotoxin (ETX)-induced injury. In the present studies, mechanisms regulating the response of these cells to ETX were analyzed. Acute endotoxemia was induced by injection of mice with repurified ETX (3 mg/kg, i.p.). Macrophages, as well as endothelial cells, isolated from the livers of ETX treated C3H/HeOuJ mice were found to be activated. Thus, these cells displayed altered morphology and increased expression of the proinflammatory genes. This correlated with activation of JNK kinases and the transcription factors, nuclear factor-kappa B (NF-kappaB) and activating protein-1 (AP-1). In contrast, ETX administration to C3H/HeJ mice, which lack functional toll-like receptor 4 (TLR-4), had no major effects on NF-kappaB and AP-1 nuclear binding activity or on proinflammatory gene expression. These data demonstrate that the responses of liver macrophages and endothelial cells to ETX are dependent on TLR-4.; Triggering receptor expressed on myeloid cells (TREM) is another receptor thought to be important in regulating inflammatory responses to microbial products. Acute endotoxemia was found to induce expression of TREM-1 and TREM-3, but to decrease TREM-2 expression in both macrophages and endothelial cells. This was dependent on TLR-4, but not TNFR1. In these cells, blockade of TREM-1 activation prevented ETX-induced upregulation of nitric oxide synthase-2, confirming the proinflammatory activities of TREM-1 protein.; In contrast to TLR-4 and TREM-1, macrophage stimulating protein (MSP) and its receptor, stem cell-derived tyrosine kinase (STK), are thought to play a role in limiting ETX-induced inflammatory responses. Treatment of macrophages, but not endothelial cells, with MSP resulted in increased manganese superoxide dismutase (MnSOD) and copper zinc superoxide dismutase (CuZnSOD) protein expression. These data, together with our findings that expression of CuZnSOD and MnSOD was significantly reduced in macrophages and endothelial cells from STK-/- mice treated with ETX provide support for the idea that MSP/STK plays a role in the regulation of oxidative defense in the liver during acute endotoxemia. Taken together, the present studies demonstrate that TLR-4, TREM and STK modulate liver macrophages and endothelial cells activities during ETX-induced responses, suggesting these receptors are important in innate immunity to microbial products in the liver. |
