We studied the effect of endotoxin (LPS), and cytokines (TNF, IL-1, and IL-6) on hepatic microsomal triglyceride transfer protein (MTP) mRNA levels in vivo in Syrian hamsters and in vitro in HepG2 cells. LPS, interleukin-1 (IL-1), and to a lesser extent tumor necrosis factor (TNF) significantly decreased MTP mRNA levels in hamster liver. These effects required several hours. Furthermore, IL-1 and IL-6 significantly decreased MTP mRNA levels in HepG2 cells. This decrease appeared soon after IL-1 administration (8 h) and at very low doses (0.1 ng/ml). MTP activity and protein levels of the large subunit of MTP also decreased modestly in HepG2 cells with prolonged cytokine treatment. IL-1 reduced the expression of an MTP promoter luciferase construct to a similar degree as seen with MTP mRNA, indicating that transcriptional regulation plays a major role in the decrease of MTP gene expression. Deletional analysis of the MTP promoter identified the region -121 to -88 bp upstream to the coding sequence as the site of the negative regulation by IL-1. This region contains an insulin response element (IRE), activating protein 1 (AP-1), hepatic nuclear factor 1 (HNF-1) and hepatic nuclear factor 4 (HNF-4) consensus sequences; mutations of the IRE and HNF-4 sites did not affect the response to IL-1. In contrast, mutating AP-1 or HNF-1 sites led to a marked decrease in basal expression and the loss of the IL-1 effect, suggesting that an intact AP-1 and/or HNF-1 regulatory element are crucial for the IL-1 regulation of MTP gene expression. However, prolonged incubation with IL-1 did not alter HepG2 apolipoprotein B secretion suggesting that MTP mRNA down-regulation does not contribute significantly to the cytokine-induced effects on lipid metabolism.