Bacterial Lipopolysaccharide Enhances Aflatoxin B1 Hepatotoxicity in Rats by A Mechanism That Depends on Tumor Necrosis Factor α

Abstract

Exposure to a nontoxic dose of bacterial endotoxin (lipopolysaccharide [LPS]) potentiates the hepatotoxicity of aflatoxin B1 (AFB1). Because some of the pathophysiologic effects associated with LPS are mediated through tumor necrosis factor α (TNF–α), this study was conducted to explore the role of TNF–α in the AFB1/LPS model. Male Sprague–Dawley rats (250–300 g) were treated with either 1 mg AFB1/kg, intraperitoneally, or its vehicle (0.5% dimethyl sulfoxide [DMSO]/water), and 4 hours later with either Escherichia coli lipopolysaccharide (7.4 × 106EU/kg, intravenously) or its saline vehicle. LPS administration resulted in a marked rise in TNF–α levels at 6 hours, which preceded the onset of liver injury. TNF–α messenger RNA (mRNA) in liver was increased by LPS treatment. The mRNA of receptors (R1 and R2) for TNF–α was also examined. R1 mRNA levels were not altered; however, R2 mRNA levels were increased by either AFB1 or LPS administration. To determine if TNF–α plays a causal role in the development of liver injury, the increase in TNF–α was attenuated by administration of either pentoxifylline or anti–TNF–α serum, and liver injury was assessed. Administration of either of these agents resulted in protection. LPS treatment resulted in the upregulation of gene transcription for cyclooxygenase–2 (COX–2). However, administration of the selective COX–2 inhibitor NS–398 did not decrease injury. TNF–α and COX–2 inhibitors did not affect hepatic sequestration of neutrophils. Furthermore, it did not appear that TNF–α contributed to injury through inhibition of tissue repair. These data support the hypothesis that LPS–induced expression of TNF–α underlies the potentiation of AFB1–induced hepatotoxicity.