TNF-α modulates expression of the tissue transglutaminase gene in liver cells

One of several postulated roles for tissue transglutaminase (tTG) is the stabilization and assembly of extracellular matrix via peptide cross-linking. We previously determined that tTG activity increased in an animal model of hepatic fibrogenesis and in human liver disease. To further study the role of tTG in liver disease, we initiated investigations into the effect of a proinflammatory mediator, tumor necrosis factor (TNF)-α, on tTG activity in cultured liver cells. Treatment of human Hep G2 cells with 1 ng/ml TNF-α increased [¹⁴C]putrescine cross-linking to cellular proteins. An increase in tTG mRNA content was observed i h after addition of TNF-α, and levels of tTG mRNA remained elevated after 24 h. Hep G2 cells, transiently transfected with a luciferase reporter containing 1.67 kb of the human tTG promoter, showed an increase in reporter activity after addition of TNF-α. Gel shift experiments using nuclear extracts from TNF-α-treated cells and oligonucleotides containing the tTG nuclear factor (NF)-κB motif revealed increased binding, concordant with mRNA data. Transient transfections with a truncated reporter construct lacking the tTG NF-κB sequence showed an attenuated response to TNF-α treatment. Similar responses were seen in stably transfected HeLa cells. Primary hepatocytes isolated from a transgenic mouse line containing the mouse tTG promoter driving the β-galactosidase reporter, show similar time-dependent increases in promoter activity when treated with TNF-α. Furthermore, Hep G2 cells are incapable of upmodulating tTG promoter reporter activity in the presence of TNF-α when those cells overexpress a transdominant, negative mutant NF-κB subunit. Because TNF-α expression is upregulated in hepatic inflammation, the data suggest TNF-α- mediated increases in tTG expression may play an important role in the process of hepatic fibrogenesis.