It has been proposed that tumor necrosis factor (TNF) is a direct regulator of postinjury hepatic protein synthesis. To test this hypothesis we investigated the total protein and specific acute phase protein synthesis response of murine hepatocytes to stimulation with mu-rTNF-alpha in vivo and in vitro. Total hepatocyte secretory protein synthesis was assessed by incorporation of [35-S] methionine into TCA-precipitated protein; and acute phase protein synthesis was assessed by induction of a 23-kD acute phase protein marker and by suppression of albumin synthesis determined by SDS-PAGE and autoradiography.
We found that rTNF in vivo (8,000 units, IP injection) was associated with reduced total hepatocyte secretory protein synthesis (29 10), increased synthesis of the 23-kD acute phase reactant (4.1 1.6-fold), and decreased albumin synthesis (0.68 0.2-fold) compared to saline-injected control animals. The in vitro stimulation of cultured murine hepatocytes directly with rTNF failed to demonstrate changes in total secretory protein synthesis or 23-kD protein; however, it did result in significant suppression of albumin synthesis (0.82 0.1-fold). In additional experiments, hepatocytes: nonparenchymal cell co-cultures stimulated with lipopolysaccharide (LPS) demonstrated protein synthesis changes similar to the in vivo TNF response including increased 23-kD protein and decreased albumin synthesis. These co-cultures demonstrated TNF production; however, addition of TNF antiserum during LPS stimulation had no effect on either 23-kD protein or albumin synthesis, despite the complete neutralization of TNF activity in the co-culture supernatants.
These results indicate that TNF induces synthesis of specific acute phase proteins (23-kD) indirectly while directly contributing to the reduction in albumin synthesis characteristic of the acute phase response. We conclude that individual hepatocyte acute phase proteins are specifically regulated and that TNF is an early, indirect mediator of the response to injury.