Tumor necrosis factor increases mitochondrial oxidant production and induces expression of uncoupling protein-2 in the regenerating rat liver

✍ Scribed by F. Y. Janet Lee; Yunbo Li; Hong Zhu; ShiQi Yang; Hui Zhi Lin; Michael Trush; Anna Mae Diehl

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 367 KB
• Volume: 29
• Category: Article
• ISSN: 0270-9139
• DOI: 10.1002/hep.510290320

✦ Synopsis

The growth-stimulatory actions of tumor necrosis factor ␣ (TNF-␣) after partial hepatectomy (PH) are difficult to reconcile with its well-established role in the genesis of liver injury. The lethal actions of TNF are thought to involve the induction of oxidant production by mitochondria. It is not known if TNF initiates mitochondrial oxidant production after PH. Furthermore, if this potentially toxic response follows PH, it is not clear how hepatocytes defend themselves sufficiently so that replication, rather than death, occurs. These studies test the hypothesis that TNF does increase mitochondrial oxidant production after PH but that these oxidants primarily promote the induction of antioxidant defenses in regenerating hepatocytes. Consistent with this concept, H 2 O 2 production by liver mitochondria increases from 5 minutes to 3 hours after PH, beginning before the transient inductions of hepatic NF kB activity (which peaks at 30 minutes post-PH) and uncoupling protein-2 (UCP-2) (which begins around 30 minutes and peaks from 6-24 hours post-PH). Pretreatment with neutralizing anti-TNF antibodies, which inhibits hepatocyte DNA synthesis after PH, also reduces post-PH hepatic mitochondrial oxidant production by 80% and inhibits NF B activation and UCP-2 induction by 50% and 80%, respectively. In contrast, pretreatment with D609, an agent that inhibits phosphatidylcholine-specific phospholipase C, neither inhibits regenerative induction of mitochondrial oxidant production, UCP-2 expression, nor hepatocyte DNA synthesis, although it inhibits NF B activation by 50%. Given published evidence that NF B is antiapoptotic and that UCP-2 may decrease mitochondrial oxidant production in some cells, these results suggest that TNF-dependent increases in oxidant production by liver mitochondria promote the induction of antioxidant defenses in the regenerating liver. (HEPATOLOGY 1999;29:677-687.)

Tumor necrosis factor (TNF), a proinflammatory cytokine that causes liver injury, is also known to be important for normal liver regeneration after partial hepatectomy (PH). Recent studies in transgenic mice with targeted deletion of the TNF receptor type-1 gene 1 support original observations that were obtained by treating rats with neutralizing anti-TNF antibodies before PH. 2 Specifically, neutralization of TNF activity by either strategy profoundly inhibits the induction of hepatocyte DNA synthesis that normally occurs within the initial 1 to 2 days after PH. Preliminary comparison of several early prereplicative events in TNF-inhibited animals and controls suggests that TNF plays an important role in activating stress-and mitogen-activated kinase cascades and inducing the nuclear accumulation of several transcription factors, including NF B, c-Jun, C/EBP ␤, and C/EBP ␦ in regenerating hepatocytes. These growth-stimulatory actions of TNF in the regenerating liver are difficult to reconcile with its well-established role in the genesis of liver injury. Indeed, the cytotoxic effects of TNF on various cell types have been studied extensively, and TNF is known to kill hepatocytes when cellular protein and/or RNA synthesis are inhibited. The apoptotic actions of TNF in hepatocytes and other cells are thought to involve increased oxidant production by mitochondria. TNF can inhibit complex III of the mitochondrial electron transport chain, mimicking the effects of antimycin A, a potent inhibitor of mitochondrial respiration. Both agents increase mitochondrial production of superoxide anion within minutes, rapidly activating apoptotic enzymes, such as caspase 3, 21 that are localized in the intermembranous compartment of mitochondria, 22 and producing cell death within hours. Although TNF activates type-1 TNF receptors after PH and this interaction appears to be necessary for the ensuing regenerative response, 1 it is not known if TNF/TNF receptor interactions initiate mitochondrial oxidant production or other apoptotic signals after PH. Furthermore, if these potentially toxic responses do occur during liver regeneration, it is not clear how hepatocytes defend themselves sufficiently so that cellular replication, rather than death, is the outcome of PH.

The ultimate role of oxidants in any cell depends on the amount produced as well as the duration of exposure. 25 Controlled production of oxidants can function as a signaling mechanism, as exemplified by oxidant induction of redoxsensitive transcription factors, including NF B. However, when the production of oxidants overwhelms cellular defenses, inhibition of mitochondrial respiration, DNA damage, and/or lipid peroxidation ensue. If mitochondrial oxidant production increases during liver regeneration, then it is likely that this process is tightly controlled to prevent Abbreviations: TNF, tumor necrosis factor; PH, partial hepatectomy; UCP-2, uncoupling protein-2; HE, hemotoxylin and eosin; PAS, periodic acid Schiff; ATP, adenosine triphosphate; BSA, bovine serum albumin; mRNA, messenger RNA.