Nuria A. Callejas

Contribution of cyclooxygenase 2 to liver regeneration after partial hepatectomy

Partial hepatectomy (PH) triggers a rapid regenerative response in the remaining tissue to reinstate the organ function and the cell numbers. Among the molecules that change in the course of regeneration is an accumulation of prostaglandin E2 in the sera of rats with PH. Analysis of the cyclooxygenase (COX) isoenzymes in the remnant liver showed the preferential expression of COX‐2 in hepatocytes. Cultured regenerating hepatocytes expressed significant levels of COX‐2, a process that was not observed in the sham counterparts. Maximal expression of COX‐2 was detected 16 h after PH with increased levels present even at 96 h. Pharmacological inhibition of COX‐2 activity with NS398 shunted the up‐regulation of cell proliferation after PH, which suggests a positive interaction of prostaglandins with the progression of the cell cycle. Similar results were obtained after PH of mice lacking the COX‐2 gene. The expression of COX‐2 in regenerating liver was concomitant with a decrease in CCAAT‐enhancer binding protein (C/EBP‐α) level and an increase in the expression of C/EBP‐β and C/EBP‐δ. These results suggest a contribution of the enhanced synthesis of prostaglandins to liver regeneration observed after PH.

Expression of cyclooxygenase‐2 in foetal rat hepatocytes stimulated with lipopolysaccharide and pro‐inflammatory cytokines

1 Cyclooxygenase‐2 (COX‐2) is involved in the biosynthesis of prostanoids in the course of inflammatory reactions. This isoenzyme is regulated at the transcription level and many cells express COX‐2 upon challenge with lipopolysaccharide (LPS) or pro‐inflammatory cytokines. 2 Since hepatocytes respond to LPS and pro‐inflammatory stimuli, we investigated the expression of COX‐2 in foetal and adult hepatocytes upon challenge with these substances. 3 COX‐2 was expressed in foetal hepatocytes incubated with LPS, tumour necrosis factor‐α and interleukin‐1β. This response rapidly decreased after birth and was absent in hepatocytes from animals aged 2 days or more and treated under identical conditions. The expression of COX‐2 was determined at the mRNA, protein and enzyme activity levels using Northern and Western blot, and following the synthesis of prostaglandin E2, respectively. The use of NS 398, a specific pharmacological inhibitor of COX‐2, confirmed the expression of this isoenzyme in activated foetal hepatocytes. 4 Synergism in COX‐2 expression was observed between LPS, tumour necrosis factor‐α and interleukin‐1β. Interleukin‐6 and permeant analogues of cyclic AMP failed to induce COX‐2 or to synergize with LPS. Also, transforming growth factor‐β inhibited the LPS‐ and pro‐inflammatory cytokines‐dependent expression of COX‐2. 5 These results indicate that foetal hepatocytes are competent to express COX‐2 upon challenge with pro‐inflammatory stimuli, a process lost completely in hepatocytes isolated from animals aged 2 days.

Nitric oxide in liver inflammation and regeneration

Hepatocytes express and release inflammatory mediators after challenge with bacterial cell wall molecules and proinflammatory cytokines. Nitric oxide synthase-2 (NOS-2) is expressed under these conditions and the high-output NO synthesis that follows contributes to the inflammatory response in this tissue and participates in the onset of several hepatopathies. However, in the course of liver regeneration, for example, after partial hepatectomy, NOS-2 is expressed at moderate levels and contributes to inhibit apoptosis and to favor progression in the cell cycle until the organ size and function are restored. The mechanisms involved in the regulation of NOS-2 expression under these conditions are revised.