Introduction During ischemic preconditioning brief

Introduction During ischemic preconditioning, brief periods of tissue ischemia are used to render tissue resistant to injury resulting from prolonged ischemia and reperfusion, so-called ischemia–reperfusion injury. Hepatic ischemia–reperfusion injury plays a major role in hepatic resection and shock, particularly during liver transplantation. Hepatic vascular control is commonly used to prevent massive hemorrhage during hepatectomy. Hepatic vascular occlusion provides a relatively bloodless operative field, enabling unhurried, meticulous, and accurate intrahepatic dissection and hemostasis. Hemeoxygenases (HOs) catalyze the formation of carbon monoxide (CO), biliverdin, and Fe2+ ions during heme degradation. HO-1 is the inducible isoform of HO and is induced for cytoprotection under various stress-related conditions including hyperemia, hypoxia, and ischemia–reperfusion. Nitric oxide (NO) plays a crucial role in maintaining the hepatic vascular tone. NO synthase (NOS) metabolically produces NO from L-arginine, oxygen, and nicotinamide VE-822 dinucleotide phosphate. In the liver, inducible NOS (iNOS) is a key molecule catalyzing NO synthesis. Proliferating cell nuclear antigen (PCNA), also called cyclin, is an auxiliary protein of DNA polymerase-δ; its expression levels are directly correlated with cell proliferation and DNA synthesis rates. PCNA is synthesized in the late G1 and S phases and remains present in the nuclei throughout the cell cycle; thus, the PCNA levels are correlated with the cell proliferative state.
Methods
Results PCNA expression did not differ significantly between Group 3 (30-minute hepatic ischemia only) and Group 4 (30-minute hepatic ischemia with preconditioning; p=0.9813). It significantly increased in Group 5 (45-minute hepatic ischemia only) compared with that in Group 3 (p=0.0007), whereas it decreased significantly in Group 6 (45-minute ischemia with preconditioning) compared with that in Group 5 (p=0.0065; Figure 1). Similarly, HO-1 expression did not differ significantly between Group 3 and Group 4 (p=0.1797). It significantly increased in Group 5 compared with that in Group 3 (p=0.0024), whereas it decreased significantly in Group 6 compared with that in Group 5 (p=0.049; Figure 2). The difference in iNOS expression of Groups 3 and 5 was nonsignificant (p=0.2131). Preconditioning did not cause any significant difference in iNOS expression in either the 30- or 45-minute ischemia groups (Group 3 vs. Group 4, p=0.2997; Group 5 vs. Group 6, p=0.0673; Figure 3). Similarly, HSP70 expression did not differ significantly between Groups 3 and 4 (p=0.6376), and preconditioning was also ineffective in both the 30- and 45-minute ischemia groups (Group 3 vs. Group 4, p=0.5041; Group 5 vs. Group 6, p=0.9124; Figure 4).
Discussion Ischemic preconditioning before a brief period of vascular occlusion can render an organ resistant to the damaging effects of ischemia–reperfusion. This protective effect was originally described for the heart and subsequently for the brain, intestine, skeletal muscle, and liver. Pang et al reported that only a brief period of hepatic ischemia is required for preconditioning. Xue et al hypothesized that the short reperfusion phase of ischemic preconditioning generates reduced reactive oxygen species (ROS) sufficient for activating redox-sensitive intracellular signaling pathways. Furthermore, the downstream consequences of such ROS-induced signaling pathways are cytoprotective to liver cells through the abrogation of cell death pathways, simulation of antioxidant and other cytoprotective mechanisms, and initiation of the cell cycle. However, the actual protective mechanisms of ischemic preconditioning remain unclear. HO is the rate-limiting enzyme in heme degradation, resulting in the formation of CO, biliverdin, and Fe2+ ions. Thus far, three distinct HO isoforms—HO-1, HO-2, and HO-3—have been identified. HO-1 is constitutively expressed in the normal gastric, intestinal, and colonic mucosa. It is also expressed by all liver and circulating nucleated cells and can be induced under several stress-related conditions such as ischemia–reperfusion. HO-1 induction is considered cytoprotective. HO-1 inducers may reduce the instances of hepatic ischemia-reperfusion injury, whereas HO-1 inhibitors may exacerbate them. The protective mechanism of HO-1 is attributable to its byproducts, CO and biliverdin, which are vasodilators that improve liver microcirculation and reduce hepatocellular apoptosis and necrosis.