Advances in ischemia and reperfusion injury: effects on liver microcirculation and therapeutic strategies for sinusoidal protection

Abstract

Ischemia/reperfusion (IR) injury is a pathological condition caused by an initial interruption of blood, its biomechanical stimulus, and O2 supply followed by the restoration of perfusion and the accompanying oxygen, nutrient supply and shear stress. Clinically, ischemia/reperfusion injury is almost unavoidable in liver resection surgery, transplantation, and in blood transfusion after hemorrhagic shock. This pathology, associated with microvascular dysfunction and the “no reflow phenomenon” causes a wide range of derangements in the liver, which may finally lead to hepatic failure in case of severe injury. Sinusoidal dysfunction is, in part, due to the reduction in vasodilator molecules, such as nitric oxide. The decrease in NO bioavailability is due to both, a reduction in its production by eNOS in endothelial cells and an increase of its - scavenging by ROS like O2 . Thus, therapies focused on increasing eNOS expression or reducing ROS production may be useful in preventing microcirculatory derangements associated with IR injury.

Despite the importance of the liver sinusoid in health and disease scarce reports investigated the pathophysiological consequences of sinusoidal cell deregulation on hepatic IR injury. This has conduced to the erroneous idea that hepatocytes should be the main target for protection and therefore most of the therapies have focused on this cell type. Although IR injury has a clear negative impact in clinical practice, nowadays no pharmacological treatment is available, possibly due to the lack of microcirculatory protection of the tested drugs. Thus, we hypothesize that therapies focused in preserving the sinusoidal endothelial phenotype during IR will conduce not only to a better microcirculation but also to a global improvement of the whole liver.

With this background, the global aim of the present PhD thesis was to characterize the liver microcirculation in the setting of ischemia-reperfusion injury and evaluate possible drugs that, through an improvement in the hepatic sinusoid, could maintain a correct hepatic phenotype during IR.

The results derived from this doctoral thesis demonstrate the deleterious effects of ischemia/reperfusion injury on hepatic microcirculation both in cold and warm ischemia situations. In particular, the in vivo effects of warm IR include acute microcirculatory injury associated with an increase in intrahepatic vascular resistance, portal hypertension and reduced hepatic perfusion. To prevent this damage affecting all cell types in the liver sinusoid two therapeutic strategies have been evaluated: a recombinant form of the antioxidant human manganese superoxide dismutase (rMnSOD) in the context of cold storage for transplantation and the vasoprotective drug simvastatin in a model of partial warm ischemia. The papers included in this doctoral thesis demonstrate how both drugs are effective in improving liver endothelial function. Those effects are due, in part, to the maintenance of KLF2 expression and the vasoprotective pathways derived of this transcription factor; the maintenance of nitric oxide bioavailability and the reduction in superoxide levels. The endothelial and microcirculatory hepatic protection achieved by those drugs further conduces to a prevention of liver inflammation mediated by adhesion molecules, and thus to a reduction in hepatic parenchymal injury and a global decrease in cell death.

In conclusion, all the findings here described support the idea that preservation of the hepatic sinusoidal phenotype and function in essential to prevent ischemia/reperfusion injury and therefore to maintain liver function.

En la present tesi doctoral es demostren els efectes deleteris del dany per isquèmia-reperfusió sobre la microcirculació hepàtica tant en condicions de preservació en fred com d’isquèmia calenta. En particular, els efectes in vivo de l’isquèmia-reperfusió en calent inclouen dany microcirculatori agut associat amb un increment de la resistència vascular intrahepàtica, hipertensió portal i reducció de la perfusió hepàtica. Per prevenir aquests danys que afecten als diferents tipus cel·lulars del sinusoid hepàtic s’han avaluat dues estratègies terapèutiques: l’antioxidant rMnSOD en el context de la preservació en fred per transplantament i el vasoprotector simvastatina en un model d’isquèmia calenta. Els articles inclosos en la present tesi doctoral demostren com ambdós fàrmacs són efectius millorant la funció endotelial hepàtica. Aquests efectes es deuen, en part, al manteniment de les vies vasoprotectores derivades del factor de transcripció KLF2 , al manteniment de la biodisponibilitat d’òxid nítric i a la reducció dels nivells del radical lliure superòxid. Aquesta protecció de l’endoteli i la microcirculació hepàtica s’associa amb una prevenció de l’inflamació mitjançada per mol·lècules d’adhesió i finalment condueix a una reducció del dany hepàtic i una menor mort cel·lular.