The liver has dual blood supply from the PV and the hepatic artery and is the organ with the most abundant blood flow in the body. The PV supplies 70% of the blood to the liver and provides necessary nutrients for maintaining liver regeneratio. Liver transplantation has been increasingly used to treat acute liver failure, fulminant hepatitis, hepatic cirrhosis and metabolic liver diseases. With the rise in the number of patients requiring liver transplanation, donor liver shortage has become an acute issue of global concern and greatly hinders the development of liver transplantation. Cohn and Herrod conceived the idea of liver perfusion using arterial blood instead of blood from the PV in 1952 and showed that normal liver blood flow could be maintained by perfusing the liver with arterial bood with the right flow rate and pressure through the PV, thus promoting liver regeneration and maintaining liver function, showing a novel approach for sovlling the issue of liver transplantation donor liver shortage. Another breakthrough in donor liver transplant site and vascular reocnstruction came from Welch  who conceived the idea of auxillary liver transplantation in 1955, which provided a theoretical basis for subsequent studies on liver transplantation and donor liver regeneration. Erhard et al. established PVA in heterotopic auxiliary liver transplantation to reuduce the risk of donor liver PV thromobsis. Margarit et al. also used the method to treat fulminant hepatic failurein 2000.
Liver regeneration is a complex process requiring the participiation of multiple cellular factors and fine modulation of HGF; when the liver returns to approxiatmely its normal size, regeneration will cease. Studies have shown that PVA promotes liver regeneration in the initial period but also suport long term growth of the liver. Shimizu et al. found that PVA markedly increased oxygen saturation of the perfused liver and greatly reduced consumption of the energy load, ultimately promoting hepatocyte regeneration. Studies have shown that IL-6 is an ipmortant signaling molecule in the initiation of liver regeneration and plays a critical role in initiating the acute phase response in hepatocytes in which multiple types of proteins are rapidly produced to participate in acute or chronic inflammatory repsonse. IL-6, which is produced by both macrophages in the liver and hepatocytes, could activate Kuppfer cells to release TNF-α, which, in turn, stimulate IL-6 production. In the pre-stimulation phase, hepatocyte regeneration is activated and hepatocytes transition from G0 to G1. In the interim, both TNF-α and IL-6 levels rise in the liver tissue and the remaining proteases in the liver are activated. The TNF→TNFR-1→NF-kB→IL-6→STAT3 cascade is activated and initates hepatocyte regeneration, which lasts for 4–6 h post hepatectomy. In addiiton, HGF is an important growth factor for hepatocytes and accelerates cell cycle progression during liver regeneration. HGF is produced by mesenchymal cells in the body and acts on hepatocytes in a paracrine and endocrine manner. HGF is considered an initiator of liver regeneration and is directly mitogentic for hepatocytes. We found that liver regeneration was mainly asosciated with HGF, TNF-α, and IL-6 levels in the liver tissues, and HGF, TNF-α, and IL-6 levels noticealy increased in partially resected donor liver tissues and showed dynamic changes post liver transplant. Improved donor liver implantation site and PVA in the partially resected liver transplant allows proper blood flow at an appropriate rate and pressure in a patent PV, assuring a steady supply of liver regeneration-associated cytokines including TNF-α, IL-6 and HGF, initiating liver regeneration.
Over the recent years, most studies on PVA in heterotopic auxiliary liver transplantation invovle kideny resection and place the donor liver in the kidney-resected area. The PVA model with end-to-end anastomosisof the renal artery and PV unnecessarily sacrifices one kideny and does not follow normal anatomic and physiological relation. In the current study, we estalibshed a novel PVA model by placing the donor liver in the right paracolic sulcus and performing end-to-side anastomosis of the donor liver PV and recipient left common iliac artery. We further investigated the effects on liver regeneration of donor liver placement sites and vascular reconstruction methods for PVA in heterotopic auxiliary liver transplantation in the two models. We found that in our modified PVA model, the volume, velocity, and pressure of blood flow in the donor liver PV was controlled and the donor liver was perfused at an appropriate volume, velocity and pressure, thus lessening the risks of over congestion and rupture of hepatic capsule. As a result, the ahepatic phase was shortened by an abundant supply of oxgenated arterial blood in a short time, thus accelerating liver regeneration. Studes showed that increased volume of blood flow and enhanced oxygen supply after PVA was established was conducive to liver energy metabolism and liver regeneration[8–10]. Other studies further revealed that PVA had no effect on normal liver function but promoted energy metabolism and recovery of hepatic reserve function[11–13]. In our study, the control group had a higher rate of moratlity than the experimental group; the exact causes are unknown, but could be due to postoperative hemorrage, PV thrombosis, or MODS. The plasma contents of ALT, AST, and total bilirubin declined to normal over time and the amplitude of decrease was significantly greater in the experimental group than the control group.
In the current study, we only used 30% donor liver which greatly reduced the volume of donor liver requried, which could help lay the foundation for solving donor liver shortgage in liver transplantation. Fan et al. [14–20] reported that PVA increased oxgyen transport to the donor liver by increasing the extent of liver resection and raising the regeneration of the remnant liver in an animal model. Liver regeneration is mainly initiated by release of cytokines while TNF-α, IL-6 and HGF stimulate hepatocytes to transit from G0 to G1. TNF-α and IL-6 are the main modulators of liver regeneration and induce hepatocyte proliferation by activating NF-kB and STAT3. Lipopolysaccharides, C3a and C5a bind to their respective receptors on macrophages in the liver and play a role in liver regeneration by modulating TNF-α and IL-6. In the partial hepatectomy model, IL-6/sIL-6R in combination with growth factors promote entry of hepatocytes into the cell cycle via PI3K/AKT signaling to promote liver regeneration. In addiiton, it was rpeorted that TNF-α and EGF promoted DNA replication during liver regeneration. NK cells and NKT cells upregulate TNF-α and IL-6/STAT3 through immune resposne, and togather with HGF, promote liver regeneration . HGF is an important cytokine in liver regeneration and acctivates IGFBP to increase the translation of insulin-like growth factor-binding protein (IGFBP). It has mitogenic activities and hepatoprotective activities. HGF is upregulated during liver regeneration and peaks at day 7 post liver transplant. Hepatocytes are the main effectors of liver regeneration, and the interdependency of cytokiens and growth factors is the main mode of hepatocyte regeneration. Our immunohistochemistry revealed that the expression of TNF-α and IL-6 was markedly increased at POD 1 in both groups and peaked at POD 5 and 7 and thereafter declined, indicating that after partial hepatectomy and PVA, liver regeneration was initated. Notably, the levels of TNF-α and IL-6 in the liver tissues of the experimental group were markedly higehr than those of the control group. In addition, the levels of HGF, a cytokine critical for liver regeneration, showed marked increase in boht groups post transplant but were significantly higher in the experimental group, indicaitng that HGF togather with TNF-α and IL-6 promoted liver regeneration.
In summary, TNF-α, IL-6 and HGF play a critical role in initiating liver regeneration post transplant and TNF-α, IL-6 and HGF levels in the liver tissues could more fully reflect the ability and extent of liver regeneration. The experimental group had significantly higher levels of TNF-α, IL-6 and HGF than the control group, indicating that the liver hemodynmaics of our modified PVA model of heterotopic auxiliary liver transplantation is more conducive liver regeneration. Higher levels of TNF-α, IL-6 and HGF promoted liver regeneration[24, 25]; enhanced liver hemodynamics may improve hepatic access to TNF-α, IL-6 and HGF Therefore, our modified PVA model of heterotopic auxiliary liver transplantation is more conducive to liver regeneration and our findings provide a theoretical basis for future studies on PVA and liver regeneration.