Hemostasis and liver regeneration

Summary/abstract

The liver is unique in its remarkable regenerative capacity, which enables use of liver resection as a treatment for specific liver diseases, including removal of neoplastic liver disease. After resection, the remaining liver tissue (i.e, liver remnant) regenerates to maintain normal hepatic function. In experimental settings as well as patients, removal of up to 2/3^rd of the liver mass stimulates a rapid and highly coordinated process resulting in regeneration of the remaining liver. Mechanisms controlling the initiation and termination of regeneration continue to be discovered, and many of the fundamental signaling pathways controlling proliferation of liver parenchymal cells (i.e., hepatocytes) have been uncovered. Interestingly, while hemostatic complications (i.e., bleeding and thrombosis) are primarily thought of as a complication of surgery itself, strong evidence suggests that components of the hemostatic system are in fact, powerful drivers of liver regeneration. This review focuses on clinical and translational evidence supporting a link between the hemostatic system and liver regeneration, and the mechanisms whereby the hemostatic system directs liver regeneration discovered using experimental settings.

Liver regeneration: Clinical application and current needs

Liver resection is considered the only curative treatment option for several neoplastic entities of the liver^{1, 2}. Approximately 50% of all patients suffering from colorectal cancer will develop metastatic disease within the liver³ and liver failure caused by extensive tumor burden is one of the leading causes of death in these patients. For patients with metastatic disease limited to the liver, surgical resection of liver metastasis can result in long-term survival and even cure, as approximately 15% of these patients will never relapse⁴. Hepatectomies are performed for eligible patients based on the resection of all radiologically and macroscopically detectable tumor, while preserving at least 20–25% of healthy total liver volume^{5, 6}. In most patients, the liver remnant regenerates to its original size to restore normal hepatic function. Importantly, despite substantial improvements in surgical techniques and peri-operative care, postoperative morbidity and mortality remain an important concern after liver resection^{7, 8}. In fact, postoperative death after liver resection is reported to occur in up to 13% of high-risk patients^{9, 10}.

Failed regeneration of the liver remnant can ultimately contribute to postoperative hepatic dysfunction and liver failure. Although extensive research has been performed on molecular mechanisms of liver regeneration, very limited translational research has been performed to document the relevance of these findings in the human setting, thereby impeding the development of new therapeutic compounds and strategies. Indeed, there is no treatment option available to promote liver regeneration for patients developing postoperative liver failure after liver resection. New surgical strategies to accelerate (postoperative) liver regeneration have been developed. Portal vein embolization (PVE) and portal vein ligation (PVL) have been widely used to increase the volume of the future liver remnant. Both techniques involve occlusion of the portal branches that provide blood to the liver fragments that will undergo resection, leading to hypertrophy of the future liver remnant¹¹. In fact, PVL is one of the first steps in the two-stage surgical hepatectomy model called ALPPS (Associating Liver Partition and Portal vein ligation for Staged hepatectomy), where rapid liver regeneration is initiated by a combination of PVL and parenchymal transection. The more rapid liver regeneration observed after ALPPS has been attributed to a reduction in portal collateral formation¹². A recently proposed method to increase the future liver remnant is the Liver Venous Deprivation (LVD) technique. It combines embolization of both the portal and hepatic vein in order to increase the damage of the embolized liver leading to rapid hypertrophy of the future liver remnant¹³. The underlying mechanisms triggering liver regeneration in these models are not completely understood. Previous studies suggest that one mechanism is an alteration in portal blood flow, which may stimulate liver regeneration via increased shear stress, increased accessibility of pro-regenerative factors, or reduced oxygen pressure¹⁴. However, even with these surgical advances, postoperative liver failure remains a frequent and critical issue without any available pharmacologic strategies for prophylaxis or treatment^{9, 10}. As postoperative liver dysfunction and failure remains the main cause of death after liver resection, identifying strategies to promote liver regeneration is critical. Such therapies ideally should reduce post-operative complications caused by failed liver regeneration, potentially accelerate patient recovery after liver resection and most importantly, reduce the incidence of postoperative mortality.

Hemostatic factors in liver regeneration: experimental and translational evidence.

The fundamental mechanisms driving liver regeneration have been discovered using the well-characterized experimental setting of partial hepatectomy (PHx) in rodents. The standard PHx model most commonly involves surgical removal of 2/3^rd of the rodent liver¹⁵. PHx is followed rapidly by a series of cell signaling events coordinating proliferation of the remaining liver parenchymal cells (i.e., hepatocytes) and reconstitution of hepatic non-parenchymal cells, ultimately restoring liver mass and function within 7–10 days^15–17. Strong experimental evidence indicates this process is mediated by a complex interplay of pathways. Hepatocyte proliferation, for example, involves a number of growth factors including hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), thrombospondin-1 (TSP-1), serotonin and transforming growth factor β (TGF-β)^18–20. Additional factors contributing to hepatocyte proliferation include non-mitogenic cytokines, bile acids, hormones, and other small molecules. The reader is referred to excellent reviews in which these processes have been detailed^{21, 22}. There is considerable redundancy in the various pathways to ensure maximal regeneration. Indeed, there are numerous examples of deficiencies in a single pathway delaying liver regeneration, but few if any examples where a single deficit fully blocks the regenerative response. It seems as if the inhibition of multiple critical pathways is needed to block liver regeneration completely²³.

Platelets and liver regeneration

Liver regeneration is initiated very quickly after surgical resection. Strong evidence suggests that components of the hemostatic system are among the earliest triggers of liver regeneration after PHx. Indeed since the first description that platelets can promote hepatocyte proliferation in 1982²⁴, significant evidence has been generated supporting a relevant role of platelets in liver regeneration. In particular, the significance of platelets during liver regeneration is supported by 1) their rapid accumulation (i.e., within 5 min) in the liver sinusoids after PHx^{25, 26} and translocation into the space of Disse ²⁶ and 2) evidence that platelet number is directly associated with successful regeneration after PHx. Administration of platelet inhibitors or platelet depletion each significantly reduced hepatocyte proliferation and delayed liver regeneration after PHx^{25, 27}. On the other hand, increasing circulating blood platelet count significantly promoted liver regeneration after 70% PHx²⁸ and counteracted liver failure induced by extended (90%) PHx^{29, 30}. Collectively, these studies suggest that platelets play a critical role as one of the earliest triggers of regeneration after PHx.

The exact molecular mechanisms whereby platelets contribute to liver regeneration need to be clarified (Figure 1). It has been proposed that local release of platelet granule contents in response to liver resection is responsible for platelet-mediated liver regeneration^31–34. Indeed, in vitro studies have shown that platelets induce hepatocyte proliferation by release of growth factors^{35, 36}. In this context, it is important to differentiate alpha and dense granules, which store different types of growth factor/molecules that may have divergent effects on liver regeneration.

Figure 1:

Mechanisms linking hemostatic factors to liver regeneration: Platelets rapidly accumulate in the liver remnant after partial hepatectomy. Accumulation of platelets in the liver appears to depend on multiple factors including von Willebrand factor (VWF), and on fibrin deposits formed as a result of intrahepatic coagulation. Experimental evidence suggests that platelets contribute to liver regeneration through multiple mechanisms including 1) degranulation and release of serotonin and other growth factors, 2) transfer of RNA by direct interactions with hepatocytes and 3) potentially through cell-cell interactions with leukocytes or other non-parenchymal cells. Among the gaps in knowledge include the source of VWF responsible for driving platelet accumulation (plasma, endothelial cells or platelets themselves), and whether fibrin drives regeneration through mechanisms independent of its role in platelet accumulation.

TF = Tissue factor

VWF = von Willebrand factor

EC = Endothelial Cell

HPC = Hepatocyte

Dense granules:

Multiple experimental and clinical studies suggest that platelet-derived serotonin could form a key component of platelet-mediated liver regeneration^{27, 37–39}. However, there is also conflicting data on the role of serotonin in liver regeneration after liver resection. Mice lacking platelet serotonin (TPH1^−/− mice) show a clear delay in liver regeneration after PHx²⁷. However, TPH1^−/− mice also have reduced platelet adhesion in vivo⁴⁰, most likely due to defects in both von Willebrand factor (VWF)-mediated platelet adhesion and secondary platelet activation^{40, 41}. As both platelet activation²⁷ and VWF-dependent platelet accumulation²⁵ are required for platelet-mediated liver regeneration, the delayed regenerative capacity in TPH1^−/− mice may also be explained by these defects in hemostasis. It is also worth noting that liver regeneration after PHx is unaffected in plasma membrane serotonin transporter (SERT) deficient rats, despite having a marked reduction in platelet serotonin levels⁴². While experimental and translational evidence is growing, the precise contribution of serotonin to liver regeneration deserves further study.

α-granules:

Platelets store a wide variety of growth factors, including insulin-like growth factor (IGF), VEGF, HGF, in their α-granules which are released upon platelet activation. Most of these are potent inducers of liver regeneration. However, thrombospondin-1 (TSP-1), an inhibitor of regeneration, is also stored in this granule subtype. Clear evidence supports a pro-regenerative role for platelets, and yet the cargo of α-granules would be anticipated to have dichotomous effects. One potential explanation for this is the distinct packaging and agonist-dependent α-granule subset release, hypothesized by Italiano et al.⁴³, which would allow for distinct platelet responses to specific stimuli^43–48. Further studies are required to evaluate this possibility after liver resection, although our recent studies suggest a connection between levels of pro- and anti-proliferative growth factors and outcome in patients after liver resection³³. Indeed, platelets from patients with postoperative liver dysfunction lacked capacity to secrete VEGF stored in their α-granules⁴⁹. It should be noted that not all studies align with this hypothesis. Kirschbaum et al. found no evidence of release and consumption of platelet-derived growth factors after partial hepatectomy in humans⁵⁰. One possible difference between these studies is differences in sample collection strategies⁵¹, highlighting both the need for standardized collection techniques and for more studies to evaluate the contribution of α-granule content to liver regeneration. Similarly, it is also important to consider that there may be general effects of major abdominal surgery on growth factor release and/or expression⁵⁰.

In addition to release of platelet growth factors, another mechanism whereby platelets could stimulate liver regeneration is via transfer of platelet RNA to hepatocytes³¹. In vitro studies have shown that platelets stimulate proliferation of hepatocytes, which required direct contact between platelets and these cells⁵². Not only was direct contact required, but platelets were also internalized by hepatocytes⁵³. Although platelets do not contain a nucleus, they do contain a wide selection of circulating RNAs⁵⁴. In recent years it has been demonstrated that platelets are capable of RNA transfer to recipient cells, including monocytic and endothelial cells⁵⁵. The Lisman group has demonstrated that RNA transfer from platelets to hepatocytes partly drives hepatocyte proliferation in vitro⁵³. However, whether functional RNA transfer also occurs in vivo during liver regeneration has yet to be demonstrated.

Although platelets seem to be able to stimulate liver regeneration directly, other mechanisms involve their interaction with liver cell types to indirectly support liver regeneration. Platelets have been shown to stimulate liver sinusoidal endothelial cells (LSECs) to release potent hepatocyte growth factors such as HGF, VEGF and IL-6⁵². This is of importance as immediately after induction of liver regeneration, LSECs are likely to contact platelets⁵⁶. Indeed, 2 hours after induction of liver regeneration in humans, abundant interaction between LSECs and platelets has been documented⁵⁶. Moreover, there is evidence to suggest a role for Kupffer cells in platelet-dependent liver regeneration, although thrombocytosis also promotes liver regeneration even after Kupffer cell depletion^{57, 58}. The possibility that platelets could stimulate liver regeneration via non-liver cell dependent mechanisms has been largely unexplored. However, one alternative hypothesis involves platelet-dependent facilitation of the inflammatory response. Leukocytes accumulate in the liver remnant after PHx and liver regeneration is delayed in leukocytopenic mice⁵⁹. Platelets are well known to recruit inflammatory cells⁶⁰, and thus, it seems plausible that platelets could promote liver regeneration by facilitating leukocyte recruitment to the remnant liver. The interaction between platelets and the inflammatory response in the setting of PHx has not been investigated extensively, but recent studies in other models have demonstrated that platelets play an important role in neutrophil accumulation to sites of inflammation and tissue repair^{61, 62}.

Platelet accumulation within the liver

Regardless of the potential mechanism(s) whereby platelets stimulate liver regeneration, accumulation of platelets within the liver remnant is likely a critical first step in platelet-directed liver regeneration (Figure 1). As mentioned before, hepatic platelet accumulation is very rapid after PHx, peaking within 30 minutes^{25, 63}, and platelet accumulation in the liver shortly after PHx is most impactful with respect to liver regeneration. Indeed, timing seems to be of critical relevance as platelet depletion prior to PHx significantly reduced hepatocyte proliferation, whereas antibody-mediated thrombocytopenia initiated 2 hours after PHx had much less impact²⁵. These studies imply that hepatic platelet accumulation in the first several hours after PHx is critical for liver regeneration. The mechanism driving platelet accumulation in the liver after PHx is likely multifactorial. Kirschbaum et al. found that administration of an anti-VWF antibody reduced hepatic platelet accumulation 30 min after PHx, and that hepatocyte proliferation was reduced in VWF-deficient mice after PHx²⁵. Besides VWF, components of the coagulation cascade may attract platelets to the liver after PHx. For example, we have shown that depletion of plasma fibrinogen with ancrod also prevented hepatic platelet accumulation⁶³. This suggests that in addition to adhesive proteins like VWF, coagulation factors are critical drivers of platelet accumulation in the liver after PHx, a potential basis for prior studies connecting blood coagulation to liver regeneration^{63, 64}.

Coagulation factors in liver regeneration

Activation of coagulation has been shown to promote liver regeneration. Indeed, hepatic fibrin(ogen) deposition is already evident in the liver remnant of mice 30 min after PHx, and this corresponded to an increase in activation of blood coagulation, indicated by increased plasma thrombin-antithrombin (TAT) levels compared to sham-operated animals⁶³. The source of intrahepatic coagulation activity after PHx appears to be activation of the extrinsic pathway. We found that liver-specific tissue factor (TF) deficiency attenuated plasma TAT levels and reduced hepatic fibrin(ogen) deposition after PHx⁶³. Furthermore, Beier et. al. reported that hepatic fibrin(ogen) deposition was increased in mice even 48 hours after PHx⁶⁴, suggesting a persistent response. In agreement with observations in liver TF-deficient mice⁶³, administration of the thrombin inhibitor lepirudin reduced hepatic fibrin(ogen) deposition⁶⁴, providing further evidence that thrombin drives fibrin(ogen) deposition in this setting. Beier et al. also discovered that plasminogen activator inhibitor-1 deficiency, anticipated to increase plasmin activity, reduced hepatic fibrinogen deposition after PHx⁶⁴. Indeed, prior studies show that activation of fibrinolysis follows shortly after fibrin deposition in mice after PHx⁶⁵. Notably, interpretation of these studies is complex as prior studies suggest a pro-regenerative role for plasmin(ogen) in liver regeneration^66–68, although remodeling of the liver may be fibrin(ogen)-independent⁶⁹. Collectively, there is strong evidence to suggest that hepatic fibrin(ogen) deposition after PHx is rapid and controlled by both procoagulant and fibrinolytic pathways. Importantly, inhibition of hepatic fibrin(ogen) deposition by liver TF-deficiency reduced hepatocyte proliferation, a result consistent with prior studies suggesting a reduction in hepatocyte proliferation after treatment with a direct thrombin inhibitor⁶⁴. Furthermore, depletion of plasma fibrinogen with ancrod prior to PHx prevented hepatic fibrin(ogen) deposition and subsequently reduced hepatocyte proliferation after PHx⁶³. Collectively, these studies suggest that coagulation activation and fibrin(ogen) deposition contributes to liver regeneration after PHx.

Importantly, not all proteolytic targets of thrombin contribute to regeneration. For example, hepatocyte proliferation was unaffected after PHx in mice lacking protease activated receptor-4 (PAR-4)⁶³, the primary receptor for thrombin on mouse platelets⁷⁰. It is also important to note that fibrin(ogen) may contribute to liver regeneration through multiple mechanisms, as it engages not only platelet integrins but also β2 integrins expressed by leukocytes, another cell type known to play a key role in regeneration after PHx^{59, 71, 72}. Further studies are needed to identify specific mechanisms underlying fibrin(ogen)-directed platelet accumulation in the liver after PHx. Notably, platelets themselves amplified hepatic fibrin(ogen) deposition after PHx⁶³, highlighting clear cross-talk contributing to amplification of intrahepatic coagulation after PHx. Similarly, additional studies are required to determine how fibrin(ogen) and VWF collectively promote hepatic platelet accumulation after PHx. Analogous to PHx in mice, an increase in hepatic fibrin(ogen) deposition was also evident in liver biopsies from liver resection patients taken 2 hours after ligation of the portal vein, perceived as the start of regeneration, compared to baseline levels⁶³. Interestingly, increased hepatic fibrin(ogen) deposition was not observed in patients that went on to develop liver dysfunction⁶³. A limitation of this study was the relatively small cohort of patients examined, and further analysis is required. However, this association suggests rapid hepatic fibrin(ogen) deposition is an essential early trigger for liver regeneration, a hypothesis supported by the aforementioned observation in mice wherein preventing hepatic fibrin(ogen) deposition reduced hepatocyte proliferation after PHx^{63, 64}. There is also evidence to suggest that changes in plasma fibrinogen shortly after surgery are associated with outcome. Although pre-operative plasma fibrinogen concentration was similar between patients that had a complication-free recovery and patients that developed liver dysfunction, two independent studies have shown that postoperative plasma fibrinogen levels can be used to distinguish patients at risk of developing post-hepatectomy liver dysfunction^{63, 73}. Overall, there is emerging evidence to suggest fibrin(ogen) has an important function in liver regeneration in mice and patients and highlight the importance of various components of the coagulation system in liver regeneration. This may lead to new opportunities to identify novel mechanisms driving intrahepatic coagulation after liver surgery.

Hemodynamic changes and hemostasis-mediated liver regeneration.

One of the earliest changes occurring after partial hepatectomy is an increase in hemodynamic forces imposed on the liver remnant. While effects on arterial blood flow seem to be minimal, the portal blood flow increases since a significant amount of blood from the extrahepatic splanchnic organs now needs to pass through a liver that is significantly reduced in size ¹⁸. Several studies have shown that the portal hyperdynamic state and the resulting increased intrahepatic shear stress is critical for liver regeneration, and may even serve as the starting point ^{72, 74–76}. However, while an initial increase in portal venous flow seems to be required for adequate liver regeneration, excessive portal venous flow (termed “portal hyperperfusion”) may also contribute to liver dysfunction and failure after extended liver resection and segmented liver transplantation¹⁴.

The sudden increase in portal pressure after resection exposes the endothelial cells to excessive shear forces. Indeed, immediate (i.e. within 10 minutes) ultrastructural changes in the endothelial lining have been observed following PHx⁷⁷. It is reasonable to speculate that platelets respond to these changes as they are known to adhere to LSECs⁷⁸. In this context, potential VWF release from activated endothelial cells in response to increased shear stress may play a role in recruiting platelets. Indeed, VWF plasma levels increase rapidly after partial hepatectomy^{56, 79}. Although these changes may be a general effect of major abdominal surgery^{79, 80}, VWF plasma levels increased within two hours after induction of liver regeneration in the liver vein of patients undergoing partial hepatectomy⁵⁶. Furthermore, this rapid VWF increase after induction of liver regeneration seemed to be required for platelet accumulation and liver regeneration in patients⁵⁶.

Although our previous studies identified that the rapid procoagulant response after PHx is initiated by hepatic TF, the trigger for hepatocyte TF activation after PHx has yet to be discovered. Upon vascular injury, subendothelial TF is exposed to its ligand factor VIIa (FVIIa), initiating coagulation. However, hepatocytes expresses a TF:FVIIa complex that is an encrypted state and requires activation via decryption to activate coagulation⁸¹. Phosphatidylserine (PS) externalization triggered by apoptosis is one potential mechanism of TF decryption in the injured/diseased liver^82–84. However, apoptosis after partial hepatectomy is minimal due to activation of the anti-apoptotic Akt pathway⁸⁵. It seems plausible that the immediate change in portal blood flow could activate intrahepatic coagulation. Indeed, in other experimental settings, it has been shown that shear stress increased TF expression and activity^{86, 87}. Whether hemodynamic changes after partial hepatectomy activates coagulation remains to be determined.

Targeting the hemostatic system to promote regeneration?

Strong evidence from experimental studies links changes in the hemostatic system with liver regeneration. Moreover, clinical studies have provided corresponding evidence in patients, wherein fibrinogen or platelet-centered measurements associate with development of liver dysfunction and outcome after liver resection. Thus, with further study it may be possible to identify strategies to accelerate liver regeneration in patients after liver resection.

Although many of the pro-regenerative factors released by platelets have very short half-lives when circulating free in blood, storage in platelets serves to protect these factors⁵¹. In this way, platelets represent very sensitive sentinels that rapidly respond to injury by releasing their cargo. Platelet activation and aggregation leads to site-specific release of granule content, which produces locally high concentrations of platelet-derived factors important for liver regeneration. Rapid availability of these growths factors is a prerequisite for functional liver regeneration processes. Thrombopoietin (TPO) has been shown to promote liver regeneration in mice, presumably by increasing platelet number⁸⁸. Therefore, TPO might represent a potential therapeutic target as TPO mimetics are approved and in clinical use. However, elevating platelet number might increase the risk for (portal vein) thrombosis, mortality, and cancer recurrence^89–91. In addition to modulation of platelet counts, pharmacological modulation of perioperative hepatic hemodynamics, via its impact on intrahepatic platelet accumulation and perhaps coagulation activation, might represent a tool to modulate liver regeneration. Indeed, pharmacologic manipulation of portal pressure using terlipressin has been shown to promote liver regeneration in mice⁹². However, the prospective randomized trial of the same group that followed this experimental trial did not show a significant improvement in patients’ outcome⁹³. Although an effect might have been missed by trial design and power, this illustrates the complexity of translating observations from experimental PHx to liver resection in patients. The heterogeneity of underlying liver disease within patients is only one of several potential reasons why translation of treatment strategies to the real-life clinical setting might fail. This, however, also demonstrates the importance of documenting the clinical relevance of experimental findings. Ultimately, more detailed analyses in this area is needed and studies examining platelet activation markers or degranulation in clinical studies measuring or modifying portal pressure could be very revealing.

Although manipulating platelet aggregation is an attractive option to drive liver regeneration, targeting platelet contents might also be an effective therapeutic approach to modulate liver regeneration. As an example, we were able to recently demonstrate that selective serotonin reuptake inhibitor (SSRI) therapy reduced intraplatelet serotonin content prior to surgery in patients undergoing liver resection³⁹. In line with our existing translational result, we observed that patients taking SSRIs displayed worse postoperative outcome after liver resection when platelet serotonin levels were compromised. Identifying creative strategies to modify granule content could represent an attractive therapeutic option to promote liver regeneration. However, as with other suggestions herein, all these potential therapeutic options require exploration and validation in clinical trials.

Identifying mechanisms driving hepatic platelet accumulation seems likely to reveal novel strategies to promote platelet-directed liver regeneration. Because platelets accumulate in the liver very rapidly after liver resection in both mice and humans, and this rapid accumulation seems of the utmost importance, interventions that boost this early platelet response may be most effective. Mechanistic studies suggest that both VWF and fibrinogen contribute to platelet accumulation after PHx in mice. Stimulating the release of VWF from endothelial cells using drugs like DDAVP (desmopressin) would be one possibility. However, as VWF is stored in endothelial cells, endothelial exhaustion may limit the efficiency of the drug. Indeed, DDAVP has limited efficacy in patients with underlying liver disease⁹⁴. Furthermore, our results indicate that the increase in VWF after liver resection is limited in patients with underlying liver disease⁵⁶. As an alternative, administration of these proteins to augment the early hepatic platelet response after liver resection seems plausible and would circumvent possible endothelial exhaustion. Both VWF and fibrinogen are available as concentrates and used clinically (i.e., Humate-P/Haemate-P, RiaSTAP/Haemocomplettan P). It seems plausible that in patients where fibrinogen or VWF levels are deemed too low to support regeneration, this defect could easily be corrected intraoperatively, or even modestly increased to further promote regeneration. It is worth noting that liver resection patients commonly receive anticoagulants approximately 6 hours after surgery to reduce the risk of thrombosis. Hepatic platelet and fibrin(ogen) deposits responsible for driving liver regeneration occur prior to this⁶³, making it unlikely that postoperative anticoagulation interferes with this mechanism of regeneration. This also keeps with the plausibility of using intraoperative interventions to promote platelet-directed liver regeneration while limiting the risk of thrombosis after surgery.

Summary and Conclusions:

In summary, strong experimental evidence links components of the hemostatic system (e.g., VWF, platelets, coagulation factors) to liver regeneration after PHx in mice. Clinical results supporting translation of these results to liver resection patients are compelling and additional details continue to emerge. Indeed, the combined strength of mechanisms discovered in experimental PHx and clinical association studies may reveal 1) novel strategies to predict post-hepatectomy liver dysfunction and 2) new therapies using components of the hemostatic system to promote liver regeneration.