The liver has an incredible ability to regenerate, which has fascinated scientists. For years, human liver hepatocytes have served as the reliable in vitro models for delineating hepatic physiology. They have vast applications in drug development and hepatic research. Despite being differentiated cells, hepatocytes shift from their quiescent state and enter the cell cycle during liver injury. The proliferation of Human Liver Hepatocytes replenishes the cells lost during injury, essentially driving tissue regeneration. This process is highly complex, involving more than one component and signaling pathways. This blog outlines the mechanisms of liver regeneration by hepatocytes.
Human Liver Hepatocytes
Hepatocytes constitute the bulk of liver cells and mass. They show a polyhedral morphology, with three distinct domains of plasma membrane depending on their location- intercellular, perisinuoidal, and pericannicular. The directional blood flow through hepatic sinusoids creates different gradients of nutrient and oxygen concentration, causing physiological variation in hepatocytes along them. The three metabolic zones - Zone 1, 2, and 3 reflect the functional distribution of hepatocytes. Anatomic distribution includes periportal, central lobular, and mid-zonal areas. These areas also produce a morphologically diverse hepatocyte population. For example, cells in the central lobular region are larger and contain higher smooth endoplasmic reticulum than those in the periportal region. A research study showed that cells in the mid-zonal region undergo proliferation following injury for the repair process.
Proliferation of Human Liver Hepatocytes
Cell-ECM Interaction
Hepatocyte proliferation is a crucial event in liver regeneration. Several studies have indicated that hepatic extracellular matrix (ECM) has a vital role in proliferation. Laminin is a key ECM component that upregulates after injury. Hepatocytes isolated just after injury attach more to the laminin surface than hepatocytes from non-injured tissue. In vitro experiments have demonstrated that laminin enhances hepatocyte proliferation. Similarly, many such ECM proteins, such as collagen and fibronectin, increase after injury, while the expression of their receptors upregulates in liver cells. These observations suggest the significance of ECM in liver regeneration. The degradation of ECM by matrix metalloproteinase (MMP) promotes hepatocyte migration and proliferation. However, tissue inhibitors of metalloproteinase (TIMP) prevent excessive growth by suppressing MMPs. Therefore, a delicate balance between the two enzymes drives adequate cell proliferation.
Hepatocyte Growth Factor
Hepatocytes express receptors for diverse growth factors such as VEGF, PDGF, FGF, etc. A key factor that is responsible for regeneration is hepatocyte growth factor or HGF. Its synthesis or release from the ECM increases in a regenerating liver. A proteoglycan, heparin sulfate, also upregulates post-injury to bind HGF and stimulate its mitogenic activity. Plasmin also activates HGF, alluding to the role of plasminogen activators (t-PA and u-PA), which convert plasminogen into plasmin. The expression of these activators increases following injury, resulting in ECM breakdown. The knockout mouse model for the activators displays problems with liver regeneration and excessive fibrin deposition.
Cell-Cell Interaction
The liver comprises a diverse cell population. Each of them has its own contributions to the regeneration process. Hepatic stellate cells are mesenchymal cells of the liver that secrete HGF, FGF, and IL6 trans-signaling to promote hepatocyte proliferation. They also induce termination of the regeneration process by building the ECM and secreting TGFβ. In some conditions, these cells also transform into hepatocytes to replenish the liver cell population.
In the same manner, biliary epithelial cells turn into oval cells, which form hepatocytes. In the absence of biliary epithelial cells, periportal Liver Hepatocytes
also transform into stem cells for biliary cells. Hepatocytes also produce growth factors for endothelial cells to repair vascular injury, like angiopoietins, VEGF, SCF, etc. In turn, stimulated endothelial cells release HGF, creating a mutual network of cooperative proliferation.
Cell Signaling
Regeneration involves diverse signaling pathways. Inferences from many studies have suggested that blocking any of the pathways does not stop regeneration. A key pathway is canonical Wnt/β-catenin. Besides Wnt, HGF and FGF also activate β-catenin. The downstream target of this pathway is the Ccnd1 gene, encoding cyclin D1, which induces hepatocyte transition from G1 to S phase.
The Hippo/YAP signaling pathway typically contributes to cell survival, proliferation, and differentiation. It triggers phosphorylation of YAP/TAZ proteins and prevents their nuclear translocation and subsequent gene expression. Unphosphorylated or activated YAP triggers hyperproliferation of hepatocytes. On the contrary, the Hippo pathway senses mechanical signals from the ECM and inhibits cell growth. Notch signaling is also essential for hepatic repair. It is a downstream target of YAP/TAZ and also coordinates with the Hippo pathway to regulate the differentiation of liver progenitors into hepatocytes.
Conclusion
Hepatocytes maintain homeostasis during normal circumstances and in the event of an injury. Hepatocyte proliferation during regeneration has been a subject of study. Several studies have deduced key cellular interactions and signaling pathways. However, the effect of spatial features and clinical translation of these findings still remains elusive. The abnormality in the process triggers fibrosis and subsequently organ failure. Therefore, more research is required on these cells to develop suitable therapies. Kosheeka offers human liver hepatocytes to fuel the research into hepatic repair and accelerate its clinical translation.
