Speeding up hepatocyte proliferation: How triiodothyronine and β-catenin join forces
Authors: Rolf Gebhardt
Journal: Hepatology (2013)

The unique regenerative capacity of the liver has fascinated humans from ancient times to today. In fact, the rather vulnerable location of the liver, downstream of the gut, may render its high regenerative potential a precondition for surviving the ongoing challenge by toxins and harmful xenobiotics delivered through the portal circulation. According to current views, liver regeneration has a tremendous redundancy with respect to the mechanisms of this process.1 Although the proliferation of highly differentiated hepatocytes may be sufficient to replace simple hepatocyte loss or to compensate for entire parts of the tissue, such as in partial hepatectomy (PHx), any impairment of this process may lead to the activation of alternative mechanisms involving cellular transdifferentiation, stem cells, and other precursors.2 Even in chronic liver disease, regenerative mechanisms appear to be active but not strong enough to overcome the various and continuous opposing insults. Under conditions of life-threatening liver diseases, orthotopic liver transplantation offers a real option for patient survival but may not always be accessible because of the scarcity of donor organs. However, promising alternatives, such as the transplantation of allogeneic hepatocytes, stem cells, and engineered tissues, are beginning to emerge.3 In these cases, the enhanced regenerative potential of the liver or hepatocytes is highly desirable for achieving rapid restoration of the complete spectrum and full capacity of liver functions. Thus, a proper understanding of the various regenerating mechanisms and their possible hazards is mandatory for the successful translation of novel regenerative concepts to clinical applications. Liver regeneration after PHx has been used as a popular paradigm for investigating complex regeneration processes, which are controlled by multiple signaling pathways triggered by growth factors and hormones that can roughly be divided into complete mitogens and auxiliary mitogens.1 Complete mitogens, such as hepatocyte growth factor and epidermal growth factor, drive hepatocyte proliferation and liver enlargement in vivo and in vitro. In contrast, auxiliary mitogens are neither mitogenic in hepatocyte cultures nor do they cause liver enlargement in vivo, yet do cause a delay in regeneration once their genes are ablated. Typical members of this group are norepinephrine, interleukin 6, insulin, and β-catenin. Although a large variety of such mitogens has been identified, their concerted action and crosstalk are not well understood. A promising example advancing our knowledge of how different proliferative signals network to efficiently drive hepatocellular proliferation is provided by Fanti et al.4 in this issue of Hepatology focusing on the crosstalk between thyroid hormones and β-catenin signaling (Fig. 1). The mitogenic properties of triiodothyronine (T3) have been extensively studied for decades in normal livers and after PHx. Nonetheless, it remains controversial whether T3 can be considered a direct mitogen because at least one criterion, the effectiveness of normal hepatocytes in vitro, has not been definitively met.5 T3 is well known for ameliorating liver regeneration after PHx or subtotal hepatectomy.5, 6 In accordance with these findings, thyroid status can be an important determinant of the regeneration process.7 However, T3 is also highly effective on normal liver when applied by way of different routes, including specially prepared diets.5, 8, 9 Despite the ongoing research in this field, the manifold mechanisms of action of T3 are not well understood. Most effects have been found to be mediated by thyroid hormone receptor β (TRβ),9 which acts as a transcription factor at T3 response elements (TRE) sites in the DNA of T3 target genes.10 In addition to such genomic actions, several nongenomic effects have also been identified (reviewed11). Among these, the rapid activation of different intracellular protein kinase cascades initiated at the plasma membrane through certain integrins is of particular interest. Examples range from the activation of a PI3-kinase-ERK1/2-MAPK-dependent pathway12 to those transmitted by way of different cell types, e.g., Kupffer cells, which secrete cytokines that trigger different kinase cascades in adjacent hepatocytes. A novel and very appealing example is added to this list by Fanti et al. Using transgenic mice with a conditional knockout of β-catenin, these authors demonstrate that β-catenin is a necessary mediator of the proliferative effect of T3.4 Furthermore, they show that T3 activates β-catenin signaling through protein kinase A (PKA)-driven SER675 phosphorylation (Fig. 1). β-Catenin is a versatile molecule: it is a structural component involved in cell-cell adhesion but can also be a crucial downstream effector of canonical Wnt signaling and, as such, is important for embryogenesis, organogenesis, and cancer (reviewed13). Activating mutations of β-catenin play a particular role in carcinogenesis; however, indirect activation of β-catenin through loss-of-function mutations in certain Wnt pathway inhibitors, such as APC or Axin, is also possible.13 In the mature liver, the majority of β-catenin is bound to cadherins at the plasma membrane; indeed, free β-catenin, which is indicative of active signaling, can be detected only in the cytoplasm of the most distal pericentral hepatocytes (reviewed14). Based on indirect evidence, it is assumed that the transcriptional activity associated with activated β-catenin forms a gradient that decreases from the central vein toward the portal tract. In elegant studies by several groups, it was convincingly demonstrated that this gradient of β-catenin-dependent signaling plays a dominant role in controlling the zonated expression of many enzymes involved in ammonia, amino acid, and carbohydrate metabolism (reviews14, 15). One such example is the heterogeneous expression of glutamine synthetase, which is localized exclusively in hepatocytes with the highest activity of β-catenin signaling, although it is not a direct target gene of β-catenin.15 Although a peculiar feature of the activity gradient of β-catenin signaling is its particular stability under most physiological conditions, a rapid and transient activation of β-catenin in all hepatocytes was observed after PHx.13 The importance of this activation for proper liver regeneration is underscored by the fact that β-catenin knockout leads to the retardation of hepatocyte proliferation13 and by the identification of cyclin D1 as a target of β-catenin.16 Remarkably, the results of Fanti et al. demonstrate that the exposure of normal mice to T3 considerably enhanced the activation of β-catenin in all zones, rendering the gradient visible by the increasing intensity of cytoplasmic and/or nuclear staining from the periportal to the pericentral zone.4 These changes were not due to the hepatocellular up-regulation of β-catenin expression but rather to the increase in β-catenin SER675 phosphorylation by way of the activation of PKA. Thus, Fanti et al. reveal a novel example of crosstalk among the signaling of T3, PKA, and β-catenin that is relevant for hepatocyte proliferation. Within this context, it should be noted that, following the subcutaneous injection of T3, Forbes et al.17 detected bromodeoxyuridine (BrdU)-labeled hepatocytes mostly in the mid-zonal region. These findings suggest that T3-driven proliferation exhibits characteristics that are different from those observed after PHx, which starts in the periportal zone. Moreover, PKA may play a much broader role than merely phosphorylating β-catenin by serving as a hub that integrates different cyclic adenosine monophosphate (cAMP)-dependent signals. For instance, the activation of PKA by way of β-adrenergic signaling might result in a similar crosstalk with β-catenin and thus explain part of the mitogenic action of norepinephrine (Fig. 1). Collectively, the results of Fanti et al.4 provide valuable insight into a complex mechanism for inducing hepatocyte proliferation in the normal mouse liver, potentially opening new avenues for therapeutic options in several types of liver disease characterized by the insufficient regeneration of liver parenchyma. However, before emerging therapies might profit from these findings, the remaining basic questions need to be addressed. For instance, it remains to be determined whether T3 truly induces hyperplasia. Although Torres et al.8 determined an increasing relative liver weight following T3 treatment of a normal liver in hypothyroid rats, it is unclear whether this was due to an increased number of hepatocytes or an abnormally high proportion of hepatocytes with increased ploidy. Indeed, earlier studies of such changes suggest at least a transient increase in ploidy after T3 treatment. Another important aspect is the robustness of the identified crosstalk among the signaling of T3, PKA, and β-catenin in different species, including humans, and under conditions of liver disease. Diverse changes in β-catenin signaling are well known in certain types of liver disease13 and may hamper or support crosstalk with T3 and PKA. Furthermore, the metabolic consequences that are associated with the hepatocellular proliferation induced by T3 application remain to be determined. Certain observations suggest that these may be considerably different from those associated with PHx. The increase in glutamine synthetase-positive hepatocytes detected by Fanti et al.4 is such a point and reveals a certain pericentralization in response to the activation of β-catenin that is comparable, although less pronounced, to what occurs in liver-specific APC-KO mice.18 This finding fits nicely with the observation that treatment with T3 is beneficial for inhibiting or reversing nonalcoholic fatty liver disease.19 Together with the finding that T3 treatment lowers the risk of tumor development and metastasis,20 the stimulation of β-catenin by T3 or similarly acting analogs may provide a promising strategy for improving and fine-tuning the current and future therapies of acute or chronic types of liver disease. Rolf Gebhardt, Ph.D. From the Institute of Biochemistry Faculty of Medicine University of Leipzig Leipzig, Germany