Characterization of hepatocyte-derived mitogenic activity on hepatic stellate cells

Intercellular crosstalk of liver sinusoidal endothelial cells in liver fibrosis, cirrhosis and hepatocellular carcinoma

Intercellular crosstalk among various liver cells plays an important role in liver fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). Capillarization of liver sinusoidal endothelial cells (LSECs) precedes fibrosis and accumulating evidence suggests that the crosstalk between LSECs and other liver cells is critical in the development and progression of liver fibrosis. LSECs dysfunction, a key event in the progression from fibrosis to cirrhosis, and subsequently obstruction of hepatic sinuses and increased intrahepatic vascular resistance (IHVR) contribute to development of portal hypertension (PHT) and cirrhosis. More importantly, immunosuppressive tumor microenvironment (TME), which is closely related to the crosstalk between LSECs and immune liver cells like CD8+ T cells, promotes advances tumorigenesis, especially HCC. However, the connections within the crosstalk between LSECs and other liver cells during the progression from liver fibrosis to cirrhosis to HCC have yet to be discussed. In this review, we first summarize the current knowledge of how different crosstalk between LSECs and other liver cells, including hepatocytes, hepatic stellate cells (HSCs), macrophoges, immune cells in liver and extra cellular matrix (ECM) contribute to the physiological function and the progrssion from liver fibrosis to cirrhosis, or even to HCC. Then we examine current treatment strategies for LSECs crosstalk in liver fibrosis, cirrhosis and HCC.

Paracrine signals from mesenchymal cell populations govern the expansion and differentiation of human hepatic stem cells to adult liver fates

The differentiation of embryonic or determined stem cell populations into adult liver fates under known conditions yields cells with some adult-specific genes but not others, aberrant regulation of one or more genes, and variations in the results from experiment to experiment. We tested the hypothesis that sets of signals produced by freshly isolated, lineage-dependent mesenchymal cell populations would yield greater efficiency and reproducibility in driving the differentiation of human hepatic stem cells (hHpSCs) into adult liver fates. The subpopulations of liver-derived mesenchymal cells, purified by immunoselection technologies, included (1) angioblasts, (2) mature endothelia, (3) hepatic stellate cell precursors, (4) mature stellate cells (pericytes), and (5) myofibroblasts. Freshly immunoselected cells of each of these subpopulations were established in primary cultures under wholly defined (serum-free) conditions that we developed for short-term cultures and were used as feeders with hHpSCs. Feeders of angioblasts yielded self-replication, stellate cell precursors caused lineage restriction to hepatoblasts, mature endothelia produced differentiation into hepatocytes, and mature stellate cells and/or myofibroblasts resulted in differentiation into cholangiocytes. Paracrine signals produced by the different feeders were identified by biochemical, immunohistochemical, and quantitative reverse-transcription polymerase chain reaction analyses, and then those signals were used to replace the feeders in monolayer and three-dimensional cultures to elicit the desired biological responses from hHpSCs. The defined paracrine signals were proved to be able to yield reproducible responses from hHpSCs and to permit differentiation into fully mature and functional parenchymal cells.

CONCLUSION

Paracrine signals from defined mesenchymal cell populations are important for the regulation of stem cell populations into specific adult fates; this finding is important for basic and clinical research as well as industrial investigations.

Endogenous hepatocyte growth factor is a niche signal for subventricular zone neural stem cell amplification and self-renewal

Neural stem cells persist in the adult mammalian brain, within the subventricular zone (SVZ). The endogenous mechanisms underpinning SVZ neural stem cell proliferation, self-renewal, and differentiation are not fully elucidated. In the present report, we describe a growth-stimulatory activity of liver explant-conditioned media on SVZ cell cultures and identify hepatocyte growth factor (HGF) as a major player in this effect. HGF exhibited a mitogenic activity on SVZ cell cultures in a mitogen-activated protein kinase (MAPK) (ERK1/2)-dependent manner as U0126, a specific MAPK inhibitor, blocked it. Combining a functional neurosphere forming assay with immunostaining for c-Met, along with markers of SVZ cells subtypes, demonstrated that HGF promotes the expansion of neural stem-like cells that form neurospheres and self-renew. Immunostaining, HGF enzyme-linked immunosorbent assay and Madin-Darby canine kidney cell scattering assay indicated that SVZ cell cultures produce and release HGF. SVZ cell-conditioned media induced proliferation on SVZ cell cultures, which was blocked by HGF-neutralizing antibodies, hence implying that endogenously produced HGF accounts for a major part in SVZ mitogenic activity. Brain sections immunostaining revealed that HGF is produced by nestin-expressing cells and c-Met is expressed within the SVZ by immature cells. HGF intracerebroventricular injection promoted SVZ cell proliferation and increased the ability of these cells exposed in vivo to HGF to form neurospheres in vitro, whereas intracerebroventricular injection of HGF-neutralizing antibodies decreased SVZ cell proliferation. The present study unravels a major role, both in vitro and in vivo, for endogenous HGF in SVZ neural stem cell growth and self-renewal.

Homogeneous differentiation of hepatocyte-like cells from embryonic stem cells: applications for the treatment of liver failure

One of the major hurdles of cellular therapies for the treatment of liver failure is the low availability of functional human hepatocytes. While embryonic stem (ES) cells represent a potential cell source for therapy, current methods for differentiation result in mixed cell populations or low yields of the cells of interest. Here we describe a rapid, direct differentiation method that yields a homogeneous population of endoderm-like cells with 95% purity. Mouse ES cells cultured on top of collagen-sandwiched hepatocytes differentiated and proliferated into a uniform and homogeneous cell population of endoderm-like cells. The endoderm-like cell population was positive for Foxa2, Sox17, and AFP and could be further differentiated into hepatocyte-like cells, demonstrating hepatic morphology, functionality, and gene and protein expression. Incorporating the hepatocyte-like cells into a bioartificial liver device to treat fulminant hepatic failure improved animal survival, thereby underscoring the therapeutic potential of these cells.

Oxygen uptake rates and liver-specific functions of hepatocyte and 3T3 fibroblast co-cultures

Bioartificial liver (BAL) devices have been developed to treat patients undergoing acute liver failure. One of the most important parameters to consider in designing these devices is the oxygen consumption rate of the seeded hepatocytes which are known to have oxygen consumption rates 10 times higher than most other cell types. Hepatocytes in various culture configurations have been tested in BAL devices including those formats that involve co-culture of hepatocytes with other cell types. In this study, we investigated, for the first time, oxygen uptake rates (OUR)s of hepatocytes co-cultured with 3T3-J2 fibroblasts at various hepatocyte to fibroblast seeding ratios. OURs were determined by measuring the rate of oxygen disappearance using a ruthenium-coated optical probe after closing and sealing the culture dish. Albumin and urea production rates were measured to assess hepatocyte function. Lower hepatocyte density co-cultures demonstrated significantly higher OURs (2 to 3.5-fold) and liver- specific functions (1.6-fold for albumin and 4.5-fold for urea production) on a per cell basis than those seeded at higher densities. Increases in OUR correlated well with increased liver-specific functions. OURs (V(m)) were modeled by fitting Michaelis-Menten kinetics and the model predictions closely correlated with the experimental data. This study provides useful information for predicting BAL design parameters that will avoid oxygen limitations, as well as maximize metabolic functions.

Liver fibrogenesis: a new role for the renin-angiotensin system

Liver fibrosis is the consequence of chronic liver injury of any etiology. When advanced, fibrosis causes portal hypertension and liver insufficiency, and is a risk factor for developing hepatocellular carcinoma. In the last decade, there have been major advances in the knowledge of the pathogenesis of hepatic fibrosis. Hepatic stellate cells (HSCs) are recognized as the main collagen-producing cells in the injured liver, and key fibrogenic factors have been identified. Among these factors, the renin-angiotensin system (RAS) appears to play a major role. Angiotensin II (Ang II) mediates key biological actions involved in hepatic tissue repair, including myofibroblast proliferation, infiltration of inflammatory cells, and collagen synthesis. Activated HSCs secrete Ang II, which induces fibrogenic actions through the activation of NADPH oxidase. Importantly, the blockade of the RAS attenuates fibrosis development in different experimental models of chronic liver injury. Based on these studies, it has been proposed that the blockade of the RAS could be effective in preventing fibrosis progression in chronic liver diseases. Although no prospective studies have evaluated the antifibrotic effect of RAS inhibitors in patients with chronic liver diseases, controlled clinical trials are under way.

The role of non-parenchymal cells in liver growth

The main non-parenchymal cells of the liver, Kupffer cells, sinusoidal endothelial cells and stellate cells, participate in liver growth with respect to both their own proliferation, and effects on hepatocyte proliferation. In the well-characterised paradigm of 70% partial hepatectomy, they undergo DNA synthesis and cell division 20-24h later than the hepatocyte population. They exert both positive and negative influences on hepatocyte proliferation, including provision of an extracellular matrix-bound reservoir of hepatocyte growth factor that is activated after damage; priming of hepatocytes for DNA synthesis through rapid generation of TNF-alpha and IL-6; and generation of factors at later time points that curb hepatocyte DNA synthesis (IL-1, TGF-beta) and initiate reconstruction and reformation of matrix proteins.

Polymorphonuclear neutrophils are a source of hepatocyte growth factor in patients with severe alcoholic hepatitis

BACKGROUND/AIMS

Hepatocyte growth factor (HGF) is a pleiotropic cytokine involved in liver regeneration. Plasma HGF levels correlate with survival and hepatocyte proliferation in alcoholic hepatitis (AH). As AH is accompanied by inflammation, neutrophilia and polymorphonuclear neutrophil (PMN) infiltration of the liver, we postulated that PMN could be a source of HGF in such patients.

METHODS

We studied 25 patients with severe AH in comparison with 20 alcoholic cirrhotic patients without AH and 20 healthy controls; the impact of a 28-day course of corticosteroids was evaluated in patients with AH.

RESULTS

On day 0, HGF plasma and homogenized liver tissue levels were markedly increased in AH patients as compared to controls. The role of PMN in HGF production during AH was confirmed by a significantly higher ex-vivo HGF production capacity of lipopolysaccharide-stimulated blood PMN from AH patients relative to both control groups. Formyl-Methionyl-Leucyl-Phenylalanine-induced PMN release of HGF (degranulation conditions) was also higher in AH patients. In this setting, we found that HGF release by PMN ex vivo correlated strongly with HGF plasma levels, and that the degree of hepatic PMN correlated strongly with hepatic HGF levels. HGF plasma levels and ex-vivo HGF release by PMN were unaffected by steroid therapy.

CONCLUSIONS

These findings suggest that, by releasing HGF, PMN could participate in liver regeneration during severe alcoholic hepatitis.

Enhanced proliferation and differentiation of rat hepatocytes cultured with bone marrow stromal cells

Liver transplantation is the only clinically effective method of treating acute liver failure. However, wider application of this therapeutic modality is restricted primarily by shortage of donor organs. In the search for alternative methods of liver replacement therapy, investigators have focused on transplantation of normal allogeneic hepatocytes and on the development of liver support systems utilizing isolated hepatocytes. Since all human livers suitable for cell harvest are being used for transplantation, hepatocyte therapy using human tissue would require growing of cells in vitro. Unfortunately, although hepatocytes have tremendous capacity to proliferate in vivo, their ability to grow in culture is severely limited. Stromal cells from bone marrow and other blood-forming organs have been found to support hematopoiesis. In this paper, we show that bone marrow-derived stromal cells (BMSCs) enhance proliferation and support differentiation of rat hepatocytes in culture. Further, we demonstrate that in hepatocyte/BMSC co-cultures, clonal expansion of small hepatocytes (SH) is increased. Using semipermeable membrane cultures, we established that direct cell-cell contact is necessary for stimulation of cell proliferation. We also show that BMSCs which are in direct contact with hepatocytes and SH colonies express Jagged1. This suggests a potential role for Notch signaling in the observed effects. Finally, we present evidence that the expression and activity of liver specific transcription factors, CCAAT/enhancer binding proteins and liver specific key enzymes such as tryptophan 2,3-dioxygenase, are improved in hepatocyte/BMSC co-cultures. In conclusion, results of this study indicate that BMSCs could facilitate proliferation and differentiation of primary rat hepatocytes and their progenitors (SH) in vitro.