Inhibition in fat-storing cell multiplication by a factor produced by normal cultured murine hepatocytes

Stimulation and proliferation of primary rat hepatic stellate cells by cytochrome P450 2E1-derived reactive oxygen species

The alcohol-inducible cytochrome P450 2E1 (CYP2E1) is expressed mainly in hepatocytes and generates reactive oxygen species (ROS). To better understand how hepatic stellate cells (HSC) become activated in the presence of oxidative stress and evaluate whether CYP2E1-derived ROS activate stellate cells, we coincubated primary stellate cells with HepG2 cells, which do (E47 cells) or do not (C34 cells) express CYP2E1. Morphologic changes and loss of lipid droplets were more apparent in the stellate cells cocultured with E47 cells. There was a more pronounced increase in alpha-smooth muscle actin (alpha-sma), intracellular and secreted collagen type I protein, and intra- and extracellular H(2)O(2) and lipid peroxidation products in stellate cells coincubated with E47 cells. Expression of collagen in stellate cells did not change when cocultured with HepG2 cells expressing a different P450, CYP3A4. Stellate cells cultured on Matrigel expressed increased alpha-sma and collagen when incubated with E47 cells. The increase in collagen production by coculture with E47 cells was prevented by antioxidants, by CYP2E1 inhibitors, and by transfected antisense CYP2E1. The addition of arachidonic acid plus ferric nitrilotriacetate (Fe-NTA), agents that potentiate oxidative stress, further induced collagen protein in the E47 coculture. Stellate cell proliferation was greater in the E47 coculture, and this was partially abrogated by catalase and vitamin E. These results show that hepatocytes containing CYP2E1 release diffusible mediators including ROS, which can activate HSC. Thus, besides perturbing the homeostasis of hepatocytes, CYP2E1-derived diffusible oxidants may also interact with stellate cells and contribute to hepatic fibrosis.

Fibrogenic effect of oxidative stress on rat hepatic stellate cells

Oxidative stress is associated with liver fibrosis and with hepatic stellate cell (HSC) activation in vivo. However, it remains controversial whether oxidative stress contributes to HSC activation either directly or through a paracrine stimulation by damaged hepatocytes. A medium containing products released from cells undergoing oxidative stress was obtained after incubation of hepatocytes with (HCM/Fe) or without (HCM) 0.1 mmol/L ferric nitrilotriacetate complex (FeNTA). Exposure of HSC to HCM/Fe for 24 hours significantly increased the number of proliferating HSC compared with HCM and to controls at all dilutions tested. The simultaneous coincubation of HSC with HCM/Fe and desferrioxamine (50 micromol/L) did not reduce the observed increase in cell proliferation, thus excluding a role for eventually contaminating iron in HCM/Fe. HCM/Fe induced also a significant increase in collagen type I accumulation in HSC culture media. To study the cellular mechanism underlying HCM/Fe effects, we evaluated the activity of the Na+/H+ exchanger, which plays a role in regulating HSC proliferation. The incubation of HSC for 24 hours with HCM/Fe significantly increased baseline intracellular pH (pHi) and Na+/H+ exchanger activity, indicating a plausible role of this antiport in mediating cell response. In conclusion, hepatocytes undergoing oxidative stress release factors which are fibrogenic for HSC, thereby, confirming what has been only hypothesized in vivo. In addition, HSC proliferation is associated with changes in the Na+/H+ exchanger activity, thus providing a useful target for the evaluation of inhibitors of this pathway for the treatment of hepatic fibrosis.

Pharmacological aspects of pentoxifylline with emphasis on its inhibitory actions on hepatic fibrogenesis

1. Pentoxifylline (PTX), a derivative of the methylxanthine theobromine, has been used for many years in the treatment of peripheral vascular diseases. Increased red blood cell flexibility, reduction of blood viscosity, and decreased potential of platelet aggregation are the basic actions of PTX, resulting in therapeutic benefits due to improved microcirculation and tissue oxygenation. 2. PTX's generally accepted mechanism of action is the inhibition of phosphodiesterases, leading to increased intracellular levels of cyclic adenosine monophosphate (cAMP). 3. A number of studies have shown PTX's effects on the cytokine network. The most relevant clinical results are the therapeutic benefits of PTX in attenuating the effects of tumor necrosis factor-alpha (TNF-alpha) in conditions such as septic shock. 4. PTX also has been found to exert antifibrogenic actions, using cultured fibroblasts or animal models of fibrosis, including liver fibrosis. 5. In hepatic stellate cell culture PTX has been shown to inhibit the basic reactions of liver fibrogenesis, being effective on cytokines and growth factors relevant in fibrogenesis of the liver, too. 6. Therefore, PTX might be an effective drug with few side effects in the treatment of liver fibrosis. Further clinical studies have to be done to establish the real therapeutic benefits of PTX in liver fibrosis and cirrhosis.

Synergism between hepatocytes and Kupffer cells in the activation of fat storing cells (perisinusoidal lipocytes)

This study concerns the cooperation of hepatocytes (PC) and Kupffer cells (KC) in the activation of rat liver fat storing cells (FSC) in culture. Various dilutions of conditioned media collected from early, serum-free cultures of both cell types were added separately and in combination, either simultaneously or sequentially, to early, non-confluent, primary cultures of FSC maintained under serum-reduced (0.5% fetal calf serum) conditions to determine the effects on proliferation (incorporations of [3H]thymidine and bromodeoxyuridine [BrdUrd], DNA-content, cell number), transformation and morphology (phase contrast microscopy, immunostainings of desmin and smooth muscle-alpha-actin), on the deposition of fibronectin and laminin and on the formation of 35S sulfated medium proteoglycans. Media of both cell types stimulated cell proliferation in a dose-dependent manner but combined PC- and KC-conditioned media was most potent and increased the incorporation of [3H]thymidine to 4-times above control values. The multiplication stimulatory effects visualized by labeling cell nuclei with BrdUrd and the increase of cell number per culture well were additive. The sequential addition of KC-conditioned medium to FSC preexposed to PC-conditioned medium increased the multiplication of FSC further and in an additive manner. The mitogenic activity of the PC-medium and the enhancing effect of KC-induced FSC proliferation was measured also when PC were damaged by incubation under anoxic conditions during generation of the conditioned medium. This observation indicates the release of the mitogen by membrane damage presumably from a cytoplasmic pool. The PC-medium did not induce either significant morphological changes or transformation of FSC towards myofibroblast-like cells. KC, however, generated transformation of FSC as indicated by more elongated cells with spindle-like cellular extensions and a reduction of retinoid droplets. Both these morphological effects were visible when PC and KC media were added simultaneously. Both media act synergistically on the deposition of fibronectin and laminin in FSC cultures and these components were found to be elevated 2.3 and 2.8-fold, respectively, if the cells were exposed to the combined media. Proteoglycan synthesis was also maximally enhanced if FSC were exposed to PC- and KC-media simultaneously. These findings suggest the involvement of (damaged) hepatocytes in the process of FSC activation. A model of sequential, spatial and time-dependent activation of FSC is suggested where cells in the immediate proximity of hepatocytes are primed to proliferate by a mitogenic signal released by membrane damage presumably from a cytoplasmic pool of injured hepatocytes into the pericellular environment. This non-inflammatory stimulation is followed by secretions of activated Kupffer cells and other inflammatory cell types which further enhance the activation of FSC.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and partial characterization of a hepatocyte-derived factor promoting proliferation of cultured fat-storing cells (parasinusoidal lipocytes)

The molecular and cellular mechanisms of activation of fat-storing cells (Ito cells or parasinusoidal lipocytes), a prerequisite of the fibrogenic response of injured liver, were studied by analysis in vitro of some aspects of the intercellular communication between parenchymal liver cells and fat-storing cells. Conditioned medium harvested from early serum-free monolayer cultures of hepatocytes isolated from normal rat liver stimulated strongly, reproducibly and dose-dependently the proliferation of nonconfluent fat-storing cells maintained under serum-reduced conditions. During exposure of fat-storing cells for 48 hr to the conditioned medium, the incorporation of [3H]thymidine into DNA was stimulated four to six times over control values, the DNA content per culture well was elevated by 40% above control values and the immunocytochemical detection of bromodeoxyuridine-labeled cell nuclei was increased from 13% stained nuclei in controls to 70% stained nuclei in treated fat-storing cells. The mitogenic effects of hepatocyte-conditioned medium were similar to or even higher than those of 10% fetal calf serum. No mitoinhibitory activity could be detected in the hepatocyte-conditioned medium when arginase, as a potential inhibitor, was excluded. Rat skin fibroblasts could not be stimulated under conditions where the proliferation activity of fat-storing cells was greatly enhanced. The occurrence of the mitogenic activity in the medium is not dependent on de novo synthesis or secretion because the media of hepatocytes cultured under anoxic conditions in the presence of cycloheximide, brefeldin A or ethylenediaminetetraacetate were highly active in promoting fat-storing cell proliferation, although hepatocyte viability was greatly reduced under some of these conditions. A significant positive correlation (r = 0.95, p < 0.01) was found between lactate dehydrogenase activity and the mitogenic potency of the conditioned medium. The proliferation factor for fat-storing cells could also be demonstrated in the lysate of freshly isolated hepatocytes from normal liver. The stimulatory activity in the medium was partially enriched by a combination of gel permeation and anion exchange fast protein liquid chromatography and characterized as a protein with an apparent molecular weight of about 60 kD that is heat and pH sensitive but insensitive to reducing agents. It does not bind to immobilized heparin; nor does soluble heparin or proteinase inhibitor affect the mitogenic activity of the factor.(ABSTRACT TRUNCATED AT 400 WORDS)