Sphingomyelinase treatment of rat hepatocytes inhibits cell-swelling-stimulated glycogen synthesis by causing cell shrinkage.
Breakdown of plasma-membrane sphingomyelin caused by TNF-alpha is known to inhibit glucose metabolism and insulin signalling in muscle and fat cells. In hepatocytes, conversion of glucose to glycogen is strongly activated by amino acid-induced cell swelling. In order to find out whether breakdown of plasma-membrane sphingomyelin also inhibits this insulin-independent process, the effect of addition of sphingomyelinase was studied in rat hepatocytes. Sphingomyelinase (but not ceramide) inhibited glycogen synthesis, caused cell shrinkage, decreased the activity of glycogen synthase a, but had no effect on phosphorylase a. Cell integrity was not affected by sphingomyelinase addition as gluconeogenesis and the intracellular concentration of ATP were unchanged. As a control, glycogen synthesis was studied in HepG2 cells. In these cells, the basal rate of glycogen production was high, could not be stimulated by amino acids, nor be inhibited by sphingomyelinase. Regarding the mechanism responsible for the inhibition of glycogen synthase a, sphingomyelinase did not affect amino acid-induced, PtdIns 3-kinase-dependent, phosphorylation of p70S6 kinase, but caused an increase in intracellular chloride, which is known to inhibit glycogen synthase phosphatase. It is concluded that the decrease in cell volume, following the breakdown of sphingomyelin in the plasma membrane of the hepatocyte, may contribute to the abnormal metabolism of glucose when TNF-alpha levels are high. (+info)
Regulation of sterol regulatory-element binding protein 1 gene expression in liver: role of insulin and protein kinase B/cAkt.
Insulin stimulates the transcription of the sterol regulatory- element binding protein (SREBP) 1/ADD1 gene in liver. Hepatocytes in primary culture were used to delineate the insulin signalling pathway for induction of SREBP1 gene expression. The inhibitors of phosphoinositide 3-kinase (PI 3-kinase), wortmannin and LY 294002, abolished the insulin-dependent increase in SREBP1 mRNA, whereas the inhibitor of the mitogen- activated protein kinase cascade, PD 98059, was without effect. To investigate the role of protein kinase B (PKB)/cAkt downstream of PI 3-kinase, hepatocytes were transduced with an adenovirus encoding a PKB--oestrogen receptor fusion protein. The PKB activity of this recombinant protein was rapidly activated in hepatocytes challenged with 4-hydroxytamoxifen (OHT), as was endogenous PKB in hepatocytes challenged with insulin. The addition of OHT to transduced hepatocytes resulted in accumulation of SREBP1 mRNA, with a time-course and magnitude similar to the effect of insulin in non-transduced cells. The level of SREBP1 mRNA was not increased by OHT in hepatocytes expressing a mutant form of the recombinant protein whose PKB activity was not activated by OHT. Thus acute activation of PKB is sufficient to induce SREBP1 mRNA accumulation in primary hepatocytes, and might be the major signalling event by which insulin induces SREBP1 gene expression in the liver. (+info)
Interferons activate the p42/44 mitogen-activated protein kinase and JAK-STAT (Janus kinase-signal transducer and activator transcription factor) signalling pathways in hepatocytes: differential regulation by acute ethanol via a protein kinase C-dependent mechanism.
Interferons (IFNs) have been used in the treatment of viral hepatitis. However, their effectiveness is much reduced (<10%) in alcoholics. The mechanism underlying this resistance remains unknown. Here, we report that IFN-alpha/beta and IFN-gamma rapidly activate the JAK-STAT1 (Janus kinase-signal transducer and activator transcription factor 1) and p42/44 mitogen-activated protein kinase (p42/44 MAPK) in freshly isolated rat hepatocytes. Treatment of hepatocytes with 25-100 mM ethanol for 30 min inhibited IFN-beta- or IFN-gamma-induced STAT1 activation and tyrosine phosphorylation. The inhibitory effect of ethanol was not reversed by pretreatment with either sodium vanadate, a non-selective tyrosine phosphatase inhibitor, or with MG132, a specific proteasome inhibitor. This suggests that protein tyrosine phosphatases or the ubiquitin-proteasome pathway are not involved in the inhibitory action of ethanol. In contrast with the JAK-STAT signalling pathway, acute ethanol exposure significantly potentiated IFN-beta or IFN-gamma-induced activation of p42/44 MAPK, and caused marked activation of protein kinase C (PKC). Inhibition of PKC partially antagonized ethanol attenuation of IFN-induced STAT1 activation, suggesting that PKC may be involved. Taken together, these findings suggest that the ability of biologically relevant concentrations of ethanol (less than 100 mM) to markedly inhibit IFN-activated STAT1 is one of the cellular mechanisms responsible for the observed resistance of IFN therapy in alcoholics. (+info)
A targeted apolipoprotein B-38.9-producing mutation causes fatty livers in mice due to the reduced ability of apolipoprotein B-38.9 to transport triglycerides.
Nonphysiological truncations of apolipoprotein (apo) B-100 cause familial hypobetalipoproteinemia (FHBL) in humans and mice. An elucidation of the mechanisms underlying the FHBL phenotypes may provide valuable information on the metabolism of apo B-containing lipoproteins and the structure-function relationship of apo B. To generate a faithful mouse model of human FHBL, a subtle mutation was introduced into the mouse apo B gene by targeting embryonic stem cells using homologous recombination followed by removal of the selection marker gene by Cre-loxP-mediated site-specific recombination. The engineered mice bear a premature stop codon at residue 1767 and a 42-base pair loxP inserted into intron 24 of the apo B gene, thus closely resembling the apo B-38.9-producing mutation in humans. Apo B-38.9 was the sole apo B protein in homozygote (apob(38.9/38.9)) plasma. In heterozygotes (apob(+/)(38. 9)), apo B-100 and apo B-48 were reduced by 75 and 40%, respectively, and apo B-38.9 represented 20% of total circulating apo B. Hepatic apo B-38.9 mRNA levels were reduced by 40%. In cultured apob(+/)(38. 9) hepatocytes, apo B-100 was produced in trace quantities, and the synthesis rate of apo B-38.9 relative to apo B-48 was reduced by 40%. However, almost equimolar amounts of apo B-38.9 and apo B-48 were secreted into the media. Pulse-chase studies revealed that apo B-38. 9 was secreted at a faster rate and more efficiently than apoB-48. Nevertheless, both apob(+/)(38.9) and apob(38.9/38.9) mice had reduced hepatic triglyceride secretion rates and fatty livers. Thus, low mRNA levels or defective secretion of apo B-38.9 may not be responsible for the FHBL phenotypes caused by the apo B-38.9 mutation. Rather, a reduced capacity of apo B-38.9 for triglyceride transport may account for the fatty livers in these mice. (+info)
Progression of hepatic stellate cell activation is associated with the level of oxidative stress rather than cytokines during CCl4-induced fibrogenesis.
In order to identify a fibrogenic factor associated with the potential of hepatic stellate cells (HSC) activation that arises during the CCl4-induced fibrogenic process, the relationship between the activation of HSC and levels of several fibrogenic factors were investigated. After isolation of HSC from the liver at different stages of CCl4 intoxication, the activation of HSC was assessed by the expression of alpha-smooth muscle actin. Levels of cytokines and oxidative stress in liver homogenates and plasma were measured by enzyme linked immunosorbent assay and the colorimetric method. In primary culture, HSC isolated from a rat liver were gradually activated in a time-dependent manner according to CCl4 administration. The progression of HSC activation was closely correlated with parameters related to oxidative stress in liver homogenates rather than the tissue levels of several cytokines. Also, the levels of antioxidants and arginase activity were inversely correlated with HSC activation. In plasma, the levels of oxidative stress and cytokines in CCl4-treated rat livers were not associated with the activation of HSC found during the CCl4-induced fibrogenic process. The relationship between HSC activation and oxidative stress was also confirmed through several factor-treated HSC cultures. In conclusion, the activation of HSC was accelerated according to CCl4 administration, and the progression of HSC activation is absolutely related to the oxidative stress. These results show that enhanced oxidative stress is an important signal for activation of HSC in experimental liver fibrogenesis. (+info)
Functional characterization of the rat gamma-glutamyl transpeptidase promoter that is expressed in transformed rat liver epithelial cells.
In the rat the gamma-glutamyl transpeptidase (GGT) gene codes for at least four different messenger RNAs (mRNA I to mRNA IV) which differ only in their 5' untranslated regions and are transcribed from a single copy gene in a tissue-specific manner. In the liver GGT expression is up-regulated in transformed cells. To understand the induction mechanisms of GGT activity by transformation, we previously cloned the 5' region of the rat GGT gene which contains the 5' untranslated leader sequence for mRNA I. In the present study, using transfection and reporter gene assays, I have demonstrated that (1) the sequence from positions -369 to +226 drives a relatively strong promoter activity in C5 and AKG cell lines, transformed rat liver epithelial cells, but a very weak one in RLE-228 cells, a normal rat liver epithelial cell line; and (2) removal of the region between -418 and -369 increases CAT activity more than 10-fold in RLE-228, C5 and AKG cells, and the DNA fragment spanning nucleotides -761 to -292 significantly reduces CAT activity driven by the adenine phosphoribosyl transferase (APRT) promoter in RLE-228 cells. (+info)
Involvement of apoptosis in the endotoxemic lesions of the liver and kidneys of piglets.
The involvement of apoptosis was evaluated in lesions of endotoxemic piglets. A single injection with E. coli O111:B4 lipopolysaccharide (LPS) induced foci of coagulative necrosis in the liver and kidneys. No significant change was observed in these organs at 1.5 hr after LPS injection, but at 6 hr, epithelial cells with chromatin condensation or fragmentation and apoptotic bodies were visible. Foci of coagulative necrosis were formed within 24 hr after LPS inoculation. In and adjacent to the necrotic foci, dead hepatocytes with nuclear condensation or fragmentation were scattered. These dead cells were positively stained by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) methods. Electronmicroscopy revealed apoptotic cells with condensed or fragmented homogeneous nuclear chromatin, and necrotic cells with irregularly destroyed nuclei and cytoplasmic membranes. Apoptotic cell death were also observed in parietal cells of the stomach and lymphocytes in the lymphatic system. DNA ladders with approximately 200-bp multimers were observed in hepatic, renal and thymic samples prepared after 6 and 24 hr of LPS injection by agarose gel electrophoresis. These results suggest that apoptosis is involved in the pathology of swine endotoxemia. (+info)
Phosphoinositide 3-kinase-dependent Ras activation by tauroursodesoxycholate in rat liver.
Ursodesoxycholic acid, widely used for the treatment of cholestatic liver disease, causes choleretic, anti-apoptotic and immunomodulatory effects. Here the effects on choleresis of its taurine conjugate tauroursodesoxycholate (TUDC), which is present in the enterohepatic circulation, were correlated with the activation of important elements of intracellular signal transduction in cultured rat hepatocytes and perfused rat liver. TUDC induced a time- and concentration-dependent activation of the small GTP-binding protein Ras and of phosphoinositide 3-kinase (PI 3-kinase) in cultured hepatocytes. Ras activation was dependent on PI 3-kinase activity, without the involvement of protein kinase C- and genistein-sensitive tyrosine kinases. Ras activation by TUDC was followed by an activation of the mitogen-activated protein kinases extracellular-signal-regulated kinase-1 (Erk-1) and Erk-2. In perfused rat liver, PI 3-kinase inhibitors largely abolished the stimulatory effect of TUDC on taurocholate excretion, suggesting an important role for a PI 3-kinase/Ras/Erk pathway in the choleretic effect of TUDC. (+info)