Effects of extracellular Ca2+ and HCO3- on epidermal growth factor-induced DNA synthesis in cultured rat hepatocytes

Pathogenesis and management of alcoholic hepatitis

Alcoholic hepatitis is a potentially life-threatening complication of alcoholic abuse, typically presenting with symptoms and signs of hepatitis in the presence of an alcohol use disorder. The definitive diagnosis requires liver biopsy, but this is not generally required. The pathogenesis is uncertain, but relevant factors include metabolism of alcohol to toxic products, oxidant stress, acetaldehyde adducts, the action of endotoxin on Kupffer cells, and impaired hepatic regeneration. Mild alcoholic hepatitis recovers with abstinence and the long-term prognosis is determined by the underlying disorder of alcohol use. Severe alcoholic hepatitis is recognized by a Maddrey discriminant function >32 and is associated with a short-term mortality rate of almost 50%. Primary therapy is abstinence from alcohol and supportive care. Corticosteroids have been shown to be beneficial in a subset of severely ill patients with concomitant hepatic encephalopathy, but their use remains controversial. Pentoxifylline has been shown in one study to improve short-term survival rates. Other pharmacological interventions, including colchicine, propylthiouracil, calcium channel antagonists, and insulin with glucagon infusions, have not been proven to be beneficial. Nutritional supplementation with available high-calorie, high-protein diets is beneficial, but does not improve mortality. Orthotopic liver transplantation is not indicated for patients presenting with alcoholic hepatitis who have been drinking until the time of admission, but may be considered in those who achieve stable abstinence if liver function fails to recover.

Membrane-initiated events account for progesterone's ability to regulate intracellular free calcium levels and inhibit rat granulosa cell mitosis

It has been proposed that the antimitogenic action of progesterone (P(4)) is mediated through a membrane receptor that has GABA(A) receptor-like characteristics. To test this hypothesis, studies were designed to compare the antimitogenic effects of P(4) with its gamma amino butyric acid(A) (GABA(A)) receptor-activating metabolite, 5alpha-pregnane-3alpha-21-diol-20-one (5alpha3alpha). These studies revealed that P(4) was more effective than 5alpha3alpha in blocking mitogen-dependent mitosis of both small granulosa cells (GCs) and spontaneously immortalized granulosa cells (SIGCs). Ligand binding studies illustrated that P(4) bound to SIGCs with an apparent dissociation constant (K(d)) of 0.32 +/- 0.09 microM, whereas 5alpha3alpha bound with an apparent K(d) of 40 +/- 19 microM. Further, the GABA(A) antagonist, bicuculline, did not attenuate P(4)'s antimitotic action in SIGCs. Finally, reverse transcriptase-polymerase chain reaction (RT-PCR) studies demonstrated that none of the 6 known alpha chains of the GABA(A) receptors to which bicuculline binds were detected in SIGCs. Taken together, these studies suggest that P(4) does not mediate its action via a GABA(A)-like receptor. Additional studies revealed that P(4) regulated intracellular free calcium levels ([Ca(2+)](i)) as part of its antimitotic action. Specifically, P(4) maintained a basal [Ca(2+)](i) level that was slightly lower than normal. Increasing extracellular calcium not only increased basal [Ca(2+)](i) but also attenuated P(4)'s antimitogenic effect. P(4)'s actions appeared to be initiated at the membrane, since horseradish peroxidase conjugated-P(4) (HP-P(4)), which is cell impermeable, was as effective in blocking mitosis as P(4). Progesterone receptor (PR) mRNA was not detected in SIGCs by RT-PCR analysis, which is consistent with the findings in GCs. However, a 60-kDa protein was detected within crude membrane fractions of both GCs and SIGCs using an antibody directed against the ligand binding domain of the PR (C-262). This antibody was also used in immunocytochemical studies to detect a protein that was associated with the plasma membrane of SIGCs. It is proposed that this 60-kDa protein mediates P(4)'s membrane-initiated actions.

Specific involvement of G(alphai2) with epidermal growth factor receptor signaling in rat hepatocytes, and the inhibitory effect of chronic ethanol

We have previously shown that chronic alcohol consumption inhibits liver regeneration by impairing epidermal growth factor receptor (EGFR)-operated phospholipase C-(gamma1) (PLC-(gamma1)) activation and the resultant rise in intracellular [Ca(2+)](i). In hepatocytes, activation of PLC-(gamma1) by EGFR requires involvement of a pertussis toxin-sensitive inhibitory guanine nucleotide-binding regulatory protein (G(alphai)) as an intermediate. In the present study, we first identified the G(alphai) protein isoform associated with the activated EGFR, and then examined whether the toxic effect of alcohol on EGFR signaling and liver cell proliferation was exerted on this association. In cultured hepatocytes from control rats, EGF rapidly induced association between EGFR and G(alphai2) but not other G(alphai) isoforms. In hepatocytes from rats fed alcohol for 16 weeks, EGF failed to stimulate this association of G(alphai2) with the EGFR. The impairment of EGFR-G(alphai2) complex formation caused by alcohol was associated with a decreased level of G(alphai2) in the plasma membrane fraction (approximately 50% control). Pertussis toxin, an inhibitor of G(alphai) function, produced an analogous disruption of the association between G(alphai2) and the EGFR, as well as inhibiting EGF-induced DNA synthesis. It is concluded that, in hepatocytes, G(alphai2) is specific among G(alphai) isoforms in coupling activation of the EGFR to other signaling pathways that control cell proliferation. Impaired coupling of G(alphai2) of EGFR could contribute to the mechanism by which chronic alcohol exposure inhibits liver regeneration.

Nonsteroidal anti-inflammatory drugs attenuate proliferation of colonic carcinoma cells by blocking epidermal growth factor-induced Ca++ mobilization

Numerous studies suggest that nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit colorectal carcinogenesis. We have previously reported that NSAIDs, in human colonic carcinoma cells (Caco-2), attenuate epidermal growth factor (EGF)-induced cellular proliferation through a process independent of their inhibitory effects on prostaglandin synthesis. Furthermore, separate studies have also suggested that NSAIDs inhibit EGF-induced store-operated Ca++ influx. Thus we developed the hypothesis that NSAIDs may limit the activity of EGF by altering intracellular Ca++ ([Ca++]i) mobilization. Serum-deprived Caco-2 cells were employed for all experimentation. [Ca++]i was measured with Fluo-3 and extracellular Ca++ influx was monitored by quenching Fluo-3 fluorescence with Mn++. Proliferation was quantitated with two assays: cellular nucleic acid and total protein content. Caco-2 cells exposed to EGF demonstrated an initial increase in [Ca++]i which was blocked by neomycin, an inhibitor of IPsubscript 3 generation, and the phospholipase C inhibitor U73122 but not U73343 (inactive control). This was followed by sustained extracellular Ca++ influx, which was attenuated with calcium-free buffer (-Ca++), the store- operated Ca++ channel blocker lanthanum, indomethacin, ibuprofen, and aspirin. In subsequent studies, cells were treated with either serum-free media or EGF +/- the aforementioned inhibitors, and again serum starved. Cells exposed to EGF +/- the inactive phospholipase C inhibitor U73343 demonstrated a significant increase in nucleic acid and protein. However, proliferation induced by EGF was not observed when [Ca++]i elevation was prevented by blocking either internal Ca++ store release via phospholipase C/IPsubscript 3 or sustained Ca++ influx through store-operated Ca++ channels. Sustained [Ca++]i elevation, as induced by EGF, appears to be required for mitogenesis. These data support our premise that one mechanism whereby NSAIDs may attenuate colonic neoplasia is by blocking EGF-induced Ca++ mobilization.

Purinoceptor-mediated calcium mobilization and cellular proliferation in cultured bovine corneal endothelial cells

In the present study, we investigated the effect of adenosine triphosphate (ATP) on cytosolic free calcium mobilization and mitogenic activity in cultured bovine corneal endothelial cells (BCEC). The [Ca2+]i was determined using a Ca2+ sensitive indicator, Fura-2/AM, and cell proliferation was evaluated by counting the cell number. ATP, its metabolites and analogs caused transient increase in [Ca2+]i in a concentration-dependent manner (10(-7) M-10(-3) M) and the potency of agonists was ordered as follows: 2-methylthio-ATP > uridine triphosphate > ATP > adenosine diphosphate. Adenosine monophosphate and adenosine did not affect [Ca2+]i. ATP (10(-4) M) also promoted the accumulation of inositol trisphosphate (IP3). The ATP-induced transient [Ca2+]i increase and IP3 accumulation were attenuated by pretreatment with a phospholipase C inhibitor, U-73122 (5 microM), for 30 min. ATP (10(-5) M) significantly enhanced the proliferation of BCEC. ATP-induced [Ca2+]i increase and cell proliferation were inhibited by a purinoceptor antagonist, suramin (10(-4) M). Thus, the present study indicates that BCEC contain P2 purinoceptors that regulate their proliferation.

Hepatocyte growth factor/scatter factor-induced intracellular signalling

Hepatocyte growth factor (HGF) identical to scatter factor (SF) is a glycoprotein involved in the development of a number of cellular phenotypes, including proliferation, mitogenesis, formation of branching tubules and, in the case of tumour cells, invasion and metastasis. This fascinating cytokine transduces its activities via its receptor encoded by the c-met oncogene, coupled to a number of transducers integrating the HGF/SF signal to the cytosol and the nucleus. The downstream transducers coupled to HGF/MET, most of which participate in overlapping pathways, determine the development of the cell's phenotype, which in most cell types is dual.

Chronic ethanol consumption disrupts complexation between EGF receptor and phospholipase C-gamma1: relevance to impaired hepatocyte proliferation

We have previously shown that chronic ethanol consumption inhibits liver regeneration by impairing EGF receptor (EGFR)-operated phospholipase C-gamma1 (PLC-gamma1) activation and resultant intracellular Ca2+ signalling. Activation of PLC-gamma1 by EGFR requires the EGFR to bind to PLC-gamma1 after its translocation from cytosol to cytoskeleton. In order to understand the mechanism by which ethanol impairs PLC-gamma1 activation, we examined the effect of alcohol on interactions between EGFR and PLC-gamma1. In cultured hepatocytes from control rats, EGF rapidly induced tyrosine phosphorylation of both the EGFR and of PLC-gamma1. EGF also stimulated PLC-gamma1 translocation from cytosol to a cytoskeletal compartment where PLC-gamma1 interacted with EGFR. In hepatocytes from rats fed ethanol for 16 weeks, the above reactions were substantially inhibited. Tyrphostin AG1478, an EGFR-specific tyrosine kinase inhibitor, mimicked the effects of chronic ethanol on EGFR phosphorylation, PLC-gamma1 translocation and interactions between EGFR and PLC-gamma1 in the cytoskeleton. Further, tyrphostin AG1478 also inhibited EGF-induced DNA synthesis. These results indicate that ethanol impairs EGFR-operated [Ca2+]i signaling by disrupting the interactions between EGFR and PLC-gamma1.

Does steatohepatitis impair liver regeneration? A study in a dietary model of non-alcoholic steatohepatitis in rats

BACKGROUND

Impaired liver regeneration is a feature of alcoholic hepatitis, but the relative importance of alcohol, nutritional imbalance and inflammatory mediators in causing this effect is unclear. Non-alcoholic steatohepatitis (NASH) is a form of liver disease with similar morphology to alcoholic hepatitis, but the effect of this disorder on liver regeneration is unclear. We, therefore, examined the status of liver regeneration in a rat nutritional model of hepatic steatosis with inflammation, which is morphologically identical to NASH in humans.

METHODS

Male Wistar rats received a methionine-choline-deficient diet (MCDD) for 4 weeks before experiments and both isocaloric pair-fed and ad libitum-fed rats were used as controls. Following partial hepatectomy (68%), the extent of hepatic regeneration was determined 24 h later using [3H]-thymidine incorporation and restitution of liver mass.

RESULTS

There was no significant difference of [3H]-thymidine incorporation in MCDD-fed, pair-fed and ad libitum-fed rats (80+/-27, 78+/-11 and 80+/-6.3 d.p.m./microg DNA, respectively). Similarly, restituted liver masses in three groups of rats were not significantly different (17+/-3.8, 18+/-1.8 and 17+/-3.1% initial liver weight, respectively).

CONCLUSIONS

The similarities in hepatic histology and cytochrome P450 2E1 induction between this nutritional model of hepatic steatohepatitis and alcoholic steatohepatitis imply that these two disorders share pathogenetic mechanisms. However, liver regeneration is not altered by NASH in rats, indicating that the nutritional and inflammatory changes that appear similar to those of alcoholic liver disease do not cause impairment of liver regeneration.

Correlation of hepatocyte growth factor-induced proliferation and calcium-activated potassium current in human gastric cancer cells

Hepatocyte growth factor (HGF) has been found to stimulate proliferation and migration of human gastric carcinoma cells. Whether the HGF-induced responses are correlated with the expressed level of HGF receptors or the changes of ionic currents is not clear. The present study investigated the effects of HGF on the proliferation and ionic currents of two human gastric adenocarcinoma cell lines, which were found to express different amounts of HGF receptor. Results showed that HGF induced a dose-dependent growth stimulation and accelerated cell cycle progression in SC-M1 cells. In patch clamp study, HGF treatment induced an outward K+ current and increased the slope conductance at -80 mV from 110+/-15 pS/pF to 207+/-15 pS/pF. The HGF-induced K+ current was abolished when tetraethylammonium chloride was added in bathing solution or a low Ca2+ solution was included in the recording pipette. Furthermore, HGF (10 ng/ml) induced an oscillatory Ca2+-activated K+ current with a lag period of 5+/-3 min in SC-M1 cells. In contrast, HGF did not induce mitogenesis, cell cycle progression and changes in ionic currents in KATO-III cells, although this cell line expressed a higher level of HGF receptors than SC-M1 cells did. These findings provide evidence that the activity of Ca2+-activated K+ channel may be involved in the HGF-induced cell proliferation in human gastric cancer cells, but it did not correlate with the density of HGF receptors.

Chronic ethanol administration to rats decreases receptor-operated mobilization of intracellular ionic calcium in cultured hepatocytes and inhibits 1,4,5-inositol trisphosphate production: relevance to impaired liver regeneration

We tested the hypothesis that ethanol impairs liver regeneration by abrogating receptor-mediated elevation of cytosolic free calcium ([Ca2+]i). In rats fed for 16 weeks with ethanol, hepatocellular proliferation induced by partial hepatectomy was greatly impaired. Similarly, EGF-induced DNA synthesis was reduced in cultured hepatocytes from ethanol-fed rats. There was no change in the number or affinity of EGF receptors on hepatocytes from ethanol-fed rats. Despite this, EGF-mediated production of inositol 1,4,5-trisphosphate (Ins[1,4,5]P3) was lower in hepatocytes from ethanol-fed rats, and the EGF-induced [Ca2+]i transient appeared to be abrogated. When vasopressin or phenylephrine were used as cell surface receptor ligands, hepatocytes cultured from ethanol-fed rats exhibited major reductions in Ins(1,4,5)P3 synthesis. This was associated with greatly truncated [Ca2+]i transients. These changes were not due to an effect on the Ins(1,4,5)P3 receptor on the endoplasmic reticulum or to a decrease in the size of the Ins(1,4,5)P3-mobilizable intracellular Ca+2 store. Further, mobilization of the same Ca2+ store by 2,5-di-tert-butylhydroquinone or thapsigargin restored the ability of hepatocytes from ethanol-fed rats to proliferate when exposed to EGF. It is concluded that chronic ethanol consumption inhibits liver regeneration by a mechanism that is, at least partly, the result of impaired receptor-operated [Ca2+]i signaling due to reduced generation of Ins(1,4,5)P3.

Antioxidant properties of omeprazole

Potential antioxidant properties of therapeutically achievable concentrations of the protonated, active form of omeprazole (OM) were investigated in vitro at specific acidic pH values to mimic intragastric conditions in the clinical setting. We found that OM is a powerful scavenger of hypochlorous acid (HOCl) even at a drug concentration of 10 microM at pH 5.3 or 3.5. This effect is also evident in the presence of the physiological HOCl scavenger ascorbate. Moreover, 10 and 50 microM OM inhibit significantly both iron- and copper-driven oxidant damage at pH 5.3 and 3.5, respectively. Since oxidative stress is involved the gastric injury of peptic ulcer and gastritis, it may be hypothesized that some therapeutical effects of OM could also be related to its antioxidant properties.