Early events in the regulation of hepatocyte DNA synthesis: the role of alpha-adrenergic stimulation

A Review on the Antimutagenic and Anticancer Effects of Cysteamine

Cancer is one of the leading causes of death worldwide. First-line treatments usually include surgery, radiotherapy, and/or systemic therapy. These methods can be associated with serious adverse events and can be toxic to healthy cells. Despite the new advances in cancer therapies, there is still a continuous need for safe and effective therapeutic agents. Cysteamine is an aminothiol endogenously synthetized by human cells during the degradation of coenzyme-A. It has been safely used in humans for the treatment of several pathologies including cystinosis and neurodegenerative diseases. Cysteamine has been shown to be a potent antimutagenic, anticarcinogenic, and antimelanoma in various in vitro and in vivo studies, but a review on these aspects of cysteamine's use in medicine is lacking in the current literature. The efficacy of cysteamine has been shown in vitro and in vivo for the treatment of different types of cancer, such as gastrointestinal cancer, pancreatic cancer, sarcomas, hepatocellular carcinoma, and melanoma, leading to the significant reduction of lesions and/or the increase of survival time. Although the mechanisms of action are not fully understood, possible explanations are (i) free radical scavenging, (ii) alteration of the tumor cell proliferation by affecting nucleic acid and protein synthesis or inhibition of DNA synthesis, and (iii) hormone regulation. In conclusion, regarding the high safety profile of cysteamine and the current literature data presented in this article, cysteamine might be considered as an interesting molecule for the prevention and the treatment of cancer. Further clinical studies should be performed to support these data in humans.

Effect of 5-HT7 receptor blockade on liver regeneration after 60-70% partial hepatectomy

Background

Serotonin exhibits a vast repertoire of actions including cell proliferation and differentiation. The effect of serotonin, as an incomplete mitogen, on liver regeneration has recently been unveiled and is mediated through 5-HT₂ receptor. The aim of the present study was to investigate the effect of 5-HT₇ receptor blockade on liver regeneration after partial hepatectomy.

Methods

Male Wistar rats were subjected to 60-70% partial hepatectomy. 5-HT₇ receptor blockade was applied by intraperitoneal administration of SB-269970 hydrochloride two hours prior to and sixteen hours after partial hepatectomy and by intraperitoneal administration of SB-258719 sixteen hours after partial hepatectomy. Animals were sacrificed at different time points until 72 h after partial hepatectomy. Liver regeneration was evaluated by [³H]-thymidine incorporation into hepatic DNA, the mitotic index in hematoxylin-eosin (HE) sections and by immunochemical detection of Ki67 nuclear antigen. Reversion of 5-HT₇ blockade was performed by intraperitoneal administration of AS-19. Serum and liver tissue lipids were also quantified.

Results

Liver regeneration peaked at 24 h ([³H]-thymidine incorporation into hepatic DNA and mitotic index by immunochemical detection of Ki67) and at 32 h (mitotic index in HE sections) in the control group of rats. 5-HT₇ receptor blockade had no effect on liver regeneration when applied 2 h prior to partial hepatectomy. Liver regeneration was greatly attenuated when blockade of 5-HT₇ receptor was applied (by SB-258719 and SB-269970) at 16 h after partial hepatectomy and peaked at 32 h ([³H]-thymidine incorporation into hepatic DNA and mitotic index by immunochemical detection of Ki67) and 40 h (mitotic index in HE sections) after partial hepatectomy. AS-19 administration totally reversed the observed attenuation of liver regeneration.

Conclusions

In conclusion, 5-HT₇ receptor is a novel type of serotonin receptor implicated in hepatocyte proliferation.

Increased 5-HT2C receptor binding in the brain stem and cerebral cortex during liver regeneration and hepatic neoplasia in rats

In the present study, serotonin 2C (5-HT(2C)) receptor binding parameters in the brainstem and cerebral cortex were investigated during liver generation after partial hepatectomy (PH) and N-nitrosodiethylamine (NDEA) induced hepatic neoplasia in male Wistar rats. The serotonin content increased significantly (p<0.01) in the cerebral cortex after PH and in NDEA induced hepatic neoplasia. Brain stem serotonin content increased significantly (p<0.05) after PH and (p<0.001) in NDEA induced hepatic neoplasia. The number and affinity of the 5-HT(2C) receptors in the crude synaptic membrane preparations of the brain stem showed a significant (p<0.001) increase after PH and in NDEA induced hepatic neoplasia. The number and affinity of 5-HT(2C) receptors increased significantly (p<0.001) in NDEA induced hepatic neoplasia in the crude synaptic membrane preparations of the cerebral cortex. There was a significant (p<0.01) increase in plasma norepinephrine in PH and (p<0.001) in NDEA induced hepatic neoplasia, indicating sympathetic stimulation. Thus, our results suggest that during active hepatocyte proliferation 5-HT(2C) receptor in the brain stem and cerebral cortex are up-regulated which in turn induce hepatocyte proliferation mediated through sympathetic stimulation.

The characterization and molecular structure of hepatoproliferin: a liver regeneration factor from rat hepatocytes

Hepatoproliferin (HPF) was purified from regenerating rat livers as an oligomeric entity (big-HPF) from which the monomeric form (small-HPF) could be obtained using disaggregating conditions. By using a solid-phase ion-exchange method, small-HPF was forced to dissociate into two charged ionic species, namely norepinephrine (NE) and a sulfonated disaccharide with a molecular structure consisting of D-glucuronic acid bound to glucosamine 2,6-disulfate by a beta-glycosidic linkage having a beta, 1 --> 4 configuration. Monomeric HPF stemmed from the formation of three electrostatic bonds between the protonated amine groups of three norepinephrines, of which two bind to the deprotonated sulfonic groups of glucosamine 2,6-disulfate and one to the deprotonated carboxylic group of glucuronic acid, to constitute a tightly associated complex with a molecular mass of 1046 Da. This represents one of the two purified isoforms of small-HPF. The other isoform, which has a lower molecular mass of 877 Da, lack one NE, leaving the weaker carboxylic group of glucuronic acid unoccupied, to constitute a more acidic form of HPF.

The effect of EGF and the comitogen, norepinephrine, on the proliferative responses of fresh and cryopreserved rat and mouse hepatocytes

The effect of cryopreservation on the proliferative response of fresh and cryopreserved (CP) rat and mouse hepatocytes was studied. Of the parameters measured, incorporation of 3H-thymidine and bromodeoxyuridine (BdrU) incorporation were the most sensitive and LDH content was the least sensitive. The optimal seeding density for epidermal growth factor (EGF)-stimulated proliferative response in fresh rat and mouse hepatocytes was 1.8 x 10(4) cells/cm2 and 2.1 x 10(4) cells/cm2, respectively. 3H-thymidine incorporation by fresh rat and mouse hepatocytes was maximal in cultures treated with 10 and 5 ng/ml EGF, respectively. The cell attachment of fresh rat hepatocytes after 48 h was higher (68%) than CP (42%), therefore, the CP hepatocyte seeding density was increased to 7.1 x 10(4) cells/cm2 so that the cell number after 48 h was the same as fresh hepatocytes. Using the adjusted seeding density, the 3H-thymidine and BdrU incorporation into fresh and CP rat hepatocytes was equivalent. The attachment efficiencies of fresh and CP mouse hepatocytes were the same, therefore, no adjustment was needed. The proliferative response (3H-thymidine incorporation and DNA content) to EGF was the same in fresh and CP mouse hepatocytes. The comitogen, norepinephrine (NE), increased the proliferative response to EGF to the same extent in both fresh and CP rat hepatocytes. In summary, cryopreserved rat and mouse hepatocytes retain their ability to proliferate in culture. Adjustment and monitoring of the seeding density is of high importance, especially with rat hepatocytes, which lose some attachment capacity after cryopreservation. The secondary mitogenic effect of NE is also retained by cryopreserved rat hepatocytes, suggesting that these cells retain alpha1-receptor function.

Effect of serotonin receptor 2 blockage on liver regeneration after partial hepatectomy in the rat liver

The effect of serotonin receptor 2 blockade (5-HT(2)) on liver regeneration after 30-34% and 60-70% partial hepatectomy in the rat liver was investigated.

MATERIALS AND METHODS

Male Wistar rats were subjected to 60-70% (group I) and 30-34% (group II) partial hepatectomy. Serotonin receptor 2 blockade was exerted by intraperitoneal administration of ketanserin at different doses and time points after partial hepatectomy. The rats of all groups were killed at different time points until 96 h after partial hepatectomy. The rate of liver regeneration was evaluated by the mitotic index in hematoxylin and eosin sections, the immunochemical detection of Ki67 and proliferating cell nuclear antigens, the rate of [(3)H]-thymidine incorporation into hepatic DNA and liver thymidine kinase enzymatic activity.

RESULTS

Liver regeneration peaked at 24 and 32 h after partial hepatectomy in 60-70% hepatectomized rats. In 30-34% hepatectomized rats liver regeneration peaked at 60 h, whereas low rates of regenerative activity were observed between 24 and 72 h after partial hepatectomy. Ketanserin administration arrested liver regeneration only when administered at 16 h after 60-70% partial hepatectomy. Ketanserin also abrogated the observed peak of regenerative activity at 60 h in 30-34% hepatectomized rats when administered at 52 h after partial hepatectomy. All indices of liver regeneration were affected by ketanserin administration.

CONCLUSIONS

Serotonin receptor 2 blockade can arrest liver regeneration only when administered close to G1/S transition point, and that while serotonin may be a cofactor for DNA synthesis, it does not play a role in initiation of liver regeneration.

Apical ballooning syndrome in a postoperative patient with normal microvascular perfusion by myocardial contrast echocardiography

Apical ballooning syndrome is classically described as transient left ventricular (LV) dysfunction, marked LV akinesia, and normal or near-normal coronary arteries. The etiology is unclear and there is limited information based on case reports and small case series. We describe a 35-year-old woman who underwent surgical hepatectomy and developed apical ballooning syndrome in the postoperative period. The novel use of myocardial contrast echocardiography (MCE) in this setting demonstrated intact microvascular perfusion and lack of coronary flow-limiting abnormalities despite apical akinesis. In select patients with similar clinical presentations, performing MCE is safe and may be pursued as an alternative to invasive coronary angiography.

Acute liver failure: targeted artificial and hepatocyte-based support of liver regeneration and reversal of multiorgan failure

Acute liver failure (ALF) still represents a major therapeutic challenge for hepatologists due to its high mortality rate as a result of multiorgan failure. Although emergency orthotopic liver transplantation represents a major advance in the management of selected patients, it is not applicable to all candidates due to limited organ availability. Therefore, new therapeutic options should be developed to bridge selected patients to transplantation or to treat patients not candidates for liver transplantation. Although new techniques for cell culture and perfusion have resulted in a number of promising devices for the provision of temporary liver support in acute liver failure, their clinical efficacy is as yet uncertain. Controlled trials on a multi-centre basis in well-defined patient groups and with standardised outcome measures, including the extent to which treatment influences cell damage and regeneration and prevents or reverses multiorgan failure, will be essential to properly evaluate the clinical value of current and evolving artificial and bioartificial devices. The same considerations must also apply to the assessment of therapeutic efficacy of hepatocyte transplantation. A better understanding of mechanisms responsible for the development of liver cell death, along with cellular and molecular mechanisms allowing surviving cells to proliferate in a hostile environment, will be required if a more targeted therapeutic approach to decreasing hepatocellular injury and enhancing liver regeneration is to be achieved. Whether extracorporeal devices or the transplantation of primary hepatocytes, stem cells or cells genetically engineered to over-express key metabolic functions, a proliferative phenotype and/or cytoprotective pathways will be best suited to meeting these demanding challenges remains to be determined.

Changes in liver blood flow after hepatectomy in conscious dogs

Hepatic circulation after hepatectomy was investigated in conscious dogs under fasting and feeding conditions. After a 40% hepatectomy, both the hepatic arterial and portal blood flow were measured simultaneously using ultrasonic transit time flowmeters. During fasting, the total hepatic blood flow (i.e., the sum of arterial and portal blood flow) changed in a biphasic pattern after hepatectomy. The first peak (517.9 +/- 42.7 ml/min; 130.1% of preoperative flow) was seen on the 1st postoperative day (POD) and the second peak (444.8 +/- 25.6 ml/min; 112.7% of preoperative flow) occurred on the 7th POD. The portal flow demonstrated the same biphasic changes as the total hepatic flow, although the hepatic arterial flow showed only the first peak. A heart rate analysis suggested that the first peak was probably due to hyperdynamic circulatory conditions, as has been previously reported. In addition, the existence of the second peak was established by the present study. The postprandial hepatic blood flow decreased during the first 2 weeks postoperatively, but exceeded the presurgical levels on PODs 21 and 28.

Growth-promoting effects of Ca(2+)-mobilizing agents in hepatocytes: lack of correlation between the acute activation of phosphoinositide-specific phospholipase C and the stimulation of DNA synthesis by angiotensin II, vasopressin, norepinephrine, and prostaglandin F2 alpha

Although several hormones that promote hepatocyte proliferation also activate phosphoinositide-specific phospholipase C (PI-PLC) and mobilize Ca2+, the role of PI-PLC in the growth-stimulating effect of these agents is not clear. We have investigated this issue further, by exposing freshly isolated adult rat hepatocytes to vasopressin, angiotensin II, norepinephrine (in the presence of the beta-adrenoceptor blocker timolol) or PGF2 alpha, and examined both acute responses and the subsequent DNA synthesis when the cells were grown in monolayer culture. All the agonists elevated the level of inositol 1,4,5-trisphosphate (InsP3) and enhanced the DNA synthesis, amplifying the response to epidermal growth factor (EGF), and this comitogenic effect could be exerted by a single exposure of the cells 24 h prior to the addition of EGF. The acute activation of PI-PLC, measured as the early rise (peak 15-60 s) in InsP3, was 8-10-fold with vasopressin or angiotensin II, 3-4-fold with norepinephrine, and approximately 2-fold with PGF2 alpha. For all the agonists, a rise in cytosolic free Ca2+ in 100% of the cells and a maximal increase in glycogen phosphorylase activity were evoked at concentrations that approximately doubled the level of InsP3. However, the growth-stimulatory effects of these agonists showed a different order of efficacy as compared to the activation of PI-PLC; in terms of the maximal stimulation of DNA synthesis, the effects were: norepinephrine approximately PGF2 alpha > angiotensin II > vasopressin. Also, norepinephrine, PGF2 alpha, and angiotensin II, but not vasopressin, further enhanced the DNA synthesis when their concentrations were increased above those yielding maximal elevation of InsP3. In experiments where vasopressin and angiotensin II were combined, their effects on the DNA synthesis were additive while the InsP3 responses were not. The results show that the extent of the initial activation of PI-PLC is not the determinant for the magnitude of the growth effects of Ca(2+)-mobilizing hormones in hepatocytes. This suggests either (a) that the proliferative response to these agents is determined by the activity of PI-PLC at a later time, or its integral over an extended part of the prereplicative period, rather than by the acute activation, or (b) that additional, PI-PLC-independent, mechanisms are required.

DNA binding by C/EBP proteins correlates with hepatocyte proliferation

The leucine zipper transcription factors C/EBP alpha and C/EBP beta exhibit growth-related variations of expression and DNA binding during liver regeneration. We examined the expression of C/EBP proteins in relation to hepatocyte proliferation by studying their DNA-binding activity in primary mouse hepatocytes in vitro. Mouse hepatocytes were dissociated by collagenase perfusion and cultured in a serum-free, defined medium containing a variety of growth factors and hormones. Cell protein extracts were collected every 24 h for up to 10 d and examined for DNA-binding activity by gel retardation analysis using a C/EBP consensus sequence oligomer (bZIP). C/EBP alpha is the major bZIP-binding protein present in the dissociated cells prior to plating. With the culture conditions we employed, little or no binding of C/EBP proteins was observed in the first 24 to 48 h of cultivation. After 48 h, C/EBP beta binding activity was elevated relative to the level seen in freshly dissociated cells. In contrast, C/EBP alpha binding continued to be greatly reduced and no C/EBP delta binding was observed. C/EBP beta binding remained elevated for the duration of the experiment. Additional growth factor treatment (EGF, FGF, TGF alpha, and HGF) of the hepatocytes did not appreciably alter the pattern of C/EBP binding. However, TGF beta treatment, known to decrease hepatocyte proliferation, increased C/EBP beta binding activity earlier and more actively than in control cells. This study confirms a negative correlation between DNA binding by the C/EBP transactivator proteins and the proliferation of primary mouse hepatocytes in vitro.

On the mechanisms of the growth-promoting effect of prostaglandins in hepatocytes: the relationship between stimulation of DNA synthesis and signaling mediated by adenylyl cyclase and phosphoinositide-specific phospholipase C

While many observations indicate that prostaglandins may act as positive regulators of hepatocyte proliferation, the underlying mechanisms are not known. We have examined some of the signal pathways in the growth response induced by prostaglandins in hepatocytes, with particular focus on adenylyl cyclase and phosphoinositide-specific phospholipase C. Adult rat hepatocytes were cultured as primary monolayers in serum-free medium in the presence of EGF and insulin. PGE2 or PGF2 alpha (added 0-3 h after plating) enhanced the incorporation of [3H]-thymidine into DNA (measured at 50 h); at 100 microM the stimulation was about threefold PGI2 and PGD2 also showed significant but smaller stimulatory effects. No significant increase in the level of cyclic AMP (cAMP) was detected in response to any of the prostaglandins. Low concentrations of glucagon (0.1-10 nM), a potent activator of hepatic adenylyl cyclase, or 8-bromo-cAMP (0.1-10 microM) enhanced the DNA synthesis. When 8-bromo-cAMP was used in maximally effective concentrations, no further stimulation was obtained by combining it with glucagon, whereas the effects of PGE2 and 8-bromo-cAMP were completely additive. All the prostaglandins also showed additivity with the effect of glucagon on the DNA synthesis. PGE2, PGF2 alpha, PGI2, and PGD2 increased intracellular inositol-1,4,5-trisphosphate (InsP3), with a relative order of efficacy roughly corresponding to their activity as stimulators of DNA synthesis. Increases in cytosolic free Ca2+, as measured in single cells, were elicited in a majority of the hepatocytes by all these prostaglandins at 1 microM. Supramaximal concentrations of vasopressin, a strong activator of phospholipase C in hepatocytes, acted additively with PGE2 on the DNA synthesis. Pretreatment of the hepatocytes with a concentration of pertussis toxin that prevented the inhibitory effect of PGE2 on glucagon-induced cAMP accumulation did not abolish the ability of PGE2 to stimulate the DNA synthesis. The results do not support a role for adenylyl cyclase activation in the stimulatory effect of prostaglandins on hepatocyte growth. While the data are compatible with an involvement of phosphoinositide-specific phospholipase C in the growth-promoting effect of prostaglandins in cultured rat hepatocytes, they suggest this may not be the sole mechanism.

Inhibition by verapamil of hepatocarcinogenesis induced by N-nitrosomorpholine in Sprague-Dawley rats

The effect of verapamil on hepatocarcinogenesis induced by N-nitrosomorpholine (NNM) was investigated in male Sprague-Dawley rats. Rats were given drinking water containing NNM for 8 weeks and received i.p. injections of verapamil or vehicle every other day for 16 weeks from the start of the experiment. Pre-neoplastic and neoplastic lesions staining positive for gamma-glutamyl transpeptidase (GGT) or the placental type of glutathione-S-transferase (GST-P) were examined histochemically at week 16. Prolonged administration of verapamil resulted in a significant decrease in the number of GGT-positive and GST-P-positive lesions. The incidence and volume as a percentage of parenchyma of hepatocellular carcinomas were also significantly less in rats treated with verapamil than in controls. Administration of verapamil significantly decreased the labelling indices of pre-neoplastic lesions and adjacent liver. These findings indicate that verapamil inhibits hepatocarcinogenesis and that this may be related to its inhibitory effect on cell proliferation in neoplastic lesions and surrounding hepatocytes.

Enhancement by isoproterenol of hepatocarcinogenesis induced by N-nitrosomorpholine in Sprague-Dawley rats

The effect of isoproterenol on hepatocarcinogenesis induced by N-nitrosomorpholine (NNM) was investigated in male Sprague-Dawley rats. Rats were given drinking water containing NNM for 8 weeks and s.c. injections of isoproterenol or vehicle every other day for 13 weeks. Pre-neoplastic and neoplastic lesions staining positive for gamma-glutamyl transpeptidase (GGT) or the placental type of glutathione-S-transferase (GST-P) were examined histochemically at week 13. Prolonged administration of isoproterenol resulted in a significant increase in the number of GGT-positive, but not GST-P-positive, lesions. The incidence, number and size of hepatocellular carcinomas were also significantly greater in rats treated with isoproterenol than in controls. Administration of isoproterenol significantly increased the intracellular cAMP and the labeling indices of pre-neoplastic lesions and adjacent liver. These findings indicate that isoproterenol enhances hepatocarcinogenesis and that this may be related to its enhancing effect, mediated by cAMP, on cell proliferation in neoplastic lesions and surrounding hepatocytes.

Intracellular signalling of epinephrine in rat hepatocytes during fetal development and hepatic regeneration

The changes in intracellular calcium concentration and IP3 production after the addition of epinephrine were analysed in adult, fetal (20th-22nd day of intrauterine life), and regenerating rat hepatocytes (4 h-24 h after partial hepatectomy) to determine whether the signal transduction is the same in quiescent proliferating and differentiating cells. The epinephrine treatment causes a significative cytosolic calcium transient in hepatocytes isolated in the last day of fetal life (22-day old) and in the early stage of regeneration (4 h). This effect is not significant in the previous stage of fetal life (20-day old) and at the onset of M phase of cell cycle after partial hepatectomy (24 h). [3H]myo inositol incorporation into IP3 and IP4 is higher in 20 day fetal and regenerating hepatocytes with respect to the control. In these cells the epinephrine does not affect basal level of IP3 and IP4, while it causes a substantial increase of these inositol phosphates in adult hepatocytes. [3H]myo inositol incorporation into PIP2 is very low at the 20th day of fetal life. Epinephrine has no effect on this parameter in fetal and regenerating hepatocytes. Our results show that the epinephrine signal is mediated differently in proliferating and in quiescent hepatocytes.

Several nongenotoxic carcinogens uncouple mitochondrial oxidative phosphorylation

A number of plasticizers and lipid-lowering drugs induce peroxisomes and cause hepatocellular carcinoma in rodents by mechanisms which remain unknown. In this study, seven structurally dissimilar peroxisome proliferating agents were shown to uncouple oxidative phosphorylation in isolated rat liver mitochondria. For example, perfluorooctanoate (0.5 mM) increased succinate-induced (state 4) mitochondrial respiration by over 50% while stimulation of state 3 respiration with ADP was minimal (i.e., uncoupling occurred). Interestingly, compounds which are potent carcinogens in vivo (e.g., Wy-14,643 and perfluorooctanoate) were more powerful uncouplers of oxidative phosphorylation in vitro than weak tumor-causing agents (e.g., valproate). Uncoupling also occurred in vivo. Basal rates of oxygen uptake in perfused livers from chronically treated rats were increased from 137 +/- 7 mumol g-1/h in pair-fed controls to 153 +/- 5 mumol g-1/h after 2.5 months of feeding Wy-14,643 (0.1% w/v in diet). Concomitantly, rates of urea synthesis from ammonia, a process highly dependent on ATP supply, were reduced almost completely from 104 +/- 10 mumol g-1/h to 13 +/- 6 mumol g-1/h. Bile flow, another energy-dependent process, was also reduced significantly by treatment with Wy-14,643 in vivo for 24 h. Taken together, these data indicate that energy supply for cellular processes such as urea synthesis and bile flow was disrupted in vivo due to uncoupling of oxidative phosphorylation by Wy-14,643. It is proposed that peroxisomal proliferators accumulate in the liver where they uncouple mitochondrial oxidative phosphorylation and interfere with cellular energetics.

Epidermal growth factor induces dose-dependent calcium oscillations in single fura-2-loaded hepatocytes

Digital imaging fluorescence microscopy has been used to investigate epidermal growth factor-induced calcium responses of fura-2-loaded hepatocytes in primary culture at the single-cell level. Epidermal growth factor induced oscillations in cytosolic free calcium ([Ca2+]i) consisting of a periodic train of spikes unlike the monophasic elevation in cell suspensions reported previously. In this study, 79% of the cells in the microscopic field responded to 0.1 nmol/L epidermal growth factor, and 78% of the responsive cells displayed oscillations. However, the frequency of oscillations differed considerably from cell to cell. [Ca2+]i measurement in a cell population was simulated using these data, but only a slightly biphasic pattern was obtained, indicating the significance of single-cell measurement of [Ca2+]i. Because considerable heterogeneity existed in the sensitivity to epidermal growth factor between the cells, single hepatocytes were stimulated sequentially with increasing concentrations of epidermal growth factor to investigate the dose dependence of the oscillations. The frequency of the oscillations increased with increasing epidermal growth factor concentration, but the amplitude was similar for all concentrations, suggesting the existence of frequency-encoded information even in the pathway through tyrosine kinase for epidermal growth factor signaling. The pattern of the oscillations with epidermal growth factor, especially the latency, was considerably different from that with phenylephrine, which is known to use the phosphatidylinositol pathway, possibly because of the difference in the pathway toward phosphatidylinositol turnover between these agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatocyte growth factor (hepatopoietin A) rapidly increases in plasma before DNA synthesis and liver regeneration stimulated by partial hepatectomy and carbon tetrachloride administration

An enzyme-linked immunosorbent assay was used to measure the level of hepatocyte growth factor in rat plasma at various times after two-thirds partial hepatectomy or CCl4 administration. An initial 17-fold rise and 13-fold rise in the level of hepatocyte growth factor was observed 2 hr after partial hepatectomy and CCl4 treatment, respectively, well before the onset of DNA synthesis in the liver. The peaks of DNA synthesis in remnant livers and livers exposed to CCl4 occurred at 24 hr and 48 hr, respectively, as determined by 5-bromo-2'-deoxyuridine labeling and [3H]thymidine uptake by the liver. A later peak level (17-fold above control) of hepatocyte growth factor at 24 hr after CCl4 treatment coincided with strong immunostaining of damaged or necrotic hepatocytes around central veins with an antibody to hepatocyte growth factor. This suggests a later intrahepatic origin of the signals for liver regeneration after hepatotoxic injury subsequent to the early extrahepatic production of hepatocyte growth factor at 2 hr after CCl4 administration. The absence of staining in the liver remnants in partially hepatectomized rats implies that the increase in hepatocyte growth factor seen in the plasma is caused by production at extrahepatic site(s). Possible sources include the pancreas, brain, thyroid and salivary glands, and Brunner's glands of the duodenum. Norepinephrine also increases in plasma as early as 2 hr after hepatectomy. In vitro, [3H]thymidine incorporation into hepatocyte DNA in the presence of hepatocyte growth factor is greater if 10(-5) mol/L norepinephrine is also present in the media.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1-adrenergic receptors in liver regeneration

The alpha 1-adrenergic receptor mediates the effects of catecholamines on DNA synthesis, as observed in rat liver following a 2/3 partial hepatectomy and in serum-free primary cultures of adult rat hepatocytes exposed to epidermal growth factor. In vitro, norepinephrine action at this receptor heterologously down-regulates epidermal growth factor receptors. In vivo, the alpha 1 receptor's effect on DNA synthesis is restricted to early time points after partial hepatectomy. alpha 1 receptor binding capacity does not vary until 48 hr after liver resection (at which time binding is reduced), but an uncoupling of receptor binding from membrane phosphoinositide turnover occurs between 8 and 16 hr after partial hepatectomy. This change is preceded by a fall in membrane-associated ras p21 detected by radioimmunoassays (46% of control levels by 2 hr after partial hepatectomy). Whether this change represents a loss of p21 protein from membranes or a modification that results in a loss of immunoreactivity is not known.

Epinephrine regulation of amino acid transport in rat hepatocytes isolated during development

The effect of epinephrine on the amino acid transport mediated by system A was investigated by determining the uptake of 2-amino [1-14C]isobutyric acid (AIB) in rat hepatocytes, freshly isolated at different stages of pre- and postnatal development. The data obtained show that the hormone increased AIB uptake, enhancing the Vmax, while Km was unchanged. This effect was evident in cells from adult, 18- to 20-day-old fetus, and neonate rat. Actinomycin D or cycloheximide abolished the hormone dependent increase. Experiments carried out with alpha- and beta-antagonists showed that the effect of epinephrine was beta-mediated in fetal life and alpha-mediated in adult life. Membrane binding experiments showed a higher value for epinephrine and beta-agonist dihydroalprenolol in the fetus versus the adult. The calcium depletion obtained after cell incubation with EGTA or calcium ionophore A23187 reduced the hormonal stimulation in the adult, and was ineffective in the prenatal period. An involvement of cAMP was present in the epinephrine modulation of AIB transport, both in adult and in fetal life.

Altered responses of regenerating hepatocytes to norepinephrine and transforming growth factor type beta

Norepinephrine (NE), acting through the alpha 1-adrenergic receptor, modules the response of rat hepatocytes in primary culture to transforming growth factor type beta 1 (TGF beta) by increasing the amount of TGF beta required for a given degree of inhibition of epidermal growth factor (EGF)-induced DNA synthesis (Houck et al., J. Cell. Physiol. 135:551-555, 1988). This effect was also found in hepatocytes isolated from regenerating livers but was greatly magnified in cells isolated between 12 and 18 hr after two-thirds partial hepatectomy (PHX). During this period of enhanced sensitivity, NE was equally potent in terms of dose but more efficacious in the regenerating hepatocytes. As it did in control hepatocytes (Cruise et al., Science 227:749-751, 1985), the alpha 1-adrenergic receptor mediated the activity of NE in regenerating hepatocytes. Vasopressin (VP) and angiotensin-II (AG) also antagonized the effect of TGF beta and showed increased activity in regenerating hepatocytes but at only 50% or less of the maximal effect reached by NE. Regenerating hepatocytes isolated 24-72 hr after PHX exhibited decreased sensitivity to inhibition by TGF beta, with a nadir in 48-hr-regenerating cells. These findings suggest that NE may be involved in triggering the early phase of DNA synthesis during liver regeneration, with the subsequent acquisition of innate resistance to TGF beta responsible for continued proliferation at a time when TGF beta mRNA is known to be increasing in the liver (Braun et al., Proc. Natl. Acad. Sci. USA 85:1539-1543, 1988). EGF induced increased DNA and protein synthesis in cultures of control hepatocytes; TGF beta inhibited the EGF-induced DNA synthesis but had no effect on protein synthesis. This may be relevant to the latter stages of liver regeneration, when high levels of TGF beta mRNA are detected in liver and cellular hypertrophy predominates over hyperplasia.

Inhibition by cysteamine of hepatocarcinogenesis induced by N-nitrosomorpholine in Sprague-Dawley rats

The effect of cysteamine (2-aminoethanethiol hydrochloride) on hepatocarcinogenesis induced by N-nitrosomorpholine (NNM) was investigated in male Sprague-Dawley rats. Rats received alternate-day s.c. injections of cysteamine, and beginning in experimental week 3 were given drinking water containing NNM for 8 weeks. Pre-neoplastic and neoplastic lesions staining positive for gamma-glutamyl transpeptidase (GGT) or glucose-6-phosphate dehydrogenase (G6PD) were examined by histochemical techniques. In week 18, quantitative histological analysis showed that prolonged administration of cysteamine resulted in a significant reduction in the number of GGT-positive and G6PD-positive hepatic lesions. Histologically, hepatocellular carcinomas were significantly fewer and smaller in GGT-positive and G6PD-positive lesions in rats treated with cysteamine than in untreated rats. Administration of cysteamine also caused a significant decrease in the liver norepinephrine concentration and in the labelling indices of pre-neoplastic lesions and the surrounding liver. Our findings indicate that cysteamine inhibits hepatocarcinogenesis; this may be related to its reducing effect on norepinephrine concentration in the liver and its subsequent inhibition of cell proliferation in neoplastic lesions and surrounding hepatocytes.

Regulation of hepatocyte growth: alpha-1 adrenergic receptor and ras p21 changes in liver regeneration

Catecholamines, acting via the alpha-1 adrenergic receptor, have been demonstrated to influence adult rat hepatocyte DNA synthesis in primary culture and in vivo during liver regeneration following partial hepatectomy (PHX). Earlier investigations have suggested that the alpha-1 effect on DNA synthesis is significant only during the first day following PHX. We examined receptor binding at several early and late time points after surgery, and we observed a significant loss of specific [3H]-prazosin binding to cells isolated from rat livers 48 and 72 hr after PHX. In contrast, the ability of norepinephrine to stimulate inositol phosphate production in isolated cells prelabeled with [3H]-myo-inositol was transiently reduced between 8 and 16 hr, when alpha-1 binding capacity was virtually unchanged. This uncoupling of phosphoinositide turnover from binding was preceded by a drop in hepatic membrane ras p21 content, as assayed by liquid competition radioimmunoassay. The loss of immunoreactive p21 from membranes was significant by 2 hr after PHX. These findings suggest a role for alpha-1 receptors and ras protein in the early events of liver regeneration.