Increased effect of adrenaline on cyclic AMP formation and positive beta-adrenergic modulation of DNA-synthesis in regenerating hepatocytes

The Role of Catecholamines in Pathophysiological Liver Processes

Over the last few years, the number of research publications about the role of catecholamines (epinephrine, norepinephrine, and dopamine) in the development of liver diseases such as liver fibrosis, fatty liver diseases, or liver cancers is constantly increasing. However, the mechanisms involved in these effects are not well understood. In this review, we first recapitulate the way the liver is in contact with catecholamines and consider liver implications in their metabolism. A focus on the expression of the adrenergic and dopaminergic receptors by the liver cells is also discussed. Involvement of catecholamines in physiological (glucose metabolism, lipids metabolism, and liver regeneration) and pathophysiological (impact on drug-metabolizing enzymes expression, liver dysfunction during sepsis, fibrosis development, or liver fatty diseases and liver cancers) processes are then discussed. This review highlights the importance of understanding the mechanisms through which catecholamines influence liver functions in order to draw benefit from the adrenergic and dopaminergic antagonists currently marketed. Indeed, as these molecules are well-known drugs, their use as therapies or adjuvant treatments in several liver diseases could be facilitated.

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.

Evidence that dl-propranolol increases thymidine kinase activity, cell mitosis, and beta-adrenoceptors during rat liver regeneration

BACKGROUND

Adrenoceptor stimulation has been described as an important factor in liver regeneration. We examined the effects of the beta-adrenoceptor antagonist, dl-propranolol, during early steps of regeneration after partial hepatectomy in rat.

METHODS

Thymidine kinase (TK) activity, cell mitosis, and beta-adrenoceptors were measured after a single intraperitoneal (i.p.) injection of dl-propranolol (10 mg/kg) immediately following 70% partial hepatectomy. Enzyme activity was measured as [3H] thymidine (dThd) monophosphate; mitotic index was determined after tissue staining, and beta-adrenoceptors were quantified by saturation binding of [3H] dihydroalprenolol. Liver regeneration was followed for 3 days.

RESULTS

dl-Propranolol increased TK activity as early as 12 h postsurgery, after which it leveled off at 24 h and declined at later times; mitotic index increased from 12 to 48 h after injection and beta-adrenoceptors augmented at 24 h compared to partial hepatectomy alone.

CONCLUSIONS

dl-Propranolol up-regulated beta-adrenoceptors early after partial hepatectomy with an increase in TK activity and augmented hepatocyte mitosis. These data may suggest that dl-propranolol modulates, probably via beta-adrenoceptor blockade, early steps of liver regeneration.

Patterns of adrenoceptor change during liver regeneration of the Wistar Kyoto rat: a binding study

BACKGROUND

Adrenoceptors have been involved in the regulation of hepatocyte proliferation after partial hepatectomy, as well as in primary culture. This report characterizes alpha 1- and beta-adrenoceptor change during the time-course of liver regeneration in adult Wistar Kyoto rats.

METHODS

Saturation binding assays with [3H]prazosin or [3H]dihydroalprenolol (for alpha 1- and beta-adrenoceptors, respectively) were done in liver plasma membranes from 6-month-old rats subjected to 70% hepatectomy followed by hepatic regeneration.

RESULTS

[3H]Prazosin and [3H]dihydroalprenolol binding gave control Bmax values of 101 +/- 10 and 12 +/- 1 fmol/mg protein and Kd of 0.50 +/- 0.10 and 4.1 +/- 0.4 nM for alpha 1- and beta-adrenoceptors, respectively. alpha 1-Adrenoceptor number and Kd increased at 24 and 48 h and returned to control values at 72 and 96 h after surgery, whereas beta-adrenoceptors augmented at 48 and 72 h, with a Kd change at 24 and 48 h posthepatectomy.

CONCLUSIONS

These results suggest that dual control of alpha 1- and beta-adrenoceptor membrane expression could be involved in different steps during hepatocyte proliferation, and that Wistar Kyoto rats have a different adrenoceptor pattern expression from other rat strains.

Proliferation of adult rat hepatocytes in primary cultures induced by platelet-derived growth factor is potentiated by phenylephrine

We investigated whether or not proliferation of adult rat hepatocytes induced by platelet-derived growth factor (PDGF) is affected by alpha1-adrenoceptor agonists such as phenylephrine during the early and late phases of primary culture. Adult rat hepatocytes underwent significant DNA synthesis after culture with 10 ng/ml of PDGF for 2 hr at a low cell density (3.3 x 10(4) cells/cm2). Under these culture conditions, the number of nuclei increased significantly during the 3.5-hr culture period. Hepatocyte DNA synthesis and proliferation induced by 10 ng/ml of PDGF decreased slightly as a result of increasing the initial plating density. An alpha1-adrenoceptor agonist, phenylephrine (10(-6) and 10(-5) M), alone did not affect hepatocyte DNA synthesis and proliferation, but markedly potentiated PDGF-induced hepatocyte DNA synthesis and proliferation. The phenylephrine effect was mimicked by phorbol myristate acetate (10(-7) M), but not by ionomycin (10(-5) M). The mitogenic effects of PDGF were almost completely blocked by treating hepatocytes with genistein (5 x 10(-6) M), U-73122 (3 x 10(-6) M), sphingosine (10(-5) M), wortmannin (10(-7) M) and rapamycin (10 ng/ml). These results demonstrate that PDGF can induce the proliferation of adult rat hepatocytes rapidly in primary culture, regardless of the initial plating density. The present results also suggest that following stimulation with PDGF, activation of tyrosine kinase, phospholipase C, phosphatidylinositol 3-kinase, protein kinase C (PKC) and p70 ribosomal protein S6 kinase is essential for the proliferation of adult rat hepatocytes. The co-mitogenic effects of phenylephrine may involve PKC activation.

Proliferation of adult rat hepatocytes in primary culture induced by insulin is potentiated by cAMP-elevating agents

We investigated whether or not insulin and cAMP-elevating agents induce the proliferation of adult rat hepatocytes during the early and late phases of primary culture. Adult rat hepatocytes synthesized a significant amount of DNA when cultured in the presence of 10(-7) M insulin for 3 h. Under these conditions, the number of nuclei increased within 4 h. Hepatocyte DNA synthesis and proliferation were not essentially affected by the initial plating densities. Other cAMP-elevating agents, such as glucagon, forskolin and dibutyryl cAMP, as well as beta-adrenoceptor agonists (i.e., metaproterenol and isoproterenol) alone had no effect on either hepatocyte DNA synthesis or proliferation in primary culture. In contrast, these agents potentiated both processes at concentrations as low as 10(-7) M when cultured in combination with 10(-7) M insulin. The stimulatory effects of beta-adrenoceptor agonists and other cAMP-elevating agents were significantly blocked by the cAMP-dependent protein kinase inhibitor, H-89 (N-[2-(p-(bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride; 10(-7) M). The mitogenic effect of insulin upon hepatocytes was almost completely suppressed by genistein (5 x 10(-6) M), wortmannin (10(-7) M) and by rapamycin (10 ng/ml). These results show that insulin rapidly induced the proliferation of adult rat hepatocytes in primary culture. The mitogenic effects of insulin were potentiated by beta-adrenoceptor agonists and cAMP-elevating agents. The effects of beta-adrenoceptor agonists and cAMP-elevating agents may be mediated through cAMP-dependent protein kinase. In addition, the activation of receptor tyrosine kinase, phosphoinositide 3-kinase and p70 ribosomal protein S6 kinase may be involved in the insulin signal transduction pathway.

Density-dependent proliferation of adult rat hepatocytes in primary culture induced by epidermal growth factor is potentiated by cAMP-elevating agents

We investigated whether or not epidermal growth factor (EGF) and cAMP-elevating agents induce the proliferation of adult rat hepatocytes during the early (4 h after adding EGF) and late phases (21 h after adding EGF) of primary cultures. Adult rat hepatocytes did not significantly proliferate after culture with 20 ng/ml EGF for 4 h at a density of 1 X 10(5) cells/cm2. In contrast, when the density was decreased by about one-third to 3.3 X 10(4) cells/cm2, the number of nuclei increased about 1.2-fold after culture with 10-20 ng/ml EGF for 4 h. Under these culture conditions, DNA synthesis began within 2-4 h of exposure to 20 ng/ml of EGF, although at the high cell density, DNA was not synthesized during this period. The beta-adrenoceptor agonists, metaproterenol and isoproterenol, and other cAMP-elevating agents, such as glucagon, forskolin, and dibutyryl cAMP, potentiated both hepatocyte DNA synthesis and proliferation about 1.4-fold when cultured in combination with 20 ng/ml EGF. The stimulatory effects of metaproterenol and other cAMP-elevating agents were specifically blocked by the cAMP-dependent protein kinase inhibitor, H-89 (10(-7) M). The effect of EGF was almost completely suppressed by genistein (5 X 10(-6) M) and rapamycin (10 ng/ml), but it was unaffected by wortmannin (10(-7) M). These results demonstrate that mature rat hepatocytes can proliferate very rapidly in low-density cultures with EGF, the effects of which were potentiated by beta-adrenoceptor agonists and cAMP-elevating agents. In addition, the activation of receptor tyrosine kinase and p70 ribosomal protein S6 kinase may be involved in EGF-induced hepatocyte DNA synthesis and proliferation.

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.

c-Myc expression is controlled by the mitogenic cAMP-cascade in thyrocytes

In dog thyroid epithelial cells in primary culture, thyrotropin (TSH), acting through cAMP, induces proliferation and differentiation expression, whereas epidermal growth factor (EGF) and phorbol esters induce proliferation and dedifferentiation. In these cells, we have detailed the regulation by cAMP of the c-myc protooncogene mRNA and protein. The cAMP signaling pathway induces a biphasic increase of c-myc mRNA and protein. c-Myc protein accumulation follows the abundance and kinetics of its mRNA expression. Using in vitro elongation of nascent transcripts to measure transcription and actinomycin D (AcD) chase experiments to study mRNA stability, we have shown that in the first phase cAMP releases a transcriptional elongation block. No modification of transcriptional initiation was observed. After 30 min of treatment with TSH, c-myc mRNA was also stabilized. During the second phase, cAMP stabilization of the mRNA disappears and transcription is again shut off. Thus, in a tissue in which it stimulates proliferation and specific gene expression, cAMP regulates biphasically c-myc expression by mechanisms operating at the transcriptional and posttranscriptional levels.

Stimulation of hepatocyte DNA synthesis by prostaglandin E2 and prostaglandin F2 alpha: additivity with the effect of norepinephrine, and synergism with epidermal growth factor

Previous data obtained in vivo and in vitro suggest that both prostaglandins (PGs) and catecholamines may have a role in promoting hepatocyte proliferation, and PGE2 and PGF2 alpha have also been implicated as mediators of the mitogenic actions of epidermal growth factor (EGF) (and transforming growth factor alpha [TGF alpha]). We have studied the effects of PGs and norepinephrine on DNA synthesis in serum-free primary cultures of rat hepatocytes, and compared the PG effects with those of norepinephrine. PGE2, PGF2 alpha, PGD2, and the synthetic analog dimethyl-PGE2 markedly enhanced the DNA synthesis. A more quantitative analysis of the effects of PGE2 and PGF2 alpha on the DNA synthesis, in the presence and absence of EGF, indicated that these PGs interacted in an essentially multiplicative manner with the effect of EGF. The effects of PGE2 and PGF2 alpha showed almost complete additivity with the stimulation of DNA synthesis produced by maximally effective concentrations of norepinephrine. The data suggest a) that PGE2 and PGF2 alpha facilitate and synergize with, rather than mediate, the actions of EGF in hepatocytes, and b) that this effect of the PGs occurs by mechanisms that are at least partly distinct from those of norepinephrine.

Biphasic regulation by N6,2'-O-dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP) of steroid 21-hydroxylase activity in rat hepatocytes

Steroid 21-hydroxylase activity has been identified in many tissues, including liver. But it is possible that the enzyme found in the liver is different from adrenal 21-hydroxylase. In the adrenal cortex, steroid 21-hydroxylase activity is increased by corticotropin (ACTH); the effect of ACTH is mediated by cyclic AMP (cAMP), and presumably involves a cAMP-dependent protein kinase (PKA). It is not yet clear, however, how extra-adrenal steroid 21-hydroxylase activity is regulated. In the present study, we examined the effect of N6,2'-O-dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP), forskolin, N-[2-(methylamino)ethyl]5-isoquinolinesulfonamide (H-8) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on steroid 21-hydroxylase activity in primary cultures of rat hepatocytes to determine the nature of regulation of extra-adrenal steroid 21-hydroxylase activity. Steroid 21-hydroxylase activity in hepatocytes incubated with 10(-11) M dbcAMP for 24 h was 1.6 times higher than that in control hepatocytes untreated with dbcAMP. On the other hand, steroid 21-hydroxylase activity decreased by 20 and 50% when the cells were incubated with 10(-5) and 10(-3) M dbcAMP, respectively. The stimulatory effect of 10(-11) M dbcAMP was not blocked by 10(-5) M H-8 (PKA inhibitor), but the inhibitory effect of 10(-5) or 10(-3) M cAMP was. TPA did not alter the activity of steroid 21-hydroxylase. These findings indicate that the steroid 21-hydroxylase in rat liver is regulated by mechanisms different from those in the adrenal glands.

Long-term inhibitory effect of cAMP on beta-adrenoceptor acquisition and nonselective attenuation of adenylyl cyclase in hepatocytes

Long-term effects of cAMP on the surface expression of beta-adrenoceptors and adenylyl cyclase activity were investigated in primary cultures of rat hepatocytes. beta-Adrenoceptor density and catecholamine-responsive adenylyl cyclase activity increased during culturing in a biphasic manner, with a plateau of 10-20 h duration occurring approximately 10 h after plating. Treatment of hepatocyte cultures with 8-bromo-cAMP during the plateau period did not affect the density of beta-adrenoceptors. In contrast, addition of 8-bromo-cAMP, 8-chlorophenylthio-cAMP, forskolin or glucagon during a period of active recruitment of surface beta-adrenoceptors resulted in a suppression of the acquisition of beta-adrenoceptors. In both experimental situations there was a partial decrease in hormone-stimulated and basal adenylyl cyclase activity. The results suggest that cAMP exerts at least two types of long-term regulation of adenylyl cyclase in hepatocytes: a suppressive effect on beta-adrenoceptor acquisition, and a partial, nonselective decrease in adenylyl cyclase activity not involving beta-adrenoceptor down-regulation.

Elevated cAMP gives short-term inhibition and long-term stimulation of hepatocyte DNA replication: roles of the cAMP-dependent protein kinase subunits

The study reports the role of the isozyme forms (cA-PKI and cA-PKII) and subunits (R and C) of cAMP-dependent protein kinase in mediating the acute depression of hepatocyte DNA replication by elevated cAMP. Combinations of cAMP analogs preferentially activating cA-PKI or II showed that either isozyme could inhibit DNA replication. The effects of glucagon and cAMP analogs were counteracted by the cAMP antagonist RpcAMPS, implicating the necessity for cA-PK dissociation in cAMP action. The effect of elevated cAMP was mimicked by microinjected C subunit, but not by the RI subunit of cA-PK. Hepatocytes under continuous cAMP challenge more than regained their replicative activity. This tardive stimulatory effect of cAMP was enhanced by insulin and blocked by dexamethasone, and was preceded by downregulation of cA-PK. In conclusion, a burst of cAMP acutely inhibits hepatocyte G1/S transition in late G1 regardless of hormonal state. In the presence of high glucocorticoid/low insulin the inhibition persists. At high insulin/low glucocorticoid the inhibitory phase is followed by a prolonged stimulation of DNA replication. Downregulation of endogenous cA-PK is a mechanism for escape from the inhibitory action of highly elevated cAMP.

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.

Enhanced proliferation of fetal rat hepatocytes in primary culture induced by ritodrine

OBJECTIVE

Although ritodrine crosses the placenta, its direct effect on fetal cell proliferation has not been reported. We hypothesized that beta 2-adrenergic receptor stimulation could promote fetal liver growth.

STUDY DESIGN

Ritodrine was added to serum- and hormone-free primary cultures of fetal, neonatal, or adult rat hepatocytes. We measured both tritiated thymidine incorporation into deoxyribonucleic acid and nucleus number. The effect of ritodrine on cell cycle was also analyzed with flow cytometry.

RESULTS

Ritodrine enhanced the proliferation of fetal rat hepatocytes. Ritodrine remarkably stimulated deoxyribonucleic acid synthesis of fetal and neonatal but not adult hepatocytes. The effect was dose dependent and was antagonized by propranolol. Analysis of the nuclear deoxyribonucleic acid content derived from flow cytometry revealed that cells stimulated by ritodrine entered S phase.

CONCLUSION

These results indicate that ritodrine may promote the proliferation of fetal hepatocytes through the stimulation of beta 2-adrenergic receptors, followed by induction of deoxyribonucleic acid synthesis.

Beta-adrenoceptor mediated responses and subtypes of beta-adrenoceptors in cultured rat Sertoli cells

Membrane particles from Sertoli cell cultures were examined for subtypes of beta-adrenoceptors with a radioligand binding technique using [125I]iodocyanopindolol and a beta 1-selective antagonist (Sandoz 204 545) or a beta 2-selective antagonist (ICI 118 551). Biphasic competition curves and modified Eddie-Hofstee plots revealed a relative distribution of approx. 80% beta 1-adrenoceptors and 20% beta 2-adrenoceptors. Only 45% of the adrenoceptor mediated stimulation of adenylyl cyclase activity was associated with beta 1-adrenoceptors, whereas the remaining 55% was mediated via beta 2-adrenoceptors. The subtype selective antagonists inhibited isoproterenol stimulated aromatization of testosterone to estradiol-17 beta in a concentration-dependent manner. Complete inhibition of beta 1-adrenoceptors resulted in a 45% reduction of estradiol-17 beta formation, whereas similar inhibition of beta 2-adrenoceptors resulted in only a 35% reduction. It is concluded that cAMP-dependent effects of beta-adrenergic agonists in Sertoli cells are mediated by activation of both beta 1- and beta 2-adrenoceptors. The discrepancy between the relative number of beta 1- and beta 2-adrenoceptors and their relative contribution to cAMP production and aromatization indicates that beta 2-adrenoceptors in Sertoli cells are more tightly coupled to the adenylyl cyclase system than beta 1-adrenoceptors. Furthermore, complete inhibition of either beta 1- or beta 2-adrenoceptors by subtype selective antagonists, demonstrates a substantial fraction of spareness between agonist activation of the adenylyl cyclase complex and aromatization.

Differential expression of guanine nucleotide-binding proteins enhances cAMP synthesis in regenerating rat liver

Events leading to cAMP accumulation after partial hepatectomy (PH) and effects of cAMP on hormonal induction of DNA synthesis in hepatocytes were characterized. Hepatic cAMP peaked biphasically post-PH and paralleled changes in adenylyl cyclase activity. Fluctuations in cyclase activity were not explained by variations in glucagon receptor kinetics, but reflected altered G-protein expression. Membrane levels of the stimulatory G-protein, Gs alpha, increased early after PH and were sustained. Levels of the inhibitory G-protein, Gi2 alpha, increased more slowly, peaked later, and quickly fell. Levels of both G-proteins correlated poorly with levels of their mRNAs, suggesting posttranscriptional factors modify their membrane concentrations. When growth factor-induced DNA synthesis was compared in hepatocyte cultures grown with or without agents that increase intracellular cAMP, DNA synthesis was inhibited by sustained high levels of cAMP but was enhanced when high cAMP levels fell. In both regenerating liver and hepatocyte cultures, the expression of a "differentiated" hepatocyte gene, phosphoenolpyruvate carboxykinase, correlated with elevated cAMP levels. These data suggest that the differential expression of G-proteins integrates signals initiated by several growth factors so that the accumulation of cAMP is tightly regulated post-PH. The ensuing variations in cAMP levels modulate both growth and differentiated functions during liver regeneration.

Stimulatory and inhibitory effects of catecholamines on DNA synthesis in primary rat hepatocyte cultures: role of alpha 1- and beta-adrenergic mechanisms

Previous studies suggest that catecholamines may be involved in the regulation of liver growth. Considerable evidence implicates alpha 1-adrenergic mechanisms in the initiation of hepatocyte proliferation, while the role of beta-adrenoceptors is less clear. We have examined further the adrenergic regulation of hepatocyte DNA synthesis, using primary monolayer cultures. In hepatocytes that were also treated with epidermal growth factor and insulin, epinephrine or norepinephrine added early after the seeding strongly accelerated the rate of S phase entry. The beta-adrenergic agonist isoproterenol and the alpha-adrenergic agonist phenylephrine also stimulated the DNA synthesis, but were less efficient than epinephrine and norepinephrine. Experiments with the alpha 1-receptor blocker prazosine and the beta-receptor blocker timolol showed that the stimulatory effect of norepinephrine consisted of both an alpha 1- and a beta-adrenergic component. The alpha 1-component was most prominent in terms of maximal response at high concentrations of the agonist, but the beta-component contributed significantly and predominated at low concentrations (less than 0.1 microM) of norepinephrine. At later stages (about 40 h) of culturing norepinephrine strongly but reversibly inhibited the cells, acting at a point late in the G1 phase. This inhibition was mimicked by isoproterenol and abolished by timolol but was unaffected by prazosine, suggesting a beta-adrenoceptor-mediated effect. The results confirm the alpha 1-adrenoceptor-mediated stimulatory effect, but also show that beta-adrenoceptors may contribute to the growth stimulation by catecholamines. Furthermore, catecholamines, via beta-adrenoceptors and cyclic AMP, inhibit the G1-S transition, and may thus play a role in the termination of hepatic proliferation.

Growth-regulatory effects of glucagon, insulin, and epidermal growth factor in cultured hepatocytes. Temporal aspects and evidence for bidirectional control by cyclic AMP

Data presented indicate that in hepatocytes insulin and glucagon promote growth by acting in a relatively early part of the prereplicative period (G0 or early G1) whereas cells (if pretreated with insulin) become more sensitive to EGF at the later stages, ie, nearer the S phase entry. The data indicate that at least two effects of glucagon (cAMP) on hepatocyte proliferation exist; in addition to a growth-promoting modulation early in the prereplicative period, there is also an inhibitory effect of glucagon (as well as other cAMP-elevating agents) that is exerted at a point shortly before the G1-to-S transition. Because both effects occur dose-dependently in the normal range of glucagon concentrations in portal blood, it is conceivable that glucagon/cAMP is involved both when liver growth is initiated and terminated.

Dual effects of glucagon and cyclic AMP on DNA synthesis in cultured rat hepatocytes: stimulatory regulation in early G1 and inhibition shortly before the S phase entry

Although several lines of evidence implicate cyclic AMP in the humoral control of liver growth, its precise role is still not clear. To explore further the role of cyclic AMP in hepatocyte proliferation, we have examined the effects of glucagon and other cyclic AMP-elevating agents on the DNA synthesis in primary cultures of adult rat hepatocytes, with particular focus on the temporal aspects. The cells were cultured in a serum-free, defined medium and treated with epidermal growth factor (EGF), insulin, and dexamethasone. Exposure of the hepatocytes to low concentrations (10 pM-1 nM) of glucagon in the early stages of culturing (usually within 6 h from plating) enhanced the initial rate of S phase entry without affecting the lag time from the plating to the onset of DNA synthesis, whereas higher concentrations inhibited it. In contrast, glucagon addition at later stages (24-45 h after plating) produced only the inhibition. Thus, if glucagon was added at a time when there was a continuous EGF/insulin-induced recruitment of cells to S phase, the rate of G1-S transition was markedly decreased within 1-3 h. This inhibitory effect occurred at low glucagon concentrations (ID50 less than 1 nM) and was mimicked by cholera toxin, forskolin, isobutyl methylxanthine, and 8-bromo cyclic AMP. The results indicate that cyclic AMP has dual effects on hepatocyte proliferation with a stimulatory modulation early in the prereplicative period (G0 or early G1), and a marked inhibition exerted immediately before the transition from G1 to S phase.

Dexamethasone inhibition of rat hepatoma cell growth and cell cycle traverse is reversed by insulin

(1) The growth of 7800 C1 Morris hepatoma cells was inhibited by dexamethasone. The inhibition was detectable at 1 nM and half-maximal effect was obtained with approx. 13 nM dexamethasone. About 80% growth inhibition was obtained with 250 nM of the hormone and the growth rate was normalized on cessation of treatment. (2) These hepatoma cells contain dexamethasone receptors with equilibrium dissociation constant of 0.24 nM and a capacity of 24 fmol/mg cell protein. Treatment of the cells with insulin did not change these dexamethasone binding properties. Binding experiments showed that 2, 10 and 100% of the receptors were occupied when the cells were incubated with 1 nM, 7 nM and 250 nM dexamethasone, respectively. (3) Insulin completely counteracted the growth inhibition by dexamethasone and antagonized the induction of peroxisomal acyl-CoA oxidase and tyrosine aminotransferase caused by the glucocorticoid. (4) Micro-flow fluorometry showed that the cultures had a major diploid DNA stem line and a minor tetraploid stem line. Changes in diploid, tetraploid and S phase cells of the diploid stem line were scored. Dexamethasone reduced the proportion of cells in S phase and of tetraploid cells. Insulin partly reversed the action of dexamethasone in S phase, but prevented the reduction in tetraploid cells caused by dexamethasone. (5) The mitotic rate was significantly reduced by dexamethasone and this effect was reversed by insulin. (6) Continuous [3H]methyl-thymidine labelling showed a growth fraction of unity in all treatment groups. (7) It is concluded that dexamethasone induces growth inhibition by reducing the G1-S transition. Insulin is able to counteract this effect and increase the rate of DNA synthesis.

A simple medium for the study of hepatocyte growth in culture under defined conditions

The combination (1:1) of Dulbecco's modified Eagle's medium and Waymouth's medium MAB 87/3 was found to provide favorable conditions for serum-free culture and growth of adult rat hepatocytes. In this simple medium, a majority of hepatocytes stimulated by epidermal growth factor plus insulin entered S phase and divided, with a normal (13 h) interval between DNA synthesis and cell division. The proliferative response did not require extra substratum or the presence of serum, even during cell isolation and plating.

Desensitization of adenylate cyclase and cyclic AMP flux during the early stages of liver regeneration

Liver regeneration is controlled by a complex network of interactions between hormones, growth factors, and a variety of hepatotrophic factors. Transient increases in cAMP in the early stages of liver regeneration that are necessary for DNA synthesis and subsequent mitosis have been reported; however, studies on the mechanisms that control cellular cAMP levels during liver regeneration, namely adenylate cyclase activity, cAMP-dependent phosphodiesterase activity, and cAMP efflux from the cell, have been generally incomplete. In this study we have shown that although there are three peaks in intracellular cAMP levels in the first 24 hours after partial hepatectomy, the adenylate cyclase activity stimulated by glucagon, prostaglandin E2, adrenaline, and fluoride in vitro decreases with time. KD and BMAX of hepatocyte glucagon and beta receptors were similar to the sham controls. Our results are consistent with a mixed homologous/heterologous desensitization of the adenylate cyclase system. There was also a loss of cAMP-dependent phosphodiesterase activity after partial hepatectomy. We speculate that even though the hormone-stimulated adenylate cyclase system has been desensitized, the system retains the ability to respond to the transient pulses of the variety of hormones secreted after partial hepatectomy and thus raise the intracellular concentration of cAMP. The decrease in cAMP-dependent phosphodiesterase may be necessary to prevent rapid breakdown of cAMP.

Temporal requirement for epidermal growth factor and insulin in the stimulation of hepatocyte DNA synthesis

Primary monolayer cultures of adult rat hepatocytes were used to study the temporal interaction of epidermal growth factor (EGF) and insulin in their stimulation of DNA synthesis. The hepatocytes were cultured both under defined conditions and with serum. EGF and insulin interacted synergistically. The entry into S phase (G1 exit) followed first-order kinetics both in untreated and hormone-stimulated cells. Addition of EGF and insulin at the time of plating did not alter the lag period before the DNA synthesis started (25-26 h), but the rate constant for the S phase entry increased five- to sixfold. Experiments where the time of hormone addition was varied indicated that insulin exerted its strongest effect at the time of plating, whereas the cells became more responsive to EGF after being cultured for up to 40-50 h. The responsiveness to EGF at these later stages required an early exposure of the hepatocytes to insulin. When the administration of EGF to insulin-pretreated hepatocytes was postponed for 44 h after plating in serum-free medium, the cellular sensitivity was increased as compared to EGF treatment at 0 h (a one-log shift of the dose-effect curve), the rate of S phase entry was more rapid, and the lag period for the onset of the EGF effect (i.e., shift of rate constant) was shortened (6-7 h vs. 26 h).

The relationship between beta-adrenoceptor regulation and beta-adrenergic responsiveness in hepatocytes. Studies on acquisition, desensitization and resensitization of isoproterenol-sensitive adenylate cyclase in primary culture

The role of beta-adrenoceptor regulation in the mechanisms controlling beta-adrenergic responsiveness in hepatocytes was explored, using primary monolayer cultures. When plated in vitro, these cells gradually acquire a strong catecholamine-sensitive adenylate cyclase activity and an enhanced ability to bind the beta-adrenoceptor ligand [125I]iodocyanopindolol (125ICYP). Examination of the time course showed that the increase in the number of 125ICYP binding sites was detectable within 1-2 h of culturing and slightly preceded the elevation of isoproterenol-responsive activity. Then the responsiveness rose steeply and between about 5-24 h it closely followed the increase in beta-receptor binding. Addition of isoproterenol (10 microM) to cells after 20 h of culturing caused a rapid homologous desensitization of the adenylate cyclase (50% after about 5 min). This was paralleled by a down-regulation of beta-adrenoceptors measured both in membrane particles and in total cell lysates. Removal of isoproterenol led to a resensitization of the adenylate cyclase, which was rapid and protein-synthesis-independent after a brief (10-min) desensitization, or slow and cycloheximide-sensitive after prolonged (4-h) exposure to the agonist. In both cases an up-regulation of the 125ICYP binding paralleled the recovery from refractoriness. In contrast, no concurring changes in 125ICYP binding were measured when the beta-adrenoceptor-linked adenylate cyclase activity was enhanced by pretreatment with pertussin toxin (islet-activating protein, IAP) or was desensitized by exposure of the cells to glucagon or 8-bromo-cAMP; however, these modulations of the adenylate cyclase were nonselective, since the pretreatments with IAP, glucagon or 8-bromo-cAMP affected both isoproterenol-sensitive and glucagon-sensitive activities. The results suggest that, in hepatocytes, regulation at the beta-adrenoceptor level is a major determinant for both short-term and long-term selective changes of the beta-adrenergic responsiveness.

Calcium, cyclic AMP and protein kinase C--partners in mitogenesis

Evidence is steadily mounting that the proto-oncogenes, whose products organize and start the programs that drive normal eukaryotic cells through their chromosome replication/mitosis cycles, are transiently stimulated by sequential signals from a multi-purpose, receptor-operated mechanism (consisting of internal surges of Ca2+ and bursts of protein kinase C activity resulting from phosphatidylinositol 4,5-bisphosphate breakdown and the opening of membrane Ca2+ channels induced by receptor-associated tyrosine-protein kinase activity) and bursts of cyclic AMP-dependent kinase activity. The bypassing or subversion of the receptor-operated Ca2+/phospholipid breakdown/protein kinase C signalling mechanism is probably the basis of the freeing of cell proliferation from external controls that characterizes all neoplastic transformations.

Hepatic regeneration and metabolism after partial hepatectomy in normal rats: effects of insulin therapy

The effect of insulin therapy on liver regeneration has been studied in normal fed rats 12, 24 and 48 h after partial hepatectomy. Dry weight of regenerating liver increased between 12 and 48 h after partial hepatectomy and was unaffected by insulin therapy. [6-3H] Thymidine uptake peaked at 24-h (24.7 +/- 2.4% of total liver cells) and insulin treatment had no additional effect. At 12-h after partial hepatectomy, hepatic [ATP] was decreased 15%, while [ADP] and [AMP] were increased 47% and 83% respectively compared with sham-operated animals. Partial hepatectomy also caused an increase in hepatic [triglyceride], a decrease in hepatic [glycogen] and an increase in the levels of glucose and several glycolytic intermediates. The hepatic redox ratios, [lactate]:[pyruvate] and [3-hydroxybutyrate]:[acetoacetate], were elevated. Insulin therapy had only minor effects on hepatic adenine nucleotide levels, intermediary metabolite concentrations or intrahepatic redox ratios after partial hepatectomy. These findings suggest a decreased hepatic intracellular energy state in regenerating liver; insulin therapy in normal rats does not influence this metabolic change nor the regenerative response.

Elevated level of beta-adrenergic receptors in hepatocytes from regenerating rat liver. Time study of [125I]iodocyanopindolol binding following partial hepatectomy and its relationship to catecholamine-sensitive adenylate cyclase

Hepatocytes from regenerating rat liver show an enhanced epinephrine-sensitive adenylate cyclase activity and cAMP response, which may be involved in triggering of the cell proliferation. We have determined adrenergic receptors and adenylate cyclase activity in hepatocytes isolated at various time points after partial hepatectomy. The number of beta-adrenergic receptors, measured by binding of [125I]iodocyanopindolol ([125I]CYP) to a particulate fraction prepared from isolated hepatocytes, increased rapidly after partial hepatectomy as compared with sham-operated or untreated controls. The maximal increase, which was observed at 48 h, was between 5- and 6-fold (from approximately 1 800 to approximately 10 500 sites per cell). Thereafter, the number of beta-adrenergic receptors decreased gradually. Competition experiments indicated beta 2-type receptors. Parallelism was found between the change in the number of beta 2-adrenergic receptors and the isoproterenol-responsive adenylate cyclase activity. The number of alpha 1-adrenergic receptors, determined by binding of [3H]prazosin, was transiently lowered by about 35% at 18-24 h, with no significant change in Kd. Although the results of this study do not exclude the possibility of post-receptor events, they suggest that the increased number of beta 2-adrenergic receptors is a major factor responsible for the enhanced catecholamine-responsive adenylate cyclase activity in regenerating liver.

Developmental alteration in adrenergic regulation of hepatic glycogen phosphorylase

The effects of development upon adrenergic regulation of glycogenolysis were characterized using isolated hepatocytes from 3 different age groups of male rats (6 week-old, 8 week-old and 30 week-old). The phosphorylase a response in isolated hepatocytes to alpha-adrenergic stimulation decreased moderately with advancing age; whereas, that to beta-adrenergic stimulation declined more rapidly and almost disappeared at the age of 30 weeks. This developmental alteration in relative contribution of alpha- and beta-adrenergic regulation of phosphorylase was further confirmed by the experiments with specific antagonists. Also, the dramatic decrease of beta-adrenergic response on glycogen phosphorylase activity was found to be closely associated with a similar change of cAMP generation. In addition, the glucagon effect on cAMP production was found to be declined with advancing animal age. These results demonstrate that the glycogenolytic response of isolated rat hepatocytes to catecholamines can be mediated by different pathways according to the age of the animal; thus, juvenile male rats exhibit both the alpha- and beta-adrenergic mechanism for activation of phosphorylase and the maturation is associated with a modest decline of alpha receptor-mediated effect and a dramatic attenuation of a beta-adrenergic/cAMP response.

Hepatic cyclic AMP generation and ornithine decarboxylase induction by glucagon and beta adrenergic agonists

The relationship of hepatic ornithine decarboxylase (ODC) activity to cyclic AMP levels and nutritional status was studied in the pre-weanling rat. Previous studies demonstrated that 2 hr without food causes a loss of hepatic ODC induction after glucagon or catecholamine injection. Isoproterenol or glucagon administration produced increased hepatic cyclic AMP and tyrosine aminotransferase activity which were not prevented by nutritional deprivation. Blockade of hepatic beta 2 receptors by the selective antagonist ICI 118,551 prevented increased cAMP levels and ODC activity after isoproterenol administration. Blockade of beta 1 receptors by atenolol did not prevent increased cAMP levels or ODC induction by isoproterenol although it did block activation of cardiac ODC. The phosphodiesterase inhibitor RO20-1724 increased hepatic cAMP levels as well as ODC and TAT activities, although the increase in ODC activity was attenuated by nutritional deprivation. RO20-1724 also potentiated the induction of hepatic ODC after glucagon or isoproterenol administration. Administration of 8-bromo cAMP elevated hepatic ODC activity regardless of nutritional status but also elevated serum levels of growth hormone and corticosterone. Hepatic ODC induction by glucagon or beta 2 agonists can be dissociated from changes in cAMP levels during nutritional deprivation.

Influence of plasma hormone levels on various stimulant-induced hepatic DNA synthesis in carbon tetrachloride-intoxicated rats

The effects of insulin, glucagon, isoproterenol and carbachol on the regeneration of injured liver were investigated in rats treated with carbon tetrachloride (CCl4). These agents effectively potentiated hepatic DNA synthesis in rats both at 48 and at 72 hr after CCl4 intoxication. The maximal stimulatory effects of the agents on the synthesis coincided in time with the peak of elevation in basal DNA synthesis following the intoxication. Plasma levels of insulin and triiodothyronine were decreased before the elevation of basal DNA synthesis in CCl4-treated rats. The possible relationship of these changes in plasma hormones to the potentiated effects of the agents on DNA synthesis was examined in rats treated with streptozotocin (STZ) or methylthiouracil (MTU). The agents caused no potentiation in STZ-treated rats. On the other hand, in MTU-treated rats, isoproterenol and carbachol significantly stimulated DNA synthesis, but this was not the case with insulin and glucagon. These results suggest that the pancreatic hormonal, beta-adrenergic and cholinergic stimulations play positive roles in regulating liver regeneration after CCl4 intoxication. Furthermore, the hypothyroid state developed in CCl4-treated rats may provide favorable conditions for the stimulation of DNA synthesis by isoproterenol and carbachol. It is unlikely, however, that insulin deficiency contributes to potentiations in the regenerative responses of the injured liver.

Action of drugs and chemical agents on rat liver regeneration

Chemical agents and drugs of widely differing pharmacological activity have been administered to partially hepatectomized male rats and the extent of liver regeneration ascertained over a period of 10 days. Of the large number investigated, the following fed ad lib in diets supplemented in a basal ration at the weight percentages indicated, proved to be hepatotrophic: anticonvulsants - mephenytoin (0.15), methsuximide (0.15), phensuximide (0.15) and primidone (0.10); benzodiazepines - clobazam (0.10), flurazepam hydrochloride (0.12), halazepam (0.070), oxazepam (0.030) and temazepam (0.10); anti-inflammatory agents - benoxaprofen (0.040), ibuprofen (0.10 and 0.20), naproxen (0.040) and sulindac (0.075); sedatives and hypnotics - ethinamate (0.75), glutethimide (0.075), methaqualone (0.030 and 0.10) and methprylon (0.30) and the analgesic and antipyretic, aminopyrine (0.15); antifungal - griseofulvin (0.50); anti-androgen - cyproterone acetate (0.020 and 0.050); uricosuric - sulfinpyrazone (0.050 and 0.20); skeletal muscle relaxant - chlorzoxazone (0.20); hydrocholeretic - florantyrone (0.30); anti-hypertensive - prazosin hydrochloride (0.010) and the thyroid inhibitor, methimazole (0.025). The feeding of several of the stimulants at the given levels to intact males elicited wet and dry liver enlargement. Most of the current test agents as screened in operated rats, had little effect on the regeneration and in fact, tended to depress the process when higher levels were fed or injected.

A comparison of the binding characteristics of the beta-adrenoceptor antagonists 3H-dihydroalprenolol and 125I-iodocyanopindolol in rat liver

The binding characteristics of 3H-dihydroalprenolol and 125I-iodocyanopindolol have been compared in a particulate fraction from regenerating rat liver. When total 3H-dihydroalprenolol binding and inhibition of total 3H-dihydroalprenolol binding by (-)isoprenaline, (-)alprenolol and (+/-)cyanopindolol was investigated, it was found that all agents were bound to two classes of saturable binding sites. In the inhibition studies, the presence of two binding components was not obvious until the data were transformed into Hofstee plots and these were decomposed, except in the case of (+/-)cyanopindolol. Only (+/-)cyanopindolol was found to distinguish clearly between the two saturable binding sites identified by 3H-dihydroalprenolol, as indicated by a broad plateau in the inhibition curve. When 125I-iodocyanopindolol was used as radioligand, only one saturable binding site was identified, even in the presence of less selective inhibiting ligands. The lower affinity component of 3H-dihydroalprenolol binding could be inhibited by 10 microM phentolamine. However, binding experiments with 3H-prazosin indicated that the lower affinity component was not identical with the alpha-adrenoceptor. Phentolamine did not influence 125I-iodocyanopindolol binding. Thus, due to its higher specific activity and a high degree of selectivity, 125I-iodocyanopindolol appears to be the ligand of choice.

Intrahepatic glucose: a requirement for neonatal ODC induction by specific hormones

We have shown previously that short-term nutritional deprivation causes a tissue-specific loss of liver ornithine decarboxylase (ODC) induction after isoproterenol, phenylephrine, or glucagon administration in rat pups. To examine the role of nutrition in the regulation of hepatic ODC, we tested the ability of intragastric nutrient administration to reverse nutritionally related deficits in the ODC response to hormonal challenge. Intragastric whole milk was effective in restoring ODC induction and accumulation of its immediate product, putrescine, in response to isoproterenol administration. Glucose was shown to mediate this effect by the ability of intragastric skimmed milk, lactose, galactose, or D-glucose to return ODC induction, and the inability of casein, sucrose, fructose, L-glucose, or pyruvate plus lactate to do so. D-Glucose also reestablished ODC induction by phenylephrine and glucagon. Parenteral administration of D-glucose produced results comparable to those obtained after intragastric administration. Isoproterenol induction of ODC was prevented when hepatic glucose uptake was blocked by phlorizin but not by blockade of central nervous system glucose uptake with 2-deoxyglucose. We conclude that intrahepatic glucose is an absolute requirement for hepatic ODC induction by isoproterenol, phenylephrine, or glucagon in preweanling rats.

Predominance of beta-adrenergic over alpha-adrenergic receptor functions involved in phosphorylase activation in liver cells of cholestatic rats

Hepatocytes isolated from normal and cholestatic rats responded to adrenergic agonists and antagonists in a quite different manner. Much greater activation of glycogen phosphorylase was caused by phenylephrine, an alpha-agonist, than by isoproterenol, a beta-agonist, in normal rat hepatocytes, and vice versa in the cholestatic rat cells. Epinephrine activation of phosphorylase was antagonized more efficiently by phenoxybenzamine, an alpha-antagonist, than by propranolol, a beta-antagonist, in normal rats, whereas it was antagonized totally by propranolol but only partially by phenoxybenzamine in cholestatic rat hepatocytes. The number of alpha-adrenergic receptors, measured by [3H]prazosin binding to membranes, as well as alpha-receptor-mediated increases in 32Pi incorporation into phosphatidylinositol and in 45Ca efflux, were reduced in hepatocytes after induction of cholestasis. The reduction of these parameters of alpha-receptor-linked functions was associated with the reciprocal increase in the number of beta-receptors and enhancement of beta-receptor-mediated accumulation of cyclic AMP in cholestatic rat hepatocytes. The affinity of epinephrine for beta-receptors was higher in cholestatic rat cells than in normal rat cells; this difference in affinity was abolished by the addition of guanylylimidodiphosphate, indicating that induction of cholestasis rendered hepatic beta-receptors more tightly coupled to the GTP-binding protein. Thus, the cascade reactions arising from beta-receptors are predominant over those from alpha-receptors, eventually leading to glycogen breakdown in cholestatic rat hepatocytes, principally because of not only the elevated beta to alpha ratio of the membrane receptor density but also the tight coupling of beta-receptors to the adenylate cyclase system via the guanine nucleotide regulatory protein.

Changes in hormone responsiveness and cyclic AMP metabolism in rat hepatocytes during primary culture and effects of supplementing the medium with insulin and dexamethasone

Primary monolayer cultures of rat hepatocytes were used for studies of long-term and acute effects of hormones on the cyclic AMP system. When hepatocyte lysates were assayed at various times after plating of the cells three major changes in the metabolism of cyclic AMP and its regulation were observed: Glucagon-sensitive adenylate cyclase activity gradually declined in culture. In contrast, catecholamine-sensitive activity, being very low in normal adult male rat liver and freshly isolated hepatocytes, showed a strong and rapid increase after seeding of the cells. Concomitantly, there was an early elevation (peak approximately equal to 6 h) and a subsequent decrease in activity of both high-Km and low-Km cyclic AMP phosphodiesterase. These enzymic changes probably explained the finding that in intact cultured cells the cyclic AMP response to glucagon was diminished for 2-24 h after seeding, followed by an increase in the responsiveness to glucagon as well as to adrenergic agents up to 48 h of culture. Supplementation of the culture media with dexamethasone and/or insulin influenced the formation and breakdown of cyclic AMP in the hepatocytes. Insulin added at the time of plating moderately increased the adenylate cyclase activity assayed at 48 h, while dexamethasone had no significant effect. In the presence of dexamethasone, insulin exerted a stronger, and dose-dependent (1 pM - 1 microM), elevation of the adenylate cyclase activity in the lysates, particularly of the glucagon responsiveness. Thus, insulin plus dexamethasone counteracted the loss of glucagon-sensitive adenylate cyclase activity occurring in vitro. Kinetic plots of the cyclic AMP phosphodiesterase activity showed three affinity regions for the substrate. Of these, the two with high and intermediate substrate affinity (Km approximately equal to 1 and approximately equal to 10 microM) were decreased in the dexamethasone-treated cells. Insulin partly prevented this effect of dexamethasone. Accumulation of cyclic AMP in intact cells in response to glucagon or beta-adrenergic agents was strongly increased in cultures pretreated with dexamethasone. The results suggest that insulin and glucocorticoids modulate the effects of glucagon and epinephrine on hepatocytes by exerting long-term influences on the cyclic AMP system.

Mechanisms for the emergence of catecholamine-sensitive adenylate cyclase and beta-adrenergic receptors in cultured hepatocytes. Dependence on protein and RNA synthesis and suppression by isoproterenol

Adult male rat hepatocytes, which normally respond poorly to beta-adrenergic agents, acquire such responsiveness during primary monolayer culture. We here show that the rise in catecholamine-sensitive adenylate cyclase activity in hepatocytes in vitro is closely paralleled by an increase in the ability to bind the beta-adrenoceptor ligand [125I]cyanopindolol. The emergence of beta-adrenergic responsiveness did not require cell attachment or serum. Addition of dexamethasone, insulin, thyroxine or dihydrotestosterone to the cultures, singly or in combination, did not prevent the augmented beta-adrenergic responsiveness. The increase in catecholamine-sensitive adenylate cyclase activity and [125I]cyanopindolol binding could be blocked by cycloheximide or actinomycin D. Exposure of the cultures to isoproterenol at 3-hourly intervals led to a dose-dependent suppression of the rise in isoproterenol-responsive adenylate cyclase and prevented the increase in beta-adrenoceptor binding.

Bidirectional concentration-dependent effects of glucagon and dibutyryl cyclic AMP on DNA synthesis in cultured adult rat hepatocytes

Glucagon and dibutyryl cyclic AMP exerted both stimulatory and inhibitory effects on hepatocyte DNA synthesis when added to primary monolayer cultures in the presence of serum, dexamethasone, insulin and epidermal growth factor. The stimulation occurred at low concentrations of glucagon (1 pM-1 nM) or dibutyryl cyclic AMP (1 nM-1 microM), while the agents inhibited DNA synthesis at higher concentrations (usually glucagon at over 10 nM or dibutyryl cyclic AMP at over 10 microM). The stimulatory effect was stronger at low cell densities (less than 20 X 10(3) hepatocytes/cm2). When the hepatocytes were cultured at higher densities, stimulatory effects were reduced or absent and the inhibition of (hormone-induced) DNA synthesis by a high concentration of glucagon was much more pronounced than at low cell densities. These results indicate dual, bidirectional, effects of cyclic AMP on hepatocyte DNA synthesis.

Adrenergic regulation of glycogenolysis in rat liver after cholestasis. Modulation of the balance between alpha 1 and beta 2 receptors

The effects of extrahepatic cholestasis upon adrenergic regulation of glycogenolysis and upon the numbers of adrenoceptors in rat liver were studied using isolated hepatocytes and plasma membranes, respectively. A 60% decrease in the number of alpha 1 adrenoceptors (285 vs. 680 fmol/mg protein) and a simultaneous 2.7-fold increase in the number of beta adrenergic sites (67 vs. 25 fmol/mg protein) were observed beginning 36 h after bile flow obstruction and persisted for at least 68 h. The reciprocal modification of the numbers of alpha 1 and beta adrenoceptors was accompanied by a change in the manner of stimulation of glycogen phosphorylase by catecholamines in hepatocytes; originally alpha 1 adrenergic in normal rats (phenylephrine Ka = 0.9 microM, isoproterenol Ka = 7.1 microM), the stimulation became predominantly beta adrenergic in cholestatic animals (phenylephrine Ka = 3.7 microM, isoproterenol Ka = 0.06 microM). In normal rats, activation of the enzyme by epinephrine was inhibited by the alpha blocker phentolamine, without inhibition by the beta blocker propranolol. In contrast, propranolol was more effective than phentolamine in cholestatic rat hepatocytes. Modification of the regulation of glycogenolysis after cholestasis did not seem to be secondary to an alteration in the metabolism of thyroid hormones or in the action of glucocorticoids. However, cholestasis provoked a 10-fold increase in the number of hepatic mitoses and in the incorporation of thymidine into liver DNA of cholestatic animals. Similar changes were observed in regenerating livers, following two-thirds hepatectomy. We propose that the changes following extrahepatic cholestasis might, as well, be explained by a regenerative process.

The proliferation response of rat liver parenchymal cells after partial hepatectomy. A methodological study comparing flow cytometry of nuclear DNA content and in vivo and in vitro uptake of thymidine

DNA synthesis in rat hepatocytes, from livers regenerating after 70% hepatectomy, was assessed by flow cytometric determination of nuclear DNA content and by incorporation of [3H]thymidine. Parenchymal liver cells were isolated by collagenase perfusion and low-speed centrifugation. Nuclei from the isolated cells were prepared for flow cytometry by a treatment with detergent, pepsin and RNase, and stained with ethidium bromide. Parallel samples of cells were incubated with [3H]thymidine and analysed for rate of incorporation of radioactivity into DNA and for labelling index determination. The flow cytometric measure of the replicative response, i.e. the presence of cells with S-phase DNA content within the diploid and tetraploid cell populations, was compared with the incorporation of [3H]thymidine. For each of fourteen animals, including two control rats and twelve partially hepatectomized animals killed either before (at 13 hr after hepatectomy), at the onset (16 and 18 hr) or at the peak (24 hr) of regenerating activity, a fairly good correlation was found between the different methods. Satisfactory resolution of the flow cytometric detection of S-phase cells was indicated by a sorting experiment using an Ortho (system 50-H) cell sorter which demonstrated that after [3H]thymidine injection in vivo 88% of the diploid and 84% of the tetraploid S-phase nuclei were labelled, while labelling in the G1-fractions was only 2 and 7%, respectively.

Stimulation by thyrotropin, cholera toxin and dibutyryl cyclic AMP of the multiplication of differentiated thyroid cells in vitro

Primary cultures of dog thyroid cells have been established. The cells originated from follicles and displayed differentiation characteristics of such cells: iodide trapping and organification, responsiveness of iodide organification and cyclic AMP accumulation to thyrotropin (TSH), induction of a two-dimensional follicular structure by TSH. TSH also stimulated the multiplication of these cells. The effect of TSH was detected with concentrations as low as 100 muU/ml and was reproduced with purified TSH. It was reproduced by cholera toxin (10 ng/ml) and dibutyryl cyclic AMP (10(-5) M). The data show that TSH, which stimulates the function of thyroid tissue, in vivo and in vitro, activates the multiplication of differentiated dog-thyroid follicular cells in primary culture, which suggests that this trophic effect is, partly at least, mediated by cyclic AMP.