Prereplicative Phase Of Liver Regeneration Research Articles

Altered regulation of EGF receptor signaling following a partial hepatectomy.

We have studied epidermal growth factor receptor (EGFR) phosphorylation and localization in the pre-replicative phase of liver regeneration induced by a 70% partial hepatectomy (PH), and how a PH affects EGFR activation and trafficking. When Western blotting was performed on livers after PH with antibodies raised against activated forms of EGFR autophosphorylation sites, no marked increase in EGFR tyrosine phosphorylation was observed. However, events associated with attenuation of EGFR signals were observed. Two hours after PH, we found increased EGFR ubiquitination and internalization, followed by receptor downregulation. Furthermore, EGFR phosphorylation following an injection of EGF was reduced after PH. This reduction correlated with an increased activation of PKC and a distinct augmentation in the phosphorylation of the PKC-regulated T654-site of EGFR. When primary cultured hepatocytes were treated with tetradecanoylphorbol acetate (TPA) to induce T654-phosphorylation of EGFR, we found colocalization of a fraction of EGFR with EEA1, downregulation of EGF-mediated EGFR autophosphorylation, altered ligand-induced intracellular sorting of EGFR, and increased mitogenic signaling through the EGFR-Ras-Raf-ERK pathway. Further, we found that both TPA and a PH enhanced EGF-induced proliferation of hepatocytes. In conclusion, our results suggest that hepatocyte priming involves modulation of EGFR that enhances its ability to mediate growth factor responses without an increase in its receptor tyrosine kinase-activity. This may be a pre-replicative competence event that increases growth factor effects during G1 progression.

Intracellular trafficking during liver regeneration: Alterations in late endocytic and transcytotic pathways

Background/Aims: Liver growth, induced by partial hepatectomy of the organ is a precisely regulated process during which a radical reorganisation of metabolism occurs as the hepatocytes become committed to enter the cell cycle. Recent studies have shown the importance of the endocytic compartment in the control of lipid and protein intracellular trafficking but also in the control of the signal transduction events, which eventually will trigger the initiation of DNA synthesis and the subsequent cell division. Methods: We isolated endosomes at different times after partial hepatectomy in male rats and compared with endosomes isolated from sham-operated animals. Also, bile was collected and analysed by 2D-gel electrophoresis. Results: The amount of late endosomes isolated from regenerating livers decreased, concomitant with decreased cathepsin D specific enzyme activity. Furthermore, secretion of horseradish peroxidase, pIgA and transferrin increased in the pre-replicative phase of liver regeneration. Conclusions: At the early stages of liver regeneration, the hepatocellular transport pathway towards degradation (late endosomes and lysosomal pathway) decreases, but the transcytosis and the bile secretion of several major proteins increases.

Changes in mitochondrial adenine nucleotides and in permeability transition in two models of rat liver regeneration

Although enhanced phosphorylative activity can be a requisite for later DNA synthesis during liver regeneration (LR), mitochondrial generation of reactive oxygen species could lead to altered mitochondrial membrane permeability during the prereplicative phase of LR. Therefore, the role of mitochondrial permeability transition (MPT) was evaluated during rat LR, induced by either partial hepatectomy (PH) or after CCl 4 administration. Parameters indicative of mitochondrial function and membrane potentials, those of oxidative stress, and in vivo changes of the intramitochondrial pool of adenine nucleotides were determined. Twelve hours after PH, mitochondrial oxidative and phosphorylative activities and adenosine diphosphate (ADP) content were increased, reaching a maximal peak at 24 hours after surgery (maximal DNA synthesis). Parameters suggestive of oxidant stress were enhanced, but mitochondrial volume and membrane electrical potential remained unaltered. Interestingly, moderate mitochondrial swelling and depolarization were found at later post-PH times (72 hours). In CCl 4-treated animals, it was found that an active liver cell necrosis delayed mitotic activity and mitochondrial uncoupled respiration. Starting 12 hours after CCl 4 intoxication, a drastic increase of inorganic phosphate occurred within swollen and strongly depolarized mitochondria, suggesting changes in the MPT. Despite expression of messenger RNA (mRNA) for mitochondrial transcription, factor A showed a similar time course in both experimental models. The so-called augmenter liver regeneration was found significantly elevated only in PH rats. In conclusion, onset of MPT could be associated with cell necrosis and inflammation after CCl 4 treatment, whereas this mitochondrial event could constitute a putative effector mechanism, through which growth or inflammatory factors inhibiting cell proliferation could initiate LR termination. (H epatology 2003;37:842-851.)

Changes in ultrastructure and the occurrence of permeability transition in mitochondria during rat liver regeneration.

Mitochondrial bioenergetic impairment has been found in the organelles isolated from rat liver during the prereplicative phase of liver regeneration. To gain insight into the mechanism underlying this impairment, we investigated mitochondrial ultrastructure and membrane permeability properties in the course of liver regeneration after partial hepatectomy, with special interest to the role played by Ca2+ in this process. The results show that during the first day after partial hepatectomy, significant changes in the ultrastructure of mitochondria in situ occur. Mitochondrial swelling and release from mitochondria of both glutamate dehydrogenase and aspartate aminotransferase isoenzymes with an increase in the mitochondrial Ca2+ content were also observed. Cyclosporin-A proved to be able to prevent the changes in mitochondrial membrane permeability properties. At 24 h after partial hepatectomy, despite alteration in mitochondrial membrane permeability properties, no release of cytochrome c was found. The ultrastructure of mitochondria, the membrane permeability properties and the Ca2+ content returned to normal values during the replicative phase of liver regeneration. These results suggest that, during the prereplicative phase of liver regeneration, the changes in mitochondrial ultrastructure observed in liver specimens were correlated with Ca2+-induced permeability transition in mitochondria.

Release of matrix proteins from mitochondria to cytosol during the prereplicative phase of liver regeneration

70% partial hepatectomy (PH) in the rat causes a release, into the cytosolic fraction, of mitochondrial matrix proteins, namely the mitochondrial isoform of aspartate aminotransferase (mAAT) and malate dehydrogenase (MDH), during the first 24 h after PH, when no growth of the residual liver is observed. After this time interval, the weight of the liver starts to increase and the normal weight is reached at 96 h after PH. This proliferative phase is characterized by a progressive recovery of the normal levels of intramitochondrial activities of mAAT and MDH. Mitochondria isolated at 24 h after PH show a membrane permeabilization to sucrose accompanied by a release of matrix enzymes; both are blocked by cyclosporin A. These results suggest an alteration of mitochondrial membrane integrity, during the prereplicative phase of liver regeneration, with the occurrence of an increased permeability that allows the passage into the cytosol of matrix enzymes.

Nonparenchymal cells from regenerating rat liver generate interleukin- 1alpha and -1beta: A mechanism of negative regulation of hepatocyte proliferation

Following experimental partial hepatectomy of 70% in the rat, there is a semisynchronized surge of hepatocyte proliferation that ceases after 48 to 72 hours. Little is known about the determinants governing the termination of the proliferative phase, although transforming growth factor (TGF) beta has been implicated as an important inhibitor of hepatocyte replication in this model. We previously reported an additional non-TGF-beta inhibitor in medium conditioned by nonparenchymal cells isolated from regenerating liver (CM-NPC-Reg) between 24 and 48 hours after partial hepatectomy, but it was not found in medium conditioned by nonparenchymal cells from unoperated control liver. CM-NPC-Reg suppressed replicative DNA synthesis of primary rat hepatocytes in response to hepatocyte growth factor (HGF), epidermal growth factor (EGF), or TGF-alpha as assessed by 3H-thymidine incorporation. We now present evidence that interleukin (IL)-1 is the major inhibitor of hepatocyte DNA synthesis present in CM-NPC-Reg. IL-1 receptor antagonist abrogated the inhibition, as did antibodies to rat IL-1alpha and -beta; a combination of both antibodies was required, implicating both IL-1alpha and IL-1beta as active constituents in CM- NPC-Reg. To investigate in vivo changes in IL-1 expression, we assessed expression of IL-1alpha messenger RNA (mRNA) in whole rat liver following partial hepatectomy; mRNA was down-regulated at 10 hours in the pre-replicative phase of liver regeneration and up-regulated at 24 hours and 48 hours when proliferation is waning. Rat hepatocytes isolated from liver 24 hours after partial hepatectomy showed increased sensitivity to the inhibitory action of IL-1. Exogenous IL-1beta, administered parenterally to a group of rats at 0 and 12 hours after partial hepatectomy significantly reduced the incorporation of the thymidine analogue, bromodeoxyuridine (BrdU), into hepatocytes at 18 hours. These data indicate that nonparenchymal cells isolated from regenerating rat liver elaborate IL-1, and support the hypothesis that IL-1 plays a role suppressing hepatocyte proliferation and terminating the surge of DNA synthesis induced after partial hepatectomy. (Hepatology 1997 Jul;26(1):49-58)

Nonparenchymal cells from regenerating rat liver generate interleukin-1? and -1?: A mechanism of negative regulation of hepatocyte proliferation

Following experimental partial hepatectomy of 70% in the rat, there is a semisynchronized surge of hepatocyte proliferation that ceases after 48 to 72 hours. Little is known about the determinants governing the termination of the proliferative phase, although transforming growth factor (TGF) β has been implicated as an important inhibitor of hepatocyte replication in this model. We previously reported an additional non-TGF-β inhibitor in medium conditioned by nonparenchymal cells isolated from regenerating liver (CM-NPC-Reg) between 24 and 48 hours after partial hepatectomy, but it was not found in medium conditioned by nonparenchymal cells from unoperated control liver. CM-NPC-Reg suppressed replicative DNA synthesis of primary rat hepatocytes in response to hepatocyte growth factor (HGF), epidermal growth factor (EGF), or TGF-α as assessed by 3H-thymidine incorporation. We now present evidence that interleukin (IL)-1 is the major inhibitor of hepatocyte DNA synthesis present in CM-NPC-Reg. IL-1 receptor antagonist abrogated the inhibition, as did antibodies to rat IL-1α and -β a combination of both antibodies was required, implicating both IL-1α and IL-1β as active constituents in CM- NPC-Reg. To investigate in vivo changes in IL-1 expression, we assessed expression of IL-1α messenger RNA (mRNA) in whole rat liver following partial hepatectomy; mRNA was down-regulated at 10 hours in the pre-replicative phase of liver regeneration and up-regulated at 24 hours and 48 hours when proliferation is waning. Rat hepatocytes isolated from liver 24 hours after partial hepatectomy showed increased sensitivity to the inhibitory action of IL-1. Exogenous IL-1β, administered parenterally to a group of rats at 0 and 12 hours after partial hepatectomy significantly reduced the incorporation of the thymidine analogue, bromodeoxyuridine (BrdU), into hepatocytes at 18 hours. These data indicate that nonparenchymal cells isolated from regenerating rat liver elaborate IL-1, and support the hypothesis that IL-1 plays a role suppressing hepatocyte proliferation and terminating the surge of DNA synthesis induced after partial hepatectomy.

Gene expression of glucokinase regulatory protein in regenerating rat liver

The activity and messenger RNA (mRNA) levels of glucokinase, and the concentration and mRNA levels of its regulatory protein, were analyzed during liver regeneration. The activity of glucokinase and the concentration of its regulatory protein decreased to 30% and 50%, respectively, after liver resection, remaining low after 1 week. No significant variations in the level of these proteins were found in sham-operated animals. The regulatory protein/glucokinase molar ratio increased during the replicative phase, to a maximum at 48 hours. The mRNA levels of glucokinase and of its regulatory protein decreased rapidly after partial hepatectomy to minimum values at 6 hours (15%) and at 12 hours (4%), respectively, returning to normal values at 24 hours and 168 hours, respectively. Sham-operated animals showed a similar decrease in mRNA levels during the prereplicative phase of liver regeneration, suggesting that the initial effects observed in the gene expression of these proteins were due to surgical stress. During the replicative phase, a specific inhibition of the regulatory protein's gene expression was observed in the regenerating liver. A decrease in the content of regulatory protein and the glucokinase activity, and an increase in the molar ratio of these two proteins correlate with the observed decrease in glycolytic flux, providing further evidence that the phosphorylation of glucose is a control point in the glycolytic/gluconeogenic flux during liver regeneration.(Hepatology 1997 Feb;25(2):324-8)

Gene expression of glucokinase regulatory protein in regenerating rat liver.

The activity and messenger RNA (mRNA) levels of glucokinase, and the concentration and mRNA levels of its regulatory protein, were analyzed during liver regeneration. The activity of glucokinase and the concentration of its regulatory protein decreased to 30% and 50%, respectively, after liver resection, remaining low after 1 week. No significant variations in the level of these proteins were found in sham-operated animals. The regulatory protein/glucokinase molar ratio increased during the replicative phase, to a maximum at 48 hours. The mRNA levels of glucokinase and of its regulatory protein decreased rapidly after partial hepatectomy to minimum values at 6 hours (15%) and at 12 hours (4%), respectively, returning to normal values at 24 hours and 168 hours, respectively. Sham-operated animals showed a similar decrease in mRNA levels during the prereplicative phase of liver regeneration, suggesting that the initial effects observed in the gene expression of these proteins were due to surgical stress. During the replicative phase, a specific inhibition of the regulatory protein's gene expression was observed in the regenerating liver. A decrease in the content of regulatory protein and the glucokinase activity, and an increase in the molar ratio of these two proteins correlate with the observed decrease in glycolytic flux, providing further evidence that the phosphorylation of glucose is a control point in the glycolytic/gluconeogenic flux during liver regeneration.

Cyclin A Is Present in the Endocytic Compartment of Rat Liver Cells and Increases during Liver Regeneration

Recent studies have implicated the cell cycle kinase cdc2 and cyclin A in the inhibition of the fusion of endocytic vesiclesin vitroduring mitosis. However, the presence of cyclins or their associated cyclin dependent kinases (cdks) in the endocytic fractions have not been reported. Using Western-blotting and immunocytochemistry approaches with different anti-cyclin A antibodies we have detected cyclin A in the endocytic compartment of the rat liver. During the pre-replicative phase of liver regeneration the amount of cyclin A in endosomes increases significantly with a peak around 12 hours after partial hepatectomy. Cyclin A-dependent kinases, cdc2 and cdk2, were also found in isolated endosome fractions, showing a distinct kinetics of accumulation during the regenerative period. Finally, histone H1 kinase activity was detected associated with cyclin A in endocytic vesicles and increased in regenerating liver. These results suggest that changes in the organization and in the function of the endocytic compartment during the hepatocellular proliferation may be modulated by proteins involved in the regulation of the cell cycle.

Mitochondrial oxidative phosphorylation and intracellular glutathione compartmentation during rat liver regeneration

The rate of mitochondrial oxidative phosphorylation and the cytosolic and mitochondrial total and oxidized glutathione concentrations were studied in regenerating rat livers after partial (70%) hepatectomy. The rate of mitochondrial oxidative phosphorylation progressively decreased during the early prereplicative phase of liver regeneration. This was accompanied by a progressive decrease in mitochondrial, but not cytosolic, glutathione concentration. Twenty-four hours after partial hepatectomy, both the rate of adenosine triphosphate (ATP) synthesis and the amount of mitochondrial glutathione were depressed by 50% with respect to controls (sham-operated animals). During the second replicative phase, both the oxidative phosphorylation rate and mitochondrial glutathione concentration were recovered; however, the kinetics of the recovery were different, being the total amount of mitochondrial glutathione completely restored 48 hours after partial hepatectomy, whereas 72 hours were needed for the recovery of oxidative phosphorylation. The decrease in the rate of oxidative phosphorylation, during the early phase of liver regeneration, appeared to be secondary to the decreased content of the catalytic subunit beta-F1 of the ATP synthase complex, which in turn was shown to be linearly related to the decrease of intramitochondrial glutathione. These observations suggest that the two phenomena may be due to the previously reported increased free radical production during the early phase of liver regeneration. The depression of mitochondrial glutathione after partial hepatectomy may play a contributory role in structural and functional alterations of mitochondria occurring in the first retrodifferential phase of liver regeneration.

Mitochondrial oxidative phosphorylation and intracellular glutathione compartmentation during rat liver regeneration

The rate of mitochondrial oxidative phosphorylation and the cytosolic and mitochondrial total and oxidized glutathione concentrations were studied in regenerating rat livers after partial (70%) hepatectomy. The rate of mitochondrial oxidative phosphorylation progressively decreased during the early prereplicative phase of liver regeneration. This was accompanied by a progressive decrease in mitochondrial, but not cytosolic, glutathione concentration. Twenty-four hours after partial hepatectomy, both the rate of adenosine triphosphate (ATP) synthesis and the amount of mitochondrial glutathione were depressed by 50% with respect to controls (shamoperated animals). During the second replicative phase, both the oxidative phosphorylation rate and mitochondrial glutathione concentration were recovered; however, the kinetics of the recovery were different, being the total amount of mitochondrial glutathione completely restored 48 hours after partial hepatectomy, whereas 72 hours were needed for the recovery of oxidative phosphorylation. The decrease in the rate of oxidative phosphorylation, during the early phase of liver regeneration, appeared to be secondary to the decreased content of the catalytic subunit β-F 1 of the ATP synthase complex, which in turn was shown to be linearly related to the decrease of intramitochondrial glutathione. These observations suggest that the two phenomena may be due to the previously reported increased free radical production during the early phase of liver regeneration. The depression of mitochondrial glutathione after partial hepatectomy may play a contributory role in structural and functional alterations of mitochondria occurring in the first retrodifferential phase of liver regeneration.

The steady-state level of hepatocyte epidermal growth factor RNA rapidly increases in the pre-replicative phase of liver regeneration

The steady-state level of hepatocyte epidermal growth factor RNA rapidly increases in the pre-replicative phase of liver regeneration

Effect of adrenergic and Ca 2+ antagonists on increased ornithine decarboxylase expression in regenerating rat liver

Partial hepatectomy (PH) (70% resection) causes within 4 hr an accumulation of ornithine decarboxylase (EC 4.1.1.17, ODC) mRNAs concomitant with an increase in ODC activity, maximum values being observed at 8 and 16 hr, respectively. In the early hours of hepatic regeneration, enhancement of transcriptional-rate of ODC gene, demonstrated by nuclear run-on analysis, can account for the accumulation of ODC mRNAs. The involvement of catecholamines in these processes is demonstrated by using prazosin and propranolol, specific antagonists of α 1 and β adrenoceptors, respectively. Prazosin reduces almost completely the rise of ODC activity at 4 hr, without affecting mRNA levels. At 16 hr, enzyme activity and mRNAs increase, however, over the values observed in regenerating liver of prazosin-untreated animals. These findings suggest that α 1-receptor activation triggers positive control signals for ODC gene expression at the early time of liver regeneration and, on the contrary, negative signals at later times by mainly post-transcriptional and transcriptional mechanisms, respectively. Propranolol reduces similarly the initial 4 hr-rise of ODC activity. These results indicate that activation of both α 1- and β-adrenoceptors causes the large increase in ODC activity. Pharmacological manipulation of intracellular Ca 2+ levels by verapamil, a Ca 2+-channel blocker, or neomycin, an inhibitor of Ca 2+ release from endogenous stores, diminishes ODC activity at 4 and 16 hr after PH. ODC mRNA levels, which are not modified at 4 hr, increase over the values of partially hepatectomized rat liver at 16 hr. Trifluoperazine inhibits both ODC activity and mRNA accumulation at the times studied. As a working hypothesis it is proposed that Ca 2+mediated processes induced by catecholamines are involved in ODC gene expression during the prereplicative phase of liver regeneration.

Pretranslational and posttranslational regulation of the EGF receptor during the prereplicative phase of liver regeneration

We studied the regulation of the epidermal growth factor receptor mRNA and the number of epidermal growth factor binding sites in subcellular compartments involved in the biosynthesis and endocytosis of the epidermal growth factor receptor during the prereplicative phase of liver regeneration. The epidermal growth factor receptor mRNA, quantified by solution hybridization, decreased after partial hepatectomy, with a nadir of about 35% 18 hr after hepatectomy. An even stronger decrease in the number of epidermal growth factor binding sites after partial hepatectomy was observed in a Golgi-enriched low-density membrane fraction, reflecting available newly synthesized epidermal growth factor receptors. It is suggested that this decrease in newly synthesized available epidermal growth factor receptors is caused primarily, but not entirely, by decreased epidermal growth factor receptor mRNA levels and the additional down-regulation of epidermal growth factor binding sites may involve posttranslational mechanisms such as intracellular occupation by transforming growth factor-alpha. The observation that the number of specific epidermal growth factor binding sites after partial hepatectomy was only moderately reduced in prelysosomal endosomes and in lysosomes, compared with the newly synthesized receptors, may indicate that a pool of receptors targeted for lysosomes exists and these receptors are regulated in a different manner than the receptor pool targeted for the cell surface. Furthermore, at least two separable endocytic subcompartments are involved in the transport of the epidermal growth factor/epidermal growth factor receptor complex in the liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Similar induction of the hepatic EGF receptor in vivo by EGF and partial hepatectomy

A 2-fold increase in the level of epidermal growth factor (EGF) receptor mRNA accompanied by a similar increase in newly synthesized ligand-binding EGF-receptors was observed 2–4 h after intraportal EGF-injections and 2–4 h after partial hepatectomy. After this initial increase, the EGF-receptor levels decreased back to control levels 6–8 h after EGF-injections and below control levels 6–8 h after partial hepatectomy. EGF was also found to influence the degradation of endocytosed [ 125I]-EGF 3 h after injection to a similar extent as partial hepatectomy. The similar effects of EGF and partial hepatectomy suggests that EGF or EGF-like factors may be mechanistically involved in the early “promotion” stage during the prereplicative phase of liver regeneration. The EGF-induced effects are, however, at least not alone responsible for the replicative process.

The expression of cAMP-dependent protein kinase subunits is differentially regulated during liver regeneration

The levels of the regulatory (RI and RII) and catalytic (C) components of cAMP-dependent protein kinase and of their messages were studied during the first 36 h of liver regeneration after 70% hepatectomy. Both RI alpha mRNA and RII alpha mRNA started to increase 4 h after the resection, reaching peak levels after 9 h. RI mRNA decreased abruptly 9-12 h after resection, whereas RII mRNA stayed elevated. C alpha mRNA was rather constant during the period of study. In accordance with the mRNA data the level of C was constant while RI and RII increased during the prereplicative phase of liver regeneration. RI increased rapidly when its message became elevated. RII, however, increased noticeably only 6-8 h after its mRNA had become elevated. The increased expression of R led to a disproportion between R and C that was most pronounced 14 h after resection, i.e. coinciding with the prereplicative cAMP burst. The increased R/C ratio at that time of regeneration diminished the concentration of active C subunit during the cAMP burst. In that way the otherwise inhibitory effect of high concentrations of active C on the DNA replication may have been decreased. The fractional saturation of RI and RII by endogenous cAMP fluctuated in parallel as a function of liver cAMP levels, although there was a tendency that RI was more highly saturated than RII at high concentrations of cAMP.

Casein kinase 2 activity increases in the prereplicative phase of liver regeneration

Cytosolic casein kinase activity increased up to 2-fold in the first 6 h after partial hepatectomy and then decreased to control values. This increase was due mainly to casein kinase 2, which reached maximal values at 6–8 h of liver regeneration. In contrast, casein kinase 1 showed a smaller increase at 4 h and then started to decrease reaching values of about 70% of control at 16 h. The increase in total casein kinase 2 was accompanied with an activation of the enzyme, as determined by the low/high β-casein activity ratio assay. Administration of an acute dose of glucagon to control rats also increased the activity ratio but failed to cause any rise in total casein kinase 2 activity.

New synthesis of cyclic AMP‐dependent protein kinases during liver regeneration

As has been previously reported one surge in cytosolic calmodulin is produced between 4 and 12 h after a partial hepatectomy. Moreover, a surge in cytosolic cyclic AMP and another in cyclic AMP-dependent protein kinase activity can be detected during the late period of the prereplicative phase of liver regeneration after a partial hepatectomy. It is known that these three surges are involved in triggering DNA synthesis. By kinetic studies and by injecting transcription (actinomycin D) and translation (cycloheximide) inhibitors into hepatectomized rats we have demonstrated that the cyclic AMP-dependent protein kinase surge is produced by new synthesis of the enzyme. In thyroparathyroidectomized rats subjected to a partial hepatectomy the calmodulin surge was similar to that observed in normal hepatectomized rats whereas the surge in cyclic AMP-dependent protein kinase activity was strongly decreased suggesting that the cyclic AMP-dependent protein kinase surge is not generated by the previous surge in calmodulin.

Increased calmodulin synthesis in the pre-replicative phase of rat liver regeneration

A cytosolic calmodulin wave has been described in the pre-replicative phase of rat liver regeneration. Here we demonstrate that this calmodulin increase is reduced by injection of cycloheximide (translation inhibitor) and actinomycin D (transcription inhibitor). In addition, we found that calmodulin mRNA levels are increased during the early pre-replicative period of liver regeneration, and this increase is affected by actinomycin D. Together, these results strongly indicate that the calmodulin wave observed in the pre-replicative phase of liver regeneration is due to de novo synthesis of this protein.