Partial purification and characterization of hepatocyte growth factor from serum of hepatectomized rats

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.

A novel growth factor in rat spleen which promotes proliferation of hepatocytes in primary culture

We have identified a factor from adult rat spleen which stimulates the proliferation of rat hepatocytes. The activity was found in the spleen soluble matrix fraction (1,300xg supernatant of inter-cellular fraction). No activity was found in the spleen cell homogenate, in the spleen insoluble matrix fraction or rat serum. After 4 days of incubation with the spleen factor, the cell number increased 4-fold higher than that at inoculation. The growth stimulation were observed in both fetal bovine serum supplemented medium and hormonally defined medium which contains insulin, epidermal growth factor, glucagon, growth hormone and prolactin. The level of activity in the spleen soluble matrix was not affected by partial hepatectomy or trypsinization. These data indicate that the spleen factor is different from previously characterized effectors of hepatocyte proliferation. The novel factor has been named spleen derived growth factor (SDGF).

Purification and partial characterization of hepatocyte growth factor from plasma of a patient with fulminant hepatic failure

Human hepatocyte growth factor (hHGF) has been purified approximately 209,000-fold with 18% yield from plasma of a patient with fulminant hepatic failure. The purification involves heat treatment of plasma, ammonium sulfate precipitation, and chromatography on Affi-Gel Blue, heparin-Sepharose, and hydroxylapatite. Purified hHGF shows several bands with molecular weights between 76,000 and 92,000. Each band shows growth-stimulating activity on cultured hepatocytes which is proportional to the intensity of the band. After reduction of the sample with 2-mercaptoethanol, SDS-PAGE yields two chains with molecular weights of 31,500-34,500 and 54,000-65,000. The effect of hHGF on DNA synthesis by hepatocytes is half-maximal at 3.5 ng/ml. hHGF stimulates proliferation of cultured hepatocytes more effectively than human epidermal growth factor (hEGF) or insulin, and the effect of hHGF is additive or synergistic with the maximal effects of hEGF and insulin. These results suggest that hHGF is a new growth factor which is different from hEGF.

Partial purification and characterization of hepatocyte proliferation stimulatory factor from liver of rats treated with D-galactosamine

The partial purification and characterization of a hepatocyte proliferation stimulatory factor (HPSF) isolated from the liver of D-galactosamine-treated rats are described. The HPSF was a heat-labile, acid-stable and trypsin-sensitive protein. The partially purified HPSF stimulated DNA synthesis and increased the labeling index of parenchymal hepatocytes at 5 micrograms/ml and maximally at 50 micrograms/ml. The effect of HPSF in stimulating DNA synthesis was synergistic with that of insulin plus epidermal growth factor (EGF). The HPSF was scarcely detected in normal rat liver. The results obtained indicate that this HPSF is distinct from insulin, multiplication-stimulating activity (MSA), EGF and other hepatocyte growth factors previously reported, and suggest a plausible role for HPSF in the regeneration of liver tissue following hepatotoxic damage.

Liver-specific growth factors

Experimental evidence of the existence of liver-specific growth factors has been collected for more than two decades. Blood-borne growth-promoting activity of hepatocytes may be separated into plasma and platelet-derived factors. Several groups have observed the stimulation of hepatocyte growth in vitro by some platelet-associated activity, which was recently isolated from rat platelets as a 27-kDa protein called platelet growth factor (PGF). There is evidence of at least two different growth factors for hepatocytes derived from platelet-poor rat plasma, 'hepatopoietin' A and B. The partial purification of several other factors has been reported. One of these factors was prepared from the plasma of patients with fulminant hepatic failure. In addition to these 'humoral' factors, cytosolic growth-promoting activity has been partially purified by several groups. While the humoral factors described so far are only active on normal hepatocytes, the cytosolic 'hepatic stimulator substance' (HSS) also promotes the proliferation of differentiated hepatoma cells. In addition, it appears to depend on the permissive action of epidermal growth factor (EGF). None of the liver-specific growth factors except PGF has been purified to homogeneity. Thus, their significance for the control of the proliferation of normal and transformed hepatocytes is still an unsettled issue.

Characterization of a hepatic proliferation inhibitor (HPI): effect of HPI on the growth of normal liver cells--comparison with transforming growth factor beta

Improvements in the purification of a hepatic proliferation inhibitor (HPI) from adult rat liver have yielded a product that has an inhibitory activity 1,000-fold greater than previously reported. The growth inhibitory activity, which could be eluted from SDS-PAGE at 17-19 kilodaltons (kD), was compared to that of transforming growth factor beta (TGF-beta). The ID50 of the HPI preparation in Fischer rat liver epithelial cells was 50 pg/ml (2.5 pM) compared to a value of 260 pg/ml (10.4 pM) obtained for pure human TGF-beta. Both inhibitors also modulated the stimulation of DNA synthesis in primary hepatocytes by either epidermal growth factor or a growth stimulatory activity prepared from serum of hepatectomized rats. The ID50s of HPI and TGF-beta in these cells were 250 pg/ml and 40 pg/ml, respectively. In contrast to TGF-beta the growth inhibitory activity of HPI was unaltered in the presence of an antibody raised against TGF-beta. The possible mechanism of action of HPI is discussed.

Purification and subunit structure of hepatocyte growth factor from rat platelets

A hepatocyte growth factor (HGF) that stimulates DNA synthesis of adult rat hepatocytes in primary culture was purified as a homogeneous material from platelets of 1000 rats by a four-step procedure: stimulation of its release from platelets by thrombin, cation-exchanger fast protein liquid chromatography (FPLC) on a Mono S column, heparin-Sepharose CL-6B chromatography, and reverse-phase HPLC on a C4 column. The purified HGF stimulated DNA synthesis of adult rat hepatocytes in primary culture at 1 ng/ml and was maximally effective at 5 ng/ml, being about twice as potent as EGF at this concentration. HGF did not stimulate DNA synthesis of Swiss 3T3 cells. It was found to be a heat- and acid-labile protein that was inactivated by reduction with dithiothreitol. The purified HGF had a molecular mass of 82 kDa, as estimated by SDS-PAGE, and was found to be a heterodimer which dissociated into a large subunit of 69 kDa and a small one of 34 kDa by SDS-PAGE under reducing conditions. These biological and chemical properties showed that HGF was not identical with any known growth factors, including platelet-derived growth factor (PDGF).

Mitogenic activity in platelet-poor plasma from rats with persistent liver nodules or liver cancer

Platelet-poor plasma (PPP) from F-344 rats with chemically-induced preneoplastic liver nodules or hepatocellular carcinoma stimulated S-phase DNA synthesis in monolayer cultures of normal rat hepatocytes. Similar mitogenic activity was detected in PPP 6 hrs to 1 week after partial hepatectomy (PH) or after necrotizing doses of CCl4 or diethylnitrosamine (DENA). Very little activity was found in PPP4 from control rats. The mitogenic activity in PPP from animals with nodules was non-dialyzable (greater than 14 kd) and bound to a heparin-sepharose affinity column. None of the mitogenic PPPs competed with [125I] epidermal growth factor (EGF) for binding sites on A431 cells or normal rat hepatocytes. These studies indicate that persistent proliferation of preneoplastic and neoplastic hepatocytes is associated with increased circulating levels of mitogenic hepatocyte growth factor.

Stimulation of growth of primary cultured adult rat hepatocytes without growth factors by coculture with nonparenchymal liver cells

DNA synthesis of adult rat parenchymal hepatocytes alone in primary culture can be stimulated only by the addition of humoral growth factors to the culture medium. However, when parenchymal hepatocytes were cocultured with nonparenchymal liver cells from adult rats, their DNA synthesis was markedly stimulated in the absence of added growth factors or calf serum. DNA synthesis of parenchymal hepatocytes was not stimulated by conditioned medium from nonparenchymal liver cells and was greatest when the parenchymal cells were plated on 24-h cultures of nonparenchymal liver cells. A dead feeder layer of nonparenchymal cells was almost as effective as a feeder layer of viable nonparenchymal cells. These results suggest that the stimulation of DNA synthesis in parenchymal hepatocytes was not due to some soluble factors secreted by nonparenchymal liver cells but to an insoluble material(s) produced by the nonparenchymal liver cells. This insoluble material(s) was collagenase- and acid-sensitive, suggesting that it was a protein containing collagen. The effect of nonparenchymal liver cells was specific: coculture with hepatoma cells, liver epithelial cells, or Swiss 3T3 cells did not stimulate DNA synthesis in parenchymal hepatocytes. Added insulin and epidermal growth factor showed additive effects with nonparenchymal cells in the cocultures. These results suggest that DNA synthesis in parenchymal hepatocytes is stimulated not only by various humoral growth factors but also by cell-cell interaction between parenchymal and nonparenchymal hepatocytes, possibly endothelial cells. This cell-cell interaction may be important in repair of liver damage and liver regeneration.

Effect of histamine H2-receptor antagonists on DNA synthesis in adult rat hepatocytes in primary culture. Cimetidine enhanced hepatocytes proliferation stimulated with insulin and epidermal growth factor

The effects of three of the most widely used histamine H2-receptor antagonists, cimetidine, ranitidine and famotidine, on liver cell growth were studied in vitro using adult rat hepatocytes in primary culture, because these antagonists are commonly given to patients with hepatic cirrhosis or fulminant hepatic failure for protection against peptic ulcers and gastrointestinal hemorrhage. At their clinically effective concentrations in the blood (0.5-5 micrograms/ml cimetidine, 0.25-2.5 micrograms/ml ranitidine and 0.05-0.5 microgram/ml famotidine), these three antagonists did not have any effect on replicative DNA synthesis either in the presence or absence of insulin plus epidermal growth factor (EGF). However, unexpectedly DNA synthesis stimulated by insulin and EGF was found to be enhanced by 0.05-0.5 mg/ml cimetidine, although it was unaffected or inhibited by ranitidine and famotidine at the concentrations tested. Cimetidine caused maximal enhancement of 1.5-2 times the control level of DNA synthesis at a concentration of 0.25 mg/ml. Cimetidine also had an enhancing effect at submaximal concentrations of insulin and EGF, but neither cimetidine nor the other antagonists had any stimulatory effect on DNA synthesis in the absence of insulin plus EGF. This enhancement of DNA synthesis by cimetidine resulted in significant increase in the total DNA content of the hepatocytes in culture. Under the conditions used, cimetidine had the lowest toxicity of these three antagonists and ranitidine the highest, as judged from data on DNA synthesis and the total protein content of cultured hepatocytes, leakage of aminotransferases from the cells and morphological observations.

A liver DNA synthesis promoter induced in rat plasma by injection of dimethylnitrosamine (DMNA) or thioacetamide

The appearance of a liver DNA synthesis promoter (HP) in rat plasma after dimethylnitrosamine (DMNA) or thioacetamide injection was investigated. After 48 h, DMNA (30 mg kg-1 body weight) produced liver (centrilobular) necrosis and intense hepatic regeneration, as assessed by microscopic observations of liver slices, as well as augmented transaminase levels; HP was detectable under these conditions. After 5 days, transaminases and HP returned to normal values (the latter undetectable), coinciding with a lack of necrotic zones. At 60 mg DMNA kg-1 body weight, necrotic areas were more marked and transaminases and HP levels higher after 48 h than with the lower dose; these increases were even more pronounced at 90 mg DMNA kg-1 body weight. After thioacetamide injection (200 mg kg-1 body wt) the situation at 48 h was very similar, with focal, centrilobular necrosis, frequent regenerative signs, high transaminases and detectable HP. Rats recovered after 7 days in a similar fashion as with DMNA. At 400 mg thioacetamide kg-1 body weight, necrotic areas and regeneration zones were more widespread and transaminases and HP higher after 48 h than with the lower dose. On account of the differing modes of action of DMNA and thioacetamide in rat liver, it is proposed that the appearance of HP activity in plasma could be related to the regenerative process that follows hepatotoxic damage.

Biological efficacy of partial hepatectomy and hepatopoietin in long-term selenium-deficient mice

After partial hepatectomy the normal low proliferation rate of hepatocytes increases dramatically. This is based on a feed-back system whose central link is a liver cell proliferation hormone, the so-called hepatopoietin. This glycoprotein is organ-specific but not species-specific, i.e. an extract from rats is also active in mice. In order to examine the influence of selenium on liver cell proliferation, male albino NMRI mice were fed a selenium-deficient diet containing less than 10 ppb Se for at least 2 months (Se-). In the plasma protein profile and in the basic DNA synthesis rate of Se(-)-animals, no significant changes were observed compared to controls. However, liver cell proliferation induced by hepatopoietin or by partial hepatectomy was increased about 3-fold in Se-deficient mice. We assume a compensated metabolic Se-deficiency state in mice under these nutritional conditions, which leads to expression of enhanced metabolic capacity when induced by stress.

Transplanted isolated hepatocytes: effect of partial hepatectomy on proliferation of long-term syngeneic implants in rat spleen

Proliferation of isolated hepatocytes in long-term splenic implants was assessed by flash labelling with a 1 pulse of tritiated thymidine (3H TdR). Cell kinetics showed that the basal labelling index was 0.9% which was greater than normal non-regenerating liver. Twenty four hours following partial hepatectomy the labelling index was 2.0%, a significant rise. These results suggest that hepatocytes transplanted to the spleen constitute a suitable model for screening putative hepatotrophic factors, and are of relevance in establishing clinically useful models of hepatocyte transplantation.

Interaction of induction, ontogenetic development and liver regeneration on the monooxygenase level

2/3 Hepatectomy (HX), but also sham operation (SO) decreased liver dry mass for the first 3-4 days after operation both in 10- and 60-day-old male Wistar rats, whereas body mass development was not altered. Both SO and HX decreased ethylmorphine N-demethylation (EN) and ethoxycoumarin O-deethylation (EO), but distinctly more pronounced after HX. In 10-day-old rats EN and EO were in the normal range 14 days after SO and HX. After 3 weeks EN even exceeded the control level. In 60-day-old rats SO alone resulted in decreased EN and EO even 3 weeks after operation. Compared with SO-controls EN was lower but EO was in the same range in HX rats 14 days after operation. Neither in 10- nor in 60-day-old rats inducibility by phenobarbital (PB) could be demonstrated 24 h after HX with cytochrome P-450 concentration (P-450), EN and EO as parameters. But inducibility by betanaphthoflavone (BNF) was significant for P-450, EN and EO in 10-day-old rats and also for P-450 and EO in 60-day-old rats. In the young adult rats EN was depressed by BNF. In rats 10 days old at HX inducibility by PB was highest 3-7 days after HX, but inducibility by BNF after 14 days. In 60-day-old rats inducibility by PB was highest for EO 1 day after HX. These results indicate that ontogenetic development can be enhanced by induction, that during development regeneration can be superimposed and both induction and regeneration can superimpose ontogenetic development. During these processes different monooxygenases are under differential control.

Partial purification and characterization of masking protein for beta-type transforming growth factor from rat platelets

beta-Transforming growth factor (TGF-beta) is stored in platelets and secreted as a high molecular weight latent form associated with a carrier protein of about 440 KD. This carrier protein could be separated from TGF-beta in 1 N acetic acid and could again mask the activity of TGF-beta under neutral conditions. Therefore, it was named the masking protein of TGF-beta. The masking protein was separated from TGF-beta by gel filtration on a Sephacryl S-300 column or by anion-exchanger FPLC on a Mono Q column in the presence of 6 M urea. Partially purified masking protein from rat platelets neutralized the activity of TGF-beta dose-dependently and was effective at 0.3 microgram/ml. This masking protein could also mask the activity of human TGF-beta, suggesting that it was not species specific. The masking protein was a heat- and acid-stable protein, but was inactivated by treatment with dithiothreitol. The Physiological role of the masking protein in the mechanisms of wound healing and liver regeneration is discussed.

Purification and characterization of a growth factor from rat platelets for mature parenchymal hepatocytes in primary cultures

A growth factor (HGF) stimulating DNA synthesis of adult rat hepatocytes in primary culture was found in rat platelets. HGF was purified from rat platelets to homogeneity by a three-step procedure: stimulation of its release from platelets by thrombin, cation-exchanger fast protein liquid chromatography on a Mono S column, and heparin-Sepharose chromatography. HGF was clearly distinguishable from the platelet-derived growth factor (PDGF) by fast protein liquid chromatography. HGF was a heat- and acid-labile cationic protein that was inactivated by reduction with dithiothreitol. Its molecular mass was estimated to be 27 kDa by NaDodSO4/PAGE and its amino acid composition was very different from that of PDGF. The purified HGF stimulated DNA synthesis in adult rat hepatocytes at 2 ng/ml and was maximally effective at 20 ng/ml; its effect was additive or synergistic with those of insulin and EGF, depending on their combinations. HGF did not stimulate DNA synthesis of Swiss 3T3 cells, while PDGF did not stimulate that of hepatocytes. Thus, HGF showed clearly different cell specificity from PDGF in its growth-promoting activities. These findings indicate that HGF is a growth factor in platelets for mature hepatocytes.

Human hepatocyte growth factor in plasma from patients with fulminant hepatic failure

Plasma from patients with fulminant hepatic failure obtained during plasma exchange therapy, like their serum, demonstrated marked stimulatory activity on DNA synthesis in cultured rat hepatocytes. Heat treatment at 56 degrees C for 30 min did not affect this activity of the plasma, but reduced that of the serum. This growth-promoting activity was confirmed by showing that the patients' serum and plasma increased the labeling index with [3H]thymidine and the total number of nuclei in hepatocyte cultures. The activity of pooled active fractions obtained by gel filtration of the heated plasma was lost completely on heat treatment at 80 degrees C for 10 min or on treatment with trypsin or chymotrypsin, which suggests that it was due to a protein. The human hepatocyte growth factor was purified about 600-fold from heated plasma of a patient by ammonium sulfate precipitation and chromatographies on Affi-Gel Blue and hydroxylapatite. The maximum effect of this partially purified factor on DNA synthesis in cultured hepatocytes was greater than that of epidermal growth factor. The molecular weight of the hepatocyte growth factor was about 85,000 as determined by SDS-PAGE.

Human serum does contain a high molecular weight hepatocyte growth factor: studies pre- and post-hepatic resection

Levels of a high molecular weight hepatotrophin were measured in human serum taken from patients before and 24 hours after undergoing major hepatic resection. In in-vitro rat hepatocyte cultures a 'hepatotrophin' enriched fraction of human serum induced the incorporation of tritiated thymidine into DNA in both pre and post-operative patients. Levels after hepatic resection were 2-3 fold higher than those achieved at the same protein concentration before operation in the same patient. The hepatotrophic factor had an apparent molecular weight of approximately 150,000 daltons, and was an anionic protein.

Prolactin administration stimulates rat hepatic DNA synthesis

Prolactin is an important growth modulatory hormone in fetal and adult tissues. Its administration stimulates enzymatic markers of the G1 phase of cell cycle in rat liver and other tissues. To determine the effects of prolactin administration on hepatic DNA synthesis (S phase), rats received prolactin at 12 hour intervals for 48 hours and DNA synthesis was assessed by [3H]-thymidine incorporation. Prolactin administration stimulated DNA synthesis 2-4 fold above controls in the livers of adult and weanling animals. Increased incorporation of radiolabel was associated with the nucleus of hepatoparenchymal cells. These data support the hypothesis that prolactin may be a physiological regulator of hepatic DNA synthesis. Further, since stress stimulates prolactin secretion, we suggest that prolactin may participate in the hepatic compensatory hyperplasia elicited by the stress associated with partial hepatectomy.

DNA synthesis in exocrine and endocrine pancreas after partial hepatectomy in Syrian golden hamsters

3H-thymidine autoradiography showed an enhanced DNA synthesis in acinar and islet cells of pancreas after partial hepatectomy in syrian golden hamsters. A significant nuclear labeling index of acinar cells was observed between 48 and 84 h and reached control levels by 120 h. An increased labeling index of islet cells was also observed, however, this increase was not statistically significant. These results indicate growth factor(s) produced after partial hepatectomy is capable of inducing DNA synthesis in pancreas.

Molecular biology of liver regeneration

Liver regeneration is a good system for studying cell proliferation in an in vivo, physiologically controlled situation. Various hepatotrophic factors, neuromediators, hormones and growth factors, presumably acting in synergy, seem necessary to induce the switch from quiescence to proliferation. As a consequence of this activation, a number of changes occurs in the hepatocyte: modifications of the plasma membrane proteins; metabolic changes such as variations in albumin and fibrinogen concentrations, and induction of the acute phase proteins; induction of several specific mRNAs; variations in cAMP concentrations, and consequently in the activity of protein kinases and several other enzymes; modifications in chromosomal proteins; induction of proteins involved in DNA replication. A model has been constructed which is more a basis for reflexion than a theoretical model. It takes into account the possible connections between the different molecular events cited above. It is hypothesized that DNA replication is at least partly uncoupled from mitosis, and that the initial events of the proliferative response may be triggered by nutritional elements.

Two types of growth inhibitor in rat platelets for primary cultured rat hepatocytes

Rat platelets contain two types of growth inhibitor of adult rat hepatocytes in primary culture. One, named platelet derived growth inhibitor (PDGI)-alpha, is a heat- and acid-labile protein with a molecular weight of over 200 KD that is not released on thrombin treatment. The other, named PDGI-beta, is a heat- and acid-stable factor with a molecular weight of 24 KD that is released by thrombin. Both PDGI-alpha and -beta were inactivated by treatment with dithiothreitol. They both caused dose-dependent inhibition of DNA synthesis stimulated by insulin plus epidermal growth factor. These inhibitions were closely correlated with marked decrease in the labeling index. Neither PDGI-alpha nor -beta had a cytotoxic effect as judged by phase-contrast microscopic examination of the cells nor inhibition of protein synthesis. The properties of PDGI-beta suggest that it may be identical with transforming growth factor-beta. These results indicate that rat platelets contain not only a growth factor (HGF), but also growth inhibitors that affect adult rat hepatocytes.

Inhibitory effect of transforming growth factor-beta on DNA synthesis of adult rat hepatocytes in primary culture

A transforming growth factor-beta (TGF-beta) found in platelets strongly inhibited DNA synthesis of adult rat hepatocytes in primary culture stimulated by insulin plus EGF or by hepatocyte growth factor (HGF) from rat platelets, but not the syntheses of secretory and intracellular proteins by the cells. TGF-beta had no cytotoxic effect, as judged by phase-contrast microscopic examination of the cell morphology. The inhibition of DNA synthesis by TGF-beta was correlated with marked decrease in the labeling index. TGF-beta did not inhibit growth of hepatoma cell line. These findings indicate that TGF-beta is a strong growth inhibitor of adult rat hepatocytes and may block their shift from the G1 phase to the S phase. The physiological role of TGF-beta in inhibiting growth of adult hepatocytes during liver regeneration is discussed.

The characterization and partial purification of hepatocyte proliferation factor

This report presents further evidence that the liver is the source of the factor responsible for the initiation and/or stimulation of hepatic regeneration. Initial experiments for the isolation and characterization of the active factor are presented. The factor was isolated from the cytosol of regenerating livers (RLC). After an in vivo exposure to RLC, hepatocytes were pulsed in vitro with 3H-thymidine to measure DNA synthesis. Rat and porcine RLC stimulated DNA synthesis in hepatocytes isolated from growing (nonhepatectomized) livers of weanling rats, or from regenerating livers of adult rats. The ability of porcine RLC to stimulate hepatocyte DNA synthesis demonstrated that the factor responsible was not species-specific. In contrast, normal non-regenerating liver cytosol did not stimulate hepatocyte DNA synthesis. Further experiments also revealed that the factor is heat stable. The activity responsible for the increased DNA synthesis was called hepatocyte proliferation factor (HPF). The assay for detecting HPF activity in the nonhepatectomized recipient will facilitate further characterization and purification of HPF.

In vitro growth of adult amphibian (Xenopus laevis) hepatocytes and characterization of hepatocyte-proliferating activity in homologous serum

Adult frog (Xenopus laevis) hepatocytes were found to proliferate in a culture medium containing adult homologous serum. Insulin and dexamethasone were required for a net proliferation of hepatocytes. Dose-response analysis showed that a low concentration of serum (greater than or equal to 0.5%) was enough to induce DNA synthesis and mitosis, but a higher concentration (5%) caused certain necrotic changes. Under optimal conditions, there was a two- to threefold increase in nuclei per culture 10 days after serum treatment. Heterologous sera (fetal bovine, calf and chick) showed less proliferative activity. Based on our results, hepatocyte-proliferating activity in adult frog serum is considered to be heat-unstable and acidic protein(s). Thus, adult frog serum may contain hepatopoietin possibly different from well-known growth factors.