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N Fausto

Publications and source records attributed to N Fausto.

At least 73 records · Page 4Linked to original sources
PubMed1991

Growth factors in liver development, regeneration and carcinogenesis.

Liver growth during regeneration is controlled by several growth factors which may be involved in the triggering, progression and termination of hepatocyte replication. It is likely that liver regeneration involves both circulating factors and those produced in hepatic tissue during the growth response. TGF alpha is an autocrine stimulator of hepatocyte proliferation which increases transiently in replicating hepatocytes both in vivo and in vitro. Constitutive TGF alpha overexpression in young transgenic mice causes liver hypertrophy and enhanced proliferation that progress to hepatic tumor development in the great majority of animals after 12 months of age. In contrast, HGF is present in normal blood in humans and animals and plasma concentrations increase after partial hepatectomy, liver injury and fulminant hepatic failure. In liver tissue, levels of HGF and its mRNA correlate better with the extent of injury than with the degree of proliferative activity. The factor is produced by nonparenchymal cells and presumably acts on hepatocytes through paracrine or endocrine mechanisms. A transient increase of TGF beta 1 in regenerating liver may promote the formation of extracellular matrix components and signal the end of hepatocyte proliferation. Prolonged overexpression of the factor in nonparenchymal cells causes liver fibrosis both in humans and experimental animals. The liver contains TGF beta 1,2 and 3, all of which inhibit hepatocyte DNA synthesis. Their mRNAs increase in the regenerating liver but with very different kinetics. Despite the enormous progress achieved in understanding the mechanisms that regulate liver regeneration, it is not known whether HGF, TGF alpha and TGF beta interact with each other or with other factors or hormones during the growth process. Further, it remains to be established how the effect of these factors may relate to the sequential changes in proto-oncogene expression that occur after partial hepatectomy.

Animals↗
PubMed1991

Transforming growth factor-beta (TGF-beta) isoforms in rat liver regeneration: messenger RNA expression and activation of latent TGF-beta.

Expression of transforming growth factor-beta s (TGF-beta s) 1-3 was studied in normal liver and during liver regeneration after partial hepatectomy in the rat to determine whether each of these isoforms might be involved in hepatocyte growth in vivo. Expression of the mRNAs for all three TGF-beta isoforms increases in the regenerating liver. In addition, the levels of expression of the mRNAs for several extracellular matrix proteins, including fibronectin, vitronectin, laminin, and collagen, also increase in the regenerating liver. Immunohistochemical staining analysis shows a similar distribution of all three TGF-beta s in normal and regenerating liver; however, in both tissues, the level of expression of TGF-beta 1 is 8- to 10-fold higher than that of TGF-beta 2 as determined by sandwich enzyme-linked immunosorbent assay. Expression of all three TGF-beta mRNAs is restricted to liver nonparenchymal cells. Although hepatocytes from normal and regenerating livers do not synthesize TGF-beta, they are sensitive to inhibition of growth by all three TGF-beta isoforms. Hepatocytes from regenerating livers are capable of activating latent TGF-beta 1 complexes in vitro, whereas normal hepatocytes are not. The different TGF-beta isoforms may function in an inhibitory paracrine mechanism that is activated during liver regeneration and may also regulate the synthesis of extracellular matrix components in the regenerating liver.

Animals↗
PubMed1991

Fetal growth factors as determinants of intrauterine hepatic growth.

To understand mechanisms at the cellular level that may lead to the selective organomegaly seen in fetuses of diabetic mothers, we examined the role of insulin and autocrine-paracrine growth factors in the regulation of hepatic growth in the fetal rat. Analyses of fetal liver from the last one-third of gestation demonstrated the presence of specific mRNAs for the transforming growth factors (TGFs) TGF-alpha and TGF-beta. TGF-alpha, a homologue of epidermal growth factor (EGF), acts through EGF receptors. Levels of mRNA for TGF-alpha increased dramatically postnatally, whereas EGF receptor number increased just before term. In contrast, levels of mRNA for TGF-beta, an inhibitor of epithelial cell growth, were greater in fetal liver than in adult liver, as was TGF-beta-receptor binding. Other analyses demonstrated increases in tyrosine kinase activities of the insulin receptor, EGF receptor, and insulinlike growth factor I receptor as term approached. Proliferation of fetal rat hepatocytes in primary culture did not require mitogens or serum, consistent with production and activity of autocrine-paracrine growth factors. TGF-beta was a potent inhibitor of fetal hepatocyte proliferation in culture, whereas insulin potentiated fetal hepatocyte growth above "mitogen-independent" levels. The regulatory mechanisms controlling fetal hepatic growth involve a complex interaction between stimulatory and inhibitory factors. Growth factor expression, receptor expression, receptor tyrosine kinase activity, and postreceptor signal transmission represent potential loci for insulin action that might be involved in the pathogenesis of fetal macrosomia seen in diabetic pregnancies.

Animals↗
PubMed1991

Oval cell proliferation and the origin of small hepatocytes in liver injury induced by D-galactosamine.

Oval cells may function as facultative liver stem cells and tumor progenitors in liver carcinogenesis. The authors determined whether oval cells proliferate and if small hepatocytes might be generated from epithelial cell progenitors in noncarcinogenic liver injury. The authors found that oval cells similar to those detected in early carcinogenesis proliferate in response to D-galactosamine (GaIN). Oval cells expressed gamma-glutamyl transpeptidase activity, bile duct-type cytokeratins and peanut agglutinin binding. Two unusual types of hepatocytes also appeared after injury: small hepatocytes (less than or equal to 16 microns in diameter) and hepatocytes lining atypical ductlike structures. In situ hybridization studies showed that the fetal form of alphafetoprotein mRNA was expressed by many oval cells, some bile duct cells, and occasional hepatocytes. By following the fate of epithelial cells labeled early after GaIN administration, the authors conclude that duct cells can generate both oval cells and small hepatocytes in response to GaIN.

Animals↗
PubMed1990

Induction of replicative competence ("priming") in normal liver.

We have used a system of nutritional manipulation to investigate whether hepatocytes of the normal liver can be primed for replication in vivo. In this system, rats that are denied protein for 3 days undergo a burst of hepatic DNA synthesis and mitosis when they are refed amino acids, while normally fed or starved rats do not respond. To determine if hepatocytes of protein deprived (PD) rats have been "primed" for replication, we examined changes in protooncogene expression in livers of PD rats to see if they would mimic the pattern of gene expression that is induced early after partial hepatectomy. c-jun, c-myc, and p53 mRNAs were elevated in livers of PD rats, while c-fos and c-ras genes were not expressed. The administration of amino acids to PD rats stimulated hepatic DNA synthesis in a shorter period than is required after partial hepatectomy and induced p53 and c-ras expression. In culture, hepatocytes from PD rats had higher levels of c-myc mRNA, underwent morphological changes more rapidly, and reached maximum rates of DNA synthesis earlier than normal hepatocytes. In both normal and primed hepatocyte cultures, transforming growth factor alpha stimulated DNA synthesis more effectively than epidermal growth factor. We conclude that hepatocytes pass through a priming stage before they proliferate and that replicative competence without DNA synthesis can be induced in hepatocytes in the normal liver.

Amino Acids↗
PubMed1990

Expression of hepatocyte and oval cell antigens in hepatocellular carcinomas produced by oncogene-transfected liver epithelial cells.

We have established an in vivo/in vitro system in which epithelial cells ("oval cells") isolated from livers of rats fed a carcinogenic diet for a very brief period are placed in culture and transfected with an oncogene. Injection s.c. into nude mice of oval cells transfected with the activated c-Ha-ras (EJ oncogene) produces tumors with morphological features of differentiated hepatocellular carcinomas. Using monoclonal antibodies that can recognize hepatocyte, oval cell, and tumor antigens, we investigated the expression of these antigens in oval cells in culture, transfected with either the EJ oncogene or the normal c-Ha-ras allele and in tumors derived from the oncogene-transfected cells. We show that EJ-transfected cells and most particularly the tumors they produce expressed hepatocyte and oval cell antigens not detectable in untransfected cells or cells transfected with the normal c-Ha-ras gene. Furthermore, we found that in cloned tumor cells, the expression of hepatocyte antigens could be induced by changes in culture conditions and was accompanied by a decrease in the expression of oval cell markers. Trabecular hepatocellular carcinomas had higher reactivity toward monoclonal antibodies recognizing hepatocyte antigens while tumors with glandular architecture reacted predominantly with monoclonal antibodies against oval cells. We conclude that, in addition to its tumorigenic effect, the EJ oncogene induced the differentiation of tumor cells toward the hepatocyte lineage. In addition, the data provide further confirmation that oval cells can serve as progenitors of differentiated hepatocellular carcinomas.

Animals↗
PubMed1990

TGF alpha overexpression in transgenic mice induces liver neoplasia and abnormal development of the mammary gland and pancreas.

To define the role of TGF alpha in normal tissue function and in pathogenesis, transgenic mice have been generated bearing a fusion gene consisting of the mouse metallothionein 1 promoter and a human TGF alpha cDNA. In these mice, human TGF alpha RNA and protein are abundant in many tissues and TGF alpha is detectable in blood and urine. The effects of TGF alpha overproduction in transgenic mice are pleiotropic and tissue specific. The liver frequently contains multifocal, well-differentiated hepatocellular carcinomas that express enhanced levels of human TGF alpha RNA. The mammary gland exhibits impeded morphogenetic penetration of epithelial duct cells into the stromal fat pad. The pancreas shows progressive interstitial fibrosis and a florid acinoductular metaplasia, during which acinar cells appear to degranulate, dedifferentiate, and assume characteristics of intercalated or centroacinar duct cells. TGF alpha therefore plays an important role in cellular proliferation, organogenesis, and neoplastic transformation.

Aging↗
PubMed1990

Transforming growth factor receptors in liver regeneration following partial hepatectomy in the rat.

Transforming growth factors alpha and beta (TGF-alpha and TGF-beta) are produced in the liver and appear to play an important role in the regulation of hepatic growth. We investigated changes in receptors for these polypeptide growth factors in regenerating liver after partial hepatectomy in the rat with comparisons to livers from normal and sham-operated animals. In normal rats, binding of 125I-epidermal growth factor (EGF) to liver membranes was fully and competitively displaced by TGF-alpha, indicating that these two growth factors share similar sites on the same hepatic receptor. Scatchard analyses revealed that EGF receptors bound EGF with 4- to 8-fold higher affinity than TGF-alpha. Following partial hepatectomy or sham operation, EGF/TGF-alpha receptor number decreased by 25, 40, and 55% at 12, 24, and 72 h, respectively. In all cases. Scatchard analysis obtained with EGF yielded a linear plot indicating a single population of binding sites with a dissociation constant (Kd) of approximately 0.9 nM. Scatchard analysis of TGF-beta binding showed that liver membranes from sham-operated and normal rats express binding sites with a Kd of approximately 35 pM. In contrast, membranes obtained from 12-, 24-, and 72-h regenerating livers were altered in a manner consistent with uncovering or appearance of a higher affinity site. Affinity labeling of liver plasma membranes with 125I-TGF-beta revealed two predominant proteins with Mr 85,000 and 66,000. In unfractionated membrane preparations, two other proteins with Mr 105,000 and approximately 150,000 were seen. Following partial hepatectomy the major change in affinity-labeled proteins was a small (10-25%) but consistent decrease in the Mr 85,000 species. These results show that receptors for TGF-alpha and TGF-beta are modulated after partial hepatectomy, a further indication that these polypeptides may have an important role in liver regeneration.

Animals↗
PubMed1990

c-myc, c-fos, and c-jun regulation in the regenerating livers of normal and H-2K/c-myc transgenic mice.

We investigated the mechanisms of regulation of c-myc, c-fos, and c-jun at the early stages of liver regeneration in mice. We show that the transient increase in steady-state levels of c-myc mRNA at the start of liver regeneration is most probably regulated by posttranscriptional mechanisms. Although there was a marked increase in c-myc transcriptional initiation shortly after partial hepatectomy, a block in elongation prevented the completion of most transcripts. To gain further information on the mechanism of regulation of c-myc expression during liver regeneration, we used transgenic mice harboring the human c-myc gene driven by the H-2K promoter. In these animals, the murine c-myc responded to the growth stimulus generated by partial hepatectomy, whereas the expression of the transgene was constitutive and did not change in the regenerating liver. However, the mRNA from both genes increased markedly after cycloheximide injection, suggesting that the regulation of c-myc mRNA abundance in the regenerating liver differs from that occurring after protein synthesis inhibition. Furthermore, we show that in normal mice c-fos and c-jun mRNA levels and transcriptional rates increase within 30 min after partial hepatectomy. c-fos transcriptional elongation was restricted in nongrowing liver, but the block was partially relieved in the regenerating liver. Nevertheless, for both c-fos and c-jun, changes in steady-state mRNA detected after partial hepatectomy were much greater than the transcriptional increase. In the regenerating liver of H-2K/c-myc mice, c-fos and c-jun expression was diminished, whereas mouse c-myc expression was enhanced in comparison with that in nontransgenic animals.

Animals↗
PubMed1990

Transforming growth factor beta 1 in liver carcinogenesis: messenger RNA expression and growth effects.

Transforming growth factor beta 1 (TGF-beta 1) is a potent inhibitor of hepatocyte proliferation. Since loss of sensitivity to growth inhibition is thought to contribute to the development of neoplasia, we analyzed the expression of TGF-beta 1 mRNA during hepatocarcinogenesis in vivo and in cultured liver epithelial cells (oval cells) obtained from carcinogen-treated animals. We found that TGF-beta 1 mRNA increases in the liver during carcinogenesis and that, at the early stages of the process, oval cells but not hepatocytes contain the growth factor mRNA. Moreover, immortalized, nontumorigenic oval cells (LE/6 cell line) continued to produce TGF-beta 1 mRNA in culture. TGF-beta 1 message markedly decreased upon cell transformation, but message levels, although generally low, were variable in various tumor cell clones. A consistent feature of the tumorigenic cell lines was a loss of sensitivity to TGF-beta 1 growth inhibition. Tumor cells could bind TGF-beta 1 with similar capacity as normal cells and had the same type of receptors (Mr 280,000, 85,000, and 65,000) capable of binding iodinated TGF-beta 1, suggesting that the loss of sensitivity to TGF-beta 1 in transformed liver epithelial cells involves postreceptor mechanisms. Further studies showed that c-myc is not a target for TGF-beta 1 in liver epithelial cells and that TGF-beta 1 no longer induces fibronectin mRNA in transformed cells. The data presented are consistent with the hypothesis that TGF-beta 1 secreted during liver carcinogenesis may inhibit the proliferation of normal cells while providing a selective advantage for the growth of cells that are "partially transformed" and are unresponsive to the factor.

Animals↗
PubMed1989

Production of hepatocellular carcinoma by oval cells: cell cycle expression of c-myc and p53 at different stages of oval cell transformation.

In rats maintained on a carcinogenic diet (choline deficient containing 0.1% ethionine), the levels of c-myc and p53 mRNAs increased by 4 wk after animals were placed on the diet. Cell isolation studies showed that the change in c-myc takes place in oval cells, while p53 increases predominantly in oval cells but also in hepatocytes. To determine whether this increase is a consequence of cell proliferation or is associated with transformation, we have developed an in vitro model of hepatocarcinogenesis using epithelial cells isolated from the livers of rats fed the carcinogenic diet. When maintained in vitro with infrequent subculture, this cell line (LE/6) undergoes spontaneous transformation. Inoculation s.c. of the transformed cells into nude mice yields tumors histologically identified as hepatocellular carcinoma. We have used these cell lines to compare the cell cycle expression of c-myc and p53 mRNAs in untransformed, partially transformed, and tumorigenic LE/6 cells. We find that the expression of both genes is under cell cycle control in untransformed and partially transformed cells. However, complete transformation of this cell line is associated with constitutive expression of myc but not p53 transcripts. On the basis of this work we suggest that constitutive expression of c-myc may be a late event in hepatocarcinogenesis.

Animals↗