A domain of methylation change at the albumin locus in rat hepatoma cell variants

Cell phenotype, binding affinity and promoter structure modulate transactivation by HNF1 and LAP

To evaluate the importance of the transcription factors known to bind to the albumin promoter as well as the parameters involved in their activity, we have used cotransfections with an albumin promoter-cat plasmid combined with expression vectors driving the expression of cDNAs coding for liver-enriched factors known to interact with this promoter. We describe the characteristics of a set of clones of hepatic origin: well differentiated, partial variants or pleiotropic dedifferentiated variants. These lines have been characterized for the accumulation of RNAs corresponding to each of the albumin promoter-binding factors. Only HNF1, and to a lesser extent C/EBP, show differences depending upon the differentiation state of the cells. Overexpression of exogenous HNF1 in these cells reveals that this factor is able to transactivate the albumin promoter only in variant cells where the endogenous protein is limiting. By contrast, if the HNF1-binding site is of weak affinity, overexpression of exogenous HNF1 stimulates the albumin promoter even in the HNF1-rich differentiated cells. Overexpression of exogenous LAP strongly transactivates an artificial promoter containing one LAP-binding site, but surprisingly in all the cell lines, it has little effect upon the albumin promoter. These results demonstrate that the transactivation potential of a given transcription factor depends on the degree of differentiation of the recipient cells, on the promoter structure, and on the affinity of the binding site for this factor.

Maintenance of liver function in long term culture of hepatocytes following in vitro or in vivo Ha-rasEJ transfection

Collagenase isolated rat hepatocytes were transfected with liposome encapsulated pEJ (LE-pEJ), a plasmid carrying the human cellular activated Ha-rasEJ oncogene. A proliferative cell line was cloned from these cells transfected in vitro. It secreted per day 0.87 micrograms albumin and 0.32 microgram transferrin per 10(6) cells, and 11.06 nmol free and conjugated bile acids (BA) per mg protein. Also, it metabolized 2-acetylaminofluorene (2-AAF) into N- and ring-hydroxylated metabolites and 2-aminofluorene at rates of 1.50, 9.73, and 1.98 nmol/mg cell protein/24 hr, respectively. Rats were i.v. injected with both LE-pEJ and LE-p17hGHneo carrying the hGH cDNA gene, and secreted hGH in the plasma which induced the synthesis of anti-hGH antibodies. A cell line was cloned from cultures of primary hepatocytes isolated from the liver of transfected rats. After 2 to 3 months in culture, this cell line secreted per day 18.9 micrograms albumin and 11.0 micrograms transferrin per 10(6) cells, 38.75 nmol total BA per mg cell protein, and up to 31 ng hGH per 10(6) cells without cloning hGH recombinant cells. A 24 hr control culture of primary hepatocytes isolated from non transfected rats secreted 25.5 micrograms albumin and 11.7 micrograms transferrin per 10(6) cells, and produced 21.64 nmol total BA and 2.13 nmol N-OH-2-AAF per mg cell protein. Hence, Ha-rasEJ transfection of either hepatocytes in vitro or liver cells in vivo, initiated cell cycles leading to presumptive proliferating hepatocytes which express liver function.

Chemistry and biology of alpha-fetoprotein

Alpha-Fetoprotein (AFP) is a product of specific fetal tissues and of neoplastic cells of hepatocyte or germ cell origin in adults. This protein belongs to a gene family that is phylogenetically most closely related to serum albumin. Its primary, secondary, and tertiary structural aspects appear similar to the three-domain concept proposed for the latter protein. The primary sequence of AFP departs most widely from serum albumin in the first 135 amino acid residues, with about 42% of the remaining 590 residues of the human proteins being identical. Some evidence exists that there are limited sequence differences in the AFP of a given animal species. AFP shows considerable charge heterogeneity that appears to relate mostly to its glycoid moiety. The proteins of some species such as the rat show more pronounced heterogeneities than that of humans. The variations in extent and type of glycosylations are evidenced by differences in the binding to various lectins. These interactions are being extensively explored in attempts to differentiate the sources of the protein produced by various normal and neoplastic cells and may provide valuable diagnostic methods. AFP, like serum albumin, shows relatively strong binding affinities for a variety of ligands. The most notable difference is the strong preferential binding of polyunsaturated fatty acids by AFP. This protein may play a role in transporting these substances to developing and to malignant cells. Various agents affect the synthesis of this protein both by specific fetal tissues and by neoplastic cells. Marked differences in the responses of cells, particularly those of neoplastic types, are indicative of variations in the genetic factors responsible for control of its synthesis. The subject of the genomic repression of the synthesis of AFP seen in fetal life upon maturation of the liver and the reoccurrence of synthesis upon malignant conversion of hepatocytes and of certain germ cells are of particular interest. The regulation of the closely related AFP and albumin genes is providing a powerful and attractive model to examine molecular events in the activation and inactivation of specific genes during development and in oncogenic processes. Extensive measurements of AFP during pregnancy and in the course of neoplasias, notably hepatoma, are being made to aid in following changes in such developments. Various specific physiological roles for this protein are also being proposed. One of these is its possible action in the regulation of immune processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Conformation and expression of the albumin gene of young and old rats

The conformation and expression of the albumin gene in the liver of young (21-) and old (85-week) rats were studied. Digestion of nuclei with MNase shows no differences in the nucleosomal organization in the coding region of the gene in the two ages. The gene has a DNase I hypersensitive site which is distinctly less sensitive in old rats. Its 5'-CCGG-3' sequences are more methylated in the old in which its rate of transcription is also lower.

Altered methylation of ras oncogenes in benzidine-induced B6C3F1 mouse liver tumors

The B6C3F1 mouse is a hybrid strain which exhibits a high (30%) spontaneous hepatoma incidence and sensitivity to chemical induction of liver tumors. The spontaneous hepatoma incidence of the paternal C3H/He strain is approximately 60%, while that of the maternal C57BL/6 strain is very low. The presence of activated oncogenes, primarily Ha-ras, and to a lesser extent, Ki-ras, has been reported in B6C3F1 mouse liver tumors. Because alterations in a gene's capacity for expression, as well as mutation, may be involved in oncogene activation, this investigation was directed toward an examination of a putative control point for transcription, i.e., the methylation state of a gene. Hypomethylation is believed to be necessary, but not sufficient, for transcription. It was therefore hypothesized that alterations in the methylation state of the Ha-ras and Ki-ras oncogenes may facilitate the aberrant expression of these genes in B6C3F1 mouse liver. Restriction enzyme analysis (MspI, HpaII, and HhaI) was used to assess the extent of DNA methylation. MspI digestion of B6C3F1 and C3H/He DNA revealed the absence of a 15-kb Ha-ras band present in MspI-digested C57BL/6 DNA, suggesting that the Ha-ras oncogene of B6C3F1 and C3H/He mouse liver lacks a methylated site. In other respects, the Ha-ras and Ki-ras oncogenes are methylated to a degree which suggests that these oncogenes have a low potential for expression in normal mouse liver. The methylation state of the Ha-ras and Ki-ras oncogenes was also assessed in benzidine-induced hepatomas and adjacent nontumor tissue from B6C3F1 mice. In four out of four cases, the Ha-ras oncogene was hypomethylated in tumor as compared to nontumor tissue and increased expression of the gene was detected in three out of four hepatomas; the Ki-ras oncogene was hypomethylated in two out of four cases. These results suggest that hypomethylation of oncogenes may provide an epigenetic mechanism for facilitating their aberrant expression. The lack of a methylated site observed in the Ha-ras oncogene in B6C3F1 and C3H/He mouse liver may indicate an increased potential for its expression which could, in part, account for the high propensity for hepatoma development in these two strains.

Helicase, DNA-binding, and immunological properties of replication-defective simian virus 40 mutant T antigens

Simian virus 40 T antigen (TAg) exhibits nonspecific and origin-specific DNA binding (ori binding) and ATPase and helicase activities, all of which are related to its roles in viral DNA replication. We have characterized some of the properties of four replication-defective but transformation-competent mutant TAgs, C6-2, T22, C11, and C8A. C6-2 and T22 TAgs were each previously determined to lack ori-binding properties, while C11 TAg was reported to lack ATPase activity. The C8A TAg did not exhibit defects in either ori-binding or ATPase functions. We have analyzed additional aspects of these mutant TAgs pertaining to their helicase, DNA-binding, and immunological properties. With the exception of the C11 TAg, all the other TAgs exhibited helicase activity. The lack of helicase activity by C11 TAg was consistent with its previously shown inability to hydrolyze ATP or to replicate viral DNA. These results therefore show that ori-binding and helicase activities are separate functions of TAg. Wild-type and mutant TAgs bound with similar efficiency to either native or denatured calf thymus DNA-cellulose, indicating no marked differences in their nonspecific DNA-binding properties. We also tested the binding of wild-type and mutant TAgs to a monoclonal antibody, PAb 100, that was previously shown to recognize an extremely small class of TAg that may represent a unique conformational form of the protein. Interestingly, while less than 10% of the wild-type, C6-2, C11, and T22 mutant TAgs were recognized by PAb 100, more than 60% of the C8A mutant TAg was bound by this antibody. Therefore, although no defect in biochemical function was observed with the C8A TAg, its deficiency in viral DNA replication may be related to an unusual conformation, as detected by its dramatically increased recognition by PAb 100. These results show that the helicase activity of TAg is not required for its transformation function.

Deletion and hypermethylation of thymidine kinase gene in V79 Chinese hamster cells resistant to bromodeoxyuridine

Previous studies on V79 Chinese hamster cells have shown that bromodeoxyuridine (BrdU) -resistant variants deficient in thymidine kinase (TK) activity arise by a multistep process which is initiated by a random event and progresses gradually during serial culture in the presence of the drug. In order to determine the molecular basis for the loss of TK activity in these cells, the TK gene was isolated from a lambda phage library of genomic V79 DNA, using a fragment of the human TK gene as a probe. One phage isolated contained the entire TK gene in a 15-kb insert, as demonstrated by the ability of the phage DNA to transform Ltk- mouse cells to the TK+ phenotype. Five fragments spanning the entire gene were then subcloned into the plasmid pUC12 for DNA methylation studies. With these probes it was shown by hybridization analysis that the copy number of the TK gene in V79 cells is about four times the copy number in CHO cells and Chinese hamster liver cells. Southern hybridization analysis of the DNA from first-stage variants partially resistant to BrdU indicated that partial resistance was accompanied by deletion of a number of copies of the TK gene in V79 cells. However, the subsequent gradual transition to full BrdU resistance and full loss of TK activity was correlated with a gradual hypermethylation of sites in the 5' region of the TK gene, with no further change in gene copy number.

Regulation of cytochrome P-450c in differentiated and dedifferentiated rat hepatoma cells: role of the Ah receptor

The induction of cytochrome P 450c mRNA and associated aryl hydrocarbon hydroxylase (AHH) activity is mediated by the Ah receptor in rodent liver and hepatic cells in vitro. In the present study we have investigated the underlying mechanisms responsible for the regulation of AHH activity in differentiated and dedifferentiated variants of the rat hepatoma cell line H4IIEC3. All of the dedifferentiated variants expressed inducible cytochrome P-450c mRNA and AHH activity following treatment with polycyclic aromatic hydrocarbons or the compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Most of the differentiated derivatives, however, were not inducible for either of these functions. Somatic cell hybridization studies revealed that the differentiated cells were AHH negative due to a defect that corresponded to the Ah receptor D gene product. 5-Azacytidine and sodium butyrate, but not mutagens, reactivated a functional Ah receptor in the differentiated line Fao, indicating that a requisite gene had been silenced by an epigenetic mechanism in this strain. Since many of the 5-azacytidine-induced revertant clones resembled dedifferentiated derivatives with respect to morphology and/or diminished expression of hepatic traits, our results support a correlation between coexpression of the dedifferentiated phenotype and AHH inducibility in these hepatoma cells.

Investigation of parameters associated with activity of the Kirsten-ras, Harvey-ras, and myc oncogenes in normal rat liver

Oncogenes have been implicated in the mechanism(s) involved in the different stages of carcinogenesis. Point mutations have been found to activate the ras family of oncogenes. However, these genes must also be expressed in order for phenotypic alterations to be seen. In addition, overproduction of the normal product of some oncogenes may be a factor in the development of cancer. This study was therefore designed to investigate the potential for activity that the Kirsten (Ki)-ras, Harvey (Ha)-ras, and myc oncogenes possess in normal rat liver; activation of these genes has previously been observed in hepatocellular carcinomas. The serum albumin gene was used as a positive control. Two possible control points of gene expression were examined: (1) gene methylation state, as there is an established positive correlation between hypomethylation and gene activity; and (2) DNase I sensitivity because transcriptionally active genes are preferentially sensitive to digestion by this enzyme. It was found that the serum albumin gene is hypomethylated and preferentially sensitive to DNase I, both states being consistent with gene activity. In contrast, Ki-ras is highly methylated and relatively resistant to DNase I digestion. The Ha-ras and myc oncogens exhibit a degree of methylation and DNase I sensitivity intermediate between the serum albumin gene and the Ki-ras oncogene. It thus appears that Ki-ras is not expressed in normal rat liver, whereas Ha-ras and myc may have the potential for activity. In addition, a correlation was found between the methylation state of a gene and its sensitivity to DNase I. These findings are compatible with the view that carcinogenesis is a multistage process. Our results indicate that possible control points for oncogene expression, as well as mutation, should be considered in investigations of the role these genes play in carcinogenesis.

The regulation of albumin and alpha-fetoprotein gene expression in mammals

Albumin and alpha-fetoprotein (AFP) are two plasma proteins synthesized by the liver and the yolk sac. The production of these major proteins is subject to considerable and characteristic variations during both the course of development and hepatic carcinogenesis. It is therefore a system of choice for the analysis of genetic expression during normal differentiation and the cancerous state of eukaryotic cells. The knowledge of regulatory mechanisms at the cellular and molecular levels of the albumin and AFP genes has recently made great progress: 1) the cells which are responsible for the synthesis of albumin and AFP in the liver and other organs have been defined by conjointly using in vitro and in vivo molecular hybridization techniques; 2) the organization of these genes and their adjoining regions has been established in the rat, the mouse and man; 3) the level at which the synthesis of these two proteins is regulated has been determined; it is the transcriptional level. The transcriptional regulation of the albumin and AFP genes could be the result of genome and/or chromatin conformation level modifications. Different groups have shown that: 1) the global structure of the albumin and AFP genes does not change during the course of development and hepatic carcinogenesis; 2) modifications at the level of the methylation of certain specific cytosines could be associated with the variations in the transcription of these genes; 3) global or local (hypersensitive sites with DNase I) changes of chromatin conformation could be correlated to the potential or the overt activity of the transcription of these genes. Very recently certain 'regulatory' regions having cis 'enhancer' or 'silencer' properties have been detected upstream from the albumin and AFP genes. These regions are hypothesized to be DNA 'target' sequences on which trans-acting regulatory factors are fixed and which control the transcription of these genes. Starting from the framework of this recent work, a model of albumin and AFP gene regulation is proposed.

Delayed DNA methylation is an integral feature of DNA replication in mammalian cells

In the majority of sites of methylation in the DNA of mammalian cells, the symmetry of methylation is restored within a few minutes of the passage of a replication fork. However, it has been shown that daughter strand methylation in immortalised cell lines is delayed in a substantial minority of sites for up to several hours after replication. We report here the results of two new approaches to the determination of the functional significance of delayed DNA methylation in mammalian cells. Firstly, we demonstrate that normal, nontransformed cells (human peripheral lymphocytes in short-term primary culture) have comparable proportions of delayed DNA methylation to many immortalised cell lines, showing that delayed DNA methylation is not just a secondary consequence of abnormally high methionine requirements commonly observed in transformed cells and that delayed DNA methylation would be unlikely not to occur in vivo. Secondly, we have used 5-aza-2'-deoxycytidine (5azadCyd) to derive subclones of cells from the Chinese hamster ovary cell line which have stably hypomethylated DNA. In three of these subclones which had lost on average one fourth of the methylation sites from their genomes, the proportion of daughter strand methylation which was delayed after replication was reduced by less than 10%. If delayed DNA methylation were site-specific, this implies that of the order of twice the number of "immediate" methylation sites than delayed methylation sites had been lost from the genomes of these hypomethylated subclones. Thus, delayed DNA methylation is an integral part of the process whereby replicating mammalian cells maintain the pattern of methylation in their genomes. These observations are discussed in relation to the significance of delayed DNA methylation for the accurate maintenance of methylation patterns in the genome and the consequent implications for the possible role of methylated deoxycytidines in mammalian gene control.