Albumin extinction followed by de novo methylation of its gene in somatic hybrids of a rat hepatoma

We have previously identified an Msp I site at the 5′ end of the rat albumin gene whose undermethylation is necessary but not sufficient for stable albumin expression in rat hepatoma cells [1]. We have also shown that the extinction of albumin expression in somatic hybrids is not the result of methylation at this site, since for two different crosses, rapid extinction was found to occur in the absence of any de novo methylation of the previously active gene[2]. In the present study, we examine albumin expression and albumin gene methylation for independent hybrid clones isolated from crosses between albumin expressing rat hepatoma cells and cells of two different non-expressing lines. The cells from hybrid clones of both crosses are characterized by stable extinction of albumin expression. Moreover, we find that de novo methylation of the “extinguished” albumin gene can occur in somatic hybrids, but only some weeks after the gene has ceased to be expressed.     

Extinction, Retention and Induction of Serum Protein Secretion in Hepatoma-fibroblast Hybrids

The production of four serum proteins has been analysed in several hepatoma-fibroblast hybrids. Extinction of albumin and alpha-foetoprotein production occurs systematically in intra and interspecific (rat X mouse) hybrids derived from mouse hepatoma cells (BW). Similar hybrids derived from two related clones of rat hepatoma cells either do not produce albumin (Fa32-derived hybrids), as the BW-derived hybrids, or retain the capacity to produce it, but at a reduced rate (Fu5-derived hybrids); some differences in the control of albumin production thus seem to exist between clonal hepatoma cell lines. The mouse hepatoma cell hybrids retain the capacity to secrete transferrin at a reduced rate, and C3 (the third component of complement) at a high rate. Further analysis of C3 production in interspecific hybrids showed that both parental genomes actively contribute to C3 production: induction of C3 secretion is thus observed in these hybrids.     

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

A well-differentiated rat hepatoma cell line, Fu5-5, yields variant clones whose rate of secretion of serum albumin ranges from 40 to less than 0.08 micrograms of albumin/mg of cell protein per 48 h. Clones were classified as high producers (10 to 40 micrograms/mg per 48 h), intermediate producers (1 to 10 micrograms/mg per 48 h), low producers (0.1 to 1.0 micrograms/mg per 48 h), and null variants (less than 0.1 micrograms/mg per 48 h). Albumin synthetic rates are proportional to secretion rates and range from 0.9 to less than 0.002% of total protein synthesis as measured by pulse-labeling. Steady-state albumin mRNA levels were measured by filter hybridization of fragmented, end-labeled mRNA and by Northern blotting. Message levels are proportional to albumin synthetic rates except for a high producer in which albumin mRNA is less elevated than the synthetic rate. The extent of methylation was quantitated at each of 24 CpG-containing sites or site clusters at the albumin locus. These sites span a region that contains the albumin gene as well as 10 kilobases of the 5' flank and 1 kilobase of the 3' flank. An 8-kilobase region is described, with boundaries in the 5' flank and in the middle of the gene, within which all 11 sites examined showed a correlation of undermethylation with the high-producer phenotype. In contrast, 12 of 13 sites outside of this region showed no phenotype correlation. Null variants derived from a high producer underwent de novo methylation of this domain. Six independent hybrid clones derived from the cross of a high producer with a null variant showed extinction of albumin production and hypermethylation of the domain. Apparently these cells retain the capacity for the de novo methylation of these specific sites.     

Albumin and alpha-fetoprotein gene transcription in rat hepatoma cell lines is correlated with specific DNA hypomethylation and altered chromatin structure in the 5'' region.

We examined DNA methylation and DNase I hypersensitivity of the alpha-fetoprotein (AFP) and albumin gene region in hepatoma cell lines which showed drastic differences in the level of expression of these genes. We assayed for methylation of the CCGG sequences by using the restriction enzyme isoschizomers HpaII and MspI. We found two methylation sites located in the 5' region of the AFP gene and one in exon 1 of the albumin gene for which hypomethylation is correlated with gene expression. Another such site, located about 4,000 base pairs upstream from the AFP gene, seems to be correlated with the tissue specificity of the cells. DNase I-hypersensitive sites were mapped by using the indirect end-labeling technique with cloned genomic DNA probes. Three tissue-specific DNase I-hypersensitive sites were mapped in the 5' flanking region of the AFP gene when this gene was transcribed. Similarly, three tissue-specific DNase I-hypersensitive sites were detected upstream from the albumin gene in producing cell lines. In both cases, the most distal sites were maintained after cessation of gene activity and appear to be correlated with the potential expression of the gene. Interestingly, specific methylation sites are localized in the same DNA region as DNase I hypersensitive sites. This suggests that specific alterations of chromatin structure and changes in methylation pattern occur in specific critical regulatory regions upstream from the albumin and AFP genes in rat hepatoma cell lines.     

A mouse minialbumin gene is specifically expressed in differentiated hepatoma cells but not in transgenic mice

A mouse genomic DNA fragment including the albumin gene in which central exons 9-12 had been deleted and flanked by 2.2 kb in 5' and 4.3 kb in 3' (minialbumin gene), was introduced into rat hepatoma cells and also into mouse embryos to produce transgenic mice. The minialbumin gene was specifically transcribed in stably transfected differentiated clones and a 47-k Da minialbumin was synthesized and secreted into the culture medium. In contrast, the transgene was not expressed in any of the seven independent transgenic mouse lines examined. This suggests that expression of the albumin gene in developing animals requires cis-regulating elements additional to those located within the immediate flanking regions of the gene, which are sufficient to elicit specific expression in differentiated hepatoma cells in culture.     

Tse-2: a trans-dominant extinguisher of albumin gene expression in hepatoma hybrid cells.

Serum albumin gene expression is generally extinguished in hepatoma x fibroblast hybrids. To define the genetic basis of this phenomenon, we screened a panel of hepatoma hybrids retaining different fibroblast chromosomes for albumin production by immunofluorescence. We report that albumin extinction in these clones was strictly correlated with the retention of mouse chromosome 1. Furthermore, albumin was systematically reexpressed in chromosome 1 segregants. These data define a tissue-specific extinguisher locus (Tse-2) that affects albumin gene expression in trans. Two other liver genes, those encoding liver alcohol dehydrogenase and liv-10, were coordinately extinguished with albumin in monochromosomal hybrids that specifically retained mouse chromosome 1.     

A cell-specific enhancer of the mouse alpha 1-antitrypsin gene has multiple functional regions and corresponding protein-binding sites.

We have previously described the isolation and characterization of genomic clones corresponding to the mouse alpha 1-antitrypsin gene (Krauter et al., DNA 5:29-36, 1986). In this report, we have analyzed the DNA sequences upstream of the RNA start site that direct hepatoma cell-specific expression of this gene when incorporated into recombinant plasmids. The 160 nucleotides 5' to the cap site direct low-level expression in hepatoma cells, and sequences between -520 and -160 bp upstream of the RNA start site functioned as a cell-specific enhancer of expression both with the alpha 1-antitrypsin promoter and when combined with a functional beta-globin promoter. Within the enhancer region, three binding sites for proteins present in hepatoma nuclear extracts were identified. The location of each site was positioned, using both methylation protection and methylation interference experiments. Each protein-binding site correlated with a functionally important region necessary for full enhancer activity. These experiments demonstrated a complex arrangement of regulatory elements comprising the alpha 1-antitrypsin enhancer. Significant qualitative differences exist between the findings presented here and the cis-acting elements operative in regulating expression of the human alpha 1-antitrypsin gene (Ciliberto et al., Cell 41:531-540, 1985; De Simone et al., EMBO J. 6:2759-2766, 1987).     

Methylation of an alpha-foetoprotein gene intragenic site modulates gene activity.

By comparing the methylation pattern of Mspl/Hpall sites in the 5' region of the mouse alpha-foetoprotein (AFP) gene of different cells (hepatoma cells, foetal and adult liver, fibroblasts), we found a correlation between gene expression and unmethylation of a site located in the first intron of the gene. Other sites did not show this correlation. In transfection experiments of unmethylated and methylated AFP-CAT chimeric constructions, we then showed that methylation of the intronic site negatively modulates expression of CAT activity. We also found that a DNA segment centered on this site binds nuclear proteins; however methylation did not affect protein binding.     

Expression of human hepatic genes in somatic cell hybrids

Four diploid human cell types (lymphocytes, fibroblasts, amniotic fluid cells, and hepatocytes) were fused to mouse hepatoma cells, HH. HH synthesized and secreted several liver-specific gene products including albumin, transferrin, and alpha-fetoprotein. The resulting interspecific hybrids were compared to determine whether or not the pattern of human hepatic gene expression was similar when these various cells were fused with the mouse hepatoma line. The expression of six human hepatic genes was examined, including albumin, alpha-fetoprotein, ceruloplasmin, transferrin, alpha-1-antitrypsin, and haptoglobin. Albumin was most frequently expressed while alpha-fetoprotein was not detected in any of the hybrids studied. The patterns of expression of human serum proteins differed between the hybrid series. Hybrids derived from human fibroblasts produced primarily albumin, while those derived from lymphoblastoid cells and amniocytes had a higher frequency of clones secreting alpha-1-antitrypsin. The findings reported here suggest that the frequency of hybrid clones expressing human hepatic gene products and the array of proteins produced are influenced by the histogenetic state of the human parental cell type.     

Conditions required for activation of the mouse albumin or α-fetoprotein gene in hybrids between mouse lymphoblastoma and rat hepatoma cells

Activation of two previously silent mouse hepatic genes has been investigated in hybrid cells between pseudodiploid mouse lymphoblastoma cells and hyperdiploid or hypertetraploid rat hepatoma cells. In this material, activation of the mouse albumin gene is a frequent event, whereas activation of mouse alpha-fetoprotein (AFP) occurs only in those cells that produce large amounts of albumin. Quantitative tests of hybrid populations for the activated proteins and their mRNAs revealed the expected sizes and structures: moreover, as in hepatoma cells, the amount of both rat and mouse albumin produced was directly proportional to the intracellular concentration of the corresponding mRNA. The cellular environment required for activation of the liver-specific genes was investigated by cell-by-cell analysis of each hybrid clone. Immunostaining for the presence of rat and mouse albumin and mouse AFP revealed unexpected heterogeneity in the phenotypes of the hybrid populations, which were found to contain cells that: (a) failed to express either of the proteins; (b) produced all three; (c) produced both rat and mouse albumin; or (d) produced rat albumin only. Karyotypic analysis indicated that the hybrid-cell phenotype depended on parental chromosome ratios rather than absolute numbers of chromosomes. It was found for albumin and mouse AFP that the fraction of immunostained cells was equal to the fraction of metaphases that contained a minimal rat-to-mouse chromosome ratio of 2.5 and 9, respectively. It is concluded that in those hybrids, expression of liver-specific genes is regulated by extinguishers, but in a dose-dependent fashion, suggesting the intervention of antagonistic activators from the rat hepatoma chromosomes.     

Undermethylation at the 5′ end of the albumin gene is necessary but not sufficient for albumin production by rat hepatoma cells in culture

We have measured methylation of the albumin gene in clones of rat hepatoma cells that vary quantitatively in their rates of synthesis of albumin and in variant and hybrid cells that produce no albumin. Although the albumin gene is undermethylated for its entire length in rat liver, only the 5′ end is ever undermethylated in hepatoma cells. Moreover, undermethylation of the 5′ end of the gene appears to be necessary for stable expression of the albumin gene in hepatoma cells. Since undermethylation of this region is found in some variant cells that fail to produce albumin, it is not a sufficient condition for albumin gene expression. Despite the excellent correlation between undermethylation of the 5′ end of the albumin gene and its stable expression, the results argue against the possibility that the methylated state of such genes during development determines whether they will or will not be expressed.     

Ability of the cells of intra- and interspecific hybrids of murine hepatoma XXIIa to synthesize the serum proteins albumin and transferrin

Independent hybrid clones resulted from the whole cell and microcell-mediated transfer of hamster or mouse fibroblast chromosomes into mouse hepatoma XXIIa cells. The fusion was promoted with PEG, ethidium bromide alone, or in combination with HAT and ouabain, was used for selecting the hybrids. Using indirect immunoautoradiography, three clones (one intra- and one interspecies microcellular; one interspecies, whole cell fusion) have been found to express their hepatic function to synthesize transferrin. The liver specific protein--albumin--was extinguished in all the hybrid combinations. Possible mechanisms of gene expression are discussed. The hybrids selected could be used for mapping chromosomes, coding proteins, as well as for studying regulation in the tandem of albumin and alpha-fetoprotein genes in the mouse genome. The microcell mediated chromosome transfer into differentiated cells has been used to construct original genetical combinations of regulatory and structural elements of the mouse genome.     

DNA methylation and the regulation of aldolase B gene expression

DNA methylation was studied as a potential factor for the regulation of tissue-specific and developmentally specific expression of the rat aldolase B gene. We examined cytosine methylation in the HpaII and HhaI recognition sequences in the aldolase B gene in aldolase expressing and nonexpressing tissues and cells. Out of the 15 methyl-sensitive restriction sites examined, the sites in the 3'-half and 3'-flanking regions were found to be heavily methylated in all the tissues or cells, regardless of the level of aldolase B gene expression. However, the methylation pattern in the region immediately upstream and in the 5'-half of the gene exhibited tissue-specificity: the site located about 0.13 kb upstream of the cap site (just next to the CCAAT box), and the sites in the first intron (intron 1) were heavily methylated in nonexpressing cells and tissues (ascites hepatoma AH130 and brain), whereas those in an expressing tissue (liver) were considerably less methylated. These results suggest that cytosine methylation at the specific sites in the 5'-flanking and 5'-half regions of the gene is associated with repression of the gene activity. However, the gene is still substantially methylated in the fetal liver on day 16 of gestation, when it is in a committed state for rapid activation in the period immediately afterwards (Numazaki et al. (1984) Eur. J. Biochem. 152, 165-170). This suggests that demethylation of the methylated cytosine residues in the specific gene region is not necessarily required before activation of the gene during development, but it may occur along with or after the activation.     

Tissue-specific expression is conferred by a sequence from the 5'' end of the rat albumin gene.

We have constructed a transient expression vector containing 400 bp of rat albumin gene immediate 5'-flanking sequences inserted 5' to the bacterial enzyme chloramphenicol acetyl transferase (CAT). We have transfected various clones of rat hepatoma cells representing different states of expression of the liver phenotype with this vector (pALB-cat) and also with two control vectors containing viral promoters (pSVE-cat and pRSV-cat), and measured activity of the bacterial enzyme CAT in cellular extracts 48 h later. The albumin flanking sequences are able to direct highly efficient CAT expression, compared with the control vectors, only in cells which express their own albumin gene: the albumin-negative hepatoma cells are at least 100 times less efficient in expressing CAT after transfection with the pALB-cat plasmid than are the albumin-positive ones. An unexpected result of our study is the total inability of the rat albumin flanking sequences to direct expression in albumin-producing mouse hepatoma cells.     

Extinction of Albumin Gene Expression in a Panel of Human Chromosome 2 Microcell Hybrids

Expression of the serum albumin gene is extinguished in rat hepatoma microcell hybrids that retain mouse chromosome 1. These data define atrans-dominant extinguisher locus,Tse-2,on mouse chromosome 1. To localize the human TSE2 locus, we prepared and characterized rat/human microcell hybrids that contained either human chromosome 1 or chromosome 2, the genetic homologues of mouse chromosome 1. Rat hepatoma microcell hybrids retaining a derivative human chromosome 1 [der 1 t(1;17)(p34.3;q11.2)] expressed their serum albumin genes at levels similar to those of parental hepatoma cells. In contrast, microcell transfer of human chromosome 2 into rat hepatoma recipients produced karyotypically heterogeneous collections of hybrid clones, some of which displayed dramatic albumin extinction phenotypes. For example, albumin mRNA levels in several extinguished microcell hybrids were reduced at least 500-fold, similar to albumin mRNA levels in hepatoma × fibroblast whole-cell hybrids. Expression of several other liver genes, including α1-antitrypsin, aldolase B, alcohol dehydrogenase, and phosphoenolpyruvate carboxykinase, was also affected in some of the microcell hybrids, but expression of these genes was not concordant with expression of albumin. Hybrid segregants were prepared from the albumin-extinguished hybrids, and reexpression of albumin mRNA and protein was observed in sublines that had lost or fragmented human chromosome 2. Finally, expression of mRNAs encoding the liver-enrichedtransactivators HNF-1, HNF-4, HNF-3α, and HNF-3β was not affected in any of the chromosome 2-containing hybrids. These data define and map a genetic locus on human chromosome 2 that extinguishes albumin gene expression intrans,and they suggest that TSE2-mediated extinction is independent of HNF-1, -4, -3α, and -3β expression.