Studies of gene transcription and translation in regenerating rat liver

Messenger RNA profiles in liver injury and stress: a comparison of lethal and nonlethal rat models

Liver damage activates processes aimed at repairing damage; simultaneously, liver functions required for survival must be maintained. The expression of genes responsible for both in rat models of lethal (lipopolysaccharide, 90% hepatectomy, and d-galactosamine) and nonlethal (turpentine, 70% hepatectomy, and acetaminophen) liver damage and stress was measured at 3, 6, 12, and 24 h after the intervention and quantitated as the area between the control curves and the test curves (AUC). The expression of genes for cell division and remodeling was upregulated most in the lethal models. The expression of most liver-specific function genes was reduced. Positive AUC was found for ARG, ASL, CPT1, Mdr1b, Mdr2, and PEPCK. It is concluded that a high expression of genes for repair of liver damage is associated with reduced expression of genes for several liver-specific functions, possibly reflecting a limited capacity for transcriptional activity. Maintained or increased expression of selected function genes indicates that the corresponding functions have high priority. The liver sustains metabolic homeostasis ensuring that other organs in the body function normally. Simultaneously, the processes required for the integrity of its own structure and function are maintained as a result of regulated expression of the genes that produce the proteins needed to perform both set of functions.

Expression of tumor-associated protein variants in chemically induced rat hepatomas and transformed rat liver cell lines determined by two-dimensional electrophoresis

High resolution two-dimensional electrophoresis (2-DE), using the rat liver as a model, was applied to study hepatocarcinogenesis induced by different chemical carcinogens. Several tumor-associated protein variants were detected by 2-DE in chemically induced rat hepatomas and transformed rat liver cell lines compared to normal rat liver tissue. Proliferation-related protein changes and/or protein alterations due to culture conditions were corrected for by comparison with 2-DE patterns of isolated cells from regenerating liver and short-term cultivated liver cells. Some of the tumor-associated variants were further characterized: (i) By peptide mapping with limited proteolysis we detected clear relationships between several variants. (ii) By studying post-translational modifications phosphorylated and glycosylated variants could be determined. (iii) A tumor-associated protein variant was identified by amino acid analysis and amino acid sequencing.

Two-dimensional electrophoretic analysis of transformation-sensitive polypeptides during chemically, spontaneously, and oncogene-induced transformation of rat liver epithelial cells

Recently, we described the establishment of a computerized database of rat liver epithelial (RLE) cellular polypeptides (Wirth et al., Electrophoresis, 1991, 12, 931-954). This database has now been expanded to include the analysis of cellular polypeptide alterations during chemically (aflatoxin B1; AFB), spontaneously, and oncogene (v-Ha-ras, v-raf, and v-myc/v-raf)-induced transformation of RLE cells. Two-dimensional mapping of [35S]methionine-labeled whole cell lysate, cell-free in vitro translation products and [32P]orthophosphate-labeled polypeptides revealed subsets of polypeptides specific for each transformation modality. A search of the RLE protein database indicated the specific subcellular location for the majority of these transformation-sensitive proteins. Significant alterations in the expression of the extracellular matrix protein, fibronectin, as well as tropomyosin- and intermediate filament-related polypeptides (vimentin, beta-tubulin, the cytokeratins, and actin) were observed among the various transformant cell lines. Immunoprecipitation and Western immunoblot analysis of tropomyosin expression in four individual AFB-, as well as four spontaneously induced, and each of the oncogene-transformed cell lines indicated that five major tropomyosin (Tm 1-5) isoforms were variably expressed in the various cell lines, including one polypeptide tentatively identified as Tm6. Whereas alterations in tropomyosin expression appeared to be transformation-specific, alterations in the individual intermediate filament polypeptides were related more to the differentiation state of the individual cell lines rather than to the transformation phenotype. These studies extend our earlier efforts toward the establishment of a comprehensive computerized database of RLE cellular proteins and demonstrates how such a database may serve as a useful source for studies concerning the regulation of growth and differentiation as well as transformation of RLE cells.

Differences in the abundance of variably spliced transcripts for the second asialoglycoprotein receptor polypeptide, H2, in normal and transformed human liver

The human hepatic asialoglycoprotein receptor comprises two homologous polypeptides designated H1 and H2. Two distinct complementary DNA clones encoding these receptor subunits have been previously isolated from the human hepatoblastoma cell line HepG2. We discovered that multiple variants of H2 transcripts exist both in HepG2 cells and in the normal human liver that, at least in part, appear to be the result of alternative splicing events. We have found that (a) the complementary DNA clone for H2 previously isolated from HepG2 cells, characterized by a 57-nucleotide insertion within the 5' end of the complementary DNA that is absent from H1, represented only one third of H2-related sequences in an unamplified normal human liver complementary DNA library and less than 10% of H2 clones in HepG2 cells; (b) the predominant message for H2 expressed in the liver and HepG2 cells, designated L-H2, appeared to represent the fully processed product of the gene encoding both L-H2 and H2; and (c) a variant H2 transcript existed in HepG2 cells, designated H2', that contained a novel, 5' 88-bp nucleotide insertion. Poly(A+) RNA analysis of the normal liver and HepG2 cells by complementary RNA hybridization and ribonuclease protection corroborated the observations made during the screening of complementary DNA libraries regarding the abundance of the various messages. A striking incongruity was found between the levels of messenger RNA containing the H2-specific 57-nucleotide sequence and the levels of polypeptide expressed in the liver and HepG2 cells as recognized by antiserum specifically raised against this sequence.

Alterations in tumor angiogenesis associated with stable expression of the HIV tat gene

Recent evidence suggests that the human immunodeficiency virus type 1 (HIV) trans-activator gene (tat) has transforming properties and may be a causative factor in the development of certain types of cancers, in particular Kaposi's sarcoma (i.e., Vogel J. et al. Nature 335:606-611, 1988). To help elucidate the potential role or roles of the HIV tat gene in neoplastic transformation, cell lines were constructed that constitutively express a functional tat gene product. HeLa cells were coelectroporated with two plasmids, one containing the HIV tat gene in an expression cassette and another containing the dominant selectable marker gene xanthine guanine phosphoribosyltransferase (XGPRT). After XGPRT selection, single-cell clones that expressed a functional tat protein were identified by measuring chloramphenicol acetyltransferase (CAT) activity after electroporating a plasmid containing the CAT gene transcriptionally controlled by HIV trans-activation-responsive region (tar). Phenotypic alterations resulting from the expression of tat were then determined. Control cells and tat-expressing cells grew at similar rates in culture. However, when grown as tumors in nude mice, tat-expressing cells produced a lower percentage of tumors, and the tumors that were produced either regressed, stopped growing, or grew at a very reduced rate compared with cells not expressing tat. These differences may have resulted from a tat-associated reduction in neovascularization in the tumors. A comparison of total cellular proteins by two-dimensional polyacrylamide gel electrophoresis indicated only one reproducible alteration in a polypeptide of approximately 44 kDa and pl of approximately 6.2 associated with tat expression. These cells may be very useful in future in vitro and in vivo studies designed to examine the effects of HIV tat on endothelial and vascular smooth-muscle cells and the role of tat in the etiology of Kaposi's sarcoma.

A two-dimensional gel database of rat liver proteins useful in gene regulation and drug effects studies

A standard two-dimensional (2-D) protein map of Fischer 344 rat liver (F344MST3) is presented, with a tabular listing of more than 1200 protein species. Sodium dodecyl sulfate (SDS) molecular mass and isoelectric point have been established, based on positions of numerous internal standards. This map has been used to connect and compare hundreds of 2-D gels of rat liver samples from a variety of studies, and forms the nucleus of an expanding database describing rat liver proteins and their regulation by various drugs and toxic agents. An example of such a study, involving regulation of cholesterol synthesis by cholesterol-lowering drugs and a high-cholesterol diet, is presented. Since the map has been obtained with a widely used and highly reproducible 2-D gel system (the Iso-Dalt system), it can be directly related to an expanding body of work in other laboratories.

Decreases in the relative concentrations of specific hepatocyte plasma membrane proteins during liver regeneration: down-regulation or dilution?

Antibodies were used to quantify seven domain-specific integral proteins of the rat hepatocyte plasma membrane during rat liver regeneration in response to two-thirds hepatectomy. Quantitative immunoblotting revealed that a subset of the plasma membrane proteins exhibited transient 30-70% decreases in relative concentration during the period of hepatocyte proliferation. The list of affected proteins included at least one representative from each of the plasma membrane domains: the apical protein HA 4, the lateral protein HA 321, and the basolateral receptors for epidermal growth factor and asialoglycoproteins. In contrast, the relative concentrations of three other plasma membrane proteins, the basolateral protein CE 9 and the two apical proteins dipeptidylpeptidase IV and aminopeptidase N, remained unchanged throughout liver regeneration. The decreases in the relative concentrations of the plasma membrane proteins were observed even when the synthesis of hepatocyte DNA was blocked by hydroxyurea, suggesting that the signalling for these two delayed consequences of two-thirds hepatectomy occurred along parallel, dependent pathways. Pulse and pulse-chase metabolic radiolabeling studies revealed that the decreases in the concentrations of the PM proteins were accomplished through protein-selective decreases in the rates of synthesis of the high-mannose precursors of the affected proteins, but not through the accelerated degradation of the mature plasma membrane proteins.

Transcriptional regulation of an aldolase gene in the regenerating rat liver

The relative abundance of rat aldolase A, B, C, alpha-fetoprotein, and albumin mRNAs was determined by Northern hybridization during liver regeneration after partial hepatectomy. Aldolase A mRNA increased more than 10-fold on the 3rd day after partial resection compared with that of normal adult rat liver. S1 analysis revealed that three species of aldolase A mRNAs (mRNA I, II & III) reappeared. However, transcriptional rate of aldolase A mRNA did not change at all during the regeneration. In contrast, aldolase B, aldolase C and albumin mRNAs did not change at all. These findings suggest that the differentiated hepatocyte maintains a differentiated state during the liver regeneration as seen in aldolase B, whereas "oncofetal" isozymes such as aldolase A resurge after partial hepatectomy under the control of post-transcriptional mechanism.

Studies in rat liver perfusion for optimal harvest of hepatocytes

Pediatric liver transplantation is successful but donor scarcity is a major limitation. We are studying hepatocyte transplantation as an alternative to provide functional hepatic replacement. This report details the study of rat liver perfusion for optimal harvest of hepatocytes and cell implantation. We performed 128 rat liver perfusions using a technique modified from the two-step enzymatic perfusion described by Seglen. We examined variations in the perfusion, rate, time, antegrade versus retrograde, pulsatile versus continuous flow, temperature, collagenase type, and variables of buffer composition. We have found optimal cell yield and viability under the following conditions: in situ perfusion, continuous flow at 25 cc/min, retrograde perfusion via the inferior vena cava, water bath temperature 38 degrees C, Boerhinger-Mannheim collagenase using a nonoxygenated HEPES based perfusion buffer, pH 7.4, for the initial perfusion and the same buffer with 4.8 mmol/L CaCl2 for the collagenase perfusion. These conditions consistently generate cell harvests of 500 to 700 x 10(5) cells/g of liver tissue with cell viability between 85% and 95%.

Studies on the hyperplasia ('regeneration') of the rat liver following partial hepatectomy. Changes in lipid peroxidation and general biochemical aspects

Using the experimental model of partial hepatectomy in the rat, we have examined the relationship between cell division and lipid peroxidation activity. In rats entrained to a regime of 12 h light/12 h dark and with a fixed 8 h feeding period in the dark phase, partial hepatectomy is followed by a rapid regeneration of liver mass with cycles of synchronized cell division at 24 h intervals. The latter phenomenon is indicated in this study by pulses of thymidine kinase activity having maxima at 24 h, 48 h and 72 h after partial hepatectomy. Microsomes prepared from regenerating livers show changes in lipid peroxidation activity (induced by NADPH/ADP/iron or by ascorbate/iron), which is significantly decreased relative to that in microsomes from sham-operated controls, again at 24 h, 48 h and 72 h after the operation. This phenomenon has been investigated with regard to possible underlying changes in the content of microsomal fatty acids, the microsomal enzymes NADPH:cytochrome c reductase and cytochrome P-450, and the physiological microsomal antioxidant alpha-tocopherol. The cycles of decreased lipid peroxidation activity are apparently due, at least in part, to changes in microsomal alpha-tocopherol content that are closely associated in time with thymidine kinase activity.

Elsie 4: quantitative computer analysis of sets of two-dimensional gel electrophoretograms

We have developed and refined a system for quantitative computer analysis of two-dimensional polyacrylamide gel electrophoretograms. The system, named Elsie 4, is based on one described by Vo et al. (Anal. Biochem. 112, 258 (1981]. It is highly automated. Elsie 4 can find, and measure the intensity of, almost any spot resolvable on two-dimensional gels, including spots visible only as shoulders off larger spots and spots so close together that there is no "valley" between them. It can automatically match the spot patterns of different gels, potentially without the need for a user to provide landmark matches. The matches between paired gels let us follow the synthesis of any spot through a set of gels. Information about a group of matched spots can be obtained by referring to any spot in the group. There is generally no need for a standard or reference gel. Data for two experiments can be combined and compared by matching any gel in one experiment with any gel in the other. There are ways to automatically find possible mismatches in sets of gels. Scans and the results of the analysis can be shown on an image displayer. The programs use function libraries; this helps ensure consistency and increase portability. The programs and functions can be linked together in many ways; this lets users build custom programs for analysis of specific experiments.

Specific modulation by ethanol of the protein synthesis pattern in the C2 rat hepatoma cell line

The effect of ethanol on protein synthesis in the C2 rat hepatoma cell line was analyzed by two-dimensional gel electrophoresis after the labeling with [35S]methionine of cells that were untreated or had been treated with 180 mM ethanol. In this cell line, this concentration of ethanol is known to induce gamma-glutamyl transpeptidase, a marker of alcoholism in man (Barouki et al., Hepatology 1983; 3: 323-329). In the present work we demonstrate that ethanol, besides causing a slight decrease in overall protein synthesis (less than 25%), primarily regulates the expression of two unique proteins among 1500 labeled products that were analyzed: one of these was induced and did not correspond to gamma-glutamyl transpeptidase, and one was repressed after 20 h of ethanol treatment. We conclude that the set of hepatic proteins altered by ethanol is likely to be very limited in number, which reflects the specificity of alcohol action on protein synthesis in the C2 cell line.