Interleukin-6 responsiveness and cell-specific expression of the rat kininogen gene

The serum concentration of rat T1 kininogen increases 20-30-fold in response to acute inflammation, an induced hepatic synthesis regulated primarily at the transcriptional level. To analyze the cis-regulatory elements responsible for the induced transcription, we fused a 1.6-kilobase segment of the rat T1 kininogen promoter to a reporter gene, chloramphenicol acetyltransferase (CAT). The resultant chimeric DNA was transfected into cultured cells. In transient transfection assays, this 5'-flanking sequence was sufficient to confer cell-specific expression: CAT activity was readily detectable when the construct was transfected into liver-derived cells, but it was not detectable in nonliver cells. Furthermore, when liver cells (Hep3B) transfected with this construct were treated with conditioned medium prepared from activated mixed lymphocyte cultures or with recombinant interleukin-6 (IL-6), a 5-fold increase in CAT activity was detected. Addition of dexamethasone to the conditioned medium or to IL-6 showed synergistic effects and resulted in a 10-fold increase in CAT activity. In contrast, when IL-1 was used with IL-6, induction of CAT activity was inhibited. Deletion analyses revealed two regions important for tissue-specific and induced regulation of T1 kininogen: sequences proximal to base pair -73 conferred enhanced expression in liver-derived cells and a distal region that conferred responsiveness to conditioned medium, recombinant IL-6, and dexamethasone. This responsive element had properties of an inducible transcriptional enhancer, and it was functional in both liver and nonliver cells when placed immediately upstream of a thymidine kinase promoter.

Two adjacent C/EBP-binding sequences that participate in the cell-specific expression of the mouse serum amyloid A3 gene

Serum amyloid A (SAA) is a major acute-phase protein synthesized primarily in the liver. Its expression, very low in normal animals, is increased several hundredfold following acute inflammation. To examine DNA sequences involved in liver-specific expression, 5'-flanking regions of the mouse SAA3 gene were analyzed by transient transfection, band shift, and DNase I protection assays. We found that a 56-bp fragment immediately 5' to the TATA box spanning the region -93 to -38 relative to the transcription start site was sufficient to confer liver cell-specific transcriptional activation onto a heterologous promoter in a dose-dependent and orientation-independent manner. This DNA fragment could form DNA-protein complexes with heat-stable nuclear proteins, and the complexes formed could be specifically competed for by excess oligomers corresponding to the C/EBP- or DBP-binding sites but not by binding sites for three other liver-specific factors, HNF1, HNF3, and HNF4. Footprint analysis using Hep3B nuclear extracts revealed two adjacent footprint regions within this 56-bp fragment, the distal region having at least fivefold-greater affinity than the proximal region. Identical footprint patterns were observed when purified recombinant C/EBP protein was used. These results indicated that binding of C/EBP to this 56-bp fragment plays an important role in vivo in enhancing expression of the mouse SAA3 gene in hepatocytes.

Regulation of mouse serum amyloid A gene expression in transfected hepatoma cells

Expression of mouse serum amyloid A (SAA1, -2, and -3) mRNAs can be induced up to 1,000-fold in the liver in response to acute inflammation. This large increase is primarily the result of a 200-fold increase in the rates of SAA gene transcription. To analyze the cis-acting regulatory element(s) responsible for regulating transcription, we fused 306 base pairs of the mouse SAA3 promoter to a reporter gene, the chloramphenicol acetyltransferase (CAT) gene, and transfected this chimeric DNA into cultured cells. In transient expression assays, this 5' sequence was sufficient to confer cell-specific expression: CAT activity was readily detectable when the construct was transfected into liver-derived cells but was not detectable in nonliver cells. Furthermore, when liver cells transfected with this construct were treated with conditioned media prepared from activated mixed lymphocyte cultures or with recombinant interleukin-1, a 10- to 15-fold increase in CAT activity was detected. Deletion analyses showed two regions of interest: a proximal region that enhanced CAT expression in a cell-specific manner and a distal region that conferred responsiveness to both conditioned media and recombinant interleukin-1. This distal responsive element had properties of an inducible transcriptional enhancer, and deletion of the proximal cell-specific region rendered the distal element responsive to stimulation by conditioned media in nonliver cells.

Molecular and cellular biology of serum amyloid A

Serum amyloid A (SAA) is one of the major acute-phase proteins in humans and mice. It is synthesized predominantly by the liver and secreted as a major component of the apolipoproteins in the high density lipoprotein particle. While the major physiological function of SAA is unclear, prolonged elevation of plasma SAA levels, as in chronic inflammation, however, results in the pathological condition amyloidosis affecting the liver, kidney and spleen. The expression of SAA mRNA is dramatically elevated in response to infection or systemic inflammation and is due primarily to the increased rate of SAA gene transcription. Studies in vitro and in vivo demonstrated that the expression of SAA genes is regulated by the inflammatory cytokine interleukin-1. Moreover, both the interleukin-1-induced expression and the enhanced liver-specific expression of the SAA gene are controlled by the binding of nuclear proteins to specific DNA sequences upstream from the structural gene.

RNA editing of apolipoprotein B mRNA. Sequence specificity determined by in vitro coupled transcription editing

Apolipoprotein (apo) B-48 mRNA is produced by in vivo RNA editing which involves a C----U conversion of the first base of the codon CAA for Gln-2153, changing it to UAA, an in-frame stop codon. We have reproduced the editing reaction in vitro using nuclear extracts. Efficient RNA editing was demonstrated by using apoB mRNA segments as substrate or in a coupled transcription-editing reaction using apoB minigenes as template. ApoB minigenes were constructed by ligating the adenovirus major late promoter to a fragment of apoB-100 DNA containing the editing site and used for the transcription-editing reaction. We defined the sequence specificity of the editing reaction using site-specific single and multiple base mutants constructed by the polymerase chain reaction. Among 22 different mutant apoB-100 minigene constructs containing mutations in the bases immediately flanking the edited C-6666, 20 were edited in the coupled transcription-editing reaction. The results suggest a relatively lax sequence specificity for apoB mRNA editing. Our observation may have important implications for apoB-48 biogenesis as well as for the editing process as a general biologic regulatory mechanism.

Stimulation of the acute-phase response in simian virus 40-hepatocyte cell lines

Seven simian virus 40 (SV40)-hepatocyte cell lines were characterized with respect to the ability to express eight liver acute-phase genes. cDNA clones corresponding to albumin, serum amyloid A, alpha 1-acid glycoprotein, haptoglobin, alpha-, beta-, and gamma-fibrinogen, and alpha 1-major-acute-phase protein mRNAs were used in Northern (RNA) or slot blot analyses. In the noninduced state, six of the seven cell lines showed significant (i.e., liverlike) levels of constitutive expression of all genes examined except that expression of haptoglobin mRNA was considerable lower than in the normal liver. To examine whether these immortalized liver cells can respond appropriately to inflammatory mediators, cells were treated with conditioned medium from activated human monocytes or mixed lymphocyte cultures. Results showed that these SV40-hepatocyte cell lines responded to the conditioned media in culture by down-regulating albumin gene expression and up-regulating other acute-phase genes in a time- and dose-dependent manner. These results indicate that the SV40-hepatocytes retained not only the ability to express a number of acute-phase genes but also the ability to respond to external stimuli. The usefulness of these cell lines for analysis of the molecular mechanisms involved in the regulation of these acute-phase genes is discussed.

Altered expression of acute-phase reactants in mouse liver tumors

We studied the expression of eight liver acute-phase genes in spontaneous and diethylnitrosamine-induced mouse liver tumors (MLTs) under basal and induced conditions. Primary spontaneous and chemically induced MLTs were used for RNA isolation and histopathologic analysis. In the noninduced state, all MLTs showed similar levels of mRNA for albumin, serum amyloid A, alpha 1-acid glycoprotein, haptoglobin, and alpha-, beta-, and gamma-fibrinogens compared with control or background livers. The mRNA for the alpha 1-major acute-phase protein, however, was consistently elevated in both spontaneous and diethylnitrosamine-induced MLTs. The expression of these acute-phase reactants in 11 MLTs was examined following exposure of the mice to turpentine. The relative expression of these mRNAs in these MLTs varied widely compared with mRNA expression in controls. Though all MLTs expressed the same three species of fibrinogen mRNAs as did the controls, no MLT demonstrated downregulation of albumin mRNA levels, and only 1 of 11 MLTs showed a marginal increase in serum amyloid A mRNA levels. Synthesis of mRNA for alpha 1-acid glycoprotein and haptoglobin was intermediate. The study of the response of MLTs to specific acute-phase stimuli may make possible a better understanding of the basis for coordinated expression of acute-phase reactants and of the variable phenotypes associated with cancers.

Effects of experimentally induced nephrosis on protein synthesis in rat liver

A nephrotic syndrome was experimentally induced in rats by a single intravenous injection of aminonucleoside of puromycin. Experimental animals were studied 8 days after the injection, at which time they exhibited marked proteinuria and hypoalbuminemia compared with control animals. The experimental animals also exhibited alterations in protein synthesis in liver as evidenced by a marked increase in the rate of albumin synthesis relative to total hepatic protein synthesis, changes in the relative concentrations of several plasma proteins, an increased protein content of plasma, an increased liver weight relative to body weight, and an increased RNA content of liver. Perfused liver preparations derived from nephrotic rats exhibited an increased release of albumin and other secretory proteins compared with control preparations. In contrast, there was no difference in the rate of synthesis of nonexported proteins between the two groups. The elevation in the relative rate of albumin synthesis was accompanied by a relative increase of the same magnitude in albumin mRNA. Furthermore, the relative amounts of several other mRNAs, including those coding for beta-fibrinogen, haptoglobin, metallothionein II, and two unidentified proteins, were increased, whereas the amount of mRNA coding for alpha 1-acid glycoprotein was decreased in livers of nephrotic rats compared to controls. These results indicate that nephrosis leads to marked alterations in the synthesis of albumin and other plasma proteins. Mechanisms responsible for these alterations include changes in the relative abundance of specific mRNAs and an increase in total cellular RNA.

Changes in plasma albumin concentration, synthesis rate, and mRNA level during acute inflammation

Induction of acute inflammation in rats by the subcutaneous injection of turpentine resulted in a marked fall in the concentration of albumin in plasma. This fall, which reached a minimum of 45% of the control level at 36 h after injection of the inflammatory agent, occurred in the presence of no significant change in the concentration of total protein in plasma. It was accompanied by a corresponding decline in the relative abundance of albumin mRNA in liver, which reached a minimum of 25% of the control level at 36 h after initiation of the inflammatory response. Perfused livers from 36-h postinjection rats exhibited albumin secretion rates that were reduced to 38% of control values. In contrast, release of total secretory proteins, secretion of nonalbumin plasma proteins, and synthesis of nonexported proteins by perfused livers were elevated to 166, 266, and 117% of the control values, respectively, as a result of the inflammatory response. These results demonstrate that acute inflammation causes a relative reduction in hepatic albumin mRNA, which leads to a corresponding decrease in albumin synthesis and secretion by liver and a fall in the concentration of albumin in plasma. The concentration of total protein in plasma is maintained during acute inflammation in part by increased synthesis and secretion by liver of nonalbumin plasma proteins, e.g. the acute-phase reactants.

Regulation of carbamoyl phosphate synthetase-aspartate transcarbamoylase-dihydroorotase gene expression in growing and arrested cells

We have studied expression of the carbamoyl-P synthetase-aspartate transcarbamoylase-dihydroorotase (CAD) gene in growing or confluent Syrian hamster cells, and also in cells arrested by depriving them of serum or restimulated by adding fresh serum. In contrast to other biosynthetic enzymes such as dihydrofolate reductase, the amount of the CAD enzyme decreased very little when growth was arrested, as judged by the small change in aspartate transcarbamoylase activity. However, the level of CAD mRNA was about 10-fold lower in arrested cells than in growing cells. The rate of transcription of CAD, measured by the nuclear run-off technique, decreased roughly in parallel with the decrease in the steady-state level of the mRNA, suggesting that control of transcription is probably an important element in the overall regulation of CAD gene expression. Expression of a different gene which lies near the 5' end of CAD changed in parallel with that of CAD, but expression of two other genes in the same region was insensitive to the growth state of the cells. Therefore, control of gene expression as a function of growth was specific rather than a reflection of more general changes in chromatin structure in the vicinity of CAD.

Expression of rat apolipoprotein A-IV and A-I genes: mRNA induction during development and in response to glucocorticoids and insulin

Rat apolipoproteins (apo-) A-IV and A-I share many structural similarities, the most notable of which is a domain of repeated docosapeptides with amphipathic helical potential. Although the genes for apo-A-IV and apo-A-I probably diverged from a common ancestor, these proteins seem to have developed different functions in their evolution. In the present study, cloned cDNAs were used to characterize the expression of apo-A-IV and apo-A-I mRNAs in a wide variety of adult rat tissues, as well as in small intestine and liver obtained from fetal, suckling, and weanling animals; comparisons were made to the expression of apo-E mRNA. The apo-A-IV and apo-A-I mRNAs were most abundant in adult small intestine and liver, with trace amounts detected in other tissues. Substantial amounts of these mRNAs were detected in the yolk sac, suggesting that this fetal tissue plays an important role in lipid metabolism during gestation. Noncoordinate accumulation of apo-A-IV and apo-A-I mRNAs was observed within and between the liver and small intestine during neonatal development. The apo-A-IV mRNA levels in the developing small intestine and liver appeared to correlate with their triglyceride secretion rates, suggesting that this protein plays an important role in the metabolism of triglyceride-rich lipoproteins. When dexamethasone (0.1 microM), insulin (0.01 microM), or insulin and dexamethasone together were incubated with primary cultures of nonproliferating adult rat hepatocytes, apo-A-IV mRNA levels were 4-, 7-, and 11-fold higher, respectively, than in non-hormone-treated control hepatocytes. Hormone administration resulted in a 2-fold greater amount of apo-A-I mRNA in each case, with no significant change in the level of apo-E mRNA. The overall results suggest that these structurally related apolipoproteins are regulated in substantially different ways.

Tissue specific expression and developmental regulation of two genes coding for rat fatty acid binding proteins

We have examined the tissue distribution and developmental regulation of two low molecular weight cytosolic fatty acid binding proteins. Based on their initial site of isolation, they have been referred to as liver and intestinal fatty acid binding proteins (FABP). Cloned cDNAs were used to probe blots of RNAs extracted from a wide variety of adult rat tissues as well as small intestine and liver RNA obtained from fetal, suckling, and weaning animals. The highest concentrations of "liver" FABP mRNA were found in small intestine and liver. "Intestinal" FABP mRNA is most abundant in small bowel RNA while only trace amounts were encountered in liver. Both mRNAs were detectable in stomach, colon, pancreas, spleen, lung, heart, testes, adrenal, and brain RNA at 1-8% the concentrations observed in small intestine. Accumulation of both mRNAs in the small intestinal epithelium increases during development. The mRNAs are first detectable between the 19th and 21st day of gestation. They undergo a coordinated 3-4-fold increase in concentration within the first 24 h after birth. Thereafter, gut levels of intestinal FABP mRNA remain constant during the suckling period while liver FABP mRNA increases an additional 2-fold. Liver FABP mRNA levels are also induced in hepatocytes during the first postnatal day but subsequently do not change during the suckling and weaning phase, despite marked alterations in hepatic fatty acid metabolism. These observations support the concept that the major role of these proteins is to facilitate the entry of lipids into cells and/or their subsequent intracellular transport and compartmentalization. The data also raise questions about the identity of extragastrointestinal FABPs.

Apolipoprotein E mRNA is abundant in the brain and adrenals, as well as in the liver, and is present in other peripheral tissues of rats and marmosets

The relative amount of apolipoprotein (apo-) E mRNA in 12 different tissues of the rat and marmoset was examined by dot blot hybridization using cloned cDNA probes. As expected, it was found to be most abundant in the liver. However, substantial amounts of apo-E mRNA were found in the brain and adrenals at relative levels about one-third of that found in the liver. Significant quantities of apo-E mRNA were detected in all of the other peripheral tissues as well. The apo-E mRNA levels in these tissues were 2-10% of that found in the liver of the rat and 10-30% of that found in the liver of the marmoset. Apo-E mRNA was also abundant in human brain and in each species examined; it was distributed throughout all major areas of this organ. In contrast, apo-A-I mRNA was detected in abundant amounts only in the small intestine and in the liver. Extrahepatic apo-E mRNA appears to be functional, generating a translation product similar or identical to that generated by the liver. During fetal and neonatal development, apo-E mRNA is rapidly induced from low levels to approximately equal to 60% of adult levels in liver at parturition. The fetal yolk sac contains more apo-E mRNA than the fetal liver, suggesting a significant role for the yolk sac as a source of apo-E during gestation.

Evidence that the folate-requiring enzymes of de novo purine biosynthesis are encoded by individual mRNAs

Isolation of the mRNAs encoding for the three folate-requiring enzymes involved in de novo purine biosynthesis followed by their in vitro translation resulted in three separate proteins electrophoretically identical with those previously isolated. The three enzymes are glycinamide ribonucleotide transformylase, 5-aminoimidazole-4-carboxamide ribonucleotide transformylase, and 5,10-methenyl-, 5,10-methylene-, and 10-formyltetrahydrofolate synthetase. Thus these enzymes do not appear to be derived from large multifunctional proteins that are then subject to proteolysis in vivo or during in vitro purification. The levels of these enzymatic activities were increased by approximately 2-fold after raising the concentration of protein in the chicken's diet. The observed response is similar to that noted for glutamine phosphoribosylpyrophosphate amidotransferase, the presumed rate-limiting enzymatic activity for this pathway. For 5-amino-imidazole-4-carboxamide ribonucleotide transformylase and the trifunctional synthetase but not glycinamide ribonucleotide transformylase the increase in enzymatic activity correlates with higher mRNA levels.

Structure of amplified DNA in different Syrian hamster cell lines resistant to N-(phosphonacetyl)-L-aspartate

Syrian hamster cell lines selected in multiple steps for resistance to high levels of N-(phosphonacetyl)-L-aspartate (PALA) contain many copies of the gene coding for the pyrimidine pathway enzyme CAD. Approximately 500 kilobases of additional DNA was coamplified with each copy of the CAD gene in several cell lines. To investigate its structure and organization, we cloned ca. 162 kilobases of coamplified DNA from cell line 165-28 and ca. 68 kilobases from cell line B5-4, using a screening method based solely on the greater abundance of amplified sequences in the resistant cells. Individual cloned fragments were then used to probe Southern transfers of genomic DNA from 12 different PALA-resistant mutants and the wild-type parents. A contiguous region of DNA ca. 44 kilobases long which included the CAD gene was amplified in all 12 mutants. However, the fragments cloned from 165-28 which were external to this region were not amplified in any other mutant, and the external fragments cloned from B5-4 were not amplified in two of the mutants. These results suggest that movement or major rearrangement of DNA may have accompanied some of the amplification events. We also found that different fragments were amplified to different degrees within a single mutant cell line. We conclude that the amplified DNA was not comprised of identical, tandemly arranged units. Its structure was much more complex and was different in different mutants. Several restriction fragments containing amplified sequences were found only in the DNA of the mutant cell line from which they were isolated and were not detected in DNA from wild-type cells or from any other mutant cells. These fragments contained novel joints created by rearrangement of the DNA during amplification. The cloned novel fragments hybridized only to normal fragments in every cell line examined, except for the line from which each novel fragment was isolated or the parental population for that line. This result argues that "hot spots" for forming novel joints are rare or nonexistent.

Biology of simian virus 40 (SV40) transplantation antigen (TrAg). X. Tumorigenic potential of mouse cells transformed by SV40 in high responder C57BL/6 mice and correlation with the persistence of SV40 TrAg, early proteins and sequences

Primary mouse embryo fibroblasts of C57Bl/6 origin and cells derived from a tumor induced by polyoma virus in a C57Bl/6 mouse were transformed with SV40. The tumorigenic potential of these cells in normal adult and SV40-immunized mice was correlated with the synthesis of SV40 tumor antigens including the transplantation rejection antigen (TrAg) and with the presence of SV40 early region DNA sequences. Primary cells transformed by SV40 (B6/WT-3) induced tumors in immunocompetent adult syngeneic mice after adaptation in the immunosuppressed host. Passage of these tumor cells (B6/WT-3-T) through SV40-immunized mice resulted in the retention of both T and t antigens and TrAg. However, passage of SV40-transformed polyoma tumor cells through SV40-immunized immunocompetent adult mice but not in nonimmunized mice resulted in the loss of expression of SV40 tumor antigens including TrAg. This loss correlated with the loss of SV40 early region sequences from these double transformed cells. These results demonstrate that the establishment of in vitro SV40-transformed primary mouse cells into a tumor capable of progressive growth in high responder mice does not lead to the selection of variants which have lost the expression of early region DNA sequences.

Rapid and selective shutoff of plasma protein production in herpes simplex virus type 2-infected hepatoma cells

The effect of herpes simplex virus type 2 (HSV-2) infection of hepatoma McA-RH7777 cells on the production of alpha-fetoprotein (AFP) and several other secreted plasma proteins was examined. Cells infected with HSV-2 were labeled with [35S]methionine for various times postinfection (p.i.). Culture media and intracellular extracts were immunoprecipitated with plasma protein antibodies and examined by polyacrylamide gel electrophoresis. The relative levels of AFP and several other plasma proteins decreased substantially in infected cells compared with mock-treated controls; however, the level of at least one secreted plasma protein was unchanged after infection and the level of another increased after infection. Hybridization analyses with AFP cDNA showed that AFP RNA decreased to 43 and 30% of controls at 3.5 and 6.5 hr p.i., respectively. These observations were supported by the results of cell-free translation of isolated poly(A)-containing RNA. HSV-2 proteins were not detectable at times p.i. when AFP synthesis, secretion, and mRNA levels were significantly diminished. No decrease in AFP levels was observed in cells infected with ultraviolet-irradiated virus. We conclude that HSV-2 infection of McA-RH7777 cells leads to a rapid decrease in synthesis and secretion of specific plasma proteins which is apparent, at least for AFP, at the level of mRNA. This shutoff is not common to all McA-RH7777 cell proteins and, although seen early after infection, most likely requires the expression of a virus gene(s).

Diabetes-induced alterations in liver protein synthesis. Changes in the relative abundance of mRNAs for albumin and other plasma proteins

Mechanisms responsible for diabetes-induced alterations in liver protein synthesis were investigated in vivo and in perfused liver using Bio-Breeding Worcester (BB/W) control rats, spontaneously diabetic BB/W rats maintained on insulin therapy, and diabetic BB/W rats withdrawn from insulin therapy for 48 h. Withdrawal of insulin therapy in the diabetic rats resulted in marked alterations in a number of parameters related to liver protein synthesis compared to BB/W control or insulin-maintained diabetic rats. Alterations seen in vivo following withdrawal of insulin included changes in the relative concentrations of several plasma proteins, a 40% reduction in total liver RNA relative to DNA, a 5-fold reduction in albumin synthesis relative to the synthesis of total liver proteins, a 5-fold reduction in albumin mRNA relative to total RNA, reductions in the relative abundance of mRNAs for at least four plasma proteins other than albumin, and a relative increase in mRNA for at least one plasma protein. Alterations observed in perfused liver included reductions in total liver protein synthesis (60% of control), albumin production (24% of control), and total secretory protein production (44% of control). All parameters studied were essentially unchanged from BB/W control values when the diabetic rats were maintained on insulin therapy. The results indicate that insulin deficiency leads to marked reductions in liver protein synthesis, particularly the synthesis of albumin and other plasma proteins. The mechanisms responsible for these alterations include changes in the relative abundance of specific mRNAs and a decrease in total cellular RNA.

The role of amino acids in the regulation of protein synthesis in perfused rat liver. II. Effects of amino acid deficiency on peptide chain initiation, polysomal aggregation, and distribution of albumin mRNA

Decreased rates of protein synthesis which occurred in rat livers perfused with amino acid-deficient medium were accompanied by a loss of polysomes and a doubling of concentrations of ribosomal subunits and monomers as compared to unperfused liver or livers perfused with amino acid-supplemented medium. The loss of polysomes was not the result of mRNA degradation because this effect could be reversed by addition of amino acids to the perfusion medium. Instead, loss of polysomes indicated an impairment in peptide chain initiation. To determine whether the block of initiation could be the result of sequestration of mRNA in untranslatable pools, the content of albumin mRNA in membrane-bound and free polysomes and in the non-polysomal fraction isolated on sucrose density gradients was determined by hybridization to an albumin cDNA. In livers perfused with an amino acid-supplemented medium, 90% of the albumin mRNA was found in bound polysomes, and with amino acid-deficient medium, this value was decreased to 80%. Thus, there was no indication of a significant pool of mRNA which was not being translated. The block of initiation was, however, accompanied by a marked change in density and methionine-binding characteristics of the 40 S ribosomal subunit in livers perfused with deficient medium as compared to livers perfused with supplemented medium or to unperfused livers. There was a relative loss of 40 S subunits in a low density form, 1.41 g/cm3, an increase in the proportion in a high density form, 1.48 g/cm3, and a decrease in binding of [35S]methionine to 40 S ribosomal subunits. These changes provide evidence of reduced rates of formation of the 40 S initiation complex.