Histone structure: asymmetric distribution of lysine residues in lysine-rich histone.

Structural studies on a very lysine-rich histone show that the carboxyl-terminal half of the molecule is enriched in lysine (and proline). which suggests that it is a site for binding to DNA. The amino-terminal half. containing most of the acidic residues. resembles small, nonhistone proteins and so might have specificity for factors other than DNA.

Human apolipoprotein B: structure of carboxyl-terminal domains, sites of gene expression, and chromosomal localization.

Apolipoprotein (apo-) B is the ligand responsible for the receptor-mediated catabolism of low density lipoproteins, the principal cholesterol-transporting lipoproteins in plasma. The primary structure of the carboxyl-terminal 30 percent (1455 amino acids) of human apo-B (apo-B100) has been deduced from the nucleotide sequence of complementary DNA. Portions of the protein structure that may relate to its receptor binding function and lipid binding properties have been identified. The apo-B100 messenger RNA is about 19 kilobases in length. The apo-B100 gene is expressed primarily in liver and, to a lesser extent, in small intestine, but in no other tissues. The gene for apo-B100 is located in the p24 region (near the tip of the short arm) of chromosome 2.

Type III hyperlipoproteinemic phenotype in transgenic mice expressing dysfunctional apolipoprotein E.

Transgenic mice were prepared that expressed a dysfunctional apo E variant, apo E (Arg-112, Cys-142), which is associated with dominant inheritance of type III hyperlipoproteinemia (type III HLP) in humans. Among eight founder mice, plasma apo E (Arg-112, Cys-142) levels varied 100-fold and directly correlated with plasma cholesterol and triglyceride levels. On a normal chow diet, mice expressing high levels (> 70 mg/dl) of the dysfunctional apo E had grossly elevated plasma lipids, with cholesterol levels of up to 410 mg/dl and triglyceride levels of up to 1,210 mg/dl. Upon agarose electrophoresis, plasma from these mice demonstrated beta-very low density lipoproteins (beta-VLDL). Mice expressing low (< 2.5 mg/dl) or intermediate (21 mg/dl) levels of the apo E variant had much less severe hyperlipidemia and did not have beta-VLDL. Although the transgenic mouse beta-VLDL were enriched in cholesteryl esters compared with normal mouse VLDL, they were not as cholesterol enriched as human beta-VLDL from type III HLP subjects. Transgenic mouse beta-VLDL injected into normal mice were cleared from plasma at a significantly slower rate than normal mouse VLDL, demonstrating the impaired catabolism of beta-VLDL. Thus, transgenic mice expressing high levels of the dysfunctional apo E (Arg-112, Cys-142) variant have many characteristics of the human type III HLP phenotype and appear to be a suitable animal model for this disorder.

Identical structural and receptor binding defects in apolipoprotein E2 in hypo-, normo-, and hypercholesterolemic dysbetalipoproteinemia.

Apolipoprotein E (apoprotein E or apo-E) from type III hyperlipoproteinemic subjects with the E2/2 homozygous phenotype displays both structural and receptor binding heterogeneity. The apo-E from all subjects thus far studied, however, has been functionally defective, though to different degrees. Although nearly every type III hyperlipoproteinemic subject has the E2/2 phenotype, 95-99% of the people with this same phenotype do not display type III hyperlipoproteinemia, nor do they have elevated plasma cholesterol levels. Consequently, it became important to determine whether the apo-E2 from hypo- and normocholesterolemic individuals with the E2/2 phenotype is also functionally abnormal. To do this, apo-E2 was isolated from two hypo-, two normo- and two hypercholesterolemic homozygous E2/2 subjects. The apo-E2 was recombined with vesicles and tested for its ability to displace (125)I-low density lipoproteins (LDL) from apo-B,E (LDL) receptors on human fibroblasts. The apo-E2 from all six subjects was found to be severely defective in receptor binding (<2% of the binding activity of normal apo-E3). In all cases, the binding activity of the apo-E2 was increased 10- to 20-fold by treating the apoproteins with cysteamine, a reagent that converts cysteine residues to positively charged lysine analogues. The cysteine content of each apo-E was determined by monitoring the change in the isoelectric focusing position of the cysteamine-treated apo-E2. Using this method, it was found that the apo-E2 from each subject contained two cysteine residues per mole. A partial sequence analysis of the cysteine-containing regions of the apo-E from three of the six subjects indicated that the two cysteine residues were at residues 112 and 158 in the amino acid sequence. The cysteine at residue 158 has previously been implicated in the severe binding defect of the apo-E2 from a type III hyperlipoproteinemic subject. Since the apo-E2 of the hypo-, normo-, and hypercholesterolemic subjects in this study all displayed a severe functional abnormality, it is apparent that factors in addition to the defective receptor binding activity of the apo-E2 are necessary for the manifestation of type III hyperlipoproteinemia.

Fat feeding in humans induces lipoproteins of density less than 1.006 that are enriched in apolipoprotein [a] and that cause lipid accumulation in macrophages.

Formula diets containing lard or lard and egg yolks were fed to six normolipidemic volunteers to investigate subsequent changes in the composition of lipoproteins of d less than 1.006 g/ml and in their ability to bind and be taken up by receptors on mouse macrophages. Both formulas induced the formation of d less than 1.006 lipoproteins that were approximately 3.5-fold more active than fasting very low density lipoproteins (VLDL) in binding to the receptor for beta-VLDL on macrophages. Subfractionation of postprandial d less than 1.006 lipoproteins by agarose chromatography yielded two subfractions, fraction I (chylomicron remnants) and fraction II (hepatic VLDL remnants), which bound to receptors on macrophages. However, fraction I lipoproteins induced a 4.6-fold greater increase in macrophage triglyceride content than fraction II lipoproteins or fasting VLDL. Fraction I lipoproteins were enriched in apolipoproteins (apo) B48, E, and [a]. Fraction II lipoproteins lacked apo[a] but possessed apo B100 and apo E. The apo[a] was absent in normal fasting VLDL, but was present in the d less than 1.006 lipoproteins (beta-VLDL) of fasting individuals with type III hyperlipoproteinemia. The apo[a] from postprandial d less than 1.006 lipoproteins was larger than either of two apo[a] subspecies obtained from lipoprotein (a) [Lp(a)] isolated at d = 1.05-1.09. However, all three apo[a] subspecies were immunochemically identical and had similar amino acid compositions: all were enriched in proline and contained relatively little lysine, phenylalanine, isoleucine, or leucine. The association of apo[a] with dietary fat-induced fraction I lipoproteins suggests that the previously observed correlation between plasma Lp(a) concentrations and premature atherosclerosis may be mediated, in part, by the effect of apo[a] on chylomicron remnant metabolism.

Identification of a new structural variant of human apolipoprotein E, E2(Lys146 leads to Gln), in a type III hyperlipoproteinemic subject with the E3/2 phenotype.

A type III hyperlipoproteinemic subject having the apolipoprotein E (apo E) phenotype E3/2 was identified. From isoelectric focusing experiments in conjunction with cysteamine treatment (a method that measures cysteine content in apo E), the E2 isoform of this subject was determined to have only one cysteine residue, in contrast to all previously studied E2 apoproteins, which had two cysteines. This single cysteine was shown to be at residue 112, the same site at which it occurs in apo E3. From amino acid and sequence analyses, it was determined that this apo E2 differed from apo E3 by the occurrence of glutamine rather than lysine at residue 146. When phospholipid X protein recombinants of the subject's isolated E3 and E2 isoforms were tested for their ability to bind to the human fibroblast apo-B,E receptor, it was found that the E3 bound normally (compared with an apo E3 control) but that the E2 had defective binding (approximately 40% of normal). Although they contained E3 as well as E2, the beta-very low density lipoproteins (beta-VLDL) from this subject were very similar in character to the beta-VLDL from an E2/2 type III hyperlipoproteinemic subject; similar subfractions could be obtained from each subject and were shown to have a similar ability to stimulate cholesteryl ester accumulation in mouse peritoneal macrophages. The new apo E2 variant has also been detected in a second type III hyperlipoproteinemic subject.

A novel electrophoretic variant of human apolipoprotein E. Identification and characterization of apolipoprotein E1.

A new apolipoprotein E (apo E) phenotype has been demonstrated in a Finnish hypertriglyceridemic subject (R.M.). At the time of this study, R.M.'s plasma triglyceride and cholesterol levels were 1,021 and 230 mg/dl, respectively. The subject's apo E isoelectric focusing pattern was characterized by two major bands, one in the E3 position and the other in the E1 position. Normally the E1 position is occupied by sialylated derivatives of apo E4, E3, or E2. The E1 band of subject R.M. is not a sialylated form, however, because it was not affected by neuraminidase digestion. The identity of the E1 variant as a genetically determined structure was established by amino acid and partial sequence analyses, confirming that the variant is an example of a previously uncharacterized apo E phenotype, E3/1. Both cysteamine modification and amino acid analysis demonstrated that this variant contains two cysteine residues per mole. Sequence analysis of two cyanogen bromide fragments and one tryptic fragment of the apo E3/1 showed that it differs from E2(Arg158----Cys) at residue 127, where an aspartic acid residue is substituted for glycine. This single amino acid interchange is sufficient to account for the one-charge difference observed on isoelectric focusing gels between E2(Arg158----Cys) and the E1 variant. The variant has been designated E1 (Gly127----Asp, Arg158----Cys). When compared with apo E3, the E1 variant demonstrated reduced ability to compete with 125I-LDL for binding to LDL (apo B,E) receptors on cultured fibroblasts (approximately 4% of the amount of binding of apo E3). This defective binding is similar to that of E2-(Arg158----Cys). Therefore, the binding defect of the variant is probably due to the presence of cysteine at residue 158, rather than aspartic acid at residue 127. In contrast, the apo E3 isoform from this subject demonstrated normal binding activity, indicating that it has a normal structure. In family studies, the vertical transmission of the apo E1 variant has been established. It is not yet clear, however, if the hypertriglyceridemia observed in the proband is associated with the presence of the E1(Gly127----Asp, Arg158----Cys) variant.

Type III hyperlipoproteinemia associated with apolipoprotein E phenotype E3/3. Structure and genetics of an apolipoprotein E3 variant.

A family has been described in which type III hyperlipoproteinemia is associated with apo E phenotype E3/3 (Havel, R. J., L. Kotite, J. P. Kane, P. Tun, and T. Bersot. 1983. J. Clin. Invest. 72:379-387). In the current study, the structure of apo E from the propositus of this family was determined using both protein and DNA analyses. The propositus is heterozygous for two different apo E alleles, one coding for normal apo E3 and one for a previously undescribed variant apo E3 in which arginine replaces cysteine at residue 112 and cysteine replaces arginine at residue 142. Apo E gene analysis of nine other family members spanning four generations indicated that only those five members having type III hyperlipoproteinemia possess the variant apo E3. Like the propositus, all five are heterozygous for this variant, suggesting that the disorder in this family is transmitted in a dominant fashion. The variant apo E3 was defective in its ability to bind to lipoprotein receptors, and this functional defect probably contributes to the expression of type III hyperlipoproteinemia in this family.

Atherogenic lipoproteins resulting from genetic defects of apolipoproteins B and E.

Accelerated atherosclerosis occurs in patients with type III hyperlipoproteinemia and familial hypercholesterolemia. These genetic disorders focus attention on specific types of lipoproteins as being responsible for the development of accelerated coronary artery heart disease. The accumulation of chylomicron remnants of intestinal origin and of VLDL remnants or IDL of hepatic origin observed in type III hyperlipoproteinemia appears to correlate with coronary disease. The presence of defective forms of apo E prevents normal receptor-mediated catabolism of these lipoproteins. Patients with familial hypercholesterolemia have an elevation of plasma LDL (and to a lesser extent an increase in VLDL remnants and IDL) secondary to defective LDL receptors that impair normal catabolism. Familial defective apo B100 is secondary to an abnormality of apo B100 that prevents the normal interaction of LDL with the LDL receptor and increases plasma LDL. However, it has not yet been established that familial defective apo B100 predisposes affected individuals to accelerated atherosclerosis. Animals fed diets high in saturated fat and cholesterol have an accumulation of beta-VLDL, IDL, and LDL that resembles the changes in lipoproteins observed in patients with these genetic disorders. Macrophages (which are presumably derived from circulating monocytes) have emerged as a likely key component in atherogenesis because they appear to be progenitors of foam cells in arterial lesions. Macrophages in the arterial wall express receptors that recognize chylomicron remnants and VLDL remnants (beta-VLDL) and chemically modified LDL. Thus, in the presence of these specific lipoproteins, macrophages are converted to cells that resemble foam cells. The precise stimulus that causes monocyte-derived macrophages to enter specific regions of the arterial wall remains to be determined.

Lipoproteins of special significance in atherosclerosis. Insights provided by studies of type III hyperlipoproteinemia.

In summary, the study of type III hyperlipoproteinemia has provided important insights into lipoprotein metabolism that have helped to elucidate several functional roles for apo E and have provided a better understanding of the mechanisms whereby specific lipoproteins may be atherogenic or anti-atherogenic. The molecular defect in type III hyperlipoproteinemia and dysbetalipoproteinemia is the presence of a mutant form of apo E, usually apo E2, that is defective in binding to both apo B,E(LDL) and apo E receptors. The receptor-defective apo E results in an impaired clearance of remnant lipoproteins (beta-VLDL). In addition, the abnormal apo E may impair the lipolytic processing of hepatic beta-VLDL through its involvement in lipid transfer or exchange processes. The accumulation of beta-VLDL may provide the most direct mechanism responsible for the accelerated atherosclerosis observed in type III hyperlipoproteinemia, a mechanism that involves the receptor mediated uptake of beta-VLDL by macrophages, which are then converted to arterial foam cells. Alterations in the HDL of patients with type III hyperlipoproteinemia further support the concept that HDL are anti-atherogenic. The increase in HDL-with apo E provides insight into the role of these cholesterol-enriched HDL in reverse cholesterol transport and in the cellular redistribution of cholesterol, processes whereby cholesterol deposition may be reversed. It should be stressed that both the accumulation of beta-VLDL and alterations in HDL (reduction in typical HDL and an increase in HDL-with apo E) are associated with accelerated atherogenesis in animals fed high levels of fat and cholesterol. Although valuable information has been gained concerning the mechanisms involved in type III hyperlipoproteinemia by the study of the disease, the clinical expression of this disorder is variable, ranging from hypocholesterolemia to marked hypercholesterolemia in subjects with the same molecular defect (E2/2). This variability in expression is more easily understood when one considers the various factors that can promote the hyperlipoproteinemia and when one considers the mechanisms of action whereby these factors may exacerbate the effects of the presence of an abnormal apo E. In most cases, development of type III hyperlipoproteinemia requires that a second event (a predisposing environmental factor or a second genetic defect) be associated with the primary genetic defect (an abnormal form of apo E).

The so20,w of unsheared DNA from whole cell lysates of Escherichia coli.

We present measurements of the sedimentation coefficients of DNA present in whole cell lysates of E. coli. The method used is a preparative version of the band sedimentation experiment of Bruner and Vinograd. We show that in order to obtain reliable data on the time dependence of sedimentation, it is necessary to accelerate and decelerate the rotor over much longer times than the standard centrifuge allows. We describe the necessary modifications to the preparative centrifuge and use them to determine the So20,W of unsheared E. coli DNA. The value for the fastest moving components in the lysate is 220 S. The molecular weight of the DNA corresponding to this sedimentation coefficient is probably 1.7 X 10(9) g/mole. However, alternative values cannot be ruled out.

Apolipoprotein E: receptor binding properties.

The E apoprotein plays a central role in lipoprotein metabolism. It is a key protein determinant responsible for the recognition of lipoproteins by specific lipoprotein receptors. Structure-function studies are providing insights into the normal function of this apoprotein in cholesterol homeostasis and into the clinical abnormalities resulting from the occurrence of specific mutants.

Apolipoprotein E: far more than a lipid transport protein.

First recognized as a major determinant in lipoprotein metabolism and cardiovascular disease, apolipoprotein (apo) E has emerged as an important molecule in several biological processes not directly related to its lipid transport function, including Alzheimer's disease and cognitive function, immunoregulation, and possibly even infectious diseases. ApoE is a polymorphic protein arising from three alleles at a single gene locus. The three major isoforms, apoE4, apoE3, and apoE2, differ from one another only by single amino acid substitutions, yet these changes have profound functional consequences at both the cellular and molecular levels. ApoE3 seems to be the normal isoform in all known functions, while apoE4 and apoE2 can each be dysfunctional. Isoform (allele)-specific effects include the association of apoE2 with the genetic disorder type III hyperlipoproteinemia and with both increased and decreased risk for atherosclerosis and the association of apoE4 with increased risk for both atherosclerosis and Alzheimer's disease, impaired cognitive function, and reduced neurite outgrowth; isoform-specific differences in cellular signaling events may also exist. Functional differences in the apoE isoforms that affect (or did affect) survival before the reproductive years probably account, at least in part, for the allele frequencies of the present day.

The role of apolipoprotein E genetic variants in lipoprotein disorders.

Apolipoprotein E plays a central role in lipoprotein metabolism by serving as a ligand for the binding of lipoproteins to lipoprotein receptors. Both common and rare variants of apoE have been described. The common variants apoE2 and apoE4 have a significant impact on interindividual variation of lipid and lipoprotein levels in normal subjects. The common variant apoE2 and more than half a dozen rare variants are defective in binding to the low-density lipoprotein (LDL) receptor, and all are causally associated with the lipid disorder type III hyperlipoproteinaemia (HLP). The mode of inheritance of the disorder can be either dominant or recessive, depending on the particular mutation(s) in apoE, although the mechanisms involved are not fully understood. The common variant apoE4 and other rare variants have been reported to be associated with a variety of other lipoprotein disorders, but a causal link has not been established.

Human apolipoprotein B-100 heparin-binding sites.

Seven distinct heparin-binding sites have been demonstrated on human apolipoprotein (apo) B-100 by using a combination of digestion with cyanogen bromide or Staphylococcus aureus V-8 protease and heparin-Sepharose affinity chromatography. Based on fragment analysis, the approximate boundaries of the seven binding sites are as follows: site A, residues 5-99; site B, residues 205-279; site C, residues 875-932; site D, residues 2016-2151; site E, residues 3134-3209; site F, 3356-3489; and site G, residues 3659-3719. In sites E and F, two short regions enriched in basic amino acids have been identified, and it is likely that they are responsible for a major portion of the heparin-binding properties of these sites. The relative binding affinity of each of the seven sites was estimated in two ways. First, the affinity was assessed in a ligand blot assay using a 125I-labeled high-reactive heparin subfraction. Second, apoB-100 fragments generated by cyanogen bromide or S. aureus V-8 protease were separated into low- and high-affinity fractions by gradient salt elution of a heparin-Sepharose column. The distribution of the seven binding sites in the two fractions was determined in an immunoblotting assay using antibodies specific to each site, i.e. antibodies raised against synthetic peptide sequences found within each of the seven sites. The results of these two approaches demonstrate that site E and, to a somewhat lesser extent, site F bind to heparin with the highest affinity. Based on the analogy with apoE, in which the high-affinity heparin-binding site coincides with the domain of the protein that interacts with apoB,E (low density lipoprotein) receptors, the results of this study indicate that site E and site F, either singly or in combination, might constitute the receptor binding domain of apoB-100.

Induction of stable protein-deoxyribonucleic acid adducts in Chinese hamster cell chromatin by ultraviolet light.

Ultraviolet (uv)-light-mediated formation of protein-DNA adducts in Chinese hamster cell chromatin was investigated in an attempt to compare chromatin alterations induced in vitro with those observed in vivo. Three independent methods of analysis indicated stable protein-DNA associations: (1) a membrane filter assay which retained DNA on the filter in the presence of high salt-detergent; (2) a Sepharose 4B column assay in which protein eluted cincident with DNA; and (3) a CsCl density gradient equilibrium assay which showed both protein and DNA banding at densities other than their respective native densities. Treatment of the irradiated chromatin with DNase provided further evidence that protein-DNA and not protein-protein adducts were being observed in the column assay. There is a fluence-dependent response of protein-DNA adduct formation when the chromatin is irradiated at low ionic strength and is linear for protein over the range studied. When the chromatin is exposed to differing conditions of pH, ionic strength, or divalent metal ion concentration, the quantity of adduct formed upon uv irradiation varies. Susceptibility to adduct formation can be partially explained in terms of the condensation state of the chromatin and other factors such as rearrangement, denaturation, and dissociation of the chromatin components. Besides providing information on the biological significance of these types of uv-induced lesions, this technique may be useful as a probe of chromatin structure.

Genetic factors precipitating type III hyperlipoproteinemia in hypolipidemic transgenic mice expressing human apolipoprotein E2.

Several factors are hypothesized to precipitate or exacerbate type III hyperlipoproteinemia (HLP) in humans. Among such factors are those that directly overload remnant lipoprotein production or disrupt removal pathways, including an increased ratio of apolipoprotein (apo) E2 to normal apoE, overproduction of apoB-containing lipoproteins, and decreased LDL receptor activity. Hypolipidemic apoE2-transgenic mice bred onto an apoE-null background had dramatically higher plasma total cholesterol (192 +/- 26 mg/dL for males, 203 +/- 40 mg/dL for females) and triglyceride (295 +/- 51 mg/dL for males, 277 +/- 58 mg/dL for females) levels than apoE2 mice with endogenous mouse apoE. Thus, eliminating normal apoE in the presence of apoE2 (thereby increasing the relative abundance of the defective ligand) can convert a hypolipidemic to a hyperlipidemic phenotype. Hypolipidemic apoE2 transgenic mice overexpressing human apoB had moderate remnant accumulation compared with apoE2-only or apoB-only transgenic mice, indicating that overproduction of apoB-containing lipoproteins in the presence of apoE2 can augment remnant production. Hypolipidemic apoE2 transgenic mice bred-onto an LDL receptor-null background had markedly higher plasma total cholesterol (288 +/- 51 mg/dL for males, 298 +/- 73 mg/dL for females) and triglyceride (356 +/- 72 mg/dL for males, 317 +/- 88 mg/dL for females) levels than apoE2-only mice, and remnant accumulation increased even in apoE2 mice with a heterozygous LDL receptor-knockout background (compared with apoE2-only mice), suggesting that reducing or eliminating a major receptor-mediated remnant-removal pathway in the presence of apoE2 can also precipitate a hyperlipidemic phenotype. In all cases where either lipoprotein remnant production or removal pathways were severely stressed, increased remnant accumulation was apparent. As judged by the chemical characteristics of the remnant lipoproteins, the lipoprotein phenotype was quite similar to that of human type III HLP, especially in the apoE2-expressing mice with no endogenous apoE or LDL receptors, and thus these mice represent improved models of the disorder.