A new class mutation of low density lipoprotein receptor with altered carbohydrate chains

In a monensin-resistant mutant (Monr-31) of Chinese hamster ovary cells, the O-linked sugar chains of the low density lipoprotein (LDL) receptor are altered, suggesting a mutation at a Golgi apparatus gene. In a compactin-resistant mutant (MF-2) of Chinese hamster V79 cells, the mature LDL receptor is apparently 5000 daltons smaller; the difference is due to altered glycosylation of O-linked sugar chains. Hybrids between MF-2 and Monr-31 still produced LDL receptor molecules with aberrant sugar chains; thus both mutants are in the same complementation group. Krieger and his colleagues (Krieger, M., Kingsley, D., Sege, R., Hobbie, L., and Kozarsky, K. (1985) Trends. Biochem. Sci. 10, 447-452) have classified Chinese hamster ovary cell mutants with altered LDL receptor structure into four groups: ldlA, ldlB, ldlC, and ldlD. Cell-cell hybrids between their ldl mutants and Monr-31 produced wild type mature LDL receptors with normal molecular sizes, suggesting that these compactin- and monensin-resistant mutants define a new class of LDL receptor mutant. Since both of our mutants are defective in internalization of LDL, we assign them as int mutants. This may imply a further etiology for hypercholesterolemia, and cases can now be examined for such a class.     

Chinese hamster cell mutants with defective endocytosis of low-density lipoprotein contain altered fatty acid composition in the membrane

Chinese hamster V79 cell mutants resistant to compactin (ML236B), a specific inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, are defective in endocytosis of low-density lipoprotein (1). Two resistant clones, MF-2 and MF-3, differ in lipid composition from the parental V79 strain. In both the total cells and membrane fraction, the ratio of palmitoleic acid (16:1)/palmitic acid (16:0) is 0.4-0.5 in MF-2 and 1.7-1.8 in MF-3 while that in V79 is 0.2-0.3. By contrast, a hybrid clone between V79 and MF-3 shows a ratio of palmitoleic acid to palmitic acid very similar to that of V79. The synthesis of palmitoleic acid from acetate in the resistant clone is higher than in V79.     

Chinese hamster cell mutant resistant to ML236B (Compactin) is defective in endocytosis of low-density lipo-protein.

A fungal metabolite, ML236B (Compactin), isolated from Penicillium citrinum, is a specific inhibitor of 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A reductase (EC 1.1.1.34). Three ML236B-resistant (ML236Br) mutants, MF-1, MF-2, and MF-3, were isolated from V79 after N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis. The fluctuation test showed 2.2 X 10(-6) mutants per cell per generation of a spontaneous mutation frequency of ML236Br clones. These ML236Br clones showed a four- to fivefold-higher resistance to the drug than did their parental V79. Radioactive acetate, but not mevalonate, incorporation into the sterol fraction increased about 10-fold in ML236Br clones in comparison with that in V79. The cellular level of HMG-coenzyme A reductase in three ML236Br mutants was found to be a few-fold higher than that of V79 when cultured in the presence of lipoproteins. The 125I-labeled low-density lipoprotein-binding assay showed binding activity in three ML236Br clones comparable to that of the parental V79 cells. By contrast, an internalization assay of 125I-labeled low-density lipoprotein into the cells showed significantly reduced activity in three ML236Br clones in comparison with V79.     

Ultrastructural changes during the enhancement of cellular 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase in a Chinese hamster cell mutant resistant to compactin (ML 236B)

Chinese hamster V79 cell mutants resistant to compactin (ML236B) were isolated. A resistant clone, MF-2, grown in the presence of 2 micrograms/ml of ML236B for 1 week showed a 30-fold increase in 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG-Co A) reductase activity compared to MF-2 grown in the absence of ML236B, and cells grown for 4 weeks showed a 53-fold increase. Apparent ultrastructural changes in thin sections of the MF-2 cells were observed after growth in ML236B: dilated cisternae in the rough endoplasmic reticulum had or did not have flocculated contents; there was significant distension of perinuclear space; and vesicular inclusion bodies were present in nuclei.     

Increased surface binding sites of insulin in ML236B (compactin)-resistant mutants of Chinese hamster cell line

Mutants resistant to ML236B (compactin) were isolated from the Chinese hamster lung V79 cell line (1). Three ML236B-resistant mutants, MF-1, MF-2 and MF-3, were enhanced in insulin-specific binding activity about 2 to 3 times over the parental V79 cell lines. Compared to V79, endocytosis of insulin was also increased 2 to 3-fold in ML236B-resistant mutants than V79. Scatchard analysis showed that 5,000 insulin binding sites per cell in V79 and 16,000 in a NL236B-resistant clone, MF-2. Insulin receptors in mutant and parental strains are down-regulated to a similar extent in the parental V79 treated with an excess insulin. This is the first somatic cell mutant with increased surface binding sites for insulin.     

Chinese hamster ovary cell mutant with defective down-regulation of low density lipoprotein receptors

A monensin-resistant clone (Monr-31) shows a related series of differences from its parental Chinese hamster ovary (CHO) cell line in the cellular response to several ligands. The uptake and metabolism of low density lipoprotein (LDL) in the mutant cells are defective. Accumulation of fluorescent-labeled LDL as well as internalization and degradation of 125I-LDL are greatly reduced in Monr-31 cells. The receptor number for LDL on the cell surface of Monr-31 is about one-third that for CHO cells, but affinity constants for both cell lines are similar. Electrophoretic analysis shows a slightly reduced molecular weight of LDL receptor in Monr-31 cells in comparison to that in CHO cells. The internalization index (internalization plus degradation per binding) of LDL of the mutant is about one-half that of CHO cells, suggesting a failure of internalization of LDL as well as LDL binding. Hybrids (hyb-1, -2, and -3) between CHO and Monr-31 cells show LDL binding and LDL internalization activities comparable to that of CHO cells, suggesting that the altered LDL response in Monr-31 cells is recessive. Addition of exogenous LDL to culture medium down-regulates the LDL receptor activity of CHO, hyb-2, and hyb-3 cells, whereas no such down-regulation is seen in Monr-31 cells. Probably as a result of the failure of down-regulation, the prominent inhibition of sterol synthesis from acetate and 3-hydroxy-3-methylglutaryl-coenzyme A reductase observed in CHO cells is scarcely detectable in Monr-31 cells. As a correlated result, sterol synthesis from acetate is 6-fold higher in the mutant. The failure of down-regulation of LDL receptors in Monr-31 cells is discussed in relation to the altered binding and internalization of LDL.     

Novel feature of metabolism of low density lipoprotein receptor in a mouse macrophage-like cell line, J774.1

Biosynthesis, processing, and degradation of low density lipoprotein (LDL) receptors were studied in a mouse macrophage-like cell line, J774.1, by immunoprecipitation and immunoblotting with an antibody directed against the COOH-terminal 14 amino acids of the LDL receptor. The molecular weight of the mature LDL receptor of J774.1 cells maintained in RPMI medium was 140,000 under nonreducing condition and 160,000 under reducing condition in sodium dodecyl sulfate-polyacrylamide gels. These sizes are 10,000-15,000 daltons larger than those of the receptor in other mouse fibroblastic cells or P388 leucocyte. However, when J774.1 cells were cultured in Dulbecco's modified Eagle's medium, the molecular weight of the mouse cell lines, 123,000 under nonreducing condition and 153,000 under reducing condition. The larger LDL receptor molecules produced by J774.1 cells cultured in RPMI were insensitive to the treatment with end-alpha-N-acetylgalactosaminidase (O-glycanase), suggesting that aberrant serine/threonine-linked (O-linked) glycosylation might account for the apparent large size. Pulse-chase experiments revealed that the rate of processing of the LDL receptor from precursor to mature form in J774.1 was similar to that in other mouse cell lines, but the rate of degradation was much faster: half-life of the LDL receptor of J774.1 was about 2 h. No significant difference in biological function or lifetime was observed between the normal and the larger LDL receptor. This novel character of molecular size and lifetime of the LDL receptor in J774.1 is discussed in relation to altered maturation and/or modification during receptor biosynthesis.     

Mechanisms of 3-hydroxy-3-methylglutaryl coenzyme A reductase overaccumulation in three compactin-resistant cell lines

We have isolated three mammalian cell lines which are resistant to compactin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase. The drug resistance in all three cell lines is due to an increase of HMG-CoA reductase activity. Two of the three cell lines overaccumulate HMG-CoA reductase messenger RNA when grown in the presence of compactin. DNA hybridization experiments indicate that both a baby hamster kidney-derived compactin-resistant cell line, C100, and a cell line derived from mouse 3T6 cells, 3T6-40, exhibit amplifications of the HMG-CoA reductase gene. A third compactin-resistant cell line derived from Chinese hamster ovary cells, ML100, does not exhibit an amplification of the HMG-CoA reductase gene, nor does it show an elevated level of HMG-CoA reductase mRNA, comparable to that seen in the other cell lines.     

Characterization of functional histamine H1 receptors on a cultured smooth muscle cell line

Cultured cells of the smooth muscle line DDT1MF-2, which was derived from a hamster vas deferens tumor, expressed histamine H1-type receptors and responded biochemically and functionally to H1-specific stimulation. The H1-receptor antagonist [3H]-pyrilamine bound specifically to 9.7 x 10(6) sites/DDT1MF-2 cell with a dissociation constant (Kd) of 219 nM. The addition of histamine to suspensions of fura-2-loaded DDT1MF-2 cells elicited a rapid, transient, and stimulus concentration-dependent increase in the intracellular concentration of Ca2+ with an EC50 of 3 x 10(-5) M, which demonstrated H1 receptor specificity. Moreover, in order to evaluate in vitro contractile response of individual DDT1MF-2 cells, the degree of intracellular actin polymerization was quantified by a DNase inhibition assay. The percentage of nonpolymerized or G-actin in DDT1MF-2 cells was reduced in a histamine concentration-dependent manner with an EC50 of 1 x 10(-5) M and H1 receptor specificity. Histamine-induced actin polymerization was accompanied by changes in cell shape that were consistent with cellular contraction, as assessed by flow cytometry. The H1-type receptors of cultured DDT1MF-2 cells thus couple histamine stimulation to a variety of functional responses of smooth muscle cells.     

DnaJA4 is a SREBP-regulated chaperone involved in the cholesterol biosynthesis pathway

Using subtractive hybridization technique in 3T3-L1 adipocytes overexpressing constitutively active SREBP2, we have identified a DnaJ/Hsp40 chaperone, DnaJA4, as a new SREBP-responsive gene. SREBP2 regulation was demonstrated by changes in DnaJA4 mRNA under conditions of altered sterol status that were strictly parallel to that of well-characterized SREBP targets (LDL receptor and HMG-CoA reductase). The role of SREBP2 was further established using adenoviral overexpression of a dominant negative SREBP2, which abolished cholesterol-regulated changes in DnaJA4 expression. To determine the functional significance of this regulation, DnaJA4 was overexpressed in COS cells, which induced a specific increase in the synthesis of cholesterol from acetate. We also observed that DnaJA4 overexpression increased the activity and the protein content of HMG-CoA reductase, the rate limiting enzyme in this pathway. At the molecular level, DnaJA4 overexpression did not alter HMG-CoA reductase stability or mRNA levels, suggesting a co-translational effect of the chaperone. In the DnaJ/Hsp40 family, DnaJA4 uniquely exhibited SREBP-regulated expression, and also responded to heat shock. Through its responsiveness to SREBP, and its stimulatory effect on cholesterol synthesis, the DnaJA4 chaperone can be viewed as a new player in cholesterol synthesis. These data suggest a link between molecular chaperones, heat stress and cholesterol synthesis.     

Low binding capacity and altered O-linked glycosylation of low density lipoprotein receptor in a monensin-resistant mutant of Chinese hamster ovary cells

We have studied function and structure of the low density lipoprotein (LDL) receptors in a monensin-resistant (Monr-31) mutant isolated from Chinese hamster ovary (CHO) cells. To assay the ability of the receptor to bind LDL, we employed three methods, 125I-LDL binding to the cells at 4 degrees C, 125I-LDL binding to the receptor-phospholipid complex (Schneider, W.J., Goldstein, J.L., and Brown, M.S. (1980) J. Biol. Chem. 255, 11442-11447), and ligand blotting (Daniel, T.O., Schneider, W.J., Goldstein, J.L., and Brown, M.S. (1983) J. Biol. Chem. 258, 4606-4611). The LDL receptor number was similar in both CHO and Monr-31, but the binding affinity was reduced in the mutant. The semi-quantitative immunoblotting assay with an antibody directed against the COOH-terminal 14 amino acids and the ligand-blotting assay with LDL also showed that the relative steady-state level of the receptor in Monr-31 was comparable to that in CHO, whereas the binding capacity of the receptor in Monr-31 was lower than that in CHO. The precursor and degradation forms of the LDL receptors produced in the mutant cells were similar in size to those in the parental cells, but the apparent molecular mass of the mature receptor protein in sodium dodecyl sulfate-polyacrylamide gels was reduced about 5000 daltons in the mutant. These results suggest a structural change at the NH2-terminal LDL binding domain. Tests of the effects of tunicamycin, endo-alpha-N-acetylgalactosaminidase (O-glycanase), and sialidase (neuraminidase) on the molecular size of the mature receptors indicated that the reduced size of the receptor in the mutant cells resulted from altered oligosaccharide chain(s) linked to serine/threonine residues in the binding domain. We compared the molecular sizes and binding activity of human LDL receptors in several clones derived from CHO and Monr-31 cells which were transfected with human LDL receptor cDNA. The human LDL receptors produced in the transfected clones of Monr-31 were also smaller in molecular size and lower in binding capacity than those produced in the transfected clones of CHO. These results suggest that both structural and functional alteration of the LDL receptor of Monr-31 is not caused by a mutation in the structural gene of the LDL receptor but by altered processing or maturation of the receptor. The correlation of the decrease in molecular size and reduced binding capacity of the LDL receptor is discussed.     

Macrophage-derived factors increase low density lipoprotein uptake and receptor number in cultured human liver cells.

Recent evidence suggests the possibility that macrophages can influence lipoprotein metabolism. Therefore we investigated the ability of cultured macrophages to alter low density lipoprotein (LDL) uptake in a human liver cell line (HepG2). Conditioned media from phlogogenic-induced mouse peritoneal macrophages or from a human macrophage cell line stimulated with endotoxin increased HepG2 LDL uptake by as much as 60-70%. The increase was due, in part, to a significant macrophage-induced 40% increase in the number of LDL receptors per cell. Although macrophage conditioned media inhibited HepG2 cholesterol synthesis, the LDL receptor up-regulation did not appear to be due to the effects on cholesterol synthesis. The LDL receptor stimulatory activity was sensitive to proteolysis and heat. Its molecular mass was approximately 20 kDa based on gel filtration. Several macrophage secretory proteins were tested in HepG2 cultures for LDL uptake stimulation. Of these, oncostatin M (approximately 18 kDa by gel filtration) gave the strongest response. The rank order for LDL uptake stimulation was oncostatin M much greater than interleukin 6 = interleukin 1 = transforming growth factor-beta 1. A neutralizing antibody directed against oncostatin M inhibited the ability of conditioned media to up-regulate LDL receptors by 85%. Thus, our results indicate that macrophages can secrete several proteins that up-regulate LDL receptors in HepG2 cells and that most of the up-regulatory activity in macrophage conditioned media appears to be due to oncostatin M.     

The effect of (-)-hydroxycitrate on the activity of the low-density-lipoprotein receptor and 3-hydroxy-3-methylglutaryl-CoA reductase levels in the human hepatoma cell line Hep G2.

(-)-Hydroxycitrate, a potent inhibitor of ATP citrate-lyase, was tested in Hep G2 cells for effects on cholesterol homoeostasis. After 2.5 h and 18 h incubations with (-)-hydroxycitrate at concentrations of 0.5 mM or higher, incorporation of [1,5-14C]citrate into fatty acids and cholesterol was strongly inhibited. This most likely reflects an effective inhibition of ATP citrate-lyase. Cholesterol biosynthesis was decreased to 27% of the control value as measured by incorporations from 3H2O, indicating a decreased flux of carbon units through the cholesterol-synthetic pathway. After 18 h preincubation with 2 mM-(-)-hydroxycitrate, the cellular low-density-lipoprotein (LDL) receptor activity was increased by 50%, as determined by the receptor-mediated association and degradation. Measurements of receptor-mediated binding versus LDL concentration suggests that this increase was due to an increase in the numbers of LDL receptors. Simultaneously, enzyme levels of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase as determined by activity measurements increased 30-fold. Our results suggest that the increases in HMG-CoA reductase and the LDL receptor are initiated by the decreased flux of carbon units in the cholesterol-synthetic pathway, owing to inhibition of ATP citratelyase. A similar induction of HMG-CoA reductase and LDL receptor was also found after preincubations of cells with 0.3 microM-mevinolin, suggesting that the underlying mechanism for this induction is identical for both drugs.     

Effects of compactin, mevalonate and low-density lipoprotein on 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity and low-density-lipoprotein-receptor activity in the human hepatoma cell line Hep G2.

Compactin, an inhibitor of HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase, decreased cholesterol synthesis in intact Hep G2 cells. However, after the inhibitor was washed away, the HMG-CoA-reductase activity determined in the cell homogenate was found to be increased. Also the high-affinity association of LDL (low-density lipoprotein) to Hep G2 cells was elevated after incubation with compactin. Lipoprotein-depleted serum, present in the incubation medium, potentiated the compactin effect compared with incubation in the presence of human serum albumin. Addition of either mevalonate or LDL prevented the compactin-induced rise in activities of both HMG-CoA reductase and LDL receptor in a comparable manner. It is concluded that in this human hepatoma cell line, as in non-transformed cells, both endogenous mevalonate or mevalonate-derived products and exogenous cholesterol are able to modulate the HMG-CoA reductase activity as well as the LDL-receptor activity.     

Regulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in mouse peritoneal macrophages.

The lipoprotein-mediated regulation of 3-hydroxy-3-methylglutaryl-(HMG-) CoA reductase in cultured mouse peritoneal macrophages has been investigated. In contrast to what has been reported for other cells, HMG-CoA reductase activity is not suppressed by normal serum or by normal low density lipoproteins (LDL) from humans or dogs. Suppression of reductase activity occurred when cells were cultured in the presence of beta-migrating very low density lipoproteins (beta-VLDL) or LDL from hypercholesterolaemic dogs, or LDL modified by acetoacetylation. Human beta-VLDL from an atypical type III hyperlipoproteinaemic patient was also effective, as was apolipoprotein (apo) E-containing high density lipoproteins (HDL) from cholesterol-fed dogs (apo-E HDLc). The results indicate that cholesterol biosynthesis in mouse peritoneal macrophages is regulated by lipoprotein cholesterol entering via receptor-mediated endocytosis. Normal LDL were not effective because of the poor binding and uptake of these lipoproteins by the apo-B, E (LDL) receptor. Only beta-VLDL, apo-E HDLc, and hypercholesterolaemic LDL were avidly taken up by this receptor and were able to suppress HMG-CoA reductase. Acetoacetylated LDL were internalized via the acetyl-LDL (scavenger) receptor. Thus, mouse macrophages differ from human fibroblasts and smooth muscle cells in their physiological regulation of cholesterogenesis.     

Effects of a new calf thymus protein on 3-hydroxy-3-methylglutaryl-CoA reductase activity in rat (Rattus bubalus) hepatocyte cells (BRL-3A).

1. In a previous paper we described the purification steps of a new calf thymus protein able to activate the LDL receptor catabolism. 2. In this paper we examine the modulatory effect of this new calf thymus protein on 3HMG-CoA reductase activity in rat hepatocyte cells to better clarify the role of this protein on cholesterol metabolism. 3. The results obtained show that the calf thymus protein inhibits the HMG-CoA reductase, and support the hypothesis that the activation of LDL receptor catabolism is mediated by a decreased amount of cellular cholesterol following HMG-CoA reductase inhibition.     

Stimulation of the LDL receptor activity in the human hepatoma cell line Hep G2 by high-density serum fractions

The regulation of the LDL receptor activity in the human hepatoma cell line Hep G2 was studied. In Hep G2 cells, in contrast with fibroblasts, the LDL receptor activity was increased 2.5-fold upon increasing the concentration of normal whole serum in the culture medium from 20 to 100% by volume. Incubation of the Hep G2 cells with physiological concentrations of LDL (up to 700 micrograms/ml) instead of incubation under serum-free conditions resulted in a maximum 2-fold decrease in LDL receptor activity (10-fold decrease in fibroblasts). Incubation with physiological concentrations of HDL with a density of between 1.16 and 1.20 g/ml (heavy HDL) resulted in an approximately 7-fold increase in LDL receptor activity (1.5-fold increase in fibroblasts). This increased LDL receptor activity is due to an increase in the number of LDL receptors. Furthermore, simultaneous incubation of Hep G2 cells with LDL and heavy HDL (both 200 micrograms/ml) resulted in a 3-fold stimulation of the LDL receptor activity as compared with incubation in serum-free medium. 3-Hydroxy-3-methylglutaryl-CoA reductase activity was also stimulated after incubation of Hep G2 with heavy HDL (up to 3-fold). The increased LDL receptor activity in Hep G2 cells after incubation with heavy HDL was independent of the action of lecithin:cholesterol acyltransferase during that incubation. However, previous modification of heavy HDL by lecithin:cholesterol acyltransferase resulted in an enhanced ability of heavy HDL to stimulate the LDL receptor activity. Our results indicate that in Hep G2 cells the heavy HDL-mediated stimulation of the LDL receptor activity overrules the LDL-mediated down-regulation and raises the suggestion that in man the presence of heavy HDL and the action of lecithin:cholesterol acyltransferase in plasma may be of importance in receptor-mediated catabolism of LDL by the liver.     

Differential regulation of the expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase, synthase, and low density lipoprotein receptor genes.

The ability of mitogenic stimulation of human T lymphocytes to alter the expression of genes involved in sterol metabolism was examined. Messenger RNA levels for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, HMG-CoA synthase, and low density lipoprotein (LDL) receptor were quantified in resting and mitogen-stimulated T lymphocytes by nuclease protection assay. Mitogenic stimulation increased HMG-CoA synthase mRNA levels by 5-fold and LDL receptor by 4-fold when cells were cultured in lipoprotein-depleted medium whereas HMG-CoA reductase gene expression was not significantly increased. When cultures were supplemented with concentrations of low density lipoprotein sufficient to saturate LDL receptors, expression of all three genes was inhibited in resting lymphocytes, as effectively as was noted with fibroblasts. Similarly, LDL down-regulated gene expression in mitogen-activated lymphocytes so that mitogenic stimulation did not increase either HMG-CoA reductase or synthase mRNA levels, although LDL receptor gene expression was enhanced. These results indicate that expression of three of the genes involved in sterol metabolism is differentially regulated by LDL and mitogenic stimulation. Moreover, the increase in rates of endogenous sterol synthesis and the activity of HMG-CoA reductase in mitogen-stimulated T lymphocytes cannot be accounted for by increases in HMG-CoA reductase mRNA levels.