Evidence for coordinate expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase ad low density lipoprotein binding activity

Cholesterol feedback: from Schoenheimer's bottle to Scap's MELADL

Cholesterol biosynthesis is among the most intensely regulated processes in biology. Synthetic rates vary over hundreds of fold depending on the availability of an external source of cholesterol. Studies of this feedback regulatory process have a rich history. The field began 75 years ago when Rudolf Schoenheimer measured cholesterol balance in mice in a bottle. He found that cholesterol feeding led to decreased cholesterol synthesis, thereby introducing the general phenomenon by which end products of biosynthetic pathways inhibit their own synthesis. Recently, cholesterol feedback has been explained at a molecular level with the discovery of membrane-bound transcription factors called sterol regulatory element-binding proteins (SREBPs), and an appreciation of the sterol-sensing role of their partner, an escort protein called Scap. The key element in Scap is a hexapeptide sequence designated MELADL (rhymes with bottle). Thus, over 75 years, Schoenheimer's bottle led to Scap's MELADL. In addition to their basic importance in membrane biology, these studies have implications for the regulation of plasma cholesterol levels and consequently for the development of atherosclerotic plaques, myocardial infarctions, and strokes. In this article we review the major milestones in the cholesterol feedback story.

Roles of endogenously synthesized sterols in the endocytic pathway

The effect(s) of endogenously synthesized cholesterol (endo-CHOL) on the endosomal system in mammalian cells has not been examined. Here we treated Chinese hamster ovary cell lines with lovastatin (a hydroxymethylglutaryl-CoA reductase inhibitor) and mevalonate (a precursor for isoprenoids) to block endo-CHOL synthesis and then examined its effects on the fate of cholesterol liberated from low density lipoprotein (LDL-CHOL). The results showed that blocking endo-CHOL synthesis for 2 h or longer does not impair the hydrolysis of cholesteryl esters but partially impairs the transport of LDL-CHOL to the plasma membrane. Blocking endo-CHOL synthesis for 2 h or longer also alters the localization patterns of the late endosomes/lysosomes and retards their motility, as monitored by time-lapse microscopy. LDL-CHOL overcomes the effect of blocking endo-CHOL synthesis on endosomal localization patterns and on endosomal motility. Overexpressing Rab9, a key late endosomal small GTPase, relieves the endosomal cholesterol accumulation in Niemann-Pick type C1 cells but does not revert the reduced endosomal motility caused by blocking endo-CHOL synthesis. Our results suggested that endo-CHOL contributes to the cholesterol content of late endosomes and controls its motility, in a manner independent of NPC1. These results also supported the concept that endosomal motility plays an important role in controlling cholesterol trafficking activities.

Distinct endosomal compartments in early trafficking of low density lipoprotein-derived cholesterol

We previously studied the early trafficking of low density lipoprotein (LDL)-derived cholesterol in mutant Chinese hamster ovary cells defective in Niemann-Pick type C1 (NPC1) using cyclodextrin (CD) to monitor the arrival of cholesterol from the cell interior to the plasma membrane (PM) (Cruz, J. C., Sugii, S., Yu, C., and Chang, T.-Y. (2000) J. Biol. Chem. 275, 4013-4021). We found that newly hydrolyzed cholesterol derived from LDL first appears in certain CD-accessible pool(s), which we assumed to be the PM, before accumulating in the late endosome/lysosome, where NPC1 resides. To determine the identity of the early CD-accessible pool(s), in this study, we performed additional experiments, including the use of revised CD incubation protocols. We found that prolonged incubation with CD (>30 min) caused cholesterol in internal membrane compartment(s) to redistribute to the PM, where it became accessible to CD. In contrast, a short incubation with CD (5-10 min) did not cause such an effect. We also show that one of the early compartments contains acid lipase (AL), the enzyme required for liberating cholesterol from cholesteryl ester in LDL. Biochemical and microscopic evidence indicates that most of the AL is present in endocytic compartment(s) distinct from the late endosome/lysosome. Our results suggest that cholesterol is liberated from LDL cholesteryl ester in the hydrolytic compartment containing AL and then moves to the NPC1-containing late endosome/lysosome before reaching the PM or the endoplasmic reticulum.

Biotinylated theta-toxin derivative as a probe to examine intracellular cholesterol-rich domains in normal and Niemann-Pick type C1 cells

BCtheta is a proteolytically nicked and biotinylated derivative of a cholesterol binding protein perfringolysin O (theta-toxin), and has been used to detect cholesterol-rich domains at the plasma membrane (PM). Here we show that by modifying the cell fixation condition, BCtheta can also be used to detect cholesterol-rich domains intracellularly. When cells were processed for PM cholesterol staining, the difference in BCtheta signals between the CT43 (CT) cell, a mutant Chinese hamster ovary cell line lacking the Niemann-Pick type C1 (NPC1) protein, and its parental cell 25RA (RA) was minimal. However, when cells were fixed with 4% paraformaldehyde, they became permeable to BCtheta. Under this condition, BCtheta mainly stained cholesterol-rich domains inside the cells, with the signal being much stronger in CT cells than in RA cells. The sensitivity of BCtheta staining was superior to that of filipin staining. The staining of cholesterol-rich domain(s) inside RA cells was sensitive to beta-cyclodextrin treatment, while most of the staining inside CT cells was relatively resistant to cyclodextrin treatment. Clear differences in intracellular BCtheta staining were also seen between the normal and mutant NPC1 fibroblasts of human or mouse origin. Thus, BCtheta is a powerful tool for visually monitoring cholesterol-rich domains inside normal and NPC cells.

Trafficking defects in endogenously synthesized cholesterol in fibroblasts, macrophages, hepatocytes, and glial cells from Niemann-Pick type C1 mice

Niemann-Pick type C1 disease (NPC1) is an inherited neurovisceral lipid storage disorder, hallmarked by the intracellular accumulation of unesterified cholesterol and glycolipids in endocytic organelles. Cells acquire cholesterol through exogenous uptake and endogenous biosynthesis. NPC1 participation in the trafficking of LDL-derived cholesterol has been well studied; however, its role in the trafficking of endogenously synthesized cholesterol (endoCHOL) has received much less attention. Previously, using mutant Chinese hamster ovary cells, we showed that endoCHOL moves from the endoplasmic reticulum (ER) to the plasma membrane (PM) independent of NPC1. After arriving at the PM, it moves between the PM and internal compartments. The movement of endoCHOL from internal membranes back to the PM and the ER for esterification was shown to be defective in NPC1 cells. To test the generality of these findings, we have examined the trafficking of endoCHOL in four different physiologically relevant cell types isolated from wild-type, heterozygous, and homozygous BALB/c NPC1NIH mice. The results show that all NPC1 homozygous cell types (embryonic fibroblasts, peritoneal macrophages, hepatocytes, and cerebellar glial cells) exhibit partial trafficking defects, with macrophages and glial cells most prominently affected. Our findings suggest that endoCHOL may contribute significantly to the overall cholesterol accumulation observed in selective tissues affected by Niemann-Pick type C disease.

Fate of endogenously synthesized cholesterol in Niemann-Pick type C1 cells

Mammalian cells obtain cholesterol via two pathways: endogenous synthesis in the endoplasmic reticulum and exogenous sources mainly through the low density lipoprotein (LDL) receptor pathway. We performed pulse-chase experiments to monitor the fate of endogenously synthesized cholesterol and showed that, after reaching the plasma membrane from the endoplasmic reticulum, the newly synthesized cholesterol eventually accumulates in an internal compartment in Niemann-Pick type C1 (NPC1) cells. Thus, the ultimate fate of endogenously synthesized cholesterol in NPC1 cells is the same as LDL-derived cholesterol. However, the time required for endogenous cholesterol to accumulate internally is much slower than LDL-derived cholesterol. Different pathways thus govern the post-plasma membrane trafficking of endogenous cholesterol and LDL-derived cholesterol to the internal compartment. Results using the inhibitor N-butyldeoxynojirimycin, which depletes cellular complex glycosphingolipids, demonstrates that the cholesterol trafficking defect in NPC1 cells is not caused by ganglioside accumulation. The ultimate cause of death in NPC disease is progressive neurological deterioration in the central nervous system, where the major source of cholesterol is derived from endogenous synthesis. Our current study provides a plausible link between defects in intracellular trafficking of endogenous cholesterol and the etiology of Niemann-Pick type C disease.

Role of Niemann-Pick type C1 protein in intracellular trafficking of low density lipoprotein-derived cholesterol

Niemann-Pick type C (NPC) is a disease that affects intracellular cholesterol-trafficking pathways. By cloning the hamster ortholog of NPC1, we identified the molecular lesions in two independently isolated Chinese hamster ovary cell mutants, CT60 and CT43. Both mutants lead to premature translational terminations of the NPC1 protein. Transfecting hamster NPC1 cDNA complemented the defects of the mutants. Investigation of the CT mutants, their parental cells, and an NPC1-stable transfectant allow us to present evidence that NPC1 is involved in a post-plasma membrane cholesterol-trafficking pathway. We found that the initial movement of low density lipoprotein (LDL)-derived cholesterol to the plasma membrane (PM) did not require NPC1. After reaching the PM and subsequent internalization, however, cholesterol trafficking back to the PM did involve NPC1. Both LDL-derived cholesterol and cholesterol originating from the PM accumulated in a dense, intracellular compartment in the CT mutants. Cholesterol movement from this compartment to the PM or endoplasmic reticulum was defective in the CT mutants. Our results functionally distinguish the dense, intracellular compartment from the early endocytic hydrolytic organelle and imply that NPC1 is involved in sorting cholesterol from the intracellular compartment back to the PM or to the endoplasmic reticulum.

Enzymatic and cellular characterization of a catalytic fragment of CTP:phosphocholine cytidylyltransferase alpha

To probe the mechanism of lipid activation of CTP:phosphocholine cytidylyltransferase (CCTalpha), we have characterized a catalytic fragment of the enzyme that lacks the membrane-binding segment. The kinetic properties of the purified fragment, CCTalpha236, were characterized, as well as the effects of expressing the fragment in cultured cells. CCTalpha236 was truncated after residue 236, which corresponds to the end of the highly conserved catalytic domain. The activity of purified CCTalpha236 was independent of lipids and about 50-fold higher than the activity of wild-type CCTalpha assayed in the absence of lipids, supporting a model in which the membrane-binding segment functions as an inhibitor of the catalytic domain. The kcat/Km values for CCTalpha236 were only slightly lower than those for lipid-activated CCTalpha. The importance of the membrane-binding segment in vivo was tested by expression of CCTalpha236 in CHO58 cells, a cell line that is temperature-sensitive for growth and CCTalpha activity. Expression of wild-type CCTalpha in these cells complemented the defective growth phenotype when the cells were cultured in complete or delipidated fetal bovine serum. Expression of CCTalpha236, however, did not complement the growth phenotype in the absence of serum lipids. These cells were capable of making phosphatidylcholine in the delipidated medium, so the inability of the cells to grow was not due to defective phosphatidylcholine synthesis. Supplementation of the delipidated medium with an unsaturated fatty acid allowed growth of CHO58 cells expressing CCTalpha236. These results indicate that the membrane-binding segment of CCTalpha has an important role in cellular lipid metabolism.

Oleate potentiates oxysterol inhibition of transcription from sterol regulatory element-1-regulated promoters and maturation of sterol regulatory element-binding proteins

Activation of genes containing SRE-1 (sterol regulatory element 1) sequences is known to be under the regulation of sterols through modulation of the proteolytic maturation of SREBPs (SRE-1-binding proteins). Previous work has demonstrated SREBP-mediated transcriptional activation of genes encoding enzymes of sterol and fatty acid biosynthesis. Because synthesis of both sterols and C18 fatty acids are required for cell growth, in the absence of exogenous supplements of these lipids, we examined the hypothesis that fatty acid can also be regulatory in SREBP maturation. Our data indicate that C18 fatty acids can potentiate the biological activities of a typical, regulatory sterol: 25-hydroxycholesterol. Inhibition of C18 fatty acid synthesis in cells cultured in serum-free medium renders them resistant to killing by 25-hydroxycholesterol. Repression of expression of reporter constructs driven by promoters bearing SRE-1 element(s) by 25-hydroxycholesterol is increased by C18 fatty acid supplementation. C18 fatty acids also increase the inhibitory effect of 25-hydroxycholesterol on proteolytic maturation and nuclear localization of SREBPs. Furthermore, we also show that C18 fatty acid supplementation can enhance the inhibitory effect of 25-hydroxycholesterol on sterol and fatty acid biosynthesis. These results demonstrate that maximal down-regulation of SREBP maturation and the consequent repression of SRE-1 promoters occurs in response to both a regulatory sterol and fatty acid.

Amyloid precursor protein processing in sterol regulatory element-binding protein site 2 protease-deficient Chinese hamster ovary cells

Amyloid peptides of 39-43 amino acids (Abeta) are the major constituents of amyloid plaques present in the brains of Alzheimer's (AD) patients. Proteolytic processing of the amyloid precursor protein (APP) by the yet unidentified beta- and gamma-secretases leads to the generation of the amyloidogenic Abeta peptides. Recent data suggest that all of the known mutations leading to early onset familial AD alter the processing of APP such that increased amounts of the 42-amino acid form of Abeta are generated by a gamma-secretase activity. Identification of the beta- and/or gamma-secretases is a major goal of current AD research, as they are prime targets for therapeutic intervention in AD. It has been suggested that the sterol regulatory element-binding protein site 2 protease (S2P) may be identical to the long sought gamma-secretase. We have directly tested this hypothesis using over-expression of the S2P cDNA in cells expressing APP and by characterizing APP processing in mutant Chinese hamster ovary cells that are deficient in S2P activity and expression. The data demonstrate that S2P does not play an essential role in the generation or secretion of Abeta peptides from cells, thus it is unlikely to be a gamma-secretase.

Sterol resistance in CHO cells traced to point mutation in SREBP cleavage-activating protein

Through expression cloning we have isolated a cDNA-encoding SREBP cleavage-activating protein (SCAP), which regulates cholesterol metabolism by stimulating cleavage of transcription factors SREBP-1 and -2, thereby releasing them from membranes. The cDNA was isolated from Chinese hamster ovary cells with a dominant mutation that renders them resistant to sterol-mediated suppression of cholesterol synthesis and uptake. Sterol resistance was traced to a G-->A transition at codon 443 of SCAP, changing aspartic acid to asparagine. The D443N mutation enhances the cleavage-stimulating ability of SCAP and renders it resistant to inhibition by sterols. SCAP has multiple membrane-spanning regions, five of which resemble the sterol-sensing domain of HMG CoA reductase, an endoplasmic reticulum enzyme whose degradation is accelerated by sterols. SCAP appears to be a central regulator of cholesterol metabolism in animal cells.

Sterol-regulated release of SREBP-2 from cell membranes requires two sequential cleavages, one within a transmembrane segment

Sterol regulatory element binding proteins (SREBPs) are transcription factors attached to the endoplasmic reticulum. The NH2-segment, which activates transcription, is connected to membranes by a hairpin anchor formed by two transmembrane sequences and a short lumenal loop. Using H-Ras-SREBP-2 fusion proteins, we show that the NH2-segment is released from membranes by two sequential cleavages. The first, regulated by sterols, occurs in the lumenal loop. The second, not regulated by sterols, occurs within the first transmembrane domain. The liberated NH2-segment enters the nucleus and activates genes controlling cholesterol synthesis and uptake. Certain mutant Chinese hamster ovary cells are auxotrophic for cholesterol because they fail to carry out the second cleavage; the NH2-segment remains membrane-bound and transcription is not activated.

Regulation and immunolocalization of acyl-coenzyme A: cholesterol acyltransferase in mammalian cells as studied with specific antibodies

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) catalyzes the formation of intracellular cholesterol esters in various tissues. We recently reported the cloning and expression of human macrophage ACAT cDNA. In the current study, we report the production of specific polyclonal antibodies against ACAT by immunizing rabbits with the recombinant fusion protein composed of glutathione S-transferase and the first 131 amino acids of ACAT protein. Immunoblot analysis showed that the antibodies cross-reacted with a 50-kDa protein band from a variety of human cell lines. These antibodies immunodepleted more than 90% of detergent-solubilized ACAT activities from six different human cell types, demonstrating that the 50-kDa protein is the major ACAT catalytic component in these cells. In multiple human tissues examined, the antibodies recognized protein bands with various molecular weights. These antibodies also cross-reacted with the ACAT protein in Chinese hamster ovary cells. Immunoblot analysis showed that the ACAT protein contents in human fibroblast cells, HepG2 cells, or Chinese hamster ovary cells were not affected by sterol in the medium, demonstrating that the main mechanism for sterol-dependent regulation of ACAT activity in these cells is not change in ACAT protein content. As revealed by indirect immunofluorescent microscopy, the ACAT protein in tissue culture cells was located in the endoplasmic reticulum. This finding, along with earlier studies, suggests that cholesterol concentration in the endoplasmic reticulum may be the major determinant for regulating ACAT activity in the intact cells.

Translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the endoplasmic reticulum for esterification may require common cellular factors involved in cholesterol egress from the acidic compartments (lysosomes/endosomes)

Using a stable cell line 25-RA derived from wild-type Chinese hamster ovary (CHO) cells as the parental cell, this laboratory previously reported the isolation and characterization of CHO cell mutants (cholesterol-trafficking or CT) defective in transporting LDL-derived cholesterol out of the acidic compartment(s) (lysosomes/endosomes) to the endoplasmic reticulum (ER) for esterification. In this report, we show that the CT mutation can be complemented by fusion with human cells; however, attempts to complement the CT defect through DNA transfection have resulted in a collection of stable cell lines designated as ST cells. Under cholesterol starvation condition, the ST cells exhibit an elevated rate of cholesterol ester biosynthesis (by 3- to 5-fold) compared to both the parental CHO cells and the CT cells. The phenotypes of the ST cells are stable. ST cells are thus new cell lines arisen from the CT cells. When the plasma membranes of the parental, CT, and ST cells are labelled with [3H]cholesterol, ST cells show rates of [3H]cholesterol esterification much higher than that observed in CT cells but lower than that observed in the parental CHO cells. This result shows that translocation of plasma membrane cholesterol to the ER for esterification is defective in the CT cells. This result also suggests that ST cells acquire increased cholesterol trafficking activity between the lysosome and the ER without mixing the plasma membrane cholesterol pool. The characteristics of CT cells and ST cells reported here suggest that translocation of both lysosomal LDL-derived cholesterol and plasma membrane cholesterol to the ER for esterification may require common cellular factors involved in cholesterol egress from the acidic compartment(s) (lysosomes/endosomes).

Somatic cell genetic analysis of two classes of CHO cell mutants expressing opposite phenotypes in sterol-dependent regulation of cholesterol metabolism

Two different classes of hamster cell mutants (25RA cells and M1 cells) express opposite phenotypes in sterol dependent regulation. In 25RA cells, sterols added in growth medium fail to cause down-regulation of sterol synthesis rate and low density lipoprotein (LDL) receptor activity, while in M1 cells, removal of lipids from growth medium fail to cause up-regulation of sterol synthesis rate and LDL receptor activity. Cell hybridization analysis showed that the 25RA phenotype is semidominant, while the M1 phenotype is recessive. Using 25RA as the parental cells, we isolated eight independent mutant cells (DM cells) and showed that all of them belong to the same genetic complementation group as the M1 mutant, indicating that the normal (unmutated) M1 gene product(s) is required to express the 25RA phenotype. We next performed gene transfer experiments using hamster cell genomic DNAs containing the functional human M1 gene as the donor, and the double mutant cell DM7 as the recipient. The resultant transfectant cells express the 25RA cell phenotype (instead of the wild-type cell phenotype). This result, along with the results obtained from cell hybridization analysis, shows that the 25RA and M1 cell phenotypes are caused by mutations at two different genes.

Somatic cell genetic and biochemical characterization of cell lines resulting from human genomic DNA transfections of Chinese hamster ovary cell mutants defective in sterol-dependent activation of sterol synthesis and LDL receptor expression

We have isolated several non-leaky mutant Chinese hamster ovary (CHO) cell clones (M4, M19, and M21) requiring cholesterol and unsaturated fatty acid for growth. These mutants belong to the same complementation group as the mutant M1 cells previously reported from this laboratory. M19 cells reverted to lipid prototrophy at very low frequency and were chosen as recipients to perform DNA-mediated gene-transfer experiments using total human genomic DNAs. Biochemical characterization of these transfectant clones indicated that, unlike their parental M19 cells, they were able to exhibit activation of cholesterol biosynthesis and LDL receptor expression in response to sterol removal from the growth medium. RNA blotting analysis indicated that these transfectants were able to increase HMG-CoA synthase gene transcripts in response to sterol removal. From the genomic DNAs of a representative secondary transfectant cells, we cloned a unique human DNA fragment (designated as h lambda 2) and showed that h lambda 2 closely linked with the presumptive human M1 gene.

8-bromoadenosine cyclic 3',5'-phosphate rapidly increases 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA in human granulosa cells: role of cellular sterol balance in controlling the response to tropic stimulation

Exposure of cultured human granulosa cells to 8-bromoadenosine cyclic 3',5'-phosphate (8-bromo-cAMP) resulted in a rapid increase in the content of the mRNA for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a rate-limiting enzyme in the de novo synthesis of cholesterol. HMG-CoA reductase mRNA levels increased within 2 h of stimulation and remained elevated for at least 6 h. Treatment of granulosa cells with 25-hydroxycholesterol, a soluble cholesterol analogue, in combination with aminoglutethimide to block conversion of cellular sterols to pregnenolone, resulted in suppression of HMG-CoA reductase mRNA. When cells were stimulated with 8-bromo-cAMP in the presence of 25-hydroxycholesterol and aminoglutethimide, the increase in HMG-CoA reductase mRNA provoked by the tropic agent was markedly attenuated. This indicates that 8-bromo-cAMP raises HMG-CoA reductase mRNA levels indirectly by accelerating steroidogenesis and depleting cellular sterol pools, thus relieving sterol-mediated negative feedback of HMG-CoA reductase gene expression. 25-Hydroxycholesterol in the presence of aminoglutethimide suppressed low-density lipoprotein (LDL) receptor mRNA, but 8-bromo-cAMP effected a significant stimulation of LDL receptor mRNA levels when added with hydroxysterol and aminoglutethimide. These findings reveal differential regulation of HMG-CoA reductase and LDL receptor mRNAs in the presence of sterol negative feedback.

Metabolism of cytotoxic hydroxysterols in cultured cells. Chemical characterization of metabolites

The metabolism of labelled 7 alpha- and 7 beta-hydroxycholesterol was investigated in two lymphoma cell lines (YAC-1, RDM-4), in murine splenocytes and in HTC hepatoma cells. The structures of the metabolites in lymphoma cells were determined as 3 beta-esters of C14-C20 fatty acids by nuclear magnetic resonance and mass spectrometric studies. In hepatoma cells, more polar metabolites of 7 alpha- and 7 beta-hydroxycholesterol were detected whereas, in non-dividing lymphocyte cells, no metabolic transformation occurs. Therefore, metabolic transformation of the hydroxycholesterol is not required for the expression of their activity and the question of the physiological role of the metabolic products is raised.

Biosynthesis of glycerolipids by hepatoma and liver microsomes. I. Fatty acyl-CoA ligase and acyl-CoA:sn-glycerol-3-phosphate acyltransferase

The intracellular membranes of hepatomas exhibit an altered content and composition of lipid compared to the membranes of normal liver. In order to elucidate the role of lipid biosynthetic enzymes in these membrane differences, we first examined the fatty acyl-CoA ligase and acyl-CoA:sn-glycerol 3-phosphate (Gro-3P) acyltransferase activities and acyl specificities of microsomes from liver, Morris hepatoma 7288C, and hepatoma tissue culture (HTC) cells. Based upon incorporation of fatty acid and Gro-3P, it is concluded that acyl-CoA:sn-Gro-3P acyltransferase activities are markedly elevated (6-30-fold) in the microsomes of Morris Hepatoma 7288C and HTC cells compared to microsomes from liver, whereas the fatty acyl-CoA ligase activity is reduced (30-50-fold). Therefore, the low phospholipid content of these tumor cells does not appear to result from reduced acyltransferase activity. Though diminished ligase activity may play a role, it appears that activation of fatty acid may not be rate-limiting, even at the low levels of fatty acyl-CoA ligase present in the tumor and HTC cells. Preliminary evidence suggests that another factor that may be responsible for the low tumor phospholipid content is the limited availability of Gro-3P, a lipid precursor. The phospholipid in hepatoma 7288C is also characterized by an elevated ratio of monenoic to dienoic fatty acid. We have found that this change does not reflect an altered specificity of acyl-CoA:sn-Gro-3P acyltransferase.

Cycloheximide sensitivity in regulation of acyl coenzyme A:cholesterol acyltransferase activity in Chinese hamster ovary cells. 2. Effect of sterol endogenously synthesized

We reported in another paper [Chang, C. C. Y., Doolittle, G. M., & Chang, T. Y. (1986) Biochemistry (preceding paper in this issue)] that in Chinese hamster ovary (CHO) cells activation of acyl coenzyme A:cholesterol acyltransferase (ACAT) activity by treating cells with cycloheximide was abolished by providing exogenous sterol in the medium. We now report that providing 20 mM DL-mevalonate to cells grown in sterol-free medium increases the ACAT activity by approximately 6-fold and diminishes the cycloheximide activation effect. The mevalonate supplement has no significant effect on the rate of triglyceride or polar lipid synthesis, [3H]cholesterol efflux, or bulk protein degradation in cells. The activation of ACAT by mevalonate is prevented by adding a specific squalene oxide cyclase inhibitor to cells, indicating the requirement for endogenous sterol synthesis to mediate the mevalonate effect. In sterol-free medium, if sterol synthesis is blocked by specific enzyme inhibitors or through mutation, the ACAT activation by cycloheximide is again abolished. These results support the hypothesis that there may exist a short-lived factor(s) serving directly or indirectly as an endogenous ACAT inhibitor(s), the inhibitory action of this (these) factor(s) is (are) abolished, and its (their) turnover rate(s) is (are) increased by either exogenous sterol or by sterol endogenously synthesized. Conversely, removing exogenous sterol coupled with blocking endogenous sterol synthesis decreases the turnover rate(s) of the inhibitor(s), rendering its (their) action insensitive to intracellular degradation over the time period studied.

Chinese hamster ovary cells depend on exogenous lipids to survive phospholipase C treatment

Chinese hamster ovary cells were maintained in culture medium supplemented with delipidated serum to make them dependent on nonlipid components for lipid synthesis. Growth in lipid-free medium resulted in an increased flux through the CDP-choline pathway for phosphatidylcholine synthesis. The increased flux appeared to be mediated by the CTP:phosphocholine cytidylyltransferase because cellular phosphocholine levels decreased in cells grown in lipid-free medium, and both cell-free cytidylyltransferase activity and membrane-associated cytidylyltransferase activity increased in cells grown in lipid-free medium. Chinese hamster ovary cells maintained in culture medium supplemented with complete serum can grow at nearly normal rates in the presence of phospholipase C for many generations, even though the treatment enhances turnover of cellular phosphatidylcholine (R. Sleight and C. Kent (1983) J. Biol. Chem. 258, 824-830). The phospholipase C treatment, however, was toxic to cells maintained in medium supplemented with delipidated serum. Lysophosphatidylcholine protected cells from phospholipase toxicity, but did not support growth. The rate of utilization of lysophosphatidylcholine for phosphatidylcholine synthesis was about 8% of the turnover rate for phosphatidylcholine, and was not increased by phospholipase C treatment. Reconstitution of the medium with fractionated serum lipids showed that the cells required specific neutral lipids, namely, fatty acids plus cholesterol, in order to grow in the presence of phospholipase C. Either oleate or cholesterol (up to 80 microM) alone did not allow growth in phospholipase C, but a combination of these two lipids supported growth effectively.

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase and lipid metabolism in a concanavalin A-resistant Chinese hamster ovary cell line

Lipid metabolism in a concanavalin A-resistant, glycosylation-defective mutant cell line was investigated by comparing growth properties, lipid composition, and lipid biosynthesis in wild-type (WT), mutant (CR-7), and revertant (RCR-7) cells. In contrast to WT and RCR-7, the mutant was auxotrophic for cholesterol, but mevalonolactone did not restore growth on lipoprotein-deficient medium. The use of R-[2-14C]mevalonolactone revealed that CR-7 was deficient in the conversion of lanosterol to cholesterol. Total lipid and phospholipid content and composition were similar in all three cell lines, but CR-7 displayed subnormal content and biosynthesis of cholesterol and unsaturated fatty acids. The mutant was hypersensitive to compactin and was unable to upregulate either 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity or the binding and internalization of 125I-labeled low-density lipoprotein (LDL) in response to lipoprotein deprivation. HMG-CoA reductase activity in all three cell lines showed similar kinetics and phosphorylation status, and the binding kinetics and degradation of 125I-LDL were also similar, suggesting that CR-7 possesses kinetically normal reductase and LDL binding sites, but is deficient in their coordinate regulation. Tunicamycin (1-2 micrograms/ml) strongly and reversibly suppressed reductase activity in WT and RCR-7. CR-7 was resistant to this inhibitor. In WT cells this suppressive effect was accompanied by inhibition of 3H-labeled mannose incorporation into cellular protein, but 3H-labeled leucine incorporation was unaffected. Immunotitration of HMG-CoA reductase activity in extracts of WT cells, cultured in the presence and absence of tunicamycin, showed that suppression of reductase activity reflected the presence of reduced amounts of reductase protein, implying that glycosylation plays an important role in the coordinate regulation of HMG-CoA reductase activity and LDL binding.

Acyl-CoA:cholesterol acyltransferase in Chinese hamster ovary cells. Enzyme activity determined after reconstitution in phospholipid/cholesterol liposomes

Acyl-CoA:cholesterol acyltransferase from Chinese hamster ovary (CHO) cells was solubilized by deoxycholate, and then reconstituted in phosphatidylcholine/cholesterol liposomes. This reconstituted activity was totally dependent upon the cholesterol content of the mixture and showed saturation for cholesterol. Analysis of the reconstituted enzyme on linear Ficoll gradients shows that the enzyme has been incorporated into phospholipid/cholesterol liposomes. The CHO cell enzyme activity as measured by conventional assay (using cellular cholesterol as the substrate) was activated approximately 20-fold by low density lipoprotein. This activation process was independent of protein synthesis. When the above cell homogenates were assayed after optimal reconstitution, the activation produced by low density lipoprotein was essentially completely abolished. There was also no change in enzyme activity measured after reconstitution when cells were switched from sterol-containing medium to sterol-free medium, in contrast to a more than 7-fold drop in enzyme activity when assayed without reconstitution. These results suggest that the enzyme activity in intact cells is controlled by the content and composition of cellular lipids associated with the enzyme molecule. Since the intracellular messenger of low density lipoprotein is known to be cholesterol, it is likely that this enzyme activity in intact cells is primarily controlled by the cholesterol content in the vicinity of the enzyme molecule.

Inhibition of protein synthesis blocks the response to 25-hydroxycholesterol by inhibiting degradation of hydroxymethylglutaryl-CoA reductase

The activity of the rate-limiting enzyme of the cholesterol biosynthetic pathway, 3-hydroxy-3-methylglutaryl coenzyme A reductase in Chinese hamster ovary (CHO) cells decreased more rapidly in cells treated with 25-hydroxycholesterol alone (t 1/2 = 1.5 h) than in those incubated with cycloheximide alone (t 1/2 = 5 h). The inhibitory action of 25-hydroxycholesterol on reductase activity was reduced when the sterol and cycloheximide were added together, and was totally abolished when cells were preincubated with cycloheximide for 30 min before the addition of 25-hydroxycholesterol. The effect of puromycin was similar to that of cycloheximide. Treatment of cells with an inhibitor of RNA synthesis, i.e., actinomycin D or cordycepin, had little effect on hydroxymethylglutaryl-CoA reductase activity; however, preincubation of cells with these reagents also decreased the ability of 25-hydroxycholesterol to suppress the reductase activity. These data are consistent with a model which suggests (a) that 25-hydroxycholesterol inhibits the activity of hydroxymethylglutaryl-CoA reductase by repressing its synthesis, (b) that cycloheximide and puromycin affect hydroxymethylglutaryl-CoA reductase activity by blocking the de novo synthesis of the enzyme and by reducing the degradation of the preexisting enzyme, (c) that actinomycin D and cordycepin affect the supply of message for the continuous synthesis of at least one component of a system which degrades hydroxymethylglutaryl-CoA reductase, and (d) that one component of the degradative system has a half-life shorter than 0.5 h.

Insensitivity of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity to low-density lipoprotein in concanavalin-A-resistant Chinese-hamster ovary cells

A concanavalin-A-resistant mutant of Chinese-hamster ovary cells has been shown to have altered cholesterol content and 3-hydroxy-3-methylglutaryl coenzyme A reductase activity compared to wild-type and spontaneous-revertant cell lines. These changes are associated with insensitivity of the mutant reductase activity to suppression by low-density lipoprotein and impaired receptor-mediated binding and uptake of 125I-labelled low-density lipoprotein.