Biomedical subjects

J A Cuthbert

Publications and source records attributed to J A Cuthbert.

At least 19 recordsLinked to original sources

PubMed1992

Regulation of hepatic sterol metabolism in the rat. Parallel regulation of activity and mRNA for 7 alpha-hydroxylase but not 3-hydroxy-3-methylglutaryl-coenzyme A reductase or low density lipoprotein receptor.

In vivo regulation of hepatic sterol metabolism was examined in the rat. Sodium cholate markedly suppressed hepatic 7 alpha-hydroxylase mRNA levels and activity when fed to rats on a low cholesterol diet. Sterol balance was maintained solely by decreasing hepatic cholesterol synthesis. Compensatory mechanisms were inadequate when cholate was fed to rats on a high cholesterol diet and massive amounts of cholesterol accumulated in the liver and plasma. Suppression of bile salt synthesis was not responsible since cholate did not suppress 7 alpha-hydroxylase activity when fed to rats on a high cholesterol diet. Moreover, total hepatic low density lipoprotein receptor activity was not suppressed even though liver cholesteryl ester levels were increased more than 350-fold. Changes in 7 alpha-hydroxylase activity were always accompanied by parallel changes in mRNA, whereas mRNA levels for 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase were reduced by 50% or less, even when cholesterol synthesis was suppressed by 98%. HMG-CoA reductase and low density lipoprotein receptor activities were regulated independently although mRNA levels for these two proteins were coordinately regulated. These findings indicate that 7 alpha-hydroxylase is controlled by mRNA levels, whereas in vivo cholesterol synthesis is predominantly controlled by posttranscriptional regulation of HMG-CoA reductase activity.

Animals↗

PubMed1992

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.

Cholesterol, LDL↗

PubMed1991

Negative regulation of cell proliferation by mevalonate or one of the mevalonate phosphates.

The role of mevalonate and its products in the regulation of cellular proliferation was examined using 6-fluoromevalonate (Fmev), a compound that blocks the conversion of mevalonate pyrophosphate to isopentenyl pyrophosphate. Fmev suppressed DNA synthesis by a variety of transformed and malignant T cell, B cell, and myeloid cell lines. In contrast to results previously reported with mitogen-stimulated human peripheral blood T cell DNA synthesis, low concentrations of low density lipoprotein (LDL) alone could not restore proliferation to these cell lines. The same concentrations of LDL were able to provide sufficient cholesterol and support the growth of all cell lines when mevalonate synthesis was blocked with a specific inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, lovastatin. Fmev-mediated inhibition was totally prevented in some but not all cell lines when the concentration of exogenous LDL was increased 5-10-fold above that required to permit proliferation of lovastatin-blocked cells. Residual HMG-CoA reductase activity of cells cultured with LDL inversely correlated with the restoration of growth to Fmev-blocked cultures. Confirmation of the critical role of HMG-CoA reductase activity and mevalonate synthesis in the inhibition of cellular proliferation by Fmev was obtained by demonstrating that the specific inhibitor of this enzyme, lovastatin, restored proliferation of Fmev-blocked cells. Furthermore, supplementation of cultures with mevalonate, the product of HMG-CoA reductase activity, markedly inhibited proliferation of Fmev-blocked cells. These findings indicate that mevalonate or one of the mevalonate phosphates, which accumulates in Fmev-blocked cells, is a critical negative regulator of cellular proliferation.

Cell Division↗

PubMed1991

Hepatitis B--molecular variants with clinical significance?

Mutations in the DNA of the hepatitis B virus have been discovered and they may have clinical significance. A single nucleotide substitution resulting in a premature stop codon in the pre-core region of the hepatitis B genome is the most common change. The premature stop codon prevents the synthesis of hepatitis B e antigen (HBeAg), a virally encoded protein normally secreted by hepatocytes. The mutant hepatitis B virus was initially discovered in patients who lacked HBeAg in the serum yet had high levels of hepatitis B viral DNA, a marker of active viral replication usually found in association with the continued presence of HBeAg. Other studies demonstrated that the mutant forms were observed with increasing frequency during the successful conversion from HBeAg positivity to anti-hepatitis B e antibody (anti-HBe) positivity. The mutant form of hepatitis B virus was not identified in patients with stable chronic hepatitis B who were positive for HBeAg or in any patients with uncomplicated acute hepatitis B, regardless of the presence of HBeAg or anti-HBe. However, mutant hepatitis B virus was detected in patients with fulminant hepatitis B who lacked both HBeAg and anti-HBe. The lack of HBeAg in the serum therefore may result in a more severe form of acute disease. Together with experimental animal studies of the normal role of HBeAg, these data suggest that serum HBeAg may be associated with immunologic tolerance, whereas clearance of or lack of HBeAg may be associated with an active immunologic response. Future studies are expected to clarify the role of mutant forms of hepatitis B virus in the natural history of hepatitis B infection.

Amino Acid Sequence↗

PubMed1991

A product of mevalonate proximal to isoprenoids is the source of both a necessary growth factor and an inhibitor of cell proliferation.

These studies demonstrate that mevalonate or the mevalonate phosphates play a number of regulatory roles in cellular proliferation. They are not only required for cell growth but are also a source of an inhibitor of cell growth. Endogenous concentrations of mevalonate and the mevalonate phosphates are thus critical determinants of cellular proliferation.

Cell Division↗

PubMed1991

HeLaTG cells have mitochondrial DNA inserted into the c-myc oncogene.

To determine whether mitochondrial DNA (mtDNA) fragments found within the nucleus are transcribed, we have differentially screened a HeLaTG cDNA library. A clone that hybridized to mtDNA as well as to c-myc was identified. Analysis of the cDNA disclosed that it contained a mtDNA sequence, encoding cytochrome-c oxidase subunit III (coxIII) that was contiguous with and 5' of a c-myc sequence corresponding to part of exon 2 and exon 3. Hybridization of ScaI-digested DNA with a 1.05 kb c-myc probe revealed a unique band in HeLaTG cells, as well as a band common to HeLaTG and 13 other cell types examined. Solution hybridization of HeLaTG RNA with a radiolabeled, single-stranded cDNA probe containing the coxIII-c-myc junction demonstrated a nuclease-resistant band that matched the full length of the junctional cDNA probe. A smaller band that equaled the size of the c-myc portion alone was also detected. Only the smaller band coinciding with the c-myc sequences was protected from nuclease digestion by RNA from other cells. When a radiolabeled probe synthesized in the opposite orientation was used, nuclease-resistant bands equal in length to the coxIII portion of the probe were detected after hybridization with RNA from all cells. These results indicate that insertion of mtDNA fragments into nuclear genes occurs and that subsequent transcription of a 'chimaeric' or 'fusion' mRNA containing both mitochondrial and nuclear sequences can ensue.

Base Sequence↗

PubMed1990

Inhibition by 6-fluoromevalonate demonstrates that mevalonate or one of the mevalonate phosphates is necessary for lymphocyte proliferation.

The sterol synthesis inhibitor 6-fluoromevalonate (Fmev) was used to explore the role of mevalonate products in lymphocyte proliferation. Fmev blocks the synthesis of isopentenyl pyrophosphate and all more distal products in the sterol pathway. When cells were cultured in lipoprotein-deficient medium, Fmev (200 microM) completely inhibited mitogen-stimulated human lymphocyte proliferation, quantified by measuring DNA synthesis. The addition of low density lipoprotein (LDL) restored lymphocyte responses to normal, whereas mevalonate was totally ineffective. Similar results were obtained with concentrations of Fmev up to 1 mM. These results contrast with those observed when sterol biosynthesis was blocked with lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase. When lymphocyte proliferation was blocked with lovastatin (5 microM), either high concentrations of mevalonate or LDL together with low concentrations of mevalonate was required to restore responses. In contrast, neither LDL nor low concentrations of mevalonate when alone was able to restore lymphocyte DNA synthesis in cultures blocked with 5 microM lovastatin. The effect of Fmev on the capacity of exogenous mevalonate to restore proliferation of lovastatin-blocked lymphocytes was directly examined. Fmev had no effect on the capacity of LDL plus low concentrations of mevalonate to restore DNA synthesis to lovastatin-blocked lymphocytes, indicating that the synthesis of the necessary factor from mevalonate was unaltered by Fmev. Fmev profoundly blocked lymphocyte endogenous sterol synthesis, decreasing incorporation of radiolabeled acetate into digitonin-precipitable sterols by up to 98%. LDL did not alter the capacity of Fmev to block sterol synthesis. The possibility that Fmev allowed shunting of endogenous mevalonate into essential lipid products was assessed by examining the incorporation of radiolabeled mevalonate. Fmev (200 microM) inhibited the incorporation of mevalonate into all lipids, including ubiquinone, dolichol, and other non-sterol lipids by up to 98%, and this was not altered by LDL. Furthermore, Fmev (200 microM) suppressed the incorporation of radiolabeled mevalonate into protein by up to 97%. These data confirm that a product of mevalonate is essential for cell proliferation. However, the results indicate that the required product is directly synthesized from mevalonate or mevalonate phosphates rather than from a more distal isoprenoid metabolite.

Humans↗

PubMed1990

Hepatitis C.

The major cause of chronic post-transfusion hepatitis, the hepatitis C virus (HCV), has been identified. HCV is a single-stranded linear RNA virus with characteristics similar to the flaviviruses. A different agent, the hepatitis E virus, is associated with epidemic (enterically-transmitted) non-A, non-B hepatitis. At present, infection with HCV is recognized by the finding of anti-HCV antibodies, positive in up to 90% of patients with chronic non-A, non-B post-transfusion hepatitis. Antibodies to HCV are detected in 1% of normal volunteer blood donors and in the majority of donors implicated in post-transfusion hepatitis. HCV antibodies are also found in patients with autoimmune liver disease and hepatocellular carcinoma. Moreover, HCV infection may contribute to the pathogenesis of liver disease in alcoholic patients. The role of HCV infection in fulminant non-A, non-B hepatitis and hepatitis-associated aplastic anemia has not been elucidated as yet. Therapy of chronic non-A, non-B hepatitis with recombinant human alpha-interferon has been shown to improve or normalize aminotransferase levels in approximately 50% of patients, most of whom have evidence of HCV infection. However, relapse after cessation of treatment is common. In the future, screening blood for evidence of HCV infection may prevent most cases of non-A, non-B post-transfusion hepatitis.

Acute Disease↗

PubMed1990

Mitogenic stimulation alters the regulation of LDL receptor gene expression in human lymphocytes.

To address the possibility that influences other than ambient cholesterol concentrations regulate low density lipoprotein (LDL) receptor expression, the effect of mitogenic activation on the levels of LDL receptor mRNA in human lymphocytes was examined. Mitogenic stimulation of freshly isolated human peripheral blood mononuclear cells (PBMC) cultured in medium containing saturating concentrations of LDL resulted in cell cycle entry as evidenced by increased levels of mRNA for the interleukin-2 receptor, and also increased LDL receptor mRNA levels by 9-fold. Whereas LDL receptor gene expression was also induced when resting control PBMC were incubated in lipoprotein-deficient medium, mitogenic activation of PBMC cultured in the absence of LDL stimulated a further 3-fold increase in LDL receptor mRNA levels. The increase in LDL receptor mRNA levels in mitogen-stimulated PBMC was dependent on continued protein synthesis, was not the result of mRNA stabilization, and therefore most likely reflected enhanced gene transcription. It was unlikely that the increase in LDL receptor mRNA levels observed in mitogen-stimulated cells related merely to sterol deprivation since suppression of endogenous cholesterol synthesis with lovastatin increased LDL receptor mRNA only modestly. Moreover, mitogen-stimulated PBMC continued to synthesize cholesteryl esters, a storage form of cholesterol, confirming that they were not functionally deprived of sterols. Although mitogenic stimulation increased LDL receptor mRNA levels in PBMC, regulation by exogenous LDL was observed. Thus, LDL down-regulated LDL receptor gene expression in both control and mitogen-stimulated PBMC. Down-regulation was less effective in the latter; however, LDL down-regulated endogenous sterol synthesis to an equivalent extent in both control and mitogen-stimulated PBMC. By contrast, the oxygenated sterol, 25-hydroxycholesterol, and mevalonate, the precursor of endogenously synthesized sterols, down-regulated LDL receptor mRNA levels comparably in mitogen-stimulated and control PBMC. These data indicate that mitogenic stimulation provides an additional stimulus for LDL receptor gene expression over and above that of ambient sterols and, therefore, suggest that signals transduced during cellular activation play a role in regulation of LDL receptor mRNA levels.

Adult↗

PubMed1990

Tissue-specific regulation of low density lipoprotein receptor gene expression.

LDL receptor mRNA levels are increased by mitogenic stimulation of PBMC and are less effectively regulated by exogenous LDL in the activated cells. Enhanced LDL receptor mRNA levels reflect increased rates of gene transcription and not stabilization of mRNA. The alteration of LDL-mediated regulation of LDL receptor gene expression in mitogen-activated PBMC is not the result of cellular proliferation alone and occurs without apparent changes in measurable regulatory pools of cholesterol. These results indicate that signals transduced during mitogenic stimulation, as well as ambient LDL concentration, play a major role in regulating LDL receptor gene transcription. Fibroblasts apparently differ from PBMC in the effect of modulatory proteins on LDL receptor mRNA levels. In fibroblasts, LDL receptor gene transcription appears to be down-regulated by a short-lived protein whereas the dominant regulatory influence in PBMC appears to be a short-lived protein with a positive effect on transcription. Differences in the expression of specific transcription factors may contribute to cell-specific alterations in the regulation of LDL receptor gene expression by lipids.

Gene Expression Regulation↗

PubMed1989

Lipoproteins may provide fatty acids necessary for human lymphocyte proliferation by both low density lipoprotein receptor-dependent and -independent mechanisms.

Human lymphocytes respond optimally to mitogenic stimulation when cultured in serum-free medium supplemented with transferrin if fatty acids necessary for maximal proliferation are provided. Either lipoproteins or exogenous fatty acids support optimal lymphocyte responses. The current studies examined the role of cell surface receptors for low density lipoprotein (LDL) in the enhancement of lymphocyte proliferation. Support of lymphocyte growth by limiting concentrations of LDL was found to involve interaction of the lipoprotein with LDL receptors. Thus, modification of LDL by reductive methylation so as to inhibit receptor-mediated interactions markedly decreased the capacity of LDL to enhance lymphocyte proliferation. Moreover, growth of lymphocytes obtained from patients with LDL receptor-negative homozygous familial hypercholesterolemia was minimal when cultures were supplemented with low concentrations of LDL (less than 10 micrograms cholesterol/ml). LDL also enhanced lymphocyte proliferation by a receptor-independent mechanism since high concentrations (greater than or equal to 50 micrograms cholesterol/ml) supported growth of both normal and familial hypercholesterolemia lymphocytes. In contrast, support of lymphocyte proliferation by high density lipoprotein (HDL) subclass 3 was completely independent of LDL receptors. Thus, HDL3 enhanced responses of both normal and familial hypercholesterolemia lymphocytes in an equivalent concentration-dependent manner; this effect was not altered by reductive methylation of HDL3. One function of lipoproteins in this system may be the provision of fatty acids since oleic and linoleic acids enhanced DNA synthesis by both normal and familial hypercholesterolemia lymphocytes in the absence of lipoproteins. These results indicate that lipoproteins may provide fatty acids necessary for optimal proliferation of human lymphocytes by both LDL receptor-mediated and LDL receptor-independent interactions.

Adult↗

PubMed1989

Regulation of low density lipoprotein receptor gene expression in human lymphocytes.

Cholesterol homeostasis is maintained by coordinate regulation of endogenous synthesis and exogenous uptake of lipoprotein cholesterol by low density lipoprotein (LDL) receptors. In the lymphocyte, limiting the availability of exogenous cholesterol is known to increase the rate of endogenous sterol biosynthesis. However, the effect of cholesterol deprivation on the expression and regulation of the LDL receptor gene has not been delineated in lymphocytes. Here, LDL receptor mRNA was detected in freshly isolated human peripheral mononuclear cells. LDL receptor mRNA levels increased by 3-fold during a one-h in vitro culture in lipoprotein-deficient medium and by 6-fold during a 2-h incubation. Actinomycin D blocked the synthesis of LDL receptor mRNA in these cultures. However, neither cycloheximide nor LDL or oxygenated sterols suppressed the increase in LDL receptor mRNA levels observed after a 2-h incubation. The increase in LDL receptor mRNA was maintained for 24 h of culture in the absence of LDL. Ongoing gene transcription and not mRNA stabilization accounted for this expression. Inhibition of protein synthesis with cycloheximide completely prevented the sustained increase in LDL receptor mRNA levels measured after 24 h. Low concentrations of LDL (5 micrograms of cholesterol/ml) and oxygenated sterols also suppressed the level of LDL receptor mRNA measured after a 24-h incubation. These data show that the initial upregulation of LDL receptor gene expression is independent of protein synthesis and not suppressed by either LDL or oxygenated sterols. In contrast, the continued transcription necessary for the maintenance of steady-state levels of LDL receptor mRNA requires synthesis of new protein and is regulated by LDL and oxygenated sterols.

Adult↗

PubMed1989

Normalization of LDL receptor function by lymphocytes of patients with heterozygous familial hypercholesterolemia after treatment with plasma cholesterol lowering agents.

Low density lipoprotein (LDL)-dependent growth of mitogen-activated lymphocytes, inhibited in their capacity to synthesize cholesterol endogenously, can be used as an assay of functional receptors for LDL. Using this technique, abnormalities can be detected in circulating lymphocytes obtained from patients with familial hypercholesterolemia (FH). Functional lymphocyte LDL receptor activity was decreased in patients with heterozygous FH. Following treatment with the specific inhibitor of cholesterol synthesis, lovastatin, alone or in combination with a bile acid-binding resin, there was increased expression of functional lymphocyte LDL receptors in five of nine patients. Plasma LDL cholesterol levels decreased in all nine patients. Three other patients who were only studied while receiving therapy also manifested increased expression of functional lymphocyte LDL receptors. The degree of improvement in plasma LDL cholesterol did not predict the effect on lymphocyte LDL receptor function. Longitudinal studies indicated that an increase in functional LDL receptor activity could be observed with 4 weeks of therapy and persisted for at least 18 months on continuous treatment. These results provide direct evidence that therapy with lovastatin and a bile acid-binding resin can lead to increased expression of functional LDL receptors by lymphocytes in the majority (eight of 12) of patients with heterozygous FH.

Anticholesteremic Agents↗

PubMed1989

Regulation of LDL receptor mRNA levels in human lymphocytes by functional demand and ambient sterols.

Mitogenic stimulation increases lymphocyte LDL receptor gene expression. Increases are dependent on protein synthesis, not explained by altered mRNA stability and subject to negative feedback regulation. Furthermore, transcription occurs for at least 24 hr and requires ongoing protein synthesis. Depletion of putative endogenous pools of cholesterol that regulate cellular sterol metabolism cannot account for the increase in LDL receptor gene expression caused by mitogenic stimulation. Mitogen-stimulated cells always contain substantially higher levels of LDL receptor messenger RNA than corresponding resting cells. Mitogenic stimulation thus provides a signal that increases LDL receptor gene expression over and above that predicted from the concentration of exogenous sterols. These studies, therefore, indicate that LDL receptor transcription is modulated by signals transduced during cellular activation as well as by negative feedback from regulatory sterols.

Feedback↗

PubMed1988

Assessment of functional low-density lipoprotein receptor activity on lymphocytes of normal subjects and patients with familial hypercholesterolemia.

In FH, abnormalities of the gene encoding the receptor for LDL lead to hypercholesterolemia and premature atherosclerosis. A method to identify LDL receptor defects using peripheral blood lymphocytes has been developed. When endogenous synthesis of cholesterol was blocked, proliferation of mitogen-stimulated normal human lymphocytes was markedly inhibited unless an exogenous source of sterol was supplied. When exogenous sterol was provided as a plasma lipoprotein, LDL receptor-mediated interaction with apolipoprotein-B or -E was essential for the provision of cholesterol to normal human lymphocytes. Thus, functional LDL receptors were necessary to permit proliferation of normal lymphocytes in these cultures. Lymphocytes from patients heterozygous for abnormalities in the LDL receptor gene can be distinguished from normal lymphocytes by their diminished functional LDL receptor activity. Of interest, following treatment with plasma cholesterol-lowering agents, functional lymphocyte LDL receptor activity normalized in some but not all patients with heterozygous FH, whereas plasma LDL cholesterol levels decreased in all patients. These results suggest that therapy with plasma cholesterol-lowering agents can lead to increased expression of LDL receptors by lymphocytes in the majority of patients with heterozygous FH. The failure of some heterozygous FH patients to increase functional LDL receptor activity after prolonged therapy indicates that there is heterogeneity in these patients despite a similar capacity of the therapy to decrease plasma LDL cholesterol. Variability in the expression of the normal LDL receptor gene in individual T lymphocytes may account for some of these findings.

Cell Division↗

PubMed1987

Provision of cholesterol to lymphocytes by high density and low density lipoproteins. Requirement for low density lipoprotein receptors.

The capacity of lipoprotein fractions to provide cholesterol necessary for human lymphocyte proliferation was examined. When endogenous synthesis of cholesterol was blocked, proliferation of mitogen-stimulated normal human lymphocytes was markedly inhibited unless an exogenous source of sterol was supplied. All lipoprotein fractions with the exception of high density lipoprotein subclass 3 were able to provide cholesterol for lymphocyte proliferation. Each of the lipoprotein subfractions capable of providing cholesterol was also able to regulate endogenous sterol synthesis in cultured human lymphocytes. Provision of cholesterol by lipoproteins required the interaction of apolipoprotein B or apolipoprotein E with specific receptors on normal lymphocytes. Apolipoprotein modification by acetylation or methylation, which markedly reduced the ability to regulate sterol biosynthesis, also diminished the capacity of lipoproteins to provide cholesterol. In addition, depletion of apolipoprotein B- and apolipoprotein E-containing particles from high density lipoprotein decreased its ability to suppress cholesterol synthesis and prevented it from providing cholesterol to proliferating lymphocytes. Monoclonal antibodies directed against the receptor-recognition sites on apolipoprotein B and apolipoprotein E were used to define the specific apolipoproteins required for the provision of cholesterol to lymphocytes by the various lipoprotein fractions. The antibody to apolipoprotein B inhibited cholesterol provision by both low density lipoprotein (LDL) and other lipoprotein fractions. The antibody to apolipoprotein E did not decrease provision of cholesterol by LDL but did inhibit the capacity of other fractions to provide cholesterol. In addition, a monoclonal antibody against the ligand binding site on the LDL receptor inhibited provision of cholesterol to normal lymphocytes by all lipoproteins. Finally, lymphocytes lacking LDL receptors were unable to obtain cholesterol from any lipoprotein fraction. These studies demonstrate that LDL receptor-mediated interaction with apolipoprotein B or apolipoprotein E is essential for the provision of cholesterol to normal human lymphocytes from all lipoprotein sources.

Acetates↗

PubMed1987

Regulation of lymphocyte proliferation by cholesterol: the role of endogenous sterol metabolism and low density lipoprotein receptors.

Cholesterol availability is a major determinant of the capacity of lymphocytes to proliferate. Either endogenously-synthesized cholesterol or that taken up from the medium can be utilized as a source for new membrane biosynthesis. Mitogenic stimulation of human lymphocytes augments the rate of endogenous sterol synthesis. This mitogen-induced increase in lymphocyte sterol synthesis can be observed within 4 h of stimulation and is prevented by suppressing the activity of 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase with the specific inhibitors ML-236B or mevinolin. The resultant inhibition of lymphocyte sterol synthesis does not affect mitogen-stimulated blast transformation or initial entry into the S phase of the cell cycle, even when no source of exogenous sterol is present. However, maximal enlargement of the stimulated blast cells is suppressed by inhibition of HMG-CoA reductase activity, and lymphocyte proliferation is completely prevented. These inhibitory effects are reversed by the addition either of mevalonate, the product of the inhibited enzyme, or of low-density-lipoprotein (LDL) cholesterol. The finding that LDL cholesterol could not support growth of lymphocytes obtained from individuals who lacked LDL receptors indicates that LDL-mediated delivery of exogenous sterols to proliferating lymphocytes requires intact LDL receptors. The data indicate that neither endogenous sterol synthesis nor a source of cholesterol is necessary for mitogen-stimulated activation and blast transformation of human lymphocytes. Subsequent enlargement and cell division requires either sterol synthesis or an exogenous source of cholesterol. When the exogenous source of cholesterol is in the form of LDL, normal LDL receptors are also necessary.

Cell Cycle↗

PubMed1986

Low-density lipoprotein (LDL) and lymphocyte responses: direct suppression by native LDL and indirect inhibition from zinc chelation by contaminating EDTA.

Low-density lipoproteins (LDL) have been shown to have a number of effects on the function of various cell types. To appreciate whether the in vitro effects of LDL have in vivo relevance, it is necessary to demonstrate that the biologic action described can be accounted for by native LDL and not by an alteration in the molecule or an addition to the preparation occurring during isolation. EDTA is frequently added to LDL during preparation to prevent oxidation. The effect of EDTA dialysis on LDL-mediated inhibition of lymphocyte responses was therefore examined. LDL alone did not inhibit mitogen-induced initial lymphocyte activation but rather suppressed lymphocyte DNA synthesis and subsequent proliferation in a transferrin-reversible manner. LDL dialysed with EDTA also inhibited lymphocyte responsiveness but the inhibition was not reversed by transferrin. Further experiments demonstrated that after dialysis EDTA in the LDL accounted for the change in its inhibitory effects. EDTA did not alter the lipoprotein but itself inhibited lymphocyte responses by chelating zinc necessary for DNA synthesis. These data indicate that LDL preparations may exhibit at least two separate effects on lymphocyte function. LDL is directly suppressive, while small amounts of contaminating EDTA can additionally be suppressive by chelating zinc. Thus, EDTA present in LDL preparations can alter their apparent biologic effects.

Adult↗