Evidence for sterol-independent regulation of low-density lipoprotein receptor activity in Hep-G2 cells

Combination of Berberine with Resveratrol Improves the Lipid-Lowering Efficacy

The natural compound berberine has been reported to exhibit anti-diabetic activity and to improve disordered lipid metabolism. In our previous study, we found that such compounds upregulate expression of sirtuin 1—a key molecule in caloric restriction, it is, therefore, of great interest to examine the lipid-lowering activity of berberine in combination with a sirtuin 1 activator resveratrol. Our results showed that combination of berberine with resveratrol had enhanced hypolipidemic effects in high fat diet-induced mice and was able to decrease the lipid accumulation in adipocytes to a level significantly lower than that in monotherapies. In the high fat diet-induced hyperlipidemic mice, combination of berberine (30 mg/kg/day, oral) with resveratrol (20 mg/kg/day, oral) reduced serum total cholesterol by 27.4% ± 2.2%, and low-density lipoprotein-cholesterol by 31.6% ± 3.2%, which was more effective than that of the resveratrol (8.4% ± 2.3%, 6.6% ± 2.1%) or berberine (10.5% ± 1.95%, 9.8% ± 2.58%) monotherapy (p < 0.05 for both). In 3T3-L1 adipocytes, the treatment of 12 µmol/L or 20 µmol/L berberine combined with 25 µmol/L resveratrol showed a more significant inhibition of lipid accumulation observed by Oil red O stain compared with individual compounds. Moreover, resveratrol could increase the amount of intracellular berberine in hepatic L02 cells. In addition, the combination of berberine with resveratrol significantly increases the low-density-lipoprotein receptor expression in HepG2 cells to a level about one-fold higher in comparison to individual compound. These results implied that the enhanced effect of the combination of berberine with resveratrol on lipid-lowering may be associated with upregulation of low-density-lipoprotein receptor, and could be an effective therapy for hyperlipidemia in some obese-associated disease, such as type II diabetes and metabolic syndrome.

Impaired liver regeneration in Ldlr-/- mice is associated with an altered hepatic profile of cytokines, growth factors, and lipids

BACKGROUND & AIMS

It is widely recognized that in the early stages of liver regeneration after partial hepatectomy, the hepatocytes accumulate a significant amount of lipids. The functional meaning of this transient steatosis and its effect on hepatocellular proliferation are not well defined. In addition, the basic mechanisms of this lipid accumulation are not well understood although some studies suggest the participation of the Low Density Lipoprotein Receptor (Ldlr).

METHODS

To address these questions, we studied the process of liver regeneration in Ldlr null mice and wild type mice following partial hepatectomy.

RESULTS

Ldlr deficiency was associated with a significant decrease in serum albumin concentration, during early stages of liver regeneration, and a delayed hepatic regeneration. Remnant livers of Ldlr(-)(/)(-) showed a time-shifted expression of interleukin-6 (IL6) and a defective activation of tumor necrosis factor-α (TNFα) and hepatocyte growth factor (HGF) expression in early phases of liver regeneration. Unexpectedly, Ldlr(-)(/)(-) showed no significant differences in the content of lipid droplets after partial hepatectomy compared to wild type mice. However, lipidomic analysis of the regenerating liver from Ldlr(-)(/)(-) revealed a lipid profile compatible with liver quiescence: high content of cholesterol esters and ceramide, and low levels of phosphatidylcholine.

CONCLUSIONS

Ldlr deficiency is associated with significant changes in the hepatic lipidome that affect cytokine-growth factor signaling and impair liver regeneration. These results suggest that the analysis of the hepatic lipidome may help predict the success of liver regeneration in the clinical environment, specifically in the context of pre-existing liver steatosis.

Chylomicron remnant uptake in the livers of mice expressing human apolipoproteins E3, E2 (Arg158→Cys), and E3-Leiden

Apolipoprotein E2 (apoE2) and apoE3-Leiden cause chylomicron remnant accumulation (type III hyperlipidemia). However, the degree of dyslipidemia and its penetrance are different in humans and mice. Remnant uptake by isolated liver from apoE-/- mice transgenic for human apoE2, apoE3-Leiden, or apoE3 was measured. In the presence of both LDL receptor (LDLR) and LDL receptor-related protein (LRP), remnant uptake was apoE3>E3-Leiden>E2 mice. Absence of LDLR reduced uptake in apoE3 and apoE3-Leiden-secreting livers but not in apoE2-secreting livers. LRP inhibition with receptor-associated protein reduced uptake in apoE3- and apoE2-secreting livers, but not in apoE3-Leiden-secreting livers, regardless of the presence of LDLR. Fluorescently labeled remnants clustered with LRP in apoE3-secreting livers only in the absence of LDLR, but clustered in livers that expressed apoE2 even in the presence of LDLR, and did not cluster with LRP in livers of apoE3-Leiden even in the absence of LDLR. Remnants were reconstituted with the three human apoE isoforms. Removal by liver of mApoe-/-/mldlr-/- mice expressing the human LDLR was slightly greater than removal in the previous experiments with apoE3>E2> E3-Leiden. Thus, in vivo, human apoE2 is cleared primarily by LRP, apoE3-Leiden is cleared only by the LDLR, and apoE3 is cleared by both.

Effects of fibrillar C-terminal fragment of cleaved alpha1-antitrypsin on cholesterol homeostasis in HepG2 cells

Amyloid fibrils of diverse origin are known to disturb vital cellular functions and induce cell death. In this study, the effects of amyloid fibrils from the C-terminal fragment (C-36) of cleaved alpha1-antitrypsin (AAT) on low-density lipoprotein (LDL) metabolism were investigated in HepG2 cells. Treatment of the cells with C-36 fibrils (10 micromol/L) enhanced 125I-LDL binding and uptake 10 to 15 times, and highly up-regulated levels of LDL receptor mRNA, as compared with control cells. Competition experiments using excess of unlabeled LDL and blockage experiments with a monoclonal LDL receptor antibody diminished or completely abolished the stimulatory effects of fibrils on LDL binding and LDL receptor mRNA levels, suggesting that fibrils act via the LDL receptor pathway. However, C-36 fibrils had no significant effect on [2-14C]acetate incorporation into cholesterol biosynthesis and cholesterol ester formation, but inhibited 125I-LDL degradation by 20% and reduced bile acid biosynthesis up to 48% in a dose-dependent manner. Preincubation of the cells with fibrils before the addition of LDL totally abolished the LDL inhibitory effect on unesterified cholesterol synthesis, further confirming the LDL receptors to be the target for C-36 fibrils. Moreover, the expression of sterol regulatory element binding protein-1 (SREBP-1) was found to increase twofold and more after 24 hours of incubation of the cells with several concentrations of C-36 fibrils. Our study suggests that the cytotoxicity of C-36 fibrils on HepG2 cells is associated with perturbed intracellular cholesterol homeostasis, induced through fibril-stimulated expression of the LDL receptors via the sterol-responsive element.

Effect of tamoxifen on cholesterol synthesis in HepG2 cells and cultured rat hepatocytes

The objective of this study was to investigate the mechanisms by which tamoxifen modifies cholesterol metabolism in cellular models of liver metabolism, HepG2 cells and rat hepatocytes. The effect of tamoxifen on cholesterol and triglyceride-palmitate synthesis was measured using isotopomer spectral analysis (ISA) and gas chromatography-mass spectrometry (GC-MS) and compared with the effects of progesterone, estradiol, the antiestrogen ICI 182,780, and an oxysterol, 25-hydroxycholesterol (25OHC). Cholesterol synthesis in cells incubated in the presence of either [1-(13)C]acetate, [U-13C]glucose, or [4,5-(13)C]mevalonate for 48 hours was reduced in the presence of 10 micromol/L tamoxifen and 12.4 micromol/L 25OHC in both HepG2 cells and rat hepatocytes. The ISA methodology allowed a clear distinction between effects on synthesis and effects on precursor enrichment, and indicated that these compounds did not affect enrichment of the precursors of squalene. Progesterone was effective in both cell types at 30 micromol/L and only in HepG2 cells at 10 micromol/L. Estradiol and ICI 182,780 at 10 micromol/L did not inhibit cholesterol synthesis. None of the compounds altered the synthesis of triglyceride-palmitate in either cell type. Treatment of cells with tamoxifen produced accumulation of three sterol precursors of cholesterol, zymosterol, desmosterol, and delta8 cholesterol. This pattern of precursors indicates inhibition of delta24,25 reduction in addition to the previously described inhibition of delta8 isomerase. We conclude that tamoxifen is an effective inhibitor of the conversion of lanosterol to cholesterol in cellular models at concentrations comparable to those present in the plasma of tamoxifen-treated individuals. Our findings indicate that this mechanism may contribute to the effect of tamoxifen in reducing plasma cholesterol in humans.

Low-density lipoprotein receptor mRNA in human breast cancer cells: influence by PKC modulators

It was reported previously that low-density lipoproteins (LDL) differentially stimulate cell growth of hormone-responsive (ER+) and hormone-unresponsive (ER-) mammary tumor cell lines. Here we examined the mRNA levels of the LDL-receptor (LDL-R) gene with RNAse protection analysis in ER- (MDA-MB-231 and HBL-100) and ER+ (MCF-7 and ZR75-1) cells, and compared them with the estrogen receptor (ER) status. Measurable amounts of ER mRNA were only found in ER+ cells as expected. LDL-R mRNA abundance was 3-5 fold higher in ER- cells as compared to ER+ cells. Incubation with phorbol-12-myristate-13-acetate led to a significant increase (p < 0.005) of LDL-R mRNA in ER+ cells, whereas in ER- cells LDL-R mRNA levels remained merely unchanged. Incubation of cells with dioctanoylglycerol, a synthetic homolog of diacylglycerol, increased LDL-R mRNA in ER+ but not in ER-. Inhibition of protein kinase C (PKC) by H7 resulted in a highly significant reduction of LDL-R mRNA both in ER+ and ER- cells. PKC seems to be an important regulator of LDL-R mRNA abundance in mammary tumor cells. It is hypothesized that in human-breast cancer the process of conversion from hormone-responsive to hormone-unresponsive status is accompanied by a change in PKC activity and PKC might exert cell specific differences on the regulation of LDL-R mRNA levels, which in turn influences the delivery of exogenous cholesterol to cancer cells.

Increased uptake of LDL by oxidized macrophages is the result of an initial enhanced LDL receptor activity and of a further progressive oxidation of LDL

Iron ions were recently shown to induce cellular lipid peroxidation in macrophages, and these oxidized cells can convert native low-density lipoprotein (LDL) to oxidized LDL (Ox-LDL). The present study demonstrates that deoxycholic acid (DCA) and angiotensin II (ANG-II) can also induce oxidative modification of macrophages via metal ions independent mechanisms. Furthermore, incubation of LDL (200 micrograms of protein/ml) for 24 h at 37 degrees C with DCA, ANG-II, as well as FeSO4-induced oxidized macrophages, resulted in oxidative modification of the lipoprotein as evidenced by increased TBARS formation in LDL (by 50, 105, and 258%, respectively), decreased TNBS reactivity (by 45, 56, and 42%, respectively), and increased cellular uptake (by 60, 166, and 230%, respectively). A positive correlation (n = .88) was found between the extent of the cellular lipid peroxidation and the increment in the cellular uptake of the LDL. The oxidative modification of LDL by oxidized macrophages was found to be a progressive process. Incubation of LDL with oxidized macrophages for increasing periods of time up to 24 h resulted in progressive increment in: (1) the electrophoretic mobility of the LDL; (2) the TBARS formation in LDL; (3) the cellular uptake of LDL by the oxidized macrophages via the Ox-LDL receptor. Upon fractionation on a heparin-sepharose column of LDL that was incubated for different periods of time with oxidized macrophages, a gradual increment in the unbound LDL fraction was obtained, up to 72% after 24 h of incubation. During the first hour of LDL incubation with the oxidized macrophages a twofold increase in the cellular uptake of LDL by these cells was detected, although no significant oxidation of the lipoprotein occurred during this short time period. This effect could be attributed to an increased number of LDL receptors on the cell surface of the oxidized macrophages. In conclusion, increased uptake of LDL by oxidized macrophages results from two routes: (1) enhanced uptake via the LDL receptor due to increased LDL receptor activity; (2) lipoprotein uptake via the Ox-LDL receptors due to cellular modification of LDL. Both of these processes lead to macrophage cholesterol accumulation and foam cell formation, and thus contribute to accelerated atherosclerosis under oxidative stress.

Involvement of tyrosine kinase activity in the low-density lipoprotein receptor expression in human lung adenocarcinoma cell line A549

In common with other tumour cell lines but in contrast to normal cells, the human adenocarcinoma cell line A549 showed a biphasic regulation of the LDL receptor activity during growth both LDL binding and metabolism (sum of internalised and degraded LDL) increased during the log exponential growth phase and decreased when the cells approached confluence. This period of increasing LDL receptor activity coincided with a high resistance to cholesterol down-regulation which suggested a sterol-independent pathway of stimulation. Since A549 cells have an autocrine loop of growth factors, two of which have tyrosine kinase activity, the LDL receptor activity was tested in the presence of the tyrosine kinase inhibitor, genistein. When cells were incubated in the absence of cholesterol (LPDS medium), the inhibition that occurred was two-fold higher during the exponential growth phase than during the confluent phase. Moreover, the residual LDL binding and metabolism after genistein inhibition were completely resistant to down-regulation by cholesterol only during the growth phase. When cholesterol was present (FCS medium). inhibition was observed only during the growth phase. The inhibition of LDL receptor activity by genistein was found to be the result of a loss in the number of LDL binding sites, while the dissociation constant was not affected. This loss was accompanied by a disappearance of mRNA as shown by RNase mapping. By comparison, LDL receptor activity of normal cells (fibroblasts) was also affected by genistein during the exponential growth phase but was much more cholesterol-dependent. Taken together, these results suggest that the tyrosine kinase pathway is essential to up-regulate LDL receptor expression in highly dividing cells and particularly in tumour cells in which the sterol regulation is deficient.

Tyrosine kinase inhibitors potentiate the induction of low density lipoprotein receptor gene expression by hepatocyte growth factor

The role of tyrosine kinase, protein kinase C, cyclic nucleotide- and Ca(2+)-calmodulin-dependent protein kinase second messenger pathways in the induction of LDL receptor gene expression by hepatocyte growth factor (HGF) was studied in the human hepatoma cell line Hep-G2. Incubation with media containing HGF increased the level of LDL receptor mRNA by 6.5-fold. Co-incubation with HGF and either of two tyrosine kinase inhibitors genistein (2.0-20.0 micrograms/ml) and herbimycin A (0.5-500.0 ng/ml) increased the level of LDL receptor mRNA above that observed with HGF alone by 40-60%. Incubation with HGF in the presence of the calmodulin antagonist W7 (10-30 microM) also super-induced the level of LDL receptor mRNA by nearly 230%. The protein kinase C and A inhibitors chelerythrine (0.1-10.0 microM) and H8 (0.5-5.0 microM), respectively, had no significant effects on the induction of LDL receptor mRNA by HGF. Taken together, these data suggest that tyrosine kinase, protein kinases C and A, and Ca(2+)-calmodulin dependent protein kinase activities are not essential for activation of LDL receptor gene expression in Hep-G2 cells by HGF.

Modulation of hepatic apolipoprotein B, 3-hydroxy-3-methylglutaryl-CoA reductase and low-density lipoprotein receptor mRNA and plasma lipoprotein concentrations by defined dietary fats. Comparison of trimyristin, tripalmitin, tristearin and triolein

Different dietary fatty acids exert specific effects on plasma lipids but the mechanism by which this occurs is unknown. Hamsters were fed on low-cholesterol diets containing triacylglycerols enriched in specific saturated fatty acids, and effects on plasma lipids and the expression of genes involved in hepatic lipoprotein metabolism were measured. Trimyristin and tripalmitin caused significant rises in low-density lipoprotein (LDL) cholesterol which were accompanied by significant reductions in hepatic LDL receptor mRNA levels. Tripalmitin also increased hepatic expression of the apolipoprotein B gene, implying an increased production of LDL via very-low-density lipoprotein (VLDL) and decreased removal of LDL in animals fed this fat. Hepatic levels of 3-hydroxy-3-methylglutaryl-CoA reductase mRNA did not vary significantly between the groups. Compared with triolein, tristearin had little effect on hepatic gene expression or total plasma cholesterol. However, it caused a marked decrease in VLDL cholesterol and a rise in LDL cholesterol such that overall it appeared to be neutral. Lipid analysis suggested a rapid desaturation of much of the dietary stearate. The differential changes in plasma lipids and hepatic mRNA levels induced by specific dietary fats suggests a role for fatty acids or a metabolite thereof in the regulation of the expression of genes involved in lipoprotein metabolism.

Enhanced expression of granulosa cell low density lipoprotein receptor activity in response to in vitro culture conditions

Previous studies have shown that the B/E (low density lipoprotein [LDL]) receptor pathway plays a minor role in cholesterol uptake in the intact rat ovary, but when granulosa cells are isolated and maintained in culture, the cells develop a fully functional B/E receptor system. In the current study we examined the development of the B/E receptor over time (96 h) in culture and compared its physiological function, expression of mRNA and protein levels, and morphological events to the upregulation induced in 24 h by hormone (human chorionic gonadotropin [hCG] or Bt2cAMP). With both protocols, increased progestin production occurs and is associated with elevated binding, uptake, and degradation of LDL in the medium although the impact of Bt2cAMP stimulation on all these measurements is several times that observed with time alone. Only the hormone-stimulated LDL receptor response was associated with an increase in receptor protein (Western blot) or mRNA levels (RNase protection assay). We conclude that unstimulated granulosa cells show posttranslational increases in B/E receptor activity with time in culture, but transcriptional changes in B/E receptor follow stimulation with trophic hormone or its second messenger, cAMP.

Stimulation of low-density lipoprotein uptake in HepG2 cells by epidermal growth factor via a tyrosine kinase-dependent, but protein kinase C-independent, mechanism

Epidermal growth factor (EGF), a potent mitogenic polypeptide, stimulated the uptake and degradation of [3H]sucrose-labelled low-density lipoprotein (LDL) by HepG2 cells. The increase in LDL uptake was prevented by the presence of the tyrosine kinase inhibitor genistein. Activation of protein kinase C with phorbol 12-myristate 13-acetate (PMA) also stimulated the uptake of [3H]LDL by HepG2 cells. When EGF and PMA were added together, PMA increased the response to EGF in an additive manner. The protein kinase C inhibitor Ro-31-8220 prevented the increase in LDL uptake caused by PMA, but did not affect EGF stimulation of LDL uptake. Similarly, down-regulation of protein kinase C activity by chronic treatment with PMA also did not affect the EGF stimulation of LDL uptake. These results suggest that the EGF stimulation of LDL uptake and degradation by HepG2 cells is mediated by a tyrosine kinase-dependent, but protein kinase C-independent, mechanism.

Ketoconazole and 25-hydroxycholesterol produce reciprocal changes in the rate of transcription of the human LDL receptor gene

Sterol-dependent regulation of low-density lipoprotein (LDL) receptor gene expression was studied in the human hepatoma HepG2 cell line. Incubation of HepG2 cells with 20 microM ketoconazole increased the level of LDL receptor mRNA. After a lag of approx. 1.0 h the level rose 6.5-fold within 8.0 h and remained elevated for up to 24 h. Incubation with 10 micrograms 25-hydroxycholesterol/ml for 24 h produced a 40-50% reduction in the level of LDL receptor mRNA. Ketoconazole- and 25-hydroxycholesterol-induced changes in LDL receptor mRNA accumulation were due to alterations in the relative rate of LDL receptor gene transcription as measured by nuclear run-on transcription. Incubation with 20 microM ketoconazole for 4 h or 10 micrograms 25-hydroxycholesterol/ml for 24 h produced a 3.6-fold increase and a 40% reduction, respectively, in the transcription rate of LDL receptor gene. Removal of the Alu-like sequence elements within the LDL receptor cDNA was required to consistently measure changes in LDL receptor gene transcription. No significant changes were noted in the half-life of LDL receptor mRNA in ketoconazole or 25-hydroxycholesterol-treated cells. These data demonstrate that sterol-dependent changes in the level of LDL receptor mRNA can be completely accounted for by changes in the rate of LDL receptor gene transcription.

Early induction of LDL receptor gene during rat liver regeneration

After partial hepatectomy in the rat, there is substantial lipid accumulation in the liver. No information is available on the possible role of receptor-mediated endocytosis in this process. Since the low density lipoprotein (LDL) receptor is stimulated as a part of an early growth response in cell culture (Ellsworth et al.: Biochem. J. 279:175-187, 1991), the metabolism of this receptor during liver regeneration was studied. The mRNA and membrane protein level of the receptor were measured in the liver and in the adrenal glands at different times after partial hepatectomy, corresponding to different phases of the cell cycle. A discontinuous pattern of receptor expression is detectable in the regenerating liver; a large increase of mRNA and membrane protein occurs at an early time (2-4 h), suggesting that there is induction of LDL receptor gene transcription shortly after partial hepatectomy. This response seems specific for the liver following injury since the adrenal receptor does not show a different pattern in partially hepatectomized rats and sham-operated controls. After returning to control levels, the LDL receptor again increases slightly above control at 24 h, a time when cell replication begins.

Dietary fatty acids regulate hepatic low density lipoprotein (LDL) transport by altering LDL receptor protein and mRNA levels

The concentration of LDL in plasma is strongly influenced by the amount and the type of lipid in the diet. Recent studies in the hamster have shown that dietary fatty acids differentially affect circulating LDL levels primarily by altering receptor-dependent LDL uptake in the liver. To investigate the mechanistic basis of this effect, rates of receptor-dependent LDL transport in the liver were correlated with LDL receptor protein and mRNA levels in hamsters fed safflower oil or coconut oil and varying amounts of cholesterol. Hepatic LDL receptor activity was significantly lower in animals fed coconut oil than in animals fed safflower oil at all levels of cholesterol intake (26, 53, and 61% lower at cholesterol intakes of 0, 0.06, and 0.12%, respectively). These fatty acid-induced changes in hepatic LDL receptor activity were accompanied by parallel changes in hepatic LDL receptor protein and mRNA levels, suggesting that dietary fatty acids regulate the LDL receptor pathway largely at the mRNA level.

Stimulation of rat hepatic low density lipoprotein receptors by glucagon. Evidence of a novel regulatory mechanism in vivo

We studied the influence of glucagon on hepatic LDL receptors and plasma lipoproteins in rats. A dose-dependent (maximum, threefold) increase in LDL-receptor binding was evident already at a dose of 2 x 4 micrograms, and detectable 3 h after injection; concomitantly, cholesterol and apolipoprotein (apo) B and apoE within LDL and large HDL decreased in plasma. LDL receptor mRNA levels were however unaltered or reduced. Hepatic microsomal cholesterol was increased and the enzymatic activities of 3-hydroxy-3-methylglutaryl coenzyme A reductase and cholesterol 7 alpha-hydroxylase in hepatic microsomes were reduced. Insulin alone increased receptor binding and receptor mRNA levels twofold, but plasma cholesterol was unchanged and plasma apoE and apoB increased. Administration of insulin to glucagon-treated animals reduced the LDL-receptor binding to control levels and apoB appeared in LDL particles. Estrogen treatment increased LDL-receptor binding and mRNA levels five- and eightfold, respectively. Combined treatment with glucagon and estrogen reduced the stimulation of LDL-receptor mRNA levels by 80% although LDL-receptor binding was unchanged. Immunoblot analysis showed that glucagon increased the number of hepatic LDL receptors. We conclude that glucagon induces the number of hepatic LDL receptors by a mechanism not related to increased mRNA levels, suggesting the presence of a posttranscriptional regulatory mechanism present in the liver in vivo.