Elevated Cholesterol Synthesis Research Articles

Cholesterol: An important actor on the cancer immune scene.

Based on the structural and signaling roles of cholesterol, which are necessary for immune cell activity, high concentrations of cholesterol and its metabolites not only trigger malignant cell activities but also impede immune responses against cancer cells. To proliferate and evade immune responses, tumor cells overcome environmental restrictions by changing their metabolic and signaling pathways. Overexpression of mevalonate pathway enzymes and low-density lipoprotein receptor cause elevated cholesterol synthesis and uptake, respectively. Accordingly, cholesterol can be considered as both a cause and an effect of cancer. Variations in the effects of blood cholesterol levels on the outcome of different types of cancer may depend on the stage of cancer. However, positive effects of cholesterol-lowering drugs have been reported in the treatment of patients with some malignancies.

Sexual dimorphism: increased sterol excretion leads to hypocholesterolaemia in female hyperbilirubinaemic Gunn rats.

Circulating bilirubin is associated with reduced serum cholesterol concentrations in humans and in hyperbilirubinaemic Gunn rats. However, mechanisms contributing to hypocholesterolaemia remain unknown. Therefore, this study aimed to investigate cholesterol synthesis, transport and excretion in mutant Gunn rats. Adult Gunn and control rats were assessed for daily faecal sterol excretion using metabolic cages, and water was supplemented with [1‐13C]‐acetate to determine cholesterol synthesis. Bile was collected to measure biliary lipid secretion. Serum and liver were collected for biochemical analysis and for gene/protein expression using RT‐qPCR and western blot, respectively. Additionally, serum was collected and analysed from juvenile rats. A significant interaction of sex, age and phenotype on circulating lipids was found with adult female Gunn rats reporting significantly lower cholesterol and phospholipids. Female Gunn rats also demonstrated elevated cholesterol synthesis, greater biliary lipid secretion and increased total faecal cholesterol and bile acid excretion. Furthermore, they possessed increased hepatic low‐density lipoprotein (LDL) receptor and SREBP2 expression. In contrast, there were no changes to sterol metabolism in adult male Gunn rats. This is the first study to demonstrate elevated faecal sterol excretion in female hyperbilirubinaemic Gunn rats. Increased sterol excretion creates a negative intestinal sterol balance that is compensated for by increased cholesterol synthesis and LDL receptor expression. Therefore, reduced circulating cholesterol is potentially caused by increased hepatic uptake via the LDL receptor. Future studies are required to further evaluate the sexual dimorphism of this response and whether similar findings occur in females with benign unconjugated hyperbilirubinaemia (Gilbert's syndrome).Key points Female adult hyperbilirubinaemic (Gunn) rats demonstrated lower circulating cholesterol, corroborating human studies that report a negative association between bilirubin and cholesterol concentrations.Furthermore, female Gunn rats had elevated sterol excretion creating a negative intestinal sterol balance that was compensated for by elevated cholesterol synthesis and increased hepatic low‐density lipoprotein (LDL) receptor expression.Therefore, elevated LDL receptor expression potentially leads to reduced circulating cholesterol levels in female Gunn rats providing an explanation for the hypocholesterolaemia observed in humans with elevated bilirubin levels.This study also reports a novel interaction of sex with the hyperbilirubinaemic phenotype on sterol metabolism because changes were only reported in females and not in male Gunn rats.Future studies are required to further evaluate the sexual dimorphism of this response and whether similar findings occur in females with benign unconjugated hyperbilirubinaemia (Gilbert's syndrome).

Using Elevated Cholesterol Synthesis as a Prognostic Marker in Wilms’ Tumor: A Bioinformatic Analysis

Background Wilms tumor is the most common renal malignancy of children. Identifying factors that could predict the prognosis of patients with Wilms tumor is clinically meaningful. Many studies found tumors with elevated cholesterol synthesis that are featured with dismal prognosis. Even in some clinical trials, people with excessive dietary cholesterol intake and high plasma low-density lipoprotein levels are observed to have increased risk for cancer. However, the role of cholesterol biosynthesis in Wilms tumor has not yet been well clarified. Methods RNA sequencing transcriptome data and all corresponding clinicopathological information used in our study were downloaded from the TARGET database. High-throughput sequencing (Fragments Per Kilobase of transcript per Million fragments mapped) data sets of 130 tumor samples and 6 normal samples were obtained for further analysis. Results Wilms tumor samples with higher activity of cholesterol synthesis are characterized with worse overall survival (P < 0.05). In addition, Wilms tumor samples with mitigated activity of cholesterol synthesis are featured with better dendritic cell (DC) function and cytolytic activity (P < 0.05). Furthermore, we constructed a prognosis model based on differential expressed cholesterol synthesis-related genes (DECSG), which could predict the OS of patients with Wilms tumor accurately. KEGG and GO analysis of differential expressed genes between tumor samples with high and low cholesterol synthesis indicated that DECSGs are highly enriched in “mitosis nuclear division,” “nuclear division,” “chromosome segregation,” “cell cycle,” “Spliceosome,” and “RNA transport.” Conclusions In conclusion, our study reported increased cholesterol synthesis in Wilms tumor predicts a worse prognosis and mitigated cytolytic activity, DC function, and MHC I signature in the tumor microenvironment. We also constructed a prognosis model for predicting the OS of patients with good accuracy, which is promising in clinical translation. Future studies should focus on the detailed mechanism that caused increasing cholesterol which promotes tumor progression and undermines patients' survival.

Desmosterol in Brain Is Elevated because DHCR24 Needs REST for Robust Expression but REST Is Poorly Expressed

Cholesterol synthesis in the fetal brain is inhibited because activity of DHCR24 (24-dehydrocholesterol reductase) is insufficient, causing concentrations of the precursor desmosterol to increase temporarily to 15-25% of total sterols at birth. We demonstrate that failure of DHCR24 to be adequately upregulated during periods of elevated cholesterol synthesis in the brain results from the presence in its promoter of the repressor element 1 (RE1) nucleotide sequence that binds the RE1-silencing transcription factor (REST) and that REST, generally reduced in neural tissues, uncharacteristically but not without precedent, enhances DHCR24 transcription. DHCR24 and REST mRNA levels are reduced 3- to 4-fold in fetal mouse brain compared to liver (p < 0.001). Chromatin immunoprecipitation assays suggested that REST binds to the human DHCR24 promoter in the vicinity of the predicted human RE1 sequence. Luminescent emission from a human DHCR24 promoter construct with a mutated RE1 sequence was reduced 2-fold compared to output from a reporter with wild-type RE1 (p < 0.005). Silencing REST in HeLa cells resulted in significant reductions of DHCR24 mRNA (2-fold) and DHCR24 protein (4-fold). As expected, relative concentrations of Δ²⁴-cholesterol precursor sterols increased 3- to 4-fold, reflecting the inhibition of DHCR24 enzyme activity. In contrast, mRNA levels of DHCR7 (sterol 7-dehydrocholesterol reductase), a gene essential for cholesterol synthesis lacking an RE1 sequence, and concentrations of HMGR (3-hydroxy-3-methyl-glutaryl-CoA reductase) enzyme protein were both unaffected. Surprisingly, a dominant negative fragment of REST consisting of just the DNA binding domain (about 20% of the protein) and full-length REST enhanced DHCR24 expression equally well. Furthermore, RE1 and the sterol response element (SRE), the respective binding sites for REST and the SRE binding protein (SREBP), are contiguous. These observations led us to hypothesize that REST acts because it is bound in close proximity to SREBP, thus amplifying its ability to upregulate DHCR24. It is likely that modulation of DHCR24 expression by REST persisted in the mammalian genome either because it does no harm or because suppressing metabolically active DHCR24 while providing abundant quantities of the multifunctional sterol desmosterol during neural development proved useful.

Anticancer Activity of the Cholesterol Exporter ABCA1 Gene

The ABCA1 protein mediates the transfer of cellular cholesterol across the plasma membrane to apolipoprotein A-I. Loss-of-function mutations in the ABCA1 gene induce Tangier disease and familial hypoalphalipoproteinemia, both cardiovascular conditions characterized by abnormally low levels of serum cholesterol, increased cholesterol in macrophages, and subsequent formation of vascular plaque. Increased intracellular cholesterol levels are also frequently found in cancer cells. Here, we demonstrate anticancer activity of ABCA1 efflux function, which is compromised following inhibition of ABCA1 gene expression by oncogenic mutations or cancer-specific ABCA1 loss-of-function mutations. In concert with elevated cholesterol synthesis foundin cancer cells, ABCA1 deficiency allows for increased mitochondrial cholesterol, inhibits release of mitochondrial cell death-promoting molecules, and thus facilitates cancer cell survival, suggesting that elevated mitochondrial cholesterol is essential to the cancer phenotype.

Tacrolimus and Rapamycin do not elevate cholesterol synthesis in post‐transplant patients, but may impair response to multi‐nutrient dietary intervention

Patients frequently experience hypercholesterolemia after transplant due to immunosuppressive drugs. Mechanisms responsible for this rise in plasma cholesterol are not clear. Dietary therapy is usually not sufficient to reduce plasma cholesterol post‐transplant, however these interventions may be limited in scope. The purpose of this study was to investigate cholesterol synthesis as a possible mechanism for hypercholesterolemia in post‐transplant patients, and test the efficacy of a multi‐faceted dietary intervention. Islet (ITx; n=8) and liver (LTx; n=7) post‐transplant patients on Tacrolimus (Tac) or Rapamycin (Rapa) were recruited with normal control subjects (Ctl; n=9) to measure 24h cholesterol fractional synthesis (FSR‐C) using deuterium. A subset of patients underwent a 4‐week dietary intervention using fish oil, phytosterols, soy, and fibers. FSR‐C was not different between Ctl, ITx and LTx groups. Diet intervention significantly lowered plasma cholesterol and triglyceride in Ctl, however was not effective in ITx or LTx. Diet produced mixed results in changes in FSR‐C across groups. In conclusion, hyperlipidemia caused by Tac or Rapa may not be due to elevated cholesterol synthesis. However, these drugs may interfere with normal responses to hypolipidemic dietary therapy effective in non‐transplant populations.Grant Funding Source: Canadian Institutes of Health Research

Obesity abrogates the concentration-dependent effect of leptin on endogenous cholesterol synthesis in human monocytes

Leptin the cytokine-like hormone is involved not only in local inflammations, but it regulates cholesterol biosynthesis in human monocytes. Since, monocyte-membrane composition in obesity shows considerable difference from control cells, our aim was to elucidate the concentration dependence of the effect of leptin in OW monocytes, and the downstream signaling of high and low leptin concentrations. Control and OW monocytes were stimulated with leptin in the presence or absence of different inhibitors. Our results are as follows: a concentration-dependent biphasic effect could only be detected in control monocytes whereas in OW cells only elevated cholesterol synthesis was found. The signal pathway of 50 ng/mL leptin stimulation involves Ca 2+ signal, activation of PI3K, MAPK and HMG CoA reductase. In the 500 ng/mL leptin-stimulated control monocytes the suppression of cholesterol synthesis was dependent on the Ca 2+ signal, the H-7 sensitive cPKC and PI3K activation, whereas in OW monocytes only PI3K was involved in increased cholesterol synthesis. We conclude that leptin-signaling in OW monocytes is characterized by Ca 2+ influx, abrogation of H-7 sensitive cPKC activation, and by PI3K mediated PKC activation.

Abstract: S3-14 REVERSE CHOLESTEROL TRANSPORT COMPONENTS ASSOCIATE WITH OOCYTE GROWTH

Emerging experimental and human evidence has linked altered hepatic cholesterol homeostasis and free cholesterol (FC) accumulation to the pathogenesis of non-alcoholic steatohepatits (NASH). This review focuses on cellular mechanisms of cholesterol toxicity involved in liver injury and on alterations in cholesterol homeostasis promoting hepatic cholesterol overload in NASH.FC accumulation injures hepatocytes directly, by disrupting mitochondrial and endoplasmic reticulum (ER) membrane integrity, triggering mitochondrial oxidative injury and ER stress, and by promoting generation of toxic oxysterols, and indirectly, by inducing adipose tissue dysfunction. Accumulation of oxidized LDL particles may also activate Kupffer and hepatic stellate cells, promoting liver inflammation and fibrogenesis.Hepatic cholesterol accumulation is driven by a deeply deranged cellular cholesterol homeostasis, characterized by elevated cholesterol synthesis and uptake from circulating lipoproteins and by a reduced cholesterol excretion. Extensive dysregulation of cellular cholesterol homeostasis by nuclear transcription factors sterol regulatory binding protein (SREBP)-2, liver X-receptor (LXR)-α and farnesoid X receptor (FXR) plays a key role in hepatic cholesterol accumulation in NASH. The therapeutic implications and opportunities for normalizing cellular cholesterol homeostasis in these patients are also discussed.

Effect of Endurance Exercise Training on Markers of Cholesterol Absorption and Synthesis

PURPOSE: Abnormal cholesterol metabolism, including low intestinal cholesterol absorption and elevated cholesterol synthesis, is prevalent in individuals with diabetes, hyperlipidemia, obesity, and the metabolic syndrome. Diet-induced weight loss improves cholesterol absorption in these populations; the purpose of this study is to investigate if endurance exercise training also improves cholesterol homeostasis. METHODS: We measured circulating levels of campesterol, sitosterol, lathosterol, and cholesterol by GC/mass spectrometry from 65 sedentary subjects (34 women, 31 men, average age = 59) with at least 1 metabolic syndrome risk factor before and after 6 months of endurance exercise training. Campesterol and sitosterol are plant sterols that correlate with intestinal cholesterol absorption, while lathosterol, a precursor in the cholesterol synthetic pathway, is a marker of whole body cholesterol synthesis. We also measured plasma glucose, insulin, VO2max, and body composition by DEXA and CT scanning before and after the intervention. RESULTS: Following the intervention, plasma levels of total plant sterols (campesterol + sitosterol) were increased by 10% (p < 0.05), but there was no change in plasma lathosterol. In addition, total and LDL-cholesterol were both reduced by ~6 mg/dL (p < 0.05 for each), while HDL-C levels increased by 3.3mg/dL (p < 0.05). Furthermore, the change in total plant sterols (campesterol + sitosterol) was positively correlated with the change in VO2max (r = 0.310, p = 0.004), independent of other metabolic syndrome risk factors. CONCLUSIONS: These data indicate that exercise training reduces plasma cholesterol despite increasing cholesterol absorption in middle-to older-aged subjects with risk factors for metabolic syndrome.

Biliary lipids, cholesterol and bile synthesis: different adaptive mechanisms to dietary cholesterol in lean and obese subjects

Increased biliary cholesterol secretion together with elevated cholesterol synthesis may predispose obese subjects to cholesterol gallstone formation. To investigate whether processing of dietary cholesterol is altered in obesity, we enrolled eight lean and seven obese subjects in a double-blind crossover study. Cholesterol consumption was 300 mg/day on low and 1300 mg/day on high cholesterol diet. After 3 weeks on either diet, hepatic bile was collected to determine biliary lipid secretion, and bile salt composition by high-performance liquid chromatography and cholesterol saturation index was calculated. Cholesterol synthesis was measured employing mass isotopomer distribution analysis. Bile acid synthesis via neutral and acidic pathway was assessed by serum levels of 7alpha-hydroxy-4-cholesten-3-one and 27-hydroxycholesterol. Cholesterol synthesis was increased in obese compared with lean and feedback inhibited only in obese. On low cholesterol diet, cholesterol secretion was doubled in obese but bile acid composition and synthesis was similar between the two groups. After high cholesterol diet, cholesterol saturation index and bile secretion were unchanged. In contrast to obese, lean increased bile acid synthesis only via the acidic pathway. Dietary cholesterol appears to preferentially induce bile acid synthesis via the acidic pathway in lean, whereas cholesterol synthesis was inhibited in obese. Thus, stable cholesterol saturation index may be achieved by different mechanisms.

Impaired cholesterol transport as a mechanism for the cell signaling events in cystic fibrosis

Cystic fibrosis (CF) is caused by a loss of CFTR function causing severe lung disease. Functionally connecting CF-related cell signaling pathway alterations to CFTR is a constant goal. We have shown that several inflammatory proteins exhibit altered expression patterns in models of CF epithelium. A commonality in these pathways is protein prenylation; a product of the isoprenoid/cholesterol pathway. We hypothesize that a loss of CFTR function causes impaired intracellular cholesterol transport resulting in increased isoprenoid/cholesterol synthesis and leading to CF-related signaling events. Two models of CF exhibit increased content of unesterified cholesterol and accumulated free cholesterol. Preliminary staining of human CF trachea also indicates altered accumulation of cholesterol. Acutely inhibiting CFTR in control epithelial cells showed increased total cholesterol accumulation. CF mouse lung also had elevated cholesterol synthesis compared to matched controls. Alterations in calcium regulation have been identified in CF cells, as well as, previously implicated in cholesterol transport regulation. CF cells pretreated with GPN, a substrate that releases calcium, reverse the cholesterol accumulation in CF cell models. Intervention within cholesterol regulatory pathways may represent a novel therapeutic option for the treatment of CF. Supported by the CFF.

Effects of peroxisome proliferators gemfibrozil and clofibrate on syntheses of dolichol and cholesterol in rat liver.

The effects of two peroxisome proliferators, gemfibrozil and clofibrate, on syntheses of dolichol and cholesterol in rat liver were investigated. Gemfibrozil did not affect the overall content of dolichyl phosphate, but it changed the chain-length distribution of dolichyl phosphate, increasing the levels of species with shorter isoprene units. Gemfibrozil suppressed synthesis of dolichyl phosphate from [(3)H]mevalonate and [(3)H]farnesyl pyrophosphate in rat liver. In contrast, clofibrate increased the content of dolichol (free and acyl ester forms). It remarkably enhanced dolichol synthesis from mevalonate, but did not affect dolichol synthesis from farnesyl pyrophosphate. Gemfibrozil elevated cholesterol synthesis from [(14)C]acetate, but did not affect the synthesis from mevalonate. Clofibrate suppressed cholesterol synthesis from acetate, but did not affect cholesterol synthesis from mevalonate. These results suggest that gemfibrozil suppresses synthesis of dolichyl phosphate by inhibiting, at the least, the pathway from farnesyl pyrophosphate to dolichyl phosphate. As a result, the chain-length pattern of dolichyl phosphate may show an increase in shorter isoprene units. Clofibrate may increase the content of dolichol by enhancing dolichol synthesis from mevalonate. Gemfibrozil may increase cholesterol synthesis by activating the pathway from acetate to mevalonate. Unlike gemfibrozil, clofibrate may decrease cholesterol synthesis by inhibiting the pathway from acetate to mevalonate.

Elevation of plasma lathosterol, as an indicator of increased cholesterol synthesis, in preterm (23-32 weeks gestation) infants given Intralipid.

Hypercholesterolemia is common in preterm infants administered 10% Intralipid perhaps because of excess phospholipid in plasma causing efflux of cholesterol from tissues. The purpose of this study was to determine if cholesterol synthesis (as measured by plasma lathosterol) is increased in preterm infants (23-32 wk gestation) during infusion of up to 4 g 10% Intralipid/kg body wt/d. Two groups of infants were studied. Intralipid intake was compared to: 1) plasma cholesterol in blood sampled over the first 100 d of life (preliminary study, n = 22) and 2) plasma cholesterol, lathosterol, and apo AI and B in blood taken at birth (cord), d 3-4 of life, and at least three additional times over the next 25 d (lathosterol study, n = 22). Lathosterol was quantitated by gas liquid chromatography and apo AI and B by immunoprecipitation. In the preliminary study, plasma cholesterol levels rose (to 4.06-10.70 mM) with Intralipid administration. Infants who received less than 2 g Intralipid/kg body wt/d were not hypercholesterolemic. In the lathosterol study, plasma cholesterol increased (1.86-2.24 mM, p = 0.06) and apo AI and B did not change, but lathosterol and the cholesterol:lathosterol ratio decreased (5.24-2.88 microM, p = 0.01, and 284-124 10(2) x mmol lathosterol:mol cholesterol, p = 0.007, respectively) from birth to d 3-4 (n = 11 paired samples). Infants followed longitudinally had increased cholesterol and lathosterol (4- to 7-fold) with increasing Intralipid administration, which decreased after discontinuation of infusion. Apo AI and B decreased upon Intralipid infusion.(ABSTRACT TRUNCATED AT 250 WORDS)