Frances T. Yen

Mechanism of plasma cholesteryl ester transfer in hypertriglyceridemia.

Plasma net cholesteryl ester (CE) transfer and optimum cholesteryl ester transfer protein (CETP) activity were determined in primary hypertriglyceridemic (n = 11) and normolipidemic (n = 15) individuals. The hypertriglyceridemic group demonstrated threefold greater net CE transfer leading to enhanced accumulation of CE in VLDL. This increased net transfer was not accompanied by a change in CETP activity. In normolipidemia, but not in hypertriglyceridemia, net CE transfer correlated with VLDL triglyceride (r = 0.92, P less than 0.001). In contrast, net CE transfer in hypertriglyceridemia, but not in normolipidemia, correlated with CETP activity (r = 0.73, P less than 0.01). Correction of hypertriglyceridemia with bezafibrate reduced net CE transfer towards normal and restored the correlation with VLDL triglyceride (r = 0.90, P less than 0.005) while suppressing the correlation with CETP activity. That net CE transfer depends on VLDL concentration was confirmed by an increase of net CE transfer in normolipidemic plasma supplemented with purified VLDL. Supplementation of purified CETP to normolipidemic plasma did not stimulate net CE transfer. In contrast, net CE transfer was enhanced by addition of CETP to both plasma supplemented with VLDL and hypertriglyceridemic plasma. Thus, in normal subjects, VLDL concentration determines the rate of net CE transfer. CETP becomes rate limiting as VLDL concentration increases, i.e., in hypertriglyceridemia.

Benzo[a]pyrene impairs β‐adrenergic stimulation of adipose tissue lipolysis and causes weight gain in mice

Benzo[a]pyrene (B[a]P) is a common food pollutant that causes DNA adduct formation and is carcinogenic. The report of a positive correlation between human plasma B[a]P levels and body mass index, together with B[a]Ps lipophilicity, led us to test for possible adverse effects of B[a]P on adipose tissue. In ex vivo experiments using primary murine adipocytes, B[a]P rapidly (within minutes) and directly inhibited epinephrine-induced lipolysis (up to 75%) in a dose-dependent manner. Half-maximum inhibition was obtained with a B[a]P concentration of 0.9 mg.L(-1) (3.5 microm). Lipolysis induced by beta(1)-, beta(2)- and beta(3)-adrenoreceptor-specific agonists, as well as ACTH, were also significantly inhibited by B[a]P, whereas forskolin-induced lipolysis was not B[a]P-sensitive. Similar inhibition of catecholamine-induced lipolysis by B[a]P was also seen in isolated human adipocytes; half-maximum inhibition of lipolysis was achieved with a B[a]P concentration of 0.02 mg.L(-1) (0.08 microm). In vivo treatment of C57Bl/6J mice with 0.4 mg.kg(-1) B[a]P inhibited epinephrine-induced release of free fatty acids by 70%. Chronic exposure of mice to B[a]P (0.5 mg.kg(-1) injected i.p. every 48 h) for 15 days also decreased lipolytic response to epinephrine and induced a 43% higher weight gain compared with controls (B[a]P: 2.23 +/- 0.12 g versus control: 1.56 +/- 0.18 g, P < 0.01) due to increased fat mass. The weight gain occurred consistently without detectable changes in food intake. These results reveal a novel molecular mechanism of toxicity for the environmental pollutant B[a]P and introduce the notion that chronic exposure of human population to B[a]P and possibly other polycyclic aromatic hydrocarbons could have an impact on metabolic disorders, such as obesity.Benzo[a]pyrene (B[a]P) is a common food pollutant that causes DNA adduct formation and is carcinogenic. The report of a positive correlation between human plasma B[a]P levels and body mass index, together with B[a]Ps lipophilicity, led us to test for possible adverse effects of B[a]P on adipose tissue. In ex vivo experiments using primary murine adipocytes, B[a]P rapidly (within minutes) and directly inhibited epinephrine‐induced lipolysis (up to 75%) in a dose‐dependent manner. Half‐maximum inhibition was obtained with a B[a]P concentration of 0.9 mg·L−1 (3.5 µm). Lipolysis induced by β1‐, β2‐ and β3‐adrenoreceptor‐specific agonists, as well as ACTH, were also significantly inhibited by B[a]P, whereas forskolin‐induced lipolysis was not B[a]P‐sensitive. Similar inhibition of catecholamine‐induced lipolysis by B[a]P was also seen in isolated human adipocytes; half‐maximum inhibition of lipolysis was achieved with a B[a]P concentration of 0.02 mg·L−1 (0.08 µm). In vivo treatment of C57Bl/6J mice with 0.4 mg·kg−1 B[a]P inhibited epinephrine‐induced release of free fatty acids by 70%. Chronic exposure of mice to B[a]P (0.5 mg·kg−1 injected i.p. every 48 h) for 15 days also decreased lipolytic response to epinephrine and induced a 43% higher weight gain compared with controls (B[a]P: 2.23 ± 0.12 g versus control: 1.56 ± 0.18 g, P < 0.01) due to increased fat mass. The weight gain occurred consistently without detectable changes in food intake. These results reveal a novel molecular mechanism of toxicity for the environmental pollutant B[a]P and introduce the notion that chronic exposure of human population to B[a]P and possibly other polycyclic aromatic hydrocarbons could have an impact on metabolic disorders, such as obesity.

Ciliary Neurotrophic Factor Cell-Based Delivery Prevents Synaptic Impairment and Improves Memory in Mouse Models of Alzheimer's Disease

The development of novel therapeutic strategies for Alzheimers disease (AD) represents one of the biggest unmet medical needs today. Application of neurotrophic factors able to modulate neuronal survival and synaptic connectivity is a promising therapeutic approach for AD. We aimed to determine whether the loco-regional delivery of ciliary neurotrophic factor (CNTF) could prevent amyloid-β (Aβ) oligomer-induced synaptic damages and associated cognitive impairments that typify AD. To ensure long-term administration of CNTF in the brain, we used recombinant cells secreting CNTF encapsulated in alginate polymers. The implantation of these bioreactors in the brain of Aβ oligomer-infused mice led to a continuous secretion of recombinant CNTF and was associated with the robust improvement of cognitive performances. Most importantly, CNTF led to full recovery of cognitive functions associated with the stabilization of synaptic protein levels in the Tg2576 AD mouse model. In vitro as well as in vivo, CNTF activated a Janus kinase/signal transducer and activator of transcription-mediated survival pathway that prevented synaptic and neuronal degeneration. These preclinical studies suggest that CNTF and/or CNTF receptor-associated pathways may have AD-modifying activity through protection against progressive Aβ-related memory deficits. Our data also encourage additional exploration of ex vivo gene transfer for the prevention and/or treatment of AD.

Inhibition of cholesteryl ester transfer protein activity by monoclonal antibody. Effects on cholesteryl ester formation and neutral lipid mass transfer in human plasma.

We have employed a neutralizing monoclonal antibody, prepared against the Mr 74,000 cholesteryl ester transfer protein (CETP), to investigate the regulation of lecithin:cholesterol acyltransferase (LCAT) activity by cholesteryl ester (CE) transfer, and also to determine which lipoproteins are substrates for LCAT in human plasma. The incubation of normolipidemic plasma led to transfer of CE from HDL to VLDL, and of triglycerides from VLDL to LDL and HDL. This net mass transfer of neutral lipids between the lipoproteins was eliminated by the monoclonal antibody. However, CE transfer inhibition had no effect on the rate of plasma cholesterol esterification in plasma incubated from 10 min to 24 h at 37 degrees C. In the absence of CE transfer, HDL and LDL exhibited cholesterol esterification activity, whereas VLDL did not. The rate of CE formation in HDL was three to four times greater than in LDL during the first hour of incubation, but CE formation in HDL decreased after 6-8 h, while that in LDL continued. Thus, (a) the Mr 74,000 CETP is responsible for all neutral lipid mass transfer in incubated human plasma, (b) the rate of CE formation in plasma is not regulated by CE transfer from HDL to other lipoproteins, and (c) HDL is the major initial substrate for LCAT; LDL assumes a more significant role only after prolonged incubation of plasma.

Lipolysis stimulated lipoprotein receptor: a novel molecular link between hyperlipidemia, weight gain, and atherosclerosis in mice.

The lipolysis-stimulated lipoprotein receptor, LSR, is a multimeric protein complex in the liver that undergoes conformational changes upon binding of free fatty acids, thereby revealing a binding site (s) that recognizes both apoB and apoE. Complete inactivation of the LSR gene is embryonic lethal in mice. Here we show that removal of a single LSR allele (LSR-/+) caused statistically significant increases in both plasma triglyceride and cholesterol levels, a 2-fold increase in plasma triglyceride changes during the post-prandial phase, and delayed clearance of lipid emulsions or a high fat meal. The longer postprandial lipoprotein clearance time observed in LSR-/+ mice was further increased in LSR-/+ mice lacking functional low density lipoprotein (LDL) receptors. LSR-/+ mice placed on a Western-type diet displayed higher plasma triglycerides and cholesterol levels, increased triglyceride-rich lipoproteins and LDL, and increased aorta lipid content, as compared with control mice on the same diet. Furthermore, a direct correlation was observed between the hyperlipidemia and weight gain but only in the LSR-/+ mice. Knockdown of LSR expression by small interfering RNA in mouse Hepa1-6 cells led to decreased internalization of both DiI-labeled cyclohexanedione-LDL and very low density lipoprotein in the presence of oleate. These data led us to conclude that LSR contributes to the physiological clearance of atherogenic triglyceride-rich lipoproteins and LDL. We propose that LSR cooperates with the LDL receptor in the final hepatic processing of apoB-containing lipoproteins and represents a novel therapeutic target for the treatment of hyperlipidemia associated with obesity and atherosclerosis.

LSR/angulin-1 is a tricellular tight junction protein involved in blood–brain barrier formation

Lipolysis-stimulated lipoprotein receptor, a component of the paracellular barrier at tricellular junctions, is necessary for proper blood–brain barrier sealing during embryogenesis.

The lipolysis stimulated receptor: a gene at last.

The lipolysis stimulated receptor is a lipoprotein receptor that was initially described in 1992. In the presence of free fatty acids, the lipolysis stimulated receptor recognizes either apolipoprotein B or apolipoprotein E, and as a consequence, leads to the internalization and degradation of the lipoprotein particles. Its affinity is highest for those lipoproteins most susceptible to lipolysis, triglyceride-rich lipoproteins. Since the initial biochemical identification and description of the lipolysis stimulated receptor, several reports have been published by our group that provide circumstantial evidence for its role in vivo for the clearance of triglyceride-rich lipid particles. In this review, we bring the readers up-to-date on the evidence for the role of the lipolysis stimulated receptor in lipoprotein metabolism, as well as the recent developments in its molecular characterization.

Critical role of cPLA2 in Aβ oligomer-induced neurodegeneration and memory deficit.

Soluble beta-amyloid (Aβ) oligomers are considered to putatively play a critical role in the early synapse loss and cognitive impairment observed in Alzheimers disease. We previously demonstrated that Aβ oligomers activate cytosolic phospholipase A(2) (cPLA(2)), which specifically releases arachidonic acid from membrane phospholipids. We here observed that cPLA(2) gene inactivation prevented the alterations of cognitive abilities and the reduction of hippocampal synaptic markers levels noticed upon a single intracerebroventricular injection of Aβ oligomers in wild type mice. We further demonstrated that the Aβ oligomer-induced sphingomyelinase activation was suppressed and that phosphorylation of Akt/protein kinase B (PKB) was preserved in neuronal cells isolated from cPLA(2)(-/-) mice. Interestingly, expression of the Aβ precursor protein (APP) was reduced in hippocampus homogenates and neuronal cells from cPLA(2)(-/-) mice, but the relationship with the resistance of these mice to the Aβ oligomer toxicity requires further investigation. These results therefore show that cPLA(2) plays a key role in the Aβ oligomer-associated neurodegeneration, and as such represents a potential therapeutic target for the treatment of Alzheimers disease.

Nonrandom variations in human cancer ESTs indicate that mRNA heterogeneity increases during carcinogenesis

Virtually all cancer biological attributes are heterogeneous. Because of this, it is currently difficult to reconcile results of cancer transcriptome and proteome experiments. It is also established that cancer somatic mutations arise at rates higher than suspected, but yet are insufficient to explain all cancer cell heterogeneity. We have analyzed sequence variations of 17 abundantly expressed genes in a large set of human ESTs originating from either normal or cancer samples. We show that cancer ESTs have greater variations than normal ESTs for >70% of the tested genes. These variations cannot be explained by known and putative SNPs. Furthermore, cancer EST variations were not random, but were determined by the composition of the substituted base (b0) as well as that of the bases located upstream (up to b − 4) and downstream (up to b + 3) of the substitution event. The replacement base was also not randomly selected but corresponded in most cases (73%) to a repetition of b − 1 or of b + 1. Base substitutions follow a specific pattern of affected bases: A and T substitutions were preferentially observed in cancer ESTs. In contrast, cancer somatic mutations [Sjoblom T, et al. (2006) Science 314:268–274] and SNPs identified in the genes of the current study occurred preferentially with C and G. On the basis of these observations, we developed a working hypothesis that cancer EST heterogeneity results primarily from increased transcription infidelity.

Disialylated apolipoprotein C-III proteoform is associated with improved lipids in prediabetes and type 2 diabetes

The apoC-III proteoform containing two sialic acid residues (apoC-III2) has different in vitro effects on lipid metabolism compared with asialylated (apoC-III0) or the most abundant monosialylated (apoC-III1) proteoforms. Cross-sectional and longitudinal associations between plasma apoC-III proteoforms (by mass spectrometric immunoassay) and plasma lipids were tested in two randomized clinical trials: ACT NOW, a study of pioglitazone in subjects with impaired glucose tolerance (n = 531), and RACED (n = 296), a study of intensive glycemic control and atherosclerosis in type 2 diabetes patients. At baseline, higher relative apoC-III2 and apoC-III2/apoC-III1 ratios were associated with lower triglycerides and total cholesterol in both cohorts, and with lower small dense LDL in the RACED. Longitudinally, changes in apoC-III2/apoC-III1 were inversely associated with changes in triglycerides in both cohorts, and with total and small dense LDL in the RACED. apoC-III2/apoC-III1 was also higher in patients treated with PPAR-γ agonists and was associated with reduced cardiovascular events in the RACED control group. Ex vivo studies of apoC-III complexes with higher apoC-III2/apoC-III1 showed attenuated inhibition of VLDL uptake by HepG2 cells and LPL-mediated lipolysis, providing possible functional explanations for the inverse association between a higher apoC-III2/apoC-III1 and hypertriglyceridemia, proatherogenic plasma lipid profiles, and cardiovascular risk.