mTert induction in p21-positive cells counteracts capillary rarefaction and pulmonary emphysema

Lung diseases develop when telomeres shorten beyond a critical point. We constructed a mouse model in which the catalytic subunit of telomerase (mTert), or its catalytically inactive form (mTertCI), is expressed from the p21Cdkn1a locus. Expression of either TERT or TERTCI reduces global p21 levels in the lungs of aged mice, highlighting TERT non-canonical function. However, only TERT reduces accumulation of very short telomeres, oxidative damage, endothelial cell (ECs) senescence and senile emphysema in aged mice. Single-cell analysis of the lung reveals that p21 (and hence TERT) is expressed mainly in the capillary ECs. We report that a fraction of capillary ECs marked by CD34 and endowed with proliferative capacity declines drastically with age, and this is counteracted by TERT but not TERTCI. Consistently, only TERT counteracts decline of capillary density. Natural aging effects are confirmed using the experimental model of emphysema induced by VEGFR2 inhibition and chronic hypoxia. We conclude that catalytically active TERT prevents exhaustion of the putative CD34 + EC progenitors with age, thus protecting against capillary vessel loss and pulmonary emphysema.

Eliminating Senescent Cells Can Promote Pulmonary Hypertension Development and Progression

BACKGROUND

Senescent cells (SCs) are involved in proliferative disorders, but their role in pulmonary hypertension remains undefined. We investigated SCs in patients with pulmonary arterial hypertension and the role of SCs in animal pulmonary hypertension models.

METHODS

We investigated senescence (p16, p21) and DNA damage (γ-H2AX, 53BP1) markers in patients with pulmonary arterial hypertension and murine models. We monitored p16 activation by luminescence imaging in p16-luciferase (p16^LUC/+) knock-in mice. SC clearance was obtained by a suicide gene (p16 promoter-driven killer gene construct in p16-ATTAC mice), senolytic drugs (ABT263 and cell-permeable FOXO4-p53 interfering peptide [FOXO4-DRI]), and p16 inactivation in p16^LUC/LUC mice. We investigated pulmonary hypertension in mice exposed to normoxia, chronic hypoxia, or hypoxia+Sugen, mice overexpressing the serotonin transporter (SM22-5-HTT⁺), and rats given monocrotaline.

RESULTS

Patients with pulmonary arterial hypertension compared with controls exhibited high lung p16, p21, and γ-H2AX protein levels, with abundant vascular cells costained for p16, γ-H2AX, and 53BP1. Hypoxia increased thoracic bioluminescence in p16^LUC/+ mice. In wild-type mice, hypoxia increased lung levels of senescence and DNA-damage markers, senescence-associated secretory phenotype components, and p16 staining of pulmonary endothelial cells (P-ECs, 30% of lung SCs in normoxia), and pulmonary artery smooth muscle cells. SC elimination by suicide gene or ABT263 increased the right ventricular systolic pressure and hypertrophy index, increased vessel remodeling (higher dividing proliferating cell nuclear antigen-stained vascular cell counts during both normoxia and hypoxia), and markedly decreased lung P-ECs. Pulmonary hemodynamic alterations and lung P-EC loss occurred in older p16^LUC/LUC mice, wild-type mice exposed to Sugen or hypoxia+Sugen, and SM22-5-HTT⁺ mice given either ABT263 or FOXO4-DRI, compared with relevant controls. The severity of monocrotaline-induced pulmonary hypertension in rats was decreased slightly by ABT263 for 1 week but was aggravated at 3 weeks, with loss of P-ECs.

CONCLUSIONS

Elimination of senescent P-ECs by senolytic interventions may worsen pulmonary hemodynamics. These results invite consideration of the potential impact on pulmonary vessels of strategies aimed at controlling cell senescence in various contexts.

Phospholipase A2 receptor 1 promotes lung cell senescence and emphysema in obstructive lung disease

BACKGROUND

Cell senescence is a key process in age-associated dysfunction and diseases, notably chronic obstructive pulmonary disease (COPD). We previously identified phospholipase A2 receptor 1 (PLA2R1) as a positive regulator of cell senescence acting via Janus kinase (JAK)/signal transducer and activator of transcription (STAT) signalling. Its role in pathology, however, remains unknown. Here, we assessed PLA2R1-induced senescence in COPD and lung emphysema pathogenesis.

METHODS

We assessed cell senescence in lungs and cultured lung cells from patients with COPD and controls subjected to PLA2R1 knockdown, PLA2R1 gene transduction and treatment with the JAK1/2 inhibitor ruxolitinib. To assess whether PLA2R1 upregulation caused lung lesions, we developed transgenic mice overexpressing PLA2R1 (PLA2R1-TG) and intratracheally injected wild-type mice with a lentiviral vector carrying the Pla2r1 gene (LV-PLA2R1 mice).

RESULTS

We found that PLA2R1 was overexpressed in various cell types exhibiting senescence characteristics in COPD lungs. PLA2R1 knockdown extended the population doubling capacity of these cells and inhibited their pro-inflammatory senescence-associated secretory phenotype (SASP). PLA2R1-mediated cell senescence in COPD was largely reversed by treatment with the potent JAK1/2 inhibitor ruxolitinib. Five-month-old PLA2R1-TG mice exhibited lung cell senescence, and developed lung emphysema and lung fibrosis together with pulmonary hypertension. Treatment with ruxolitinib induced reversal of lung emphysema and fibrosis. LV-PLA2R1-treated mice developed lung emphysema within 4 weeks and this was markedly attenuated by concomitant ruxolitinib treatment.

CONCLUSIONS

Our data support a major role for PLA2R1 activation in driving lung cell senescence and lung alterations in COPD. Targeting JAK1/2 may represent a promising therapeutic approach for COPD.

Health relay students’ goals, training and assessment: result from a French national consensus

Issue

Health relay students (HRS) is one of the prevention policy rapidly increasing for the French students. It is mainly based on peer effects, peer to peer communication, but the state of play show very various practices.

Description of the problem

The variety of practices may imply heterogeneity, and actions that are too disparate. In October 2016, the national association of directors of Health Services for Students (SSU) decided to make a formal consensus process (based on single scripting strategies developed in parallel by 10 to 20 people) involving 61 persons (physician, nurses, prevention officer, members of prevention associations) from 29 French universities and based on three axes: goals; training and assessment.

Results

No disagreement was left. The goals must be validated by the SSU, as it is the unit that is able to link students ’associations wishes and health policy objectives. This need a constant dialogue with the university board, students associations, local authorities and health administration. The HRS are also important to help to know the students practices, notably thanks to their presence on social networks. Institutional policies for HRS must be consistent to the goals, and HRS must be managed by specific prevention officers. Training always associate health education topics and health prevention knowledge. It may be more or less intensive according to the goals, but need the help of association of health prevention and a validation by the SSU. HRS must be diverse, in genders and type of studies. The training must encourage HRS autonomy and creativity in their actions, while accepting to respect the University health policy. Creativity means also to open any way of communication wanted by HRS, such as social networks.

Assessment is based on lean management, HRS satisfaction and University satisfaction (institution as well as teachers, administrative workers and students). HRS empowerment is perhaps the main criteria of assessment.

Key messages

Health relay students policies are very varied and need a consensual framework under the control of health services for students. Training must associate health education and prevention objectives, develop students’ empowerment.

The antioxidant N-acetylcysteine protects from lung emphysema but induces lung adenocarcinoma in mice

Oxidative stress is a major contributor to chronic lung diseases. Antioxidants such as N-acetylcysteine (NAC) are broadly viewed as protective molecules that prevent the mutagenic effects of reactive oxygen species. Antioxidants may, however, increase the risk of some forms of cancer and accelerate lung cancer progression in murine models. Here, we investigated chronic NAC treatment in aging mice displaying lung oxidative stress and cell senescence due to inactivation of the transcription factor JunD, which is downregulated in diseased human lungs. NAC treatment decreased lung oxidative damage and cell senescence and protected from lung emphysema but concomitantly induced the development of lung adenocarcinoma in 50% of JunD-deficient mice and 10% of aged control mice. This finding constitutes the first evidence to our knowledge of a carcinogenic effect of antioxidant therapy in the lungs of aged mice with chronic lung oxidative stress and warrants the utmost caution when considering the therapeutic use of antioxidants.

Baleen as a biomonitor of mercury content and dietary history of North Atlantic minke whales (Balaenopetra acutorostrata): combining elemental and stable isotope approaches

Baleen is an incrementally-growing tissue of balaenopteran whales which preserves relatively well over time in museums and some archeological sites, and, therefore might be useful for studies examining long-term changes of metal levels in whales. This study examined Hg and stable C and N isotopic composition of baleen plates of the North Atlantic minke whale (Balaenoptera acutorostrata), which continues to be a food source for people in Greenland and elsewhere. We compared the Hg levels and stable isotopes of major tissues (kidney, liver and muscle) with those of baleen plates to see whether baleen could be used as a biomonitor of variations of Hg intake and diet both between individuals and within individuals over time. Mercury was significantly correlated with concentrations in all tissues (kidney, liver and muscle). Stable C and N isotopes in baleen were generally similar to those of muscle, which reflects the recent (approximately one month) feeding of the whale, but in some individuals there were significant differences between baleen and muscle. Sectioning of baleen into 1 cm longitudinal increments showed that these differences were due to marked dietary shifts by some individuals over time that had been recorded in the baleen but were lost from the muscle record. Whole baleen C and N isotopes were better correlated with tissue Hg levels, suggesting that baleen may provide a more reliable indicator of long-term average diet, which in turn may be better related to Hg accumulation in tissues than the shorter-term diet record contained in muscle.

Regulation of sterol regulatory element-binding proteins by cholesterol flux in CaCo-2 cells

The regulation of sterol regulatory element-binding proteins (SREBP) by cholesterol flux was studied in the intestinal cell line CaCo-2. CaCo-2 cells were incubated for 18 h with micelles containing 5 mM taurocholate and 500 microM oleic acid or micelles containing either 200 microM cholesterol or 150 microM lysophosphatidylcholine. In some incubations, an ACAT inhibitor was added or 25-hydroxycholesterol was substituted for cholesterol. The SREBP-1a transcript was 2-fold more abundant than the SREBP-1c transcript. In cells incubated with micelles containing cholesterol, rates of cholesterol synthesis were decreased and rates of esterification were increased. Cholesterol synthesis was decreased further by ACAT inhibition. Cholesterol influx decreased mRNA levels of SREBP-2, HMG-CoA synthase, HMG-CoA reductase, and fatty acid synthase. ACAT inhibition modestly suppressed gene expression further. Neither SREBP-1a nor SREBP-1c mRNA levels were altered by cholesterol. Despite decreases in gene expression of the sterol-responsive genes by cholesterol, the amounts of precursor and mature forms of SREBP-1 and SREBP-2 were not altered. In contrast, if 25-hydroxycholesterol was substituted for cholesterol, both the precursor and mature forms of SREBP-2 were decreased. The polar sterol decreased the mature form of SREBP-1 but the amount of the precursor form was unchanged. In cells incubated with micelles containing lysophosphatidylcholine, which causes cholesterol to efflux from cells, sterol-responsive gene expression was increased. The amounts of precursor and mature forms of SREBP-1 and SREBP-2, however, were not altered. In contrast, if the cells were depleted of cholesterol by incubating them with lovastatin and cyclodextrin, the mature forms of SREBP-1 and SREBP-2 were increased, as were mRNA levels for the sterol-responsive genes. The data would suggest that cholesterol influx/efflux regulates mRNA levels of sterol-responsive genes independently of changes in the amount of mature SREBP. In contrast, 25-hydroxycholesterol influx or cholesterol depletion alters the amount of mature SREBP, leading to the regulation of sterol-responsive gene expression.

Regulation of sterol regulatory element-binding proteins in hamster intestine by changes in cholesterol flux

A control chow diet or diets containing 1% cholesterol (cholesterol-enriched) or 4% cholestyramine and 0.15% lovastatin (cholesterol-depletion) were fed to hamsters for 2 weeks. Sterol regulatory element-binding protein (SREBP)-1a, SREBP-1c, SREBP-2, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, 3-hydroxy-3-methylglutaryl-coenzyme A synthase, and LDL receptor mRNA levels and SREBP-1 and -2 protein expression were estimated in villus cell populations from duodenum, jejunum, and ileum. SREBP-1a was a minor transcript in hamster intestine, and its gene expression was not altered by changes in dietary cholesterol flux. In contrast, SREBP-1c gene expression was increased by dietary cholesterol and decreased by cholesterol depletion. mRNA levels for SREBP-2 and the other sterol-responsive genes were increased in intestines of animals on the cholesterol depletion diet but minimally suppressed if at all, by the diet enriched in cholesterol. In general, the amount of the precursor form of SREBP-1 was higher in cells of the upper villus and lower in cells of the lower villus. SREBP-2 precursor was higher in cells of the lower villus and lower in cells of the upper villus. Protein expression of precursor correlated with the location of gene expression for SREBPs. The amount of precursor mass of SREBP-2 was not altered by cholesterol feeding but was increased by cholesterol depletion. The mature form of SREBP-2 was in very low abundance and difficult to detect in intestines of animals fed control chow or cholesterol. It was readily detectable and increased in intestines of animals on the cholesterol-depletion diet. The diets did not significantly alter the amount of precursor or mature forms of SREBP-1. Cholesterol feeding had no effect on cholesterol or fatty acid synthesis, whereas synthesis of these lipids was increased in intestines of hamsters on the cholesterol-depleted diet. These results suggest that SREBP-1a has little or no role in regulating intestinal cholesterol synthesis. It is postulated that under basal conditions, SREBP-1c regulates intestinal fatty acid synthesis and SREBP-2 regulates cholesterol synthesis. Following marked changes in cholesterol flux across the intestine, SREBP-2 assumes the role of SREBP-1 and regulates both cholesterol and fatty acid synthesis in intestine.

Gene expression of sterol regulatory element-binding proteins in hamster small intestine

Gene expression of sterol regulatory element-binding proteins 1a, 1c, and 2 (SREBP-1a, -1c, and -2) and of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase, HMG-CoA reductase, and the low density lipoprotein (LDL) receptor was examined in hamster small intestine. SREBP-1c transcript predominated over SREBP-1a. mRNA levels for SREBP-1a, -1c, and -2, LDL receptor, and HMG-CoA synthase were highest in jejunum and ileum. Expression of SREBP-1a and SREBP-1c was highest in cells of the upper villus and decreased in cells of the lower villus. Gene expression of SREBP-2 was lowest in cells of the upper villus and increased in cells of the lower villus. Ileal SREBP-2 gene expression was highest in cells of the midvillus. mRNA levels for HMG-CoA synthase and the LDL receptor followed a pattern similar to that of SREBP-2. A positive correlation existed between SREBP-2 gene expression and rates of cholesterol synthesis. Fatty acid synthesis was highest in jejunum and ileum, correlating positively with the expression of SREBP-1c. Sterol influx into intestinal cells decreased mRNA levels of SREBP-2, HMG-CoA reductase, HMG-CoA synthase, and LDL receptor. In ileum, sterol influx decreased gene expression of SREBP-1a and increased expression of SREBP-1c. The results suggest that SREBP-2 regulates cholesterol synthesis in the small intestine. SREBP-1a is a minor transcript and its expression does not correlate with cholesterol-synthesizing activity. SREBP-1c is a major transcript in small intestine and its expression along the length of the gut correlates with fatty acid synthesis. Sterols regulate gene expression of sterol-responsive genes, including SREBP-2, in small intestine. - Field, F. J., E. Born, S. Murthy, and S. N. Mathur. Gene expression of sterol regulatory element-binding proteins in hamster small intestine. J. Lipid Res. 2001. 42: 1;-8.

Caveolin is present in intestinal cells: role in cholesterol trafficking?

It was postulated that specialized microdomains of the plasma membrane, consistent with caveolae, might play a role in cholesterol trafficking in intestinal cells. The existence, therefore, of caveolin and the role of detergent-resistant microdomains of the plasma membrane in cholesterol trafficking were investigated in human small intestine and CaCo-2 cells. Caveolin mRNA was detected by RT-PCR in small intestinal brushings and biopsies and in CaCo-2 cells. Northern hybridization of caveolin mRNA detected 3 kb and 0.8 kb transcripts in CaCo-2 cells. From brushings of distal duodenum and in CaCo-2 cells, Western analysis for detection of caveolin protein demonstrated a 21 kDa-sized protein and a 600 kDa homooligomer. In CaCo-2 cells, caveolin was demonstrated by immunofluorescence in apical membranes as well as within cells. Using sucrose-density gradients, caveolin was localized to detergent-resistant microdomains of the plasma membrane. As determined by cholesterol oxidase-accessible cholesterol, 3-5% of plasma membrane cholesterol in CaCo-2 cells was estimated to be in these detergent-resistant microdomains. After the absorption of cholesterol from bile-salt micelles, more plasma membrane cholesterol moved to these specialized microdomains within the plasma membrane and was esterified. In CaCo-2 cells, filipin, N-ethyl maleimide, and cholesterol depletion, treatments that disrupt caveolar function, interfered with the transport of plasma membrane cholesterol to the endoplasmic reticulum, whereas okadaic acid, sphingomyelinase, and cholesterol oxidase did not. Changes in cholesterol flux at the apical membrane of the cell did not alter mRNA levels or mass of caveolin. The results suggest that caveolin is present in intestinal and CaCo-2 cells and is associated with detergent-resistant microdomains of cellular membranes. With the influx of micellar cholesterol from the lumen, plasma membrane cholesterol moves or "clusters" to these microdomains and is transported to the endoplasmic reticulum for esterification and eventual transport. Caveolin/caveolae may play a role in cholesterol trafficking in intestinal cells.

13-hydroxy octadecadienoic acid (13-HODE) inhibits triacylglycerol-rich lipoprotein secretion by CaCo-2 cells

Oxidized lipids present in atherogenic lipoproteins are derived, in part, from the diet. To address the effects of an oxidized lipid on intestinal lipoprotein assembly and secretion, CaCo-2 cells were incubated with 13-HODE or its native fatty acid, linoleic acid, and triacylglycerol-rich lipoprotein synthesis and secretion were investigated. 13-HODE was readily taken up by cells and esterified to lipids. Although both fatty acids were largely esterified to neutral lipids, in comparison to neutral lipids containing linoleic acid, a greater proportion of cellular neutral lipids containing 13-HODE and/or its metabolites was secreted. Compared to linoleic acid, however, 13-HODE caused less triacylglycerol, derived from de novo synthesis, and less triacylglycerol mass to be secreted. Cells incubated with both linoleic acid and 13-HODE together secreted less triacylglycerol mass than did cells incubated with linoleic acid alone. Less newly synthesized apoB and apoB mass were secreted by cells incubated with 13-HODE without altering the abundance of apoB mRNA. The fraction of newly synthesized apoB translocated into the secretory pathway of cells exposed to 13-HODE was significantly less than that observed in cells incubated with linolenic acid, suggesting that 13-HODE interfered with the assembly and secretion of triacylglycerol-rich lipoprotein particles.

Transport of cholesterol from the endoplasmic reticulum to the plasma membrane is constitutive in CaCo-2 cells and differs from the transport of plasma membrane cholesterol to the endoplasmic reticulum

The transport of newly synthesized cholesterol from its site of synthesis, the endoplasmic reticulum, to the plasma membrane was studied in CaCo-2 cells. The appearance of newly synthesized cholesterol on the cell surface was rapid. By 30 min, 50% of the total labeled cholesterol was observed in the plasma membrane. The arrival of cholesterol at the plasma membrane was independent of new protein synthesis, a functional Golgi apparatus, or microtubular function. Progesterone, verapamil, and trifluoperazine, inhibitors of p-glycoprotein which are known to inhibit cholesterol transport from the plasma membrane to the endoplasmic reticulum, reduced the amount of newly synthesized cholesterol reaching the plasma membrane. The p-glycoprotein inhibitors, however, caused the accumulation of sterol intermediates in the plasma membrane, suggesting that sterol trafficking to the plasma membrane remained intact, but that trafficking from the plasma membrane to the endoplasmic reticulum was disrupted. In contrast, nigericin, another potent inhibitor of cholesterol movement from the plasma membrane to the endoplasmic reticulum, did not alter the transport of newly synthesized cholesterol to the plasma membrane. Moreover, promoting cholesterol transport from the plasma membrane to the endoplasmic reticulum by sphingomyelin hydrolysis or by micellar cholesterol influx did not alter the percent of newly synthesized cholesterol transported to the plasma membrane. Likewise, preventing plasma membrane cholesterol from reaching the endoplasmic reticulum by incubating cells with lysophosphatidylcholine, filipin, or digitonin did not alter the arrival of newly synthesized cholesterol to the plasma membrane. The results suggest that the amount of cholesterol moving to the plasma membrane from the endoplasmic reticulum is constitutive and regulated at the level of cholesterol synthesis and not at the level of the transport process. The pathways of cholesterol transport to and from the plasma membrane are distinct.

Effect of micellar beta-sitosterol on cholesterol metabolism in CaCo-2 cells

CaCo-2 cells were used to address the effect of the plant sterol, beta-sitosterol, on cholesterol trafficking, cholesterol metabolism, and apoB secretion. Compared to cells incubated with micelles (5 mM taurocholate and 250 microM oleic acid) containing cholesterol, which caused an increase in the influx of plasma membrane cholesterol to the endoplasmic reticulum and increased the secretion of cholesteryl esters derived from the plasma membrane, beta-sitosterol did not alter cholesterol trafficking or cholesteryl ester secretion. Including beta-sitosterol in the micelle together with cholesterol attenuated the influx of plasma membrane cholesterol and prevented the secretion of cholesteryl esters derived from the plasma membrane. Stigmasterol and campesterol had effects similar to beta-sitosterol, although campesterol did not promote a modest influx of plasma membrane cholesterol. Including beta-sitosterol in the micelle with cholesterol decreased the uptake of cholesterol. Compared to cholesterol, 60% less beta-sitosterol was taken up by CaCo-2 cells. No observable esterification of beta-sitosterol was appreciated and the transport of the plant sterol to the basolateral medium was negligible. Cholesterol synthesis and HMG-CoA reductase activities were decreased in cells incubated with beta-sitosterol. This was associated with a decrease in reductase mass and mRNA levels. Cholesteryl ester synthesis and ACAT activities were unaltered by beta-sitosterol. Both stigmasterol and campesterol decreased reductase activity, but only campesterol increased ACAT activity. beta-sitosterol did not affect the secretion of apoB mass. The results suggest that beta-sitosterol does not promote cholesterol trafficking from the plasma membrane to the endoplasmic reticulum. beta-sitosterol interferes with the uptake of micellar cholesterol causing less plasma membrane cholesterol to influx and less cholesteryl ester to be secreted. Despite its lack of effect on cholesterol trafficking, beta-sitosterol decreases cholesterol synthesis at the level of HMG-CoA reductase gene expression.

Microsomal triglyceride transfer protein in CaCo-2 cells: characterization and regulation

As microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apoB-containing lipoproteins by intestinal epithelial cells, the characterization and regulation of MTP in human intestinal cells, CaCo-2, was studied. CaCo-2 cells express MTP mRNA of 4.2 kb and a 97 kDa subunit of MTP protein. Similar to the expression of apoB mRNA, MTP mRNA expression was dependent upon cell differentiation and was directly related to the ability of the cells to assemble and secrete apoB-containing lipoproteins. MTP mRNA expression was highest in fully differentiated cells, with small but detectable amounts found in undifferentiated cells. Under conditions of increased apoB secretion by oleic acid, phosphatidylcholine, or lysophosphatidylcholine, MTP mass, MTP activity, and MTP gene expressions were not altered. In cells treated with calcium ionophore or phorbol 12-myristate 13-acetate, no relationship could be established between apoB secretion and MTP mRNA or activity. Similarly, in cells treated with sphingomyelinase, trifluoperazine, verapamil, okadaic acid, vanadate, or monensin, agents that decrease apoB secretion, no corresponding decrease in MTP activity or mass was observed. The results suggest that the various mediators of apoB secretion alter steps in lipoprotein assembly and secretion that are not dependent on MTP activity. CaCo-2 cells have an abundant supply of MTP for the assembly of lipoproteins when apoB secretion is stimulated by dietary lipids.

Lysophosphatidylcholine increases 3-Hydroxy-3-methylglutaryl-coenzyme A reductase gene expression in CaCo-2 cells

BACKGROUND & AIMS

The small intestine plays an important role in cholesterol homeostasis. The aim of this study was to examine the regulation of cholesterol synthesis by lysophosphatidylcholine in intestinal cells.

METHODS

CaCo-2 cells cultured on semipermeable supports were incubated with taurocholate and lysophosphatidylcholine, and cholesterol synthesis rate, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity, mass, and messenger RNA abundance were estimated.

RESULTS

Lysophosphatidylcholine increased the rate of cholesterol synthesis as estimated by HMG-CoA reductase activity and acetate or water incorporation into sterols. Reductase was also increased by lysophosphatidylinositol or lysophosphatidylethanolamine but not by lysophosphatidylserine. Lysophosphatidylcholine increased HMG-CoA reductase messenger RNA and mass, suggesting that lysophosphatidylcholine regulated reductase at the level of gene expression. The various lysophospholipids caused the efflux of cellular cholesterol into the apical medium, and the amount effluxed correlated with the observed increase in reductase activity. Adding cholesterol to micelles containing lysophosphatidylcholine prevented the increase in HMG-CoA reductase activity and mass.

CONCLUSIONS

Lysophosphatidylcholine increased cholesterol synthesis by increasing the expression of HMG-CoA reductase at the level of the gene and protein. Efflux of cellular cholesterol and the need to replace this lost cholesterol account for the observed changes in cholesterol metabolism.

Phosphatidylcholine increases the secretion of triacylglycerol-rich lipoproteins by CaCo-2 cells

The regulation of lipid synthesis and secretion by phosphatidylcholine was investigated in CaCo-2 cells grown on semi-permeable filters. In cells incubated with 1 mM taurocholate and 100-500 microM phosphatidylcholine, cholesteryl ester synthesis was decreased, triacylglycerol synthesis was increased modestly, whereas phospholipid synthesis was unaffected. Acyl-CoA-cholesterol acyltransferase activity was decreased secondary to a decrease in the substrate (cholesterol) supply. The basolateral secretion of newly synthesized triacyglycerol and triacyglycerol mass was significantly increased by phosphatidylcholine, whereas cellular triacylglycerol mass decreased. This effect was no specific for phosphatidylcholine as phosphatidylethanolamine and phosphatidylserine also increased the secretion of newly synthesized triacylglycerols. Dioleoylphosphatidylcholine was as effective as egg phosphatidylcholine in increasing triacylglycerol transport. Dipalmitoylphosphatidylcholine, in contrast, was without effect. Phosphatidylcholine also increased the basolateral secretion of apolipoprotein B (apoB) mass without altering apoB mRNA levels. Disruption of the Golgi apparatus by monensin or brefeldin A prevented the increase in apoB secretion by phosphatidylcholine. Compared with microsomes prepared from control cells, those from cells incubated with phosphatidylcholine contained more newly synthesized apoB. The percentage of new synthesized apoB isolated from teh lumen of microsomes (as an estimate of apoB destined for secretion), however, was similar in the two preparations. Thus in CaCo-2 cells incubated with phosphatidylcholine, the transport of apoB and triacylglycerols is increased whereas cholesteryl ester synthesis and secretion are decreased. A normally functioning secretory pathway is required for phosphatidylcholine to increase triacylglycerol-rich lipoprotein secretion.

Triacylglycerol-rich lipoprotein cholesterol is derived from the plasma membrane in CaCo-2 cells

The source for triacylglycerol-rich lipoprotein cholesterol was investigated in CaCo-2 cells grown on filters separating an upper and a lower well. Oleic acid, a fatty acid that promotes triacylglycerol-rich lipoprotein synthesis and secretion in CaCo-2 cells, increased the vesicular-mediated influx of plasma membrane cholesterol to the endoplasmic reticulum. Unesterified and esterified cholesterol derived from the plasma membrane were increased in triacylglycerol-rich lipoproteins secreted by cells incubated with oleic acid. Fatty acids, which increased the number of lipoprotein particles secreted (increased apoB secretion), increased plasma membrane cholesterol influx and secretion. Oleic acid caused a modest increase in the synthesis of cholesterol and a two-fold increase in cholesteryl esters. The amount of newly synthesized cholesterol secreted in lipoproteins of density < 1.006 g/ml represented a small fraction of that present within the cell; however, oleic acid did increase the amount of both newly synthesized cholesterol and cholesteryl esters in triacylglycerol-rich lipoproteins. Oleic acid did not affect the fraction of newly synthesized cholesterol trafficking to the plasma membrane. Compared to cholesterol delivered to cells in micelles, plasma membrane cholesterol was the much preferred substrate for acyl-CoA:cholesterol acyltransferase. Micellar cholesterol displaced cholesterol from the plasma membrane causing more of it to influx intracellularly for esterification and secretion. We propose that plasma membrane cholesterol is the major source for triacylglycerol-rich lipoprotein cholesterol in CaCo-2 cells. Micellar cholesterol and newly synthesized cholesterol replenish the plasma membrane cholesterol that is being used for the transport of lipids.

Esterification of plasma membrane cholesterol and triacylglycerol-rich lipoprotein secretion in CaCo-2 cells: possible role of p-glycoprotein

Acylcoenzyme A:cholesterol acyltransferase (ACAT) and/or cholesteryl esters have been implicated as important factors in the normal assembly of apolipoprotein (apoB)-containing lipoproteins. The predominant substrate for ACAT is believed to originate from cholesterol contained within the plasma membrane. To investigate a possible role of intestinal plasma membrane cholesterol in triacylglycerol-rich lipoprotein synthesis and secretion, CaCo-2 cells were incubated with agents that are known to interfere with cholesterol transport from the plasma membrane to the ER. Progesterone, verapamil, and trifluoperazine significantly decreased the movement of cholesterol from plasma membrane to endoplasmic reticulum (ER) in CaCo-2 cells. Without altering the synthesis of apoB and independent of their effects on cellular cholesterol esterification, progesterone, verapamil, and trifluoperazine decreased the basolateral secretion of triacylglycerols, cholesteryl esters, and immunoreactive and newly synthesized apoB. The three agents also interfered with the esterification of cholesterol absorbed from taurocholate micelles. As progesterone, verapamil, and trifluoperazine are recognized inhibitors of p-glycoprotein, a variety of agents that have been shown to interfere with p-glycoprotein function were tested to investigate their effects on cholesterol transport and apoB secretion. All the agents significantly decreased in parallel both cholesterol transport and apoB secretion. In contrast, methotrexate, an antimetabolite that does not interact with p-glycoprotein, had no effect. Nigericin, a potassium ionophore, which causes alkalinization of intracellular vesicles, also caused a profound inhibition of cholesterol transport and apoB secretion. Preventing plasma membrane cholesterol from arriving at the ER, or inhibiting the esterification of plasma membrane cholesterol, does not alter apoB secretion. However, the results suggest a possible role for p-glycoprotein in normal cholesterol trafficking and triacylglycerol-rich lipoprotein secretion in CaCo-2 cells. It is postulated that p-glycoprotein might function to maintain the acidic environment of transport vesicles, and therefore, could play a role in the transport of lipids by the intestine.

Lysophosphatidylcholine increases the secretion of cholesteryl ester-poor triacylglycerol-rich lipoproteins by CaCo-2 cells

To address the effect of lysophosphatidylcholine on triacylglycerol transport in intestine, CaCo-2 cells, grown on semipermeable supports, were incubated with lysophosphatidylcholine solubilized in 1 mM taurocholate. [14C]Palmitoyllysophosphatidylcholine was readily taken up and incorporated predominantly into cellular phospholipids, particularly phosphatidylcholine. Twenty-five percent of the label was found in triacylglycerols. Compared with labelled cellular phospholipids, labelled triacylglycerols were preferentially secreted. Lysophosphatidylcholine caused a profound decrease in cholesteryl ester synthesis and secretion, whereas cellular triacylglycerol mass and triacylglycerol synthesis and secretion were increased. The effect was more pronounced with oleoyllysophosphatidylcholine than with either palmitoyl- or stearyl-lysophosphatidylcholine. Lysophosphatidylcholine increased the secretion of immunoreactive and newly-synthesized apoprotein B (apoB) without altering the rate of apoB synthesis. Thus, luminal lysophosphatidylcholine and/or its uptake decreases cholesterol esterification and secretion, but increases triacylglycerol synthesis and secretion, triacylglycerol mass accumulation and the secretion of apoB by CaCo-2 cells.

Regulation of apolipoprotein B secretion by biliary lipids in CaCo-2 cells

The regulation of apoB synthesis and secretion by lipids present within bile was investigated in CaCo-2 cells grown on semipermeable filters. Bile acids decreased the basolateral secretion of immunoreactive apoB. Taurocholic acid decreased the secretion of newly synthesized apoB by increasing the rate of apoB degradation, but had no effect on the synthesis and secretion of apoA-I or trichloroacetic acid-precipitable proteins. The calcium ionophore, A23187, decreased apoB secretion similar to that observed for taurocholate. The addition of the ionophore and taurocholate together did not cause a further decrease in apoB secretion. Cholesterol or its hydroxylated derivative, 25-hydroxycholesterol, did not alter secretion of immunoreactive or newly synthesized apoB. Phosphatidylcholine increased apoB synthesis and secretion without affecting the synthesis or secretion of apoA-I. Phosphatidylcholine also reversed the effect of A23187 on apoB secretion. When phosphatidylcholine was added to the basolateral medium, apoB secretion was not altered. ApoB secretion was not increased by phospholipids of other classes. Dioleoylphosphatidylcholine increased apoB secretion, whereas dipalmitoylphosphatidylcholine did not. Fatty acid-labeled phosphatidylcholine was not hydrolyzed in the apical medium. Only 2% of the added phosphatidylcholine was cell-associated, and of this, 80% of the label remained as phosphatidylcholine with most of the remainder in triacylglycerols, fatty acids, and phosphatidylethanolamine. The results suggest that bile acids decrease apoB secretion by increasing its rate of degradation. This effect may be related to their ionophoric property. Cholesterol flux does not regulate apoB secretion. Phosphatidylcholine, independent of triacylglycerol flux and independent of its hydrolysis, increases the secretion of apoB by increasing apoB synthesis. Luminal phosphatidylcholine may play a role in apoB secretion in the intestine.

Release of ceramide after membrane sphingomyelin hydrolysis decreases the basolateral secretion of triacylglycerol and apolipoprotein B in cultured human intestinal cells

The effect of sphingomyelin hydrolysis on triacylglycerol-rich lipoprotein secretion was examined in the human intestinal cell line, CaCo-2. Addition of sphingomyelinase decreased sphingomyelin and phosphatidylethanolamine by 60 and 20%, respectively. Sphingomyelin hydrolysis decreased the basolateral secretion of triacylglycerol mass, newly synthesized triacylglycerol, and apo B mass. Pulse-chase experiments with [35S]methionine demonstrated a decrease in apo B synthesis and a marked decrease in apo B100 and apo B48 secretion without altering apo A1 secretion. Sphingomyelin hydrolysis did not change apo B mRNA levels nor apo B turnover. Phosphatidylcholine-specific phospholipase C did not decrease apo B synthesis or its basolateral secretion. Membrane protein kinase C (PKC) activity was decreased twofold after sphingomyelin hydrolysis. The PKC inhibitor staurosporine decreased apo B mass and newly synthesized apo B secretion. Sphingomyelinase and staurosporine together caused an additional decrease in apo B secretion suggesting that sphingomyelin hydrolysis decreased apo B secretion independently of its effect on PKC activity. Moreover, conditions that increase PKC activity did not increase apo B secretion. Cell-permeable analogs of ceramide decreased immunoreactive apo B secretion. Sphingosine was without effect. The hydrolysis of membrane sphingomyelin by intestinal or pancreatic neutral sphingomyelinase may lead to the accumulation of cellular ceramide, which, in turn, could inhibit triacylglycerol-rich lipoprotein secretion.

Cholesterol and sphingomyelin syntheses are regulated independently in cultured human intestinal cells, CaCo-2: role of membrane cholesterol and sphingomyelin content

There is a presumed association between cellular cholesterol and sphingomyelin metabolism. To study this relationship in the intestine, the activity of the rate controlling enzyme of sphingolipid synthesis, serine palmitoyltransferase (SPT), and the biosynthesis of long-chain bases were characterized in cultured human intestinal cells, CaCo-2. Cells were then incubated with substances known to alter cholesterol biosynthesis, and the effect of these mediators on SPT activity and long-chain base synthesis was determined and compared with their effects on HMG-CoA reductase activity and cholesterol synthesis. The polar sterol, 25-hydroxycholesterol, the squalene epoxide inhibitor, U18666A, and the inhibitor of HMG-CoA reductase, lovastatin, all significantly inhibited the synthesis of cholesterol without altering either SPT activity or long-chain base synthesis. Mevalonate, which increased cholesterol production 3-fold, also had no affect on SPT activity or sphingoid base synthesis. Serine, which significantly increased the synthesis of long-chain bases, did not alter cholesterol biosynthesis. Moreover, the suicide inhibitors of SPT, beta-chloroalanine and cycloserine, did not alter cholesterol synthesis while markedly decreasing long chain base synthesis. Cells were incubated with palmitic, oleic, linoleic, and eicosapentaenoic acids. Only palmitic acid, the preferred substrate for SPT, increased the production of long-chain bases. Both palmitic and oleic acids, however, increased the synthesis of cholesterol. Cells enriched in sphingomyelin had higher rates of synthesis of both cholesterol and long-chain bases compared to their controls. In contrast, cholesterol and long-chain base syntheses were significantly decreased in cells enriched in cholesterol. Control cells incubated with phospholipid liposomes alone had higher rates of synthesis of both lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of okadaic acid on apo B and apo A-I secretion by CaCo-2 cells

The effect of protein phosphorylation on the synthesis and secretion of apo B and apo A-I by CaCo-2 cells was investigated. Okadaic acid, a potent inhibitor of protein serine/threonine phosphatases 1 and 2A, caused a significant increase in total cellular protein phosphorylation. Apo B-48 was phosphorylated in control cells and this was increased significantly in the presence of okadaic acid. Under the experimental conditions, the phosphorylation of apo B-100 or apo A-I was not observed. No evidence of tyrosine phosphorylation of apo B-100, B-48, or apo A-I was found. Okadaic acid did not change the amount of apo B mass within cells but apo B mass secreted into the basolateral medium was decreased by 40%. Apo A-I mass within cells or in the basolateral medium was unaffected by okadaic acid. Despite causing an 18% decrease in total protein synthesis, okadaic acid did not alter the rate of synthesis of apo B-100, apo B-48, or apo A-I. Cellular turnover of labeled apo B-100 in cells incubated with okadaic acid was similar to controls, whereas apo B-48 and apo A-I turnover were slowed by okadaic acid. Compared to controls, however, 1 microM okadaic acid caused a 75% and 50% decrease in the secretion of newly synthesized apo B-100 and apo B-48, respectively, while decreasing labeled apo A-I secretion by 35%. In contrast to apo A-I mRNA levels, which were not altered by okadaic acid, apo B mRNA levels were significantly decreased by the polyether fatty acid. Despite differences observed in the phosphorylation state of apo B-100 and apo B-48, okadaic acid decreased the secretion of both forms of apo B without altering their synthesis. Okadaic acid, by increasing cellular protein phosphorylation, significantly disrupts the secretory processing of apo B by CaCo-2 cells.

Regulation of LDL receptor expression by luminal sterol flux in CaCo-2 cells

The regulation of expression of the intestinal low density lipoprotein (LDL) receptor by luminal (apical) sterol flux was investigated in the human intestinal cell line CaCo-2. Cells were cultured on semipermeable micropore filters, which separated an upper and lower well. To the apical media were added solutions containing either taurocholate micelles alone or micelles containing sterols. Because of an efflux of cholesterol, which occurred from cells incubated with micelles alone, LDL receptor mRNA levels increased threefold. With an influx of micellar sterols, receptor mRNA levels decreased in a dose-dependent manner. Synthesis and degradation of the LDL receptor were addressed by pulse-chase experiments. In cells incubated with micelles containing 25-hydroxycholesterol, the rate of receptor synthesis was significantly decreased, whereas the rate of receptor turnover remained unchanged. As assessed by immunoblots and steady-state labeling of proteins followed by immunoprecipitation of the LDL receptor, cells incubated with micellar 25-hydroxycholesterol contained substantially less receptor protein. These cells also bound and degraded less LDL. In contrast, in cells incubated with micelles alone, the rate of receptor synthesis was increased and cells contained more LDL receptor protein, although this was not reflected in an increased in LDL binding. The results suggest that LDL receptor expression in CaCo-2 cells is regulated by luminal sterol flux and that this regulation occurs at the level of transcription.

Sphingomyelin content of intestinal cell membranes regulates cholesterol absorption. Evidence for pancreatic and intestinal cell sphingomyelinase activity

Micellar cholesterol uptake and secretion were investigated in the human intestinal cell line CaCo-2 following depletion of apical membrane sphingomyelin. The addition of exogenous sphingomyelinase, which hydrolysed 60% of prelabelled sphingomyelin, resulted in a 50% decrease in the uptake of cholesterol from bile salt micelles. The flux of membrane cholesterol into the cell by the hydrolysis of membrane sphingomyelin decreased the rate of cholesterol synthesis by 43% and inhibited hydroxymethylglutaryl-CoA reductase activity by 54%. Moreover, the rate of cholesterol esterification was increased 4-fold. Total cellular cholesterol mass was unchanged by the addition of sphingomyelinase; however, cholesteryl esters increased by 50% and the amount of unesterified cholesterol decreased significantly. The basolateral secretion of cholesterol mass was also decreased following sphingomyelin hydrolysis. Human pancreatic juice was found to contain neutral sphingomyelinase activity which required taurocholate for full expression. The presence of neutral sphingomyelinase activity was also documented in membranes prepared from CaCo-2 cells and in whole homogenates from human duodenal biopsies. The data suggest that the amount of sphingomyelin present in the apical membrane of the intestinal absorptive cell regulates cholesterol uptake from bile salt micelles. Sphingomyelinase activity within intestinal cells and in pancreatic juice could alter the sphingomyelin content of brush-border membranes of small intestinal absorptive cells and thus regulate the amount of cholesterol absorbed by the gut.