Exposure to prenatal infection and the development of internalizing and externalizing problems in children: a longitudinal population-based study

BACKGROUND

A large body of work has reported a link between prenatal exposure to infection and increased psychiatric risk in offspring. However, studies to date have focused primarily on exposure to severe prenatal infections and/or individual psychiatric diagnoses in clinical samples, typically measured at single time points, and without accounting for important genetic and environmental confounders. In this study, we investigated whether exposure to common infections during pregnancy is prospectively associated with repeatedly assessed child psychiatric symptoms in a large population-based study.

METHODS

Our study was embedded in a prospective pregnancy cohort (Generation R; n = 3,598 mother-child dyads). We constructed a comprehensive prenatal infection score comprising common infections for each trimester of pregnancy. Child total, internalizing, and externalizing problems were assessed repeatedly using the parent-rated Child Behavioral Checklist (average age: 1.5, 3, 6, 10, and 14 years). Linear mixed-effects models were run adjusting for a range of confounders, including child polygenic scores for psychopathology, maternal chronic illness, birth complications, and infections during childhood. We also investigated trimester-specific effects and child sex as a potential moderator.

RESULTS

Prenatal exposure to infections was associated with higher child total, internalizing, and externalizing problems, showing temporally persistent effects, even after adjusting for important genetic and environmental confounders. We found no evidence that prenatal infections were associated with changes in child psychiatric symptoms over time. Moreover, in our trimester-specific analysis, we did not find evidence of significant timing effects of prenatal infection on child psychiatric symptoms. No interactions with child sex were identified.

CONCLUSIONS

Our research adds to evidence that common prenatal infections may be a risk factor for psychiatric symptoms in children. We also extend previous findings by showing that these associations are present early on, and that rather than changing over time, they persist into adolescence. However, unmeasured confounding may still explain in part these associations. In the future, employing more advanced causal inference designs will be crucial to establishing the degree to which these effects are causal.

HIV-1 infection of genetically engineered iPSC-derived central nervous system-engrafted microglia in a humanized mouse model

IMPORTANCE

Our mouse model is a powerful tool for investigating the genetic mechanisms governing central nervous system (CNS) human immunodeficiency virus type-1 (HIV-1) infection and latency in the CNS at a single-cell level. A major advantage of our model is that it uses induced pluripotent stem cell-derived microglia, which enables human genetics, including gene function and therapeutic gene manipulation, to be explored in vivo, which is more challenging to study with current hematopoietic stem cell-based models for neuroHIV. Our transgenic tracing of xenografted human cells will provide a quantitative medium to develop new molecular and epigenetic strategies for reducing the HIV-1 latent reservoir and to test the impact of therapeutic inflammation-targeting drug interventions on CNS HIV-1 latency.

HIV-1 infection of genetically engineered iPSC-derived central nervous system-engrafted microglia in a humanized mouse model

The central nervous system (CNS) is a major human immunodeficiency virus type 1 reservoir. Microglia are the primary target cell of HIV-1 infection in the CNS. Current models have not allowed the precise molecular pathways of acute and chronic CNS microglial infection to be tested with in vivo genetic methods. Here, we describe a novel humanized mouse model utilizing human-induced pluripotent stem cell-derived microglia to xenograft into murine hosts. These mice are additionally engrafted with human peripheral blood mononuclear cells that served as a medium to establish a peripheral infection that then spread to the CNS microglia xenograft, modeling a trans-blood-brain barrier route of acute CNS HIV-1 infection with human target cells. The approach is compatible with iPSC genetic engineering, including inserting targeted transgenic reporter cassettes to track the xenografted human cells, enabling the testing of novel treatment and viral tracking strategies in a comparatively simple and cost-effective way vivo model for neuroHIV.

The influence of structural racism, pandemic stress, and SARS-CoV-2 infection during pregnancy with adverse birth outcomes

BACKGROUND

Structural racism and pandemic-related stress from the COVID-19 pandemic may increase the risk of adverse birth outcomes.

OBJECTIVE

Our objective was to examine associations between neighborhood measures of structural racism and pandemic stress with 3 outcomes: SARS-CoV-2 infection, preterm birth, and delivering small-for-gestational-age newborns. Our secondary objective was to investigate the joint association of SARS-CoV-2 infection during pregnancy and neighborhood measures with preterm birth and delivering small-for-gestational-age newborns.

STUDY DESIGN

We analyzed data of 967 patients from a prospective cohort of pregnant persons in New York City, comprising 367 White (38%), 169 Black (17%), 293 Latina (30%), and 87 Asian persons (9%), 41 persons of other race or ethnicity (4%), and 10 of unknown race or ethnicity (1%). We evaluated structural racism (social/built structural disadvantage, racial-economic segregation) and pandemic-related stress (community COVID-19 mortality, community unemployment rate increase) in quartiles by zone improvement plan code. SARS-CoV-2 serologic enzyme-linked immunosorbent assay was performed on blood samples from pregnant persons. We obtained data on preterm birth and small-for-gestational-age newborns from an electronic medical record database. We used log-binomial regression with robust standard error for clustering by zone improvement plan code to estimate associations of each neighborhood measure separately with 3 outcomes: SARS-CoV-2 infection, preterm birth, and small-for-gestational-age newborns. Covariates included maternal age, parity, insurance status, and body mass index. Models with preterm birth and small-for-gestational-age newborns as the dependent variables additionally adjusted for SARS-CoV-2 infection.

RESULTS

A total of 193 (20%) persons were SARS-CoV-2-seropositive, and the overall risks of preterm birth and small-for-gestational-age newborns were 8.4% and 9.8%, respectively. Among birthing persons in neighborhoods in the highest quartile of structural disadvantage (n=190), 94% were non-White, 50% had public insurance, 41% were obese, 32% were seropositive, 11% delivered preterm, and 12% delivered a small-for-gestational-age infant. Among birthing persons in neighborhoods in the lowest quartile of structural disadvantage (n=360), 39% were non-White, 17% had public insurance, 15% were obese, 9% were seropositive, 6% delivered preterm, and 10% delivered a small-for-gestational-age infant. In adjusted analyses, structural racism measures and community unemployment were associated with both SARS-CoV-2 infection and preterm birth, but not small-for-gestational-age infants. High vs low structural disadvantage was associated with an adjusted relative risk of 2.6 for infection (95% confidence interval, 1.7-3.9) and 1.7 for preterm birth (95% confidence interval, 1.0-2.9); high vs low racial-economic segregation was associated with adjusted relative risk of 1.9 (95% confidence interval, 1.3-2.8) for infection and 2.0 (95% confidence interval, 1.3-3.2) for preterm birth; high vs low community unemployment increase was associated with adjusted relative risk of 1.7 (95% confidence interval, 1.2-1.5) for infection and 1.6 (95% confidence interval, 1.0-2.8) for preterm birth. COVID-19 mortality rate was associated with SARS-CoV-2 infection but not preterm birth or small-for-gestational-age infants. SARS-CoV-2 infection was not independently associated with birth outcomes. We found no interaction between SARS-CoV-2 infection and neighborhood measures on preterm birth or small-for-gestational-age infants.

CONCLUSION

Neighborhood measures of structural racism were associated with both SARS-CoV-2 infection and preterm birth, but these associations were independent and did not have a synergistic effect. Community unemployment rate increases were also associated with an increased risk of preterm birth independently of SARS-CoV-2 infection. Mitigating these factors might reduce the impact of the pandemic on pregnant people.

SARS-CoV-2 during pregnancy and associated outcomes: Results from an ongoing prospective cohort

BACKGROUND

The COVID-19 pandemic is an ongoing global health threat, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Questions remain about how SARS-CoV-2 impacts pregnant individuals and their children.

OBJECTIVE

To expand our understanding of the effects of SARS-CoV-2 infection during pregnancy on pregnancy outcomes, regardless of symptomatology, by using serological tests to measure IgG antibody levels.

METHODS

The Generation C Study is an ongoing prospective cohort study conducted at the Mount Sinai Health System. All pregnant individuals receiving obstetrical care at the Mount Sinai Healthcare System from 20 April 2020 onwards are eligible for participation. For the current analysis, we included participants who had given birth to a liveborn singleton infant on or before 22 September 2020. For each woman, we tested the latest prenatal blood sample available to establish seropositivity using a SARS-CoV-2 serologic enzyme-linked immunosorbent assay. Additionally, RT-PCR testing was performed on a nasopharyngeal swab taken during labour. Pregnancy outcomes of interest (i.e., gestational age at delivery, preterm birth, small for gestational age, Apgar scores, maternal and neonatal intensive care unit admission, and length of neonatal hospital stay) and covariates were extracted from medical records. Excluding individuals who tested RT-PCR positive at delivery, we conducted crude and adjusted regression models to compare antibody positive with antibody negative individuals at delivery. We stratified analyses by race/ethnicity to examine potential effect modification.

RESULTS

The SARS-CoV-2 seroprevalence based on IgG measurement was 16.4% (95% confidence interval 13.7, 19.3; n=116). Twelve individuals (1.7%) were SARS-CoV-2 RT-PCR positive at delivery. Seropositive individuals were generally younger, more often Black or Hispanic, and more often had public insurance and higher pre-pregnancy BMI compared with seronegative individuals. None of the examined pregnancy outcomes differed by seropositivity, overall or stratified by race/ethnicity.

CONCLUSION

Seropositivity for SARS-CoV-2 without RT-PCR positivity at delivery (suggesting that infection occurred earlier during pregnancy) was not associated with selected adverse maternal or neonatal outcomes among live births in a cohort sample from New York City.

Dysregulation of mitochondrial and proteolysosomal genes in Parkinson's disease myeloid cells

An increasing number of identified Parkinson's disease (PD) risk loci contain genes highly expressed in innate immune cells, yet their role in pathology is not understood. We hypothesize that PD susceptibility genes modulate disease risk by influencing gene expression within immune cells. To address this, we have generated transcriptomic profiles of monocytes from 230 individuals with sporadic PD and healthy subjects. We observed a dysregulation of mitochondrial and proteasomal pathways. We also generated transcriptomic profiles of primary microglia from brains of 55 subjects and observed discordant transcriptomic signatures of mitochondrial genes in PD monocytes and microglia. We further identified 17 PD susceptibility genes whose expression, relative to each risk allele, is altered in monocytes. These findings reveal widespread transcriptomic alterations in PD monocytes, with some being distinct from microglia, and facilitate efforts to understand the roles of myeloid cells in PD as well as the development of biomarkers.

Low molecular weight fibrinogen degradation products stimulate the release of growth factors from endothelial cells

Cultured porcine aortic endothelial cells (PAEC) constitutively produce and secrete in their culture medium mitogens collectively called endothelial cell-derived growth factors (EDGFs). Incubation of PAEC with fibrinogen-degradation products (FDPs) obtained by plasmin digestion of highly purified fibrinogen caused an increased release of EDGFs, as assessed by [3H]-thymidine incorporation in 3T3 mouse fibroblasts. The effect was time-dependent and correlated with the degree of fibrinogenolysis. It was accompanied by elongation of the cells. Neither increase in EDGFs release nor cell damage was observed when non-degraded fibrinogen was incubated with endothelial cells. Low molecular weight fibrinogen degradation products (LMWFDPs) (M(r) less than or equal to 10,000), and the higher molecular weight fibrinogen fragments D and E were tested under the same conditions. Only the LMWFDPs caused elongation and damage to PAEC and a marked stimulation (up to 12 fold) of EDGFs release. A low density growth assay revealed that the released EDGFs were mitogenically active on the same PAEC. The activity of the released EDGFs was time and dose dependent on both 3T3 fibroblasts and PAEC, indicating that LMWFDPs caused enhanced release of EDGFs that can act in paracrine and autocrine fashion. This study suggests an additional role for fibrinogenolysis contributing to wound healing, and possibly to atherosclerosis.

Tumor necrosis factor induced release of endothelial cell lipoprotein lipase

The major functional pool of lipoprotein lipase (LPL) that hydrolyzes triglycerides in circulating lipoproteins is located on the vascular endothelium. The macrophage-secreted cytokine tumor necrosis factor (TNF), a molecule known to affect endothelial cell functions, was used to test the hypothesis that alterations of endothelial cell metabolism regulate the binding of LPL to these cells. TNF addition induced rapid (maximum release at 45 minutes) dissociation of LPL protein and activity from its binding sites on cultured porcine aortic endothelial cells. LPL release by TNF required endothelial cell metabolic event(s) which involved cell secretion. In addition, LPL release was inhibited by pertussis toxin, suggesting the involvement of guanine nucleotide regulatory protein(s). Addition of arachidonic acid, a molecule known to be released by endothelial cells due to phospholipase A2 activation by TNF treatment, released LPL from the cell surface. Furthermore, direct modulation of cellular phospholipase A2 activity also led to changes in the release of LPL. Our studies demonstrate that alterations in the cellular metabolism of endothelial cells, for example, by TNF, may release functional pools of LPL from the vascular endothelium. This decrease in LPL on endothelial cell surfaces might be involved in the development of hypertriglyceridemia and redirection of energy flow during infections and inflammation.

Unesterified fatty acids inhibit the binding of low density lipoproteins to the human fibroblast low density lipoprotein receptor

Micromolar concentrations of oleate were found to inhibit reversibly the binding of low density lipoprotein (LDL) to the human fibroblast LDL receptor. The decrease in LDL binding caused a parallel reduction of both 125I-LDL uptake and degradation at 37 degrees C. At 4 degrees C, oleate was also found to displace 125I-LDL already bound to the LDL receptor. The effect of oleate was rapid, reaching 70-80% of maximum displacement with 5-10 min of incubation, and was closely correlated to oleate-albumin molar ratios. Partition analysis of unesterified fatty acids between cells and LDL showed that the inhibitory effect of oleate resulted mainly from an interaction of unesterified fatty acids with the cell surface rather than with the LDL particles. Using different unesterified fatty acids and fatty acid analogs, we found that the inhibitory effect was modulated by both the length and the conformation of the monomeric carbon chain and was directly dependent on the presence of a negative charge on the carboxylic group. At 4 degrees C, the inhibitory effect of oleate never exceeded half of maximum binding capacity. This limitation was associated with the ability of oleate to interact only with part of the population of LDL receptors which spontaneously recycles in the absence of ligand, as demonstrated by the fact that oleate did not induce any reduction of LDL binding after cell treatment with monensin in the absence of LDL. Our results indicate that unesterified fatty acids could participate in the control of LDL catabolism in vivo by direct modulation of the ability of LDL receptor to bind LDL.

Perturbation of endothelial functions by ionizing irradiation: effects on prostaglandins, chemoattractants and mitogens

Numerous studies have shown that early radiation injury is characterized by vascular damage and that the initial site of damage appears to be the EC lining of the vessel wall. Chronic irreversible tissue reactions to radiation include thrombotic occlusion of capillaries, enhanced atherosclerosis in larger vessels, inflammatory changes, and late tissue fibrosis. These processes may be mediated by endothelial products released as a result of cellular injury. Using EC cultures, we show that ionizing irradiation affects one of the major vascular defense mechanisms against platelet activation, thrombosis, and atherosclerosis--the capacity to produce PGI2. Dose- and time-related damage to enzymes of the arachidonic acid cascade were demonstrated. Radiation damage is associated with oxidant stress and production of free radicals. The oxygen radical scavenger, vitamin C, was found to protect the capacity of irradiated ECs to produce PGI2. Radiation injury often induces an acute inflammatory response. We found that irradiated ECs release a chemotactic factor for neutrophils, which is a lipid product of the lipoxygenase pathway. Late radiation-induced tissue fibrosis and the capacity of radiation to enhance arteriosclerosis may involve participation of mitogens released from perturbed and damaged ECs. We show that conditioned medium of irradiated ECs contain larger amounts of newly synthesized mitogens capable of stimulating the proliferation of fibroblasts, SMCs, and ECs. Hence, it may be assumed that the mitogenic activity released by irradiated ECs includes both PDGF and FGF-like mitogens.

Release of endothelial cell lipoprotein lipase by plasma lipoproteins and free fatty acids

Lipoprotein lipase (LPL) bound to the lumenal surface of vascular endothelial cells is responsible for the hydrolysis of triglycerides in plasma lipoproteins. Studies were performed to investigate whether human plasma lipoproteins and/or free fatty acids would release LPL which was bound to endothelial cells. Purified bovine milk LPL was incubated with cultured porcine aortic endothelial cells resulting in the association of enzyme activity with the cells. When the cells were then incubated with media containing chylomicrons or very low density lipoproteins (VLDL), a concentration-dependent decrease in the cell-associated LPL enzymatic activity was observed. In contrast, incubation with media containing low density lipoproteins or high density lipoproteins produced a much smaller decrease in the cell-associated enzymatic activity. The addition of increasing molar ratios of oleic acid:bovine serum albumin to the media also reduced enzyme activity associated with the endothelial cells. To determine whether the decrease in LPL activity was due to release of the enzyme from the cells or inactivation of the enzyme, studies were performed utilizing radioiodinated bovine LPL. Radiolabeled LPL protein was released from endothelial cells by chylomicrons, VLDL, and by free fatty acids (i.e. oleic acid bound to bovine serum albumin). The release of radiolabeled LPL by VLDL correlated with the generation of free fatty acids from the hydrolysis of VLDL triglyceride by LPL bound to the cells. Inhibition of LPL enzymatic activity by use of a specific monoclonal antibody, reduced the extent of release of 125I-LPL from the endothelial cells by the added VLDL. These results demonstrated that LPL enzymatic activity and protein were removed from endothelial cells by triglyceride-rich lipoproteins (chylomicrons and VLDL) and oleic acid. We postulate that similar mechanisms may be important in the regulation of LPL activity at the vascular endothelium.

Effects of mevinolin and mevalonate on cell growth in several transformed cell lines

Studies were conducted to explore the effects of mevinolin, a competitive inhibitor of HMG CoA reductase, on the growth and morphology of normal and transformed murine fibroblasts. Mevinolin is known to block DNA synthesis and cell growth in a number of kinds of non-transformed cells. Eight cell lines were studied, including two normal fibroblast cell lines (C3H 10T 1/2 and NIH 3T3) and derivatives of these cell lines transformed by chemical carcinogens, X-irradiation or the H-ras oncogene. All of the eight cell lines displayed appreciable growth inhibition by 5 microM mevinolin and marked inhibition by 30 microM mevinolin. Mevinolin also induced a marked rounding in the morphology of all of the cell lines. These effects of mevinolin on cell growth and morphology were blocked or reversed by the addition of mevalonic acid. Thus, both normal and transformed cells require mevalonate, or an as yet unidentified metabolite of mevalonate for their growth, even though some transformed cells have become relatively autonomous of other growth factors. Whereas mevinolin acted primarily as a cytostatic agent for most of the cell lines studied, with the transformed cell line MCA/10T 1/2, which ordinarily grows to a very high cell density, prolonged exposure to mevinolin caused marked cytotoxicity. Thus mevinolin might be useful as an anti-tumor agent for specific tumors.

Effects of platelet-derived growth factor on 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in cultured human fibroblasts

Studies were conducted to delineate in detail the effects of platelet-derived growth factor (PDGF) upon 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase activity in cultured human fibroblasts. PDGF has a variety of coordinated effects on low density lipoprotein (LDL) and cholesterol metabolism and on DNA synthesis in these cells. We have previously shown that there is a critical time period, 10 to 20 hours after PDGF addition to quiescent cells, when mevalonate (MVA) is required for DNA synthesis to occur. The present studies demonstrate that PDGF produces a biphasic stimulation of reductase activity in cultured fibroblasts: a peak at 4 to 6 hours, followed by a decline, and then a second smaller peak at 24 hours, concurrent with DNA synthesis. The stimulation of both peaks was PDGF concentration-dependent, although quantitative differences were observed. Proportionally similar, LDL concentration-dependent reductions in both peaks of reductase activity were also seen. PDGF stimulated cholesterol synthesis from acetate in intact cells reaching peak values after 24 to 28 hours. The two peaks of reductase activity stimulated by PDGF neither coincide with the critical time period when mevalonate is needed for later DNA synthesis to occur, nor reflect the pattern of cholesterol synthesis within the cells.

Platelet-derived growth factor labeled to colloidal gold for use as a mitogenic receptor probe

Studies by others utilizing 125I-PDGF have indicated that target cells express a high affinity surface receptor for PDGF. We have bound purified platelet-derived growth factor (PDGF) to gold colloid particles to explore the interaction of PDGF with mouse 3T3 cells. The gold-PDGF complex consists of approximately 26 PDGF molecules electrostatically absorbed to gold colloid (approximately 14.1 nm). The gold-PDGF complex induced mitogenic stimulation similar to unbound PDGF, although a 5 to 6 fold greater amount of complexed PDGF was required for the same effect. Incubation of the gold-PDGF complex with 3T3 cells for 4 h at 4 degrees C revealed that 98% of the membrane binding was randomly distributed on the cell surface with respect to coated pits, with each cell binding 7000 to 11000 complexes. Addition of a 20-fold excess of unlabeled PDGF reduced surface binding of the gold-PDGF complex by 87% (1230 probes/cell). Warming to 37 degrees C followed by time-interval fixation permitted visualization of endocytosis of the complexes in coated vesicles (1-3 min), internalization (3-15 min) and lysosomal accumulation (15-60 min). Pretreatment of cultures with monensin (2 h, 10 microM) abolished receptor binding, internalization and subsequent mitogenesis of the gold-PDGF complex. These studies support the suggestion that PDGF requires a surface receptor to elicit mitogenesis.

Relationship between mevalonate and mitogenesis in human fibroblasts stimulated with platelet-derived growth factor

Relationships between mevalonate and DNA synthesis were explored in quiescent human fibroblasts stimulated with human platelet-derived growth factor (PDGF). Studies of others have indicated that mevalonate, or a product of mevalonate other than cholesterol, is essential for DNA replication. The present studies were designed to determine whether there was a critical time in the cell cycle when mevalonate was necessary for later DNA synthesis to occur. Compactin and mevinolin, inhibitors of hydroxymethylglutaryl CoA reductase, were employed to block both the synthesis of mevalonate and of DNA. Compactin inhibited the sharp peak of DNA synthesis which occurs in cells 24 h after PDGF addition in a concentration-dependent manner. This suppression of DNA synthesis was not prevented by low density lipoprotein but was fully reversed by mevalonate. Compactin inhibited DNA synthesis when the inhibitor was present during the time interval 10-20 h after PDGF addition. Its presence only in the interval before 10 h or after 20 h had no effect. Conversely, mevalonate could fully overcome the compactin block in DNA synthesis when present during the period of from 10-20 h after PDGF addition. Mevalonate present only before 10 h or after 20 h had no effect. When mevalonate was added to mevinolin-blocked cells for the interval 10-15 h after PDGF, the mevinolin block of DNA synthesis was 68% overcome; in contrast, only 20% of the reversal of the mevinolin block was seen when mevalonate was added from 15-20 h. Addition of mevalonate for only the 2-h interval of from 10-12 h after PDGF overcame the mevinolin block of DNA synthesis (assayed at 24 h) by 50%. The results show that there is a critical time period, several h before S phase, when PDGF-stimulated cells require mevalonate in order for DNA synthesis to proceed at 24 h. This critical period comprised the interval of approximately 10-20 h after PDGF addition and especially the early part of this interval.

Inhibition of LDL degradation in cultured human fibroblasts induced by endothelial cell-conditioned medium

Cultured porcine aortic endothelial cells (EC) secrete a potent mitogen into their serum-free culture medium which stimulates DNA synthesis in quiescent human fibroblasts. Previous work has shown that endothelial cell-conditioned medium (ECCM) also stimulates parallel, concentration-dependent increases in low density lipoprotein (LDL) cell surface binding and internalization. LDL degradation, however, was inhibited in ECCM-treated cells. Studies were conducted to determine whether the EC-derived growth factor (EDGF) and the endothelial-derived inhibitor of LDL degradation (EDID) are separate and distinct components, and to further delineate some of the characteristics of EDID. Time-course studies showed that the mitogenic activity released by EC into ECCM increased steadily during the first 24 hours and came to a plateau at about 48 hours of conditioning, whereas inhibitory activity increased linearly in ECCM for at least 6 days. Mitogenic activity in ECCM was stable under the same conditions. Ultrafiltration experiments showed that EDGF activity has an apparent molecular weight in excess of 30,000; in contrast EDID activity was not retained by filters with a 2000 dalton retention limit. EDID activity also failed to bind to a Blue Sepharose affinity column, while EDGF was bound and could be eluted with 1M NaCl in the presence of ethylene glycol. The ammonium ion concentration in ECCM was measured to determine the possible contribution of this ion to EDID activity. Confluent cultures of porcine aortic EC in serum-free culture medium produced substantial amounts of NH4+. After 96 hours of conditioning, the level of NH4+ in ECCM rose from near zero to 2-4 mM. When equivalent amounts of NH4Cl were added to fresh nonconditioned medium, a level of inhibitory (EDID) activity similar to that found with ECCM was observed. These studies indicate that EDGF and EDID activities are distinct and separate components of ECCM. Since NH4+ ion is present in ECCM in concentrations that are sufficient to account for all the observed EDID activity, it is likely that NH4+ is responsible for this activity.

Ultrastructural visualization of low-density lipoproteins during receptor binding and cellular endocytosis

Human fibroblasts possess surface receptors which have a high affinity for low-density lipoproteins (LDL). However, previous studies have not provided direct ultrastructural visualization of LDL bound to the receptor. To permit direct observation of unlabeled LDL during receptor binding and cellular endocytosis, we examined several fixative regimens which employ lipophilic stains. Staining with tannic acid, an oxidized form of ruthenium red, or potassium ferrocyanide imparted sufficient contrast to individual molecules of LDL to permit high-resolution electron microscopy of receptor binding and endocytosis. The LDL molecule was observed in immediate contact with the receptor and the coated vesicle, indicating that receptor-ligand binding occurs by short-range interactions.

Role of apolipoprotein E-containing lipoproteins in abetalipoproteinemia

Detailed studies of apolipoprotein E (apoE)-containing lipoproteins in abetalipoproteinemia have been performed in an attempt to resolve the apparent paradox of a suppressed low density lipoprotein (LDL) receptor pathway in the absence of apoB-containing lipoproteins. It was hypothesized that apoE-containing high density lipoproteins (HDL) in abetalipoproteinemia might functionally substitute for LDL in regulation of cholesterol metabolism in these patients. The mean (+/-standard deviation) plasma concentration of apoE in nine patients with abetalipoproteinemia was 44.8+/-8.2 mug/ml, slightly higher than the corresponding value for a group of 50 normal volunteers, 36.3+/-11 mug/ml. Fractionation of plasma lipoproteins by agarose column chromatography or by ultracentrifugation indicated that in abetalipoproteinemia, plasma apoE was restricted to a subfraction of HDL. This was in contrast to the results obtained with plasma from 30 normal volunteers, in whom apoE was distributed between very low density lipoproteins (VLDL) and HDL. Consequently, the mean apoE content of HDL in abetalipoproteinemia (44.8 mug/ml) was more than twice that found in the normal volunteers (20.3 mug/ml).ApoE-rich and apoE-poor subfractions of HDL(2) were isolated by heparin-agarose affinity chromatography. ApoE comprised a mean of 81% of the protein mass of the apoE-rich subfraction. Compared with the apoE-poor subfraction, the apoE-rich HDL(2) was of larger mean particle diameter (141+/-7 vs. 115+/-15 A) and had a higher ratio of total cholesterol/protein (1.01+/-0.11 vs. 0.63+/-0.14). Plasma and HDL fractions from three patients were studied with respect to their ability to compete with (125)I-LDL in specific binding to receptors on cultured human fibroblasts. The binding activity of plasma from patients (per milligram of protein) was about half that of plasma from normal volunteers. All binding activity in the patients' plasma was found to reside in the HDL fraction. The binding activity of the patients' HDL (on a total protein basis) was intermediate between that of normal HDL and normal LDL. However, the large differences in binding between patients' HDL and normal HDL entirely disappeared when data were expressed in terms of the apoE content of these lipoproteins. This suggested that the binding activity was restricted to that subfraction of HDL particles that contain apoE. These apoE-rich HDL particles had calculated binding potencies per milligram of protein 10-25 times that of normal LDL. Direct binding studies using (125)I-apoE-rich HDL(2) and (125)I-apoE-poor HDL(2), confirmed the suggestion that binding is restricted to the subfraction of HDL particles containing apoE. The apoE-rich HDL(2) were found to be very potent inhibitors of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase activity in cultured fibroblasts, providing direct evidence of the ability of these lipoproteins to regulate cholesterol metabolism. On the basis of binding potencies of apoE-rich HDL, apoE concentrations, and the composition of apoE-rich HDL, it could be calculated that apoE-rich HDL in abetalipoproteinemia have a capacity to deliver cholesterol to tissues via the LDL receptor pathway equivalent to an LDL concentration of 50-150 mg/dl of cholesterol. Thus, these apoE-rich lipoproteins are capable of producing the suppression of cholesterol synthesis and LDL receptor activity previously observed in abetalipoproteinemia.

Stimulation of arterial endothelial cell prostacyclin synthesis by high density lipoproteins

Prostacyclin (PGI2) is a vasoactive prostaglandin synthesized by vascular endothelial and smooth muscle cells. In order to investigate whether plasma lipoproteins influence the biosynthesis and release of prostacyclin by vascular tissues, human high density lipoproteins (HDL) and low density lipoproteins (LDL) were incubated with porcine arterial endothelial cells grown in tissue culture. PGI2 production was measured by radioimmunoassay of its stable metabolite, 6-keto-PGF1 alpha. In incubations of HDL with endothelial cells for 24 h, levels of 6-keto-PGF1 alpha in the medium increased significantly in a dose-dependent fashion to values 5-fold above control. This effect was less pronounced in confluent than in subconfluent cultures and did not occur in the presence of an inhibitor of cyclooxygenase. No significant stimulation of 6-keto-PGF1 alpha was observed when the endothelial cells were incubated with LDL. In time course experiments with HDL, 6-keto-PGF1 alpha levels increased continuously over 24 h. Rat HDL, containing a high content of arachidonate in its cholesterol ester fatty acids, caused a 2-4-fold greater release of 6-keto-PGF1 alpha than human HDL. The delipidated apoprotein of both human and rat HDL caused similar stimulation of 6-keto-PGF1 alpha production, but much less than intact HDL. The data indicate that HDL stimulates PGI2 synthesis by cultured arterial endothelial cells, possibly by providing the cells with arachidonate.

Effect of platelet-derived and endothelial cell-derived growth factors on the low density lipoprotein receptor pathway in cultured human fibroblasts

Human platelet-derived growth factor (PDGF) has been previously shown to stimulate low density lipoprotein (LDL) receptor activity in cultured cells. Studies were conducted to delineate in detail the effects of PDGF on the LDL receptor pathway in normal human fibroblasts and to explore relationships between the effects of PDGF on LDL metabolism, on cholesterol metabolism, and on DNA synthesis. Increasing concentrations of PDGF stimulated parallel increases in both DNA synthesis and 125I-LDL cell surface binding. The effect of PDGF was due entirely to an increase (up to 4.3-fold) in the number of receptor sites per cell, and not to a change in receptor affinity (Kd approximately 2.0 nM). Parallel PDGF concentration-dependent increases in 125I-LDL binding, internalization, and degradation at 37 degrees C were observed. The results indicate that PDGF-stimulated cells metabolize receptor-bound LDL in a manner that is identical with that seen with quiescent cells. A single study with highly purified PDGF demonstrated that it was PDGF itself, and not some other component in the partially purified PDGF preparation used in most of this work, that was responsible for the observed effects. Studies were conducted on the effects of PDGF on hydroxymethylglutaryl CoA reductase activity, on cholesterol esterification, and on down-regulation by LDL of the LDL receptor. These studies indicated that LDL cholesterol taken into the PDGF-stimulated cell via the receptor pathway, appeared to become available normally and to have metabolic effects within the cell similar to those seen in quiescent cells. Fibroblasts from subjects with familial hypercholesterolemia showed a normal mitogenic response to PDGF, despite the absence or near absence of an effect on the LDL receptor pathway. Finally, studies were also conducted with endothelial cell-conditioned medium (ECCM), used as a source of the endothelial cell-derived growth factor. ECCM was similar to PDGF in stimulating LDL binding, but differed strikingly from PDGF in that the degradation of internalized LDL was inhibited. As a result, ECCM-treated cells did not effectively increase cholesterol esterification or suppress hydroxymethylglutaryl CoA reductase activity when LDL was present. These findings with substances produced by endothelial cells may have important implications for atherogenesis.

Colloidal gold--low density lipoprotein conjugates as membrane receptor probes

We have developed a method for conjugating low density lipoproteins (LDL) with colloidal gold. Conjugation, complete after 1 min, occurs by electrostatic adsorption of the LDL to the negatively charged gold particle. Each conjugate consists of approximately eight biologically active LDL molecules clustered around a central 19-nm gold granule. Acidic (pH 4), alkaline (pH 9), or high ionic (600 milliosmolar NaCl) environments do not dissociate the conjugate. Colloidal gold is an electron-dense, nondegradable marker that is easily identified within the cell and serves as a valuable probe for studying receptor binding and endocytosis. By using a modified method of ruthenium red staining, the LDL molecules of the conjugate can be directly visualized when they are bound to the cell surface receptor. Receptor binding (4 degrees C) of the conjugate by cultured human fibroblasts reveals that the gold granule is positioned 18-21 nm from the coated pit region of the membrane. This distance, similar to the diameter of LDL, suggests concomitant internalization of the receptor during vesicular endocytosis and early lysosomal incorporation (10 min at 37 degrees C). Continued internalization (30-60 min at 37 degrees C) results in the formation of free pools of gold within the lysosome.

Platelet-derived growth factor stimulates low density lipoprotein receptor activity in cultured human fibroblasts

Human platelets contain a mitogen, the platelet-derived growth factor (PDGF), that stimulates the proliferation of a variety of cell types in culture and that may play a role in atherogenesis. Studies were conducted to explore the effects of PDGF on low density lipoprotein (LDL) receptor activity of cultured human fibroblasts. The PDGF utilized in these studies was partially purified from human platelet-rich plama by ion exchange chromatography and gel filtration. LDL receptor activity was assessed by both specific binding of 125I-labeled LDL to the fibroblast's surface at 4 degrees C, and the incorporation of [14C]oleate into cholesteryl esters. Exposure of normal human fibroblasts to increasing amounts of PDGF (0.1-10 microgram/ml) for 48 hr caused a dose-related increase in 125I-labeled LDL binding to a maximum of approximately 300%. In the presence of added LDL, this increase in LDL binding was not seen. Cholesterol esterifiction was also stimulated following a 48-hr exposure to PDGF. Following a conditioning period in LDL- and PDGF-depleted medium, cholesterol esterification was greatly increased during a 48-hr exposure to LDL alone; a smaller but significant increase occurred with PDGF alone. However, both PDGF and LDL were required to return the level of esterification to that observed with whole human serum. Fibroblasts from a patient with homozygous familial hypercholesterolemia, which lack the LDL receptor, also showed a significant increase in cholesteol esterification with PDGF alone, whereas LDL had no effect. These studies demonstrate that PDGF can stimulate the LDL receptor activity in cultured human fibroblasts. The effect on other related activities of the LDL receptor system and the mechanism involved remain to be defined.

Inhibition of lymphocyte proliferation stimulated by lectins and allogeneic cells by normal plasma lipoproteins

Lipoproteins, isolated by sequential flotation at densities 1.006, 1.019, 1.063, and 1.21, were examined for their ability to inhibit human lymphocytes stimulated by allogeneic cells and by lectins (phytohemagglutinin-P and concanavalin A). All the classes of normal plasma lipoproteins inhibited lymphoproliferation when peripheral blood lymphocytes were cultured in autologous, heterologous, or lipoprotein-deficient plasma (d greater than 1.21). The rank order of inhibitory potency was intermediate density lipoprotein (IDL) greater than very low density lipoproteins (VLDL) greater than low density lipoproteins (LDL) greater than high density lipoproteins (HDL), regardless of the mode of stimulation. The concentrations of IDL, VLDL, and LDL required for complete inhibition of stimulated lymphoproliferation were considerably below the levels of each of these lipoproteins normally found in human plasma. In addition, the concentration of HDL required for 50-90% inhibition was in the range of HDL levels normally found in human plasma. Moreover, at relatively higher concentrations, lipoproteins suppressed the incorporation of [3H]thymidine into DNA below the levels seen with reseting, unstimulated lymphocytes. The results suggest that circulating lymphocytes may normally be highly suppressed by the combined effects of all the endogenous lipoproteins and that the lipoproteins may play important roles in vivo in modulating lymphocyte functions and responses.