Expression of the human apolipoprotein E gene suppresses steroidogenesis in mouse Y1 adrenal cells

Apolipoprotein E Isoform-specific changes related to stress and trauma exposure

Post-Traumatic Stress Disorder (PTSD) is a highly prevalent mental health disorder. Due to the high level of variability in susceptibility and severity, PTSD therapies are still insufficient. In addition to environmental exposures, genetic risks play a prominent role and one such factor is apolipoprotein E. The protein (apoE) is functionally involved in cholesterol transport and metabolism and exists as 3 major isoforms in humans: E2, E3, and E4. To model the role of apolipoprotein E isoform in stress-related changes in behavior and cognition, female and male mice (3-5 months of age) expressing E2, E3, or E4 were used. Mice were either placed into control groups or exposed to chronic variable stress (CVS), which has been shown to induce PTSD-like behavioral and neuroendocrine changes. E2 mice showed a unique response to CVS compared to E3 and E4 mice that included impaired spatial learning and memory, increased adrenal gland weight, and no increase in glucocorticoid receptor protein levels (normalized to apoE levels). In addition, the cholesterol metabolite 7-ketocholesterol was elevated in the cortex after CVS in E3 and E4, but not E2 female mice. E2 confers unique changes in behavioral, cognitive, and biomarker profiles after stress exposure and identify 7-ketocholesterol as a possible novel biomarker of the traumatic stress response. We further explored the relationship between E2 and PTSD in an understudied population by genotyping 102 patients of Cambodian and Vietnamese ethnicity. E2 carriers demonstrated a higher odds ratio of having a PTSD diagnosis compared to E3/E3 carriers, supporting that the E2 genotype is associated with PTSD diagnosis after trauma exposure in this population.

VLDL/LDL serves as the primary source of cholesterol in the adrenal glucocorticoid response to food deprivation

The contribution of individual lipoprotein species to the generation of the adrenal cholesterol pool used for the synthesis of anti-inflammatory glucocorticoid species remains unknown. Here we examined the impact of specific lowering of very low-density lipoprotein (VLDL) and low-density (LDL) levels on adrenal cholesterol and glucocorticoid homeostasis. Hereto, lethally-irradiated hypercholesterolemic apolipoprotein E (APOE) knockout mice received APOE-containing bone marrow from wild-type mice (n = 6) or APOE knockout control bone marrow (n = 10) and were subsequently fed a regular chow diet. Transplantation with wild-type bone marrow was associated with a 10-fold decrease in VLDL/LDL-cholesterol levels. No changes were observed in adrenal weights, adrenal cholesterol content, or basal plasma corticosterone levels. However, food deprivation-induced corticosterone secretion was 64% lower (P < 0.05) in wild-type bone marrow recipients as compared to APOE knockout bone marrow recipients, in the context of similar plasma adrenocorticotropic hormone (ACTH) levels. A parallel 19-29% decrease in adrenal relative mRNA expression levels of ACTH-responsive genes SR-BI (P < 0.01), STAR (P < 0.05), and CYP11A1 (P < 0.05) was detected. In support of relative glucocorticoid insufficiency, blood lymphocyte and eosinophil concentrations were respectively 2.4-fold (P < 0.01) and 8-fold (P < 0.001) higher in wild-type bone marrow recipients under food deprivation stress conditions. In conclusion, we have shown that a selective lowering of VLDL/LDL levels in APOE knockout mice through a transplantation with APOE-containing wild-type bone marrow is associated with a decreased maximal adrenal glucocorticoid output. Our studies provide experimental support for the hypothesis that, in vivo, VLDL/LDL serves as the primary source of cholesterol used for glucocorticoid synthesis during food deprivation stress.

Molecular mechanisms of tributyltin-induced alterations in cholesterol homeostasis and steroidogenesis in hamster testis: In vivo and in vitro studies

Tributyltins (TBT) are ubiquitous and persistent environmental contaminants that disturb normal endocrine function including gonadal function in humans and marine organisms. TBT was administered through oral route to male Syrian hamsters at daily doses of 50, 100, and 150 ppm/kg for 65 days. Changes in testis morphology, immunohistochemistry of iNOS, 3β-HSD, and 17β-HSD, cholesterol transport receptor, nuclear receptors, and transcription factors were analyzed. TBT treatment affected each of these parameters to significant levels in a dose-dependent manner compared to vehicle treated control. Real-time PCR and protein analyses revealed that expression levels of ApoE and LDL-R mRNA were up-regulated in the testis of TBT-treated animals while the expression levels of SR-B1, LXR, PPARs α, β, and γ, SCAP, SREBP 1 and 2, 3β-HSD, 17β-HSD, CYP17A1, and P450_SCC were down-regulated. Leydig cells were isolated and separated adopting percoll gradient centrifugation under aseptic condition. The viability of Leydig cell was affected by TBT treatment in a dose- and time-dependent manner. Further, the mechanism of action of TBT was ascertained by siRNA transfection of ApoE, which was upregulated, and SREBP, which was down-regulated. These observations led us to infer that exposure to TBT hinders intracellular cholesterol transport resulting in abnormal sex steroid biosynthesis and alteration of steroidogenic enzyme activities. Finally, we could recognize ApoE and SREBP as the major factors regulating genes that control cholesterol biosynthesis and steroidogenesis that ultimately inhibit the synthesis of testosterone. Therefore, ApoE is one of the important molecular targets that can be intercepted in context of male infertility/male contraception.

ApoE2 Exaggerates PTSD-Related Behavioral, Cognitive, and Neuroendocrine Alterations

Apolipoprotein E (apoE) is an essential component of lipoprotein particles in both the brain and periphery, and exists in three isoforms in the human population: E2, E3, and E4. ApoE has numerous, well-established roles in neurobiology. Most notably, E4 is associated with earlier onset and increased risk of Alzheimer's disease (AD). Although possession of E2 is protective in the context of AD, E2 appears to confer an increased incidence and severity of posttraumatic stress disorder (PTSD). However, the biological processes underlying this link remain unclear. In this study, we began to elucidate these associations by examining the effects of apoE on PTSD severity in combat veterans, and on PTSD-like behavior in mice with human apoE. In a group of 92 veterans with PTSD, we observed significantly higher Clinician-Administered PTSD Scale and PTSD Checklist scores in E2+ individuals, as well as alterations in salivary cortisol levels. Furthermore, we measured behavioral and biological outcomes in mice expressing human apoE after a single stressful event as well as following a period of chronic variable stress, a model of combat-related trauma. Mice with E2 showed impairments in fear extinction, and behavioral, cognitive, and neuroendocrine alterations following trauma. To the best of our knowledge, these data constitute the first translational demonstration of PTSD severity in men and PTSD-like symptoms in mice with E2, and point to apoE as a novel biomarker of susceptibility, and potential therapeutic target, for PTSD.

Obesity occurring in apolipoprotein E-knockout mice has mild effects on fertility

The Apolipoprotein (Apo) family is implicated in lipid metabolism. There are five types of Apo: Apoa, Apob, Apoc, Apod, and Apoe. Apoe has been demonstrated to play a central role in lipoprotein metabolism and to be essential for efficient receptor-mediated plasma clearance of chylomicron remnants and VLDL remnant particles by the liver. Apoe-deficient (Apoe−/−) mice develop atherosclerotic plaques spontaneously, followed by obesity. In this study, we investigated whether lipid deposition caused by Apoe knockout affects reproduction in female mice. The results demonstrated that Apoe−/− mice were severely hypercholesterolemic, with their cholesterol metabolism disordered, and lipid accumulating in the ovaries causing the ovaries to be heavier compared with the WT counterparts. In addition, estrogen and progesterone decreased significantly at D 100. Quantitative PCR analysis demonstrated that at D 100 the expression of cytochromeP450 aromatase (Cyp19a1), 3β-hydroxysteroid dehydrogenase (Hsd3b), mechanistic target of rapamycin (Mtor), and nuclear factor-κB (Nfkb) decreased significantly, while that of BCL2-associated agonist of cell death (Bad) and tuberous sclerosis complex 2 (Tsc2) increased significantly in the Apoe−/− mice. However, there was no difference in the fertility rates of the Apoe−/− and WT mice; that is, obesity induced by Apoe knockout has no significant effect on reproduction. However, the deletion of Apoe increased the number of ovarian follicles and the ratio of ovarian follicle atresia and apoptosis. We believe that this work will augment our understanding of the role of Apoe in reproduction.

Follicle-stimulating hormone regulation of ovarian transcripts for steroidogenesis-related proteins and cell survival, growth and differentiation factors in vitro during early secondary oocyte growth in coho salmon

Little is known about follicle-stimulating hormone (FSH) function during oocyte growth in fishes. The goal of this study was to gain a fundamental understanding of FSH action on ovarian follicles during early secondary oocyte growth by examining changes in ovarian gene expression and steroidogenesis in response to FSH. Coho salmon (Oncorhynchus kisutch) mid to late cortical alveolus stage follicles were incubated with or without salmon FSH in time-course and concentration-response experiments. Steroid levels were determined in the culture medium by immunoassay and levels of target ovarian mRNAs were determined by quantitative RT-PCR. Medium estradiol-17β (E2) levels increased in response to FSH and plateaued by 36h, while testosterone levels increased similarly but were lower and more variable than E2. Gonadotropin receptor transcripts were differentially regulated, with fshr and lhcgr being down- and up- regulated, respectively. Transcripts encoding proteins involved in steroidogenesis, such as star and hsd3b were significantly upregulated by FSH, whereas aromatase (cyp19a1a) mRNA was unaffected by FSH and declined over time in culture. A recently identified teleost gene, bmp16, was suppressed by FSH and an anti-apoptotic factor, clusterin 1 (clu1), was upregulated by FSH. Lesser FSH effects were observed on igf2, cyp11a1 and cyp17a1, which were stimulated, and igf1ra, inhbb, amh and apoe, which were suppressed. As evident by the significant increases in steroid production and transcripts for specific steroidogenesis-related proteins, FSH influences steroidogenesis during early secondary growth and may be a critical signal for puberty onset. Effects of FSH on ovarian anti-apoptotic and growth factor genes suggest roles for FSH in cell survival, growth and differentiation in teleosts.

Hyperglycemia and advanced glycosylation end products suppress adipocyte apoE expression: implications for adipocyte triglyceride metabolism

Endogenous adipocyte apolipoprotein E (apoE) plays an important role in adipocyte lipoprotein metabolism and lipid flux. A potential role for hyperglycemia in regulating adipocyte apoE expression and triglyceride metabolism was examined. Exposure of adipocytes to high glucose or advanced glycosylation end product-BSA significantly suppressed apoE mRNA and protein levels. This suppression was significantly attenuated by antioxidants or inhibitors of the NF-κB transcription pathway. Hyperglycemia in vivo led to adipose tissue oxidant stress and significant reduction in adipose tissue and adipocyte apoE mRNA level. Incubation with antioxidant in organ culture completely reversed this suppression. Hyperglycemia also reduced adipocyte triglyceride synthesis, and this could be completely reversed by adenoviral-mediated increases in apoE. To more specifically evaluate an in vivo role for adipocyte apoE expression on organismal triglyceride distribution in vivo, WT or apoE knockout (EKO) adipose tissue was transplanted in EKO recipient mice. After 12 wk, WT adipocytes transplanted in EKO mice accumulated more triglyceride compared with transplanted EKO adipocytes. In addition, EKO recipients of WT adipose tissue had reduced hepatic triglyceride content compared with EKO recipients transplanted with EKO adipose tissue. Our results demonstrate that hyperglycemia and advanced glycosylation end products suppress the expression of adipocyte apoE in vitro and in vivo and thereby reduce adipocyte triglyceride synthesis. In vivo results using adipose tissue transplantation suggest that reduction of adipocyte apoE, and subsequent reduction of adipocyte triglyceride accumulation, could influence lipid accumulation in nonadipose tissue.

Endogenous ApoE expression modulates adipocyte triglyceride content and turnover

Apolipoprotein E (apoE) is highly expressed in adipose tissue and adipocytes in which its expression is regulated by peroxisome proliferator-activated receptor (PPAR)-gamma agonists and tumor necrosis factor-alpha. There is, however, no information regarding a role for endogenous apoE in differentiated adipocyte function. In this report, we define a novel role for apoE in modulating adipocyte lipid metabolism. ApoE(-/-) mice have less body fat and smaller adipocytes compared with wild-type controls. Freshly isolated adipose tissue from apoE(-/-) mice contains lower levels of triglyceride and free fatty acid, and these differences are maintained in cultured adipocytes derived from preadipocytes. Adenoviral expression of apoE in apoE(-/-)-cultured adipocytes increases triglyceride and fatty acid content. During incubation with apoE-containing triglyceride-rich lipoproteins, apoE(-/-) adipose tissue accumulates less triglyceride than wild type. The absence of apoE expression in primary cultured adipocytes also leads to changes in the expression of genes involved in the metabolism/turnover of fatty acids and the triglyceride droplet. Markers of adipocyte differentiation were lower in freshly isolated and cultured apoE(-/-) adipocytes. Importantly, PPAR-gamma-mediated changes in lipid content and gene expression are markedly altered in cultured apoE(-/-) adipocytes. These results establish a novel role for endogenous apoE in adipocyte lipid metabolism and have implications for constructing an integrated model of adipocyte physiology in health and disease.

Apolipoprotein E isoforms increase intracellular Ca2+ differentially through a omega-agatoxin IVa-sensitive Ca2+-channel

Apolipoprotein E (apoE) is the major apolipoprotein in the brain and is known for its important role in plasticity and neurodegeneration. We show that apoE dose-dependently increases intracellular free Ca2+ in rat hippocampal astrocytes and neurons. This effect varies with isoforms in the order E4 > E3 > E2. It is insensitive to blockade of action potentials by tetrodotoxin or inhibition of binding of apoE by heparinase, by the LRP ligand lactoferrin and by low density lipoprotein. ApoE evoked Ca2+-increases are blocked in zero [Ca]o and by the Ca-channel antagonists nickel and omega-Agatoxin-IVa but not by nifedipine and omega-Conotoxin-GVIa, demonstrating an isoform-specific activation of P/Q type Ca2+-channels. This novel mechanism is discussed with respect to Alzheimer's disease, that is linked for most cases to the apoE epsilon-allelic variation (epsilon4 > epsilon3 > epsilon2).

Subphysiologic Apolipoprotein E (ApoE) Plasma Levels Inhibit Neointimal Formation After Arterial Injury in ApoE-Deficient Mice

OBJECTIVE

Apolipoprotein E (apoE) reduces mouse atherosclerosis progression independent of plasma cholesterol level effects. A mouse artery injury model was used to examine whether apoE exhibits beneficial lipid-independent effects on neointimal formation.

METHODS AND RESULTS

ApoE-deficient (apoE-/-), wild-type (WT), and transgenic apoE-/- mice (secreting apoE at different levels from adrenal glands) underwent femoral artery injury. Mice with low expression of plasma apoE (0.1% of WT) had cholesterol levels approximately half those of apoE-/- littermates (but still approximately 6x >WT). Mice with higher expression (HE; 2% to 3% of WT) of plasma apoE had cholesterol levels approximately twice those of WT. Injured WT mouse (versus apoE-/-) arteries had a smaller mean intima-to-media (I/M) ratio (0.87 versus 1.96; P<0.05). HE mice tended to have lower mean I/M ratios (1.3; P>0.05 versus apoE-/- mice). Multiple regression analysis indicated that apoE levels were significantly associated with reduced I/M ratios, but plasma cholesterol levels were not, before or after adjusting for apoE. In addition, foam cell content of the neointima and media of injured arteries, a negative prognostic indicator in postangioplasty human lesions, was inversely related to plasma apoE levels.

CONCLUSIONS

Similar to its effects on atherosclerosis progression, in a mouse model of restenosis, a subphysiological level of apoE was associated with beneficial effects on lesion size/composition.

Differential reproductive efficiency associated with common apolipoprotein e alleles in postreproductive-aged subjects

OBJECTIVE

To investigate the possible impact of apolipoprotein E (APOE) polymorphism on reproductive efficiency.

DESIGN

Population study.

SETTING

University Departments and a Laboratory of National Research Council.

PATIENT(S)

One hundred sixty healthy unrelated subjects of postreproductive age.

INTERVENTION(S)

Peripheral blood collection and questionnaire administration.

MAIN OUTCOME MEASURE(S)

Apolipoprotein E genotypes were detected after PCR amplification and CfoI digestion; plasma total cholesterol was assayed.

RESULT(S)

The mean number of children of e*2 allele carriers (2.4) was lower than that of e*3/e*3 and e*4/e*3 subjects (3.9). The trend was similar (2.8 vs. 4.8) when the number of pregnancies was considered. Moreover, there was a clear inverse relationship between number of children and e*2-carrying genotype proportions (chi(2) for trend = 6.3). Conversely, the e*3/e*3 genotype was associated with the highest number of children and pregnancies (3.9 and 4.9, respectively), and the e*4/e*3 genotype, with intermediate values (3.7 and 4.4). Carriers of e*2 allele also showed the lowest levels of total cholesterol.

CONCLUSION(S)

The e*2 allele seems to be associated with the lowest reproductive efficiency and the e*3 allele, with the highest. The different total cholesterol levels associated with APOE genotypes could have an effect on steroidogenesis and determine as a consequence the observed differential fertility.

Testing the role of apoA-I, HDL, and cholesterol efflux in the atheroprotective action of low-level apoE expression

Low levels of transgenic mouse apolipoprotein E (apoE) suppress atherosclerosis in apoE knockout (apoE-/-) mice without normalizing plasma cholesterol. To test whether this is due to facilitation of cholesterol efflux from the vessel wall, we produced apoA-I-/-/apoE-/- mice with or without the transgene. Even without apoA-I and HDL, apoA-I-/-/apoE-/- mice had the same amount of aorta cholesteryl ester as apoE-/- mice. Low apoE in the apoA-I-/-/apoE-/- transgenic mice reduced aortic lesions by 70% versus their apoA-I-/-/apoE-/- siblings. To define the free cholesterol (FC) efflux capacity of lipoproteins from the various genotypes, sera were assayed on macrophages expressing ATP-binding cassette transporter A1 (ABCA1). Surprisingly, ABCA1 FC efflux was twice as high to sera from the apoA-I-/-/apoE-/- or apoE-/- mice compared with wild-type mice, and this activity correlated with serum apoA-IV. Immunodepletion of apoA-IV from apoA-I-/-/apoE-/- serum abolished ABCA1 FC efflux, indicating that apoAI-V serves as a potent acceptor for FC efflux via ABCA1. With increasing apoE expression, apoA-IV and FC acceptor capacity decreased, indicating a reciprocal relationship between plasma apoE and apoA-IV. Low plasma apoE (1-3 x 10(-8) M) suppresses atherosclerosis by as yet undefined mechanisms, not dependent on the presence of apoA-I or HDL or an increased capacity of serum acceptors for FC efflux.

Altered adrenal gland cholesterol metabolism in the apoE-deficient mouse

Previous studies suggest the hypothesis that apoE produced by adrenocortical cells modulates cellular cholesterol metabolism to enhance the storage of esterified cholesterol (EC) at the expense of cholesterol delivery to the steroidogenic pathway. In the present study, parameters of adrenal cholesterol metabolism and corticosteroid production were examined in wild type and apoE-deficient (apoe(-/-)) mice. Adrenal gland EC content and the EC/free cholesterol (FC) ratio in mice stressed by adrenocorticotropin (ACTH) treatment or saline injection were reduced in apoe(-/-) compared to apoe(+/+) mice. Relative to apoe(+/+) mice, apoE deficiency also resulted in increased levels of plasma corticosterone in the basal state, in response to acute or long-term ACTH treatment, and after a swim-induced neuroendocrine-directed stress test. Measurements of adrenal gland scavenger receptor class B, type I (SR-BI), LDL receptor, and LDL receptor related protein (LRP) levels and the activities of ACAT or HMG-CoA reductase showed no difference between genotypes. Apoe(-/-) and apoe(+/+) mice showed similar quantitative increases in LDL receptors, SR-BI, adrenal weight gain, and ACAT activities in response to ACTH, and both genotypes had similar basal plasma ACTH concentrations. These results suggest that the effects of apoE deficiency reflect events at the level of the adrenal gland and are specific to changes in cholesterol accumulation and corticosterone production. Further, these findings support the hypothesis that apoE acts to enhance adrenocortical EC accumulation and diminish corticosterone production.

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

Apolipoprotein E peptide stimulation of rat ovarian theca cell androgen synthesis is mediated by members of the low density lipoprotein receptor superfamily

Ovarian androgen production is rate limiting for follicular maturation and can induce follicular atresia. Thus, it is important to define the actions of the intraovarian agents, such as apolipoprotein (apo) E, that modulate theca cell androgen production. Theca cell androgen production is stimulated at low concentrations and inhibited at higher concentrations of native apo E. The apo E peptide, acetyl-Y(LRKLRKRLLRDADDL)(2)C or acetyl-Y(141-155)(2)C, has low density lipoprotein (LDL) receptor and LDL receptor-related protein-binding activity, and it mimics the activity of native apo E in the theca-interstitial cell system. To define the role of members of the LDL receptor superfamily in the apo E peptide-mediated responses, we found that receptor-associated protein prevented the stimulation without altering the inhibition of androstenedione production. The apo E peptide (129-162), which has no LDL receptor-binding activity, did not stimulate androstenedione production. The apo E peptide acetyl-Y(141-155)(2)C did not stimulate androstenedione production when cell surface heparan sulfate proteoglycans were degraded with heparinase. The apo E peptide acetyl-Y(141-155)(2)C bound to heparin, a property of LDL receptor ligands, and in this complex the peptide had no effect on androstenedione production. These observations support the conclusion that apo E-mediated stimulation, but not inhibition, of ovarian theca cell androstenedione production was mediated by members of the LDL receptor superfamily.

Apolipoprotein E and atherosclerosis: insight from animal and human studies

Major advances have been made in our understanding of the role of apolipoprotein E (apoE) in the onset and development of atherosclerosis. Increasing evidence from both animal and human studies suggests that apoE is able to protect against atherosclerosis by: a) promoting efficient uptake of triglyceride-rich lipoproteins from the circulation; b) maintaining normal macrophage lipid homeostasis; c) playing a role in cellular cholesterol efflux and reverse cholesterol transport; d) acting as an antioxidant; e) inhibiting platelet aggregation; and f) modulating immune function. In humans, apoE is polymorphic, and this genetic variation has a strong effect on its antiatherogenic characteristics. Thus, compared to the epsilon3 allele, the epsilon4 allele promotes atherosclerosis, whereas the epsilon2 allele is either pro- or anti-atherogenic, depending on the influence of both environmental and genetic factors. ApoE and its gene are prime targets for therapeutic intervention aimed at preventing or treating atherosclerotic vascular disease.

The evolution of Alzheimer disease, the reproductive schedule, and apoE isoforms

Alzheimer disease (AD)-like neuropathology increases progressively during aging in most primates, and, in some species, is concurrent with reproductive decline in females and cognitive impairments. We consider how the schedule of AD may have evolved in early humans in relation to the apolipoprotein E (apoE) allele system, which is not found in other primates, and to the increasing duration of postnatal care. The delay of independence and the increasing length of maturation required that the schedule of AD-like neurodegeneration be slowed, otherwise parental caregivers would already have become impaired. We hypothesize that the uniquely human apoE epsilon3 allele evolved from the epsilon4 of primate ancestors during human evolution in relation to the rapid increases of brain size and the emergence of grandmothering. In discussing theses possibilities, we review the diverse bioactivities of apoE, which include involvement in hormone systems. The evolution of menopause is also considered in relation to the protective effect of estrogen on AD.

Inducible expression of protein kinase Calpha suppresses steroidogenesis in Y-1 adrenocortical cells

We have previously shown that protein kinase C (PKC) suppresses steroidogenesis in Y-1 adrenocortical cells. To ask directly if the PKCalpha isoform mediates this suppression, we have developed Y-1 cell lines in which PKCalpha is expressed from a tetracycline-regulated promoter. Induction of PKCalpha expression in these cell lines results in decreased P450 cholesterol side-chain cleavage enzyme (P450-SCC) activity as judged by the conversion of hydroxycholesterol to pregnenolone. Transcription of a P450-SCC promoter-luciferase construct is also reduced when PKCalpha expression is increased. However, expression of PKCalpha has no effect on 8-bromo-cAMP induction of steroidogenesis, indicating that these pathways function independently to regulate steroidogenesis. To determine the relationship between endogenous PKC activity and steroidogenesis, we examined 12 Y-1 subclones that were isolated by limited dilution cloning. In each of these subclones, steroid production correlates inversely with total PKC activity and with the expression of PKCalpha but not PKCepsilon or PKCzeta. These studies define for the first time the role of a specific PKC isoform (PKCalpha) in regulating steroidogenesis and P450-SCC activity in adrenocortical cells.

Apolipoprotein E inhibits platelet-derived growth factor-induced vascular smooth muscle cell migration and proliferation by suppressing signal transduction and preventing cell entry to G1 phase

The anti-atherogenic effects of apolipoprotein (apo) E have been attributed to its ability to reduce plasma cholesterol level and to limit foam cell formation. The purpose of this study was to ascertain if apoE also may have cytostatic functions that could attenuate vascular occlusive diseases. Purified apoE inhibited smooth muscle cell migration directed to platelet-derived growth factor (PDGF) or oxidized LDL (oxLDL) (p < 0.0001). The purified apoE also suppressed PDGF- and oxLDL-induced smooth muscle cell proliferation (p < 0.001). These apoE inhibitory effects were not because of suppression of PDGF binding to its receptors on the smooth muscle cells, but was correlated with a significant reduction in agonist-stimulated mitogen-activated protein kinase activity (p < 0.01). ApoE also inhibited PDGF-induced cyclin D1 mRNA expression, suggesting that the apoE effect was mediated by growth arrest at the G0 to G1 phase. Taken together, these results suggest that apoE has cytostatic functions in the vessel wall and may protect against vascular diseases through inhibition of cell signaling events associated with growth factor-induced smooth muscle cell migration and proliferation.

Selective uptake of low density lipoprotein-cholesteryl ester is enhanced by inducible apolipoprotein E expression in cultured mouse adrenocortical cells

Apolipoprotein (apo) E is expressed at high levels by steroidogenic cells of the adrenal gland, ovary, and testis. The cell surface location of apoE in adrenocortical cells suggests that apoE may facilitate the uptake of lipoprotein cholesterol by either the endocytic or the selective uptake pathways, or both. To examine these possibilities, the human apoE gene was expressed in murine Y1 adrenocortical cells under control of an inducible tetracycline-regulated promoter. The results show that induction of apoE yielded a 2-2.5-fold increase in the uptake of low density lipoprotein-cholesteryl ester (LDL-CE) but had little effect on high density lipoprotein-CE uptake. Analysis of lipoprotein uptake pathways showed that apoE increased LDL-CE uptake by both endocytic and selective uptake pathways. In terms of cholesterol delivery to the adrenal cell, the apoE-mediated enhancement of LDL-CE selective uptake was quantitatively more important. Furthermore, the predominant effect of apoE expression was on the low affinity component of LDL-CE selective uptake. LDL particles incubated with apoE-expressing cells contained 0.92 +/- 0.11 apoE molecules/apoB after gel filtration chromatography, indicating stable complex formation between apoE and LDL. ApoE expression by Y1 cells was necessary for enhanced LDL-CE selective uptake. This result may indicate an interaction between apoE-containing LDL and cell surface apoE. These data suggest that apoE produced locally by steroidogenic cells facilitates cholesterol acquisition by the LDL selective uptake pathway.

A minimally lipidated form of cell-derived apolipoprotein E exhibits isoform-specific stimulation of neurite outgrowth in the absence of exogenous lipids or lipoproteins

Within the central nervous system, apolipoprotein E (apoE) synthesis is increased in response to nerve injury, a finding that may reflect a role for apoE in neuronal remodeling. Recent studies show that apoE3 promotes and apoE4 inhibits neurite outgrowth in cultured neuronal cells. Interestingly, these isoform-specific effects are observed only when apoE is presented to cells in the presence of an exogenous lipid source such as rabbit beta-very low density lipoprotein (beta-VLDL), making it difficult to discern the biologically active form of apoE or to understand the role of the lipid source. In the present study we tested whether a cell-derived lipidated form of apoE can alter neurite outgrowth in the absence of beta-VLDL by constructing Neuro-2a cell lines expressing high levels of apoE. Our results showed that endogenous apoE3 stimulated neurite outgrowth, whereas the endogenous apoE4 isoform was neutral. Furthermore, beta-VLDL antagonized the stimulatory effects of the endogenous apoE3. Characterization of the secreted apoE3 indicated that the neurite outgrowth-stimulating activity could be recovered from culture medium with an anti-apoE immunoaffinity column and was present in a poorly lipidated particle with a density between 1.19 and 1.26 g/ml. These results indicated that the biological activity of apoE3 in stimulating neurite outgrowth was inherent in the cell-derived apoE particle and was not dependent on either (a) an interaction of apoE3 with an artificial lipid source or (b) independent actions of apoE3 and beta-VLDL.

Rapid elevation of neuronal cytoplasmic calcium by apolipoprotein E peptide

Apolipoprotein E (apoE) and certain peptides derived from it have been shown to exert neurotoxic effects in vitro, and apoE has been linked to the etiology of Alzheimer's disease. The mechanisms underlying these toxic and pathological effects are, however, not known. To approach this question, we have studied the effects of apoE peptides on the cytoplasmic calcium ([Ca2+]i) homeostasis of cultured cortical neurons. A tandem dimer repeat peptide (apoEdp) derived from the receptor binding domain of apoE was found to have a potent effect on elevation of [Ca2+]i calcium. The pathway by which apoEdp exerted this effect was shown to involve both the mobilization of intracellular calcium and the influx of extracellular calcium, although the effect on influx was more pronounced. Calcium mobilization occurs via a G-protein-linked phospholipase C (PLC) pathway, whereas calcium influx appears to involve a novel Co2+-sensitive channel. Both the mobilization and the influx of calcium require the binding of the apoE peptide to a membrane receptor because both pathways are blocked by antibody to low-density-lipoprotein receptor-related protein. The data suggest that the neurotoxic effects of apoE may be mediated by a persistent elevation of [Ca2+]i.

Lipoproteins, lipid droplets, lysosomes, and adrenocortical steroid hormone synthesis: morphological studies

Recent studies concerning cellular cholesterol homeostasis suggest that there is a relationship between the serum lipoproteins (low density and high density lipoproteins: LDL and HDL), the intracellular storage of cholesterol (lipid droplets), lysosomes, and the steroidogenic activity of adrenocortical cells. This review surveys the current knowledge on cholesterol import from LDL/HDL by adrenocortical cells, its regulation, and the participation of lipid droplets and lysosomes in this process. The possible role of adrenocortical cell microvilli in the uptake of LDL/HDL is discussed. Under certain physiological, experimental, and pathological circumstances lysosomes accumulate unesterified and/or esterified cholesterol in the form of lipid-lysosome complexes. As suggested by the data presented in this review, lipid-lysosome complexes appear to be involved in cholesterol homeostasis, via altering lipid compartmentalization. Since previous reports do not clearly demonstrate a positive correlation between the volume of lipid- and lysosome-compartments and the rate of steroid hormone synthesis [for review, see Nussdorfer (1986) Int. Rev. Cytol., 98:1-405], the objective of this review is to provide a better understanding of the interactions of plasma lipoproteins, lipid droplets, lysosomes, and steroidogenesis.

Effects of apoprotein E on intracellular metabolism of model triglyceride-rich particles are distinct from effects on cell particle uptake

Apoprotein E (apoE) enhances uptake of triglyceride-rich lipoprotein particles (TGRP). We questioned whether apoE would also modulate intracellular metabolism of TGRP in addition to its effects on particle uptake. We prepared model TGRP with triolein and cholesteryl oleate (1:1, w/w) as the core lipids, emulsified by egg yolk phosphatidylcholine, and containing a non-degradable marker, [3H]cholesteryl hexadecyl ether. Particles were intermediate density lipoprotein-sized as determined by core lipid/phospholipid ratios (2.0-3.0/1) and gel filtration chromatography on Sepharose CL-2B. Emulsions were incubated with J774 macrophages for 5 min to 6 h at core lipid concentrations of 300-1200 micrograms/ml and 0-0.2 microgram recombinant apoE/mg core lipid. Particle uptake was determined by [3H]cholesteryl ether uptake and fluorescence microscopy in the absence and presence of apoE. Similar uptake of particles with and without apoE was achieved by utilizing a 4 times higher particle concentration in the absence of apoE. At equivalent levels of uptake, particles with apoE lead to one-half of the triglyceride mass accumulation and twice the triglyceride utilization as compared to particles without apoE. Further, apoE doubles cell cholesteryl ester hydrolysis and to a lesser extent (approximately 30%) increases cholesteryl ester resynthesis by acyl-CoA cholesterol acyltransferase. Particles, both with and without apoE, reach the lysosomal compartment as determined by colocalization with fluorescein-labeled alpha 2-macroglobulin. These results suggest that, in addition to its role in enhancing TGRP uptake, apoE has additional effects on modulating the cellular metabolism of both triglyceride and cholesteryl ester, after particle internalization.

Apolipoprotein E expression in Y1 adrenal cells is associated with increased intracellular cholesterol content and reduced free cholesterol efflux

The expression of apoE mRNA in the adrenal gland is inversely correlated to steroidogenesis and directly correlated to the level of cholesteryl ester stores. To further investigate the relationship between apoE and cellular cholesterol homeostasis, several parameters of cholesterol metabolism in the murine Y1 adrenal cell line engineered to constitutively express human apoE (Y1-E cells) have been studied. It is reported here that Y1-E cells have increased cellular cholesterol content and markedly reduced efflux of free cholesterol as compared to control Y1 cells that do not express apoE. Y1-E cells have increases in both free and esterified cholesterol. However, Y1 and Y1-E cells incorporate [14C]oleate into cholesteryl ester at similar rates and have similar levels of maximal ACAT activity in isolated microsomes. Turnover of cholesteryl ester stores prelabeled with [14C]oleate occurred at similar rates in Y1-E and control Y1 cells, suggesting that increased cholesteryl ester stores in Y1-E cells do not result from reduced cholesteryl ester hydrolysis. Y1-E cells showed reduced cholesterol efflux as compared to control Y1 cells with either native high-density lipoprotein or cholesterol-free reconstituted particles as extracellular acceptors. Cholesterol efflux was not altered by inhibition of ACAT, suggesting that cholesterol esterification in Y1-E cells is not inhibiting efflux. These results suggest that reduced cholesterol efflux is responsible, at least in part, for the cholesterol accumulation in Y1-E cells. In comparison to the rat adrenal gland in vivo, Y1-E cells resemble adrenocortical cells under conditions where steroidogenesis is suppressed and apoE expression and cholesteryl ester storage are increased.(ABSTRACT TRUNCATED AT 250 WORDS)