Abstract MP31: Bmal1 Regulates Production Of Larger Lipoproteins By Modulating Crebh And Apolipoprotein A 4

High plasma lipid and lipoprotein levels are risk factors for various metabolic diseases, such as diabetes, obesity, metabolic syndrome, and atherosclerosis. We previously showed that circadian rhythms regulate plasma lipids, and deregulation of these rhythms cause hyperlipidemia and atherosclerosis in mice. Here, we show that global and liver-specific Bmal1-deficient mice maintained on a chow or a Western diet developed hyperlipidemia, which was denoted by the presence of higher amounts of triglyceride- and apoAIV-rich larger chylomicron and very-low-density lipoprotein, due to overproduction. Bmal1 deficiency decreased Shp and increased MTP, a key protein that facilitates primordial lipoprotein assembly and secretion. Moreover, we show that Bmal1 regulates Crebh to modulate apoAIV expression and the assembly of larger lipoproteins. This is supported by the observation that Crebh- and apoAIV-deficient mice, along with Bmal1-deficient mice with knockdown of Crebh, had smaller lipoproteins. Further, overexpression of Bmal1 in Crebh-deficient mice had no effect on apoAIV expression and lipoprotein size. These studies indicate that regulation of apoAIV and assembly of larger lipoproteins by Bmal1 requires Crebh. Mechanistic studies showed that Bmal1 regulates Crebh expression by two mechanisms. First, Bmal1 interacts with the Crebh promoter to control circadian regulation. Second, Bmal1 increases Rev-erbα expression, and Rev-erbα interacts with the Crebh promoter to repress expression. In short, Bmal1 modulates both the synthesis of primordial lipoproteins and their subsequent expansion into larger lipoproteins by regulating two different proteins, MTP and apoAIV, via two different transcription factors, Shp and Crebh. It is likely that disruptions in circadian mechanisms contribute to hyperlipidemia, and synchronization of circadian rhythms may limit/prevent hyperlipidemia and atherosclerosis.

What we know and don't know: a mapping review of available evidence, and evidence gaps, on adolescent sexual and reproductive health in Bangladesh

Across low- and middle-income countries, investment in adolescent sexual and reproductive health (ASRH) is growing. However, the lack of comprehensive ASRH data hinders programmes. This mapping review examines the available evidence on ASRH in Bangladesh and points out the areas where critical information gaps exist. National surveys, research studies, grey literature, and reports on ASRH in Bangladesh published between 2011 and 2021 were reviewed. Data were extracted into categories, and topical summaries were presented. Research gaps were identified using an analytical framework informed by the Guttmacher Institute's global summary of ASRH research gaps. The gaps identified were synthesised according to relevance against three of the framework's categories: coverage, under-reporting and substantive. We also explored the extent to which human rights dimensions of ASRH have been addressed in the literature. While some of the issues covered, such as access to ASRH information, bodily autonomy and self-determination regarding marriage and childbearing choices, clearly address dimensions of human rights, very few studies were found that explored ASRH through a human rights lens. Furthermore, many of the same research gaps identified globally were also evident in the Bangladesh-specific literature. We assert that an expanded ASRH research agenda in Bangladesh that aims to fill the identified evidence gaps would inform more robust, targeted ASRH programming.

Cranio-cervical junction malformation causing cord compression in infant with achondroplasia: a bigger picture

Introduction: Achondroplasia is a genetic disorder known for short stature and skeletal abnormalities.Case Report: We present a case of narrowing of the foramen magnum from a large opisthion extending to the spinal canal.Conclusion: Foramen magnum stenosis and cervicomedullary stenosis are potentially life threatening neurological manifestations of achondroplasia.

Mucinous Carcinoma of The Breast: Report of Two Cases

Mucinous carcinoma of the breast is one of 16 rare varieties of malignant lesions of epithelial origin Inbreast. They account for 1-6% of all breast cancer. Variations Inpresentation, cllnlcal course and prognosis has been observed in these patients. In this communication we report two cases of pure mucinous carcinoma of the breast .Both the cases were unusual, first one of them having early presentation with pulmonary and bone metastasis less commonly seen In this type of breast carcinoma and the second case had lymph node involvement that is seen in less than one fifth of the cases of mucinous carcinoma of breast. A brief review about mucinous carcinoma is supplemented with the report. Journal of Surgical Sciences (2013) Vol. 17 (1) : 45-49

Abstract 431: Impact of Liver-Specific Transgenic Overexpression of Drosophila and Human MTP on Diet-Induced Hepatic Steatosis and Lipoprotein Secretion

Microsomal triglyceride transfer protein (MTP) is essential for the formation of apolipoprotein B (apoB)-containing lipoproteins. Previous results established that invertebrate forms of MTP (e.g., Drosophila ) are capable of phospholipid (PL) transfer, whereas vertebrate forms of MTP (e.g., human) engage in both PL and triglyceride (TG) transfer. To determine, in vivo, whether the PL and TG transfer activities of MTP play different roles in lipoprotein formation and protection from hepatic steatosis, we created B6D2F1 mice that express human MTP (hMTP) or Drosophila MTP (dMTP) under the control of the apoE 5’ proximal promoter and 3’ hepatic control region in the plasmid pLIV11. Immunoblot analysis confirmed that transgenic expression was limited to the liver and that expression of the transgenes did not impact the abundance of endogenous mouse MTP. To assess the ability of MTP to protect mice from hepatic steatosis, dMTP transgenic mice, hMTP transgenic, and wild type (WT) littermates were placed on a high fat (45% from lard) and cholesterol (0.2%) diet for eight weeks. Mice displayed no significant differences in plasma TG or total plasma cholesterol; however, hMTP mice displayed a trend toward reduced liver TG content, indicating that chronic MTP overexpression may protect the liver from hepatic lipid accumulation. Surprisingly, however, a 2-fold increase in liver TG was observed in two separate founder lines of dMTP transgenic mice. H&E staining confirmed an increase in neutral lipid accumulation in mice expressing the dMTP transgene, relative to both WT and hMTP mice. Quantitative PCR analysis did not reveal changes in expression of genes responsible for hepatic lipogenesis or lipid oxidation. These data complement earlier studies using adenovirus-mediated expression, indicating that acute hMTP and dMTP expression in liver-specific MTP knockout mice can reverse hepatic steatosis caused by MTP deficiency. In contrast, chronic hepatic overexpression of dMTP in a wild type background appears to interfere with endogenous lipoprotein formation and secretion. Understanding the basis for this phenotype may provide insights into how the specific lipid transfer and other activities of MTP contribute to apoB lipoprotein formation.

Genetic diversity in merozoite surface protein-1 and 2 among Plasmodium falciparum isolates from malarious districts of tribal dominant state of Jharkhand, India

INTRODUCTION

The genetic make-up of malaria parasite is potent for understanding the parasite virulence, designing antimalarial vaccine and evaluating the impact of malaria control measures. There is a paucity of information on genetic structure of Plasmodium falciparum in Jharkhand, India where malaria is rampant and this study aimed to establish molecular characterization of P. falciparum field isolates from Jharkhand measured with two highly polymorphic genetic markers, i.e. the merozoite surface proteins (MSPs) 1 and 2.

METHODS

The genetic diversity of P. falciparum population from low transmission area, Ranchi, Bokaro and Hazaribagh and highly malarious area, Latehar and Palamau districts of Jharkhand were evaluated by polymerase chain reaction-sequencing analyzing msp-1 and msp-2 genes to explore the genetic structure of parasite from this understudied region.

RESULTS

A total of 134 P. falciparum isolates were analyzed by polymorphic regions of msp-1 and msp-2 and classified according to prevalence of allelic families. The majority of patients from all the five sites had mean monoclonal infections of 67·1 and 60·4% of P. falciparum for msp-1 and msp-2, respectively, whereas, mean multiple genotypes of 32·8 and 39·5% for msp-1 and msp-2, respectively. Interestingly, we observed higher multiclonal infection in low transmission area as compared to highly malarious area in the case of msp-1 genotypes, whereas in msp-2 higher multiclonal infection was observed in highly malarious area compared to low transmission area. The overall multiplicities of infection of msp-1 and msp-2 were 1·38 and 1·39, respectively.

CONCLUSION

This is the first report on molecular characterization of P. falciparum field isolates from Jharkhand. The genetic diversity and allelic distribution found in this study is somewhat similar to other reports from India and Southeast Asian countries. However, P. falciparum infection can be highly complex and diverse in these disease-endemic regions of Jharkhand, suggesting continual genetic mixing that could have significant implications for the use of antimalarial drugs and vaccines.

Diurnal expression of the thrombopoietin gene is regulated by CLOCK

BACKGROUND

Most physiologic processes exhibit diurnal fluctuations controlled by the circadian regulation of sleep-wake behavior and feeding cycles. In addition, many cell types express endogenous circadian rhythms that affect cell-specific processes. Independent reports support the hypothesis that thrombopoietin (TPO) is under circadian control.

OBJECTIVES

The current study tested the hypothesis that CLOCK, a circadian transcription factor, may regulate Thpo, the gene encoding TPO.

METHODS

Circadian gene expression patterns were analyzed in mice and in human cell lines, Small interfering RNA was used to knock down CLOCK expression in cell lines, and gene expression was also examined in Clock(Δ19/Δ19) mutant mice.

RESULTS

It was found that there was a diurnal rhythm in the expression of Thpoin vivo in mice, and that this was associated with concomitant rhythms of protein abundance. Thpo was rhythmically expressed in human cell lines, consistent with the gene being directly or indirectly regulated by the circadian clock. Silencing of CLOCK in the Huh7 human hepatoma cell line led to a significant reduction in the rhythmicity of Thpo expression. The expression of Mpl in murine marrow also displayed diurnal rhythmicity in vivo. In Clock(Δ19/Δ19) mutant mice, Thpo and Mpl expression was disrupted and there was an increase in the number of mature megakaryocytes, but no change in the ploidy distribution within the megakaryocyte population.

CONCLUSIONS

These findings establish that Clock regulates Thpo and Mpl expression in vivo, and demonstrate an important link between the body's circadian timing mechanisms and megakaryopoiesis.

Evaluation of existing curriculum (2002) of undergraduate medical education in Bangladesh

This study was conducted for evaluation of existing MBBS curriculum (2002) of undergraduate medical education in Bangladesh. The specific objectives of this study were: i) to assess the subject wise course content coverage in the new MBBS curriculum, ii) to assess different examination system for evaluation of MBBS students, iii) to evaluate the effectiveness of teaching and learning activities under the curriculum, iv) to explore students opinions regarding improvement of new curriculum. This was a descriptive cross-sectional study. The study was conducted among the students of Dhaka medical colleges of Bangladesh in 2008. Data was collected by self administered structured questioner adopting convenient sampling method. About ninety percent students opined that the coverage of course content of subjects in the curriculum in Phase I was enough. In case of the subjects in phase II except community medicine more than four fifth of the students expressed their opinion about coverage of course content in the curriculum as enough. In case of phase III it was mentioned by most of the students that coverage of course content was enough. Study revealed that teaching methods were perceived suitable by about three fourth of the respondents, to achieve learning objectives. Most of the students expressed their positive views regarding practice of block posting teaching. More than three fourth of the students perceived that formative assessment was encouraging for students to become time bound learner and Structured Oral Examination (SOE) was fair on an average. Only 31(8.6%) of the respondents had opinion that Objective Structured Practical Examination (OSPE)/Objective Structured Clinical Examination (OSCE) was not well organized. About half of the students opined that 20% marks in written test should be allocated for Multiple Choice Question (MCQ). Students' suggestions regarding teaching were: there should be smaller group sessions; more interactive sessions; more clinical and practical sessions; more problem oriented sessions; more sessions with senior and experienced teachers; teachers should follow the curriculum properly; and should be well prepared for class. Regarding assessment suggestions were: written script of the formative examination should be returned to students with feedback; teachers should not be biased. Study recommended that training of the teachers on teaching methodology and assessment system is needed; teachers should provide feedback to the students according to the performance of the formative assessment at the individual level; to maintain the standards of assessment proper planning, designing, conduction and evaluation of assessment should be taken into consideration; subject wise review and updating is essential to make the curriculum more need based, user friendly and applicable considering context of Bangladesh.

A 30 year old female with lower abdominal lump

A female of 30 years of age, hailing from Narayangonj, Dhaka got admitted herself into Dhaka medical College & Hospital on 25.10.08 with the complaints of a lump in the lower abdomen for 1 year and pain in the lower abdomen for the same duration. Local examination reveals a lump in the lower abdomen which is intra abdominal, 10cm × 12cm in size. CT scan of Abdomen reveals a solid mass measuring (12 × 9 × 8.5) cm is seen in left side of abdominal cavity extending up to pelvic cavity. FNAC of abdominal lump shows sheets of mesothelial cells and few small clusters of regular epithelial like cells. Laparotomy was done on 27.11.08 under G/A & reveals a solid mass measuring about 10 × 8cm size & was well encapsulated, adherent with omentum found in the lower abdomen. Few mesenteric lymph nodes were enlarged. The mass was gently separated from the omentum ensuring adequate haemostasis. Post-operative period was uneventful. Histopathology report shows neoplasm composed of plump of fibroblasts arranged in broad sweeping fascicles and infiltrate into adjacent tissue consistent with fibromatosis. The patient was discharged with advice on 10th post operative day.

A 40 year old man with acute abdomen

A 40 years old man with the complaints of abdominal pain and per rectal bleeding for 2 days and abdominal distension for 1 day. There was an ill defined, tender intra abdominal solid mass at the left side of the lower abdomen measuring about 3 x 3cm2 on deep palpation. Direct rectal examination could not be done due to excessive pain at the anal region and non co operation of the patient. Plain X-ray of the abdomen A/P view in erect posture including both domes of the diaphragm shows distended large intestinal gas shadow & bottle like radiopaque shadow at the lower mid abdomen. Ultrasonogram (USG) of the abdomen could not be done due to the restlessness of the patient. With all available aseptic precaution the abdomen was opened by a lower midline incision. After exploration of the sigmoid colon two glass bottles (One measuring 10cm x 5 cm, the other 12cm x 7cm) were removed and the colon was closed primarily. There was also a small piece of stone found in the peritoneal cavity measuring 2 cm in diameter which was sent for histopathology. Post operative period was uneventful. The patient was discharged with advice on the 8th POD.

Management of external hernias: analysis of 1020 [corrected] cases

A prospective study was done to find out the incidence of different external hernias, identify the predisposing factors and type of repair performed. Over a period of 8 years, 1020 patients were admitted with different varieties of external hernias in Chittagong Medical College Hospital. Among them 743 patients had inguinal hernia, 130 had incisional hernia, 58% had indirect inguinal hernia, 29% had direct inguinal hernia, Bilateral inguinal hernia was present in 13% cases. 272 (32.08%) presented with complications like, irreducibility, obstruction, strangulation, 576 (67.92%) was admitted from out-patient department for elective operation. 97.04% of patients with inguinal hernia were male, 2.96% of patients were female. But among incisional hernia 77.70% are female and 22.30% are male patients. The highest incidence of both inguinal hernia and incisional hernia were found in 4th decade of life. Predisposing factors could be identified in 42.06% cases. 90.76% patients approached doctors for advice after 6 months of noticing hernia only 36.37% could mention the cause of delay. Out of 1020 patients (812) 72.18% patients underwent different operations. 2.5% patients were inoperable due to associated disease like Ca-liver, IHD, renal transplant, CLD, uncontrolled DM. Others refused or absconded, being afraid of operation. Effort for post operative follow up was not very satisfactory as only a small number could be followed for a limited period.

Comparative pharmacokinetics of diphenhydramine in camels and horses after intravenous administration

The pharmacokinetics of diphenhydramine (DPHM) was compared in camels (n = 8) and horses (n = 6) following intravenous (i.v.) administration of a dose of 0.625 mg/kg body weight. In addition, the metabolism and urinary detection time of DPHM was evaluated in camels. The data obtained (median and range in brackets) in camels and horses, respectively, were as follows. The terminal elimination half lives (h) were 1.58 (1.13-2.58) and 6.11 (4.80-14.1), and the total body clearances (L/h per kg) were 1.42 (1.13-1.74) and 0.79 (0.66-0.90). The volumes of distribution at steady state (L/kg) were 2.38 (1.58-4.43) and 5.98 (4.60-8.31) and the volumes of the central compartment of the two compartment pharmacokinetic model were 1.58 (0.80-2.54) and 2.48 (1.79-3.17). All the pharmacokinetic parameters in camels were significantly different from those of horses. Five metabolites of DPHM were tentatively identified in the camel's urine. Two metabolites, diphenylmethoxyacetic acid and 1-(4-hydroxyphenyl)-phenylmethoxyacetic acid, were present in the acid fraction. Two metabolites, desamino-DPHM and diphenylmethanol, were identified in the basic fraction, in addition to DPHM itself, which was present mainly as a conjugate. Even after enzymatic hydrolysis, DPHM could be detected for up to 24 h in camels after an i.v. dose of 0.625 mg/kg body weight.

The disposition of diclofenac in camels after intravenous administration

The pharmacokinetics of diclofenac was studied in camels (Camelus dromedarus) (n=6) following intravenous (i.v.) administration of a dose of 2.5 mg kg(-1) body weight. The metabolism and urinary detection time were also studied. The results obtained (median and range) were as follows: the terminal elimination half-life (t(1/2beta)) was 2.35 (1.90-2.73)h, total body clearance (Cl(T)) was 0.17 (0.16-0.21)lh kg(-1). The volume of distribution at steady state (V(SS)) was 0.31 (0.21-0.39)l(-1)kg(-1), the volume of the central compartment of the two compartment pharmacokinetic model (V(C)) was 0.15 (0.11-0.17)l kg(-1). Five metabolites of diclofenac were tentatively identified in urine and were excreted mainly in conjugate form. The main metabolite was identified as hydroxy diclofenac. Both diclofenac and hydroxy diclofenac, appear to be the main elimination route for diclofenac when administered i.v. in camels. Diclofenac could be identified up to 4 days following i.v. administration in camels using a sensitive gas chromatography/mass spectrometry (GC/MS) method.

Hypoxia and pulmonary acclimatisation at 4578 m altitude: the role of acetazolamide and dexamethasone

OBJECTIVES

To quantify the changes in ventilatory response and arterial blood gases in healthy male volunteers on acute ascent to 4578 meters altitude along with evaluating the role of acetazolamide and dexamethasone prophylaxis in acclimatisation.

METHODS

Forty four lowlander male subjects participated as two groups in the study. Twenty four lowlander healthy male adults (age mean +/- SE 27.8 +/- 1.24 years) comprised the non-acclimatised group. They were sub grouped in a double blind fashion into four. Each subgroup (n=6) received placebo (multivitamin) or acetazolamide (250 mg) or dexamethasone (4 mg) or combined regimen of the two drugs twice daily for 5 days commencing 24 hours before ascent. The volunteers reached the altitude of 4578 meters within a span of one day. The second group (acclimatised) comprised of age and height matched twenty volunteers who had arrived 4 and 8 weeks earlier at the same altitude. Arterial PO2, PCO2, SO2 and minute respiratory rate pH were measured. The pulmonary functions (FVC, %FEV1 PEF, FEF(25-75%) and MVV) were recorded by Compact Spirometer (Vitalograph). Pre and post ascent measurements were carried out in non-acclimatised group whereas one measurement of the same parameters were made in acclimatised group.

RESULTS

The study revealed a significant increase in respiratory rate min-1 after 24 hours of ascent along with reduction in PaO2 and SaO2. The gradual increase in PaO2 and SaO2 and reduction in PaCO2 was observed after 4 weeks at the same altitude. The reduction in FVC, %FEV1, PEF, FEF(25-75%) and MW were recorded in our subjects after 24 hours of acute ascent. Nevertheless, all pulmonary parameters in volunteers having 8 weeks of stay at 4578 m altitude, were found equivalent to the non-acclimatised group. The greater increase in minute respiratory rate, PaO2, SaO2 and pulmonary functions was found in volunteers taking acetazolamide.

CONCLUSION

It is concluded that hyperventilation and increase in lung volumes are the adaptive pulmonary responses which help in improving levels of PaO2 and SaO2. The acetazolamide-dexamethasone prophylaxis appears beneficial in promoting pulmonary acclimatisation during hypobaric hypoxia.

Prevalence, risk factors and genotype distribution of HCV and HBV infection in the tribal population: a community based study in south India

Viral hepatitis caused by the hepatitis C virus (HCV) and hepatitis B virus (HBV) represents a major public health problem in India. These viruses share common modes of transmission, such as parenteral routes. We aimed to assess the exposure of a tribal population to these viruses in south India. The present study was carried out on serum samples from 890 individuals (526 males and 324 females) belonging to the Lambada tribe residing in the state of Andhra Pradesh, south India. Anti-HCV antibody and hepatitis B surface antigen (HBsAg) status in the sera were analyzed using commercially available enzyme immunoassays (Abbott Labs, Chicago, IL). HCV-RNA and HBV-DNA in the sera was tested by reverse transcriptase polymerase chain reaction (RT-PCR) and PCR, respectively. The infecting genotype of HCV was determined using type-specific primers corresponding to the NS5 region of the virus. Out of the 890 samples, 18 (2.02%; male 11/526; female 7/364) were positive for HCV-RNA by RT-PCR and, 17 of them were positive for anti-HCV antibody. Genotyping of HCV isolates from the 18 individuals positive for HCV-RNA revealed that 66.67% (12/18) were infected with type 1 of HCV and its variants; while in the remaining (6/18), the infecting genotype was found to be type 3 and its variants. A total of 46 samples (5.16%; males 28/526; female 18/364) were positive for HBsAg; while 11 were positive only for HBV-DNA, 9 were positive for both hepatitis B e antigen (HBeAg) and HBV-DNA. Cultural practices such as tattooing, traditional medicine (e.g. blood-letting), rituals (e.g. scarification), body-piercing etc are the potential sources of spread of infection in this tribe. None of the samples analyzed revealed co-infection with the 2 viruses.

Binding of microsomal triglyceride transfer protein to lipids results in increased affinity for apolipoprotein B: evidence for stable microsomal MTP-lipid complexes

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are known to interact with each other. We evaluated the effect of different lipids on the protein-protein interactions between MTP and apoB100 or its C-terminally truncated forms. Negatively charged lipids decreased protein-protein interactions between apoB and MTP. In contrast, zwitterionic phospholipids enhanced (2-4-fold) the binding of apoB100 to MTP by increasing affinity (1.5-3-fold) between these proteins without affecting the number of binding sites. Similarly, phospholipids augmented (1.5-4-fold) the binding of various C-terminally truncated apoB peptides to MTP. The increased binding was greater for apoB peptides containing lipid-binding domains, such as apoB28 and apoB42. Surprisingly, preincubation of apoB28 with lipid vesicles had no effect on MTP binding. In contrast, incubation of MTP with lipid vesicles resulted in a stable association of MTP with vesicles, and MTP-lipid vesicles bound better (5-fold increase) to LDL than did lipid-free MTP. To determine whether MTP exists stably associated with lipids in cells, microsomal contents from COS cells expressing MTP, HepG2 cells, and mouse liver were ultracentrifuged, and MTP was visualized in different density fractions. MTP was found associated and unassociated with lipids. In contrast, apoB17 and apoB:270-570 were present unassociated with lipids in COS cells. These studies show that the binding of MTP to lipids results in increased affinity for apoB and that stable MTP-lipid complexes exist in the lumen of the endoplasmic reticulum. Protein-protein interactions between apoB and MTP may juxtapose lipids associated with MTP to lipid-binding domains of apoB and facilitate hydrophobic interactions leading to enhance affinity. We speculate that MTP-lipid complexes may serve as nuclei to form "primordial lipoproteins" and may also play a role in the bulk addition of lipids during the "core expansion" of these lipoproteins.

Metabolism of equilenin in MCF-7 and MDA-MB-231 human breast cancer cells

Sulfate conjugates of the B-ring unsaturated estrogens, equilin, equilenin, and 8-dehydroestrone, and their 17alpha- and 17beta-dihydro analogues, constitute about 54% of Premarin (Wyeth-Ayerst), the most commonly prescribed estrogen formulation in estrogen replacement therapy. Despite the wide clinical use of Premarin, there have been very few studies on the metabolism of the B-ring unsaturated estrogens in humans and there is no information regarding the fate of these compounds in breast tissue or tumors. In this study, we investigated the metabolism of equilenin in two lines of human breast-cancer cells, MCF-7 and MDA-MB-231. MCF-7 cells respond to treatment with Ah-receptor agonists with induction of cytochromes P450 1A1 and 1B1, whereas in MDA-MB-231 cells P450 1B1 is predominantly induced. Metabolites of equilenin were identified and quantified by GC/MS utilizing a series of synthetic metabolite standards and deuterium-labeled analogues as internal standards. In the two cell lines, the same pathways of equilenin metabolism were observed. Equilenin was reduced at C-17 to the 17beta-dihydro form, with minimal production of the 17alpha-dihydro isomer. Both equilenin and 17beta-dihydroequilenin were hydroxylated at the C-4 position, and the resultant catechol metabolites were methylated to form 4-methoxyequilenin and 4-methoxy-17beta-dihydroequilenin. Rates of equilenin metabolism were markedly elevated in cultures exposed to the Ah-receptor agonists, 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3,4,4',5-tetrachlorobiphenyl, implicating the activities of P450s 1A1 and 1B1 in the metabolism. The 2-hydroxylation pathways of equilenin and 17beta-dihydroequilenin metabolism were not observed. In microsomal reactions with cDNA-expressed human enzymes, both P450s 1A1 and 1B1 catalyzed the 4-hydroxylation of 17beta-dihydroequilenin, whereas with 17beta-estradiol as substrate P450 1A1 catalyzes predominantly 2-hydroxylation and P450 1B1 predominantly 4-hydroxylation. Since P450 1B1 is constitutively expressed and both P450s 1A1 and 1B1 are inducible in many extrahepatic tissues including the mammary epithelium, these results indicate the potential for 4-hydroxylation of equilenin and 17beta-dihydroequilenin in extrahepatic, estrogen-responsive tissues.

Mechanisms involved in the intestinal digestion and absorption of dietary vitamin A

Dietary retinyl esters are hydrolyzed in the intestine by the pancreatic enzyme, pancreatic triglyceride lipase (PTL), and intestinal brush border enzyme, phospholipase B. Recent work on the carboxylester lipase (CEL) knockout mouse suggests that CEL may not be involved in dietary retinyl ester digestion. The possible roles of the pancreatic lipase-related proteins (PLRP) 1 and 2 and other enzymes require further investigation. Unesterified retinol is taken up by the enterocytes, perhaps involving both diffusion and protein-mediated facilitated transport. Once in the cell, retinol is complexed with cellular retinol-binding protein type 2 (CRBP2) and the complex serves as a substrate for reesterification of the retinol by the enzyme lecithin:retinol acyltransferase (LRAT). Retinol not bound to CRBP2 is esterified by acyl-CoA acyltransferase (ARAT). The retinyl esters are incorporated into chylomicrons, intestinal lipoproteins that transport other dietary lipids such as triglycerides, phospholipids, and cholesterol. Chylomicrons containing newly absorbed retinyl esters are then secreted into the lymph.

Acute mountain sickness score and hypoxemia

BACKGROUND

Hypoxemia is the immediate consequence of hyobaric hypoxia, which is the crucial starting mechanism of acute mountain sickness (AMS). The AMS is generally a benign and self-limiting condition which can be prevented by gradual ascent. However, ascent rates recommended for prophylaxis of AMS are far slower than those attempted during military operations and by climbers.

OBJECTIVE

The current study was carried out to quantify the relationship between AMS and hypoxemia alongwith evaluating the benefits of acetazolamide-dexamethasone chemoprophylaxis during acute ascent.

SUBJECTS AND METHODS

Twenty-four low lander male adults (age mean +/- SE 27.8 +/- 1.24 years) were selected. They were grouped in a double-blind fashion into four groups and each group (n = 6) received placebo (multivitamin) or acetazolamide (250 mg) or dexamethasone (4 mg) or a combined regimen of the two drugs twice daily for 5 days, commencing 24 hours before ascent. The volunteers reached the altitude of 4578 meters within a span of one day. Their AMS symptoms were recorded on modified environmental symptoms questionnaire (ESQ), after 24 and 72 hours of ascent. Arterial PO2, SO2 and PCO2 were measured by GEMSTAT blood-gas analyzer (Mallincrodt-USA).

RESULTS

The ESQ, AMS-C (cerebral) and AMS-R (respiratory) scores of combined therapy group were significantly lower as compared to the other groups on the symptom rating scale. The significant finding amongst the volunteers taking acetazolamide was mild to moderate diuresis whereas severity of headache was markedly less in dexamethasone group. The commonest feature of combined therapy was that none of the volunteers complained of headache, dysponea, irritability and more than mild disturbance of sleep. The ESQ scores of volunteers were inversely correlated to PaO2 and SaO2 after 24 hours of ascent to 4578 meters.

CONCLUSION

The study concludes that severity of AMS is closely related to hypoxemia and combination therapy of acetazolamide-dexamethasone may be effective in preventing AMS.

Structural, biochemical and signaling properties of the low-density lipoprotein receptor gene family

The low-density lipoprotein (LDL) receptor (LDL-R) family members (LDL-R, LRP, megalin, VLDL-R, apoER2) bind several extra-cellular structurally dissimilar ligands and internalize them for degradation by lysosomes by a process called receptor-mediated endocytosis. The receptor-mediated endocytosis involves immobilization of circulating ligands onto the cell-surface followed by their internalization and degradation. All the receptors can perform both of these functions. However, in the majority of the cases, other proteins immobilize ligands on to the cell-surface and subsequent internalization is mediated by these receptors. The LDL-R and LRP play important roles in plasma cholesterol homeostasis and fetal development. Megalin is an antigenic determinant for Heymann nephritis in rats and may be important for re-absorption of various molecules by the kidney. VLDL-R homologue in chicken is essential for female fertility. This receptor and apoER2 are critical for the proper development of the brain in mice. The members of the LDL-R gene family contain several complement-type and EGF precursor-like repeats, and single transmembrane and cytoplasmic domain. Cysteine-rich complement-type repeats containing DxSDE sequences at the C-termini constitute ligand-binding domains. In contrast to the ligand binding domains, receptor-binding domains in different ligands do not share sequence homology. It has been proposed that positive electrostatic surface potentials, not the primary sequences, in different ligands constitute receptor-binding domains. The EGF precursor homology repeats in receptors are important for the dissociation of ligands from receptors in endocytic vesicles. The transmembrane domain is necessary for anchoring to membranes and the cytoplasmic domain is required for their targeting to coated pits and subsequent internalization. The receptor-mediated endocytosis involves recognition of the NPXY motif by clathrin. Recently, this motif has also been implicated in signaling pathways that are crucial in brain development. The signaling process involves the recognition of the NPXY motif by Disabled-1 protein and possibly other proteins involved in intracellular signaling cascade. The LDL-R gene family has provided important insights into the mechanisms of ligand catabolism and may serve as new targets for the treatment of different cardiovascular and neuronal disorders. In the future, their role in signaling may provide novel insights into brain development and neuronal layering.

Signposts in the assembly of chylomicrons

Intestinal cells synthesize and secrete chylomicrons in the postprandial state. Synthesis of these particles is defective in abetalipoproteinemia and chylomicron retention disease. Chylomicrons are very large, heterogeneous, lipid-rich particles ranging in diameters from 75 to 450 nm and function to transport dietary fat and fat-soluble vitamins to blood. The size heterogeneity of the secreted particles depends on the rate of fat absorption, type and amount of fat absorbed. The fatty acid composition of triglycerides present in chylomicrons reflects the composition of dietary fat, whereas the fatty acid composition of chylomicron phospholipids does not. The differences in the fatty acid compositions are also observed when lipids are labeled with glycerol. Thus, the differences are not due to differential incorporation of dietary fatty acids into different lipids but are mainly due to different pools of lipids used for chylomicron assembly. It has been suggested that preformed phospholipids and nascent triglycerides are preferentially used for intestinal lipoprotein assembly. Biosynthesis of chylomicrons requires apoB48. ApoB48 is translated from apoB mRNA that is post-transcriptionally edited in the intestinal cells to incorporate a stop codon. Nascent apoB48 may be cotranslationally lipidated and this process is critically dependent on the presence of microsomal triglyceride transfer protein. Two different models have been proposed for the assembly of chylomicrons. In the independent model, intestinal cells are hypothesized to synthesize VLDL and chylomicron by two independent pathways. The chylomicron assembly pathway is hypothesized to be sensitive to a surfactant, Pluronic L81, but that of VLDL assembly is not. In the sequential assembly model, synthesis of all lipoproteins is hypothesized to begin with the assembly of apoB-containing primordial lipoprotein particles. The primordial particles are suggested to fuse with triglyceride-rich lipid droplets that are synthesized independently of apoB. This process results in the core expansion of primordial particles and the synthesis of nascent lipoproteins. Differences in the size of secreted lipoproteins may be due to differences in the size of triglyceride-rich lipid droplets. Pluronic L81 is hypothesized to inhibit the formation of large triglyceride-rich droplets that serve as precursors for chylomicron assembly. In this review, we have discussed some signposts that might be unique to different steps in the assembly of chylomicrons. First, it is proposed that the association of preformed phospholipids with nascent apoB in the endoplasmic reticulum may serve as a signpost for the very early steps in the assembly of chylomicrons. Second, association of large amounts of newly synthesized triglycerides compared to preformed triglycerides may serve as a signpost for the assembly of larger lipoproteins. Third, the incorporation of retinyl esters may serve as markers for the final stages of chylomicron assembly. These signposts may be helpful in the identification and characterization of various intermediates in the assembly of chylomicrons. The knowledge about the molecular assembly of chylomicrons may lead to better therapeutic agents for controlling various hyperlipidemias, obesity, and atherosclerosis.

Retinyl ester secretion by intestinal cells: a specific and regulated process dependent on assembly and secretion of chylomicrons

Retinyl esters (RE) have been used extensively as markers to study chylomicron (CM) catabolism because they are secreted in the postprandial state with CM and do not exchange with other lipoproteins in the plasma. To understand the mechanism of secretion of RE by the intestine under the fasting and postprandial states, differentiated Caco-2 cells were supplemented with radiolabeled retinol under conditions that support or do not support CM secretion. We observed that these cells assimilate vitamin A by a rapid uptake mechanism. After uptake, cells store retinol in both esterified and unesterified forms. Under fasting conditions, cells do not secrete RE but secrete free retinol unassociated with lipoproteins. Under postprandial conditions, cells secrete significant amounts of RE only with CM. The secretion of RE with CM was independent of the rate of uptake of retinol and intracellular free and esterified retinol levels, and was absolutely dependent on the assembly and secretion of CM. The secretion of RE was correlated with the secretion of CM and not with the secretion of total apolipoprotein B. Inhibition of CM secretion by Pluronic L81 decreased the secretion of RE and did not result in their increased secretion with smaller lipoproteins. These data strongly suggest that RE secretion by the intestinal cells is a specific and regulated process that occurs in the postprandial state and is dependent on the assembly and secretion of CM. We propose that RE are added to CM during final stages of lipoprotein assembly and may serve as signposts for these steps.

SULT1A1 catalyzes 2-methoxyestradiol sulfonation in MCF-7 breast cancer cells

In a previous study of nine human breast-derived cell lines, rates of metabolism of 17beta-estradiol (E(2)) were greatly enhanced when cultures were exposed to the aromatic hydrocarbon receptor agonist, 2,3,7,8-tetrachlorodibenzo-p-dioxin. Elevated rates of E(2) hydroxylation at the C-2, -4, -6alpha and -15alpha positions were observed concomitant with the induction of cytochromes P450 1A1 and 1B1. In each cell line, 2- and 4-hydroxyestradiol (2- and 4-OHE(2)) were converted to 2- and 4-methoxyestradiol (2- and 4-MeOE(2)) by the action of catechol O:-methyltransferase. In this study, conjugation of these estrogen metabolites was investigated. A comparison of the levels of metabolites determined with and without prior treatment of the media with a crude beta-glucuronidase/sulfatase preparation showed that most of the 2-MeOE(2) present was in conjugated form, whereas 4-MeOE(2), 6alpha-OHE(2) and 15alpha-OHE(2) were minimally conjugated. Inhibitor studies suggested that it was the sulfatase activity of the preparation that hydrolyzed the 2-MeOE(2) conjugates in MCF-7 cell media; the presence of 2-MeOE(2)-3-sulfate in MCF-7 culture media was confirmed by electrospray ion-trap mass spectrometry. To identify the enzyme catalyzing this conjugation, the expression of mRNAs encoding five sulfotransferases (SULT1A1, SULT1A2, SULT1A3, SULT1E1 and SULT2A1) was evaluated in the nine cell lines by use of the reverse transcription-polymerase chain reaction. Only expression of SULT1A1 mRNA correlated with the observed conjugation of nanomolar levels of 2-MeOE(2) in these cell lines. Cloning and sequencing of SULT1A1 cDNA from MCF-7 cells revealed that mRNAs encoding two previously identified allelic variants, SULT1A1*1 ((213)Arg) and SULT1A1*2 ((213)His), were expressed in these cells. Heterologous cDNA-directed expression of either variant in MDA-MB-231 cells, which do not normally express SULT1A1, conferred 2-MeOE(2) sulfonation activity. The SULT1A1 allelic variants were also expressed in SF:9 insect cells, from which post-microsomal supernatants were used to determine K:(m) values of 0.90 +/- 0.12 and 0.81 +/- 0.06 microM for SULT1A1*1 and SULT1A1*2, respectively, with 2-MeOE(2) as substrate. These results show that SULT1A1 is an efficient and selective catalyst of 2-MeOE(2) sulfonation and, as such, may be important in modulating the anticarcinogenic effects of 2-MeOE(2) that have been described recently.

High affinity binding between lipoprotein lipase and lipoproteins involves multiple ionic and hydrophobic interactions, does not require enzyme activity, and is modulated by glycosaminoglycans

Lipoprotein lipase (LPL) physically associates with lipoproteins and hydrolyzes triglycerides. To characterize the binding of LPL to lipoproteins, we studied the binding of low density lipoproteins (LDL), apolipoprotein (apo) B17, and various apoB-FLAG (DYKDDDDK octapeptide) chimeras to purified LPL. LDL bound to LPL with high affinity (K(d) values of 10(-12) m) similar to that observed for the binding of LDL to its receptors and 1D1, a monoclonal antibody to LDL, and was greater than its affinity for microsomal triglyceride transfer protein. LDL-LPL binding was sensitive to both salt and detergents, indicating the involvement of both hydrophobic and hydrophilic interactions. In contrast, the N-terminal 17% of apoB interacted with LPL mainly via ionic interactions. Binding of various apoB fusion peptides suggested that LPL bound to apoB at multiple sites within apoB17. Tetrahydrolipstatin, a potent enzyme activity inhibitor, had no effect on apoB-LPL binding, indicating that the enzyme activity was not required for apoB binding. LDL-LPL binding was inhibited by monoclonal antibodies that recognize amino acids 380-410 in the C-terminal region of LPL, a region also shown to interact with heparin and LDL receptor-related protein. The LDL-LPL binding was also inhibited by glycosaminoglycans (GAGs); heparin inhibited the interactions by approximately 50% and removal of trace amounts of heparin from LPL preparations increased LDL binding. Thus, we conclude that the high affinity binding between LPL and lipoproteins involves multiple ionic and hydrophobic interactions, does not require enzyme activity and is modulated by GAGs. It is proposed that LPL contains a surface exposed positively charged amino acid cluster that may be important for various physiological interactions of LPL with different biologically important molecules. Moreover, we postulate that by binding to this cluster, GAGs modulate the association between LDL and LPL and the in vivo metabolism of LPL.

NADPH- and hydroperoxide-supported 17beta-estradiol hydroxylation catalyzed by a variant form (432L, 453S) of human cytochrome P450 1B1

Human cytochrome P450 1B1 (CYP1B1) catalyzes the hydroxylation of 17beta-estradiol (E(2)) at C-4, with a lesser activity at C-2. The E(2) 4-hydroxylase activity of human CYP1B1 was first observed in studies of MCF-7 breast cancer cells. Sequencing of polymerase chain reaction products revealed that CYP1B1 expressed in MCF-7 cells was not the previously characterized enzyme but a polymorphic form with leucine substituted for valine at position 432 and serine substituted for asparagine at position 453. To investigate the NADPH- and organic hydroperoxide-supported E(2) hydroxylase activities of the 432L, 453S form of human CYP1B1, the MCF-7 CYP1B1 cDNA was cloned and the enzyme was expressed in Sf9 insect cells. In microsomal assays supplemented with human NADPH:cytochrome P450 oxidoreductase, the expressed 432L, 453S form catalyzed NADPH-supported E(2) hydroxylation with a similar preference for 4-hydroxylation as the 432V, 453N form, with maximal rates of 1.97 and 0.37 nmol (min)(-1)(nmol cytochrome P450)(-1) for 4- and 2-hydroxylation, respectively. Cumeme hydroperoxide efficiently supported E(2) hydroxylation by both the 432V, 453N and 432L, 453S forms at several-fold higher rates than the NADPH-supported activities and with a lesser preference for E(2) 4- versus 2-hydroxylation (2:1). The hydroperoxide-supported activities of both forms were potently inhibited by the CYP1B1 inhibitor, 3,3',4, 4',5,5'-hexachlorobiphenyl. These results indicate that the 432V, 453N and 432L, 453S forms of CYP1B1 have similar catalytic properties for E(2) hydroxylation, and that human CYP1B1 is very efficient in catalyzing the hydroperoxide-dependent formation of catecholestrogens.

Decreased secretion of ApoB follows inhibition of ApoB-MTP binding by a novel antagonist

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are essential for the efficient assembly of triglyceride-rich lipoproteins. Evidence has been presented for physical interactions between these proteins. To study the importance of apoB-MTP binding in apoB secretion, we have identified a compound, AGI-S17, that inhibited (60-70% at 40 microM) the binding of various apoB peptides to MTP but not to an anti-apoB monoclonal antibody, 1D1, whose epitope overlaps with an MTP binding site in apoB. AGI-S17 had no significant effect on the lipid transfer activity of the purified MTP. In contrast, another antagonist, BMS-200150, did not affect apoB-MTP binding but inhibited MTP's lipid transfer activity. The differential effects of these inhibitors suggest two functionally independent, apoB binding and lipid transfer, domains in MTP. AGI-S17 was then used to study its effect on the lipid transfer and apoB binding activities of MTP in HepG2 cells. AGI-S17 had no effect on cellular lipid transfer activities, but it inhibited coimmunoprecipitation of apoB with MTP. These studies indicate that AGI-S17 inhibits apoB-MTP binding but has no effect on MTP's lipid transfer activity. Experiments were then performed to study the effect of inhibition of apoB-MTP binding on apoB secretion in HepG2 cells. AGI-S17 (40 microM) did not affect cell protein levels but decreased the total mass of apoB secreted by 70-85%. Similarly, AGI-S17 inhibited the secretion of nascent apoB by 60-80%, but did not affect albumin secretion. These studies indicate that AGI-S17 decreases apoB secretion most likely by inhibiting apoB-MTP interactions. Thus, the binding of MTP to apoB may be important for the assembly and secretion of apoB-containing lipoproteins and can be a potential target for the development of lipid-lowering drugs. It is proposed that the apoB binding may represent MTP's chaperone activity that assists in the transfer from the membrane to the lumen of the endoplasmic reticulum and in the net lipidation of nascent apoB, and may be essential for lipoprotein assembly and secretion.

A proposed model for the assembly of chylomicrons

The intestine synthesizes very low density lipoproteins (VLDL) and chylomicrons (CM) to transport fat and fat-soluble vitamins into the blood. VLDL assembly occurs constitutively whereas CM assembly is a characteristic property of the enterocytes during the postprandial state. The secretion of CM is specifically inhibited by Pluronic L81. CM are very heterogeneously-sized particles that consist of a core of triglycerides (TG) and cholesterol esters and a monolayer of phospholipids (PL), cholesterol and proteins. The fatty acid composition of TG, but not PL, in CM mirrors the fatty acid composition of fat in the diet. CM assembly is deficient in abetalipoproteinemia and CM retention disease. Abetalipoproteinemia results due to mutation in the mttp gene and is characterized by the virtual absence of apoB-containing lipoproteins in the plasma. Patients suffer from neurologic disorders, visual impairment, and exhibit acanthocytosis. CM retention disease, an inherited recessive disorder, is characterized by chronic diarrhea with steatorrhea in infancy, abdominal distention and failure to thrive. It is caused by a specific defect in the secretion of intestinal lipoproteins; secretion of lipoproteins by the liver is not affected. Besides human disorders, mice that do not assemble intestinal lipoproteins have been developed. These mice are normal at birth, but defective in fat and fat-soluble vitamin absorption, and fail to thrive. Thus, fat and fat-soluble vitamin transport by the intestinal lipoproteins is essential for proper growth and development of neonates. Recently, differentiated Caco-2 cells and rabbit primary enterocytes have been described that synthesize and secrete CM. These cells can be valuable in distinguishing between the two different models proposed for the assembly of CM. In the first model, the assembly of VLDL and CM is proposed to occur by two 'independent' pathways. Second, CM assembly is proposed to be a product of 'core expansion' that results in the synthesis of lipoproteins of different sizes. According to this model, intestinal lipoprotein assembly begins with the synthesis of 'primordial' lipoprotein particles and involves release of the nascent apoB with PL derived from the endoplasmic reticulum (ER) membrane. In addition, TG-rich 'lipid droplets' of different sizes are formed independent of apoB synthesis. The fusion of lipid droplets and primordial lipoproteins results in the formation of different size lipoproteins due to the 'core expansion' of the primordial lipoproteins.

The mammalian low-density lipoprotein receptor family

The low-density lipoprotein (LDL) receptor (LDL-R) family consists of cell-surface receptors that recognize extracellular ligands and internalize them for degradation by lysosomes. The LDL-R is the prototype of this family, which also contains very-low-density lipoprotein receptors (VLDL-R), apolipoprotein E receptor 2, LRP, and megalin. The family members contain four major structural modules: the cysteine-rich complement-type repeats, epidermal growth factor precursor-like repeats, a transmembrane domain, and a cytoplasmic domain. Each structural module serves distinct and important functions. These receptors bind several structurally dissimilar ligands. It is proposed that instead of a primary sequence, positive electrostatic potential in different ligands constitutes a receptor binding domain. This family of receptors plays crucial roles in various physiologic functions. LDL-R plays an important role in cholesterol homeostasis. Mutations cause familial hypercholesterolemia and premature coronary artery disease. LDL-R-related protein plays an important role in the clearance of plasma-activated alpha 2-macroglobulin and apolipoprotein E-enriched lipoproteins. It is essential for fetal development and has been associated with Alzheimer's disease. Megalin is the major receptor in absorptive epithelial cells of the proximal tubules and an antigenic determinant for Heymann nephritis in rats. Mutations in a chicken homolog of VLDL-R cause female sterility and premature atherosclerosis. This receptor is not expressed in liver tissue; however, transgenic expression of VLDL-R in liver corrects hypercholesterolemia in experiment animals, which suggests that it can be a candidate for gene therapy for various hyperlipidemias. The functional importance of individual receptors may lie in their differential tissue expression. The regulation of expression of these receptors occurs at the transcriptional level. Expression of the LDL-R is regulated by intracellular sterol levels involving novel membrane-bound transcription factors. Other members of the family are not regulated by sterols. All the members are, however, regulated by hormones and growth factors, but the mechanisms of regulation by hormones have not been elucidated. Studies of these receptors have provided important insights into receptor structure-function and mechanisms of ligand removal and catabolism. It is anticipated that increased knowledge about the LDL-R family members will open new avenues for the treatment of many disorders.

Assembly and secretion of chylomicrons by differentiated Caco-2 cells. Nascent triglycerides and preformed phospholipids are preferentially used for lipoprotein assembly

To develop a cell culture model for chyclomicron (CM) assembly, the apical media of differentiated Caco-2 cells were supplemented with oleic acid (OA) together with either albumin or taurocholate (TC). The basolateral media were subjected to sequential density gradient ultracentrifugations to obtain large CM, small CM, and very low density lipoproteins (VLDL), and the distribution of apoB in these fractions was quantified. In the absence of OA, apoB was secreted as VLDL/LDL size particles. Addition of OA (>/=0.8 mM) with TC, but not with albumin, resulted in the secretion of one-third of apoB as CM. Lipid analysis revealed that half of the secreted phospholipids (PL) and triglycerides (TG) were associated with CM. In CM, TG were 7-11-fold higher than PL indicating that CM were TG-rich particles. Secreted CM contained apoB100, apoB48, and other apolipoproteins. Secretion of large CM was specifically inhibited by Pluronic L81, a detergent known to inhibit CM secretion in animals. These studies demonstrate that differentiated Caco-2 cells assemble and secrete CM in a manner similar to enterocytes in vivo. Next, experiments were performed to identify the sources of lipids used for lipoprotein assembly. Cells were labeled with [3H]glycerol for 12 h, washed, and supplemented with OA, TC, and [14C] glycerol for various times to induce CM assembly and to radiolabel nascent lipids. TG and PL were extracted from cells and media and the association of preformed and nascent lipids with lipoproteins was determined. All the lipoproteins contained higher amounts of preformed PL compared with nascent PL. VLDL contained equal amounts of nascent and preformed TG, whereas CM contained higher amounts of nascent TG even when nascent TG constituted a small fraction of the total cellular pool. These studies indicate that nascent TG and preformed PL are preferentially used for CM assembly and provide a molecular explanation for the in vivo observations that the fatty acid composition of TG, but not PL, of secreted CM reflects the composition of dietary fat. It is proposed that in the intestinal cells the preformed PL from the endoplasmic reticulum bud off with apoB as primordial particles and the assembly of larger lipoproteins is dependent on the synthesis and delivery of nascent TG to these particles.

Tetradecylthioacetic acid (a 3-thia fatty acid) impairs secretion of oleic acid-induced triacylglycerol-rich lipoproteins in CaCo-2 cells

The fatty acid analogue tetradecylthioacetic acid (TTA) has previously been shown to decrease triacylglycerol secretion in CaCo-2 cells (Gedde-Dahl et al., J. Lipid Res. 36 (1995) 535-543). The present study was designed to further elucidate the effect of TTA on lipoprotein production in CaCo-2 cells. TTA did not affect oleic acid-induced triacylglycerol synthesis, but it significantly decreased secretion of newly synthesized triacylglycerol when compared to cells incubated with oleic acid alone or oleic acid in combination with palmitic acid. In contrast, pulse-chase experiments showed no difference in the amount of labeled triacylglycerol secreted from cells exposed to either fatty acid combination during the chase period, indicating that TTA did not affect the secretory process in general. Cells incubated with TTA alone secreted triacylglycerol present at 1.025<rho<1.073 g/ml, corresponding to the low density lipoprotein/intermediate density lipoprotein density range. In contrast, cells supplemented with oleic acid or oleic acid in combination with TTA secreted triacylglycerol mainly in the very low density lipoprotein/chylomicron density range (rho<1.006 g/ml). Despite a marked decrease in triacylglycerol secretion, TTA treatment did not change secretion of apolipoprotein B nor the activity of microsomal triacylglycerol transfer protein (MTP) in the cells. Furthermore, the presence of TTA in cellular triacylglycerol had no effect on the ability of purified MTP to transfer triacylglycerol from donor to acceptor vesicles. Together, the above observations suggest that TTA interferes with other MTP-independent factors that regulate the intestinal lipoprotein secretion.

Amino acids 430-570 in apolipoprotein B are critical for its binding to microsomal triglyceride transfer protein

Several studies have demonstrated protein-protein interactions between microsomal triglyceride transfer protein (MTP) and apolipoprotein B (apoB). However, the binding sites involved in these interactions have not been elucidated. To identify an MTP binding site in apoB, we have expressed several apoB sequences as fusion proteins with the eight-amino acid FLAG peptide. The chimeras were transiently expressed in COS cells, and conditioned media were used to study the binding of these sequences to either immobilized or soluble MTP. A polypeptide containing amino acids 270-570 (B:270-570), but not 1-300, bound to MTP. AGI-S17, an antagonist of apoB-MTP binding, inhibited the binding of B:270-570 to MTP but not to M2, a monoclonal antibody that recognizes the FLAG peptide. These data indicated that B:270-570 contains an MTP binding site. Next, sequences within 270-570 were subjected to C-terminal truncations at natural proline residues. B:270-509 bound less efficiently than B:270-570, whereas, B:270-430 and other shorter chimeras did not bind to MTP. Furthermore, truncations at amino acids 502 and 509 decreased MTP binding by 73 and 42%, respectively. These data indicate that B:430-570 in the alpha1-globular domain of apoB plays a crucial role in MTP binding and presumably in the initiation and maturation of apoB-containing lipoproteins.

Lysine and arginine residues in the N-terminal 18% of apolipoprotein B are critical for its binding to microsomal triglyceride transfer protein

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are essential for the efficient assembly and secretion of triglyceride-rich lipoproteins. We have presented evidence for a high-affinity interaction between these proteins [Hussain, M. M., et al. (1997) Biochemistry 36, 13060-13067]. In this study, we used chemically modified low-density lipoproteins (LDL) and recombinant human apoB18 to identify amino acid residues in apoB that are critical for its interactions with MTP. Acetoacetylation of 74% of lysine residues and cyclohexanedione modification of 54% of arginine residues completely abolished the interactions between LDL and MTP. Regeneration of lysine and arginine residues by hydroxylamine treatment completely restored the binding of modified LDL to MTP. Carboxyethylation of all the histidine residues decreased, but did not abolish, apoB-MTP interactions. In contrast, glycine methyl ester modifications of aspartic and glutamic acid residues, up to 38-44%, had no effect on LDL-MTP interactions. Furthermore, modification of lysine and arginine, but not the aspartic and glutamic acid, residues in apoB18 also completely abolished its interactions with MTP. These studies indicated that lysine and arginine, but not aspartic and glutamic acid, residues are critical for apoB-MTP interactions, whereas histidine residues are not as critical. Since lysine and arginine residues in apoB are known to interact with the LDL receptors and heparin, we studied the effect of different glycosaminoglycans on apoB-MTP interactions. Glycosaminoglycans had no significant inhibitory effect on apoB-MTP interactions, suggesting that the lysine and arginine residues crucial for apoB-MTP interactions are different from those that interact with the LDL receptor and heparin. The lysine and arginine residues in apoB18 may directly interact with negatively charged residues in the MTP molecule, or they may function to maintain the conformation of the recognition site.

Lysophosphatidylcholine increases apolipoprotein B secretion by enhancing lipid synthesis and decreasing its intracellular degradation in HepG2 cells

Free fatty acids and lysophosphatidylcholine (lysoPC) are the major lipids bound to human plasma albumin. The effects of fatty acids on the hepatic production of Apolipoprotein B (apo B) have been studied but those of lysoPC have not. In HepG2 cells, lysoPC increased apo B secretion in different experiments by 50-120%, but did not affect the flotation properties of secreted lipoproteins. LysoPC affected neither the cellular protein levels nor apo A-I secretion suggesting that its effect was specific to apo B. Apo B secretion was maximum after incubating cells for 6 h with 0.2 mM lysoPC as equimolar fatty acid free bovine serum albumin (BSA) complexes. LysoPC was metabolized by cells and its fatty acids were used for the synthesis of phosphatidylcholine and triglycerides (TG). Experiments were performed to understand the mechanism of lysoPC action. LysoPC increased the incorporation of 3H-glycerol into newly synthesized cellular (3-fold) and secreted (4-fold) triglycerides, and increased the synthesis (40%) and secretion (4-fold) of phospholipids. LysoPC did not affect apo B synthesis, but inhibited the intracellular degradation of apo B and increased its secretion. Triacsin C (5 microM), an inhibitor of long chain acyl-CoA synthase, completely inhibited the induction of lipid synthesis and abolished the effect of lysoPC on apo B secretion. These studies indicated that lysoPC increased apo B secretion by inducing lipid synthesis; newly synthesized lipids probably protected apo B from intracellular degradation and enhanced secretion. These studies are consistent with the hypothesis that physiologic concentrations of lysoPC can be an important modulator for hepatic apo B secretion.

Assembly and secretion of VLDL in nondifferentiated Caco-2 cells stably transfected with human recombinant ApoB48 cDNA

Intestinal cells secrete apoB48-containing very low density lipoproteins (VLDLs) and chylomicrons for the transport of biliary and dietary lipids. The molecular mechanisms regulating the assembly of intestinal lipoproteins are not known due to a lack of reliable and specific cell culture models. Caco-2 (a human colon carcinoma) cells have been used to study intestinal lipid metabolism. These cells have been shown to secrete both apoB100- and apoB48-containing triglyceride (TG)-rich lipoproteins only after differentiation into enterocyte-like cells. To study lipoprotein assembly in nondifferentiated Caco-2 cells, we stably expressed human recombinant apoB48 cDNA under the control of a constitutive cytomegalovirus promoter. Pulse-chase analysis revealed that the majority (> 50%) of apoB48 synthesized was degraded intracellularly in the presence or absence of oleic acid. Transfected nondifferentiated cells secreted lipoproteins with flotation densities similar to those of plasma HDL or LDL when cultured in serum-free or serum-containing media, respectively. Incubation of cells with media containing serum and oleic acid resulted in the secretion of VLDL-like particles. Secretion of VLDL was inhibited (> 80%) by triacsin C due to > 60% inhibition of oleate-induced TG synthesis. However, inhibition of cholesteryl ester synthesis by 70% with an acyl coenzyme A:cholesterol acyltransferase inhibitor did not affect VLDL secretion. Efficient assembly of lipoproteins usually requires the microsomal TG transfer protein (MTP). The presence of MTP in transfected Caco-2 cells was investigated by measuring TG transfer activity in microsomal fractions. Microsomal fractions had 0.2% TG transfer activity per hour per microgram of protein, which corresponds to 30% to 60% of the MTP activity present in liver-derived cells. To determine whether MTP activity was required for lipoprotein assembly, transfected cells were incubated in the presence of the MTP inhibitor CP-10,447. This compound completely abolished the secretion of apoB. These data show that the transfected cell lines secrete lipoproteins of different densities under different culture conditions and that the assembly of larger VLDL particles requires active TG synthesis and MTP activity. Thus, in nondifferentiated Caco-2 cells, the amount of apoB secreted and not the MTP activity is the limiting factor for lipoprotein assembly.

Apolipoprotein B binding to microsomal triglyceride transfer protein decreases with increases in length and lipidation: implications in lipoprotein biosynthesis

Microsomal triglyceride transfer protein (MTP), a heterodimer of 97 kDa and protein disulfide isomerase, is required for the assembly of apolipoprotein B (apoB)-containing triglyceride-rich lipoproteins. These proteins have been shown to interact with each other during early stages of lipoprotein biosynthesis. Our studies indicated that binding between apoB and heterodimeric MTP was of high affinity (Kd 10-30 nM) due to ionic interactions. In contrast to MTP, protein disulfide isomerase alone interacted very poorly with lipoproteins, indicating the importance of the heterodimer in these bindings. Preincubation of lipoproteins with detergents enhanced their interaction with MTP. Native VLDL bound poorly to MTP, but its preincubation with Tween-20 resulted in significantly increased binding to MTP. Furthermore, binding of LDL was enhanced by preincubation with taurocholate, indicating that partial delipidation of apoB-containing lipoproteins results in increased binding to MTP. Subsequently, attempts were made to study interactions between C-terminally truncated apoB polypeptides and MTP. Binding of all the polypeptides to MTP was enhanced in the presence of taurocholate. Comparisons revealed that the binding of different apoB polypeptides to MTP was in the order of apoB18 > apoB28 > apoB42 > apoB100. These studies indicated that optimum interactions occur between apoB18 and MTP, and that the increase in apoB length beyond apoB18 has a negative effect on these interactions. Since apoB18 does not assemble triglyceride-rich lipoproteins, these studies suggest that apoB may interact with MTP before its lipidation. It is proposed that steps in lipoprotein biosynthesis may be dictated by the sequential display of different functional domains on the apoB polypeptide.

Measurement of apolipoprotein B in various cell lines: correlation between intracellular levels and rates of secretion

We have standardized simple but sensitive enzyme-linked immunoassays to understand a relationship between intracellular levels and secretion rates of apoB. The assays were based on commercially available antibodies and were specific to human apoB. A monoclonal antibody, 1D1, was immobilized on microtiter wells and incubated with different amounts of low density lipoproteins to obtain a standard curve. Conditioned media were added to other wells in parallel, and the amount of apoB was quantitated from a linear regression curve. To standardize conditions for the measurement of intracellular apoB, cells were homogenized and solubilized with different concentrations of taurocholate. We found that 0.5% taurocholate was sufficient to solubilize all the apoB in HepG2, Caco-2, and McA-RH7777 cells. Next, a standard curve was prepared in the presence of taurocholate and used to determine intracellular levels of apoB in different cell lines. The intracellular levels (pmol/mg cell protein) and the rates of secretion (pmol/mg/h) of apoB100 were positively correlated (r2 = 0.81, P = 0.0009) in HepG2 cells. Furthermore, a positive correlation (r2 = 0.88, P < 0.0001) was found between intracellular and secreted apoB42 in stably transfected McA-RH7777 cells. In contrast, no correlation was observed for human apoB28 and apoB18 in stably transfected cells that were secreted either partially associated or completely unassociated with lipoproteins. These studies indicated that the rate of secretion of lipid-associated apoB, but not the lipid-free apoB, was tightly controlled.

Uptake of chylomicrons by the liver, but not by the bone marrow, is modulated by lipoprotein lipase activity

We have shown that chylomicrons are catabolized by the liver and bone marrow in rabbits and marmosets. In the present investigation, we studied the role of various apolipoproteins and lipoprotein lipase in the clearance of these particles by the liver and bone marrow in rabbits. Incubation of chylomicrons with purified apolipoprotein (apo) E or C-II resulted in more rapid clearance of these particles from the plasma, whereas incubation of chylomicrons with apoA-I, apoC-I, apoC-III1, or apoC-III2, did not affect their clearance rates. Analysis of tissue uptake revealed that the increased plasma clearance rate of chylomicrons enriched with apoE or apoC-II was primarily due to enhanced uptake by the liver. The uptake of chylomicrons by the bone marrow increased after their enrichment with apoA-I but decreased after their enrichment with apoC-II. Because apoC-II is a cofactor for lipoprotein lipase, we hypothesized that the increased clearance rates were due to faster hydrolysis of chylomicrons and rapid generation of chylomicron remnants. To test this hypothesis, lipoprotein lipase activity was inhibited by injection of an antilipoprotein lipase monoclonal antibody. Inhibition of lipoprotein lipase retarded clearance of chylomicrons from the plasma and decreased their uptake by the liver but did not affect their uptake by the bone marrow. These studies suggest that bone marrow can take up chylomicrons in the absence of lipoprotein lipase activity and provide an explanation for the presence of foam cells in the bone marrow of type I hyperlipoproteinemic patients.

Nickel is a specific antagonist for the catabolism of activated alpha 2-macroglobulin

The multifunctional low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) binds and degrades several ligands involved in protease and lipoprotein metabolism. We previously reported that nickel (Ni2+) specifically inhibits the binding of activated alpha 2-macroglobulin (alpha 2 M*) at 4 degrees C to LRP and had no effect on the binding of other ligands to the receptor (Hussain et al. (1995) Biochem. 34, 16074-16081). In the current investigation, we have examined the effect of Ni2+ on the catabolism of 125 I-labeled alpha 2M*, receptor-associated protein (RAP) and lactoferrin at physiologic temperatures by fibroblasts. Nickel completely inhibited the degradation of alpha 2M* over a wide range of concentrations (0.3-2.4 nM); 50% inhibition for the degradation of 1.2 nM alpha 2M* was observed at 0.5 mM Ni2+. Furthermore, nickel inhibited the binding, internalization and degradation of 125I-alpha 2M* in a dose- and time-dependent manner. In contrast, the degradation of several concentrations of 125I-RAP by fibroblasts was not affected by different amounts of Ni2+ for various times. Similarly, Ni2+ did not inhibit the degradation of lactoferrin either before or after treating the cells with heparitinase to remove cell-surface proteoglycans. The degradation of lactoferrin was, however, inhibited by the RAP indicating that lactoferrin degradation was mediated by the LRP. These data suggest that Ni2+ is a specific inhibitor for the degradation of alpha 2M*.

Up-regulation of the low density lipoprotein receptor-related protein by dexamethasone in HepG2 cells

Dexamethasone has been shown to decrease the expression of the low density lipoprotein (LDL) receptor, but its effect on other members of the LDL receptor family is not known. We studied the effect of dexamethasone in HepG2 cells on the expression of the LDL receptor family members using radiolabeled receptor associated protein (RAP) which binds to all the members of the family. Treatment of HepG2 cells with increasing concentrations of dexamethasone resulted in a 2-fold increase in the binding and degradation of RAP. To identify the receptor responsible for the increased binding and degradation of RAP, we used specific ligands. For LDL receptor, we used LDL itself. For the LDL receptor-related protein/alpha 2-macroglobulin receptor, we used activated alpha 2-macroglobulin. The binding of LDL to HepG2 cells was decreased, whereas binding and degradation of activated alpha 2-macroglobulin was increased by 2-fold suggesting that dexamethasone increased LRP expression. Increased LRP expression was positively correlated with the increase in the steady-state levels and transcript numbers of the LRP mRNA; no changes in RAP or gamma-actin mRNA levels were observed. Increased mRNA levels were not due to an increased rate of transcription of the gene as assessed by nuclear run-on experiments. These studies indicate that dexamethasone increases cell-surface LRP activity in HepG2 cells by increasing the steady state mRNA levels and suggest that post-transcriptional mechanisms play a role in controlling LRP mRNA levels.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

In situ hybridization studies of alpha 2-macroglobulin receptor and receptor-associated protein in human prostate carcinoma

We have utilized in situ hybridization and immunocytochemistry techniques to examine the expression of both alpha 2-macroglobulin (alpha 2-MR) and the 39 kDa receptor-associated protein (RAP) in 8 benign (BPH) and 34 malignant human prostate tissues, including 4 metastases. The levels of alpha 2-MR mRNA expression (but not RAP) increased significantly in high Gleason score carcinomas ( > 8) and in metastatic lesions, suggesting that alpha 2-MR expression is associated with advanced cancer. Semi-quantitative analysis with computer-assisted system analysis (CASA) confirmed this interpretation. This is the first report of alpha 2-MR expression being associated with advanced prostate cancer.

Corynebacterium pseudotuberculosis infection and lymphadenitis (taloa or mala) in the camel

Pure cultures of Corynebacterium pseudotuberculosis were obtained from 11 cases of lymphadenitis (known locally as taloa or mala) in camels. Camel isolates produced typical taloa in camels experimentally inoculated subcutaneously at the base of the external ear with 10(10) colony forming units. A sheep strain of C. pseudotuberculosis inoculated into camels produced a local abscess at the site of inoculation but did not produce taloa. Re-infection of camels recovered from experimental inoculation did not produce taloa suggesting the possibility of the development of a vaccine against lymphadenitis in camels.

Nickel is a specific inhibitor for the binding of activated alpha 2-macroglobulin to the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor

The low density receptor-related protein/alpha 2-macroglobulin receptor (LRP/alpha 2-MR) binds to several ligands involved in lipoprotein and protease clearance. The receptor-associated protein (RAP) inhibits the binding of all known ligands. We studied the inhibition by Ni2+ of the binding of different ligands to cells and to the purified LRP/alpha 2-MR. Ni2+ inhibited all of the specific binding of radiolabeled methylamine-activated alpha 2-macroglobulin (125I-alpha 2-M*) to rabbit aortic smooth muscle cells (SMC), rat hepatoma Fu5AH, and mouse fibroblast L cells. Ni2+ also inhibited the binding of trypsin-activated alpha 2-macroglobulin to SMC but did not affect the binding of RAP, Pseudomonas exotoxin A, or low-density lipoproteins. The inhibition of alpha 2-M* binding by Ni2+ was not due to its interaction with alpha 2-M*. Preincubation of SMC with Ni2+ followed by ligand binding suggested that Ni2+ binds to cell-surface molecules and inhibits the binding of alpha 2-M* but does not affect RAP binding. Most of the binding of alpha 2-M* to SMC was due to its binding to the LRP/alpha 2-MR, as opposed to the recently described signaling receptor, as demonstrated by the inhibition of this binding by the RAP. Moreover, the inhibition of alpha 2-M* binding to the LRP/alpha 2-MR by Ni2+ was demonstrated using purified receptor immobilized on microtiter plates. Two to three molecules of 63Ni2+ bound to the immobilized receptor with equal affinity but not to alpha 2-M*. The specific binding of alpha 2-M* to the immobilized receptor was inhibited in the presence of nickel.(ABSTRACT TRUNCATED AT 250 WORDS)

Chylomicron metabolism in normal, cholesterol-fed, and Watanabe heritable hyperlipidemic rabbits. Saturation of the sequestration step of the remnant clearance pathway

The plasma clearance of radiolabeled chylomicrons was compared in normal, cholesterol-fed, and Watanabe heritable hyperlipidemic (WHHL) rabbits. Chylomicron clearance was rapid in normal rabbits but was significantly retarded in cholesterol-fed and WHHL rabbits. At 40 min after the injection of chylomicrons, 14-17% of the injected dose remained in the plasma of normal rabbits, whereas approximately 40-50% of the injected dose remained in the plasma of cholesterol-fed and WHHL rabbits. The differences were reflected in the reduced plasma clearance by the liver and bone marrow of the cholesterol-fed and WHHL rabbits. The hyperlipidemic rabbits expressed normal levels of low density lipoprotein (LDL) receptor-related protein/alpha 2-macroglobulin receptor in the liver. In contrast, the hepatic levels of LDL receptors were lower in hyperlipidemic rabbits; as expected, they were significantly lower in WHHL rabbits compared with normal and cholesterol-fed rabbits. Furthermore, it was demonstrated that lipoproteins accumulating in the plasma of the hyperlipidemic rabbits competed for and retarded the clearance of chylomicrons from the plasma. Competition was demonstrated by cross-circulation of normal and cholesterol-fed or normal and WHHL rabbits, in which the rapid influx of plasma containing the accumulated plasma lipoproteins from cholesterol-fed or WHHL rabbits was shown to impair the uptake of chylomicrons by the liver and bone marrow of normal rabbits. These observations were extended by infusing isolated lipoproteins into normal rabbits. The rabbit d < 1.02 g/ml (remnant) fraction and the canine cholesterol-rich high density lipoproteins (HDL) with apolipoprotein E (HDLc) inhibited chylomicron clearance, whereas human LDL and HDL from humans and rabbits did not. We conclude that the low LDL receptor activity in the cholesterol-fed and WHHL rabbits may contribute, at least in part, to the impaired clearance by decreasing remnant uptake and causing the accumulation of chylomicron and/or very low density lipoprotein remnants. The accumulated remnant lipoproteins then compete for and saturate the mechanism responsible for the initial rapid clearance of chylomicrons from the plasma. We speculate that saturation of the initial rapid clearance may occur at the sequestration step, which involves the binding of remnants to heparan sulfate proteoglycans in the space of Disse.

Characterization of recombinant human apoB-48-containing lipoproteins in rat hepatoma McA-RH7777 cells transfected with apoB-48 cDNA. Overexpression of apoB-48 decreases synthesis of endogenous apoB-100

We studied the effect of overexpression of apolipoprotein (apo) B-48 on the synthesis and secretion of endogenous apoB-100 in rat hepatoma McA-RH7777 cell lines stably transfected with human apoB-48 cDNA under the control of the cytomegalovirus promoter. Three cell lines that secrete 40 to 60 ng human apoB.mg cell protein-1.h-1 were used. The recombinant human apoB-48 exhibited physicochemical characteristics (buoyant density, 1.06 to 1.21 g/mL; beta-electrophoretic mobility and diameters, 16 to 20 nm) indistinguishable from those of endogenous rat apoB-48. Overexpression of the recombinant human apoB-48 resulted in a 50% decrease in the secretion of endogenous apoB-100 but did not affect the secretion of apoE or apoA-I. Several possible mechanisms for the decreased secretion of apoB-100 were evaluated. First, recruitment of lipids into lipoproteins was shown to be unaffected since no major changes in the physicochemical properties of apoB-100-containing lipoproteins were observed. Second, the intracellular degradation of apoB-100 was not altered as the intracellular retention half-time and secretion efficiency remained unaffected by apoB-48 overexpression. Third, the posttranslational regulatory mechanisms for apoB-100 remained normal, as demonstrated by a twofold increase in apoB-100 secretion after supplementation with oleic acid. Unexpectedly, a 35% to 50% decrease in the steady-state synthesis of endogenous apoB-100 was observed in apoB-48-transfected cells compared with control cells. These data suggested that decreased secretion of apoB-100 was secondary to decreased synthesis. The decreased apoB-100 synthesis was not due to decreased steady-state levels of rat apoB-100 mRNA. These results suggest that overexpression of recombinant human apoB-48 may interfere with posttranscriptional events, possibly at the translation-translocation level, and decrease translational yield of apoB-100. These posttranscriptional events prior to the complete synthesis of the apoB-100 polypeptide can be important in the control of apoB-100 secretion.

Role of heparan sulfate proteoglycans in the binding and uptake of apolipoprotein E-enriched remnant lipoproteins by cultured cells

Addition of apolipoprotein (apo) E to rabbit beta-very low density lipoproteins (beta-VLDL) has been shown to result in a marked enhancement of their binding and uptake by various cell types. Apolipoprotein E binds to lipoprotein receptors and proteoglycans. To distinguish between apoE binding to these sites, cells were treated with heparinase. Heparinase treatment of receptor-negative familial hypercholesterolemic (FH) fibroblasts and human hepatoma cells (HepG2) released 30-40% of newly synthesized cell surface 35S-labeled proteoglycans and decreased the binding of beta-VLDL+apoE to FH and normal fibroblasts and HepG2 cells by more than 80%. Furthermore, heparinase treatment significantly decreased the uptake of fluorescently labeled beta-VLDL+apoE by HepG2 cells and decreased cholesteryl ester synthesis in FH fibroblasts by 75%. Likewise, canine chylomicron remnants enriched in apoE demonstrated enhanced binding that was 80% inhibited by heparinase treatment of HepG2 cells. Heparinase treatment did not affect beta-VLDL (without added apoE) or low density lipoprotein (LDL) binding to these cells or the binding activity of beta-VLDL+apoE to the LDL receptor-related protein (LRP) or to the LDL receptor on ligand blots. Chinese hamster ovary (CHO) mutant cells lacking the synthesis of either heparan sulfate (pgsD-677) or all proteoglycans (pgsA-745) did not display any enhanced binding of the beta-VLDL+apoE. By comparison, wild-type CHO cells demonstrated enhanced binding of beta-VLDL+apoE that could be abolished by treatment with heparinase. These mutant cells and wild-type CHO cells possessed a similar amount of LRP, as determined by ligand blot analyses and by alpha 2-macroglobulin binding, and possessed a similar amount of LDL receptor activity, as determined by LDL binding. Therefore, we would interpret these data as showing that heparan sulfate proteoglycan may be involved in the initial binding of the apoE-enriched remnants with the subsequent involvement of the LRP in the uptake of these lipoproteins. It remains to be determined whether the heparan sulfate proteoglycan can function by itself in both the binding and internalization of the apoE-enriched remnants or whether the proteoglycan is part of a complex with LRP that mediates a two-step process, i.e. binding and subsequent internalization by the receptor.

Clearance of chylomicron remnants by the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor

The involvement of the low density lipoprotein receptor-related protein (LRP) in chylomicron remnant (CR) catabolism was investigated. Ligand blot analyses demonstrated that beta-very low density lipoproteins (beta-VLDL) incubated with apolipoprotein E (beta-VLDL+E) bound to the LRP and low density lipoprotein receptors, whereas active (receptor-binding) alpha 2-macroglobulin (alpha 2M) bound only to LRP partially purified from rat liver membranes. Iodinated beta-VLDL+E and active alpha 2M showed high affinity binding to the LRP/alpha 2M receptor of low density lipoprotein receptor-negative fibroblasts. The binding and degradation of radiolabeled alpha 2M by these cells were partially inhibited by beta-VLDL+E. Furthermore, alpha 2M interfered with the internalization of beta-VLDL+E and subsequent induction in the cholesterol esterification by these cells. These studies suggested that remnant lipoproteins and active alpha 2M compete for binding to the LRP/alpha 2M receptor. Next, we examined whether the LRP/alpha 2M receptor plays a role, in the presence of low density lipoprotein receptors, in the in vivo catabolism of CR in mice. In vivo studies demonstrated that the unlabeled active, but not the native, alpha 2M partially inhibited the plasma clearance and hepatic uptake of radiolabeled CR or apoE-enriched radiolabled CR. Likewise, apoE-enriched CR retarded the plasma clearance and hepatic uptake of radiolabeled active alpha 2M. These studies provide physiological evidence that the LRP/alpha 2M receptor may function as a CR receptor that removes CR from the plasma.

Secretion of lipid-poor nascent human apolipoprotein apoAI, apoCIII, and apoE by cell clones expressing the corresponding genes

The human apolipoprotein apoAI, apoCIII, and apoE genes were placed under the control of the mouse metallothionein 1 promoter in a bovine papilloma virus vector that also contained the human metallothionein 1A gene. Following transfection of mouse C127 cells with the expression vector, cell clones resistant to Cd2+ were selected and found to express in high abundance specific apolipoprotein genes. Individual cell clones expressing apoAI, apoCIII, or apoE genes were used further to study the isoprotein composition and the flotation properties of the corresponding nascent apolipoproteins. It was found that the lipoproteins secreted by cell clones expressing the apoAI, apoCIII, and apoE genes consisted of the proapoAI disialylated form of apoCIII (apoCIIIS2) and mainly sialylated forms of apoE. Separation of the secreted apolipoproteins by density gradient ultracentrifugation resulted in limited flotation of nascent apoAI, apoE and apoCIII in the high density lipoprotein (HDL) fraction. Similar analysis in the presence of human serum increased the flotation of apoAI, apoE, and apoCIII to 6.5-, 4.5-, and 5.5-fold, respectively, and resulted in their redistribution to various lipoprotein fractions. HDL increased the flotation of apoAI to 12-fold and very low density lipoprotein (VLDL) increased the flotation of apoCIII and apoE to 6.5- and 5.5-fold, respectively. These findings suggest that in the cell system used, the majority of nascent apoAI, apoCIII and apoE is secreted in the lipid-poor form, which then associates extracellularly with preexisting lipoproteins.

Protective effect of fungal growth product (6MFA), assessed therapeutically against Ehrlich's ascites tumor in Swiss mice

6MFA is a growth product of the fungus Aspergillus ochraceus (ATCC 28706) obtained by fermentation in stationary culture. It has both interferon inducing and antiviral properties, in vivo and in vitro, with a relatively high margin of safety (9, 17, 18). Ehrlich's ascites tumor bearing Swiss albino male mice were treated with 0.5 ml of acqueous preparation of 6MFA (0.75 mg total solids) i.p. in a therapeutic regimen schedule; the sham treated mice received only PBS. 6MFA treatment produced an increase in mean survival time over the untreated controls, restricted the body weight increase due to ascites and decreased the rate of mortality. As much as 100% of survival response was obtained in the group treated with 0.5 ml of 6MFA at the rate of one inoculation per week for 5 weeks. In general a dose-dependent response was seen in the antitumor effect of 6MFA against Ehrlich's ascites tumor in Swiss mice. Delay in administration of 6MFA to tumor bearing mice affected the survival rate.

Intracellular modification of human apolipoprotein AII (apoAII) and sites of apoAII mRNA synthesis: comparison of apoAII with apoCII and apoCIII isoproteins

We have studied the intracellular modifications of human apoAII by pulse-chase labeling of HepG2 cell cultures with [35S]methionine or [3H]arginine followed by two-dimensional analysis and autoradiography of the radiolabeled apoAII isoproteins. A short (5.0-min) pulse showed the presence of a precursor form of apoAII (pI = 5.75) designated proapoAII or apoAII3. A 5-10-min chase resulted in a decrease in the relative concentration of the proapoAII coupled with an increase in the relative concentration of a new form (pI = 5.3) designated modified proapoAII or apoAII1. Longer chase resulted in the appearance of the plasma apoAII form and at least five other acidic apoAII isoproteins in the cell lysate and the culture medium. Labeling with [3H]arginine showed that apoAII isoproteins designated 3, 1, -1, and -3 contained the prosegment whereas isoproteins designated 1a, 0, -1a, -2a, -3a, and -4a did not. Comparison of nascent apoAII, apoCII, and apoCIII isoproteins revealed that they were distinctly different on the two-dimensional gels. Neuraminidase treatment converted the acidic apoAII isoproteins to isoproteins 1a and 0 (modified and plasma apoAII forms). The combined data are consistent with the following intra- and/or extracellular modifications of apoAII: (a) modification of the apoAII which results in the net loss of two positive charges; (b) glycosylation of the modified proapoAII with carbohydrate chains containing sialic acid; (c) proteolytic removal of the prosegment and cyclization of the N-terminal glutamine. Analysis of apoAII mRNA distribution in 13 fetal human tissues as well as in cell lines of human origin showed abundance of apoAII mRNA in liver and HepG2 cells and only traces in the intestine.