Hepatic lipoprotein and cholesterol metabolism. Report of a conference.
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Acyl Coenzyme A↗
Biomedical subjects
R A Davis
Publications and source records attributed to R A Davis.
Adolescent pregnancy and infant mortality: isolating the effects of race.
Researchers continue to accept the untested assumption that the difference between black and white infant mortality rates is largely attributable to race-linked differences in teenage pregnancies (Wise, 1984). The basic notion is that the inordinately high black teenage pregnancy rate (because of its association with low birth weight births) accounts for this difference. This paper tests this key assumption directly and finds only partial support for it. It then argues that the key to the black-white difference in infant mortality is poverty, not teenage pregnancy. Using data from the state of North Carolina, the results suggest that poverty, not race, plays the crucial role in infant mortality.
Adolescent↗
Developmental coordinate expression of triacylglycerol and small molecular weight apoB synthesis and secretion by rat hepatocytes.
Glycerolipid and apoB synthesis and secretion were examined in hepatocytes obtained from fetal, suckling (day 6), and adult rats in order to examine the developmental regulation of lipoprotein production. The capacity to synthesize [3H]triacylglycerol (from [3H]glycerol) followed the order: adult greater than day 6 greater than fetal. Addition of 1 mM oleic acid to the incubation media stimulated the incorporation of [3H]glycerol into triacylglycerol 6.7- and 3.6-fold by fetal and adult hepatocytes, respectively. After maximal stimulation by 1 mM oleic acid, triacylglycerol secretion by fetal cells was still only 39% of the amount secreted by adult cells that had been treated similarly. Fetal cells stimulated with 1 mM oleic acid synthesized the same amount of triacylglycerol as adult cells that had been treated with 0.1 mM oleic acid. However, the fetal cells secreted only one-third as much triacylglycerol, further demonstrating relatively impaired secretion of triacylglycerol. In order to determine whether low triacylglycerol secretion was associated with differences in apoB metabolism, cells were incubated with [35S]methionine and apoB was quantified after immunoprecipitation. Fetal cells synthesized and secreted nearly equal amounts of large molecular weight and small molecular weight apoB. In contrast, adult cells synthesized and secreted nearly twice as much small molecular weight apoB as large molecular weight apoB. Moreover, although fetal and adult cells secreted large molecular weight apoB at similar rates, adult cells synthesized and secreted small molecular weight apoB at rates that were nearly two times higher than fetal cells. These data suggest that the ability to assemble and secrete VLDL varies in parallel with the developmental expression of small molecular weight apoB. These studies also show the usefulness of the cultured rat hepatocyte model for examining the ontogeny and regulation of lipoprotein assembly/secretion.
Age Factors↗
Examination of bile acid negative feedback regulation in rats.
Recent data obtained using cultured rat hepatocytes showed that bile acids do not inhibit bile acid synthesis, whereas cholesterol concentrations vary in parallel with bile acid synthesis (Davis et al. (1983. J. Biol. Chem. 258: 4079-4082). This led us to re-evaluate in vivo experiments upon which the consensus that bile acid synthesis is primarily regulated by bile acid "negative feedback" is based. Infusion of taurocholate into either the jugular vein or duodenum of bile-diverted rats stimulated biliary cholesterol secretion and bile flow, but it did not inhibit bile acid synthesis. The lack of an inhibitory effect was evident using several different infusion rates of taurocholate. Even at the greatest rate of taurocholate infusion (25 mumol/(100 g.hr] there was no significant inhibition of bile acid synthesis. In contrast, infusing mevinolin (1 mg/hr), a potent competitive inhibitor of HMG-CoA reductase, almost completely inhibited bile acid synthesis and biliary cholesterol secretion. Since mevinolin did not affect bile flow, these results cannot be ascribed to bile secretory failure. Thus, while these studies suggest that taurocholate may not regulate bile acid synthesis directly via negative feedback, cholesterol is likely to act as a positive effector of bile acid synthesis.
Animals↗
Intrahepatic assembly of very low density lipoproteins. Rate of transport out of the endoplasmic reticulum determines rate of secretion.
To identify the rate-limiting step(s) in the hepatic production of very low density lipoproteins (VLDL), we investigated the intracellular distribution and rate of intracellular transport of de novo synthesized apolipoprotein B (apoB). For all secretory proteins examined (i.e. albumin, large molecular weight apoB, and small molecular weight apoB) the rough and smooth microsomes contained the majority of intracellular de novo synthesized protein, while the Golgi subfraction contained 10% or less. Pulse-chase analysis of the intracellular movement of apoB and albumin showed that the first order rate constant (in terms of half-life) describing the rate of movement out of the smooth and rough microsomes determined the overall rate of movement out of the cell. These data suggest that movement out of the endoplasmic reticulum, the site where VLDL is assembled, determines the overall rate of secretion. Furthermore, compared to albumin, the rate of intracellular transport of apoB was approximately two times slower suggesting that processing unique to VLDL apoB occurring in the endoplasmic reticulum was responsible. Additional studies show that essentially all of the de novo synthesized 35S-labeled albumin (produced from a pulse of [35S]methionine) lost from the cell during the chase period could be recovered in the culture medium. In contrast, much less of large molecular weight apoB (36%) and small molecular weight apoB (60%) was recovered in the culture medium. Since these cultured rat hepatocytes do not take up or degrade newly secreted apoB, these data suggest that a significant amount of apoB is degraded intracellularly.
Animals↗
Creativity in neurosurgical publications.
The publications of 50 of the first American neurosurgeons were studied, and a citation analysis was done for the period 1965 through 1979. Because creative authors and their papers influence the scholarship and technical course of a discipline such as neurosurgery, the number of citations to an author or publication is a good measure of creativity (the production of original ideas that are lasting and eminent). The 10 authors who had the largest number of citations of papers, chapters, and books were Penfield (3228), Cushing (2588), Dandy (1732), Bailey (1061), White (968), Matson (932), Ingraham (651), Scoville (604), Davidoff (540), and Elsberg (414). The most cited book was Epilepsy and the Functional Anatomy of the Human Brain, by Penfield and Jasper, and the most cited paper was Scoville and Milner's "Loss of recent memory after bilateral hippocampal lesions." Authors who wrote more papers were found to have a greater total number of citations for these papers, but did not average more citations per paper than less productive colleagues. Neurosurgeons with research experience were more productive but not more creative than were more clinically oriented authors. The 5-year creative peak of 7 of the 10 most creative neurosurgeons occurred after the age of 50. The subjects on which the 50 neurosurgeons wrote the most papers were central nervous system tumors, trauma, and the basic neurosciences; these subjects also were cited most frequently. The proximity of the date of publication to the period of citation analysis resulted in a moderately better chance of a paper being cited, but more recent papers did not average more total citations.
Creativity↗
Regulation of bile acid synthesis in cultured rat hepatocytes: stimulation by apoE-rich high density lipoproteins.
Cultured rat hepatocytes obtained by liver perfusion with collagenase in the presence of soybean trypsin inhibitor were used to examine the role of high density lipoproteins (HDL) in supplying cholesterol to the hepatocyte for bile acid synthesis. Within 6 hr of adding HDL (d 1.07-1.21 g/ml) obtained from rat serum there was a significant stimulation of bile acid synthesis and secretion that reached 2-fold after 24 hr. The stimulation by HDL occurred at normal plasma concentrations (i.e., 500 micrograms/ml) and showed further stimulation in a dose-dependent manner reaching a maximum stimulation of 2- to 2.5-fold. The stimulation of bile acid synthesis was dependent on the cholesteryl ester content of the HDL. Several lines of evidence show that the HDL is taken up by a receptor-mediated process dependent on apoE. These include: 1) at the same concentration (500 micrograms/ml) apoE-poor HDL (not retained by heparin affinity chromatography of HDL isolated from the plasma of rats fasted for 72 hr stimulated bile acid synthesis by 48%, whereas apoE-rich HDL stimulated bile acid synthesis by 110%; 2) reductive methylation totally blocked the stimulation of bile acid synthesis by HDL; 3) HDLC, which contained apoE as its major protein component, also maximally stimulated bile acid synthesis; and 4) human HDL, which contained no detectable apoE, failed to stimulate bile acid synthesis. Additional studies showed that apoE-enriched HDL and HDLC both inhibited cholesterol synthesis (determined by the incorporation of 3H2O) and caused a net accumulation of cholesteryl esters in hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Regulation of bile acid synthesis via direct effects on the microsomal membrane.
Rats treated with ethinylestradiol (5 mg kg-1 day-1 for 5 days) secrete de novo synthesized bile acids at a markedly reduced rate (-57%). Administration of the nonionic detergent Triton WR-1339 to estradiol-treated rats rapidly restored the rate of secretion of de novo synthesized bile acids to control levels. In contrast, when Triton was administered to control rats, the secretion rate of bile acids was unaffected. The reduction in bile acid synthesis displayed by estradiol-treated rats was similar to the 50% decrease in the activity of hepatic microsomal 7 alpha-hydroxylase. The activity of 7 alpha-hydroxylase was also restored to control levels by the administration of Triton to estradiol-treated rats. We examined the possibility that estradiol acts directly on the hepatic microsomes. Adding increasing amounts of estradiol to microsomes obtained from control rats resulted in decreasing activities of 7 alpha-hydroxylase. The inhibition by estradiol of 7 alpha-hydroxylase obtained in vitro occurred with amounts of estradiol that were found to accumulate in the liver via in vivo treatment. Double-reciprocal analysis showed that at and below 50 micrograms of estradiol/0.5 mg of protein uncompetitive inhibition was displayed. Additional experiments showed that adding Triton to microsomes obtained from estradiol-treated rats increased the activity of 7 alpha-hydroxylase to control levels. In contrast, Triton did not increase the activity of 7 alpha-hydroxylase when it was added to control microsomes. These data show for the first time that the estrogenic steroid estradiol acts directly on the microsomes and inhibits both the activity of 7 alpha-hydroxylase and the rate of bile acid synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
The charge polymorphism of rat apoprotein E.
Rat apolipoprotein E (apo-E) exists in plasma as four unique isoelectric forms (designated E-1, E-2, E-3, or E-4 from acidic to basic, respectively). We have examined the processes accounting for this polymorphism using intact rats or cultured rat hepatocytes. Intrahepatic precursors of rat apo-E were isolated and analyzed on isoelectric focusing gels. The primary translation product of rat liver apo-E mRNA focused as two isoproteins with more basic pI values than the isoproteins of plasma apo-E. The microsome-processed translation product also focused as two isoproteins having pI values corresponding to apo-E-4 and apo-E-3 isoproteins of plasma apo-E. Following a bolus injection of [3H]leucine into the portal vein, intrahepatic isoproteins corresponding to plasma apo-E-2 and apo-E-1 isoproteins were first detected in the rough endoplasmic reticulum (RER) and Golgi fractions, respectively. The apparent molecular weight of intrahepatic apo-E increased as it passed from the RER to the Golgi. Only the most acidic isoform, apo-E-1, of plasma apo-E was sensitive to neuraminidase treatment indicating that sialic acid residues are responsible, in part, for the polymorphism of rat apo-E. Using cultured hepatocytes, tunicamycin (1 microgram/ml) inhibited the incorporation of [3H]glucosamine into both molecular weight forms of apolipoprotein B but did not influence the synthesis, glycosylation (as measured by [3H]glucosamine incorporation), or secretion of apo-E. Tunicamycin-inhibited hepatocytes secreted the normal complement of apo-E isoforms including apo-E-1, thus confirming that apo-E-1 is not an N-linked glycoprotein. These results suggest that post-translational modifications involving both RER and Golgi-specific reactions contribute to the polymorphism of rat apo-E.
Animals↗
Lipoprotein synthesis and secretion by cultured hepatocytes.
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Acetates↗
Phosphorylation-dephosphorylation of apolipoprotein B.
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Animals↗
The determination of nicotine and cotinine in plasma.
A method to determine plasma nicotine and cotinine simultaneously is described. After a simple extraction procedure, the amounts of nicotine and cotinine present in a sample are determined by gas chromatography using nitrogen-sensitive detection and a fused silica capillary column. The method has been demonstrated to detect nicotine and cotinine plasma concentrations within 1 and 9 ng/ml, respectively. The accuracy and precision of the method was demonstrated using spiked calf and human sera. Data are presented for the direct comparison of the present method with other methods of determination for plasma nicotine and cotinine.
Chromatography, Gas↗
Testing Kohn's self-reliance hypothesis among high school adolescents.
Melvin Kohn and his associates have established that self-reliance promotes a sense of well-being (lower levels of fatalism) among adults. The present study attempts to test this proposition among adolescents. Instead of the usual job-related activities, however, it incorporates an alternative measure of self-reliance--high school curriculum assignment. Consistent with previous findings among adults, the results of this study show that self-reliance among adolescents also promotes a sense of well-being: students enrolled in tracks where close supervision is kept to a minimum and the exercise of initiative is emphasized tended to exhibit lower levels of fatalism. The implications of this finding for adolescents are discussed.
Achievement↗
Intrahepatic assembly of very low density lipoproteins. Varied synthetic response of individual apolipoproteins to fasting.
Hepatocytes obtained from rats fed for 3 days chow (control) or drinking water only (fasted) were used to examine how metabolic state affects lipogenesis, apolipoprotein synthesis, and the capacity to secrete de novo synthesized triacylglycerol. The secretion of triacylglycerol (mass and 3H-labeled via 3H2O incorporation) by both groups of cells was constant for 30 h. Moreover, cells from fasted rats secreted triacylglycerol at rates which were markedly reduced (mass -84%; 3H-labeled -91%). To assess the relative capacities of the two groups of hepatocytes to augment triacylglycerol secretion in response to stimulated lipogenesis, cells were incubated with increasing concentrations of glucose. Control cells responded to glucose by increasing equally the synthesis and secretion of [3H] triacylglycerol. When cells from fasted rats were challenged with glucose, triacylglycerol secretion was not increased. Rather, it accumulated intracellularly. Double-reciprocal plot analysis of the capacity to augment triacylglycerol secretion in response to glucose showed that cells from fasted rats had a greater than 10-fold decrease in V'max. Moreover, fasting changed the synthesis and secretion of apolipoproteins selectively: secretion of low molecular weight apo-B was decreased 50%, large molecular weight apo-B was unchanged, and apo-E was increased 2-4-fold. Analysis of the lipoproteins from both groups of cells on Bio-Gel A-50m showed that the very low density lipoprotein secreted by cells from fasted rats was smaller. In addition, all of the increased de novo synthesized apo-E secreted by cells from fasted rats eluted after the triacylglycerol-rich lipoproteins. The combined data show that: 1) the synthesis of individual very low density lipoprotein apolipoproteins is independently regulated, and 2) the synthesis (availability) of apo-B determines the capacity of the hepatocyte to assemble/secrete triacylglycerol-rich very low density lipoprotein.
Animals↗
Dietary cholesterol does not affect the synthesis of apolipoproteins B and E by rat hepatocytes.
To examine the unproved hypothesis that dietary cholesterol affects the synthesis of apolipoprotein B and E, we fed rats a cholesterol-rich diet that has been shown to alter dramatically the serum concentrations of these apolipoproteins. Rats fed for 4 weeks on a cholesterol-rich diet accumulate increased concentrations of low Mr apolipoprotein B (+2.7-fold) and decreased concentrations of apolipoprotein E (-40%) in their serum. Hepatocytes obtained from similarly treated rats were placed in monolayer culture and the rate of synthesis de novo of apolipoproteins was determined. Although cells from cholesterol-fed rats remained filled with lipid droplets throughout the experimental period, there was no difference in plating efficiency or viability, compared with cells obtained from chow-fed control rats. Both groups of cells synthesized and secreted immunoprecipitable apolipoproteins B and E at similar rates throughout the 18 h experiment. Thus there was a discordance between the effects of dietary cholesterol on serum apolipoprotein concentrations and hepatocyte synthesis and secretion. The data indicate that altered hepatic apolipoprotein synthesis cannot account for the changes in serum apolipoprotein concentrations caused by dietary cholesterol.
Animals↗
Lipoprotein and apolipoprotein distribution in Zucker rats following an endurance running program.
We examined the response to 12 weeks of endurance running of the obese Zucker rat and its lean littermate with regard to changes in serum lipids, lipoproteins and apoproteins. The obese Zucker rat is hyperlipoproteinemic, characterized by elevated serum triglyceride and cholesterol levels primarily associated with the very low density lipoprotein (VLDL) fraction. In lean Zucker rats, training did not affect the concentrations of serum lipids or apolipoproteins. In marked contrast, obese Zucker rats that were trained had significant decreases in serum concentrations of triglycerides, free and esterified cholesterol, and apolipoprotein B compared to their sedentary counterparts. Training obese rats caused an increase in the serum concentration of apolipoprotein E (HDL fraction). In contrast, training did not affect the concentration of Apo E in lean rats. The VLDL fraction was most affected by training obese rats showing marked 50-65% decreases in VLDL triglyceride and VLDL cholesterol. HDL cholesterol was unchanged in lean rats whereas training prompted a 29% increase in obese rats. These data show that exercise training altered the metabolic abnormalities of obese Zucker rats which are responsible for the accumulation in serum of VLDL, lipids and apolipoproteins.
Animals↗
The use of computerized axial tomography versus peritoneal lavage in the evaluation of blunt abdominal trauma: a prospective study.
Computerized axial tomography (CAT) of the abdomen was prospectively compared with peritoneal lavage (PL) in 19 patients who sustained acute blunt abdominal trauma. All were selected carefully and were deemed stable, never having been in shock, and all required diagnostic PL. Seven patients underwent exploratory laparotomy on the basis of CAT and PL findings. There were no negative findings at laparotomy. Three significant splenic injuries, one hepatic laceration, and two hemoperitoneums were undetected by CAT. All seven cases explored were preceded by a grossly positive PL. Thus no major injury would have been missed if PL had been used alone. There were no complications of PL but one patient aspirated oral contrast medium and one patient developed hypotension during CAT. Open PL required one half the time (20.6 minutes) as CAT (47.4 minutes). The total cost of CAT was approximately eight times that of PL ($900.01 versus $116.38). In our hands, PL would seem to be significantly more sensitive with fewer false negative results than CAT of the abdomen in acute blunt abdominal trauma, a deviation from results of earlier reported series. CAT alone would have added cost, time, some risk, and very little information of use that would not be obtained by PL followed by surgery. Therefore before CAT replaces PL in the evaluation of adult patients with blunt abdominal trauma, we feel that additional prospective studies are needed to better define the accuracy and sensitivity of CAT as compared with PL with regard to intraperitoneal injuries.
Abdominal Injuries↗
Intrahepatic assembly of very low density lipoproteins. Phosphorylation of small molecular weight apolipoprotein B.
The possibility that apo-B is phosphorylated was examined using cultured rat hepatocytes. Rabbit antiserum prepared against rat apo-B was found to specifically react with both large and small molecular weight apo-B (by electroblotting assay and by immunoprecipitation of [35S]methionine-labeled proteins synthesized and secreted by hepatocytes). Following a 4-h incubation with [35P]orthophosphate, immunoprecipitation, and sodium dodecyl sulfate electrophoresis, an autoradiographic band corresponding to small molecular weight apo-B was obtained from cells and medium. Compared to the relative abundance of 32P which was associated with secreted small molecular weight apo-B, there was little (if any) detected in large molecular weight apo-B. Addition of excess unlabeled apo-B (obtained from rat serum) totally competed with the specific antiserum for this radioactive protein, indicating it was antigenically related to apo-B. Moreover, isolation of the 32P-labeled apo-B electrophoretic band, followed by acid hydrolysis and phosphoamino acid analysis, showed that at least 20% of the 32P originally associated with small molecular weight apo-B was in the form of phosphoserine. Control experiments ruled out the possible contamination of apo-B with phospholipid as well as the possibility that the phosphoserine produced by acid hydrolysis could have been derived from phosphatidylserine. To examine the relevance of these data to the in vivo state, rats were injected with [32P]orthophosphate. Immunoprecipitation of their livers followed by autoradiographic analysis showed the presence of 32P in small molecular weight apo-B. These data show for the first time that small molecular weight apo-B is synthesized as a phosphoserine containing protein.
Amino Acids↗