Biomedical subjects

S D Clarke

Publications and source records attributed to S D Clarke.

At least 73 records · Page 4Linked to original sources

PubMed1990

Suppression of rat hepatic fatty acid synthase and S14 gene transcription by dietary polyunsaturated fat.

The objective of this research was to determine whether dietary polyunsaturated fatty acids suppress hepatic fatty acid synthase (FAS) mRNA levels by altering FAS gene transcription. Male Sprague-Dawley rats were meal-fed for 10 d a high glucose diet supplemented with 20% digestible energy as menhaden oil or tripalmitin. The transcription rate for FAS was determined by nuclear run-on analysis in hepatic nuclei isolated from rats 2 h postmeal. The values for transcription rates of FAS and S14 (a putative lipogenic protein) in rats fed menhaden oil were only 6 and 21%, respectively, of the rates in rats fed the tripalmitin diet (p less than 0.02). Gene transcription for beta-actin and phosphoenolpyruvate carboxykinase did not differ between treatments. The reduction in hepatic FAS mRNA levels caused by dietary polyunsaturated fats appears to be caused primarily by an inhibition of FAS transcription. The control of transcription by polyunsaturated fats appears not to be mediated by cAMP because the transcription rate for phosphoenolpyruvate carboxykinase (whose gene is very sensitive to cAMP stimulation) was unaffected by the source of dietary fat.

Animals↗

PubMed1990

Nutritional control of rat liver fatty acid synthase and S14 mRNA abundance.

The objective of this research was to evaluate the change in abundance of S14 and fatty acid synthase (FAS) mRNAs under a variety of nutritional conditions to evaluate the hypothesis that the regulation of the S14 gene is similar to that of other proteins involved in lipid metabolism and that changes in S14 expression are comparable to those that occur in FAS expression. Livers from rats fed a high carbohydrate diet were found to contain 350- and 100-fold more S14 and FAS mRNA than livers from rats fasted for 48 h. Although feeding a high fat diet increased S14 and FAS mRNA above fasting (P less than 0.05), the level of S14 and FAS mRNAs was only 5% and 4%, respectively, of the amount in the high carbohydrate group. Both S14 and FAS mRNAs accumulated quickly upon intubation of fasted rats with a solution of sucrose. The earliest rise in these mRNAs occurred within 60 min; by 240 min after gavage, each mRNA had increased 30-fold. The rapid induction of FAS and S14 mRNAs was also observed during ingestion of a high glucose meal. Hepatic FAS and S14 mRNA decreased 80-90% and 60%, respectively, during the 21-h interval between meals. This degree of mRNA loss was estimated to require a half-life for FAS and S14 mRNA of less than 8 h and less than 12 h, respectively. Regression analysis of the three dietary studies revealed a correlation coefficient for the relationship between S14 and FAS mRNA abundance ranging between 0.88 and 0.96.(ABSTRACT TRUNCATED AT 250 WORDS)

Adipose Tissue↗

PubMed1990

Dietary polyunsaturated fats uniquely suppress rat liver fatty acid synthase and S14 mRNA content.

The objective of these studies was to demonstrate that dietary polyunsaturated, but not saturated, fatty acids decrease mRNA abundance for fatty acid synthase (FAS) and S14. A series of experiments involving adult and weanling rats was designed to examine the ability of saturated (tripalmitin), (n-9) monounsaturated (triolein), (n-6) di-unsaturated (safflower oil), and (n-3) polyenic (fish oil) fatty acids to suppress the gene expression of FAS and S14. Dietary polyunsaturated fats reduced by 75-90% the hepatic abundance of FAS and S14 mRNA. Fish oil, rich in 20- and 22-carbon polyenic fatty acids, was more effective than safflower oil, whereas tripalmitin and triolein were without effect. Polyunsaturated fats were also very effective at preventing the rise in FAS and S14 mRNA associated with weaning. The inhibitory action of polyunsaturated fat was rapidly (less than 3 h) removed by deleting the fat from the meal. Regression analysis revealed a high correlation (0.81-0.94) between FAS and S14 expression among the various dietary studies. These data support the hypothesis that dietary polyunsaturated fats uniquely regulate the gene expression of lipogenic enzymes and that the mediator is likely a specific entity derived from the long-chain polyenic fatty acids. Moreover, the high correlation between FAS and S14 expression supports the hypothesis that S14 is a member of the lipogenic protein family and has potential as a model gene for the study of FAS expression.

Animals↗

PubMed1990

Potency of polyunsaturated and saturated fats as short-term inhibitors of hepatic lipogenesis in rats.

Three dietary studies using male Sprague-Dawley rats conditioned to meal-eat a high glucose, fat-free diet and one in vitro study with isolated rat hepatocytes were designed to examine the hypothesis that polyunsaturated fats (i.e., safflower oil or linoleate) are more potent acute inhibitors of liver fatty acid synthesis than are saturated fats (i.e., beef tallow or palmitate). Fat in the first in vivo study was administered via intubation (1500 mg/rat) whereas in the second and third in vivo studies fat was added to the meal in amounts of 50, 100, 250 or 500 mg/g fat-free diet. When the rats were in a postprandial condition, significant suppression of hepatic lipogenesis required the meal to contain 38% of its energy as fat (i.e., 250 mg/g fat-free diet). At this level of fat, safflower oil was more inhibitory than beef tallow (p less than 0.05). The inhibition constant (Ki) for palmitate inhibition of fatty acid synthesis by isolated hepatocytes was fourfold greater than linoleate's Ki (fatty acid/albumin ratio of 1.4/1). When fat constituted 50% of the ingested energy, beef tallow was equivalent to safflower oil as an inhibitor of lipogenesis. Although a single meal containing 50 mg safflower oil/g fat-free diet did not decrease fatty acid synthesis, it effectively delayed the induction of lipogenesis during the first 30 min of the adaptive decrease in lipogenic enzymes attributed to polyunsaturated fats extends to short-term regulatory mechanisms.

Animals↗

PubMed1990

Inhibition of triiodothyronine's induction of rat liver lipogenic enzymes by dietary fat.

The objective of these studies was to demonstrate that the reduction in lipogenic enzymes caused by ingestion of dietary polyunsaturated fat can in part be attributed to an inhibition of triiodothyronine's induction of hepatic lipogenic enzymes. A 3 x 4 factorial design was employed to examine the effect of diets containing no fat, beef tallow or safflower oil on the triiodothyronine-mediated (0, 2, 4 or 8 micrograms/100 g injected daily intraperitoneally) induction of rat liver lipogenic enzymes. Triiodothyronine (T3) administration induced (p less than 0.05) the activity of malic enzyme, fatty acid synthase and glucose-6-phosphate dehydrogenase in a dose-dependent manner. Malic enzyme activity was increased five- to sevenfold by 8 micrograms T3/100 g daily. Fatty acid synthase activity at the 8 micrograms dose had increased two- to threefold whereas glucose-6-phosphate dehydrogenase activity was elevated only 15-35%. Beef tallow and safflower oil supplementation of the high glucose, fat-free diet significantly reduced the T3 induction of all the enzymes. Safflower oil was more effective than tallow as a repressor of T3 action. The effect of dietary fat, particularly safflower oil, was to increase the amount of T3 required to induce the activity of lipogenic enzymes. In a second study, with a 2 x 2 factorial design, daily injection of 15 micrograms T3/100 g was found to overcome the safflower oil inhibition of lipogenic enzymes. These data support the hypothesis that polyunsaturated fats uniquely suppress the gene expression of lipogenic enzymes by functioning as competitive inhibitors of T3 action, possibly at the nuclear receptor level.

Animals↗

PubMed1990

Limits of medium-chain and long-chain triacylglycerol utilization by neonatal piglets.

Fifteen unsuckled neonatal piglets from three litters were fasted for 2 h after birth then allotted within litter to a 2 x 3 factorial arrangement of treatments and given by stomach tube 2.36, 4.91 or 11.32 g of either (1-14C)-triolein or (1-14C)-trioctanoin/BW (kg).75. The hourly 14CO2 production was measured for 12 h for estimating the oxidation rate of dosed triacylglycerols. The contents of each segment of the digestive tract were collected for 14C and lipid analyses for calculating absorption of the dosed triacylglycerols. Grams of trioctanoin absorbed (P less than .01) and oxidized (P less than .001) by piglets were higher than grams of triolein. The maximum amounts of trioctanoin and triolein absorbed in 12 h were 2.77 +/- .29 and 1.83 +/- .36 g/BW (kg).75, respectively. The maximum amounts of trioctanoin and triolein oxidized in 12 h were 1.60 +/- .14 and .33 +/- .17 g/BW (kg).75, respectively. The maximum amounts of trioctanoin and triolein oxidization during 1-h periods were .231 +/- .017 and .049 +/- .021 g/BW (kg).75, respectively. The calculated maximum percentage of piglets' daily maintenance energy needs supplied by trioctanoin oxidation was 42.0%, compared with 10.6% from triolein. The maximum utilization of trioctanoin occurred at the intermediate dose. Results indicated that trioctanoin was used more effectively than triolein; it may be useful as a source of supplemental energy to improve the survival of piglets. A single dose should provide about 6 g/body wt (kg).75.

Animals↗

PubMed1990

Focal brain lesions on computed tomography in patients with acquired immune deficiency syndrome.

Neurologic involvement is common in patients with acquired immune deficiency syndrome (AIDS). To assess the computed tomographic findings in AIDS, the authors reviewed 12 consecutive patients found to have focal brain lesions representing AIDS complications on computed tomography and having histologic or clinical proof of the diagnosis. Five patients had toxoplasmosis, six had lymphoma and one had both conditions sequentially. The finding of multiple lesions and mass effects with edema was more common with toxoplasmosis. In six patients who had toxoplasmosis 33 nodules were observed compared with only 8 in seven patients who had lymphoma. Disease extending linearly along the ventricular wall was typically seen in patients who had lymphoma.

Acquired Immunodeficiency Syndrome↗

PubMed1989

Digestion and absorption of fish oil by neonatal piglets.

In order to establish the digestibility and absorption of fish oil, 18 unsuckled, newborn piglets were tube-fed an amount of fish oil calculated to provide the piglets' maintenance energy needs for 12 h. After fat administration the piglets were either intubated with 25 ml of fat-free milk replacer per h, or fasted. The digesta of the stomach, small intestine, and large intestine (digesta plus feces) were collected 12 h after fish oil intubation. Disappearance of total extractable fat was 95 +/- 1 and 94 +/- 1% for fed and fasted piglets, respectively. True absorption of the lipid was calculated from the disappearance of eicosapentaenoic acid and was 99% for both fed and fasted piglets. Nearly 50% of the stomach lipid was as diglycerides and free fatty acids, suggesting that hydrolysis of the fish oil was initiated in the stomach. Small intestinal lipid was 63-65% free fatty acids, indicating that the piglet pancreatic lipase was of sufficient activity to hydrolyze fish oil triglyceride. Neonatal piglets appeared to have ample capacity to digest and absorb fish oil, which supports the concept that lipid supplementation could improve the energy status of the newborn piglet.

Absorption↗

PubMed1989

Lack of specificity of polyunsaturated fats in the inhibition of rat liver glucose-6-phosphate dehydrogenase.

The mechanism of the effect of polyunsaturated fatty acids (PUFA) on glucose-6-phosphate dehydrogenase (EC 1.1.1.49) (G6PDH) was studied in young, male Wistar rats. Starvation-refeeding increased G6PDH level above that seen in ad libitum-fed animals (enzyme overshoot). A second episode of starvation-refeeding produced even higher levels of G6PDH activity (induction increment). Interposing a high fat diet (containing PUFA) between starvation and feeding the inducer diet abolished one-half to two-thirds of the overshoot. Feeding a high fat diet between the two starvations abolished the induction increment. Inhibitors of arachidonic acid metabolism were not able to reverse the PUFA effect. In another set of experiments it was shown that both linoleic and linolenic acid are equally effective in either reducing the overshoot or abolishing the induction increment. The evidence was interpreted as supporting a hypothesis that the PUFA effect does not require the formation of a specific end product of arachidonic metabolism in a direct way.

Animals↗

PubMed1989

Cellular lipid binding proteins: expression, function, and nutritional regulation.

The membrane transport and cytosolic solubilization of hydrophobic ligands, including sterols, fatty acids, retinoids, and certain hydrophobic carcinogens, are facilitated by a group of similar low molecular weight proteins: plasma membrane transport protein, fatty acid binding proteins, sterol carrier protein, and retinoid binding proteins. The cellular content of these proteins, which establishes the capacity of a cell to utilize the various ligands, is determined by events regulating transcription and translation, e.g., the mRNA abundance of liver- and gut-type FABPs is increased by dietary fat, and translation of hepatic FABP appears to be stimulated by insulin. Functions attributable to these lipid binding proteins remain unclear, but data are presented that indicate physiological roles in 1) fatty acid transport, esterification, and oxidation, 2) steroidogenesis, and 3) retinoid uptake, retinaldehyde reduction, and retinol esterification. An exciting and novel prospect for cellular trafficking proteins is the role they may play in regulating gene expression. In this respect, cellular lipid binding proteins, e.g., retinoid binding proteins, may deliver their ligands to nuclear trans-acting proteins, and thereby modulate genes coding for key proteins involved in lipid metabolism or differentiation. Even though the functions of these proteins still need to be unequivocally established, it is clear that they are important in the overall homeostasis of lipid metabolism.

Animals↗

PubMed1989

Growth hormone acutely increases glucose output by hepatocytes isolated from hypophysectomized rats.

A series of experiments using isolated rat hepatocytes was carried out to establish rat liver cells in suspension as a physiological model for examining GH responses, and to determine whether acute recombinant bovine GH (rbGH) treatment of rat liver cells increased glucose output and/or suppressed fatty acid synthesis from lactate. Rat liver cells were isolated by collagenase perfusion and incubated in short-term (less than 60 min) suspension. The amount of insulin, glucagon or vasopressin required to elicit a half-maximal response was within the physiological range of the circulating hormone. When hepatocytes from normal rats were acutely (less than 60 min) treated with 0, 0.1, 10, 100 or 1000 nmol rbGH/l, rates of hepatocyte glucose output and fatty acid synthesis were unaltered. In addition, acute rbGH treatment (1000 nmol/l) did not alter hepatocyte responsiveness to insulin or vasopressin. However, acute rbGH treatment of hepatocytes isolated from hypophysectomized rats significantly (P less than 0.05) increased the rate of glucose output twofold and moderately (P less than 0.10) enhanced fatty acid synthesis. The accelerated rate of glucose production was not accompanied by an increase in the amount of glycogen phosphorylase-a. The observations with liver cells from hypophysectomized rats are not consistent with a GH receptor-transducing mechanism which is like that for glucagon (adenylate cyclase-linked) or insulin (tyrosine kinase-linked).

Animals↗

PubMed1988

Free fatty acid inhibition of the insulin induction of glucose-6-phosphate dehydrogenase in rat hepatocyte monolayers.

Rat hepatocytes in monolayer culture were utilized to determine if the decrease in glucose-6-phosphate dehydrogenase (G6PD) activity resulting from the ingestion of fat can be mimicked by the addition of fatty acids to a chemically, hormonally defined medium. G6PD activity in cultured hepatocytes was induced several-fold by insulin. Dexamethasone or T3 did not amplify the insulin induction of G6PD. Glucose alone increased G6PD activity in cultured hepatocytes from fasted donors by nearly 500%. Insulin in combination with glucose induced G6PD an additional two-fold. The increase in G6PD activity caused by glucose was greater in hepatocytes isolated from 72 hr-fasted rats as compared to fed donor rats. Such a response was reminiscent of the "overshoot" phenomenon in which G6PD activity is induced well above the normal level by fasting-refeeding rats a high glucose diet. Addition of linoleate to the medium resulted in a significant suppression of insulin's ability to induce G6PD, but linoleate had no effect on the induction of G6PD activity by glucose alone. A shift to the right in the insulin-response curve for the induction of G6PD also was detected for the induction of malic enzyme and acetyl-CoA carboxylase. Arachidonate (0.25 mM) was a significantly more effective inhibitor of the insulin action than linoleate was. Apparently rat hepatocytes in monolayer culture can be utilized as a model to investigate the molecular mechanism by which fatty acids inhibit the production of lipogenic enzymes. In part, this mechanism of fatty acid inhibition involves desensitization of hepatocytes to the lipogenic action of insulin.

Acetyl-CoA Carboxylase↗

PubMed1988

Fetal growth and fetal lung phospholipid content in rats fed safflower oil, menhaden oil, or hydrogenated coconut oil.

The objective was to determine if dietary fish oil decreased the degree of fatty acid saturation in rat lung phosphatidylcholine (PC). A diet containing 12% of its energy as fat was fed for 3 wk to growing male Sprague-Dawley rats (trial I) or to pregnant rats for days 8-21 of gestation (trial II). The dietary fat treatments in trial I were safflower oil (SO), menhaden oil (MO), or hydrogenated coconut oil (HCO) and in trial II were SO, MO, HCO, or SO-MO (75%:25%). Polyunsaturated fatty acids reduced (p less than 0.05) hepatic fatty acid synthetase (MO greater than SO) in growing rats but the dietary lipids had no effect on lung palmitate content. Maternal consumption of MO vs SO reduced (p less than 0.05) fetal body weight and lung weight but not lung:body wt ratio. Dietary MO and SO-MO increased (p less than 0.05) disaturated PC content of fetal lungs. The fetal lung data indicate that maternal ingestion of fish oil improve fetal lung maturation.

Animals↗

PubMed1988

Phrenic nerve function in patients with diaphragmatic weakness and systemic lupus erythematosus.

Diaphragmatic weakness has been identified as one of the pulmonary manifestations of systemic lupus erythematosus. Whether this weakness results from a neuropathic or myopathic process has not been established. Thirty patients with SLE were screened for the presence of inspiratory muscle (IM) weakness. Detailed studies were performed in nine with IM weakness. All nine were found to have diaphragmatic weakness (mean +/- SD, maximal transdiaphragmatic pressure 50 +/- 12 cmH2O). Phrenic nerve latencies, evaluated using transcutaneous stimulation, were normal in all individuals excluding a demyelinating neuropathy. Compound diaphragm action potential (CDAP) with phrenic nerve stimulation was normal in six of these nine patients. Reduced CDAP in three of nine patients was consistent either with axonal degeneration of the phrenic nerve or diaphragm myopathy. Nerve conduction and electromyographic studies on peripheral nerves and muscles respectively failed to demonstrate an associated generalized neuropathy or myopathy. We conclude that diaphragmatic weakness in patients with SLE is both common and is very unlikely to be caused by a phrenic neuropathy.

Action Potentials↗

PubMed1987

Modification of fatty acid composition of membrane phospholipid in hepatocyte monolayer with n-3, n-6 and n-9 fatty acids and its relationship to triacylglycerol production.

The objective of these studies with rat hepatocytes in primary culture was to establish that: (a) membrane phospholipids would become enriched with the specific fatty acid supplemented to the media and (b) hepatocyte monolayer triacylglycerol synthetic rates were dependent on the type of fatty acid enrichment of the membrane phospholipids. Hepatocytes cultured in the absence of media lipid developed a phospholipid fatty acid composition which is indicative of an essential fatty acid deficiency. The extensive rise in 18:1(n - 9) content indicated that delta 9-desaturase was active. The fatty acid composition of phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol in the microsomal- and mitochondrial-enriched fractions was highly dependent upon the type of fatty acid supplemented to the medium. Incorporation of fatty acids into phospholipids was rapid, and a new steady-state in fatty acid composition was achieved within approx. 36 h. Changes in the fatty acid composition of these hepatocyte phospholipid subclasses resulting from media supplementation with 18:2/20:4(n-6) or 20:5(n-3) were similar, but not identical, to changes which occurred in vivo as a result of consuming diets rich in 18:2(n-6) or 20:5(n-3). Hepatocyte lipogenesis was highly dependent upon the type of fatty acid supplemented to the medium. Prior conditioning with 16:0 increased triacylglycerol synthesis and secretion. Secretion of triacylglycerol was reduced by polyenoic fatty acid enrichment with 20:5(n-3) greater than 20:4/18:2(n-6). The suppression of triacylglycerol synthesis by 20:5(n-3) was due to an increased (P less than 0.05) diacylglycerol specific activity, which indicates that 20:5(n-3) suppression of hepatic triacylglycerol production may be caused in part by the inhibition of diacylglycerol acyltransferase.

Animals↗

PubMed1986

Hormonal triggering of the diurnal variation of sterol carrier protein.

Rat liver sterol carrier protein (SCP) is a major intracellular protein regulating lipid metabolism and transport. During a dark-light cycle, SCP undergoes a dramatic diurnal variation in synthesis and level, reflecting translational events. Several hormones participate in the control of SCP synthesis. Insulin was implicated when the circadian rhythm of SCP was lost in both diabetes and fasting, states where insulin is low. After a 12-h fast the amplitude of the diurnal rhythm is diminished; after a 48-h fast it disappears, although SCP synthesis and level remain high. When endogenous insulin secretion is increased in fasted rats by glucose administration, SCP increases 2-fold in less than 30 min. When food intake is manipulated, but the dark-light cycle is unchanged, the circadian rhythm of SCP corresponds to feeding patterns and not light cycling. During feeding, increases in SCP are triggered following the expected increase in serum insulin. However, SCP is rapidly and significantly elevated in response to insulin only when glucocorticoids are normally high or increased by injection of the synthetic glucocorticoid, dexamethasone. Hepatocyte SCP levels are also induced by a combination of insulin and dexamethasone (2.3-fold) or insulin alone (1.3-fold). Dexamethasone alone causes a striking depression of SCP (2.4-fold). Thus, insulin is a major regulator of the diurnal variation of SCP synthesis. Glucocorticoids and other hormones (e.g. triiodothyronine) are also essential for maximum induction of SCP but play permissive roles.

Animals↗

PubMed1986

Fatty acid inhibition of hormonal induction of acetyl-coenzyme A carboxylase in hepatocyte monolayers.

Primary cultures of adult rat hepatocytes were utilized to ascertain the impact of free fatty acids on the insulin plus dexamethasone induction of acetyl-CoA carboxylase. Lipogenesis was induced threefold by the combination of insulin and dexamethasone. The rise in fatty acid synthesis was accompanied by a comparable increase in the rate-determining enzyme acetyl-CoA carboxylase. Dexamethasone was required for the insulin induction of acetyl-CoA carboxylase. Under the permissive action of glucocorticoid, 10(-7) M insulin maximally increased enzyme activity. Half-maximum stimulation occurred with 5 X 10(-9) M insulin. Media containing 0.2 mM palmitate, oleate, linoleate, arachidonate, or docosahexaenoate significantly suppressed the hormonal induction of acetyl-CoA carboxylase. The extent of suppression was only 30-35% and did not vary with chain length or degree of unsaturation. Carboxylase activity was not suppressed further by raising the concentration of linoleate to 0.5 mM; however, 0.5 mM palmitate depleted the cells of ATP and abolished acetyl-CoA carboxylase activity. Therefore, based upon the inhibitory characteristics of the various fatty acids and the lack of a concentration dependency of the fatty acid inhibition, it would appear that fatty acid inhibition of the induction of acetyl-CoA carboxylase activity may not be a direct, physiological regulatory mechanism.

Acetyl-CoA Carboxylase↗

PubMed1986

Inhibition of liver lipogenesis by dietary polyunsaturated fat in severely diabetic rats.

Diabetic and nondiabetic rats were used to ascertain if dietary polyunsaturated fats inhibited hepatic acetyl-CoA carboxylase and fatty acid synthetase in insulin-insufficient rats as had been previously shown for normal rats. Male rats were rendered diabetic (400-600) mg glucose/100 mL blood) with streptozotocin and were fed a high fructose fat-free diet. Safflower oil or palmitate (or tallow) was added to the basal fructose diet at a level to supply 12,24 or 36% additional digestible energy. Compared with normal rats, diabetic rats had significantly lower hepatic fatty acid biosynthesis, but the proportion of acetyl-CoA carboxylase expressing catalytic activity as determined by the avidin-inactivation technique was unaffected by diabetes. Diabetes did not lower the maximal maximal activities of carboxylase and fatty acid synthetase. Moreover, the activities of these enzymes greatly exceeded the rate of fatty acid synthesis. At all levels of fat supplementation, the high linoleate safflower oil consistently resulted in a 50% lower rate of fatty acid biosynthesis than did comparable levels of tallow or palmitate. Safflower oil was also a more effective suppressor of the activities of acetyl-CoA carboxylase and fatty acid synthetase than the saturated fats. Our data suggest that the greater inhibition of hepatic fatty acid biosynthesis by polyenoic fatty acids is an insulin-independent mechanism.

Acetyl-CoA Carboxylase↗