The use of in vivo-in vitro labeling techniques to study phospholipid fatty acid turnover and fatty acid esterification into triglycerides in adipose tissue of aging mice

We are interested in membrane phospholipid and triglyceride synthesis and turnover in aging cells. As a preliminary, short-term feasibility study we have used an established in vivo-in vitro technique to estimate the initial rates of [1-14C] palmitate (complexed to albumin) esterification to triglycerides and phospholipids in adipocytes and non-adipocytes in the epididymal fat pads of aging mice (8-92 weeks). We have expressed our data in terms of unit cell, unit triglyceride mass and unit (membrane) phospholipid mass. Fat pad and adipocyte size, cell surface area, and adipocyte volume changes were measured and found to follow the same relations as reported in the literature, with some exceptions in very old mice (retired breeders). Rates of fatty acid esterification to triglycerides were about 100 times faster than those to phospholipids in adipocytes. Aging caused a marked fall in the rates of triglyceride fatty acid formation from added palmitate; thus, the rate of fatty acid esterification to triglycerides fell from 0.75 to 0.13 nequiv. fatty acid per min per fat pad (youngest most active group, cf. oldest group). Esterification of fatty acids into phospholipids in adipocytes of the oldest mice was significantly lower than in those of the young and middle-aged groups. Contamination of adipocytes by non-adipocytes was observed in fat pads from old, but not from young, mice. The non-adipocytes accounted for about half of the phospholipid fatty acid esterification. The rate of phospholipid esterification was so slow in adipocytes (all ages) and so relatively fast in non-adipocytes that further studies of phospholipid fatty acid turnover in adipocytes using this system are not considered feasible, especially as a means for studying removal rates of autoxidized fatty acids from membrane phospholipids in vivo during aging.

Essential and nonessential fatty acid oxidation in mice bearing Ehrlich ascites carcinoma

We tested the hypothesis that mobilized (essential) free fatty acids (FFA) are spared from oxidation in cancer-bearing animals. We injected tracers [1-14C] linoleate, [1-14C] palmitate and NaH14CO3 intravenously as single rapid doses in separate groups of mice bearing Ehrlich ascites tumor (EAT) and controls, and measured breath 14CO2. The data from NaH14CO3 injections were used to develop kinetic, compartmental models of the HCO3--CO2 systems. These models were integrated with our earlier model of plasma FFA turnover for control and EAT-bearing mice. The integrated multicompartmental models were then fitted to breath 14CO2 data from mice injected with tracer FFA to compare the rates of FFA oxidation. FFA were not spared from an oxidative fate in our cancer-bearing vs normal animals; moreover, essential FFA were not preferentially spared from oxidation compared to non-essential FFA in the cancer-bearing mice.

Delirium induced by topical application of podophyllin: a case report

The authors describe a patient who developed delirium secondary to the topical absorption of podophyllin; this may be the first known case of delirium after topical application of the drug. Other serious medical and neurological complications of podophyllin poisoning are also discussed.

Hepatic contribution to newly made fatty acids in adipose tissue in rats and inhibition of hepatic and extrahepatic lipogenesis from glucose by dietary corn oil

We have reexamined an earlier rat study in which the authors concluded that 60 min after [U-14C]-glucose injection half of labeled fatty acids found in adipose tissue had been made in liver and then transported to the adipose tissue. We have shown that even under conditions in which the lipogenic role of the liver is optimized (fed-refed rats on a fat-free, high-carbohydrate diet), almost none of the labeled fatty acids found in adipose tissue of rats 60 min after they were fed a labeled glucose test meal was derived from the liver. This conclusion was based experimentally on (a) the use of the blocking agent Triton WR 1339 to measure the total labeled triglyceride fatty acids (TGFA) synthesized and secreted by the liver in 60 min and (b) comparison of plasma TGFA-14C data with radioactivity found in liver and in adipose tissue in 60 min. Without using Triton WR 1339, mathematical, analysis of plasma TFGA-14C following the glucose test-meal leads one to the same conclusion: 97% of 14C-labeled fatty acids found in adipose tissue at 60 min was made in situ. Additional studies in rats established that the source of error in the earlier studies was an incorrect assumption that dietary corn oil could inhibit hepatic lipogenesis from glucose C without inhibiting fatty acid synthesis in adipose tissue. In our studies, 10% corn oil inhibited equally both hepatic and adipose tissue fatty acid synthesis from glucose C under conditions that precluded any significant transport of labeled TGFA-14C from liver to adipose tissue.

Secretion and turnover of very low density lipoprotein triacylglycerols in rats fed chronically diets rich in glucose and fructose

Very low density lipoproteins (VLDL) were isolated from serum after intravenous injection of rats with 1-14C-palmitic acid. These lipoproteins were in turn injected into tail veins of rats which had been fed ad libitum for 21 days on fat-free diets in which the source of carbohydrates was glucose or fructose. Groups of rats were killed at intervals up to 10 minutes after injection and the rates of decline of serum triacylglycerol (TG) and of serum VLDL-TG specific radioactivity were measured. The half-lives of VLDL-TG turnover were very short (approximately 1 minute in both groups) compared to those described previously for rats fed conventional diets or for fasted animals, but the higher plasma TG concentrations in fructose-fed rats were as reported elsewhere. From this information and the serum VLDL-TG concentrations in the two dietary groups, it was possible to estimate the rate of VLDL secretion from the liver which was found to be 75% greater in the fructose-fed rats. No differences were found in the total lipoprotein lipase activity in acetone powders of white adipose tissue from other rats fed fructose and glucose.

Apolipoprotein B is structurally and metabolically heterogeneous in the rat

Electrophoresis of rat apolipoprotein B (apoB) on 5% polyacrylamide gels in the presence of NaDodSO4 separates three major components: PI, which comigrates with human low density lipoprotein (LDL) apoB; PII, a slightly faster-moving satellite band; and PIII, which migrates somewhat more slowly than myosin heavy chain. The proportion of PIII decreases with increasing density of the parent rat lipoprotein, from 90% an 70%, respectively, in chylomicrons and very low density lipoproteins (VLDL), to 7% in the major LDL2 (density 1.038-1.063 g/ml) fraction. A major component that comigrates with rat PIII is a marker for human chylomicron apoB, being absent from human VLDL, intermediate density lipoprotein (IDL), and LDL. Preliminary immunological and peptide mapping data show that rat apoB PI and PIII are closely related structurally, with the latter possibly being a large fragment of the former. Both peptides are synthesized in rat liver and found in Golgi secretory vesicles. Kinetic tracer experiments show that rat PI and PIII are present on separate VLDL particles, both of which are extensively removed from the circulation at the remnant stage, and that the declining PIII-to-PI/II ratios in IDL and LDL may be attributed to the more rapid turnover of PIII-containing lipoproteins at all levels, particularly within the LDL density range.

Comparison of lipogenic responses to dietary glucose in selected mouse adipose tissues

Rates of fatty acid synthesis from glucose-carbon (glucose-C) and all other 2-carbon (2-C) units were measured in three white adipose tissues sites of fasted and fed mice, which were given a test meal containing [U-14C]glucose. Total fatty acid synthesis was measured in all mice by intraperitoneal injection of 3H2O. In fasted-refed mice the rates of lipogenesis from glucose-C and all 2-C sources were much faster in popliteal than in epididymal fat. Most of the newly synthesized fatty acids were derived from glucose-C. However, in fed-refed mice these differences between the sites were minimal, and all the absolute rates were much higher than those found in fasted-refed mice. This suggested that the adipose tissue in the three sites did not have different physiological roles. Variability in the actual rates of lipogenesis, from one experiment to another, in fasted-refed mice could be attributed to small differences in the periods of fasting, before the mice were given the test meal.

Dietary control of lipogenesis in vivo in host tissues and tumors of mice bearing Ehrlich ascites carcinoma

We have determined rates of fatty acid (FA) synthesis from glucose carbon and all two-carbon units in control mice and in mice bearing Ehrlich ascites carcinomas. Using [U-14C]glucose and 3H2O as tracers under three nutritional conditions (24-hr fasted, 24-hr fasted-refed, and ad libitum fed-refed), we found that lipogenic regulatory mechanisms in adipose tissue and livers of mice bearing advanced tumors were similar to those of control mice. FA synthesis from glucose carbon and from all two-carbon units in livers of tumorous (8-day tumors) mice was at least as fast as that in control mice in the fasted and fasted-refed states but only about one-half that of controls in the fed-refed condition. The rate of FA synthesis from two-carbon units in carcasses of mice with 8-day tumors was not significantly different from that of controls in any of the 3 dietary states studied; however, in fed-refed mice with 8-day tumors, the rate of FA synthesis in the whole body was only one-half that of controls. The rate of FA synthesis from glucose carbon in carcasses of these tumorous mice was significantly depressed compared to that of controls in both the 24-hr-fasted and the fed-refed states. In well-nourished mice with early (5-day) tumors, the whole-body lipogenic rate from all two-carbon units was not depressed. Thus, decreased lipogenesis observed in host tissues of mice with advanced tumors is due to malnourishment; this secondarily depressed lipogenic activity probably contributes significantly to the loss of body fat that may occur at later stages of tumor growth. De novo FA synthesis in Ehrlich ascites cells, although small compared to that of the whole-body rate, was substantial in relation to lipids needed for tumor nutrition.

Fatty acid synthesis in vivo and hepatic contribution to whole-body lipogenic rates in obese Zucker rats

We have re-examined the claim by Godbole and York, based on the effect of surgical hepatectomy (Diabetologia 14:191, 1978), that liver contributed more than 90% of the newly synthesized FA found in adipose tissue of obese rats at the end of a 1-hr pulse of 3H2O. The amount of newly synthesized FA transported via plasma VLDL from liver to adipose tissue was estimated in lean and obese Zucker rats by determining the effects of Triton WR-1339, which blocks the uptake of VLDL-TGFA into tissues. Triton treatment was found not to cause any significant change in the amount of radioactive FA found in subcutaneous/perimetrial fat tissues, carcass or liver in either chow-fed or high-glucose, fed-refed lean or obese rats, although in the fed-refed dietary state the proportion found in the liver was increased over that in the chow-fed groups. Furthermore, the amounts of newly made FA which accumulated in the plasma of Triton-treated, chow-fed and glucose-fed refed animals during this period constituted only a few percentages of those found in the adipose tissue of these animals. Thus, in contrast to the claims of Godbole and York, no significant transfer of newly made FA from liver to adipose tissue occurs during a 1-hr experiment; it follows that the amount of these FA found in different tissues at the end of that period are valid measurements of their actual lipogenic activities in situ. It is suggested that the Godbole and York results are artifacts of their surgical hepatectomy procedure.

Very low density lipoprotein metabolism in domestic pigs

Patterns of triacylglycerol (TG) turnover in plasma and liver, and the hepatic secretion of very density lipoprotein triacylglycerol (VLDL TG) into the circulation, have been studied in young Large White female pigs, using i.v. tracer [1,3-14C]- and [2-3H]glycerol. Serial measurements were made of plasma [14C]glycerol and [14C]glucose and of liver TG and plasma VLDL TG specific activities. In other studies VLDL TG obtained from a donor pig was reinjected into recipient animals to measure the early disappearance (dilution) of VLDL TG. Multicompartmental analysis revealed a mean rate of hepatic TG turnover somewhat slower than the rate of VLDL TG turnover, suggesting that almost all of the hepatic TG turnover was due to secretion of VLDL TG, and that the intestine probably contributed an appreciable part of the newly synthesized plasma VLDL TG. The t1/2 of reinjected VLDL TG, whether prepared by the most rapid possible techniques and reinjected immediately or stored for several days, was approximately 10 min. This was much faster than the t1/2 of the falling limb of the plasma VLDL TG curve seen after injection of labelled glycerol (t1/2 approximately 2 H). Thus, in these respects, the pig resembled all other species studied rather than human subjects as described by Farquhar et al.

Compartmental analysis of linoleate and palmitate turnover in a murine carcinoma

We have carried out a balance study in Ehrlich ascites carcinoma in mice to determine whether large amounts of free fatty acids (FFA) could be diverted to an oxidative fate as suggested by earlier workers. At least 90% of the FFA tha turn over in the Ehrlich ascites tumor fluid are incorporated into the cell lipid esters of this carcinoma. Simultaneous with our balance study, we have compared the metabolic fate of essential and nonessential fatty acids (FA) in vivo in mouse Ehrlich ascites tumors using [1-14C]linoleic acid and [9,10-3H]palmitic acid complexed to mouse serum albumin. We followed the early disappearance of labeled FFA from the tumor system and the appearance of radioactivity in various tumor lipids and calculated rates of esterification and recycling of FA esters to FFA by cancer cells in vivo using multicompartmental analysis. We also estimated rates of "irreversible" disposal of FFA (combined rates of oxidation and transfer to host) in this tumor system. All rates for essential FA were found to be very similar to those for nonessential FA; however, some subtle differences seemed to exist; e.g., linoleate tended to disappear from the extracellular FFA pool faster than did palmitate and to appear in cellular phospholipids more rapidly than did palmitate, but the differences were not statistically significant. The major metabolic pathway for both classes of FFA was participation in an extremely rapid "futile cycle" of FA esterification (primarily into phospholipids) and hydrolysis. This cycle operates approximately 40 to 60 times faster than the rate of net FA esterification required for tumor growth (400 to 600 versus 10 nmol FA per min per 7-ml tumor). The "irreversible" disposal of FFA, based upon tracer studies with both essential and nonessential FFA, was approximately 6 times faster than the rate of FFA utilization for net growth.

Evaluation of impaired triglyceride fatty acid transport and oxidation for the detection of cancer in mice

We have tested the hypothesis proposed by Costa, Lyles, and Ullrich. (Effect of Human and Experimental Cancer on the Conversion of 14C Tripalmitin to 14CO2. Cancer (Phila.), 38:1259-1265, 1976) that the transport and/or oxidation of triglyceride fatty acids is markedly impaired in rodents bearing a growing s.c. carcinoma. Specifically, we have tested whether oxidation of triglyceride fatty acids is depressed in cancer-bearing animals. Mice inoculated s.c. with Ehrlich carcinoma cells were given injections (i.v. and i.p.) of 14C-labeled triglyceride fatty acids prepared as very-low-density lipoproteins by physiological methods or (i.p.) with [-14C]tripalmitin dissolved in peanut oil during both early (3 to 4 days) and advanced (7 to 8 weeks) stages of tumor growth. Specific activity of the expired 14CO2 was measured for periods ranging from 1 to 7 hr following injections. Because cancer-bearing mice can become severely hypertriglyceridemic, plasma triglyceride pool sizes were also measured during each experiment to account for the effects of possible differential dilution of the tracers. At no instance did we find any significant differences between specific activities of expired 14CO2 or plasma triglyceride pool sizes of the cancer-bearing animals and controls. Thus, a cancer-induced impairment of triglyceride fatty acid transport and metabolism to CO2, such as reported by Costa et al., does not seem to be a universal phenomenon in rodents.

Fatty acid oxidation to H2O by Ehrlich ascites carcinoma in mice

Oxidation of free fatty acids (FFA) by Ehrlich ascites tumor in mice was studied in vivo by the direct measurement of 3H2O formed from [9,10-(3)]palmitate. The FFA tracer complexed to serum albumin was injected i.p. into unanesthetized mice, and blood plasma 3H2O was measured at different time points for 30 min. The contribution of 3H2O by desaturation of labeled palmitate to monounsaturated fatty acids in the tumor was estimated by the use of [1-14C]palmitate and was shown to be negligible during the course of our experiments. In order to estimate the rates of tumor FFA oxidation, the kinetics of the tumor-host water distribution system was studied by injecting tracer 3H2O i.p. and following the disappearance of 3H2O in the blood plasma at different time points for 30 min. The results of these experiments were used to compute the tumor FFA oxidation rate by multicompartmental analyses and SAAM. Despite the nearly anaerobic state of the ascites tumor fluid in vivo, cancer cells suspended in this fluid oxidized FFA at least as fast as they do in vitro under aerobic conditions. Moreover, according to our current estimate, the need of the tumor for FFA as a metabolic fuel appears to be much greater than its net lipid needs for growth.

The role of dietary fat and hepatic triglyceride secretion in cancer-induced hypertriglyceridemia

Growth of Ehrlich ascites carcinoma induces hyperlipemia in mice. In the present study using male Swiss-Webster mice, we examined whether the usual elevations of plasma triglyceride levels in cancerous mice would occur in the absence of dietary fat. Hypertiglyceridemia developed at a similar rate and to a comparable degree in tumerous mice eating a fat-free (58% glucose) diet and in those fed Purina chow. Maximal hyperlipidemia was observed on day 6 or day 8 in tumorous mice fed either diet. To determine whether the endogenous cancer-induced hyperlipidemia was due to hypersecretion of triglycerides by the liver, triglyceride secretion rates were studied 0, 2, 4, 6, 8, 10, and 12 days after tumor inoculation using Triton WR-1339. The secretory rates did not increase prior to or during the development of hypertriglyceridemia in tumorous mice and were not significantly different from those of control mice. On days 10 and 12, triglyceride secretion actually decreased in tumorous mice. Other possible causes for hypertriglyceridemia are discussed in light of the present findings of undetectable differences in triglyceride secretion rates accompanying growth of Ehrlich ascites carcinoma in mice.

Re-evaluation of lipogenesis from dietary glucose carbon in liver and carcass of mice

We have estimated rates of fatty acid synthesis from glucose carbon and from all 2-carbon units in liver and carcass of mice using [U-14C] glucose and 3H2O under four different nutritional states. The liver synthesized only a small fraction (2--9%) of the fatty acids that were formed from glucose carbon in mice that were fasted 24 hr, fasted--refed, or fed ad libitum. However, in fed-refed mice, the liver's role increased and now accounted for 40% of the fatty acids that were formed from glucose carbon. Under the latter conditions (fed-refed), the liver synthesized 50% of the fatty acids that were formed from all 2-carbon units. At least five-sixths of all the fatty acids synthesized de novo in the fed-refed mouse were derived from carbon fed in the glucose test meal. These studies, in contrast to most earlier studies, provide direct evidence in mice of the major contribution that dietary carbohydrate makes, especially in the liver, to the synthesis of fatty acids. In addition, we have shown that lipogenic inhibition (fasting) and activation (feeding) are most marked in liver and greater for glucose than for non-glucose-carbon. Possible implications for dietary control of carbohydrate-induced hyperlipemia and obesity are discussed.

Regulation of plasma-free fatty acid mobilization by dietary glucose in Ehrlich ascites tumor-bearing mice

We studied the ability of dietary glucose to cause an abrupt inhibition of free fatty acid (FFA) mobilization in mice bearing advanced Ehrlich ascites carcinoma. FFA irreversible disposal rates were estimated after i.v. injection of tracer [1-14C]palmitate complexed to mouse serum albumin. Four groups of mice were studied: 16-hr-fasted mice versus 16-hr-fasted mice refed a 58% glucose, fat-free test meal for 10 min; and control versus tumorous mice. Plasma FFA fell significantly [from 0.97 +/- 0.06 (S.E.) to 0.37 +/- 0.02 muEq/ml (n = 30 and 134, respectively)] following the ingestion of the small test meal. The lowered plasma FFA pool size remained approximately constant between t = 15 and 45 min after the mice began to eat. Tracer studies in the fasted-refed mice, carried out during that interval, showed that the plasma FFA irreversible disposal rate was reduced by 50% in both control and tumor-bearing mice. Although cancerous mice tended to have elevated plasma FFA levels in the early morning, these animals appear to have normal control mechanisms for inhibiting FFA mobilization following ingestion of carbohydrate.

Re-evaluation of effects of meal feeding on lipogenic activation by glucose in rats

Based upon analyses of epididymal fat pads, gorging rats have been reported to synthesize fatty acids from glucose carbon 200 times faster than nibbling rats. This contrasts with our earlier study in mice in which no such adaptation was found in gorgers. Three methods were used to re-evaluate lipogenesis from glucose carbon in fasted-refed nibbling and gorging rats. Two methods in which [U-14C]glucose was injected intraperitoneally before or after different test-meals confirmed an apparent 100- to 200-fold increase in lipogenesis (14C incorporation into fatty acids) in epididymal fat pads of gorgers; however, incorporation of 14C into total fatty acids in the whole body of gorgers was only five times greater than in nibblers. Quantitative tracer techniques (intravenous and oral [U-14C]-glucose) were used to evaluate glucose carbon flux (22-hour fasted) and lipogenic activation following the ingestion of a labeled glucose test-meal. Glucose carbon conversion to total fatty acids (whole rat) increased from 2.7 (24-hour fasted) to 11 microng C/minute/200 g body weight, a fourfold activation, within 15 minutes after feeding the test-meal to nibbling rats. The corresponding increase in gorging rats was from 3.7 to 53 micrgong C/minute/200 g body weight, a 14-fold activation. These data indicate a species difference exists between rats and mice during adaptation to a gorging food-intake pattern.

Measurement of glucose recycling and liver glycogen synthesis in mice using doubly labeled substrates

Tracer experiments have been carried out using gorging and nibbling mice to study several related aspects of carbohydrate metabolism: 1) inhibition of gluconeogenesis shortly after animals ingest a glucose-rich meal; 2) the extent to which dietary glucose carbon is recycled by way of 3C compounds after dietary glucose is absorbed; and 3) recycling of glucose by exchange between free and a hypothetical, "bound" glucose pool. Fasted, gorging mice were allowed to eat 120 mg [U-14C,6-T]glucose (58% glucose diet) in 4 min. Plasma glucose-C specific activity rapidly reached that of the dietary glucose-C. Superficially, this suggested nearly complete inhibition of hepatic gluconeogenesis. However, plasma [6-T]glucose, glycogen-[14C, 3H]glucose analyses, and [14C]-glycerol conversion to glucose showed that hepatic gluconeogenesis continued during alimentary hyperglycemia. Half of liver glycogen seemed to be formed from a hepatic G-6-P pool that was never labeled. Indirect kinetic evidence of a large, bound exchangeable glucose pool was presented. Since no direct evidence of such a pool has been obtained, the possibility is raised that a serious artifact of the tracer technique exists or else some unconventional model of carbohydrate metabolism is required to explain our data.