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Biomedical subjects

A H Merrill

Publications and source records attributed to A H Merrill.

At least 73 records · Page 4Linked to original sources
PubMed1995

Role of dietary sphingolipids and inhibitors of sphingolipid metabolism in cancer and other diseases.

Sphingolipids are found in all eukaryotic and some prokaryotic organisms and participate in the regulation of cell growth, differentiation, and diverse cell functions including cell-cell communication, cell-substratum interactions and intracellular signal transduction. Nonetheless, the field of nutrition has given scant attention to these compounds so that little is known about the following fundamental questions: What is the fate of sphingolipids that are consumed in food? Does consumption of dietary sphingolipids affect the behavior of cells in the gastrointestinal tract or other organs? How do other factors in the diet affect sphingolipid metabolism? Several recent findings underscore the importance of these questions, for examples: 1) Sphingolipids are digested throughout the GI tract to ceramide and sphingosine, which are highly bioactive compounds that affect cellular regulatory pathways; 2) addition of sphingomyelin to a standard AIN diet (which is essentially devoid of sphingolipids) reduces the appearance of aberrant colonic crypts, and perhaps the number of tumors, in mice treated with a colon carcinogen; and 3) an enzyme of sphingolipid metabolism has been discovered to be the target of a class of toxic and carcinogenic mycotoxins called fumonisins. Given these recent findings, it is possible that some of the confusion that has arisen regarding the relationships between dietary fat and disease might be due to the lack of consideration of the sphingolipids that are also present.

Animals↗
PubMed1994

Disruption of sphingolipid metabolism and stimulation of DNA synthesis by fumonisin B1. A molecular mechanism for carcinogenesis associated with Fusarium moniliforme.

Consumption of grains contaminated with Fusarium moniliforme (Sheldon) causes liver cancer in rats and has been correlated with esophageal cancer in humans. The causative agents are believed to be a family of compounds known as fumonisins, which bear remarkable structural resemblances to sphingosine and sphinganine, the long-chain (sphingoid) base backbones of sphingolipids. Recently, fumonisin B1 has been shown to block de novo synthesis of sphingolipids by inhibiting sphingosine (sphinganine) N-acyltransferase, which leads to accumulation of sphingoid bases. Because the exogenous addition of sphingosine and sphingosine 1-phosphate to Swiss 3T3 cells has been shown to stimulate DNA synthesis (Zhang, H., Buckley, N.E., Gibson, K., and Spiegel, S. (1990) J. Biol. Chem. 265, 76-81; Zhang, H., Desai, N.N., Olivera, A., Seki, T., Brooker, G., and Spiegel, S. (1991) J. Cell Biol. 114, 155-167), we hypothesized that fumonisins might stimulate DNA synthesis by disrupting sphingolipid metabolism. Fumonisin B1 caused accumulation of sphinganine and sphingosine in Swiss 3T3 fibroblasts and, as occurred when these sphingoid bases were added exogenously, stimulated thymidine incorporation into DNA and augmented the mitogenic effect of insulin in a concentration-dependent manner. The mechanism underlying the mitogenic effect of fumonisin B1 was further investigated by using beta-fluoroalanine to block the initial step of sphingolipid biosynthesis catalyzed by serine palmitoyltransferase. beta-Fluoroalanine reduced sphingoid base accumulation in fumonisin B1-treated fibroblasts and inhibited fumonisin B1-stimulated DNA synthesis, but had no effect on mitogenesis when added alone. Fumonisin B1 did not cause accumulation of sphinganine 1-phosphate; therefore, it appears that sphingoid bases per se can stimulate DNA synthesis. To prove that the 1-phosphate is not obligatory, a 1-deoxysphinganine was synthesized, and it was as potent as sphinganine in stimulating DNA synthesis. These results establish that fumonisin B1 is mitogenic via accumulation of sphingoid bases rather than inhibition of complex sphingolipid biosynthesis per se. Because mitogens can often affect cell transformation, this provides a plausible molecular mechanism to explain the carcinogenicity of fumonisins.

3T3 Cells↗
PubMed1994

Dietary fumonisin B1 induces disruption of sphingolipid metabolism in Sprague-Dawley rats: a new mechanism of nephrotoxicity.

Fumonisins are potent inhibitors of sphingolipid biosynthesis produced by several Fusarium species. Consumption of corn or corn products infected with F. moniliforme, or high levels of fumonisins, is associated with several animal diseases. In a 4-wk feeding study, the concentration of fumonisin B1 that caused nephrotoxicity in Sprague-Dawley rats was much less than that required to cause hepatotoxicity. This retrospective study shows a close correlation between the extent and severity of ultrastructural lesions and the degree of disruption of sphingolipid metabolism. The kidney was more sensitive to fumonisin B1-induced disruption of sphingolipid metabolism than liver with significant elevation of free sphingosine, free sphinganine, and the free sphinganine:free sphingosine ratio in rats fed 15, 50 and 150 micrograms/g fumonisin B1. Accumulation of free sphinganine and elevation of the free sphinganine:free sphingosine ratio in urine closely reflected the changes that occurred in kidney. The accumulated sphinganine and elevation of the free sphinganine:free sphingosine ratio was associated with accumulation of cells in urine. Thus, urine rather than serum is the fluid of choice for detecting elevated free sphingoid bases generated as a consequence of fumonisin-induced kidney damage.

Animals↗
PubMed1994

Dietary sphingomyelin inhibits 1,2-dimethylhydrazine-induced colon cancer in CF1 mice.

Sphingolipids are in all eukaryotic cells and modulate cell growth, differentiation, and transformation; however, little is known about the physiological effects of their consumption. Mice were fed diets supplemented with milk sphingomyelin to determine effects on colon carcinogenesis. Cancer was initiated in CF1 mice by 1,2-dimethylhydrazine. Mice were then fed AIN76A diets supplemented with 0.025 to 0.1 g sphingomyelin/100 g for 28 wk until the supply of sphingomyelin was depleted and then fed unsupplemented diet for 24 wk. Sphingomyelin did not affect weight gain. Mice fed sphingomyelin had a 20% incidence of colon tumors compared with 47% in controls (P = 0.08 for all sphingomyelin-fed mice vs. controls). Tumors were adenomas or adenocarcinomas and located in the distal third of the colon. In shorter-term studies, colonic epithelial cell proliferation was significantly greater than controls in mice fed 0.025 g sphingomyelin/100 g diet, but not in those fed higher amounts of sphingomyelin. The number of aberrant crypts was significantly lower in 1,2-dimethylhydrazine-treated mice fed 0.05 g sphingomyelin/100 g diet than in controls. These results demonstrate that consumption of sphingomyelin affects the behavior of colonic cells. Because sphingolipids are present in food, the reduction in 1,2-dimethylhydrazine-induced premalignant lesions and the incidence of colon tumors in CF1 mice implies that these compounds may be another important class of nutritional modulators of carcinogenesis.

1,2-Dimethylhydrazine↗
PubMed1994

Uptake and metabolism of sphingolipids in isolated intestinal loops of mice.

Sphingolipids are found in all eukaryotic organisms. However, little is known about the digestion, uptake and subsequent metabolism of these constituents of food. In this study, radiolabeled sphingolipids were placed in isolated intestinal segments of female CF1 mice, and the metabolism and distribution of the radiolabel were followed. Most of the sphingomyelin was degraded to ceramide and other products in all regions of the intestine, and increasing amounts of several [3H]-labeled sphingolipids appeared in the tissues. Small amounts of the radiolabel disappeared from the intestinal loops and appeared in liver within the first 30 to 60 min implying that neither intact sphingomyelin nor its metabolites are transported very efficiently from the intestine to other organs. There were different degrees of uptake and metabolism of sphingomyelin, [4,5-3H-sphinganyl]ceramide, and [3H]sphingosine. The [3H]sphingomyelin was also administered by gavage and the appearance along the intestine measured. After 90 min, 12% was found in the cecum and colon. These results establish that some of the sphingomyelin that enters the gastrointestinal tract is hydrolyzed and taken up by the intestine, with the lipid backbone being degraded or reutilized for complex sphingolipid synthesis; however, at least a portion passes into the large intestine. The appearance of bioactive compounds throughout the gastrointestinal tract may alter the behavior of intestinal cells.

Administration, Oral↗
PubMed1993

Fumonisin B1 inhibits sphingosine (sphinganine) N-acyltransferase and de novo sphingolipid biosynthesis in cultured neurons in situ.

Fumonisins, mycotoxins produced by Fusarium moniliforme and a number of other fungi, cause neuronal degeneration, liver and renal toxicity, cancer, and other injury to animals. Recent work with rat hepatocytes (Wang, E., Norred, W. P., Bacon, C. W., Riley, R. T., and Merrill, A. H., Jr. (1991) J. Biol. Chem. 266, 14486-14490) found that fumonisins block sphingosine biosynthesis by inhibiting the conversion of sphinganine to dihydroceramides, which precedes introduction of the 4,5-trans-double bond of sphingosine. The current study utilized mouse cerebellar neurons in culture to evaluate how this affects the distribution of newly synthesized ceramides among different complex sphingolipids. Fumonisin B1 inhibited ceramide synthase in mouse brain microsomes with a competitive-like kinetic behavior with respect to both sphinganine and stearoyl-CoA. Fumonisin B1 inhibited sphingolipid biosynthesis in cultured cerebellar neurons in situ as reflected by accumulation of free sphinganine, a reduction in the mass of total sphingolipids, reductions in the incorporation of [14C]serine into glucosylceramide, lactosylceramide, sphingomyelin, and gangliosides (GM1, GD3, GD1a, GD1b, GT1b, and GQ1b), and inhibition of the incorporation of [14C]galactose and [3H]sphinganine into complex sphingolipids. Dose-response studies revealed that the labeling of sphingomyelin (IC50 of 0.7 microM) was more sensitive to inhibition by fumonisin B1 than was glycolipid formation (IC50 of approximately 7 microM) in these cells. A similar effect was seen when beta-fluoroalanine was added to inhibit the activity of serine palmitoyltransferase, the first enzyme of the pathway. The inhibition of complex sphingolipid synthesis was reversible, and nearly normal labeling profiles were obtained 48 h after removing the mycotoxin. These studies establish that fumonisin B1 inhibits de novo sphingolipid biosynthesis by neuronal cells and, moreover, that limiting ceramide synthesis differentially affects the formation of sphingomyelin versus glycosphingolipids.

Acyltransferases↗
PubMed1993

Regulation of phosphatidate phosphatase activity from the yeast Saccharomyces cerevisiae by sphingoid bases.

The regulation of Saccharomyces cerevisiae membrane-associated phosphatidate phosphatase (3-sn-phosphatidate phosphohydrolase, EC 3.1.3.4) activity by sphingoid bases was examined using Triton X-100/lipid-mixed micelles. Sphingosine, phytosphingosine, and sphinganine inhibited purified preparations of the 104- and 45-kDa forms of phosphatidate phosphatase in a dose-dependent manner. The structural requirements for the sphingoid base inhibition of phosphatidate phosphatase activity were a free amino group and a long chain hydrocarbon. A detailed kinetic analysis was performed to determine the mechanism of phosphatidate phosphatase inhibition by sphingoid bases. The phosphatidate phosphatase dependence on phosphatidate was cooperative (Hill numbers of approximately 2) in the absence and presence of sphingoid bases. Sphingosine, phytosphingosine, and sphinganine were parabolic competitive inhibitors of phosphatidate phosphatase activity. This indicated that more than one inhibitor molecule contributed to the exclusion of phosphatidate from the enzyme. The aKi values (inhibitor constants) for sphingosine, phytosphingosine, and sphinganine were 1.5, 0.4, and 0.2 mol %, respectively, and the Km value for phosphatidate was 2.2 mol %. The cellular concentrations of free phytosphingosine and sphinganine were 0.16 and 0.53 mol %, respectively, relative to the total phospholipids in S. cerevisiae. The cellular concentrations of phytosphingosine and sphinganine were in the range of the aKi values for these sphingoid bases. These results raised the suggestion that phosphatidate phosphatase activity may be regulated in vivo by sphingoid bases.

Amines↗
PubMed1993

Alteration of tissue and serum sphinganine to sphingosine ratio: an early biomarker of exposure to fumonisin-containing feeds in pigs.

Fumonisins are a group of naturally occurring compounds produced by the fungus Fusarium moniliforme. They are believed to be the etiologic agent of several animal diseases associated with consumption of corn-based feeds including porcine pulmonary edema. Recently it was shown in vitro that fumonisins are specific inhibitors of sphingosine and sphinganine N-acyltransferases. Inhibition of these enzymes in cultured cells results in the accumulation of free long chain sphingoid bases, specifically sphingosine and sphinganine, and the depletion of complex sphingolipids. In this study, tissues and serum from male SPF pigs fed a nutritionally balanced diet containing corn or corn screenings naturally contaminated with fumonisins for up to 14 days were analyzed for free sphingoid bases and complex sphingolipids. Total fumonisins (B1 and B2) in the diets were analyzed at 0 (< 1), 5, 23, 39, 101, and 175 ppm. Pulmonary edema only occurred at 175 ppm, while histologic liver damage was present at > or = 23 ppm, and serum liver enzymes were significantly elevated at > or = 101 ppm. The results of this study show that free sphinganine is elevated in liver, lung, and kidney, from pigs consuming feeds containing fumonisins at total fumonisin concentrations of 23 ppm or greater. Sphingosine is also elevated in a dose-dependent manner, but to a lesser extent than sphinganine. The consequence of this differential inhibition is that the ratio of sphinganine to sphingosine increases, suggesting that sphinganine N-acyltransferase is the preferred target for fumonisins. Elevation of free sphinganine and free sphingosine in serum paralleled the increases in tissues. Statistically significant increases in the ratio were observed at feed concentrations as low as 5 ppm total fumonisins and in pigs (at higher concentrations) in which other serum biochemistry parameters and tissue morphology were not altered. Elevated ratios were also observed in serum from pigs fed pure fumonisin B1. The sensitivity of the ratio indicates that it could serve as an effective biomarker for consumption of fumonisin-containing feeds. In addition, the data supports the hypothesis that inhibition of sphingosine and sphinganine N-acyltransferase plays an important role in the pathogenesis of animal diseases associated with consumption of feed containing fumonisins.

Animal Feed↗
PubMed1993

Apo A-IV: a new satiety signal.

Chylomicrons have a suppressive effect on food consumption, which is attributed to apolipoprotein A-IV (apo A-IV). This protein is found in cerebrospinal fluid in vivo, and its infusion into the third ventricle of the brain reduces food intake. These findings suggest that this apolipoprotein operates through the central nervous system to regulate food intake.

Animals↗
PubMed1993

Effects of feeding Fusarium moniliforme culture material, containing known levels of fumonisin B1, on the young broiler chick.

The effects of feeding Fusarium moniliforme culture material, containing known concentrations of fumonisin B1 (FB1), were studied in broiler chicks. Day-old chicks were allotted randomly to dietary treatments containing 0, 1.02, 2.04, 3.06, 4.08, 5.10, 6.12, and 7.14% fumonisin culture material (FCM). These levels of FCM supplied 0, 75, 150, 225, 300, 375, 450, and 525 mg of FB1/kg of feed. Each dietary treatment was fed to four pen replicates of six birds each for 21 days. Chicks fed FCM that supplied 450 and 525 mg FB1/kg diet had lower (P < .05) feed intakes and BW gains; increased (P < .05) liver and kidney weights; and increased (P < .05) mean cell hemoglobin, and mean cell hemoglobin concentrations. Compared with controls, chicks fed FCM had increased (P < .05) free sphinganine levels and sphinganine:sphingosine ratios. Treatment-associated histological lesions were only observed in the liver of chicks fed diets containing FCM that supplied 225 mg FB1/kg or higher. Diets containing FCM that supplied levels as low as 75 mg FB1/kg affected the physiology of chicks by increasing free sphinganine levels and sphinganine:sphingosine ratios. Because inhibition of sphingolipid biosynthesis has been hypothesized as the mechanism of action of FB1, this suggests that diets containing 75 mg FB1/kg FCM may be toxic to young broiler chicks.

Animal Feed↗
PubMed1992

Influences of sphingosine on two-stage skin tumorigenesis in Sencar mice.

Sphingosine (SPH) was studied as an inhibitor of skin tumor promotion in skin cancer initiated by 7,12-dimethylbenz[a]-anthracene (DMBA) and promoted by 12-O-tetradecanoylphorbol-13-acetate (TPA). Two tumorigenesis studies were conducted using female Sencar mice treated with 10 nmol DMBA in 0.2 ml acetone at 8 weeks of age and promoted beginning 1 week later with 3.2 nmol TPA applied twice per week. In the high-dose study, 10 mumol SPH was applied 30 min before each TPA treatment. The low-dose study used 0.5, 0.05, or 0.01 mumol treatments with SPH 30 min before TPA. In the high-dose study SPH treatment alone following initiation by DMBA, and SPH treatment preceding TPA in DMBA-initiated mice accelerated the development of papillomas in comparison with the DMBA/TPA-treated group. The low-dose experiment showed no consistent alteration of tumorigenesis by SPH in the DMBA/TPA-treated groups, and low doses of SPH following DMBA did not promote skin tumorigenesis. SPH treatment did not alter body weight in either experiment.

9,10-Dimethyl-1,2-benzanthracene↗
PubMed1992

In vitro toxicology of fumonisins and the mechanistic implications.

The effects of fumonisins B1(FB1), B2(FB2), and the backbone of fumonisin B1 remaining after hydrolysis of the tricarballylic groups with base (HFB1) on sphingolipid biosynthesis were studied in both primary rat hepatocytes and pig kidney epithelial cells (LLC-PK1). Fumonisins were potent inhibitors of sphingolipid biosynthesis in hepatocytes (IC50 of FB1 = 0.1 microM), but overt toxicity was not observed. In renal cells, fumonisins also inhibited sphingosine biosynthesis (IC50 for FB1 = 35 microM), and caused decreased cell proliferation as well. Higher doses (greater than or equal to 70 microM) killed renal cells after exposure for 3 days. The inhibition of de novo sphingolipid biosynthesis was specific, and appeared to be at the site of ceramide synthase, which catalyzes the formation of dihydroceramide or ceramide by the addition of the amide-linked fatty acid to sphinganine or sphingosine. These results may account for the ability of fumonisins to cause equine leucoencephalomalacia and to promote tumor formation.

Animals↗
PubMed1992

Fumonisin inhibition of de novo sphingolipid biosynthesis and cytotoxicity are correlated in LLC-PK1 cells.

Fumonisins are a group of structurally related compounds produced by Fusarium moniliforme. Recently, it has been shown that fumonisins B1 and B2 are the first naturally occurring inhibitors of sphingosine and sphinganine N-acyltransferase (ceramide synthase) in rat primary hepatocytes (Wang et al. J. Biol. Chem. 266, 14, 486-14, 490, 1991). These enzymes are key components in the pathways for de novo sphingolipid biosynthesis and sphingolipid turnover. The results of the present study show that fumonisins B1 and B2 inhibit proliferation and are cytotoxic to LLC-PK1 cells. Concentrations of fumonisin B1 and B2 between 10 and 35 microM inhibited cell proliferation, whereas higher concentrations (greater than 35 microM) killed cells. Inhibition of cell proliferation and cell death were preceded by a lag period of at least 24 hr during which cells appeared to be functioning normally. Cells exposed to fumonisin B1 exhibited normal growth kinetics and morphology soon after fumonisin B1 was removed; thus, the effects of fumonisin B1 were reversible. The EC50 for alterations in sphingolipid biosynthesis was 10 to 15 microM. Inhibition of de novo sphingolipid biosynthesis occurred before inhibition of cell proliferation or cytotoxicity, and the dose response for the decrease in the [3H]sphingosine to [3H]sphinganine ratio at 7 hr closely paralleled the dose response for effects on proliferation and cytotoxicity at 3-5 days. In addition, the level of free sphinganine, and to a lesser extent sphingosine, increased in fumonisin-treated cells in a dose-dependent manner. During the 24-hr lag period preceding inhibition of cell proliferation, the free sphinganine content increased by 12,800% in cells exposed to 35 microM fumonisin B1. Whereas a mechanistic relationship between the inhibition of de novo sphingolipid biosynthesis and inhibition of proliferation and cell death has not been demonstrated, the results of this study support the hypothesis that inhibition of de novo sphingolipid biosynthesis is an early event in the toxicity of fumonisins to LLC-PK1 cells.

Acyltransferases↗