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A large body of research has demonstrated that human stearoyl-CoA desaturase 1 (SCD1), a universally expressed fatty acid Δ9-desaturase that converts saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA), is a central regulator of metabolic and signaling pathways involved in cell proliferation, differentiation, and survival. Unlike SCD1, stearoyl-CoA desaturase 5 (SCD5), a second SCD isoform found in a variety of vertebrates, including humans, has received considerably less attention but new information on the catalytic properties, regulation and biological functions of this enzyme has begun to emerge. This review will examine the new evidence that supports key metabolic and biological roles for SCD5, as well as the potential implication of this desaturase in the mechanisms of human diseases.

The processes of cell proliferation, cell death and differentiation involve an intricate array of biochemical and morphological changes that require a finely tuned modulation of metabolic pathways, chiefly among them is fatty acid metabolism. The critical participation of stearoyl CoA desaturase-1 (SCD1), the fatty acyl Δ9-desaturing enzyme that converts saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA), in the mechanisms of replication and survival of mammalian cells, as well as their implication in the biological alterations of cancer have been actively investigated in recent years. This review examines the growing body of evidence that argues for a role of SCD1 as a central regulator of the complex synchronization of metabolic and signaling events that control cellular metabolism, cell cycle progression, survival, differentiation and transformation to cancer.

Cancer cells require high levels of lipid synthesis to produce structural, signaling and energetic lipids to support continuous replication. We and others have reported that constitutively increased lipogenesis, mainly by the tandem activation of acetyl-CoA carboxylase, fatty acid synthase and stearoyl-CoA desaturase-1 (SCD1), is critical to sustain the biological features of cancer cells, making this metabolic pathway a potential anticancer target for nutritional and pharmacological interventions. Isoflavones are biologically potent botanical compounds that possess clear antilipogenic and anticancer properties; however, the regulatory effects of these nutraceutical agents on lipid biosynthesis in cancer cells are still not well understood. Here we show that genistein, an isoflavone abundant in soybeans, decreased the levels of SCD1 protein in H460 human lung adenocarcinoma cells, consequently reducing the rate of biosynthesis of oleic acid as well as its presence in cancer cell lipids. Moreover, genistein promoted a marked reduction in de novo synthesis of major phospholipids, triacylglycerol and cholesterolesters. Finally, cancer cells treated with genistein displayed a dramatic reduction in cell proliferation as a result of a blockade in cell cycle progression through G2/M phases. As a whole, our data suggest that, by globally downregulating lipid biosynthesis, genistein suppresses cancer cell growth, emphasizing the relevance of this botanical compound as a potential therapeutic agent against lung cancer, a disease for which therapeutic choices remain limited.

Aims Normal human cells in culture progressively lose their capacity for replication, ending in an irreversible arrested state known as replicative senescence. Senescence has been functionally associated to the process of organismal ageing and is also considered a major tumor-suppressing mechanism. Although a great deal of knowledge has uncovered many of the molecular aspects of senescence, little is known about the regulation of lipid synthesis, particularly the biosynthesis and Δ9-desaturation of fatty acids, during the senescence process. Main methods By using immunoblotting and metabolic radiolabeling, we determined the senescence-associated changes in major lipogenic pathways. Key findings The levels of fatty acid synthase and stearoyl-CoA desaturase-1 and, consequently, the formation of monounsaturated fatty acids, were notably decreased in senescent cells when compared to proliferating (young) fibroblasts. Moreover, we detected a reduction in the de novo synthesis of phospholipids with a concomitant increase in the formation of cholesterol in senescent cells compared to young fibroblasts. Finally, it was found that exogenous fatty acids were preferentially incorporated into the triacylglycerol pool of senescent cells. Significance This set of observations is the first demonstration of a profound modification in lipid metabolism, particularly fatty acid biosynthesis and desaturation, caused by the senescence process and contributes to the increasing body of evidence linking de novo lipogenesis with cellular proliferation.

The precise role of monounsaturated fatty acid (MUFA) synthesis in cell proliferation and programmed cell death remains unknown. The strong correlation of high levels of MUFA and neoplastic phenotype suggest that the regulation of stearoyl CoA desaturase (SCD) must play a significant role in cancer development. In this study, the levels of SCD protein and activity were investigated in normal (WI38) and SV40-transformed (SV40-WI38) human lung fibroblasts. Thus, the activity of SCD on exogenous [ 14 C]stearic acid and endogenous [ 14 C]acetate-labeled fatty acids was increased by 2.2- and 2.6-fold, respectively, in SV40-WI38 compared to WI38 fibroblasts. Concomitantly, a 3.3-fold increase in SCD protein content was observed in SV40-transformed cells. Cell transformation also led to high levels of MUFA, which was paralleled by a more fluid membrane environment. Furthermore, the levels of PPAR-γ, a well-known activator of SCD expression, were highly increased in SV40-transformed fibroblasts. SCD activity appeared linked to the events of programmed cell death, since incubations with 40 μM etoposide induced apoptosis in SV40 cells, and led to a decrease in fatty acid synthesis, SCD activity and in MUFA cellular levels. Taken together, these results suggest that SCD protein and activity levels are associated with the events of neoplastic cell transformation and programmed cell death.

We studied the regulation of triacylglycerol (TAG) metabolism by phosphatidylcholine (PC) in CHO MT58 cells, which are deficient in PC synthesis because of a temperature-sensitive CTP:phosphocholine cytidylyltransferase. At the permissive growth temperature (34°C), these cells contained 49% less TAG and 30% less PC than wild-type CHO K1 cells. Treatment with dipalmitoylphosphatidylcholine normalized both the PC and TAG levels. Despite low TAG levels, the incorporation of [14C] oleate into TAG was increased in CHO MT58 cells. The in vitro de novo synthesis of TAG and the activity of diacylglycerol acyltransferase were 90% and 34% higher, respectively. Two other key enzyme activities in TAG synthesis, acyl-CoA synthetase and mitochondrial glycerol-3-phosphate acyltransferase (GPAT), increased by 48% and 2-fold, respectively, and mitochondrial GPAT mRNA increased by ∼4-fold. Additionally, TAG hydrolysis was accelerated in CHO MT58 cells, and in vitro lipolytic activity increased by 68%. These studies suggest that a homeostatic mechanism increases TAG synthesis and recycling in response to PC deficiency. TAG recycling produces diacylglycerol and fatty acids that can be substrates for de novo PC synthesis and for lysophosphatidylcholine (lysoPC) acylation. In CHO MT58 cells, in which de novo PC synthesis is blocked, lysoPC acylation with fatty acid originating from TAG may represent the main pathway for generating PC., Instituto de Investigaciones Bioquímicas de La Plata

Stearoyl-CoA desaturase-1 (SCD1), the main enzyme that converts saturated fatty acids into monounsaturated fatty acids, is a key factor in the mechanisms of cancer cell proliferation, survival and tumorigenesis. Evidence indicates that SCD1 activity regulates these events in part by targeting the phosphatidylinositol-3 phosphate kinase/Akt and Ras/extracellular signalregulated kinase (ERK) pathways, but the molecular mechanisms remain unknown. We now show that in H460 lung cancer cells, the suppression of SCD activity with CVT-11127, a specific small molecule SCD inhibitor, impairs the ligand-induced phosphorylation of epidermal growth factor (EGF) receptor, causing the inactivation of its downstream targets Akt, ERK and mammalian target of rapamycin. Importantly, the mitogenic response to EGF was markedly defective in SCD-depleted cancer cells. The inactivation of EGF receptor (EGFR) promoted by SCD inhibition may be caused by perturbations in the lipid microenvironment surrounding the receptor, since we detected significant alterations in the lateral mobility of plasma lipid microdomains. Finally, incubation of lung cancer cells with SCD blockers potentiated the antigrowth effect of gefitinib, an EGFR inhibitor employed in cancer treatment. Altogether, our data indicate that SCD activity may control cancer cell metabolism, proliferation and survival by modulating the EGFR→Akt/ERK signaling platforms. Our studies also suggest a value for SCD inhibitors as novel pharmacological agents in lung cancer, one of the most common and lethal forms of cancer for which therapeutic options remain very limited. © 2008 Society for Experimental Biology and Medicine.

As part of a shift toward macromolecule production to support continuous cell proliferation, cancer cells coordinate the activation of lipid biosynthesis and the signaling networks that stimulate this process. A ubiquitous metabolic event in cancer is the constitutive activation of the fatty acid biosynthetic pathway, which produces saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) to sustain the increasing demand of new membrane phospholipids with appropriate acyl composition. In cancer cells, the tandem activation of the fatty acid biosynthetic enzymes adenosine triphosphate citrate lyase, acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) leads to increased synthesis of SFA and their further conversion into MUFA by stearoyl-CoA desaturase (SCD) 1. The roles of adenosine triphosphate citrate lyase, ACC and FAS in the pathogenesis of cancer have been a subject of extensive investigation. However, despite early experimental and epidemiological observations reporting elevated levels of MUFA in cancer cells and tissues, the involvement of SCD1 in the mechanisms of carcinogenesis remains surprisingly understudied. Over the past few years, a more detailed picture of the functional relevance of SCD1 in cell proliferation, survival and transformation to cancer has begun to emerge. The present review addresses the mounting evidence that argues for a key role of SCD1 in the coordination of the intertwined pathways of lipid biosynthesis, energy sensing and the transduction signals that influence mitogenesis and tumorigenesis, as well as the potential value of this enzyme as a target for novel pharmacological approaches in cancer interventions.

Lung cancer is the most frequent form of cancer. The survival rate for patients with metastatic lung cancer is approximately 5%, hence alternative therapeutic strategies to treat this disease are critically needed. Recent studies suggest that lipid biosynthetic pathways, particularly fatty acid synthesis and desaturation, are promising molecular targets for cancer therapy. We have previously reported that inhibition of stearoylCoA desaturase-1 (SCD1), the enzyme that produces monounsaturated fatty acids (MUFA), impairs lung cancer cell proliferation, survival and invasiveness, and dramatically reduces tumor formation in mice. In this report, we show that inhibition of SCD activity in human lung cancer cells with the small molecule SCD inhibitor CVT-11127 reduced lipid synthesis and impaired proliferation by blocking the progression of cell cycle through the G(1)/S boundary and by triggering programmed cell death. These alterations resulting from SCD blockade were fully reversed by either oleic (18:1n-9), palmitoleic acid (16:1n-7) or cis-vaccenic acid (18:1n-7) demonstrating that cis-MUFA are key molecules for cancer cell proliferation. Additionally, co-treatment of cells with CVT-11127 and CP-640186, a specific acetylCoA carboxylase (ACC) inhibitor, did not potentiate the growth inhibitory effect of these compounds, suggesting that inhibition of ACC or SCD1 affects a similar target critical for cell proliferation, likely MUFA, the common fatty acid product in the pathway. This hypothesis was further reinforced by the observation that exogenous oleic acid reverses the anti-growth effect of SCD and ACC inhibitors. Finally, exogenous oleic acid restored the globally decreased levels of cell lipids in cells undergoing a blockade of SCD activity, indicating that active lipid synthesis is required for the fatty acid-mediated restoration of proliferation in SCD1-inhibited cells. Altogether, these observations suggest that SCD1 controls cell cycle progression and apoptosis and, consequently, the overall rate of proliferation in cancer cells through MUFA-mediated activation of lipid synthesis.

Saturated (SFA) and monounsaturated (MUFA) fatty acids, the most abundant fatty acid species, have many divergent biological effects including the regulation of cell proliferation, programmed cell death and lipid-mediated cytotoxicity. Their distribution is regulated by Stearoyl-CoA Desaturases (SCD), the enzymes that convert SFA into MUFA. A positive correlation between high levels of tissue MUFA and several types of cancer has been reported, but a causal relationship between the function of SCD1, the main human SCD isoform, and cancer development has not yet been firmly established. Here we report that the stable knockdown of SCD1 gene expression in A549 human lung adenocarcinoma cells decreased the ratio MUFA/SFA in total lipids and inhibited the incorporation of glucose into cell lipids. Cell proliferation and anchorage-independent growth were considerably decreased in SCD1-depleted cells, whereas the rate of apoptosis was elevated, with respect to control A549 cells. In addition, phosphorylation of Akt-Ser473 and GSK-3beta-Ser9 was found notably impaired in SCD1-ablated A549 cells. Interestingly, the effects of SCD1 blockade on Akt activation, cancer cell growth and apoptosis could not be reversed by exogenously added oleic acid. Remarkably, the reduction of SCD1 expression in lung cancer cells significantly delayed the formation of tumors and reduced the growth rate of tumor xenografts in mice. Our study demonstrates that SCD1 activity regulates Akt activation and determines the rate of cell proliferation, survival and invasiveness in A549 cancer cells and shows, for the first time, that SCD1 is a key factor in the regulation of tumorigenesis in vivo.

Saturated and monounsaturated fatty acids are the most abundant fatty acid species in mammalian organisms, and their distribution is regulated by stearoyl-CoA desaturase, the enzyme that converts saturated into monounsaturated fatty acids. A positive correlation between high monounsaturated fatty acid levels and neoplastic transformation has been reported, but little is still known about the regulation of stearoyl-CoA desaturase in cell proliferation and apoptosis, as well as in cancer development. Here we report that simian virus 40-transformed human lung fibroblasts bearing a knock-down of human stearoyl-CoA desaturase by stable antisense cDNA transfection (hSCDas cells) showed a considerable reduction in monounsaturated fatty acids, cholesterol, and phospholipid synthesis, compared with empty vector transfected-simian virus 40 cell line (control cells). hSCDas cells also exhibited high cellular levels of saturated free fatty acids and triacylglycerol. Interestingly, stearoyl-CoA desaturase-depleted cells exhibited a dramatic decrease in proliferation rate and abolition of anchorage-independent growth. Prolonged exposure to exogenous oleic acid did not reverse either the slower proliferation or loss of anchorage-independent growth of hSCDas cells, suggesting that endogenous synthesis of monounsaturated fatty acids is essential for rapid cell replication and invasiveness, two hallmarks of neoplastic transformation. Moreover, apoptosis was increased in hSCDas cells in a ceramide-independent manner. Finally, stearoyl-CoA desaturase-deficient cells were more sensitive to palmitic acid-induced apoptosis compared with control cells. Our data suggest that, by globally regulating lipid metabolism, stearoyl-CoA desaturase activity modulates cell proliferation and survival and emphasize the important role of endogenously synthesized monounsaturated fatty acids in sustaining the neoplastic phenotype of transformed cells., Instituto de Investigaciones Bioquímicas de La Plata

Both diabetes mellitus type 1 and diabetes mellitus type 2 are widespread diseases that alter carbohydrate and lipid metabolism. e Stilmann-Salgado (eSS) rats are experimental animals that spontaneously evolve to a state similar to that of young people affected by non-insulin-dependent diabetes mellitus (NIDDM; type 2). Using 6-mon-old eSS rats that, according to the literature [Martinez, S.M., Tarrés, M.C., Montenegro, S., Milo, R., Picena, J.C., Figueroa, N., and Rabasa, S.R. (1988) Spontaneous Diabetes in eSS Rats, Acta Diabetol. Lat. 25, 303-313], had already developed insulin resistance, we investigated the changes evoked on delta9, delta6, and delta5 liver desaturases. The abundance of mRNA and enzymatic activities were measured, as well as the FA composition of liver microsomal lipids. Compared to control rats, the mRNA content and activity of SCD-1 (stearoyl CoA-desaturase, isoform of the delta9 desaturase) were significantly higher, whereas the mRNA and activities of delta6 and delta5 desaturases were not significantly modified. Correspondingly, the proportion of 18:1n-9 and the ratios of 18:1n-9/18:0 and 16:1/16:0 in lipids were significantly increased, whereas the proportion of 20:4n-6 was unaltered. These effects were found while glycemia was constant or increased. The results are completely opposite those described in insulin-dependent diabetes mellitus (type 1), in which a depression of all the desaturases is found. They suggest that in eSS rats, the activities of the desaturases were not modified by an insulin-resistance effect. Moreover, we suggest that the enhancement of SCD-1 activity might be considered as another typical sign of the NIDDM syndrome, because it has also been found in other animal models of NIDDM, for example, the ones evoked by the sucrose-rich diet and in the Zucker rat.

Diacylglycerol (DAG) is a versatile molecule that participates as substrate in the synthesis of structural and energetic lipids, and acts as the physiological signal that activates protein kinase C. Diacylglycerol acyltransferase (DGAT), the last committed enzyme in triacylglycerol synthesis, could potentially regulate the content and use of both signaling and glycerolipid substrate DAG by converting it into triacylglycerol. To test this hypothesis, we stably overexpressed the DGAT1 mouse gene in human lung SV40-transformed fibroblasts (DGAT cells), which contains high levels of DAG. DGAT cells exhibited a 3.9-fold higher DGAT activity and a 3.2-fold increase in triacylglycerol content, whereas DAG and phosphatidylcholine decreased by 70 and 20%, respectively, compared with empty vector-transfected SV40 cells (Control cells). Both acylation and de novo synthesis of phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin were reduced by 30-40% in DGAT cells compared with controls, suggesting that DGAT used substrates for triacylglycerol synthesis that had originally been destined to produce phospholipids. The incorporation of [14C]DAG and [ 14C]fatty acids released from plasma membrane by additions of either phospholipase C or phospholipase A2 into triacylglycerol was increased by 6.2- and 2.8-fold, respectively, in DGAT cells compared with control cells, indicating that DGAT can attenuate signaling lipids. Finally, DGAT overexpression reversed the neoplastic phenotype because it dramatically reduced the cell growth rate and suppressed the anchorage-independent growth of the SV40 cells. These results strongly support the view that DGAT participates in the regulation of membrane lipid synthesis and lipid signaling, thereby playing an important role in modulating cell growth properties., Instituto de Investigaciones Bioquímicas de La Plata

Both diabetes mellitus type 1 and diabetes mellitus type 2 are widespread diseases that alter carbohydrate and lipid metabolism. e Stilmann-Salgado (eSS) rats are experimental animals that spontaneously evolve to a state similar to that of young people affected by non-insulin-dependent diabetes mellitus (NIDDM; type 2). Using 6-mon-old eSS rats that, according to the literature [Martinez, S.M., Tarres, M.C., Montenegro, S., Milo, R., Picena, J.C., Figueroa, N., and Rabasa, S.R. (1988) Spontaneous Diabetes in eSS Rats, Acta Diabetol. Lat. 25, 303-313], had already developed insulin resistance, we investigated the changes evoked on delta9, delta6, and delta5 liver desaturases. The abundance of mRNA and enzymatic activities were measured, as well as the FA composition of liver microsomal lipids. Compared to control rats, the mRNA content and activity of SCD-1 (stearoyl CoA-desaturase, isoform of the delta9 desaturase) were significantly higher, whereas the mRNA and activities of delta6 and delta5 desaturases were not significantly modified. Correspondingly, the proportion of 18:1n-9 and the ratios of 18:1n-9/18:0 and 16:1/16:0 in lipids were significantly increased, whereas the proportion of 20:4n-6 was unaltered. These effects were found while glycemia was constant or increased. The results are completely opposite those described in insulin-dependent diabetes mellitus (type 1), in which a depression of all the desaturases is found. They suggest that in eSS rats, the activities of the desaturases were not modified by an insulin-resistance effect. Moreover, we suggest that the enhancement of SCD-1 activity might be considered as another typical sign of the NIDDM syndrome, because it has also been found in other animal models of NIDDM, for example, the ones evoked by the sucrose-rich diet and in the Zucker rat., Instituto de Investigaciones Bioquímicas de La Plata

The diacylglycerol (DAG) signal generated from membrane phospholipids by hormone-activated phospholipases is attenuated by mechanisms that include lipolysis or phospholipid resynthesis. To determine whether the DAG signal might also be terminated by incorporation of DAG into triacylglycerol (TAG), we studied the direct formation of TAG from endogenous DAG generated by bacterial phospholipase C (PLC). When Chinese hamster ovary (CHO) cells prelabeled with [¹⁴C]oleate were treated with PLC from Clostridium perfringens for 6 h, [¹⁴C]phospholipid decreased 15% and labeled TAG increased 60%. This transfer of ¹⁴C label was even greater when the cells were simultaneously exposed to PLC and 100 μM oleic acid. PLC as well as oleate treatment concomitantly increased the TAG mass within the cell. Moreover, when phospholipids were prelabeled with [³H]glycerol, a subsequent increase in [³H]TAG indicated that an intact DAG moiety was channeled into the TAG structure. Incubating CHO cells with the diacylglycerol kinase inhibitor R59022 enhanced the formation of TAG from phospholipids hydrolyzed by PLC or by PLC in the presence of 100 μM oleate, but not by incubation with oleate alone, indicating that the DAG released from plasma membrane phospholipids does not require the formation of a phosphatidic acid precursor for TAG synthesis. Similarly, the diacylglycerol lipase inhibitor RHC 80267 did not alter TAG synthesis from plasma membrane DAG, further supporting direct incorporation of DAG into TAG. These studies indicate that DAG derived from plasma membrane phospholipid is largely used for TAG formation, and support the view that this mechanism can terminate DAG signals. The studies also suggest that a transport mechanism exists to move plasma membrane-derived DAG to the endoplasmic reticulum., Facultad de Ciencias Médicas, Instituto de Investigaciones Bioquímicas de La Plata

The mitochondrial isoform of glycerol-3-phosphate acyltransferase (GPAT), the first step in glycerolipid synthesis, is up-regulated by insulin and by high carbohydrate feeding via SREBP-1c, suggesting that it plays a role in triacylglycerol synthesis. To test this hypothesis, we overexpressed mitochondrial GPAT in Chinese hamster ovary (CHO) cells. When GPAT was overexpressed 3.8-fold, triacylglycerol mass was 2.7-fold higher than in control cells. After incubation with trace [14C]oleate (∼3 μM), control cells incorporated 4.7-fold more label into phospholipid than triacylglycerol, but GPAT-overexpressing cells incorporated equal amounts of label into phospholipid and triacylglycerol. In GPAT-overexpressing cells, the incorporation of label into phospholipid, particularly phosphatidylcholine, decreased 30%, despite normal growth rate and phospholipid content, suggesting that exogenous oleate was directed primarily toward triacylglycerol synthesis. Transiently transfected HEK293 cells that expressed a 4.4-fold increase in GPAT activity incorporated 9.7-fold more [14C]oleate into triacylglycerol compared with control cells, showing that the effect of GPAT overexpression was similar in two different cell types that had been transfected by different methods. When the stable, GPAT-overexpressing CHO cells were incubated with 100 μM oleate to stimulate triacylglycerol synthesis, they incorporated 1.9-fold more fatty acid into triacylglycerol than did the control cells. Confocal microscopy of CHO and HEK293 cells transfected with the GPAT-FLAG construct showed that GPAT was located correctly in mitochondria and was not present elsewhere in the cell. These studies indicate that overexpressed mitochondrial GPAT directs incorporation of exogenous fatty acid into triacylglycerol rather than phospholipid and imply that (a) mitochondrial GPAT and lysophosphatidic acid acyltransferase produce a separate pool of lysophosphatidic acid and phosphatidic acid that must be transported to the endoplasmic reticulum where the terminal enzymes of triacylglycerol synthesis are located, and (b) this pool remains relatively separate from the pool of lysophosphatidic acid and phosphatidic acid that contributes to the synthesis of the major phospholipid species., Instituto de Investigaciones Bioquímicas de La Plata

Liver is one of the tissues most actively involved in triacylglycerol synthesis and secretion. Hypertriglyceridemia is commonly associated with the diabetic state which has been detected in very young rats after the induction of experimental diabetes. In the present work, acylglycerol synthesis in liver of streptozotocin-treated rats, fed a diet supplemented with n-3 and n-6 fatty acids, was studied. At the onset of the experiment, plasma triacylglycerol levels increased significantly in diabetic animals when compared to controls. Two weeks after the dietary treatment, the aforementioned parameter decreased in diabetic animals consuming either n-6 or n-3 fatty acids. In control rats, n-3 fatty acids depressed triacylglycerol synthesis in liver microsomes. In the diabetic group both diets increased diacylglycerol and triacylglycerol synthesis. The addition of liver cytosolic fraction from control rats to the incubation medium, stimulated the triacylglycerol synthesis in all the groups. Nevertheless, the radioactivity recovered in the neutral lipid fractions was lower in the samples from rats fed n-3 fatty acids compared to n-6. We conclude that dietary n-3 fatty acids decreased significantly triacylglycerol plasma levels in diabetic rats probably through the inhibition of liver triacylglycerol secretion. In addition, there probably is an n-3 fatty acid sensitive factor in the liver cytosolic fraction able to depress triglyceride synthesis.

Neutral lipid storage disease (NLSD) is an autosomal recessive disorder in which excess triacylglycerol (TG) accumulates in most cells. Although it has been hypothesized that the TG accumulation is caused by a functional defect in cytosolic lipase activity, we were able to expose TG hydrolysis in NLSD cells by using triacsin C, an inhibitor of acyl-CoA synthetase that blocks the reincorporation of hydrolyzed fatty acids into glycerolipids. Our data suggest that TG lipolysis in NLSD cells is masked by rapid TG resynthesis, occurring because released acylglycerols cannot be used for phospholipid synthesis. In uptake studies, triacsin C blocked the incorporation of [3H]glycerol into glycerolipids, incorporation of [14C]oleate into TG, but not incorporation of [14C]oleate into phospholipid. Thus, the drug inhibited both de novo synthesis of glycerolipids via the glycerol-3-phosphate pathway and the synthesis of TG from diacylglycerol. The drug did not appear to block reacylation of lysophospholipids. Triacsin C caused a loss of about 60% of the TG mass from both NLSD and oleate-loaded control cells. Rates of TG lipolysis were similar in NLSD cells and oleate-loaded control cells labeled with [6-(7-nitro-2,1,3-benzoxadiazol-4-yl)-amino]hexanoic acid or labeled with [14C]oleate or [3H]glycerol and chased in the presence of triacsin C. During a 96-h chase, [14C]oleate reincorporation into the different phospholipid species increased only in control cells. Similar results were observed when NLSD, and control cells were chased after labeling with [3H]glycerol. These data strongly suggest that normal human fibroblasts mobilize stored TG for phospholipid synthesis and that recycling to PC occurs via a TG-derived mono- or diacylglycerol intermediate. Normal recycling to phosphatidylethanolamine may primarily involve TG-derived acyl groups rather than an acylglycerol precursor. NLSD cells appear to have a block in this recycling pathway with the result that both hydrolyzed fatty acids and the acylglycerol backbone are re-esterified to form TG. Because the NLSD phenotype includes ichthyosis, fatty liver, myopathy, cardiomyopathy, and mental retardation, the recycling pathway appears to be critical for the normal function of skin, liver, muscle, heart, and the central nervous system.