Independent arachidonic acid-mediated gene regulatory pathways in lymphocytes

Dietary n-6 PUFA deprivation downregulates arachidonate but upregulates docosahexaenoate metabolizing enzymes in rat brain

BACKGROUND

Dietary n-3 polyunsaturated fatty acid (PUFA) deprivation increases expression of arachidonic acid (AA 20:4n-6)-selective cytosolic phospholipase A(2) (cPLA(2)) IVA and cyclooxygenase (COX)-2 in rat brain, while decreasing expression of docosahexaenoic acid (DHA 22:6n-3)-selective calcium-independent iPLA(2) VIA. Assuming that these enzyme changes represent brain homeostatic responses to deprivation, we hypothesized that dietary n-6 PUFA deprivation would produce changes in the opposite directions.

METHODS

Brain expression of PUFA-metabolizing enzymes and their transcription factors was quantified in male rats fed an n-6 PUFA adequate or deficient diet for 15weeks post-weaning.

RESULTS

The deficient compared with adequate diet increased brain mRNA, protein and activity of iPLA(2) VIA and 15-lipoxygenase (LOX), but decreased cPLA(2) IVA and COX-2 expression. The brain protein level of the iPLA(2) transcription factor SREBP-1 was elevated, while protein levels were decreased for AP-2α and NF-κB p65, cPLA(2) and COX-2 transcription factors, respectively.

CONCLUSIONS

With dietary n-6 PUFA deprivation, rat brain PUFA metabolizing enzymes and some of their transcription factors change in a way that would homeostatically dampen reductions in brain n-6 PUFA concentrations and metabolism, while n-3 PUFA metabolizing enzyme expression is increased. The changes correspond to reported in vitro enzyme selectivities for AA compared with DHA.

POLYUNSATURATED FATTY ACID REGULATION OF GENES OF LIPID METABOLISM

Apart from being an important macronutrient, dietary fat has recently gained much prominence for its role in regulating gene expression. Polyunsaturated fatty acids (PUFAs) affect gene expression through various mechanisms including, but not limited to, changes in membrane composition, intracellular calcium levels, and eicosanoid production. Furthermore, PUFAs and their various metabolites can act at the level of the nucleus, in conjunction with nuclear receptors and transcription factors, to affect the transcription of a variety of genes. Several of these transcription mediators have been identified and include the nuclear receptors peroxisome proliferator-activated receptor (PPAR), hepatocyte nuclear factor (HNF)-4alpha, and liver X receptor (LXR) and the transcription factors sterol-regulatory element binding protein (SREBP) and nuclear factor-kappaB (NFkappaB). Their interaction with PUFAs has been shown to be critical to the regulation of several key genes of lipid metabolism. Working out the mechanisms by which these interactions and consequent effects occur is proving to be complicated but is invaluable to our understanding of the role that dietary fat can play in disease management and prevention.

Polyunsaturated Fatty Acid Regulation of Gene Expression

Polyunsaturated fatty acids (PUFAs), specifically the n-3 series, have been implicated in the prevention of various human diseases, including obesity, diabetes, coronary heart disease and stroke, and inflammatory and neurologic diseases. PUFAs function mainly by altering membrane lipid composition, cellular metabolism, signal transduction, and regulation of gene expression. PUFAs regulate the expression of genes in various tissues, including the liver, heart, adipose tissue, and brain. The role of transcription factors such as SREBP1c and nuclear receptors such as PPAR-alpha, HNF-4alpha, and LXRalpha in mediating the nuclear effects of PUFAs are addressed.

Polyunsaturated fatty acids and gene expression in mammalian systems

Over the last 30 years it has become apparent that specific dietary fatty acids are capable of regulating, either directly or indirectly through various signal pathways, the expression of numerous genes, either positively or negatively. Such nutrient-gene interactions have important effects on cell metabolism, differentiation and growth, and ultimately on disease processes. The present review describes some of the more important fatty acid-gene interactions in relation to health and disease in mammalian species, and focuses on the underlying cell signal mechanisms, including various transcription factors, affected by fatty acids and some of their oxygenated derivatives, e.g. the eicosanoids. The review also attempts to clarify some of the complexities of the effects of fatty acids by suggesting a possible overriding regulation by the redox status of the cell. The latter will at least stimulate controversy in this exciting area of lipid research.

Monohydroxylated fatty acid content in peripheral blood mononuclear cells and immune status of people at long times after the Chernobyl accident

The monohydroxylated fatty acid content of peripheral blood mononuclear cells from 23 cleanup workers and 16 unexposed individuals was studied in relation to their immune status after the Chernobyl accident. Men with absorbed doses below 0.32 Gy showed higher levels of free and esterified 12-hydroxyeicosatetraenoic acid (12-HETE) than unexposed men, whereas 15-HETE and the 17-hydroxy derivative of C22 fatty acid (17-OH 22), either free or esterified in phospholipids, were increased in a dose-dependent manner. The percentage of CD4-positive cells was also increased significantly in heavily irradiated men, whereas the percentage of CD8-positive cells tended to decrease with dose. Furthermore, the absolute count of CD4-positive cells was correlated positively with the amount of esterified 15-HETE in the phospholipid fraction of the mononuclear cells and with the total 15-HETE. These results show for the first time that the accumulation of autoxidized/lipoxygenase products of polyunsaturated fatty acids in the mononuclear cells of irradiated individuals was associated with immune imbalance. This may be the basis for certain late effects of radiation such as autoimmune disorders, somatic and neoplastic diseases, and early aging.

Effect of shigatoxin-1 on arachidonic acid release by human glomerular epithelial cells

BACKGROUND

Altered arachidonic acid (AA) metabolism has been implicated in the pathogenesis of renal injury in the hemolytic uremic syndrome (HUS). However, there is very little information of the effect of shigatoxin (Stx; the putative mediator of renal damage in HUS) on AA release or metabolism by renal cells. Since recent studies have demonstrated that glomerular epithelial cells (GECs) may be important early targets of Stx, the current study was undertaken to examine the effects of Stx on AA release and metabolism by GECs.

METHODS

Cultured human GECs were exposed to Stx1 +/- lipopolysaccharide (LPS) for 4 to 48 hours followed by determination of (3)H-arachidonate release, thromboxane A(2) (TxA(2)) and prostacyclin (PGI(2)) production, cyclooxygenase (COX) activity, and Western and Northern analyses for phospholipase A(2) (PLA(2)) and COX protein and mRNA levels, respectively.

RESULTS

Stx1 increased arachidonate release by GECs. LPS alone had no such effect, but increased arachidonate release in response to Stx1. Stx1-stimulated arachidonate release correlated with elevations in cPLA(2) and sPLA(2) protein and cPLA(2) mRNA levels. Stx1 also increased both TxA(2) and PGI(2) production by GECs; LPS alone did not alter eicosanoid production, but augmented Stx1 effects. Both Stx1 and LPS stimulated COX activity; however, these effects were not additive. Although there was an accompanying elevation of COX-1 and COX-2 mRNA, Stx1 decreased and LPS did not change COX1 and COX2 protein levels.

CONCLUSIONS

Stx1 alone or in conjunction with LPS increases arachidonate release and eicosanoid production by human GECs; this effect correlates with increased PLA(2) protein and mRNA levels. To our knowledge, this is the first study identifying the mechanisms of Stx1-stimulated AA release. These results raise the possibility that arachidonate release and metabolism by GECs, and conceivably other renal cell types, are involved in renal injury in HUS.

Incorporation and distribution of saturated and unsaturated fatty acids into nuclear lipids of hepatic cells

Liver nuclear incorporation of stearic (18:0), linoleic (18:2n-6), and arachidonic (20:4n-6) acids was studied by incubation in vitro of the [1-14C] fatty acids with nuclei, with or without the cytosol fraction at different times. The [1-14C] fatty acids were incorporated into the nuclei as free fatty acids in the following order: 18:0 > 20:4n-6 >> 18:2n-6, and esterified into nuclear lipids by an acyl-CoA pathway. All [1-14C] fatty acids were esterified mainly to phospholipids and triacylglycerols and in a minor proportion to diacylglycerols. Only [1-14C]18:2n-6-CoA was incorporated into cholesterol esters. The incorporation was not modified by cytosol addition. The incorporation of 20:4n-6 into nuclear phosphatidylcholine (PC) pools was also studied by incubation of liver nuclei in vitro with [1-14C]20:4n-6-CoA, and nuclear labeled PC molecular species were determined. From the 15 PC nuclear molecular species determined, five were labeled with [1-14C]20:4n-6-CoA: 18:0-20:4, 16:0-20:4, 18:1-20:4, 18:2-20:4, and 20:4-20:4. The highest specific radioactivity was found in 20:4-20:4 PC, which is a minor species. In conclusion, liver cell nuclei possess the necessary enzymes to incorporate exogenous saturated and unsaturated fatty acids into lipids by an acyl-CoA pathway, showing specificity for each fatty acid. Liver cell nuclei also utilize exogenous 20:4n-6-CoA to synthesize the major molecular species of PC with 20:4n-6 at the sn-2 position. However, the most actively synthesized is 20:4-20:4 PC, which is a quantitatively minor component. The labeling pattern of 20:4-20:4 PC would indicate that this molecular species is synthesized mainly by the de novo pathway.

Polyunsaturated fatty acid regulation of gene expression

For the past three decades, polyunsaturated fatty acids (PUFA) have been recognized as important energy sources and membrane components. PUFA also play key roles in many cellular events, such as gene regulation. Most recently, research has focused on identifying the mechanisms by which PUFA modulate gene transcription, mRNA stability and cellular differentiation. It is the purpose of this review to examine the effects of PUFA on gene expression in lipogenic as well as other tissues. Because the (n-3) and (n-6) series of PUFA are intimately involved in gene regulation, they will be the focus of review. The effects of other fatty acid families on gene expression are reviewed elsewhere.

Specificity of hydroperoxy fatty acid inhibition of cell growth and the lack of effect on tumour necrosis factor-induced cytotoxicity in WEHI clone 13 cells

We have examined whether different omega6-hydroperoxy fatty acids affect tumour cell growth or modulate TNF-induced toxicity in a fatty acid specific way in WEHI clone 13 fibrosarcoma cells. The omega6-hydroperoxides were synthesized from 8 different n - 6 and n - 3 PUFAs by soybean lipoxygenase. The omega6-hydroperoxy fatty acids inhibited cell growth in a concentration-dependent way by a mechanism that is related to the hydroperoxy moiety. Intracellular GSH seemed to protect since the GSH synthase inhibitor L-buthionine-S,R-sulfoximine (BSO) increased cell growth inhibition further. The antioxidants butylated hydroxyanisole (BHA), butylated hydroxytoluene and alpha-tocopherol did not affect the toxicity. The extent of growth inhibition varied between the hydroperoxides, but the difference was relatively small. The most toxic was hydroperoxy-alpha-linolenic acid which reduced cell survival by 56% after 44 h incubation at 35 microM, while the least toxic, hydroperoxy-gamma-linolenic acid, reduced cell survival by only 10%. The data also show that there is no correlation between toxicity and degree of unsaturation of the hydroperoxy fatty acids. None of the 8 different hydroperoxy fatty acids potentiated TNF-induced toxicity. This, together with the differential effects of BHA and BSO on TNF- and hydroperoxy fatty acid toxicity, indicate that neither the hydroperoxides nor their metabolites are involved in mediating or modulating the TNF-effect.

Evidence for the involvement of 5-lipoxygenase products in the regulation of the expression of the proenkephalin gene in cultured astroglial cells

Cultured astroglial cells secrete eicosanoids which are produced by the cyclooxygenase and lipoxygenases. These cells also transcribe the proenkephalin gene. In the present study, it was investigated whether agents which inhibit the metabolism of arachidonic acid affect the basal and stimulated expression of the gene. Tetradecanoyl phorbol acetate (TPA; 1-1000 nmol/l) increases the concentration of proenkephalin mRNA in these cells by activating protein kinase C. The enhancement in proenkephalin mRNA caused by TPA (10 nmol/l) was not affected by the cyclooxygenase inhibitor indomethacin (5 mumol/l). However, nordihydroguaiaretic acid, which blocks cyclooxygenase and lipoxygenases, potentiated the effect of TPA on proenkephalin mRNA, when used at concentrations of 0.5-50 mumol/l. Two selective inhibitors of 5-lipoxygenase, i.e. MK886 (5 mumol/l) and BAY X1005 (1 mumol/l), also enhanced the effect of TPA (10 nmol/l) without affecting the basal expression of the gene. When added to the incubation medium, leukotriene E4 (10-1000 nmol/l) diminished in a dose-dependent manner the basal and TPA-induced expression of the proenkephalin gene. It is concluded that in astroglial cells derived from cortex of new-born rats products of 5-lipoxygenase can diminish the action of protein kinase C on the proenkephalin gene.

Evidence that dietary arachidonic acid increases circulating triglycerides

Male Syrian hamsters and male CD-1 mice were fed diets supplemented with ethyl esters of oleic, linoleic, arachidonic, and eicosapentaenoic acids (1.1-5%, w/w) for 3-4 wk. Plasma and serum triglycerides were significantly higher in the arachidonic acid-supplemented animals compared to those in the other supplementation groups. Changes in serum insulin and glucose levels did not appear to be related to the changes in circulating triglycerides observed in the arachidonic acid-supplemented group. These data indicate that dietary arachidonic acid elevates circulating triglyceride levels compared to other unsaturated fatty acids in hamsters and mice by unknown mechanisms.

Redox regulation of programmed cell death in lymphocytes

A redox imbalance caused by an over-production of prooxidants or a decrease in antioxidants seems to play a role in the programmed cell death that occurs in various developmental programs. Such a physiological function for oxidative stress is particularly applicable to the immune system, wherein individual lymphocytes undergo continuous scrutiny to determine if they should be preserved or programmed to die. Following activation, lymphocytes produced increased levels of reactive oxygen species (ROS) which may serve as intracellular signaling molecules. The ultimate outcome of this increased ROS formation, i.e., lymphocyte proliferation versus programmed cell death, may be dictated by macrophage-derived costimulatory molecules that bolster or diminish lymphocyte antioxidant defenses. HIV-1-infected individuals display multiple symptoms of redox imbalance consistent with their being in oxidative stress, and lymphocytes from such individuals are more prone to undergo apoptosis in vitro. It is suggested that oxidative stress, and lymphocytes from such individuals are more prone to undergo apoptosis in vitro. It is suggested that oxidative stress is a physiological mediator of programmed cell death in lymphoid cells, and that HIV disease represents an extreme case of what can happen when regulatory safeguards are compromised.

The regulation of stearoyl-CoA desaturase (SCD)

The stearoyl-CoA desaturase gene family encodes stearoyl-CoA desaturase, the key enzyme involved in the biosynthesis of unsaturated fatty acids, as well as in the regulation of this process. Because of the important role that the SCD gene product plays in fat cell metabolism, future studies on SCD1 gene expression could provide new insights into the role of fatty acids in cellular regulation, metabolism, and gene expression both in normal and disease states. In addition, the SCD gene family can be used as a model to study mechanisms of cellular differentiation, tissue-specific gene expression, and dietary and hormonal regulation of gene expression.