Effect of fatty acids on physical properties of microsomes from isolated perfused rat liver

Excess dietary carbohydrate affects mitochondrial integrity as observed in brown adipose tissue

Hyperglycemia affects over 400 million individuals worldwide. The detrimental health effects are well studied at the tissue level, but the in vivo effects at the organelle level are poorly understood. To establish such an in vivo model, we used mice lacking TXNIP, a negative regulator of glucose uptake. Examining mitochondrial function in brown adipose tissue, we find that TXNIP KO mice have a lower content of polyunsaturated fatty acids (PUFAs) in their membrane lipids, which affects mitochondrial integrity and electron transport chain efficiency and ultimately results in lower mitochondrial heat output. This phenotype can be rescued by a ketogenic diet, confirming the usefulness of this model and highlighting one facet of early cellular damage caused by excess glucose influx.

Liver function and dysfunction - a unique window into the physiological reach of ER stress and the unfolded protein response

The unfolded protein response (UPR) improves endoplasmic reticulum (ER) protein folding in order to alleviate stress. Yet it is becoming increasingly clear that the UPR regulates processes well beyond those directly involved in protein folding, in some cases by mechanisms that fall outside the realm of canonical UPR signaling. These pathways are highly specific from one cell type to another, implying that ER stress signaling affects each tissue in a unique way. Perhaps nowhere is this more evident than in the liver, which-beyond being a highly secretory tissue-is a key regulator of peripheral metabolism and a uniquely proliferative organ upon damage. The liver provides a powerful model system for exploring how and why the UPR extends its reach into physiological processes that occur outside the ER, and how ER stress contributes to the many systemic diseases that involve liver dysfunction. This review will highlight the ways in which the study of ER stress in the liver has expanded the view of the UPR to a response that is a key guardian of cellular homeostasis outside of just the narrow realm of ER protein folding.

Enhanced synthesis of saturated phospholipids is associated with ER stress and lipotoxicity in palmitate treated hepatic cells

High levels of saturated FAs (SFAs) are acutely toxic to a variety of cell types, including hepatocytes, and have been associated with diseases such as type 2 diabetes and nonalcoholic fatty liver disease. SFA accumulation has been previously shown to degrade endoplasmic reticulum (ER) function leading to other manifestations of the lipoapoptotic cascade. We hypothesized that dysfunctional phospholipid (PL) metabolism is an initiating factor in this ER stress response. Treatment of either primary hepatocytes or H4IIEC3 cells with the SFA palmitate resulted in dramatic dilation of the ER membrane, coinciding with other markers of organelle dysfunction. This was accompanied by increased de novo glycerolipid synthesis, significant elevation of dipalmitoyl phosphatidic acid, diacylglycerol, and total PL content in H4IIEC3 cells. Supplementation with oleate (OA) reversed these markers of palmitate (PA)-induced lipotoxicity. OA/PA cotreatment modulated the distribution of PA between lipid classes, increasing the flux toward triacylglycerols while reducing its incorporation into PLs. Similar trends were demonstrated in both primary hepatocytes and the H4IIEC3 hepatoma cell line. Overall, these findings suggest that modifying the FA composition of structural PLs can protect hepatocytes from PA-induced ER stress and associated lipotoxicity.

Disruption of endoplasmic reticulum structure and integrity in lipotoxic cell death

Cell dysfunction and death induced by lipid accumulation in nonadipose tissues, or lipotoxicity, may contribute to the pathogenesis of obesity and type 2 diabetes. However, the mechanisms leading to lipotoxic cell death are poorly understood. We recently reported that, in Chinese hamster ovary (CHO) cells and in H9c2 cardiomyoblasts, lipid overload induced by incubation with 500 muM palmitate leads to intracellular accumulation of reactive oxygen species, which subsequently induce endoplasmic reticulum (ER) stress and cell death. Here, we show that palmitate also impairs ER function through a more direct mechanism. Palmitate was rapidly incorporated into saturated phospholipid and triglyceride species in microsomal membranes of CHO cells. The resulting membrane remodeling was associated with dramatic dilatation of the ER and redistribution of protein-folding chaperones to the cytosol within 5 h, indicating compromised ER membrane integrity. Increasing beta-oxidation, through the activation of AMP-activated protein kinase, decreased palmitate incorporation into microsomes, decreased the escape of chaperones to the cytosol, and decreased subsequent caspase activation and cell death. Thus, palmitate rapidly increases the saturated lipid content of the ER, leading to compromised ER morphology and integrity, suggesting that impairment of the structure and function of this organelle is involved in the cellular response to fatty acid overload.

Dietary modifications of phospholipid composition and biophysical properties of the brush border membrane along the trout intestine

Brush border membranes (BBM) are isolated from middle and posterior intestine of trout fed either an essential fatty acid-rich diet or a saturated one. The different phospholipid classes are separated, and their fatty acid composition is determined. Fluorescence anisotropy studies are performed using two lipid fluorophores, namely diphenylhexatriene (DPH) and trimethylamino-diphenylhexatriene (TMA-DPH). The results indicate that the usual parameters affecting the lipid fluidity such as the phospholipid:protein (PL:PROT), cholesterol:phospholipid (CHOL:PL), and sphingomyelin:phosphatidylcholine (SP:PC) ratios and the unsaturation of the acyl chains are sufficient to explain the fluidity values determined using DPH, but not those obtained with TMA-DPH as a probe. This fluorophore is assessed to be localized only in the external leaflet of the membrane. Hence, it will be affected by the composition of the major phospholipids of this leaflet, sphingomyelin and phosphatidylcholine.

Lipid domains in plasma membranes from rat liver

The existence of fluid and solid lipid domains in isolated rat-liver plasma membranes was evaluated using the fluorescent fatty acids trans-parinaric and cis-parinaric acid as probe molecules for solid and fluid membrane areas, respectively. The fluorescence probe 1,6-diphenyl-1,3,5-hexatriene indicated that a phase transition was present in the liver plasma membrane between 18 degrees C and 30 degrees C. At intermediate temperatures, cis-parinaric acid, which partitioned approximately equally into fluid and solid lipid areas, detected two lipid domains: the mole fractions of fluid and solid lipid domains at 24 degrees C were 0.32 and 0.68 while the mole fractions of cis-parinaric acid in each domain were 0.34 and 0.66, respectively. The dissociation constant, aqueous to membrane lipid partition coefficient, and bound to free ratio for trans-parinaric acid were 7.0 +/- 0.7 microM, 4.0 +/- 0.6 x 10(6), and 83:17, respectively. The affinity of the membrane for cis-parinaric acid was twofold lower than for trans-parinaric acid. The trans-parinaric acid partitioned preferentially into solid lipid, Ksp/f = 3.30, while the cis-parinaric acid partitioned equally between fluid and solid phases Ksp/f = 0.92. Thus, the data demonstrate the coexistence of fluid and solid domains in rat liver plasma membranes.

Calcium modulates fatty acid dynamics in rat liver plasma membranes

Modulation of free fatty acid binding in isolated rat liver plasma membranes was evaluated using the fluorescent fatty acids trans-parinaric and cis-parinaric acid as analogues for saturated and unsaturated fatty acids, respectively. Binding of trans-parinarate but not cis-parinarate was inhibited by physiological levels of Ca2+. The effect was reversed by addition of excess EGTA. Calcium decreased the aqueous to lipid partition coefficient, Kp, of trans-parinaric acid for liver plasma membranes while increasing the Kp for cis-parinaric acid. In addition, Ca2+ also altered the fluorescence lifetime, the quantum yield, and the relative partitioning of trans-parinaric and cis-parinaric acid into fluid and solid phases. Calcium and EGTA did not affect the binding of 1,6-diphenyl-1,3,5-hexatriene. The effect of Ca2+ on the liver plasma membrane structure was to increase the rigidity of the membrane, primarily the solid domain. The fluorescence polarization of trans-parinarate, cis-parinarate, and 1,6-diphenyl-1,3,5-hexatriene at 24 degrees C in liver plasma membranes in the absence of Ca2+ was 0.295 +/- 0.008, 0.253 +/- 0.007, and 0.284 +/- 0.005, respectively. Calcium (2.4 mM) increased the polarization of these probe molecules in liver plasma membranes by 8-10%. EGTA (3.4 mM) reversed or abolished the increase in polarization. Thus, the fluorescent fatty acids trans-parinarate and cis-parinarate may be used to monitor fatty acid binding by isolated membranes, to evaluate factors such as Ca2+ which modulate fatty acid binding, and to investigate the microenvironment in which the fatty acids residue. The data suggest that Ca2+ may be an important regulator of fatty acid uptake by the liver plasma membrane, and thereby interact with intermediary metabolism of lipids at a step not involving lipolytic or synthetic enzymes.

Hepatic microsomal composition studies in the Gunn rat

The homozygous jaundiced (jj) Gunn rat exhibits hepatic microsomal enzyme activities which vary from markedly decreased to normal when compared with the non-jaundiced (JJ) Gunn rat. In order to determine if an alteration in microsomal lipid might be related to these observations, cholesterol, phospholipid, fatty acid and fluorescence polarization determinations were carried out in Gunn rats of both genotypes. Significant differences in microsomal palmitic, stearic and arachidonic acid composition were present, but these were not striking. Fluorescence polarization data best fit a two-phase linear model for both genotypes with no significant differences in breakpoint temperatures. In jj rats, the anisotrophy parameter ((r0/r)-1)-1 was significantly greater than that seen in JJ rats at both 25 and 37 degrees C, indicating a decreased membrane fluidity in the jaundiced animals. Alterations in enzyme microenvironment due to subtle changes in lipid composition may be related to the different enzyme activities observed in Gunn rats.

Thermodynamics and membrane processes

This review represents a personal view of membrane thermodynamics. I do not intend to deal at all with the irreversible thermodynamics of membrane mass transfer processes. This aspect has been covered far more competently and completely by other people (Bittar, 1970; Paterson, 1970; Rottenberg, Caplan & Essig, 1970; Mitchell, 1970; Rothschildet al.1980; Oster, Perelson & Katchalsky, 1973; Kedem & Katchalsky, 1958; Schwartz, 1971). The recent review on osmosis by Hill (1979) is a particularly succinct appraisal of a facet of irreversible membrane thermodynamics. Arata & Nishimura (1980) have considered the coupling of electron transfer to vectorial processes in biological membranes.