Nuclear translocation of cytosolic phospholipase A2 is induced by ATP depletion

Phospholipase A(2) (PLA(2)) enzymes may play a role in cellular injury due to ATP depletion. Renal Madin-Darby canine kidney cells were subjected to ATP depletion to assess the effects of cellular energy metabolism on cytosolic PLA(2) (cPLA(2)) regulation. ATP depletion results in a decrease in soluble cPLA(2) activity and an increase in membrane-associated activity, which is reversed upon restoration of ATP levels by addition of dextrose. In ATP-depleted cells cPLA(2) mass shifts from cytosol to nuclear fractions. GFP-cPLA(2) is localized at the nuclear membrane of stably transfected ATP-depleted LLC-PK(1) cells under conditions where [Ca(2+)](i) is known to increase. cPLA(2) translocation does not occur if the increase in [Ca(2+)](i) increase is inhibited. If [Ca(2+)](i) is allowed to increase when ATP is depleted and the cells are then lysed, cPLA(2) remains associated with nuclear fractions even if the homogenate [Ca(2+)] is markedly reduced. In contrast, cPLA(2), which becomes associated with the nucleus when [Ca(2+)](i) is increased using ionophore, readily dissociates from the nuclear fractions of ATP-replete cells upon reduction of homogenate [Ca(2+)]. Okadaic acid inhibits the ATP depletion-induced association of cPLA(2) with nuclear fractions. Thus energy deprivation results in [Ca(2+)]-induced nuclear translocation, which is partially prevented by a phosphatase inhibitor.

PLIP, a novel splice variant of Tip60, interacts with group IV cytosolic phospholipase A(2), induces apoptosis, and potentiates prostaglandin production

The group IV cytosolic phospholipase A(2) (cPLA(2)) has been localized to the nucleus (M. R. Sierra-Honigmann, J. R. Bradley, and J. S. Pober, Lab. Investig. 74:684-695, 1996) and is known to translocate from the cytosolic compartment to the nuclear membrane (S. Glover, M. S. de Carvalho, T. Bayburt, M. Jonas, E. Chi, C. C. Leslie, and M. H. Gelb, J. Biol. Chem. 270:15359-15367, 1995; A. R. Schievella, M. K. Regier, W. L. Smith, and L. L. Lin, J. Biol. Chem. 270:30749-30754, 1995). We hypothesized that nuclear proteins interact with cPLA(2) and participate in the functional effects of this translocation. We have identified a nuclear protein, cPLA(2)-interacting protein (PLIP), a splice variant of human Tip60, which interacts with the amino terminal region of cPLA(2). Like Tip60, PLIP cDNA includes the MYST domain containing a C2HC zinc finger and well-conserved similarities to acetyltransferases. Both PLIP and Tip60 coimmunoprecipitate and colocalize with cPLA(2) within the nuclei of transfected COS cells. A polyclonal antibody raised to PLIP recognizes both PLIP and Tip60. Endogenous Tip60 and/or PLIP in rat mesangial cells is localized to the nucleus in response to serum deprivation. Nuclear localization coincides temporally with apoptosis. PLIP expression, mediated by adenoviral gene transfer, potentiates serum deprivation-induced prostaglandin E(2) (PGE(2)) production and apoptosis in mouse mesangial cells from cPLA(2)(+/+) mice but not in mesangial cells derived from cPLA(2)(-/-) mice. Thus PLIP, a splice variant of Tip60, interacts with cPLA(2) and potentiates cPLA(2)-mediated PGE(2) production and apoptosis.

Cell biology and molecular mechanisms of injury in ischemic acute renal failure

The pathogenesis of acute renal failure has been attributed to persistent vasoconstriction and leukocyte-endothelial interactions, resulting in inflammation and compromise of local blood flow to the outer medulla, the loss of tubular epithelial cell polarity with multiple functional sequelae, necrosis or apoptosis of epithelial cells, and the de-differentiation, migration and proliferation of surviving cells. In this paper, the authors present their views of pathophysiology of ischemic acute renal failure.

Fatty acid-induced cytotoxicity: differences in susceptibility between MDCK cells and primary cultures of proximal tubular cells

We have compared the cytotoxicity of exogenously added fatty acid (oleic acid) and that of endogenous free fatty acids released from cell membranes by phospholipase A2 in primary cultures of mouse proximal tubular (MPT) cells and in Madine-Darby canine kidney (MDCK) cells. Exposure of MPT cell monolayers to oleic acid (125 mmol/L) for 2 hours resulted in severe irreversible injury to 70% +/- 4% of MPT cells. In striking contrast, only 8% +/- 3% of MDCK cells were killed by the same insult. This striking difference in the response to exogenous oleate by MPT and MDCK cells was associated with modest and comparable reductions in cell adenosine triphosphate (ATP) content in both cell types. Chemical anoxia induced by cyanide plus deoxyglucose (CN-DOG) in the absence of glucose was associated with greater injury in MPT cells (45% +/- 6% killed) than in MDCK cells (16% +/- 5% cells killed) despite severe and comparable depletion of cell ATP content in both MPT cells (96.0% +/- 0.6% reduction) and MDCK cells (96.0% +/- 0.5% reduction). The release of endogenous fatty acids by the exposure of cells to exogenous phospholipase A2 caused mild injury in both cell types that was more severe in MPT cells than in MDCK cells. The combined insult of phospholipase A2 and chemical anoxia for 2 hours caused substantially greater cell injury in both MPT and MDCK cells than either intervention alone, but the combined insult was still more damaging to MPT cells (73% +/- 4% killed) than to MDCK cells (30% +/- 4% killed). We conclude that the cell membrane in MDCK cells is intrinsically more resistant to fatty acid-induced injury than the lipid membrane in MPT cells.

Lipid peroxidation contributes to hydrogen peroxide induced cytotoxicity in renal epithelial cells

We have examined the role of lipid peroxidation in the cytotoxicity of H2O2 in OK cells containing markedly differing amounts of cell membrane polyunsaturated fatty acids (PUFA). In OK cells grown in a serum free medium, PUFA were undetectable. The membranes of these cells contained predominantly oleic, stearic and palmitic acids. When cultured in medium containing 10% calf serum, OK cells contained measurable amounts of PUFA [linoleic (5 +/- 1%) and arachidonic acids (8 +/- 1%)]. When the serum containing medium was supplemented with 60 mM linoleic acid, the membrane content of both linoleic (21 +/- 1%) as well as arachidonic acid (15 +/- 1%) as substantially increased. The severity of injury induced by H2O2 in OK cells was substantially altered by the PUFA content of the cell membrane. Exposure of OK cells to 1.25 mM H2O2 for one hour resulted in more cell death (determined by a trypan blue assay) in cells grown in serum supplemented with linoleic acid with "normal" PUFA content (90 +/- 2%) than in cells with "reduced" levels of PUFA grown in unsupplemented calf serum (81 +/- 3%). Cells gown in defined, serum free medium with undetectable levels of PUFA suffered the least H2O2-induced lethal cell injury (47 +/- 8%). Comparable differences in the cytotoxicity of H2O2 among cells with differing PUFA content were found using a clonogenic assay of cell viability. Malondialdehyde (MDA) accumulation induced by 1.25 mM H2O2 was greater in cells with "normal" PUFA content (702 +/- 103 pM/microgram cell DNA/hr) than in cells with "reduced" PUFA (328 +/- 112 pM/100 microgram DNA/hr) and was undetectable in cells grown in defined, serum free medium. In summary, the content of PUFA of cells in culture is profoundly influenced by culture conditions. Our data provide novel and direct evidence that peroxidation of cell membranes contributes directly to the severity of cell injury and death induced by H2O2.

Functional and cytoskeletal changes induced by sublethal injury in proximal tubular epithelial cells

Mouse proximal tubular (MPT) cells in culture were subjected to ATP depletion by incubating them with cyanide in the absence of dextrose for 1 h. This insult resulted in marked alterations in the actin cytoskeleton. These changes were not associated with a decrease in cell viability and thus reflected sublethal injury. The effect of sublethal injury on the functional integrity of the intercellular tight junction (TJ) was then examined in MPT cell monolayers grown on permeable supports. During chemical anoxia, monolayer permeability to the paracellular marker mannitol progressively increased to 297 +/- 62% of baseline after 1 h. Chemical anoxia also caused a reversible loss in cell-substrate adhesion when MPT cells were studied as confluent monolayers or as single cells. Thus disruption of the actin cytoskeleton in nonlethally injured cells results in important reversible alterations in renal epithelial function characterized by impairment of the "gate" function of the TJ as well as impaired cell-substrate adhesion. We hypothesize that sublethal epithelial cell injury without accompanying necrosis may contribute to the decrement in renal function characteristic of ischemic renal injury.

Renal mouse proximal tubular cells are more susceptible than MDCK cells to chemical anoxia

To elucidate the mechanisms responsible for the resistance of continuous cell lines to anoxic injury, we have compared the effects of ATP depletion induced by chemical anoxia on primary cultures of mouse proximal tubular (MPT) cells and on Madin-Darby canine kidney (MDCK) cells. Inhibition of ATP production by cyanide and 2-deoxyglucose (CN+DOG) in the absence of dextrose reduced cell ATP content to < 5% of control values in MPT cells and caused progressive deterioration in mitochondrial function as well as loss of cell viability in these cells. Cell free fatty acid (FFA) content rose from 4.3 +/- 0.9 to 23.7 +/- 2.0 micrograms/mg of total lipid weight after 4 h of CN + DOG (P < 0.05). The mitochondrial injury and cell death induced by CN + DOG in MPT cells was ameliorated by the addition of fatty acid-free bovine albumin to the cell medium, which reduced cell FFA content during chemical anoxia from 25.0 +/- 3.0 to 10.4 +/- 2.0 micrograms/mg (P < 0.05). The phospholipase A2 (PLA2) inhibitor, mepacrine, also resulted in functional protection and reduction of cell FFA content from 20.2 +/- 2.3 to 15.9 +/- 1.7 micrograms/mg (P < 0.05). These data suggest a role for phospholipase activation and accumulation of toxic lipid metabolites in the pathophysiology of MPT cell injury. We then compared cell injury induced by CN + DOG in MPT and MDCK cells. Despite comparable reduction in cell ATP content in the two cell types, injury was far more severe in MPT than MDCK cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effect of atrial natriuretic factor and mannitol following renal ischemia

We have examined the effect of atrial natriuretic factor (ANF) administered with and without mannitol on renal function following ischemic injury in both the isolated erythrocyte-perfused rat kidney and in the rat in vivo. ANF, administered alone after 25 min ischemia in the isolated kidney, reversed postischemic vasoconstriction but did not improve glomerular filtration rate (GFR). Mannitol alone had no effect on either renal vascular resistance or GFR. However, in isolated kidneys treated with the combination of both ANF and mannitol following reflow, GFR (0.65 +/- 0.04 ml.min-1.g-1) was markedly improved compared with GFR in the untreated ischemia group (0.20 +/- 0.04 ml.min-1.g-1) and was not different from GFR in the nonischemic controls (0.68 +/- 0.05 ml.min-1.g-1). Comparable results were obtained in studies performed in vivo. In rats subjected to 45 min ischemia, GFR (0.15 +/- 0.05 ml/min) was reduced compared with the GFR in sham-operated animals (0.95 +/- 0.07 ml/min). ANF or mannitol administered alone following ischemia and reflow did not improve GFR compared with the untreated ischemic group. However, in rats subjected to ischemia and treated with a combination of ANF and mannitol postreflow, GFR (0.69 +/- 0.10 ml/min) was 4.6-fold higher than GFR in the untreated ischemic group. Thus the combination of ANF and mannitol appear to act synergistically to improve GFR following ischemic injury.

5-Hydroxytryptophan and carbidopa in spontaneously hypertensive rats

Serial measurements of blood pressure, body weight, food and water intake, and salt and water excretion were compared in two groups of spontaneously hypertensive rats (SHR) over a 12-day period: control SHR (n = 11) and a group (n = 9) which received supplementary 5-hydroxytryptophan (5-HTP; 2 mg/ml) in its drinking water. During the final 4 days of study, both groups received additional oral carbidopa (50 mg/kg twice a day) to inhibit peripheral, but not brain aromatic L-amino-acid decarboxylase (LAAD), an enzyme necessary to the formation of 5-hydroxytryptamine (5-HT, serotonin) from 5-HTP. 5-Hydroxytryptophan increased urinary 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) markedly; following carbidopa, urinary 5-HT, and to a lesser degree urinary 5-HIAA, decreased, whereas brain 5-HT and 5-HIAA increased. Spontaneously hypertensive rats treated with 5-HTP plus carbidopa had significantly lower blood pressure levels, lower pulse rates, reductions in food and water intake, salt and water excretion, and a loss of body weight, when compared with the control SHR. These data indicate that enhanced brain formation of 5-HT can give rise to metabolic and circulatory responses with a resultant lowering of blood pressure.