An oxidized metabolite of linoleic acid increases intracellular calcium in rat adrenal glomerulosa cells

Abstract

-EKODE, an epoxy-keto derivative of linoleic acid, was previously shown to stimulate aldosterone secretion in rat adrenal glomerulosa cells (15). In the present study, we investigated the effect of exogenous EKODE on cytosolic [Ca 2ϩ ] increase and aimed to elucidate the mechanism involved in this process. Through the use of the fluorescent Ca 2ϩ -sensitive dye Fluo-4, EKODE was shown to rapidly increase intracellular [Ca 2ϩ ] ( [Ca 2ϩ ]i) along a bell-shaped dose-response relationship with a maximum peak at 5 M. Experiments performed in the presence or absence of Ca 2ϩ revealed that this increase in [Ca 2ϩ ]i originated exclusively from intracellular pools. EKODEinduced [Ca 2ϩ ]i increase was blunted by prior application of angiotensin II, Xestospongin C, and cyclopiazonic acid, indicating that inositol trisphosphate (InsP3)-sensitive Ca 2ϩ stores can be mobilized by EKODE despite the absence of InsP3 production. Accordingly, EKODE response was not sensitive to the phospholipase C inhibitor U-73122. EKODE mobilized a Ca 2ϩ store included in the thapsigargin (TG)-sensitive stores, although the interaction between EKODE and TG appears complex, since EKODE added at the plateau response of TG induced a rapid drop in [Ca 2ϩ ]i. 9-Oxo-octadecadienoic acid, another oxidized derivative of linoleic acid, also increases [Ca 2ϩ ]i, with a dose-response curve similar to EKODE. However, arachidonic and linoleic acids at 10 M failed to increase [Ca 2ϩ ]i but did reduce the amplitude of the response to EKODE. It is concluded that EKODE mobilizes Ca 2ϩ from an InsP3-sensitive store and that this [Ca 2ϩ ]i increase is responsible for aldosterone secretion by glomerulosa cells. Similar bell-shaped dose-response curves for aldosterone and [Ca 2ϩ ]i increases reinforce this hypothesis. intracellular calcium ion concentration LINOLEIC ACID is the most prevalent polyunsaturated fatty acid in mammals, found mostly in adipocyte triglycerides. It is a potential source of metabolic energy through ␤-oxidation but is also vulnerable to addition of molecular oxygen at its double bonds. Reactions that add oxygen to linoleic acid may be catalyzed by lipoxygenases, cytochromes, or free metal ions. The initial products of this addition are hydroperoxides, which rapidly undergo rearrangement and modification to epoxides, hydroxides, and ketones, some of which are relatively stable in aqueous solution. These oxidation reactions probably play a role in the pathological state called "oxidative stress," and, accordingly, some stable oxidation products of linoleic acid have toxic effects on mammalian cells and organelles (28).In earlier reports, we tested some of the compounds formed from linoleic acid on rat adrenal cells and demonstrated that at least three of these reaction products stimulate secretion of aldosterone by glomerulosa cells and corticosterone by fasciculata cells (7,14,15). The experiments reported in the present study were aimed at investigating the signaling mechanism by which one ...

New Mechanisms to Control Aldosterone Synthesis

“…However, sensitization of adrenocortical glomerulosa cells to the action of Ang II also can be mediated by low-density lipoproteins [70] . The association between fat metabolism and elevated plasma aldosterone levels in obesity was also observed by other groups, and it is proposed that linoleic acid oxidation products stimulate the production of aldosterone by rat adrenal cells via a calcium-dependent mechanism [71] .…”

From aldosteronism to oxidative stress: the role of excessive intracellular calcium accumulation

“…Yet another mechanism for a sustained ALDO-like state has implicated ROS in targeting and activating mineralocorticoid receptors. [9][10][11] In keeping with the pathophysiological roles, aldosteronism and oxidative stress are salutary responses to mineralocorticoid receptor antagonism. [12][13][14][15][16] AN EXPERIMENTAL MODEL OF ALDOSTERONISM Aldosterone/salt treatment We have worked extensively with the ALDOST model, wherein eightweek-old male Sprague-Dawley rats are first uninephrectomized to reduce renal mass available for Na + excretion.…”

Synthesis of Six Epoxyketooctadecenoic Acid (EKODE) Isomers, Their Generation from Nonenzymatic Oxidation of Linoleic Acid, and Their Reactivity with Imidazole Nucleophiles

“…A search of the literature revealed that various isomers of α,β-unsaturated epoxyketooctadecenoic acid (EKODE) 7 with this molecular weight are generated naturally from linoleic acid oxidation, and we suspected they would be capable of undergoing reaction with protein-based His nucleophiles. Although no indication of such covalent binding potential is evident in the literature, these EKODE isomers have been shown, in a series of studies, to have pronounced biological activity in stimulating steroid hormone production and secretion 8 and in increasing cellular calcium . Thus, regardless of whether any EKODE biological activity is associated with their covalent binding capacities, access to the individual isomers is crucial to advancing structure−activity knowledge on these highly potent bioactive molecules.…”