Fatty acids may regulate aldosterone secretion and mediate some of insulin's effects on blood pressure

The discovery, isolation and identification of aldosterone: reflections on emerging regulation and function

This paper has a focus on the early history of aldosterone. The Taits take us on a chronological trawl through the history in which they had a first hand role and made a major contribution-their bioassay was in many ways the key. The gifted Swiss chemists made a critical contribution to the scale and isolation of larger amounts. This was international collaboration at its best. Developing technologies were utilised as crucial cutting edge applications in the advancing front, technology transfer before the word was invented. Measurement of aldosterone and angiotensin were crucial advances to the understanding of the regulation of the hormone. In the period 1960-2003, some 30,000 papers mentioned aldosterone as a keyword, even so advances on a larger scale were slow. I have indicated some of my own work with the Howard Florey team using the adrenal autotransplant in the conscious sheep. Recently, the understanding of the role of induced proteins, the flow on from the RALES trial and the development of eplerenone has revitalised the aldosterone field.

Epoxy-keto derivative of linoleic acid stimulates aldosterone secretion

Plasma levels of aldosterone are not always predictable from the activity of renin and the concentration of potassium. Among the unexplained are elevated levels of aldosterone in some obese humans. Obesity is characterized by increased plasma fatty acids and oxidative stress. We postulated that oxidized fatty acids stimulate aldosteronogenesis. The most readily oxidized fatty acids are the polyunsaturated, and the most abundant of those is linoleic acid. We tested oxidized derivatives of linoleic acid for effects on rat adrenal cells. One derivative, 12,13-epoxy-9-keto-10(trans)-octadecenoic acid (EKODE), was particularly potent. EKODE stimulated aldosteronogenesis at concentrations from 0.5 to 5 micromol/L, and inhibited aldosteronogenesis at higher doses. EKODE's stimulatory effect was most prominent when angiotensin and potassium effects were submaximal. The lipid's mechanism of action was on the early pathway leading to pregnenolone; its action was inhibited by atrial natriuretic peptide. Plasma EKODE was measured by liquid chromatography/mass spectrometry. All human plasmas tested contained EKODE in concentrations ranging from 10(-9) to 5x10(-7) mol/L. In samples from 24 adults, levels of EKODE correlated directly with aldosterone (r=0.53, P=0.007). In the 12 blacks in that cohort, EKODE also correlated with body mass index and systolic pressure. Those other correlations were not seen in white subjects. The results suggest that oxidized derivatives of polyunsaturated fatty acids other than arachidonic are biologically active. Compounds like EKODE, derived from linoleic acid, may affect adrenal steroid production in humans and mediate some of the deleterious effects of obesity and oxidative stress, especially in blacks.

An oxidized derivative of linoleic acid affects aldosterone secretion by adrenal cells in vitro

Based on the clinical observation that humans with visceral adiposity have higher plasma aldosterone levels than controls, we postulated that endogenous fatty acids can be oxidized by the liver to form stimuli of the adrenal cortex. Although we could show that hepatocytes produced adrenal stimuli from linoleic acid in vitro, the yield was very small. To facilitate the elucidation of chemical structures, we incubated a large amount of linoleic acid with lipoxygenase, then treated the hydroperoxide with cysteine and iron. The major product of this process was 12,13-epoxy-9-keto-10-trans-octadecenoic acid. This epoxy-keto compound stimulated aldosterone production at concentrations from 0.5 to 15 microm. At higher concentrations, it was inhibitory. The epoxy-keto-octadecenoic acid exhibited the chromatographic characteristics of one product of the incubation of linoleic acid with hepatocytes. The results are consistent with the postulated conversion of linoleic acid to stimuli of aldosterone production. This may be a mechanistic link between visceral obesity and hypertension in humans.

Neonatal hypoxic hyperlipidemia in the rat: effects on aldosterone and corticosterone synthesis in vitro

Neonatal hypoxia increases aldosterone production and plasma lipids. Because fatty acids can inhibit aldosterone synthesis, we hypothesized that increases in plasma lipids restrain aldosteronogenesis in the hypoxic neonate. We exposed rats to 7 days of hypoxia from birth to 7 days of age (suckling) or from 28 to 35 days of age (weaned at day 21). Plasma was analyzed for lipid content, and steroidogenesis was studied in dispersed whole adrenal glands untreated and treated to wash away lipids. Hypoxia increased plasma cholesterol, triglycerides, and nonesterified fatty acids in the suckling neonatal rat only. Washing away lipids increased aldosterone production in cells from 7-day-old rats exposed to hypoxia, but not in cells from normoxic 7-day-old rats or from normoxic or hypoxic 35-day-old rats. Addition of oleic or linolenic acid to washed cells inhibited both aldosterone and corticosterone production, although cells from hypoxic 7-day-old rats were less sensitive. We conclude that hypoxia induces hyperlipidemia in the suckling neonate and that elevated nonesterified fatty acids inhibit aldosteronogenesis.

Plasma aldosterone, plasma lipoproteins, obesity and insulin resistance in humans

Aldosterone production in vitro can be affected by many hormones, autacoids, ions, and lipids, but regulation in humans is incompletely understood. We measured plasma aldosterone in adult subjects with a wide range of obesity and insulin resistance. Aldosterone levels correlated with measures of visceral obesity in one predominantly male cohort and in the women of a second cohort. In the same subjects, aldosterone correlated with insulin resistance. Aldosterone also correlated with plasma cortisol in men and women, and with DHEA-S in women. The data suggested that visceral fat stimulates adrenal steroidogenesis. We found that certain fatty acids stimulated aldosterone production in vitro by rat adrenal cells incubated with rat hepatocytes, but not adrenal cells alone. The results suggested that fatty acids from visceral adipocytes induce hepatic formation of an adrenal secretagogue. This may explain the correlation of plasma steroids with visceral obesity. Aldosterone may contribute to vascular diseases that complicate obesity.

Late steps of aldosterone biosynthesis: sheep are not rats

1. The last three steps of aldosterone biosynthesis have been demonstrated to be catalysed by a single enzyme, referred to as CYP11B (or P450(11) beta) in cow, pig, sheep and bullfrog and as CYP11B2 (or P450aldo) in rat, human, mouse and hamster. 2. The related enzyme CYP11B1 (also referred to as P450(11) beta) in rat, human, mouse and hamster does not have aldosterone synthesis activity, but no such enzyme has been reported in the cow, pig or sheep to date. 3. Exclusive aldosterone secretion in the zona glomerulosa (ZG) of the adrenal cortex in species such as rat, human, mouse and hamster could be ascribed to the restricted distribution of CYP11B2 to the same region in the adrenal cortex. 4. In other species, such as cow, pig and sheep, the CYP11B enzyme is expressed throughout the adrenal cortex and, thus, the exclusive aldosterone biosynthesis in the ZG could not be explained simply by the distribution of the enzyme. 5. We have shown in the sheep that potassium loading and acute sodium depletion stimulate the CYP11B transcript levels, which are not further increased by chronic sodium depletion. 6. The predominant CYP11B in the sheep adrenal cortex catalyses the synthesis of aldosterone from deoxycorticosterone (DOC) in vitro, is expressed throughout the adrenal cortex and the corresponding transcript levels are increased by K+ loading or sodium depletion. In short, as far as the last step of aldosterone biosynthesis is concerned, sheep are different from rats. In the rat, the CYP11B2 transcript or protein is elevated by K+ loading or sodium depletion, but not the CYP11B1 transcript or protein. 7. We propose that during severe sodium deficiency there is a switch in the aldosterone pathway to one preferentially involving 18-OH-DOC and not corticosterone.

Aldosterone in obesity

Plasma aldosterone levels were measured in adults whose body mass index ranged from lean to obese. Blood was drawn while subjects rested supine for 30-90 minutes. Aldosterone was higher in obese subjects, but could not be explained by renin or K+. The best predictors of plasma aldosterone were abdominal obesity measured as waist/hip ratio or by CT scan, and insulin resistance measured by insulin or oral glucose tolerance tests, or euglycemic clamp. In one cohort, these correlations were limited to women; in the other, they were also found in men. In the women with a strong correlation between aldosterone and visceral fat, aldosterone also correlated with cortisol and DHEA-S. The data are consistent with an effect of visceral fat on adrenal steroidogenesis. Visceral adipocytes have a high rate of triglyceride turnover, and their circulation drains directly to the liver. In an experiment based on these characteristics, rat hepatocytes responded to fatty acids by releasing an unidentified secretagogue that stimulated aldosterone production by rat adrenal glomerulosa cells. The clinical data suggest that aldosterone participates in hypertension associated with the "Insulin Resistance Syndrome". The adrenal in viscerally obese subjects may be driven by a secretagogue released from the liver by fatty acids from abdominal adipocytes.

Hypothesis: aldosterone is synthesized by an alternative pathway during severe sodium depletion. 'A new wine in an old bottle'

1. The last three steps of aldosterone biosynthesis, 11 beta-hydroxylation, 18-hydroxylation and 18-oxidation, have been demonstrated to be catalysed by one enzyme, which is the cytochrome P450(11 beta) (CYP11B) in cow, pig, sheep and bullfrog or cytochrome P450aldo (CYP11B2) in rat, human, mouse and hamster. 2. The related enzyme P450(11 beta) (CYP11B1) from rat, human, mouse and hamster adrenals displays 11 beta-hydroxylation and 18-hydroxylation activities, but not 18-oxidation activity in vitro. No such enzyme has been reported in the cow, pig or sheep to date. 3. Data showing the dissociation of aldosterone secretion from plasma angiotensin II (AngII) levels indicate the presence of other factor(s) that regulate aldosterone biosynthesis in response to changes in body sodium status. Thus, we propose the existence of a 'sodium status factor' that regulates aldosterone biosynthesis in addition to AngII, K+, adrenocorticotropic hormone and atrial natriuretic peptide. 4. We propose that during severe sodium deficiency there is a switch in the aldosterone pathway to a pathway using 18-hydroxy-deoxycorticosterone (18-OH-DOC) rather than corticosterone as an intermediate. This switch may be mediated via the putative 'sodium status factor'. 5. Two models of the hypothesis will be discussed in this paper: (i) a 'one-enzyme' model; and (ii) a 'two-enzyme' model. 6. The one-enzyme model proposes that P450aldo (P450(11 beta) as in the case of the cow, sheep and pig) changes its enzymatic activity during severe sodium deficiency (i.e. switching to the alternative aldosterone biosynthesis pathway). 7. The two-enzyme model proposes that, under normal circumstances, P450aldo synthesizes aldosterone from deoxycorticosterone, while during severe sodium deficiency the P450(11 beta) provides the substrate (i.e. 18-OH-DOC) for the P450aldo.

Fatty acids involved in signal cross-talk between cell membrane and nucleus

The results presented underline the fact that the nature and the concentration of the non-esterified fatty acids (NEFAs) liberated from membrane lipids, particularly the essential ones issued from lipid nutrition, clearly belong to a large group of factors (hormones, retinoids, growth factors, cytokines...) which control the shift between cell multiplication and differentiation. NEFAs act on this shift, per se or after being metabolized, by influencing, as second messengers or modulators, the intertwined mechanisms of action of growth factors and steroid hormones. These results may explain the molecular links which exist between endocrinology, oncology and nutrition.

Biological complexity is under the 'strange attraction' of non-esterified fatty acids

It is now clear from numerous data that non-esterified fatty acids (NEFAs) can act without any metabolic modification as second messengers or modulators of the complex signalling network which is characteristic of mammals. This network can respond differently to adapt the organism to the various endogenous and exogenous environmental situations. NEFAs have a wide range of molecular structures, and thus can exert different specific modulatory actions on this signalling network, such as amplification, inhibition or signal redirection. We have chosen the term 'strange attractions' to describe these signalling modulations by analogy with the 'strange attractions' concept introduced in deterministic chaos theory. NEFAs can modulate the functions of mammals at all levels of organization (molecular, cellular, tissue, organ, etc).

Specificity and mechanism of fatty acid inhibition of aldosterone secretion

We have shown that unesterified, unsaturated long-chain fatty acids inhibit angiotensin II (AII) binding to receptors in adrenal glomerulosa cells. In this report, we show that oleic and arachidonic acids are specific inhibitors of the AT1 subtype of angiotensin receptor, and exert no effect on receptors of the AT2 subtype. By contrast, decanoic acid is a weak inhibitor of the AT2 subtype only. Our previous work on a post-receptor locus of inhibition by fatty acids of aldosterone biosynthesis showed that the 18-oxidase step is uniquely sensitive. In brief, the first and last steps involved in angiotensin-stimulated aldosterone secretion are particularly sensitive to inhibition by fatty acids. These results suggest a specific role for unesterified fatty acids in regulation of salt and water metabolism.

Regulation of pituitary-adrenocortical activity by free fatty acids in vivo and in vitro

Virtually every metabolic disorder characterized by elevated plasma free fatty acid (FFA) levels is also associated with hypercorticoidism. For example, the glucocorticoid response to insulin-hypoglycemia is shown in this report to be greatly potentiated in Type I diabetic rats. Since glucocorticoids (corticosterone, in rats) potentiate lipolysis and promote gluconeogenesis, they exacerbate diabetes. We found that elevation of circulating FFA levels in normal rats (via Intralipid/heparin infusion) increased plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone, and resulted in hyperglycemia. In vitro, however, cultured pituitary cells were relatively unaffected by FFA except at very high concentrations. Neither basal ACTH secretion nor the ACTH response to corticotropin-releasing hormone (CRH) was affected by pathophysiological molar ratios of FFA:BSA. Thus, the ACTH secretory response to FFA in vivo likely is mediated via neuroendocrine activation. Cultured adrenocortical cells, however, were stimulated by oleic acid and, to a lesser extent, by linoleic acid; saturated fatty acids were without effect. The latencies of oleic acid-induced steroidogenesis in vitro and Intralipid-induced corticosterone secretion in vivo were both about 60 min. We conclude that pathophysiological levels of circulating FFA (typical of diabetes, obesity, starvation, and consumption of high-fat diets) initiate a positive feedback loop between the adipocyte and the HPA axis, which ultimately exacerbates the symptoms of these disorders.

Fatty acids in cell signalling: modulation by lipid binding proteins

Long-chain fatty acids and several of their metabolites have now been shown to be involved as primary or secondary messengers in specific cell signalling pathways. In view of their extremely low aqueous solubility, the extracellular as well as intracellular transport of these compounds is assumed to be facilitated by specific lipid binding proteins, such as cytoplasmic fatty acid-binding protein (FABP). In this paper a survey is given on the biological significance and possible modulatory action of intracellular lipid binding proteins for fatty acid-mediated signal transduction pathways.

Relationships among plasma aldosterone, high-density lipoprotein cholesterol, and insulin in humans

To investigate the pathogenesis of hypertension in patients with obesity and insulin resistance and to explore the role of plasma lipids, we studied 30 subjects at the end of 7 days of low (20 mEq/d) then high (200 mEq/d) sodium diets. Glucose and insulin tolerance tests were performed at the end of each week and blood and urine collected for measurements of plasma aldosterone, renin activity, electrolytes, insulin, and lipoproteins. There was a strong negative correlation between plasma aldosterone and high-density lipoprotein cholesterol during both diets. There were weaker positive correlations between plasma aldosterone and insulin or triglycerides. When the aldosterone-renin ratio was the dependent variable and the correlation controlled for serum potassium, the inverse relationship with high-density lipoprotein cholesterol and the positive correlation with insulin remained, but only during the high salt diet. Subjects were divided into three groups based on high-density lipoprotein cholesterol. Subjects with the lowest high-density lipoprotein cholesterol levels showed the highest aldosterone, plasma triglycerides, body mass index, and waist-to-hip ratio. Those subjects also demonstrated the greatest resistance to insulin action on glucose and plasma unesterified fatty acids. There was a weak direct correlation between plasma aldosterone and systolic blood pressure during the high salt diet. These data suggest that high aldosterone levels may be a link between dyslipidemia, insulin resistance, and hypertension, a relationship made more evident by high salt intake.

The effects of gamma-linolenic acid on breast pain and diabetic neuropathy: possible non-eicosanoid mechanisms

Gamma-linolenic acid (GLA) has recently been found to be beneficial in the management of breast pain and of diabetic neuropathy. GLA is a precursor of unsaturated fatty acids which are important in membrane structures, as second messengers in their own right and as precursors of eicosanoids. While the mechanisms of GLA action are likely to be complex, non-eicosanoid effects are probably of substantial importance. These effects include modification of membrane fluidity and of the functions of lipid-associated receptors and changes in the inositol cycle.