Regulation of aldosterone secretion by mineralocorticoid receptor-mediated signaling

First-in-human study evaluating safety, pharmacokinetics, and pharmacodynamics of lorundrostat, a novel and highly selective aldosterone synthase inhibitor

Dysregulation of the mineralocorticoid hormone aldosterone is an increasingly prevalent cause of hypertension. Aldosterone synthase (CYP11B2) shares 93% homology to 11β-hydroxylase (CYP11B1), which produces cortisol. Lorundrostat, a highly selective inhibitor of CYP11B2, is a potential safe and effective treatment for aldosterone-dependent, uncontrolled hypertension, including treatment-resistant hypertension. Lorundrostat showed highly selective inhibition of CYP11B2 in vitro, with 374-fold selectivity for CYP11B2 vs. CYP11B1. A first-in-human study of single ascending doses ranging from 5 to 800 mg and multiple ascending doses ranging from 40 to 360 mg once daily was conducted in healthy participants. After single- and multiple-dose administration, lorundrostat plasma levels peaked 1-3 h after administration with a t1/2 of 10-12 h. Plasma aldosterone decreased up to 40% with single 100-mg to 200-mg doses and up to 70% with single 400 to 800-mg doses. Plasma aldosterone returned to baseline within 16 h after single 100-mg doses and multiple once-daily 120-mg doses. Lorundrostat demonstrated a favorable safety profile in healthy participants. Dose- and exposure-dependent inhibition of renal tubular sodium reabsorption was observed across a clinically relevant dose range with no suppression of basal or cosyntropin-stimulated cortisol production and only a modest increase in mean serum potassium.

Effects of very low-calorie ketogenic diet on hypothalamic-pituitary-adrenal axis and renin-angiotensin-aldosterone system

BACKGROUND

The hypothalamic-pituitary-adrenal (HPA) axis is a neuroendocrine system involved in controlling stress responses in humans under physiological and pathological conditions; cortisol is the main hormone produced by the HPA axis. It is known that calorie restriction acts as a stressor and can lead to an increase in cortisol production. Renin-angiotensin-aldosterone system (RAAS) is a complex endocrine network regulating blood pressure and hydrosaline metabolism, whose final hormonal effector is aldosterone. RAAS activation is linked to cardiometabolic diseases, such as heart failure and obesity. Obesity has become a leading worldwide pandemic, associated with serious health outcomes. Calorie restriction represents a pivotal strategy to tackle obesity. On the other hand, it is well known that an increased activity of the HPA may favour visceral adipose tissue expansion, which may jeopardize a successful diet-induced weight loss. Very low-calorie ketogenic diet (VLCKD) is a normoprotein diet with a drastic reduction of the carbohydrate content and total calorie intake. Thanks to its sustained protein content, VLCKD is extremely effective to reduce adipose tissue while preserving lean body mass and resting metabolic rate.

PURPOSE

The purpose of this narrative review is to gain more insights on the effects of VLCKD on the HPA axis and RAAS, in different phases of weight loss and in different clinical settings.

Molecular and Epigenetic Control of Aldosterone Synthase, CYP11B2 and 11-Hydroxylase, CYP11B1

Aldosterone and cortisol serve important roles in the pathogenesis of cardiovascular diseases and metabolic disorders. Epigenetics is a mechanism to control enzyme expression by genes without changing the gene sequence. Steroid hormone synthase gene expression is regulated by transcription factors specific to each gene, and methylation has been reported to be involved in steroid hormone production and disease. Angiotensin II or potassium regulates the aldosterone synthase gene, CYP11B2. The adrenocorticotropic hormone controls the 11b-hydroxylase, CYP11B1. DNA methylation negatively controls the CYP11B2 and CYP11B1 expression and dynamically changes the expression responsive to continuous stimulation of the promoter gene. Hypomethylation status of the CYP11B2 promoter region is seen in aldosterone-producing adenomas. Methylation of recognition sites of transcription factors, including cyclic AMP responsive element binding protein 1 or nerve growth factor-induced clone B, diminish their DNA-binding activity. A methyl-CpG-binding protein 2 cooperates directly with the methylated CpG dinucleotides of CYP11B2. A low-salt diet, treatment with angiotensin II, and potassium increase the CYP11B2 mRNA levels and induce DNA hypomethylation in the adrenal gland. A close association between a low DNA methylation ratio and an increased CYP11B1 expression is seen in Cushing's adenoma and aldosterone-producing adenoma with autonomous cortisol secretion. Epigenetic control of CYP11B2 or CYP11B1 plays an important role in autonomic aldosterone or cortisol synthesis.

Aldosterone and cardiovascular diseases

Aldosterone's role in the kidney and its pathophysiologic actions in hypertension are well known. However, its role or that of its receptor [minieralocorticoid receptor (MR)] in other cardiovascular (CV) disease are less well described. To identify their potential roles in six CV conditions (heart failure, myocardial infarction, atrial fibrillation, stroke, atherosclerosis, and thrombosis), we assessed these associations in the following four areas: (i) mechanistic studies in rodents and humans; (ii) pre-clinical studies of MR antagonists; (iii) clinical trials of MR antagonists; and (iv) genetics. The data were acquired from an online search of the National Library of Medicine using the PubMed search engine from January 2011 through June 2021. There were 3702 publications identified with 200 publications meeting our inclusion and exclusion criteria. Data strongly supported an association between heart failure and dysregulated aldosterone/MR. This association is not surprising given aldosterone/MR's prominent role in regulating sodium/volume homeostasis. Atrial fibrillation and myocardial infarction are also associated with dysregulated aldosterone/MR, but less strongly. For the most part, the data were insufficient to determine whether there was a relationship between atherosclerosis, stroke, or thrombosis and aldosterone/MR dysregulation. This review clearly documented an expanding role for aldosterone/MR's dysregulation in CV diseases beyond hypertension. How expansive it might be is limited by the currently available data. It is anticipated that with an increased focus on aldosterone/MR's potential roles in these diseases, additional clinical and pre-clinical data will clarify these relationships, thereby, opening approaches to use modulators of aldosterone/MR's action to more precisely treat these CV conditions.

Lysine-specific demethylase 1 deficiency modifies aldosterone synthesis in a sex-specific manner

Biologic sex influences the development of cardiovascular disease and modifies aldosterone (ALDO) and blood pressure (BP) phenotypes: females secrete more ALDO, and their adrenal glomerulosa cell is more sensitive to stimulation. Lysine-specific demethylase 1 (LSD1) variants in Africans and LSD1 deficiency in mice are associated with BP and/or ALDO phenotypes. This study, in 18- and 40-week-old wild type (WT) and LSD1+/- mice, was designed to determine whether (1) sex modifies ALDO biosynthetic enzymes; (2) LSD1 deficiency disrupts the effect of sex on these enzymes; (3) within each genotype, there is a positive relationship between ALDO biosynthesis (proximate phenotype), plasma ALDO (intermediate phenotype) and BP levels (distant phenotype); and (4) sex and LSD1 genotype interact on these phenotypes. In WT mice, female sex increases the expression of early enzymes in ALDO biosynthesis but not ALDO levels or systolic blood pressure (SBP). However, enzyme expressions are shifted downward in LSD1+/- females vs males, so that early enzyme levels are similar but the late enzymes are substantially lower. In both age groups, LSD1 deficiency modifies the adrenal enzyme expressions, circulating ALDO levels, and SBP in a sex-specific manner. Finally, significant sex/LSD1 genotype interactions modulate the three phenotypes in mice. In conclusion, biologic sex in mice interacts with LSD1 deficiency to modify several phenotypes: (1) proximal (ALDO biosynthetic enzymes); (2) intermediate (circulating ALDO); and (3) distant (SBP). These results provide entry to better understand the roles of biological sex and LSD1 in (1) hypertension heterogeneity and (2) providing more personalized treatment.

cyp21a2 Knockout Tadpoles Survive Metamorphosis Despite Low Corticosterone

Corticosteroids are so vital for organ maturation that reduced corticosteroid signaling during postembryonic development causes death in terrestrial vertebrates. Indeed, death occurs at metamorphosis in frogs lacking proopiomelanocortin (pomc) or the glucocorticoid receptor (GR; nr3c1). Some residual corticosteroids exist in pomc mutants to activate the wild-type (WT) GR and mineralocorticoid receptor (MR), and the elevated corticosteroids in GR mutants may activate MR. Thus, we expected a more severe developmental phenotype in tadpoles with inactivation of 21-hydroxylase, which should eliminate all interrenal corticosteroid biosynthesis. Using CRISPR/Cas9 in Xenopus tropicalis, we produced an 11-base pair deletion in cyp21a2, the gene encoding 21-hydroxylase. Growth and development were delayed in cyp21a2 mutant tadpoles, but unlike the other frog models, they survived metamorphosis. Consistent with an absence of 21-hydroxylase, mutant tadpoles had a 95% reduction of aldosterone in tail tissue, but they retained some corticosterone (∼40% of WT siblings), an amount, however, too low for survival in pomc mutants. Decreased corticosteroid signaling was evidenced by reduced expression of corticosteroid-response gene, klf9, and by impaired negative feedback in the hypothalamus-pituitary-interrenal axis with higher messenger RNA expression levels of crh, pomc, star, and cyp11b2 and an approximately 30-fold increase in tail content of progesterone. In vitro tail-tip culture showed that progesterone can transactivate the frog GR. The inadequate activation of GR by corticosterone in cyp21a2 mutants was likely compensated for by sufficient corticosteroid signaling from other GR ligands to allow survival through the developmental transition from aquatic to terrestrial life.

Effects of salinity and a glucocorticoid antagonist, RU486, on waterborne aldosterone and corticosterone of northern leopard frog larvae

Increased salinity is an emerging contaminant of concern for aquatic taxa. For amphibians exposed to salinity, there is scarce information about the physiological effects and changes in osmoregulatory hormones such as corticosterone (CORT) and aldosterone (ALDO). Recent studies have quantified effects of salinity on CORT physiology of amphibians based on waterborne hormone collection methods, but much less is known about ALDO in iono- and osmoregulation of amphibians. We re-assayed waterborne hormone samples from a previous study to investigate effects of salinity (sodium chloride, NaCl) and a glucocorticoid receptor antagonist (RU486) on ALDO of northern leopard frog (Rana pipiens) larvae. We also investigated relationships between ALDO and CORT. Waterborne ALDO marginally decreased with increasing salinity and was, unexpectedly, positively correlated with baseline and stress-induced waterborne CORT. Importantly, ALDO increased when larvae were exposed to RU486, suggesting that RU486 may also suppress mineralocorticoid receptors or that negative feedback of ALDO is mediated through glucocorticoid receptors. Alternatively, CORT increases with RU486 treatment and might be a substrate for ALDO synthesis, which could account for increases in ALDO with RU486 treatment and the correlation between CORT and ALDO. ALDO was negatively correlated with percent water, such that larvae secreting more ALDO retained less water. Although sample sizes were limited and further validation and studies are warranted, our findings expand our understanding of adrenal steroid responses to salinization in amphibians and proposes new hypotheses regarding the co-regulation of ALDO and CORT.

Genetic renal disease classification by hormonal axes

The kidneys, which regulate many homeostatic pathways, are also a major endocrinological target organ. Many genetic renal diseases can be classified according to the affected protein along such endocrinological pathways. In this review, we examine the hypothesis that a more severe phenotype is expected as the affected protein is located more distally along such pathways. Thus, the location of a defect along its endocrinological pathway should be taken into consideration, in addition to the mutation type, when assessing genetic renal disease severity.

Mineralocorticoid Receptor Antagonists Decrease the Rates of Positive Screening for Primary Aldosteronism

OBJECTIVE

Mineralocorticoid receptor antagonists (MRAs) are effective in patients with resistant hypertension and/or primary aldosteronism (PA). Screening for PA should ideally be conducted after stopping medications that might interfere with the renin-angiotensin-aldosterone system, but this is challenging in patients with recalcitrant hypertension or hypokalemia. Herein, we aimed to evaluate the impact of MRAs on PA screening in clinical practice.

METHODS

We conducted a retrospective cohort study of patients with hypertension who had plasma aldosterone and renin measurements before and after MRA use in a tertiary referral center, over 19 years.

RESULTS

A total of 146 patients, 91 with PA, were included and followed for up to 18 months. Overall, both plasma renin and aldosterone increased after MRA initiation (from median, interquartile range: 0.5 [0.1, 0.8] to 1.2 [0.6, 4.8] ng/mL/hour and from 19.1 [12.9, 27.7] to 26.4 [17.1, 42.3] ng/dL, respectively; P<.0001 for both), while the aldosterone/renin ratio (ARR) decreased from 40.3 (18.5, 102.7) to 23.1 (8.6, 58.7) ng/dL per ng/mL/hour (P<.0001). Similar changes occurred irrespective of the MRA treatment duration and other antihypertensives used. Positive PA screening abrogation after MRA initiation was found in 45/94 (48%) patients. Conversely, 17% of patients had positive PA screening only after MRA treatment, mostly due to correction of hypokalemia. An initially positive screening test was more likely altered by high MRA doses and more likely persistent in patients with confirmed PA or taking beta-blockers.

CONCLUSION

MRAs commonly reduce ARR and the proportion of positive PA screening results. When PA is suspected, screening should be repeated off MRAs.

ABBREVIATIONS

ACEI = angiotensin-converting enzyme inhibitor; ARB = angiotensin receptor blocker; ARR = aldosterone/renin ratio; DRC = direct renin concentration; MRA = mineralocorticoid receptor antagonist; PA = primary aldosteronism; PAC = plasma aldosterone concentration; PRA = plasma renin activity; RAAS = renin-angiotensin-aldosterone system.

Gut microbiota modulates stress-induced hypertension through the HPA axis

Stress is associated with an increased risk of hypertension, and the incidence of stress-related hypertension has risen rapidly in recent years; however, the underlying mechanisms remain elusive. Gut dysbiosis has been demonstrated to contribute to hypertension and hyperactivation of the hypothalamus-pituitary-adrenal (HPA) axis. Based on our previous findings showing the altered gut microbiota in the rats of stress-induced hypertension (SIH), the present study aims to investigate whether the stress-induced alteration in gut microbiota can lead to the dysfunction of the HPA axis which contributes to the development of SIH. SIH was developed in rats subjected to electric foot-shock combined with buzzer noise stressors. The gut microbiota of rats were deleted by administering an antibiotic cocktail containing ampicillin (1 g/L), vancomycin (500 mg/L), neomycin (1 g/L), and metronidazole (1 g/L) in drinking water. The serum levels of adrenocorticotropic hormone (ACTH) and corticosterone (CORT) were tested using enzyme-linked immunosorbent assay (ELISA). The mRNA expression of glucocorticoid receptor (GR) and corticotropin-releasing factor (CRF), CRFR1 and CRFR2 was detected by quantitative reverse transcription polymerase chain reaction (qRT-PCR). The cellular protein expressions of corticotropin-releasing hormone (CRH), c-fos, and GR were examined by immunohistochemical staining. In the present study, SIH rats showed a hyperactive HPA axis as indicated by the increased CRH expression in the paraventricular nucleus (PVN) of the hypothalamus, the elevated serum ACTH or CORT concentrations, and increased adrenal gland index. The decreased GR expression and increased CRFR1 in the hypothalamus might underlie the hyperactivation of the HPA axis. The microbial deletion by antibiotics mitigated the hyperactivation of the HPA axis and attenuated the stress-induced elevation of blood pressure, indicating that the causal link of gut microbiota to SIH is mediated, at least in part, by the HPA axis activity. Our findings shed new light on the mechanisms underlying SIH.

Lysine-Specific Demethylase-1 Deficiency Increases Agonist Signaling Via the Mineralocorticoid Receptor

LSD1 (lysine-specific demethylase-1) is an epigenetic regulator of gene transcription. LSD1 risk allele in humans and LSD1 deficiency (LSD1^+/-) in mice confer increasing salt-sensitivity of blood pressure with age, which evolves into salt-sensitive hypertension in older individuals. However, the mechanism underlying the relationship between LSD1 and salt-sensitivity of blood pressure remains elusive. Here, we show that LSD1 genotype (in humans) and LSD1 deficiency (in mice) lead to similar associations with increased blood pressure and urine potassium levels but with decreased aldosterone levels during a liberal salt diet. Thus, we hypothesized that LSD1 deficiency leads to an MR (mineralocorticoid receptor)-dependent hypertensive state. Yet, further studies in LSD1^+/- mice treated with the MR antagonist eplerenone demonstrate that hypertension, kaliuria, and albuminuria are substantially improved, suggesting that the ligand-independent activation of the MR is the underlying cause of this LSD1 deficiency-mediated phenotype. Indeed, while MR and epithelial sodium channel expression levels were increased in LSD1^+/- mouse kidney tissues, aldosterone secretion from LSD1^+/- glomerulosa cells was significantly lower. Collectively, these data establish that LSD1 deficiency leads to an inappropriate activation and increased levels of the MR during a liberal salt regimen and suggest that inhibiting the MR pathway is a useful strategy for treatment of hypertension in human LSD1 risk allele carriers.

Steroidogenic acute regulatory protein/aldosterone synthase mediates angiotensin II-induced cardiac fibrosis and hypertrophy

Aldosterone produced in adrenal glands by angiotensin II (Ang II) is known to elicit myocardial fibrosis and hypertrophy. This study was designed to test the hypothesis that Ang II causes cardiac morphological changes through the steroidogenic acute regulatory protein (StAR)/aldosterone synthase (AS)-dependent aldosterone synthesis primarily initiated in the heart. Sprague-Dawley rats were randomized to following groups: Ang II infusion for a 4-week period, treatment with telmisartan, spironolactone or adrenalectomy during Ang II infusion. Sham-operated rats served as control. Relative to Sham rats, Ang II infusion significantly increased the protein levels of AT1 receptor, StAR, AS and their tissue expression in the adrenal glands and heart. In coincidence with reduced aldosterone level in the heart, telmisartan, an AT1 receptor blocker, significantly down-regulated the protein level and expression of StAR and AS. Ang II induced changes in the expression of AT1/StAR/AS were not altered by an aldosterone receptor antagonist spironolactone. Furthermore, Ang II augmented migration of macrophages, protein level of TGFβ1, phosphorylation of Smad2/3 and proliferation of myofibroblasts, accompanied by enhanced perivascular/interstitial collagen deposition and cardiomyocyte hypertrophy, which all were significantly abrogated by telmisartan or spironolactone. However, adrenalectomy did not fully suppress Ang II-induced cell migration/proliferation and fibrosis/hypertrophy, indicating a role of aldosterone synthesized within the heart in pathogenesis of Ang II induced injury. These results indicate that myocardial fibrosis and hypertrophy stimulated by Ang II is associated with tissue-specific activation of aldosterone synthesis, primarily mediated by AT1/StAR/AS signaling pathways.

Sex-specific differences in endoplasmic reticulum aminopeptidase 1 modulation influence blood pressure and renin-angiotensin system responses

Salt sensitivity of blood pressure (SSBP) and hypertension are common, but the underlying mechanisms remain unclear. Endoplasmic reticulum aminopeptidase 1 (ERAP1) degrades angiotensin II (ANGII). We hypothesized that decreasing ERAP1 increases BP via ANGII-mediated effects on aldosterone (ALDO) production and/or renovascular function. Compared with WT littermate mice, ERAP1-deficient (ERAP1+/-) mice had increased tissue ANGII, systolic and diastolic BP, and SSBP, indicating that ERAP1 deficiency leads to volume expansion. However, the mechanisms underlying the volume expansion differed according to sex. Male ERAP1+/- mice had increased ALDO levels and normal renovascular responses to volume expansion (decreased resistive and pulsatility indices and increased glomerular volume). In contrast, female ERAP1+/- mice had normal ALDO levels but lacked normal renovascular responses. In humans, ERAP1 rs30187, a loss-of-function gene variant that reduces ANGII degradation in vitro, is associated with hypertension. In our cohort from the Hypertensive Pathotype (HyperPATH) Consortium, there was a significant dose-response association between rs30187 risk alleles and systolic and diastolic BP as well as renal plasma flow in men, but not in women. Thus, lowering ERAP1 led to volume expansion and increased BP. In males, the volume expansion was due to elevated ALDO with normal renovascular function, whereas in females the volume expansion was due to impaired renovascular function with normal ALDO levels.

Sex differences in renal hemodynamics and renin-angiotensin system activity post-CPAP therapy in humans with obstructive sleep apnea

Men have faster loss of kidney function and greater renal renin-angiotensin system (RAS) activity compared with women. Obstructive sleep apnea (OSA) is common in chronic kidney disease; the vascular effects of OSA differ by sex, and OSA-associated glomerular hyperfiltration can be reversed by continuous positive airway pressure (CPAP) therapy. We evaluated sex differences in the effect of CPAP on renal hemodynamics and the renal RAS in OSA. Twenty-nine Na⁺-replete, otherwise healthy study participants with OSA (10 women and 19 men) with nocturnal hypoxemia were studied pre- and post-CPAP (>4 h/night for 4 wk). Renal hemodynamics [renal plasma flow (RPF), glomerular filtration rate (GFR), and filtration fraction(FF)] were measured at baseline and in response to ANG II challenge, as a marker of renal RAS activity, pre- and post-CPAP therapy for 1 mo. In women, CPAP was associated with increased RPF (626 ± 22 vs. 718 ± 43 mL/min, P = 0.007, pre- vs. post-CPAP), maintained GFR (108 ± 2 vs. 105 ± 3 mL/min, P = 0.8), and reduced FF (17.4 ± 0.8% vs. 15.0 ± 0.7%, P = 0.017). In men, CPAP was associated with maintained RPF (710 ± 37 vs. 756 ± 38 mL/min, P = 0.1), maintained GFR (124 ± 8 vs. 113 ± 6 mL/min, P = 0.055), and reduced FF (18.6 ± 1.7% vs. 15.5 ± 1.1%, P = 0.035). Pre-CPAP, there were no sex differences in renal hemodynamic responses to ANG II. CPAP use was associated with a greater renovasoconstrictive response to ANG II in women (RPF at Δ30 min: -100 ± 27 vs. -161 ± 25 mL/min, P = 0.007, and RPF at Δ60 min: -138 ± 27 vs. -206 ± 32 mL/min, P = 0.007) but not men. CPAP use was associated with improved renal hemodynamics in both sexes and downregulated renal RAS activity in women but not men.

Histone demethylase LSD1 deficiency and biological sex: impact on blood pressure and aldosterone production

Human risk allele carriers of lysine-specific demethylase 1 (LSD1) and LSD1-deficient mice have salt-sensitive hypertension for unclear reasons. We hypothesized that LSD1 deficiency causes dysregulation of aldosterone's response to salt intake resulting in increased cardiovascular risk factors (blood pressure and microalbumin). Furthermore, we determined the effect of biological sex on these potential abnormalities. To test our hypotheses, LSD1 male and female heterozygote-knockout (LSD1+/-) and WT mice were assigned to two age groups: 18 weeks and 36 weeks. Plasma aldosterone levels and aldosterone production from zona glomerulosa cells studied ex vivo were greater in both male and female LSD1+/- mice consuming a liberal salt diet as compared to WT mice consuming the same diet. However, salt-sensitive blood pressure elevation and increased microalbuminuria were only observed in male LSD1+/- mice. These data suggest that LSD1 interacts with aldosterone's secretory response to salt intake. Lack of LSD1 causes inappropriate aldosterone production on a liberal salt diet; males appear to be more sensitive to this aldosterone increase as males, but not females, develop salt sensitivity of blood pressure and increased microalbuminuria. The mechanism responsible for the cardiovascular protective effect in females is uncertain but may be related to estrogen modulating the effect of mineralocorticoid receptor activation.

Aldosterone and Mineralocorticoid Receptor System in Cardiovascular Physiology and Pathophysiology

The mineralocorticoid hormone aldosterone (Aldo) has been intensively studied for its ability to influence both the physiology and pathophysiology of the cardiovascular system. Indeed, although research on Aldo actions for decades has mainly focused on its effects in the kidney, several lines of evidence have now demonstrated that this hormone exerts disparate extrarenal adverse effects, especially in the circulatory system. Accordingly, in the last lusters, a number of studies in preclinical models (in vitro and in vivo) and in humans have established that Aldo, following the interaction with its receptor-the mineralocorticoid receptor (MR)-is able to activate specific intracellular genomic and nongenomic pathways, thus regulating the homeostasis of the cardiovascular system. Importantly, through this mechanism of action, this hormone becomes a crucial regulator of the function and growth of different types of cells, including fibroblasts, cardiomyocytes, and vascular cells. For this main reason, it is plausible that when Aldo is present at high levels in the blood, it profoundly modifies the physiology of these cells, therefore being at the foundation of several cardiovascular disorders, such as heart failure (HF). On these grounds, in this review, we will provide an updated account on the current knowledge concerning Aldo activity in the cardiovascular system and the most recent preclinical studies and clinical trials designed to test better approaches able to counter the hyperactivity of the Aldo/MR signaling pathway in the setting of cardiovascular diseases.

Role of glucocorticoid negative feedback in the regulation of HPA axis pulsatility

The hypothalamic-pituitary-adrenal (HPA) axis is the major neuroendocrine axis regulating homeostasis in mammals. Glucocorticoid hormones are rapidly synthesized and secreted from the adrenal gland in response to stress. In addition, under basal conditions glucocorticoids are released rhythmically with both a circadian and an ultradian (pulsatile) pattern. These rhythms are important not only for normal function of glucocorticoid target organs, but also for the HPA axis responses to stress. Several studies have shown that disruption of glucocorticoid rhythms is associated with disease both in humans and in rodents. In this review, we will discuss our knowledge of the negative feedback mechanisms that regulate basal ultradian synthesis and secretion of glucocorticoids, including the role of glucocorticoid and mineralocorticoid receptors and their chaperone protein FKBP51. Moreover, in light of recent findings, we will also discuss the importance of intra-adrenal glucocorticoid receptor signaling in regulating glucocorticoid synthesis.

Caloric restriction improves glucose homeostasis, yet increases cardiometabolic risk in caveolin-1-deficient mice

BACKGROUND AND PURPOSE

The plasma membrane protein caveolin-1 (CAV-1) has been shown to be involved in modulating glucose homeostasis and the actions of the renin-angiotensin-aldosterone system (RAAS). Caloric restriction (CR) is widely accepted as an effective therapeutic approach to improve insulin sensitivity and reduce the severity of diabetes. Recent data indicate that polymorphisms of the CAV-1 gene are strongly associated with insulin resistance, hypertension and metabolic abnormalities in non-obese individuals. Therefore, we sought to determine whether CR improves the metabolic and cardiovascular (CV) risk factors in the lean CAV-1 KO mice.

MATERIALS/METHODS

Twelve- to fourteen-week-old CAV-1 knockout (KO) and genetically matched wild-type (WT) male mice were randomized by genotype to one of two dietary regimens: ad libitum (ad lib) food intake or 40% CR for 4 weeks. Three weeks following the onset of dietary restriction, all groups were assessed for insulin sensitivity. At the end of the study, all groups were assessed for fasting glucose, insulin, HOMA-IR, lipids, corticosterone levels and blood pressure (BP). Aldosterone secretion was determined from acutely isolated Zona Glomerulosa cells.

RESULTS

We confirmed that the CAV-1 KO mice on the ad lib diet display a phenotype consistent with the cardiometabolic syndrome, as shown by higher systolic BP (SBP), plasma glucose, HOMA-IR and aldosterone levels despite lower body weight compared with WT mice on the ad lib diet. CAV-1 KO mice maintained their body weight on the ad lib diet, but had substantially greater weight loss with CR, as compared to caloric restricted WT mice. CR-mediated changes in weight were associated with dramatic improvements in glucose and insulin tolerance in both genotypes. These responses to CR, however, were more robust in CAV-1KO vs. WT mice and were accompanied by reductions in plasma glucose, insulin and HOMA-IR in CAV-1KO but not WT mice. Surprisingly, in the CAV-1 KO, but not in WT mice, CR was associated with increased SBP and aldosterone levels, suggesting that in CAV-1 KO mice CR induced an increase in some CV risk factors.

CONCLUSIONS

CR improved the metabolic phenotype in CAV-1 KO mice by increasing insulin sensitivity; nevertheless, this intervention also increased CV risk by inappropriate adaptive responses in the RAAS and BP.

Biological Sex Modulates the Adrenal and Blood Pressure Responses to Angiotensin II

The relationship between biological sex and aldosterone on blood pressure (BP) is unclear. We hypothesized that sex would modify the interaction between aldosterone and vascular responses to salt intake and angiotensin II (AngII). To test this hypothesis, in 1592 subjects from the well-controlled Hypertensive Pathotype cohort, we compared responses of women and men to chronic (BP and aldosterone levels in response to dietary salt) and acute (BP, renal plasma flow, and aldosterone responses to AngII infusion) manipulations. Women had a 30% higher salt sensitivity of BP than men (P<0.0005) regardless of age or hypertension status, a greater BP response to AngII, and a 15% greater aldosterone response to AngII on both restricted and liberal salt diets (P<0.005). Furthermore, there was an interaction (P=0.003) between sex and aldosterone on BP response to AngII. Women also had a greater (P<0.01) increment in renal plasma flow in response to AngII than men. To assess potential mechanisms for this sex effect, we compared aldosterone responses to AngII or potassium from rat zona glomerulosa cells and observed greater aldosterone production in female than male zona glomerulosa cells basally and in response to both agonists (P<0.0001). In a rodent model of aldosterone-mediated cardiovascular disease induced by increased AngII and low NO, circulating aldosterone levels (P<0.01), myocardial damage (P<0.001), and proteinuria (P<0.05) were greater in female than male rats despite having similar BP responses. Thus, increased aldosterone production likely contributes to sex differences in cardiovascular disease, suggesting that women may be more responsive to mineralocorticoid receptor blockade than men.

Combined Salt and Caloric Restrictions: Potential Adverse Outcomes

BACKGROUND

We hypothesized that caloric restriction (CR) and salt restriction (ResS) would have similar effects on reducing cardiovascular risk markers and that combining CR and ResS would be synergistic in modulating these markers.

METHODS AND RESULTS

To test our hypothesis, rats were randomized into 2 groups: ad libitum liberal salt diet (ad libitum/high-sodium, 1.6% sodium) or ResS diet (ad libitum/ResS, 0.03% sodium). CR was initiated in half of the rats in each group by reducing caloric intake to 60% while maintaining sodium intake constant (CR/high-sodium, 2.7% sodium or CR/ResS, 0.05% sodium) for 4 weeks. CR in rats on a high-sodium diet improved metabolic parameters, renal transforming growth factor-β and collagen-1α1 and increased plasma adiponectin and renal visfatin and NAD+ protein levels. Although CR produced some beneficial cardiovascular effects (increased sodium excretion and reduced blood pressure), it also was associated with potentially adverse cardiovascular effects. Adrenal zona glomerulosa cell responsiveness and aldosterone levels and activation were inappropriately increased for the volume state of the rodent. Like CR on HS, CR on a ResS diet also produced relative increased zona glomerulosa responsiveness and an increased blood pressure with no improvement in metabolic parameters.

CONCLUSIONS

These results suggest that combining CR and ResS may decrease the beneficial effects of each alone. Furthermore, CR, regardless of dietary salt intake, inappropriately activates aldosterone production. Thus, caution should be used in combining ResS and CR because the combination may lead to increased cardiovascular risk.

Dynamic responses of the adrenal steroidogenic regulatory network

The hypothalamic-pituitary-adrenal axis is a dynamic system regulating glucocorticoid hormone synthesis in the adrenal glands. Many key factors within the adrenal steroidogenic pathway have been identified and studied, but little is known about how these factors function collectively as a dynamic network of interacting components. To investigate this, we developed a mathematical model of the adrenal steroidogenic regulatory network that accounts for key regulatory processes occurring at different timescales. We used our model to predict the time evolution of steroidogenesis in response to physiological adrenocorticotropic hormone (ACTH) perturbations, ranging from basal pulses to larger stress-like stimulations (e.g., inflammatory stress). Testing these predictions experimentally in the rat, our results show that the steroidogenic regulatory network architecture is sufficient to respond to both small and large ACTH perturbations, but coupling this regulatory network with the immune pathway is necessary to explain the dissociated dynamics between ACTH and glucocorticoids observed under conditions of inflammatory stress.