Stimulation of the binding properties of adult rat corticosteroid-binding globulin by a lipolysis-induced rise in plasma free fatty acids

Hyperlipidaemia impairs the circadian clock and physiological homeostasis of vascular smooth muscle cells via the suppression of Smarcd1

Many mammalian physiological processes show diurnal oscillation and are controlled by a circadian clock. Disruption of the circadian clock has been implicated in the pathogenesis of cardiovascular disorders, but the mechanism through which clock and vessel function are integrated is unclear. Here we show that the rhythmicity of key clock genes and Smarcd1, a member of the SWI/SNF chromatin remodelling complex family, is suppressed in the layer of vascular smooth muscle cells (VSMCs) of the thoracic aorta of hyperlipidaemic rats fed a high-fat diet (HFD). Smarcd1 stimulates the transcription of clock genes, notably bmal1, through co-activation of the nuclear orphan receptor RORα in VSMCs. The co-activation of Smarcd1 and RORα is dependent on the mediation of PGC-1α, a transcriptional co-activator. Pathophysiologically, Smarcd1 inhibits VSMC proliferation and migration by blocking cell cycle re-entry and via the activation of kinase signalling pathways. Our results demonstrate that Smarcd1 is a critical node integrating the circadian clock and VSMC physiological homeostasis.

PGC-1alpha inhibits oleic acid induced proliferation and migration of rat vascular smooth muscle cells

Background

Oleic acid (OA) stimulates vascular smooth muscle cell (VSMC) proliferation and migration. The precise mechanism is still unclear. We sought to investigate the effects of peroxisome proliferator-activated receptor gamma (PPARγ) coactivator-1 alpha (PGC-1α) on OA-induced VSMC proliferation and migration.

Principal Findings

Oleate and palmitate, the most abundant monounsaturated fatty acid and saturated fatty acid in plasma, respectively, differently affect the mRNA and protein levels of PGC-1α in VSMCs. OA treatment resulted in a reduction of PGC-1α expression, which may be responsible for the increase in VSMC proliferation and migration caused by this fatty acid. In fact, overexpression of PGC-1α prevented OA-induced VSMC proliferation and migration while suppression of PGC-1α by siRNA enhanced the effects of OA. In contrast, palmitic acid (PA) treatment led to opposite effects. This saturated fatty acid induced PGC-1α expression and prevented OA-induced VSMC proliferation and migration. Mechanistic study demonstrated that the effects of PGC-1α on VSMC proliferation and migration result from its capacity to prevent ERK phosphorylation.

Conclusions

OA and PA regulate PGC-1α expression in VSMCs differentially. OA stimulates VSMC proliferation and migration via suppression of PGC-1α expression while PA reverses the effects of OA by inducing PGC-1α expression. Upregulation of PGC-1α in VSMCs provides a potential novel strategy in preventing atherosclerosis.

Corticosteroid-binding globulin synthesis and distribution in rat white adipose tissue

Corticosterone binding (CB) capacity was determined in visceral and subcutaneous white adipose tissue (WAT), as well as in plasma of lean Zucker rats. Perfusion of rats with saline eliminated most liver and kidney corticosterone binding but did not affect CB in WAT. The cytosol extracts of isolated cells, however, did not bind corticosterone in detectable amounts. By means of a RT-PCR procedure it was found that corticosterone-binding globulin (CBG) was expressed in WAT. By immunohistochemical detection in WAT sections, CBG was seen in a thin layer surrounding the cells near the plasma membrane. These data suggest that the CBG layer surrounding the cells may act as a protective barrier limiting the access of glucocorticoids to adipocytes.

The insulin resistance syndrome and the binding capacity of cortisol binding globulin (CBG) in men and women

BACKGROUND

Both insulin resistance and cortisol binding globulin (CBG) capacity have been found to correlate with plasma free fatty acid (FFA) concentration.

OBJECTIVE

To examine the changes in CBG binding with varying degrees of insulin resistance and plasma FFA levels.

SUBJECTS AND METHODS

Anthropometric parameters, serum cortisol levels, plasma CBG, CBG binding and insulin sensitivity (using the frequently sampled intravenous glucose tolerance test with minimal model analysis) were measured in a group of 38 healthy subjects (19 men, mean age 36.2 +/- 1.9; body mass index (BMI) 28.8 +/- 1.2, range 22.2-35.7), and 19 women, age 34.9 +/- 1.4; BMI 28.1 +/- 0.8, range 19-37.9)].

RESULTS

Plasma CBG levels did not differ between men and women. In men, CBG binding was associated with several parameters of the insulin resistance syndrome, including area under the curve for glucose during an oral glucose tolerance test (MBG, r = 0.45, P = 0.04), fasting insulin (r = 0.66, P = 0. 002), plasma triglycerides (r = 0.75, P < 0.0001), VLDL-triglycerides (r = 0.59, P = 0.007), fasting FFA (r = 0.72, P = 0.002), uric acid (r = 0.57 (P = 0.01) and insulin sensitivity (SI, r = - 0.58, P = 0.008). Free cortisol (estimated as the ratio of cortisol to CBG) was not associated with waist-to-hip ratio (WHR) or parameters of insulin sensitivity. In contrast to men, CBG binding was not associated with MBG, fasting insulin, plasma triglycerides, VLDL-triglycerides, FFA, uric acid or SI (all P = NS) in women. Serum free cortisol, however, correlated positively with WHR (r = 0. 62, P = 0.02) and negatively with SI (r = - 0.68, P = 0.01) in obese women. A multiple linear regression to predict CBG binding was constructed, with plasma CBG concentration and insulin sensitivity as independent variables. In this model, only SI entered the equation at a statistically significant level (P = 0.0012) contributing to 52% of the variance in CBG binding in men. When plasma FFA levels were added to the model, both SI (P = 0.04) and FFA levels (P = 0.039) contributed to 66% of the variance of CBG binding in men. In women, both plasma CBG concentration (P = 0.0005) and insulin sensitivity (P = 0.047) entered the equation at a statistically significant level, contributing to 60% of the variance in CBG binding. When plasma FFA levels were added to the model, only plasma CBG concentration (P = 0.043) was found to significantly contribute to 38% of the variance in CBG binding. The latter finding suggests that FFA levels constituted a confounding variable in the association between SI and CBG binding in women.

CONCLUSIONS

Both plasma free fatty acid and insulin sensitivity influence cortisol binding globulin binding capacity in men. Whether cortisol binding globulin binding is a factor implicated in the pathophysiology of insulin resistance or represents an adaptative tool in this situation awaits further studies.

Responses of serum corticosterone and corticosteroid-binding globulin to acute and prolonged stress in the rat

Responses of serum corticosterone (B) and corticosteroid-binding globulin (CBG) to ether anesthesia (a "classic" acute stress) and to a number of stressors influencing metabolic homeostasis--fasting, physical exercise, cold exposure, and water deprivation--were studied in male and female rats. Metabolic stressors included placing in an ice bath, physical exercise (swimming), fasting for 2 d, swimming after fasting for 2 d, cold-room (4 degrees C) exposure for 2 d, fasting in combination with cold-room exposure for 1 d, and water deprivation for 2 d. The study demonstrated clear differences between males and females in basal B levels and B responses to some stressors. Only ether anesthesia and fasting resulted in similar B levels in males and females whereas in control and all other groups serum B levels were higher in females. Serum CBG was considerably higher in females. In females, ether, swimming, swimming after fasting, fasting, and fasting during cold exposure resulted in a decrease in circulating CBG. Ice bathing and cold exposure did not influence CBG, and water deprivation elevated serum CBG. In males, animals subjected to fasting and fasting during cold exposure had CBG levels lower than control animals. Other groups did not differ from the control. Higher CBG levels in females counterbalanced higher total B in setting circulating free B: significant sex differences in free B were observed only after swimming or fasting during cold exposure. Stress-responsive changes in CBG levels seem to contribute little to changes in free B; the main contributing factor is the rise in total B. However, CBG may play a special role, independent of the functions of corticosteroids. It is proposed that the need for substantial mobilization of spare fuel (as it takes place during physical exercise or fasting) is critical in involving CBG in the stress response.

Inhibition of corticosteroid-binding globulin caused by a severe stressor is apparently mediated by the adrenal but not by glucocorticoid receptors

The effect of stress on serum corticosteroid-binding globulin (CBG) was studied in adult male Sprague-Dawley rats. CBG was measured either by a homologous radioimmunoassay (RIA) or by a binding assay (BA) using 3H-corticosterone. Exposure of adult male rats to a severe stressor such as immobilization (IMO) for 1 h did not alter serum CBG levels, but a significant decrease was found after 6 and especially 24 h IMO. This decrease was not observed after 24 h exposure to a milder treatment such as food and water deprivation. The effect of different periods of exposure to two stressors, IMO or restraint, was also studied. The following results were obtained:serum CBG levels were reduced by IMO, but only by restraint; IMO-induced reduction of CBG levels was always observed 24 h after starting exposure to IMO, independently of the actual period of exposure to the stressor; and IMO-induced inhibition of CBG was proportional to the hours of exposure to the stressor. Although IMO-induced inhibition of CBG was prevented by adrenalectomy, a role for glucocorticoid acting through their classical type II receptors is unclear as far as treatment of rats with the glucocorticoid receptor antagonist RU486 (100 mg/kg) did not prevent the inhibition caused by IMO. The present data clearly indicate that acute exposure to a stressor is able to decrease CBG levels provided that duration of exposure to the stressor and its intensity are high and that the effect is tested at least 6 h after the onset of stress. The effect appears to be mediated by some adrenal factor(s) other than glucocorticoids.

Postprandial free fatty acids stimulate activity of human corticosteroid binding globulin

The effect of postprandial variation of free fatty acids (FFA) on serum corticosteroid binding globulin (CBG) properties and cortisol (hydrocortisone) concentrations were explored in 11 women (20-30 yr) during 8 h after an oral load of tallow (26% C16:0, 18% C18:0, and 43% C18:1), oleic-sunflower (oleic-SF; 73% C18:1), sunflower (SF; 67% C18:2), and mixed oil (MO; 39% C18:1 and 48% C18:2). Serum FFA increased little after SF and MO but more than doubled in the late postprandial period (6 and 8 h) after oleic-SF (due to monounsaturated FFA) or tallow (due to saturated and monounsaturated FFA). CBG concentrations remained unchanged, but in relation with the postprandial elevation of serum FFA, CBG binding activity was increased after tallow or oleic-SF as a result of a combined two- to threefold increase in affinity constant and a 50% reduction in binding sites. Immunological and in vitro binding studies showed the changes in CBG behavior to be conformational and to be mediated mainly by monounsaturated FFA, especially C18:1. The modifications of CBG properties were associated with sustained high concentrations of cortisol (suppression of midday decrease) 6 and 8 h after tallow or oleic-SF. Thus dietary FFA may have an impact on bioavailability of glucocorticoids.