Stearoyl-CoA desaturase activity in primary culture of chicken hepatocytes. Influence of insulin, glucocorticoid, fatty acids and cordycepin

Dexamethasone in the era of COVID-19: friend or foe? An essay on the effects of dexamethasone and the potential risks of its inadvertent use in patients with diabetes

BACKGROUND

The disclosure in the media of a benefit with the use of dexamethasone in patients with COVID-19 infection sets precedents for self-medication and inappropriate use of corticosteroids.

METHODS

This is a critical interpretive synthesis of the data available in the literature on the effects of the use of corticosteroids and the impact that their indiscriminate use may have on patients with diabetes. Reviews and observational and experimental studies published until June 18, 2020 were selected.

RESULTS

Corticosteroids are substances derived from cholesterol metabolism that interfere with multiple aspects of glucose homeostasis. Interactions between corticoid receptors and target genes seem to be among the mechanisms responsible for the critical functions of glucocorticoids for survival and anti-inflammatory effects observed with these medications. Corticosteroids increase hepatic gluconeogenesis, reduce peripheral use of glucose and increase insulin levels. Previous studies have shown that glucocorticoids have a pro-adipogenic function, increasing deposition of abdominal fat, and lead to glucose intolerance and hypertriglyceridemia. In addition, these drugs play a role in controlling liver metabolism and can lead to the development of hepatic steatosis. Glucocorticoids reduce the recruitment of osteoblasts and increase the number of osteoclasts, which results in increased bone resorption and greater bone fragility. Moreover, these medications cause water and sodium retention and increase the response to circulating vasoconstrictors, which results in increased blood pressure levels. Chronic or high-dose use of corticosteroids can, by itself, lead to the onset of diabetes. For those who were already diagnosed with diabetes, studies show that chronic use of corticosteroids leads to a 94% higher risk of hospitalization due to diabetes complications. In addition to the direct effects on glycemic control, the effects on arterial pressure control, lipids and bone metabolism also have a potential for severe consequences in patients with diabetes.

CONCLUSION

Fear and uncertainty toward a potentially serious infection may lead people to self-medication and the inappropriate and abusive use of corticosteroids. More than ever, it is necessary for health professionals to be alert and able to predict damages related to the use of these drugs, which is the first step to minimize the potential damages to come.

Interactive effects of maternal and weaning high linoleic acid intake on hepatic lipid metabolism, oxylipins profile and hepatic steatosis in offspring

Non-alcoholic fatty liver disease (NAFLD) has been described as a hepatic manifestation of the metabolic syndrome. When several studies correlated maternal linoleic acid (LA) intake with the development of obesity, only few links have been made between n-6 fatty acid (FA) and NAFLD. Herein, we investigated the influence of both maternal and weaning high LA intake on lipid metabolism and susceptibility to develop later metabolic diseases in offspring. Pregnant rats were fed a control-diet (2% LA) or a LA-rich diet (12% LA) during gestation and lactation. At weaning, offspring was assigned to one of the two diets, i.e., either maintained on the same maternal diet or fed the other diet for 6 months. Physiological, biochemical parameters and hepatic FA metabolism were analyzed. We demonstrated that the interaction between the maternal and weaning LA intake altered metabolism in offspring and could lead to hepatic steatosis. This phenotype was associated with altered hepatic FA content and lipid metabolism. Interaction between maternal and weaning LA intake led to a specific pattern of n-6 and n-3 oxylipins that could participate to the development of hepatic steatosis in offspring. Our findings highlight the significant interaction between maternal and weaning high LA intake to predispose offspring to later metabolic disease and support the predictive adaptive response hypothesis.

Influence of prednisolone on parameters of de novo lipogenesis and indices for stearoyl-CoA- and Δ6- desaturase activity in healthy males: A Post-hoc analysis of a randomized, placebo-controlled, double-blind trial

Glucocorticoid treatment decreases liver insulin sensitivity and may modify fatty acid metabolism. We investigated the influence of oral prednisolone on indices for de novo lipogenesis (DNLi), stearoyl-CoA desaturase (SCDi) and Δ6-desaturase (D6Di) activity in healthy males. In addition, we explored whether the changes may be associated with prednisolone-induced changes in glucose and lipid metabolism and insulin sensitivity. Thirty-two healthy young males (mean ± SD age 22 ± 3 years, BMI 22.4 ± 1.7 kg/m²) were allocated to receive prednisolone 7.5 mg/day (PRED7.5; n = 12), prednisolone 30 mg/day (PRED30; n = 12), or placebo (n = 8) in a randomized double-blind fashion for 2 weeks. Fatty acid compositions of plasma cholesteryl esters (CE), phospholipids (PL) and triglycerides (TG) were measured at baseline and on day 14. DNLi, SCDi and D6Di were estimated from product/precursor ratios in CE, with DNLi primary deriving from 16:1ω7/18:2ω6, SCDi from 16:1ω7/16:0 and D6Di from 22:6ω3/20:5ω3. Ratios were also assessed in PL and TG. In CE, PRED30 increased DNLi by 51.2 [95%CI 14.8; 87.6]%, increased SCDi by 48.6 [18.7; 78.5]%, and decreased D6Di by 57.7 [-91.8; -23.5]% (p ≤ 0.01 for all, compared to placebo). The prednisolone-induced increases in DNLi and SCDi were positively correlated with insulin sensitivity (r = 0.35 and 0.50, respectively). Similar results were found in PL and TG. Prednisolone dose-dependently increases DNLi and SCDi and decreases D6Di in plasma CE, PL and TG in healthy males after 2 weeks. The observed unfavorable effects on fatty acid metabolism were related to the induction of glucocorticoid-induced insulin resistance.

Glucocorticoid hormones and energy homeostasis

Glucocorticoids (GC) and their cognate intracellular receptor, the glucocorticoid receptor (GR), have been characterised as critical checkpoints in the endocrine control of energy homeostasis in mammals. Indeed, aberrant GC action has been linked to a variety of severe metabolic diseases, including obesity, insulin resistance and type 2 diabetes. As a steroid-binding member of the nuclear receptor superfamily of transcription factors, the GR translocates into the cell nucleus upon GC binding where it serves as a transcriptional regulator of distinct GC-responsive target genes that are - in many cases - associated with glucose and lipid regulatory pathways and thereby intricately control both physiological and pathophysiological systemic energy homeostasis. Here, we summarize the current knowledge of GC/GR function in energy metabolism and systemic metabolic dysfunction, particularly focusing on glucose and lipid metabolism.

How Do Glucocorticoids Regulate Lipid Metabolism?

Glucocorticoids (GCs) and their cognate, intracellular receptor, the glucocorticoid receptor (GR) have been characterized as critical checkpoints in the hormonal control of energy homeostasis in mammals. Whereas physiological levels of GCs are required for proper metabolic control, aberrant GC action has been linked to a variety of severe metabolic diseases, including type 2 diabetes and obesity. As a member of the nuclear receptor superfamily of transcription factors, the GR translocates into the cell nucleus upon GC binding where it serves as a transcriptional regulator of distinct GC-responsive target genes that are in many cases associated with lipid regulatory pathways and thereby intricately control both physiological and pathophysiological systemic lipid homeostasis. Thus, this chapter focuses on the current knowledge of GC/GR function in lipid handling and its implications for systemic metabolic dysfunction.

Regulation of triglyceride metabolism by glucocorticoid receptor

Glucocorticoids are steroid hormones that play critical and complex roles in the regulation of triglyceride (TG) homeostasis. Depending on physiological states, glucocorticoids can modulate both TG synthesis and hydrolysis. More intriguingly, glucocorticoids can concurrently affect these two processes in adipocytes. The metabolic effects of glucocorticoids are conferred by intracellular glucocorticoid receptors (GR). GR is a transcription factor that, upon binding to glucocorticoids, regulates the transcriptional rate of specific genes. These GR primary target genes further initiate the physiological and pathological responses of glucocorticoids. In this article, we overview glucocorticoid-regulated genes, especially those potential GR primary target genes, involved in glucocorticoid-regulated TG metabolism. We also discuss transcriptional regulators that could act with GR to participate in these processes. This knowledge is not only important for the fundamental understanding of steroid hormone actions, but also are essential for future therapeutic interventions against metabolic diseases associated with aberrant glucocorticoid signaling, such as insulin resistance, dyslipidemia, central obesity and hepatic steatosis.

Effects of introducing linseed in livestock diet on blood fatty acid composition of consumers of animal products

Reducing the ratio between essential fatty acids: C18:2 n-6/C18:3 n-3 down to 5 is recommended by Nutritional Guidelines. We studied the fatty acid (FA) changes in consumers' plasma following changes in livestock diet. First, a zootechnical study introduced 5% of extruded linseed into the diet of livestock to replace other oleaginous ingredients, and on an iso-nutritional values basis. The products from linseed-fed animals contained more n-3 fatty acids (precursor alpha-linolenic and derivatives obtained by elongations and desaturations) than control animal products (issued from animals fed without linseed), and more conjugated linoleic acids (CLA). The n-6/n-3 ratio was reduced by 54% in butter, 60% in meat and 86% in eggs. Following this, a double-blind, randomised, cross-over clinical study involving 75 healthy volunteers compared plasma and erythrocyte FA profiles in consumers of animal products (from livestock fed the linseed diet or from livestock fed standard diet). It showed modifications in the FA composition of the experimental human regimen with more C18:3 n-3 (1.65 vs. 0.75 g/day), and more n-3 derivatives. The C18:2 n-6/C18:3 n-3 ratio decreased (7 vs. 15). In volunteers' plasma, C18:3 n-3 increased in the essay group (0.93 vs. 0.44% of the FA), so did n-3 derivatives and CLA. The n-6/n-3 ratio decreased from 14.3 to 10.2. In erythrocytes, C20:5 n-3 increased in the essay group (0.59 vs. 0.45%) and so did C22:6 n-3. The n-6/n-3 ratio decreased in parallel from 4.2 to 3.8. Without any changes in consumers' eating habits, foodstuffs from animals fed linseed diets induced significant modifications of human plasma and erythrocyte fatty acid composition (comparable to that noted under the 'Cretan' diet) and a sharp increase in CLA.

Role of stearoyl-coenzyme A desaturase in lipid metabolism

Stearoyl-CoA desaturase (SCD) (EC 1.14.99.5) is an endoplasmic reticulum-bound enzyme that catalyzes the delta9-cis desaturation of saturated fatty acyl-CoAs, the preferred substrates being palmitoyl- and stearoyl-CoA, which are converted to palmitoleoyl- and oleoyl-CoA, respectively. These monounsaturated fatty acids are used as substrates for the synthesis of triglycerides, wax esters, cholesteryl esters and membrane phospholipids. The saturated to monounsaturated fatty acid ratio affects membrane phospholipid composition and alteration in this ratio has been implicated in a variety of disease states including cardiovascular disease, obesity, diabetes, neurological disease, skin disorders and cancer. Thus, the expression of SCD is of physiological importance in normal and disease states. Several mammalian SCD genes have been cloned. A single human, three mouse and two rat are the best characterized SCD genes. The physiological role of each SCD isoform and the reason for having three or more SCD gene isoforms in the rodent genome are currently unknown. A clue as to the physiological role of the SCD, at least SCD1 gene and its endogenous products came from recent studies of asebia mouse strains that have a natural mutation in the SCD1 gene and a mouse model with a targeted disruption of the SCD1 gene. In this review we discuss our current understanding of the physiological role of SCD in lipid synthesis and metabolism.

Erythrocyte membrane fatty acids and subsequent breast cancer: a prospective Italian study

BACKGROUND

The relationship between erythrocyte membrane fatty acids and postmenopausal breast cancer risk was analyzed previously only by retrospective studies, which suggested a protective effect of increased saturation index (SI), i.e., the ratio of membrane stearic to oleic acid. We investigated the relationships in a prospective study of hormones, diet, and prediagnostic breast cancer (the ORDET study) conducted in northern Italy.

METHODS

A total of 4052 postmenopausal women were followed for an average of 5.5 years; 71 cases of invasive breast cancer were identified. For each case subject, two matched control subjects were chosen randomly from among cohort members. The various fatty acids in erythrocyte membranes were measured as a percentage of total fatty acids. Conditional logistic regression analysis evaluated the association between membrane fatty acid composition and breast cancer risk. The SI, which is influenced by the activity of the enzyme delta 9 desaturase (Delta 9-d), was also investigated. All statistical tests were two-sided.

RESULTS

Oleic (highest versus lowest tertile of percentage of total fatty acids, odds ratio [OR] = 2.79; 95% confidence interval [CI] = 1.24 to 6.28) and monounsaturated fatty acids (highest to lowest tertile, OR = 5.21; 95% CI = 1.95 to 13.91) were positively associated with breast cancer risk. The SI (highest to lowest tertile, OR = 0.29; 95% CI = 0.13 to 0.64) was inversely associated with breast cancer risk. The analysis suggested an inverse association between total polyunsaturated fatty acids and breast cancer risk, but individual polyunsaturated fatty acids behaved differently. There was no association between saturated fatty acids and breast cancer risk.

CONCLUSIONS

We have found that monounsaturated fats and SI in erythrocyte membranes are predictors of postmenopausal breast cancer. Both of these variables depend on the activity of the enzyme Delta 9-d. The dietary, metabolic, and hormonal factors acting on Delta 9-d expression and activity and, therefore, on patterns of fatty acid metabolism, should be further investigated as possible determinants of breast cancer.

Effect of sucrose addition to drinking water, that induces hypertension in the rats, on liver microsomal Delta9 and Delta5-desaturase activities

This study was undertaken with the aim of investigating the effect of sucrose addition to the drinking water of rats who were fed with the same diet as a control group, on Delta9- and Delta5-desaturase activities and on the fatty acid composition of serum and liver microsomes. Weanling male Wistar rats had 30% sucrose in their drinking water for 20 weeks. An increase in total calories consumed, visceral fat accumulation, insulin, triglycerides and blood pressure and a decrease in the food intake were observed in the sucrose-fed group as compared with the control group. A decrease in linoleic and alpha-linolenic acid (essential fatty acids) in all serum lipid fractions of sucrose-fed rats was found. This observation correlated with a low food intake by sucrose-fed rats. The conversion of [1 (14)C]-palmitic to [1 (14)C]-palmitoleic acid by Delta9-desaturase activity was increased in sucrose-fed compared with control rats, while the conversion of [1 (14)C]-dihomo-gamma-linolenic acids by Delta5-desaturase activity was depressed. In sucrose-fed as compared to control rats, the proportion of palmitoleic and oleic fatty acids was increased. Arachidonic acid was decreased in sucrose-fed rats. The 1,6-diphenylhexatriene fluorescence polarization of the microsomal membranes was significantly lower in the sucrose-fed group compared to the control group. These results indicate that the sucrose addition to the drinking water of the rats increased microsomal Delta9-desaturase activity and membrane disorder and decreased the activity of the Delta5-desaturase, a key enzyme in the biosynthesis of arachidonic acid, implicated in hypertension.

Myristic acid, unlike palmitic acid, is rapidly metabolized in cultured rat hepatocytes

This study was designed to examine and compare the metabolism of myristic and palmitic acids in cultured rat hepatocytes. [1-(14)C]-Labeled fatty acids were solubilized with albumin at 0.1 mmol/L in culture medium. Incubation with 24-hr cultured hepatocytes was carried out for 12 hr. Myristic acid was more rapidly (P < 0.05) taken up by the cells than was palmitic acid (86.9 +/- 0.9% and 68.3 +/- 5.7%, respectively, of the initial radioactivity was cleared from the medium after 4 hr incubation). Incorporation into cellular lipids, however, was similar after the same time (33.4 +/- 2.8% and 34.9 +/- 9.3%, respectively, of initial radioactivity). In the early phase of the incubation (30 min), myristic acid was more rapidly incorporated into cellular triglycerides than was palmitic acid (7.4 +/- 0.9% and 3.6 +/- 1.9%, respectively, of initial radioactivity). However, after 12 hr incubation, the radioactivity of cellular triglycerides, cellular phospholipids, and secreted triglycerides was significantly higher with palmitic acid as precursor. Myristic acid oxidation was significantly higher than that of palmitic acid (14.9 +/- 2.2% and 2.3 +/- 0.6%, respectively, of the initial radioactivity was incorporated into the beta-oxidation products after 4 hr). Myristic acid was also more strongly elongated to radiolabeled palmitic acid (12.2 +/- 0.8% of initial radioactivity after 12 hr) than palmitic acid was to stearic acid (5.1 +/- 1.3% of initial radioactivity after 12 hr). The combination of elongation and beta-oxidation results in the rapid disappearance of C14:0 in hepatocytes whereas C16:0 is esterified to form glycerolipids. This study provides evidence that myristic acid is more rapidly metabolized in cultured hepatocytes than is palmitic acid.

Fatty-acid composition in serum phospholipids and risk of breast cancer: an incident case-control study in Sweden

The study of the relationship between dietary intake of fatty acids and the risk of breast cancer has not yielded definite conclusions with respect to causality, possibly because of methodological issues inherent to nutritional epidemiology. To evaluate the hypothesis of possible protection of n-3 polyunsaturated fatty acids (PUFA) against breast cancer in women, we examined the fatty-acid composition of phospholipids in pre-diagnostic sera of 196 women who developed breast cancer, and of 388 controls matched for age at recruitment and duration of follow-up, in a prospective cohort study in Umeâ, northern Sweden. Individual fatty acids were measured as a percentage of total fatty acids, using capillary gas chromatography. Conditional logistic-regression models showed no significant association between n-3 PUFA and breast-cancer risk. In contrast, women in the highest quartile of stearic acid had a relative risk of 0.49 (95% confidence interval, 0.22-1.08) compared with women in the lowest quartile (trend p = 0.047), suggesting a protective role of stearic acid in breast-cancer risk. Besides stearic acid, women in the highest quartile of the 18:0/18:1 n-9c ratio had a relative risk of 0.50 (95% confidence interval, 0.23-1.10) compared with women in the lowest quartile (trend p = 0.064), suggesting a decrease in breast-cancer risk in women with low activity of the enzyme delta 9-desaturase (stearoyl CoA desaturase), which may reflect an underlying metabolic profile characterized by insulin resistance and chronic hyper-insulinemia.

Hormonal regulation of stearoyl coenzyme-A desaturase 1 activity and gene expression in primary cultures of chicken hepatocytes

Previous studies have provided evidence for the important role of liver stearoyl-CoA desaturase (SCD) in excessive adiposity in the chicken and suggest that the difference in SCD activity between fat and lean chickens could be explained by a difference in SCD1 gene expression. In the present study, the regulation of SCD1 gene expression was analyzed as the result of insulin and glucagon action, using primary cultures of 6-week-old chicken hepatocytes. Insulin increased SCD1 activity and mRNA levels, whereas glucagon decreased dramatically both the enzyme activity and the mRNA levels. Nuclear run-on transcription assays and mRNA stability investigations demonstrated that insulin and glucagon effects on SCD1 gene expression was primarily transcriptional. Furthermore, the results indicated that the glucagon-mediated inhibition of SCD1 gene transcription was more potent than just counteracting the insulin-mediated effect. These data represent the first demonstration that the glucagon effect on the SCD1 gene expression is primarily transcriptional. Moreover, among hepatic genes involved in lipid metabolism in chicken, SCD1 is the first gene shown to be regulated at the transcriptional level by insulin, in the absence of triiodothyronine. These data point out the potency of the growing chicken hepatocyte culture model in contrast with the embryonic cell culture model as regards the investigations of the insulin effect on gene expression.

Octanoate metabolism in primary culture of chicken hepatocytes: evidence for high-capacity octanoate esterification

[14C]Octanoate metabolism was measured in chicken primary hepatocytes, as a function of time in culture. [14C]Octanoate was essentially converted to oxidative products (CO2 and ketone bodies) in freshly isolated cells while [14C]palmitate was exclusively esterified in triacylglycerols (TG). In contrast, in cultured cells, [14C] octanoate was exclusively recovered in TG as radiolabeled octanoate (33%), but also as newly synthetised fatty acids mainly palmitate (37%), stearate (18%), oleate (4.4%), decanoate (2.4%), and myristate (2.2%). Therefore, it may be suggested that the enhanced lipogenic conditions induced by culture enhance octanoate flux toward TG synthesis in this model. It is further suggested that chicken hepatocytes in primary culture could be a useful model to study regulatory mechanisms of several lipogenic enzymes.

Inhibiting delta9-desaturase activity impairs triacylglycerol secretion in cultured chicken hepatocytes

The relationship between endogenous oleic acid produced by hepatic delta9-desaturase and the secretion of VLDL-triglycerides was investigated in a primary culture of chicken hepatocytes. When the fatty acid compositions of the secreted and intracellular triglycerides (TG) (or triacylglycerols) were compared, an imbalance between monoenes and saturated fatty acids was observed, with the secreted TG being significantly more unsaturated than the intracellular TG. The addition of a mixture of cyclopropenic fatty acids (specific inhibitors of fatty acid desaturation) to the culture medium of cells 24 h before measurement of their delta9-desaturase activity and TG secretion rate caused a significant impairment of both desaturase activity and TG secretion, without affecting total TG synthesis. However, the addition of oleic acid to the culture medium of cells treated with cyclopropenic fatty acids restored the TG secretion rate. Palmitic acid did not restore the TG secretion rate and linoleic acid partly restored the TG secretion rate. Finally, even in the presence of oleic acid in the culture medium of secreting cells, those which had been treated with cyclopropenic fatty acids had a significantly lower TG secretion rate than nontreated cells. Taken together, these results show that TG secretion is highly dependent on the delta9-desaturase activity and suggest that oversecretion of VLDL-TG in chickens and subsequent fattening could originate in a high hepatic delta9-desaturation of saturated fatty acids.