Laura M. Frago

Astrocytes are known to have key roles in the neuroendocrine control of metabolism. The spectrum of actions of this cell population continues to expand as their heterogeneity is deciphered and new study approaches are employed. Astrocytes were first implicated in metabolic control when they were shown to participate in hypothalamic inflammation induced by high-fat diets, which leads to complications such as continued weight gain and glucose intolerance. In addition to their participation in pathophysiological processes associated with poor nutrition and obesity, these glial cells are now accepted as fundamental players in the physiological control of energy homeostasis. Along with their effect on neurons controlling appetite and energy expenditure, glial cells are implicitly involved in the generation of circadian rhythms, including eating and sleep patterns that affect metabolism and may link metabolism with other neuroendocrine systems such as puberty. The evolving concepts regarding astrocytes in metabolic control, including their participation in responses to currently used obesity drugs, highlight them as potential therapeutic targets. Here, we review advances in our understanding of how astrocytes participate in these processes, describe the mechanisms involved and discuss future directions.

Background/Objectives: Carbohydrate-restricted diets (CHRs) are increasingly employed in the treatment of obesity. We aimed to investigate the effects of a CHR on hepatic lipid anabolism and its association with changes in the proinflammatory environment and insulin signaling. Methods: Forty-eight C57BL/6J female mice were used in this study. We aimed to analyze the impact of a CHR on the hepatic proinflammatory profile and its relationship with changes in insulin signaling and fatty acid anabolism in obese female mice after two months on a high-fat diet. We also examined the impact of a one-month chow diet after CHR. Blood samples were collected, and the liver was processed during all-time study periods for analyses of biochemical, hormonal, and inflammatory markers, as well as possible changes in leptin and insulin signaling pathways. Results: Compared with chow-fed mice, CHR mice showed increased interleukin (IL)-1β and IL-2 levels, as well as leptin-related signaling in the liver. There was also a decrease in the expression of fatty acid synthase and the phosphorylation of ATP-citrate lyase, which was associated with a reduction in the activation of the insulin receptor, Akt, the mammalian target of rapamycin, cAMP-response element-binding protein, and glycogen synthase kinase 3β. The subsequent reintroduction of a chow diet after CHR resulted in lower hepatic free fatty acid and triglyceride levels than in obese mice without previous CH restriction. Conclusions: This study suggests that CHR inhibits de novo hepatic lipogenesis in obese mice by attenuating insulin signaling in a low-grade proinflammatory state.

The efficacy of ketogenic diets (KD) in inducing weight loss is well documented, but the changes in peripheral and hepatic insulin sensitivity and their associated effects on fatty acid anabolism remain to be elucidated. The present study examined the impact of a chow diet (CD) versus a one-month KD on peripheral and hepatic insulin sensitivity in male mice with obesity due to two months on a high-fat diet. The subsequent effect of instauration of one-month CD in both subgroups was also investigated. KD mice presented higher weight, serum leptin and high-molecular weight adiponectin levels along with reduced HOMA-IR index. Phosphorylation of STAT3 was increased, while the phosphorylation of insulin receptor, Akt and cAMP response element-binding protein, the activation of ATP citrate lyase and levels of fatty acid synthase, were reduced in KD mice. After instauration of CD in both subgroups, those mice previously fed KD exhibited lower HOMA-IR index and serum and hepatic triglycerides. These findings suggest that KD intervention improves peripheral insulin sensitivity through the reduction in hepatic insulin signaling that inhibits fatty acid anabolism, thereby reducing hepatic triglyceride content.

Background/Objectives: Males and females have different propensities to develop obesity and its complications and understanding how sex affects the responds to specific nutritional signals is of great interest for developing personalized dietary interventions. Our aim was to study the impact of specific dietary lipids on energy balance and parameters of lipid metabolism in the hypothalamus and brown adipose tissue (BAT). Subjects/Methods: The metabolic responses of male and female mice to 8 weeks of diets enriched in sunflower seed oil (SSO) or palm kernel oil (PKO) were analysed. Results No change in energy intake was observed, but weight gain was induced by the fatty acid enriched diets, reaching statistical significance in response to SSO in females. The fatty acid enriched diets increased glycemia, reaching statistical significance in both sexes with SSO. In the hypothalamus citrate synthase activity was induced by both diets in both sexes, but carnitine palmitoyl transferase 1a (Cpt1a) mRNA levels increased only in females on the SSO diet. In BAT, uncoupling protein 1 and Cpt1a mRNA levels increased in both sexes with both diets, while fatty acid synthase mRNA levels decreased. Activation of p-AMPK was induced and the levels of p-mTOR decreased with both diets, although overall greater effects were observed with SSO. Conclusions Saturated and unsaturated fatty acid-enriched diets elicit distinct metabolic responses in males and females, with greater overall effects found with increased unsaturated fatty acid intake. Indeed, despite similar caloric intake, females fed the unsaturated fat-enriched diet (SSO) had greater weight gain and energy efficiency, while in BAT this was accompanied by favourable metabolic adaptations, including increased thermogenesis and fat oxidation, with less disruption of autophagy. These findings highlight the importance of diet composition and sex in shaping metabolic outcomes and emphasize the need to understand these differences when designing dietary strategies.

Alzheimer’s disease (AD) is characterized by the deposition in the brain of senile plaques composed of amyloid-β peptides (Aβs) that increase inflammation. An endogenous peptide derived from the insulin-like growth factor (IGF)-I, glycine-proline-glutamate (GPE), has IGF-I-sensitizing and neuroprotective actions. Here, we examined the effects of GPE on Aβ levels and hippocampal inflammation generated by the intracerebroventricular infusion of Aβ25-35 for 2 weeks (300 pmol/day) in ovariectomized rats and the signaling-related pathways and levels of Aβ-degrading enzymes associated with these GPE-related effects. GPE prevented the Aβ-induced increase in the phosphorylation of p38 mitogen-activated protein kinase and the reduction in activation of signal transducer and activator of transcription 3, insulin receptor substrate-1, and Akt, as well as on interleukin (IL)-2 and IL-13 levels in the hippocampus. The functionality of somatostatin, measured as the percentage of inhibition of adenylate cyclase activity and the levels of insulin-degrading enzyme, was also preserved by GPE co-treatment. These findings indicate that GPE co-administration may protect from Aβ insult by changing hippocampal cytokine content and somatostatin functionality through regulation of leptin- and IGF-I-signaling pathways that could influence the reduction in Aβ levels through modulation of levels and/or activity of Aβ proteases.

Alzheimer´s disease (AD) is characterized by the brain deposition of senile plaques composed by amyloid-β peptides (Aβ) that increase inflammation. An endogenous peptide derived from the insulin-like growth factor (IGF)-I, glycine-proline-glutamate (GPE) has IGF-I-sensitizing and neuroprotective actions. Here, we examined the effects of GPE on Aβ levels and hippocampal inflammation generated by the intracerebroventricular infusion of Aβ25-35 during 2 weeks (300 pmol/day) in ovariectomized rats and the signaling-related pathways and levels of Aβ-degrading enzymes associated with these GPE-related effects. GPE prevented the Aβ-induced increase in the phosphorylation of p38 mitogen-activated protein kinase and the reduction in activation of signal transducer and activator of transcription 3, insulin receptor substrate-1 and Akt, as well as on interleukin (IL)-2 and IL-13 levels in the hippocampus. The functionality of somatostatin, measured as the percentage of inhibition of adenylate cyclase activity and the levels of insulin-degrading enzyme were also preserved by GPE co-treatment. These findings indicate that GPE co-administration may protect from Aβ insult by changing hippocampal cytokine content and somatostatin functionality through regulation of leptin- and IGF-I-signaling pathways that could influence the reduction in Aβ levels through modulation of levels and/or activity of Aβ proteases.

Leptin inhibits food intake and reduces the size of body fat depots, changing adipocyte sensitivity to insulin to restrain lipid accrual. This adipokine may modulate the production of cytokines that could diminish insulin sensitivity, particularly in visceral adipose tissue. To explore this possibility, we examined the effects of chronic central administration of leptin on the expression of key markers of lipid metabolism and its possible relationship with changes in inflammatory- and insulin-signaling pathways in epididymal adipose tissue. Circulating non-esterified fatty acids and pro- and anti-inflammatory cytokines were also measured. Fifteen male rats were divided into control (C), leptin (L, icv, 12 μg/day for 14 days), and pair-fed (PF) groups. We found a decrease in the activity of glucose-6-phosphate dehydrogenase and malic enzyme in the L group, with no changes in the expression of lipogenic enzymes. A reduction in the expression of lipoprotein lipase and carnitine palmitoyl-transferase-1A, together with a decrease in the phosphorylation of insulin-signaling targets and a low-grade inflammatory pattern, were detected in the epididymal fat of L rats. In conclusion, the decrease in insulin sensitivity and increased pro-inflammatory environment could regulate lipid metabolism, reducing epididymal fat stores in response to central leptin infusion.

Excess body weight is frequently associated with low-grade inflammation. Evidence indicates a relationship between obesity and cancer, as well as with other diseases, such as diabetes and non-alcoholic fatty liver disease, in which inflammation and the actions of various adipokines play a role in the pathological mechanisms involved in these disorders. Leptin is mainly produced by adipose tissue in proportion to fat stores, but it is also synthesized in other organs, where leptin receptors are expressed. This hormone performs numerous actions in the brain, mainly related to the control of energy homeostasis. It is also involved in neurogenesis and neuroprotection, and central leptin resistance is related to some neurological disorders, e.g., Parkinson’s and Alzheimer’s diseases. In peripheral tissues, leptin is implicated in the regulation of metabolism, as well as of bone density and muscle mass. All these actions can be affected by changes in leptin levels and the mechanisms associated with resistance to this hormone. This review will present recent advances in the molecular mechanisms of leptin action and their underlying roles in pathological situations, which may be of interest for revealing new approaches for the treatment of diseases where the actions of this adipokine might be compromised.

Leptin is involved in the modulation of insulin signaling in peripheral tissues, being closely associated with changes in lipid metabolism. This adipokine modifies inflammatory pathways that can interact with insulin targets in peripheral organs; however, the mechanisms remain unclear. Inflammatory and insulin signaling targets, cytokines, adiponectin, irisin and non-esterified fatty acid (NEFA) levels and enzymes of fatty acid anabolism were studied in the gastrocnemius of chronic centrally infused leptin (L), pair-fed and control rats. The phosphorylation of signal transducer and activator of transcription 3 (STAT3) and c-Jun N-terminal kinase (JNK) was reduced in L rats (59% and 58%, respectively). The phosphorylation of the insulin receptor and Akt and adiponectin and irisin content was increased in L rats (154%, 157%, 308% and 329%, respectively). The levels of glucose-6-phosphate dehydrogenase, the mRNA content of acetyl Co-A carboxylase and NEFA concentrations were diminished in the muscles of L rats (59%, 50% and 61%, respectively). The activation of JNK correlated positively with STAT3 phosphorylation, tumoral necrosis factor-α and NEFA and negatively with irisin and Akt phosphorylation. These data suggest that the activation of insulin signaling targets and a decrease in NEFA content are associated with a reduction in muscle inflammation parameters, suggesting that leptin may integrate these pathways.

Leptin modulates insulin signaling and this involves the Akt pathway, which is influenced by changes in the inflammatory environment and with leptin regulating cytokine synthesis. We evaluated the association between activation of the insulin-signaling pathway and alterations in pro- and anti-inflammatory cytokine levels in inguinal fat and liver of chronic central leptin infused (L), pair-fed (PF), and control rats. Signal transducer and activator of transcription 3 (STAT3) phosphorylation was increased in inguinal fat and reduced in liver of L rats. Phosphorylation of c-Jun N-terminal kinase (JNK) and nuclear factor kappa B (NFkB) was increased in inguinal fat of L rats, together with a pro-inflammatory cytokine profile, while in the liver activation of JNK and NFkB were reduced and an anti-inflammatory pattern was found. Phosphorylation of the insulin receptor, Akt and mechanistic target of rapamycin was decreased in inguinal fat and increased in liver of L rats. There was a direct relationship between pSTAT3 and JNK and a negative correlation of Akt with pSTAT3 and JNK in both tissues. These results indicate that the effects of chronically increased leptin on insulin-related signaling are tissue-specific and suggest that inflammation plays a relevant role in the crosstalk between leptin and insulin signaling.