Leptin: Less Is More

β-Cell Dysfunction, Hepatic Lipid Metabolism, and Cardiovascular Health in Type 2 Diabetes: New Directions of Research and Novel Therapeutic Strategies

Cardiovascular disease (CVD) remains a major problem for people with type 2 diabetes mellitus (T2DM), and dyslipidemia is one of the main drivers for both metabolic diseases. In this review, the major pathophysiological and molecular mechanisms of β-cell dysfunction and recovery in T2DM are discussed in the context of abnormal hepatic lipid metabolism and cardiovascular health. (i) In normal health, continuous exposure of the pancreas to nutrient stimulus increases the demand on β-cells. In the long term, this will not only stress β-cells and decrease their insulin secretory capacity, but also will blunt the cellular response to insulin. (ii) At the pre-diabetes stage, β-cells compensate for insulin resistance through hypersecretion of insulin. This increases the metabolic burden on the stressed β-cells and changes hepatic lipoprotein metabolism and adipose tissue function. (iii) If this lipotoxic hyperinsulinemic environment is not removed, β-cells start to lose function, and CVD risk rises due to lower lipoprotein clearance. (iv) Once developed, T2DM can be reversed by weight loss, a process described recently as remission. However, the precise mechanism(s) by which calorie restriction causes normalization of lipoprotein metabolism and restores β-cell function are not fully established. Understanding the pathophysiological and molecular basis of β-cell failure and recovery during remission is critical to reduce β-cell burden and loss of function. The aim of this review is to highlight the link between lipoprotein export and lipid-driven β-cell dysfunction in T2DM and how this is related to cardiovascular health. A second aim is to understand the mechanisms of β-cell recovery after weight loss, and to explore new areas of research for developing more targeted future therapies to prevent T2DM and the associated CVD events.

Adiponectin, Leptin and Cardiovascular Disorders

The landmark discoveries of leptin and adiponectin firmly established adipose tissue as a sophisticated and highly active endocrine organ, opening a new era of investigating adipose-mediated tissue crosstalk. Both obesity-associated hyperleptinemia and hypoadiponectinemia are important biomarkers to predict cardiovascular outcomes, suggesting a crucial role for adiponectin and leptin in obesity-associated cardiovascular disorders. Normal physiological levels of adiponectin and leptin are indeed essential to maintain proper cardiovascular function. Insufficient adiponectin and leptin signaling results in cardiovascular dysfunction. However, a paradox of high levels of both leptin and adiponectin is emerging in the pathogenesis of cardiovascular disorders. Here, we (1) summarize the recent progress in the field of adiponectin and leptin and its association with cardiovascular disorders, (2) further discuss the underlying mechanisms for this new paradox of leptin and adiponectin action, and (3) explore the possible application of partial leptin reduction, in addition to increasing the adiponectin/leptin ratio as a means to prevent or reverse cardiovascular disorders.

The Leptin System and Diet: A Mini Review of the Current Evidence

Leptin promotes satiety and modulates energy balance and weight. Diet-induced obesity leads to leptin resistance, exacerbating overeating. We reviewed the literature on the relationship between diet and leptin, which suggests that addressing leptin resistance through dietary interventions can contribute counteracting obesity. Albeit some limitations (e.g., limited rigor, small samples sizes), studies in animals and humans show that diets high in fat, carbohydrates, fructose, and sucrose, and low in protein are drivers of leptin resistance. Despite methodological heterogeneity pertaining to this body of literature, experimental studies show that energy-restricted diets can reduce leptinemia both in the short and long term and potentially reverse leptin resistance in humans. We also discuss limitations of this evidence, future lines of research, and implications for clinical and public health translations. Main limitations include the lack of a single universally-accepted definition of leptin resistance, and of adequate ways to accurately measure it in humans. The use of leptin sensitizers (drugs) and genetically individualized diets are alternatives against leptin resistance that should be further researched in humans. The tested very-low-energy intervention diets are challenging to translate into wide clinical or population recommendations. In conclusion, the link between nutritional components and leptin resistance, as well as research indicating that this condition is reversible, emphasizes the potential of diet to recover sensitivity to this hormone. A harmonized definition of leptin resistance, reliable methods to measure it, and large-scale, translational, clinical, and precision nutrition research involving rigorous methods are needed to benefit populations through these approaches.

A feed-forward regulatory loop in adipose tissue promotes signaling by the hepatokine FGF21

In this study, Han et al. demonstrate that JNK signaling in adipocytes causes an increased circulating concentration of the hepatokine fibroblast growth factor 21 (FGF21) that regulates systemic metabolism. This regulatory loop represents a novel signaling paradigm that connects autocrine and endocrine signaling modes of the same hormone in different tissues.

The cJun NH₂-terminal kinase (JNK) signaling pathway is activated by metabolic stress and promotes the development of metabolic syndrome, including hyperglycemia, hyperlipidemia, and insulin resistance. This integrated physiological response involves cross-talk between different organs. Here we demonstrate that JNK signaling in adipocytes causes an increased circulating concentration of the hepatokine fibroblast growth factor 21 (FGF21) that regulates systemic metabolism. The mechanism of organ crosstalk is mediated by a feed-forward regulatory loop caused by JNK-regulated FGF21 autocrine signaling in adipocytes that promotes increased expression of the adipokine adiponectin and subsequent hepatic expression of the hormone FGF21. The mechanism of organ cross-talk places circulating adiponectin downstream of autocrine FGF21 expressed by adipocytes and upstream of endocrine FGF21 expressed by hepatocytes. This regulatory loop represents a novel signaling paradigm that connects autocrine and endocrine signaling modes of the same hormone in different tissues.

Leptin modulates pancreatic β-cell membrane potential through Src kinase-mediated phosphorylation of NMDA receptors

The adipocyte-derived hormone leptin increases trafficking of KATP and Kv2.1 channels to the pancreatic β-cell surface, resulting in membrane hyperpolarization and suppression of insulin secretion. We have previously shown that this effect of leptin is mediated by the NMDA subtype of glutamate receptors (NMDARs). It does so by potentiating NMDAR activity, thus enhancing Ca2+ influx and the ensuing downstream signaling events that drive channel trafficking to the cell surface. However, the molecular mechanism by which leptin potentiates NMDARs in β-cells remains unknown. Here, we report that leptin augments NMDAR function via Src kinase-mediated phosphorylation of the GluN2A subunit. Leptin-induced membrane hyperpolarization diminished upon pharmacological inhibition of GluN2A but not GluN2B, indicating involvement of GluN2A-containing NMDARs. GluN2A harbors tyrosine residues that, when phosphorylated by Src family kinases, potentiate NMDAR activity. We found that leptin increases phosphorylation of Tyr-418 in Src, an indicator of kinase activation. Pharmacological inhibition of Src or overexpression of a kinase-dead Src mutant prevented the effect of leptin, whereas a Src kinase activator peptide mimicked it. Using mutant GluN2A overexpression, we show that Tyr-1292 and Tyr-1387 but not Tyr-1325 are responsible for the effect of leptin. Importantly, β-cells from db/db mice, a type 2 diabetes mouse model lacking functional leptin receptors, or from obese diabetic human donors failed to respond to leptin but hyperpolarized in response to NMDA. Our study reveals a signaling pathway wherein leptin modulates NMDARs via Src to regulate β-cell excitability and suggests NMDARs as a potential target to overcome leptin resistance.

Adipose tissue in health and disease

Adipose, or fat, tissue (AT) was once considered an inert tissue that primarily existed to store lipids, and was not historically recognized as an important organ in the regulation and maintenance of health. With the rise of obesity and more rigorous research, AT is now recognized as a highly complex metabolic organ involved in a host of important physiological functions, including glucose homeostasis and a multitude of endocrine capabilities. AT dysfunction has been implicated in several disease states, most notably obesity, metabolic syndrome and type 2 diabetes. The study of AT has provided useful insight in developing strategies to combat these highly prevalent metabolic diseases. This review highlights the major functions of adipose tissue and the consequences that can occur when disruption of these functions leads to systemic metabolic dysfunction.

Glucagon-like peptide-1 receptor co-agonists for treating metabolic disease

Background

Glucagon-like peptide-1 receptor (GLP-1R) agonists are approved to treat type 2 diabetes and obesity. They elicit robust improvements in glycemic control and weight loss, combined with cardioprotection in individuals at risk of or with pre-existing cardiovascular disease. These attributes make GLP-1 a preferred partner for next-generation therapies exhibiting improved efficacy yet retaining safety to treat diabetes, obesity, non-alcoholic steatohepatitis, and related cardiometabolic disorders. The available clinical data demonstrate that the best GLP-1R agonists are not yet competitive with bariatric surgery, emphasizing the need to further improve the efficacy of current medical therapy.

Scope of review

In this article, we discuss data highlighting the physiological and pharmacological attributes of potential peptide and non-peptide partners, exemplified by amylin, glucose-dependent insulinotropic polypeptide (GIP), and steroid hormones. We review the progress, limitations, and future considerations for translating findings from preclinical experiments to competitive efficacy and safety in humans with type 2 diabetes and obesity.

Major conclusions

Multiple co-agonist combinations exhibit promising clinical efficacy, notably tirzepatide and investigational amylin combinations. Simultaneously, increasing doses of GLP-1R agonists such as semaglutide produces substantial weight loss, raising the bar for the development of new unimolecular co-agonists. Collectively, the available data suggest that new co-agonists with robust efficacy should prove superior to GLP-1R agonists alone to treat metabolic disorders.

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GLP-1 is a preferred partner for co-agonist development.

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Co-agonist combinations must exhibit improved weight loss beyond GLP-1 alone.

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Unimolecular coagonists must exhibit retained or improved cardioprotection.

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Obesity represents an optimal condition for the development of new GLP-1 co-agonists.

Administration of small-molecule guanabenz acetate attenuates fatty liver and hyperglycemia associated with obesity

Nonalcoholic fatty liver disease (NAFLD) is characterized by excessive accumulation of hepatic triglycerides (TG) and hyperglycemia arising due to persistent insulin resistance, and is profoundly linked to obesity. However, there is currently no established treatment for NAFLD in obese human subjects. We previously isolated Helz2, the expression of which was upregulated in human and mouse NAFLD, and its deletion activated the hepatic expression of functional leptin receptor long form (Leprb) and suppressed NAFLD development and body weight (BW) gain in obese mice. A high-throughput assay of small-molecule drugs revealed that guanabenz acetate (Ga), originally used to treat hypertension, possesses a high affinity constant against HELZ2, and its administration activates LEPRB expression in HepG2 cells in vitro. The chronic oral administration of Ga shows the selective leptin sensitization in the liver via upregulation of hepatic Leprb expression, which affects expression of genes involved in lipogenesis and fatty acid β-oxidation and diminishes hepatocyte hypertrophy with droplets enriched in TG in high-fat diet-induced obese mice. This activity significantly improves insulin resistance to decrease hyperglycemia and hepatocyte and adipocyte weights, resulting in BW reduction without reducing food intake. Regarding drug repositioning, Ga has the potential to effectively treat NAFLD and hyperglycemia in obese patients.

Partial leptin deficiency confers resistance to diet-induced obesity in mice

Objective

Hyperleptinemia per se is sufficient to promote leptin resistance in the obese state. Leptin sensitivity can be restored by reducing circulating leptin levels within a physiologically healthy range and is a viable antiobesity and antidiabetic strategy. However, a previous study suggests that partial leptin deficiency favors diet-induced obesity and related metabolic disorders in mice, arguing that a lower leptin level may indeed promote diet-induced obesity and its associated metabolic disorders. Here, we aim to elucidate what the impact of partial leptin deficiency is on fat mass and insulin sensitivity.

Methods

We used two different mouse models of partial leptin deficiency: an adipocyte-specific congenital heterozygous leptin knockout mouse line (LepHZ) and the well-established whole body heterozygous leptin knockout mouse (OBHZ). The metabolic studies of OBHZ and LepHZ mice were performed both on normal carbohydrate-rich chow diet and on a high-fat diet (HFD). Male and female mice were included in the study to account for sex-specific differences. Body weight, food intake, glucose tolerance, and insulin tolerance were tested. Histology of adipose tissue and liver tissue allowed insights into adipose tissue inflammation and hepatic triglyceride content. Immunohistochemistry was paired with RT-PCR analysis for expression levels of inflammatory markers.

Results

Both OBHZ and LepHZ mice displayed reduced circulating leptin levels on the chow diet and HFD. On chow diet, male OBHZ and LepHZ mice showed elevated fat mass and body weight, while their glucose tolerance and insulin sensitivity remained unchanged. However, the inability in partially leptin-deficient mice to fully induce circulating leptin during the development of diet-induced obesity results in reduced food intake and leaner mice with lower body weight compared to their littermate controls. Importantly, a strong reduction of adipose tissue inflammation is observed along with improvements in insulin sensitivity and enhanced glucose tolerance. Additionally, partial leptin deficiency protects the mice from fatty liver and liver fibrosis. Chronically HFD-fed OBHZ and LepHZ mice remain more sensitive to exogenous leptin injection, as reflected by their reduced food intake upon an acute leptin treatment.

Conclusion

In response to HFD feeding, the inability to upregulate leptin levels due to partial leptin deficiency protects mice from diet-induced obesity and metabolic dysregulation. Thus, in an obesogenic environment, maintaining lower leptin levels is highly beneficial for both obesity and diabetes management. Chronic leptin reduction represents a viable preventive strategy whose efficacy awaits clinical testing.

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Partial leptin deficiency protects from diet-induced obesity.

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Reduced leptin protects from diet-induced obesity independent of sex.

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Reduction of circulating leptin levels inhibits HFD-induced adipose tissue inflammation.

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Partial leptin deficiency confers resistance to HFD-induced liver fibrosis.

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Partial leptin deficiency enhances leptin sensitivity.