Adropin

Adropin was identified in 2008 through a bioinformatics screen for secreted factors regulated by dietary macronutrient intake. The ENHO (Energy Homeostasis Associated) gene encodes a 76-amino acid precursor; after signal peptide and propeptide cleavage, the active secreted form is adropin34-76 — a 43-amino acid peptide. ENHO is expressed most highly in the liver, brain, and heart, with lower expression in adipose tissue and skeletal muscle.

The discovery paper showed that ENHO-null mice on a high-fat diet developed severe insulin resistance, obesity, and dyslipidaemia, while ENHO transgenic overexpression protected against diet-induced metabolic syndrome. Subsequent studies established that adropin acts as a hepatokine (liver-secreted) and neurokine (brain-secreted) that coordinates peripheral energy metabolism with central satiety signals.

Clinically, reduced circulating adropin is consistently observed in obesity, type 2 diabetes, non-alcoholic fatty liver disease (NAFLD), and coronary artery disease. It rises with caloric restriction, exercise, and insulin-sensitizing therapies. This inverse correlation with metabolic disease burden has generated strong interest in adropin as a diagnostic biomarker and as a therapeutic target — either through exogenous adropin administration or via ENHO-upregulating interventions.

// FAT OXIDATION REGULATION IN SKELETAL MUSCLE

Adropin suppresses the expression of pyruvate dehydrogenase kinase 4 (PDK4) in skeletal muscle — an enzyme that inhibits glucose oxidation by inactivating the pyruvate dehydrogenase complex (PDC). By reducing PDK4, adropin shifts muscle substrate utilisation from fat toward glucose, improving glucose disposal and insulin sensitivity. This mechanism is particularly relevant during high-fat feeding where PDK4 is pathologically elevated.

// ENDOTHELIAL PROTECTION & NITRIC OXIDE

Adropin activates endothelial nitric oxide synthase (eNOS) and upregulates nitric oxide (NO) production in vascular endothelium — a key mechanism underlying its cardioprotective effects. Studies show adropin protects against ischaemia-reperfusion injury, reduces oxidative stress in endothelial cells, attenuates TNF-α-induced adhesion molecule expression, and preserves vascular reactivity in obese/diabetic models.

// INSULIN SENSITISATION

Adropin enhances insulin signalling through PI3K/Akt and AMPK pathways in hepatic and skeletal muscle tissue. In obese rodent models, exogenous adropin administration normalises fasting glucose, improves oral glucose tolerance, and reduces hepatic fat content without altering food intake or body weight — indicating a direct metabolic effect independent of appetite modulation.

// CNS ENERGY SENSING

ENHO expression in the brain — particularly hypothalamus and brainstem — suggests adropin participates in central energy homeostasis regulation. Brain-specific ENHO knockout mice develop increased adiposity, implicating adropin in hypothalamic circuits that sense and respond to nutrient status. The CNS contribution to adropin's overall metabolic effects is an active area of investigation.

Discovery (Kumar et al., Cell Metabolism 2008): ENHO was identified by screening gene expression data for secreted proteins regulated by fat and carbohydrate intake. ENHO-null mice showed: obesity, insulin resistance, hypertriglyceridaemia, and elevated fatty acid oxidation markers. Transgenic overexpression reversed all these phenotypes. Circulating adropin correlated inversely with body mass index in human subjects.

Cardioprotection studies (2014–2022): Multiple groups demonstrated that intravenous adropin administration before myocardial ischaemia-reperfusion significantly reduced infarct size in rodent models, associated with eNOS activation and oxidative stress attenuation. Circulating adropin is reduced in coronary artery disease patients and inversely correlates with MACE (major adverse cardiovascular events) risk.

Human observational data: Lower serum adropin is consistently observed in T2D, NAFLD, PCOS, and hypertension. Adropin levels correlate positively with HDL-cholesterol and inversely with triglycerides, BMI, HOMA-IR, and CRP. These associations position adropin as a potential cardiometabolic risk biomarker.

No approved pharmaceutical formulation of exogenous adropin exists. Peptide synthesis of Adropin34-76 is feasible and used in research settings. Stability and oral bioavailability are low — injectable research protocols extrapolate from rodent effective doses (typically 0.1–1 nmol/kg IV or SQ).