Unveiling the Mechanisms of Leptin Resistance and Obesity: A Key Breakthrough for Scientific Weight Loss

Obesity and Leptin Resistance, Insulin Resistance

The UKPD survey indicates that no significant mutations in the OB or OB-R genes have been found in obese individuals. Like normal-weight individuals, serum leptin concentration in obese and diabetic patients is positively correlated with body mass index (BMI). Therefore, most obese patients have high serum leptin levels, not due to leptin deficiency, but rather hyperleptinemia, which may indicate leptin resistance. Leptin resistance may be due to defects in the central transport of leptin in obese individuals, preventing leptin from entering the cerebrospinal fluid (CSF) via OB-R mediation. This may be because serum leptin levels are saturated, preventing continuous entry into the CSF.

Many epidemiological studies indicate that obese individuals often have hyperinsulinemia and hyperleptinemia, which may be accompanied by insulin resistance due to post-insulin receptor defects. The UKPD survey shows that plasma leptin is related to sex, MBI, and fasting insulin concentration.

β-Adrenergic Receptor (β₃-AR)

The β₃-AR gene was cloned in 1989. The β₃-AR gene structure shares 50% homology with β-AR and β₂-AR. β₃-AR is a polypeptide composed of 402 amino acids. β₃-AR is a membrane receptor, and its transmembrane signal transduction requires coupling with G proteins.

The physiological function of β₃-AR is primarily to act on adipose tissue. In humans, β₃-AR is mainly distributed in brown adipose tissue, while only small amounts are found in white adipose tissue, the gallbladder, and the colon. Brown adipose tissue contains a large number of mitochondria and uncoupling proteins, which can stimulate the oxidation and phosphorylation of fatty acids, generating heat and consuming excess body fat. Although the amount of brown adipose tissue in the human body is small, its thermogenic effect is high; only 15g of this tissue can increase the body's heat production by 10%–15%. Experiments have also shown that β-AR agonists can induce the differentiation of preadipocytes into brown adipocytes by increasing the expression of uncoupling protein genes.

Molecular biological methods were used to detect the expression of β-AR mRNA in various parts of the human body. β-AR mRNA expression was found in human omental cells, subcutaneous adipose tissue, and biopsy tissues of the gallbladder and colon, but not in muscle, heart, liver, lung, thyroid, or lymphocytes.

Brown adipose tissue is closely related to obesity. β₃-AR is mainly distributed in brown adipose tissue, and abnormalities or absence of its receptors can cause obesity. Experiments show that β₃-AR expression is reduced in genetically obese mice, while β-AR agonists have effects on obesity, anti-diabetic effects, and improved insulin resistance in animal models of obesity and diabetes. Epidemiological studies have found that β₃-AR gene variations (the replacement of Trp at position 64 with Arg, forming the Trp64Arg allele) are associated with obesity and insulin resistance.

Tumor Necrosis Factor (TNF-α)

TNF-α regulates appetite, thermogenesis, and lipid metabolism through direct and indirect mechanisms (hypothalamic feedback), and simultaneously mediates two extreme metabolic states: cachexia and obesity.

1. Direct Regulation of Appetite and Thermogenesis by TNF-α

TNF-α, in addition to directly inhibiting gastric emptying and regulating appetite, can also affect appetite by stimulating interleukin-1 release and regulating insulin and glucocorticoids.

TNF-α primarily mediates non-shivering thermogenesis in brown adipose tissue.

2. Direct Regulation of Lipid Metabolism by TNF-α

TNF-α significantly inhibits lipoprotein lipase activity, thereby inhibiting the uptake of exogenous lipids by adipocytes and reducing lipid synthesis in adipocytes. TNF-α can also promote lipolysis and inhibit the expression of glucose transporter-4, reducing fat synthesis.

3. TNF-α Regulation via the Hypothalamus

TNF-α is synthesized and secreted into the bloodstream by adipocytes. It rapidly reaches the hypothalamus via periventricular nerve cells and axons, transmitting information about fat storage and thus regulating hypothalamic functions such as appetite and thermogenesis.

The two extremes of energy metabolism are cachexia and obesity. TNF-α is not only closely related to cachexia but also plays an important role in obesity. Therefore, the mechanism by which TNF-α regulates energy metabolism is complex, and its specific role in obesity and cachexia requires further investigation.