From Leptin to Insulin: Unveiling the Secrets of Fat Accumulation and Scientific Weight Loss

Factors Affecting Fat Accumulation

Adipose tissue grows in two ways: one is through the division of preadipocytes, increasing the number of adipocytes; the other is through the large-scale deposition of fat within cells, increasing adipocyte volume. Adipose tissue growth regulation is a complex process involving the nervous system, endocrine system, and the adipose tissue's own regulation.

1. Endocrine Regulation

Catecholamines, prostaglandins, lipogenic hormones, thyroxine, adrenocorticotropic hormone (ACTH), adrenocorticotropic hormone (ACTH), glucagon, etc., are all hormones that promote lipolysis. They activate hormone-sensitive lipases by acting on specific receptors on the adipocyte membrane via the second messenger cAMP, phosphorylating the lipase and enhancing lipolysis.

Insulin is the main physiological factor controlling the uptake and utilization of glucose and the synthesis of fat from glucose in adipose tissue; it is a lipogenic hormone. Insulin promotes lipogenesis and has a strong anti-lipolytic effect. Insulin exerts its effect by lowering cAMP levels and by activating lipoprotein phosphatase, causing dephosphorylation of hormone-sensitive lipase and reducing its activity, thereby inhibiting triglyceride degradation.

Leptin is a factor produced by adipose tissue and present in the bloodstream, regulating energy balance. Body weight and obmRNA levels in adipose tissue are positively correlated. Leptin can suppress appetite and reduce energy intake. This effect is achieved by inhibiting the neuropeptide-γ in the hypothalamus, thus reducing appetite in the central nervous system and decreasing energy intake. Simultaneously, leptin can promote energy expenditure. Its mechanism of action may involve acting on the central nervous system, increasing the activity of the sympathetic nervous system, leading to increased peripheral norepinephrine release, activating β₃ receptors on adipocyte membranes, and increasing the expression of uncoupled proteins, thereby converting stored energy into heat. Leptin also inhibits fat synthesis. Adding leptin to cultured preadipocytes inhibits the expression of the acetyl-CoA carboxylase gene, indicating that leptin can directly inhibit lipid synthesis in adipose tissue.

2. Regulation by Cytokines

Peroxidase proliferator-activated receptor (PPAR) is a lipid-activating transcription factor. Its target genes encode proteins involved in lipid homeostasis. PPAR-α stimulates β-oxidation of fatty acids. PPAR-γ induces adipocyte differentiation by stimulating the expression of several genes related to adipocyte proliferation. The ligand for PPAR-γ is prostaglandin, a high-affinity ligand that can induce the activity and RNA expression of lipoprotein lipases in adipose tissue. PPARs also participate in regulating high-density lipoprotein and cholesterol levels in vivo, primarily mediating the transcription of apo-AI and apo-AIII.

Adipose tissue contains abundant mRNA of retinoic acid, retinol, and vitamin D receptors. Vitamin A and vitamin D can inhibit adipocyte differentiation, possibly through the regulation of the PPAR receptor family activated by retinol, thyroxine, and fatty acids. In addition, many cytokines participate in regulating the differentiation and growth of adipose tissue, such as insulin-like growth factor-1 and epidermal growth factor.

3. Autoregulation of Adipose Tissue

Adipose tissue can produce certain cytokines that play a local regulatory role in its growth, such as tumor necrosis factor (TNF). In some genetically defective obesity models, TNF expression in adipocytes is also increased. This suggests that adipocytes secrete TNF as a local regulator to limit fat accumulation. However, excessive TNF secretion can cause insulin resistance in obese patients and those with type 2 diabetes.

In addition to cytokine regulation, interactions exist between microvascular endothelial cells and preadipocytes, and between adipocytes and preadipocytes, enabling coordinated growth of various components of adipose tissue.