Adipose tissue plays a crucial role in energy homeostasis not only in storing
triglyceride, but also responding to nutrient, neural, and hormonal signals, and producing factors which control feeding, thermogenesis, immune and neuroendocrine function, and
glucose and lipid metabolism. Adipose tissue secretes
leptin,
steroid hormones,
adiponectin, inflammatory
cytokines,
resistin,
complement factors, and vasoactive
peptides. The endocrine function of adipose tissue is typified by
leptin. An increase in
leptin signals satiety to neuronal targets in the hypothalamus.
Leptin activates Janus-activating kinase2 (Jak2) and STAT 3, resulting in stimulation of anorexigenic
peptides, e.g.,
alpha-MSH and CART, and inhibition of orexigenic
peptides, e.g., NPY and AGRP. The reduction in
leptin levels during fasting stimulates appetite, decreases thermogenesis, thyroid and reproductive
hormones, and increases
glucocorticoids.
Leptin also stimulates
fatty acid oxidation,
insulin release, and peripheral
insulin action. These effects involve regulation of
PI-3 kinase, PTP-1B, suppressor of
cytokine signaling-3 (SOCS-3), and
AMP-activated protein kinase in the brain and peripheral organs. There is emerging evidence that
leptin,
adiponectin, and
resistin act through overlapping pathways. Understanding the signal transduction of adipocyte
hormones will provide novel insights on the pathogenesis and treatment of
obesity, diabetes, and various metabolic disorders.