| Research Abstract |
Leptin, the protein product of the recently cloned obese gene, is secreted from adipose tissues and plays an important role in regulating body weight and energy expenditure through its receptor in the hypothalamus, the center of energy homeostasis. In addition to its hypothalamic action, several lines of evidence have shown that leptin acts directly on some extrahypothalamic and peripheral tissues such as pancreatic islets and T-lymphocytes.
In the present study, we investigated the specific binding sites of leptin and examined the effects of leptin on the secretion and synthesis of catecholamines in cultured bovine adrenal medullary cells. 1) The plasma membranes isolated from bovine adrenal medulla showed a single class of specific binding sites of^<125>I-leptin with an apparent Kd of 6.6nM and Bmax of 62 fmol/mg protein. Leptin receptor type a (ObRa) but not type b (ObRb) mRNA was detected in the tissue by reverse transcriptase-polymerase chain reaction. 2) Leptin (3-30nM) significan
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tly stimulated ^<14>C-catecholamine synthesis from [^<14>C]tyrosine, but not from [^<14>C] DOPA although it had little effect on catecholamine secretion in cultured bovine adrenal medullary cells. 3) The stimulation of catecholamine synthesis in the cells was associated with the phosphorylation and activation of tyrosine hydroxylase, the late-limiting enzyme of catecholamine synthesis. 4) The incubation of cells with leptin resulted in a rapid activation of the mitogen-activated protein kinases (MAPKs). An inhibitor of MAPK kinase, UO126, nullified the stimulatory effect of leptin on the synthesis of ^<14>C-catecholamines. 5) Leptin potentiated the stimulatory effect of acetylcholine on ^<14>C-catecholamine synthesis, whereas leptin failed to enhance the phosphorylation and activation of tyrosine hydroxylase induced by acetylcholine.
These findings suggest that leptin stimulates catecholamine synthesis via the activation of tyrosine hydroxylase by two different mechanisms, i.e., one is dependent on tyrosine hydroxylase phosphorylation mediated through the MAPK pathway and the second is independent on enzyme phosphorylation. This information helps to unveil the biochemical basis of the mechanism by which leptin acts on metabolic homeostasis. Less
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