Fat Cells, Mechanism of Lipogenesis and Lipolysis

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  • OKUDA Hiromichi
    2nd Department of Medical Biochemistry, School of Medicine, Ehime University

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  • 脂肪細胞, 脂肪の合成と分解のメカニズム
  • シボウ サイボウ シボウ ノ ゴウセイ ト ブンカイ ノ メカニズム

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Abstract

In fat cells there are lipogenic and lipolytic pathways. In the lipogenic pathway, blood glucose is incorporated into fat cells by insulin and converted to triglyceride through α-glycerophosphate and citrate. Human fat cells contain ATP-citrate lyase which changes citrate to acetylcoA and oxaloacetic acid. The triglyceride portion of chylomicron or VLDL is hydrolyzed by lipoprotein lipase to glycerol and fatty acids which are then incorporated into fat cells and converted to triglyceride with α-glycerophosphate derived from glucose. Triglyceride thus synthesized in fat cells, has the form of lipid droplets whose surface are covered with phospholipids including lecithin and proteins. <BR>Lipolysis induced by catecholamines and ACTH may possibly be mediated by the cyclic AMP cascade. Lipolytic hormones activate adenylate cyclase, to increase cyclic AMP formation. Cyclic AMP promotes lipolytic activity by elevating cyclic AMP-dependent protein kinase activity which in turn activates hormone-sensitive lipase (HSL) through phosphorylation, resulting in the hydrolysis of stored triglyceride to glycerol and free fatty acids. However, this theory is attended with some problems. For instance, it was found that norepinephrine induced lipolysis in fat cells without a concomitant increase in HSL activity. Neither ACTH-induced lipolysis in fat cells nor its inhibition by insulin was accompanied by matching fluctuations in HSL activity. Thus, another mechanism of lipolysis than the cyclic AMP cascade is proposed.

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