Lipolysis and Lipogenesis

Lipolysis and lipogenesis are the opposing arms of the body's fat-storage cycle. Lipogenesis builds and stores triacylglycerol in adipose tissue when fuel is plentiful, while lipolysis hydrolyzes that stored fat to release free fatty acids and glycerol when fuel is needed. Insulin, catecholamines, and the cellular energy sensor AMP-activated protein kinase switch the balance between the two according to nutritional state.

Definition

Lipolysis is the sequential hydrolysis of stored triacylglycerol by adipose triglyceride lipase, hormone-sensitive lipase, and monoacylglycerol lipase to release free fatty acids and glycerol; lipogenesis is the synthesis of fatty acids and their esterification into triacylglycerol for storage. The two are reciprocally regulated by insulin and counter-regulatory hormones.

Scope

The entry focuses on the adipocyte fat-storage cycle: the hormone-regulated lipase cascade that mobilizes triacylglycerol (lipolysis) and the synthesis and esterification of fatty acids into stored triacylglycerol (lipogenesis), together with the hormonal control linking them. It treats hepatic de novo lipogenesis as a connected process but cross-references the dedicated synthesis entry. It is a biochemical and physiological reference, not advice on weight or metabolic disease management.

Core questions

• Which lipases hydrolyze stored triacylglycerol, and in what order?
• How do insulin and catecholamines switch fat storage and mobilization on and off?
• How is lipogenesis coordinated with lipolysis so they do not run simultaneously?
• What signals couple whole-body energy status to adipose fat turnover?

Key concepts

• Adipose triglyceride lipase (ATGL)
• Hormone-sensitive lipase (HSL)
• Monoacylglycerol lipase
• Perilipin and lipid droplet regulation
• Insulin-driven lipogenesis and antilipolysis
• Catecholamine/cAMP/PKA activation of lipolysis
• Free fatty acid and glycerol release
• AMP-activated protein kinase as energy sensor

Key theories

Sequential lipase cascade of lipolysis

Triacylglycerol breakdown proceeds in defined steps: adipose triglyceride lipase performs the rate-limiting first hydrolysis to diacylglycerol, hormone-sensitive lipase acts next, and monoacylglycerol lipase completes the reaction, releasing three fatty acids and glycerol under hormonal control.

Reciprocal hormonal control of storage and mobilization

Insulin promotes lipogenesis and suppresses lipase activity in the fed state, while catecholamines and glucagon activate lipolysis through cyclic-AMP and protein kinase A signaling in the fasted state, with AMP-activated protein kinase integrating cellular energy status.

Mechanisms

In the fed, insulin-rich state, adipocytes take up glucose and fatty acids, synthesize and esterify triacylglycerol, and store it in lipid droplets coated by perilipin; insulin simultaneously suppresses lipase activity, favouring net storage. During fasting or stress, catecholamines and glucagon raise cyclic AMP and activate protein kinase A, which phosphorylates perilipin and hormone-sensitive lipase and triggers the lipase cascade: adipose triglyceride lipase removes the first fatty acid (the rate-limiting step), hormone-sensitive lipase the second, and monoacylglycerol lipase the third, releasing free fatty acids to the blood for oxidation in other tissues and glycerol for hepatic gluconeogenesis. The synthesis side is governed transcriptionally by the insulin-responsive SREBP-1c program and acutely by AMP-activated protein kinase, which inhibits the lipogenic enzyme acetyl-CoA carboxylase when energy is scarce, ensuring that storage and mobilization are reciprocally, not simultaneously, active.

Clinical relevance

The lipolysis-lipogenesis balance determines how the body stores and releases energy and provides the biochemical background for insulin resistance, obesity, and the elevated free fatty acids seen in uncontrolled diabetes. This entry describes the normal regulatory cycle for reference and education and is not a basis for managing body weight or metabolic conditions in any individual.

History

Hormone-sensitive lipase was long thought to be the sole rate-limiting enzyme of fat mobilization, but the discovery of adipose triglyceride lipase in 2004 by Zechner and colleagues revised the model into a sequential cascade and clarified the rate-limiting first step. In parallel, the transcriptional control of the lipogenic side was tied to the insulin-responsive SREBP-1c pathway, and AMP-activated protein kinase was established as the cellular energy sensor coordinating the two arms.

Key figures

• Rudolf Zechner
• Salih Wakil
• Jay Horton
• D. Grahame Hardie

Seminal works

• zechner-2012
• zechner-2009
• horton-2002

Frequently asked questions

What is the difference between lipolysis and lipogenesis?

Lipogenesis is the synthesis and storage of fat as triacylglycerol, favoured in the fed state, whereas lipolysis is the hydrolysis of stored triacylglycerol to release fatty acids and glycerol, favoured during fasting; insulin and counter-regulatory hormones reciprocally control the two.

Which enzyme starts the breakdown of stored fat?

Adipose triglyceride lipase catalyzes the first and rate-limiting step, converting triacylglycerol to diacylglycerol, before hormone-sensitive lipase and monoacylglycerol lipase complete the hydrolysis.