UC Berkeley

Lipolysis is the biochemical process during which triacylglycerol (TAG) stored in cellular lipid droplets undergoes stepwise hydrolysis to release fatty acids (FAs). FAs produced from lipolysis in adipose tissue are released into circulation, whereas, in other tissues, FAs are used intracellularly. There are 3 major lipases involved in intracellular

TAG hydrolysis; Desnutrin/adipose triglyceride lipase (ATGL) catalyzes hydrolysis of TAG to form diacylglycerol (DAG). DAG is then hydrolyzed by hormone-sensitive lipase (HSL) to monoacylglycerol, which is further hydrolyzed by monoglyceride lipase (MGL).

By employing adipose-specific conditional desnutrin knockout strategy (desnutrin ASKO mice), we show that desnutrin ASKO mice not only manifests in obesity, but converts BAT to a WAT-like tissue. Desnutrin ASKO mice exhibit impaired thermogenesis with decreased expression of UCP-1, with lower PPAR alpha binding to its promoter, revealing the

requirement of desnutrin-catalyzed lipolysis in the maintenance of a BAT phenotype. We also found that ablation of desnutrin in islet beta cells by using RIP-Cre or RIP-CreER impairs glucose-stimulated insulin secretion (GSIS) and induces TAG accumulation in islets, leading to pronounced hyperglycemia. Furthermore, we show that desnutrin-catalyzed lipolysis activates PPAR delta, rather than PPAR alpha, which in turn is critical for expression of genes involved in mitochondrial function, and thus ATP production, required for GSIS.

We next examined the mode of desnutrin regulation. We found that desnutrin is phosphorylated on Ser406 by AMP-activated protein kinase (AMPK) to increase its TAG hydrolase activity. To study the in vivo significance for AMPK phosphorylation, adipose-specific ablation of AMPK catalytic subunits alpha1 and alpha 2 (AMPK alpha 1/alpha 2 ASKO) were performed. We detected decreased phosphorylation of desnutrin on Ser406, accompanied with a lower TAG hydrolase activity. Furthermore, the inhibitory phosphorylation of hormone sensitive-lipase (HSL) on Ser565 by AMPK was reduced in AMPK alpha 1/alpha 2 ASKO WAT, resulting in an increased activating phosphorylation of HSL on Ser563 and Ser660 by PKA. Overall, AMPK alpha 1/alpha 2 ASKO mice manifest lean phenotype, with decreased fat mass, as well as reduced TAG and DAG content in WAT. We conclude that, when desnutrin activity is low, HSL dominates the lipolytic rate. These in vivo 2 transgenic mice studies reveal a novel function of lipolysis to provide specific ligands for the maintenance and function in adipose and non-adipose tissues. These studies may offer promising therapeutic approaches for obesity, diabetes, and related disorders.