A main function of white adipose tissue is to release fatty acids from triacylglycerol for other tissues to use as an energy source. While endocrine regulation of lipolysis has been extensively studied, autocrine/paracrine regulation is not well understood. Here, we describe the role of AdPLA, the newly identified major adipocyte phospholipase A2, in the regulation of lipolysis and adiposity. AdPLA null mice have a markedly higher rate of lipolysis, due to increased cAMP levels arising from the marked reduction in adipose PGE2 that binds the Gαi-coupled receptor, EP3. AdPLA null mice have drastically reduced adipose tissue mass and triglyceride content, with normal adipogenesis. They also have higher energy expenditure with higher fatty acid oxidation within adipocytes. AdPLA deficient ob/ob mice remain hyperphagic but lean, with increased energy expenditure, yet have ectopic triglyceride storage and insulin resistance. AdPLA is a major regulator of adipocyte lipolysis and critical for the development of obesity.

Ethanolamine kinase (ATP:ethanolamine O-phosphotransferase, EC 2.7.1.82) catalyzes the committed step of phosphatidylethanolamine synthesis via the CDP-ethanolamine pathway. The gene encoding ethanolamine kinase (EKI1) was identified from the Saccharomyces Genome Data Base (locus YDR147W) based on its homology to the Saccharomyces cerevisiae CKI1-encoded choline kinase, which also exhibits ethanolamine kinase activity. The EKI1 gene was isolated and used to construct eki1⌬ and eki1⌬ cki1⌬ mutants. A multicopy plasmid containing the EKI1 gene directed the overexpression of ethanolamine kinase activity in wild-type, eki1⌬ mutant, cki1⌬ mutant, and eki1⌬ cki1⌬ double mutant cells. The heterologous expression of the S. cerevisiae EKI1 gene in Sf-9 insect cells resulted in a 165,500-fold overexpression of ethanolamine kinase activity relative to control insect cells. The EKI1 gene product also exhibited choline kinase activity. Biochemical analyses of the enzyme expressed in insect cells, in eki1⌬ mutants, and in cki1⌬ mutants indicated that ethanolamine was the preferred substrate. The eki1⌬ mutant did not exhibit a growth phenotype. Biochemical analyses of eki1⌬, cki1⌬, and eki1⌬ cki1⌬ mutants showed that the EKI1 and CKI1 gene products encoded all of the ethanolamine kinase and choline kinase activities in S. cerevisiae. In vivo labeling experiments showed that the EKI1 and CKI1 gene products had overlapping functions with respect to phospholipid synthesis. Whereas the EKI1 gene product was primarily responsible for phosphatidylethanolamine synthesis via the CDP-ethanolamine pathway, the CKI1 gene product was primarily responsible for phosphatidylcholine synthesis via the CDP-choline pathway. Unlike cki1⌬ mutants, eki1⌬ mutants did not suppress the essential function of Sec14p.

. Ethanolamine was a poor substrate for the purified choline kinase, and it was also poor inhibitor of choline kinase activity. ADP inhibited choline kinase activity (IC 50 ‫؍‬ 0.32 mM) in a positive cooperative manner (n ‫؍‬ 1.5), and the mechanism of inhibition with respect to ATP and choline was complex. The regulation of choline kinase activity by ATP and ADP may be physiologically relevant.

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