The objective of our research is to understand the molecular and cellular mechanisms whereby intracellular membrane lipid composition regulates lipid and glucose metabolism in health and disease. Phosphatidylcholines (PC) are the most abundant lipid class in membrane bilayers. PC consist of two fatty acids (FA) covalently linked by ester bonds to a glycerol molecule, with the third carbon of glycerol esterified to a polar phosphorylcholine head group. The molecular species of PC, as defined by fatty acyl chain length and saturation, is dynamic under physiological stress such as fasting and in common pathophysiological states, including non-alcoholic fatty liver disease (NAFLD). Whereas variations in PC molecular species regulate membrane fluidity, it is unclear whether these changes per se exert metabolic control. Phosphatidylcholine transfer protein (PC-TP; synonym StarD2), a 25 kDa soluble cytosolic protein with exquisite specificity for binding PC, is enriched in liver and oxidative tissues. Our work has demonstrated that PC-TP functions as a sensor of membrane PC composition. PC-TP in turn regulates hepatic lipid and glucose metabolism, principally by binding and activating a mitochondria-associated long chain acyl-CoA thioesterase (Acot), thioesterase superfamily member 2 (Them2; synonym Acot13), which is also enriched in oxidative tissues. Them2 is homotetrameric enzyme that hydrolyzes long chain fatty acyl-CoA molecules into free FA plus CoASH, thereby controlling metabolic fates of FA with cells. Whereas mice lacking PC-TP (Pctp-/-) and Them2 (Them2-/-) are protected against high fat diet (HFD)-induced hepatic insulin resistance, only Them2-/- mice are also resistant to hepatic steatosis. Recent data indicate that Them2 expression in skeletal muscle is a key driver of both insulin resistance and hepatic steatosis, which is particularly relevant in light of the emerging connection between altered muscle metabolism and NAFLD. In closely related studies, we have demonstrated that Them1 (synonyms Acot11 and StarD14), a brown-fat enriched protein with both lipid-binding and acyl-CoA thioesterase domains, is markedly upregulated by cold ambient temperatures and functions to suppress thermogenesis in mice. Owing to increased energy expenditure, Them1-/- mice are highly resistant to diet-induced obesity and associated metabolic disorders. The lipid binding domain binds components of PC molecules (i.e. FA and lysophosphatidylcholines) that regulate its enzymatic activity. In addition to elucidating the biological, cellular, transcriptional and molecular regulation of these proteins, we are actively developing small molecule inhibitors that hold promise for the management of obesity-related disorders including NAFLD and type 2 diabetes.