The short-chain dehydrogenases/reductases (SDR) family is a large family of NAD- or NADP-dependent oxidoreductase enzymes. 17-beta-hydroxysteroid dehydrogenase 13 (HSD17B13) is a recently-discovered member of this family whose physiological function is unknown (Liu et al. 2007). It has, however, been found to associate with lipid droplets and is significantly upregulated in patients with nonalcoholic fatty liver disease (NAFLD) which is characterised by an accumulation of lipid droplets in the liver. In a murine model of fatty liver and diabetes, mice with fatty livers show upregulated expression of HSD17B13, suggesting a pathogenic role for HSD17B13 in NAFLD development (Su et al. 2014).
Lipid droplets (LDs) are cytosolic structures found in cells of all eukaryotes, comprising a monolayer of phospholipids surrounding a core of uncharged lipids such as triglyceride (TAG) and sterol esters. CIDEA, CIDEB and CIDEC were first studied for their roles in promotion of apoptosis but they are also known to play a role in energy metabolism. CIDEA and C bind to lipid droplets and regulate their enlargement, thereby restricting lipolysis and favouring storage (by promoting net neutral lipid transfer from smaller to larger lipid droplets) (Gao & Goodman 2015). LD formation involves the partitioning of neutral lipids from their site of synthesis at the endoplasmic reticulum (ER) to the cytosol. The fat storage-inducing transmembrane proteins 1 and 2 (FITM1 and FITM2), associated with the ER membrane, mediate binding and partitioning of TAGs into LDs. The short-chain dehydrogenases/reductases (SDR) family is a large family of NAD- or NADP-dependent oxidoreductase enzymes. 17-beta-hydroxysteroid dehydrogenase 13 (HSD17B13) is a recently-discovered enzyme of unknown physiological function that is associated with lipid droplets and significantly upregulated in patients with nonalcoholic fatty liver disease. Hypoxia-inducible lipid droplet-associated protein (HILPDA) is a lipid droplet protein and stimulates intracellular lipid accumulation.