The fat-specific protein 27 (Fsp27), a protein localized to lipid droplets (LDs), plays an important role in controlling lipid storage and mitochondrial activity in adipocytes. Fsp27-null mice display increased energy expenditure and are resistant to high fat diet-induced obesity and diabetes. However, little is known about how the Fsp27 protein is regulated. Here, we show that Fsp27 stability is controlled by the ubiquitin-dependent proteasomal degradation pathway in adipocytes. The ubiquitination of Fsp27 is regulated by three lysine residues located in the C-terminal region. Substitution of these lysine residues with alanines greatly increased Fsp27 stability and enhanced lipid storage in adipocytes. Furthermore, Fsp27 was stabilized and rapidly accumulated following treatment with ␤-agonists that induce lipolysis and fatty acid re-esterification in adipocytes. More importantly, Fsp27 stabilization was dependent on triacylglycerol synthesis and LD formation, because knockdown of diacylglycerol acyltransferase in adipocytes significantly reduced Fsp27 accumulation in adipocytes. Finally, we observed that increased Fsp27 during ␤-agonist treatment preferentially associated with LDs. Taken together, our data revealed that Fsp27 can be stabilized by free fatty acid availability, triacylglycerol synthesis, and LD formation. The stabilization of Fsp27 when free fatty acids are abundant further enhances lipid storage, providing positive feedback to regulate lipid storage in adipocytes.Cell death-inducing DNA fragmentation factor-45-like effector (Cide) proteins, including Cidea, Cideb, and fat-specific protein 27 (Fsp27, 3 also known as Cidec in humans), are a family of proteins shown to play critical roles in controlling metabolism homeostasis (1). Our previous work demonstrated that mice with a deficiency in Cidea or Cideb have higher energy expenditure and enhanced insulin sensitivity and are resistant to high fat diet-induced obesity and diabetes (2, 3). Fsp27 is enriched in adipocytes, in both white adipose tissue and brown adipose tissue (2, 4). The Fsp27 protein is detected in the lipid droplet (LD)-enriched fraction (5), and its overexpression can promote triacylglycerol (TAG) storage (6, 7). Interestingly, Fsp27 and Cidea mRNAs have also been detected in fatty livers, where an excess amount of lipids accumulate and large LDs form (8 -10). More recently, Fsp27 was demonstrated to be a direct mediator of peroxisome proliferator-activated receptor ␥-dependent hepatic steatosis (10). In accordance with a role for Fsp27 in LD formation, Fsp27 deficiency results in dramatically reduced white adipose tissue deposits and the acquisition of a brown fat-like morphology in these white adipose tissues, which is characterized by the appearance of smaller LDs and increased mitochondrial size and activity (11, 12). Furthermore, both Fsp27-deficient and Fsp27/leptin double-deficient mice display improved insulin sensitivity and lean phenotype (12). Except for one study showing that Cidea is degraded through the ubiquitin-m...

Obesity is characterized by aberrant fat accumulation. However, the intracellular signaling pathway that senses dietary fat and leads to fat storage remains elusive. Here, we have observed that the levels of histone deacetylase 6 (HDAC6) and the related family member HDAC10 are markedly reduced in adipose tissues of obese animals and humans. Mice with adipocyte-specific depletion of Hdac6 exhibited increased fat accumulation and reduced insulin sensitivity. In normal adipocytes, we found that reversal of P300/CBP-associated factor-induced (PCAF-induced) acetylation at K56 on cell death-inducing DFFA-like effector C (CIDEC, also known as FSP27) critically regulated lipid droplet fusion and lipid storage. Importantly, HDAC6 deacetylates CIDEC, leading to destabilization and reduced lipid droplet fusion. Accordingly, we observed elevated levels of CIDEC and its acetylated form in HDAC-deficient adipocytes as well as the adipose tissue of obese animals and humans. Fatty acids (FAs) prevented CIDEC deacetylation by promoting the dissociation of CIDEC from HDAC6, which resulted in increased association of CIDEC with PCAF on the endoplasmic reticulum. Control of CIDEC acetylation required the conversion of FAs to triacylglycerols. Thus, we have revealed a signaling axis that is involved in the coordination of nutrient availability, protein acetylation, and cellular lipid metabolic responses.

Adipocytes are among the most important cells for energy storage, metabolism and balance in mammals. Dysfunctional adipocytes are closely associated with type 2 diabetes and obesity, and thus with insulin resistance and inflammation. Recent studies show that endoplasmic reticulum (ER) stress may be responsible for obesity-induced insulin resistance. In this study, we investigated an ER-associated degradation-related protein in adipocytes, namely osteosarcoma amplified 9 (OS-9), a protein that is reported to have lectin activity and to be involved in ER quality control. We found that OS-9 is expressed at high levels in both types of fat tissues - brown and white adipose tissues - and we examined the subcellular localization of OS-9 in 3T3-L1 adipocytes. Furthermore, we observed that the expression levels of the OS-9 protein gradually increased as 3T3-L1 adipocytes differentiated. Most notably, we found that the depletion of this protein influences adipocyte differentiation and lipid storage, although the mechanism behind this is unclear.