Adipose differentiation-related protein (ADFP; also known as ADRP or adipophilin), is a lipid droplet (LD) protein found in most cells and tissues. ADFP expression is strongly induced in cells with increased lipid load. We have inactivated the Adfp gene in mice to better understand its role in lipid accumulation. The Adfpdeficient mice have unaltered adipose differentiation or lipolysis in vitro or in vivo. Importantly, they display a 60% reduction in hepatic triglyceride (TG) and are resistant to diet-induced fatty liver. To determine the mechanism for the reduced hepatic TG content, we measured hepatic lipogenesis, very-low-density lipoprotein (VLDL) secretion, and lipid uptake and utilization, all of which parameters were shown to be similar between mutant and wild-type mice. The finding of similar VLDL output in the presence of a reduction in total TG in the Adfp-deficient liver is explained by the retention of TG in the microsomes where VLDL is assembled. Given that lipid droplets are thought to form from the outer leaflet of the microsomal membrane, the reduction of TG in the cytosol with concomitant accumulation of TG in the microsome of Adfp ؊/؊ cells suggests that ADFP may facilitate the formation of new LDs. In the absence of ADFP, impairment of LD formation is associated with the accumulation of microsomal TG but a reduction in TG in other subcellular compartments.Adipose differentiation-related protein (ADFP) was first isolated by differential hybridization screening of 1246 cells during their differentiation to adipocytes (29). Its mRNA is induced 100 fold during the process. Using 3T3L1 cells, Brasaemle et al. (3) showed that Adfp gene expression is induced early, at day 1 of adipocyte differentiation, and that mRNA levels are maintained throughout differentiation. In contrast, ADFP protein levels, initially upregulated, gradually go down after day 4 (3), suggesting that these levels are subject to significant translational or posttranslational regulation. At the same time, upregulation of perilipin (PLIN), another lipid droplet (LD) protein, is observed at day 4 of differentiation; it has been postulated that perilipin and ADFP might compete for LD localization during the differentiation of 3T3L1 cells (3, 36). ADFP protein is localized to the surface of the LD, though it also has been detected in the LD core by freeze fracture electron microscopy (49, 50).ADFP shares sequence homology with other LD proteins including perilipin and Tip47, collectively known as PAT domain-containing proteins (36, 43), as well as with another LD protein, S3-12 (53). S3-12 shares a 11-amino-acid repeat motif with ADFP, in addition to another region of homology to both ADFP and Tip47 at its carboxyl terminus (6,24,36). With the exception of perilipin, which is expressed only in fat and steroidogenic tissues, the other LD proteins are detected in a variety of cells and tissues (22). Perilipin is phosphorylated by protein kinase A during lipolysis, resulting in an altered conformation to allow hormone-sensitive lipase ...

Adipose differentiation-related protein (ADRP) is a lipid droplet-associated protein that is expressed early during adipose differentiation. The present study was undertaken to reveal the role of ADRP in adipose differentiation. In murine fibroblasts infected with green fluorescent protein (GFP)-ADRP fusion protein expression adenovirus vector, confocal microscopic analysis showed the number and size of lipid droplets apparently increased comparing with those of control cells. Overexpressed GFP-ADRP were mainly located at the surface of lipid droplets and appeared to be “ring-shaped.” Triacylglycerol content was also significantly ( P < 0.001) increased in GFP-ADRP-overexpressed cells compared with control cells. ADRP-induced lipid accumulation did not depend on adipocyte-specific gene induction, such as peroxisome proliferator-activated receptor-γ, lipoprotein lipase, or other lipogenic genes, including acyl-CoA synthetase, fatty acid-binding protein, and fatty acid transporter. In conclusion, ADRP stimulated lipid accumulation and lipid droplet formation without induction of other adipocyte-specific genes or other lipogenic genes in murine fibroblasts. The detailed molecular mechanisms of ADRP on lipid accumulation remain to be elucidated.

No abstract