Accumulation of Caveolin in the Endoplasmic Reticulum Redirects the Protein to Lipid Storage Droplets

Abstract

Caveolin-1 is normally localized in plasma membrane caveolae and the Golgi apparatus in mammalian cells. We found three treatments that redirected the protein to lipid storage droplets, identified by staining with the lipophilic dye Nile red and the marker protein ADRP. Caveolin-1 was targeted to the droplets when linked to the ER-retrieval sequence, KKSL, generating Cav–KKSL. Cav–ΔN2, an internal deletion mutant, also accumulated in the droplets, as well as in a Golgi-like structure. Third, incubation of cells with brefeldin A caused caveolin-1 to accumulate in the droplets. This localization persisted after drug washout, showing that caveolin-1 was transported out of the droplets slowly or not at all. Some overexpressed caveolin-2 was also present in lipid droplets. Experimental reduction of cellular cholesteryl ester by 80% did not prevent targeting of Cav–KKSL to the droplets. Cav–KKSL expression did not grossly alter cellular triacylglyceride or cholesteryl levels, although droplet morphology was affected in some cells. These data suggest that accumulation of caveolin-1 to unusually high levels in the ER causes targeting to lipid droplets, and that mechanisms must exist to ensure the rapid exit of newly synthesized caveolin-1 from the ER to avoid this fate.

PrPc and reggies/flotillins are contained in and released via lipid-rich vesicles in Jurkat T cells

“…Release of these proteins by lipid-rich vesicles appears rather unconventional, but osmium staining of membranes in ultrathin sections and apparent split membranes in cryosections around electron-lucent vesicles hint toward the fact that they are surrounded by a bona fide membrane, i. e. a lipid bilayer, as is required for release by exocytosis. Reports on caveolin-1 and cholesterol transport in FRT cells and hepatocytes have shown the association of mutant caveolin-1 with Nile Red-labeled 'lipid droplets' [15,16]. Lipid droplets, however, are thought to be surrounded by a phospholipid monolayer [25] enriched in certain proteins, such as a caveolin-1 mutant form [21].…”

Raft association and lipid droplet targeting of flotillins are independent of caveolin

“…However, this study showed that flotillins were not recruited to lipid bodies, but the authors used a flotillin-2-myc fusion construct to study the targeting of flotillins to lipid droplets, consistent with our finding that flotillin-2 remains unassociated with lipid bodies upon OA treatment. Interestingly, an increasing number of studies have revealed that caveolins can redistribute to lipid droplets under various conditions, including OA treatment (Fujimoto et al, 2001;Ostermeyer et al, 2001;Pol et al, 2001Pol et al, , 2004Brasaemle et al, 2004). This lipid-body targeting of caveolins is thought to regulate cellular lipid balance, especially cellular cholesterol homeostasis Pol et al, 2005;Le Lay et al, 2006).…”

Regulation of adipose tissue lipolysis revisited

“…Caveolae have several putative functions, including participation in signal transduction (73) , membrane trafficking pathways and NEFA binding and transport (74) . Interestingly, caveolin-1 also associates with LD (75)(76)(77)(78) , hinting at a role for caveolin-1 in lipolysis. Accordingly, caveolin-1-deficient mice display a blunted response to pharmacological and physiological lipolytic stimuli (79) .…”