Ceramide Selectively Displaces Cholesterol from Ordered Lipid Domains (Rafts)

Abstract

Ceramide is a membrane lipid involved in a number of crucial biological processes. Recent evidence suggests that ceramide is likely to reside and function within lipid rafts; ordered sphingolipid and cholesterol-rich lipid domains believed to exist within many eukaryotic cell membranes. Using lipid vesicles containing co-existing raft domains and disordered fluid domains, we find that natural and saturated synthetic ceramides displace sterols from rafts. Other raft lipids remain raft-associated in the presence of ceramide, showing displacement is relatively specific for sterols. Like cholesterol-containing rafts, ceramide-rich "rafts" remain in a highly ordered state. Comparison of the sterol-displacing abilities of natural ceramides with those of saturated diglycerides and an unsaturated ceramide demonstrates that tight lipid packing is critical for sterol displacement by ceramide. Based on these results, and the fact that cholesterol and ceramides both have small polar headgroups, we propose that ceramides and cholesterol compete for association with rafts because of a limited capacity of raft lipids with large headgroups to accommodate small headgroup lipids in a manner that prevents unfavorable contact between the hydrocarbon groups of the small headgroup lipids and the surrounding aqueous environment. Minimizing the exposure of cholesterol and ceramide to water may be a strong driving force for the association of other molecules with rafts. Furthermore, displacement of sterol from rafts by ceramide is very likely to have marked effects upon raft structure and function, altering liquid ordered properties as well as molecular composition. In this regard, certain previously observed physiological processes may be a result of displacement. In particular, a direct connection to the previously observed sphingomyelinase-induced displacement of cholesterol from plasma membranes in cells is proposed.

Ceramide and raft signaling are linked with each other in UVA radiation-induced gene expression

“…As shown in Table 1, signaling occurred when ceramide concentration was high enough to displace the Chol from ordered lipid domains. These observations are in line with studies using lipid vesicles containing coexisting raft domains and disordered fluid domains where natural and synthetic ceramides displaced sterols from rafts (Megha and London, 2004). According to these studies, ceramides and Chol compete for association with rafts because of a limited capacity of raft lipids with large headgroups to accommodate small headgroup lipids in a manner that prevents unfavorable contact between the hydrocarbon groups of the small lipids and the surrounding aqueous environment (Megha and London, 2004).…”

“…These observations are in line with studies using lipid vesicles containing coexisting raft domains and disordered fluid domains where natural and synthetic ceramides displaced sterols from rafts (Megha and London, 2004). According to these studies, ceramides and Chol compete for association with rafts because of a limited capacity of raft lipids with large headgroups to accommodate small headgroup lipids in a manner that prevents unfavorable contact between the hydrocarbon groups of the small lipids and the surrounding aqueous environment (Megha and London, 2004). Accordingly, addition of synthetic ceramides mimicked the decrease of Chol observed after UVA irradiation (Figures 1b and 7d) and preloading of keratinocytes with Chol could partially inhibit ceramide-induced gene expression (Figure 7e).…”

“…In this regard, in Chinese Hamster Ovary cells a decrease of sphingomyelin levels has been shown to enhance the cellular Chol efflux, indicating that sphingomyelin has a function in retaining Chol in the plasma membrane (Fukasawa et al, 2000). In addition, ceramide has recently been shown to selectively displace Chol from lipid rafts (Megha and London, 2004). We therefore propose that in irradiated keratinocytes, the decrease of Chol levels represents the consequence of two processes associated with the UVA radiation-induced hydrolysis of raft-associated sphingomyelin: the hydrolysis induced decrease in sphingomyelin facilitates Chol efflux and the subsequently formed ceramides replace Chol in rafts.…”

Heterozygous caveolin-3 mice show increased susceptibility to palmitate-induced insulin resistance

“…2001). In addition, ceramide has been shown to compete with and displace cholesterol in synthetic unilamellar membrane systems (Megha, 2004). Therefore, increased de novo synthesis and accumulation of ceramide, driven by the exogenous saturated fatty acid, palmitate, may be sufficient to impair signal transduction of IR by disrupting lipid membrane integrity and caveolae structure.…”

Conversion of apical plasma membrane sphingomyelin to ceramide attenuates the intoxication of host cells by cholera toxin

“…Certain sphingolipids, such as sphingomyelin (SM), hydrogen bond with cholesterol, promoting the formation of lipid microdomains (Bittman et al, 1994). The precursor of SM, ceramide, however, has a lower affinity for cholesterol (Megha and London, 2004), and yet still forms distinct ordered microdomains in vitro (Kolesnick et al, 2000;Wang and Silvius, 2003) and large, lipid raft-like structures in the outer leaflet of the PM in vivo (Grassme et al, 2003). The content of SM and ceramide in eukaryotic cells is dynamic and can be rapidly altered by the enzymatic activities of sphingomyelinases (SMases) and SM synthases.…”