Rapid transmembrane diffusion of ceramide and dihydroceramide spin-labelled analogues in the liquid ordered phase

In order to study the basic physical phenomena underlying complex lipid transbilayer movement in biological membranes, we have measured the transmembrane diffusion of spin-labelled analogues of sphingolipids in phosphatidylcholine (PC) large unilamellar vesicles in the absence or presence of cholesterol, going from a fluid ( liquid disordered) l_d, phase to a more viscous, liquid ordered (l_o), phase. We have found cholesterol to reduce the transverse diffusion of glucosylceramide (GlcCer) and galactosylceramide (GalCer) in a concentration-dependent manner. However, surprisingly, we could neither detect any influence of cholesterol on the rapid flip-flop of ceramide nor on the flip-flop of dihydroceramide, for which the τ_1/2 of flip-flop remains in the order of 1 minute at 20°C in the presence of cholesterol. As a consequence of rapid flip-flop of ceramide in both the l_o and the l_d phase, ceramide is likely to distribute between the two monolayers of a membrane, and could in principle partition into segregated domains in each side of the plasma membrane of eukaryotic cells.     

Dihydroceramides: From Bit Players to Lead Actors

Sphingolipid synthesis involves a highly conserved biosynthetic pathway that produces fundamental precursors of complex sphingolipids. The final reaction involves the insertion of a double bond into dihydroceramides to generate the more abundant ceramides, which are converted to sphingomyelins and glucosylceramides/gangliosides by the addition of polar head groups. Although ceramides have long been known to mediate cellular stress responses, the dihydroceramides that are transiently produced during de novo sphingolipid synthesis were deemed inert. Evidence published in the last few years suggests that these dihydroceramides accumulate to a far greater extent in tissues than previously thought. Moreover, they have biological functions that are distinct and non-overlapping with those of the more prevalent ceramides. Roles are being uncovered in autophagy, hypoxia, and cellular proliferation, and the lipids are now implicated in the etiology, treatment, and/or diagnosis of diabetes, cancer, ischemia/reperfusion injury, and neurodegenerative diseases. This minireview summarizes recent findings on this emerging class of bioactive lipids.     

Dihydroceramide desaturase activity in tumors

Dihydroceramide desaturase activity in the transplantable mouse hepatoma-22, rat hepatoma-27, M1 sarcoma, and RS1 rat cholangiocellular carcinoma has been investigated. It was found that the dihydroceramide desaturase activity in mouse hepatoma-22 is lower than that in normal mouse liver. However, the activity of this enzyme in subcutaneously and intrahepatically transplanted rat hepatoma-27 is increased compared to normal value. Dihydroceramide desaturase activity in subcutaneously and intrahepatically transplanted M1 sarcoma as well as in hepatoma-27 is dependent on the tumor microenvironment. The enzyme activity in RS1 tumor was not revealed. The data indicate that dihydroceramide desaturase activity depends on the tumor type and its microenvironment.     

Regulation of neural progenitor cell motility by ceramide and potential implications for mouse brain development

We provide evidence that the sphingolipid ceramide, in addition to its pro-apoptotic function, regulates neural progenitor (NP) motility in vitro and brain development in vivo . Ceramide ( N -palmitoyl d -erythro sphingosine and N -oleoyl d -erythro sphingosine) and the ceramide analog N -oleoyl serinol (S18) stimulate migration of NPs in scratch (wounding) migration assays. Sphingolipid depletion by inhibition of de novo ceramide biosynthesis, or ceramide inactivation using an anti-ceramide antibody, obliterates NP motility, which is restored by ceramide or S18. These results suggest that ceramide is crucial for NP motility. Wounding of the NP monolayer activates neutral sphingomyelinase indicating that ceramide is generated from sphingomyelin. In membrane processes, ceramide is co-distributed with its binding partner atypical protein kinase C ζ/λ (aPKC), and Cdc42, α/β-tubulin, and β-catenin, three proteins involved in aPKC-dependent regulation of cell polarity and motility. Sphingolipid depletion by myriocin prevents membrane translocation of aPKC and Cdc42, which is restored by ceramide or S18. These results suggest that ceramide-mediated membrane association of aPKC/Cdc42 is important for NP motility. In vivo , sphingolipid depletion leads to ectopic localization of mitotic or post-mitotic neural cells in the embryonic brain, while S18 restores the normal brain organization. In summary, our study provides novel evidence that ceramide is critical for NP motility and polarity in vitro and in vivo .     

高碳酸抑制兔离体基底动脉缺氧性收缩

在离休兔基底动脉观察了ＣＯ＿２浓度、ｐＨ值对血管缺氧反应的影响。在以５－羟色胺（５－ＨＴ,１０￣（－６）ｍｏｌ／Ｌ）预收缩后,缺氧使血管张力升高（Ｔ＿Ｈ）１１４．８７±４０．７５ｍｇ,其占总张力变化的百分数Ｔ（％）为４８．６２±１２．１１（ｎ＝５１）。去内皮对缺氧性收缩无影响。随浴槽液中ＣＯ＿２浓度增加,缺氧性收缩幅度减弱,在Ｐｃｏ＿２为８．００、９．３３和１０．６７ｋＰ８时,Ｔ（％）仅分别为３８．３０±５．３６（ｎ＝１１）、３２．２５±７．７６（ｎ＝１３）和２９．９０±９．２６（ｎ＝１１）,较对照值（Ｐｃｏ＿２＝５．３３ｋＰａ时）均有显著差异（Ｐ＜０．０１）。ｐＨ７．２－７．７时血管缺氧反应无显著变化。一氧化氮合成酶抑制剂Ｌ－ＮＮＡ（１０￣（－６）ｍｏｌ／Ｌ可抑制血管的缺氧收缩反应,也抑制了不同ＣＯ＿２浓度对缺氧反应的影响。结果提示ＣＯ＿２可能经促血管分泌一氧化氮的功能而抑制了缺氧性收缩。