Intracellular lipids

Lipid phosphate phosphohydrolases (LPPs), formerly called type 2 phosphatidate phosphohydrolases (PAP-2), catalyse the dephosphorylation of bioactive phospholipids (phosphatidic acid, ceramide-1-phosphate) and lysophospholipids (lysophosphatidic acid, sphingosine-1-phosphate). The substrate selectivity of individual LPPs is broad in contrast to the related sphingosine-1-phosphate phosphatase. LPPs are characterized by a lack of requirement for Mg2+ and insensitivity to N-ethylmaleimide. Three subtypes (LPP-1, LPP-2, LPP-3) have been identified in mammals. These enzymes have six putative transmembrane domains and three highly conserved domains that are characteristic of a phosphatase superfamily. Whether LPPs cleave extracellular mediators or rather have an influence on intracellular lipid phosphate concentrations is still a matter of debate. 

Soccio, R. E. and Breslow, J. L. 2003. StAR-related lipid transfer (START) proteins Mediators of intracellular lipid metabolism. J. Biol. Chem., 278(25) 22183-22186. 

As is known, hydrolysis of intracellular lipids does not lead to a storage of glycerol and fatty acids. This indicates that the hydrolysis rate for the lipids is balanced against the rate of their intracellular oxidation. In the adipose tissue, glycerol and fatty acids as produced by triacylglycejride hydrolysis are not subject to oxidation and are released into the blood to be supplied to other organs. 

Barrett, J., Saghir, N., Timanova, A., Clarke, K and Brophy, P.M. (1997) Characterisation and properties of an intracellular lipid-binding protein from the tapeworm Moniezia expansa. European Journal of Biochemistry 250, 269-275. 

Simpson, MA, LiCata, V.J., Coe, N.R. and Bernlohr, D.A. (1999) Biochemical and biophysical analysis of the intracellular lipid binding proteins of adipocytes. Molecular and Cellular Biochemistry 192, 33—40. 

The relatively nonpolar squaraine rotaxane 14c was found to interact with cells in a very similar way to the well-known lipophilic dye Nile Red this probe rapidly accumulates at lipophilic sites inside a living cell, such as the endoplasmic reticulum and intracellular lipid droplets [55], The red emission band for probe 14c is quite narrow and permits the acquisition of multicolor images. It displayed high chemical stability and low toxicity. 

Some evidence indicates that long-term use of topical antimicrobial agents may alter skin flora. Water content, humidity, pH, intracellular lipids, and rates of shedding help retain the protective barrier properties of the skin. When the barrier is compromised (e.g., by hand hygiene practices such as scrubbing), skin dryness, irritation, cracking, and other problems may result. Although the palmar surface of the hand has twice as many cell layers and the cells are >30 times thicker than on the rest of the skin, palms are quite permeable to water. 

When the diet contains more fatty acids than are needed immediately as fuel, they are converted to triacylglycerols in the liver and packaged with specific apolipoproteins into very-low-density lipoprotein (VLDL). Excess carbohydrate in the diet can also be converted to triacylglycerols in the liver and exported as VLDLs (Fig. 21-40a). In addition to triacylglycerols, VLDLs contain some cholesterol and cholesteryl esters, as well as apoB-100, apoC-I, apoC-II, apoC-III, and apo-E (Table 21-3). These lipoproteins are transported in the blood from the liver to muscle and adipose tissue, where activation of lipoprotein lipase by apoC-II causes the release of free fatty acids from the VLDL triacylglycerols. Adipocytes take up these fatty acids, reconvert them to triacylglycerols, and store the products in intracellular lipid droplets myocytes, in contrast, primarily oxidize the fatty acids to supply energy. Most VLDL remnants are removed from the circulation by hepatocytes. The uptake, like that for chylomicrons, is... 

Table 10.3 Distribution of Major Phophollpids in Endoplasmic Reticulum, Intracellular Lipid Droplets, Plasma Membrane, and Milk Lipid Globule Membrane Fractions from Bovine Mammary Tissue.

Intracellular lipid droplets Cytoplasmic lipid droplets 14.6 45.8 20.9 11.9 6.8... 

Hood, L. F. and Patton, S. 1973. Isolation and characterization of intracellular lipid droplets from bovine mammary tissue. J. Dairy Sci. 56, 858-863. 

Vibrational imaging of living cells has been applied for studying intracellular lipid droplet organelles [86, 89, 90, 113, 114] (cf. Fig. 6.10A), monitoring receptor-mediated endocytosis [92], analyzing mouse embryonic stem cells... 

Fig. 6.10. In vivo multiplex CARS microspectroscopy of a NIH 3T3-L1 fibroblast cell in the high-wavenumber region where C-H stretch vibrations reside. A CARS image revealing the intracellular distribution of constituents with high densities of lipids, such as the membrane envelope of the nucleus and intracellular lipid droplet (LD) organelles. Typical MEM-reconstructed Raman spectra taken for (B) a single LD organelle that is indicated by the arrow in A, (C) the nucleus, and (D) the cytoplasm. The spectrum exposure time was 0.3 s...

Penetration enhancers have different mechanisms of action depending on their physicochemical properties. Some examples of penetration enhancers and their mechanisms are bile salts (micellization and solubilization of epithelial lipids), fatty acids such as oleic acid (perturbation of intracellular lipids) [25,26], azone (l-dodecylazacycloheptan-2-one) (increasing fluidity of intercellular lipids), and surfactants such as sodium lauryl sulfate (expansion of intracellular spaces). The complete list of enhancers and their mechanism of actions are discussed in detail in Chapter 10. 

The regulatory mechanism of cellular uptake of fatty acids appears to be limited and so the composition of the intracellular lipids is likely to reflect the availability of the fatty acids in the medium. This was shown for the CC9C10 hybridoma (Butler et al., 1997) and for BHK and CHO cells (Schmid et al., 1991). Thus, cells growing in serum-supplemented cultures are likely to attain a fatty acid composition reflecting that of serum, in which the predominant fatty acids are palmitic, stearic, oleic, and linoleic acids at a ratio 2 1 3 1, respectively. 

That other proteins are associated with the MFGM coat is probable, particularly proteins associated with the surface of intracellular lipid droplets. However, several of the proteins identified as being associated with intracellular lipid droplets (Wu et al., 2000) have yet to be identified as constituents of the MFGM coat. Two proteins associated with intracellular lipid droplets, protein disulfide isomerase (Ghosal et al., 1994) and the nuclear coactivator protein plOO (Keenan et al., 2000) are absent from MFGM preparations. Thus, there apparently is some selectivity in which of the proteins associated with intracellular lipid droplets are secreted. 

ADPH was not identified as a major MFGM protein until recently because it co-migrates with the glycoprotein known as PAS 6/7. ADPH was known previously as adipocyte differentiation-related protein (ADRP) because it is expressed early during adipocyte differentiation and was believed to be expressed only in adipocytes (Jiang and Serrero, 1992). Since then, ADPH has been detected in a large number of tissues and cell types, where it invariably is associated with intracellular lipid droplets (Heid et al., 1996 1998 Brasaemle et al., 1997). 

Our current view of the MFGM is that it is a true bilayer membrane with a dense protein coat 10 to 50 nm thick oriented on the cytoplasmic membrane face (the face contacting the core lipids of the globule) and an innermost layer derived from material that coated the lipid droplets before secretion. The bilayer membrane of the MFGM almost certainly is derived from specialized regions of apical plasma membrane. The dense protein coat is most probably the complex formed from interaction of cytosolic XDH with the cytoplasmic tails of BTN and ADPH and perhaps other proteins of the intracellular lipid droplet surface (Figure 4.9). 

Heid et al., 1996), so it is likely that this protein originates from the surface of intracellular lipid droplets and interacts with BTN, XDH, or perhaps other proteins or protein complexes on the inner face of the MFGM. Phospholipids and glycosphingolipids are known to be asymmetrically organized in cellular membranes but we have no specific information as to how these constituents are oriented in the MFGM. 

The intracellular lipid is then cleared from the cells by the HDL-mediated reverse cholesterol transport system. 

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