Lipoprotein uptake localization

The action of lipoprotein lipase lining the blood vessels degrades the triacyl-glycerols, releasing fatty acids locally for cellular uptake. 

It has been reported that BPD-MA associated with lipoproteins in vivo and that this uptake abets the documented localization in tumor cell membranes [80]. Furthermore, BPD-MA absorbs light strongly at 690 nm (in MeOH, a =... 

The preferential localization of mucosal LDL uptake to the cells of the intestinal crypt and lower villus regions suggests that this lipoprotein supplies cholesterol for membrane formation and differentiation. However, two features of this system indicate that LDL uptake is not quantitatively important in this regard. First, dietary and drug interventions that dramatically alter the rate of cholesterol synthesis in these cells have little effect upon the rate of LDL transport. Second, it can be calculated that approximately 4 times more cholesterol is acquired by the intestinal... 

The relative importance of each of these contributions to pool C is likely to be different in epithelial cells located at different points along the villus-crypt axis. The fact that cholesterol derived from synthesis and from the uptake of LDL is critically important for membrane formation and differentiation is suggested by the finding that 70-80% of total mucosal sterol synthetic activity and LDL transport activity are localized to the immature cells of the lower villus and crypt regions in both the proximal and distal intestine. In the mature absorptive cells of the upper villus in the jejunum, where most sterol absorption takes place, the rate of cholesterol synthesis appears to be suppressed. In the absence of fat absorption, cholesterol newly synthesized in these cells apparently is sloughed into the lumen and not reabsorbed. However, with active triglyceride absorption cholesterol synthesis in these cells is increased and a portion of this sterol appears in the intestinal lymph. Only under this condition does pool B apparently supply sterol for lipoprotein formation. 

Pearse and Bretscher (1981) have discussed the role of coated vesicles in membrane synthesis and function. Eukaryotic cells are able to specifically take up macromolecules by absorptive endocytosis. The macromolecules are usually transferred to lyso-somes where they may be degraded. The first stage of the process involves the binding of macromolecules to receptors which are localized in coated pits. The latter are indented sites on the plasma membrane and the coated pit buds into the cytoplasm to form a coated vesicle in which lie the endocytosed macromolecules. The coated vesicle sheds its coat rapidly and the endocytic vesicles fuse with each other. This allows receptors to be returned to the plasma membrane while the contents are transferred to the lyso-somes. In order to explain how lysosomal and plasma membranes remain different, it was suggested that the coated pits are able to accept certain macromolecules while excluding others. The accepted proteins enter the coated pit and were presumed to bind directly or indirectly to clathrin. Clathrin, a 180000-dalton protein on the cytoplasmic face of coated pits, provides the polyhedron skeleton for the coated vesicles. Examples of the use of coated vesicles for mediated endocytosis are in the uptake of low-density lipoprotein from the blood and in humans for the transport of immunoglobulins from the mother to the child. For other mammals such as the rat the antibodies are selectively absorbed from the mother s milk by the intestinal epithelium. Coated vesicles also provide a mechanism for virus transport into cells. 

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