In addition to the CDP-choline pathway for phosphatidylcholine (PC) synthesis, the liver has a unique phosphatidylethanolamine (PE) methyltransferase activity for PC synthesis via three methylations of the ethanolamine moiety of PE. Previous studies indicate that the two pathways are functionally different and not interchangeable even though PC is the common product of both pathways. This study was designed to test the hypothesis that these two pathways produce different profiles of PC species. The PC species from these two pathways were labeled with specific stable isotope precursors, D9-choline and D4-ethanolamine, and analyzed by electrospray tandem mass spectrometry. Our studies revealed a profound distinction in PC profiles between the CDP-choline pathway and the PE methylation pathway. PC molecules produced from the CDP-choline pathway were mainly comprised of medium chain, saturated (e.g. 16:0/18:0) species. On the other hand, PC molecules from the PE methylation pathway were much more diverse and were comprised of significantly more long chain, polyunsaturated (e.g. 18:0/20:4) species. PC species from the methylation pathway contained a higher percentage of arachidonate and were more diverse than those from the CDP-choline pathway. This profound distinction of PC profiles may contribute to the different functions of these two pathways in the liver.Phosphatidylcholine (PC) 1 is a major group of phospholipid in all mammalian cells (1). PC is comprised of hydrocarbon chains attached to glycerophosphocholine via acyl, alkyl, or alkenyl linkages. The molecular diversity of PC and other phospholipids is dictated by the combination of different lengths, number of double bonds, and types of linkages of hydrocarbon chains. As a result, a single mammalian cell contains at least a thousand species of phospholipids (2). In most mammalian cells, PC is synthesized mainly via the CDPcholine pathway (3). This pathway uses choline as an initial substrate and is catalyzed by three enzymes: choline kinase, CTP:phosphocholine cytidylyltransferase (CT), and cholinephosphate transferase, with CT as the rate-limiting enzyme (1). Hepatocytes are unique because they also possess a high activity of phosphatidylethanolamine methyltransferase (PEMT) that converts PE to PC via three sequential steps of methylation (4) in addition to a high level of CDP-choline pathway activity. The significance of the PEMT pathway is not completely understood (5).The PEMT pathway seems redundant because its product, PC, is also synthesized by the CDP-choline pathway in hepatocytes. Therefore, the PEMT pathway is traditionally considered a backup pathway for PC synthesis in hepatocytes (3). However, recent studies indicate that these two pathways apparently have opposite effects on proliferative characteristics of the liver and liver-derived cell lines (6 -10). These studies suggest that the higher activity of the CDP-choline pathway favors the faster proliferation of hepatocytes. Conversely, expression of PEMT strongly inhibits the growth of hepatom...
The stiffening and thickening of the arterial wall after subarachnoid hemorrhage may reflect increased connective tissue. The purpose of this study was to examine the nature of collagen synthesis in response to periarterial blood. Rat femoral arteries were exposed to periarterial blood for varying lengths of time (control, 1, 3, 7, and 14 d). Dot-blot analysis of total ribonucleic acid extracted from the arteries (n = 10 to 15 animals each) demonstrated that the expression of procollagen Types I and III messenger ribonucleic acid increased at 7 (threefold) and 14 days. The expression of transforming growth factor-beta (TGF-beta), an important regulator of collagen synthesis, was markedly increased by 3 days (threefold), followed by a gradual decline. There were marked differences in procollagen Types I and III and TGF-beta gene expression between arteries exposed to blood and sham-operated arteries for a period of 7 days (n = 25 animals). Northern blot analysis of total ribonucleic acid extracted from cultured vascular smooth muscle cells showed that the treatment with a higher concentration of serum for 48 hours increased the expression of procollagen Types I and III and TGF-beta, whereas exposure to oxyhemoglobin did not. After exposure to periarterial blood, arterial walls show increased synthesis of procollagen Types I and III, perhaps a response to the increased secretion of TGF-beta, which in turn could be the result of exposure to serum factors.
The stiffening and thickening of the arterial wall after subarachnoid hemorrhage may reflect increased connective tissue. The purpose of this study was to examine the nature of collagen synthesis in response to periarterial blood. Rat femoral arteries were exposed to periarterial blood for varying lengths of time (control, 1, 3, 7, and 14 d). Dot-blot analysis of total ribonucleic acid extracted from the arteries (n = 10 to 15 animals each) demonstrated that the expression of procollagen Types I and III messenger ribonucleic acid increased at 7 (threefold) and 14 days. The expression of transforming growth factor-beta (TGF-beta), an important regulator of collagen synthesis, was markedly increased by 3 days (threefold), followed by a gradual decline. There were marked differences in procollagen Types I and III and TGF-beta gene expression between arteries exposed to blood and sham-operated arteries for a period of 7 days (n = 25 animals). Northern blot analysis of total ribonucleic acid extracted from cultured vascular smooth muscle cells showed that the treatment with a higher concentration of serum for 48 hours increased the expression of procollagen Types I and III and TGF-beta, whereas exposure to oxyhemoglobin did not. After exposure to periarterial blood, arterial walls show increased synthesis of procollagen Types I and III, perhaps a response to the increased secretion of TGF-beta, which in turn could be the result of exposure to serum factors.
Polyploidy is a profound phenotype found in tumors and its mechanism is unknown. We report here that when B-cell lymphoma gene-2 (Bcl-2) was overexpressed in a Chinese hamster ovary cell line that was deficient in CTP:phosphocholine cytidylyltransferase (CT), cellular DNA content doubled. The higher DNA content was due to a permanent conversion from diploid cells to tetraploid cells. The mechanism of polyploid formation could be attributed to the duplication of 18 parental chromosomes. The rate of conversion from diploid to tetraploid was Bcl-2 dose dependent. The diploid genome was not affected by Bcl-2 expression or by CT deficiency alone. Endogenous CT or expression of recombinant rat liver CTalpha prior to Bcl-2 expression prevented the formation of polyploid cells. This conversion was irreversible even when both initiating factors were removed. In this study, we have identified Bcl-2 as a positive regulator and CTalpha as a negative regulator of polyploid formation.
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