Low-density lipoprotein and oxysterols suppress the transcription of CTP:Phosphoethanolamine cytidylyltransferase in vitro

Transcription of cytochrome P450 46A1 in NIH3T3 cells is negatively regulated by FBS

Extracellular administration of side-chain oxysterols, such as 24S-hydroxycholesterol (24S-HC), 27-hydroxycholesterol (27-HC) and 25-hydroxycholesterol (25-HC) to cells suppresses HMG-CoA reductase (Hmgcr) and CTP:phosphoethanolamine cytidylyltransferase (Pcyt2) mRNA levels. Oxysterols are enzymatically produced in cells from cholesterol by cytochrome P450 46A1 (Cyp46A1), Cyp27A1, Cyp3A11 and cholesterol 25-hydroxylase (Ch25h). We analyzed which of these oxysterol-producing enzymes are expressed in NIH3T3 cells and found that only Cyp46A1 was expressed. When Cyp46A1 was overexpressed in NIH3T3 cells, intrinsic oxysterols increased in the order 24S-HC > 25-HC > 27-HC. We investigated the mechanism regulating the production of endogenous oxysterols in NIH3T3 cells by Cyp46A1 and found that the mRNA, relative protein levels and enzymatic activity of Cyp46A1, and the amounts of 24S-HC, 25-HC and 27-HC significantly increased under serum-starved conditions, and these increases were suppressed by FBS supplementation. The aqueous phase of FBS obtained by the Bligh & Dyer method significantly suppressed Cyp46A1 mRNA levels. Fractionation of the aqueous phase by HPLC and analysis of the inhibiting fractions by nanoLC and TripleTOF MS/MS identified insulin-like factor-II (IGF-II). Cyp46A1 mRNA levels in serum-starved NIH3T3 cells were significantly suppressed by the addition of IGFs and insulin and endogenous oxysterol levels were decreased. CYP46A1 mRNA levels in the T98G human glioblastoma cell line were also increased by serum starvation but not by FBS supplementation, and the aqueous phase did not inhibit the increase. These results suggest that mRNA levels of Cyp46A1 are regulated by factors in FBS.

Side-chain oxysterols suppress the transcription of CTP: Phosphoethanolamine cytidylyltransferase and 3-hydroxy-3-methylglutaryl-CoA reductase by inhibiting the interaction of p300 and NF-Y, and H3K27 acetylation

CTP: phosphoethanolamine cytidylyltransferase (Pcyt2) is the rate-limiting enzyme in mammalian phosphatidylethanolamine (PE) biosynthesis. Previously, we reported that increasedPcyt2 mRNA levels after serum starvation are suppressed by 25-hydroxycholesterol (HC) (25-HC), and that nuclear factor-Y (NF-Y) is involved in the inhibitory effects. Transcription of Hmgcr, which encodes 3-hydroxy-3-methylglutaryl-CoA reductase, is suppressed in the same manner. However, no typical sterol regulatory element (SRE) was detected in the Pcyt2 promoter. We were therefore interested in the effect of 25-HC on the modification of histones and thus treated cells with histone acetyltransferase inhibitor (anacardic acid) or histone deacetylase inhibitor (trichostatin A). The suppressive effect of 25-HC on Pcyt2 and Hmgcr mRNA transcription was ameliorated by trichostatin A. Anacardic acid, 25-HC and 24(S)-HC suppressed their transcription by inhibiting H3K27 acetylation in their promoters as evaluated by chromatin immunoprecipitation (ChIP) assays. 27-HC, 22(S)-HC and 22(R)-HC also suppressed their transcription, but 7α-HC, 7β-HC, the synthetic LXR agonist T0901317 and cholesterol did not. Furthermore, 25-HC inhibited p300 recruitment to the Pcyt2 and Hmgcr promoters, and suppressed H3K27 acetylation. 25-HC in the medium was easily conducted into cells. Based on these results, we concluded that 25-HC (and other side-chain oxysterols) in the medium was easily transferred into cells, suppressed H3K27 acetylation via p300 recruitment on the NF-Y complex in the Pcyt2 and Hmgcr promoters, and then suppressed transcription of these genes although LXR is not involved.

Transcriptional suppression of CTP:phosphoethanolamine cytidylyltransferase by 25-hydroxycholesterol is mediated by nuclear factor-Y and Yin Yang 1

Nuclear factor-Y and Yin Yang 1 are important for basal transcription of CTP:phosphoethanolamine cytidylyltransferase for phosphatidylethanolamine biosynthesis. Nuclear factor-Y is involved in the inhibitory effect of 25-hydroxycholesterol on this enzyme transcription by inhibiting the interaction between nuclear factor-Y and RNA polymerase II.

Homeostasis of phospholipids - The level of phosphatidylethanolamine tightly adapts to changes in ethanolamine plasmalogens

Ethanolamine plasmalogens constitute a group of ether glycerophospholipids that, due to their unique biophysical and biochemical properties, are essential components of mammalian cellular membranes. Their importance is emphasized by the consequences of defects in plasmalogen biosynthesis, which in humans cause the fatal disease rhizomelic chondrodysplasia punctata (RCDP). In the present lipidomic study, we used fibroblasts derived from RCDP patients, as well as brain tissue from plasmalogen-deficient mice, to examine the compensatory mechanisms of lipid homeostasis in response to plasmalogen deficiency. Our results show that phosphatidylethanolamine (PE), a diacyl glycerophospholipid, which like ethanolamine plasmalogens carries the head group ethanolamine, is the main player in the adaptation to plasmalogen insufficiency. PE levels were tightly adjusted to the amount of ethanolamine plasmalogens so that their combined levels were kept constant. Similarly, the total amount of polyunsaturated fatty acids (PUFAs) in ethanolamine phospholipids was maintained upon plasmalogen deficiency. However, we found an increased incorporation of arachidonic acid at the expense of docosahexaenoic acid in the PE fraction of plasmalogen-deficient tissues. These data show that under conditions of reduced plasmalogen levels, the amount of total ethanolamine phospholipids is precisely maintained by a rise in PE. At the same time, a shift in the ratio between ω-6 and ω-3 PUFAs occurs, which might have unfavorable, long-term biological consequences. Therefore, our findings are not only of interest for RCDP but may have more widespread implications also for other disease conditions, as for example Alzheimer's disease, that have been associated with a decline in plasmalogens.

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PE accurately compensates for the lack of plasmalogens in vitro and in vivo.

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PE levels decrease to adapt to excess of ethanolamine plasmalogens (PlsEtn).

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Plasmalogen deficiency favors incorporation of arachidonic acid into PE.

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Docosahexaenoic acid in ethanolamine phospholipids decreases upon PlsEtn depletion.

Regulation of Phosphatidylethanolamine Homeostasis—The Critical Role of CTP:Phosphoethanolamine Cytidylyltransferase (Pcyt2)

Phosphatidylethanolamine (PE) is the most abundant lipid on the protoplasmatic leaflet of cellular membranes. It has a pivotal role in cellular processes such as membrane fusion, cell cycle regulation, autophagy, and apoptosis. CTP:phosphoethanolamine cytidylyltransferase (Pcyt2) is the main regulatory enzyme in de novo biosynthesis of PE from ethanolamine and diacylglycerol by the CDP-ethanolamine Kennedy pathway. The following is a summary of the current state of knowledge on Pcyt2 and how splicing and isoform specific differences could lead to variations in functional properties in this family of enzymes. Results from the most recent studies on Pcyt2 transcriptional regulation, promoter function, autophagy, and cell growth regulation are highlighted. Recent data obtained from Pcyt2 knockout mouse models is also presented, demonstrating the essentiality of this gene in embryonic development as well as the major physiological consequences of deletion of one Pcyt2 allele. Those include development of symptoms of the metabolic syndrome such as elevated lipogenesis and lipoprotein secretion, hypertriglyceridemia, liver steatosis, obesity, and insulin resistance. The objective of this review is to elucidate the nature of Pcyt2 regulation by linking its catalytic function with the regulation of lipid and energy homeostasis.

Breast cancer cells adapt to metabolic stress by increasing ethanolamine phospholipid synthesis and CTP:ethanolaminephosphate cytidylyltransferase-Pcyt2 activity

The significance of phosphatidylethanolamine (PE) in breast cancer cell metabolism was investigated under stress conditions caused by serum deficiency. Serum deficient MCF-7 cells adapt to stress conditions by increasing synthesis and content of PE and diacylglycerol (DAG). The biosynthesis of PE from DAG and ethanolamine was regulated at the level of formation of CDP-ethanolamine, the metabolic step catalyzed by Pcyt2. The catalytic activity of Pcyt2 was elevated 2-3-fold, yet the enzyme remained rate-limiting in serum-deficient cells. Contributions to the elevated Pcyt2 activity included transcriptional and translational components. The mRNA levels of two splice variants, Pcyt2α and Pcyt2β, were 1.5-3-fold higher in deficient cells. The total amounts of Pcyt2 and Pcyt2α proteins were similarly elevated 1.5-2.5-fold. In vivo [γ(32)Pi] radiolabeling revealed that Pcyt2 was additionally regulated by phosphorylation. Under unfavorable metabolic conditions, both endogenous and His/Myc-tagged Pcyt2 were increasingly phosphorylated at Ser residues. The results established that elevated DAG formation and the increased activity of the rate-regulatory enzyme Pcyt2 were critical modulators of the PE Kennedy pathway, and total PE content in serum deprived breast cancer cells. Therefore, as an essential gene sensitive to nutritional microenvironment, Pcyt2 could represent a legitimate target in novel metabolic strategies for cancer.