Characterization of a phosphonium analog of choline as a probe in 31P NMR studies of phospholipid metabolism

Phosphorous metabolites and steady-state energetics of transformed fibroblasts during three-dimensional growth

Rat1-T1 and MR1 spheroids represent separate transformed phenotypes originated from the same rat fibroblasts that differ in three-dimensional (3D) growth kinetics, histological structure, and oxygenation status. In the present study, (31)P-NMR spectroscopy of perfused spheroid suspensions was used to investigate cellular energetics relative to 3D growth, development of necrosis, and cell cycle distribution. Both spheroid types were characterized by a remarkably low amount of free (inorganic) phosphate (P(i)) and a low phosphocreatine peak. The ratio of nucleoside triphosphate (NTP) to P(i) ranged between 1.5 and 2.0. Intracellular pH, NTP-to-P(i) ratio, and NTP/cell remained constant throughout spheroid growth, being unaffected by the emergence of oxygen deficiency, cell quiescence, and necrosis. However, a 50% decrease in the ratio of the lipid precursors phosphorylcholine and phosphorylethanolamine (PC/PE) was observed with increasing spheroid size and was correlated with an increased G(1)/G(0) phase cell fraction. In addition, the ratio of the phospholipid degradation products glycerophosphorylcholine and glycerophosphorylethanolamine (GPC/GPE) increased with spheroid diameter in Rat1-T1 aggregates. We conclude that changes in phospholipid metabolism, rather than alterations in energy-rich phosphates, reflect cell quiescence in spheroid cultures, because cells in the inner oxygen-deficient zones seem to adapt their energy metabolism to the environmental conditions before necrotic cell destruction.

Tumour phospholipid metabolism

Following the impetus of early clinical and experimental investigations, in vivo and in vitro MRS studies of tumours pointed in the eighties to the possible significance of signals arising from phospholipid (PL) precursors and catabolites as novel biochemical indicators of in vivo tumour progression and response to therapy. In the present decade, MRS analyses of individual components contributing to the 31P PME (phosphomonoester) and PDE (phosphodiester) resonances, as well as to the 1H 'choline peak', have reinforced some of these expectations. Moreover, the absolute quantification of these signals provided the basis for addressing more specific (although still open) questions on the biochemical mechanisms responsible for the formation of intracellular pools of PL derivatives in tumours, under different conditions of cell proliferative status and/or malignancy level. This article is aimed at providing an overview on: (a) quantitative MRS measurements on the contents of phosphocholine (PCho), phosphoethanolamine (PEtn) and their glycerol derivatives ģlycerol 3-phosphocholine (GPC) and glycerol 3-phosphoethanolamine (GPE)[ in human tumours and cells (with particular attention to breast and brain cancer and lymphomas), as well as in normal mammalian tissues (including developing organs and rapidly proliferating tissues); (b) possible correlations of MRS parameters like PEtn/PCho and PCho/GPC ratios with in vitro cell growth status and/or cell tumorigenicity; and (c) current and new hypotheses on the role and interplay of biosynthetic and catabolic pathways of the choline and ethanolamine cycles in modulating the intracellular sizes of PCho and PEtn pools, either in response to mitogenic stimuli or in relation to malignant transformation.

Study of the metabolism of choline and phosphatidylcholine in tumors in vivo using phosphonium-choline

The results of an initial study on the feasibility of using the phosphonium analog of choline to follow the metabolism of phosphatidylcholine in tumors in vivo using 31P NMR are reported. C3H/He mice bearing a mammary carcinoma tumor on the foot pad were fed a choline-free diet supplemented with the phosphonium analog of choline. Metabolites of this compound, including the phosphonium analogs of phosphatidylcholine, phosphocholine, glycerophosphocholine, and betaine were observed noninvasively in vivo in tumors by 31P NMR after 2-3 weeks of feeding. Clearance of these phosphonium-labeled metabolites from tumors was measured after a change to a choline-containing diet. Significant decreases were seen in the levels of the analogs of betaine (P < 0.003) and phosphatidylcholine (P < 0.004) by Day 4. A significant increase in the level of authentic phosphocholine (P < 0.003) occurred over the same time period.

Quantitation of resonances in biological 31P NMR spectra via principal component analysis: potential and limitations

This paper examines the potential and limitations of peak area quantitation of biological NMR spectra using principal component analysis (PCA), including its requirement for prior knowledge. The principles of the method are presented without in-depth mathematical treatment. PCA is illustrated for simulated data, 31P NMR spectra obtained consecutively over 1-2.5 days from perfused Rat-2 cells metabolizing the choline analogue phosphoniumcholine (Chop) and in vivo proton-decoupled, NOE-enhanced, three-dimensional CSI localized 31P NMR spectra of the liver of healthy volunteers. The results show that PCA can be used to quantitate strongly overlapping peaks without prior knowledge of the peak shapes or positions and to reconstruct spectra with significantly reduced noise variance. Two major limitations of PCA are presented: (1) PCA cannot separate peaks whose intensities are well correlated; (2) PCA is sensitive to differences in chemical shift and line-width of peaks between spectra. The discussion focuses on what knowledge of the biological and spectroscopic features of the samples and the principles of PCA is necessary for peak area quantitation via PCA.

Application of 31P NMR spectroscopy to monitor chemotherapy-associated changes of serum phospholipids in patients with malignant lymphomas

31P spectra were obtained from 22 healthy volunteers and 35 patients with malignant lymphomas. Sera from patients were collected at the time of diagnosis and at several time-points during therapy. Long-term follow-up studies showed a good correlation between the 31P NMR spectra of sera and the clinically evident response of the disease to the chemotherapy. During therapy leading to remission resonance from phospholipids increased progressively resulting in spectra similar to those seen in normal sera. By contrast, in patients who did not respond to therapy the intensities of the phospholipid peaks remained relatively low or became progressively reduced as the disease progressed. To understand the source of the spectral differences, we also examined the concentrations of high-density lipoprotein, low-density lipoprotein, cholesterol, and triglycerides. In individuals responding to the treatment, both high-density lipoprotein and cholesterol increased to the point where they were statistically equivalent to those from healthy volunteers.