Correlation between the occurrence of ¹H‐MRS lipid signal, necrosis and lipid droplets during C6 rat glioma development

Abstract

The aim of this study was to investigate the possible correlation between the 1H MRS mobile lipid signal, necrosis and lipid droplets in C6 rat glioma. First, the occurrence of necrosis and lipid droplets was determined during tumor development, by a histological analysis performed on 34 rats. Neither necrosis nor lipid droplets were observed before 18 days post-implantation. At later stages of development, both necrosis and lipid droplets were apparent, the lipid droplets being mainly located within the necrotic areas. Using a second group of eight rats, a temporal correlation was evidenced between mobile lipid signal detected by in vivo single-voxel one- (136 ms echo time) and two-dimensional J-resolved 1H MR spectroscopy, and the presence of necrosis and lipid droplets on the histological sections obtained from the brains of the same rats. Finally, spatial distribution of the mobile lipid signal was analyzed by chemical-shift imaging performed on a third group of eight animals, at the end of the tumor growth. The spectroscopic image corresponding to the resonance of mobile lipids had its maximum intensity in the center of the tumor where necrotic regions were observed on the histological sections. These necrotic areas contained large amounts of lipid droplets. All these results suggest that mobile lipids detected in vivo by 1H MRS (136 ms echo time) in C6 rat brain glioma arise mainly from lipid droplets located in necrosis.

An investigation of human brain tumour lipids by high‐resolution magic angle spinning 1H MRS and histological analysis

“…Therefore, it may be possible that the use of anti-angiogenic therapies on these tumours can reverse the hypoxic state by vascular normalisation (25), resulting in reduced formation of lipid droplets. Lipid droplet formation is, however, consistent with the observation that, in rodent brain C6 gliomas, not all hypoxic cells contained intracellular lipid droplets, but only those on the periphery of necrosis that were highly hypoxic and so most likely to then progress to necrosis (10). The distribution of lipid pseudo-droplet sizes is remarkably similar for non-necrotic and necrotic tumours ( Fig.…”

“…As previous studies were carried out on C6 glioma cells in vitro, or on tumours grown in rodent brain (5)(6)(7)(8)(9)(10), there is still the need to validate a similar interpretation for the lipid signals of human brain tumours observed by 1 H MRS. We therefore performed high-resolution magic angle spinning (HRMAS) 1 H MRS on whole-biopsy samples obtained from patients with low-and high-grade astrocytomas, to evaluate their NMR-visible lipid content. By using HRMAS, we can obtain high-resolution 1 H spectra from whole-biopsy tissue that can be easily and directly related to in vivo 1 H MRS data (16), but without the confounding factors associated with performing a biochemical extraction of lipids for their assessment.…”

“…Ex vivo studies have further supported this, showing not only a correlation between NMR-visible mobile lipids and percentage of necrosis, but also the presence of mobile lipids in tumours with no necrosis (4). NMR-visible mobile lipids have also been detected by 1 H MRS in cultured brain tumour cells (5)(6)(7), animal brain tumours (8)(9)(10)(11), and human brain tumour biopsy specimens (12). However, the exact metabolic origin and localisation of these lipids has been vigorously debated.…”

“…However, most subsequent studies have shown evidence of the formation of lipid droplets in the cytosol of intact glioma cells (5)(6)(7) or their extracellular presence in the necrotic core of experimental tumours (8)(9)(10). These 1 H MRS lipid resonances are believed to originate from the mobile fatty acyl chains, mainly of TGs, with lesser contributions from free fatty acids and cholesteryl esters (14).…”

“…It has further been suggested that hypoxic cells may produce lipid droplets as a temporary or final storage compartment of fatty acids in the form of TG and that lipid droplets would accelerate the necrotic process (10). In vivo studies of rodent tumour models have also shown an accumulation of NMR-mobile lipids in response to drug-induced apoptosis (11).…”