Probing hepatic metabolism of [2-13C]dihydroxyacetone in vivo with 1H-decoupled hyperpolarized 13C-MR

OBJECTIVES

To enhance detection of the products of hyperpolarized [2-13C]dihydroxyacetone metabolism for assessment of three metabolic pathways in the liver in vivo. Hyperpolarized [2-13C]DHAc emerged as a promising substrate to follow gluconeogenesis, glycolysis and the glycerol pathways. However, the use of [2-13C]DHAc in vivo has not taken off because (i) the chemical shift range of [2-13C]DHAc and its metabolic products span over 144 ppm, and (ii) 1H decoupling is required to increase spectral resolution and sensitivity. While these issues are trivial for high-field vertical-bore NMR spectrometers, horizontal-bore small-animal MR scanners are seldom equipped for such experiments.

METHODS

Real-time hepatic metabolism of three fed mice was probed by 1H-decoupled 13C-MR following injection of hyperpolarized [2-13C]DHAc. The spectra of [2-13C]DHAc and its metabolic products were acquired in a 7 T small-animal MR scanner using three purpose-designed spectral-spatial radiofrequency pulses that excited a spatial bandwidth of 8 mm with varying spectral bandwidths and central frequencies (chemical shifts).

RESULTS

The metabolic products detected in vivo include glycerol 3-phosphate, glycerol, phosphoenolpyruvate, lactate, alanine, glyceraldehyde 3-phosphate and glucose 6-phosphate. The metabolite-to-substrate ratios were comparable to those reported previously in perfused liver.

DISCUSSION

Three metabolic pathways can be probed simultaneously in the mouse liver in vivo, in real time,  using hyperpolarized DHAc.

Myocardial energy shortage and unmet anaplerotic needs in the fasted long-chain acyl-CoA dehydrogenase knockout mouse

AIMS

The aim of this animal study is to assess fasting-induced changes in myocardial substrate metabolism and energy status as a consequence of mitochondrial long-chain fatty acid β-oxidation deficiency, using magnetic resonance spectroscopy (MRS).

METHODS AND RESULTS

Carbon-13 ((13)C) MRS of hyperpolarized [1-(13)C]pyruvate was used to assess in vivo pyruvate dehydrogenase (PDH) activity in fed and fasted wild-type (WT) mice and long-chain acyl-CoA dehydrogenase knockout (LCAD KO) mice. PDH activity decreased after fasting in both genotypes, but was 2.7-fold higher in fasted LCAD KO mice compared with fasted WT mice. Incorporation of the (13)C label into the myocardial malate and aspartate pools in fasted LCAD KO mice demonstrates enhanced activity of anaplerotic pathways in fasted LCAD KO hearts. These findings were corroborated by ex vivo assays revealing partially depleted pools of citric acid cycle intermediates in fasted LCAD KO myocardium, suggesting an increased, but unmet need for anaplerosis. The in vivo myocardial energy status, assessed using phosphorous-31 ((31)P) MRS, was lower in fasted LCAD KO mice than in fasted WT mice.

CONCLUSION

This study revealed that the heart of fasted LCAD KO mice has an elevated reliance on glucose oxidation, in combination with an unmet demand for myocardial anaplerosis. Due to a lack of substrate availability, the sustained myocardial glucose uptake and PDH activity in LCAD KO mice are ineffective to maintain metabolic homeostasis during fasting, which is reflected by an impaired myocardial energy status in fasted LCAD KO mice.

In vivo magnetic resonance spectroscopy of liver tumors and metastases

Primary liver cancer is the fifth most common malignancy in men and the eighth in women worldwide. The liver is also the second most common site for metastatic spread of cancer. To assist in the diagnosis of these liver lesions non-invasive advanced imaging techniques are desirable. Magnetic resonance (MR) is commonly used to identify anatomical lesions, but it is a very versatile technique and also can provide specific information on tumor pathophysiology and metabolism, in particular with the application of MR spectroscopy (MRS). This may include data on the type, grade and stage of tumors, and thus assist in further management of the disease. The purpose of this review is to summarize and discuss the available literature on proton, phosphorus and carbon-13-MRS as performed on primary liver tumors and metastases, with human applications as the main perspective. Upcoming MRS approaches with potential applications to liver tumors are also included. Since knowledge of some technical background is indispensable to understand the results, a basic introduction of MRS and some technical issues of MRS as applied to tumors and metastases in the liver are described as well. In vivo MR spectroscopy of tumors in a metabolically active organ such as the liver has been demonstrated to provide important information on tumor metabolism, but it also is challenging as compared to applications on some other tissues, in particular in humans, mostly because of its abdominal location where movement may be a disturbing factor.