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BOLD fMRI ( 16) provides dynamic and non-invasive measurements of blood oxygenation as a proxy for adipose tissue metabolic activity, but it is unable to dissociate changes in blood flow from changes in adipose tissue metabolism. Hyperpolarized 13C MRI ( 14) and hyperpolarized xenon MRI ( 15) are promising for providing dynamic biochemical measurements in adipose tissue, but these techniques require costly and complex hyperpolarization procedures. Computed tomography scanning ( 13) provides a static snapshot of body composition, including differences between adipose tissue and other tissues, but as a macroscopic technique it has limited ability to assess precise biochemical properties and as a static technique it is unable to capture dynamic changes to adipose tissue metabolism.
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Although FDG PET is widely available, ionizing radiation exposure, cost, procedure duration, and poor reproducibility of images are its key limitations ( 10– 12). Positron emission tomography (PET) with fluorodeoxyglucose (FDG) ( 6– 9) is the primary method to assess BAT in humans. Numerous tomographic imaging techniques have been used to determine the metabolic activity of adipose tissue. Besides displaying increased energy expenditure, mice with increased BAT or WAT metabolic activity also show increased insulin sensitivity ( 5), suggesting reduced risk of type 2 diabetes. A variety of pharmacological agents and prolonged cold exposure increase the metabolic rate of adipose tissue, either by increasing classical non-shivering thermogenic activity within the BAT or by inducing BAT-like thermogenic activity within certain adipocytes (“beige” or “brite” cells) within the WAT ( 2– 4). For this reason, increasing metabolic activity within the BAT, and encouraging BAT-like metabolic activity within the WAT, a prime target for increasing adipose tissue energy expenditure and thus promoting weight loss and healthy weight maintenance in humans ( 1). Spatially resolved FTIR imaging is a promising technique to quantify cold-induced biochemical changes in BAT and s-WAT in a label-free manner.īrown adipose tissue (BAT), defined by the presence of uncoupling protein 1 (UCP-1), has the potential for a higher metabolic rate than the far more prevalent white adipose tissue (WAT) in mammals, due to its propensity for thermogenesis. Principal component analysis applied to FTIR spectra revealed pronounced differences in overall spectral characteristics between 30, 24, and 10☌ BAT and s-WAT. Complementary 1H NMR measurements confirmed the findings from this ratio in BAT.
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The degree of unsaturation, calculated from the ratio of the integrated area from 2,992 to 3,020 cm −1 (unsaturated lipids) to the integrated area from 2,830 to 2,980 cm −1 (saturated lipids), showed stepwise decreases going from colder-exposed to warmer-exposed BAT. Greater protein to lipid ratio was associated with greater UCP-1 expression level in the BAT ( p = 0.021) and s-WAT ( p = 0.032) and greater Dio2 expression in s-WAT ( p = 0.033). Protein to lipid ratio, calculated from the ratio of the integrated area from 1,600 to 1,700 cm −1 (amide I) to the integrated area from 2,830 to 2,980 cm −1 (saturated lipids), was elevated in 10☌ BAT and s-WAT compared to 24☌ ( p = 0.004 and p < 0.0001) and 30☌ ( p = 0.0033 and p < 0.0001). Fat exposed to colder temperatures demonstrated greater thermogenic activity as indicated by increased messenger RNA expression levels of a panel of thermogenic marker genes including uncoupling protein 1 (UCP-1) and Dio2. In this study, we used spatially resolved Fourier transform infrared (FTIR) imaging to quantify biochemical changes caused by cold exposure in the brown and subcutaneous white adipose tissues (BAT and s-WAT) of 6 week-old C57BL6 mice exposed to 30☌ ( N = 5), 24☌ ( N = 5), and 10☌ ( N = 5) conditions for 10 days. Stimulating increased thermogenic activity in adipose tissue is an important biological target for obesity treatment, and label-free imaging techniques with the potential to quantify stimulation-associated biochemical changes to the adipose tissue are highly sought after. 2Pennington Biomedical Research Center, Baton Rouge, LA, United States.1Department of Physics, University of Wisconsin-Milwaukee, Milwaukee, WI, United States.