Natural abundance isotope ratios to differentiate sources of carbon used during tumor growth in vivo

Background: Radioactive or stable isotopic labeling of metabolites is a strategy that is routinely used to map the cellular fate of a selected labeled metabolite after it is added to cell culture or to the circulation of an animal. However, the transformation of the labeled metabolite by cellular metabolism within organs complicates the use of this experimental strategy to quantify and understand metabolite transfer between organs. These methods are also technically demanding, expensive and potentially toxic. To allow quantification of the bulk movement of metabolites between organs, we have developed a novel application of stable isotope ratio mass spectrometry (SIRMS). Results: We exploit natural differences in 13C/12C ratios of plant nutrients for a low-cost and non-toxic carbon labeling, allowing a measurement of bulk carbon transfer between organs in vivo. SIRMS measurements were found to be sufficiently sensitive to measure organs from individual Drosophila melanogaster larvae, giving robust measurements down to 2.5 ug per sample. We apply the method to determine if carbon incorporated into a growing solid tumor is ultimately derived from food or host tissues. Conclusion: Measuring tumor growth in a D.melanogaster larvae tumor model reveals that these tumors derive a majority of carbon from host sources. We believe the low cost and non-toxic nature of this methodology gives it broad applicability to study carbon flows between organs also in other animals and for a range of other biological questions.