Abstract
Many cancers adeptly modulate metabolism to thrive in fluctuating oxygen conditions; however, current tools fail to image metabolic and vascular endpoints at spatial resolutions needed to visualize these adaptations in vivo. We demonstrate a high-resolution intravital microscopy technique to quantify glucose uptake, mitochondrial membrane potential (MMP), and SO2 to characterize the in vivo phentoypes of three distinct murine breast cancer lines. Tetramethyl rhodamine, ethyl ester (TMRE) was thoroughly validated to report on MMP in normal and tumor-bearing mice. Imaging MMP or glucose uptake together with vascular endpoints revealed that metastatic 4T1 tumors maintained increased glucose uptake across all SO2 (“Warburg effect”), and also showed increased MMP relative to normal tissue. Non-metastatic 67NR and 4T07 tumor lines both displayed increased MMP, but comparable glucose uptake, relative to normal tissue. The 4T1 peritumoral areas also showed a significant glycolytic shift relative to the tumor regions. During a hypoxic stress test, 4T1 tumors showed significant increases in MMP with corresponding significant drops in SO2, indicative of intensified mitochondrial metabolism. Conversely, 4T07 and 67NR tumors shifted toward glycolysis during hypoxia. Our findings underscore the importance of imaging metabolic endpoints within the context of a living microenvironment to gain insight into a tumor’s adaptive behavior.
Highlights
Many cancers adeptly modulate metabolism to thrive in fluctuating oxygen conditions; current tools fail to image metabolic and vascular endpoints at spatial resolutions needed to visualize these adaptations in vivo
Many cancer types have been shown to rely on mitochondrial metabolism in combination with glycolysis to meet the increased energy demands required for proliferation and metastasis[2,3,4]
Hypoxia has been associated with an increase in mitochondrial mass in metastatic murine breast cancer[15], and an increase in mitochondrial size mediated by hypoxia-inducible factor 1α (HIF-1α) has been shown to prevent mitochondrial apoptosis in colon carcinoma[13]
Summary
Many cancers adeptly modulate metabolism to thrive in fluctuating oxygen conditions; current tools fail to image metabolic and vascular endpoints at spatial resolutions needed to visualize these adaptations in vivo. In the so-called “Reverse Warburg Effect” (RWE), glycolytic stromal cells excrete lactate, and this micro-environmental “waste” is taken in by cancer cells and used to fuel oxidative phosphorylation (OXPHOS)[17] It follows that observing the regional interplay between multiple metabolic and vascular endpoints aids understanding of a tumor’s phenotype. Considering the importance of glycolysis, MMP, and the oxygen gradients within blood vessels to tumor bioenergetics, there are surprisingly no techniques to image in vivo these three endpoints with a single technology Used techniques such as cellular metabolic flux analyzers and metabolomics provide comprehensive information about cancer metabolism, but are limited to in vitro assays[18] or ex vivo assays[19] and neither provides spatial information. Magnetic resonance spectral imaging (MR(S)I) can report on a host of important endpoints related to both mitochondrial metabolism and glycolysis[23,24] as well as vasculature[25], yet spatial and temporal resolution are limiting[22]
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