Cancer Biology

Once thought to be a mere consequence of the state of a cell, intermediary metabolism is now recognized as a key regulator of mammalian cell fate and function. In addition, cell metabolism is often disturbed in malignancies such as cancer, and targeting metabolic pathways can provide new therapeutic options. Cell metabolism is mostly studied in cell cultures in vitro, using techniques such as metabolomics, stable isotope tracing, and biochemical assays. Increasing evidence however shows that the metabolic profile of cells is highly dependent on the microenvironment, and metabolic vulnerabilities identified in vitro do not always translate to in vivo settings. Here, we provide a detailed protocol on how to perform in vivo stable isotope tracing in leukemia cells in mice, focusing on glutamine metabolism in acute myeloid leukemia (AML) cells. This method allows studying the metabolic profile of leukemia cells in their native bone marrow niche.

An important component of this methodology is to assess the role of the tumor microenvironment on tumor growth and survival. To tackle this problem, we have adapted the original approach of Warburg (Warburg, 1923), by combining thin tissue slices with Stable Isotope Resolved Metabolomics (SIRM) to determine detailed metabolic activity of human tissues. SIRM enables the tracing of metabolic transformations of source molecules such as glucose or glutamine over defined time periods, and is a requirement for detailed pathway tracing and flux analysis. In our approach, we maintain freshly resected tissue slices (both cancerous and non- cancerous from the same organ of the same subject) in cell culture media, and treat with appropriate stable isotope-enriched nutrients, e.g., ¹³C₆-glucose or ¹³C₅, ¹⁵N₂-glutamine. These slices are viable for at least 24 h, and make it possible to eliminate systemic influence on the target tissue metabolism while maintaining the original 3D cellular architecture. It is therefore an excellent pre-clinical platform for assessing the effect of therapeutic agents on target tissue metabolism and their therapeutic efficacy on individual patients (Xie et al., 2014; Sellers et al., 2015).

Mice are widely used for human tumor xenograft studies of cancer development and drug efficacy and toxicity. Stable isotope tracing coupled with metabolomic analysis is an emerging approach for assaying metabolic network activity. In mouse models there are several routes of tracer introduction, which have particular advantages and disadvantages that depend on the model and the questions addressed. This protocol describes the bolus i.v. route via repeated tail vein injections of solutions of stable isotope enriched tracers including ¹³C₆-glucose and ¹³C₅,¹⁵N₂-glutamine. Repeated injections give higher enrichments and over longer labeling periods than a single bolus. Multiple injections of glutamine are necessary to achieve adequate enrichment in engrafted tumors.

Therapy-induced hypoxia drives changes in the tumor microenvironment that contribute to the poor response to therapy. Hypoxia is capable of driving the expression and/or activation of specific signaling cascades (e.g., c-Met, Axl, CTGF), the recruitment of tumor promoting immune cells, and the induction of cell survival pathways including autophagy (Phan et al., 2013; Hu et al., 2012; Ye et al., 2010). We have recently shown that anti-VEGF therapy-induced hypoxia can result in changes in the extracellular matrix that contribute to the aggressiveness of tumors post therapy (Aguilera et al., 2014). Importantly, therapies that induce hypoxia do not always increase epithelial plasticity and tumor aggressiveness (Ostapoff et al., 2013; Cenik et al., 2013). We have used pimonidazole to evaluate hypoxia in tumors and herein provide a detailed protocol for this useful tool to interrogate the levels of hypoxia in vivo.

The utility of the Hypoxyprobe^TM (pimonidazole hydrochloride) immunohistochemical analysis approach allows for the assessment of hypoxia in different tissues as well as cell types. Pimonidazole is a 2-nitroimidazole that is reductively activated specifically in hypoxic cells and forms stable adducts with thiol groups in proteins, peptides, and amino acids (Cenik et al., 2013; Arnold et al., 2010; Raleigh and Koch, 1990; Raleigh et al., 1998). Furthermore, the amount of pimonidazole that is detected is directly proportional to the level of hypoxia within tumors.