Beyond glucose and Warburg: finding the sweet spot in cancer metabolism models

Abstract

Advances in cancer biology have highlighted metabolic reprogramming as an essential aspect of tumorigenesis and progression. However, recent efforts to study tumor metabolism in vivo have identified some disconnects between in vitro and in vivo biology. This is due, at least in part, to the simplified nature of cell culture models and highlights a growing need to utilize more physiologically relevant approaches to more accurately assess tumor metabolism. In this review, we outline the evolution of our understanding of cancer metabolism and discuss some discrepancies between in vitro and in vivo conditions. We describe how the development of physiological media, in combination with advanced culturing methods, can bridge the gap between in vitro and in vivo metabolism.

Fig. 1. Central carbon metabolism fuels energy and biosynthesis.

Glucose metabolism fuels several biosynthetic and energy-generating pathways (red). Glycolytic intermediates can be re-directed from glycolysis to other biosynthetic pathways, supporting DNA and RNA synthesis (pentose phosphate pathway), glycan generation (hexosamine biosynthesis), or one-carbon metabolism (serine biosynthesis). Pyruvate entry to the TCA cycle results in both high levels of ATP generation and biosynthesis pathways such as fatty acid synthesis. OAA oxaloacetate, α-KG alpha-ketoglutarate.

Fig. 2. Utility and readouts of metabolomics and stable isotope tracing.

A In metabolomics, samples are obtained and processed using mass spectrometry to identify the abundance of various metabolites. As such, individual metabolites are readily identified, and differences in the sizes of different metabolite pools can be identified (shown here by the size of various circles). While more sophisticated analyses can be used to model the activity of various metabolic pathways, metabolomic studies cannot directly assess pathway activity. B Stable isotope tracing requires the addition of a mass-labeled metabolite (e.g., ¹³C-glucose,¹⁵-N-glutamine) to the system (represented here by blue overlays of various models). After a set incubation period, samples are obtained and processed for mass spectrometry. This technique provides a greater depth of information regarding the utilization of different nutrients and metabolites by incorporating labeled atoms into different metabolites (represented here by blue shading of circles). Thus, pathway activity is directly assessed.

Fig. 3. Timeline of tissue culture media development and formulation.

In the early 1900s, biological fluids such as plasma were common practice for culturing cells. This natural media period continued until the development of BME in 1955. This was shortly followed by the rapid generation of other synthetic media designed for different cell types that have now become standard for in vitro work today. However, a recent physiologic media revolution initiated in 2015 with SMEM has ushered in a new wave of media development modeling in vivo environments.