Oncogenic mutations can drive the dependency of tumours on specific nutrients, such as glucose and glutamine, as well as impose changes in the wiring of metabolic pathways. This phenomenon is often referred to as 'metabolic reprogramming'. One function of metabolic reprogramming in cancer is to provide biosynthetic building blocks (such as amino acids, lipids and nucleotides) to support the generation of daughter cells. (Figure 1).
Metabolic reprogramming is, in part, dictated by expression of alternative enzyme isoforms with distinct regulatory properties, as illustrated with the glycolytic enzyme pyruvate kinase. Unlike the M1 isoform of pyruvate kinase (PKM1), which is found in most normal tissues, cancer cells selectively express the alternatively spliced isoform PKM2.
We have shown that PKM2, but not PKM1, can be inhibited by oxidation. Under conditions of oxidative stress, which are often encountered during tumourigenesis, inhibition of PKM2 allows the utilisation of glucose for the production of antioxidants, thereby promoting cancer cell survival (Figure 2). As oncogenic events can alter metabolism before uncontrolled tumour growth commences, our work suggests that, in addition to promoting proliferation, metabolic reprogramming can also support survival under stress during tumourigenesis.
We have also contributed to the development of small molecules that can activate specifically PKM2, but no other mammalian pyruvate kinase isoform, and have shown that these compounds sensitise cells to oxidant-induced death. Furthermore, we have demonstrated that PKM2 activators limit xenograft tumour growth by impeding the biosynthetic capacity of cancer cells (Anastasiou et al., under revision). Overall, our results provide evidence that isoform-specific targeting of metabolic enzymes is a feasible and effective strategy for limiting tumour growth.
Metabolic reactions are driven by enzymes that, in general, are considered amenable to targeting by small molecule compounds ('druggable'), a notion also supported by our past work. Indeed, some of the early cancer therapies were proven to target metabolic pathways. Yet rational design of therapeutic strategies that target tumour metabolism remains poorly developed. In part, this is because much of our current knowledge of metabolic networks is based on studies with animal organs, a favourite source of biomaterial in the days before cell culture became routine. As such, most charted metabolic pathways reflect the functional needs of terminally differentiated, rather than proliferating cells. Furthermore, whole organs comprise of several cell types each with distinct functions and metabolic characteristics.
Thus, better understanding of cancer metabolism necessitates studies in proliferating cells, within the natural context of the tumour microenvironment, and at the level of individual cells. The lab is undertaking a variety of projects that investigate how oncogenic signal transduction pathways drive metabolic reprogramming; and what the molecular mechanisms are that determine the dependency of cancer cells on specific nutrients for proliferation and survival. Towards these goals, we primarily employ NMR- and mass spectrometry-based metabolomics, combined with biochemistry and metabolic engineering in cell and organotypic cultures.
To gain insights into the influence of the tumour microenvironment on cancer metabolism, we manipulate the activity of metabolic pathways in mice by knocking-down and overexpressing metabolic enzymes, or by introducing rationally designed metabolic enzyme mutants with altered regulatory properties, in a tissue-specific manner. We are also combining these approaches with genetic and chemically-induced mouse cancer models to examine the contribution of specific metabolic pathways to tumourigenesis.
We capitalise on our detailed understanding of metabolic enzyme regulation to design and develop metabolic biosensors using protein engineering. Such tools allow quantitative studies of metabolic pathway activities at single-cell level in a non-invasive manner. We are complementing this approach by isotope-labelling metabolomics experiments to study dynamic changes in metabolism during various stages of tumourigenesis and in response to therapeutic treatments.