Functional Importance of Metabolic Pathways in The Tumour Microenvironment to Tumour Development and Progression

Abstract

Altered energy metabolism is one of the hallmarks of cancer and is receiving much attention as a therapeutic target. The changes affect a wide range of metabolic pathways. In this study, we focussed our attention on fatty acid and amino acid metabolism, as two less well studied pathways with clear translational potential. We used the Drosophila eye epithelium as a cancer model. Knockdown of the metabolism genes found to be essential to eye development showed that the metabolic regulator Carnitine palmitoyltransferase 2 (CPT-2) in either immune cells or RasV12/S100A4- induced tumours was the most effective suppressor of tumour growth and invasion. We therefore focussed on CPT-2 in our translational studies. Remarkably a role for CPT-2 in the TME is consistent with clinical data that has been mined in collaboration with Dr Vijay Sharma, showing that CPT-2 expression in tumour or immune cells is associated with poor prognosis, especially when there is low mutation burden. In a separate collection of breast cancer patient samples, the association between CPT-2 expression and patient outcome was significant. High expression of CPT-2 prevented recurrence of DCIS. The findings show that CPT-2 has a protective effect at the in-situ stage in preventing recurrence of in-situ disease. These are surprising findings as CPT-2 has been a hitherto neglected component of the fatty acid oxidation pathway in cancer studies. It may have a role both as a biomarker of progression and as a therapeutic target, although its effects are complex, switching from a protective effect at the in-situ stage to a broadly negative effect at the invasive stage. The latter effect is in keeping with our experimental observations. In the second study, we probed the amino acid metabolism pathways using a dietary manipulation approach. Using an experimental model of tumourigenesis in Drosophila, we removed non-essential amino acids (NEAAs) from the food and we found that, of all the amino acids we tested,glycine deprivation suppressed the tumour growth the most. Knockdown of the glycine transporter in neural cells had the same effect. We hypothesise that these effects may be mediated by circuitry in the brain controlling circadian clock because it has previously been shown that circadian neurobiology is a key target of glycinergic signalling. Interestingly, our preliminary mass spectrometry findings confirmed that Ecdysone is upregulated in animals with neuronal glycine transporter (GlyT ) knockdown. This raises the possibility that nutritional inputs mediate changes in the neuronal circadian clock, ultimately impacting on tumour growth and progression, via steroid hormone production. Overall, glycine represents a metabolic vulnerability in rapidly proliferating cancer cells that could in principle be translated into human studies and be targeted for therapeutic benefit. In summary, we have, through these studies, identified and characterised two therapeutic targets, CPT-2 and glycine metabolism, which showed the strongest effects in our experimental models, and can now be further investigated in translational studies.