It is well established that altered metabolism in cancer cells supports survival and growth. Understanding these biological changes could lead to treatments able to specifically target tumourigenic cells. However, despite the known variability in tumour biology, the majority of research is based on the same available cell lines. In renal cell carcinoma (RCC) research these lines frequently carry a VHL mutation: VHL gene alterations are associated with hereditary RCC in VHL syndrome and up to 91% of sporadic clear cell RCCs (Nickerson et al., 2008). The loss of pVHL results in aberrant HIF signalling and changes in metabolism (Keefe et al., 2013).
New research from Wettersten et al. (2015) used metabolomics and proteomics to study metabolic changes in human RCC samples of defined grades (Fuhrman et al., 1982), validating findings in two RCC cell lines – 786-O (VHLm/-)* and Caki-1 (VHL+/+). They found that there are tumour grade-dependent metabolic changes with higher grade tumours showing increased levels of glutamine, fatty acid and glutathione metabolic reprogramming.
Normally pyruvate produced by glycolysis is converted to oxaloacetate and fed into the TCA cycle as the major source of cellular energy. Tumour cells show increased glycolysis but direct the resulting pyruvate towards lactate metabolism (Warburg, 1956). Wettersten et al., confirmed that in higher grade RCC tumours there were increased glycolysis metabolites but a decrease in pyruvate carboxylase and oxaloacetate levels. Instead increased lactate dehydrogenase (LDHA) and lactate concentrations indicated that lactose fermentation was the major energy source. Reduced viability of 786-O and Caki-1 cells in glucose deprivation culture conditions or in the presence of glycolysis inhibitor 2-DG demonstrated the high dependence of tumour cells on glucose and glycolysis respectively.
Under normal conditions fatty acid β-oxidation produces acetyl-CoA that can be utilised in the TCA cycle for energy production. Wettersten et al. reported a reduction in short fatty acids levels in higher grade tumours, indicative of increased metabolism, but a reduction in β-oxidation enzymes resulting in reduced oxidation of intermediate acyl-CoA and build-up of acylcarnitines. Previously high levels of carnitine and acylcarnitines have been report in RCC patient urine (Ganti et al., 2012).
Glutamine can feed into the TCA cycle, into the urea cycle, and into the glutathione (GSH) pathway (Smith, 1990). In the RCC tumour samples the TCA and urea cycle enzymes were downregulated. Instead the markedly higher levels of GSH indicated that more glutamine was being used to alleviate oxidative stress providing the cells with a survival advantage. This reliance on glutamine was demonstrated by reduced viability of 786-O and Caki-1 cells in glutamine deprivation culture conditions associated with reduced levels of GSH and its redox partner GSSG.
RCC in BHD patients also show increased lactate production and decreased fatty acid oxidation indicative of a metabolic shift (Preston et al., 2011). In BHD and other subtypes of RCC tumourigenesis is associated with mutations in other genes such as FLCN (BHD), TSC1 or TSC2 (TSC) and fumarate hydratase (HLRCC). Identifying distinct metabolic and signalling changes in these tumours could determine the impact of different mutations and identify specific treatment targets.
Wettersten et al. suggest that understanding more about the distinct metabolic changes in different cancer grades could lead to targeted grade-based treatments. Current RCC targeted treatments are based on kinase inhibition to repress aberrant signalling pathways. Further understanding of different tumour subtypes could enable future patients to receive highly effective mutation- and grade-based treatments.
* 786-O cells harbor an inactivating mutation in one VHL allele while the second allele is deleted.
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