Developing a model of acid-driven evolution for Pancreatic Ductal Adenocarcinoma (PDAC)
Keywords: lactate
pH
glycolysis
acidosis
pancreatic cancer
PDAC
heterogeneity
metabolism
monocarboxylate transporters

Pancreatic ductal adenocarcinomas (PDAC) are characterised by poor prognosis and rising incidence. Their hallmarks include rapid progression, which suggests efficient metabolic reprogramming, and poor response to therapy, corresponding to accelerated somatic evolution. The elevated metabolic rate, necessary to sustain growth, creates an acidic microenvironment which may lead to more aggressive phenotypes. The high degree of heterogeneity in PDAC tumours sets up conditions for extensive feedbacks between metabolism, selection, and somatic evolution. The trajectory of the disease could be influenced by targeting these relationships, but, at present, understanding of these processes is incomplete. The aim of this doctoral project was to elucidate the role of acidity in PDAC with focus on the metabolic sources of acidity and feedback circuits that control metabolism. Furthermore, the role of low pH in selection and adaptation, and in maintaining population-level heterogeneity was explored.
A panel of PDAC cell lines was characterised for metabolic profiles using a specifically- designed fluorometric assay. Glycolytic metabolism, contrary to oxidative phosphorylation, was found to be highly sensitive to pH, which is a key negative feedback cue regulating the glycolytic output. Rapid or progressive acidification of media resulted in a reversible decrease in metabolic rate, arguing for an adaptive response rather than irreversible selection. Membrane permeability to lactic acid, the end-product of glycolysis, was assessed at single- cell resolution using fluorescence imaging. This readout revealed significant heterogeneity among cells of the same cell line. Surprisingly, the chemical inhibitors of lactic acid transport decreased the permeability of the transporters, but failed to reduce glycolytic flux, which may be due to autoregulation. Sorting according to lactic acid permeability showed that clones with high permeability manifested increased metabolic rates versus the low-permeability subgroup. However, the difference between these subpopulations was abolished with time, indicating a dynamic equilibrium. Transcriptomic analyses of the two populations identified a distinct pattern of expression, including elevated levels of IL6/STAT3 signalling in the high permeability cells. Pharmacological and genetic manipulations of this pathway revealed negative feedback loops between IL6 receptor-mediated activation of STAT3 and IL6R expression. This produced an oscillating system that periodically leads to STAT3 activation. This metabolic oscillation can explain the metabolic heterogeneity among PDAC cells, which is maintained even at uniform in vitro conditions. Evidence for the relevance of heterogeneity in PDAC was confirmed in xenografts in vivo.
Overall, these findings indicate that PDAC cell lines harbour a small population of cells characterised by increased metabolic rates due to elevated IL6R expression. This highly glycolytic phenotype, controlled by an IL6-mediated feedback circuit is transient. This has the advantage of favouring tumour progression at the cellular level in terms of growth and proliferation, without excessively acidifying their milieu.