Adventures of a pH Regulatory Protein and How pH Potentiates Lipid Uptake in Cancer

This thesis is based on two original articles that both relate to pH in the cellular environment. Paper I is focused on the pH regulatory protein NBCn1 which functions as a net acid extruder. In Paper II we studied the consequences of low pH in cancer cells with respect to their lipid uptake. Paper III is a review on the regulation of cancer cell metabolism, particularly lipid metabolism, caused by tumour microenvironmental acidosis.

Paper I:
The Na+/HCO3- co-transporter NBCn1 (SLC4A7) plays a key role in epithelial pH homeostasis. However, the mechanisms governing its subcellular localization remain unclear. In this study we found that both a predicted N-terminal β-sheet and in particular a short, predicted C-terminal α-helical motif were essential for the plasma membrane localization of NBCn1 in epithelial cells. We found that disruption of cell–cell contacts abolished this localization and that NBCn1 interacted with adherens junction protein Ecadherin as well as cell polarity protein DLG1. Additionally, NBCn1 interacted with RhoA and localized to lamellipodia and filopodia during cell migration. Surprisingly, we found that NBCn1 also localized to centrosomes and primary cilia in non-dividing, polarized epithelial cells, as well as to the cell cycle machinery during mitosis. We suggest a model in which NBCn1 traffics between these localizations, at least in part in a Rab11 dependent manner.

Paper II:
Cancer cells in the acidic tumor microenvironment exhibit increased lipid metabolism and -accumulation. We found that in mammary and pancreatic cancer cells both acute acidosis and adaptation to acidic growth led to rapid uptake of fatty acids (FAs). These FAs were subsequently converted into triacylglycerols and stored within lipid droplets (LDs). Notably, FA uptake was largely independent of other protein-facilitated lipid uptake mechanisms, including CD36, FATP2, and caveolin- and clathrin-dependent endocytosis. We suggest that the acidic microenvironment drives FA uptake primarily by increasing the protonation of negatively charged FAs, potentially serving as a major trigger for acidosis-driven metabolic rewiring.