Golgi pH and glycosylation

Abstract Glycans, as a part of glycoproteins, glycolipids and other glycoconjugates, are involved in many vital intra- and inter-cellular tasks, such as protein folding and sorting, protein quality control, vesicular trafficking, cell signalling, immunological defence, cell motility and adhesion. Therefore, their correct construction is crucial for the normal functioning of eukaryotic cells and organisms they form. Most cellular glycans are constructed in the Golgi, and abnormalities in their structure may derive, for instance, from alkalinization of the Golgi lumen. In this work we show that Golgi pH is generally higher and more variable in abnormally glycosylating, i.e. strongly T-antigen (Gal-β1,3-GalNAc-ser/thr) expressing cancer cells, than in non-T-antigen expressing cells. We also confirmed that the Golgi pH alterations detected in cancer cells have the potential to induce glycosylation changes. A mere 0.2 pH unit increase in Golgi pH is able to induce T-antigen expression and inhibit terminal N-glycosylation in normally glycosylating cells. The mechanism of inhibition involves mislocalization of the corresponding glycosyltransferases. We also studied potential factors that can promote Golgi pH misregulation in health and disease, and found that cultured cancer cells, despite variation and elevation in Golgi pH, are fully capable of acidifying the Golgi lumen under the normal Golgi pH. Moreover, we introduce a Golgi localized Cl-/HCO3- exchanger, AE2a, that participates in Golgi pH regulation by altering luminal bicarbonate concentration and thus also buffering capacity. Participation of AE2a in Golgi pH regulation is especially intriguing, because it also provides a novel mechanism for expelling protons from the Golgi lumen

SLC4A2 anion exchanger promotes tumour cell malignancy via enhancing net acid efflux across golgi membranes

Abstract Proper functioning of each secretory and endocytic compartment relies on its unique pH micro-environment that is known to be dictated by the rates of V-ATPase-mediated H+ pumping and its leakage back to the cytoplasm via an elusive “H+ leak” pathway. Here, we show that this proton leak across Golgi membranes is mediated by the AE2a (SLC4A2a)-mediated bicarbonate-chloride exchange, as it is strictly dependent on bicarbonate import (in exchange for chloride export) and the expression level of the Golgi-localized AE2a anion exchanger. In the acidic Golgi lumen, imported bicarbonate anions and protons then facilitate a common buffering reaction that yields carbon dioxide and water before their egress back to the cytoplasm via diffusion or water channels. The flattened morphology of the Golgi cisternae helps this process, as their high surface-volume ratio is optimal for water and gas exchange. Interestingly, this net acid efflux pathway is often upregulated in cancers and established cancer cell lines, and responsible for their markedly elevated Golgi resting pH and attenuated glycosylation potential. Accordingly, AE2 knockdown in SW-48 colorectal cancer cells was able to restore these two phenomena, and at the same time, reverse their invasive and anchorage-independent growth phenotype. These findings suggest a possibility to return malignant cells to a benign state by restoring Golgi resting pH