Sodium homeostasis in the tumour microenvironment

Abstract

The concentration of sodium ions (Na⁺) is raised in solid tumours and can be measured at the cellular, tissue and patient levels. At the cellular level, the Na⁺ gradient across the membrane powers the transport of H⁺ ions and essential nutrients for normal activity. The maintenance of the Na⁺ gradient requires a large proportion of the cell's ATP. Na⁺ is a major contributor to the osmolarity of the tumour microenvironment, which affects cell volume and metabolism as well as immune function. Here, we review evidence indicating that Na⁺ handling is altered in tumours, explore our current understanding of the mechanisms that may underlie these alterations and consider the potential consequences for cancer progression. Dysregulated Na⁺ balance in tumours may open opportunities for new imaging biomarkers and re-purposing of drugs for treatment.

Keywords: Channels; MRI; Microenvironment; Sodium; Transporters; Tumours.

Figure 1

Accumulation of Na⁺ in tumours. Converse to the reported decrease in pO₂ and pH, many tumours (such as breast cancer) exhibit elevated [Na⁺] (9). This elevated tumour [Na⁺] may be due to increases in the extracellular volume fraction relative to the intracellular volume fraction, or due to increases in the [Na⁺] concentration within either compartment. Moreover, tumour [Na⁺] is likely influenced by the heterogeneity of the tumour microenvironment. Factors that could increase the extracellular volume fraction (interstitial volume) include increases in colloidal and interstitial pressure (13) due to vasculature permeabilisation, blood plasma protein release and the formation of fibrin (11, 12), and cancer cell death as a result of targeted chemotherapy or poor vascularisation within the necrotic tumour core. Moreover, Na⁺ binding to protein sequestered within the desmoplastic tumour microenvironment (e.g. glycosaminoglycans) could contribute to an increase in the extracellular Na⁺ content (287). Alternatively, the increased cellularity observed within poorly vascularised tumours (193) suggests that increases in intracellular volume fraction can contribute to elevated tumour [Na⁺]; indeed, [Na⁺]_i has been reported to be elevated in cultured cancer cells (20, 22, 23), potentially due to altered transporter expression (Table 1).

Figure 2

Cellular Na⁺ import and export mechanisms. Cells exhibit a diverse repertoire of Na⁺ channels and transporters, many of which exhibit altered expression in cancer (Table 1) and are being explored as potential therapeutic targets (Table 2). The activity and conductance of these channels is regulated by [Na⁺]_i, [Na⁺]_e, membrane potential and auxiliary regulatory proteins. Channels that facilitate Na⁺ influx include voltage gated Na⁺ channels (VGSC), epithelial Na⁺ channels (ENaC), acid-sensing channels (ASIC), glutamate-activated N-methyl-D-aspartate receptors (NMDA), ATP-activated P2X purinoceptor 7 (P2X7) and the G protein-coupled Na⁺ leak channel, non-selective (NALCN). The inward Na⁺ gradient and a hyperpolarised membrane potential are maintained by the ATP-driven Na⁺/K⁺ ATPase. Na⁺ influx is also linked to the transport of numerous other ions and substrates, namely H⁺ efflux (Na⁺/H⁺ exchanger 1, NHE1), Cl^- and K⁺ influx (Na⁺-K⁺-Cl^- cotransporter, NKCC), cytosolic and mitochondrial Ca²⁺ efflux (Na⁺/Ca²⁺ exchanger, NCX and mitochondrial Na⁺-Ca²⁺(Li⁺) exchanger, NCLX, respectively) glucose uptake (sodium-glucose linked transporter, SGLT) and amino acid (AA) uptake. Na⁺/H⁺ exchangers (NHE) are also present on both mitochondria and lysosomes, the latter of which achieve Na⁺ efflux into the cytosol via two-pore channels (TPC) and transient receptor potential mucolipin (TRPML) channels.

Figure 3

Physiological consequences of Na⁺ accumulation within tumours. Dashed arrows indicate putative mechanisms which remain to be fully characterised. Red arrows indicate denote movement/conversion of metabolites. A: Elevated [Na⁺] is linked to alterations in tumour metabolism and pH regulation. Elevations in [Na⁺]_i activate the Na⁺/K⁺ ATPase, thereby raising ATP demand and driving a high glycolytic rate (34). To maintain a high pH_i, the resulting H⁺ is rapidly extruded by NHE, which is driven by the inward Na⁺ gradient. Increased [Na⁺]_i could also facilitate depletion of mitochondrial Ca²⁺ ([Ca²⁺]_m) via NCLX, thereby altering mitochondrial metabolism. Conversely, changes to the Na⁺ gradient across the plasma membrane will likely alter the driving force for transporters importing key metabolic substrates such as glutamine (SLC1A5) and glucose (SGLT), thereby influencing anabolic and anapleurotic processes. B: Elevated tumour Na⁺ and membrane potential (V_m). V_m is generated by the electrogenic Na⁺/K⁺ ATPase; Na⁺ influx via VGSC/ENaC/ASICs results in a depolarised membrane potential (V_m) in cancer cells. A depolarised V_m can lead to the activation of proliferative signalling cascades (such as KRas), cytoskeletal reorganisation facilitating migration, and accelerates the cell cycle. Conversely, most healthy differentiated cells exhibit a more hyperpolarised V_m that tightly controls cell cycle progression. C: Cell volume regulation by Na⁺-linked transport mechanisms. Elevations in tumour [Na⁺] are linked to changes in cell volume regulation. NKCC sequentially facilitates the accumulation of intracellular Cl^-, H₂O uptake (aquaporins, AQP, and osmosis) and cell swelling (288). Conversely, K⁺-Cl^- cotransporters (KCC) mediate Cl^- efflux, promoting H₂O exit from the cell (289). NHE1 activity results in a net osmotic gain due to Na⁺ influx; the osmolar contribution of intracellular H⁺ ions is compensated due to the dissociation of intracellular buffers. The resulting increase in pH_i and [HCO₃ ^-]_i drives Cl^- import via anion exchangers (AE), leading to H₂O uptake and cell swelling (290). NHE1 can also operate to in reverse mode to resist compressive forces (118).

Figure 4

Effect of elevated Na⁺ on cancer progression and the tumour microenvironment. Dashed arrows indicate putative mechanisms which remain to be fully characterised. A: Elevated tumour Na⁺ and migration/invasion. VGSCs have been correlated with activation of a proinvasive gene transcription network upregulating Wnt, MAPK and Ca²⁺ signalling (161, 164). NHE1 localises to the leading edge of invading cells (136); VGSC colocalisation with NHE1 within cavaeolae leads to activation of NHE1, acidification of the extracellular environment and digestion of the extracellular matrix by cathepsins and matrix metalloproteinases (120, 142, 152). Interestingly, the β subunit of VGSCs acts as an adhesion molecule that interacts with the extracellular matrix to regulate migration and invasion (291). Extracellular acidification can also activate ASIC and ENaC channels (58, 59), leading to further increases in [Na⁺]_i. Na⁺-linked Ca²⁺ influx via reverse-mode NCX action has been linked to cancer cell motility via Ca²⁺ signalling-activated TGF-β signalling (151). NKCC regulates cell swelling required for migratory behaviour by facilitating [Cl^-]_i accumulation and H₂O uptake via osmosis and aquaporins (288). NKCC also acts as a scaffold for cofilin within invadopodia, which facilitates cytoskeletal remodelling (292). B: Elevated [Na⁺]_e and cancer cell proliferation. The inward Na⁺ gradient drives the uptake of anabolic substrates such as glucose and glutamine (SGLT and SLC1A5), respectively (–46); altered tumour [Na⁺] might regulate the uptake of these substrates. Via glycolysis/pentose phosphate pathway (PPP) and glutaminolysis, glucose and glutamine are utilised as substrates for redox homeostasis, biosynthesis, and cell proliferation (i.e. reducing equivalents, nucleotides, and fatty acids) (293). SLC1A5 also regulates mTORC1, a key regulator of protein translation and cell growth (254). Moreover, salt inducible kinase 3 is activated by elevated [Na⁺]_e, promoting G1/S phase transition (129), and [Cl^-] accumulation due to upregulated NKCC activity can promote cell cycle progression (48, 117, 130). Elevated [Na⁺]_e leads to DNA breaks with significant implications for oncogenic mutations/tumour suppressor silencing (132), and a high osmolality and VGSC activity also activates the MAPK signalling cascade, potentially via Rac1 activation (127, 128). C: Elevated Na⁺ and osmolality drives inflammation in the tumour microenvironment. Increased [Na⁺]_e and osmolality promote proliferative and antiapoptotic signalling in tumour associated macrophages (171) and by increasing the production of proinflammatory cytokines by local endothelial cells and Th-17 helper cells (177). Together these factors also promote the further recruitment of proinflammatory immune cells (171). These factors lead to extracellular matrix breakdown, neovascularisation and tumour remodelling, thereby promoting tumour progression and metastasis (129, 179, 187, 294).