Characterization of the Intracellular Acidity Regulation of Brain Tumor Cells and Consequences for Therapeutic Optimization of Temozolomide

Abstract

A well-known feature of tumor cells is high glycolytic activity, leading to acidification of the tumor microenvironment through extensive lactate production. This acidosis promotes processes such as metastasis, aggressiveness, and invasiveness, which have been associated with a worse clinical prognosis. Moreover, the function and expression of transporters involved in regulation of intracellular pH might be altered. In this study, the capacity of tumor cells to regulate their intracellular pH when exposed to a range of pH from very acidic to basic was characterized in two glioma cell lines (F98 and U87) using a new recently published method of fluorescence imaging. Our results show that the regulation of acidity in tumors is not the same for the two investigated cell lines; U87 cells are able to reduce their intracellular acidity, whereas F98 cells do not exhibit this property. On the other hand, F98 cells show a higher level of resistance to acidity than U87 cells. Intracellular regulation of acidity appears to be highly cell-dependent, with different mechanisms activated to preserve cell integrity and function. This characterization was performed on 2D monolayer cultures and 3D spheroids. Spatial heterogeneities were exhibited in 3D, suggesting a spatially modulated regulation in this context. Based on the corpus of knowledge available in the literature, we propose plausible mechanisms to interpret our results, together with some new lines of investigation to validate our hypotheses. Our results might have implications on therapy, since the activity of temozolomide is highly pH-dependent. We show that the drug efficiency can be enhanced, depending on the cell type, by manipulating the extracellular pH. Therefore, personalized treatment involving a combination of temozolomide and pH-regulating agents can be considered.

On the Importance of Acidity in Cancer Cells and Therapy

“…We validated this methodology by measuring the pH drift observed in the cell culture medium [127]. We then applied it to monitor simultaneously the intra and extracellular pH in two glioma cell lines (murine F98 and human U87 gliomas) that were submitted to a range of acute pH changes from 5.0 to 8.4 [19]. We showed that the two cell lines reacted differently to acidity.…”

“…While many experiments have investigated cells within their native environments, understanding their intrinsic functioning often requires exposure to controlled conditions outside the norm, for instance, exploring the dynamic capacity of cells to regulate the intracellular pH necessitates subjecting them to a broad range of pH levels, from highly acidic to basic, in a systematic manner. Our research with two glioma cell lines demonstrated the potential to optimize temozolomide efficacy through this approach [19], although it has faced criticism for exposing cells to extreme conditions not typically encountered in vivo.…”

“…Therefore, by experimentally characterizing the cell's ability to regulate its pH i as a function of the pH e , the model highlights potential ways to improve the efficacy of TMZ on tumor cells. We subsequently characterized the pH regulation ability of two glioma cell lines and were indeed able to show that different optimal pHs for TMZ efficacy were identified [19].…”

“…Normal cells are rarely observed in acidic conditions, but, if artificially submitted to acidic conditions, they also activate the same regulatory mechanisms. Reciprocally, tumor cells immersed in alkaline pH are also very efficient in restoring an intracellular physiological pH [19].…”

Intracellular pH regulates β-catenin with low pHi increasing adhesion and signaling functions

“…Constitutively increased pHi is an emerging hallmark of cancer that is associated with several cancer cell behaviors (White et al, 2017). While pHi has been shown to increase with metastatic potential in tissue-matched cell lines (Spear and White, 2023), spatial pHi heterogeneity exists in 3D cancer cell cultures (Dovmark et al, 2017; Tafech et al, 2023). Concurrent work in our lab has revealed that microenvironment can alter pHi and concomitantly modulate β-catenin abundance in cancer cell models.…”