Selective decrease of SN1(SNAT3) mRNA expression in human and rat glioma cells adapted to grow in acidic medium

The system N glutamine (Gln) transporter SN1(SNAT3) is overexpressed in human malignant glioma cells in situ as compared to the adjacent brain tissue or metastases from different organs [Sidoryk, M., Matyja, E., Dybel, A., Zielińska, M., Bogucki, J., Jaskólski, D.J., Liberski, P.P., Kowalczyk, P., Albrecht, J., 2004]. Increased expression of a glutamine transporter SNAT3 is a marker of malignant gliomas. NeuroReport 15, 575–578], but its role in tumor growth as compared to the other Gln transporters is unknown. One of the profound, growth-promoting effects of glial tumor in situ is acidification of the extracellular space. In the kidney SN1(SNAT3) mRNA participates in the adaptation to acidosis. In this study therefore, expression of mRNAs coding for SN1(SNAT3) and other Gln transporters was measured in human (T98G) and rat (C6) glioma cells incubated for 4 h in an acidic medium (AI) (pH 6.5). MTT assay revealed no cell loss in AI cells, and intracellular pH (pHi) as measured by a fluorescent probe (BCECF-AM) was slightly alkaline in C6 and T98G cells, indicating that the cells have adapted to AI. AI significantly decreased the SN1(SNAT3) mRNA expression in C6 (a 60% decrease) and T98G cells (a 50% decrease). The decrease retreated in C6 cells 4 h after transferring them back to the neutral medium. The expression of ASCT2 mRNA (system ASC), ATA1 mRNA (system A) and SN2(SNAT5) mRNA (system N) were not affected by AI in either of the cell lines. [3H]Gln uptake in C6 or T98G cells grown in neutral medium was mainly mediated by system ASCT2: system N contributed to only ∼7% of the uptake. AI did not affect the total Gln uptake, and only slightly decreased the system N-mediated component of the uptake. Hence, SN1(SNAT3) does not seem to be involved in the adaptation of cultured glioma cells to acidic millieu.