An acidic environment changes cyclin D1 localization and alters colony forming ability in gliomas

The human glioma cell lines, U87 and T98G, were evaluated for their ability to survive and form colonies in an acidic environment of pH_ext 6.0. In contrast to U87, which showed an 80–90% survival rate, only 40% of T98G cells survived 6 days at pH_ext 6.0 and lost their colony forming ability when returned to a normocidic environment. Although both U87 and T98G cells maintain an intracellular pH (pH_i) of 7.0 at pH_ext 6.0 and arrest mostly in G1 phase of the cell cycle, only T98G demonstrated a major loss of cyclin D1 that was prevented by the proteasome inhibitor MG132. Colony forming ability was restored by stably transfecting T98G cells with a cyclin D1-expressing plasmid. Both U87 and T98G cells demonstrated increased cytoplasmic localization of cyclin D1 during exposure at pH_ext 6.0. Upon prolonged (24 h) incubation at pH_ext 6.0, nuclear cyclin D1 was nearly absent in T98G in contrast to U87 cells. Thus, an acidic environment triggers cytoplasmic localization and proteasomal degradation of cyclin D1.