Temozolomide Induces the Acquisition of Invasive Phenotype by O6-Methylguanine-DNA Methyltransferase (MGMT)+ Glioblastoma Cells in a Snail-1/Cx43-Dependent Manner

Glioblastoma multiforme (GBM) recurrences after temozolomide (TMZ) treatment result from the expansion of drug-resistant and potentially invasive GBM cells. This process is facilitated by O6-Methylguanine-DNA Methyltransferase (MGMT), which counteracts alkylating TMZ activity. We traced the expansion of invasive cell lineages under persistent chemotherapeutic stress in MGMT^low (U87) and MGMT^high (T98G) GBM populations to look into the mechanisms of TMZ-induced microevolution of GBM invasiveness. TMZ treatment induced short-term, pro-invasive phenotypic shifts of U87 cells, in the absence of Snail-1 activation. They were illustrated by a transient induction of their motility and followed by the hypertrophy and the signs of senescence in scarce U87 sub-populations that survived long-term TMZ stress. In turn, MGMT^high T98G cells reacted to the long-term TMZ treatment with the permanent induction of invasiveness. Ectopic Snail-1 down-regulation attenuated this effect, whereas its up-regulation augmented T98G invasiveness. MGMT^low and MGMT^high cells both reacted to the long-term TMZ stress with the induction of Cx43 expression. However, only in MGMT^high T98G populations, Cx43 was directly involved in the induction of invasiveness, as manifested by the induction of T98G invasiveness after ectopic Cx43 up-regulation and by the opposite effect after Cx43 down-regulation. Collectively, Snail-1/Cx43-dependent signaling participates in the long-term TMZ-induced microevolution of the invasive GBM front. High MGMT activity remains a prerequisite for this process, even though MGMT-related GBM chemoresistance is not necessary for its initiation.