The failure of malignant cells to undergo apoptosis is a major obstacle in cancer therapy, and thus identifying the underlining molecules involved therein is imperative for improving patient survival. An important mechanism of drug resistance is cell adhesion-mediated drug resistance (CAM-DR). In this study we identify a novel switch by which glioblastoma multiforme (GBM) cells alter the mode of CAM-DR. In the absence of a microenvironmental cue provided by components of the extracellular matrix (ECM), GBM cells are able to employ an alternative, but equally effective, mode of CAM-DR by forming spheres via cell-cell interactions. Intriguingly, when inhibiting cell-cell interactions in the absence of ECM components, either by low cell density or by inhibition of gap junctions (intercellular connexin tunnels) through chemical inhibition with carbenoxyolone or co-incubation with the connexin-mimicking Gap 27 Cx37,43 peptide, GBM cells were sensitized to tumor necrosis factor-related apoptosis-inducing ligand- and CD95-induced apoptosis. By demonstrating that GBM cells can alternate from one form of CAM-DR (cell-substrate tethering) to another (homocellular cell-cell adhesion) and that inhibition of both forms is necessary for apoptosis sensitization, our findings not only have important implications for novel approaches to restore defective apoptosis programs, but also reveal a novel role of gap junctions in GBM.