Proteasome inhibition is a promising approach for cancer therapy. However, the mechanisms involved have not been fully elucidated. Gap junctions play important roles in the regulation of tumor cell phenotypes and mediation of the bystander effect in cancer therapy. Because the degradation of gap junction proteins involves the proteasome, we speculated that altered gap junctions might contribute to the antitumor activities of proteasome inhibition. Incubation of Hepa-1c1c7 cells with the proteasome inhibitor MG132 elevated the levels of gap junction protein connexin 43 (Cx43) and promoted gap junctional intercellular communication. This was associated with a marked accumulation of ubiquitylated Cx43 and a significantly decreased rate of Cx43 degradation. The elevated Cx43 contributed to MG132-induced cell apoptosis. This is shown by the observations that: (i) overexpression of Cx43 in the gap junction-deficient LLC-PK1 cells rendered them vulnerable to MG132-elicited cell injury; (ii) fibroblasts derived from Cx43-null mice were more resistant to MG-132 compared with Cx43 wild-type control; and (iii) the gap junction inhibitor flufenamic acid significantly attenuated cell damage caused by MG132 in Hepa-1c1c7 cells. Further studies demonstrated that MG132 activates endoplasmic reticulum stress. Exposure of cells to the endoplasmic reticulum stress inducers thapsigargin and tunicamycin also led to cell apoptosis, which was modulated by Cx43 levels in a way similar to MG132. These results suggested that elevated Cx43 sensitizes cells to MG132-induced cell apoptosis. Regulation of gap junctions could be an important mechanism behind the antitumor activities of proteasome inhibitors.