Proteasome inhibition by MG132 induces growth inhibition and death of human pulmonary fibroblast cells in a caspase-independent manner

MG132 as a proteasome inhibitor that can induce apoptotic cell death in various cell types including lung cancer cells. We investigated the cellular effects of MG132 on human pulmonary fibroblast (HPF) cells in relation to cell growth inhibition and death, and described the molecular mechanisms of MG132 in HPF cell death. This agent dose-dependently inhibited the growth of HPF cells with an IC₅₀ of approximately 20 μM at 24 h and induced cell death accompanied by the loss of mitochondrial membrane potential (MMP; ΔΨ_m) and an increase in caspase-3 and -8 activities. MG132 increased intracellular ROS levels and GSH-depleted cell numbers. However, all the tested caspase inhibitors intensified HPF growth inhibition by MG132 and caspase-9 inhibitor also enhanced cell death and MMP (ΔΨ_m) loss. Moreover, the administration of Bcl-2 siRNA augmented HPF cell death by MG132 whereas p53, Bax, caspase-3 and -8 siRNAs did not strongly affect cell death. In addition, each caspase inhibitor and siRNA differently affects ROS levels including O₂^.- regardless of cell growth inhibition and cell death levels. Caspase-8 and -9 inhibitors increased the number of GSH-depleted cells in MG132-treated HPF cells. In conclusion, MG132 induced growth inhibition and death in HPF cells in a caspase-independent manner. The growth inhibition and death of HPF cells by MG132 and/or each caspase inhibitor or apoptosis-related siRNA were not tightly related to the changes in ROS levels.