Apoptosis induced by the fungal pathogen gliotoxin requires a triple phosphorylation of Bim by JNK

We previously reported that gliotoxin (GT), the major virulence factor of the mold Aspergillus fumigatus causing invasive aspergillosis (IA) in immunocompromised patients, induces apoptosis in a Bak-dependent manner. The signaling pathway leading to Bak activation and subsequent mitochondrial outer membrane permeabilization (MOMP) is elusive. Here, we show that GT and the supernatant of A. fumigatus (but not its GT-defective mutant) activate the JNK pathway and require a co-operative JNK-mediated Bim(EL) phosphorylation at three sites (S100, T112 and S114) to induce apoptosis in mouse fibroblasts, human bronchial and mouse alveolar epithelial cells. Cells (i) treated with the JNK inhibitor SP600125, (ii) deleted or knocked down for JNK1/2 or Bim or (iii) carrying the Bim(EL) triple phosphomutant S100A/T112A/S114A instead of wild-type Bim(EL) are similarly resistant to GT-induced apoptosis. Triple-phosphorylated Bim(EL) is more stable, redistributes from a cytoskeletal to a membrane fraction, better interacts with Bcl-2 and Bcl-x(L) and more effectively activates Bak than the unphosphorylated mutant. These data indicate that JNK-mediated Bim(EL) phosphorylation at S100, T112 and S114 constitutes a novel regulatory mechanism to activate Bim in response to apoptotic stimuli.