Activation of caspase-2 mediates the apoptosis induced by GTP-depletion in insulin-secreting (HIT-T15) cells

This study investigated the possible involvement of a specific caspase(s) (a family of aspartate-specific cysteine proteases) in programmed cell death of islet beta-cells due to sustained GTP depletion. Treatment (up to 48 h) with 3 mug/ml mycophenolic acid (MPA), which specifically depletes intracellular guanine nucleotides, reduced cell-cycle progression from G1 phase into S and G2/M phases (as assessed by flow cytometry) and, subsequently, induced apoptosis of HIT-15 cells (transformed pancreatic beta-cells). The latter was accompanied by a marked increase of caspase-2 activity (+343%) and moderate activation of caspase-9 (+150%) and caspase-3 (+145%). Importantly, only caspase-2 activation preceded induction of apoptosis. There was no change in activity of caspase-1, -4, -5, -6, and -8. Release of the mitochondrial protein cytochrome c into cytosol was also observed at a late stage. Cotreatment of cells with a permeable pan-caspase inhibitor (Z-VAD-FMK) blocked GTP depletion-induced cell death in a dose-dependent manner. A specific caspase-2 inhibitor (Z-VDVAD-FMK), but not a caspase-3 inhibitor (DEVD-CHO), was also capable of restoring cell viability. Interestingly, activation of caspase-2 leads to caspase-3 activation because the caspase-2 inhibitor abrogated caspase-3 activity. Our results indicate that, while activation of multiple caspases are involved in the execution phase of GTP depletion-induced apoptosis, caspase-2 appears to play the major role in the initiation of this program. This study revealed a novel, caspase-2 mediated form of apoptosis that may be consequent to impaired mitogenesis.