Calpains are cysteine proteases that have been implicated as both effectors and suppressors of apoptosis. Previously, we showed that c-myc transformation regulated calpain activity and sensitized cells to apoptosis induced by calpain inhibition. The objective of this study was to investigate the role of calpain in the Eμ-myc transgenic model of B-cell lymphoma. Calpain activity assays, apoptosis, cell cycle assays, and expression measurements were used to determine the activity and role of calpain in vitro and in vivo. We found that Eμ-myc transgenic cells have highly elevated calpain activity. Calpastatin, the negative calpain regulator, was expressed at much lower levels in Eμ-myc lymphoma cells compared to normal splenic B cells. The primary isoform in Eμ-myc lymphoma is calpain 1. Treatment of Eμ-myc lymphoma cells with the calpain inhibitors PD150606 or calpain inhibitor III induced caspase-3-dependent apoptosis in vitro. General caspase inhibitors or caspase-3/7 inhibitor protected cells from death induced by calpain inhibitor, whereas caspase-9 inhibitors failed to rescue cells. Human Burkitt's lymphoma (BL2) cells display a pattern of sensitivity and caspase-3 dependence similar to calpain inhibition. Treatment of Eμ-myc lymphoma-bearing mice with PD150606 inhibited calpain activity in vivo and induced cell death in these cells as determined by terminal deoxynucleotide transferase-mediated dUTP nick-end labeling staining. Multiple daily treatments resulted in reduced tumor load, particularly in combination with etoposide. In conclusion, calpain is highly elevated in the Eμ-myc lymphoma and calpain inhibition has therapeutic potential.
