Fig. 4. Enhanced activity of caspase-9 after treatment with a low dose of STP combined with IR. Caspase-9 (A), caspase-8 (B), and caspase-1 (C) activities were determined 6 h after treatment using caspase-specific peptide substrates (Ac-LEHD-pNA, Ac-IETD-pNA, and Ac-YVAD-pNA, respectively) in cellular extracts derived from control cells (CTL) or from cells treated with STP (20 nM), IR (5 Gy), or both.