Sequence-based discovery of a synthetic peptide inhibitor of caspase 6

We recently demonstrated that tumor necrosis factor alpha activates caspase 6, which in turn cleaves transcription factor AP-2 alpha. We mapped the cleavage site at 19 amino acids from the N-terminus at the sequence aspartate-argenine-histidine-aspartate (DRHD). Mutating aspartic acid at position 19 abrogated the cleavage site. From these observations, we hypothesized that the DRHD peptide could act as a caspase 6 inhibitor. To test this hypothesis, the peptide zAsp(Ome)-Arg-His-Asp(Ome)-fluoromethyl ketone (zDRHDfmk) was synthesized. Here we show that zDRHDfmk inhibits TNFalpha-induced caspase 6 activity and apoptosis in breast cancer cells. When compared to other caspase inhibitors, zDRHDfmk inhibited caspase 6 activity more effectively than the general caspase inhibitor zVal-Ala-Lys(Ome)-fluoromethy ketone (zVADfmk) or the caspase 6 inhibitor zVal-Glu-Ile-Asp-(Ome)-fluoromethyl ketone (zVEIDfmk). However, it was less effective in inhibiting TNFalpha-induced apoptosis than zVADfmk or zVEIDfmk, presumably because caspase 6 is only one of at least three effector caspases, the others being caspase 3 and 7, that are active during caspase-dependent apoptosis. The discovery of this sequence-based caspase 6 inhibitor provides a new tool for studying caspase 6. More importantly, it could be used, in combination with other agents, as a drug to inhibit apoptosis in neurodegenrative diseases such as Alzheimer's, Parkinson and amyotrophic lateral sclerosis.