Inhibition of platelet formation by caspase inhibitors.

CD34⁺ cells purified from marrow were cultured in the presence of TPO. CD34⁺CD41⁺ cells were FACS-purified, as detailed in “Materials and methods.” at day 8 of culture. The sorted cells were further cultured in the presence of TPO with or without caspase inhibitors (Z-VAD.fmk, Z-LEDH.fmk, or Z-DEVD.fmk) or the apoptosis inducer STS. Representative micrographs of MKs at day 8 in the presence of TPO (A); at day 12 in the presence of TPO (B,C [C illustrates a proplatelet-forming MK at day 12 at a higher magnification]); and at day 12 in the presence of TPO plus 50 μM z-VAD.fmk (D). Original magnification, × 100. The kinetics of proplatelet formation in the presence or absence of different caspase inhibitors and STS is also shown (E). Results are means of 3 independent determinations; 3 experiments gave identical results. In panel F, the effects of calpeptin, Z-VAD, DEVD, and LEHD on proplatelet formation are compared at day 12 of culture in 3 experiments.