Survivin regulates hematopoietic progenitor cell proliferation through p21(WAF1/Cip1)-dependent and -independent pathways

The cyclin-dependent kinase inhibitor p21(WAF1/Cip1) and Survivin enhance granulocyte macrophage colony-forming unit (CFU-GM) cell cycle and proliferation and have been implicated as antiapoptotic proteins. We investigated the relationships between p21 and Survivin in primary CFU-GM and c-kit(+), lineage-negative (Lin(-)) cells and demonstrate p21-dependent and -independent pathways whereby Survivin regulates progenitor cell proliferation. Ectopic Survivin enhanced p21(+/+) CFU-GM formation and expansion of c-kit(+), Lin(-) cells, whereas p21 gene loss abrogated these effects, indicating a p21 requirement. A dominant-negative form of Survivin and p21 gene deletion accelerated the loss of CFU-GM upon growth factor deprivation, and wild-type Survivin overexpression inhibited apoptosis of p21(+/+) CFU-GM and c-kit(+), Lin(-) cells but not p21(-/-) cells, suggesting that both Survivin and p21 block apoptosis of progenitors and that Survivin-mediated antiapoptosis requires p21. In contrast to the p21-dependent antiapoptotic effects, Survivin increased the proportion of CFU-GM in S-phase in both p21(+/+) and p21(-/-) cells. Furthermore, modulating Survivin expression increased polyploidy in c-kit(+), Lin(-) cells, which was accentuated by p21 deficiency. These results suggest that the Survivin-p21 axis plays an important role in the proliferation of normal hematopoietic cells and that Survivin regulates apoptosis through a p21(WAF1/Cip1)-dependent pathway but may control S-phase entry independent of p21.