The role of p21waf1/cip1 and p27Kip1 in HDACi-mediated tumor cell death and cell cycle arrest in the Eμ-myc model of B-cell lymphoma

Following the establishment of histone deacetylases (HDACs) as promising therapeutic targets for the reversal of aberrant epigenetic states associated with cancer, the development of HDAC inhibitors (HDACi) and their underlying mechanisms of action has been a significant area of scientific interest. HDACi induce diverse biological responses including the inhibition of cell proliferation by blocking progression through the G1 or G2/M phases of the cell cycle. As a putative tumor-suppressor protein, p21waf1/cip1 influences cell proliferation by inhibiting the activity of cyclin–cyclin-dependent kinase (CDK) complexes at the G1/S and G2/M cell cycle checkpoints. HDACi transcriptionally activate CDKN1A, and it has been proposed that induction of p21waf1/cip1 can determine if a cell undergoes apoptosis or cell cycle arrest following HDACi treatment. In the Eμ-myc transgenic mouse model of B-cell lymphoma, knockout of cdkn1a had no effect on disease latency, indicating that p21waf1/cip1 did not function as a tumor suppressor in this system. Although HDACi robustly induced expression of p21waf1/cip1 in wild-type Eμ-myc lymphomas, deletion of cdkn1a did not sensitize the lymphoma cells to HDACi-induced apoptosis and HDACi-induced cell cycle arrest still occurred. However, knockdown of cdkn1b in cdkn1a knockout lymphomas resulted in defective vorinostat-mediated arrest at G1/S indicating an essential role of p27Kip1 in mediating this biological response to vorinostat. These data demonstrate that induction of cdkn1a does not regulate HDACi-mediated tumor cell apoptosis and refute the notion that p21waf1/cip1 is an obligate mediator of HDACi-induced cell cycle arrest.