Apoptosis and autophagy have opposite roles on imatinib-induced K562 leukemia cell senescence

Imatinib, the anti-Abl tyrosine kinase inhibitor used as first-line therapy in chronic myeloid leukemia (CML), eliminates CML cells mainly by apoptosis and induces autophagy. Analysis of imatinib-treated K562 cells reveals a cell population with cell cycle arrest, p27 increase and senescence-associated beta galactosidase (SA-β-Gal) staining. Preventing apoptosis by caspase inhibition decreases annexin V-positive cells, caspase-3 cleavage and increases the SA-β-Gal-positive cell population. In addition, a concomitant increase of the cell cycle inhibitors p21 and p27 is detected emphasizing the senescent phenotype. Inhibition of apoptosis by targeting Bim expression or overexpression of Bcl2 potentiates senescence. The inhibition of autophagy by silencing the expression of the proteins ATG7 or Beclin-1 prevents the increase of SA-β-Gal staining in response to imatinib plus Z-Vad. In contrast, in apoptotic-deficient cells (Bim expression or overexpression of Bcl2), the inhibition of autophagy did not significantly modify the SA-β-Gal-positive cell population. Surprisingly, targeting autophagy by inhibiting ATG5 is accompanied by a strong SA-β-Gal staining, suggesting a specific inhibitory role on senescence. These results demonstrate that in addition to apoptosis and autophagy, imatinib induced senescence in K562 CML cells. Moreover, apoptosis is limiting the senescent response to imatinib, whereas autophagy seems to have an opposite role.