Loss of p21 disrupts p14ARF-induced G1 cell cycle arrest but augments p14ARF-induced apoptosis in human carcinoma cells

The human INK4a locus encodes two structurally unrelated tumor suppressor proteins, p16(INK4a) and p14(ARF) (p19(ARF) in the mouse), which are frequently inactivated in human cancer. Both the proapoptotic and cell cycle-regulatory functions of p14ARF were initially proposed to be strictly dependent on a functional p53/mdm-2 tumor suppressor pathway. However, a number of recent reports have implicated p53-independent mechanisms in the regulation of cell cycle arrest and apoptosis induction by p14ARF. Here, we show that the G1 cell cycle arrest induced by p14ARF entirely depends on both p53 and p21 in human HCT116 and DU145 carcinoma cells. In contrast, neither loss of p53 nor p21 impaired apoptosis induction by p14ARF as evidenced by nuclear DNA fragmentation, phosphatidyl serine exposure, and caspase activation, which included caspase-3/7- and caspase-9-like activities. However, lack of functional p21 resulted in the accumulation of cells in G2/Mphase of the cell cycle and markedly enhanced p14(ARF)-induced apoptosis that was, nevertheless, efficiently inhibited by the cell permeable broad-spectrum caspase inhibitor zVAD-fmk (valyl-alanyl-aspartyl( O)-methyl)-fluoromethylketone). Thus, loss of cell cycle restriction point control in the absence of p21 may interfere with p14(ARF)-induced apoptosis. Finally, these data indicate that the signaling events required for G1 cell cycle arrest and apoptosis induction by p14ARF dissociate upstream of p53.