Natural killer (NK) cell neoplasms, including extranodal NK/T-cell lymphoma, nasal type (ENKL) and aggressive NK cell leukemia (ANKL), show a highly aggressive clinical course with poor response to chemotherapy, and new treatment approaches are urgently needed to improve cure rates.
Patients with NK cell neoplasms cluster in Asia and Latin American countries, and the frequency of p53 mutations has been reported to be various by district.
We have demonstrated that MDM2 protein was overexpressed in aggressive subclasses of NK cell neoplasms (Sugimoto et al. Jap J Cancer Res. 2002), which suggests that wild-type p53 expressing malignant NK cells may be a good candidate for biologic therapies that abrogate MDM2-p53 interactions and lead to cell death.
Nutlin-3 is a small-molecule antagonist of MDM2 that efficiently blocks the MDM2-p53 interaction. In this study, we investigated the effects of nutlin-3 in 3 cell lines of ENKL and ANKL with known p53 mutation status (wt-p53: NK-YS, HANK-1; mt-p53: KHYG-1). Since aggressive NK-cell neoplasms arise in hypoxic environments and usually show an angiodestructive-infiltration pattern resulting in the tissue necrosis, we tried to assess the anti-proliferative effects and molecular mechanisms of nutlin-3 in the hypoxic condition. For hypoxia experiments, cells were cultured under 1.0% O2 for at least 14 days to assure their continuous proliferation and survival. Under hypoxia, more cells were positive to Annexin V than in normoxia, indicating that hypoxic conditions promote apoptosis in NK cell neoplasms. Nutlin-3 treatment in normoxia resulted in a reduction of cell proliferation with G0/G1 cell cycle arrest in a time and concentration-dependent manner in wt-p53 cells (IC50 at 48 hrs; 3.2 μM for NK-YS and 5.0 μM for HANK-1, MTT test). In hypoxia, nutlin-3 further enhanced cell growth inhibition and G0/G1 cell cycle arrest. An increase in the specific apoptosis (sub G1 and annexin V positivity) by nutlin-3 was observed with similar level between normoxia and hypoxia. The mt-p53 KHYG-1 cells demonstrated neither cell cycle arrest nor increase in the apoptotic cell fraction after nutlin-3 treatment. In the wt-p53 NK-YS and HANK-1 cells, nutlin-3 treatment increased the cellular levels of p53, and p53 dependent proteins including p21, MDM2 itself and the proapoptotic BH3-only proteins Noxa and Puma followed by the activation of caspase-9 and caspase-3 regardless of foxygen level. We observed no significant increase in the p53 targets in the mt-p53 overexpressing KHYG-1 cells.
L-asparaginase has been demonstrated to induce apoptosis in aggressive NK cell neopplasms. To determine if inhibition of the TP53-MDM2 interaction by nutlin-3 in NK cell neoplasms might potentiate the effects of L-asparaginase, we assessed the effect of combining the two drugs. However, L-asparaginase induced apoptosis only in NK-YS cells, and no synergistic anti-proliferative effect was observed in any of the cell lines analyzed.
These findings demonstrate that nutlin-3 successfully activates wt-p53 in NK cell neoplasms leading to the upregulation of traditional targets such as p21 and proapoptotic proteins including Noxa and Puma, and result in apoptotic cell death regardless of oxygen concentration. The data suggest that p53 activators such as nutlin-3 may be considerable for selected patients with wt-p53 NK cell neoplasms.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.