NPM-ALK Modulates the p53 Tumour Suppressor Pathway in a JNK and PI 3-Kinase Dependent Manner: MDM-2 Is a Potential Therapeutic Target for the Treatment of ALK-Expressing Malignancies

Anaplastic large cell lymphoma (ALCL) is in the majority of cases a paediatric disease of a T- or null-cell phenotype and is characterised by the presence of the t(2;5)(p23;q35) or variant translocations involving the ALK gene on chromosome 2. This chromosomal translocation generates the Nucleophosmin-Anaplastic Lymphoma Kinase (NPM-ALK) fusion protein, a hyperactive kinase with transforming properties. The p53 tumour suppressor gene is rarely mutated in ALK-expressing ALCL, perhaps one reason why this disease has a good prognosis. However, the mechanism controlling p53 activity in ALCL has not been fully elucidated. We show in patient-derived ALCL cell lines and NPM-ALK transformed BaF3 cells that NPM-ALK induces post-translational modification of the p53 antagonist MDM2, leading to inactivation of the p53 tumour suppressor pathway. Furthermore, we demonstrate that the PI 3-Kinase-Akt pathway downstream of NPM-ALK is responsible for this activity. It therefore follows that inactivation of MDM2 with the specific inhibitor nutlin-3 results in a decrease in proliferation and subsequently apoptosis of NPM-ALK-expressing ALCL cells, a response that is enhanced when cells are exposed to nutlin-3 in conjunction with the PI 3-Kinase inhibitor LY294003. We also demonstrate that NPM-ALK activates JNK by phosphorylation in turn leading to JNK-mediated sequestration and degradation of p53. This activity can be attenuated following administration of a specific JNK inhibitor. We conclude that NPM-ALK regulates the activity of the p53 tumour suppressor pathway via sequestration by JNK and MDM2 leading to its degradation. MDM2 antagonists in combination with JNK/PI 3-Kinase inhibitors may therefore be potential targets for the treatment of ALK-expressing malignancies such as ALCL.