B-cell chronic lymphocytic leukemia (B-CLL IntroductionThe tumor suppressor TP53 plays an important role in the control of key genes involved in the regulation of DNA repair, cell cycle, and apoptosis. 1,2 p53 is activated in response to DNA damage or other forms of stress, protecting cells from malignant transformation. This is the reason why p53 is frequently inactivated in human cancer. p53 is a short-lived protein, and its cellular level is controlled by the rate at which it is degraded. Although several U3 ubiquitin ligases have been implicated in p53 ubiquitylation and degradation, MDM2 appears to function as a master regulator of p53. 3,4 MDM2 not only facilitates p53 degradation, but it also binds p53 and inhibits its transcriptional activity. Therefore, inhibitors of p53-MDM2 binding are expected to stabilize and activate p53. Recently, the first potent and selective small-molecule antagonists of MDM2, the nutlins, have been shown to activate the p53 pathway in cancer cells with wild-type p53 in vitro and in vivo. 5 B-cell chronic lymphocytic leukemia (B-CLL) is characterized by the accumulation of long-lived CD5 ϩ B lymphocytes. 6 TP53 is mutated in only 5% to 10% of B-CLL cases at diagnosis, but in nearly 30% in chemotherapy-resistant tumors. TP53 mutation is associated with poor clinical outcome, shorter survival, and lack of response to therapy with purine nucleoside analogs or alkylating agents. [7][8][9][10][11] In fact, alterations in the TP53 gene are among the worst prognostic indicators for B-CLL. [12][13][14] Most of the chemotherapeutic drugs currently used induce cell cycle arrest or apoptosis through activation of p53, and p53 inactivation leads to chemoresistance. 1,2 Chemotherapeutic drugs, including purine analogs, topoisomerase inhibitors, and alkylating agents, have been shown to effectively increase p53 levels in B-CLL. 15,16 Thus, p53 activation is considered among the critical molecular events in chemotherapy-induced apoptosis in B-CLL cells. Although TP53 is mutated in only 5% to 10% of patients, the p53 pathway could be altered at a higher frequency, thus effectively attenuating p53 function. One of the mechanisms involved in p53 stabilization in response to DNA damage is its phosphorylation by ataxia telangiectasia mutated (ATM) protein. 1,2 Interestingly, ATM is inactivated in 10% to 20% of B-CLL cases, thus providing an alternative way to disable p53 function. [17][18][19][20] Tumors with alterations upstream of p53 would not respond adequately to genotoxic chemotherapeutics that act through the p53 pathway (eg, alkylating agents such as chlorambucil and cyclophosphamide; purine nucleosides such as fludarabine and cladribine; or topoisomerase inhibitors such as doxorubicin and mitoxantrone). Therefore, new therapies that overcome these For personal use only. on May 11, 2018. by guest www.bloodjournal.org From defects by acting directly on p53 stability may benefit these patients. Nutlins activate p53 by releasing it from MDM2-mediated negative control and thus compensate for d...

Glioblastoma multiforme (GBM) is the most common and aggressive primary brain tumor in adults. Despite concerted efforts to improve current therapies and develop novel clinical approaches, patient survival remains poor. As such, increasing attention has focused on developing new therapeutic strategies that specifically target the apoptotic pathway in order to improve treatment responses. Recently, nutlins, small-molecule antagonists of MDM2, have been developed to inhibit p53-MDM2 interaction and activate p53 signaling in cancer cells. Glioma cell lines and primary cultured glioblastoma cells were treated with nutlin-3a. Nutlin-3a induced p53-dependent G1- and G2-M cell cycle arrest and apoptosis in glioma cell lines with normal TP53 status. In addition, nutlin-arrested glioma cells show morphological features of senescence and persistent induction of p21 protein. Furthermore, senescence induced by nutlin-3a might be depending on mTOR pathway activity. In wild-type TP53 primary cultured cells, exposure to nutlin-3a resulted in variable degrees of apoptosis as well as cellular features of senescence. Nutlin-3a-induced apoptosis and senescence were firmly dependent on the presence of functional p53, as revealed by the fact that glioblastoma cells with knockdown p53 with specific siRNA, or cells with mutated or functionally impaired p53 pathway, were completely insensitive to the drug. Finally, we also found that nutlin-3a increased response of glioma cells to radiation therapy. The results provide a basis for the rational use of MDM2 antagonists as a novel treatment option for glioblastoma patients.