Ras-induzierte Differenzierung nach Schädigung der DNA in Zellen der akuten myeloischen Leukämie

Ras proteins belong to a family of proto-oncogens that encode small GTPases, found to be involved in many cellular processes like cell division, apoptosis and differentiation. Remarkably, 20-30% of all human tumours contain activating mutations in the RAS genes. A retrospective study of a part of patients with acute myeloid leukaemia treated within CALGB 8525 revealed a correlation between RAS-mutations and chemotherapeutic treatment (cytarabine). AML patients harbouring oncogenic RAS showed significantly less cumulative incidence of relapse upon treatment with highdose cytarabine in the post-induction chemotherapy, when compared to AML patients with oncogenic RAS treated with low-dose cytarabine. In contrast, dose escalation had a much weaker effect on the response to cytarabine in patients that harbour wildtype RAS. This study showed that an oncogenic mutation can mediate a beneficial effect towards chemotherapeutic treatment. However, the molecular basis of this observation was not understood. Therefore, the aim of this work was to investigate the effect of oncogenic RAS on response and the molecular mechanisms that mediate this effect in a cell system in vitro. To this end, we took advantage of mouse bone marrow cells immortalized by the MLLENL oncogene. MLL-ENL expressing cells define a leukemia-initiating cell population resembling acute myeloid leukaemia in humans. These cells were co-infected with either an empty vector or a vector expressing oncogenic RAS and subsequently their response to chemotherapeutic substances like cytarabine was tested. The cytotoxic effect of cytarabine is due to replication fork stalling which leads to the activation of the DNA-damage response followed by cell cycle arrest and apoptosis. Therefore, cytarabine targets mainly proliferating cells. There was no difference in response to cytarabine between control cells and RASinfected cells in suspension with regard to cell survival, cell cycle and apoptosis. However, when cells were cultured in semisolid media to evaluate colony formation, the treatment with chemotherapeutic drugs (cytarabine, etoposide, daunorubicine) led to a compromised colony formation only in RAS-infected cells. In order to identify the process responsible for elimination of clonogenic cells upon treatment, the status of apoptosis, senescence and differentiation was checked in these cells. Expression- and flow cytometry-analysis revealed no impact of cytarabine treatment on apoptosis of RAS-infected cells. They rather showed less apoptosis after cytarabine treatment compared to control cells. Therefore, in the RAS expressing cells apoptosis did not account for the reduced clonogenicity observed. Previous work has showed that oncogenic Ras expressing cells activate a DNAdamage response, which subsequently leads to a senescent phenotype. Senescent cells show high expression of the tumour suppressors p53, p21Cip1, p16Ink4a, p19Arf and p15Ink4b and express SA-β-Galactosidase whose activity can be visualized. The present work shows that the checkpoint proteins Chk1, H2A.x and ATM were activated (phosphorylated) only in RAS-infected cells and that treatment with cytotoxic drugs enhanced the phosphorylation of these proteins. Similarly, the expression of p53, p21Cip1, p16Ink4a, p19Arf and p15Ink4b was induced in RAS-infected cells and the levels of p53 and p21Cip1 were further enhanced upon cytarabine treatment. In contrast, cytarabine treatment led to an activation of SA-β-Galactosidase in both control cells and RAS-infected cells, suggesting that cytarabine induces senescence in control cell. The above-mentioned proteins are not only highly expressed in senescent cells but also in differentiated cells. It is also known that, both oncogenic Ras and cytarabine can induce differentiation in hematopoietic cells. Expression- and flow cytometry-analysis for the differentiation markers ly6g (Gr1) and itgam (Mac1) revealed that RAS-infected cells were more differentiated than control cells and that the differentiation was further induced upon cytarabine treatment. Of note, the Ras- and cytarabine induced differentiation was inhibited upon co-treatment with the AMT/R Inhibitor caffeine, suggesting that activation of the DNA damage response pathway is critical for Ras- and cytarabine induced apoptosis. The data of this study point out, that conventional cytostatic drugs can activate a further fail-safe mechanism, differentiation, besides inhibition of proliferation and induction of apoptosis. These in vitro data are consistent with the data obtained in acute promyelocytic leukaemia where induction of differentiation, by given high-doses of retinoic acid concomitantly with chemotherapy, is associated with higher cure rates and therefore is an attractive goal for anticancer therapy. Induction of differentiation is of particular interest to the eradication of tumour-initiating cells, since conventional therapeutics often do not eliminate those cells, therefore causing resistance. Screening of new compounds, which activates the differentiation of tumour-initiating cells, would be an important tool in order to counteract the resistance of tumour cells towards chemotherapeutic drugs.