Sorafenib induces cell death in chronic lymphocytic leukemia by translational downregulation of Mcl-1.

Chronic lymphocytic leukemia (CLL) has a high prevalence in western countries and remains incurable to date. Here, we provide evidence that the multikinase inhibitor sorafenib induces apoptosis in primary CLL cells. This strong pro-apoptotic effect is not restricted to any subgroup of patients, based on Binet stage and the expression of ZAP70 or CD38. Mechanistically, sorafenib-induced cell death is preceded by a rapid downregulation of Mcl-1 through the inhibition of protein translation. Subsequently, the cell intrinsic apoptotic pathway is activated, indicated by destabilization of the mitochondrial membrane potential and activation of caspase-3 and -9. In contrast to sorafenib, the monoclonal vascular epidermal growth factor (VEGF)-antibody bevacizumab failed to induce apoptosis in CLL cells, suggesting that sorafenib induces cell death irrespectively of VEGF signalling. Notably, although sorafenib inhibits phosphorylation of the Scr-kinase Lck, knock-down of Lck did not induce apoptosis in CLL cells. Of note, the pro-apoptotic effect of sorafenib is not restricted to cell-cycle arrested cells, but is also maintained in proliferating CLL cells. In addition, we provide evidence that sorafenib can overcome drug resistance in CLL cells protected by microenvironmental signals from stromal cells. Conclusively, sorafenib is highly active in CLL and may compose a new therapeutic option for patients who relapse after immunochemotherapy.

Recruitment of PKC-betaII to lipid rafts mediates apoptosis-resistance in chronic lymphocytic leukemia expressing ZAP-70.

ZAP-70 is a key signaling molecule in T cells. It couples the antigen-activated T-cell receptor to downstream signaling pathways. Its expression in leukemic B-cells derived from a subgroup of patients with chronic lymphocytic leukemia (CLL) is associated with an aggressive course of the disease. However, its implication for the pathogenesis of aggressive CLL is still unclear. In this study, we show that the expression of ZAP-70 enhances the signals associated with the B-cell receptor, recruiting protein kinase C-betaII (PKC-betaII) into lipid raft domains. Subsequently, PKC-betaII is activated and shuttles from the plasma membrane to the mitochondria. We unravel that the antiapoptotic protein Bcl-2 and its antagonistic BH3-protein Bim(EL) are putative substrates for PKC-betaII. PKC-betaII-mediated phosphorylation of Bcl-2 augments its antiapoptotic function by increasing its ability to sequester more pro-apoptotic Bim(EL.) In addition, the phosphorylation of Bim(EL) by PKC-betaII leads to its proteasomal degradation. These changes confer leukemic cells to a more antiapoptotic state with aggressiveness of the disease. Most importantly, these molecular changes can be therapeutically targeted with the small molecule inhibitor Enzastaurin. We provide evidence that this compound is highly active in leukemic cells and augments the cytotoxic effects of standard chemotherapeutic drugs.

Mechanisms of apoptosis-induction by rottlerin: therapeutic implications for B-CLL.

Constitutively activated signaling pathways contribute to the apoptosis-defect of B-CLL cells. Protein kinase C-delta is a permanently activated kinase and a putative downstream target of phosphatidylinositol-3 kinase in B-CLL. Blockade of protein kinase C-delta (PKC-delta) by the highly specific inhibitor rottlerin induces apoptosis in chronic lymphocytic leukaemia (CLL) cells. By co-culturing bone marrow stromal and CLL cells, we determined that the proapoptotic effect of rottlerin is not abolished in the presence of survival factors, indicating that a targeted therapy against PKC-delta might be a powerful approach for the treatment of CLL patients. The downstream events following rottlerin treatment engage mitochondrial and non-mitochondrial pathways and ultimately activate caspases that execute the apoptotic cell death. Herein we report that the inhibition of PKC-delta decreases the expression of the important antiapoptotic proteins Mcl-1 and XIAP accompanied by a loss of the mitochondrial membrane potential Deltapsi. In addition, we discovered that ZAP-70-expressing cells are significantly more susceptible to rottlerin-induced cell death than ZAP-70 negative cells. We finally observed that rottlerin can augment cell toxicity induced by standard chemotherapeutic drugs. Conclusively, PKC-delta is a promising new target in the combat against CLL.

Constitutive activation of the MAPkinase p38 is critical for MMP-9 production and survival of B-CLL cells on bone marrow stromal cells.

In the present work we investigated the role and biological significance of mitogen activated protein kinases (MAPK) in B-cell chronic lymphocytic leukaemia (B-CLL). The MAPK p38 was constitutively activated in B-CLL, but not in normal peripheral B cells. In addition, we demonstrated that the upstream kinases of p38, MKK3/6 were also constitutively activated in B-CLL cells. Furthermore, we determined by EMSA that the p38 MAP kinase pathway was not linked to the constitutive high expression of NF-kappaB, a critical survival factor of B-CLL cells. Recently, it has been shown that serum levels of angiogenic factors like VEGF, bFGF and MMP-9 are elevated in the serum of CLL patients and correlate with an unfavorable prognosis. We showed that the constitutive expression of MMP-9 was dependent on p38-activity and inhibition of p38 strongly downregulated MMP-9 expression. Coculture of B-CLL cells and stromal cells can protect spontaneous apoptosis of leukemic B cells. To determine the role of permanently activated p38 and MMP-9 expression, we cocultured B-CLL cells with bone marrow stromal cells. Survival of B-CLL cells on stroma was severely impaired when p38 was inhibited. Furthermore, blockade of MMP-9 activity also antagonized the antiapoptotic effect of stromal cells.

Cyclin-dependent kinase inhibitor Roscovitine induces apoptosis in chronic lymphocytic leukemia cells.

A new class of cell cycle inhibitors is currently entering clinical trials. These drugs exert their activity by inhibition of cyclin-dependent kinases (cdk) and induce cell cycle arrest and apoptosis in cancer cells. Roscovitine, a cdk2-inhibitor that is in preclinical evaluation, induced apoptosis in B-CLL cells at doses that were not cytotoxic for normal human B cells. At 20 microM, Roscovitine induced apoptosis in 21 of 28 B-CLL samples and was equally effective in zap-70-positive or -negative samples. Caspase-3 was cleaved in B-CLL cells exposed to Roscovitine and the pancaspase inhibitor z.VAD.fmk-blocked Roscovitine-induced apoptosis. Expression of the proapoptotic protein Bak was increased and Bax cleavage and conformational change was observed in Roscovitine-treated B-CLL cells. Antiapoptotic proteins Mcl-1 and XIAP were downregulated, but the expression of Bcl-2 remained unchanged. In contrast to previous reports in cancer cell lines, Roscovitine treatment was not accompanied by nuclear accumulation of p53. Cyc202 (R-Roscovitine) is in early clinical trials in cancer patients. Given its powerful effects on zap-70-positive and -negative B-CLL cells, but not on normal lymphocytes, Roscovitine might be an attractive drug to be tested in this incurable disease.