Susanne Schnabl

Diverse marrow stromal cells protect CLL cells from spontaneous and drug-induced apoptosis: development of a reliable and reproducible system to assess stromal cell adhesion-mediated drug resistance

Marrow stromal cells (MSCs) provide important survival and drug resistance signals to chronic lymphocytic leukemia (CLL) cells, but current models to analyze CLL-MSC interactions are heterogeneous. Therefore, we tested different human and murine MSC lines and primary human MSCs for their ability to protect CLL cells from spontaneous and drug-induced apoptosis. Our results show that both human and murine MSCs are equally effective in protecting CLL cells from fludarabine-induced apoptosis. This protective effect was sustained over a wide range of CLL-MSC ratios (5:1 to 100:1), and the levels of protection were reproducible in 4 different laboratories. Human and murine MSCs also protected CLL cells from dexamethasone- and cyclophosphamide-induced apoptosis. This protection required cell-cell contact and was virtually absent when CLL cells were separated from the MSCs by micropore filters. Furthermore, MSCs maintained Mcl-1 and protected CLL cells from spontaneous and fludarabine-induced Mcl-1 and PARP cleavage. Collectively, these studies define common denominators for CLL cocultures with MSCs. They also provide a reliable, validated tool for future investigations into the mechanism of MSC-CLL cross talk and for drug testing in a more relevant fashion than the commonly used suspension cultures.

Reconstitution of PTEN activity by CK2 inhibitors and interference with the PI3-K/Akt cascade counteract the antiapoptotic effect of human stromal cells in chronic lymphocytic leukemia

Evidence suggests that tumor microenvironment is critically involved in supporting survival of chronic lymphocytic leukemia (CLL) cells. However, the molecular mechanisms of this effect and the clinical significance are not fully understood. We applied a microenvironment model to explore the interaction between CLL cells and stromal cells and to elucidate the role of phosphatidylinositol 3 kinase (PI3-K)/Akt/phosphatase and tensin homolog detected on chromosome 10 (PTEN) cascade in this process and its in vivo relevance. Primary human stromal cells from bone marrow, lymph nodes, and spleen significantly inhibited spontaneous apoptosis of CLL cells. Pan-PI3-K inhibitors (LY294002, wortmannin, PI-103), isotype-specific inhibitors of p110α, p110β, p110γ, and small interfering RNA against PI3-K and Akt1 counteracted the antiapoptotic effect of the stromal cells. Induction of apoptosis was associated with a decrease in phosphatidylinositol-3,4,5-triphosphate, PI3-K-p85, and dephosphorylation of phosphatidylinositol-dependent kinase-1 (PDK-1), Akt1, and PTEN. Freshly isolated peripheral blood mononuclear cells from patients with CLL (n = 44) showed significantly higher levels of phosphorylated Akt1, PDK-1, PTEN, and CK2 than healthy persons (n = 8). CK2 inhibitors (4,5,6,7-tetrabromo-1H-benzotriazole, apigenin, and 5,6-dichloro-1-β-D-ribofuranosylbenzimidazol) decreased phosphorylation of PTEN and Akt, induced apoptosis in CLL cells, and enhanced the response to fludarabine. In conclusion, bone marrow microenvironment modulates the PI3-K/Akt/PTEN cascade and prevents apoptosis of CLL cells. Combined inhibition of PI3-K/Akt and recovery of PTEN activity may represent a novel therapeutic concept for CLL.

Gliotoxin is a potent NOTCH2 transactivation inhibitor and efficiently induces apoptosis in chronic lymphocytic leukaemia (CLL) cells

Chronic lymphocytic leukaemia (CLL) cells express constitutively activated NOTCH2 in a protein kinase C (PKC)– dependent manner. The transcriptional activity of NOTCH2 correlates not only with the expression of its target gene FCER2 (CD23) but is also functionally linked with CLL cell viability. In the majority of CLL cases, DNA‐bound NOTCH2 complexes are less sensitive to the γ‐secretase inhibitor (GSI) DAPT. Therefore, we searched for compounds that interfere with NOTCH2 signalling at the transcription factor level. Using electrophoretic mobility shift assays (EMSA), we identified the Aspergillum‐derived secondary metabolite gliotoxin as a potent NOTCH2 transactivation inhibitor. Gliotoxin completely blocked the formation of DNA‐bound NOTCH2 complexes in CLL cells independent of their sensitivity to DAPT. The inhibition of NOTCH2 signalling by gliotoxin was associated with down regulation of CD23 (FCER) expression and induction of apoptosis. Short time exposure of CLL cells indicated that the early apoptotic effect of gliotoxin is independent of proteasome regulated nuclear factor κB activity, and is associated with up regulation of NOTCH3 and NR4A1 expression. Gliotoxin could overcome the supportive effect of primary bone marrow stromal cells in an ex vivo CLL microenvironment model. In conclusion, we identified gliotoxin as a potent NOTCH2 inhibitor with a promising therapeutic potential in CLL.

NOTCH2 links protein kinase C delta to the expression of CD23 in chronic lymphocytic leukaemia (CLL) cells

One characteristic of chronic lymphocytic leukaemia (CLL) lymphocytes is high expression of CD23, which has previously been identified as a downstream target for NOTCH2 signalling. The mechanisms regulating NOTCH2‐dependent CD23 expression, however, are largely unknown. This study showed that peripheral CLL cells overexpressed transcriptionally active NOTCH2 (N2IC), irrespective of their prognostic marker profile. When placed in culture, NOTCH2 activity was spontaneously decreased in 25 out of 31 CLL cases (81%) within 24 h. DNA‐bound N2IC complexes could be maintained by the protein kinase C (PKC) activator phorbol 12‐myristate 13‐acetate (PMA) or by γ‐interferon (IFN‐γ), two CLL characteristic inducers of CD23 expression. Inhibition of PKC‐δ by RNA interference or by rottlerin antagonised PMA‐induced NOTCH2 activation and also suppressed NOTCH2 activity in CLL cases with constitutively activated NOTCH2 signalling. In 23 out of 29 CLL cases tested (79%), DNA‐bound N2IC complexes were found to be resistant to the γ‐secretase inhibitor (GSI) DAPT, suggesting that GSIs will be only effective in a subset of CLL cases. These data suggest that deregulation of NOTCH2 signalling is critically involved in maintaining the malignant phenotype of CLL lymphocytes and point to a link between PKC‐δ and NOTCH2 signalling in the leukemic cells.

Gliotoxin Targets Nuclear NOTCH2 in Human Solid Tumor Derived Cell Lines In Vitro and Inhibits Melanoma Growth in Xenograft Mouse Model

Deregulation of NOTCH2 signaling is implicated in a wide variety of human neoplasias. The current concept of targeting NOTCH is based on using gamma secretase inhibitors (GSI) to regulate the release of the active NOTCH intracellular domain. However, the clinical outcome of GSI remains unsatisfactory. Therefore we analyzed human solid tumor derived cell lines for their nuclear NOTCH activity and evaluated the therapeutic potential of the NOTCH2 transactivation inhibitor gliotoxin in comparison to the representative GSI DAPT. Electrophoretic mobility shift assays (EMSA) were used as a surrogate method for the detection of NOTCH/CSL transcription factor complexes. The effect of gliotoxin on cell viability and its clinical relevance was evaluated in vitro and in a melanoma xenograft mouse model. Cell lines derived from melanoma (518A2), hepatocellular carcinoma (SNU398, HCC-3, Hep3B), and pancreas carcinoma (PANC1) express high amounts of nuclear NOTCH2. Gliotoxin efficiently induced apoptosis in these cell lines whereas the GSI DAPT was ineffective. The specificity of gliotoxin was demonstrated in the well differentiated nuclear NOTCH negative cell line Huh7, which was resistant to gliotoxin treatment in vitro. In xenotransplanted 518A2 melanomas, a single day dosing schedule of gliotoxin was well tolerated without any study limiting side effects. Gliotoxin significantly reduced the tumor volume in early (83 mm3 vs. 115 mm3, p = 0.008) as well as in late stage (218 mm3 vs. 576 mm3, p = 0.005) tumor models. In conclusion, NOTCH2 appears to be a key target of gliotoxin in human neoplasias and gliotoxin deserves further evaluation as a potential therapeutic agent in cancer management.

Critical assessment of the efficiency of CD34 and CD133 antibodies for enrichment of rabbit hematopoietic stem cells

Rabbits have many hereditary diseases common to humans and are therefore a valuable model for regenerative disease and hematopoietic stem cell (HSC) therapies. Currently, there is no substantial data on the isolation and/or enrichment of rabbit HSCs. This study was initiated to evaluate the efficiency of the commercially available anti‐CD34 and anti‐CD133 antibodies for the detection and potential enrichment of rabbit HSCs from peripheral blood. PBMCs from rabbit and human blood were labelled with different clones of anti‐human CD34 monoclonal antibodies (AC136, 581, and 8G12) and rabbit polyclonal CD34 antibody (pCD34) and anti‐human CD133 monoclonal antibodies (AC133 and 293C3). Flow cytometry showed a higher percentage of rabbit CD34+ cells labelled by AC136 in comparison to the clone 581 and pCD34 (P < 0.01). A higher percentage of rabbit CD133+ cells were also detected by 293C3 compared to the AC133 clone (P < 0.01). Therefore, AC136 clone was used for the indirect immunomagnetic enrichment of rabbit CD34+ cells using magnetic‐activated cell sorting (MACS). The enrichment of the rabbit CD34+ cells after sorting was low in comparison to human samples (2.4% vs. 39.6%). PCR analyses confirmed the efficient enrichment of human CD34+ cells and the low expression of CD34 mRNA in rabbit positive fraction. In conclusion, the tested antibodies might be suitable for detection, but not for sorting the rabbit CD34+ HSCs and new specific anti‐rabbit CD34 antibodies are needed for efficient enrichment of rabbit HSCs.