Simon D. Wagner

CD154 induces a switch in pro-survival Bcl-2 family members in chronic lymphocytic leukaemia

Chronic lymphocytic leukaemia cells survive and proliferate in patients but rapidly die in culture. The microenvironment that sustains leukaemic cells in vivo contains both stromal cell elements and T cells. We defined changes in Bcl‐2 family protein expression on culture with CD40 ligand (CD154) expressed on mouse fibroblast L cells, and interleukin‐4 (IL‐4; CD154/IL‐4 system): conditions that support survival and proliferation. Unexpectedly, Bcl‐2 protein expression decreased whilst pro‐survival Bcl‐xL (as well as A1 and Mcl‐1) increased. However, the CD154‐L cell/IL‐4 system also increased the pro‐apoptotic proteins, Bid and Noxa, suggesting that an increased pool of pro‐survival factors and not the effects of a single protein mediate survival. Most pro‐apoptotic proteins were not induced in drug or spontaneous apoptosis, but expression of Bcl‐xS, a pro‐apoptotic BCL2L1 isoform, was associated with cell death. This was post‐transcriptionally controlled, and, therefore, alternative splicing at the Bcl‐x locus appears to have a role in the regulation of chronic lymphocytic leukaemia (CLL) cell survival. This study demonstrated a switch in pro‐survival proteins associated with the transition from quiescence to CD154‐driven proliferation. CLL therapies targeting Bcl‐2 may need to be modified to antagonize proliferation centre‐specific pro‐survival proteins.

BCL-6 is expressed in breast cancer and prevents mammary epithelial differentiation

Appropriately timed proliferation, differentiation and apoptosis are essential to the normal functions of the mammary epithelium. Here, we report that the transcription factor BCL-6 is expressed in mammary epithelium in nonpregnant animals as well as during early pregnancy. When overexpressed in the nontransformed EpH4 mammary epithelial cell line, BCL-6 prevents the STAT-driven expression of the milk protein β-casein and duct formation, and prevents apoptosis. Consistent with an antiapoptotic function, we demonstrate that BCL-6 is expressed in 68% of histologically high-grade ductal breast carcinomas, which are clinically the most aggressive. BCL-6 has previously been characterized as a regulator of B lymphocyte growth and development, but our work identifies a novel role for it in mammary epithelial differentiation, which may also implicate it in carcinogenesis.

A peptide aptamer to antagonize BCL-6 function

BCL-6 is a transcription factor essential for germinal centre B-cell development. The BCL-6 gene is involved in diffuse large-cell lymphoma and overexpressed in other types of non-Hodgkins lymphoma and in high-grade breast cancer. BCL-6 is a transcriptional repressor whose N-terminal POZ domain mediates protein–protein interactions to exert its effects. Reasoning that disruption of POZ domain-mediated interactions may be an effective route to antagonizing the effects of BCL-6 in lymphoma, we screened a library for peptide aptamers that specifically bind to BCL-6 POZ and not the POZ domains of related proteins and describe here the first of these reagents, Apt48. Apt48 binds BCL-6 POZ in a manner distinct from the transcriptional corepressor SMRT, yet was found to prevent BCL-6-mediated repression of a luciferase reporter gene. Apt48 also reproduced several previously validated effects of BCL-6 inhibition. Notably, expression of the differentiation markers CD69, Blimp-1 and cyclin D2 was increased in B-cell lines when Apt48 was expressed. We also show that expression of Apt48 restores cytokine-mediated growth arrest to BCL-6 overexpressing cells. Thus, we have identified a peptide aptamer that affects a function of BCL-6 that is required to prevent differentiation of proliferating B cells.

CD49d is an independent prognostic marker that is associated with CXCR4 expression in CLL

The world of chronic lymphocytic leukemia (CLL) research is awash with prognostic markers. However, very few of the current group play a clearly defined role in the pathology of this disease and even fewer represent a tractable therapeutic target. One such marker that fulfils both of these criteria is the integrin CD49d. This molecule been implicated in the capacity of CLL cells to migrate into lymphoid tissues and there is a CD49d blocking antibody, Natalizumab, currently in clinical trials. Here we carried out the largest multi-centre evaluation of CD49d as a prognostic marker in 652 primary CLL samples. We confirm that CD49d is predictive for time to first treatment (P<0.0001) and overall survival (P<0.0001) and increases the prognostic power of CD38, ZAP-70 and IGHV gene mutation status in concordant cases. Furthermore, CD49d retained independent prognostic significance in multivariate analysis. In contrast to previous studies, we showed no correlation between CD49d expression and in vitro resistance to fludarabine in liquid cultures (P=0.28) but CD49d(hi) cells were significantly more resistant than CD49d(lo) cells when assays were carried out on fibronectin-coated plates (P=0.03). Furthermore, we showed for the first time that the expression of CD49d is strongly associated with expression of the chemokine receptor CXCR4 suggesting a co-ordinated role for these molecules in the trafficking of CLL cells to the lymphoid tissues. Taken together, our data support the introduction of CD49d into routine immunophenotyping panels for CLL and indicate that the therapeutic targeting of this molecule may prove useful in this disease.

miR-125b and miR-155 contribute to BCL2 repression and proliferation in response to CD40 ligand (CD154) in human leukemic B-cells.

Background: BCL2 repression occurs in normal and leukemic B-lymphocytes in lymph nodes. Results: miR-155 and miR-125b repress BCL2 translation and, in part, mediate the proliferative response to CD40 ligand (CD154). Conclusion: miR-155 and miR-125b contribute to proliferative responses to CD154 and one of their targets is BCL2. Significance: CD154 drives proliferation of mature B-cell malignancies and abrogating the effects of miRNA induced by CD154 may be therapeutically useful. Developmental stage-specific regulation of BCL2 occurs during B-cell maturation and has a role in normal immunity. CD40 signaling promotes proliferation and rescues B-cells from apoptosis, partly through induction of BCL2L1 and BCL2A1 and repression of BCL2. We previously showed that a stromal cell/CD40 ligand (CD154) culture system reproduced this switch in survival protein expression in primary human leukemic B-cells and we employed this model system to investigate BCL2 repression. BCL2 was post-transcriptionally regulated and the repressed BCL2 mRNA was associated with non-polysomal, but dense fractions on sucrose density gradients. Microarrays identified a set of miRNA that were induced by culture conditions and potentially able to bind to the BCL2 3′-UTR. Luciferase reporter assays demonstrated that miR-125b and miR-155 repressed BCL2 mRNA but while stromal cell contact alone was sufficient to induce strongly miR-125b this did not cause BCL2 repression. miR-155, which is the most abundant miRNA under basal conditions, specifically required CD154 for further induction above a threshold to exert its full repressive effects. Anti-miR-125b and anti-miR-155 prevented CD154-mediated repression of BCL2 and reduced CD154-mediated proliferation in the MEC1 B-cell line. We suggest that miR-155 and miR-125b, which are induced by CD154 and stromal cell signals, contribute to regulating proliferation and that BCL2 is one of their target mRNAs.

Enhancement of CD154/IL4 proliferation by the T follicular helper (Tfh) cytokine, IL21 and increased numbers of circulating cells resembling Tfh cells in chronic lymphocytic leukaemia.

Chronic lymphocytic leukaemia (CLL) cells encounter T‐cells and proliferate in response to T‐cell signals in the lymph node microenvironment. In this report we determined interleukin 21 (IL21) function in CLL and showed that IL21 and interleukin 4 (IL4) act co‐operatively to promote leukaemic cell proliferation without apoptosis or differentiation We further show that IL21 increased side population (SP) cells, which are associated with resistance to chemotherapy and increased self‐renewal capacity in CLL. IL21 and IL4 are the major cytokines produced by the recently described CD4+ T follicular helper (Tfh) cell subset. Determination of Tfh cells in peripheral blood showed that patients had significantly increased numbers as compared to normal subjects although no association was found between Tfh numbers and IGHV gene mutational status or clinical stage. Our data suggests that the Tfh cytokines, IL4 and IL21, contribute to driving leukaemic cell proliferation in the lymph node microenvironment, and may contribute to the specific production of cells resistant to conventional chemotherapy. We suggest that increased circulating Tfh cells is a component of T‐cell dysregulation in CLL. Our findings have implications for the therapeutic use of IL21.

The role of BCL6 in lymphomas and routes to therapy

BCL6 is a transcription factor that has essential B‐cell and T‐cell roles in normal antibody responses. It is involved in chromosomal translocations in diffuse large B‐cell lymphoma (DBCL; including primary mediastinal B‐cell lymphoma) and nodular lymphocyte predominant Hodgkin lymphoma, and is expressed in follicular lymphoma and Burkitts lymphoma. The neoplastic T‐cells of angioimmunoblastic T‐cell lymphoma also express BCL6. BCL6 prevents terminal B‐cell differentiation largely through repression of PRDM1. In the “cell of origin” classification of DLBCL BCL6 is associated with the germinal centre subtype, which carries a good response to modern treatments. More recently, specific BCL6 antagonists, including small molecule inhibitors, have been developed. These antagonists have demonstrated that DLBCL cells, in which BCL6 is transcriptionally active, are dependent on this gene for survival. BCL6 antagonists are active against primary DLBCL and may find future application in the treatment of lymphomas.

Poly-(ADP-ribose) polymerase-1 (Parp-1) binds in a sequence-specific manner at the Bcl-6 locus and contributes to the regulation of Bcl-6 transcription

Bcl-6 is a transcription factor that is normally expressed in germinal centre B cells. It is essential for the formation of germinal centres and the production of high-affinity antibodies. Transcriptional downregulation of Bcl-6 occurs on terminal differentiation to plasma cells. Bcl-6 is highly expressed in B-cell non-Hodgkins lymphoma and, in a subset of cases of diffuse large cell lymphoma, the mechanism of Bcl-6 overexpression involves interruption of normal transcriptional controls. Transcriptional control of Bcl-6 is, therefore, important for normal antibody responses and lymphomagenesis, but little is known of the cis-acting control elements. This report focuses on a region of mouse/human sequence homology in the first intron of Bcl-6, which is a candidate site for such a control element. We demonstrate that poly-(ADP-ribose) polymerase-1 (Parp-1) binds in vitro and in vivo to specific sequences in this region. We further show that PARP inhibitors, and Parp-1 knockdown by siRNA induce Bcl-6 mRNA expression in Bcl-6 expressing cell lines. We speculate that Parp-1 activation plays a role in switching off Bcl-6 transcription and subsequent B-cell exit from the germinal centre.

Regulation of CD38 in proliferating chronic lymphocytic leukemia cells stimulated with CD154 and interleukin-4

Background and Objectives Chronic lymphocytic leukemia (CLL) cells, like normal B-cells, exist in two populations in vivo: quiescent cells in the peripheral circulation and proliferating cells in lymph nodes. The surface marker CD38 has roles in cell adhesion and signaling. Its expression correlates with poor clinical outcome and is associated with expression of the signaling intermediate ZAP-70, which is also a marker of poor prognosis. We investigated the regulation of CD38 and ZAP-70 in proliferating CLL cells. Design and Methods We cultured CLL cells on a stromal cell layer that maintains viability and also with some stromal cells expressing CD40 ligand (CD154) in order to measure changes in expression of CD38 and ZAP-70. Results We demonstrated up-regulation of CD38 expression by CD154. The degree of up-regulation did not correlate with clinical stage or mutational status. In addition in the majority of cases tested ZAP-70 expression increased in parallel with up-regulation of CD38 although discordant cases were also observed. Interpretation and Conclusions Overall we demonstrated that regulation of CD38 in CLL is dynamic and dependent on signals from CD154 and a stromal cell layer. We speculate that CD38 and ZAP-70 are expressed in lymph node leukemic cells in both good and poor prognosis patients, but, in cases with good clinical outcome, these molecules are down-regulated in the peripheral blood whereas in cases with poor prognosis their expression is maintained.

The role of BCL6 in lymphomas and routes to therapy.

BCL6 is a transcription factor that has essential B‐cell and T‐cell roles in normal antibody responses. It is involved in chromosomal translocations in diffuse large B‐cell lymphoma (DBCL; including primary mediastinal B‐cell lymphoma) and nodular lymphocyte predominant Hodgkin lymphoma, and is expressed in follicular lymphoma and Burkitts lymphoma. The neoplastic T‐cells of angioimmunoblastic T‐cell lymphoma also express BCL6. BCL6 prevents terminal B‐cell differentiation largely through repression of PRDM1. In the “cell of origin” classification of DLBCL BCL6 is associated with the germinal centre subtype, which carries a good response to modern treatments. More recently, specific BCL6 antagonists, including small molecule inhibitors, have been developed. These antagonists have demonstrated that DLBCL cells, in which BCL6 is transcriptionally active, are dependent on this gene for survival. BCL6 antagonists are active against primary DLBCL and may find future application in the treatment of lymphomas.