TGFβ-mediated apoptosis of Burkitt's lymphoma BL41 cells is associated with the relocation of mitochondrial BimEL

Caspase-3 triggers a TPCK-sensitive protease pathway leading to degradation of the BH3-only protein puma

The protein Puma (p53-upregulated modulator of apoptosis) belongs to the BH3-only group of the Bcl-2 family and is a major regulator of apoptosis. Although the transcriptional regulation of Puma is clearly established, little is known about the regulation of its expression at the protein levels. We show here that various signals--including the cytokine TGFβ, the death effector TRAIL or chemical drugs such as anisomycin--downregulate Puma protein levels via a novel pathway based on the sequential activation of caspase-3 and a protease inhibited by the serpase inhibitor N-tosyl-L-phenylalanine chloromethyl ketone. This pathway is specific for Puma because (1) the levels of other BH3-only proteins, such as Bim and Noxa were not modified by these stimuli and (2) this caspase-mediated degradation was dependent on both the BH3 and C-terminal domains of Puma. Our data also show that Puma is regulated during the caspase-3-dependent differentiation of murine embryonic stem cells and suggest that this pathway may be relevant and important during caspase-mediated cell differentiation not associated with apoptosis.

Targeting the BCL-2 family in malignancies of germinal centre origin

The germinal centre is a dynamic microenvironment where B-cell responses are honed. Antigen-specific cells undergo clonal expansion followed by antibody affinity maturation and class switching through somatic hypermutation and recombination of immunoglobulin genes respectively. The huge proliferative capacity of the B-cells and the potential for generating non-functional or autoreactive immunoglobulins, necessitate strict control measures. Pro-apoptotic signalling pathways via B-cell receptors, FAS and the TGF-beta receptor, ALK5, ensure that apoptosis of germinal centre B-cells is the norm and cells only survive to differentiate fully if they receive sufficient pro-survival signals to overcome their 'primed for death' status. Several of the B-cell signalling pathways converge on the intrinsic apoptotic machinery to control expression of the BCL-2 family of apoptosis regulators including BIM, the pro-survival factor BCL-X(L) and the BH3-only protein, BIK (recently identified as a mediator of a TGF-beta-induced default apoptosis pathway in human B-cells). It is a foreseeable hazard that cells undergoing genetic mutation and recombination events might unintentionally target some of these factors, resulting in defective programmed cell death. Here we discuss the function of BCL-2 family proteins in germinal centre reactions, their deregulation in malignancies of germinal centre origin, and the potential for targeting BCL-2-related proteins therapeutically in lymphomas.

Mcl-1(128-350) fragment induces apoptosis through direct interaction with Bax

Mcl-1 full-length (Mcl-1(1-350)), a tightly regulated protein, plays an important role in protecting cells against apoptosis. Cleavage of Mcl-1 at Asp127 by caspase (Mcl-1(C1)) contributes to the regulation of Mcl-1 expression, but its pro-apoptotic function remains controversial. Here, we reported that Mcl-1(128-350) expression induced caspase-dependent apoptosis. We demonstrated that Mcl-1(128-350) but not Mcl-1(1-350) interacts with Bax. This interaction required an intact BH3 Mcl-1(128-350) domain and leads to Bax activation and translocation to mitochondria. The silencing of Bax, but not of Bak, prevented Mcl-1(128-350) induced apoptosis. In conclusion, Mcl-1(128-350) exerts a pro-apoptotic function governed by its capacity to interact with Bax.

BimL upregulation induced by BCR cross-linking in BL41 Burkitt's lymphoma results from a splicing mechanism of the BimEL mRNA

B lymphocyte receptor-mediated apoptosis is associated with increased expression of the BimL isoform of Bim. The mechanisms involved in the regulation of BimL protein expression are still unknown. We report that BimL expression following BCR activation is not associated with a specific increase of BimL mRNA but rather to the intron retention structure of the BimEL mRNA. Indeed, expression of a BimEL cDNA leads in Hela cells leads to the production of both BimEL and BimL proteins. Mutation of the intron-splicing GT sequence present in the exon 3 results in the production of only BimEL protein. Ectopic expression of BimEL cDNA resulted in a large increase of BimL expression upon BCR-stimulation, whereas cells transfected with the GT/AA mutated form of BimEL only produced BimEL proteins upon BCR-activation. These data showed that BimL expression induced by BCR activation may result from the splicing of BimEL mRNA independently of Bim promoter regulation.

Activation of the lytic program of the Epstein-Barr virus in Burkitt's lymphoma cells leads to a two steps downregulation of expression of the proapoptotic protein BimEL, one of which is EBV-late-gene expression dependent

The Epstein-Barr virus (EBV) generally latently infects its target cells with expression of genes conferring resistance to apoptosis. However, the modulation of apoptotic signals during lytic cycle remains poorly understood. We show here that resulting from viral reactivation in the EBV-positive Mutu-I and Akata Burkitt's lymphoma cell lines, a two steps proteasome-dependent downregulation of expression of the proapoptotic protein BimEL occurs. The first drop might be EBV-independent, is ERK 1/2 dependent, and BimEL is phosphorylated on Ser69. A second dramatic drop of the BimEL level observed during the lytic cycle is dependent of EBV-late-gene expression, ERK 1/2 independent, and no further phosphorylation of BimEL on Ser69 occurred. These results demonstrate for the first time, that the lytic cycle contributes to downregulation of BimEL and then could add to protection against apoptosis.