cytochrome-c-t and Lymphoma--Follicular

Despite the wide use of anti-CD20 antibody rituximab in the cancer treatment of B cell malignancies, the signalling pathways of CD20-induced apoptosis are still not understood. By using dominant negative (DN)-caspase-9 overexpressing follicular lymphoma cells we demonstrated that the activation of caspase-9 was essential for rituximab-mediated apoptosis. The death receptor pathway mediated by caspase-8 activation was not involved in rituximab-mediated apoptosis since overexpression of FLIP(short) or FLIP(long) proteins, inhibitors of caspase-8 activation, could not inhibit rituximab-induced apoptosis. However, the death receptor pathway activation by anti-Fas antibodies showed an additive effect on rituximab-induced apoptosis. The stabilisation of the mitochondrial outer membrane by Bcl-x(L) overexpression inhibited cell death, showing the important role of mitochondria in rituximab-induced apoptosis. Interestingly, the rituximab-induced release of cytochrome c and collapse of mitochondrial membrane potential were regulated by caspase-9. We suggest that caspase-9 and downstream caspases may feed back to mitochondria to amplify mitochondrial disruption during intrinsic apoptosis.

Topics: Antibodies, Monoclonal; Antibodies, Monoclonal, Murine-Derived; Apoptosis; bcl-X Protein; Caspase 9; CD40 Antigens; Cell Line, Tumor; Cytochromes c; Enzyme Activation; fas Receptor; Humans; Kinetics; Lymphoma, Follicular; Mitochondria; Receptors, Death Domain; Rituximab

Glucocorticoids are commonly used in the treatment of various lymphoid malignancies. In the present study, we show that dexamethasone (Dex) induced depolarization of mitochondrial membrane, release of cytochrome c and DNA fragmentation in a human follicular lymphoma cell line, HF28RA. New protein synthesis was required before Dex-induced mitochondrial changes, and the kinetics of the apoptotic events correlated with the upregulation of the Bim protein. Furthermore, we studied whether specific inhibitors of known survival pathways would potentiate Dex-induced apoptosis. Our results show that inhibition of PKC and ERK pathways had no effect on apoptosis. In contrast, inhibition of PI3-kinase or Akt markedly enhanced Dex-induced apoptosis. The enhancement was seen at the mitochondrial level, and the kinetics of apoptosis was notably accelerated. In addition, inhibition of PI3-kinase did not alter levels of Bax, Bcl-2, Bcl-X(L) or Bim proteins in mitochondria but caused translocation of the pro-apoptotic protein Bad to mitochondria. However, inhibition of PI3-kinase-Akt pathway and subsequent translocation of Bad to mitochondria did not induce apoptosis itself. Based on these results and our current understanding of Bim and Bad action, it seems that both proteins play a synergistic role in this process. Thus, these results indicate that inhibitors of PI3-kinase-Akt pathway might be combined in future with glucocorticoids to improve the treatment of lymphoid malignancies.

Topics: Anti-Inflammatory Agents; Apoptosis; Apoptosis Regulatory Proteins; bcl-2-Associated X Protein; Bcl-2-Like Protein 11; bcl-Associated Death Protein; bcl-X Protein; Caspases; Cytochromes c; Dexamethasone; Enzyme Activation; Humans; Lymphoma, Follicular; Membrane Potentials; Membrane Proteins; Mitochondria; Mitogen-Activated Protein Kinases; Phosphatidylinositol 3-Kinases; Phosphoinositide-3 Kinase Inhibitors; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-akt; Proto-Oncogene Proteins c-bcl-2; Signal Transduction; Tumor Cells, Cultured