p53-Independent induction of Fas and apoptosis in leukemic cells by an adenosine derivative, Cl-IB-MECA

A(3) adenosine receptor (A(3)AR) agonists have been reported to influence cell death and survival. The effects of an A(3)AR agonist, 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxy-N-methyl-beta-D-ribofuranonamide (Cl-IB-MECA), on apoptosis in two human leukemia cell lines, HL-60 and MOLT-4, were investigated. Cl-IB-MECA (> or =30 microM) increased the apoptotic fractions, as determined using fluorescence-activated cell sorting (FACS) analysis, and activated caspase 3 and poly-ADP-ribose-polymerase. Known messengers coupled to A(3)AR (phospholipase C and intracellular calcium) did not seem to play a role in the induction of apoptosis. Neither dantrolene nor BAPTA-AM affected the Cl-IB-MECA-induced apoptosis. Cl-IB-MECA failed to activate phospholipase C in HL-60 cells, while UTP activated it through endogenous P2Y(2) receptors. Induction of apoptosis during a 48hr exposure to Cl-IB-MECA was not prevented by the A(3)AR antagonists [5-propyl-2-ethyl-4-propyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate] (MRS 1220) or N-[9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-yl]benzeneacetamide (MRS 1523). Furthermore, higher concentrations of MRS 1220, which would also antagonize A(1) and A(2A) receptors, were ineffective in preventing the apoptosis. Although Cl-IB-MECA has been shown in other systems to cause apoptosis through an A(3)AR-mediated mechanism, in these cells it appeared to be an adenosine receptor-independent effect, which required prolonged incubation. In both HL-60 and MOLT-4 cells, Cl-IB-MECA induced the expression of Fas, a death receptor. This induction of Fas was not dependent upon p53, because p53 is not expressed in an active form in either HL-60 or MOLT-4 cells. Cl-IB-MECA-induced apoptosis in HL-60 cells was augmented by an agonistic Fas antibody, CH-11, and this increase was suppressed by the antagonistic anti-Fas antibody ZB-4. Therefore, Cl-IB-MECA induced apoptosis via a novel, p53-independent up-regulation of Fas.

Activation of Fas by FasL induces apoptosis by a mechanism that cannot be blocked by Bcl-2 or Bcl-x(L)

Fas activation triggers apoptosis in many cell types. Studies with anti-Fas antibodies have produced conflicting results on Fas signaling, particularly the role of the Bcl-2 family in this process. Comparison between physiological ligand and anti-Fas antibodies revealed that only extensive Fas aggregation, by membrane bound FasL or aggregated soluble FasL consistently triggered apoptosis, whereas antibodies could act as death agonists or antagonists. Studies on Fas signaling in cell lines and primary cells from transgenic mice revealed that FADD/MORT1 and caspase-8 were required for apoptosis. In contrast, Bcl-2 or Bcl-x(L) did not block FasL-induced apoptosis in lymphocytes or hepatocytes, demonstrating that signaling for cell death induced by Fas and the pathways to apoptosis regulated by the Bcl-2 family are distinct.

Inhibition of CD95 (Fas/Apo1)-mediated apoptosis by vaccinia virus WR

Stimulation of the CD95 (Apo-1/Fas) molecule either by the CD95 ligand or by monoclonal antibodies induces programmed cell death by apoptosis in a variety of cell lines and primary cells. In this study we observed that infection of B lymphoblast and T lymphoblast cell lines with vaccinia virus strain WR and recombinant vaccinia WR constructs, but not strain Copenhagen, rendered cells refractor to CD95-medicated apoptosis. In particular, vaccinia virus infection suppressed anti-CD95 antibody-induced membrane disintegration, apoptotic nuclear morphology of cells, and DNA fragmentation. Inhibition of apoptosis was not mediated by CD95 down-regulation or reduced binding of anti-CD95 antibody to infected cells, and occurred at a time point when cellular metabolism was not yet affected by the lytic vaccinia virus infection. Vaccinia virus (WR)-infected cells were resistant to CD95 ligand-CD95-mediated lysis by CD4+ and CD8+ T lymphocytes. Because cytolysis mediated by CD95 is one of two major mechanisms used by cytotoxic T lymphocytes to kill target cells, inhibition of CD95-mediated apoptosis may constitute a novel immune escape mechanism for this virus. Additionally, this mechanism may contribute to the higher pathogenicity of vaccinia virus strain WR compared with strain Copenhagen.