The PML gene of acute promyelocytic leukaemia (APL) encodes a cell growth and tumour suppressor, however, the mechanisms by which PML suppresses tumorigenesis are poorly understood. We show here that Pml is required for Fas- and caspase-dependent DNA-damage-induced apoptosis. We also found that Pml is essential for induction of programmed cell death by Fas, tumour necrosis factor alpha (TNF), ceramide and type I and II interferons (IFNs). As a result, Pml-/- mice and cells are protected from the lethal effects of ionizing radiation and anti-Fas antibody. Pml is required for caspase 1 and caspase 3 activation upon exposure to these stimuli. The PML-RAR alpha fusion protein of APL renders haemopoietic progenitor cells resistant to Fas-, TNF- and IFN-induced apoptosis with a lack of caspase 3 activation, thus acting as a Pml dominant-negative product. These results demonstrate that Pml is a mediator of multiple apoptotic signals, and implicate inhibition of apoptosis in the pathogenesis of APL.

Abstract. Human umbilical vein endothelial cells (ECs) adhere in vitro to proteins of the extracellular matrix including fibronectin (fn) and vitronectin (vn). Specific receptors for fn and vn have been previously characterized. These receptors belong to a family of membrane glycoproteins characterized (a) by being a transmembrane complex of two noncovalently linked subunits and (b) by recognizing the tripeptide ArgGly-Asp on their respective ligands. In this paper we investigated how vn and fn control the organization of their respective receptors over the surface of ECs. It was found that the clustering of individual receptors and the organization thereafter of focal contacts occurred only when ECs were exposed to the specific ligand and did not occur on the opposite ligand. The shape of receptor clusters was slightly different and a colocalization of the two receptors was found when ECs were cultured on a mixed matrix of fn plus vn. Adhesion was selectively inhibited by vn or fn receptor antibodies on their respective substrates. The clustering of both receptors preceded the association of vinculin with focal contacts and stress fiber formation. Also, the vn receptor, in the absence of associated fn receptor, was capable of inducing the organization of the membrane-microfilament interaction complex. Overall, these results indicate that individual matrix ligands induce only the clustering of their respective membrane receptors. The clustering of only one receptor is capable of supporting the subsequent formation of focal contacts and the local assembly of related cytoskeletal proteins.H UMAN endothelial cells (ECs) ~ adhere, spread, and organize their cytoskeleton on different molecules of the extracellular matrix such as fibronectin (fn), vitronectin (vn), and collagen (for review see references 8, 32). The reasons for such multiple recognition may be found in the fact that ECs express and expose on their surface several receptor molecules that, on the outer side of the membrane, specifically recognize and bind different components of the extracellular matrix and, on the cytoplasmic side, link a chain of proteins of the membrane-microfilament interaction complex involved in the mechanism of adhesion and cytoskeletal organization (for review see reference 9).Recently, a family of cell adhesion receptors that recognizes a number of extracellular matrix components has been described (15, 27). These receptors have several structural and functional homologies. They consist of two noncovalently linked subunits (denominated a and 13 chains) and are capable of recognizing the sequence Arg-Gly-Asp (RGD) which is present in many extracelllar matrix proteins and is : EC, endothelial cell; fn, fibronectin; vn, vitronectin. believed to play a key role in cell adhesion. Within this family fn-and vn-specific receptors have been isolated (25,26). Although they similarly recognize the RGD sequence on their targets, fn and vn receptors have mutually exclusive specificities. Indeed, liposomes containing these receptors do s...

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The PML gene is fused to the retinoic acid receptor alpha (RARalpha) gene in chromosomal translocations associated with acute promyelocytic leukemia (APL). Ablation of murine PML protein by homologous recombination revealed that PML regulates hemopoietic differentiation and controls cell growth and tumorigenesis. PML function was essential for the tumor-growth-suppressive activity of retinoic acid (RA) and for its ability to induce terminal myeloid differentiation of precursor cells. PML was needed for the RA-dependent transactivation of the p21WAF1/CIP1 gene, which regulates cell cycle progression and cellular differentiation. These results indicate that PML is a critical component of the RA pathway and that disruption of its activity by the PML-RARalpha fusion protein may be important in APL pathogenesis.