DNA damage can activate the oncosuppressor protein ataxia telangiectasia mutated (ATM), which phosphorylates the histone H2AX within characteristic DNA damage foci. Here, we show that ATM undergoes an activating phosphorylation in syncytia elicited by the envelope glycoprotein complex (Env) of human immunodeficiency virus-1 (HIV-1) in vitro. This was accompanied by aggregation of ATM in discrete nuclear foci that also contained phospho-histone H2AX. DNA damage foci containing phosphorylated ATM and H2AX were detectable in syncytia present in the brain or lymph nodes from patients with HIV-1 infection, as well as in a fraction of blood leukocytes, correlating with viral status. Knockdown of ATM or of its obligate activating factor NBS1 (Nijmegen breakage syndrome 1 protein), as well as pharmacological inhibition of ATM with KU-55933, inhibited H2AX phosphorylation and prevented Env-elicited syncytia from undergoing apoptosis. ATM was found indispensable for the activation of MAP kinase p38, which catalyzes the activating phosphorylation of p53 on serine 46, thereby causing p53 dependent apoptosis. Both wild type HIV-1 and an HIV-1 mutant lacking integrase activity induced syncytial apoptosis, which could be suppressed by inhibiting ATM. HIV-1-infected T lymphoblasts from patients with inactivating ATM or NBS1 mutations also exhibited reduced syncytial apoptosis. Altogether these results indicate that apoptosis induced by a fusogenic HIV-1 Env follows a pro-apoptotic pathway involving the sequential activation of ATM, p38MAPK and p53.

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C4.4A is a member of the Ly6 family, with low homology to uPAR. It has been detected mainly on metastasizing carcinoma cells and proposed to be involved in wound healing. So far, C4.4A has been observed as an orphan receptor, and its functional activity has not been explored. Using recombinant rat C4.4A (rrC4.4A) made in a eukaryotic expression system, we demonstrate by immunohistology that C4.4A ligands are strongly expressed in tissues adjacent to squamous epithelia of, e.g., tongue and esophagus, the expression pattern partly overlapping with laminin (LN) and complementing the C4.4A expression that is found predominantly on the basal layers of squamous epithelium. ELISA screening of several components of the extracellular matrix revealed selective binding of rrC4.4A to LN1 and LN5 and that transfection of the BSp73AS tumor line with C4.4A cDNA (BSp73AS-1B1) promoted LN1 and LN5 binding. Binding of BSp73AS-1B1 to LN5 and, less markedly, LN1 induced spreading, lamellipodia formation and migration. C4.4A also associates with galectin-3 in nontransformed tissues and tumor lines. There is evidence that the association of C4.4A with galectin-3 influences LN adhesion. C4.4A was described originally as a metastasis-associated molecule. Our findings that LN1 and LN5 are C4.4A ligands, that galectin-3 associates with C4.4A and that C4.4A ligand binding confers a migratory phenotype are well in line with the supposed metastasis association. ' 2005 Wiley-Liss, Inc.Key words: C4.4A; laminin; galectin; cell migration LNs are adhesive glycoproteins found in the basement membrane. Fifteen LN isoforms have been identified. 1 LN isoforms are tissue-specific and expressed in a developmentally regulated pattern. 2,3 Cell adhesion to LN plays an important role in maintaining normal tissue organization. LNs promote cell adhesion and migration, 4 are involved in the control of cell proliferation and gene expression 5 and stimulate neurite outgrowth, 6 the multitude of functions being brought about by a large array of receptors. 7 The most prominent LN receptors are integrins, but distinct receptors have also been described, e.g., a-dystroglycan, syndecans, the 67 kDa LN receptor and galectins. 7,8 a-Dystroglycan provides a link between the basement membrane and the dystrophin-actin cytoskeleton. 9 Its binding to LN is Ca 2þ -dependent 10 and requires carbohydrate moieties. 11 Binding of syndecans 2 and 4 to LN5 induces expression of MMP-1. 12 Moreover, syndecan 1 interacts with unprocessed LN5 in migrating keratinocytes, and this interaction depends on the presence of the glycosaminoglycan chain. 13 Galectins belong to the family of galactose-specific lectins and bind the lactosamine-type sugars on LN1. 8 However, at high concentrations, galectins 3 and 1 can downregulate adhesion to LN, probably by blocking the cellular attachment sites. 14,15 The 67 kDa LN receptor can act in concert with a6b4 to promote cell adhesion to LN1. 16 It is also involved in the degradation of LN1 and the release of motility fragments. 17 The GPI-ancho...