Pro-apoptotic function of checkpoint kinase-2 in syncytia elicited by the HIV-1 envelope

Fusogenic HIV-1 isolates induce the fusion of infected and bystander cells. Such syncytia can be found as "multinucleated giant cells" in the brain from HIV-1-infected individuals, as well as in lymphoid tissues. Syncytia elicited by the HIV-1 envelope glycoprotein (Env) manifest the aggregation of PML in discrete nuclear bodies and the recruitment of TopBP1, NBS1 and ATM to DNA damage foci containing phosphorylated ATM and histone H2AX (γ-H2AX). This DNA damage response then culminates in p53-dependent activation of the mitochondrial pathway of apoptosis. Here, we show that Env-elicited syncytia also manifest activating phosphorylations of the checkpoint kinases 1 and 2 (Chk1 and Chk2), and both Chk1 and Chk2 colocalize with γ-H2AX foci. However, only the siRNA-mediated knockdown of Chk2, not the depletion of Chk1, inhibits mitochondrial outer membrane permeabilization and subsequent syncytial apoptosis. Depletion of PML, TopBP1, NBS1 or ATM inhibit the activating phosphorylation of Chk2. Altogether, these results indicate that Chk2 (but not Chk1) participates in the DNA damage-elicited proapoptotic cascade that leads to the demise of Env-elicited syncytia.