Herpes simplex virus 1 blocks caspase-3-independent and caspase-dependent pathways to cell death

Earlier reports have shown that herpes simplex virus 1 (HSV-1) mutants indu ce programmed cell death and that wild-type HSV blocks the execution of the cell death program triggered by viral gene products, by the effecters of t he immune system such as the Fas and tumor necrosis factor pathways, or by nonspecific stress agents such as either osmotic shock induced by sorbitol or thermal shock. A report from this laboratory showed that caspase inhibit ors do not block DNA fragmentation induced by infection with the HSV-1 d120 mutant. To identify the events in programmed cell death induced and blocke d by HSV-1, we examined cells infected with wild-type virus or the d120 mut ant or cells infected and exposed to sorbitol. We report that: (i) the HSV- 1 d120 mutant induced apoptosis by a caspase-3-independent pathway inasmuch as caspase 3 was not activated and DNA fragmentation was not blocked by ca spase inhibitors even though the virus caused cytochrome c release and depo larization of the inner mitochondrial membrane. (ii) Cells infected with wi ld-type HSV-1 exhibited none of the manifestations associated with programm ed cell death assayed in these studies. (iii) Uninfected cells exposed to o smotic shock succumbed to caspase-dependent apoptosis inasmuch as cytochrom e c was released, the inner mitochondrial potential was lost, caspase-3 was activated, and chromosomal DNA was fragmented. (iv) Although caspase-3 was activated in cells infected with wild-type HSV-1 and exposed to sorbitol, cytochrome c outflow, depolarization of the inner mitochondrial membrane, a nd DNA fragmentation were blocked. We conclude that although d120 induces a poptosis by a caspase-3-independent pathway, the wild-type virus blocks apo ptosis induced by this pathway and also blocks the caspase-dependent pathwa y induced by osmotic shock. The block in the caspase-dependent pathway may occur downstream of caspase-3 activation.