Regulation of caspase 8- and caspase 9-induced apoptosis by the herpes simplex virus type 1 latency-associated transcript.

The latency-associated transcript (LAT) is the only herpes simplex virus type 1 (HSV-1) gene that is abundantly transcribed during latency. Plasmids expressing LAT inhibit apoptosis induced by etoposide and ceramide in transiently transfected cells. LAT also inhibits apoptosis in trigeminal ganglia of rabbits and promotes spontaneous reactivation, suggesting these events are coupled. In this study, we compared caspase cleavage (activation) in cells infected with dLAT2903 (LAT-null mutant) versus wild-type McKrae or the rescued LAT-null mutant (dLAT2903R). Neuro-2A cells (mouse neuroblastoma), but not NIH3T3 cells infected with dLAT2903, contained higher levels of cleaved caspase 9 compared to cells infected with McKrae. Cleaved caspase 9 was also readily detected in neuro-2A cells, but not NIH3T3 cells, after ultraviolet (UV) light treatment, suggesting that the ability of cells to process caspases and undergo apoptosis influences the antiapoptotic properties of LAT. HSV-1 expresses numerous genes in addition to LAT that can block apoptosis during productive infection of cultured cells. Because these genes may mask the effects of LAT on apoptosis, transient transfection assays were performed to test whether LAT can inhibit caspase 8- and caspase 9-induced apoptosis. A plasmid expressing nucleotides 1 to 4658 of LAT efficiently inhibited caspase 8- and caspase 9-induced apoptosis in transiently transfected neuro-2A cells. These studies indicate that LAT has the potential to inhibit the two major pathways of apoptosis in the absence of other viral genes. Furthermore, these studies support a role for the antiapoptotic properties of LAT in the latency-reactivation cycle.