The herpes simplex virus type 1 (HSV-1) genome contains three origins of DNA replication, one copy of oriL and two copies of oriS. Although oriL and oriS are structurally different, they have extensive nucleotide sequence similarity and can substitute for each other to initiate
viral DNA replication. A fundamental question that remains to be answered is why the HSV-1 genome contains two types of origin. We have recently identified a novel
glucocorticoid response element (GRE) within oriL that is not present in oriS and have shown by gel mobility shift assays that purified
glucocorticoid receptor (GR), as well as GR present in cellular extracts, can bind to the GRE in oriL. To determine whether
glucocorticoids and the GRE affect the efficiency of oriL-dependent DNA replication, we performed transient DNA replication assays in the presence and absence of
dexamethasone (DEX). Because HSV-1 is a neurotropic virus and establishes latency in cells of neural origin, these tests were conducted in PC12 cells, which assume the properties of sympathetic neurons when differentiated with
nerve growth factor (
NGF). In
NGF-differentiated PC12 cells, oriL-dependent DNA replication was enhanced 5-fold by DEX, whereas in undifferentiated cells, DEX enhanced replication approximately 2-fold. Notably, the enhancement of oriL function by DEX was abolished when the GRE was mutated.
NGF-induced differentiation alone had no effect. In contrast to oriL, oriS-dependent DNA replication was reduced approximately 5-fold in
NGF-differentiated PC12 cells and an additional 4-fold in differentiated cells treated with DEX. In undifferentiated PC12 cells, DEX had only a minor inhibitory effect (approximately 2-fold) on oriS function. Although the cis-acting elements that mediate the
NGF- and DEX-specific repression of oriS-dependent DNA replication are unknown, a functional GRE is critical for the DEX-induced enhancement of oriL function in
NGF-differentiated PC12 cells. The enhancement of oriL-dependent DNA replication by DEX in differentiated PC12 cells suggests the possibility that
glucocorticoids, agents long recognized to enhance reactivation of latent
herpesvirus infections, act through the GRE in oriL to stimulate
viral DNA replication and reactivation in terminally differentiated neurons in vivo.