Identification of a region on the adenovirus E1A gene responsible for induction by phorbol ester tumor promoter

NAD-linked mechanisms of gene de-repression and a novel role for CtBP in persistent adenovirus infection of lymphocytes

BACKGROUND

Adenovirus (AdV) infection is ubiquitous in the human population and causes acute infection in the respiratory and gastrointestinal tracts. In addition to lytic infections in epithelial cells, AdV can persist in a latent form in mucosal lymphocytes, and nearly 80% of children contain viral DNA in the lymphocytes of their tonsils and adenoids. Reactivation of latent AdV is thought to be the source of deadly viremia in pediatric transplant patients. Adenovirus latency and reactivation in lymphocytes is not well studied, though immune cell activation has been reported to promote productive infection from latency. Lymphocyte activation induces global changes in cellular gene expression along with robust changes in metabolic state. The ratio of free cytosolic NAD+/NADH can impact gene expression via modulation of transcriptional repressor complexes. The NAD-dependent transcriptional co-repressor C-terminal Binding Protein (CtBP) was discovered 25 years ago due to its high affinity binding to AdV E1A proteins, however, the role of this interaction in the viral life cycle remains unclear.

METHODS

The dynamics of persistently- and lytically-infected cells are evaluated. RT-qPCR is used to evaluate AdV gene expression following lymphocyte activation, treatment with nicotinamide, or disruption of CtBP-E1A binding.

RESULTS

PMA and ionomycin stimulation shifts the NAD+/NADH ratio in lymphocytic cell lines and upregulates viral gene expression. Direct modulation of NAD+/NADH by nicotinamide treatment also upregulates early and late viral transcripts in persistently-infected cells. We found differential expression of the NAD-dependent CtBP protein homologs between lymphocytes and epithelial cells, and inhibition of CtBP complexes upregulates AdV E1A expression in T lymphocyte cell lines but not in lytically-infected epithelial cells.

CONCLUSIONS

Our data provide novel insight into factors that can regulate AdV infections in activated human lymphocytes and reveal that modulation of cellular NAD+/NADH can de-repress adenovirus gene expression in persistently-infected lymphocytes. In contrast, disrupting the NAD-dependent CtBP repressor complex interaction with PxDLS-containing binding partners paradoxically alters AdV gene expression. Our findings also indicate that CtBP activities on viral gene expression may be distinct from those occurring upon metabolic alterations in cellular NAD+/NADH ratios or those occurring after lymphocyte activation.

PMA induces expression from the herpes simplex virus thymidine kinase promoter via the activation of JNK and ERK in the presence of adenoviral E1A proteins

The herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) promoter contains elements involved in both constitutive and induced expression. We determined that phorbol 12-myristate 13-acetate (PMA) induces the HSV-1 TK promoter in HEK293 cells. However, PMA did not induce expression from the promoter in HeLa cells and did not result in a globally increased gene expression in HEK293 cells. Induction of HSV-1 TK promoter required activation of both of JNK and ERK pathways. However, activation of the two pathways alone was not sufficient for induction of HSV-1 TK promoter. By transiently transfecting into HeLa cells the adenoviral E1A gene, which exists as an integrant in HEK293 genome, we demonstrated that E1A proteins are necessary for induction of HSV-1 TK promoter by PMA. We propose mechanisms by which signaling pathways activated by the tumor-promoter PMA cooperate with the oncogene E1A to stimulate a eukaryotic promoter, namely the HSV-1 TK promoter.