Induction of the RelB NF-kappaB subunit by the cytomegalovirus IE1 protein is mediated via Jun kinase and c-Jun/Fra-2 AP-1 complexes.

We recently demonstrated that the cytomegalovirus (CMV) immediate-early 1 (IE1) protein induces transcription of the gene encoding the RelB NF-kappaB subunit. The mechanism of this activation has been explored here. We report that the induction of the relB promoter by IE1 protein is mediated via activation of JNK and AP-1. The region controlling relB promoter induction was mapped to the upstream approximately 600-bp region between -1694 and -1096 bp. IE1 stimulated AP-1 activity in NIH 3T3 cells. Competition electrophoretic mobility shift assay (EMSA) confirmed the presence of one bona fide AP-1 element centered at -1503 bp. Introduction of a G-to-C mutation in the AP-1 binding site within the distal region of the relB promoter eliminated its activation by IE1 in both NIH 3T3 fibroblasts and vascular smooth muscle cells (SMCs). Supershift EMSA identified c-Jun, Fra-2, and c-Fos in AP-1 binding complexes in IE1 transfected NIH 3T3 cells. IE1 induced c-Jun phosphorylation, and treatment with SP600125, a selective JNK inhibitor, as well as overexpression of JNK-binding domain of JIP1, blocked IE1-mediated induction of AP-1 and relB promoter activity in NIH 3T3 cells and SMCs. Ectopic expression of c-Jun plus Fra-2, but not c-Fos, induced relB promoter activity. The relB promoter has two proximal NF-kappaB elements, and c-Jun/Fra-2 worked in synergy with p50/p65 NF-kappaB complexes. Overall, these findings demonstrate for the first time the role of AP-1 in transcriptional regulation of a gene encoding an NF-kappaB subunit, and its involvement in induction of RelB activity by the CMV IE1 protein.