RelB-p50 NF-κB complexes are selectively induced by cytomegalovirus immediate-early protein 1:: Differential regulation of Bcl-xL promoter activity by NF-κB family members

The NF-kappaB/Rel family has been implicated in control of transcription of the Bcl-x(L) gene, a target which mediates cell survival signals. The cytomegalovirus (CMV) immediate-early protein 1 (IE1) was previously shown to induce NF-kappaB activity. Here, we report that in both vascular smooth muscle cells (SMCs) and NIH 3T3 cells, surprisingly, IE1 failed to induce Bcl-x(L) promoter activity, although it induced activity of E8-CAT, a reporter construct driven by two copies of the NF-kappaB element upstream of the c-myc promoter (upstream regulatory element [URE]). Thus, the subunit nature of the NF-kappaB/Rel factors induced by IE1 was examined using immunofluorescence and immunoblotting. IE1 was found to selectively induce nuclear RelB and p50 in SMCs and NIH 3T3 cells. An increase in RelB protein mediated by IE1 could, in part, be related to an increase in steady-state relB mRNA levels. Consistent with this subunit identification, IE1 was unable to induce E8-CAT activity in relB(-/-) murine embryonic fibroblast cells. In cotransfection analysis of SMCs and NIH 3T3 cells, RelB and p50 proteins failed to induce Bcl-x(L) promoter activity while inducing E8-CAT. Furthermore, the NF-kappaB element of the Bcl-xL promoter only weakly bound RelB-p50 complexes compared to the URE NF-kappaB element. Overall, these findings demonstrate in SMCs and NIH 3T3 cells that the CMV IE1 protein selectively induces ROB and p50, which fail to activate the Bcl-x(L) promoter, indicating a strong specificity of binding and activity for the RelB member of the NF-kappaB family. Furthermore, our results implicate ROB in CMV infection of cells such as vascular SMCs.