Distinct promoters mediate constitutive and inducible Bcl-XL expression in malignant lymphocytes

Bcl-XL is a Bcl-2-related survival protein that is essential for normal development. Bcl-XL expression is rapidly induced by a wide range of survival signals and many cancer cells constitutively express high levels. The Bcl-X gene has a complex organization with multiple promoters giving rise to RNAs with alternate 5′ non-coding exons. Here we have investigated the mechanisms that control basal and induced expression of Bcl-XL in B-lymphoma cells. Antisense experiments demonstrated that Bcl-XL was essential for survival of Akata6 B-lymphoma cells. The levels of RNAs containing the IB Bcl-X non-coding exon, derived from the distal 1B promoter, correlated with basal expression of Bcl-XL in primary malignant B cells and this promoter was highly active in B-cell lines. The activity of this promoter was largely dependent on a single Ets binding site and Ets family proteins were bound at this promoter in intact cells. CD40 ligand (CD40L)-induced cell survival was associated with increased Bcl-XL expression and accumulation of exon IA-containing RNAs, derived from the proximal 1A promoter. Nuclear factor-kappaB (NF-κB) inhibition prevented induction of Bcl-XL protein and exon IA-containing RNAs by CD40L. Therefore, the distal Bcl-X 1B promoter plays a critical role in driving constitutive expression-mediated via Ets family proteins in malignant B cells, whereas NF-κB plays a central role in the induction of Bcl-XL in response to CD40 signalling via the proximal 1A promoter.