Distinct promoters mediate constitutive and inducible Bcl-XL expression in malignant lymphocytes.
Bcl-X(L) is a Bcl-2-related survival protein that is essential for normal development. Bcl-X(L) 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-X(L) in B-lymphoma cells. Antisense experiments demonstrated that Bcl-X(L) 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-X(L) 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-X(L) expression and accumulation of exon IA-containing RNAs, derived from the proximal 1A promoter. Nuclear factor-kappaB (NF-kappaB) inhibition prevented induction of Bcl-X(L) 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-kappaB plays a central role in the induction of Bcl-X(L) in response to CD40 signalling via the proximal 1A promoter.
Habens
F
F
Cancer Research UK Clinical Centre, Cancer Sciences Division, University of Southampton School of Medicine, Southampton General Hospital, Southampton, UK.
Lapham
A S
AS
Dallman
C L
CL
Pickering
B M
BM
Michels
J
J
Marcusson
E G
EG
Johnson
P W M
PW
Packham
G
G
D001483:Base Sequence; D002051:Burkitt Lymphoma / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D002470:Cell Survival; D047369:Chromatin Immunoprecipitation; D017931:DNA Primers; D006801:Humans; D011401:Promoter Regions, Genetic; D020133:Reverse Transcriptase Polymerase Chain Reaction; D051020:bcl-X Protein / Q000378:metabolism*
D017931:DNA Primers; D051020:bcl-X Protein