Bryostatin 1 is known to exhibit in vitro and in vivo activity against
chronic lymphocytic leukemia (CLL) cells by inducing their further maturation into plasma-like cells. Signal transducer and activator of transcription (STAT)
proteins play a central role in B-lymphocyte growth and function and are aberrantly phosphorylated on
serine residues in CLL cells. To determine whether
STAT transcription factors are important in
Bryostatin 1-induced differentiation of CLL cells, primary CLL cells were examined for signaling events following exposure to
Bryostatin 1 in vitro. Western analysis and electrophoretic mobility shift assays revealed that
Bryostatin 1 induced
tyrosine phosphorylation and
DNA binding of STAT1, yet there was no effect on constitutive
serine phosphorylation of STAT1.
Bryostatin 1-induced STAT1 activation occurred in a manner that was dependent on
protein kinase C (PKC),
mitogen-activated protein kinase (MAPK), and Janus
tyrosine kinase (JAK) activation. Evidence indicates that
Bryostatin 1 induces STAT1 activation through an
interferon gamma (IFN gamma) autocrine loop. However, STAT1 activation by IFN gamma stimulation alone was not sufficient to induce differentiation. This insufficiency is due to the broader effect on gene expression caused by
Bryostatin 1 compared with IFN gamma, as demonstrated by microarray analysis. Both up-regulation of CD22 expression and
immunoglobulin M (
IgM) production, markers of CLL differentiation, were inhibited by a decoy
oligonucleotide for STAT1, indicating that STAT1 is necessary for
Bryostatin 1-induced differentiation of CLL cells. This study implicates
STAT transcription factors as important mediators of
Bryostatin 1-induced differentiation of CLL cells and could possibly lead to improved therapeutic approaches for the treatment of CLL.