Phosphatase and tensin homolog deleted on
chromosome 10 (PTEN) is rarely
mutated in
pancreatic cancers, but its regulation by
transforming growth factor (TGF)-beta might mediate growth suppression and other
oncogenic actions. Here, we examined the role of
TGFbeta and the effects of
oncogenic K-RAS/ERK upon PTEN expression in the absence of
SMAD4. We utilized two SMAD4-null pancreatic
cell lines, CAPAN-1 (K-RAS
mutant) and BxPc-3 (WT-K-RAS), both of which express
TGFbeta surface receptors. Cells were treated with TGFbeta1 and separated into
cytosolic/nuclear fractions for
western blotting with phospho-SMAD2, SMAD 2, 4 phospho-ATP-dependent
tyrosine kinases (
Akt),
Akt and PTEN
antibodies. PTEN
mRNA levels were assessed by reverse transcriptase-polymerase
chain reaction. The
MEK1 inhibitor, PD98059, was used to block the downstream action of
oncogenic K-RAS/ERK, as was a dominant-negative (DN) K-RAS construct.
TGFbeta increased phospho-SMAD2 in both
cytosolic and nuclear fractions. PD98059 treatment further increased phospho-SMAD2 in the nucleus of both pancreatic
cell lines, and DN-K-RAS further improved SMAD
translocation in K-RAS
mutant CAPAN cells.
TGFbeta treatment significantly suppressed PTEN protein levels
concomitant with
activation of
Akt by 48 h through
transcriptional reduction of PTEN
mRNA that was evident by 6 h. TGFbeta-induced PTEN suppression was reversed by PD98059 and DN-K-RAS compared with treatments without
TGFbeta. TGFbeta-induced PTEN expression was inversely related to cellular proliferation. Thus,
oncogenic K-RAS/ERK in pancreatic
adenocarcinoma facilitates TGFbeta-induced
transcriptional down-regulation of the
tumor suppressor PTEN in a SMAD4-independent manner and could constitute a signaling switch mechanism from growth suppression to growth promotion in
pancreatic cancers.
