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1 Urologic Laboratory,
Mechanical induction of growth factor synthesis
may mediate adaptive responses of smooth muscle cells (SMC) to
increases in physical load. We previously demonstrated that cyclic
mechanical stretch induces expression of the SMC, fibroblast, and
epithelial cell mitogen heparin-binding epidermal growth factor-like
growth factor (HB-EGF) in bladder SMC, an observation that suggests
that this growth factor may be involved in compensatory bladder
hypertrophy. In the present study we provide evidence that the
activator protein-1 (AP-1) transcription factor plays a critical role
in this mechanoinduction process. Rat bladder SMC were transiently
transfected with a series of 5' deletion mutants of a promoter-reporter
construct containing 1.7 kb of the mouse HB-EGF promoter that was
previously shown to be stretch responsive. The stretch-mediated
increase in promoter activity was completely ablated with deletion of
nucleotide positions
1301 to
881. Binding of AP-1, as
evaluated by electrophoretic mobility shift assay, to a synthetic
oligonucleotide containing an AP-1 binding site increased in response
to stretch, and binding was inhibited by excess unlabeled DNA
corresponding to nucleotides
993 to
973 from the HB-EGF
promoter, a region that contains a previously recognized composite
AP-1/Ets site. Stretch-induced promoter activity was significantly
inhibited by site-directed mutagenesis of the AP-1 or Ets components of
this site. Consistent with the promoter and gel-shift studies,
curcumin, an inhibitor of AP-1 activation, suppressed the HB-EGF mRNA
induction after stretch. Stretch also specifically increased mRNA
levels for matrix metalloproteinase (MMP)-1, the promoter of which
contains a functional AP-1 element, but not for MMP-2, the promoter of
which does not contain an AP-1 element. The stretch response of the
MMP-1 gene was also completely inhibited by curcumin. Collectively,
these findings indicate that AP-1-mediated transcription plays an
important role in the regulation of gene expression in bladder muscle
in response to mechanical forces.
heparin-binding epidermal growth factor; mechanical signaling; gene expression; activator protein
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