Regulation of EMT and metastasis by mutual inhibition of AP4 and miR-15a/16-1 in colorectal cancer

We recently identified the AP4 transcription factor as a
c-MYC-inducible mediator of epithelial-mesenchymal transition/EMT
in colorectal cancer (CRC). Here we report that AP4 is
down-regulated after DNA damage in a p53-dependent manner. p53
repressed AP4 via inducing the microRNAs miR-15a and miR-16-1,
which are encoded by the DLEU2 gene and represent tumor suppressors
in chronic lymphocytic leukemia. miR-15a/16-1 acted via a conserved
seed-matching sequence in the AP4 3’-UTR. Ectopic miR-15a/16-1 also
down-regulated AP4 expression and induced mesenchymal-epithelial
transition (MET) of CRC cells. Induction of miR-15a/16-1 was
necessary for p53-induced MET and mediated, at least in part,
inhibition of migration by p53. Down-regulation of AP4 was
necessary for miR-15a/16-1-induced MET and suppression of migration
and cell cycle progression. Furthermore, constitutive ectopic
expression of miR-15a/16-1 in xeno-transplanted CRC cells
suppressed the formation of lung metastases in NOD/SCID mice. In
addition, ectopic AP4 suppressed the expression of miR-15a/16-1 by
direct occupancy of E-boxes in the vicinity of the two alternative
DLEU2 promoters. Finally, expression of miR-15a and DLEU2 was
significantly down-regulated in primary colon and colorectal cancer
samples, which displayed elevated AP4 protein levels, a marker
previously shown to correlate with distant metastasis and poor
survival. In conclusion, miR-15a/16-1 and AP4 therefore constitute
a double-negative feed-back loop, which may serve to stabilize
epithelial and mesenchymal states, respectively. During CRC
progression, and presumably in other tumor entities, the
deregulation of this circuitry by inactivation of p53 or/and DLEU2,
or activation of AP4 may contribute to metastasis.