Colorectal
cancer (CRC) is the second highest cause of cancer death and the third most
common malignancy. Aberrant Notch signaling in CRC promotes epithelial to
mesenchymal transition, chemoresistance and metastasis. Its effects are
mediated through transcription factors (TFs), in particular HES1 and HEY1,
which influence COX2, NF-B and Wnt signaling. In CRC, Notch signaling can
be activated by retinoic acid, through the nuclear receptor (NR) RARa and the early developmental TF SOX9, which
activates HES1. Using CRC tissue microarrays (TMAs), we found that SLIRP, a
nuclear receptor (NR) corepressor, is a good prognostic factor in CRC, with a
tumor suppressor phenotype. High tumor SLIRP expression in a TMA cohort of 967
patients correlated with improved 5 year survival (p<0.01) and inversely
with tumor stage and lymph node invasion. In two CRC cDNA microarray sets, high
SLIRP mRNA expression correlated with improved disease free survival over three
years following surgery (p<0.05). Here we investigated the mechanism for
this clinical advantage and the hypothesis that SLIRP is a repressor Notch
signaling. In human CRC cell transfection studies using luciferase (Luc)
reporters, siRNA mediated depletion of SLIRP increased the activity of
ligand-stimulated RAR, HES1 and SOX9 signaling, resulting in
increased expression of downstream targets, including NOTCH2, NOTCH3, NFKB1,
NFKB2, LMO2, HES1 and SOX9. In ChIP studies, SLIRP was recruited (with RARa) to the HES1 and SOX9 promoters. When SLIRP was
depleted from the cells with siRNA, ChIP studies revealed an increase in RARa recruitment to the HES1 and SOX9 promoters,
along with increased binding of SOX9 to the HES1 promoter, and a decrease in
binding of HES1 to its own promoter. Further, we found that siRNA mediated
SLIRP depleted CRC cells were more invasive in matrigel assays and more
resistant to the standard CRC chemotherapeutic agents, 5-Fluorouracil and
irinotecan. Taken together, these data suggest that SLIRP is a potent
suppressor of Notch and RARa signaling in CRC and provide insight into the
mechanisms by which SLIRP functions as a tumor suppressor to reduce invasion
and enhance sensitivity to chemotherapy in this disease.