The Notch signaling pathway is an evolutionarily conserved pathway essential
for regulation of cell development and differentiation. Upregulation and
activation of Notch signaling enhances the oncogenic potential of cancer
cells through apoptosis resistance. The NOTCH1 expression pattern in
hepatocellular cancer (HCC) and its role in apoptosis attenuation was
determined. Immunohistostaining identified intensive positive staining of
NOTCH1 in human HCC tissues as compared to control tissues. RT-PCR and
Western blot quantification data showed that NOTCH1 and its downstream
target transcription factor Hes1 were significantly upregulated in HCC
cells. Based on these findings, we separated a population of CD44+
tumor-initiating cells (HepG2: >7%; SNU449: >6%) from HCC cell lines to
ascertain the role of NOTCH1 in tumorigenesis. After NOTCH1-specific small
interfering RNA (siRNA) transfection of tumor-initiating cells (TICs),
NOTCH1 was significantly downregulated, and efficient uptake of
DNA-targeting chemotherapeutic drugs was observed. Meanwhile, by flow
cytometry analysis we found that the rate of apoptosis induction was
significantly higher (P<0.01) and that cell viability was reduced
(HepG2<23%; SNU449<28%) in siRNA transfected cells. In addition, the release
of cytochrome C and activation of caspase 9 in CD44+ TICs was observed after
siRNA transfection, confirming the induction of the mitochondrial-dependent
intrinsic apoptotic pathway. Western blot analysis revealed inhibition of
the PI3-Akt signaling pathway in siRNA-transfected TICs. These data suggest
that activated NOTCH1 plays a significant role in liver cancer progression
through apoptosis inhibition via regulation of PI3- Akt signaling.
Therefore, pharmacological inactivation of NOTCH1 represents a clinically
relevant therapeutic target for treating HCC.
Small Molecule Antagonists of the Wnt/Beta-Catenin Signaling Pathway Target Breast Tumor-Initiating Cells in a Her2/Neu Mouse Model of Breast Cancer
