Inhibition of Notch by gamma-secretase inhibitors induces apoptosis through activated caspase-3 in LS-1034 colon cancer model

The Notch pathway is an evolutionarily conserved regulator of cellular differentiation in many tissues during development and later through adult life. It has been reported that particular cancer types depend on the Notch pathway for survival. Due to tissue specificities exhibited by the Notch pathway, dependency on Notch activation as a tumor survival mechanism is still being evaluated in many tumor types. Adenomatous polyposis coli (Apc) is a tumor suppressor gene which is mutated in many human colon cancers and also in the LS-1034 colon cancer cell line. Notch pathway dependency has been reported previously in the adenomas of ApcMin mice. We hypothesized that, due to its Apc mutation, inhibition of Notch pathway activation by gamma-secretase inhibitor (GSI) may inhibit tumor growth in LS-1034 cells. To test this hypothesis, we examined the ability of GSIs to both inhibit growth and induce apoptosis of LS-1034 cells. In-vitro, GSI treatment significantly inhibits growth of LS-1034 cells by inducing cell cycle arrest and apoptosis as assayed by cleavage of caspase-3. These effects were significantly diminished in colon cancer cell line HT-29 which lacks an Apc mutation. In order to evaluate the in-vivo effects of gamma-secretase inhibition, mice bearing xenografted LS-1034 cells were treated with the same GSI. In them, we observed Notch pathway inhibition, in-vivo activation of caspase-3, and significantly inhibited tumor growth progression. Our data suggest that Apc mutations may lend tumors to greater dependence on the Notch pathway for survival and therefore significant sensitivity to GSIs. This data also suggests that Notch pathway inhibition may provide a novel strategy for treatment of human colon cancer patients where Apc is mutated.