Bax and APPL1 are involved in DCC-ADD induced colorectal ...

III. Discussion
The pro-apoptotic activity of DCC-ADD has long
been discussed. In the present study, we constructed the
recombinant plasmid pIRES2-EGFP-ADD, and transfected
it into human colorectal cancer cells SW1116. The results
from precise methods of CCK-8 cell proliferation assay,
AO/EB fluorescence staining assay and TUNEL assay
confirm that point once again.
The possible mechanism of DCC-induced
apoptosis is related to the cleavage of DCC at Asp1290 by
caspase3 leading to the exposure of ADD, which activates
caspase9 and the downstream apoptotic signals (Mehlen P,
et al., 1998).
Bax is a member of the Bcl-2 family and plays a
central role in the mitochondria-dependent apoptotic
pathway. When receiving a death signal, Bax can promote
the release of cytochrom c from mitochondria, which can
combine with Apaf-1 and activate caspase9 (Lalier L, et al.,
2007; Renault TT, et al., 2011). Our datas revealed that the
Bax expression of colorectal carcinoma cells was increased
obviously post DCC-ADD gene transfection, which gives
new clues that DCC may induce apoptosis through the
mitochondria-dependent apoptotic pathway.
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It has been demonstrated that APPL1 (also called
DCC-interacting protein-13, DIP13) can mediate the proapoptotic
activity of DCC through the interaction between
its DCC-interacting domain and DCC-ADD. Co-expression
of DCC and APPL1 can increase DCC induced apoptosis
obviously. And removal of the DCC-interacting domain on
APPL1 or inhibition of APPL1 expression blocks DCCinduced
apoptosis (Liu J, et al., 2002).
In the present study, the result showed that DCC-
ADD gene transfection induced colorectal carcinoma cells
apoptosis and enhanced the expression level of APPL1.
ADD can interact with initiator of caspase9 and promote the
activation of caspases leading to cell apoptosis (Mehlen P,
et al., 1998; Forcet C, et al., 2001). As the adapter protein
between ADD and caspase9, APPL1 plays an important
role in DCC-induced apoptosis (Liu J, et al., 2002).
In summary, we surmise the pathway of DCC inducing
apoptosis may due to its ADD being exposed by cleavage
of caspase3. And Bax and APPL1 mediate the activating of
caspase9 by ADD, which promotes the apoptosis cascade at
last. Among them, the expression and activity changes of
the caspase9 need further experimental validation.