As we and others previously reported, inhibition of the function
or expression of FAS results in apoptosis of tumor cells (7–11). This
cell death is considered to be caused by the suppression of CPT1
followed by accumulation of ceramide, which in turn activates
proapoptotic genes such as BNIP3(13). It should be noted that
BNIP3was found to be one of proapoptotic genes induced by
hypoxia, and that specific blocking of the FAS expression by siRNA
significantly increased the expression of BNIP3followed by
apoptosis (13, 51). In fact, we have shown that the expressions of
FAS and BNIP3are indeed inversely correlated in breast cancer
patients (13). Therefore, FAS may act as an ‘‘antiapoptotic’’ gene
under hypoxia. This notion is consistent with the previous
observations of immunohistochemical analysis on human tumor
samples where overexpression of FAS was found to be a relatively
early event (7–11). We also reported that the expression of FAS was
inversely related to that of PTEN in human breast tumor
specimens, and the expression of higher FAS and lower PTEN is
correlated to poor survival of patients, suggesting that the PTEN
inactivation followed by Akt activation induced the FAS expression
(22). Although the direct involvement of FAS in the initial step of
tumorigenesis is yet to be determined, overexpression of FAS in
tumors seems to be a survival strategy of the cancer cells to block
apoptosis caused by hypoxic condition