Therefore, the activation of the Akt pathway followed by induction
of SREBP-1 is considered to be one of the major pathways of
reactivation of the FAS gene in cancer cell, and this reactivation is
triggered at least by the hypoxic condition of tumor microenvironment.
This notion is also strongly supported by our results of
immunohistochemical analysis on clinical samples where FAS
expression was significantly colocalized with the CA9-stained
hypoxic area. It is known that Akt is quickly phosphorylated under
hypoxic condition and that this activation of Akt results in upregulation
of HIF1 (45–47). Our results indeed showed that LY294002
inhibited hypoxia-induced HIF1 as well as the expression of FAS and
p-SERBP-1 (Fig. 3B). Our results also indicate that HIF1 inhibitor,
YC-1, strongly blocked phosphorylation of SREBP-1 (Fig. 3A), which
is in good agreement with the recent finding by Li et al. (48) that
HIF1 plays a key role in activation of SREBP-1 in vivo. Therefore, the
hypoxia-induced FAS expression is considered to be mediated via
phosphorylation of Akt followed by activation of HIF1 and SREBP-1