ect of TSA, which might be resulted in the reduction of TSA-mediated cell death. Our results showed that TSA increased the number of GFP-HIF-1α-positive cells in Figure [1I](#F1){ref-type="fig"}, which might be associated with the acetylation of K674 in HIF-1α (Figure [5](#F5){ref-type="fig"}). VEGF-HRE activity was much more enhanced by HIF-1α overexpression (Figure [6B](#F6){ref-type="fig"}), which was inhibited by mutant HIF-1α (K674R) overexpression. This is why the percentage of tumor cell death by TSA treatment looks like decreased by HIF-1α overexpression, which was overcome by mutant HIF-1α (K674R) overexpression (Figure [6C](#F6){ref-type="fig"}). So, antitumor effect of TSA might be increased by the inhibition of endogenous HIF-1α level. It implicates that GFP-HIF-1α-positive cells are more resistant to TSA-mediated cell death. These results demonstrate that drug resistance could be regulated by K674 acetylation in HIF-1α by various types of HDAC inhibitors, such as TSA.

It has been reported that silent mating type information regulation 2 homolog 1 (SIRT1), NAD-dependent deacetylase, binds to HIF-1α and deacetylates it at K674 \[[@R32]\]. So, it can\'t be ruled out the possibility that TSA might inhibit SIRT1 to increase the acetylation of K674 in HIF-1α. Except the acetylation of K674 in HIF-1α, K532 is another acetylation site. ARD1 protein specifically acetylates K532 in the oxygen-dependent degradation (ODD) domain of HIF-1 residues 401--603, which regulates a HIF-1α stability \[[@R33]\]. K532 acetylation in HIF-1α increases the stability of Von Hippel-Landau (VHL) binding to HIF-1α and leads to the rapid proteasomal degradation of HIF-1α via ubiquitination \[[@R33]\]. Therefore, it will be further studied whether TSA might inhibit ARD1 and prevent VHL binding to and the degradation of HIF-1α through K532 acetylation.

VEGF is essential for the blood vessel formation to maintain oxygen supply. HIF-1α increased by TSA-mediated acetylation enhances VEGF expression with HRE, which m