To address the suppression issue further, we made retroviral
HIF1α expression vectors in which the amount of
HIF1α produced can be regulated by the addition of doxycycline.
VHL–/– renal carcinoma cells were then infected with
the viruses and maintained in pools. Immunoblot analysis
of these cells grown in the presence or absence of 1 μg/mL of
doxycycline confirmed that HIF1α expression was induced
by doxycycline and that the HIF1α levels achieved were
similar to those observed after treating HK2 immortalized
renal epithelial cells with the prolyl hydroxylase inhibitor
dimethyloxalylglycine (DMOG) or the proteasomal inhibitor
MG132 (Fig. 3A; Supplementary Fig. S5). Re-expression of
HIF1α in the VHL–/– renal carcinoma cell lines A498, 769-P,
UOK101, and SLR24—all of which produce HIF2α, but not
wild-type HIF1α—impaired proliferation in vitro (Fig. 3B).
This effect was specific because induction of HIF1α did not
diminish the proliferation of RCC4 and UMRC-2 VHL–/– renal
carcinoma cells, which express both HIF1α and HIF2α
(Fig. 3B; Supplementary Fig. S5). Therefore, HIF1α can suppress
the proliferation of VHL–/– renal carcinoma cells when
expressed at levels approximating those achieved after VHL
inactivation