Human VHL Knockout Cell Line-A549

The von Hippel-Lindau (VHL) protein binds and degrades hypoxia-inducible factor (HIF), which is hydroxylated by prolyl hydroxylases under normoxic conditions. Although initially described as a tumor suppressor, increasing evidence suggests that VHL may instead promote tumor growth. Here, researchers found that VHL interacts with p53, preventing its tetramerization, promoter binding, and expression of its target genes p21, PUMA, and Bax. VHL limits the decreased proliferation and increased apoptosis caused by p53 activation, independent of prolyl hydroxylation and HIF activity, and its presence in tumors leads to resistance to p53-inducing chemotherapy in vivo. This study suggests that VHL has both antitumor functions through HIF degradation and novel protumor functions through inhibition of p53 targets (p21, PUMA, Bax).

The researchers created two models to study the effects of VHL on p53 activity in vivo: a VHL elimination model (A549 parental cells vs. A549 VHL knockout cells) and a VHL rescue model (−VHL 786-O vs. +VHL 786-O). In each model, VHL-deficient tumor cells were injected into the left flank of nude mice, while VHL-expressing tumor cells were injected into the right flank of the same animals (Figure 1a). These tumors were grown for 4 weeks, their size was measured, and then weekly doxorubicin treatment was initiated for an additional 4 weeks. VHL-deficient tumors initially grew larger than VHL-expressing tumors, likely due to VHL's effects on HIF. However, after induction of p53 with doxorubicin, VHL-deficient tumors responded to treatment to a greater extent than VHL-expressing tumors, as measured by the ratio of post-treatment tumor volume to pre-treatment tumor volume for each animal (Figure 1b, c). Furthermore, p21 protein levels were reduced in VHL-expressing tumors after treatment compared with VHL-deficient tumors, despite similar p53 protein induction in response to doxorubicin (Figure 1d, e). Proliferation was significantly reduced, while apoptosis was significantly increased, in VHL-deficient tumors after treatment. These results suggest that VHL is attenuating the therapeutic effects of anthracyclines on tumors in vivo.

Figure 1. VHL attenuates response to anthracycline chemotherapy in vivo. (Kinnaird A, et al., 2020)