Regulation of HIF-1α Stability through S-Nitrosylation

Abstract

Hypoxia-inducible factor 1 (HIF-1) is a master transcriptional factor. Under normal oxygen tension, HIF-1 activity is usually suppressed due to the rapid, oxygen-dependent degradation of one of its two subunits, HIF-1alpha. Here we report that normoxic HIF-1 activity can be upregulated through NO-mediated S-nitrosylation and stabilization of HIF-1alpha. In murine tumors, exposure to ionizing radiation stimulated the generation of NO in tumor-associated macrophages. As a result, the HIF-1alpha protein is S-nitrosylated at Cys533 (through "biotin switch" assay) in the oxygen-dependent degradation domain, which prevents its destruction. Importantly, this mechanism appears to be independent of the prolylhydroxylase-based pathway that is involved in oxygen-dependent regulation of HIF-1alpha. Selective disruption of this S-nitrosylation significantly attenuated both radiation-induced and macrophage-induced activation of HIF-1alpha. This interaction between NO and HIF-1 sheds new light on their involvement in tumor response to treatment as well as mammalian inflammation process in general.

Cycling hypoxia and free radicals regulate angiogenesis and radiotherapy response

“…HIF1 has been referred to as the "master regulator of oxygen homeostasis" 7 , but it is regulated by many factors aside from hypoxia, including oncogenes, growth factors and free radicals [27][28][29] . Free radicals are chemical species that contain unpaired electrons; of particular importance here are oxygen-containing radicals, such as the superoxide anion (O 2 -).…”

“…Exposure to NO has been shown to nitrosylate thiols in the HIF1α protein leading to HIF1α stabilization under aerobic conditions 39 . Recently, the specific target has been identified to be a cysteine residue in the ODD of HIF1α, which blocks its binding to the VHl complex for subsequent degradation 27 . NO can promote binding of HIF1 to HREs in HIF1 target genes by binding to an adjacent HIF1 ancillary sequence (HAs) and hence acting as a transcriptional co-activator 40 .…”

How widespread is stable protein S-nitrosylation as an end-effector of protein regulation?

“…Many proteins that are reported to be regulated by S-nitrosothiol formation in the cardiovascular system have also been shown, most often previously, to be regulated by disulfides, often at the same cysteine residue. For example, S-nitrosation and disulfide formation has been shown to induce the same functional change in hypoxia-inducible factor 1α-subunit [92,93], caspase-3 [60,94], Na + /K + ATPase [95,96] and actin [84,97]. A summary of proteins that form S-nitrosothiols as well as disulfides is provided in Table 1, some of which are discussed in more detail below.…”

Is vasculogenesis crucial for the regrowth of irradiated tumours?

“…Although studies have shown that BMDCs increasingly infiltrate into tumours after irradiation and facilitate relapse 3–5 , they do not incorporate into the vasculature. Thus, these BMDCs do not participate in vasculogenesis as defined by Begg and colleagues and in the literature in general 1,6 .…”

“…Thus, these BMDCs do not participate in vasculogenesis as defined by Begg and colleagues and in the literature in general 1,6 . Indeed, the BMDCs that are recruited by irradiated tumours are generally of myeloid lineage — including ‘alternatively’ activated macrophages and tyrosine protein kinase receptor (TIE2)-expressing monocytes (TEMs) — that facilitate post-irradiation tumour revascularization through paracrine mechanisms 2–5 . Distorting the definition of vasculogenesis not only confuses the literature but also misrepresents the biology — in this case by implying that the cells are doing something that they are not.…”