AbstractInflammation and hypoxia, or decreased oxygen availability, are two relevant conditions involved in a large variety of physiological and pathological processes, such as immune diseases and cancer. At molecular level, these stress responses are strictly orchestrated by evolutionary conserved signaling cascades, mainly driven by the transcription factors NF-kB and Hypoxia Inducible Factors (HIFs), respectively. However, over the past years, the inflammatory response has been associated with hypoxia both in immune cells and in cancer cells. The crosstalk between NF-kB and HIF is extensive, including shared activating stimuli and regulators, as well as downstream target genes.
While how NF-kB regulates HIF has been already deeply studied following TNF-α induction, the activation of HIF pathway following other inflammatory stimuli was quite unexplored. In this study, the non-canonical stimulus LIGHT (TNFSF14) was included in the list of common inducers of NF-kB and HIF. LIGHT was discovered to control HIF induction under normal oxygen levels. In particular, induction of HIF-2α, at mRNA and protein levels, occurred through activation of the non-canonical NF-kB pathway and recruitment of p52 to the HIF-2α promoter.
While a role for HIF-1α in restricting the NF-kB activity already emerged, the regulatory role of HIF-1β or HIF-2α in the control of the inflammatory response was poorly understood. Here, it is shown that both HIF-1β and HIF-2α are required for the full activation of the NF-kB pathways, under different inflammatory contexts. In particular, it was demonstrated that HIF-1β modulates activators and subunits involved in both canonical and non-canonical NF-kB signalling cascades at multiple levels. Interestingly, HIF-1β depletion in TNF-α- or LIGHT-induced cells triggered the apoptotic pathway, suggesting that HIF-1β is critical for cell survival. Also, HIF-1β knock-down dramatically reduced p100 levels, through repression of TRAF6, in basal conditions and upon inflammatory stimulations. Importantly, our findings depict TRAF6, a known regulator of the NF-kB activity, as a novel direct target of HIF-1β, establishing a further level of crosstalk between HIF and NF-kB.
Finally, it was demonstrated that the ability of HIF-1β to control the immune response is importantly conserved among species. In Drosophila melanogaster, HIF-1β controls the longevity of flies in unstimulated conditions, as well as it regulates their susceptibility to bacterial infections and exposure to hypoxic stress.
This work uncovered novel mechanisms by which HIF-1β controls the inflammatory response, suggesting a relevance for situations often characterized by hypoxia and inflammation, such as tumours or auto-immune disorders. Although pharmacological interventions modulating HIF or NF-kB in such pathological scenarios are already available, targeting specifically the pro-inflammatory HIF-1β/HIF-2α dimers might lead to the development of novel targeted therapies against the activation of specific NF-kB dimers.
|Date of Award||2018|
|Supervisor||Sonia Rocha (Supervisor) & Ronald Hay (Supervisor)|