Recent progress

Our research in the past has shown that HIFs respond to growth and coagulation factors, hormones and cytokines under normoxic conditions by using ROS as mediators. Thereby, different cellular compartments and therein localized enzymes such as NOX4, an ER-based member of the NADPH oxidases, and a ROS-generating Fenton reaction at the ER can modulate HIF-1α levels. At the same time hypoxia and ROS can also activate the HIF-1α promoter by involving phosphatidylinositol 3-kinase and a functional NFkappaB site in the hif-1α gene promoter. Further, we found that the ROS inducer arsenite (As(III) could increase HIF-1α protein levels under normoxia while the hypoxia-mediated induction of HIF1α was reduced. Thereby, the As(III) effects on HIF-1α were dependent on both, transcriptional regulation via the transcription factor Nrf2 mediated by NOX4, PI3K/Akt, and ERK1/2 as well as by modulation of HIF-1α protein stability.

Moreover, we were the first to discover the molecular mechanisms by which hypoxia activates transcription of plasminogen activator inhibitor-1 (PAI-1), which is a breast cancer marker as well as being associated with obesity. Resveratrol, a constituent of grapes and berries, was proposed to improve obesity-related health problems, and by using SGBS adipocytes and a model of human adipose tissue inflammation we showed that treatment of SGBS adipocytes with resveratrol reduced PAI-1 gene expression. Although signaling via PI3K, Sirt1, AMPK, ROS and Nrf2 appeared to play a significant role in the modulation of PAI-1 gene expression under non-inflammatory conditions, these signaling components were not involved in mediating the effects of resveratrol on PAI-1 production under inflammatory conditions. Instead, we demonstrated that the effects of resveratrol on PAI-1 induction under inflammatory conditions were mediated via inhibition of the NFkB pathway. Thus, resveratrol can act as an NFkB inhibitor in adipocytes and the subsequently reduced PAI-1 expression in inflamed adipose tissue might provide new insight as regards novel options in the treatment of obesity.

In addition to the established finding that PAI-1 serves as a breast cancer marker indicating a bad prognosis, we found that the epidermal growth factor (EGF) receptor Her1 adaptor protein CIN85 is expressed at very high levels in samples from patients with invasive breast adenocarcinomas. Recently we were able to identify a direct molecular link between EGFR and the hypoxia signaling system where the oxygen sensor hypoxia-inducible factor prolyl hydroxylase 2 (PHD2) is considered to be the main HIF-1α regulator. By analyzing samples from 313 breast cancer patients, we found that EGFR is a first clinicopathological parameter positively correlating with PHD2. Mechanistically, we identified PHD2 as a direct binding partner of EGFR and show that PHD2 regulates EGFR stability as well as its subsequent signaling in breast carcinoma cells.

We also found that HIF-1 signaling is important for the insulin-(protein kinase B/Akt)-dependent induction of PAI-1. In line with insulin-dependent HIF α-subunit regulation, we showed that the PKB/Akt target GSK-3 initiates VHL-independent HIF-1α degradation. In line with these observations, we identified a new pathway which can degrade GSK-3 phosphorylated HIF-1α via recruitment of the ubiquitin ligase and tumour suppressor F-box and WD protein Fbw7. This process could be antagonized by ubiquitin-specific protease-28 (USP28).

Last updated: 19.1.2017