Regulation of hypoxia-inducible factor 1 and the loss of the cellular response to hypoxia in diabetes

Diabetes is frequently associated with hypoxia and is known to impair ischaemia-induced neovascularisation and other forms of adaptive cell and tissue responses to low oxygen levels. Hyperglycaemia appears to be the driving force of such deregulation. Recent data indicate that destabilisation of hypoxia-inducible factor 1 (HIF-1) is most likely the event that transduces hyperglycaemia into the loss of the cellular response to hypoxia in most diabetic complications. HIF-1 is a critical transcription factor involved in oxygen homeostasis that regulates a variety of adaptive responses to hypoxia, including angiogenesis, metabolic reprogramming and survival. Thus, destabilisation of HIF-1 is likely to have a negative impact on cell and tissue adaptation to low oxygen. Indeed, destabilisation of HIF-1 by high glucose levels has serious consequences in various organs and tissues, including myocardial collateralisation, wound healing, renal, neural and retinal function, as a result of poor cell and tissue responses to low oxygen. This review aims to integrate and summarise some of the most recent developments, including new proposed molecular models, on this research topic, particularly in terms of their implications for potential therapeutic approaches for the prevention or treatment of some of the diabetic complications characterised by impaired cellular and tissue responses to hypoxia.

Regulation of hypoxia-inducible factor 1 and the loss of the cellular response to hypoxia in diabetes

Diabetes is frequently associated with hypoxia and is known to impair ischaemia-induced neovascularisation and other forms of adaptive cell and tissue responses to low oxygen levels. Hyperglycaemia appears to be the driving force of such deregulation. Recent data indicate that destabilisation of hypoxia-inducible factor 1 (HIF-1) is most likely the event that transduces hyperglycaemia into the loss of the cellular response to hypoxia in most diabetic complications. HIF-1 is a critical transcription factor involved in oxygen homeostasis that regulates a variety of adaptive responses to hypoxia, including angiogenesis, metabolic reprogramming and survival. Thus, destabilisation of HIF-1 is likely to have a negative impact on cell and tissue adaptation to low oxygen. Indeed, destabilisation of HIF-1 by high glucose levels has serious consequences in various organs and tissues, including myocardial collateralisation, wound healing, renal, neural and retinal function, as a result of poor cell and tissue responses to low oxygen. This review aims to integrate and summarise some of the most recent developments, including new proposed molecular models, on this research topic, particularly in terms of their implications for potential therapeutic approaches for the prevention or treatment of some of the diabetic complications characterised by impaired cellular and tissue responses to hypoxia.

Methylglyoxal-induced imbalance in the ratio of vascular endothelial growth factor to angiopoietin 2 secreted by retinal pigment epithelial cells leads to endothelial dysfunction

Progressive microvascular complications are a main feature of diabetes and are associated with impairment of the angiogenic response. Methylglyoxal (MGO) has been implicated in the molecular events that lead to endothelial dysfunction in diabetes. In this study, we hypothesize that increased levels of MGO disrupt the ratio of vascular endothelial growth factor (VEGF) to angiopoietin 2 (Ang 2) secreted by retinal pigment epithelial (RPE) cells, which provides a key destabilizing signal that leads to apoptosis and decreased proliferation of retinal endothelial cells. Indeed, we show that MGO increases the levels of Ang 2 and dramatically decreases the levels of VEGF secreted by RPE cells in response to hypoxia. Downregulation of VEGF is likely to be related to decreased hypoxia-inducible factor-1alpha (HIF-1alpha) protein levels and HIF-1 transcriptional activity. Data further show that MGO-induced imbalance in the VEGF/Ang II ratio significantly changes the levels of BAX and Bcl-2 in endothelial cells. Moreover, this imbalance is accompanied by an increase in the activity of caspase-3 and decreased proliferation of endothelial cells. Data obtained in cell culture systems are consistent with observations in retinas of diabetic animals, where increased availability of MGO is associated with changes in distribution and levels of HIF-1alpha, VEGF and Ang 2 and increased microvascular permeability. In conclusion, the MGO-induced imbalance in the VEGF/Ang 2 ratio secreted by retinal epithelial cells activates apoptosis and decreases proliferation of retinal endothelial cells, which are likely to contribute to endothelial dysfunction in diabetic retinopathy.