KRIT1 loss-of-function induces a chronic Nrf2-mediated adaptive homeostasis that sensitizes cells to oxidative stress: Implication for Cerebral Cavernous Malformation disease

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Abstract

KRIT1 (CCM1) is a disease gene responsible for Cerebral Cavernous Malformations (CCM), a major cerebrovascular disease of proven genetic origin affecting 0.3–0.5% of the population.

Previously, we demonstrated that KRIT1 loss-of-function is associated with altered redox homeostasis and abnormal activation of the redox-sensitive transcription factor c-Jun, which collectively result in pro-oxidative, pro-inflammatory and pro-angiogenic effects, suggesting a novel pathogenic mechanism for CCM disease and raising the possibility that KRIT1 loss-of-function exerts pleiotropic effects on multiple redox-sensitive mechanisms.

To address this possibility, we investigated major redox-sensitive pathways and enzymatic systems that play critical roles in fundamental cytoprotective mechanisms of adaptive responses to oxidative stress, including the master Nrf2 antioxidant defense pathway and its downstream target Glyoxalase 1 (Glo1), a pivotal stress-responsive defense enzyme involved in cellular protection against glycative and oxidative stress through the metabolism of methylglyoxal (MG). This is a potent post-translational protein modifier that may either contribute to increased oxidative molecular damage and cellular susceptibility to apoptosis, or enhance the activity of major apoptosis-protective proteins, including heat shock proteins (Hsps), promoting cell survival.

Experimental outcomes showed that KRIT1 loss-of-function induces a redox-sensitive sustained upregulation of Nrf2 and Glo1, and a drop in intracellular levels of MG-modified Hsp70 and Hsp27 proteins, leading to a chronic adaptive redox homeostasis that counteracts intrinsic oxidative stress but increases susceptibility to oxidative DNA damage and apoptosis, sensitizing cells to further oxidative challenges. While supporting and extending the pleiotropic functions of KRIT1, these findings shed new light on the mechanistic relationship between KRIT1 loss-of-function and enhanced cell predisposition to oxidative damage, thus providing valuable new insights into CCM pathogenesis and novel options for the development of preventive and therapeutic strategies.

Graphical abstract

Schematic models representing adaptive redox responses associated with KRIT1 loss-of-function.

Graphical abstract

KRIT1 loss-of-function causes a persistent activation of the redox-sensitive transcription factors c-Jun and Nrf2 and consequent upregulation of downstream targets, including cycloxygenase-2 (COX-2), heme oxygenase-1 (HO-1) and glyoxalase 1 (GLO1). While the c-Jun/COX-2 axis promotes pro-oxidant and pro-inflammatory effects, the Nrf2/HO-1 and Nrf2/GLO1 pathways mediate adaptive antioxidant responses that counteract these effects by limiting ROS* and MG intracellular accumulation, thus contributing to reduce a vicious cycle of oxidative stress and providing an adaptive defense for long term cell survival. However, this sustained adaptive redox homeostasis occurs at the expense of other cytoprotective mechanisms, including the MG-dependent formation of cytoprotective AP-Hsp70 and AP-Hsp27 protein adducts, leading to enhanced cell susceptibility to oxidative DNA damage and apoptosis, and sensitizing cells to additional stressful insults. Inter-individual differences in Nrf2-mediated adaptive defense mechanisms might influence susceptibility to CCM disease onset and progression. *The generic ROS term refers to O2•−and H2O2as well as to putative secondary oxidative products that might be implicated without certainty.

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