PIN1’s Crucial Role in NRF2 Expression and Activation
Nuclear factor erythroid 2-related factor 2 (NRF2) is a transcription factor that plays a crucial role in cellular defense mechanisms, particularly in response to oxidative stress, by transactivating a large number of genes associated with the stress response, including antioxidant, anti-inflammatory, and detoxification proteins [209]. NRF2 activation leads to increased synthesis of the potent antioxidant glutathione. The activation of NRF2 typically involves its translocation to the nucleus, where it binds to antioxidant response elements (AREs) in the promoters of target genes, initiating their transcription [210].
In a review study published in 2021, collectively involving 87 studies with 4928 children with autism and 4181 controls, it was reported that the levels of reduced glutathione, total glutathione, methionine, cysteine, folate, vitamin D and cobalamin were consistently significantly reduced in children with ASD compared to the controls [211]. In a post-mortem study, glutathione synthase expression was significantly decreased in frontal cortex brain tissue from seven autism subjects compared to 8 controls. NRF2 gene expression was also decreased in the frontal cortex, and this downregulation corresponded to a decrease in the abundance of methylcobalamin and total cobalamin, as well as S-adenosylmethionine (SAMe) [212].
Kelch-like ECH-associated protein 1 (KEAP1) is a ubiquitin ligase that senses oxidative stress and tightly regulates the activity of NRF2. Under normoxic conditions, NRF2 is bound to KEAP1, and this binding keeps it in the cytoplasm and prevents it from entering the nucleus. Also, intracellular levels of NRF2 are kept low because proteasomal degradation (ubiquitination) of NRF2 persists when it is tied up in a KEAP1/NRF2 complex [213]. However, under elevated intracellular levels of ROS, the cysteine residues of KEAP1 sensor domains become oxidized, and this triggers conformational alterations of the KEAP1 tertiary structure, which results in the release of NRF2 from the protein complex. This renders NRF2 free to enter the nucleus and proceed with its antioxidant and cytoprotective activities by inducing the expression of the NRF2-ARE related gene targets [214].
Paradoxically, acute oxidative stress can suppress NRF2 protein synthesis, through the global inhibition of protein synthesis [215]. Supraphysiologic levels of stress can even induce misfolding of both KEAP1 and NRF2, a maladaptive event that impairs protein function. Treatment of both yeast and mammalian cells with hydrogen peroxide results in the formation of misfolded protein inclusions, dose dependently [214].
PIN1 stabilizes NRF2 by competing with KEAP1 for NRF2 binding [216]. Furthermore, the common heat shock protein HSP90α is a molecular chaperone that is essential for transport of NRF2 to the nucleus, but this also critically depends on PIN1. Prolyl isomerization of NRF2 by PIN1 allows importin α5 to associate with the Hsp90α-PIN1-NRF2 complex. A dynein motor system transports this complex along microtubules toward the nuclear pore complex, achieving the import of NRF2 into the nucleus, as illustrated in Figure 2 [217]. The BTBR (Black and Tan BRachyury) mouse strain is a commonly used mouse model of autism [218]. Sulforaphane is a natural molecule derived from cruciferous vegetables, and it has been shown to activate NRF2 [219]. BTBR mice treated with sulforaphane had reduced repetitive behaviors and improved socialization. Glutathione peroxidase and glutathione reductase activity were increased in both the periphery and the brain of these mice. Oxidative stress parameters such as the NF-κB transcription factor and lipid peroxides were reduced in their neutrophils [220]. This implies that impaired NRF2 transactivation due to insufficient PIN1 is a feature of autism.