Hypoxia Inducible Factor (HIF)‐1α is a Transcriptional Mediator for Thiamine Insufficiency Induced Neurotoxicity

Abstract

Age-dependent decline in vitamin B1 (thiamine) levels has a devastating impact on the brain parenchyma. The neurotoxicity promoted by chronic thiamine insufficiency (TI) is a well-established comorbidity of Alzheimer's Disease (AD), the most prevalent age-related metabolic neurodegenerative disease. Inadequate levels of blood thiamine and derivates highly correlate with senile dementia prognosis as predictive peripheral AD biomarker. Impaired activity of thiamine-dependent enzymes due to TI, dramatically decreased cerebral oxidative metabolism promoting oxidative stress, mitochondrial damage, inflammation, and eventually neuronal loss. Furthermore, age-related decline in TI potentiates the AD neuropathology triggering the amyloid plaque formation and hyperphosphorylation of tau. We previously showed that TI stabilizes Hypoxia Inducible Factor -1α (HIF-1α), the main transcription factor involved in hypoxic stress, leading to HIF-1α induced pro-apoptotic and pro-inflammatory response. Among the transcriptionally activated HIF-1α target genes, TI triggered the expression of the pro-apoptotic protein BCL2/adenovirus E1B 19 kDa protein-interacting protein-3 (BNIP-3). Induction of BNIP-3 leads to its mitochondria migration where eventually triggers cell death via loss of membrane potential and increase in reactive oxygen species. To date, despite the well-established correlation between TI and neuropathogenesis, the molecular mechanism underlying the TI mediated neurotoxicity is still unknown. Therefore, we hypothesize that HIF-1α may be a critical upstream mediator between thiamine insufficiency and neurotoxicity. In this study, hippocampal murine cells (HT22) treated in TI conditions exhibited a significant increase in protein and mRNA levels of HIF-1α. Increase in BNIP-3 and other HIF-1α target genes (LDHA, VEGF, BACE-1, MCP-1) implicated in pro-inflammatory and pro-apoptotic response was also detected. Treatment with 50 μM and 100 μM of the thiamine antagonist Pyrithiamine (PT) up to 5 days significantly decrease HT22 viability to 30% and 20%, respectively. HIF-1α silencing via shRNA attenuated the neurotoxic response improving viability up to 60% with the same PT doses. In order to better characterize the HIF-1α response, BNIP-3 was also silenced via shRNA. In congruency with HIF-1α shRNA result, we found a 2-fold decrease in PT-induced toxicity in BNIP-3 shRNA transduced cells. Localization of BNIP-3 in mitochondria and loss of mitochondrial potential after PT treatment further highlighted the involvement of the HIF-1α - BNIP-3 response in TI induced neuronal death. Overall, these findings suggest a central role for HIF-1α as upstream regulator in TI mediated neurotoxicity.