Putative Modulation of OATP1A2 and P-gp Expression at the Blood–Brain Barrier by Nrf2–PXR: An Associative Hypothesis for Amyloid-β Transport in Alzheimer’s Disease

Impaired amyloid-β (Aβ) clearance across the blood–brain barrier (BBB) is a major contributor to Aβ accumulation in Alzheimer’s disease (AD). P-glycoprotein (P-gp) has been identified as a key BBB efflux transporter involved in Aβ clearance, whereas emerging evidence suggests that organic anion transporting polypeptide 1A2 (OATP1A2) and its rodent counterparts, such as Oatp1a4, may participate in the influx component of Aβ transport. Nuclear factor erythroid 2–related factor 2 (Nrf2) and pregnane X receptor (PXR) are important transcriptional regulators of oxidative stress responses, xenobiotic metabolism, and transporter expression and may therefore modulate OATP1A2 and P-gp expression at the BBB. However, the mechanisms by which Nrf2–PXR crosstalk may regulate OATP1A2/P-gp expression in BBB endothelial cells under AD-relevant pathological conditions, as well as the consequences of this regulation for Aβ transport homeostasis, remain incompletely understood. This review summarizes current evidence linking P-gp, OATP1A2/Oatp1a4, Nrf2, and PXR to BBB transporter homeostasis in AD. A “net-effect” model is further proposed, in which Nrf2–PXR crosstalk may shift the BBB transporter balance toward enhanced P-gp-mediated efflux and reduced OATP1A2-associated influx. Because several key components of this model, particularly OATP1A2-mediated Aβ influx and BBB-specific Nrf2–PXR regulation, remain insufficiently validated, this model should be regarded as a mechanistic framework for future experimental investigation rather than as an established pathogenic pathway. Clarifying this regulatory axis may provide new insights into BBB dysfunction and impaired Aβ clearance in AD and may help identify potential molecular targets for therapeutic intervention.