Retinol Binding Protein 4 Levels Are Not Altered in Preclinical Alzheimer's Disease and Not Associated with Cognitive Decline or Incident Dementia

Accumulating evidence suggests that disparate pathways from systemic metabolism to retinoic acid/vitamin A signaling can contribute to Alzheimer's disease (AD) pathobiology. Retinol binding protein 4 (RBP4) is an adipocyte-secreted hormone (adipokine) that regulates insulin signaling and is also a key transporter of retinoic acid and its derivatives. While earlier studies found alterations in the brain and cerebrospinal fluid (CSF) levels of RBP4 in later stages of AD, it is not known if circulating RBP4 is altered in preclinical AD or if it can be a useful biomarker for cognitive decline and dementia. In this study, we used ELISA to measure plasma RBP4 levels in cognitively normal individuals (Clinical Dementia Rating, CDR 0). Subjects with preclinical AD were identified by previously established CSF criteria (preclinical AD: 20 men, 18 women; control: 45 men, 73 women). Plasma RBP4 levels were similar between preclinical AD and control subjects in men (preclinical AD: 30.0 +/- 7.4 mu g/mL; control: 30.0 +/- 8.7 mu g/mL; p=0.97) and women (preclinical AD 30.9 +/- 7.9 mu g/mL; control: 31.7 +/- 8.5 mu g/mL; p=0.72). Additionally, RBP4 levels were not related to body mass index or CSF AD biomarkers levels of amyloid-beta(42), tau, or phosphorylated tau. Baseline plasma RBP4 levels were not associated with the incidence of CDR >= 0.5, all-cause dementia, or AD diagnosis. Collectively, these results do not support peripheral RBP4 as a clinical biomarker or therapeutic target in the early stages of AD.