Stroke-associated carotid atherosclerosis : a role for FABP4 in atheroma vulnerability and cardiometabolic risk

Carotid artery disease is a common cause of stroke, with approximately one in five ischemic strokes in the carotid artery distribution stemming from thromboembolisms caused by an unstable atherosclerotic carotid plaque (CP). Despite the fact that carotid atherosclerosis can lead to cerebrovascular complications, some patients afflicted by a high-grade carotid stenosis (>70%) remain asymptomatic. This divergence in clinical courses captures one of the most pressing questions in cardiovascular medicine: what are the key molecules and pathways decisive to local atheroma vulnerability and connected with disabling clinical outcomes? In the first paper, we utilized a genome-wide approach to screen for gene expression changes in stroke-associated CPs to identify genes and pathways related to atheroma vulnerability and symptom generation. This approach was successful in identifying several significant expression changes in novel genes and in genes already connected with atherosclerosis development, suggesting that the local atheroma-derived gene expression patterns are closely connected to the patient s clinical phenotype. A detailed data-driven approach, using three independent data pre-processing methods, identified fatty acid-binding protein 4 (FABP4) as the lead hit overexpressed in stroke-associated CPs, thus linking FABP4 and its transcriptional activity to atheroma vulnerability. Immunohistochemical analysis revealed the most prominent expression of FABP4 protein in the histological regions of atheroma vulnerability, coinciding with the abundance of lipid-laden macrophages and thus connecting the overexpression of FABP4 with lipid accumulation and inflammation within the CPs. In the second paper, we focused on the molecular mechanisms through which increased FABP4 expression could regulate atheroma vulnerability. As causality is difficult to infer from associative expression data we adapted a genetic approach searching for variants that could regulate the expression of FABP4. Using a naturally occurring low-expression variant (rs77878271) we were able to link genetically reduced atheroma expression of FABP4 to endoplasmic reticulum (ER) stress signaling, and the attenuation of apoptosis, with high expression contributing to increased lipid burden and inflammation. We discovered that the low-expression variant of FABP4 was associated with lower circulating total cholesterol levels, with the lowest levels in obese (body mass index, BMI ≥ 30) minor allele homozygotes. Furthermore, the variant allele carriers showed obesity-related reduction in subclinical markers of atherosclerosis, manifested as reduced carotid intima-media thickness (IMT) and lower prevalence of CPs. Most importantly, we found that the local atheroma effects of FABP4 penetrated to end point level, as the minor allele homozygotes showed 8-fold lower odds for myocardial infarction (MI) and enrichment of the variant in patients with clinically asymptomatic carotid artery disease. Obesity-related reduction of total cholesterol levels observed in the low-expression variant carriers pointed to a metabolic component mediated by genetically reduced FABP4 expression. Based on the discoveries made in FABP4-deficient mouse models, we hypothesized that genetically reduced FABP4 expression could lead to sustained enhancement of adipocentric de novo lipogenesis (DNL), which could in part explain the obesity-related phenotypes also in man. We found that the low-expression variant of FABP4 was associated with circulating markers of enhanced lipolysis and reduced circulating FABP4 protein levels. The variant allele carriers showed a propensity for higher BMI, with evidence for enhancement of DNL, improved beta-cell function, and reduced obesity-related type 2 diabetes (T2D) risk. These results imply that FABP4-regulated production of adipocyte-derived lipid signals during DNL in the low-expression variant carriers may in part lead to enhanced beta-cell function and reduced obesity-related T2D risk. In this thesis work, we identified FABP4 as a key molecule at the crossroads of metabolic and inflammatory responses, potentially involved in local atheroma vulnerability and connected with cardiometabolic outcomes. Most likely the role of FABP4 in atherosclerosis is multifaceted, converging with the passage of time within the artery atheroma, and culminating in the regulation of inflammatory responses affecting cell survival, acute plaque vulnerability and ultimately cardiovascular risk. These data implicate FABP4 as a potential pharmacological target for prevention of cardiometabolic complications connected with atherosclerosis and obesity.